lucene/core/src/java/org/apache/lucene/codecs/compressing/CompressingStoredFieldsWriter.java

/*
 * 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.compressing;


import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.lucene.codecs.CodecUtil;
import org.apache.lucene.codecs.StoredFieldsReader;
import org.apache.lucene.codecs.StoredFieldsWriter;
import org.apache.lucene.codecs.compressing.CompressingStoredFieldsReader.SerializedDocument;
import org.apache.lucene.index.CorruptIndexException;
import org.apache.lucene.index.DocIDMerger;
import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.index.FieldInfos;
import org.apache.lucene.index.IndexFileNames;
import org.apache.lucene.index.IndexableField;
import org.apache.lucene.index.MergeState;
import org.apache.lucene.index.SegmentInfo;
import org.apache.lucene.store.ByteBuffersDataOutput;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.IOContext;
import org.apache.lucene.store.IndexInput;
import org.apache.lucene.store.IndexOutput;
import org.apache.lucene.util.ArrayUtil;
import org.apache.lucene.util.BitUtil;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.packed.PackedInts;

import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;

/**
 * {@link StoredFieldsWriter} impl for {@link CompressingStoredFieldsFormat}.
 * @lucene.experimental
 */
public final class CompressingStoredFieldsWriter extends StoredFieldsWriter {

  /** Extension of stored fields file */
  public static final String FIELDS_EXTENSION = "fdt";
  /** Extension of stored fields index */
  public static final String INDEX_EXTENSION = "fdx";
  /** Extension of stored fields meta */
  public static final String META_EXTENSION = "fdm";
  /** Codec name for the index. */
  public static final String INDEX_CODEC_NAME = "Lucene85FieldsIndex";

  static final int         STRING = 0x00;
  static final int       BYTE_ARR = 0x01;
  static final int    NUMERIC_INT = 0x02;
  static final int  NUMERIC_FLOAT = 0x03;
  static final int   NUMERIC_LONG = 0x04;
  static final int NUMERIC_DOUBLE = 0x05;

  static final int TYPE_BITS = PackedInts.bitsRequired(NUMERIC_DOUBLE);
  static final int TYPE_MASK = (int) PackedInts.maxValue(TYPE_BITS);

  static final int VERSION_START = 1;
  static final int VERSION_OFFHEAP_INDEX = 2;
  /** Version where all metadata were moved to the meta file. */
  static final int VERSION_META = 3;
  /** Version where numChunks is explicitly recorded in meta file and a dirty chunk bit is recorded in each chunk */
  static final int VERSION_NUM_CHUNKS = 4;
  static final int VERSION_CURRENT = VERSION_NUM_CHUNKS;
  static final int META_VERSION_START = 0;

  private final String segment;
  private FieldsIndexWriter indexWriter;
  private IndexOutput metaStream, fieldsStream;

  private Compressor compressor;
  private final CompressionMode compressionMode;
  private final int chunkSize;
  private final int maxDocsPerChunk;

  private final ByteBuffersDataOutput bufferedDocs;
  private int[] numStoredFields; // number of stored fields
  private int[] endOffsets; // end offsets in bufferedDocs
  private int docBase; // doc ID at the beginning of the chunk
  private int numBufferedDocs; // docBase + numBufferedDocs == current doc ID
  

  private long numChunks;
  private long numDirtyChunks; // number of incomplete compressed blocks written
  private long numDirtyDocs; // cumulative number of missing docs in incomplete chunks

