Moved BlockSegmentPostings to a different file.

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"

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};
@@ -109,6 +112,9 @@ pub use self::tweak_score_top_collector::{ScoreSegmentTweaker, ScoreTweaker};
 
 mod facet_collector;
 pub use self::facet_collector::FacetCollector;
+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.
@@ -116,6 +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.
 ///
@@ -154,6 +162,20 @@ pub trait Collector: Sync {
     /// Combines the fruit associated to the collection of each segments
     /// into one fruit.
     fn merge_fruits(&self, segment_fruits: Vec<Self::Fruit>) -> crate::Result<Self::Fruit>;
+
+    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)
+    }
 }
 
 /// The `SegmentCollector` is the trait in charge of defining the
@@ -161,7 +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;
@@ -171,6 +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

@@ -56,7 +56,7 @@ impl<T: PartialOrd, D: PartialOrd> PartialEq for ComparableDoc<T, D> {
 impl<T: PartialOrd, D: PartialOrd> Eq for ComparableDoc<T, D> {}
 
 pub(crate) struct TopCollector<T> {
-    limit: usize,
+    pub limit: usize,
     _marker: PhantomData<T>,
 }
 
@@ -69,9 +69,7 @@ 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.");
-        }
+        assert!(limit > 0, "Limit must be strictly greater than 0.");
         TopCollector {
             limit,
             _marker: PhantomData,
@@ -124,13 +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),
@@ -161,6 +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

src/collector/top_score_collector.rs

@@ -8,12 +8,13 @@ use crate::collector::{
 };
 use crate::fastfield::FastFieldReader;
 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::fmt;
+use crate::query::{Weight, PruningScorerIfPossible};
 
 /// The `TopDocs` collector keeps track of the top `K` documents
 /// sorted by their score.
@@ -417,6 +418,42 @@ 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)>>,
@@ -428,6 +465,12 @@ impl Collector for TopDocs {
 /// 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)>;
 

src/core/searcher.rs

@@ -4,7 +4,6 @@ use crate::core::Executor;
 use crate::core::InvertedIndexReader;
 use crate::core::SegmentReader;
 use crate::query::Query;
-use crate::query::Scorer;
 use crate::query::Weight;
 use crate::schema::Document;
 use crate::schema::Schema;
@@ -24,17 +23,9 @@ fn collect_segment<C: Collector>(
     segment_reader: &SegmentReader,
 ) -> crate::Result<C::Fruit> {
     let mut scorer = weight.scorer(segment_reader, 1.0f32)?;
-    let mut segment_collector = collector.for_segment(segment_ord as u32, segment_reader)?;
-    if let Some(delete_bitset) = segment_reader.delete_bitset() {
-        scorer.for_each(&mut |doc, score| {
-            if delete_bitset.is_alive(doc) {
-                segment_collector.collect(doc, score);
-            }
-        });
-    } else {
-        scorer.for_each(&mut |doc, score| segment_collector.collect(doc, score));
-    }
-    Ok(segment_collector.harvest())
+    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.

