Fix global score update bug in MultiLeafKnnCollector

lucene/CHANGES.txt

@@ -259,7 +259,9 @@ Optimizations
 
 Bug Fixes
 ---------------------
-(No changes)
+
+* GITHUB#13463: Address bug in MultiLeafKnnCollector causing #minCompetitiveSimilarity to stay artificially low in
+  some corner cases. (Greg Miller)
 
 Other
 ---------------------

lucene/core/src/java/org/apache/lucene/search/knn/MultiLeafKnnCollector.java

@@ -41,6 +41,7 @@ public final class MultiLeafKnnCollector implements KnnCollector {
   private final float greediness;
   // the queue of the local similarities to periodically update with the global queue
   private final FloatHeap updatesQueue;
+  private final float[] updatesScratch;
   // interval to synchronize the local and global queues, as a number of visited vectors
   private final int interval = 0xff; // 255
   private boolean kResultsCollected = false;
@@ -62,6 +63,7 @@ public MultiLeafKnnCollector(
     this.globalSimilarityQueue = globalSimilarityQueue;
     this.nonCompetitiveQueue = new FloatHeap(Math.max(1, Math.round((1 - greediness) * k)));
     this.updatesQueue = new FloatHeap(k);
+    this.updatesScratch = new float[k];
   }
 
   @Override
@@ -103,9 +105,18 @@ public boolean collect(int docId, float similarity) {
     if (kResultsCollected) {
       // as we've collected k results, we can start do periodic updates with the global queue
       if (firstKResultsCollected || (subCollector.visitedCount() & interval) == 0) {
-        cachedGlobalMinSim = globalSimilarityQueue.offer(updatesQueue.getHeap());
-        updatesQueue.clear();
-        globalSimUpdated = true;
+        // BlockingFloatHeap#offer requires input to be sorted in ascending order, so we can't
+        // pass in the underlying updatesQueue array as-is since it is only partially ordered
+        // (see GH#13462):
+        int len = updatesQueue.size();
+        if (len > 0) {
+          for (int i = 0; i < len; i++) {
+            updatesScratch[i] = updatesQueue.poll();
+          }
+          assert updatesQueue.size() == 0;
+          cachedGlobalMinSim = globalSimilarityQueue.offer(updatesScratch, len);
+          globalSimUpdated = true;
+        }
       }
     }
     return localSimUpdated || globalSimUpdated;

lucene/core/src/java/org/apache/lucene/util/hnsw/BlockingFloatHeap.java

@@ -72,12 +72,13 @@ public float offer(float value) {
    * <p>Values must be sorted in ascending order.
    *
    * @param values a set of values to insert, must be sorted in ascending order
+   * @param len number of values from the {@code values} array to insert
    * @return the new 'top' element in the queue.
    */
-  public float offer(float[] values) {
+  public float offer(float[] values, int len) {
     lock.lock();
     try {
-      for (int i = values.length - 1; i >= 0; i--) {
+      for (int i = len - 1; i >= 0; i--) {
         if (size < maxSize) {
           push(values[i]);
         } else {

lucene/core/src/test/org/apache/lucene/search/knn/TestMultiLeafKnnCollector.java

@@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.search.knn;
+
+import org.apache.lucene.search.TopKnnCollector;
+import org.apache.lucene.tests.util.LuceneTestCase;
+import org.apache.lucene.util.hnsw.BlockingFloatHeap;
+
+public class TestMultiLeafKnnCollector extends LuceneTestCase {
+
+  /** Validates a fix for GH#13462 */
+  public void testGlobalScoreCoordination() {
+    int k = 7;
+    BlockingFloatHeap globalHeap = new BlockingFloatHeap(k);
+    MultiLeafKnnCollector collector1 =
+        new MultiLeafKnnCollector(k, globalHeap, new TopKnnCollector(k, Integer.MAX_VALUE));
+    MultiLeafKnnCollector collector2 =
+        new MultiLeafKnnCollector(k, globalHeap, new TopKnnCollector(k, Integer.MAX_VALUE));
+
+    // Collect k (7) hits in collector1 with scores [100, 106]:
+    for (int i = 0; i < k; i++) {
+      collector1.collect(0, 100f + i);
+    }
+
+    // The global heap should be updated since k hits were collected, and have a min score of
+    // 100:
+    assertEquals(100f, globalHeap.peek(), 0f);
+    assertEquals(100f, collector1.minCompetitiveSimilarity(), 0f);
+
+    // Collect k (7) hits in collector2 with only two that are competitive (200 and 300),
+    // which also forces an update of the global heap with collector2's hits. This is a tricky
+    // case where the heap will not be fully ordered, so it ensures global queue updates don't
+    // incorrectly short-circuit (see GH#13462):
+    collector2.collect(0, 10f);
+    collector2.collect(0, 11f);
+    collector2.collect(0, 12f);
+    collector2.collect(0, 13f);
+    collector2.collect(0, 200f);
+    collector2.collect(0, 14f);
+    collector2.collect(0, 300f);
+
+    // At this point, our global heap should contain [102, 103, 104, 105, 106, 200, 300] since
+    // values 200 and 300 from collector2 should have pushed out 100 and 101 from collector1.
+    // The min value on the global heap should be 102:
+    assertEquals(102f, globalHeap.peek(), 0f);
+    assertEquals(102f, collector2.minCompetitiveSimilarity(), 0f);
+  }
+}