Reduce sorting in TopDocs by not sorting the portion skipped by the offset.

Author: stuhood

src/collector/top_collector.rs

@@ -114,9 +114,7 @@ where T: PartialOrd + Clone
         }
 
         Ok(top_collector
-            .into_sorted_vec()
-            .into_iter()
-            .skip(self.offset)
+            .into_sorted_after(self.offset)
             .map(|cdoc| (cdoc.feature, cdoc.doc))
             .collect())
     }

src/collector/top_score_collector.rs

@@ -850,6 +850,24 @@ where
         self.buffer
     }
 
+    /// Returns the elements between `offset` and `top_n` in sorted order.
+    pub fn into_sorted_after(
+        mut self,
+        offset: usize,
+    ) -> impl Iterator<Item = ComparableDoc<Score, D, R>> {
+        if self.buffer.len() > self.top_n {
+            self.truncate_top_n();
+        }
+
+        if offset >= self.buffer.len() {
+            return vec![].into_iter().skip(0);
+        }
+
+        let (_, _, remainder) = self.buffer.select_nth_unstable(offset);
+        remainder.sort_unstable();
+        self.buffer.into_iter().skip(offset)
+    }
+
     /// Returns the top n elements in stored order.
     /// Useful if you do not need the elements in sorted order,
     /// for example when merging the results of multiple segments.