benwtrent commented on code in PR #11860:
URL: https://github.com/apache/lucene/pull/11860#discussion_r1038415190


##########
lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsWriter.java:
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@@ -0,0 +1,753 @@
+/*
+ * 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.lucene95;
+
+import static 
org.apache.lucene.codecs.lucene95.Lucene95HnswVectorsFormat.DIRECT_MONOTONIC_BLOCK_SHIFT;
+import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import org.apache.lucene.codecs.CodecUtil;
+import org.apache.lucene.codecs.KnnFieldVectorsWriter;
+import org.apache.lucene.codecs.KnnVectorsWriter;
+import org.apache.lucene.codecs.lucene90.IndexedDISI;
+import org.apache.lucene.index.*;
+import org.apache.lucene.index.Sorter;
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.store.IndexInput;
+import org.apache.lucene.store.IndexOutput;
+import org.apache.lucene.util.*;
+import org.apache.lucene.util.hnsw.HnswGraph;
+import org.apache.lucene.util.hnsw.HnswGraph.NodesIterator;
+import org.apache.lucene.util.hnsw.HnswGraphBuilder;
+import org.apache.lucene.util.hnsw.NeighborArray;
+import org.apache.lucene.util.hnsw.OnHeapHnswGraph;
+import org.apache.lucene.util.packed.DirectMonotonicWriter;
+
+/**
+ * Writes vector values and knn graphs to index segments.
+ *
+ * @lucene.experimental
+ */
+public final class Lucene95HnswVectorsWriter extends KnnVectorsWriter {
+
+  private final SegmentWriteState segmentWriteState;
+  private final IndexOutput meta, vectorData, vectorIndex;
+  private final int M;
+  private final int beamWidth;
+
+  private final List<FieldWriter<?>> fields = new ArrayList<>();
+  private boolean finished;
+
+  Lucene95HnswVectorsWriter(SegmentWriteState state, int M, int beamWidth) 
throws IOException {
+    this.M = M;
+    this.beamWidth = beamWidth;
+    segmentWriteState = state;
+    String metaFileName =
+        IndexFileNames.segmentFileName(
+            state.segmentInfo.name, state.segmentSuffix, 
Lucene95HnswVectorsFormat.META_EXTENSION);
+
+    String vectorDataFileName =
+        IndexFileNames.segmentFileName(
+            state.segmentInfo.name,
+            state.segmentSuffix,
+            Lucene95HnswVectorsFormat.VECTOR_DATA_EXTENSION);
+
+    String indexDataFileName =
+        IndexFileNames.segmentFileName(
+            state.segmentInfo.name,
+            state.segmentSuffix,
+            Lucene95HnswVectorsFormat.VECTOR_INDEX_EXTENSION);
+    boolean success = false;
+    try {
+      meta = state.directory.createOutput(metaFileName, state.context);
+      vectorData = state.directory.createOutput(vectorDataFileName, 
state.context);
+      vectorIndex = state.directory.createOutput(indexDataFileName, 
state.context);
+
+      CodecUtil.writeIndexHeader(
+          meta,
+          Lucene95HnswVectorsFormat.META_CODEC_NAME,
+          Lucene95HnswVectorsFormat.VERSION_CURRENT,
+          state.segmentInfo.getId(),
+          state.segmentSuffix);
+      CodecUtil.writeIndexHeader(
+          vectorData,
+          Lucene95HnswVectorsFormat.VECTOR_DATA_CODEC_NAME,
+          Lucene95HnswVectorsFormat.VERSION_CURRENT,
+          state.segmentInfo.getId(),
+          state.segmentSuffix);
+      CodecUtil.writeIndexHeader(
+          vectorIndex,
+          Lucene95HnswVectorsFormat.VECTOR_INDEX_CODEC_NAME,
