antonha commented on code in PR #13149:
URL: https://github.com/apache/lucene/pull/13149#discussion_r1516508079


##########
lucene/core/src/java/org/apache/lucene/util/bkd/DocIdsWriter.java:
##########
@@ -36,6 +36,66 @@ final class DocIdsWriter {
 
   private final int[] scratch;
 
+  private final ScratchDocIdSetIterator scratchDocIdSetIterator = new 
ScratchDocIdSetIterator();
+
+  /**
+   * DocIdSetIterator to be used to iterate over the scratch buffer. A single 
instance is reused to
+   * avoid re-allocating the object. The reset method should be called before 
each use with the
+   * count.
+   *
+   * <p>The main reason for existing is to be able to call the {@link
+   * IntersectVisitor#visit(DocIdSetIterator)} method rather than the {@link
+   * IntersectVisitor#visit(int)} method. This seems to make a difference in 
performance, probably
+   * due to fewer virtual calls then happening (once per read call rather than 
once per doc).
+   */
+  private class ScratchDocIdSetIterator extends DocIdSetIterator {
+
+    private int index = -1;
+    private int count = -1;
+
+    @Override
+    public int docID() {
+      if (index < 0) {
+        return -1;
+      }
+      if (index >= count) {
+        return NO_MORE_DOCS;
+      }
+      return scratch[index];
+    }
+
+    @Override
+    public int nextDoc() throws IOException {
+      index++;
+      if (index >= count) {
+        return NO_MORE_DOCS;
+      }
+      return scratch[index];
+    }
+
+    @Override
+    public int advance(int target) throws IOException {
+      while (index < count && scratch[index] < target) {

Review Comment:
   I tried changing the advance method to just throw an `throw new 
UnsupportedOperationException()` - and tests do pass - I can commit that if you 
prefer that. I guess we should add some kind of comment of why it is 
unsupported though?



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