fix directio reader

Author: ChrisHegarty

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOLucene99FlatVectorsReader.java

@@ -46,6 +46,7 @@
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
+import java.util.Map;
 
 import static org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader.readSimilarityFunction;
 import static org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader.readVectorEncoding;
@@ -169,6 +170,12 @@ public long ramBytesUsed() {
         return SHALLOW_SIZE + fields.ramBytesUsed();
     }
 
+    @Override
+    public Map<String, Long> getOffHeapByteSize(FieldInfo fieldInfo) {
+        final FieldEntry entry = getFieldEntryOrThrow(fieldInfo.name);
+        return Map.of(DirectIOLucene99FlatVectorsFormat.VECTOR_DATA_EXTENSION, entry.vectorDataLength());
+    }
+
     @Override
     public void checkIntegrity() throws IOException {
         CodecUtil.checksumEntireFile(vectorData);
@@ -185,12 +192,17 @@ public FlatVectorsReader getMergeInstance() {
         }
     }
 
-    private FieldEntry getFieldEntry(String field, VectorEncoding expectedEncoding) {
+    private FieldEntry getFieldEntryOrThrow(String field) {
         final FieldInfo info = fieldInfos.fieldInfo(field);
-        final FieldEntry fieldEntry;
-        if (info == null || (fieldEntry = fields.get(info.number)) == null) {
+        final FieldEntry entry;
+        if (info == null || (entry = fields.get(info.number)) == null) {
             throw new IllegalArgumentException("field=\"" + field + "\" not found");
         }
+        return entry;
+    }
+
+    private FieldEntry getFieldEntry(String field, VectorEncoding expectedEncoding) {
+        final FieldEntry fieldEntry = getFieldEntryOrThrow(field);
         if (fieldEntry.vectorEncoding != expectedEncoding) {
             throw new IllegalArgumentException(
                 "field=\"" + field + "\" is encoded as: " + fieldEntry.vectorEncoding + " expected: " + expectedEncoding