+          Lucene95HnswVectorsFormat.VERSION_CURRENT,
+          state.segmentInfo.getId(),
+          state.segmentSuffix);
+      success = true;
+    } finally {
+      if (success == false) {
+        IOUtils.closeWhileHandlingException(this);
+      }
+    }
+  }
+
+  @Override
+  public KnnFieldVectorsWriter<?> addField(FieldInfo fieldInfo) throws 
IOException {
+    FieldWriter<?> newField =
+        FieldWriter.create(fieldInfo, M, beamWidth, 
segmentWriteState.infoStream);
+    fields.add(newField);
+    return newField;
+  }
+
+  @Override
+  public void flush(int maxDoc, Sorter.DocMap sortMap) throws IOException {
+    for (FieldWriter<?> field : fields) {
+      if (sortMap == null) {
+        writeField(field, maxDoc);
+      } else {
+        writeSortingField(field, maxDoc, sortMap);
+      }
+    }
+  }
+
+  @Override
+  public void finish() throws IOException {
+    if (finished) {
+      throw new IllegalStateException("already finished");
+    }
+    finished = true;
+
+    if (meta != null) {
+      // write end of fields marker
+      meta.writeInt(-1);
+      CodecUtil.writeFooter(meta);
+    }
+    if (vectorData != null) {
+      CodecUtil.writeFooter(vectorData);
+      CodecUtil.writeFooter(vectorIndex);
+    }
+  }
+
+  @Override
+  public long ramBytesUsed() {
+    long total = 0;
+    for (FieldWriter<?> field : fields) {
+      total += field.ramBytesUsed();
+    }
+    return total;
+  }
+
+  private void writeField(FieldWriter<?> fieldData, int maxDoc) throws 
IOException {
+    // write vector values
+    long vectorDataOffset = vectorData.alignFilePointer(Float.BYTES);
+    switch (fieldData.fieldInfo.getVectorEncoding()) {
+      case BYTE -> writeByteVectors(fieldData);
+      case FLOAT32 -> writeFloat32Vectors(fieldData);
+    }
+    long vectorDataLength = vectorData.getFilePointer() - vectorDataOffset;
+
+    // write graph
+    long vectorIndexOffset = vectorIndex.getFilePointer();
+    OnHeapHnswGraph graph = fieldData.getGraph();
+    int[][] graphLevelNodeOffsets = writeGraph(graph);
+    long vectorIndexLength = vectorIndex.getFilePointer() - vectorIndexOffset;
+
+    writeMeta(
+        fieldData.fieldInfo,
+        maxDoc,
+        vectorDataOffset,
+        vectorDataLength,
+        vectorIndexOffset,
+        vectorIndexLength,
+        fieldData.docsWithField,
+        graph,
+        graphLevelNodeOffsets);
+  }
+
+  private void writeFloat32Vectors(FieldWriter<?> fieldData) throws 
IOException {
+    final ByteBuffer buffer =
+        ByteBuffer.allocate(fieldData.dim * 
Float.BYTES).order(ByteOrder.LITTLE_ENDIAN);
+    final BytesRef binaryValue = new BytesRef(buffer.array());
+    for (Object v : fieldData.vectors) {
+      buffer.asFloatBuffer().put((float[]) v);
+      vectorData.writeBytes(binaryValue.bytes, binaryValue.offset, 
binaryValue.length);
+    }
+  }
+
+  private void writeByteVectors(FieldWriter<?> fieldData) throws IOException {
+    for (Object v : fieldData.vectors) {
+      BytesRef vector = (BytesRef) v;
+      vectorData.writeBytes(vector.bytes, vector.offset, vector.length);
+    }
+  }
+
+  private void writeSortingField(FieldWriter<?> fieldData, int maxDoc, 
Sorter.DocMap sortMap)
+      throws IOException {
+    final int[] docIdOffsets = new int[sortMap.size()];
