Use hppc IntIntHashMap to avoid Integer box/unbox when remapping vect…

…or ordinals during merge (#12950)

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

@@ -19,7 +19,6 @@
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 
 import java.io.IOException;
-import java.util.Map;
 import org.apache.lucene.codecs.HnswGraphProvider;
 import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
@@ -30,9 +29,9 @@
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.util.BitSet;
 import org.apache.lucene.util.Bits;
-import org.apache.lucene.util.CollectionUtil;
 import org.apache.lucene.util.FixedBitSet;
 import org.apache.lucene.util.InfoStream;
+import org.apache.lucene.util.hppc.IntIntHashMap;
 
 /**
  * This selects the biggest Hnsw graph from the provided merge state and initializes a new
@@ -172,7 +171,7 @@ protected final int[] getNewOrdMapping(
             "Unexpected vector encoding: " + fieldInfo.getVectorEncoding());
     }
 
-    Map<Integer, Integer> newIdToOldOrdinal = CollectionUtil.newHashMap(initGraphSize);
+    IntIntHashMap newIdToOldOrdinal = new IntIntHashMap(initGraphSize);
     int oldOrd = 0;
     int maxNewDocID = -1;
     for (int oldId = initializerIterator.nextDoc();
@@ -192,9 +191,10 @@ protected final int[] getNewOrdMapping(
     for (int newDocId = mergedVectorIterator.nextDoc();
         newDocId <= maxNewDocID;
         newDocId = mergedVectorIterator.nextDoc()) {
-      if (newIdToOldOrdinal.containsKey(newDocId)) {
+      int hashDocIndex = newIdToOldOrdinal.indexOf(newDocId);
+      if (newIdToOldOrdinal.indexExists(hashDocIndex)) {
         initializedNodes.set(newOrd);
-        oldToNewOrdinalMap[newIdToOldOrdinal.get(newDocId)] = newOrd;
+        oldToNewOrdinalMap[newIdToOldOrdinal.indexGet(hashDocIndex)] = newOrd;
       }
       newOrd++;
     }