Simplify HnswGraph#search. (#627)

Currently the contract on `bound` is that it holds the score of the top of the
`results` priority queue. It means that a candidate is only considered if its
score is better than the bound *or* if less than `topK` results have been
accumulated so far. I think it would be simpler if `bound` would always hold
the minimum score that is required for a candidate to be considered? This would
also be more consistent with how our WAND support works, by trusting
`setMinCompetitiveScore` alone, instead of having to check whether the priority
queue is full as well.

Author: jpountz

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

@@ -167,17 +167,17 @@ static NeighborQueue searchLevel(
       }
     }
 
-    // Set the bound to the worst current result and below reject any newly-generated candidates
-    // failing to exceed this bound
+    // A bound that holds the minimum similarity to the query vector that a candidate vector must
+    // have to be considered.
     BoundsChecker bound = BoundsChecker.create(similarityFunction.reversed);
-    bound.set(results.topScore());
+    if (results.size() >= topK) {
+      bound.set(results.topScore());
+    }
     while (candidates.size() > 0) {
       // get the best candidate (closest or best scoring)
       float topCandidateScore = candidates.topScore();
-      if (results.size() >= topK) {
-        if (bound.check(topCandidateScore)) {
-          break;
-        }
+      if (bound.check(topCandidateScore)) {
+        break;
       }
       int topCandidateNode = candidates.pop();
       graphValues.seek(level, topCandidateNode);
@@ -189,11 +189,12 @@ static NeighborQueue searchLevel(
         }
 
         float score = similarityFunction.compare(query, vectors.vectorValue(friendOrd));
-        if (results.size() < topK || bound.check(score) == false) {
+        if (bound.check(score) == false) {
           candidates.add(friendOrd, score);
           if (acceptOrds == null || acceptOrds.get(friendOrd)) {
-            results.insertWithOverflow(friendOrd, score);
-            bound.set(results.topScore());
+            if (results.insertWithOverflow(friendOrd, score) && results.size() >= topK) {
+              bound.set(results.topScore());
+            }
           }
         }
       }