Moved BlockSegmentPostings to a different file.

CHANGELOG.md

@@ -3,20 +3,6 @@ Tantivy 0.13.0
 - Bugfix in `FuzzyTermQuery` not matching terms by prefix when it should (@Peachball)
 - Relaxed constraints on the custom/tweak score functions. At the segment level, they can be mut, and they are not required to be Sync + Send.
 - `MMapDirectory::open` does not return a `Result` anymore.
-- Change in the DocSet and Scorer API. (@fulmicoton). 
-A freshly created DocSet point directly to their first doc. A sentinel value called TERMINATED marks the end of a DocSet.
-`.advance()` returns the new DocId. `Scorer::skip(target)` has been replaced by `Scorer::seek(target)` and returns the resulting DocId.
-As a result, iterating through DocSet now looks as follows
-```rust
-let mut doc = docset.doc();
-while doc != TERMINATED {
-   // ...
-   doc = docset.advance();
-}
-```
-The change made it possible to greatly simplify a lot of the docset's code.
-- Misc internal optimization and introduction of the `Scorer::for_each_pruning` function. (@fulmicoton)
-- Added an offset option to the Top(.*)Collectors. (@robyoung)
 
 Tantivy 0.12.0
 ======================

Cargo.toml

@@ -50,6 +50,7 @@ murmurhash32 = "0.2"
 chrono = "0.4"
 smallvec = "1.0"
 rayon = "1"
+# ordered-float = "1"
 
 [target.'cfg(windows)'.dependencies]
 winapi = "0.3"
@@ -58,6 +59,8 @@ winapi = "0.3"
 rand = "0.7"
 maplit = "1"
 matches = "0.1.8"
+proptest = "0.9"
+float-cmp = "0.6"
 
 [dev-dependencies.fail]
 version = "0.4"

examples/iterating_docs_and_positions.rs

@@ -10,7 +10,7 @@
 // ---
 // Importing tantivy...
 use tantivy::schema::*;
-use tantivy::{doc, DocSet, Index, Postings, TERMINATED};
+use tantivy::{doc, DocId, DocSet, Index, Postings};
 
 fn main() -> tantivy::Result<()> {
     // We first create a schema for the sake of the
@@ -62,11 +62,12 @@ fn main() -> tantivy::Result<()> {
         {
             // this buffer will be used to request for positions
             let mut positions: Vec<u32> = Vec::with_capacity(100);
-            let mut doc_id = segment_postings.doc();
-            while doc_id != TERMINATED {
+            while segment_postings.advance() {
+                // the number of time the term appears in the document.
+                let doc_id: DocId = segment_postings.doc(); //< do not try to access this before calling advance once.
+
                 // This MAY contains deleted documents as well.
                 if segment_reader.is_deleted(doc_id) {
-                    doc_id = segment_postings.advance();
                     continue;
                 }
 
@@ -85,7 +86,6 @@ fn main() -> tantivy::Result<()> {
                 // Doc 2: TermFreq 1: [0]
                 // ```
                 println!("Doc {}: TermFreq {}: {:?}", doc_id, term_freq, positions);
-                doc_id = segment_postings.advance();
             }
         }
     }

src/collector/custom_score_top_collector.rs

@@ -11,13 +11,13 @@ impl<TCustomScorer, TScore> CustomScoreTopCollector<TCustomScorer, TScore>
 where
     TScore: Clone + PartialOrd,
 {
-    pub(crate) fn new(
+    pub fn new(
         custom_scorer: TCustomScorer,
-        collector: TopCollector<TScore>,
+        limit: usize,
     ) -> CustomScoreTopCollector<TCustomScorer, TScore> {
         CustomScoreTopCollector {
             custom_scorer,
-            collector,
+            collector: TopCollector::with_limit(limit),
         }
     }
 }

src/collector/facet_collector.rs

@@ -1,5 +1,6 @@
 use crate::collector::Collector;
 use crate::collector::SegmentCollector;
+use crate::docset::SkipResult;
 use crate::fastfield::FacetReader;
 use crate::schema::Facet;
 use crate::schema::Field;
@@ -187,11 +188,6 @@ pub struct FacetSegmentCollector {
     collapse_facet_ords: Vec<u64>,
 }
 
-enum SkipResult {
-    Found,
-    NotFound,
-}
-
 fn skip<'a, I: Iterator<Item = &'a Facet>>(
     target: &[u8],
     collapse_it: &mut Peekable<I>,
@@ -201,14 +197,14 @@ fn skip<'a, I: Iterator<Item = &'a Facet>>(
             Some(facet_bytes) => match facet_bytes.encoded_str().as_bytes().cmp(target) {
                 Ordering::Less => {}
                 Ordering::Greater => {
-                    return SkipResult::NotFound;
+                    return SkipResult::OverStep;
                 }
                 Ordering::Equal => {
-                    return SkipResult::Found;
+                    return SkipResult::Reached;
                 }
             },
             None => {
-                return SkipResult::NotFound;
+                return SkipResult::End;
             }
         }
         collapse_it.next();
@@ -285,7 +281,7 @@ impl Collector for FacetCollector {
                     // is positionned on a term that has not been processed yet.
                     let skip_result = skip(facet_streamer.key(), &mut collapse_facet_it);
                     match skip_result {
-                        SkipResult::Found => {
+                        SkipResult::Reached => {
                             // we reach a facet we decided to collapse.
                             let collapse_depth = facet_depth(facet_streamer.key());
                             let mut collapsed_id = 0;
@@ -305,7 +301,7 @@ impl Collector for FacetCollector {
                             }
                             break;
                         }
-                        SkipResult::NotFound => {
+                        SkipResult::End | SkipResult::OverStep => {
                             collapse_mapping.push(0);
                             if !facet_streamer.advance() {
                                 break;

src/collector/mod.rs

@@ -84,7 +84,7 @@ See the `custom_collector` example.
 
 */
 
-use crate::DocId;
+use crate::{DocId, Searcher, Executor};
 use crate::Score;
 use crate::SegmentLocalId;
 use crate::SegmentReader;
@@ -100,6 +100,9 @@ mod top_collector;
 
 mod top_score_collector;
 pub use self::top_score_collector::TopDocs;
+#[cfg(test)]
+pub(crate) use self::top_score_collector::TopScoreSegmentCollector;
+
 
 mod custom_score_top_collector;
 pub use self::custom_score_top_collector::{CustomScorer, CustomSegmentScorer};
@@ -109,7 +112,9 @@ pub use self::tweak_score_top_collector::{ScoreSegmentTweaker, ScoreTweaker};
 
 mod facet_collector;
 pub use self::facet_collector::FacetCollector;
-use crate::query::Weight;
+use crate::fastfield::DeleteBitSet;
+use crate::query::{Scorer, Weight};
+use std::borrow::BorrowMut;
 
 /// `Fruit` is the type for the result of our collection.
 /// e.g. `usize` for the `Count` collector.
@@ -117,6 +122,8 @@ pub trait Fruit: Send + downcast_rs::Downcast {}
 
 impl<T> Fruit for T where T: Send + downcast_rs::Downcast {}
 
+
+
 /// Collectors are in charge of collecting and retaining relevant
 /// information from the document found and scored by the query.
 ///
@@ -156,27 +163,18 @@ pub trait Collector: Sync {
     /// into one fruit.
     fn merge_fruits(&self, segment_fruits: Vec<Self::Fruit>) -> crate::Result<Self::Fruit>;
 
-    /// Created a segment collector and
-    fn collect_segment(
-        &self,
-        weight: &dyn Weight,
-        segment_ord: u32,
-        reader: &SegmentReader,
-    ) -> crate::Result<<Self::Child as SegmentCollector>::Fruit> {
-        let mut segment_collector = self.for_segment(segment_ord as u32, reader)?;
-
-        if let Some(delete_bitset) = reader.delete_bitset() {
-            weight.for_each(reader, &mut |doc, score| {
-                if delete_bitset.is_alive(doc) {
-                    segment_collector.collect(doc, score);
-                }
-            })?;
-        } else {
-            weight.for_each(reader, &mut |doc, score| {
-                segment_collector.collect(doc, score);
-            })?;
-        }
-        Ok(segment_collector.harvest())
+    fn collect_weight(&self, searcher: &Searcher, weight: &dyn Weight, executor: &Executor) -> crate::Result<Self::Fruit> {
+        let segment_readers = searcher.segment_readers();
+        let fruits = executor.map(
+            |(segment_ord, segment_reader)| {
+                let mut scorer = weight.scorer(segment_reader, 1.0f32)?;
+                let segment_collector =
+                    self.for_segment(segment_ord as u32, segment_reader)?;
+                Ok(segment_collector.collect_scorer(scorer.borrow_mut(), segment_reader.delete_bitset()))
+            },
+            segment_readers.iter().enumerate(),
+        )?;
+        self.merge_fruits(fruits)
     }
 }
 
@@ -185,7 +183,7 @@ pub trait Collector: Sync {
 ///
 /// `.collect(doc, score)` will be called for every documents
 /// matching the query.
-pub trait SegmentCollector: 'static {
+pub trait SegmentCollector: 'static + Sized {
     /// `Fruit` is the type for the result of our collection.
     /// e.g. `usize` for the `Count` collector.
     type Fruit: Fruit;
@@ -195,6 +193,19 @@ pub trait SegmentCollector: 'static {
 
     /// Extract the fruit of the collection from the `SegmentCollector`.
     fn harvest(self) -> Self::Fruit;
+
+    fn collect_scorer(mut self, scorer: &mut dyn Scorer, delete_bitset: Option<&DeleteBitSet>) -> Self::Fruit {
+        if let Some(delete_bitset) = delete_bitset {
+            scorer.for_each(&mut |doc, score| {
+                if delete_bitset.is_alive(doc) {
+                    self.collect(doc, score);
+                }
+            });
+        } else {
+            scorer.for_each(&mut |doc, score| self.collect(doc, score));
+        }
+        self.harvest()
+    }
 }
 
 // -----------------------------------------------

src/collector/top_collector.rs

@@ -18,9 +18,9 @@ use std::collections::BinaryHeap;
 /// Two elements are equal if their feature is equal, and regardless of whether `doc`
 /// is equal. This should be perfectly fine for this usage, but let's make sure this
 /// struct is never public.
-pub(crate) struct ComparableDoc<T, D> {
-    pub feature: T,
-    pub doc: D,
+struct ComparableDoc<T, D> {
+    feature: T,
+    doc: D,
 }
 
 impl<T: PartialOrd, D: PartialOrd> PartialOrd for ComparableDoc<T, D> {
@@ -57,7 +57,6 @@ impl<T: PartialOrd, D: PartialOrd> Eq for ComparableDoc<T, D> {}
 
 pub(crate) struct TopCollector<T> {
     pub limit: usize,
-    pub offset: usize,
     _marker: PhantomData<T>,
 }
 
@@ -70,23 +69,15 @@ where
     /// # Panics
     /// The method panics if limit is 0
     pub fn with_limit(limit: usize) -> TopCollector<T> {
-        if limit < 1 {
-            panic!("Limit must be strictly greater than 0.");
-        }
-        Self {
+        assert!(limit > 0, "Limit must be strictly greater than 0.");
+        TopCollector {
             limit,
-            offset: 0,
             _marker: PhantomData,
         }
     }
 
-    /// Skip the first "offset" documents when collecting.
-    ///
-    /// This is equivalent to `OFFSET` in MySQL or PostgreSQL and `start` in
-    /// Lucene's TopDocsCollector.
-    pub fn and_offset(mut self, offset: usize) -> TopCollector<T> {
-        self.offset = offset;
-        self
+    pub fn limit(&self) -> usize {
+        self.limit
     }
 
     pub fn merge_fruits(
@@ -99,7 +90,7 @@ where
         let mut top_collector = BinaryHeap::new();
         for child_fruit in children {
             for (feature, doc) in child_fruit {
-                if top_collector.len() < (self.limit + self.offset) {
+                if top_collector.len() < self.limit {
                     top_collector.push(ComparableDoc { feature, doc });
                 } else if let Some(mut head) = top_collector.peek_mut() {
                     if head.feature < feature {
@@ -111,7 +102,6 @@ where
         Ok(top_collector
             .into_sorted_vec()
             .into_iter()
-            .skip(self.offset)
             .map(|cdoc| (cdoc.feature, cdoc.doc))
             .collect())
     }
@@ -121,23 +111,7 @@ where
         segment_id: SegmentLocalId,
         _: &SegmentReader,
     ) -> crate::Result<TopSegmentCollector<F>> {
-        Ok(TopSegmentCollector::new(
-            segment_id,
-            self.limit + self.offset,
-        ))
-    }
-
-    /// Create a new TopCollector with the same limit and offset.
-    ///
-    /// Ideally we would use Into but the blanket implementation seems to cause the Scorer traits
-    /// to fail.
-    #[doc(hidden)]
-    pub(crate) fn into_tscore<TScore: PartialOrd + Clone>(self) -> TopCollector<TScore> {
-        TopCollector {
-            limit: self.limit,
-            offset: self.offset,
-            _marker: PhantomData,
-        }
+        Ok(TopSegmentCollector::new(segment_id, self.limit))
     }
 }
 
@@ -148,13 +122,13 @@ where
 /// The theorical complexity for collecting the top `K` out of `n` documents
 /// is `O(n log K)`.
 pub(crate) struct TopSegmentCollector<T> {
-    limit: usize,
+    pub limit: usize,
     heap: BinaryHeap<ComparableDoc<T, DocId>>,
     segment_id: u32,
 }
 
 impl<T: PartialOrd> TopSegmentCollector<T> {
-    fn new(segment_id: SegmentLocalId, limit: usize) -> TopSegmentCollector<T> {
+    pub fn new(segment_id: SegmentLocalId, limit: usize) -> TopSegmentCollector<T> {
         TopSegmentCollector {
             limit,
             heap: BinaryHeap::with_capacity(limit),
@@ -185,6 +159,10 @@ impl<T: PartialOrd + Clone> TopSegmentCollector<T> {
         self.heap.len() >= self.limit
     }
 
+    pub fn pruning_score(&self) -> Option<T> {
+        self.heap.peek().map(|head| head.feature.clone())
+    }
+
     /// Collects a document scored by the given feature
     ///
     /// It collects documents until it has reached the max capacity. Once it reaches capacity, it
@@ -211,7 +189,7 @@ impl<T: PartialOrd + Clone> TopSegmentCollector<T> {
 
 #[cfg(test)]
 mod tests {
-    use super::{TopCollector, TopSegmentCollector};
+    use super::TopSegmentCollector;
     use crate::DocAddress;
 
     #[test]
@@ -272,48 +250,6 @@ mod tests {
             top_collector_limit_3.harvest()[..2].to_vec(),
         );
     }
-
-    #[test]
-    fn test_top_collector_with_limit_and_offset() {
-        let collector = TopCollector::with_limit(2).and_offset(1);
-
-        let results = collector
-            .merge_fruits(vec![vec![
-                (0.9, DocAddress(0, 1)),
-                (0.8, DocAddress(0, 2)),
-                (0.7, DocAddress(0, 3)),
-                (0.6, DocAddress(0, 4)),
-                (0.5, DocAddress(0, 5)),
-            ]])
-            .unwrap();
-
-        assert_eq!(
-            results,
-            vec![(0.8, DocAddress(0, 2)), (0.7, DocAddress(0, 3)),]
-        );
-    }
-
-    #[test]
-    fn test_top_collector_with_limit_larger_than_set_and_offset() {
-        let collector = TopCollector::with_limit(2).and_offset(1);
-
-        let results = collector
-            .merge_fruits(vec![vec![(0.9, DocAddress(0, 1)), (0.8, DocAddress(0, 2))]])
-            .unwrap();
-
-        assert_eq!(results, vec![(0.8, DocAddress(0, 2)),]);
-    }
-
-    #[test]
-    fn test_top_collector_with_limit_and_offset_larger_than_set() {
-        let collector = TopCollector::with_limit(2).and_offset(20);
-
-        let results = collector
-            .merge_fruits(vec![vec![(0.9, DocAddress(0, 1)), (0.8, DocAddress(0, 2))]])
-            .unwrap();
-
-        assert_eq!(results, vec![]);
-    }
 }
 
 #[cfg(all(test, feature = "unstable"))]

src/collector/top_score_collector.rs

@@ -1,21 +1,20 @@
 use super::Collector;
 use crate::collector::custom_score_top_collector::CustomScoreTopCollector;
+use crate::collector::top_collector::TopCollector;
 use crate::collector::top_collector::TopSegmentCollector;
-use crate::collector::top_collector::{ComparableDoc, TopCollector};
 use crate::collector::tweak_score_top_collector::TweakedScoreTopCollector;
 use crate::collector::{
     CustomScorer, CustomSegmentScorer, ScoreSegmentTweaker, ScoreTweaker, SegmentCollector,
 };
 use crate::fastfield::FastFieldReader;
-use crate::query::Weight;
 use crate::schema::Field;
-use crate::DocAddress;
+use crate::{DocAddress, Executor, Searcher};
 use crate::DocId;
 use crate::Score;
 use crate::SegmentLocalId;
 use crate::SegmentReader;
-use std::collections::BinaryHeap;
 use std::fmt;
+use crate::query::{Weight, PruningScorerIfPossible};
 
 /// The `TopDocs` collector keeps track of the top `K` documents
 /// sorted by their score.
@@ -59,11 +58,7 @@ pub struct TopDocs(TopCollector<Score>);
 
 impl fmt::Debug for TopDocs {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(
-            f,
-            "TopDocs(limit={}, offset={})",
-            self.0.limit, self.0.offset
-        )
+        write!(f, "TopDocs({})", self.0.limit())
     }
 }
 
@@ -107,45 +102,6 @@ impl TopDocs {
         TopDocs(TopCollector::with_limit(limit))
     }
 
-    /// Skip the first "offset" documents when collecting.
-    ///
-    /// This is equivalent to `OFFSET` in MySQL or PostgreSQL and `start` in
-    /// Lucene's TopDocsCollector.
-    ///
-    /// ```rust
-    /// use tantivy::collector::TopDocs;
-    /// use tantivy::query::QueryParser;
-    /// use tantivy::schema::{Schema, TEXT};
-    /// use tantivy::{doc, DocAddress, Index};
-    ///
-    /// let mut schema_builder = Schema::builder();
-    /// let title = schema_builder.add_text_field("title", TEXT);
-    /// let schema = schema_builder.build();
-    /// let index = Index::create_in_ram(schema);
-    ///
-    /// let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-    /// index_writer.add_document(doc!(title => "The Name of the Wind"));
-    /// index_writer.add_document(doc!(title => "The Diary of Muadib"));
-    /// index_writer.add_document(doc!(title => "A Dairy Cow"));
-    /// index_writer.add_document(doc!(title => "The Diary of a Young Girl"));
-    /// index_writer.add_document(doc!(title => "The Diary of Lena Mukhina"));
-    /// assert!(index_writer.commit().is_ok());
-    ///
-    /// let reader = index.reader().unwrap();
-    /// let searcher = reader.searcher();
-    ///
-    /// let query_parser = QueryParser::for_index(&index, vec![title]);
-    /// let query = query_parser.parse_query("diary").unwrap();
-    /// let top_docs = searcher.search(&query, &TopDocs::with_limit(2).and_offset(1)).unwrap();
-    ///
-    /// assert_eq!(top_docs.len(), 2);
-    /// assert_eq!(&top_docs[0], &(0.5204813, DocAddress(0, 4)));
-    /// assert_eq!(&top_docs[1], &(0.4793185, DocAddress(0, 3)));
-    /// ```
-    pub fn and_offset(self, offset: usize) -> TopDocs {
-        TopDocs(self.0.and_offset(offset))
-    }
-
     /// Set top-K to rank documents by a given fast field.
     ///
     /// ```rust
@@ -326,7 +282,7 @@ impl TopDocs {
         TScoreSegmentTweaker: ScoreSegmentTweaker<TScore> + 'static,
         TScoreTweaker: ScoreTweaker<TScore, Child = TScoreSegmentTweaker>,
     {
-        TweakedScoreTopCollector::new(score_tweaker, self.0.into_tscore())
+        TweakedScoreTopCollector::new(score_tweaker, self.0.limit())
     }
 
     /// Ranks the documents using a custom score.
@@ -440,7 +396,7 @@ impl TopDocs {
         TCustomSegmentScorer: CustomSegmentScorer<TScore> + 'static,
         TCustomScorer: CustomScorer<TScore, Child = TCustomSegmentScorer>,
     {
-        CustomScoreTopCollector::new(custom_score, self.0.into_tscore())
+        CustomScoreTopCollector::new(custom_score, self.0.limit())
     }
 }
 
@@ -462,80 +418,64 @@ impl Collector for TopDocs {
         true
     }
 
+    fn collect_weight(&self, searcher: &Searcher, weight: &dyn Weight, executor: &Executor) -> crate::Result<Self::Fruit> {
+        let segment_readers = searcher.segment_readers();
+        let fruits = executor.map(
+            |(segment_ord, segment_reader)| {
+                match weight.pruning_scorer(segment_reader, 1.0f32)? {
+                    PruningScorerIfPossible::NonPruning(mut scorer) => {
+                        let segment_collector =
+                            self.for_segment(segment_ord as u32, segment_reader)?;
+                        let fruit =
+                            segment_collector.collect_scorer(scorer.as_mut(), segment_reader.delete_bitset());
+                        Ok(fruit)
+                    }
+                    PruningScorerIfPossible::Pruning(mut pruning_scorer) => {
+                        let limit = self.0.limit;
+                        let mut segment_collector =
+                            self.for_segment(segment_ord as u32, segment_reader)?;
+                        for _ in 0..limit {
+                            if !pruning_scorer.advance() {
+                                return Ok(segment_collector.harvest());
+                            }
+                            segment_collector.collect(pruning_scorer.doc(), pruning_scorer.score());
+                        }
+                        let mut pruning_score = segment_collector.0.pruning_score().unwrap_or(0.0f32);
+                        while pruning_scorer.advance_with_pruning(pruning_score) {
+                            segment_collector.0.collect(pruning_scorer.doc(), pruning_scorer.score());
+                            pruning_score = segment_collector.0.pruning_score().unwrap_or(0.0f32);
+                        }
+                        Ok(segment_collector.harvest())
+                    }
+                }
+            },
+            segment_readers.iter().enumerate(),
+        )?;
+        self.merge_fruits(fruits)
+    }
+
     fn merge_fruits(
         &self,
         child_fruits: Vec<Vec<(Score, DocAddress)>>,
     ) -> crate::Result<Self::Fruit> {
         self.0.merge_fruits(child_fruits)
     }
-
-    fn collect_segment(
-        &self,
-        weight: &dyn Weight,
-        segment_ord: u32,
-        reader: &SegmentReader,
-    ) -> crate::Result<<Self::Child as SegmentCollector>::Fruit> {
-        let heap_len = self.0.limit + self.0.offset;
-        let mut heap: BinaryHeap<ComparableDoc<Score, DocId>> = BinaryHeap::with_capacity(heap_len);
-
-        if let Some(delete_bitset) = reader.delete_bitset() {
-            let mut threshold = f32::MIN;
-            weight.for_each_pruning(threshold, reader, &mut |doc, score| {
-                if delete_bitset.is_deleted(doc) {
-                    return threshold;
-                }
-                let heap_item = ComparableDoc {
-                    feature: score,
-                    doc,
-                };
-                if heap.len() < heap_len {
-                    heap.push(heap_item);
-                    if heap.len() == heap_len {
-                        threshold = heap.peek().map(|el| el.feature).unwrap_or(f32::MIN);
-                    }
-                    return threshold;
-                }
-                *heap.peek_mut().unwrap() = heap_item;
-                threshold = heap.peek().map(|el| el.feature).unwrap_or(std::f32::MIN);
-                threshold
-            })?;
-        } else {
-            weight.for_each_pruning(f32::MIN, reader, &mut |doc, score| {
-                let heap_item = ComparableDoc {
-                    feature: score,
-                    doc,
-                };
-                if heap.len() < heap_len {
-                    heap.push(heap_item);
-                    // TODO the threshold is suboptimal for heap.len == heap_len
-                    if heap.len() == heap_len {
-                        return heap.peek().map(|el| el.feature).unwrap_or(f32::MIN);
-                    } else {
-                        return f32::MIN;
-                    }
-                }
-                *heap.peek_mut().unwrap() = heap_item;
-                heap.peek().map(|el| el.feature).unwrap_or(std::f32::MIN)
-            })?;
-        }
-
-        let fruit = heap
-            .into_sorted_vec()
-            .into_iter()
-            .map(|cid| (cid.feature, DocAddress(segment_ord, cid.doc)))
-            .collect();
-        Ok(fruit)
-    }
 }
 
 /// Segment Collector associated to `TopDocs`.
 pub struct TopScoreSegmentCollector(TopSegmentCollector<Score>);
 
+impl TopScoreSegmentCollector {
+    pub fn new(segment_id: SegmentLocalId, limit: usize) -> Self {
+        TopScoreSegmentCollector(TopSegmentCollector::new(segment_id, limit))
+    }
+}
+
 impl SegmentCollector for TopScoreSegmentCollector {
     type Fruit = Vec<(Score, DocAddress)>;
 
     fn collect(&mut self, doc: DocId, score: Score) {
-        self.0.collect(doc, score);
+        self.0.collect(doc, score)
     }
 
     fn harvest(self) -> Vec<(Score, DocAddress)> {
@@ -549,10 +489,10 @@ mod tests {
     use crate::collector::Collector;
     use crate::query::{AllQuery, Query, QueryParser};
     use crate::schema::{Field, Schema, FAST, STORED, TEXT};
+    use crate::DocAddress;
     use crate::Index;
     use crate::IndexWriter;
     use crate::Score;
-    use crate::{DocAddress, DocId, SegmentReader};
 
     fn make_index() -> Index {
         let mut schema_builder = Schema::builder();
@@ -592,21 +532,6 @@ mod tests {
         );
     }
 
-    #[test]
-    fn test_top_collector_not_at_capacity_with_offset() {
-        let index = make_index();
-        let field = index.schema().get_field("text").unwrap();
-        let query_parser = QueryParser::for_index(&index, vec![field]);
-        let text_query = query_parser.parse_query("droopy tax").unwrap();
-        let score_docs: Vec<(Score, DocAddress)> = index
-            .reader()
-            .unwrap()
-            .searcher()
-            .search(&text_query, &TopDocs::with_limit(4).and_offset(2))
-            .unwrap();
-        assert_eq!(score_docs, vec![(0.48527452, DocAddress(0, 0))]);
-    }
-
     #[test]
     fn test_top_collector_at_capacity() {
         let index = make_index();
@@ -628,27 +553,6 @@ mod tests {
         );
     }
 
-    #[test]
-    fn test_top_collector_at_capacity_with_offset() {
-        let index = make_index();
-        let field = index.schema().get_field("text").unwrap();
-        let query_parser = QueryParser::for_index(&index, vec![field]);
-        let text_query = query_parser.parse_query("droopy tax").unwrap();
-        let score_docs: Vec<(Score, DocAddress)> = index
-            .reader()
-            .unwrap()
-            .searcher()
-            .search(&text_query, &TopDocs::with_limit(2).and_offset(1))
-            .unwrap();
-        assert_eq!(
-            score_docs,
-            vec![
-                (0.5376842, DocAddress(0u32, 2)),
-                (0.48527452, DocAddress(0, 0))
-            ]
-        );
-    }
-
     #[test]
     fn test_top_collector_stable_sorting() {
         let index = make_index();
@@ -762,50 +666,6 @@ mod tests {
         }
     }
 
-    #[test]
-    fn test_tweak_score_top_collector_with_offset() {
-        let index = make_index();
-        let field = index.schema().get_field("text").unwrap();
-        let query_parser = QueryParser::for_index(&index, vec![field]);
-        let text_query = query_parser.parse_query("droopy tax").unwrap();
-        let collector = TopDocs::with_limit(2).and_offset(1).tweak_score(
-            move |_segment_reader: &SegmentReader| move |doc: DocId, _original_score: Score| doc,
-        );
-        let score_docs: Vec<(u32, DocAddress)> = index
-            .reader()
-            .unwrap()
-            .searcher()
-            .search(&text_query, &collector)
-            .unwrap();
-
-        assert_eq!(
-            score_docs,
-            vec![(1, DocAddress(0, 1)), (0, DocAddress(0, 0)),]
-        );
-    }
-
-    #[test]
-    fn test_custom_score_top_collector_with_offset() {
-        let index = make_index();
-        let field = index.schema().get_field("text").unwrap();
-        let query_parser = QueryParser::for_index(&index, vec![field]);
-        let text_query = query_parser.parse_query("droopy tax").unwrap();
-        let collector = TopDocs::with_limit(2)
-            .and_offset(1)
-            .custom_score(move |_segment_reader: &SegmentReader| move |doc: DocId| doc);
-        let score_docs: Vec<(u32, DocAddress)> = index
-            .reader()
-            .unwrap()
-            .searcher()
-            .search(&text_query, &collector)
-            .unwrap();
-
-        assert_eq!(
-            score_docs,
-            vec![(1, DocAddress(0, 1)), (0, DocAddress(0, 0)),]
-        );
-    }
-
     fn index(
         query: &str,
         query_field: Field,

src/collector/tweak_score_top_collector.rs

@@ -14,11 +14,11 @@ where
 {
     pub fn new(
         score_tweaker: TScoreTweaker,
-        collector: TopCollector<TScore>,
+        limit: usize,
     ) -> TweakedScoreTopCollector<TScoreTweaker, TScore> {
         TweakedScoreTopCollector {
             score_tweaker,
-            collector,
+            collector: TopCollector::with_limit(limit),
         }
     }
 }

src/common/bitset.rs

@@ -33,10 +33,6 @@ impl TinySet {
         TinySet(0u64)
     }
 
-    pub fn clear(&mut self) {
-        self.0 = 0u64;
-    }
-
     /// Returns the complement of the set in `[0, 64[`.
     fn complement(self) -> TinySet {
         TinySet(!self.0)
@@ -47,11 +43,6 @@ impl TinySet {
         !self.intersect(TinySet::singleton(el)).is_empty()
     }
 
-    /// Returns the number of elements in the TinySet.
-    pub fn len(self) -> u32 {
-        self.0.count_ones()
-    }
-
     /// Returns the intersection of `self` and `other`
     pub fn intersect(self, other: TinySet) -> TinySet {
         TinySet(self.0 & other.0)
@@ -118,12 +109,22 @@ impl TinySet {
     pub fn range_greater_or_equal(from_included: u32) -> TinySet {
         TinySet::range_lower(from_included).complement()
     }
+
+    pub fn clear(&mut self) {
+        self.0 = 0u64;
+    }
+
+    pub fn len(self) -> u32 {
+        self.0.count_ones()
+    }
 }
 
 #[derive(Clone)]
 pub struct BitSet {
     tinysets: Box<[TinySet]>,
-    len: usize,
+    len: usize, //< Technically it should be u32, but we
+    // count multiple inserts.
+    // `usize` guards us from overflow.
     max_value: u32,
 }
 
@@ -203,7 +204,7 @@ mod tests {
 
     use super::BitSet;
     use super::TinySet;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::DocSet;
     use crate::query::BitSetDocSet;
     use crate::tests;
     use crate::tests::generate_nonunique_unsorted;
@@ -277,13 +278,11 @@ mod tests {
         }
         assert_eq!(btreeset.len(), bitset.len());
         let mut bitset_docset = BitSetDocSet::from(bitset);
-        let mut remaining = true;
         for el in btreeset.into_iter() {
-            assert!(remaining);
+            bitset_docset.advance();
             assert_eq!(bitset_docset.doc(), el);
-            remaining = bitset_docset.advance() != TERMINATED;
         }
-        assert!(!remaining);
+        assert!(!bitset_docset.advance());
     }
 
