Add raw OffHeapByteSize to IVFVectorsReader (#128451)

Author: ChrisHegarty

server/src/main/java/org/elasticsearch/index/codec/vectors/IVFVectorsReader.java

@@ -35,6 +35,7 @@
 import org.elasticsearch.search.vectors.IVFKnnSearchStrategy;
 
 import java.io.IOException;
+import java.util.Map;
 import java.util.function.IntPredicate;
 
 import static org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader.SIMILARITY_FUNCTIONS;
@@ -323,6 +324,22 @@ abstract NeighborQueue scorePostingLists(
         int nProbe
     ) throws IOException;
 
+    @Override
+    public Map<String, Long> getOffHeapByteSize(FieldInfo fieldInfo) {
+        var raw = rawVectorsReader.getOffHeapByteSize(fieldInfo);
+        FieldEntry fe = fields.get(fieldInfo.number);
+        if (fe == null) {
+            assert fieldInfo.getVectorEncoding() == VectorEncoding.BYTE;
+            return raw;
+        }
+        return raw;  // for now just return the size of raw
+
+        // TODO: determine desired off off-heap requirements
+        // var centroids = Map.of(EXTENSION, fe.xxxLength());
+        // var clusters = Map.of(EXTENSION, fe.yyyLength());
+        // return KnnVectorsReader.mergeOffHeapByteSizeMaps(raw, centroids, clusters);
+    }
+
     @Override
     public void close() throws IOException {
         IOUtils.close(rawVectorsReader, ivfCentroids, ivfClusters);