  /** Sole constructor. */
  CompressingStoredFieldsWriter(Directory directory, SegmentInfo si, String segmentSuffix, IOContext context,
      String formatName, CompressionMode compressionMode, int chunkSize, int maxDocsPerChunk, int blockShift) throws IOException {
    assert directory != null;
    this.segment = si.name;
    this.compressionMode = compressionMode;
    this.compressor = compressionMode.newCompressor();
    this.chunkSize = chunkSize;
    this.maxDocsPerChunk = maxDocsPerChunk;
    this.docBase = 0;
    this.bufferedDocs = ByteBuffersDataOutput.newResettableInstance();
    this.numStoredFields = new int[16];
    this.endOffsets = new int[16];
    this.numBufferedDocs = 0;

    boolean success = false;
    try {
      metaStream = directory.createOutput(IndexFileNames.segmentFileName(segment, segmentSuffix, META_EXTENSION), context);
      CodecUtil.writeIndexHeader(metaStream, INDEX_CODEC_NAME + "Meta", VERSION_CURRENT, si.getId(), segmentSuffix);
      assert CodecUtil.indexHeaderLength(INDEX_CODEC_NAME + "Meta", segmentSuffix) == metaStream.getFilePointer();

      fieldsStream = directory.createOutput(IndexFileNames.segmentFileName(segment, segmentSuffix, FIELDS_EXTENSION), context);
      CodecUtil.writeIndexHeader(fieldsStream, formatName, VERSION_CURRENT, si.getId(), segmentSuffix);
      assert CodecUtil.indexHeaderLength(formatName, segmentSuffix) == fieldsStream.getFilePointer();

      indexWriter = new FieldsIndexWriter(directory, segment, segmentSuffix, INDEX_EXTENSION, INDEX_CODEC_NAME, si.getId(), blockShift, context);

      metaStream.writeVInt(chunkSize);
      metaStream.writeVInt(PackedInts.VERSION_CURRENT);

      success = true;
    } finally {
      if (!success) {
        IOUtils.closeWhileHandlingException(metaStream, fieldsStream, indexWriter);
      }
    }
  }

  @Override
  public void close() throws IOException {
    try {
      IOUtils.close(metaStream, fieldsStream, indexWriter, compressor);
    } finally {
      metaStream = null;
      fieldsStream = null;
      indexWriter = null;
      compressor = null;
    }
  }

  private int numStoredFieldsInDoc;

  @Override
  public void startDocument() throws IOException {
  }

  @Override
  public void finishDocument() throws IOException {
    if (numBufferedDocs == this.numStoredFields.length) {
      final int newLength = ArrayUtil.oversize(numBufferedDocs + 1, 4);
      this.numStoredFields = ArrayUtil.growExact(this.numStoredFields, newLength);
      endOffsets = ArrayUtil.growExact(endOffsets, newLength);
    }
    this.numStoredFields[numBufferedDocs] = numStoredFieldsInDoc;
    numStoredFieldsInDoc = 0;
    endOffsets[numBufferedDocs] = Math.toIntExact(bufferedDocs.size());
    ++numBufferedDocs;
    if (triggerFlush()) {
      flush(false);
    }
  }

  private static void saveInts(int[] values, int length, DataOutput out) throws IOException {
    assert length > 0;
    if (length == 1) {
      out.writeVInt(values[0]);
    } else {
      boolean allEqual = true;
      for (int i = 1; i < length; ++i) {
        if (values[i] != values[0]) {
          allEqual = false;
          break;
        }
      }
      if (allEqual) {
        out.writeVInt(0);
        out.writeVInt(values[0]);
      } else {
        long max = 0;
        for (int i = 0; i < length; ++i) {
          max |= values[i];
        }
        final int bitsRequired = PackedInts.bitsRequired(max);
        out.writeVInt(bitsRequired);
        final PackedInts.Writer w = PackedInts.getWriterNoHeader(out, PackedInts.Format.PACKED, length, bitsRequired, 1);
        for (int i = 0; i < length; ++i) {
          w.add(values[i]);
        }
        w.finish();
      }
    }
  }

  private void writeHeader(int docBase, int numBufferedDocs, int[] numStoredFields,
                           int[] lengths, boolean sliced, boolean dirtyChunk) throws IOException {
    final int slicedBit = sliced ? 1 : 0;
    final int dirtyBit = dirtyChunk ? 2 : 0;

    // save docBase and numBufferedDocs
    fieldsStream.writeVInt(docBase);
    fieldsStream.writeVInt((numBufferedDocs << 2) | dirtyBit | slicedBit);

    // save numStoredFields
    saveInts(numStoredFields, numBufferedDocs, fieldsStream);