     #[test]

src/core/index.rs

@@ -24,10 +24,8 @@ use crate::IndexWriter;
 use std::borrow::BorrowMut;
 use std::collections::HashSet;
 use std::fmt;
-
 #[cfg(feature = "mmap")]
-use std::path::Path;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
 fn load_metas(

src/core/inverted_index_reader.rs

@@ -7,6 +7,7 @@ use crate::schema::FieldType;
 use crate::schema::IndexRecordOption;
 use crate::schema::Term;
 use crate::termdict::TermDictionary;
+use owned_read::OwnedRead;
 
 /// The inverted index reader is in charge of accessing
 /// the inverted index associated to a specific field.
@@ -96,7 +97,8 @@ impl InvertedIndexReader {
         let offset = term_info.postings_offset as usize;
         let end_source = self.postings_source.len();
         let postings_slice = self.postings_source.slice(offset, end_source);
-        block_postings.reset(term_info.doc_freq, postings_slice);
+        let postings_reader = OwnedRead::new(postings_slice);
+        block_postings.reset(term_info.doc_freq, postings_reader);
     }
 
     /// Returns a block postings given a `Term`.
@@ -125,7 +127,7 @@ impl InvertedIndexReader {
         let postings_data = self.postings_source.slice_from(offset);
         BlockSegmentPostings::from_data(
             term_info.doc_freq,
-            postings_data,
+            OwnedRead::new(postings_data),
             self.record_option,
             requested_option,
         )

src/core/searcher.rs

@@ -1,8 +1,10 @@
 use crate::collector::Collector;
+use crate::collector::SegmentCollector;
 use crate::core::Executor;
 use crate::core::InvertedIndexReader;
 use crate::core::SegmentReader;
 use crate::query::Query;
+use crate::query::Weight;
 use crate::schema::Document;
 use crate::schema::Schema;
 use crate::schema::{Field, Term};
@@ -14,6 +16,18 @@ use crate::Index;
 use std::fmt;
 use std::sync::Arc;
 
+fn collect_segment<C: Collector>(
+    collector: &C,
+    weight: &dyn Weight,
+    segment_ord: u32,
+    segment_reader: &SegmentReader,
+) -> crate::Result<C::Fruit> {
+    let mut scorer = weight.scorer(segment_reader, 1.0f32)?;
+    let segment_collector =
+        collector.for_segment(segment_ord as u32, segment_reader)?;
+    Ok(segment_collector.collect_scorer(&mut scorer, segment_reader.delete_bitset()))
+}
+
 /// Holds a list of `SegmentReader`s ready for search.
 ///
 /// It guarantees that the `Segment` will not be removed before
@@ -140,7 +154,12 @@ impl Searcher {
         let segment_readers = self.segment_readers();
         let fruits = executor.map(
             |(segment_ord, segment_reader)| {
-                collector.collect_segment(weight.as_ref(), segment_ord as u32, segment_reader)
+                collect_segment(
+                    collector,
+                    weight.as_ref(),
+                    segment_ord as u32,
+                    segment_reader,
+                )
             },
             segment_readers.iter().enumerate(),
         )?;

src/docset.rs

@@ -1,47 +1,58 @@
+use crate::common::BitSet;
 use crate::fastfield::DeleteBitSet;
 use crate::DocId;
 use std::borrow::Borrow;
 use std::borrow::BorrowMut;
+use std::cmp::Ordering;
 
-/// Sentinel value returned when a DocSet has been entirely consumed.
-///
-/// This is not u32::MAX as one would have expected, due to the lack of SSE2 instructions
-/// to compare [u32; 4].
-pub const TERMINATED: DocId = std::i32::MAX as u32;
+/// Expresses the outcome of a call to `DocSet`'s `.skip_next(...)`.
+#[derive(PartialEq, Eq, Debug)]
+pub enum SkipResult {
+    /// target was in the docset
+    Reached,
+    /// target was not in the docset, skipping stopped as a greater element was found
+    OverStep,
+    /// the docset was entirely consumed without finding the target, nor any
+    /// element greater than the target.
+    End,
+}
 
 /// Represents an iterable set of sorted doc ids.
 pub trait DocSet {
     /// Goes to the next element.
-    ///
-    /// The DocId of the next element is returned.
-    /// In other words we should always have :
-    /// ```ignore
-    /// let doc = docset.advance();
-    /// assert_eq!(doc, docset.doc());
-    /// ```
-    ///
-    /// If we reached the end of the DocSet, TERMINATED should be returned.
-    ///
-    /// Calling `.advance()` on a terminated DocSet should be supported, and TERMINATED should
-    /// be returned.
-    /// TODO Test existing docsets.
-    fn advance(&mut self) -> DocId;
+    /// `.advance(...)` needs to be called a first time to point to the correct
+    /// element.
+    fn advance(&mut self) -> bool;
 
-    /// Advances the DocSet forward until reaching the target, or going to the
-    /// lowest DocId greater than the target.
+    /// After skipping, position the iterator in such a way that `.doc()`
+    /// will return a value greater than or equal to target.
     ///
-    /// If the end of the DocSet is reached, TERMINATED is returned.
+    /// SkipResult expresses whether the `target value` was reached, overstepped,
+    /// or if the `DocSet` was entirely consumed without finding any value
+    /// greater or equal to the `target`.
     ///
-    /// Calling `.seek(target)` on a terminated DocSet is legal. Implementation
-    /// of DocSet should support it.
+    /// WARNING: Calling skip always advances the docset.
+    /// More specifically, if the docset is already positionned on the target
+    /// skipping will advance to the next position and return SkipResult::Overstep.
     ///
-    /// Calling `seek(TERMINATED)` is also legal and is the normal way to consume a DocSet.
-    fn seek(&mut self, target: DocId) -> DocId {
-        let mut doc = self.doc();
-        while doc < target {
-            doc = self.advance();
+    /// If `.skip_next()` oversteps, then the docset must be positionned correctly
+    /// on an existing document. In other words, `.doc()` should return the first document
+    /// greater than `DocId`.
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        if !self.advance() {
+            return SkipResult::End;
+        }
+        loop {
+            match self.doc().cmp(&target) {
+                Ordering::Less => {
+                    if !self.advance() {
+                        return SkipResult::End;
+                    }
+                }
+                Ordering::Equal => return SkipResult::Reached,
+                Ordering::Greater => return SkipResult::OverStep,
+            }
         }
-        doc
     }
 
     /// Fills a given mutable buffer with the next doc ids from the
@@ -60,38 +71,38 @@ pub trait DocSet {
     /// use case where batching. The normal way to
     /// go through the `DocId`'s is to call `.advance()`.
     fn fill_buffer(&mut self, buffer: &mut [DocId]) -> usize {
-        if self.doc() == TERMINATED {
-            return 0;
-        }
         for (i, buffer_val) in buffer.iter_mut().enumerate() {
-            *buffer_val = self.doc();
-            if self.advance() == TERMINATED {
-                return i + 1;
+            if self.advance() {
+                *buffer_val = self.doc();
+            } else {
+                return i;
             }
         }
         buffer.len()
     }
 
     /// Returns the current document
-    /// Right after creating a new DocSet, the docset points to the first document.
-    ///
-    /// If the DocSet is empty, .doc() should return `TERMINATED`.
     fn doc(&self) -> DocId;
 
     /// Returns a best-effort hint of the
     /// length of the docset.
     fn size_hint(&self) -> u32;
 
+    /// Appends all docs to a `bitset`.
+    fn append_to_bitset(&mut self, bitset: &mut BitSet) {
+        while self.advance() {
+            bitset.insert(self.doc());
+        }
+    }
+
     /// Returns the number documents matching.
     /// Calling this method consumes the `DocSet`.
     fn count(&mut self, delete_bitset: &DeleteBitSet) -> u32 {
         let mut count = 0u32;
-        let mut doc = self.doc();
-        while doc != TERMINATED {
-            if !delete_bitset.is_deleted(doc) {
+        while self.advance() {
+            if !delete_bitset.is_deleted(self.doc()) {
                 count += 1u32;
             }
-            doc = self.advance();
         }
         count
     }
@@ -103,42 +114,22 @@ pub trait DocSet {
     /// given by `count()`.
     fn count_including_deleted(&mut self) -> u32 {
         let mut count = 0u32;
-        let mut doc = self.doc();
-        while doc != TERMINATED {
+        while self.advance() {
             count += 1u32;
-            doc = self.advance();
         }
         count
     }
 }
 
-impl<'a> DocSet for &'a mut dyn DocSet {
-    fn advance(&mut self) -> u32 {
-        (**self).advance()
-    }
-
-    fn seek(&mut self, target: DocId) -> DocId {
-        (**self).seek(target)
-    }
-
-    fn doc(&self) -> u32 {
-        (**self).doc()
-    }
-
-    fn size_hint(&self) -> u32 {
-        (**self).size_hint()
-    }
-}
-
 impl<TDocSet: DocSet + ?Sized> DocSet for Box<TDocSet> {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         let unboxed: &mut TDocSet = self.borrow_mut();
         unboxed.advance()
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
         let unboxed: &mut TDocSet = self.borrow_mut();
-        unboxed.seek(target)
+        unboxed.skip_next(target)
     }
 
     fn doc(&self) -> DocId {
@@ -160,4 +151,9 @@ impl<TDocSet: DocSet + ?Sized> DocSet for Box<TDocSet> {
         let unboxed: &mut TDocSet = self.borrow_mut();
         unboxed.count_including_deleted()
     }
+
+    fn append_to_bitset(&mut self, bitset: &mut BitSet) {
+        let unboxed: &mut TDocSet = self.borrow_mut();
+        unboxed.append_to_bitset(bitset);
+    }
 }

src/indexer/index_writer.rs

@@ -10,7 +10,7 @@ use crate::core::SegmentMeta;
 use crate::core::SegmentReader;
 use crate::directory::TerminatingWrite;
 use crate::directory::{DirectoryLock, GarbageCollectionResult};
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::DocSet;
 use crate::error::TantivyError;
 use crate::fastfield::write_delete_bitset;
 use crate::indexer::delete_queue::{DeleteCursor, DeleteQueue};
@@ -112,15 +112,15 @@ fn compute_deleted_bitset(
         if let Some(mut docset) =
             inverted_index.read_postings(&delete_op.term, IndexRecordOption::Basic)
         {
-            let mut deleted_doc = docset.doc();
-            while deleted_doc != TERMINATED {
+            while docset.advance() {
+                let deleted_doc = docset.doc();
                 if deleted_doc < limit_doc {
                     delete_bitset.insert(deleted_doc);
                     might_have_changed = true;
                 }
-                deleted_doc = docset.advance();
             }
         }
+
         delete_cursor.advance();
     }
     Ok(might_have_changed)
@@ -346,7 +346,7 @@ impl IndexWriter {
 
     fn drop_sender(&mut self) {
         let (sender, _receiver) = channel::bounded(1);
-        self.operation_sender = sender;
+        mem::replace(&mut self.operation_sender, sender);
     }
 
     /// If there are some merging threads, blocks until they all finish their work and

src/indexer/log_merge_policy.rs

@@ -54,6 +54,10 @@ impl LogMergePolicy {
 
 impl MergePolicy for LogMergePolicy {
     fn compute_merge_candidates(&self, segments: &[SegmentMeta]) -> Vec<MergeCandidate> {
+        if segments.is_empty() {
+            return Vec::new();
+        }
+
         let mut size_sorted_tuples = segments
             .iter()
             .map(SegmentMeta::num_docs)
@@ -63,35 +67,27 @@ impl MergePolicy for LogMergePolicy {
 
         size_sorted_tuples.sort_by(|x, y| y.1.cmp(&(x.1)));
 
-        if size_sorted_tuples.len() <= 1 {
-            return Vec::new();
-        }
-
         let size_sorted_log_tuples: Vec<_> = size_sorted_tuples
             .into_iter()
             .map(|(ind, num_docs)| (ind, f64::from(self.clip_min_size(num_docs)).log2()))
             .collect();
 
-        if let Some(&(first_ind, first_score)) = size_sorted_log_tuples.first() {
-            let mut current_max_log_size = first_score;
-            let mut levels = vec![vec![first_ind]];
-            for &(ind, score) in (&size_sorted_log_tuples).iter().skip(1) {
-                if score < (current_max_log_size - self.level_log_size) {
-                    current_max_log_size = score;
-                    levels.push(Vec::new());
-                }
-                levels.last_mut().unwrap().push(ind);
+        let (first_ind, first_score) = size_sorted_log_tuples[0];
+        let mut current_max_log_size = first_score;
+        let mut levels = vec![vec![first_ind]];
+        for &(ind, score) in (&size_sorted_log_tuples).iter().skip(1) {
+            if score < (current_max_log_size - self.level_log_size) {
+                current_max_log_size = score;
+                levels.push(Vec::new());
             }
-            levels
-                .iter()
-                .filter(|level| level.len() >= self.min_merge_size)
-                .map(|ind_vec| {
-                    MergeCandidate(ind_vec.iter().map(|&ind| segments[ind].id()).collect())
-                })
-                .collect()
-        } else {
-            return vec![];
+            levels.last_mut().unwrap().push(ind);
         }
+
+        levels
+            .iter()
+            .filter(|level| level.len() >= self.min_merge_size)
+            .map(|ind_vec| MergeCandidate(ind_vec.iter().map(|&ind| segments[ind].id()).collect()))
+            .collect()
     }
 }
 
@@ -183,7 +179,6 @@ mod tests {
         let result_list = test_merge_policy().compute_merge_candidates(&test_input);
         assert_eq!(result_list.len(), 2);
     }
-
     #[test]
     fn test_log_merge_policy_small_segments() {
         // segments under min_layer_size are merged together
@@ -199,17 +194,6 @@ mod tests {
         assert_eq!(result_list.len(), 1);
     }
 
-    #[test]
-    fn test_log_merge_policy_all_segments_too_large_to_merge() {
-        let eight_large_segments: Vec<SegmentMeta> =
-            std::iter::repeat_with(|| create_random_segment_meta(100_001))
-                .take(8)
-                .collect();
-        assert!(test_merge_policy()
-            .compute_merge_candidates(&eight_large_segments)
-            .is_empty());
-    }
-
     #[test]
     fn test_large_merge_segments() {
         let test_input = vec![

src/indexer/merger.rs

@@ -2,7 +2,7 @@ use crate::common::MAX_DOC_LIMIT;
 use crate::core::Segment;
 use crate::core::SegmentReader;
 use crate::core::SerializableSegment;
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::DocSet;
 use crate::fastfield::BytesFastFieldReader;
 use crate::fastfield::DeleteBitSet;
 use crate::fastfield::FastFieldReader;
@@ -574,12 +574,10 @@ impl IndexMerger {
                     let inverted_index = segment_reader.inverted_index(indexed_field);
                     let mut segment_postings = inverted_index
                         .read_postings_from_terminfo(term_info, segment_postings_option);
-                    let mut doc = segment_postings.doc();
-                    while doc != TERMINATED {
-                        if !segment_reader.is_deleted(doc) {
+                    while segment_postings.advance() {
+                        if !segment_reader.is_deleted(segment_postings.doc()) {
                             return Some((segment_ord, segment_postings));
                         }
-                        doc = segment_postings.advance();
                     }
                     None
                 })
@@ -589,45 +587,57 @@ impl IndexMerger {
             // of all of the segments containing the given term.
             //
             // These segments are non-empty and advance has already been called.
-            if segment_postings.is_empty() {
-                continue;
-            }
-            // If not, the `term` will be entirely removed.
+            if !segment_postings.is_empty() {
+                // If not, the `term` will be entirely removed.
 
-            // We know that there is at least one document containing
-            // the term, so we add it.
-            let to_term_ord = field_serializer.new_term(term_bytes)?;
+                // We know that there is at least one document containing
+                // the term, so we add it.
+                let to_term_ord = field_serializer.new_term(term_bytes)?;
 
-            if let Some(ref mut term_ord_mapping) = term_ord_mapping_opt {
-                for (segment_ord, from_term_ord) in merged_terms.matching_segments() {
-                    term_ord_mapping.register_from_to(segment_ord, from_term_ord, to_term_ord);
-                }
-            }
-
-            // We can now serialize this postings, by pushing each document to the
-            // postings serializer.
-            for (segment_ord, mut segment_postings) in segment_postings {
-                let old_to_new_doc_id = &merged_doc_id_map[segment_ord];
-
-                let mut doc = segment_postings.doc();
-                while doc != TERMINATED {
-                    // deleted doc are skipped as they do not have a `remapped_doc_id`.
-                    if let Some(remapped_doc_id) = old_to_new_doc_id[doc as usize] {
-                        // we make sure to only write the term iff
-                        // there is at least one document.
-                        let term_freq = segment_postings.term_freq();
-                        segment_postings.positions(&mut positions_buffer);
-
-                        let delta_positions = delta_computer.compute_delta(&positions_buffer);
-                        field_serializer.write_doc(remapped_doc_id, term_freq, delta_positions)?;
+                if let Some(ref mut term_ord_mapping) = term_ord_mapping_opt {
+                    for (segment_ord, from_term_ord) in merged_terms.matching_segments() {
+                        term_ord_mapping.register_from_to(segment_ord, from_term_ord, to_term_ord);
                     }
-
-                    doc = segment_postings.advance();
                 }
-            }
 
-            // closing the term.
-            field_serializer.close_term()?;
+                // We can now serialize this postings, by pushing each document to the
+                // postings serializer.
+                for (segment_ord, mut segment_postings) in segment_postings {
+                    let old_to_new_doc_id = &merged_doc_id_map[segment_ord];
+                    loop {
+                        let doc = segment_postings.doc();
+
+                        // `.advance()` has been called once before the loop.
+                        //
+                        // It was required to make sure we only consider segments
+                        // that effectively contain at least one non-deleted document
+                        // and remove terms that do not have documents associated.
+                        //
+                        //  For this reason, we cannot use a `while segment_postings.advance()` loop.
+
+                        // deleted doc are skipped as they do not have a `remapped_doc_id`.
+                        if let Some(remapped_doc_id) = old_to_new_doc_id[doc as usize] {
+                            // we make sure to only write the term iff
+                            // there is at least one document.
+                            let term_freq = segment_postings.term_freq();
+                            segment_postings.positions(&mut positions_buffer);
+
+                            let delta_positions = delta_computer.compute_delta(&positions_buffer);
+                            field_serializer.write_doc(
+                                remapped_doc_id,
+                                term_freq,
+                                delta_positions,
+                            )?;
+                        }
+                        if !segment_postings.advance() {
+                            break;
+                        }
+                    }
+                }
+
+                // closing the term.
+                field_serializer.close_term()?;
+            }
         }
         field_serializer.close()?;
         Ok(term_ord_mapping_opt)

src/lib.rs

@@ -156,7 +156,7 @@ mod snippet;
 pub use self::snippet::{Snippet, SnippetGenerator};
 
 mod docset;
-pub use self::docset::{DocSet, TERMINATED};
+pub use self::docset::{DocSet, SkipResult};
 pub use crate::common::{f64_to_u64, i64_to_u64, u64_to_f64, u64_to_i64};
 pub use crate::core::{Executor, SegmentComponent};
 pub use crate::core::{Index, IndexMeta, Searcher, Segment, SegmentId, SegmentMeta};
@@ -285,7 +285,7 @@ mod tests {
 
     use crate::collector::tests::TEST_COLLECTOR_WITH_SCORE;
     use crate::core::SegmentReader;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::DocSet;
     use crate::query::BooleanQuery;
     use crate::schema::*;
     use crate::DocAddress;
@@ -381,12 +381,19 @@ mod tests {
             index_writer.commit().unwrap();
         }
         {
-            index_writer.add_document(doc!(text_field=>"a"));
-            index_writer.add_document(doc!(text_field=>"a a"));
+            {
+                let doc = doc!(text_field=>"a");
+                index_writer.add_document(doc);
+            }
+            {
+                let doc = doc!(text_field=>"a a");
+                index_writer.add_document(doc);
+            }
             index_writer.commit().unwrap();
         }
         {
-            index_writer.add_document(doc!(text_field=>"c"));
+            let doc = doc!(text_field=>"c");
+            index_writer.add_document(doc);
             index_writer.commit().unwrap();
         }
         {
@@ -465,12 +472,10 @@ mod tests {
     }
 
     fn advance_undeleted(docset: &mut dyn DocSet, reader: &SegmentReader) -> bool {
-        let mut doc = docset.advance();
-        while doc != TERMINATED {
-            if !reader.is_deleted(doc) {
+        while docset.advance() {
+            if !reader.is_deleted(docset.doc()) {
                 return true;
             }
-            doc = docset.advance();
         }
         false
     }
@@ -636,8 +641,9 @@ mod tests {
             .inverted_index(term.field())
             .read_postings(&term, IndexRecordOption::Basic)
             .unwrap();
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 0);
-        assert_eq!(postings.advance(), TERMINATED);
+        assert!(!postings.advance());
     }
 
     #[test]
@@ -659,8 +665,9 @@ mod tests {
             .inverted_index(term.field())
             .read_postings(&term, IndexRecordOption::Basic)
             .unwrap();
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 0);
-        assert_eq!(postings.advance(), TERMINATED);
+        assert!(!postings.advance());
     }
 
     #[test]
@@ -682,8 +689,9 @@ mod tests {
             .inverted_index(term.field())
             .read_postings(&term, IndexRecordOption::Basic)
             .unwrap();
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 0);
-        assert_eq!(postings.advance(), TERMINATED);
+        assert!(!postings.advance());
     }
 
     #[test]
@@ -752,8 +760,10 @@ mod tests {
         {
             // writing the segment
             let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-            let doc = doc!(text_field=>"af af af bc bc");
-            index_writer.add_document(doc);
+            {
+                let doc = doc!(text_field=>"af af af bc bc");
+                index_writer.add_document(doc);
+            }
             index_writer.commit().unwrap();
         }
         {
@@ -769,9 +779,10 @@ mod tests {
             let mut postings = inverted_index
                 .read_postings(&term_af, IndexRecordOption::WithFreqsAndPositions)
                 .unwrap();
+            assert!(postings.advance());
             assert_eq!(postings.doc(), 0);
             assert_eq!(postings.term_freq(), 3);
-            assert_eq!(postings.advance(), TERMINATED);
+            assert!(!postings.advance());
         }
     }
 

src/positions/mod.rs

@@ -37,9 +37,9 @@ const LONG_SKIP_INTERVAL: u64 = (LONG_SKIP_IN_BLOCKS * COMPRESSION_BLOCK_SIZE) a
 #[cfg(test)]
 pub mod tests {
 
-    use super::PositionSerializer;
+    use super::{PositionReader, PositionSerializer};
     use crate::directory::ReadOnlySource;
-    use crate::positions::reader::PositionReader;
+    use crate::positions::COMPRESSION_BLOCK_SIZE;
     use std::iter;
 
     fn create_stream_buffer(vals: &[u32]) -> (ReadOnlySource, ReadOnlySource) {
@@ -68,7 +68,7 @@ pub mod tests {
         let mut position_reader = PositionReader::new(stream, skip, 0u64);
         for &n in &[1, 10, 127, 128, 130, 312] {
             let mut v = vec![0u32; n];
-            position_reader.read(0, &mut v[..]);
+            position_reader.read(&mut v[..n]);
             for i in 0..n {
                 assert_eq!(v[i], i as u32);
             }
@@ -76,19 +76,19 @@ pub mod tests {
     }
 
     #[test]
-    fn test_position_read_with_offset() {
-        let v: Vec<u32> = (0..1000).collect();
+    fn test_position_skip() {
+        let v: Vec<u32> = (0..1_000).collect();
         let (stream, skip) = create_stream_buffer(&v[..]);
         assert_eq!(skip.len(), 12);
         assert_eq!(stream.len(), 1168);
+
         let mut position_reader = PositionReader::new(stream, skip, 0u64);
-        for &offset in &[1u64, 10u64, 127u64, 128u64, 130u64, 312u64] {
-            for &len in &[1, 10, 130, 500] {
-                let mut v = vec![0u32; len];
-                position_reader.read(offset, &mut v[..]);
-                for i in 0..len {
-                    assert_eq!(v[i], i as u32 + offset as u32);
-                }
+        position_reader.skip(10);
+        for &n in &[10, 127, COMPRESSION_BLOCK_SIZE, 130, 312] {
+            let mut v = vec![0u32; n];
+            position_reader.read(&mut v[..n]);
+            for i in 0..n {
+                assert_eq!(v[i], 10u32 + i as u32);
             }
         }
     }
@@ -103,12 +103,11 @@ pub mod tests {
         let mut position_reader = PositionReader::new(stream, skip, 0u64);
         let mut buf = [0u32; 7];
         let mut c = 0;
-
-        let mut offset = 0;
         for _ in 0..100 {
-            position_reader.read(offset, &mut buf);
-            position_reader.read(offset, &mut buf);
-            offset += 7;
+            position_reader.read(&mut buf);
+            position_reader.read(&mut buf);
+            position_reader.skip(4);
+            position_reader.skip(3);
             for &el in &buf {
                 assert_eq!(c, el);
                 c += 1;
@@ -116,58 +115,6 @@ pub mod tests {
         }
     }
 
-    #[test]
-    fn test_position_reread_anchor_different_than_block() {
-        let v: Vec<u32> = (0..2_000_000).collect();
-        let (stream, skip) = create_stream_buffer(&v[..]);
-        assert_eq!(skip.len(), 15_749);
-        assert_eq!(stream.len(), 4_987_872);
-        let mut position_reader = PositionReader::new(stream.clone(), skip.clone(), 0);
-        let mut buf = [0u32; 256];
-        position_reader.read(128, &mut buf);
-        for i in 0..256 {
-            assert_eq!(buf[i], (128 + i) as u32);
-        }
-        position_reader.read(128, &mut buf);
-        for i in 0..256 {
-            assert_eq!(buf[i], (128 + i) as u32);
-        }
-    }
-
-    #[test]
-    #[should_panic(expected = "offset arguments should be increasing.")]
-    fn test_position_panic_if_called_previous_anchor() {
-        let v: Vec<u32> = (0..2_000_000).collect();
-        let (stream, skip) = create_stream_buffer(&v[..]);
-        assert_eq!(skip.len(), 15_749);
-        assert_eq!(stream.len(), 4_987_872);
-        let mut buf = [0u32; 1];
-        let mut position_reader = PositionReader::new(stream.clone(), skip.clone(), 200_000);
-        position_reader.read(230, &mut buf);
-        position_reader.read(9, &mut buf);
-    }
-
-    #[test]
-    fn test_positions_bug() {
-        let mut v: Vec<u32> = vec![];
-        for i in 1..200 {
-            for j in 0..i {
-                v.push(j);
-            }
-        }
-        let (stream, skip) = create_stream_buffer(&v[..]);
-        let mut buf = Vec::new();
-        let mut position_reader = PositionReader::new(stream.clone(), skip.clone(), 0);
-        let mut offset = 0;
-        for i in 1..24 {
-            buf.resize(i, 0);
-            position_reader.read(offset, &mut buf[..]);
-            offset += i as u64;
-            let r: Vec<u32> = (0..i).map(|el| el as u32).collect();
-            assert_eq!(buf, &r[..]);
-        }
-    }
-
     #[test]
     fn test_position_long_skip_const() {
         const CONST_VAL: u32 = 9u32;
@@ -177,7 +124,7 @@ pub mod tests {
         assert_eq!(stream.len(), 1_000_000);
         let mut position_reader = PositionReader::new(stream, skip, 128 * 1024);
         let mut buf = [0u32; 1];
-        position_reader.read(0, &mut buf);
+        position_reader.read(&mut buf);
         assert_eq!(buf[0], CONST_VAL);
     }
 
@@ -196,7 +143,7 @@ pub mod tests {
         ] {
             let mut position_reader = PositionReader::new(stream.clone(), skip.clone(), offset);
             let mut buf = [0u32; 1];
-            position_reader.read(0, &mut buf);
+            position_reader.read(&mut buf);
             assert_eq!(buf[0], offset as u32);
         }
     }

src/positions/reader.rs

@@ -3,6 +3,7 @@ use crate::directory::ReadOnlySource;
 use crate::positions::COMPRESSION_BLOCK_SIZE;
 use crate::positions::LONG_SKIP_INTERVAL;
 use crate::positions::LONG_SKIP_IN_BLOCKS;
+use crate::postings::compression::compressed_block_size;
 /// Positions works as a long sequence of compressed block.
 /// All terms are chained one after the other.
 ///
@@ -61,20 +62,22 @@ impl Positions {
 
     fn reader(&self, offset: u64) -> PositionReader {
         let long_skip_id = (offset / LONG_SKIP_INTERVAL) as usize;
+        let small_skip = (offset % LONG_SKIP_INTERVAL) as usize;
         let offset_num_bytes: u64 = self.long_skip(long_skip_id);
         let mut position_read = OwnedRead::new(self.position_source.clone());
         position_read.advance(offset_num_bytes as usize);
         let mut skip_read = OwnedRead::new(self.skip_source.clone());
         skip_read.advance(long_skip_id * LONG_SKIP_IN_BLOCKS);
-        PositionReader {
+        let mut position_reader = PositionReader {
             bit_packer: self.bit_packer,
             skip_read,
             position_read,
+            inner_offset: 0,
             buffer: Box::new([0u32; 128]),
-            block_offset: std::i64::MAX as u64,
-            anchor_offset: (long_skip_id as u64) * LONG_SKIP_INTERVAL,
-            abs_offset: offset,
-        }
+            ahead: None,
+        };
+        position_reader.skip(small_skip);
+        position_reader
     }
 }
 
@@ -82,12 +85,51 @@ pub struct PositionReader {
     skip_read: OwnedRead,
     position_read: OwnedRead,
     bit_packer: BitPacker4x,
-    buffer: Box<[u32; COMPRESSION_BLOCK_SIZE]>,
+    inner_offset: usize,
+    buffer: Box<[u32; 128]>,
+    ahead: Option<usize>, // if None, no block is loaded.
+                          // if Some(num_blocks), the block currently loaded is num_blocks ahead
+                          // of the block of the next int to read.
+}
 
-    block_offset: u64,
-    anchor_offset: u64,
-
-    abs_offset: u64,
+// `ahead` represents the offset of the block currently loaded
+// compared to the cursor of the actual stream.
+//
+// By contract, when this function is called, the current block has to be
+// decompressed.
+//
+// If the requested number of els ends exactly at a given block, the next
+// block is not decompressed.
+fn read_impl(
+    bit_packer: BitPacker4x,
+    mut position: &[u8],
+    buffer: &mut [u32; 128],
+    mut inner_offset: usize,
+    num_bits: &[u8],
+    output: &mut [u32],
+) -> usize {
+    let mut output_start = 0;
+    let mut output_len = output.len();
+    let mut ahead = 0;
+    loop {
+        let available_len = COMPRESSION_BLOCK_SIZE - inner_offset;
+        // We have enough elements in the current block.
+        // Let's copy the requested elements in the output buffer,
+        // and return.
+        if output_len <= available_len {
+            output[output_start..].copy_from_slice(&buffer[inner_offset..][..output_len]);
+            return ahead;
+        }
+        output[output_start..][..available_len].copy_from_slice(&buffer[inner_offset..]);
+        output_len -= available_len;
+        output_start += available_len;
+        inner_offset = 0;
+        let num_bits = num_bits[ahead];
+        bit_packer.decompress(position, &mut buffer[..], num_bits);
+        let block_len = compressed_block_size(num_bits);
+        position = &position[block_len..];
+        ahead += 1;
+    }
 }
 
 impl PositionReader {
@@ -99,65 +141,57 @@ impl PositionReader {
         Positions::new(position_source, skip_source).reader(offset)
     }
 
