FieldInfosFormat translation should be independent of VectorSimilartyFunction enum

lucene/core/src/java/org/apache/lucene/codecs/lucene94/Lucene94FieldInfosFormat.java

@@ -18,6 +18,7 @@
 
 import java.io.IOException;
 import java.util.Collections;
+import java.util.List;
 import java.util.Map;
 import org.apache.lucene.codecs.CodecUtil;
 import org.apache.lucene.codecs.DocValuesFormat;
@@ -111,6 +112,8 @@
  *         <li>0: EUCLIDEAN distance. ({@link VectorSimilarityFunction#EUCLIDEAN})
  *         <li>1: DOT_PRODUCT similarity. ({@link VectorSimilarityFunction#DOT_PRODUCT})
  *         <li>2: COSINE similarity. ({@link VectorSimilarityFunction#COSINE})
+ *         <li>3: MAXIMUM_INNER_PRODUCT similarity. ({@link
+ *             VectorSimilarityFunction#MAXIMUM_INNER_PRODUCT})
  *       </ul>
  * </ul>
  *
@@ -284,10 +287,38 @@ private static VectorEncoding getVectorEncoding(IndexInput input, byte b) throws
   }
 
   private static VectorSimilarityFunction getDistFunc(IndexInput input, byte b) throws IOException {
-    if (b < 0 || b >= VectorSimilarityFunction.values().length) {
-      throw new CorruptIndexException("invalid distance function: " + b, input);
+    try {
+      return distOrdToFunc(b);
+    } catch (IllegalArgumentException e) {
+      throw new CorruptIndexException("invalid distance function: " + b, input, e);
     }
-    return VectorSimilarityFunction.values()[b];
+  }
+
+  // List of vector similarity functions. This list is defined here, in order
+  // to avoid an undesirable dependency on the declaration and order of values
+  // in VectorSimilarityFunction. The list values and order have been chosen to
+  // match that of VectorSimilarityFunction in, at least, Lucene 9.10. Values
+  static final List<VectorSimilarityFunction> SIMILARITY_FUNCTIONS =
+      List.of(
+          VectorSimilarityFunction.EUCLIDEAN,
+          VectorSimilarityFunction.DOT_PRODUCT,
+          VectorSimilarityFunction.COSINE,
+          VectorSimilarityFunction.MAXIMUM_INNER_PRODUCT);
+
+  static VectorSimilarityFunction distOrdToFunc(byte i) {
+    if (i < 0 || i >= SIMILARITY_FUNCTIONS.size()) {
+      throw new IllegalArgumentException("invalid distance function: " + i);
+    }
+    return SIMILARITY_FUNCTIONS.get(i);
+  }
+
+  static byte distFuncToOrd(VectorSimilarityFunction func) {
+    for (int i = 0; i < SIMILARITY_FUNCTIONS.size(); i++) {
+      if (SIMILARITY_FUNCTIONS.get(i).equals(func)) {
+        return (byte) i;
+      }
+    }
+    throw new IllegalArgumentException("invalid distance function: " + func);
   }
 
   static {
@@ -378,7 +409,7 @@ public void write(
         }
         output.writeVInt(fi.getVectorDimension());
         output.writeByte((byte) fi.getVectorEncoding().ordinal());
-        output.writeByte((byte) fi.getVectorSimilarityFunction().ordinal());
+        output.writeByte(distFuncToOrd(fi.getVectorSimilarityFunction()));
       }
       CodecUtil.writeFooter(output);
     }

lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java

@@ -22,6 +22,7 @@
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import org.apache.lucene.codecs.CodecUtil;
 import org.apache.lucene.codecs.FlatVectorsReader;
@@ -171,15 +172,25 @@ private void validateFieldEntry(FieldInfo info, FieldEntry fieldEntry) {
     }
   }
 
