Add a de-duplicating flat vector format

lucene/core/src/java/module-info.java

@@ -25,6 +25,7 @@
   exports org.apache.lucene.analysis.tokenattributes;
   exports org.apache.lucene.analysis;
   exports org.apache.lucene.codecs.compressing;
+  exports org.apache.lucene.codecs.dedup;
   exports org.apache.lucene.codecs.lucene90.compressing;
   exports org.apache.lucene.codecs.lucene90;
   exports org.apache.lucene.codecs.lucene94;
@@ -86,7 +87,8 @@
   provides org.apache.lucene.codecs.KnnVectorsFormat with
       org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat,
       org.apache.lucene.codecs.lucene104.Lucene104ScalarQuantizedVectorsFormat,
-      org.apache.lucene.codecs.lucene104.Lucene104HnswScalarQuantizedVectorsFormat;
+      org.apache.lucene.codecs.lucene104.Lucene104HnswScalarQuantizedVectorsFormat,
+      org.apache.lucene.codecs.dedup.DedupFlatVectorsFormat;
   provides org.apache.lucene.codecs.PostingsFormat with
       org.apache.lucene.codecs.lucene104.Lucene104PostingsFormat;
   provides org.apache.lucene.index.SortFieldProvider with

lucene/core/src/java/org/apache/lucene/codecs/dedup/DedupFlatVectorsFormat.java

@@ -0,0 +1,94 @@
+/*
+ * 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.dedup;
+
+import java.io.IOException;
+import org.apache.lucene.codecs.hnsw.FlatVectorScorerUtil;
+import org.apache.lucene.codecs.hnsw.FlatVectorsFormat;
+import org.apache.lucene.codecs.hnsw.FlatVectorsReader;
+import org.apache.lucene.codecs.hnsw.FlatVectorsScorer;
+import org.apache.lucene.codecs.hnsw.FlatVectorsWriter;
+import org.apache.lucene.index.SegmentReadState;
+import org.apache.lucene.index.SegmentWriteState;
+
+/**
+ * A {@link FlatVectorsFormat} that de-duplicates stored vectors across all fields in a segment.
+ * Unique vectors are stored once in a shared dictionary (.dvd), and each field maintains a compact
+ * ordinal mapping from its document ords to dictionary ords (.dvm/.dvd).
+ *
+ * <h2>.dvd (dedup vector data) file</h2>
+ *
+ * <ul>
+ *   <li>Shared dictionary: unique vectors stored contiguously, accessible by dictOrd
+ *   <li>Per-field ord-to-dictOrd mappings (vint encoded)
+ *   <li>Per-field OrdToDoc DISI data (for sparse fields)
+ * </ul>
+ *
+ * <h2>.dvm (dedup vector meta) file</h2>
+ *
+ * <ul>
+ *   <li>Dictionary size (number of unique vectors), dimension, vector encoding
+ *   <li>Per-field: field number, size, ordToDoc config, map offset/length
+ *   <li>-1 sentinel marking end of fields
+ * </ul>
+ *
+ * <p>During merge, vectors are streamed one at a time. Only a hash map of {@code long hash → int
+ * dictOrd} is held in memory (~12 bytes per unique vector). Vector data is never buffered.
+ *
+ * @lucene.experimental
+ */
+public final class DedupFlatVectorsFormat extends FlatVectorsFormat {
+
+  public static final String NAME = "DedupFlatVectorsFormat";
+
+  static final String META_CODEC_NAME = "DedupFlatVectorsFormatMeta";
+  static final String DATA_CODEC_NAME = "DedupFlatVectorsFormatData";
+  static final String META_EXTENSION = "dvm";
+  static final String DATA_EXTENSION = "dvd";
+
+  static final int VERSION_START = 0;
+  static final int VERSION_CURRENT = VERSION_START;
+
+  private final FlatVectorsScorer vectorsScorer;
+
+  /** Creates a dedup format with the default scorer. */
+  public DedupFlatVectorsFormat() {
+    this(FlatVectorScorerUtil.getLucene99FlatVectorsScorer());
+  }
+
+  /** Creates a dedup format with the given scorer. */
+  public DedupFlatVectorsFormat(FlatVectorsScorer vectorsScorer) {
+    super(NAME);
+    this.vectorsScorer = vectorsScorer;
+  }
+
+  @Override
+  public FlatVectorsWriter fieldsWriter(SegmentWriteState state) throws IOException {
+    return new DedupFlatVectorsWriter(state, vectorsScorer);
+  }
+
+  @Override
+  public FlatVectorsReader fieldsReader(SegmentReadState state) throws IOException {
+    return new DedupFlatVectorsReader(state, vectorsScorer);
+  }
+
+  @Override
+  public String toString() {
+    return "DedupFlatVectorsFormat(vectorsScorer=" + vectorsScorer + ")";
+  }
+}