always rerank at least the best result found so that caller will have something with which to compare this index's results to others'

jvector-base/src/main/java/io/github/jbellis/jvector/graph/NodeQueue.java

@@ -141,30 +141,50 @@ public int[] nodesCopy() {
     /**
      * Rerank results and return the worst approximate score that made it into the topK.
      * The topK results will be placed into `reranked`, and the remainder into `unused`.
+     * <p>
+     * Only the best result or results whose approximate score is at least `rerankFloor` will be reranked.
      */
     public float rerank(int topK, ScoreFunction.Reranker reranker, float rerankFloor, NodeQueue reranked, NodesUnsorted unused) {
         // Rescore the nodes whose approximate score meets the floor.  Nodes that do not will be marked as -1
         int[] ids = new int[size()];
         float[] exactScores = new float[size()];
         var approximateScoresById = new Int2ObjectHashMap<Float>();
+        float bestScore = Float.NEGATIVE_INFINITY;
+        int bestIndex = -1;
+        int scoresAboveFloor = 0;
         for (int i = 0; i < size(); i++) {
             long heapValue = heap.get(i + 1);
             float score = decodeScore(heapValue);
             var nodeId = decodeNodeId(heapValue);
+            // track the best score found so far in case nothing is above the floor
+            if (score > bestScore) {
+                bestScore = score;
+                bestIndex = i;
+            }
+
             if (score >= rerankFloor) {
+                // rerank this one
                 ids[i] = nodeId;
                 exactScores[i] = reranker.similarityTo(ids[i]);
                 approximateScoresById.put(ids[i], Float.valueOf(score));
+                scoresAboveFloor++;
             } else {
-                // if it didn't qualify for reranking, add it to the unused pile
-                unused.add(nodeId, score);
+                // mark it unranked
                 ids[i] = -1;
             }
         }
 
+        if (scoresAboveFloor == 0 && bestIndex >= 0) {
+            // if nothing was above the floor, then rerank the best one found
+            ids[bestIndex] = decodeNodeId(heap.get(bestIndex + 1));
+            exactScores[bestIndex] = reranker.similarityTo(ids[bestIndex]);
+            approximateScoresById.put(ids[bestIndex], Float.valueOf(bestScore));
+        }
+
         // go through the entries and add to the appropriate collection
         for (int i = 0; i < ids.length; i++) {
             if (ids[i] == -1) {
+                unused.add(decodeNodeId(heap.get(i + 1)), decodeScore(heap.get(i + 1)));
                 continue;
             }