-    fn advance_num_blocks(&mut self, num_blocks: usize) {
-        let num_bits: usize = self.skip_read.as_ref()[..num_blocks]
-            .iter()
-            .cloned()
-            .map(|num_bits| num_bits as usize)
-            .sum();
-        let num_bytes_to_skip = num_bits * COMPRESSION_BLOCK_SIZE / 8;
-        self.skip_read.advance(num_blocks as usize);
-        self.position_read.advance(num_bytes_to_skip);
-    }
-
-    /// Fills a buffer with the positions `[offset..offset+output.len())` integers.
-    ///
-    /// `offset` is required to have a value >= to the offsets given in previous calls
-    /// for the given `PositionReaderAbsolute` instance.
-    pub fn read(&mut self, mut offset: u64, mut output: &mut [u32]) {
-        offset += self.abs_offset;
-        assert!(
-            offset >= self.anchor_offset,
-            "offset arguments should be increasing."
-        );
-        let delta_to_block_offset = offset as i64 - self.block_offset as i64;
-        if delta_to_block_offset < 0 || delta_to_block_offset >= 128 {
-            // The first position is not within the first block.
-            // We need to decompress the first block.
-            let delta_to_anchor_offset = offset - self.anchor_offset;
-            let num_blocks_to_skip =
-                (delta_to_anchor_offset / (COMPRESSION_BLOCK_SIZE as u64)) as usize;
-            self.advance_num_blocks(num_blocks_to_skip);
-            self.anchor_offset = offset - (offset % COMPRESSION_BLOCK_SIZE as u64);
-            self.block_offset = self.anchor_offset;
-            let num_bits = self.skip_read.get(0);
-            self.bit_packer
-                .decompress(self.position_read.as_ref(), self.buffer.as_mut(), num_bits);
-        } else {
-            let num_blocks_to_skip =
-                ((self.block_offset - self.anchor_offset) / COMPRESSION_BLOCK_SIZE as u64) as usize;
-            self.advance_num_blocks(num_blocks_to_skip);
-            self.anchor_offset = self.block_offset;
-        }
-
-        let mut num_bits = self.skip_read.get(0);
-        let mut position_data = self.position_read.as_ref();
-
-        for i in 1.. {
-            let offset_in_block = (offset as usize) % COMPRESSION_BLOCK_SIZE;
-            let remaining_in_block = COMPRESSION_BLOCK_SIZE - offset_in_block;
-            if remaining_in_block >= output.len() {
-                output.copy_from_slice(&self.buffer[offset_in_block..][..output.len()]);
-                break;
-            }
-            output[..remaining_in_block].copy_from_slice(&self.buffer[offset_in_block..]);
-            output = &mut output[remaining_in_block..];
-            offset += remaining_in_block as u64;
-            position_data = &position_data[(num_bits as usize * COMPRESSION_BLOCK_SIZE / 8)..];
-            num_bits = self.skip_read.get(i);
+    /// Fills a buffer with the next `output.len()` integers.
+    /// This does not consume / advance the stream.
+    pub fn read(&mut self, output: &mut [u32]) {
+        let skip_data = self.skip_read.as_ref();
+        let position_data = self.position_read.as_ref();
+        let num_bits = self.skip_read.get(0);
+        if self.ahead != Some(0) {
+            // the block currently available is not the block
+            // for the current position
             self.bit_packer
                 .decompress(position_data, self.buffer.as_mut(), num_bits);
-            self.block_offset += COMPRESSION_BLOCK_SIZE as u64;
+            self.ahead = Some(0);
         }
+        let block_len = compressed_block_size(num_bits);
+        self.ahead = Some(read_impl(
+            self.bit_packer,
+            &position_data[block_len..],
+            self.buffer.as_mut(),
+            self.inner_offset,
+            &skip_data[1..],
+            output,
+        ));
+    }
+
+    /// Skip the next `skip_len` integer.
+    ///
+    /// If a full block is skipped, calling
+    /// `.skip(...)` will avoid decompressing it.
+    ///
+    /// May panic if the end of the stream is reached.
+    pub fn skip(&mut self, skip_len: usize) {
+        let skip_len_plus_inner_offset = skip_len + self.inner_offset;
+
+        let num_blocks_to_advance = skip_len_plus_inner_offset / COMPRESSION_BLOCK_SIZE;
+        self.inner_offset = skip_len_plus_inner_offset % COMPRESSION_BLOCK_SIZE;
+
+        self.ahead = self.ahead.and_then(|num_blocks| {
+            if num_blocks >= num_blocks_to_advance {
+                Some(num_blocks - num_blocks_to_advance)
+            } else {
+                None
+            }
+        });
+
+        let skip_len_in_bits = self.skip_read.as_ref()[..num_blocks_to_advance]
+            .iter()
+            .map(|num_bits| *num_bits as usize)
+            .sum::<usize>()
+            * COMPRESSION_BLOCK_SIZE;
+        let skip_len_in_bytes = skip_len_in_bits / 8;
+        self.skip_read.advance(num_blocks_to_advance);
+        self.position_read.advance(skip_len_in_bytes);
     }
 }

src/postings/block_max_postings.rs

@@ -0,0 +1,16 @@
+use crate::postings::Postings;
+use crate::DocId;
+
+/// Inverted list with additional information about the maximum term frequency
+/// within a block, as well as globally within the list.
+pub trait BlockMaxPostings: Postings {
+    /// Returns the maximum frequency in the entire list.
+    fn max_term_freq(&self) -> u32;
+    /// Returns the maximum frequency in the current block.
+    fn block_max_term_freq(&mut self) -> u32;
+    /// Returns the document with the largest frequency.
+    fn max_doc(&self) -> DocId;
+    /// Returns the document with the largest frequency within the current
+    /// block.
+    fn block_max_doc(&self) -> DocId;
+}

src/postings/block_max_segment_postings.rs

@@ -0,0 +1,76 @@
+use crate::postings::{BlockMaxPostings, Postings, SegmentPostings};
+use crate::{DocId, DocSet, SkipResult};
+
+/// A wrapper over [`SegmentPostings`](./struct.SegmentPostings.html)
+/// with max block frequencies.
+pub struct BlockMaxSegmentPostings {
+    postings: SegmentPostings,
+    max_blocks: SegmentPostings,
+    doc_with_max_term_freq: DocId,
+    max_term_freq: u32,
+}
+
+impl BlockMaxSegmentPostings {
+    /// Constructs a new segment postings with block-max information.
+    pub fn new(
+        postings: SegmentPostings,
+        max_blocks: SegmentPostings,
+        doc_with_max_term_freq: DocId,
+        max_term_freq: u32,
+    ) -> Self {
+        Self {
+            postings,
+            max_blocks,
+            doc_with_max_term_freq,
+            max_term_freq,
+        }
+    }
+}
+
+impl DocSet for BlockMaxSegmentPostings {
+    fn advance(&mut self) -> bool {
+        self.postings.advance()
+    }
+
+    fn doc(&self) -> DocId {
+        self.postings.doc()
+    }
+
+    fn size_hint(&self) -> u32 {
+        self.postings.size_hint()
+    }
+
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.postings.skip_next(target)
+    }
+}
+
+impl Postings for BlockMaxSegmentPostings {
+    fn term_freq(&self) -> u32 {
+        self.postings.term_freq()
+    }
+    fn positions_with_offset(&mut self, offset: u32, output: &mut Vec<u32>) {
+        self.postings.positions_with_offset(offset, output);
+    }
+    fn positions(&mut self, output: &mut Vec<u32>) {
+        self.postings.positions(output);
+    }
+}
+
+impl BlockMaxPostings for BlockMaxSegmentPostings {
+    fn max_term_freq(&self) -> u32 {
+        self.max_term_freq
+    }
+    fn block_max_term_freq(&mut self) -> u32 {
+        if let SkipResult::End = self.max_blocks.skip_next(self.doc()) {
+            panic!("Max blocks corrupted: reached end of max block");
+        }
+        self.max_blocks.term_freq()
+    }
+    fn max_doc(&self) -> DocId {
+        self.doc_with_max_term_freq
+    }
+    fn block_max_doc(&self) -> DocId {
+        self.max_blocks.doc()
+    }
+}

src/postings/block_search.rs

@@ -87,7 +87,6 @@ fn exponential_search(arr: &[u32], target: u32) -> (usize, usize) {
     (begin, end)
 }
 
-#[inline(never)]
 fn galloping(block_docs: &[u32], target: u32) -> usize {
     let (start, end) = exponential_search(&block_docs, target);
     start + linear_search(&block_docs[start..end], target)
@@ -130,18 +129,23 @@ impl BlockSearcher {
     ///
     /// If SSE2 instructions are available in the `(platform, running CPU)`,
     /// then we use a different implementation that does an exhaustive linear search over
-    /// the block regardless of whether the block is full or not.
-    ///
-    /// Indeed, if the block is not full, the remaining items are TERMINATED.
-    /// It is surprisingly faster, most likely because of the lack of branch misprediction.
-    pub(crate) fn search_in_block(self, block_docs: &AlignedBuffer, target: u32) -> usize {
+    /// the full block whenever the block is full (`len == 128`). It is surprisingly faster, most likely because of the lack
+    /// of branch.
+    pub(crate) fn search_in_block(
+        self,
+        block_docs: &AlignedBuffer,
+        len: usize,
+        start: usize,
+        target: u32,
+    ) -> usize {
         #[cfg(target_arch = "x86_64")]
         {
-            if self == BlockSearcher::SSE2 {
+            use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
+            if self == BlockSearcher::SSE2 && len == COMPRESSION_BLOCK_SIZE {
                 return sse2::linear_search_sse2_128(block_docs, target);
             }
         }
-        galloping(&block_docs.0[..], target)
+        start + galloping(&block_docs.0[start..len], target)
     }
 }
 
@@ -162,7 +166,6 @@ mod tests {
     use super::exponential_search;
     use super::linear_search;
     use super::BlockSearcher;
-    use crate::docset::TERMINATED;
     use crate::postings::compression::{AlignedBuffer, COMPRESSION_BLOCK_SIZE};
 
     #[test]
@@ -193,12 +196,19 @@ mod tests {
     fn util_test_search_in_block(block_searcher: BlockSearcher, block: &[u32], target: u32) {
         let cursor = search_in_block_trivial_but_slow(block, target);
         assert!(block.len() < COMPRESSION_BLOCK_SIZE);
-        let mut output_buffer = [TERMINATED; COMPRESSION_BLOCK_SIZE];
+        let mut output_buffer = [u32::max_value(); COMPRESSION_BLOCK_SIZE];
         output_buffer[..block.len()].copy_from_slice(block);
-        assert_eq!(
-            block_searcher.search_in_block(&AlignedBuffer(output_buffer), target),
-            cursor
-        );
+        for i in 0..cursor {
+            assert_eq!(
+                block_searcher.search_in_block(
+                    &AlignedBuffer(output_buffer),
+                    block.len(),
+                    i,
+                    target
+                ),
+                cursor
+            );
+        }
     }
 
     fn util_test_search_in_block_all(block_searcher: BlockSearcher, block: &[u32]) {

src/postings/block_segment_postings.rs

@@ -1,11 +1,26 @@
-use crate::common::{BinarySerializable, VInt};
-use crate::directory::ReadOnlySource;
-use crate::postings::compression::{
-    AlignedBuffer, BlockDecoder, VIntDecoder, COMPRESSION_BLOCK_SIZE,
-};
-use crate::postings::{BlockInfo, FreqReadingOption, SkipReader};
+use crate::DocId;
+use tantivy_fst::Streamer;
+use crate::postings::{SkipReader, FreqReadingOption, USE_SKIP_INFO_LIMIT};
+use owned_read::OwnedRead;
+use crate::postings::compression::{BlockDecoder, COMPRESSION_BLOCK_SIZE, VIntDecoder, compressed_block_size, AlignedBuffer};
 use crate::schema::IndexRecordOption;
-use crate::{DocId, TERMINATED};
+use crate::common::{VInt, BinarySerializable};
+
+
+fn split_into_skips_and_postings(
+    doc_freq: u32,
+    mut data: OwnedRead,
+) -> (Option<OwnedRead>, OwnedRead) {
+    if doc_freq >= USE_SKIP_INFO_LIMIT {
+        let skip_len = VInt::deserialize(&mut data).expect("Data corrupted").0 as usize;
+        let mut postings_data = data.clone();
+        postings_data.advance(skip_len);
+        data.clip(skip_len);
+        (Some(data), postings_data)
+    } else {
+        (None, data)
+    }
+}
 
 /// `BlockSegmentPostings` is a cursor iterating over blocks
 /// of documents.
@@ -15,65 +30,30 @@ use crate::{DocId, TERMINATED};
 /// While it is useful for some very specific high-performance
 /// use cases, you should prefer using `SegmentPostings` for most usage.
 pub struct BlockSegmentPostings {
-    pub(crate) doc_decoder: BlockDecoder,
-    loaded_offset: usize,
+    doc_decoder: BlockDecoder,
     freq_decoder: BlockDecoder,
     freq_reading_option: FreqReadingOption,
 
     doc_freq: usize,
+    doc_offset: DocId,
 
-    data: ReadOnlySource,
+    num_vint_docs: usize,
+
+    remaining_data: OwnedRead,
     skip_reader: SkipReader,
 }
 
-fn decode_bitpacked_block(
-    doc_decoder: &mut BlockDecoder,
-    freq_decoder_opt: Option<&mut BlockDecoder>,
-    data: &[u8],
-    doc_offset: DocId,
-    doc_num_bits: u8,
-    tf_num_bits: u8,
-) {
-    let num_consumed_bytes = doc_decoder.uncompress_block_sorted(data, doc_offset, doc_num_bits);
-    if let Some(freq_decoder) = freq_decoder_opt {
-        freq_decoder.uncompress_block_unsorted(&data[num_consumed_bytes..], tf_num_bits);
-    }
-}
 
-fn decode_vint_block(
-    doc_decoder: &mut BlockDecoder,
-    freq_decoder_opt: Option<&mut BlockDecoder>,
-    data: &[u8],
-    doc_offset: DocId,
-    num_vint_docs: usize,
-) {
-    doc_decoder.clear();
-    let num_consumed_bytes = doc_decoder.uncompress_vint_sorted(data, doc_offset, num_vint_docs);
-    if let Some(freq_decoder) = freq_decoder_opt {
-        freq_decoder.uncompress_vint_unsorted(&data[num_consumed_bytes..], num_vint_docs);
-    }
-}
-
-fn split_into_skips_and_postings(
-    doc_freq: u32,
-    data: ReadOnlySource,
-) -> (Option<ReadOnlySource>, ReadOnlySource) {
-    if doc_freq < COMPRESSION_BLOCK_SIZE as u32 {
-        return (None, data);
-    }
-    let mut data_byte_arr = data.as_slice();
-    let skip_len = VInt::deserialize(&mut data_byte_arr)
-        .expect("Data corrupted")
-        .0 as usize;
-    let vint_len = data.len() - data_byte_arr.len();
-    let (skip_data, postings_data) = data.slice_from(vint_len).split(skip_len);
-    (Some(skip_data), postings_data)
+#[derive(Debug, Eq, PartialEq)]
+pub enum BlockSegmentPostingsSkipResult {
+    Terminated,
+    Success(u32), //< number of term freqs to skip
 }
 
 impl BlockSegmentPostings {
     pub(crate) fn from_data(
         doc_freq: u32,
-        data: ReadOnlySource,
+        data: OwnedRead,
         record_option: IndexRecordOption,
         requested_option: IndexRecordOption,
     ) -> BlockSegmentPostings {
@@ -85,22 +65,21 @@ impl BlockSegmentPostings {
 
         let (skip_data_opt, postings_data) = split_into_skips_and_postings(doc_freq, data);
         let skip_reader = match skip_data_opt {
-            Some(skip_data) => SkipReader::new(skip_data, doc_freq, record_option),
-            None => SkipReader::new(ReadOnlySource::empty(), doc_freq, record_option),
+            Some(skip_data) => SkipReader::new(skip_data, record_option),
+            None => SkipReader::new(OwnedRead::new(&[][..]), record_option),
         };
-
         let doc_freq = doc_freq as usize;
-        let mut block_segment_postings = BlockSegmentPostings {
-            doc_decoder: BlockDecoder::with_val(TERMINATED),
-            loaded_offset: std::usize::MAX,
+        let num_vint_docs = doc_freq % COMPRESSION_BLOCK_SIZE;
+        BlockSegmentPostings {
+            num_vint_docs,
+            doc_decoder: BlockDecoder::new(),
             freq_decoder: BlockDecoder::with_val(1),
             freq_reading_option,
+            doc_offset: 0,
             doc_freq,
-            data: postings_data,
+            remaining_data: postings_data,
             skip_reader,
-        };
-        block_segment_postings.advance();
-        block_segment_postings
+        }
     }
 
     // Resets the block segment postings on another position
@@ -113,16 +92,17 @@ impl BlockSegmentPostings {
     // # Warning
     //
     // This does not reset the positions list.
-    pub(crate) fn reset(&mut self, doc_freq: u32, postings_data: ReadOnlySource) {
+    pub(crate) fn reset(&mut self, doc_freq: u32, postings_data: OwnedRead) {
         let (skip_data_opt, postings_data) = split_into_skips_and_postings(doc_freq, postings_data);
-        self.data = ReadOnlySource::new(postings_data);
-        self.loaded_offset = std::usize::MAX;
-        self.loaded_offset = std::usize::MAX;
+        let num_vint_docs = (doc_freq as usize) & (COMPRESSION_BLOCK_SIZE - 1);
+        self.num_vint_docs = num_vint_docs;
+        self.remaining_data = postings_data;
         if let Some(skip_data) = skip_data_opt {
-            self.skip_reader.reset(skip_data, doc_freq);
+            self.skip_reader.reset(skip_data);
         } else {
-            self.skip_reader.reset(ReadOnlySource::empty(), doc_freq);
+            self.skip_reader.reset(OwnedRead::new(&[][..]))
         }
+        self.doc_offset = 0;
         self.doc_freq = doc_freq as usize;
     }
 
@@ -143,13 +123,12 @@ impl BlockSegmentPostings {
         self.doc_decoder.output_array()
     }
 
-    #[inline(always)]
-    pub(crate) fn docs_aligned(&self) -> &AlignedBuffer {
+    pub(crate) fn docs_aligned(&self) -> (&AlignedBuffer, usize) {
         self.doc_decoder.output_aligned()
     }
 
     /// Return the document at index `idx` of the block.
-    #[inline(always)]
+    #[inline]
     pub fn doc(&self, idx: usize) -> u32 {
         self.doc_decoder.output(idx)
     }
@@ -172,256 +151,166 @@ impl BlockSegmentPostings {
     /// except the last block that may have a length
     /// of any number between 1 and `NUM_DOCS_PER_BLOCK - 1`
     #[inline]
-    pub fn block_len(&self) -> usize {
+    pub(crate) fn block_len(&self) -> usize {
         self.doc_decoder.output_len
     }
 
-    pub(crate) fn position_offset(&self) -> u64 {
-        self.skip_reader.position_offset()
-    }
-
-    /// Position on a block that may contains `target_doc`.
+    /// position on a block that may contains `doc_id`.
+    /// Always advance the current block.
     ///
-    /// If all docs are smaller than target, the block loaded may be empty,
-    /// or be the last an incomplete VInt block.
-    pub fn seek(&mut self, target_doc: DocId) {
-        self.skip_reader.seek(target_doc);
-        self.load_block();
-    }
+    /// Returns true if a block that has an element greater or equal to the target is found.
+    /// Returning true does not guarantee that the smallest element of the block is smaller
+    /// than the target. It only guarantees that the last element is greater or equal.
+    ///
+    /// Returns false iff all of the document remaining are smaller than
+    /// `doc_id`. In that case, all of these document are consumed.
+    ///
+    pub fn skip_to(&mut self, target_doc: DocId) -> BlockSegmentPostingsSkipResult {
+        let mut skip_freqs = 0u32;
+        while self.skip_reader.advance() {
+            if self.skip_reader.doc() >= target_doc {
+                // the last document of the current block is larger
+                // than the target.
+                //
+                // We found our block!
+                let num_bits = self.skip_reader.doc_num_bits();
+                let num_consumed_bytes = self.doc_decoder.uncompress_block_sorted(
+                    self.remaining_data.as_ref(),
+                    self.doc_offset,
+                    num_bits,
+                );
+                self.remaining_data.advance(num_consumed_bytes);
+                let tf_num_bits = self.skip_reader.tf_num_bits();
+                match self.freq_reading_option {
+                    FreqReadingOption::NoFreq => {}
+                    FreqReadingOption::SkipFreq => {
+                        let num_bytes_to_skip = compressed_block_size(tf_num_bits);
+                        self.remaining_data.advance(num_bytes_to_skip);
+                    }
+                    FreqReadingOption::ReadFreq => {
+                        let num_consumed_bytes = self
+                            .freq_decoder
+                            .uncompress_block_unsorted(self.remaining_data.as_ref(), tf_num_bits);
+                        self.remaining_data.advance(num_consumed_bytes);
+                    }
+                }
+                self.doc_offset = self.skip_reader.doc();
+                return BlockSegmentPostingsSkipResult::Success(skip_freqs);
+            } else {
+                skip_freqs += self.skip_reader.tf_sum();
+                let advance_len = self.skip_reader.total_block_len();
+                self.doc_offset = self.skip_reader.doc();
+                self.remaining_data.advance(advance_len);
+            }
+        }
 
-    fn load_block(&mut self) {
-        let offset = self.skip_reader.byte_offset();
-        if self.loaded_offset == offset {
-            return;
-        }
-        self.loaded_offset = offset;
-        match self.skip_reader.block_info() {
-            BlockInfo::BitPacked {
-                doc_num_bits,
-                tf_num_bits,
-                ..
-            } => {
-                decode_bitpacked_block(
-                    &mut self.doc_decoder,
-                    if let FreqReadingOption::ReadFreq = self.freq_reading_option {
-                        Some(&mut self.freq_decoder)
-                    } else {
-                        None
-                    },
-                    &self.data.as_slice()[offset..],
-                    self.skip_reader.last_doc_in_previous_block,
-                    doc_num_bits,
-                    tf_num_bits,
-                );
+        // we are now on the last, incomplete, variable encoded block.
+        if self.num_vint_docs > 0 {
+            let num_compressed_bytes = self.doc_decoder.uncompress_vint_sorted(
+                self.remaining_data.as_ref(),
+                self.doc_offset,
+                self.num_vint_docs,
+            );
+            self.remaining_data.advance(num_compressed_bytes);
+            match self.freq_reading_option {
+                FreqReadingOption::NoFreq | FreqReadingOption::SkipFreq => {}
+                FreqReadingOption::ReadFreq => {
+                    self.freq_decoder
+                        .uncompress_vint_unsorted(self.remaining_data.as_ref(), self.num_vint_docs);
+                }
             }
-            BlockInfo::VInt(num_vint_docs) => {
-                decode_vint_block(
-                    &mut self.doc_decoder,
-                    if let FreqReadingOption::ReadFreq = self.freq_reading_option {
-                        Some(&mut self.freq_decoder)
+            self.num_vint_docs = 0;
+            return self
+                .docs()
+                .last()
+                .map(|last_doc| {
+                    if *last_doc >= target_doc {
+                        BlockSegmentPostingsSkipResult::Success(skip_freqs)
                     } else {
-                        None
-                    },
-                    &self.data.as_slice()[offset..],
-                    self.skip_reader.last_doc_in_previous_block,
-                    num_vint_docs as usize,
-                );
-            }
+                        BlockSegmentPostingsSkipResult::Terminated
+                    }
+                })
+                .unwrap_or(BlockSegmentPostingsSkipResult::Terminated);
         }
+        BlockSegmentPostingsSkipResult::Terminated
     }
 
     /// Advance to the next block.
     ///
     /// Returns false iff there was no remaining blocks.
     pub fn advance(&mut self) -> bool {
-        if !self.skip_reader.advance() {
-            return false;
+        if self.skip_reader.advance() {
+            let num_bits = self.skip_reader.doc_num_bits();
+            let num_consumed_bytes = self.doc_decoder.uncompress_block_sorted(
+                self.remaining_data.as_ref(),
+                self.doc_offset,
+                num_bits,
+            );
+            self.remaining_data.advance(num_consumed_bytes);
+            let tf_num_bits = self.skip_reader.tf_num_bits();
+            match self.freq_reading_option {
+                FreqReadingOption::NoFreq => {}
+                FreqReadingOption::SkipFreq => {
+                    let num_bytes_to_skip = compressed_block_size(tf_num_bits);
+                    self.remaining_data.advance(num_bytes_to_skip);
+                }
+                FreqReadingOption::ReadFreq => {
+                    let num_consumed_bytes = self
+                        .freq_decoder
+                        .uncompress_block_unsorted(self.remaining_data.as_ref(), tf_num_bits);
+                    self.remaining_data.advance(num_consumed_bytes);
+                }
+            }
+            // it will be used as the next offset.
+            self.doc_offset = self.doc_decoder.output(COMPRESSION_BLOCK_SIZE - 1);
+            true
+        } else if self.num_vint_docs > 0 {
+            let num_compressed_bytes = self.doc_decoder.uncompress_vint_sorted(
+                self.remaining_data.as_ref(),
+                self.doc_offset,
+                self.num_vint_docs,
+            );
+            self.remaining_data.advance(num_compressed_bytes);
+            match self.freq_reading_option {
+                FreqReadingOption::NoFreq | FreqReadingOption::SkipFreq => {}
+                FreqReadingOption::ReadFreq => {
+                    self.freq_decoder
+                        .uncompress_vint_unsorted(self.remaining_data.as_ref(), self.num_vint_docs);
+                }
+            }
+            self.num_vint_docs = 0;
+            true
+        } else {
+            false
         }
-        self.load_block();
-        true
     }
 
     /// Returns an empty segment postings object
     pub fn empty() -> BlockSegmentPostings {
         BlockSegmentPostings {
-            doc_decoder: BlockDecoder::with_val(TERMINATED),
-            loaded_offset: std::usize::MAX,
+            num_vint_docs: 0,
+
+            doc_decoder: BlockDecoder::new(),
             freq_decoder: BlockDecoder::with_val(1),
             freq_reading_option: FreqReadingOption::NoFreq,
+
+            doc_offset: 0,
             doc_freq: 0,
-            data: ReadOnlySource::new(vec![]),
-            skip_reader: SkipReader::new(ReadOnlySource::new(vec![]), 0, IndexRecordOption::Basic),
+
+            remaining_data: OwnedRead::new(vec![]),
+            skip_reader: SkipReader::new(OwnedRead::new(vec![]), IndexRecordOption::Basic),
         }
     }
 }
 
-#[cfg(test)]
-mod tests {
-    use super::BlockSegmentPostings;
-    use crate::common::HasLen;
-    use crate::core::Index;
-    use crate::docset::{DocSet, TERMINATED};
-    use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
-    use crate::postings::postings::Postings;
-    use crate::postings::SegmentPostings;
-    use crate::schema::IndexRecordOption;
-    use crate::schema::Schema;
-    use crate::schema::Term;
-    use crate::schema::INDEXED;
-    use crate::DocId;
+impl<'a> Streamer<'a> for BlockSegmentPostings {
+    type Item = &'a [DocId];
 
-    #[test]
-    fn test_empty_segment_postings() {
-        let mut postings = SegmentPostings::empty();
-        assert_eq!(postings.advance(), TERMINATED);
-        assert_eq!(postings.advance(), TERMINATED);
-        assert_eq!(postings.len(), 0);
-    }
-
-    #[test]
-    fn test_empty_postings_doc_returns_terminated() {
-        let mut postings = SegmentPostings::empty();
-        assert_eq!(postings.doc(), TERMINATED);
-        assert_eq!(postings.advance(), TERMINATED);
-    }
-
-    #[test]
-    fn test_empty_postings_doc_term_freq_returns_0() {
-        let postings = SegmentPostings::empty();
-        assert_eq!(postings.term_freq(), 1);
-    }
-
-    #[test]
-    fn test_empty_block_segment_postings() {
-        let mut postings = BlockSegmentPostings::empty();
-        assert!(!postings.advance());
-        assert_eq!(postings.doc_freq(), 0);
-    }
-
-    #[test]
-    fn test_block_segment_postings() {
-        let mut block_segments = build_block_postings(&(0..100_000).collect::<Vec<u32>>());
-        let mut offset: u32 = 0u32;
-        // checking that the `doc_freq` is correct
-        assert_eq!(block_segments.doc_freq(), 100_000);
-        loop {
-            let block = block_segments.docs();
-            for (i, doc) in block.iter().cloned().enumerate() {
-                assert_eq!(offset + (i as u32), doc);
-            }
-            offset += block.len() as u32;
-            if block_segments.advance() {
-                break;
-            }
+    fn next(&'a mut self) -> Option<&'a [DocId]> {
+        if self.advance() {
+            Some(self.docs())
+        } else {
+            None
         }
     }
-
-    #[test]
-    fn test_skip_right_at_new_block() {
-        let mut doc_ids = (0..128).collect::<Vec<u32>>();
-        // 128 is missing
-        doc_ids.push(129);
-        doc_ids.push(130);
-        {
-            let block_segments = build_block_postings(&doc_ids);
-            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
-            assert_eq!(docset.seek(128), 129);
-            assert_eq!(docset.doc(), 129);
-            assert_eq!(docset.advance(), 130);
-            assert_eq!(docset.doc(), 130);
-            assert_eq!(docset.advance(), TERMINATED);
-        }
-        {
-            let block_segments = build_block_postings(&doc_ids);
-            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
-            assert_eq!(docset.seek(129), 129);
-            assert_eq!(docset.doc(), 129);
-            assert_eq!(docset.advance(), 130);
-            assert_eq!(docset.doc(), 130);
-            assert_eq!(docset.advance(), TERMINATED);
-        }
-        {
-            let block_segments = build_block_postings(&doc_ids);
-            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
-            assert_eq!(docset.doc(), 0);
-            assert_eq!(docset.seek(131), TERMINATED);
-            assert_eq!(docset.doc(), TERMINATED);
-        }
-    }
-
-    fn build_block_postings(docs: &[DocId]) -> BlockSegmentPostings {
-        let mut schema_builder = Schema::builder();
-        let int_field = schema_builder.add_u64_field("id", INDEXED);
-        let schema = schema_builder.build();
-        let index = Index::create_in_ram(schema);
-        let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-        let mut last_doc = 0u32;
-        for &doc in docs {
-            for _ in last_doc..doc {
-                index_writer.add_document(doc!(int_field=>1u64));
-            }
-            index_writer.add_document(doc!(int_field=>0u64));
-            last_doc = doc + 1;
-        }
-        index_writer.commit().unwrap();
-        let searcher = index.reader().unwrap().searcher();
-        let segment_reader = searcher.segment_reader(0);
-        let inverted_index = segment_reader.inverted_index(int_field);
-        let term = Term::from_field_u64(int_field, 0u64);
-        let term_info = inverted_index.get_term_info(&term).unwrap();
-        inverted_index.read_block_postings_from_terminfo(&term_info, IndexRecordOption::Basic)
-    }
-
-    #[test]
-    fn test_block_segment_postings_skip2() {
-        let mut docs = vec![0];
-        for i in 0..1300 {
-            docs.push((i * i / 100) + i);
-        }
-        let mut block_postings = build_block_postings(&docs[..]);
-        for i in vec![0, 424, 10000] {
-            block_postings.seek(i);
-            let docs = block_postings.docs();
-            assert!(docs[0] <= i);
-            assert!(docs.last().cloned().unwrap_or(0u32) >= i);
-        }
-        block_postings.seek(100_000);
-        assert_eq!(block_postings.doc(COMPRESSION_BLOCK_SIZE - 1), TERMINATED);
-    }
-
-    #[test]
-    fn test_reset_block_segment_postings() {
-        let mut schema_builder = Schema::builder();
-        let int_field = schema_builder.add_u64_field("id", INDEXED);
-        let schema = schema_builder.build();
-        let index = Index::create_in_ram(schema);
-        let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-        // create two postings list, one containg even number,
-        // the other containing odd numbers.
-        for i in 0..6 {
-            let doc = doc!(int_field=> (i % 2) as u64);
-            index_writer.add_document(doc);
-        }
-        index_writer.commit().unwrap();
-        let searcher = index.reader().unwrap().searcher();
-        let segment_reader = searcher.segment_reader(0);
-
-        let mut block_segments;
-        {
-            let term = Term::from_field_u64(int_field, 0u64);
-            let inverted_index = segment_reader.inverted_index(int_field);
-            let term_info = inverted_index.get_term_info(&term).unwrap();
-            block_segments = inverted_index
-                .read_block_postings_from_terminfo(&term_info, IndexRecordOption::Basic);
-        }
-        assert_eq!(block_segments.docs(), &[0, 2, 4]);
-        {
-            let term = Term::from_field_u64(int_field, 1u64);
-            let inverted_index = segment_reader.inverted_index(int_field);
-            let term_info = inverted_index.get_term_info(&term).unwrap();
-            inverted_index.reset_block_postings_from_terminfo(&term_info, &mut block_segments);
-        }
-        assert!(block_segments.advance());
-        assert_eq!(block_segments.docs(), &[1, 3, 5]);
-    }
 }

src/postings/compression/mod.rs

@@ -1,5 +1,4 @@
 use crate::common::FixedSize;
-use crate::docset::TERMINATED;
 use bitpacking::{BitPacker, BitPacker4x};
 
 pub const COMPRESSION_BLOCK_SIZE: usize = BitPacker4x::BLOCK_LEN;
@@ -18,12 +17,6 @@ pub struct BlockEncoder {
     pub output_len: usize,
 }
 