    // save lengths
    saveInts(lengths, numBufferedDocs, fieldsStream);
  }

  private boolean triggerFlush() {
    return bufferedDocs.size() >= chunkSize || // chunks of at least chunkSize bytes
        numBufferedDocs >= maxDocsPerChunk;
  }

  private void flush(boolean force) throws IOException {
    assert triggerFlush() != force;
    numChunks++;
    if (force) {
      numDirtyChunks++; // incomplete: we had to force this flush
      numDirtyDocs += numBufferedDocs;
    }
    indexWriter.writeIndex(numBufferedDocs, fieldsStream.getFilePointer());

    // transform end offsets into lengths
    final int[] lengths = endOffsets;
    for (int i = numBufferedDocs - 1; i > 0; --i) {
      lengths[i] = endOffsets[i] - endOffsets[i - 1];
      assert lengths[i] >= 0;
    }
    final boolean sliced = bufferedDocs.size() >= 2 * chunkSize;
    final boolean dirtyChunk = force;
    writeHeader(docBase, numBufferedDocs, numStoredFields, lengths, sliced, dirtyChunk);

    // compress stored fields to fieldsStream
    //
    // TODO: do we need to slice it since we already have the slices in the buffer? Perhaps
    // we should use max-block-bits restriction on the buffer itself, then we won't have to check it here.
    byte [] content = bufferedDocs.toArrayCopy();
    bufferedDocs.reset();

    if (sliced) {
      // big chunk, slice it
      for (int compressed = 0; compressed < content.length; compressed += chunkSize) {
        compressor.compress(content, compressed, Math.min(chunkSize, content.length - compressed), fieldsStream);
      }
    } else {
      compressor.compress(content, 0, content.length, fieldsStream);
    }

    // reset
    docBase += numBufferedDocs;
    numBufferedDocs = 0;
    bufferedDocs.reset();
  }
  
  @Override
  public void writeField(FieldInfo info, IndexableField field)
      throws IOException {

    ++numStoredFieldsInDoc;

    int bits = 0;
    final BytesRef bytes;
    final String string;

    Number number = field.numericValue();
    if (number != null) {
      if (number instanceof Byte || number instanceof Short || number instanceof Integer) {
        bits = NUMERIC_INT;
      } else if (number instanceof Long) {
        bits = NUMERIC_LONG;
      } else if (number instanceof Float) {
        bits = NUMERIC_FLOAT;
      } else if (number instanceof Double) {
        bits = NUMERIC_DOUBLE;
      } else {
        throw new IllegalArgumentException("cannot store numeric type " + number.getClass());
      }
      string = null;
      bytes = null;
    } else {
      bytes = field.binaryValue();
      if (bytes != null) {
        bits = BYTE_ARR;
        string = null;
      } else {
        bits = STRING;
        string = field.stringValue();
        if (string == null) {
          throw new IllegalArgumentException("field " + field.name() + " is stored but does not have binaryValue, stringValue nor numericValue");
        }
      }
    }

    final long infoAndBits = (((long) info.number) << TYPE_BITS) | bits;
    bufferedDocs.writeVLong(infoAndBits);

    if (bytes != null) {
      bufferedDocs.writeVInt(bytes.length);
      bufferedDocs.writeBytes(bytes.bytes, bytes.offset, bytes.length);
    } else if (string != null) {
      bufferedDocs.writeString(string);
    } else {
      if (number instanceof Byte || number instanceof Short || number instanceof Integer) {
        bufferedDocs.writeZInt(number.intValue());
      } else if (number instanceof Long) {
        writeTLong(bufferedDocs, number.longValue());
      } else if (number instanceof Float) {
        writeZFloat(bufferedDocs, number.floatValue());
      } else if (number instanceof Double) {
        writeZDouble(bufferedDocs, number.doubleValue());
      } else {
        throw new AssertionError("Cannot get here");
      }
    }
  }

  // -0 isn't compressed.
  static final int NEGATIVE_ZERO_FLOAT = Float.floatToIntBits(-0f);
  static final long NEGATIVE_ZERO_DOUBLE = Double.doubleToLongBits(-0d);