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_segment_postings.rs

@@ -0,0 +1,316 @@
+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::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.
+///
+/// # Warning
+///
+/// While it is useful for some very specific high-performance
+/// use cases, you should prefer using `SegmentPostings` for most usage.
+pub struct BlockSegmentPostings {
+    doc_decoder: BlockDecoder,
+    freq_decoder: BlockDecoder,
+    freq_reading_option: FreqReadingOption,
+
+    doc_freq: usize,
+    doc_offset: DocId,
+
+    num_vint_docs: usize,
+
+    remaining_data: OwnedRead,
+    skip_reader: SkipReader,
+}
+
+
+#[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: OwnedRead,
+        record_option: IndexRecordOption,
+        requested_option: IndexRecordOption,
+    ) -> BlockSegmentPostings {
+        let freq_reading_option = match (record_option, requested_option) {
+            (IndexRecordOption::Basic, _) => FreqReadingOption::NoFreq,
+            (_, IndexRecordOption::Basic) => FreqReadingOption::SkipFreq,
+            (_, _) => FreqReadingOption::ReadFreq,
+        };
+
+        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, record_option),
+            None => SkipReader::new(OwnedRead::new(&[][..]), record_option),
+        };
+        let doc_freq = doc_freq as usize;
+        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,
+            remaining_data: postings_data,
+            skip_reader,
+        }
+    }
+
+    // Resets the block segment postings on another position
+    // in the postings file.
+    //
+    // This is useful for enumerating through a list of terms,
+    // and consuming the associated posting lists while avoiding
+    // reallocating a `BlockSegmentPostings`.
+    //
+    // # Warning
+    //
+    // This does not reset the positions list.
+    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);
+        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);
+        } else {
+            self.skip_reader.reset(OwnedRead::new(&[][..]))
+        }
+        self.doc_offset = 0;
+        self.doc_freq = doc_freq as usize;
+    }
+
+    /// Returns the document frequency associated to this block postings.
+    ///
+    /// This `doc_freq` is simply the sum of the length of all of the blocks
+    /// length, and it does not take in account deleted documents.
+    pub fn doc_freq(&self) -> usize {
+        self.doc_freq
+    }
+
+    /// Returns the array of docs in the current block.
+    ///
+    /// Before the first call to `.advance()`, the block
+    /// returned by `.docs()` is empty.
+    #[inline]
+    pub fn docs(&self) -> &[DocId] {
+        self.doc_decoder.output_array()
+    }
+
+    pub(crate) fn docs_aligned(&self) -> (&AlignedBuffer, usize) {
+        self.doc_decoder.output_aligned()
+    }
+
+    /// Return the document at index `idx` of the block.
+    #[inline]
+    pub fn doc(&self, idx: usize) -> u32 {
+        self.doc_decoder.output(idx)
+    }
+
+    /// Return the array of `term freq` in the block.
+    #[inline]
+    pub fn freqs(&self) -> &[u32] {
+        self.freq_decoder.output_array()
+    }
+
+    /// Return the frequency at index `idx` of the block.
+    #[inline]
+    pub fn freq(&self, idx: usize) -> u32 {
+        self.freq_decoder.output(idx)
+    }
+
+    /// Returns the length of the current block.
+    ///
+    /// All blocks have a length of `NUM_DOCS_PER_BLOCK`,
+    /// except the last block that may have a length
+    /// of any number between 1 and `NUM_DOCS_PER_BLOCK - 1`
+    #[inline]
+    pub(crate) fn block_len(&self) -> usize {
+        self.doc_decoder.output_len
+    }
+
+    /// position on a block that may contains `doc_id`.
+    /// Always advance the current 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);
+            }
+        }
+
+        // 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);
+                }
+            }
+            self.num_vint_docs = 0;
+            return self
+                .docs()
+                .last()
+                .map(|last_doc| {
+                    if *last_doc >= target_doc {
+                        BlockSegmentPostingsSkipResult::Success(skip_freqs)
+                    } else {
+                        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() {
+            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
+        }
+    }
+
+    /// Returns an empty segment postings object
+    pub fn empty() -> BlockSegmentPostings {
+        BlockSegmentPostings {
+            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,
+
+            remaining_data: OwnedRead::new(vec![]),
+            skip_reader: SkipReader::new(OwnedRead::new(vec![]), IndexRecordOption::Basic),
+        }
+    }
+}
+
+impl<'a> Streamer<'a> for BlockSegmentPostings {
+    type Item = &'a [DocId];
+
+    fn next(&'a mut self) -> Option<&'a [DocId]> {
+        if self.advance() {
+            Some(self.docs())
+        } else {
+            None
+        }
+    }
+}

src/postings/mod.rs

@@ -2,6 +2,8 @@
 Postings module (also called inverted index)
 */
 
+mod block_max_postings;
+mod block_max_segment_postings;
 mod block_search;
 pub(crate) mod compression;
 /// Postings module
@@ -12,6 +14,7 @@ mod postings;
 mod postings_writer;
 mod recorder;
 mod segment_postings;
+mod block_segment_postings;
 mod serializer;
 mod skip;
 mod stacker;
@@ -27,7 +30,11 @@ pub use self::postings::Postings;
 pub(crate) use self::skip::SkipReader;
 pub use self::term_info::TermInfo;
 