+    int offset = 1; // 0 means no vector for this (field, document)
+    DocIdSetIterator iterator = fieldData.docsWithField.iterator();
+    for (int docID = iterator.nextDoc();
+        docID != DocIdSetIterator.NO_MORE_DOCS;
+        docID = iterator.nextDoc()) {
+      int newDocID = sortMap.oldToNew(docID);
+      docIdOffsets[newDocID] = offset++;
+    }
+    DocsWithFieldSet newDocsWithField = new DocsWithFieldSet();
+    final int[] ordMap = new int[offset - 1]; // new ord to old ord
+    final int[] oldOrdMap = new int[offset - 1]; // old ord to new ord
+    int ord = 0;
+    int doc = 0;
+    for (int docIdOffset : docIdOffsets) {
+      if (docIdOffset != 0) {
+        ordMap[ord] = docIdOffset - 1;
+        oldOrdMap[docIdOffset - 1] = ord;
+        newDocsWithField.add(doc);
+        ord++;
+      }
+      doc++;
+    }
+
+    // write vector values
+    long vectorDataOffset =
+        switch (fieldData.fieldInfo.getVectorEncoding()) {
+          case BYTE -> writeSortedByteVectors(fieldData, ordMap);
+          case FLOAT32 -> writeSortedFloat32Vectors(fieldData, ordMap);
+        };
+    long vectorDataLength = vectorData.getFilePointer() - vectorDataOffset;
+
+    // write graph
+    long vectorIndexOffset = vectorIndex.getFilePointer();
+    OnHeapHnswGraph graph = fieldData.getGraph();
+    int[][] graphLevelNodeOffsets = graph == null ? new int[0][] : new 
int[graph.numLevels()][];
+    HnswGraph mockGraph = reconstructAndWriteGraph(graph, ordMap, oldOrdMap, 
graphLevelNodeOffsets);
+    long vectorIndexLength = vectorIndex.getFilePointer() - vectorIndexOffset;
+
+    writeMeta(
+        fieldData.fieldInfo,
+        maxDoc,
+        vectorDataOffset,
+        vectorDataLength,
+        vectorIndexOffset,
+        vectorIndexLength,
+        newDocsWithField,
+        mockGraph,
+        graphLevelNodeOffsets);
+  }
+
+  private long writeSortedFloat32Vectors(FieldWriter<?> fieldData, int[] 
ordMap)
+      throws IOException {
+    long vectorDataOffset = vectorData.alignFilePointer(Float.BYTES);
+    final ByteBuffer buffer =
+        ByteBuffer.allocate(fieldData.dim * 
Float.BYTES).order(ByteOrder.LITTLE_ENDIAN);
+    final BytesRef binaryValue = new BytesRef(buffer.array());
+    for (int ordinal : ordMap) {
+      float[] vector = (float[]) fieldData.vectors.get(ordinal);
+      buffer.asFloatBuffer().put(vector);
+      vectorData.writeBytes(binaryValue.bytes, binaryValue.offset, 
binaryValue.length);
+    }
+    return vectorDataOffset;
+  }
+
+  private long writeSortedByteVectors(FieldWriter<?> fieldData, int[] ordMap) 
throws IOException {
+    long vectorDataOffset = vectorData.alignFilePointer(Float.BYTES);
+    for (int ordinal : ordMap) {
+      BytesRef vector = (BytesRef) fieldData.vectors.get(ordinal);
+      vectorData.writeBytes(vector.bytes, vector.offset, vector.length);
+    }
+    return vectorDataOffset;
+  }
+
+  /**
+   * Reconstructs the graph given the old and new node ids.
+   *
+   * <p>Additionally, the graph node connections are written to the 
vectorIndex.
+   *
+   * @param graph The current on heap graph
+   * @param newToOldMap the new node ids indexed to the old node ids
+   * @param oldToNewMap the old node ids indexed to the new node ids
+   * @param levelNodeOffsets where to place the new offsets for the nodes in 
the vector index.