  // for compression of timestamps
  static final long SECOND = 1000L;
  static final long HOUR = 60 * 60 * SECOND;
  static final long DAY = 24 * HOUR;
  static final int SECOND_ENCODING = 0x40;
  static final int HOUR_ENCODING = 0x80;
  static final int DAY_ENCODING = 0xC0;

  /** 
   * Writes a float in a variable-length format.  Writes between one and 
   * five bytes. Small integral values typically take fewer bytes.
   * <p>
   * ZFloat --&gt; Header, Bytes*?
   * <ul>
   *    <li>Header --&gt; {@link DataOutput#writeByte Uint8}. When it is
   *       equal to 0xFF then the value is negative and stored in the next
   *       4 bytes. Otherwise if the first bit is set then the other bits
   *       in the header encode the value plus one and no other
   *       bytes are read. Otherwise, the value is a positive float value
   *       whose first byte is the header, and 3 bytes need to be read to
   *       complete it.
   *    <li>Bytes --&gt; Potential additional bytes to read depending on the
   *       header.
   * </ul>
   */
  static void writeZFloat(DataOutput out, float f) throws IOException {
    int intVal = (int) f;
    final int floatBits = Float.floatToIntBits(f);

    if (f == intVal
        && intVal >= -1
        && intVal <= 0x7D
        && floatBits != NEGATIVE_ZERO_FLOAT) {
      // small integer value [-1..125]: single byte
      out.writeByte((byte) (0x80 | (1 + intVal)));
    } else if ((floatBits >>> 31) == 0) {
      // other positive floats: 4 bytes
      out.writeInt(floatBits);
    } else {
      // other negative float: 5 bytes
      out.writeByte((byte) 0xFF);
      out.writeInt(floatBits);
    }
  }
  
  /** 
   * Writes a float in a variable-length format.  Writes between one and 
   * five bytes. Small integral values typically take fewer bytes.
   * <p>
   * ZFloat --&gt; Header, Bytes*?
   * <ul>
   *    <li>Header --&gt; {@link DataOutput#writeByte Uint8}. When it is
   *       equal to 0xFF then the value is negative and stored in the next
   *       8 bytes. When it is equal to 0xFE then the value is stored as a
   *       float in the next 4 bytes. Otherwise if the first bit is set
   *       then the other bits in the header encode the value plus one and
   *       no other bytes are read. Otherwise, the value is a positive float
   *       value whose first byte is the header, and 7 bytes need to be read
   *       to complete it.
   *    <li>Bytes --&gt; Potential additional bytes to read depending on the
   *       header.
   * </ul>
   */
  static void writeZDouble(DataOutput out, double d) throws IOException {
    int intVal = (int) d;
    final long doubleBits = Double.doubleToLongBits(d);
    
    if (d == intVal &&
        intVal >= -1 && 
        intVal <= 0x7C &&
        doubleBits != NEGATIVE_ZERO_DOUBLE) {
      // small integer value [-1..124]: single byte
      out.writeByte((byte) (0x80 | (intVal + 1)));
      return;
    } else if (d == (float) d) {
      // d has an accurate float representation: 5 bytes
      out.writeByte((byte) 0xFE);
      out.writeInt(Float.floatToIntBits((float) d));
    } else if ((doubleBits >>> 63) == 0) {
      // other positive doubles: 8 bytes
      out.writeLong(doubleBits);
    } else {
      // other negative doubles: 9 bytes
      out.writeByte((byte) 0xFF);
      out.writeLong(doubleBits);
    }
  }

