LUCENE-9004: KNN vector search using NSW graphs

lucene/core/src/java/org/apache/lucene/codecs/VectorWriter.java

@@ -227,6 +227,11 @@ public SearchStrategy searchStrategy() {
       return subs.get(0).values.searchStrategy();
     }
 
+    @Override
+    public TopDocs search(float[] target, int k, int fanout) throws IOException {
+      throw new UnsupportedOperationException();
+    }
+
     class MergerRandomAccess implements VectorValues.RandomAccess {
 
       private final List<RandomAccess> raSubs;
@@ -272,12 +277,6 @@ public float[] vectorValue(int target) throws IOException {
       public BytesRef binaryValue(int targetOrd) throws IOException {
         throw new UnsupportedOperationException();
       }
-
-      @Override
-      public TopDocs search(float[] target, int k, int fanout) throws IOException {
-        throw new UnsupportedOperationException();
-      }
-
     }
   }
 }

lucene/core/src/java/org/apache/lucene/codecs/lucene90/Lucene90VectorFormat.java

@@ -24,6 +24,7 @@
 import org.apache.lucene.codecs.VectorWriter;
 import org.apache.lucene.index.SegmentReadState;
 import org.apache.lucene.index.SegmentWriteState;
+import org.apache.lucene.index.VectorValues;
 
 /**
  * Lucene 9.0 vector format, which encodes dense numeric vector values.
@@ -33,15 +34,16 @@ public final class Lucene90VectorFormat extends VectorFormat {
 
   static final String META_CODEC_NAME = "Lucene90VectorFormatMeta";
   static final String VECTOR_DATA_CODEC_NAME = "Lucene90VectorFormatData";
-
+  static final String VECTOR_INDEX_CODEC_NAME = "Lucene90VectorFormatIndex";
   static final String META_EXTENSION = "vem";
   static final String VECTOR_DATA_EXTENSION = "vec";
+  static final String VECTOR_INDEX_EXTENSION = "vex";
 
   static final int VERSION_START = 0;
   static final int VERSION_CURRENT = VERSION_START;
 
   /** Sole constructor */
-  public Lucene90VectorFormat() {
+  Lucene90VectorFormat() {
   }
 
   @Override
@@ -54,4 +56,10 @@ public VectorReader fieldsReader(SegmentReadState state) throws IOException {
     return new Lucene90VectorReader(state);
   }
 
+  static boolean isHnswStrategy(VectorValues.SearchStrategy searchStrategy) {
+    return searchStrategy == VectorValues.SearchStrategy.DOT_PRODUCT_HNSW ||
+        searchStrategy == VectorValues.SearchStrategy.EUCLIDEAN_HNSW;
+  }
+
+
 }