+   * @return The graph
+   * @throws IOException if writing to vectorIndex fails
+   */
+  private HnswGraph reconstructAndWriteGraph(
+      OnHeapHnswGraph graph, int[] newToOldMap, int[] oldToNewMap, int[][] 
levelNodeOffsets)
+      throws IOException {
+    if (graph == null) return null;
+
+    List<int[]> nodesByLevel = new ArrayList<>(graph.numLevels());
+    nodesByLevel.add(null);
+
+    int maxOrd = graph.size();
+    int maxConnOnLevel = M * 2;
+    NodesIterator nodesOnLevel0 = graph.getNodesOnLevel(0);
+    levelNodeOffsets[0] = new int[nodesOnLevel0.size()];
+    while (nodesOnLevel0.hasNext()) {
+      int node = nodesOnLevel0.nextInt();
+      NeighborArray neighbors = graph.getNeighbors(0, newToOldMap[node]);
+      long offset = vectorIndex.getFilePointer();
+      reconstructAndWriteNeigbours(neighbors, oldToNewMap, maxConnOnLevel, 
maxOrd);
+      levelNodeOffsets[0][node] = Math.toIntExact(vectorIndex.getFilePointer() 
- offset);
+    }
+
+    maxConnOnLevel = M;
+    for (int level = 1; level < graph.numLevels(); level++) {
+      NodesIterator nodesOnLevel = graph.getNodesOnLevel(level);
+      int[] newNodes = new int[nodesOnLevel.size()];
+      int n = 0;
+      while (nodesOnLevel.hasNext()) {
+        newNodes[n++] = oldToNewMap[nodesOnLevel.nextInt()];
+      }
+      Arrays.sort(newNodes);
+      nodesByLevel.add(newNodes);
+      levelNodeOffsets[level] = new int[newNodes.length];
+      int nodeOffsetIndex = 0;
+      for (int node : newNodes) {
+        NeighborArray neighbors = graph.getNeighbors(level, newToOldMap[node]);
+        long offset = vectorIndex.getFilePointer();
+        reconstructAndWriteNeigbours(neighbors, oldToNewMap, maxConnOnLevel, 
maxOrd);
+        levelNodeOffsets[level][nodeOffsetIndex++] =
+            Math.toIntExact(vectorIndex.getFilePointer() - offset);
+      }
+    }
+    return new HnswGraph() {
+      @Override
+      public int nextNeighbor() {
+        throw new UnsupportedOperationException("Not supported on a mock 
graph");
+      }
+
+      @Override
+      public void seek(int level, int target) {
+        throw new UnsupportedOperationException("Not supported on a mock 
graph");
+      }
+
+      @Override
+      public int size() {
+        return graph.size();
+      }
+
+      @Override
+      public int numLevels() {
+        return graph.numLevels();
+      }
+
+      @Override
+      public int entryNode() {
+        throw new UnsupportedOperationException("Not supported on a mock 
graph");
+      }
+
+      @Override
+      public NodesIterator getNodesOnLevel(int level) {
+        if (level == 0) {
+          return graph.getNodesOnLevel(0);
+        } else {
+          return new NodesIterator(nodesByLevel.get(level), 
nodesByLevel.get(level).length);
+        }
+      }
+    };
+  }
+
+  private void reconstructAndWriteNeigbours(
+      NeighborArray neighbors, int[] oldToNewMap, int maxConnOnLevel, int 
maxOrd)
+      throws IOException {
+    int size = neighbors.size();
+    vectorIndex.writeVInt(size);
+
+    // Destructively modify; it's ok we are discarding it after this
+    int[] nnodes = neighbors.node();
+    for (int i = 0; i < size; i++) {
+      nnodes[i] = oldToNewMap[nnodes[i]];
+    }
+    Arrays.sort(nnodes, 0, size);
+    // Now that we have sorted, do delta encoding to minimize the required 
bits to store the
+    // information
+    for (int i = size - 1; i > 0; --i) {
+      assert nnodes[i] < maxOrd : "node too large: " + nnodes[i] + ">=" + 
maxOrd;
+      nnodes[i] -= nnodes[i - 1];