  /** 
   * Writes a long in a variable-length format.  Writes between one and 
   * ten bytes. Small values or values representing timestamps with day,
   * hour or second precision typically require fewer bytes.
   * <p>
   * ZLong --&gt; Header, Bytes*?
   * <ul>
   *    <li>Header --&gt; The first two bits indicate the compression scheme:
   *       <ul>
   *          <li>00 - uncompressed
   *          <li>01 - multiple of 1000 (second)
   *          <li>10 - multiple of 3600000 (hour)
   *          <li>11 - multiple of 86400000 (day)
   *       </ul>
   *       Then the next bit is a continuation bit, indicating whether more
   *       bytes need to be read, and the last 5 bits are the lower bits of
   *       the encoded value. In order to reconstruct the value, you need to
   *       combine the 5 lower bits of the header with a vLong in the next
   *       bytes (if the continuation bit is set to 1). Then
   *       {@link BitUtil#zigZagDecode(int) zigzag-decode} it and finally
   *       multiply by the multiple corresponding to the compression scheme.
   *    <li>Bytes --&gt; Potential additional bytes to read depending on the
   *       header.
   * </ul>
   */
  // T for "timestamp"
  static void writeTLong(DataOutput out, long l) throws IOException {
    int header; 
    if (l % SECOND != 0) {
      header = 0;
    } else if (l % DAY == 0) {
      // timestamp with day precision
      header = DAY_ENCODING;
      l /= DAY;
    } else if (l % HOUR == 0) {
      // timestamp with hour precision, or day precision with a timezone
      header = HOUR_ENCODING;
      l /= HOUR;
    } else {
      // timestamp with second precision
      header = SECOND_ENCODING;
      l /= SECOND;
    }

    final long zigZagL = BitUtil.zigZagEncode(l);
    header |= (zigZagL & 0x1F); // last 5 bits
    final long upperBits = zigZagL >>> 5;
    if (upperBits != 0) {
      header |= 0x20;
    }
    out.writeByte((byte) header);
    if (upperBits != 0) {
      out.writeVLong(upperBits);
    }
  }

  @Override
  public void finish(FieldInfos fis, int numDocs) throws IOException {
    if (numBufferedDocs > 0) {
      flush(true);
    } else {
      assert bufferedDocs.size() == 0;
    }
    if (docBase != numDocs) {
      throw new RuntimeException("Wrote " + docBase + " docs, finish called with numDocs=" + numDocs);
    }
    indexWriter.finish(numDocs, fieldsStream.getFilePointer(), metaStream);
    metaStream.writeVLong(numChunks);
    metaStream.writeVLong(numDirtyChunks);
    metaStream.writeVLong(numDirtyDocs);
    CodecUtil.writeFooter(metaStream);
    CodecUtil.writeFooter(fieldsStream);
    assert bufferedDocs.size() == 0;
  }
  
  // bulk merge is scary: its caused corruption bugs in the past.
  // we try to be extra safe with this impl, but add an escape hatch to
  // have a workaround for undiscovered bugs.
  static final String BULK_MERGE_ENABLED_SYSPROP = CompressingStoredFieldsWriter.class.getName() + ".enableBulkMerge";
  static final boolean BULK_MERGE_ENABLED;
  static {
    boolean v = true;
    try {
      v = Boolean.parseBoolean(System.getProperty(BULK_MERGE_ENABLED_SYSPROP, "true"));
    } catch (SecurityException ignored) {}
    BULK_MERGE_ENABLED = v;
  }

  private void copyOneDoc(CompressingStoredFieldsReader reader, int docID)
      throws IOException {
    assert reader.getVersion() == VERSION_CURRENT;
    SerializedDocument doc = reader.document(docID);
    startDocument();
    bufferedDocs.copyBytes(doc.in, doc.length);
    numStoredFieldsInDoc = doc.numStoredFields;
    finishDocument();
  }

  private void copyChunks(
      final MergeState mergeState,
      final CompressingStoredFieldsMergeSub sub,
      final int fromDocID,
      final int toDocID)
      throws IOException {
    final CompressingStoredFieldsReader reader =
        (CompressingStoredFieldsReader) mergeState.storedFieldsReaders[sub.readerIndex];
    assert reader.getVersion() == VERSION_CURRENT;
    assert reader.getChunkSize() == chunkSize;
    assert reader.getCompressionMode() == compressionMode;
    assert !tooDirty(reader);
    assert mergeState.liveDocs[sub.readerIndex] == null;

    int docID = fromDocID;
    final FieldsIndex index = reader.getIndexReader();