-impl Default for BlockEncoder {
-    fn default() -> Self {
-        BlockEncoder::new()
-    }
-}
-
 impl BlockEncoder {
     pub fn new() -> BlockEncoder {
         BlockEncoder {
@@ -61,13 +54,11 @@ pub struct BlockDecoder {
     pub output_len: usize,
 }
 
-impl Default for BlockDecoder {
-    fn default() -> Self {
+impl BlockDecoder {
+    pub fn new() -> BlockDecoder {
         BlockDecoder::with_val(0u32)
     }
-}
 
-impl BlockDecoder {
     pub fn with_val(val: u32) -> BlockDecoder {
         BlockDecoder {
             bitpacker: BitPacker4x::new(),
@@ -99,18 +90,14 @@ impl BlockDecoder {
     }
 
     #[inline]
-    pub(crate) fn output_aligned(&self) -> &AlignedBuffer {
-        &self.output
+    pub(crate) fn output_aligned(&self) -> (&AlignedBuffer, usize) {
+        (&self.output, self.output_len)
     }
 
     #[inline]
     pub fn output(&self, idx: usize) -> u32 {
         self.output.0[idx]
     }
-
-    pub fn clear(&mut self) {
-        self.output.0.iter_mut().for_each(|el| *el = TERMINATED);
-    }
 }
 
 pub trait VIntEncoder {
@@ -147,9 +134,9 @@ pub trait VIntDecoder {
     /// For instance, if delta encoded are `1, 3, 9`, and the
     /// `offset` is 5, then the output will be:
     /// `5 + 1 = 6, 6 + 3= 9, 9 + 9 = 18`
-    fn uncompress_vint_sorted(
+    fn uncompress_vint_sorted<'a>(
         &mut self,
-        compressed_data: &[u8],
+        compressed_data: &'a [u8],
         offset: u32,
         num_els: usize,
     ) -> usize;
@@ -159,7 +146,7 @@ pub trait VIntDecoder {
     ///
     /// The method takes a number of int to decompress, and returns
     /// the amount of bytes that were read to decompress them.
-    fn uncompress_vint_unsorted(&mut self, compressed_data: &[u8], num_els: usize) -> usize;
+    fn uncompress_vint_unsorted<'a>(&mut self, compressed_data: &'a [u8], num_els: usize) -> usize;
 }
 
 impl VIntEncoder for BlockEncoder {
@@ -173,9 +160,9 @@ impl VIntEncoder for BlockEncoder {
 }
 
 impl VIntDecoder for BlockDecoder {
-    fn uncompress_vint_sorted(
+    fn uncompress_vint_sorted<'a>(
         &mut self,
-        compressed_data: &[u8],
+        compressed_data: &'a [u8],
         offset: u32,
         num_els: usize,
     ) -> usize {
@@ -183,7 +170,7 @@ impl VIntDecoder for BlockDecoder {
         vint::uncompress_sorted(compressed_data, &mut self.output.0[..num_els], offset)
     }
 
-    fn uncompress_vint_unsorted(&mut self, compressed_data: &[u8], num_els: usize) -> usize {
+    fn uncompress_vint_unsorted<'a>(&mut self, compressed_data: &'a [u8], num_els: usize) -> usize {
         self.output_len = num_els;
         vint::uncompress_unsorted(compressed_data, &mut self.output.0[..num_els])
     }
@@ -199,7 +186,7 @@ pub mod tests {
         let vals: Vec<u32> = (0u32..128u32).map(|i| i * 7).collect();
         let mut encoder = BlockEncoder::new();
         let (num_bits, compressed_data) = encoder.compress_block_sorted(&vals, 0);
-        let mut decoder = BlockDecoder::default();
+        let mut decoder = BlockDecoder::new();
         {
             let consumed_num_bytes = decoder.uncompress_block_sorted(compressed_data, 0, num_bits);
             assert_eq!(consumed_num_bytes, compressed_data.len());
@@ -212,9 +199,9 @@ pub mod tests {
     #[test]
     fn test_encode_sorted_block_with_offset() {
         let vals: Vec<u32> = (0u32..128u32).map(|i| 11 + i * 7).collect();
-        let mut encoder = BlockEncoder::default();
+        let mut encoder = BlockEncoder::new();
         let (num_bits, compressed_data) = encoder.compress_block_sorted(&vals, 10);
-        let mut decoder = BlockDecoder::default();
+        let mut decoder = BlockDecoder::new();
         {
             let consumed_num_bytes = decoder.uncompress_block_sorted(compressed_data, 10, num_bits);
             assert_eq!(consumed_num_bytes, compressed_data.len());
@@ -229,11 +216,11 @@ pub mod tests {
         let mut compressed: Vec<u8> = Vec::new();
         let n = 128;
         let vals: Vec<u32> = (0..n).map(|i| 11u32 + (i as u32) * 7u32).collect();
-        let mut encoder = BlockEncoder::default();
+        let mut encoder = BlockEncoder::new();
         let (num_bits, compressed_data) = encoder.compress_block_sorted(&vals, 10);
         compressed.extend_from_slice(compressed_data);
         compressed.push(173u8);
-        let mut decoder = BlockDecoder::default();
+        let mut decoder = BlockDecoder::new();
         {
             let consumed_num_bytes = decoder.uncompress_block_sorted(&compressed, 10, num_bits);
             assert_eq!(consumed_num_bytes, compressed.len() - 1);
@@ -249,11 +236,11 @@ pub mod tests {
         let mut compressed: Vec<u8> = Vec::new();
         let n = 128;
         let vals: Vec<u32> = (0..n).map(|i| 11u32 + (i as u32) * 7u32 % 12).collect();
-        let mut encoder = BlockEncoder::default();
+        let mut encoder = BlockEncoder::new();
         let (num_bits, compressed_data) = encoder.compress_block_unsorted(&vals);
         compressed.extend_from_slice(compressed_data);
         compressed.push(173u8);
-        let mut decoder = BlockDecoder::default();
+        let mut decoder = BlockDecoder::new();
         {
             let consumed_num_bytes = decoder.uncompress_block_unsorted(&compressed, num_bits);
             assert_eq!(consumed_num_bytes + 1, compressed.len());
@@ -264,11 +251,6 @@ pub mod tests {
         }
     }
 
-    #[test]
-    fn test_block_decoder_initialization() {
-        let block = BlockDecoder::with_val(TERMINATED);
-        assert_eq!(block.output(0), TERMINATED);
-    }
     #[test]
     fn test_encode_vint() {
         {
@@ -278,7 +260,7 @@ pub mod tests {
             for offset in &[0u32, 1u32, 2u32] {
                 let encoded_data = encoder.compress_vint_sorted(&input, *offset);
                 assert!(encoded_data.len() <= expected_length);
-                let mut decoder = BlockDecoder::default();
+                let mut decoder = BlockDecoder::new();
                 let consumed_num_bytes =
                     decoder.uncompress_vint_sorted(&encoded_data, *offset, input.len());
                 assert_eq!(consumed_num_bytes, encoded_data.len());

src/postings/compression/vint.rs

@@ -42,7 +42,7 @@ pub(crate) fn compress_unsorted<'a>(input: &[u32], output: &'a mut [u8]) -> &'a
 }
 
 #[inline(always)]
-pub fn uncompress_sorted(compressed_data: &[u8], output: &mut [u32], offset: u32) -> usize {
+pub fn uncompress_sorted<'a>(compressed_data: &'a [u8], output: &mut [u32], offset: u32) -> usize {
     let mut read_byte = 0;
     let mut result = offset;
     for output_mut in output.iter_mut() {

src/postings/mod.rs

@@ -2,13 +2,19 @@
 Postings module (also called inverted index)
 */
 
+mod block_max_postings;
+mod block_max_segment_postings;
 mod block_search;
-mod block_segment_postings;
 pub(crate) mod compression;
+/// Postings module
+///
+/// Postings, also called inverted lists, is the key datastructure
+/// to full-text search.
 mod postings;
 mod postings_writer;
 mod recorder;
 mod segment_postings;
+mod block_segment_postings;
 mod serializer;
 mod skip;
 mod stacker;
@@ -19,17 +25,22 @@ pub(crate) use self::block_search::BlockSearcher;
 pub(crate) use self::postings_writer::MultiFieldPostingsWriter;
 pub use self::serializer::{FieldSerializer, InvertedIndexSerializer};
 
+use self::compression::COMPRESSION_BLOCK_SIZE;
 pub use self::postings::Postings;
-pub(crate) use self::skip::{BlockInfo, SkipReader};
+pub(crate) use self::skip::SkipReader;
 pub use self::term_info::TermInfo;
 
-pub use self::block_segment_postings::BlockSegmentPostings;
 pub use self::segment_postings::SegmentPostings;
 
+pub use self::block_segment_postings::BlockSegmentPostings;
+pub use self::block_max_postings::BlockMaxPostings;
+pub use self::block_max_segment_postings::BlockMaxSegmentPostings;
+
 pub(crate) use self::stacker::compute_table_size;
 
 pub use crate::common::HasLen;
 
+pub(crate) const USE_SKIP_INFO_LIMIT: u32 = COMPRESSION_BLOCK_SIZE as u32;
 pub(crate) type UnorderedTermId = u64;
 
 #[cfg_attr(feature = "cargo-clippy", allow(clippy::enum_variant_names))]
@@ -47,7 +58,7 @@ pub mod tests {
     use crate::core::Index;
     use crate::core::SegmentComponent;
     use crate::core::SegmentReader;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::{DocSet, SkipResult};
     use crate::fieldnorm::FieldNormReader;
     use crate::indexer::operation::AddOperation;
     use crate::indexer::SegmentWriter;
@@ -111,12 +122,29 @@ pub mod tests {
             let mut postings = inverted_index
                 .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
                 .unwrap();
-            assert_eq!(postings.doc(), 0);
+            postings.advance();
             postings.positions(&mut positions);
             assert_eq!(&[0, 1, 2], &positions[..]);
             postings.positions(&mut positions);
             assert_eq!(&[0, 1, 2], &positions[..]);
-            assert_eq!(postings.advance(), 1);
+            postings.advance();
+            postings.positions(&mut positions);
+            assert_eq!(&[0, 5], &positions[..]);
+        }
+        {
+            let mut postings = inverted_index
+                .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
+                .unwrap();
+            postings.advance();
+            postings.advance();
+            postings.positions(&mut positions);
+            assert_eq!(&[0, 5], &positions[..]);
+        }
+        {
+            let mut postings = inverted_index
+                .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
+                .unwrap();
+            assert_eq!(postings.skip_next(1), SkipResult::Reached);
             assert_eq!(postings.doc(), 1);
             postings.positions(&mut positions);
             assert_eq!(&[0, 5], &positions[..]);
@@ -125,25 +153,7 @@ pub mod tests {
             let mut postings = inverted_index
                 .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
                 .unwrap();
-            assert_eq!(postings.doc(), 0);
-            assert_eq!(postings.advance(), 1);
-            postings.positions(&mut positions);
-            assert_eq!(&[0, 5], &positions[..]);
-        }
-        {
-            let mut postings = inverted_index
-                .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
-                .unwrap();
-            assert_eq!(postings.seek(1), 1);
-            assert_eq!(postings.doc(), 1);
-            postings.positions(&mut positions);
-            assert_eq!(&[0, 5], &positions[..]);
-        }
-        {
-            let mut postings = inverted_index
-                .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
-                .unwrap();
-            assert_eq!(postings.seek(1002), 1002);
+            assert_eq!(postings.skip_next(1002), SkipResult::Reached);
             assert_eq!(postings.doc(), 1002);
             postings.positions(&mut positions);
             assert_eq!(&[0, 5], &positions[..]);
@@ -152,8 +162,8 @@ pub mod tests {
             let mut postings = inverted_index
                 .read_postings(&term, IndexRecordOption::WithFreqsAndPositions)
                 .unwrap();
-            assert_eq!(postings.seek(100), 100);
-            assert_eq!(postings.seek(1002), 1002);
+            assert_eq!(postings.skip_next(100), SkipResult::Reached);
+            assert_eq!(postings.skip_next(1002), SkipResult::Reached);
             assert_eq!(postings.doc(), 1002);
             postings.positions(&mut positions);
             assert_eq!(&[0, 5], &positions[..]);
@@ -278,21 +288,22 @@ pub mod tests {
                     .read_postings(&term_a, IndexRecordOption::WithFreqsAndPositions)
                     .unwrap();
                 assert_eq!(postings_a.len(), 1000);
+                assert!(postings_a.advance());
                 assert_eq!(postings_a.doc(), 0);
                 assert_eq!(postings_a.term_freq(), 6);
                 postings_a.positions(&mut positions);
                 assert_eq!(&positions[..], [0, 2, 4, 6, 7, 13]);
-                assert_eq!(postings_a.advance(), 1u32);
+                assert!(postings_a.advance());
                 assert_eq!(postings_a.doc(), 1u32);
                 assert_eq!(postings_a.term_freq(), 1);
                 for i in 2u32..1000u32 {
-                    assert_eq!(postings_a.advance(), i);
+                    assert!(postings_a.advance());
                     assert_eq!(postings_a.term_freq(), 1);
                     postings_a.positions(&mut positions);
                     assert_eq!(&positions[..], [i]);
                     assert_eq!(postings_a.doc(), i);
                 }
-                assert_eq!(postings_a.advance(), TERMINATED);
+                assert!(!postings_a.advance());
             }
             {
                 let term_e = Term::from_field_text(text_field, "e");
@@ -302,6 +313,7 @@ pub mod tests {
                     .unwrap();
                 assert_eq!(postings_e.len(), 1000 - 2);
                 for i in 2u32..1000u32 {
+                    assert!(postings_e.advance());
                     assert_eq!(postings_e.term_freq(), i);
                     postings_e.positions(&mut positions);
                     assert_eq!(positions.len(), i as usize);
@@ -309,9 +321,8 @@ pub mod tests {
                         assert_eq!(positions[j], (j as u32));
                     }
                     assert_eq!(postings_e.doc(), i);
-                    postings_e.advance();
                 }
-                assert_eq!(postings_e.doc(), TERMINATED);
+                assert!(!postings_e.advance());
             }
         }
     }
@@ -325,8 +336,16 @@ pub mod tests {
         let index = Index::create_in_ram(schema);
         {
             let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-            index_writer.add_document(doc!(text_field => "g b b d c g c"));
-            index_writer.add_document(doc!(text_field => "g a b b a d c g c"));
+            {
+                let mut doc = Document::default();
+                doc.add_text(text_field, "g b b d c g c");
+                index_writer.add_document(doc);
+            }
+            {
+                let mut doc = Document::default();
+                doc.add_text(text_field, "g a b b a d c g c");
+                index_writer.add_document(doc);
+            }
             assert!(index_writer.commit().is_ok());
         }
         let term_a = Term::from_field_text(text_field, "a");
@@ -336,6 +355,7 @@ pub mod tests {
             .inverted_index(text_field)
             .read_postings(&term_a, IndexRecordOption::WithFreqsAndPositions)
             .unwrap();
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 1u32);
         postings.positions(&mut positions);
         assert_eq!(&positions[..], &[1u32, 4]);
@@ -357,8 +377,11 @@ pub mod tests {
             let index = Index::create_in_ram(schema);
             {
                 let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-                for i in 0u64..num_docs as u64 {
-                    let doc = doc!(value_field => 2u64, value_field => i % 2u64);
+                for i in 0..num_docs {
+                    let mut doc = Document::default();
+                    doc.add_u64(value_field, 2);
+                    doc.add_u64(value_field, (i % 2) as u64);
+
                     index_writer.add_document(doc);
                 }
                 assert!(index_writer.commit().is_ok());
@@ -375,10 +398,11 @@ pub mod tests {
                     .inverted_index(term_2.field())
                     .read_postings(&term_2, IndexRecordOption::Basic)
                     .unwrap();
-                assert_eq!(segment_postings.seek(i), i);
+
+                assert_eq!(segment_postings.skip_next(i), SkipResult::Reached);
                 assert_eq!(segment_postings.doc(), i);
 
-                assert_eq!(segment_postings.seek(j), j);
+                assert_eq!(segment_postings.skip_next(j), SkipResult::Reached);
                 assert_eq!(segment_postings.doc(), j);
             }
         }
@@ -390,16 +414,17 @@ pub mod tests {
                 .unwrap();
 
             // check that `skip_next` advances the iterator
+            assert!(segment_postings.advance());
             assert_eq!(segment_postings.doc(), 0);
 
-            assert_eq!(segment_postings.seek(1), 1);
+            assert_eq!(segment_postings.skip_next(1), SkipResult::Reached);
             assert_eq!(segment_postings.doc(), 1);
 
-            assert_eq!(segment_postings.seek(1), 1);
-            assert_eq!(segment_postings.doc(), 1);
+            assert_eq!(segment_postings.skip_next(1), SkipResult::OverStep);
+            assert_eq!(segment_postings.doc(), 2);
 
             // check that going beyond the end is handled
-            assert_eq!(segment_postings.seek(num_docs), TERMINATED);
+            assert_eq!(segment_postings.skip_next(num_docs), SkipResult::End);
         }
 
         // check that filtering works
@@ -410,7 +435,7 @@ pub mod tests {
                 .unwrap();
 
             for i in 0..num_docs / 2 {
-                assert_eq!(segment_postings.seek(i * 2), i * 2);
+                assert_eq!(segment_postings.skip_next(i * 2), SkipResult::Reached);
                 assert_eq!(segment_postings.doc(), i * 2);
             }
 
@@ -420,7 +445,7 @@ pub mod tests {
                 .unwrap();
 
             for i in 0..num_docs / 2 - 1 {
-                assert!(segment_postings.seek(i * 2 + 1) > (i * 1) * 2);
+                assert_eq!(segment_postings.skip_next(i * 2 + 1), SkipResult::OverStep);
                 assert_eq!(segment_postings.doc(), (i + 1) * 2);
             }
         }
@@ -432,7 +457,6 @@ pub mod tests {
             assert!(index_writer.commit().is_ok());
         }
         let searcher = index.reader().unwrap().searcher();
-        assert_eq!(searcher.segment_readers().len(), 1);
         let segment_reader = searcher.segment_reader(0);
 
         // make sure seeking still works
@@ -443,11 +467,11 @@ pub mod tests {
                 .unwrap();
 
             if i % 2 == 0 {
-                assert_eq!(segment_postings.seek(i), i);
+                assert_eq!(segment_postings.skip_next(i), SkipResult::Reached);
                 assert_eq!(segment_postings.doc(), i);
                 assert!(segment_reader.is_deleted(i));
             } else {
-                assert_eq!(segment_postings.seek(i), i);
+                assert_eq!(segment_postings.skip_next(i), SkipResult::Reached);
                 assert_eq!(segment_postings.doc(), i);
             }
         }
@@ -462,16 +486,12 @@ pub mod tests {
             let mut last = 2; // start from 5 to avoid seeking to 3 twice
             let mut cur = 3;
             loop {
-                let seek = segment_postings.seek(cur);
-                if seek == TERMINATED {
-                    break;
-                }
-                assert_eq!(seek, segment_postings.doc());
-                if seek == cur {
-                    assert_eq!(segment_postings.doc(), cur);
-                } else {
-                    assert_eq!(segment_postings.doc(), cur + 1);
+                match segment_postings.skip_next(cur) {
+                    SkipResult::End => break,
+                    SkipResult::Reached => assert_eq!(segment_postings.doc(), cur),
+                    SkipResult::OverStep => assert_eq!(segment_postings.doc(), cur + 1),
                 }
+
                 let next = cur + last;
                 last = cur;
                 cur = next;
@@ -557,7 +577,7 @@ pub mod tests {
     }
 
     impl<TDocSet: DocSet> DocSet for UnoptimizedDocSet<TDocSet> {
-        fn advance(&mut self) -> DocId {
+        fn advance(&mut self) -> bool {
             self.0.advance()
         }
 
@@ -583,22 +603,30 @@ pub mod tests {
         for target in targets {
             let mut postings_opt = postings_factory();
             let mut postings_unopt = UnoptimizedDocSet::wrap(postings_factory());
-            let skip_result_opt = postings_opt.seek(target);
-            let skip_result_unopt = postings_unopt.seek(target);
+            let skip_result_opt = postings_opt.skip_next(target);
+            let skip_result_unopt = postings_unopt.skip_next(target);
             assert_eq!(
                 skip_result_unopt, skip_result_opt,
                 "Failed while skipping to {}",
                 target
             );
-            assert!(skip_result_opt >= target);
-            assert_eq!(skip_result_opt, postings_opt.doc());
-            if skip_result_opt == TERMINATED {
-                return;
+            match skip_result_opt {
+                SkipResult::Reached => assert_eq!(postings_opt.doc(), target),
+                SkipResult::OverStep => assert!(postings_opt.doc() > target),
+                SkipResult::End => {
+                    return;
+                }
             }
-            while postings_opt.doc() != TERMINATED {
-                assert_eq!(postings_opt.doc(), postings_unopt.doc());
-                assert_eq!(postings_opt.advance(), postings_unopt.advance());
+            while postings_opt.advance() {
+                assert!(postings_unopt.advance());
+                assert_eq!(
+                    postings_opt.doc(),
+                    postings_unopt.doc(),
+                    "Failed while skipping to {}",
+                    target
+                );
             }
+            assert!(!postings_unopt.advance());
         }
     }
 }
@@ -607,7 +635,7 @@ pub mod tests {
 mod bench {
 
     use super::tests::*;
-    use crate::docset::TERMINATED;
+    use crate::docset::SkipResult;
     use crate::query::Intersection;
     use crate::schema::IndexRecordOption;
     use crate::tests;
@@ -625,7 +653,7 @@ mod bench {
                 .inverted_index(TERM_A.field())
                 .read_postings(&*TERM_A, IndexRecordOption::Basic)
                 .unwrap();
-            while segment_postings.advance() != TERMINATED {}
+            while segment_postings.advance() {}
         });
     }
 
@@ -657,7 +685,7 @@ mod bench {
                 segment_postings_c,
                 segment_postings_d,
             ]);
-            while intersection.advance() != TERMINATED {}
+            while intersection.advance() {}
         });
     }
 
@@ -673,10 +701,11 @@ mod bench {
             .unwrap();
 
         let mut existing_docs = Vec::new();
+        segment_postings.advance();
         for doc in &docs {
             if *doc >= segment_postings.doc() {
                 existing_docs.push(*doc);
-                if segment_postings.seek(*doc) == TERMINATED {
+                if segment_postings.skip_next(*doc) == SkipResult::End {
                     break;
                 }
             }
@@ -688,7 +717,7 @@ mod bench {
                 .read_postings(&*TERM_A, IndexRecordOption::Basic)
                 .unwrap();
             for doc in &existing_docs {
-                if segment_postings.seek(*doc) == TERMINATED {
+                if segment_postings.skip_next(*doc) == SkipResult::End {
                     break;
                 }
             }
@@ -727,9 +756,8 @@ mod bench {
                 .read_postings(&*TERM_A, IndexRecordOption::Basic)
                 .unwrap();
             let mut s = 0u32;
-            while segment_postings.doc() != TERMINATED {
+            while segment_postings.advance() {
                 s += (segment_postings.doc() & n) % 1024;
-                segment_postings.advance()
             }
             s
         });

src/postings/segment_postings.rs

@@ -1,19 +1,51 @@
+use crate::common::BitSet;
 use crate::common::HasLen;
-
-use crate::docset::DocSet;
+use crate::docset::{DocSet, SkipResult};
 use crate::positions::PositionReader;
-
 use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
 use crate::postings::serializer::PostingsSerializer;
-use crate::postings::BlockSearcher;
-
+use crate::postings::{BlockSearcher, BlockSegmentPostings};
 use crate::postings::Postings;
-
 use crate::schema::IndexRecordOption;
 use crate::DocId;
+use owned_read::OwnedRead;
+use std::cmp::Ordering;
+use crate::postings::block_segment_postings::BlockSegmentPostingsSkipResult;
 
-use crate::directory::ReadOnlySource;
-use crate::postings::BlockSegmentPostings;
+struct PositionComputer {
+    // store the amount of position int
+    // before reading positions.
+    //
+    // if none, position are already loaded in
+    // the positions vec.
+    position_to_skip: usize,
+    position_reader: PositionReader,
+}
+
+impl PositionComputer {
+    pub fn new(position_reader: PositionReader) -> PositionComputer {
+        PositionComputer {
+            position_to_skip: 0,
+            position_reader,
+        }
+    }
+
+    pub fn add_skip(&mut self, num_skip: usize) {
+        self.position_to_skip += num_skip;
+    }
+
+    // Positions can only be read once.
+    pub fn positions_with_offset(&mut self, offset: u32, output: &mut [u32]) {
+        self.position_reader.skip(self.position_to_skip);
+        self.position_to_skip = 0;
+        self.position_reader.read(output);
+        let mut cum = offset;
+        for output_mut in output.iter_mut() {
+            cum += *output_mut;
+            *output_mut = cum;
+        }
+    }
+}
 
 /// `SegmentPostings` represents the inverted list or postings associated to
 /// a term in a `Segment`.
@@ -23,17 +55,18 @@ use crate::postings::BlockSegmentPostings;
 pub struct SegmentPostings {
     block_cursor: BlockSegmentPostings,
     cur: usize,
-    position_reader: Option<PositionReader>,
+    position_computer: Option<PositionComputer>,
     block_searcher: BlockSearcher,
 }
 
 impl SegmentPostings {
     /// Returns an empty segment postings object
     pub fn empty() -> Self {
+        let empty_block_cursor = BlockSegmentPostings::empty();
         SegmentPostings {
-            block_cursor: BlockSegmentPostings::empty(),
-            cur: 0,
-            position_reader: None,
+            block_cursor: empty_block_cursor,
+            cur: COMPRESSION_BLOCK_SIZE,
+            position_computer: None,
             block_searcher: BlockSearcher::default(),
         }
     }
@@ -59,13 +92,15 @@ impl SegmentPostings {
         }
         let block_segment_postings = BlockSegmentPostings::from_data(
             docs.len() as u32,
-            ReadOnlySource::from(buffer),
+            OwnedRead::new(buffer),
             IndexRecordOption::Basic,
             IndexRecordOption::Basic,
         );
         SegmentPostings::from_block_postings(block_segment_postings, None)
     }
+}
 
+impl SegmentPostings {
     /// Reads a Segment postings from an &[u8]
     ///
     /// * `len` - number of document in the posting lists.
@@ -74,12 +109,12 @@ impl SegmentPostings {
     ///   frequencies and/or positions
     pub(crate) fn from_block_postings(
         segment_block_postings: BlockSegmentPostings,
-        position_reader: Option<PositionReader>,
+        positions_stream_opt: Option<PositionReader>,
     ) -> SegmentPostings {
         SegmentPostings {
             block_cursor: segment_block_postings,
-            cur: 0, // cursor within the block
-            position_reader,
+            cur: COMPRESSION_BLOCK_SIZE, // cursor within the block
+            position_computer: positions_stream_opt.map(PositionComputer::new),
             block_searcher: BlockSearcher::default(),
         }
     }
@@ -89,52 +124,134 @@ impl DocSet for SegmentPostings {
     // goes to the next element.
     // next needs to be called a first time to point to the correct element.
     #[inline]
-    fn advance(&mut self) -> DocId {
-        if self.cur == COMPRESSION_BLOCK_SIZE - 1 {
-            self.cur = 0;
-            self.block_cursor.advance();
-        } else {
-            self.cur += 1;
+    fn advance(&mut self) -> bool {
+        if self.position_computer.is_some() && self.cur < COMPRESSION_BLOCK_SIZE {
+            let term_freq = self.term_freq() as usize;
+            if let Some(position_computer) = self.position_computer.as_mut() {
+                position_computer.add_skip(term_freq);
+            }
         }
-        self.doc()
+        self.cur += 1;
+        if self.cur >= self.block_cursor.block_len() {
+            self.cur = 0;
+            if !self.block_cursor.advance() {
+                self.cur = COMPRESSION_BLOCK_SIZE;
+                return false;
+            }
+        }
+        true
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        if self.doc() == target {
-            return target;
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        if !self.advance() {
+            return SkipResult::End;
+        }
+        match self.doc().cmp(&target) {
+            Ordering::Equal => {
+                return SkipResult::Reached;
+            }
+            Ordering::Greater => {
+                return SkipResult::OverStep;
+            }
+            _ => {
+                // ...
+            }
         }
-        self.block_cursor.seek(target);
 
-        // At this point we are on the block, that might contain our document.
-        let output = self.block_cursor.docs_aligned();
+        // In the following, thanks to the call to advance above,
+        // we know that the position is not loaded and we need
+        // to skip every doc_freq we cross.
 