+    }
+    for (int i = 0; i < size; i++) {
+      vectorIndex.writeVInt(nnodes[i]);
+    }
+  }
+
+  @Override
+  public void mergeOneField(FieldInfo fieldInfo, MergeState mergeState) throws 
IOException {
+    long vectorDataOffset = vectorData.alignFilePointer(Float.BYTES);
+    VectorValues vectors = MergedVectorValues.mergeVectorValues(fieldInfo, 
mergeState);
+
+    IndexOutput tempVectorData =
+        segmentWriteState.directory.createTempOutput(
+            vectorData.getName(), "temp", segmentWriteState.context);
+    IndexInput vectorDataInput = null;
+    boolean success = false;
+    try {
+      // write the vector data to a temporary file
+      DocsWithFieldSet docsWithField =
+          writeVectorData(tempVectorData, vectors, 
fieldInfo.getVectorEncoding().byteSize);
+      CodecUtil.writeFooter(tempVectorData);
+      IOUtils.close(tempVectorData);
+
+      // copy the temporary file vectors to the actual data file
+      vectorDataInput =
+          segmentWriteState.directory.openInput(
+              tempVectorData.getName(), segmentWriteState.context);
+      vectorData.copyBytes(vectorDataInput, vectorDataInput.length() - 
CodecUtil.footerLength());
+      CodecUtil.retrieveChecksum(vectorDataInput);
+      long vectorDataLength = vectorData.getFilePointer() - vectorDataOffset;
+      long vectorIndexOffset = vectorIndex.getFilePointer();
+      // build the graph using the temporary vector data
+      // we use Lucene95HnswVectorsReader.DenseOffHeapVectorValues for the 
graph construction
+      // doesn't need to know docIds
+      // TODO: separate random access vector values from DocIdSetIterator?
+      int byteSize = vectors.dimension() * 
fieldInfo.getVectorEncoding().byteSize;
+      OffHeapVectorValues offHeapVectors =
+          new OffHeapVectorValues.DenseOffHeapVectorValues(
+              vectors.dimension(), docsWithField.cardinality(), 
vectorDataInput, byteSize);
+      OnHeapHnswGraph graph = null;
+      int[][] vectorIndexNodeOffsets = null;
+      if (offHeapVectors.size() != 0) {
+        // build graph
+        HnswGraphBuilder<?> hnswGraphBuilder =
+            HnswGraphBuilder.create(
+                offHeapVectors,
+                fieldInfo.getVectorEncoding(),
+                fieldInfo.getVectorSimilarityFunction(),
+                M,
+                beamWidth,
+                HnswGraphBuilder.randSeed);
+        hnswGraphBuilder.setInfoStream(segmentWriteState.infoStream);
+        graph = hnswGraphBuilder.build(offHeapVectors.copy());
+        vectorIndexNodeOffsets = writeGraph(graph);
+      }
+      long vectorIndexLength = vectorIndex.getFilePointer() - 
vectorIndexOffset;
+      writeMeta(
+          fieldInfo,
+          segmentWriteState.segmentInfo.maxDoc(),
+          vectorDataOffset,
+          vectorDataLength,
+          vectorIndexOffset,
+          vectorIndexLength,
+          docsWithField,
+          graph,
+          vectorIndexNodeOffsets);
+      success = true;
+    } finally {
+      IOUtils.close(vectorDataInput);
+      if (success) {
+        segmentWriteState.directory.deleteFile(tempVectorData.getName());
+      } else {
+        IOUtils.closeWhileHandlingException(tempVectorData);
+        IOUtils.deleteFilesIgnoringExceptions(
+            segmentWriteState.directory, tempVectorData.getName());
+      }
+    }
+  }
+
+  /**
+   * @param graph Write the graph in a compressed format
+   * @return The non-cumulative offsets for the nodes. Should be used to 
create cumulative offsets.
