BaseVectorSimilarityQueryTestCase assumes connected hnsw graph

lucene/core/src/test/org/apache/lucene/search/BaseVectorSimilarityQueryTestCase.java

@@ -37,6 +37,7 @@
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.tests.index.RandomIndexWriter;
 import org.apache.lucene.tests.util.LuceneTestCase;
+import org.apache.lucene.tests.util.hnsw.HnswTestUtil;
 
 @LuceneTestCase.SuppressCodecs("SimpleText")
 abstract class BaseVectorSimilarityQueryTestCase<
@@ -165,6 +166,7 @@ public void testRandomFilter() throws IOException {
 
     try (Directory indexStore = getIndexStore(getRandomVectors(numDocs, dim));
         IndexReader reader = DirectoryReader.open(indexStore)) {
+      assumeTrue("graph is disconnected", HnswTestUtil.graphIsConnected(reader, vectorField));
       IndexSearcher searcher = newSearcher(reader);
 
       Query query =
@@ -289,6 +291,7 @@ public void testSomeDeletes() throws IOException {
       w.commit();
 
       try (IndexReader reader = DirectoryReader.open(indexStore)) {
+        assumeTrue("graph is disconnected", HnswTestUtil.graphIsConnected(reader, vectorField));
         IndexSearcher searcher = newSearcher(reader);
 
         Query query =

lucene/test-framework/src/java/module-info.java

@@ -51,6 +51,7 @@
   exports org.apache.lucene.tests.store;
   exports org.apache.lucene.tests.util.automaton;
   exports org.apache.lucene.tests.util.fst;
+  exports org.apache.lucene.tests.util.hnsw;
   exports org.apache.lucene.tests.util;
 
   provides org.apache.lucene.codecs.Codec with

lucene/test-framework/src/java/org/apache/lucene/tests/codecs/asserting/AssertingKnnVectorsFormat.java

@@ -18,6 +18,7 @@
 package org.apache.lucene.tests.codecs.asserting;
 
 import java.io.IOException;
+import org.apache.lucene.codecs.HnswGraphProvider;
 import org.apache.lucene.codecs.KnnFieldVectorsWriter;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.KnnVectorsReader;
@@ -34,6 +35,7 @@
 import org.apache.lucene.search.KnnCollector;
 import org.apache.lucene.tests.util.TestUtil;
 import org.apache.lucene.util.Bits;
+import org.apache.lucene.util.hnsw.HnswGraph;
 
 /** Wraps the default KnnVectorsFormat and provides additional assertions. */
 public class AssertingKnnVectorsFormat extends KnnVectorsFormat {
@@ -100,7 +102,7 @@ public long ramBytesUsed() {
     }
   }
 
-  static class AssertingKnnVectorsReader extends KnnVectorsReader {
+  static class AssertingKnnVectorsReader extends KnnVectorsReader implements HnswGraphProvider {
     final KnnVectorsReader delegate;
     final FieldInfos fis;
 
@@ -173,5 +175,10 @@ public void close() throws IOException {
     public long ramBytesUsed() {
       return delegate.ramBytesUsed();
     }
+
+    @Override
+    public HnswGraph getGraph(String field) throws IOException {
+      return ((HnswGraphProvider) delegate).getGraph(field);
+    }
   }
 }

lucene/test-framework/src/java/org/apache/lucene/tests/util/hnsw/HnswTestUtil.java

@@ -0,0 +1,132 @@
+/*
+ * 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.tests.util.hnsw;
+
+import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+import org.apache.lucene.codecs.HnswGraphProvider;
+import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
+import org.apache.lucene.index.CodecReader;
+import org.apache.lucene.index.FilterLeafReader;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.util.FixedBitSet;
+import org.apache.lucene.util.hnsw.HnswGraph;
+
+/** Utilities for use in tests involving HNSW graphs */
+public class HnswTestUtil {
+
+  /**
+   * Returns true iff level 0 of the graph is fully connected - that is every node is reachable from
+   * any entry point.
+   */
+  public static boolean isFullyConnected(HnswGraph knnValues) throws IOException {
+    return componentSizes(knnValues).size() < 2;
+  }
+
+  /**
+   * Returns the sizes of the distinct graph components on level 0. If the graph is fully-connected
+   * there will only be a single component. If the graph is empty, the returned list will be empty.
+   */
+  public static List<Integer> componentSizes(HnswGraph hnsw) throws IOException {
+    List<Integer> sizes = new ArrayList<>();
+    FixedBitSet connectedNodes = new FixedBitSet(hnsw.size());
+    assert hnsw.size() == hnsw.getNodesOnLevel(0).size();
+    int total = 0;
+    while (total < connectedNodes.length()) {
+      int componentSize = traverseConnectedNodes(hnsw, connectedNodes);
+      assert componentSize > 0;
+      sizes.add(componentSize);
+      total += componentSize;
+    }
+    return sizes;
+  }
+
+  // count the nodes in a connected component of the graph and set the bits of its nodes in
+  // connectedNodes bitset
+  private static int traverseConnectedNodes(HnswGraph hnswGraph, FixedBitSet connectedNodes)
+      throws IOException {
+    // Start at entry point and search all nodes on this level
+    int entryPoint = nextClearBit(connectedNodes, 0);
+    if (entryPoint == NO_MORE_DOCS) {
+      return 0;
+    }
+    Deque<Integer> stack = new ArrayDeque<>();
+    stack.push(entryPoint);
+    int count = 0;
+    while (!stack.isEmpty()) {
+      int node = stack.pop();
+      if (connectedNodes.get(node)) {
+        continue;
+      }
+      count++;
+      connectedNodes.set(node);
+      hnswGraph.seek(0, node);
+      int friendOrd;
+      while ((friendOrd = hnswGraph.nextNeighbor()) != NO_MORE_DOCS) {
+        stack.push(friendOrd);
+      }
+    }
+    return count;
+  }
+
+  private static int nextClearBit(FixedBitSet bits, int index) {
+    // Does not depend on the ghost bits being clear!
+    long[] barray = bits.getBits();
+    assert index >= 0 && index < bits.length() : "index=" + index + ", numBits=" + bits.length();
+    int i = index >> 6;
+    long word = ~(barray[i] >> index); // skip all the bits to the right of index
+
+    if (word != 0) {
+      return index + Long.numberOfTrailingZeros(word);
+    }
+
+    while (++i < barray.length) {
+      word = ~barray[i];
+      if (word != 0) {
+        int next = (i << 6) + Long.numberOfTrailingZeros(word);
+        if (next >= bits.length()) {
+          return NO_MORE_DOCS;
+        } else {
+          return next;
+        }
+      }
+    }
+    return NO_MORE_DOCS;
+  }
+
+  public static boolean graphIsConnected(IndexReader reader, String vectorField)
+      throws IOException {
+    for (LeafReaderContext ctx : reader.leaves()) {
+      CodecReader codecReader = (CodecReader) FilterLeafReader.unwrap(ctx.reader());
+      HnswGraph graph =
+          ((HnswGraphProvider)
+                  ((PerFieldKnnVectorsFormat.FieldsReader) codecReader.getVectorReader())
+                      .getFieldReader(vectorField))
+              .getGraph(vectorField);
+      if (isFullyConnected(graph) == false) {
+        return false;
+      }
+    }
+    return true;
+  }
+}

lucene/test-framework/src/java/org/apache/lucene/tests/util/hnsw/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * 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.
+ */
+
+/** Support for HNSW testing. */
+package org.apache.lucene.tests.util.hnsw;