    // copy docs that belong to the previous chunk
    while (docID < toDocID && reader.isLoaded(docID)) {
      copyOneDoc(reader, docID++);
    }
    if (docID >= toDocID) {
      return;
    }
    // copy chunks
    long fromPointer = index.getStartPointer(docID);
    final long toPointer =
        toDocID == sub.maxDoc ? reader.getMaxPointer() : index.getStartPointer(toDocID);
    if (fromPointer < toPointer) {
      if (numBufferedDocs > 0) {
        flush(true);
      }
      final IndexInput rawDocs = reader.getFieldsStream();
      rawDocs.seek(fromPointer);
      do {
        final int base = rawDocs.readVInt();
        final int code = rawDocs.readVInt();
        final int bufferedDocs = code >>> 2;
        if (base != docID) {
          throw new CorruptIndexException(
              "invalid state: base=" + base + ", docID=" + docID, rawDocs);
        }
        // write a new index entry and new header for this chunk.
        indexWriter.writeIndex(bufferedDocs, fieldsStream.getFilePointer());
        fieldsStream.writeVInt(docBase); // rebase
        fieldsStream.writeVInt(code);
        docID += bufferedDocs;
        docBase += bufferedDocs;
        if (docID > toDocID) {
          throw new CorruptIndexException(
              "invalid state: base=" + base + ", count=" + bufferedDocs + ", toDocID=" + toDocID,
              rawDocs);
        }
        // copy bytes until the next chunk boundary (or end of chunk data).
        // using the stored fields index for this isn't the most efficient, but fast enough
        // and is a source of redundancy for detecting bad things.
        final long endChunkPointer;
        if (docID == sub.maxDoc) {
          endChunkPointer = reader.getMaxPointer();
        } else {
          endChunkPointer = index.getStartPointer(docID);
        }
        fieldsStream.copyBytes(rawDocs, endChunkPointer - rawDocs.getFilePointer());
        ++numChunks;
        final boolean dirtyChunk = (code & 2) != 0;
        if (dirtyChunk) {
          assert bufferedDocs < maxDocsPerChunk;
          ++numDirtyChunks;
          numDirtyDocs += bufferedDocs;
        }
        fromPointer = endChunkPointer;
      } while (fromPointer < toPointer);
    }

    // copy leftover docs that don't form a complete chunk
    assert reader.isLoaded(docID) == false;
    while (docID < toDocID) {
      copyOneDoc(reader, docID++);
    }
  }

  @Override
  public int merge(MergeState mergeState) throws IOException {
    final MatchingReaders matchingReaders = new MatchingReaders(mergeState);
    final MergeVisitor[] visitors = new MergeVisitor[mergeState.storedFieldsReaders.length];
    final List<CompressingStoredFieldsMergeSub> subs =
        new ArrayList<>(mergeState.storedFieldsReaders.length);
    for (int i = 0; i < mergeState.storedFieldsReaders.length; i++) {
      final StoredFieldsReader reader = mergeState.storedFieldsReaders[i];
      reader.checkIntegrity();
      MergeStrategy mergeStrategy = getMergeStrategy(mergeState, matchingReaders, i);
      if (mergeStrategy == MergeStrategy.VISITOR) {
        visitors[i] = new MergeVisitor(mergeState, i);
      }
      subs.add(new CompressingStoredFieldsMergeSub(mergeState, mergeStrategy, i));
    }
    int docCount = 0;
    final DocIDMerger<CompressingStoredFieldsMergeSub> docIDMerger =
        DocIDMerger.of(subs, mergeState.needsIndexSort);
    CompressingStoredFieldsMergeSub sub = docIDMerger.next();
    while (sub != null) {
      assert sub.mappedDocID == docCount : sub.mappedDocID + " != " + docCount;
      final StoredFieldsReader reader = mergeState.storedFieldsReaders[sub.readerIndex];
      if (sub.mergeStrategy == MergeStrategy.BULK) {
        final int fromDocID = sub.docID;
        int toDocID = fromDocID;
        final CompressingStoredFieldsMergeSub current = sub;
        while ((sub = docIDMerger.next()) == current) {
          ++toDocID;
          assert sub.docID == toDocID;
        }
        ++toDocID; // exclusive bound
        copyChunks(mergeState, current, fromDocID, toDocID);
        docCount += (toDocID - fromDocID);
      } else if (sub.mergeStrategy == MergeStrategy.DOC) {
        copyOneDoc((CompressingStoredFieldsReader) reader, sub.docID);
        ++docCount;
        sub = docIDMerger.next();
      } else if (sub.mergeStrategy == MergeStrategy.VISITOR) {
        assert visitors[sub.readerIndex] != null;
        startDocument();
        reader.visitDocument(sub.docID, visitors[sub.readerIndex]);
        finishDocument();
        ++docCount;
        sub = docIDMerger.next();
      } else {
        throw new AssertionError("Unknown merge strategy [" + sub.mergeStrategy + "]");
      }
    }
    finish(mergeState.mergeFieldInfos, docCount);
    return docCount;
  }
  