+  // List of vector similarity functions. This list is defined here, in order
+  // to avoid an undesirable dependency on the declaration and order of values
+  // in VectorSimilarityFunction. The list values and order have been chosen to
+  // match that of VectorSimilarityFunction in, at least, Lucene 9.10. Values
+  // should only be appended to the end of the list.
+  public static final List<VectorSimilarityFunction> SIMILARITY_FUNCTIONS =
+      List.of(
+          VectorSimilarityFunction.EUCLIDEAN,
+          VectorSimilarityFunction.DOT_PRODUCT,
+          VectorSimilarityFunction.COSINE,
+          VectorSimilarityFunction.MAXIMUM_INNER_PRODUCT);
+
   public static VectorSimilarityFunction readSimilarityFunction(DataInput input)
       throws IOException {
-    int similarityFunctionId = input.readInt();
-    if (similarityFunctionId < 0
-        || similarityFunctionId >= VectorSimilarityFunction.values().length) {
-      throw new CorruptIndexException(
-          "Invalid similarity function id: " + similarityFunctionId, input);
+    int i = input.readInt();
+    if (i < 0 || i >= SIMILARITY_FUNCTIONS.size()) {
+      throw new IllegalArgumentException("invalid distance function: " + i);
     }
-    return VectorSimilarityFunction.values()[similarityFunctionId];
+    return SIMILARITY_FUNCTIONS.get(i);
   }
 
   public static VectorEncoding readVectorEncoding(DataInput input) throws IOException {

lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsWriter.java

@@ -18,6 +18,7 @@
 package org.apache.lucene.codecs.lucene99;
 
 import static org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat.DIRECT_MONOTONIC_BLOCK_SHIFT;
+import static org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader.SIMILARITY_FUNCTIONS;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -33,6 +34,7 @@
 import org.apache.lucene.index.MergeState;
 import org.apache.lucene.index.SegmentWriteState;
 import org.apache.lucene.index.Sorter;
+import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.TaskExecutor;
 import org.apache.lucene.store.IndexOutput;
@@ -436,7 +438,7 @@ private void writeMeta(
       throws IOException {
     meta.writeInt(field.number);
     meta.writeInt(field.getVectorEncoding().ordinal());
-    meta.writeInt(field.getVectorSimilarityFunction().ordinal());
+    meta.writeInt(distFuncToOrd(field.getVectorSimilarityFunction()));
     meta.writeVLong(vectorIndexOffset);
     meta.writeVLong(vectorIndexLength);
     meta.writeVInt(field.getVectorDimension());
@@ -500,6 +502,15 @@ public void close() throws IOException {
     IOUtils.close(meta, vectorIndex, flatVectorWriter);
   }
 
+  static int distFuncToOrd(VectorSimilarityFunction func) {
+    for (int i = 0; i < SIMILARITY_FUNCTIONS.size(); i++) {
+      if (SIMILARITY_FUNCTIONS.get(i).equals(func)) {
+        return (byte) i;
+      }
+    }
+    throw new IllegalArgumentException("invalid distance function: " + func);
+  }
+
   private static class FieldWriter<T> extends KnnFieldVectorsWriter<T> {
 
     private static final long SHALLOW_SIZE =

lucene/core/src/test/org/apache/lucene/codecs/lucene94/TestLucene94FieldInfosFormat.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.codecs.lucene94;
+
+import java.util.Arrays;
+import org.apache.lucene.codecs.Codec;
+import org.apache.lucene.index.VectorSimilarityFunction;
+import org.apache.lucene.tests.index.BaseFieldInfoFormatTestCase;
+import org.apache.lucene.tests.util.TestUtil;
+
+public class TestLucene94FieldInfosFormat extends BaseFieldInfoFormatTestCase {
+  @Override
+  protected Codec getCodec() {
+    return TestUtil.getDefaultCodec();
+  }
+
+  // Ensures that all expected vector similarity functions are translatable
+  // in the format.
+  public void testVectorSimilarityFuncs() {
+    // This does not necessarily have to be all similarity functions, but
+    // differences should be considered carefully.
+    var expectedValues = Arrays.stream(VectorSimilarityFunction.values()).toList();
+
+    assertEquals(Lucene94FieldInfosFormat.SIMILARITY_FUNCTIONS, expectedValues);
+  }
+}

lucene/core/src/test/org/apache/lucene/codecs/lucene99/TestLucene99HnswQuantizedVectorsFormat.java

@@ -19,6 +19,7 @@
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.FilterCodec;
@@ -186,4 +187,13 @@ public void testLimits() {
             new Lucene99HnswScalarQuantizedVectorsFormat(
                 20, 100, 1, null, new SameThreadExecutorService()));
   }
+
+  // Ensures that all expected vector similarity functions are translatable
+  // in the format.
+  public void testVectorSimilarityFuncs() {
+    // This does not necessarily have to be all similarity functions, but
+    // differences should be considered carefully.
+    var expectedValues = Arrays.stream(VectorSimilarityFunction.values()).toList();
+    assertEquals(Lucene99HnswVectorsReader.SIMILARITY_FUNCTIONS, expectedValues);
+  }
 }