fix lucene91 back-compat test failure; add CHANGES entry

Author: Michael Sokolov

lucene/CHANGES.txt

@@ -184,6 +184,8 @@ Improvements
 
 * GITHUB#15332: Add PhraseQuery.Builder.setMaxTerms() method to limit the maximum number of terms and excessive memory use (linyunanit)
 
+* GITHUB#15425: Refactoring internal HnswGraph.NodesIterator to avoid unneeded copying and sorting (Mike Sokolov)
+
 Optimizations
 ---------------------
 * GITHUB#15140: Optimize TopScoreDocCollector with TernaryLongHeap for improved performance over Binary-LongHeap. (Ramakrishna Chilaka)

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene91/Lucene91OnHeapHnswGraph.java

@@ -175,7 +175,7 @@ public NodesIterator getNodesOnLevel(int level) {
     if (level == 0) {
       return new DenseNodesIterator(size());
     } else {
-      return new ArrayNodesIterator(nodesByLevel.get(level));
+      return new ArrayNodesIterator(nodesByLevel.get(level), graph.get(level).size());
     }
   }
 }

lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsWriter.java

@@ -45,7 +45,6 @@
 import org.apache.lucene.index.MergeState;
 import org.apache.lucene.index.SegmentWriteState;
 import org.apache.lucene.index.Sorter;
-import org.apache.lucene.index.VectorEncoding;
 import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.search.TaskExecutor;
 import org.apache.lucene.store.IndexOutput;
@@ -61,7 +60,6 @@
 import org.apache.lucene.util.hnsw.IncrementalHnswGraphMerger;
 import org.apache.lucene.util.hnsw.NeighborArray;
 import org.apache.lucene.util.hnsw.OnHeapHnswGraph;
-import org.apache.lucene.util.hnsw.UpdateableRandomVectorScorer;
 import org.apache.lucene.util.hnsw.RandomVectorScorerSupplier;
 import org.apache.lucene.util.packed.DirectMonotonicWriter;
 

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

@@ -191,9 +191,14 @@ public static class ArrayNodesIterator extends NodesIterator {
     private final int[] nodes;
     private int cur = 0;
 
-    /** Sole constructor */
+    /** Normal constructor */
     public ArrayNodesIterator(int[] nodes) {
-      super(nodes.length);
+      this(nodes, nodes.length);
+    }
+
+    /** Constructor that allows overriding size, used only for back-compat */
+    public ArrayNodesIterator(int[] nodes, int size) {
+      super(size);
       this.nodes = nodes;
     }