-        self.cur = self.block_searcher.search_in_block(&output, target);
+        // skip blocks until one that might contain the target
+        // check if we need to go to the next block
+        let mut sum_freqs_skipped: u32 = 0;
+        if !self
+            .block_cursor
+            .docs()
+            .last()
+            .map(|doc| *doc >= target)
+            .unwrap_or(false)
+        // there should always be at least a document in the block
+        // since advance returned.
+        {
+            // we are not in the right block.
+            //
+            // First compute all of the freqs skipped from the current block.
+            if self.position_computer.is_some() {
+                sum_freqs_skipped = self.block_cursor.freqs()[self.cur..].iter().sum();
+                match self.block_cursor.skip_to(target) {
+                    BlockSegmentPostingsSkipResult::Success(block_skip_freqs) => {
+                        sum_freqs_skipped += block_skip_freqs;
+                    }
+                    BlockSegmentPostingsSkipResult::Terminated => {
+                        return SkipResult::End;
+                    }
+                }
+            } else if self.block_cursor.skip_to(target)
+                == BlockSegmentPostingsSkipResult::Terminated
+            {
+                // no positions needed. no need to sum freqs.
+                return SkipResult::End;
+            }
+            self.cur = 0;
+        }
 
-        // The last block is not full and padded with the value TERMINATED,
-        // so that we are guaranteed to have at least doc in the block (a real one or the padding)
-        // that is greater or equal to the target.
-        debug_assert!(self.cur < COMPRESSION_BLOCK_SIZE);
+        let cur = self.cur;
+
+        // we're in the right block now, start with an exponential search
+        let (output, len) = self.block_cursor.docs_aligned();
+        let new_cur = self
+            .block_searcher
+            .search_in_block(&output, len, cur, target);
+        if let Some(position_computer) = self.position_computer.as_mut() {
+            sum_freqs_skipped += self.block_cursor.freqs()[cur..new_cur].iter().sum::<u32>();
+            position_computer.add_skip(sum_freqs_skipped as usize);
+        }
+        self.cur = new_cur;
 
         // `doc` is now the first element >= `target`
-
-        // If all docs are smaller than target the current block should be incomplemented and padded
-        // with the value `TERMINATED`.
-        //
-        // After the search, the cursor should point to the first value of TERMINATED.
-        let doc = output.0[self.cur];
+        let doc = output.0[new_cur];
         debug_assert!(doc >= target);
-        doc
+        if doc == target {
+            SkipResult::Reached
+        } else {
+            SkipResult::OverStep
+        }
     }
 
     /// Return the current document's `DocId`.
+    ///
+    /// # Panics
+    ///
+    /// Will panics if called without having called advance before.
     #[inline]
     fn doc(&self) -> DocId {
-        self.block_cursor.doc(self.cur)
+        let docs = self.block_cursor.docs();
+        debug_assert!(
+            self.cur < docs.len(),
+            "Have you forgotten to call `.advance()` at least once before calling `.doc()`                                      ."
+        );
+        docs[self.cur]
     }
 
     fn size_hint(&self) -> u32 {
         self.len() as u32
     }
+
+    fn append_to_bitset(&mut self, bitset: &mut BitSet) {
+        // finish the current block
+        if self.advance() {
+            for &doc in &self.block_cursor.docs()[self.cur..] {
+                bitset.insert(doc);
+            }
+            // ... iterate through the remaining blocks.
+            while self.block_cursor.advance() {
+                for &doc in self.block_cursor.docs() {
+                    bitset.insert(doc);
+                }
+            }
+        }
+    }
 }
 
 impl HasLen for SegmentPostings {
@@ -168,19 +285,9 @@ impl Postings for SegmentPostings {
 
     fn positions_with_offset(&mut self, offset: u32, output: &mut Vec<u32>) {
         let term_freq = self.term_freq() as usize;
-        if let Some(position_reader) = self.position_reader.as_mut() {
-            let read_offset = self.block_cursor.position_offset()
-                + (self.block_cursor.freqs()[..self.cur]
-                    .iter()
-                    .cloned()
-                    .sum::<u32>() as u64);
+        if let Some(position_comp) = self.position_computer.as_mut() {
             output.resize(term_freq, 0u32);
-            position_reader.read(read_offset, &mut output[..]);
-            let mut cum = offset;
-            for output_mut in output.iter_mut() {
-                cum += *output_mut;
-                *output_mut = cum;
-            }
+            position_comp.positions_with_offset(offset, &mut output[..]);
         } else {
             output.clear();
         }
@@ -189,31 +296,197 @@ impl Postings for SegmentPostings {
 
 #[cfg(test)]
 mod tests {
-
+    use super::BlockSegmentPostings;
+    use super::BlockSegmentPostingsSkipResult;
     use super::SegmentPostings;
     use crate::common::HasLen;
-
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::core::Index;
+    use crate::docset::DocSet;
     use crate::postings::postings::Postings;
+    use crate::schema::IndexRecordOption;
+    use crate::schema::Schema;
+    use crate::schema::Term;
+    use crate::schema::INDEXED;
+    use crate::DocId;
+    use crate::SkipResult;
+    use tantivy_fst::Streamer;
 
     #[test]
     fn test_empty_segment_postings() {
         let mut postings = SegmentPostings::empty();
-        assert_eq!(postings.advance(), TERMINATED);
-        assert_eq!(postings.advance(), TERMINATED);
+        assert!(!postings.advance());
+        assert!(!postings.advance());
         assert_eq!(postings.len(), 0);
     }
 
     #[test]
-    fn test_empty_postings_doc_returns_terminated() {
-        let mut postings = SegmentPostings::empty();
-        assert_eq!(postings.doc(), TERMINATED);
-        assert_eq!(postings.advance(), TERMINATED);
+    #[should_panic(expected = "Have you forgotten to call `.advance()`")]
+    fn test_panic_if_doc_called_before_advance() {
+        SegmentPostings::empty().doc();
     }
 
     #[test]
-    fn test_empty_postings_doc_term_freq_returns_0() {
-        let postings = SegmentPostings::empty();
-        assert_eq!(postings.term_freq(), 1);
+    #[should_panic(expected = "Have you forgotten to call `.advance()`")]
+    fn test_panic_if_freq_called_before_advance() {
+        SegmentPostings::empty().term_freq();
+    }
+
+    #[test]
+    fn test_empty_block_segment_postings() {
+        let mut postings = BlockSegmentPostings::empty();
+        assert!(!postings.advance());
+        assert_eq!(postings.doc_freq(), 0);
+    }
+
+    #[test]
+    fn test_block_segment_postings() {
+        let mut block_segments = build_block_postings(&(0..100_000).collect::<Vec<u32>>());
+        let mut offset: u32 = 0u32;
+        // checking that the block before calling advance is empty
+        assert!(block_segments.docs().is_empty());
+        // checking that the `doc_freq` is correct
+        assert_eq!(block_segments.doc_freq(), 100_000);
+        while let Some(block) = block_segments.next() {
+            for (i, doc) in block.iter().cloned().enumerate() {
+                assert_eq!(offset + (i as u32), doc);
+            }
+            offset += block.len() as u32;
+        }
+    }
+
+    #[test]
+    fn test_skip_right_at_new_block() {
+        let mut doc_ids = (0..128).collect::<Vec<u32>>();
+        doc_ids.push(129);
+        doc_ids.push(130);
+        {
+            let block_segments = build_block_postings(&doc_ids);
+            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
+            assert_eq!(docset.skip_next(128), SkipResult::OverStep);
+            assert_eq!(docset.doc(), 129);
+            assert!(docset.advance());
+            assert_eq!(docset.doc(), 130);
+            assert!(!docset.advance());
+        }
+        {
+            let block_segments = build_block_postings(&doc_ids);
+            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
+            assert_eq!(docset.skip_next(129), SkipResult::Reached);
+            assert_eq!(docset.doc(), 129);
+            assert!(docset.advance());
+            assert_eq!(docset.doc(), 130);
+            assert!(!docset.advance());
+        }
+        {
+            let block_segments = build_block_postings(&doc_ids);
+            let mut docset = SegmentPostings::from_block_postings(block_segments, None);
+            assert_eq!(docset.skip_next(131), SkipResult::End);
+        }
+    }
+
+    fn build_block_postings(docs: &[DocId]) -> BlockSegmentPostings {
+        let mut schema_builder = Schema::builder();
+        let int_field = schema_builder.add_u64_field("id", INDEXED);
+        let schema = schema_builder.build();
+        let index = Index::create_in_ram(schema);
+        let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
+        let mut last_doc = 0u32;
+        for &doc in docs {
+            for _ in last_doc..doc {
+                index_writer.add_document(doc!(int_field=>1u64));
+            }
+            index_writer.add_document(doc!(int_field=>0u64));
+            last_doc = doc + 1;
+        }
+        index_writer.commit().unwrap();
+        let searcher = index.reader().unwrap().searcher();
+        let segment_reader = searcher.segment_reader(0);
+        let inverted_index = segment_reader.inverted_index(int_field);
+        let term = Term::from_field_u64(int_field, 0u64);
+        let term_info = inverted_index.get_term_info(&term).unwrap();
+        inverted_index.read_block_postings_from_terminfo(&term_info, IndexRecordOption::Basic)
+    }
+
+    #[test]
+    fn test_block_segment_postings_skip() {
+        for i in 0..4 {
+            let mut block_postings = build_block_postings(&[3]);
+            assert_eq!(
+                block_postings.skip_to(i),
+                BlockSegmentPostingsSkipResult::Success(0u32)
+            );
+            assert_eq!(
+                block_postings.skip_to(i),
+                BlockSegmentPostingsSkipResult::Terminated
+            );
+        }
+        let mut block_postings = build_block_postings(&[3]);
+        assert_eq!(
+            block_postings.skip_to(4u32),
+            BlockSegmentPostingsSkipResult::Terminated
+        );
+    }
+
+    #[test]
+    fn test_block_segment_postings_skip2() {
+        let mut docs = vec![0];
+        for i in 0..1300 {
+            docs.push((i * i / 100) + i);
+        }
+        let mut block_postings = build_block_postings(&docs[..]);
+        for i in vec![0, 424, 10000] {
+            assert_eq!(
+                block_postings.skip_to(i),
+                BlockSegmentPostingsSkipResult::Success(0u32)
+            );
+            let docs = block_postings.docs();
+            assert!(docs[0] <= i);
+            assert!(docs.last().cloned().unwrap_or(0u32) >= i);
+        }
+        assert_eq!(
+            block_postings.skip_to(100_000),
+            BlockSegmentPostingsSkipResult::Terminated
+        );
+        assert_eq!(
+            block_postings.skip_to(101_000),
+            BlockSegmentPostingsSkipResult::Terminated
+        );
+    }
+
+    #[test]
+    fn test_reset_block_segment_postings() {
+        let mut schema_builder = Schema::builder();
+        let int_field = schema_builder.add_u64_field("id", INDEXED);
+        let schema = schema_builder.build();
+        let index = Index::create_in_ram(schema);
+        let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
+        // create two postings list, one containg even number,
+        // the other containing odd numbers.
+        for i in 0..6 {
+            let doc = doc!(int_field=> (i % 2) as u64);
+            index_writer.add_document(doc);
+        }
+        index_writer.commit().unwrap();
+        let searcher = index.reader().unwrap().searcher();
+        let segment_reader = searcher.segment_reader(0);
+
+        let mut block_segments;
+        {
+            let term = Term::from_field_u64(int_field, 0u64);
+            let inverted_index = segment_reader.inverted_index(int_field);
+            let term_info = inverted_index.get_term_info(&term).unwrap();
+            block_segments = inverted_index
+                .read_block_postings_from_terminfo(&term_info, IndexRecordOption::Basic);
+        }
+        assert!(block_segments.advance());
+        assert_eq!(block_segments.docs(), &[0, 2, 4]);
+        {
+            let term = Term::from_field_u64(int_field, 1u64);
+            let inverted_index = segment_reader.inverted_index(int_field);
+            let term_info = inverted_index.get_term_info(&term).unwrap();
+            inverted_index.reset_block_postings_from_terminfo(&term_info, &mut block_segments);
+        }
+        assert!(block_segments.advance());
+        assert_eq!(block_segments.docs(), &[1, 3, 5]);
     }
 }

src/postings/serializer.rs

@@ -6,6 +6,7 @@ use crate::directory::WritePtr;
 use crate::positions::PositionSerializer;
 use crate::postings::compression::{BlockEncoder, VIntEncoder, COMPRESSION_BLOCK_SIZE};
 use crate::postings::skip::SkipSerializer;
+use crate::postings::USE_SKIP_INFO_LIMIT;
 use crate::schema::Schema;
 use crate::schema::{Field, FieldEntry, FieldType};
 use crate::termdict::{TermDictionaryBuilder, TermOrdinal};
@@ -390,7 +391,7 @@ impl<W: Write> PostingsSerializer<W> {
             }
             self.block.clear();
         }
-        if doc_freq >= COMPRESSION_BLOCK_SIZE as u32 {
+        if doc_freq >= USE_SKIP_INFO_LIMIT {
             let skip_data = self.skip_write.data();
             VInt(skip_data.len() as u64).serialize(&mut self.output_write)?;
             self.output_write.write_all(skip_data)?;

src/postings/skip.rs

@@ -1,8 +1,7 @@
 use crate::common::BinarySerializable;
-use crate::directory::ReadOnlySource;
-use crate::postings::compression::{compressed_block_size, COMPRESSION_BLOCK_SIZE};
+use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
 use crate::schema::IndexRecordOption;
-use crate::{DocId, TERMINATED};
+use crate::DocId;
 use owned_read::OwnedRead;
 
 pub struct SkipSerializer {
@@ -51,143 +50,80 @@ impl SkipSerializer {
 }
 
 pub(crate) struct SkipReader {
-    last_doc_in_block: DocId,
-    pub(crate) last_doc_in_previous_block: DocId,
+    doc: DocId,
     owned_read: OwnedRead,
+    doc_num_bits: u8,
+    tf_num_bits: u8,
+    tf_sum: u32,
     skip_info: IndexRecordOption,
-    byte_offset: usize,
-    remaining_docs: u32, // number of docs remaining, including the
-    // documents in the current block.
-    block_info: BlockInfo,
-
-    position_offset: u64,
-}
-
-#[derive(Clone, Eq, PartialEq, Copy, Debug)]
-pub(crate) enum BlockInfo {
-    BitPacked {
-        doc_num_bits: u8,
-        tf_num_bits: u8,
-        tf_sum: u32,
-    },
-    VInt(u32),
-}
-
-impl Default for BlockInfo {
-    fn default() -> Self {
-        BlockInfo::VInt(0)
-    }
 }
 
 impl SkipReader {
-    pub fn new(data: ReadOnlySource, doc_freq: u32, skip_info: IndexRecordOption) -> SkipReader {
+    pub fn new(data: OwnedRead, skip_info: IndexRecordOption) -> SkipReader {
         SkipReader {
-            last_doc_in_block: 0u32,
-            last_doc_in_previous_block: 0u32,
-            owned_read: OwnedRead::new(data),
+            doc: 0u32,
+            owned_read: data,
             skip_info,
-            block_info: BlockInfo::default(),
-            byte_offset: 0,
-            remaining_docs: doc_freq,
-            position_offset: 0u64,
+            doc_num_bits: 0u8,
+            tf_num_bits: 0u8,
+            tf_sum: 0u32,
         }
     }
 
-    pub fn reset(&mut self, data: ReadOnlySource, doc_freq: u32) {
-        self.last_doc_in_block = 0u32;
-        self.last_doc_in_previous_block = 0u32;
-        self.owned_read = OwnedRead::new(data);
-        self.block_info = BlockInfo::default();
-        self.byte_offset = 0;
-        self.remaining_docs = doc_freq;
+    pub fn reset(&mut self, data: OwnedRead) {
+        self.doc = 0u32;
+        self.owned_read = data;
+        self.doc_num_bits = 0u8;
+        self.tf_num_bits = 0u8;
+        self.tf_sum = 0u32;
     }
 
-    #[cfg(test)]
-    #[inline(always)]
-    pub(crate) fn last_doc_in_block(&self) -> DocId {
-        self.last_doc_in_block
+    pub fn total_block_len(&self) -> usize {
+        (self.doc_num_bits + self.tf_num_bits) as usize * COMPRESSION_BLOCK_SIZE / 8
     }
 
-    pub fn position_offset(&self) -> u64 {
-        self.position_offset
+    pub fn doc(&self) -> DocId {
+        self.doc
     }
 
-    pub fn byte_offset(&self) -> usize {
-        self.byte_offset
+    pub fn doc_num_bits(&self) -> u8 {
+        self.doc_num_bits
     }
 
-    fn read_block_info(&mut self) {
-        let doc_delta = u32::deserialize(&mut self.owned_read).expect("Skip data corrupted");
-        self.last_doc_in_block += doc_delta as DocId;
-        let doc_num_bits = self.owned_read.get(0);
-        match self.skip_info {
-            IndexRecordOption::Basic => {
-                self.owned_read.advance(1);
-                self.block_info = BlockInfo::BitPacked {
-                    doc_num_bits,
-                    tf_num_bits: 0,
-                    tf_sum: 0,
-                };
-            }
-            IndexRecordOption::WithFreqs => {
-                let tf_num_bits = self.owned_read.get(1);
-                self.block_info = BlockInfo::BitPacked {
-                    doc_num_bits,
-                    tf_num_bits,
-                    tf_sum: 0,
-                };
-                self.owned_read.advance(2);
-            }
-            IndexRecordOption::WithFreqsAndPositions => {
-                let tf_num_bits = self.owned_read.get(1);
-                self.owned_read.advance(2);
-                let tf_sum = u32::deserialize(&mut self.owned_read).expect("Failed reading tf_sum");
-                self.block_info = BlockInfo::BitPacked {
-                    doc_num_bits,
-                    tf_num_bits,
-                    tf_sum,
-                };
-            }
-        }
-    }
-
-    pub fn block_info(&self) -> BlockInfo {
-        self.block_info
-    }
-
-    /// Advance the skip reader to the block that may contain the target.
+    /// Number of bits used to encode term frequencies
     ///
-    /// If the target is larger than all documents, the skip_reader
-    /// then advance to the last Variable In block.
-    pub fn seek(&mut self, target: DocId) {
-        while self.last_doc_in_block < target {
-            self.advance();
-        }
+    /// 0 if term frequencies are not enabled.
+    pub fn tf_num_bits(&self) -> u8 {
+        self.tf_num_bits
+    }
+
+    pub fn tf_sum(&self) -> u32 {
+        self.tf_sum
     }
 
     pub fn advance(&mut self) -> bool {
-        match self.block_info {
-            BlockInfo::BitPacked {
-                doc_num_bits,
-                tf_num_bits,
-                tf_sum,
-            } => {
-                self.remaining_docs -= COMPRESSION_BLOCK_SIZE as u32;
-                self.byte_offset += compressed_block_size(doc_num_bits + tf_num_bits);
-                self.position_offset += tf_sum as u64;
-            }
-            BlockInfo::VInt(num_vint_docs) => {
-                self.remaining_docs -= num_vint_docs;
-            }
-        }
-        self.last_doc_in_previous_block = self.last_doc_in_block;
-        if self.remaining_docs >= COMPRESSION_BLOCK_SIZE as u32 {
-            self.read_block_info();
-            true
+        if self.owned_read.as_ref().is_empty() {
+            false
         } else {
-            self.last_doc_in_block = TERMINATED;
-            self.block_info = BlockInfo::VInt(self.remaining_docs);
-            self.remaining_docs > 0
+            let doc_delta = u32::deserialize(&mut self.owned_read).expect("Skip data corrupted");
+            self.doc += doc_delta as DocId;
+            self.doc_num_bits = self.owned_read.get(0);
+            match self.skip_info {
+                IndexRecordOption::Basic => {
+                    self.owned_read.advance(1);
+                }
+                IndexRecordOption::WithFreqs => {
+                    self.tf_num_bits = self.owned_read.get(1);
+                    self.owned_read.advance(2);
+                }
+                IndexRecordOption::WithFreqsAndPositions => {
+                    self.tf_num_bits = self.owned_read.get(1);
+                    self.owned_read.advance(2);
+                    self.tf_sum =
+                        u32::deserialize(&mut self.owned_read).expect("Failed reading tf_sum");
+                }
+            }
+            true
         }
     }
 }
@@ -195,11 +131,9 @@ impl SkipReader {
 #[cfg(test)]
 mod tests {
 
-    use super::BlockInfo;
     use super::IndexRecordOption;
     use super::{SkipReader, SkipSerializer};
-    use crate::directory::ReadOnlySource;
-    use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
+    use owned_read::OwnedRead;
 
     #[test]
     fn test_skip_with_freq() {
@@ -211,34 +145,15 @@ mod tests {
             skip_serializer.write_term_freq(2u8);
             skip_serializer.data().to_owned()
         };
-        let doc_freq = 3u32 + (COMPRESSION_BLOCK_SIZE * 2) as u32;
-        let mut skip_reader = SkipReader::new(
-            ReadOnlySource::new(buf),
-            doc_freq,
-            IndexRecordOption::WithFreqs,
-        );
+        let mut skip_reader = SkipReader::new(OwnedRead::new(buf), IndexRecordOption::WithFreqs);
         assert!(skip_reader.advance());
-        assert_eq!(skip_reader.last_doc_in_block(), 1u32);
-        assert_eq!(
-            skip_reader.block_info(),
-            BlockInfo::BitPacked {
-                doc_num_bits: 2u8,
-                tf_num_bits: 3u8,
-                tf_sum: 0
-            }
-        );
+        assert_eq!(skip_reader.doc(), 1u32);
+        assert_eq!(skip_reader.doc_num_bits(), 2u8);
+        assert_eq!(skip_reader.tf_num_bits(), 3u8);
         assert!(skip_reader.advance());
-        assert_eq!(skip_reader.last_doc_in_block(), 5u32);
-        assert_eq!(
-            skip_reader.block_info(),
-            BlockInfo::BitPacked {
-                doc_num_bits: 5u8,
-                tf_num_bits: 2u8,
-                tf_sum: 0
-            }
-        );
-        assert!(skip_reader.advance());
-        assert_eq!(skip_reader.block_info(), BlockInfo::VInt(3u32));
+        assert_eq!(skip_reader.doc(), 5u32);
+        assert_eq!(skip_reader.doc_num_bits(), 5u8);
+        assert_eq!(skip_reader.tf_num_bits(), 2u8);
         assert!(!skip_reader.advance());
     }
 
@@ -250,60 +165,13 @@ mod tests {
             skip_serializer.write_doc(5u32, 5u8);
             skip_serializer.data().to_owned()
         };
-        let doc_freq = 3u32 + (COMPRESSION_BLOCK_SIZE * 2) as u32;
-        let mut skip_reader = SkipReader::new(
-            ReadOnlySource::from(buf),
-            doc_freq,
-            IndexRecordOption::Basic,
-        );
+        let mut skip_reader = SkipReader::new(OwnedRead::new(buf), IndexRecordOption::Basic);
         assert!(skip_reader.advance());
-        assert_eq!(skip_reader.last_doc_in_block(), 1u32);
-        assert_eq!(
-            skip_reader.block_info(),
-            BlockInfo::BitPacked {
-                doc_num_bits: 2u8,
-                tf_num_bits: 0,
-                tf_sum: 0u32
-            }
-        );
+        assert_eq!(skip_reader.doc(), 1u32);
+        assert_eq!(skip_reader.doc_num_bits(), 2u8);
         assert!(skip_reader.advance());
-        assert_eq!(skip_reader.last_doc_in_block(), 5u32);
-        assert_eq!(
-            skip_reader.block_info(),
-            BlockInfo::BitPacked {
-                doc_num_bits: 5u8,
-                tf_num_bits: 0,
-                tf_sum: 0u32
-            }
-        );
-        assert!(skip_reader.advance());
-        assert_eq!(skip_reader.block_info(), BlockInfo::VInt(3u32));
-        assert!(!skip_reader.advance());
-    }
-
-    #[test]
-    fn test_skip_multiple_of_block_size() {
-        let buf = {
-            let mut skip_serializer = SkipSerializer::new();
-            skip_serializer.write_doc(1u32, 2u8);
-            skip_serializer.data().to_owned()
-        };
-        let doc_freq = COMPRESSION_BLOCK_SIZE as u32;
-        let mut skip_reader = SkipReader::new(
-            ReadOnlySource::from(buf),
-            doc_freq,
-            IndexRecordOption::Basic,
-        );
-        assert!(skip_reader.advance());
-        assert_eq!(skip_reader.last_doc_in_block(), 1u32);
-        assert_eq!(
-            skip_reader.block_info(),
-            BlockInfo::BitPacked {
-                doc_num_bits: 2u8,
-                tf_num_bits: 0,
-                tf_sum: 0u32
-            }
-        );
+        assert_eq!(skip_reader.doc(), 5u32);
+        assert_eq!(skip_reader.doc_num_bits(), 5u8);
         assert!(!skip_reader.advance());
     }
 }

src/query/all_query.rs

@@ -1,6 +1,6 @@
 use crate::core::Searcher;
 use crate::core::SegmentReader;
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::DocSet;
 use crate::query::boost_query::BoostScorer;
 use crate::query::explanation::does_not_match;
 use crate::query::{Explanation, Query, Scorer, Weight};
@@ -25,6 +25,7 @@ pub struct AllWeight;
 impl Weight for AllWeight {
     fn scorer(&self, reader: &SegmentReader, boost: f32) -> crate::Result<Box<dyn Scorer>> {
         let all_scorer = AllScorer {
+            state: State::NotStarted,
             doc: 0u32,
             max_doc: reader.max_doc(),
         };
@@ -39,20 +40,39 @@ impl Weight for AllWeight {
     }
 }
 
+enum State {
+    NotStarted,
+    Started,
+    Finished,
+}
+
 /// Scorer associated to the `AllQuery` query.
 pub struct AllScorer {
+    state: State,
     doc: DocId,
     max_doc: DocId,
 }
 
 impl DocSet for AllScorer {
-    fn advance(&mut self) -> DocId {
-        if self.doc + 1 >= self.max_doc {
-            self.doc = TERMINATED;
-            return TERMINATED;
+    fn advance(&mut self) -> bool {
+        match self.state {
+            State::NotStarted => {
+                self.state = State::Started;
+                self.doc = 0;
+            }
+            State::Started => {
+                self.doc += 1u32;
+            }
+            State::Finished => {
+                return false;
+            }
+        }
+        if self.doc < self.max_doc {
+            true
+        } else {
+            self.state = State::Finished;
+            false
         }
-        self.doc += 1;
-        self.doc
     }
 
     fn doc(&self) -> DocId {
@@ -73,7 +93,6 @@ impl Scorer for AllScorer {
 #[cfg(test)]
 mod tests {
     use super::AllQuery;
-    use crate::docset::TERMINATED;
     use crate::query::Query;
     use crate::schema::{Schema, TEXT};
     use crate::Index;
@@ -101,16 +120,18 @@ mod tests {
         {
             let reader = searcher.segment_reader(0);
             let mut scorer = weight.scorer(reader, 1.0f32).unwrap();
+            assert!(scorer.advance());
             assert_eq!(scorer.doc(), 0u32);
-            assert_eq!(scorer.advance(), 1u32);
+            assert!(scorer.advance());
             assert_eq!(scorer.doc(), 1u32);
-            assert_eq!(scorer.advance(), TERMINATED);
+            assert!(!scorer.advance());
         }
         {
             let reader = searcher.segment_reader(1);
             let mut scorer = weight.scorer(reader, 1.0f32).unwrap();
+            assert!(scorer.advance());
             assert_eq!(scorer.doc(), 0u32);
-            assert_eq!(scorer.advance(), TERMINATED);
+            assert!(!scorer.advance());
         }
     }
 
@@ -123,11 +144,13 @@ mod tests {
         let reader = searcher.segment_reader(0);
         {
             let mut scorer = weight.scorer(reader, 2.0f32).unwrap();
+            assert!(scorer.advance());
             assert_eq!(scorer.doc(), 0u32);
             assert_eq!(scorer.score(), 2.0f32);
         }
         {
             let mut scorer = weight.scorer(reader, 1.5f32).unwrap();
+            assert!(scorer.advance());
             assert_eq!(scorer.doc(), 0u32);
             assert_eq!(scorer.score(), 1.5f32);
         }

src/query/automaton_weight.rs

@@ -6,8 +6,8 @@ use crate::query::{Scorer, Weight};
 use crate::schema::{Field, IndexRecordOption};
 use crate::termdict::{TermDictionary, TermStreamer};
 use crate::DocId;
-use crate::Result;
 use crate::TantivyError;
+use crate::{Result, SkipResult};
 use std::sync::Arc;
 use tantivy_fst::Automaton;
 
@@ -51,13 +51,10 @@ where
             let term_info = term_stream.value();
             let mut block_segment_postings = inverted_index
                 .read_block_postings_from_terminfo(term_info, IndexRecordOption::Basic);
-            loop {
+            while block_segment_postings.advance() {
                 for &doc in block_segment_postings.docs() {
                     doc_bitset.insert(doc);
                 }
-                if !block_segment_postings.advance() {
-                    break;
-                }
             }
         }
         let doc_bitset = BitSetDocSet::from(doc_bitset);
@@ -67,7 +64,7 @@ where
 
     fn explain(&self, reader: &SegmentReader, doc: DocId) -> Result<Explanation> {
         let mut scorer = self.scorer(reader, 1.0f32)?;
-        if scorer.seek(doc) == doc {
+        if scorer.skip_next(doc) == SkipResult::Reached {
             Ok(Explanation::new("AutomatonScorer", 1.0f32))
         } else {
             Err(TantivyError::InvalidArgument(
@@ -80,7 +77,6 @@ where
 #[cfg(test)]
 mod tests {
     use super::AutomatonWeight;
-    use crate::docset::TERMINATED;
     use crate::query::Weight;
     use crate::schema::{Schema, STRING};
     use crate::Index;
@@ -145,12 +141,13 @@ mod tests {
         let mut scorer = automaton_weight
             .scorer(searcher.segment_reader(0u32), 1.0f32)
             .unwrap();
+        assert!(scorer.advance());
         assert_eq!(scorer.doc(), 0u32);
         assert_eq!(scorer.score(), 1.0f32);
-        assert_eq!(scorer.advance(), 2u32);
+        assert!(scorer.advance());
         assert_eq!(scorer.doc(), 2u32);
         assert_eq!(scorer.score(), 1.0f32);
-        assert_eq!(scorer.advance(), TERMINATED);
+        assert!(!scorer.advance());
     }
 
     #[test]
@@ -163,6 +160,7 @@ mod tests {
         let mut scorer = automaton_weight
             .scorer(searcher.segment_reader(0u32), 1.32f32)
             .unwrap();
+        assert!(scorer.advance());
         assert_eq!(scorer.doc(), 0u32);
         assert_eq!(scorer.score(), 1.32f32);
     }

src/query/bitset/mod.rs

@@ -1,6 +1,7 @@
 use crate::common::{BitSet, TinySet};
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::{DocSet, SkipResult};
 use crate::DocId;
+use std::cmp::Ordering;
 
 /// A `BitSetDocSet` makes it possible to iterate through a bitset as if it was a `DocSet`.
 ///
@@ -32,50 +33,75 @@ impl From<BitSet> for BitSetDocSet {
         } else {
             docs.tinyset(0)
         };
-        let mut docset = BitSetDocSet {
+        BitSetDocSet {
             docs,
             cursor_bucket: 0,
             cursor_tinybitset: first_tiny_bitset,
             doc: 0u32,
-        };
-        docset.advance();
-        docset
+        }
     }
 }
 
 impl DocSet for BitSetDocSet {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         if let Some(lower) = self.cursor_tinybitset.pop_lowest() {
             self.doc = (self.cursor_bucket as u32 * 64u32) | lower;
-            return self.doc;
+            return true;
         }
         if let Some(cursor_bucket) = self.docs.first_non_empty_bucket(self.cursor_bucket + 1) {
             self.go_to_bucket(cursor_bucket);
             let lower = self.cursor_tinybitset.pop_lowest().unwrap();
             self.doc = (cursor_bucket * 64u32) | lower;
-            self.doc
+            true
         } else {
-            self.doc = TERMINATED;
-            TERMINATED
+            false
         }
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        // skip is required to advance.
+        if !self.advance() {
+            return SkipResult::End;
+        }
         let target_bucket = target / 64u32;
 
         // Mask for all of the bits greater or equal
         // to our target document.
-        if target_bucket > self.cursor_bucket {
-            self.go_to_bucket(target_bucket);
-            let greater_filter: TinySet = TinySet::range_greater_or_equal(target);
-            self.cursor_tinybitset = self.cursor_tinybitset.intersect(greater_filter);
-            self.advance();
+        match target_bucket.cmp(&self.cursor_bucket) {
+            Ordering::Greater => {
+                self.go_to_bucket(target_bucket);
+                let greater_filter: TinySet = TinySet::range_greater_or_equal(target);
+                self.cursor_tinybitset = self.cursor_tinybitset.intersect(greater_filter);
+                if !self.advance() {
+                    SkipResult::End
+                } else if self.doc() == target {
+                    SkipResult::Reached
+                } else {
+                    debug_assert!(self.doc() > target);
+                    SkipResult::OverStep
+                }
+            }
+            Ordering::Equal => loop {
+                match self.doc().cmp(&target) {
+                    Ordering::Less => {
+                        if !self.advance() {
+                            return SkipResult::End;
+                        }
+                    }
+                    Ordering::Equal => {
+                        return SkipResult::Reached;
+                    }
+                    Ordering::Greater => {
+                        debug_assert!(self.doc() > target);
+                        return SkipResult::OverStep;
+                    }
+                }
+            },
+            Ordering::Less => {
+                debug_assert!(self.doc() > target);
+                SkipResult::OverStep
+            }
         }
-        let mut doc = self.doc();
-        while doc < target {
-            doc = self.advance();
-        }
-        doc
     }
 
     /// Returns the current document
@@ -96,7 +122,7 @@ impl DocSet for BitSetDocSet {
 mod tests {
     use super::BitSetDocSet;
     use crate::common::BitSet;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::{DocSet, SkipResult};
     use crate::DocId;
 
     fn create_docbitset(docs: &[DocId], max_doc: DocId) -> BitSetDocSet {
@@ -107,24 +133,19 @@ mod tests {
         BitSetDocSet::from(docset)
     }
 