+   * @throws IOException if writing to vectorIndex fails
+   */
+  private int[][] writeGraph(OnHeapHnswGraph graph) throws IOException {
+    if (graph == null) return new int[0][0];
+    // write vectors' neighbours on each level into the vectorIndex file
+    int countOnLevel0 = graph.size();
+    int[][] offsets = new int[graph.numLevels()][];
+    for (int level = 0; level < graph.numLevels(); level++) {
+      NodesIterator nodesOnLevel = graph.getNodesOnLevel(level);
+      offsets[level] = new int[nodesOnLevel.size()];
+      int nodeOffsetId = 0;
+      while (nodesOnLevel.hasNext()) {
+        int node = nodesOnLevel.nextInt();
+        NeighborArray neighbors = graph.getNeighbors(level, node);
+        int size = neighbors.size();
+        // Write size in VInt as the neighbors list is typically small
+        long offsetStart = vectorIndex.getFilePointer();
+        vectorIndex.writeVInt(size);
+        // Destructively modify; it's ok we are discarding it after this
+        int[] nnodes = neighbors.node();
+        Arrays.sort(nnodes, 0, size);
+        // Now that we have sorted, do delta encoding to minimize the required 
bits to store the
+        // information
+        for (int i = size - 1; i > 0; --i) {
+          assert nnodes[i] < countOnLevel0 : "node too large: " + nnodes[i] + 
">=" + countOnLevel0;
+          nnodes[i] -= nnodes[i - 1];
+        }
+        for (int i = 0; i < size; i++) {
+          vectorIndex.writeVInt(nnodes[i]);
+        }
+        offsets[level][nodeOffsetId++] =
+            Math.toIntExact(vectorIndex.getFilePointer() - offsetStart);
+      }
+    }
+    return offsets;
+  }
+
+  private void writeMeta(
+      FieldInfo field,
+      int maxDoc,
+      long vectorDataOffset,
+      long vectorDataLength,
+      long vectorIndexOffset,
+      long vectorIndexLength,
+      DocsWithFieldSet docsWithField,
+      HnswGraph graph,
+      int[][] graphLevelNodeOffsets)
+      throws IOException {
+    meta.writeInt(field.number);
+    meta.writeInt(field.getVectorEncoding().ordinal());
+    meta.writeInt(field.getVectorSimilarityFunction().ordinal());
+    meta.writeVLong(vectorDataOffset);
+    meta.writeVLong(vectorDataLength);
+    meta.writeVLong(vectorIndexOffset);
+    meta.writeVLong(vectorIndexLength);
+    meta.writeVInt(field.getVectorDimension());
+
+    // write docIDs
+    int count = docsWithField.cardinality();
+    meta.writeInt(count);
+    if (count == 0) {
+      meta.writeLong(-2); // docsWithFieldOffset
+      meta.writeLong(0L); // docsWithFieldLength
+      meta.writeShort((short) -1); // jumpTableEntryCount
+      meta.writeByte((byte) -1); // denseRankPower
+    } else if (count == maxDoc) {
+      meta.writeLong(-1); // docsWithFieldOffset
+      meta.writeLong(0L); // docsWithFieldLength
+      meta.writeShort((short) -1); // jumpTableEntryCount
+      meta.writeByte((byte) -1); // denseRankPower
+    } else {
+      long offset = vectorData.getFilePointer();
+      meta.writeLong(offset); // docsWithFieldOffset
+      final short jumpTableEntryCount =
+          IndexedDISI.writeBitSet(
+              docsWithField.iterator(), vectorData, 
IndexedDISI.DEFAULT_DENSE_RANK_POWER);
+      meta.writeLong(vectorData.getFilePointer() - offset); // 
docsWithFieldLength
+      meta.writeShort(jumpTableEntryCount);
+      meta.writeByte(IndexedDISI.DEFAULT_DENSE_RANK_POWER);
+
+      // write ordToDoc mapping
