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   /*
    * Copyright 2014 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.handler.codec.compression;
  
  import static io.netty.handler.codec.compression.Bzip2Constants.HUFFMAN_DECODE_MAX_CODE_LENGTH;
  import static io.netty.handler.codec.compression.Bzip2Constants.HUFFMAN_SYMBOL_RUNA;
  import static io.netty.handler.codec.compression.Bzip2Constants.HUFFMAN_SYMBOL_RUNB;
  import static io.netty.handler.codec.compression.Bzip2Constants.MAX_BLOCK_LENGTH;
  
  /**
   * Reads and decompresses a single Bzip2 block.<br><br>
   *
   * Block decoding consists of the following stages:<br>
   * 1. Read block header<br>
   * 2. Read Huffman tables<br>
   * 3. Read and decode Huffman encoded data - {@link #decodeHuffmanData(Bzip2HuffmanStageDecoder)}<br>
   * 4. Run-Length Decoding[2] - {@link #decodeHuffmanData(Bzip2HuffmanStageDecoder)}<br>
   * 5. Inverse Move To Front Transform - {@link #decodeHuffmanData(Bzip2HuffmanStageDecoder)}<br>
   * 6. Inverse Burrows Wheeler Transform - {@link #initialiseInverseBWT()}<br>
   * 7. Run-Length Decoding[1] - {@link #read()}<br>
   * 8. Optional Block De-Randomisation - {@link #read()} (through {@link #decodeNextBWTByte()})
   */
  final class Bzip2BlockDecompressor {
      /**
       * A reader that provides bit-level reads.
       */
      private final Bzip2BitReader reader;
  
      /**
       * Calculates the block CRC from the fully decoded bytes of the block.
       */
      private final Crc32 crc = new Crc32();
  
      /**
       * The CRC of the current block as read from the block header.
       */
      private final int blockCRC;
  
      /**
       * {@code true} if the current block is randomised, otherwise {@code false}.
       */
      private final boolean blockRandomised;
  
      /* Huffman Decoding stage */
      /**
       * The end-of-block Huffman symbol. Decoding of the block ends when this is encountered.
       */
      int huffmanEndOfBlockSymbol;
  
      /**
       * Bitmap, of ranges of 16 bytes, present/not present.
       */
      int huffmanInUse16;
  
      /**
       * A map from Huffman symbol index to output character. Some types of data (e.g. ASCII text)
       * may contain only a limited number of byte values; Huffman symbols are only allocated to
       * those values that actually occur in the uncompressed data.
       */
      final byte[] huffmanSymbolMap = new byte[256];
  
      /* Move To Front stage */
      /**
       * Counts of each byte value within the {@link Bzip2BlockDecompressor#huffmanSymbolMap} data.
       * Collected at the Move To Front stage, consumed by the Inverse Burrows Wheeler Transform stage.
       */
      private final int[] bwtByteCounts = new int[256];
  
      /**
       * The Burrows-Wheeler Transform processed data. Read at the Move To Front stage, consumed by the
       * Inverse Burrows Wheeler Transform stage.
       */
      private final byte[] bwtBlock;
  
      /**
       * Starting pointer into BWT for after untransform.
       */
      private final int bwtStartPointer;
  
      /* Inverse Burrows-Wheeler Transform stage */
      /**
       * At each position contains the union of :-
       *   An output character (8 bits)
       *   A pointer from each position to its successor (24 bits, left shifted 8 bits)
       * As the pointer cannot exceed the maximum block size of 900k, 24 bits is more than enough to
       * hold it; Folding the character data into the spare bits while performing the inverse BWT,
      * when both pieces of information are available, saves a large number of memory accesses in
      * the final decoding stages.
      */
     private int[] bwtMergedPointers;
 
     /**
      * The current merged pointer into the Burrow-Wheeler Transform array.
      */
     private int bwtCurrentMergedPointer;
 
     /**
      * The actual length in bytes of the current block at the Inverse Burrows Wheeler Transform
      * stage (before final Run-Length Decoding).
      */
     private int bwtBlockLength;
 
     /**
      * The number of output bytes that have been decoded up to the Inverse Burrows Wheeler Transform stage.
      */
     private int bwtBytesDecoded;
 