  /** 
   * Returns true if we should recompress this reader, even though we could bulk merge compressed data 
   * <p>
   * The last chunk written for a segment is typically incomplete, so without recompressing,
   * in some worst-case situations (e.g. frequent reopen with tiny flushes), over time the 
   * compression ratio can degrade. This is a safety switch.
   */
  boolean tooDirty(CompressingStoredFieldsReader candidate) {
    // A segment is considered dirty only if it has enough dirty docs to make a full block
    // AND more than 1% blocks are dirty.
    return candidate.getNumDirtyDocs() > maxDocsPerChunk
        && candidate.getNumDirtyChunks() * 100 > candidate.getNumChunks();
  }

  private enum MergeStrategy {
    /** Copy chunk by chunk in a compressed format */
    BULK,

    /** Copy document by document in a decompressed format */
    DOC,

    /** Copy field by field of decompressed documents */
    VISITOR
  }

  private MergeStrategy getMergeStrategy(
      MergeState mergeState, MatchingReaders matchingReaders, int readerIndex) {
    final StoredFieldsReader candidate = mergeState.storedFieldsReaders[readerIndex];
    if (matchingReaders.matchingReaders[readerIndex] == false
        || candidate instanceof CompressingStoredFieldsReader == false
        || ((CompressingStoredFieldsReader) candidate).getVersion() != VERSION_CURRENT) {
      return MergeStrategy.VISITOR;
    }
    CompressingStoredFieldsReader reader = (CompressingStoredFieldsReader) candidate;
    if (BULK_MERGE_ENABLED
        && reader.getCompressionMode() == compressionMode
        && reader.getChunkSize() == chunkSize
        && reader.getPackedIntsVersion() == PackedInts.VERSION_CURRENT
        // its not worth fine-graining this if there are deletions.
        && mergeState.liveDocs[readerIndex] == null
        && !tooDirty(reader)) {
      return MergeStrategy.BULK;
    } else {
      return MergeStrategy.DOC;
    }
  }

  private static class CompressingStoredFieldsMergeSub extends DocIDMerger.Sub {
    private final int readerIndex;
    private final int maxDoc;
    private final MergeStrategy mergeStrategy;
    int docID = -1;

    CompressingStoredFieldsMergeSub(
        MergeState mergeState, MergeStrategy mergeStrategy, int readerIndex) {
      super(mergeState.docMaps[readerIndex]);
      this.readerIndex = readerIndex;
      this.mergeStrategy = mergeStrategy;
      this.maxDoc = mergeState.maxDocs[readerIndex];
    }

    @Override
    public int nextDoc() {
      docID++;
      if (docID == maxDoc) {
        return NO_MORE_DOCS;
      } else {
        return docID;
      }
    }
  }

  @Override
  public long ramBytesUsed() {
    return bufferedDocs.ramBytesUsed() + numStoredFields.length * Integer.BYTES + endOffsets.length * Integer.BYTES;
  }
}