-pub use self::segment_postings::{BlockSegmentPostings, SegmentPostings};
+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;
 

src/postings/segment_postings.rs

@@ -1,21 +1,16 @@
 use crate::common::BitSet;
 use crate::common::HasLen;
-use crate::common::{BinarySerializable, VInt};
 use crate::docset::{DocSet, SkipResult};
 use crate::positions::PositionReader;
-use crate::postings::compression::{compressed_block_size, AlignedBuffer};
-use crate::postings::compression::{BlockDecoder, VIntDecoder, COMPRESSION_BLOCK_SIZE};
+use crate::postings::compression::COMPRESSION_BLOCK_SIZE;
 use crate::postings::serializer::PostingsSerializer;
-use crate::postings::BlockSearcher;
-use crate::postings::FreqReadingOption;
+use crate::postings::{BlockSearcher, BlockSegmentPostings};
 use crate::postings::Postings;
-use crate::postings::SkipReader;
-use crate::postings::USE_SKIP_INFO_LIMIT;
 use crate::schema::IndexRecordOption;
 use crate::DocId;
 use owned_read::OwnedRead;
 use std::cmp::Ordering;
-use tantivy_fst::Streamer;
+use crate::postings::block_segment_postings::BlockSegmentPostingsSkipResult;
 
 struct PositionComputer {
     // store the amount of position int
@@ -299,313 +294,6 @@ impl Postings for SegmentPostings {
     }
 }
 