-    #[test]
-    fn test_empty() {
-        let bitset = BitSet::with_max_value(1000);
-        let mut empty = BitSetDocSet::from(bitset);
-        assert_eq!(empty.advance(), TERMINATED)
-    }
-
     fn test_go_through_sequential(docs: &[DocId]) {
         let mut docset = create_docbitset(docs, 1_000u32);
         for &doc in docs {
+            assert!(docset.advance());
             assert_eq!(doc, docset.doc());
-            docset.advance();
         }
-        assert_eq!(docset.advance(), TERMINATED);
+        assert!(!docset.advance());
+        assert!(!docset.advance());
     }
 
     #[test]
     fn test_docbitset_sequential() {
+        test_go_through_sequential(&[]);
         test_go_through_sequential(&[1, 2, 3]);
         test_go_through_sequential(&[1, 2, 3, 4, 5, 63, 64, 65]);
         test_go_through_sequential(&[63, 64, 65]);
@@ -135,64 +156,64 @@ mod tests {
     fn test_docbitset_skip() {
         {
             let mut docset = create_docbitset(&[1, 5, 6, 7, 5112], 10_000);
-            assert_eq!(docset.seek(7), 7);
+            assert_eq!(docset.skip_next(7), SkipResult::Reached);
             assert_eq!(docset.doc(), 7);
-            assert_eq!(docset.advance(), 5112);
+            assert!(docset.advance(), 7);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[1, 5, 6, 7, 5112], 10_000);
-            assert_eq!(docset.seek(3), 5);
+            assert_eq!(docset.skip_next(3), SkipResult::OverStep);
             assert_eq!(docset.doc(), 5);
-            assert_eq!(docset.advance(), 6);
+            assert!(docset.advance());
         }
         {
             let mut docset = create_docbitset(&[5112], 10_000);
-            assert_eq!(docset.seek(5112), 5112);
+            assert_eq!(docset.skip_next(5112), SkipResult::Reached);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[5112], 10_000);
-            assert_eq!(docset.seek(5113), TERMINATED);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert_eq!(docset.skip_next(5113), SkipResult::End);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[5112], 10_000);
-            assert_eq!(docset.seek(5111), 5112);
+            assert_eq!(docset.skip_next(5111), SkipResult::OverStep);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[1, 5, 6, 7, 5112, 5500, 6666], 10_000);
-            assert_eq!(docset.seek(5112), 5112);
+            assert_eq!(docset.skip_next(5112), SkipResult::Reached);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), 5500);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 5500);
-            assert_eq!(docset.advance(), 6666);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 6666);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[1, 5, 6, 7, 5112, 5500, 6666], 10_000);
-            assert_eq!(docset.seek(5111), 5112);
+            assert_eq!(docset.skip_next(5111), SkipResult::OverStep);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), 5500);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 5500);
-            assert_eq!(docset.advance(), 6666);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 6666);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
         {
             let mut docset = create_docbitset(&[1, 5, 6, 7, 5112, 5513, 6666], 10_000);
-            assert_eq!(docset.seek(5111), 5112);
+            assert_eq!(docset.skip_next(5111), SkipResult::OverStep);
             assert_eq!(docset.doc(), 5112);
-            assert_eq!(docset.advance(), 5513);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 5513);
-            assert_eq!(docset.advance(), 6666);
+            assert!(docset.advance());
             assert_eq!(docset.doc(), 6666);
-            assert_eq!(docset.advance(), TERMINATED);
+            assert!(!docset.advance());
         }
     }
 }
@@ -202,7 +223,6 @@ mod bench {
 
     use super::BitSet;
     use super::BitSetDocSet;
-    use crate::docset::TERMINATED;
     use crate::test;
     use crate::tests;
     use crate::DocSet;
@@ -237,7 +257,7 @@ mod bench {
         }
         b.iter(|| {
             let mut docset = BitSetDocSet::from(bitset.clone());
-            while docset.advance() != TERMINATED {}
+            while docset.advance() {}
         });
     }
 }

src/query/block_max_scorer.rs

@@ -0,0 +1,31 @@
+use crate::docset::DocSet;
+use crate::DocId;
+use crate::Score;
+use downcast_rs::impl_downcast;
+use crate::query::Scorer;
+
+/// A set of documents matching a query within a specific segment
+/// and having a maximum score within certain blocks.
+///
+/// See [`Query`](./trait.Query.html) and [`Scorer`](./trait.Scorer.html).
+pub trait BlockMaxScorer: downcast_rs::Downcast + DocSet + Scorer + 'static {
+    /// Returns the maximum score within the current block.
+    ///
+    /// The blocks are defined when indexing. For example, blocks can be
+    /// have a specific number postings each, or can be optimized for
+    /// retrieval speed. Read more in
+    /// [Faster BlockMax WAND with Variable-sized Blocks][vbmw]
+    ///
+    /// This method will perform a bit of computation and is not cached.
+    ///
+    /// [vbmw]: https://dl.acm.org/doi/abs/10.1145/3077136.3080780
+    fn block_max_score(&mut self) -> Score;
+
+    /// Returns the last document in the current block.
+    fn block_max_doc(&mut self) -> DocId;
+
+    /// Returns the maximum possible score within the entire document set.
+    fn max_score(&self) -> Score;
+}
+
+impl_downcast!(BlockMaxScorer);

src/query/block_max_wand.rs

@@ -0,0 +1,613 @@
+use crate::docset::{DocSet, SkipResult};
+use crate::query::score_combiner::ScoreCombiner;
+use crate::query::{BlockMaxScorer, Scorer};
+use crate::DocId;
+use crate::Score;
+use crate::query::weight::PruningScorer;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+struct Pivot {
+    position: usize,
+    first_occurrence: usize,
+    doc: DocId,
+}
+
+
+/// Find the position in the sorted list of posting lists of the **pivot**.
+///
+/// docsets need to be advanced, and are required to be sorted by the doc they point to.
+///
+/// The pivot is then defined as the lowest DocId that has a chance of matching our condition.
+fn find_pivot_position<'a, TScorer>(
+    mut docsets: impl Iterator<Item = &'a TScorer>,
+    lower_bound_score: Score,
+) -> Option<Pivot>
+    where TScorer: BlockMaxScorer
+{
+    let mut position = 0;
+    let mut upper_bound = Score::default();
+    while let Some(docset) = docsets.next() {
+        upper_bound += docset.max_score();
+        if lower_bound_score < upper_bound {
+            let pivot_doc = docset.doc();
+            let first_occurrence = position;
+            while let Some(docset) = docsets.next() {
+                if docset.doc() != pivot_doc {
+                    break;
+                } else {
+                    position += 1;
+                }
+            }
+            return Some(Pivot {
+                position,
+                doc: pivot_doc,
+                first_occurrence,
+            });
+        }
+        position += 1;
+    }
+    None
+}
+
+/// Given an iterator over all ordered lists up to the pivot (inclusive) and the following list (if
+/// exists), it returns the next document ID that can be possibly relevant, based on the block max
+/// scores.
+fn find_next_relevant_doc<T, TScorer>(
+    docsets_up_to_pivot: &mut [T],
+    pivot_docset: &mut T,
+    docset_after_pivot: Option<&mut T>,
+) -> DocId
+    where
+        T: AsMut<TScorer>,
+        TScorer: BlockMaxScorer + Scorer,
+{
+    let pivot_docset = pivot_docset.as_mut();
+    let mut next_doc = 1 + docsets_up_to_pivot
+        .iter_mut()
+        .map(|docset| docset.as_mut().block_max_doc())
+        .chain(std::iter::once(pivot_docset.block_max_doc()))
+        .min()
+        .unwrap();
+    if let Some(docset) = docset_after_pivot {
+        let doc = docset.as_mut().doc();
+        if doc < next_doc {
+            next_doc = doc;
+        }
+    }
+    if next_doc <= pivot_docset.doc() {
+        pivot_docset.doc() + 1
+    } else {
+        next_doc
+    }
+}
+
+/// Sifts down the first element of the slice.
+///
+/// `docsets[1..]` are assumed sorted.
+/// This function swaps `docsets[0]` with its right
+/// neighbor successively -bubble sort style- until it reaches the first
+/// position such that `docsets` is sorted.
+fn sift_down<T, TScorer>(docsets: &mut [T])
+    where
+        T: AsRef<TScorer>,
+        TScorer: BlockMaxScorer + Scorer,
+{
+    for idx in 1..docsets.len() {
+        if docsets[idx].as_ref().doc() >= docsets[idx - 1].as_ref().doc() {
+            return;
+        }
+        docsets.swap(idx, idx - 1);
+    }
+}
+
+/// Creates a `DocSet` that iterates through the union of two or more `DocSet`s,
+/// applying [BlockMaxWand] dynamic pruning.
+///
+/// [BlockMaxWand]: https://dl.acm.org/doi/10.1145/2009916.2010048
+pub struct BlockMaxWand<TScorer, TScoreCombiner> {
+    docsets: Vec<Box<TScorer>>,
+    doc: DocId,
+    score: Score,
+    combiner: TScoreCombiner,
+}
+
+impl<TScorer, TScoreCombiner> BlockMaxWand<TScorer, TScoreCombiner>
+    where
+        TScoreCombiner: ScoreCombiner,
+        TScorer: BlockMaxScorer + Scorer,
+{
+    fn new(
+        docsets: Vec<TScorer>,
+        combiner: TScoreCombiner,
+    ) -> BlockMaxWand<TScorer, TScoreCombiner> {
+        let mut non_empty_docsets: Vec<_> = docsets
+            .into_iter()
+            .flat_map(|mut docset| {
+                if docset.advance() {
+                    Some(Box::new(docset))
+                } else {
+                    None
+                }
+            })
+            .collect();
+        non_empty_docsets.sort_by_key(Box::<TScorer>::doc);
+        BlockMaxWand {
+            docsets: non_empty_docsets,
+            combiner,
+            doc: 0u32,
+            score: 0f32
+        }
+    }
+
+    /// Find the position in the sorted list of posting lists of the **pivot**.
+    fn find_pivot_position(&self, lower_bound_score: Score) -> Option<Pivot> {
+        find_pivot_position(
+            self.docsets.iter().map(|docset| docset.as_ref()),
+            lower_bound_score)
+    }
+
+    fn advance_with_pivot(&mut self, pivot: Pivot, lower_bound_score: Score) -> SkipResult {
+        let block_upper_bound: Score = self.docsets[..=pivot.position]
+            .iter_mut()
+            .map(|docset| docset.block_max_score())
+            .sum();
+        if block_upper_bound > lower_bound_score {
+            if pivot.doc == self.docsets[0].doc() {
+                // Since self.docsets is sorted by their current doc, in this branch, all
+                // docsets in [0..=pivot] are positioned on pivot.doc.
+                //
+                // Lets compute the actual score for this doc.
+                //
+                // NOTE(elshize): One additional check needs to be done to improve performance:
+                // update block-wise bound while accumulating score with the actual score,
+                // and check each time if still above threshold.
+                self.combiner.clear();
+                for idx in (0..=pivot.position).rev() {
+                    self.combiner.update(self.docsets[idx].as_mut());
+                    if !self.docsets[idx].advance() {
+                        self.docsets.swap_remove(idx);
+                    }
+                }
+                self.score = self.combiner.score();
+                self.doc = pivot.doc;
+                self.docsets.sort_by_key(Box::<TScorer>::doc);
+                SkipResult::Reached
+            } else {
+                // The substraction does not underflow because otherwise we would go to the other
+                // branch.
+                //
+                // `advanced_idx` is the last idx that is not positionned on the pivot yet.
+                let advanced_idx = pivot.first_occurrence - 1;
+                if !self.docsets[advanced_idx].advance() {
+                    self.docsets.swap_remove(advanced_idx);
+                }
+                if self.docsets.is_empty() {
+                    return SkipResult::End;
+                }
+                sift_down(&mut self.docsets[advanced_idx..]);
+                SkipResult::OverStep
+            }
+        } else {
+            let (up_to_pivot, pivot_and_rest) = self.docsets.split_at_mut(pivot.position as usize);
+            let (pivot, after_pivot) = pivot_and_rest.split_first_mut().unwrap();
+            let next_doc = find_next_relevant_doc(up_to_pivot, pivot, after_pivot.first_mut());
+            // NOTE(elshize): It might be more efficient to advance the list with the higher
+            // max score, but let's advance the first one for now for simplicity.
+            if self.docsets[0].skip_next(next_doc) == SkipResult::End {
+                self.docsets.swap_remove(0);
+            }
+            if self.docsets.is_empty() {
+                return SkipResult::End;
+            }
+            sift_down(&mut self.docsets[..]);
+            SkipResult::OverStep
+        }
+    }
+}
+
+impl<TScorer, TScoreCombiner> PruningScorer
+for BlockMaxWand<TScorer, TScoreCombiner>
+    where
+        TScoreCombiner: ScoreCombiner,
+        TScorer: Scorer + BlockMaxScorer,
+{
+    fn doc(&self) -> DocId {
+        self.doc
+    }
+
+    fn score(&self) -> Score {
+        self.score
+    }
+
+    fn advance_with_pruning(&mut self, lower_bound_score: f32) -> bool {
+        while let Some(pivot) = self.find_pivot_position(lower_bound_score) {
+            match self.advance_with_pivot(pivot, lower_bound_score) {
+                SkipResult::End => { return false },
+                SkipResult::Reached=> { return true; }
+                SkipResult::OverStep => {}
+            }
+        }
+        false
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+    use crate::common::HasLen;
+    use crate::docset::DocSet;
+    use crate::query::score_combiner::SumCombiner;
+    use crate::query::Union;
+    use crate::query::{BlockMaxScorer, Scorer};
+    use crate::{DocId, Score};
+    use float_cmp::approx_eq;
+    use proptest::strategy::Strategy;
+    use std::cmp::Ordering;
+    use std::num::Wrapping;
+    use crate::collector::{SegmentCollector, TopScoreSegmentCollector};
+
+    /*
+    #[derive(Debug, Clone)]
+    pub struct VecDocSet {
+        postings: Vec<(DocId, Score)>,
+        cursor: Wrapping<usize>,
+        block_max_scores: Vec<(DocId, Score)>,
+        max_score: Score,
+        block_size: usize,
+    }
+
+    impl VecDocSet {
+        fn new(postings: Vec<(DocId, Score)>, block_size: usize) -> VecDocSet {
+            let block_max_scores: Vec<(DocId, f32)> = postings
+                .chunks(block_size)
+                .into_iter()
+                .map(|block| {
+                    (
+                        block.iter().last().unwrap().0,
+                        block
+                            .iter()
+                            .map(|(_, s)| *s)
+                            .fold(-f32::INFINITY, |left, right| left.max(right))
+                    )
+                })
+                .collect();
+            let max_score = block_max_scores
+                .iter()
+                .copied()
+                .map(|(_, s)| s)
+                .fold(-f32::INFINITY, |left, right| left.max(right));
+            VecDocSet {
+                postings,
+                cursor: Wrapping(0_usize) - Wrapping(1_usize),
+                block_max_scores,
+                max_score,
+                block_size,
+            }
+        }
+        /// Constructs a new set and advances it.
+        fn started(postings: Vec<(DocId, Score)>, block_size: usize) -> VecDocSet {
+            let mut docset = VecDocSet::new(postings, block_size);
+            docset.advance();
+            docset
+        }
+    }
+
+    impl DocSet for VecDocSet {
+        fn advance(&mut self) -> bool {
+            self.cursor += Wrapping(1);
+            self.postings.len() > self.cursor.0
+        }
+
+        fn doc(&self) -> DocId {
+            self.postings[self.cursor.0].0
+        }
+
+        fn size_hint(&self) -> u32 {
+            self.len() as u32
+        }
+    }
+
+    impl HasLen for VecDocSet {
+        fn len(&self) -> usize {
+            self.postings.len()
+        }
+    }
+
+    impl BlockMaxScorer for VecDocSet {
+        fn max_score(&self) -> Score {
+            self.max_score
+        }
+        fn block_max_score(&mut self) -> Score {
+            self.block_max_scores[self.cursor.0 / self.block_size].1
+        }
+        fn block_max_doc(&mut self) -> DocId {
+            self.block_max_scores[self.cursor.0 / self.block_size].0
+        }
+    }
+
+    impl Scorer for VecDocSet {
+        fn score(&mut self) -> Score {
+            self.postings[self.cursor.0].1
+        }
+    }
+
+    #[derive(Debug)]
+    struct ComparableDoc<T, D> {
+        feature: T,
+        doc: D,
+    }
+
+    impl<T: PartialOrd, D: PartialOrd> PartialOrd for ComparableDoc<T, D> {
+        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+            Some(self.cmp(other))
+        }
+    }
+
+    impl<T: PartialOrd, D: PartialOrd> Ord for ComparableDoc<T, D> {
+        #[inline]
+        fn cmp(&self, other: &Self) -> Ordering {
+            // Reversed to make BinaryHeap work as a min-heap
+            let by_feature = other
+                .feature
+                .partial_cmp(&self.feature)
+                .unwrap_or(Ordering::Equal);
+
+            let lazy_by_doc_address =
+                || self.doc.partial_cmp(&other.doc).unwrap_or(Ordering::Equal);
+
+            // In case of a tie on the feature, we sort by ascending
+            // `DocAddress` in order to ensure a stable sorting of the
+            // documents.
+            by_feature.then_with(lazy_by_doc_address)
+        }
+    }
+
+    impl<T: PartialOrd, D: PartialOrd> PartialEq for ComparableDoc<T, D> {
+        fn eq(&self, other: &Self) -> bool {
+            self.cmp(other) == Ordering::Equal
+        }
+    }
+
+    impl<T: PartialOrd, D: PartialOrd> Eq for ComparableDoc<T, D> {}
+
+    fn union_vs_bmw(posting_lists: Vec<VecDocSet>) {
+        let mut union = Union::<VecDocSet, SumCombiner>::from(posting_lists.clone());
+        let mut top_union = TopScoreSegmentCollector::new(0, 10);
+        while union.advance() {
+            top_union.collect(union.doc(), union.score());
+        }
+        let top_bmw = TopScoreSegmentCollector::new(0, 10 );
+        let mut bmw = BlockMaxWand::new(posting_lists, SumCombiner::default());
+        let top_docs_bnw = top_bmw.collect_scorer(&mut bmw, None);
+        for ((expected_score, expected_doc), (actual_score, actual_doc)) in
+        top_union.harvest().into_iter().zip( top_docs_bnw )
+        {
+            assert!(approx_eq!(
+                f32,
+                expected_score,
+                actual_score,
+                epsilon = 0.0001
+            ));
+            assert_eq!(expected_doc, actual_doc);
+        }
+    }
+
+    #[test]
+    fn test_bmw_0() {
+        union_vs_bmw(vec![
+            VecDocSet {
+                postings: vec![
+                    (0, 1.0),
+                    (23, 1.0),
+                    (28, 1.0),
+                    (56, 1.0),
+                    (59, 1.0),
+                    (66, 1.0),
+                    (93, 1.0),
+                ],
+                cursor: Wrapping(0_usize) - Wrapping(1_usize),
+                block_max_scores: vec![(93, 1.0)],
+                max_score: 1.0,
+                block_size: 16,
+            },
+            VecDocSet {
+                postings: vec![
+                    (2, 1.6549665),
+                    (43, 2.6958032),
+                    (53, 3.5309567),
+                    (71, 2.7688136),
+                    (87, 3.4279852),
+                    (96, 3.9028034),
+                ],
+                cursor: Wrapping(0_usize) - Wrapping(1_usize),
+                block_max_scores: vec![(96, 3.9028034)],
+                max_score: 3.9028034,
+                block_size: 16,
+            },
+        ])
+    }
+
+    #[test]
+    fn test_bmw_1() {
+        union_vs_bmw(vec![
+            VecDocSet {
+                postings: vec![(73, 1.0), (82, 1.0)],
+                cursor: Wrapping(0_usize) - Wrapping(1_usize),
+                block_max_scores: vec![(82, 1.0)],
+                max_score: 1.0,
+                block_size: 16,
+            },
+            VecDocSet {
+                postings: vec![
+                    (21, 3.582513),
+                    (23, 1.6928024),
+                    (27, 3.887647),
+                    (42, 1.5469292),
+                    (61, 1.7317574),
+                    (62, 1.2968783),
+                    (82, 2.4040694),
+                    (85, 3.1487892),
+                ],
+                cursor: Wrapping(0_usize) - Wrapping(1_usize),
+                block_max_scores: vec![(85, 3.887647)],
+                max_score: 3.887647,
+                block_size: 16,
+            },
+        ])
+    }
+
+    proptest::proptest! {
+        #[test]
+        fn test_union_vs_bmw(postings in proptest::collection::vec(
+            proptest::collection::vec(0_u32..100, 1..10)
+                .prop_flat_map(|v| {
+                    let scores = proptest::collection::vec(1_f32..4_f32, v.len()..=v.len());
+                    scores.prop_map(move |s| {
+                        let mut postings: Vec<_> = v.iter().copied().zip(s.iter().copied()).collect();
+                        postings.sort_by_key(|p| p.0);
+                        postings.dedup_by_key(|p| p.0);
+                        VecDocSet::new(postings, 16)
+                    })
+                }),
+            2..5)
+        ) {
+            union_vs_bmw(postings);
+        }
+    }
+
+    #[test]
+    fn test_find_pivot_position() {
+        let postings = vec![
+            VecDocSet::started(vec![(0, 2.0)], 1),
+            VecDocSet::started(vec![(1, 3.0)], 1),
+            VecDocSet::started(vec![(2, 4.0)], 1),
+            VecDocSet::started(vec![(3, 5.0)], 1),
+            VecDocSet::started(vec![(3, 6.0)], 1),
+        ];
+        assert_eq!(
+            find_pivot_position(postings.iter(), 2.0f32),
+            Some(Pivot {
+                position: 1,
+                doc: 1,
+                first_occurrence: 1,
+            })
+        );
+        assert_eq!(
+            find_pivot_position(postings.iter(), 5.0f32),
+            Some(Pivot {
+                position: 2,
+                doc: 2,
+                first_occurrence: 2,
+            })
+        );
+        assert_eq!(
+            find_pivot_position(postings.iter(), 9.0f32),
+            Some(Pivot {
+                position: 4,
+                doc: 3,
+                first_occurrence: 3,
+            })
+        );
+        assert_eq!(
+            find_pivot_position(postings.iter(), 20.0f32),
+            None
+        );
+    }
+
+    #[test]
+    fn test_find_next_relevant_doc_before_pivot() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (3, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (4, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(2, 0.0), (6, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(6, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (8, 0.0)], 2)),
+        ];
+        let (up_to_pivot, rest) = postings.split_at_mut(2);
+        let (pivot, after_pivot) = rest.split_first_mut().unwrap();
+        let next_doc = find_next_relevant_doc(up_to_pivot, pivot, Some(&mut after_pivot[0]));
+        assert_eq!(next_doc, 4);
+    }
+
+    #[test]
+    fn test_find_next_relevant_doc_prefix_smaller_than_pivot() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (3, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (4, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(5, 0.0), (8, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(6, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (8, 0.0)], 2)),
+        ];
+        let (up_to_pivot, rest) = postings.split_at_mut(2);
+        let (pivot, after_pivot) = rest.split_first_mut().unwrap();
+        let next_doc = find_next_relevant_doc(up_to_pivot, pivot, Some(&mut after_pivot[0]));
+        assert_eq!(next_doc, 6);
+    }
+
+    #[test]
+    fn test_find_next_relevant_doc_after_pivot() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(2, 0.0), (8, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(5, 0.0), (7, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (7, 0.0)], 2)),
+        ];
+        let (up_to_pivot, rest) = postings.split_at_mut(2);
+        let (pivot, after_pivot) = rest.split_first_mut().unwrap();
+        let next_doc = find_next_relevant_doc(up_to_pivot, pivot, Some(&mut after_pivot[0]));
+        assert_eq!(next_doc, 5);
+    }
+
+    #[test]
+    fn test_sift_down_already_sifted() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(2, 0.0), (8, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(5, 0.0), (7, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (7, 0.0)], 2)),
+        ];
+        sift_down(&mut postings[2..]);
+        assert_eq!(
+            postings.into_iter().map(|p| p.doc()).collect::<Vec<_>>(),
+            vec![0, 1, 2, 5, 6]
+        );
+    }
+
+    #[test]
+    fn test_sift_down_sift_one_down() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (8, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(5, 0.0), (7, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(7, 0.0), (7, 0.0)], 2)),
+        ];
+        sift_down(&mut postings[2..]);
+        assert_eq!(
+            postings.into_iter().map(|p| p.doc()).collect::<Vec<_>>(),
+            vec![0, 1, 5, 6, 7]
+        );
+    }
+
+    #[test]
+    fn test_sift_down_to_bottom() {
+        let mut postings = vec![
+            Box::new(VecDocSet::started(vec![(0, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(1, 0.0), (8, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(7, 0.0), (8, 0.0)], 2)), // pivot
+            Box::new(VecDocSet::started(vec![(5, 0.0), (7, 0.0)], 2)),
+            Box::new(VecDocSet::started(vec![(6, 0.0), (7, 0.0)], 2)),
+        ];
+        sift_down(&mut postings[2..]);
+        assert_eq!(
+            postings.into_iter().map(|p| p.doc()).collect::<Vec<_>>(),
+            vec![0, 1, 5, 6, 7]
+        );
+    }
+    
+     */
+}

src/query/boolean_query/boolean_weight.rs

@@ -2,7 +2,6 @@ use crate::core::SegmentReader;
 use crate::query::explanation::does_not_match;
 use crate::query::score_combiner::{DoNothingCombiner, ScoreCombiner, SumWithCoordsCombiner};
 use crate::query::term_query::TermScorer;
-use crate::query::weight::{for_each_pruning_scorer, for_each_scorer};
 use crate::query::EmptyScorer;
 use crate::query::Exclude;
 use crate::query::Occur;
@@ -11,21 +10,16 @@ use crate::query::Scorer;
 use crate::query::Union;
 use crate::query::Weight;
 use crate::query::{intersect_scorers, Explanation};
-use crate::{DocId, Score};
+use crate::{DocId, SkipResult};
 use std::collections::HashMap;
 
-enum SpecializedScorer<TScoreCombiner: ScoreCombiner> {
-    TermUnion(Union<TermScorer, TScoreCombiner>),
-    Other(Box<dyn Scorer>),
-}
-
-fn scorer_union<TScoreCombiner>(scorers: Vec<Box<dyn Scorer>>) -> SpecializedScorer<TScoreCombiner>
+fn scorer_union<TScoreCombiner>(scorers: Vec<Box<dyn Scorer>>) -> Box<dyn Scorer>
 where
     TScoreCombiner: ScoreCombiner,
 {
     assert!(!scorers.is_empty());
     if scorers.len() == 1 {
-        return SpecializedScorer::Other(scorers.into_iter().next().unwrap()); //< we checked the size beforehands
+        return scorers.into_iter().next().unwrap(); //< we checked the size beforehands
     }
 
     {
@@ -35,21 +29,14 @@ where
                 .into_iter()
                 .map(|scorer| *(scorer.downcast::<TermScorer>().map_err(|_| ()).unwrap()))
                 .collect();
-            return SpecializedScorer::TermUnion(Union::<TermScorer, TScoreCombiner>::from(
-                scorers,
-            ));
+            let scorer: Box<dyn Scorer> =
+                Box::new(Union::<TermScorer, TScoreCombiner>::from(scorers));
+            return scorer;
         }
     }
-    SpecializedScorer::Other(Box::new(Union::<_, TScoreCombiner>::from(scorers)))
-}
 
-impl<TScoreCombiner: ScoreCombiner> Into<Box<dyn Scorer>> for SpecializedScorer<TScoreCombiner> {
-    fn into(self) -> Box<dyn Scorer> {
-        match self {
-            Self::TermUnion(union) => Box::new(union),
-            Self::Other(scorer) => scorer,
-        }
-    }
+    let scorer: Box<dyn Scorer> = Box::new(Union::<_, TScoreCombiner>::from(scorers));
+    scorer
 }
 
 pub struct BooleanWeight {
@@ -85,50 +72,41 @@ impl BooleanWeight {
         &self,
         reader: &SegmentReader,
         boost: f32,
-    ) -> crate::Result<SpecializedScorer<TScoreCombiner>> {
+    ) -> crate::Result<Box<dyn Scorer>> {
         let mut per_occur_scorers = self.per_occur_scorers(reader, boost)?;
 
-        let should_scorer_opt: Option<SpecializedScorer<TScoreCombiner>> = per_occur_scorers
+        let should_scorer_opt: Option<Box<dyn Scorer>> = per_occur_scorers
             .remove(&Occur::Should)
             .map(scorer_union::<TScoreCombiner>);
 
         let exclude_scorer_opt: Option<Box<dyn Scorer>> = per_occur_scorers
             .remove(&Occur::MustNot)
-            .map(scorer_union::<TScoreCombiner>)
-            .map(Into::into);
+            .map(scorer_union::<TScoreCombiner>);
 
         let must_scorer_opt: Option<Box<dyn Scorer>> = per_occur_scorers
             .remove(&Occur::Must)
             .map(intersect_scorers);
 
