FieldInfosFormat translation should be independent of VectorSimilartyFunction enum (#13119)

This commit updates the FieldInfosFormat translation of vector similarity functions to be independent of the VectorSimilartyFunction enum.

The VectorSimilartyFunction enum lives outside of the codec format, and the format should not inadvertently depend upon the declaration order or values in VectorSimilartyFunction. The format should be in charge of the translation of similarity function to format ordinal (and visa versa). In reality, and for now, the translation remains the same as the declaration order, but this may not be the case in the future.

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

@@ -18,6 +18,7 @@ package org.apache.lucene.codecs.lucene94;
 
 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 @@ import org.apache.lucene.store.IndexOutput;
  *         <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 @@ public final class Lucene94FieldInfosFormat extends FieldInfosFormat {
   }
 
   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 final class Lucene94FieldInfosFormat extends FieldInfosFormat {
         }
         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 static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 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 +172,24 @@ public final class Lucene99HnswVectorsReader extends KnnVectorsReader
     }
   }
 
+  // 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 must be identical
+  // to that of {@link o.a.l.c.l.Lucene94FieldInfosFormat#SIMILARITY_FUNCTIONS}.
+  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.IndexFileNames;
 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 @@ public final class Lucene99HnswVectorsWriter extends KnnVectorsWriter {
       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 final class Lucene99HnswVectorsWriter extends KnnVectorsWriter {
     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 @@ package org.apache.lucene.codecs.lucene99;
 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 class TestLucene99HnswQuantizedVectorsFormat extends BaseKnnVectorsFormat
             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);
+  }
 }