-/// `BlockSegmentPostings` is a cursor iterating over blocks
-/// of documents.
-///
-/// # Warning
-///
-/// While it is useful for some very specific high-performance
-/// use cases, you should prefer using `SegmentPostings` for most usage.
-pub struct BlockSegmentPostings {
-    doc_decoder: BlockDecoder,
-    freq_decoder: BlockDecoder,
-    freq_reading_option: FreqReadingOption,
-
-    doc_freq: usize,
-    doc_offset: DocId,
-
-    num_vint_docs: usize,
-
-    remaining_data: OwnedRead,
-    skip_reader: SkipReader,
-}
-
-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)
-    }
-}
-
-#[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: OwnedRead,
-        record_option: IndexRecordOption,
-        requested_option: IndexRecordOption,
-    ) -> BlockSegmentPostings {
-        let freq_reading_option = match (record_option, requested_option) {
-            (IndexRecordOption::Basic, _) => FreqReadingOption::NoFreq,
-            (_, IndexRecordOption::Basic) => FreqReadingOption::SkipFreq,
-            (_, _) => FreqReadingOption::ReadFreq,
-        };
-
-        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, record_option),
-            None => SkipReader::new(OwnedRead::new(&[][..]), record_option),
-        };
-        let doc_freq = doc_freq as usize;
-        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,
-            remaining_data: postings_data,
-            skip_reader,
-        }
-    }
-
-    // Resets the block segment postings on another position
-    // in the postings file.
-    //
-    // This is useful for enumerating through a list of terms,
-    // and consuming the associated posting lists while avoiding
-    // reallocating a `BlockSegmentPostings`.
-    //
-    // # Warning
-    //
-    // This does not reset the positions list.
-    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);
-        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);
-        } else {
-            self.skip_reader.reset(OwnedRead::new(&[][..]))
-        }
-        self.doc_offset = 0;
-        self.doc_freq = doc_freq as usize;
-    }
-
-    /// Returns the document frequency associated to this block postings.
-    ///
-    /// This `doc_freq` is simply the sum of the length of all of the blocks
-    /// length, and it does not take in account deleted documents.
-    pub fn doc_freq(&self) -> usize {
-        self.doc_freq
-    }
-
-    /// Returns the array of docs in the current block.
-    ///
-    /// Before the first call to `.advance()`, the block
-    /// returned by `.docs()` is empty.
-    #[inline]
-    pub fn docs(&self) -> &[DocId] {
-        self.doc_decoder.output_array()
-    }
-
-    pub(crate) fn docs_aligned(&self) -> (&AlignedBuffer, usize) {
-        self.doc_decoder.output_aligned()
-    }
-
-    /// Return the document at index `idx` of the block.
-    #[inline]
-    pub fn doc(&self, idx: usize) -> u32 {
-        self.doc_decoder.output(idx)
-    }
-
-    /// Return the array of `term freq` in the block.
-    #[inline]
-    pub fn freqs(&self) -> &[u32] {
-        self.freq_decoder.output_array()
-    }
-
-    /// Return the frequency at index `idx` of the block.
-    #[inline]
-    pub fn freq(&self, idx: usize) -> u32 {
-        self.freq_decoder.output(idx)
-    }
-
-    /// Returns the length of the current block.
-    ///
-    /// All blocks have a length of `NUM_DOCS_PER_BLOCK`,
-    /// except the last block that may have a length
-    /// of any number between 1 and `NUM_DOCS_PER_BLOCK - 1`
-    #[inline]
-    fn block_len(&self) -> usize {
-        self.doc_decoder.output_len
-    }
-
-    /// position on a block that may contains `doc_id`.
-    /// Always advance the current 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);
-            }
-        }
-
-        // 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);
-                }
-            }
-            self.num_vint_docs = 0;
-            return self
-                .docs()
-                .last()
-                .map(|last_doc| {
-                    if *last_doc >= target_doc {
-                        BlockSegmentPostingsSkipResult::Success(skip_freqs)
-                    } else {
-                        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() {
-            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
-        }
-    }
-
-    /// Returns an empty segment postings object
-    pub fn empty() -> BlockSegmentPostings {
-        BlockSegmentPostings {
-            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,
-
-            remaining_data: OwnedRead::new(vec![]),
-            skip_reader: SkipReader::new(OwnedRead::new(vec![]), IndexRecordOption::Basic),
-        }
-    }
-}
-
-impl<'b> Streamer<'b> for BlockSegmentPostings {
-    type Item = &'b [DocId];
-
-    fn next(&'b mut self) -> Option<&'b [DocId]> {
-        if self.advance() {
-            Some(self.docs())
-        } else {
-            None
-        }
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::BlockSegmentPostings;

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/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/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;

src/query/union.rs

@@ -1,7 +1,7 @@
 use crate::common::TinySet;
 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;
@@ -9,6 +9,99 @@ 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.
@@ -39,6 +132,7 @@ pub struct Union<TScorer, TScoreCombiner = DoNothingCombiner> {
     score: Score,
 }
 
+
 impl<TScorer, TScoreCombiner> From<Vec<TScorer>> for Union<TScorer, TScoreCombiner>
 where
     TScoreCombiner: ScoreCombiner,
@@ -126,6 +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>

src/query/weight.rs

@@ -1,7 +1,26 @@
 use super::Scorer;
 use crate::core::SegmentReader;
 use crate::query::Explanation;
-use crate::DocId;
+use crate::{DocId, Score};
+
+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)
+    }
+}
+
+pub enum PruningScorerIfPossible {
+    Pruning(Box<dyn PruningScorer>),
+    NonPruning(Box<dyn Scorer>)
+}
 
 /// A Weight is the specialization of a Query
 /// for a given set of segments.
@@ -15,6 +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>;