     /* Run-Length Encoding and Random Perturbation stage */
     /**
      * The most recently RLE decoded byte.
      */
     private int rleLastDecodedByte = -1;
 
     /**
      * The number of previous identical output bytes decoded. After 4 identical bytes, the next byte
      * decoded is an RLE repeat count.
      */
     private int rleAccumulator;
 
     /**
      * The RLE repeat count of the current decoded byte. When this reaches zero, a new byte is decoded.
      */
     private int rleRepeat;
 
     /**
      * If the current block is randomised, the position within the RNUMS randomisation array.
      */
     private int randomIndex;
 
     /**
      * If the current block is randomised, the remaining count at the current RNUMS position.
      */
     private int randomCount = Bzip2Rand.rNums(0) - 1;
 
     /**
      * Table for Move To Front transformations.
      */
     private final Bzip2MoveToFrontTable symbolMTF = new Bzip2MoveToFrontTable();
 
     // This variables is used to save current state if we haven't got enough readable bits
     private int repeatCount;
     private int repeatIncrement = 1;
     private int mtfValue;
 
     Bzip2BlockDecompressor(final int blockSize, final int blockCRC, final boolean blockRandomised,
                            final int bwtStartPointer, final Bzip2BitReader reader) {
 
         bwtBlock = new byte[blockSize];
 
         this.blockCRC = blockCRC;
         this.blockRandomised = blockRandomised;
         this.bwtStartPointer = bwtStartPointer;
 
         this.reader = reader;
     }
 
     /**
      * Reads the Huffman encoded data from the input stream, performs Run-Length Decoding and
      * applies the Move To Front transform to reconstruct the Burrows-Wheeler Transform array.
      */
     boolean decodeHuffmanData(final Bzip2HuffmanStageDecoder huffmanDecoder) {
         final Bzip2BitReader reader = this.reader;
         final byte[] bwtBlock = this.bwtBlock;
         final byte[] huffmanSymbolMap = this.huffmanSymbolMap;
         final int streamBlockSize = this.bwtBlock.length;
         final int huffmanEndOfBlockSymbol = this.huffmanEndOfBlockSymbol;
         final int[] bwtByteCounts = this.bwtByteCounts;
         final Bzip2MoveToFrontTable symbolMTF = this.symbolMTF;
 
         int bwtBlockLength = this.bwtBlockLength;
         int repeatCount = this.repeatCount;
         int repeatIncrement = this.repeatIncrement;
         int mtfValue = this.mtfValue;
 
         for (;;) {
             if (!reader.hasReadableBits(HUFFMAN_DECODE_MAX_CODE_LENGTH)) {
                 this.bwtBlockLength = bwtBlockLength;
                 this.repeatCount = repeatCount;
                 this.repeatIncrement = repeatIncrement;
                 this.mtfValue = mtfValue;
                 return false;
             }
             final int nextSymbol = huffmanDecoder.nextSymbol();
 
             if (nextSymbol == HUFFMAN_SYMBOL_RUNA) {
                 repeatCount += repeatIncrement;
                 repeatIncrement <<= 1;
             } else if (nextSymbol == HUFFMAN_SYMBOL_RUNB) {
                 repeatCount += repeatIncrement << 1;
                 repeatIncrement <<= 1;
             } else {
                 if (repeatCount > 0) {
                     if (bwtBlockLength + repeatCount > streamBlockSize) {
                         throw new DecompressionException("block exceeds declared block size");
                     }
                     final byte nextByte = huffmanSymbolMap[mtfValue];
                     bwtByteCounts[nextByte & 0xff] += repeatCount;
                     while (--repeatCount >= 0) {
                         bwtBlock[bwtBlockLength++] = nextByte;
                     }
 
                     repeatCount = 0;
                     repeatIncrement = 1;
                 }
 
                 if (nextSymbol == huffmanEndOfBlockSymbol) {
                     break;
                 }
 
                 if (bwtBlockLength >= streamBlockSize) {
                     throw new DecompressionException("block exceeds declared block size");
                 }
 
                 mtfValue = symbolMTF.indexToFront(nextSymbol - 1) & 0xff;
 
                 final byte nextByte = huffmanSymbolMap[mtfValue];
                 bwtByteCounts[nextByte & 0xff]++;
                 bwtBlock[bwtBlockLength++] = nextByte;
             }
         }
         if (bwtBlockLength > MAX_BLOCK_LENGTH) {
             throw new DecompressionException("block length exceeds max block length: "
                     + bwtBlockLength + " > " + MAX_BLOCK_LENGTH);
         }
 
         this.bwtBlockLength = bwtBlockLength;
         initialiseInverseBWT();
         return true;
     }
 