+      long start = vectorData.getFilePointer();
+      meta.writeLong(start);
+      meta.writeVInt(DIRECT_MONOTONIC_BLOCK_SHIFT);
+      // dense case and empty case do not need to store ordToMap mapping
+      final DirectMonotonicWriter ordToDocWriter =
+          DirectMonotonicWriter.getInstance(meta, vectorData, count, 
DIRECT_MONOTONIC_BLOCK_SHIFT);
+      DocIdSetIterator iterator = docsWithField.iterator();
+      for (int doc = iterator.nextDoc();
+          doc != DocIdSetIterator.NO_MORE_DOCS;
+          doc = iterator.nextDoc()) {
+        ordToDocWriter.add(doc);
+      }
+      ordToDocWriter.finish();
+      meta.writeLong(vectorData.getFilePointer() - start);
+    }
+
+    meta.writeVInt(M);
+    // write graph nodes on each level
+    if (graph == null) {
+      meta.writeVInt(0);
+    } else {
+      meta.writeVInt(graph.numLevels());
+      long valueCount = 0;
+      for (int level = 0; level < graph.numLevels(); level++) {
+        NodesIterator nodesOnLevel = graph.getNodesOnLevel(level);
+        valueCount += nodesOnLevel.size();
+        if (level > 0) {
+          int[] nol = nodesOnLevel.copy();
+          meta.writeVInt(nol.length); // number of nodes on a level
+          for (int i = nodesOnLevel.size() - 1; i > 0; --i) {
+            nol[i] -= nol[i - 1];
+          }
+          for (int n : nol) {
+            assert n >= 0 : "delta encoding for nodes failed; expected nodes 
to be sorted";
+            meta.writeVInt(n);
+          }
+        } else {
+          assert nodesOnLevel.size() == count : "Level 0 expects to have all 
nodes";
+        }
+      }
+      long start = vectorIndex.getFilePointer();
+      meta.writeLong(start);
+      meta.writeVInt(DIRECT_MONOTONIC_BLOCK_SHIFT);
+      final DirectMonotonicWriter memoryOffsetsWriter =
+          DirectMonotonicWriter.getInstance(
+              meta, vectorIndex, valueCount, DIRECT_MONOTONIC_BLOCK_SHIFT);
+      long cumulativeOffsetSum = 0;
+      for (int[] levelOffsets : graphLevelNodeOffsets) {
+        for (int v : levelOffsets) {
+          memoryOffsetsWriter.add(cumulativeOffsetSum);
+          cumulativeOffsetSum += v;
+        }
+      }
+      memoryOffsetsWriter.finish();
+      meta.writeLong(vectorIndex.getFilePointer() - start);
+    }
+  }
+
+  /**
+   * Writes the vector values to the output and returns a set of documents 
that contains vectors.
+   */
+  private static DocsWithFieldSet writeVectorData(
+      IndexOutput output, VectorValues vectors, int scalarSize) throws 
IOException {
+    DocsWithFieldSet docsWithField = new DocsWithFieldSet();
+    for (int docV = vectors.nextDoc(); docV != NO_MORE_DOCS; docV = 
vectors.nextDoc()) {
+      // write vector
+      BytesRef binaryValue = vectors.binaryValue();
+      assert binaryValue.length == vectors.dimension() * scalarSize;
+      output.writeBytes(binaryValue.bytes, binaryValue.offset, 
binaryValue.length);
+      docsWithField.add(docV);
+    }
+    return docsWithField;
+  }
+
+  @Override
+  public void close() throws IOException {
+    IOUtils.close(meta, vectorData, vectorIndex);
+  }
+
+  private abstract static class FieldWriter<T> extends 
KnnFieldVectorsWriter<T> {

Review Comment:
   @jpountz my major refactor corrects this. There are many places where 
generics are used, but don't provide a ton of value. I can fix that in the big 
refactor.



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