-        let positive_scorer: SpecializedScorer<TScoreCombiner> =
-            match (should_scorer_opt, must_scorer_opt) {
-                (Some(should_scorer), Some(must_scorer)) => {
-                    if self.scoring_enabled {
-                        SpecializedScorer::Other(Box::new(RequiredOptionalScorer::<
-                            Box<dyn Scorer>,
-                            Box<dyn Scorer>,
-                            TScoreCombiner,
-                        >::new(
-                            must_scorer, should_scorer.into()
-                        )))
-                    } else {
-                        SpecializedScorer::Other(must_scorer)
-                    }
+        let positive_scorer: Box<dyn Scorer> = match (should_scorer_opt, must_scorer_opt) {
+            (Some(should_scorer), Some(must_scorer)) => {
+                if self.scoring_enabled {
+                    Box::new(RequiredOptionalScorer::<_, _, TScoreCombiner>::new(
+                        must_scorer,
+                        should_scorer,
+                    ))
+                } else {
+                    must_scorer
                 }
-                (None, Some(must_scorer)) => SpecializedScorer::Other(must_scorer),
-                (Some(should_scorer), None) => should_scorer,
-                (None, None) => {
-                    return Ok(SpecializedScorer::Other(Box::new(EmptyScorer)));
-                }
-            };
+            }
+            (None, Some(must_scorer)) => must_scorer,
+            (Some(should_scorer), None) => should_scorer,
+            (None, None) => {
+                return Ok(Box::new(EmptyScorer));
+            }
+        };
 
         if let Some(exclude_scorer) = exclude_scorer_opt {
-            let positive_scorer_boxed: Box<dyn Scorer> = positive_scorer.into();
-            Ok(SpecializedScorer::Other(Box::new(Exclude::new(
-                positive_scorer_boxed,
-                exclude_scorer,
-            ))))
+            Ok(Box::new(Exclude::new(positive_scorer, exclude_scorer)))
         } else {
             Ok(positive_scorer)
         }
@@ -148,16 +126,14 @@ impl Weight for BooleanWeight {
             }
         } else if self.scoring_enabled {
             self.complex_scorer::<SumWithCoordsCombiner>(reader, boost)
-                .map(Into::into)
         } else {
             self.complex_scorer::<DoNothingCombiner>(reader, boost)
-                .map(Into::into)
         }
     }
 
     fn explain(&self, reader: &SegmentReader, doc: DocId) -> crate::Result<Explanation> {
         let mut scorer = self.scorer(reader, 1.0f32)?;
-        if scorer.seek(doc) != doc {
+        if scorer.skip_next(doc) != SkipResult::Reached {
             return Err(does_not_match(doc));
         }
         if !self.scoring_enabled {
@@ -174,51 +150,6 @@ impl Weight for BooleanWeight {
         }
         Ok(explanation)
     }
-
-    fn for_each(
-        &self,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score),
-    ) -> crate::Result<()> {
-        let scorer = self.complex_scorer::<SumWithCoordsCombiner>(reader, 1.0f32)?;
-        match scorer {
-            SpecializedScorer::TermUnion(mut union_scorer) => {
-                for_each_scorer(&mut union_scorer, callback);
-            }
-            SpecializedScorer::Other(mut scorer) => {
-                for_each_scorer(scorer.as_mut(), callback);
-            }
-        }
-        Ok(())
-    }
-
-    /// Calls `callback` with all of the `(doc, score)` for which score
-    /// is exceeding a given threshold.
-    ///
-    /// This method is useful for the TopDocs collector.
-    /// For all docsets, the blanket implementation has the benefit
-    /// of prefiltering (doc, score) pairs, avoiding the
-    /// virtual dispatch cost.
-    ///
-    /// More importantly, it makes it possible for scorers to implement
-    /// important optimization (e.g. BlockWAND for union).
-    fn for_each_pruning(
-        &self,
-        threshold: f32,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score) -> Score,
-    ) -> crate::Result<()> {
-        let scorer = self.complex_scorer::<SumWithCoordsCombiner>(reader, 1.0f32)?;
-        match scorer {
-            SpecializedScorer::TermUnion(mut union_scorer) => {
-                for_each_pruning_scorer(&mut union_scorer, threshold, callback);
-            }
-            SpecializedScorer::Other(mut scorer) => {
-                for_each_pruning_scorer(scorer.as_mut(), threshold, callback);
-            }
-        }
-        Ok(())
-    }
 }
 
 fn is_positive_occur(occur: Occur) -> bool {

src/query/boolean_query/mod.rs

@@ -207,6 +207,7 @@ mod tests {
             let mut boolean_scorer = boolean_weight
                 .scorer(searcher.segment_reader(0u32), 1.0f32)
                 .unwrap();
+            assert!(boolean_scorer.advance());
             assert_eq!(boolean_scorer.doc(), 0u32);
             assert_nearly_equals(boolean_scorer.score(), 0.84163445f32);
         }
@@ -214,6 +215,7 @@ mod tests {
             let mut boolean_scorer = boolean_weight
                 .scorer(searcher.segment_reader(0u32), 2.0f32)
                 .unwrap();
+            assert!(boolean_scorer.advance());
             assert_eq!(boolean_scorer.doc(), 0u32);
             assert_nearly_equals(boolean_scorer.score(), 1.6832689f32);
         }

src/query/boost_query.rs

@@ -1,7 +1,8 @@
+use crate::common::BitSet;
 use crate::fastfield::DeleteBitSet;
 use crate::query::explanation::does_not_match;
 use crate::query::{Explanation, Query, Scorer, Weight};
-use crate::{DocId, DocSet, Searcher, SegmentReader, Term};
+use crate::{DocId, DocSet, Searcher, SegmentReader, SkipResult, Term};
 use std::collections::BTreeSet;
 use std::fmt;
 
@@ -71,7 +72,7 @@ impl Weight for BoostWeight {
 
     fn explain(&self, reader: &SegmentReader, doc: u32) -> crate::Result<Explanation> {
         let mut scorer = self.scorer(reader, 1.0f32)?;
-        if scorer.seek(doc) != doc {
+        if scorer.skip_next(doc) != SkipResult::Reached {
             return Err(does_not_match(doc));
         }
         let mut explanation =
@@ -98,12 +99,12 @@ impl<S: Scorer> BoostScorer<S> {
 }
 
 impl<S: Scorer> DocSet for BoostScorer<S> {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         self.underlying.advance()
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        self.underlying.seek(target)
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.underlying.skip_next(target)
     }
 
     fn fill_buffer(&mut self, buffer: &mut [DocId]) -> usize {
@@ -118,6 +119,10 @@ impl<S: Scorer> DocSet for BoostScorer<S> {
         self.underlying.size_hint()
     }
 
+    fn append_to_bitset(&mut self, bitset: &mut BitSet) {
+        self.underlying.append_to_bitset(bitset)
+    }
+
     fn count(&mut self, delete_bitset: &DeleteBitSet) -> u32 {
         self.underlying.count(delete_bitset)
     }

src/query/empty_query.rs

@@ -1,5 +1,4 @@
 use super::Scorer;
-use crate::docset::TERMINATED;
 use crate::query::explanation::does_not_match;
 use crate::query::Weight;
 use crate::query::{Explanation, Query};
@@ -49,12 +48,15 @@ impl Weight for EmptyWeight {
 pub struct EmptyScorer;
 
 impl DocSet for EmptyScorer {
-    fn advance(&mut self) -> DocId {
-        TERMINATED
+    fn advance(&mut self) -> bool {
+        false
     }
 
     fn doc(&self) -> DocId {
-        TERMINATED
+        panic!(
+            "You may not call .doc() on a scorer \
+             where the last call to advance() did not return true."
+        );
     }
 
     fn size_hint(&self) -> u32 {
@@ -70,15 +72,18 @@ impl Scorer for EmptyScorer {
 
 #[cfg(test)]
 mod tests {
-    use crate::docset::TERMINATED;
     use crate::query::EmptyScorer;
     use crate::DocSet;
 
     #[test]
     fn test_empty_scorer() {
         let mut empty_scorer = EmptyScorer;
-        assert_eq!(empty_scorer.doc(), TERMINATED);
-        assert_eq!(empty_scorer.advance(), TERMINATED);
-        assert_eq!(empty_scorer.doc(), TERMINATED);
+        assert!(!empty_scorer.advance());
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_empty_scorer_panic_on_doc_call() {
+        EmptyScorer.doc();
     }
 }

src/query/exclude.rs

@@ -1,37 +1,41 @@
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::{DocSet, SkipResult};
 use crate::query::Scorer;
 use crate::DocId;
 use crate::Score;
 
+#[derive(Clone, Copy, Debug)]
+enum State {
+    ExcludeOne(DocId),
+    Finished,
+}
+
 /// Filters a given `DocSet` by removing the docs from a given `DocSet`.
 ///
 /// The excluding docset has no impact on scoring.
 pub struct Exclude<TDocSet, TDocSetExclude> {
     underlying_docset: TDocSet,
     excluding_docset: TDocSetExclude,
+    excluding_state: State,
 }
 
 impl<TDocSet, TDocSetExclude> Exclude<TDocSet, TDocSetExclude>
 where
-    TDocSet: DocSet,
     TDocSetExclude: DocSet,
 {
     /// Creates a new `ExcludeScorer`
     pub fn new(
-        mut underlying_docset: TDocSet,
+        underlying_docset: TDocSet,
         mut excluding_docset: TDocSetExclude,
     ) -> Exclude<TDocSet, TDocSetExclude> {
-        while underlying_docset.doc() != TERMINATED {
-            let target = underlying_docset.doc();
-            if excluding_docset.seek(target) != target {
-                // this document is not excluded.
-                break;
-            }
-            underlying_docset.advance();
-        }
+        let state = if excluding_docset.advance() {
+            State::ExcludeOne(excluding_docset.doc())
+        } else {
+            State::Finished
+        };
         Exclude {
             underlying_docset,
             excluding_docset,
+            excluding_state: state,
         }
     }
 }
@@ -47,7 +51,28 @@ where
     /// increasing `doc`.
     fn accept(&mut self) -> bool {
         let doc = self.underlying_docset.doc();
-        self.excluding_docset.seek(doc) != doc
+        match self.excluding_state {
+            State::ExcludeOne(excluded_doc) => {
+                if doc == excluded_doc {
+                    return false;
+                }
+                if excluded_doc > doc {
+                    return true;
+                }
+                match self.excluding_docset.skip_next(doc) {
+                    SkipResult::OverStep => {
+                        self.excluding_state = State::ExcludeOne(self.excluding_docset.doc());
+                        true
+                    }
+                    SkipResult::End => {
+                        self.excluding_state = State::Finished;
+                        true
+                    }
+                    SkipResult::Reached => false,
+                }
+            }
+            State::Finished => true,
+        }
     }
 }
 
@@ -56,24 +81,27 @@ where
     TDocSet: DocSet,
     TDocSetExclude: DocSet,
 {
-    fn advance(&mut self) -> DocId {
-        while self.underlying_docset.advance() != TERMINATED {
+    fn advance(&mut self) -> bool {
+        while self.underlying_docset.advance() {
             if self.accept() {
-                return self.doc();
+                return true;
             }
         }
-        TERMINATED
+        false
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        let underlying_seek_result = self.underlying_docset.seek(target);
-        if underlying_seek_result == TERMINATED {
-            return TERMINATED;
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        let underlying_skip_result = self.underlying_docset.skip_next(target);
+        if underlying_skip_result == SkipResult::End {
+            return SkipResult::End;
         }
         if self.accept() {
-            return underlying_seek_result;
+            underlying_skip_result
+        } else if self.advance() {
+            SkipResult::OverStep
+        } else {
+            SkipResult::End
         }
-        self.advance()
     }
 
     fn doc(&self) -> DocId {
@@ -113,9 +141,8 @@ mod tests {
             VecDocSet::from(vec![1, 2, 3, 10, 16, 24]),
         );
         let mut els = vec![];
-        while exclude_scorer.doc() != TERMINATED {
+        while exclude_scorer.advance() {
             els.push(exclude_scorer.doc());
-            exclude_scorer.advance();
         }
         assert_eq!(els, vec![5, 8, 15]);
     }

src/query/fuzzy_query.rs

@@ -117,7 +117,7 @@ impl FuzzyTermQuery {
         }
     }
 
-    /// Creates a new Fuzzy Query of the Term prefix
+    /// Creates a new Fuzzy Query that treats transpositions as cost one rather than two
     pub fn new_prefix(term: Term, distance: u8, transposition_cost_one: bool) -> FuzzyTermQuery {
         FuzzyTermQuery {
             term,
@@ -188,8 +188,6 @@ mod test {
         }
         let reader = index.reader().unwrap();
         let searcher = reader.searcher();
-
-        // passes because Levenshtein distance is 1 (substitute 'o' with 'a')
         {
             let term = Term::from_field_text(country_field, "japon");
 
@@ -202,18 +200,6 @@ mod test {
             assert_nearly_equals(1f32, score);
         }
 
-        // fails because non-prefix Levenshtein distance is more than 1 (add 'a' and 'n')
-        {
-            let term = Term::from_field_text(country_field, "jap");
-
-            let fuzzy_query = FuzzyTermQuery::new(term, 1, true);
-            let top_docs = searcher
-                .search(&fuzzy_query, &TopDocs::with_limit(2))
-                .unwrap();
-            assert_eq!(top_docs.len(), 0, "Expected no document");
-        }
-
-        // passes because prefix Levenshtein distance is 0
         {
             let term = Term::from_field_text(country_field, "jap");
 

src/query/intersection.rs

@@ -1,4 +1,4 @@
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::{DocSet, SkipResult};
 use crate::query::term_query::TermScorer;
 use crate::query::EmptyScorer;
 use crate::query::Scorer;
@@ -20,14 +20,12 @@ pub fn intersect_scorers(mut scorers: Vec<Box<dyn Scorer>>) -> Box<dyn Scorer> {
     if scorers.len() == 1 {
         return scorers.pop().unwrap();
     }
-    scorers.sort_by_key(|scorer| scorer.size_hint());
-    let doc = go_to_first_doc(&mut scorers[..]);
-    if doc == TERMINATED {
-        return Box::new(EmptyScorer);
-    }
     // We know that we have at least 2 elements.
-    let left = scorers.remove(0);
-    let right = scorers.remove(0);
+    let num_docsets = scorers.len();
+    scorers.sort_by(|left, right| right.size_hint().cmp(&left.size_hint()));
+    let left = scorers.pop().unwrap();
+    let right = scorers.pop().unwrap();
+    scorers.reverse();
     let all_term_scorers = [&left, &right]
         .iter()
         .all(|&scorer| scorer.is::<TermScorer>());
@@ -36,12 +34,14 @@ pub fn intersect_scorers(mut scorers: Vec<Box<dyn Scorer>>) -> Box<dyn Scorer> {
             left: *(left.downcast::<TermScorer>().map_err(|_| ()).unwrap()),
             right: *(right.downcast::<TermScorer>().map_err(|_| ()).unwrap()),
             others: scorers,
+            num_docsets,
         });
     }
     Box::new(Intersection {
         left,
         right,
         others: scorers,
+        num_docsets,
     })
 }
 
@@ -50,34 +50,22 @@ pub struct Intersection<TDocSet: DocSet, TOtherDocSet: DocSet = Box<dyn Scorer>>
     left: TDocSet,
     right: TDocSet,
     others: Vec<TOtherDocSet>,
-}
-
-fn go_to_first_doc<TDocSet: DocSet>(docsets: &mut [TDocSet]) -> DocId {
-    let mut candidate = 0;
-    'outer: loop {
-        for docset in docsets.iter_mut() {
-            let seek_doc = docset.seek(candidate);
-            if seek_doc > candidate {
-                candidate = docset.doc();
-                continue 'outer;
-            }
-        }
-        return candidate;
-    }
+    num_docsets: usize,
 }
 
 impl<TDocSet: DocSet> Intersection<TDocSet, TDocSet> {
     pub(crate) fn new(mut docsets: Vec<TDocSet>) -> Intersection<TDocSet, TDocSet> {
         let num_docsets = docsets.len();
         assert!(num_docsets >= 2);
-        docsets.sort_by_key(|docset| docset.size_hint());
-        go_to_first_doc(&mut docsets);
-        let left = docsets.remove(0);
-        let right = docsets.remove(0);
+        docsets.sort_by(|left, right| right.size_hint().cmp(&left.size_hint()));
+        let left = docsets.pop().unwrap();
+        let right = docsets.pop().unwrap();
+        docsets.reverse();
         Intersection {
             left,
             right,
             others: docsets,
+            num_docsets,
         }
     }
 }
@@ -92,44 +80,128 @@ impl<TDocSet: DocSet> Intersection<TDocSet, TDocSet> {
     }
 }
 
+impl<TDocSet: DocSet, TOtherDocSet: DocSet> Intersection<TDocSet, TOtherDocSet> {
+    pub(crate) fn docset_mut(&mut self, ord: usize) -> &mut dyn DocSet {
+        match ord {
+            0 => &mut self.left,
+            1 => &mut self.right,
+            n => &mut self.others[n - 2],
+        }
+    }
+}
+
 impl<TDocSet: DocSet, TOtherDocSet: DocSet> DocSet for Intersection<TDocSet, TOtherDocSet> {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         let (left, right) = (&mut self.left, &mut self.right);
-        let mut candidate = left.advance();
+
+        if !left.advance() {
+            return false;
+        }
+
+        let mut candidate = left.doc();
+        let mut other_candidate_ord: usize = usize::max_value();
 
         'outer: loop {
             // In the first part we look for a document in the intersection
             // of the two rarest `DocSet` in the intersection.
-
             loop {
-                let right_doc = right.seek(candidate);
-                candidate = left.seek(right_doc);
-                if candidate == right_doc {
-                    break;
+                match right.skip_next(candidate) {
+                    SkipResult::Reached => {
+                        break;
+                    }
+                    SkipResult::OverStep => {
+                        candidate = right.doc();
+                        other_candidate_ord = usize::max_value();
+                    }
+                    SkipResult::End => {
+                        return false;
+                    }
+                }
+                match left.skip_next(candidate) {
+                    SkipResult::Reached => {
+                        break;
+                    }
+                    SkipResult::OverStep => {
+                        candidate = left.doc();
+                        other_candidate_ord = usize::max_value();
+                    }
+                    SkipResult::End => {
+                        return false;
+                    }
                 }
             }
-
-            debug_assert_eq!(left.doc(), right.doc());
             // test the remaining scorers;
-            for docset in self.others.iter_mut() {
-                let seek_doc = docset.seek(candidate);
-                if seek_doc > candidate {
-                    candidate = left.seek(seek_doc);
-                    continue 'outer;
+            for (ord, docset) in self.others.iter_mut().enumerate() {
+                if ord == other_candidate_ord {
+                    continue;
+                }
+                // `candidate_ord` is already at the
+                // right position.
+                //
+                // Calling `skip_next` would advance this docset
+                // and miss it.
+                match docset.skip_next(candidate) {
+                    SkipResult::Reached => {}
+                    SkipResult::OverStep => {
+                        // this is not in the intersection,
+                        // let's update our candidate.
+                        candidate = docset.doc();
+                        match left.skip_next(candidate) {
+                            SkipResult::Reached => {
+                                other_candidate_ord = ord;
+                            }
+                            SkipResult::OverStep => {
+                                candidate = left.doc();
+                                other_candidate_ord = usize::max_value();
+                            }
+                            SkipResult::End => {
+                                return false;
+                            }
+                        }
+                        continue 'outer;
+                    }
+                    SkipResult::End => {
+                        return false;
+                    }
                 }
             }
-
-            return candidate;
+            return true;
         }
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        self.left.seek(target);
-        let mut docsets: Vec<&mut dyn DocSet> = vec![&mut self.left, &mut self.right];
-        for docset in &mut self.others {
-            docsets.push(docset);
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        // We optimize skipping by skipping every single member
+        // of the intersection to target.
+        let mut current_target: DocId = target;
+        let mut current_ord = self.num_docsets;
+
+        'outer: loop {
+            for ord in 0..self.num_docsets {
+                let docset = self.docset_mut(ord);
+                if ord == current_ord {
+                    continue;
+                }
+                match docset.skip_next(current_target) {
+                    SkipResult::End => {
+                        return SkipResult::End;
+                    }
+                    SkipResult::OverStep => {
+                        // update the target
+                        // for the remaining members of the intersection.
+                        current_target = docset.doc();
+                        current_ord = ord;
+                        continue 'outer;
+                    }
+                    SkipResult::Reached => {}
+                }
+            }
+            if target == current_target {
+                return SkipResult::Reached;
+            } else {
+                assert!(current_target > target);
+                return SkipResult::OverStep;
+            }
         }
-        go_to_first_doc(&mut docsets[..])
     }
 
     fn doc(&self) -> DocId {
@@ -156,7 +228,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::Intersection;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::{DocSet, SkipResult};
     use crate::postings::tests::test_skip_against_unoptimized;
     use crate::query::VecDocSet;
 
@@ -166,18 +238,20 @@ mod tests {
             let left = VecDocSet::from(vec![1, 3, 9]);
             let right = VecDocSet::from(vec![3, 4, 9, 18]);
             let mut intersection = Intersection::new(vec![left, right]);
+            assert!(intersection.advance());
             assert_eq!(intersection.doc(), 3);
-            assert_eq!(intersection.advance(), 9);
+            assert!(intersection.advance());
             assert_eq!(intersection.doc(), 9);
-            assert_eq!(intersection.advance(), TERMINATED);
+            assert!(!intersection.advance());
         }
         {
             let a = VecDocSet::from(vec![1, 3, 9]);
             let b = VecDocSet::from(vec![3, 4, 9, 18]);
             let c = VecDocSet::from(vec![1, 5, 9, 111]);
             let mut intersection = Intersection::new(vec![a, b, c]);
+            assert!(intersection.advance());
             assert_eq!(intersection.doc(), 9);
-            assert_eq!(intersection.advance(), TERMINATED);
+            assert!(!intersection.advance());
         }
     }
 
@@ -186,8 +260,8 @@ mod tests {
         let left = VecDocSet::from(vec![0]);
         let right = VecDocSet::from(vec![0]);
         let mut intersection = Intersection::new(vec![left, right]);
+        assert!(intersection.advance());
         assert_eq!(intersection.doc(), 0);
-        assert_eq!(intersection.advance(), TERMINATED);
     }
 
     #[test]
@@ -195,7 +269,7 @@ mod tests {
         let left = VecDocSet::from(vec![0, 1, 2, 4]);
         let right = VecDocSet::from(vec![2, 5]);
         let mut intersection = Intersection::new(vec![left, right]);
-        assert_eq!(intersection.seek(2), 2);
+        assert_eq!(intersection.skip_next(2), SkipResult::Reached);
         assert_eq!(intersection.doc(), 2);
     }
 
@@ -238,7 +312,7 @@ mod tests {
         let a = VecDocSet::from(vec![1, 3]);
         let b = VecDocSet::from(vec![1, 4]);
         let c = VecDocSet::from(vec![3, 9]);
-        let intersection = Intersection::new(vec![a, b, c]);
-        assert_eq!(intersection.doc(), TERMINATED);
+        let mut intersection = Intersection::new(vec![a, b, c]);
+        assert!(!intersection.advance());
     }
 }

src/query/mod.rs

@@ -3,6 +3,8 @@
 mod all_query;
 mod automaton_weight;
 mod bitset;
+mod block_max_scorer;
+mod block_max_wand;
 mod bm25;
 mod boolean_query;
 mod boost_query;
@@ -35,6 +37,7 @@ pub use self::vec_docset::VecDocSet;
 pub use self::all_query::{AllQuery, AllScorer, AllWeight};
 pub use self::automaton_weight::AutomatonWeight;
 pub use self::bitset::BitSetDocSet;
+pub use self::block_max_scorer::BlockMaxScorer;
 pub use self::boolean_query::BooleanQuery;
 pub use self::boost_query::BoostQuery;
 pub use self::empty_query::{EmptyQuery, EmptyScorer, EmptyWeight};
@@ -56,6 +59,8 @@ pub use self::scorer::Scorer;
 pub use self::term_query::TermQuery;
 pub use self::weight::Weight;
 pub use tantivy_query_grammar::Occur;
+pub use self::weight::PruningScorerIfPossible;
+
 
 #[cfg(test)]
 mod tests {

src/query/phrase_query/mod.rs

@@ -60,8 +60,8 @@ pub mod tests {
                 .map(|docaddr| docaddr.1)
                 .collect::<Vec<_>>()
         };
-        assert_eq!(test_query(vec!["a", "b"]), vec![1, 2, 3, 4]);
         assert_eq!(test_query(vec!["a", "b", "c"]), vec![2, 4]);
+        assert_eq!(test_query(vec!["a", "b"]), vec![1, 2, 3, 4]);
         assert_eq!(test_query(vec!["b", "b"]), vec![0, 1]);
         assert!(test_query(vec!["g", "ewrwer"]).is_empty());
         assert!(test_query(vec!["g", "a"]).is_empty());

src/query/phrase_query/phrase_scorer.rs

@@ -1,4 +1,4 @@
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::{DocSet, SkipResult};
 use crate::fieldnorm::FieldNormReader;
 use crate::postings::Postings;
 use crate::query::bm25::BM25Weight;
@@ -25,12 +25,12 @@ impl<TPostings: Postings> PostingsWithOffset<TPostings> {
 }
 
 impl<TPostings: Postings> DocSet for PostingsWithOffset<TPostings> {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         self.postings.advance()
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        self.postings.seek(target)
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.postings.skip_next(target)
     }
 
     fn doc(&self) -> DocId {
@@ -149,7 +149,7 @@ impl<TPostings: Postings> PhraseScorer<TPostings> {
                 PostingsWithOffset::new(postings, (max_offset - offset) as u32)
             })
             .collect::<Vec<_>>();
-        let mut scorer = PhraseScorer {
+        PhraseScorer {
             intersection_docset: Intersection::new(postings_with_offsets),
             num_terms: num_docsets,
             left: Vec::with_capacity(100),
@@ -158,11 +158,7 @@ impl<TPostings: Postings> PhraseScorer<TPostings> {
             similarity_weight,
             fieldnorm_reader,
             score_needed,
-        };
-        if scorer.doc() != TERMINATED && !scorer.phrase_match() {
-            scorer.advance();
         }
-        scorer
     }
 
     pub fn phrase_count(&self) -> u32 {
@@ -229,21 +225,31 @@ impl<TPostings: Postings> PhraseScorer<TPostings> {
 }
 
 impl<TPostings: Postings> DocSet for PhraseScorer<TPostings> {
-    fn advance(&mut self) -> DocId {
-        loop {
-            let doc = self.intersection_docset.advance();
-            if doc == TERMINATED || self.phrase_match() {
-                return doc;
+    fn advance(&mut self) -> bool {
+        while self.intersection_docset.advance() {
+            if self.phrase_match() {
+                return true;
             }
         }
+        false
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        let doc = self.intersection_docset.seek(target);
-        if doc == TERMINATED || self.phrase_match() {
-            return doc;
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        if self.intersection_docset.skip_next(target) == SkipResult::End {
+            return SkipResult::End;
+        }
+        if self.phrase_match() {
+            if self.doc() == target {
+                return SkipResult::Reached;
+            } else {
+                return SkipResult::OverStep;
+            }
+        }
+        if self.advance() {
+            SkipResult::OverStep
+        } else {
+            SkipResult::End
         }
-        self.advance()
     }
 
     fn doc(&self) -> DocId {

src/query/phrase_query/phrase_weight.rs

@@ -9,8 +9,8 @@ use crate::query::Weight;
 use crate::query::{EmptyScorer, Explanation};
 use crate::schema::IndexRecordOption;
 use crate::schema::Term;
-use crate::Result;
 use crate::{DocId, DocSet};
+use crate::{Result, SkipResult};
 
 pub struct PhraseWeight {
     phrase_terms: Vec<(usize, Term)>,
@@ -99,7 +99,7 @@ impl Weight for PhraseWeight {
             return Err(does_not_match(doc));
         }
         let mut scorer = scorer_opt.unwrap();
-        if scorer.seek(doc) != doc {
+        if scorer.skip_next(doc) != SkipResult::Reached {
             return Err(does_not_match(doc));
         }
         let fieldnorm_reader = self.fieldnorm_reader(reader);
@@ -114,7 +114,6 @@ impl Weight for PhraseWeight {
 #[cfg(test)]
 mod tests {
     use super::super::tests::create_index;
-    use crate::docset::TERMINATED;
     use crate::query::PhraseQuery;
     use crate::{DocSet, Term};
 
@@ -133,11 +132,12 @@ mod tests {
             .phrase_scorer(searcher.segment_reader(0u32), 1.0f32)
             .unwrap()
             .unwrap();
+        assert!(phrase_scorer.advance());
         assert_eq!(phrase_scorer.doc(), 1);
         assert_eq!(phrase_scorer.phrase_count(), 2);
-        assert_eq!(phrase_scorer.advance(), 2);
+        assert!(phrase_scorer.advance());
         assert_eq!(phrase_scorer.doc(), 2);
         assert_eq!(phrase_scorer.phrase_count(), 1);
-        assert_eq!(phrase_scorer.advance(), TERMINATED);
+        assert!(!phrase_scorer.advance());
     }
 }

src/query/query_parser/query_parser.rs

@@ -113,9 +113,8 @@ fn trim_ast(logical_ast: LogicalAST) -> Option<LogicalAST> {
 /// The language covered by the current parser is extremely simple.
 ///
 /// * simple terms: "e.g.: `Barack Obama` are simply tokenized using
-///   tantivy's [`SimpleTokenizer`](tantivy::tokenizer::SimpleTokenizer), hence
-///   becoming `["barack", "obama"]`. The terms are then searched within
-///   the default terms of the query parser.
+///   tantivy's `StandardTokenizer`, hence becoming `["barack", "obama"]`.
+///   The terms are then searched within the default terms of the query parser.
 ///
 ///   e.g. If `body` and `title` are default fields, our example terms are
 ///   `["title:barack", "body:barack", "title:obama", "body:obama"]`.

src/query/range_query.rs

@@ -10,7 +10,7 @@ use crate::schema::Type;
 use crate::schema::{Field, IndexRecordOption, Term};
 use crate::termdict::{TermDictionary, TermStreamer};
 use crate::DocId;
-use crate::Result;
+use crate::{Result, SkipResult};
 use std::collections::Bound;
 use std::ops::Range;
 
@@ -300,13 +300,10 @@ impl Weight for RangeWeight {
             let term_info = term_range.value();
             let mut block_segment_postings = inverted_index
                 .read_block_postings_from_terminfo(term_info, IndexRecordOption::Basic);
-            loop {
+            while block_segment_postings.advance() {
                 for &doc in block_segment_postings.docs() {
                     doc_bitset.insert(doc);
                 }
-                if !block_segment_postings.advance() {
-                    break;
-                }
             }
         }
         let doc_bitset = BitSetDocSet::from(doc_bitset);
@@ -315,7 +312,7 @@ impl Weight for RangeWeight {
 
     fn explain(&self, reader: &SegmentReader, doc: DocId) -> Result<Explanation> {
         let mut scorer = self.scorer(reader, 1.0f32)?;
-        if scorer.seek(doc) != doc {
+        if scorer.skip_next(doc) != SkipResult::Reached {
             return Err(does_not_match(doc));
         }
         Ok(Explanation::new("RangeQuery", 1.0f32))

src/query/reqopt_scorer.rs

@@ -1,8 +1,9 @@
-use crate::docset::DocSet;
+use crate::docset::{DocSet, SkipResult};
 use crate::query::score_combiner::ScoreCombiner;
 use crate::query::Scorer;
 use crate::DocId;
 use crate::Score;
+use std::cmp::Ordering;
 use std::marker::PhantomData;
 
 /// Given a required scorer and an optional scorer
@@ -16,6 +17,7 @@ pub struct RequiredOptionalScorer<TReqScorer, TOptScorer, TScoreCombiner> {
     req_scorer: TReqScorer,
     opt_scorer: TOptScorer,
     score_cache: Option<Score>,
+    opt_finished: bool,
     _phantom: PhantomData<TScoreCombiner>,
 }
 
@@ -27,12 +29,14 @@ where
     /// Creates a new `RequiredOptionalScorer`.
     pub fn new(
         req_scorer: TReqScorer,
-        opt_scorer: TOptScorer,
+        mut opt_scorer: TOptScorer,
     ) -> RequiredOptionalScorer<TReqScorer, TOptScorer, TScoreCombiner> {
+        let opt_finished = !opt_scorer.advance();
         RequiredOptionalScorer {
             req_scorer,
             opt_scorer,
             score_cache: None,
+            opt_finished,
             _phantom: PhantomData,
         }
     }
@@ -44,7 +48,7 @@ where
     TReqScorer: DocSet,
     TOptScorer: DocSet,
 {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         self.score_cache = None;
         self.req_scorer.advance()
     }
@@ -72,8 +76,22 @@ where
         let doc = self.doc();
         let mut score_combiner = TScoreCombiner::default();
         score_combiner.update(&mut self.req_scorer);
-        if self.opt_scorer.seek(doc) == doc {
-            score_combiner.update(&mut self.opt_scorer);
+        if !self.opt_finished {
+            match self.opt_scorer.doc().cmp(&doc) {
+                Ordering::Greater => {}
+                Ordering::Equal => {
+                    score_combiner.update(&mut self.opt_scorer);
+                }
+                Ordering::Less => match self.opt_scorer.skip_next(doc) {
+                    SkipResult::Reached => {
+                        score_combiner.update(&mut self.opt_scorer);
+                    }
+                    SkipResult::End => {
+                        self.opt_finished = true;
+                    }
+                    SkipResult::OverStep => {}
+                },
+            }
         }
         let score = score_combiner.score();
         self.score_cache = Some(score);
@@ -84,7 +102,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::RequiredOptionalScorer;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::DocSet;
     use crate::postings::tests::test_skip_against_unoptimized;
     use crate::query::score_combiner::{DoNothingCombiner, SumCombiner};
     use crate::query::ConstScorer;
@@ -101,9 +119,8 @@ mod tests {
                 ConstScorer::from(VecDocSet::from(vec![])),
             );
         let mut docs = vec![];
-        while reqoptscorer.doc() != TERMINATED {
+        while reqoptscorer.advance() {
             docs.push(reqoptscorer.doc());
-            reqoptscorer.advance();
         }
         assert_eq!(docs, req);
     }
@@ -116,45 +133,46 @@ mod tests {
                 ConstScorer::new(VecDocSet::from(vec![1, 2, 7, 11, 12, 15]), 1.0f32),
             );
         {
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 1);
             assert_eq!(reqoptscorer.score(), 2f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 3);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 3);
             assert_eq!(reqoptscorer.score(), 1f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 7);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 7);
             assert_eq!(reqoptscorer.score(), 2f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 8);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 8);
             assert_eq!(reqoptscorer.score(), 1f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 9);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 9);
             assert_eq!(reqoptscorer.score(), 1f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 10);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 10);
             assert_eq!(reqoptscorer.score(), 1f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 13);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 13);
             assert_eq!(reqoptscorer.score(), 1f32);
         }
         {
-            assert_eq!(reqoptscorer.advance(), 15);
+            assert!(reqoptscorer.advance());
             assert_eq!(reqoptscorer.doc(), 15);
             assert_eq!(reqoptscorer.score(), 2f32);
         }
-        assert_eq!(reqoptscorer.advance(), TERMINATED);
+        assert!(!reqoptscorer.advance());
     }
 
