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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:
*
* 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 io.netty.handler.codec.compression;
import static io.netty.handler.codec.compression.Bzip2Constants.*;
/**
* A decoder for the Bzip2 Huffman coding stage.
*/
final class Bzip2HuffmanStageDecoder {
/**
* A reader that provides bit-level reads.
*/
private final Bzip2BitReader reader;
/**
* The Huffman table number to use for each group of 50 symbols.
*/
byte[] selectors;
/**
* The minimum code length for each Huffman table.
*/
private final int[] minimumLengths;
/**
* An array of values for each Huffman table that must be subtracted from the numerical value of
* a Huffman code of a given bit length to give its canonical code index.
*/
private final int[][] codeBases;
/**
* An array of values for each Huffman table that gives the highest numerical value of a Huffman
* code of a given bit length.
*/
private final int[][] codeLimits;
/**
* A mapping for each Huffman table from canonical code index to output symbol.
*/
private final int[][] codeSymbols;
/**
* The Huffman table for the current group.
*/
private int currentTable;
/**
* The index of the current group within the selectors array.
*/
private int groupIndex = -1;
/**
* The byte position within the current group. A new group is selected every 50 decoded bytes.
*/
private int groupPosition = -1;
/**
* Total number of used Huffman tables in range 2..6.
*/
final int totalTables;
/**
* The total number of codes (uniform for each table).
*/
final int alphabetSize;
/**
* Table for Move To Front transformations.
*/
final Bzip2MoveToFrontTable tableMTF = new Bzip2MoveToFrontTable();
// For saving state if end of current ByteBuf was reached
int currentSelector;
/**
* The Canonical Huffman code lengths for each table.
*/
final byte[][] tableCodeLengths;
// For saving state if end of current ByteBuf was reached
int currentGroup;
int currentLength = -1;
int currentAlpha;
boolean modifyLength;
Bzip2HuffmanStageDecoder(final Bzip2BitReader reader, final int totalTables, final int alphabetSize) {
this.reader = reader;
this.totalTables = totalTables;
this.alphabetSize = alphabetSize;
minimumLengths = new int[totalTables];
codeBases = new int[totalTables][HUFFMAN_DECODE_MAX_CODE_LENGTH + 2];
codeLimits = new int[totalTables][HUFFMAN_DECODE_MAX_CODE_LENGTH + 1];
codeSymbols = new int[totalTables][HUFFMAN_MAX_ALPHABET_SIZE];
tableCodeLengths = new byte[totalTables][HUFFMAN_MAX_ALPHABET_SIZE];
}
/**
* Constructs Huffman decoding tables from lists of Canonical Huffman code lengths.
*/
void createHuffmanDecodingTables() {
final int alphabetSize = this.alphabetSize;
for (int table = 0; table < tableCodeLengths.length; table++) {
final int[] tableBases = codeBases[table];
final int[] tableLimits = codeLimits[table];
final int[] tableSymbols = codeSymbols[table];
final byte[] codeLengths = tableCodeLengths[table];
int minimumLength = HUFFMAN_DECODE_MAX_CODE_LENGTH;
int maximumLength = 0;
// Find the minimum and maximum code length for the table
for (int i = 0; i < alphabetSize; i++) {
final byte currLength = codeLengths[i];
maximumLength = Math.max(currLength, maximumLength);
minimumLength = Math.min(currLength, minimumLength);
}
minimumLengths[table] = minimumLength;
// Calculate the first output symbol for each code length
for (int i = 0; i < alphabetSize; i++) {
tableBases[codeLengths[i] + 1]++;
}
for (int i = 1, b = tableBases[0]; i < HUFFMAN_DECODE_MAX_CODE_LENGTH + 2; i++) {
b += tableBases[i];
tableBases[i] = b;
}
// Calculate the first and last Huffman code for each code length (codes at a given
// length are sequential in value)
for (int i = minimumLength, code = 0; i <= maximumLength; i++) {
int base = code;
code += tableBases[i + 1] - tableBases[i];
tableBases[i] = base - tableBases[i];
tableLimits[i] = code - 1;
code <<= 1;
}
// Populate the mapping from canonical code index to output symbol
for (int bitLength = minimumLength, codeIndex = 0; bitLength <= maximumLength; bitLength++) {
for (int symbol = 0; symbol < alphabetSize; symbol++) {
if (codeLengths[symbol] == bitLength) {
tableSymbols[codeIndex++] = symbol;
}
}
}
}
currentTable = selectors[0];
}
/**
* Decodes and returns the next symbol.
* @return The decoded symbol
*/
int nextSymbol() {
// Move to next group selector if required
if (++groupPosition % HUFFMAN_GROUP_RUN_LENGTH == 0) {
groupIndex++;
if (groupIndex == selectors.length) {
throw new DecompressionException("error decoding block");
}
currentTable = selectors[groupIndex] & 0xff;
}
final Bzip2BitReader reader = this.reader;
final int currentTable = this.currentTable;
final int[] tableLimits = codeLimits[currentTable];
final int[] tableBases = codeBases[currentTable];
final int[] tableSymbols = codeSymbols[currentTable];
int codeLength = minimumLengths[currentTable];
// Starting with the minimum bit length for the table, read additional bits one at a time
// until a complete code is recognised
int codeBits = reader.readBits(codeLength);
for (; codeLength <= HUFFMAN_DECODE_MAX_CODE_LENGTH; codeLength++) {
if (codeBits <= tableLimits[codeLength]) {
// Convert the code to a symbol index and return
return tableSymbols[codeBits - tableBases[codeLength]];
}
codeBits = codeBits << 1 | reader.readBits(1);
}
throw new DecompressionException("a valid code was not recognised");
}
}