Fix flaky accelerated HNSW gaps test

java/cuvs-lucene/src/test/java/com/nvidia/cuvs/lucene/TestCuVSAcceleratedHNSWGaps.java

@@ -116,10 +116,12 @@ public void testVectorSearchWithAlternatingDocuments() throws IOException {
 
     // Use the first vector (from document 0) as query
     float[] queryVector = dataset[0];
-    int topK = random.nextInt(5, TOP_K_LIMIT);
+    int vectorCount = (datasetSize + 1) / 2;
+    int topK = Math.min(random.nextInt(5, TOP_K_LIMIT), vectorCount);
 
     Query query = new KnnFloatVectorQuery("vector", queryVector, topK);
     ScoreDoc[] hits = searcher.search(query, topK).scoreDocs;
+    List<Integer> acceptableIds = calculateAcceptableNeighbors(queryVector, topK, dataset);
 
     // Verify we get exactly TOP_K results
     assertEquals("Should return exactly " + topK + " results", topK, hits.length);
@@ -129,17 +131,18 @@ public void testVectorSearchWithAlternatingDocuments() throws IOException {
       String docId = reader.storedFields().document(hit.doc).get("id");
       int id = Integer.parseInt(docId);
       assertEquals("All results should be even-numbered (have vectors)", 0, id % 2);
+      float expectedScore = VectorSimilarityFunction.EUCLIDEAN.compare(queryVector, dataset[id]);
+      assertEquals(
+          "Score should match the vector for document " + id,
+          expectedScore,
+          hit.score,
+          expectedScore * 1e-4f);
+      assertTrue(
+          "Result " + id + " was not among the closest " + (topK * 3) + " documents",
+          acceptableIds.contains(id));
       log.log(Level.FINE, "Document ID: " + id + ", Score: " + hit.score);
     }
 
-    // Verify the results match expected top-k based on Euclidean distance
-    List<Integer> expectedIds = calculateExpectedTopK(queryVector, topK, dataset);
-    for (int i = 0; i < hits.length; i++) {
-      String docId = reader.storedFields().document(hits[i].doc).get("id");
-      int id = Integer.parseInt(docId);
-      assertTrue("Result " + id + " should be in expected top-k results", expectedIds.contains(id));
-    }
-
     log.log(Level.FINE, "Alternating document test passed with " + hits.length + " results");
   }
 
@@ -169,23 +172,24 @@ public void testVectorSearchWithFilterAndAlternatingDocuments() throws IOExcepti
         "Filtered alternating document test passed with " + filteredHits.length + " results");
   }
 
-  public static List<Integer> calculateExpectedTopK(float[] query, int topK, float[][] dataset) {
+  private static List<Integer> calculateAcceptableNeighbors(
+      float[] query, int topK, float[][] dataset) {
     Map<Integer, Double> distances = new TreeMap<>();
 
-    // Calculate distances only for documents that have vectors (even-numbered)
+    // Only even-numbered documents have vectors.
     for (int i = 0; i < dataset.length; i += 2) {
       double distance = 0;
-      for (int j = 0; j < dataset[0].length; j++) {
+      for (int j = 0; j < query.length; j++) {
         distance += (query[j] - dataset[i][j]) * (query[j] - dataset[i][j]);
       }
       distances.put(i, distance);
     }
 
-    // Sort by distance and return top-k
+    // HNSW search is approximate, so accept results within three times the requested K.
     return distances.entrySet().stream()
         .sorted(Map.Entry.comparingByValue())
+        .limit(topK * 3L)
         .map(Map.Entry::getKey)
-        .limit(topK)
         .toList();
   }
 }