     #[test]

src/query/scorer.rs

@@ -1,4 +1,5 @@
-use crate::docset::DocSet;
+use crate::common::BitSet;
+use crate::docset::{DocSet, SkipResult};
 use crate::DocId;
 use crate::Score;
 use downcast_rs::impl_downcast;
@@ -12,6 +13,14 @@ pub trait Scorer: downcast_rs::Downcast + DocSet + 'static {
     ///
     /// This method will perform a bit of computation and is not cached.
     fn score(&mut self) -> Score;
+
+    /// Iterates through all of the document matched by the DocSet
+    /// `DocSet` and push the scored documents to the collector.
+    fn for_each(&mut self, callback: &mut dyn FnMut(DocId, Score)) {
+        while self.advance() {
+            callback(self.doc(), self.score());
+        }
+    }
 }
 
 impl_downcast!(Scorer);
@@ -20,6 +29,11 @@ impl Scorer for Box<dyn Scorer> {
     fn score(&mut self) -> Score {
         self.deref_mut().score()
     }
+
+    fn for_each(&mut self, callback: &mut dyn FnMut(DocId, Score)) {
+        let scorer = self.deref_mut();
+        scorer.for_each(callback);
+    }
 }
 
 /// Wraps a `DocSet` and simply returns a constant `Scorer`.
@@ -47,12 +61,12 @@ impl<TDocSet: DocSet> From<TDocSet> for ConstScorer<TDocSet> {
 }
 
 impl<TDocSet: DocSet> DocSet for ConstScorer<TDocSet> {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         self.docset.advance()
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        self.docset.seek(target)
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.docset.skip_next(target)
     }
 
     fn fill_buffer(&mut self, buffer: &mut [DocId]) -> usize {
@@ -66,6 +80,10 @@ impl<TDocSet: DocSet> DocSet for ConstScorer<TDocSet> {
     fn size_hint(&self) -> u32 {
         self.docset.size_hint()
     }
+
+    fn append_to_bitset(&mut self, bitset: &mut BitSet) {
+        self.docset.append_to_bitset(bitset);
+    }
 }
 
 impl<TDocSet: DocSet + 'static> Scorer for ConstScorer<TDocSet> {

src/query/term_query/block_max_term_scorer.rs

@@ -0,0 +1,98 @@
+use crate::docset::{DocSet, SkipResult};
+use crate::query::{Explanation, Scorer};
+use crate::DocId;
+use crate::Score;
+
+use crate::fieldnorm::FieldNormReader;
+use crate::postings::Postings;
+use crate::postings::{BlockMaxPostings, BlockMaxSegmentPostings};
+use crate::query::bm25::BM25Weight;
+use crate::query::BlockMaxScorer;
+
+pub struct BlockMaxTermScorer {
+    postings: BlockMaxSegmentPostings,
+    fieldnorm_reader: FieldNormReader,
+    similarity_weight: BM25Weight,
+}
+
+impl BlockMaxTermScorer {
+    pub fn new(
+        postings: BlockMaxSegmentPostings,
+        fieldnorm_reader: FieldNormReader,
+        similarity_weight: BM25Weight,
+    ) -> Self {
+        Self {
+            postings,
+            fieldnorm_reader,
+            similarity_weight,
+        }
+    }
+}
+
+impl BlockMaxTermScorer {
+    fn _score(&self, fieldnorm_id: u8, term_freq: u32) -> Score {
+        self.similarity_weight.score(fieldnorm_id, term_freq)
+    }
+
+    pub fn term_freq(&self) -> u32 {
+        self.postings.term_freq()
+    }
+
+    pub fn fieldnorm_id(&self) -> u8 {
+        self.fieldnorm_reader.fieldnorm_id(self.doc())
+    }
+
+    pub fn explain(&self) -> Explanation {
+        let fieldnorm_id = self.fieldnorm_id();
+        let term_freq = self.term_freq();
+        self.similarity_weight.explain(fieldnorm_id, term_freq)
+    }
+}
+
+impl DocSet for BlockMaxTermScorer {
+    fn advance(&mut self) -> bool {
+        self.postings.advance()
+    }
+
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.postings.skip_next(target)
+    }
+
+    fn doc(&self) -> DocId {
+        self.postings.doc()
+    }
+
+    fn size_hint(&self) -> u32 {
+        self.postings.size_hint()
+    }
+}
+
+impl Scorer for BlockMaxTermScorer {
+    fn score(&mut self) -> Score {
+        self._score(
+            self.fieldnorm_reader.fieldnorm_id(self.doc()),
+            self.postings.term_freq(),
+        )
+    }
+}
+
+impl BlockMaxScorer for BlockMaxTermScorer {
+    fn block_max_score(&mut self) -> Score {
+        self._score(
+            self.fieldnorm_reader
+                .fieldnorm_id(self.postings.block_max_doc()),
+            self.postings.term_freq(),
+        )
+    }
+
+    fn block_max_doc(&mut self) -> DocId {
+        self.postings.block_max_doc()
+    }
+
+    fn max_score(&self) -> Score {
+        self._score(
+            self.fieldnorm_reader.fieldnorm_id(self.postings.max_doc()),
+            self.postings.max_term_freq(),
+        )
+    }
+}

src/query/term_query/mod.rs

@@ -1,7 +1,9 @@
+mod block_max_term_scorer;
 mod term_query;
 mod term_scorer;
 mod term_weight;
 
+pub use self::block_max_term_scorer::BlockMaxTermScorer;
 pub use self::term_query::TermQuery;
 pub use self::term_scorer::TermScorer;
 pub use self::term_weight::TermWeight;
@@ -26,8 +28,10 @@ mod tests {
         {
             // writing the segment
             let mut index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
-            let doc = doc!(text_field => "a");
-            index_writer.add_document(doc);
+            {
+                let doc = doc!(text_field => "a");
+                index_writer.add_document(doc);
+            }
             assert!(index_writer.commit().is_ok());
         }
         let searcher = index.reader().unwrap().searcher();
@@ -38,6 +42,7 @@ mod tests {
         let term_weight = term_query.weight(&searcher, true).unwrap();
         let segment_reader = searcher.segment_reader(0);
         let mut term_scorer = term_weight.scorer(segment_reader, 1.0f32).unwrap();
+        assert!(term_scorer.advance());
         assert_eq!(term_scorer.doc(), 0);
         assert_eq!(term_scorer.score(), 0.28768212);
     }

src/query/term_query/term_scorer.rs

@@ -1,4 +1,4 @@
-use crate::docset::DocSet;
+use crate::docset::{DocSet, SkipResult};
 use crate::query::{Explanation, Scorer};
 use crate::DocId;
 use crate::Score;
@@ -45,12 +45,12 @@ impl TermScorer {
 }
 
 impl DocSet for TermScorer {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         self.postings.advance()
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        self.postings.seek(target)
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        self.postings.skip_next(target)
     }
 
     fn doc(&self) -> DocId {

src/query/term_query/term_weight.rs

@@ -4,13 +4,12 @@ use crate::docset::DocSet;
 use crate::postings::SegmentPostings;
 use crate::query::bm25::BM25Weight;
 use crate::query::explanation::does_not_match;
-use crate::query::weight::{for_each_pruning_scorer, for_each_scorer};
 use crate::query::Weight;
 use crate::query::{Explanation, Scorer};
 use crate::schema::IndexRecordOption;
-use crate::Result;
+use crate::DocId;
 use crate::Term;
-use crate::{DocId, Score};
+use crate::{Result, SkipResult};
 
 pub struct TermWeight {
     term: Term,
@@ -26,7 +25,7 @@ impl Weight for TermWeight {
 
     fn explain(&self, reader: &SegmentReader, doc: DocId) -> Result<Explanation> {
         let mut scorer = self.scorer_specialized(reader, 1.0f32)?;
-        if scorer.seek(doc) != doc {
+        if scorer.skip_next(doc) != SkipResult::Reached {
             return Err(does_not_match(doc));
         }
         Ok(scorer.explain())
@@ -44,39 +43,6 @@ impl Weight for TermWeight {
                 .unwrap_or(0))
         }
     }
-
-    /// Iterates through all of the document matched by the DocSet
-    /// `DocSet` and push the scored documents to the collector.
-    fn for_each(
-        &self,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score),
-    ) -> crate::Result<()> {
-        let mut scorer = self.scorer_specialized(reader, 1.0f32)?;
-        for_each_scorer(&mut scorer, callback);
-        Ok(())
-    }
-
-    /// Calls `callback` with all of the `(doc, score)` for which score
-    /// is exceeding a given threshold.
-    ///
-    /// This method is useful for the TopDocs collector.
-    /// For all docsets, the blanket implementation has the benefit
-    /// of prefiltering (doc, score) pairs, avoiding the
-    /// virtual dispatch cost.
-    ///
-    /// More importantly, it makes it possible for scorers to implement
-    /// important optimization (e.g. BlockWAND for union).
-    fn for_each_pruning(
-        &self,
-        threshold: f32,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score) -> Score,
-    ) -> crate::Result<()> {
-        let mut scorer = self.scorer(reader, 1.0f32)?;
-        for_each_pruning_scorer(&mut scorer, threshold, callback);
-        Ok(())
-    }
 }
 
 impl TermWeight {

src/query/union.rs

@@ -1,13 +1,107 @@
 use crate::common::TinySet;
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::{DocSet, SkipResult};
 use crate::query::score_combiner::{DoNothingCombiner, ScoreCombiner};
-use crate::query::Scorer;
+use crate::query::{Scorer, BlockMaxScorer};
 use crate::DocId;
 use crate::Score;
+use std::cmp::Ordering;
 
 const HORIZON_NUM_TINYBITSETS: usize = 64;
 const HORIZON: u32 = 64u32 * HORIZON_NUM_TINYBITSETS as u32;
 
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+struct Pivot {
+    position: usize,
+    first_occurrence: usize,
+    doc: DocId,
+}
+
+/// Find the position in the sorted list of posting lists of the **pivot**.
+///
+/// docsets need to be advanced, and are required to be sorted by the doc they point to.
+///
+/// The pivot is then defined as the lowest DocId that has a chance of matching our condition.
+fn find_pivot_position<'a, TScorer>(
+    mut docsets: impl Iterator<Item = &'a TScorer>,
+    lower_bound_score: Score,
+) -> Option<Pivot>
+    where TScorer: BlockMaxScorer
+{
+    let mut position = 0;
+    let mut upper_bound = Score::default();
+    while let Some(docset) = docsets.next() {
+        upper_bound += docset.max_score();
+        if lower_bound_score < upper_bound {
+            let pivot_doc = docset.doc();
+            let first_occurrence = position;
+            while let Some(docset) = docsets.next() {
+                if docset.doc() != pivot_doc {
+                    break;
+                } else {
+                    position += 1;
+                }
+            }
+            return Some(Pivot {
+                position,
+                doc: pivot_doc,
+                first_occurrence,
+            });
+        }
+        position += 1;
+    }
+    None
+}
+
+/// Sifts down the first element of the slice.
+///
+/// `docsets[1..]` are assumed sorted.
+/// This function swaps `docsets[0]` with its right
+/// neighbor successively -bubble sort style- until it reaches the first
+/// position such that `docsets` is sorted.
+fn sift_down<TScorer>(docsets: &mut [TScorer])
+    where
+        TScorer: BlockMaxScorer + Scorer,
+{
+    for idx in 1..docsets.len() {
+        if docsets[idx].doc() >= docsets[idx - 1].doc() {
+            return;
+        }
+        docsets.swap(idx, idx - 1);
+    }
+}
+
+
+/// Given an iterator over all ordered lists up to the pivot (inclusive) and the following list (if
+/// exists), it returns the next document ID that can be possibly relevant, based on the block max
+/// scores.
+fn find_next_relevant_doc<TScorer>(
+    docsets_up_to_pivot: &mut [TScorer],
+    pivot_docset: &mut TScorer,
+    docset_after_pivot: Option<&mut TScorer>,
+) -> DocId
+    where
+        TScorer: BlockMaxScorer + Scorer,
+{
+    let mut next_doc = 1 + docsets_up_to_pivot
+        .iter_mut()
+        .map(|docset| docset.block_max_doc())
+        .chain(std::iter::once(pivot_docset.block_max_doc()))
+        .min()
+        .unwrap();
+    if let Some(docset) = docset_after_pivot {
+        let doc = docset.doc();
+        if doc < next_doc {
+            next_doc = doc;
+        }
+    }
+    if next_doc <= pivot_docset.doc() {
+        pivot_docset.doc() + 1
+    } else {
+        next_doc
+    }
+}
+
 // `drain_filter` is not stable yet.
 // This function is similar except that it does is not unstable, and
 // it does not keep the original vector ordering.
@@ -38,6 +132,7 @@ pub struct Union<TScorer, TScoreCombiner = DoNothingCombiner> {
     score: Score,
 }
 
+
 impl<TScorer, TScoreCombiner> From<Vec<TScorer>> for Union<TScorer, TScoreCombiner>
 where
     TScoreCombiner: ScoreCombiner,
@@ -46,9 +141,17 @@ where
     fn from(docsets: Vec<TScorer>) -> Union<TScorer, TScoreCombiner> {
         let non_empty_docsets: Vec<TScorer> = docsets
             .into_iter()
-            .filter(|docset| docset.doc() != TERMINATED)
+            .flat_map(
+                |mut docset| {
+                    if docset.advance() {
+                        Some(docset)
+                    } else {
+                        None
+                    }
+                },
+            )
             .collect();
-        let mut union = Union {
+        Union {
             docsets: non_empty_docsets,
             bitsets: Box::new([TinySet::empty(); HORIZON_NUM_TINYBITSETS]),
             scores: Box::new([TScoreCombiner::default(); HORIZON as usize]),
@@ -56,13 +159,7 @@ where
             offset: 0,
             doc: 0,
             score: 0f32,
-        };
-        if union.refill() {
-            union.advance();
-        } else {
-            union.doc = TERMINATED;
         }
-        union
     }
 }
 
@@ -83,7 +180,7 @@ fn refill<TScorer: Scorer, TScoreCombiner: ScoreCombiner>(
             let delta = doc - min_doc;
             bitsets[(delta / 64) as usize].insert_mut(delta % 64u32);
             score_combiner[delta as usize].update(scorer);
-            if scorer.advance() == TERMINATED {
+            if !scorer.advance() {
                 // remove the docset, it has been entirely consumed.
                 return true;
             }
@@ -96,7 +193,6 @@ impl<TScorer: Scorer, TScoreCombiner: ScoreCombiner> Union<TScorer, TScoreCombin
         if let Some(min_doc) = self.docsets.iter().map(DocSet::doc).min() {
             self.offset = min_doc;
             self.cursor = 0;
-            self.doc = min_doc;
             refill(
                 &mut self.docsets,
                 &mut *self.bitsets,
@@ -124,6 +220,76 @@ impl<TScorer: Scorer, TScoreCombiner: ScoreCombiner> Union<TScorer, TScoreCombin
         }
         false
     }
+
+
+}
+
+
+impl<TScorer: BlockMaxScorer, TScoreCombiner: ScoreCombiner> Union<TScorer, TScoreCombiner> {
+    fn advance_with_pivot(&mut self, pivot: Pivot, lower_bound_score: Score) -> SkipResult {
+        let block_upper_bound: Score = self.docsets[..=pivot.position]
+            .iter_mut()
+            .map(|docset| docset.block_max_score())
+            .sum();
+        if block_upper_bound > lower_bound_score {
+            if pivot.doc == self.docsets[0].doc() {
+                // Since self.docsets is sorted by their current doc, in this branch, all
+                // docsets in [0..=pivot] are positioned on pivot.doc.
+                //
+                // Lets compute the actual score for this doc.
+                //
+                // NOTE(elshize): One additional check needs to be done to improve performance:
+                // update block-wise bound while accumulating score with the actual score,
+                // and check each time if still above threshold.
+                let mut combiner = TScoreCombiner::default();
+                for idx in (0..=pivot.position).rev() {
+                    combiner.update(&mut self.docsets[idx]);
+                    if !self.docsets[idx].advance() {
+                        self.docsets.swap_remove(idx);
+                    }
+                }
+                self.score = combiner.score();
+                self.doc = pivot.doc;
+                self.docsets.sort_by_key(TScorer::doc);
+                SkipResult::Reached
+            } else {
+                // The substraction does not underflow because otherwise we would go to the other
+                // branch.
+                //
+                // `advanced_idx` is the last idx that is not positionned on the pivot yet.
+                let advanced_idx = pivot.first_occurrence - 1;
+                if !self.docsets[advanced_idx].advance() {
+                    self.docsets.swap_remove(advanced_idx);
+                }
+                if self.docsets.is_empty() {
+                    return SkipResult::End;
+                }
+                sift_down(&mut self.docsets[advanced_idx..]);
+                SkipResult::OverStep
+            }
+        } else {
+            let (up_to_pivot, pivot_and_rest) = self.docsets.split_at_mut(pivot.position as usize);
+            let (pivot, after_pivot) = pivot_and_rest.split_first_mut().unwrap();
+            let next_doc = find_next_relevant_doc(up_to_pivot, pivot, after_pivot.first_mut());
+            // NOTE(elshize): It might be more efficient to advance the list with the higher
+            // max score, but let's advance the first one for now for simplicity.
+            if self.docsets[0].skip_next(next_doc) == SkipResult::End {
+                self.docsets.swap_remove(0);
+            }
+            if self.docsets.is_empty() {
+                return SkipResult::End;
+            }
+            sift_down(&mut self.docsets[..]);
+            SkipResult::OverStep
+        }
+    }
+
+    /// Find the position in the sorted list of posting lists of the **pivot**.
+    fn find_pivot_position(&self, lower_bound_score: Score) -> Option<Pivot> {
+        find_pivot_position(
+            self.docsets.iter().map(|docset| docset),
+            lower_bound_score)
+    }
 }
 
 impl<TScorer, TScoreCombiner> DocSet for Union<TScorer, TScoreCombiner>
@@ -131,23 +297,30 @@ where
     TScorer: Scorer,
     TScoreCombiner: ScoreCombiner,
 {
-    fn advance(&mut self) -> DocId {
+    fn advance(&mut self) -> bool {
         if self.advance_buffered() {
-            return self.doc;
+            return true;
         }
-        if !self.refill() {
-            self.doc = TERMINATED;
-            return TERMINATED;
+        if self.refill() {
+            self.advance();
+            true
+        } else {
+            false
         }
-        if !self.advance_buffered() {
-            return TERMINATED;
-        }
-        self.doc
     }
 
-    fn seek(&mut self, target: DocId) -> DocId {
-        if self.doc >= target {
-            return self.doc;
+    fn skip_next(&mut self, target: DocId) -> SkipResult {
+        if !self.advance() {
+            return SkipResult::End;
+        }
+        match self.doc.cmp(&target) {
+            Ordering::Equal => {
+                return SkipResult::Reached;
+            }
+            Ordering::Greater => {
+                return SkipResult::OverStep;
+            }
+            Ordering::Less => {}
         }
         let gap = target - self.offset;
         if gap < HORIZON {
@@ -165,11 +338,18 @@ where
 
             // Advancing until we reach the end of the bucket
             // or we reach a doc greater or equal to the target.
-            let mut doc = self.doc();
-            while doc < target {
-                doc = self.advance();
+            while self.advance() {
+                match self.doc().cmp(&target) {
+                    Ordering::Equal => {
+                        return SkipResult::Reached;
+                    }
+                    Ordering::Greater => {
+                        return SkipResult::OverStep;
+                    }
+                    Ordering::Less => {}
+                }
             }
-            doc
+            SkipResult::End
         } else {
             // clear the buffered info.
             for obsolete_tinyset in self.bitsets.iter_mut() {
@@ -183,42 +363,45 @@ where
             // advance all docsets to a doc >= to the target.
             #[cfg_attr(feature = "cargo-clippy", allow(clippy::clippy::collapsible_if))]
             unordered_drain_filter(&mut self.docsets, |docset| {
-                docset.seek(target) == TERMINATED
+                if docset.doc() < target {
+                    if docset.skip_next(target) == SkipResult::End {
+                        return true;
+                    }
+                }
+                false
             });
 
             // at this point all of the docsets
             // are positionned on a doc >= to the target.
-            if !self.refill() {
-                self.doc = TERMINATED;
-                return TERMINATED;
+            if self.refill() {
+                self.advance();
+                if self.doc() == target {
+                    SkipResult::Reached
+                } else {
+                    debug_assert!(self.doc() > target);
+                    SkipResult::OverStep
+                }
+            } else {
+                SkipResult::End
             }
-            self.advance()
         }
     }
 
-    // TODO Also implement `count` with deletes efficiently.
+    // TODO implement `count` efficiently.
 
     fn doc(&self) -> DocId {
         self.doc
     }
 
     fn size_hint(&self) -> u32 {
-        self.docsets
-            .iter()
-            .map(|docset| docset.size_hint())
-            .max()
-            .unwrap_or(0u32)
+        0u32
     }
 
     fn count_including_deleted(&mut self) -> u32 {
-        if self.doc == TERMINATED {
-            return 0;
-        }
         let mut count = self.bitsets[self.cursor..HORIZON_NUM_TINYBITSETS]
             .iter()
             .map(|bitset| bitset.len())
-            .sum::<u32>()
-            + 1;
+            .sum::<u32>();
         for bitset in self.bitsets.iter_mut() {
             bitset.clear();
         }
@@ -248,7 +431,7 @@ mod tests {
 
     use super::Union;
     use super::HORIZON;
-    use crate::docset::{DocSet, TERMINATED};
+    use crate::docset::{DocSet, SkipResult};
     use crate::postings::tests::test_skip_against_unoptimized;
     use crate::query::score_combiner::DoNothingCombiner;
     use crate::query::ConstScorer;
@@ -277,12 +460,12 @@ mod tests {
         };
         let mut union: Union<_, DoNothingCombiner> = make_union();
         let mut count = 0;
-        while union.doc() != TERMINATED {
+        while union.advance() {
+            assert!(union_expected.advance());
             assert_eq!(union_expected.doc(), union.doc());
-            assert_eq!(union_expected.advance(), union.advance());
             count += 1;
         }
-        assert_eq!(union_expected.advance(), TERMINATED);
+        assert!(!union_expected.advance());
         assert_eq!(count, make_union().count_including_deleted());
     }
 
@@ -310,7 +493,9 @@ mod tests {
     fn test_aux_union_skip(docs_list: &[Vec<DocId>], skip_targets: Vec<DocId>) {
         let mut btree_set = BTreeSet::new();
         for docs in docs_list {
-            btree_set.extend(docs.iter().cloned());
+            for &doc in docs.iter() {
+                btree_set.insert(doc);
+            }
         }
         let docset_factory = || {
             let res: Box<dyn DocSet> = Box::new(Union::<_, DoNothingCombiner>::from(
@@ -325,10 +510,10 @@ mod tests {
         };
         let mut docset = docset_factory();
         for el in btree_set {
+            assert!(docset.advance());
             assert_eq!(el, docset.doc());
-            docset.advance();
         }
-        assert_eq!(docset.doc(), TERMINATED);
+        assert!(!docset.advance());
         test_skip_against_unoptimized(docset_factory, skip_targets);
     }
 
@@ -351,10 +536,10 @@ mod tests {
             ConstScorer::from(VecDocSet::from(vec![0u32, 5u32])),
             ConstScorer::from(VecDocSet::from(vec![1u32, 4u32])),
         ]);
+        assert!(docset.advance());
         assert_eq!(docset.doc(), 0u32);
-        assert_eq!(docset.seek(0u32), 0u32);
-        assert_eq!(docset.seek(0u32), 0u32);
-        assert_eq!(docset.doc(), 0u32)
+        assert_eq!(docset.skip_next(0u32), SkipResult::OverStep);
+        assert_eq!(docset.doc(), 1u32)
     }
 
     #[test]

src/query/vec_docset.rs

@@ -1,8 +1,9 @@
 #![allow(dead_code)]
 
 use crate::common::HasLen;
-use crate::docset::{DocSet, TERMINATED};
+use crate::docset::DocSet;
 use crate::DocId;
+use std::num::Wrapping;
 
 /// Simulate a `Postings` objects from a `VecPostings`.
 /// `VecPostings` only exist for testing purposes.
@@ -11,30 +12,26 @@ use crate::DocId;
 /// No positions are returned.
 pub struct VecDocSet {
     doc_ids: Vec<DocId>,
-    cursor: usize,
+    cursor: Wrapping<usize>,
 }
 
 impl From<Vec<DocId>> for VecDocSet {
     fn from(doc_ids: Vec<DocId>) -> VecDocSet {
-        VecDocSet { doc_ids, cursor: 0 }
+        VecDocSet {
+            doc_ids,
+            cursor: Wrapping(usize::max_value()),
+        }
     }
 }
 
 impl DocSet for VecDocSet {
-    fn advance(&mut self) -> DocId {
-        self.cursor += 1;
-        if self.cursor >= self.doc_ids.len() {
-            self.cursor = self.doc_ids.len();
-            return TERMINATED;
-        }
-        self.doc()
+    fn advance(&mut self) -> bool {
+        self.cursor += Wrapping(1);
+        self.doc_ids.len() > self.cursor.0
     }
 
     fn doc(&self) -> DocId {
-        if self.cursor == self.doc_ids.len() {
-            return TERMINATED;
-        }
-        self.doc_ids[self.cursor]
+        self.doc_ids[self.cursor.0]
     }
 
     fn size_hint(&self) -> u32 {
@@ -52,21 +49,22 @@ impl HasLen for VecDocSet {
 pub mod tests {
 
     use super::*;
-    use crate::docset::DocSet;
+    use crate::docset::{DocSet, SkipResult};
     use crate::DocId;
 
     #[test]
     pub fn test_vec_postings() {
         let doc_ids: Vec<DocId> = (0u32..1024u32).map(|e| e * 3).collect();
         let mut postings = VecDocSet::from(doc_ids);
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 0u32);
-        assert_eq!(postings.advance(), 3u32);
+        assert!(postings.advance());
         assert_eq!(postings.doc(), 3u32);
-        assert_eq!(postings.seek(14u32), 15u32);
+        assert_eq!(postings.skip_next(14u32), SkipResult::OverStep);
         assert_eq!(postings.doc(), 15u32);
-        assert_eq!(postings.seek(300u32), 300u32);
+        assert_eq!(postings.skip_next(300u32), SkipResult::Reached);
         assert_eq!(postings.doc(), 300u32);
-        assert_eq!(postings.seek(6000u32), TERMINATED);
+        assert_eq!(postings.skip_next(6000u32), SkipResult::End);
     }
 
     #[test]

src/query/weight.rs

@@ -1,44 +1,25 @@
 use super::Scorer;
 use crate::core::SegmentReader;
 use crate::query::Explanation;
-use crate::{DocId, Score, TERMINATED};
+use crate::{DocId, Score};
 
-/// Iterates through all of the document matched by the DocSet
-/// `DocSet` and push the scored documents to the collector.
-pub(crate) fn for_each_scorer<TScorer: Scorer + ?Sized>(
-    scorer: &mut TScorer,
-    callback: &mut dyn FnMut(DocId, Score),
-) {
-    let mut doc = scorer.doc();
-    while doc != TERMINATED {
-        callback(doc, scorer.score());
-        doc = scorer.advance();
+pub trait PruningScorer {
+
+    fn doc(&self) -> DocId;
+
+    fn score(&self) -> Score;
+
+    /// Advance to the next document that has a score strictly greater than
+    /// `lower_bound_score`.
+    fn advance_with_pruning(&mut self, score_lower_bound: f32) -> bool;
+    fn advance(&mut self) -> bool {
+        self.advance_with_pruning(std::f32::NEG_INFINITY)
     }
 }
 
-/// Calls `callback` with all of the `(doc, score)` for which score
-/// is exceeding a given threshold.
-///
-/// This method is useful for the TopDocs collector.
-/// For all docsets, the blanket implementation has the benefit
-/// of prefiltering (doc, score) pairs, avoiding the
-/// virtual dispatch cost.
-///
-/// More importantly, it makes it possible for scorers to implement
-/// important optimization (e.g. BlockWAND for union).
-pub(crate) fn for_each_pruning_scorer<TScorer: Scorer + ?Sized>(
-    scorer: &mut TScorer,
-    mut threshold: f32,
-    callback: &mut dyn FnMut(DocId, Score) -> Score,
-) {
-    let mut doc = scorer.doc();
-    while doc != TERMINATED {
-        let score = scorer.score();
-        if score > threshold {
-            threshold = callback(doc, score);
-        }
-        doc = scorer.advance();
-    }
+pub enum PruningScorerIfPossible {
+    Pruning(Box<dyn PruningScorer>),
+    NonPruning(Box<dyn Scorer>)
 }
 
 /// A Weight is the specialization of a Query
@@ -53,6 +34,11 @@ pub trait Weight: Send + Sync + 'static {
     /// See [`Query`](./trait.Query.html).
     fn scorer(&self, reader: &SegmentReader, boost: f32) -> crate::Result<Box<dyn Scorer>>;
 
+    fn pruning_scorer(&self, reader: &SegmentReader, boost: f32) -> crate::Result<PruningScorerIfPossible> {
+        let scorer = self.scorer(reader, boost)?;
+        Ok(PruningScorerIfPossible::NonPruning(Box::new(scorer)))
+    }
+
     /// Returns an `Explanation` for the given document.
     fn explain(&self, reader: &SegmentReader, doc: DocId) -> crate::Result<Explanation>;
 
@@ -65,37 +51,4 @@ pub trait Weight: Send + Sync + 'static {
             Ok(scorer.count_including_deleted())
         }
     }
-
-    /// Iterates through all of the document matched by the DocSet
-    /// `DocSet` and push the scored documents to the collector.
-    fn for_each(
-        &self,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score),
-    ) -> crate::Result<()> {
-        let mut scorer = self.scorer(reader, 1.0f32)?;
-        for_each_scorer(scorer.as_mut(), callback);
-        Ok(())
-    }
-
-    /// Calls `callback` with all of the `(doc, score)` for which score
-    /// is exceeding a given threshold.
-    ///
-    /// This method is useful for the TopDocs collector.
-    /// For all docsets, the blanket implementation has the benefit
-    /// of prefiltering (doc, score) pairs, avoiding the
-    /// virtual dispatch cost.
-    ///
-    /// More importantly, it makes it possible for scorers to implement
-    /// important optimization (e.g. BlockWAND for union).
-    fn for_each_pruning(
-        &self,
-        threshold: f32,
-        reader: &SegmentReader,
-        callback: &mut dyn FnMut(DocId, Score) -> Score,
-    ) -> crate::Result<()> {
-        let mut scorer = self.scorer(reader, 1.0f32)?;
-        for_each_pruning_scorer(scorer.as_mut(), threshold, callback);
-        Ok(())
-    }
 }

src/schema/field.rs

@@ -12,13 +12,13 @@ pub struct Field(u32);
 
 impl Field {
     /// Create a new field object for the given FieldId.
-    pub const fn from_field_id(field_id: u32) -> Field {
+    pub fn from_field_id(field_id: u32) -> Field {
         Field(field_id)
     }
 
     /// Returns a u32 identifying uniquely a field within a schema.
     #[allow(clippy::trivially_copy_pass_by_ref)]
-    pub const fn field_id(&self) -> u32 {
+    pub fn field_id(&self) -> u32 {
         self.0
     }
 }