issue/57 simplified intersection, benchmark on a smaller amount of data.

src/postings/docset.rs

@@ -36,7 +36,9 @@ pub trait DocSet {
     /// skipping will advance to the next position and return SkipResult::Overstep.
     ///
     fn skip_next(&mut self, target: DocId) -> SkipResult {
-        self.advance();
+        if !self.advance() {
+            return SkipResult::End;
+        }
         loop {
             match self.doc().cmp(&target) {
                 Ordering::Less => {

src/postings/intersection.rs

@@ -38,16 +38,9 @@ impl<TDocSet: DocSet> DocSet for IntersectionDocSet<TDocSet> {
             return false;
         }
         let num_docsets = self.docsets.len();
-        let mut count_matching = 1;
-        let mut doc_candidate = {
-            let mut first_docset = &mut self.docsets[0];
-            if !first_docset.advance() {
-                self.finished = true;
-                return false;
-            }
-            first_docset.doc()
-        };
-        let mut ord = 1;
+        let mut count_matching = 0;
+        let mut doc_candidate = 0;
+        let mut ord = 0;
         loop {
             let mut doc_set = &mut self.docsets[ord];
             match doc_set.skip_next(doc_candidate) {

src/postings/mod.rs

@@ -231,7 +231,7 @@ mod tests {
             let index = Index::create_in_ram(schema);
             {
                 let mut index_writer = index.writer_with_num_threads(1, 40_000_000).unwrap();
-                for _ in 0 .. 15_000_000 {
+                for _ in 0 .. 1_500_000 {
                     let mut doc = Document::default();
                     if rng.gen_weighted_bool(15) {
                         doc.add_text(text_field, "a");