     /**
      * Set up the Inverse Burrows-Wheeler Transform merged pointer array.
      */
     private void initialiseInverseBWT() {
         final int bwtStartPointer = this.bwtStartPointer;
         final byte[] bwtBlock  = this.bwtBlock;
         final int[] bwtMergedPointers = new int[bwtBlockLength];
         final int[] characterBase = new int[256];
 
         if (bwtStartPointer < 0 || bwtStartPointer >= bwtBlockLength) {
             throw new DecompressionException("start pointer invalid");
         }
 
         // Cumulative character counts
         System.arraycopy(bwtByteCounts, 0, characterBase, 1, 255);
         for (int i = 2; i <= 255; i++) {
             characterBase[i] += characterBase[i - 1];
         }
 
         // Merged-Array Inverse Burrows-Wheeler Transform
         // Combining the output characters and forward pointers into a single array here, where we
         // have already read both of the corresponding values, cuts down on memory accesses in the
         // final walk through the array
         for (int i = 0; i < bwtBlockLength; i++) {
             int value = bwtBlock[i] & 0xff;
             bwtMergedPointers[characterBase[value]++] = (i << 8) + value;
         }
 
         this.bwtMergedPointers = bwtMergedPointers;
         bwtCurrentMergedPointer = bwtMergedPointers[bwtStartPointer];
     }
 
     /**
      * Decodes a byte from the final Run-Length Encoding stage, pulling a new byte from the
      * Burrows-Wheeler Transform stage when required.
      * @return The decoded byte, or -1 if there are no more bytes
      */
     public int read() {
         while (rleRepeat < 1) {
             if (bwtBytesDecoded == bwtBlockLength) {
                 return -1;
             }
 
             int nextByte = decodeNextBWTByte();
             if (nextByte != rleLastDecodedByte) {
                 // New byte, restart accumulation
                 rleLastDecodedByte = nextByte;
                 rleRepeat = 1;
                 rleAccumulator = 1;
                 crc.updateCRC(nextByte);
             } else {
                 if (++rleAccumulator == 4) {
                     // Accumulation complete, start repetition
                     int rleRepeat = decodeNextBWTByte() + 1;
                     this.rleRepeat = rleRepeat;
                     rleAccumulator = 0;
                     crc.updateCRC(nextByte, rleRepeat);
                 } else {
                     rleRepeat = 1;
                     crc.updateCRC(nextByte);
                 }
             }
         }
         rleRepeat--;
 
         return rleLastDecodedByte;
     }
 
     /**
      * Decodes a byte from the Burrows-Wheeler Transform stage. If the block has randomisation
      * applied, reverses the randomisation.
      * @return The decoded byte
      */
     private int decodeNextBWTByte() {
         int mergedPointer = bwtCurrentMergedPointer;
         int nextDecodedByte =  mergedPointer & 0xff;
         bwtCurrentMergedPointer = bwtMergedPointers[mergedPointer >>> 8];
 
         if (blockRandomised) {
             if (--randomCount == 0) {
                 nextDecodedByte ^= 1;
                 randomIndex = (randomIndex + 1) % 512;
                 randomCount = Bzip2Rand.rNums(randomIndex);
             }
         }
         bwtBytesDecoded++;
 
         return nextDecodedByte;
     }
 
     public int blockLength() {
         return bwtBlockLength;
     }
 
     /**
      * Verify and return the block CRC. This method may only be called
      * after all of the block's bytes have been read.
      * @return The block CRC
      */
     int checkCRC() {
         final int computedBlockCRC = crc.getCRC();
         if (blockCRC != computedBlockCRC) {
             throw new DecompressionException("block CRC error");
         }
         return computedBlockCRC;
     }
 }