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Java example source code file (Bzip2HuffmanStageEncoder.java)

This example Java source code file (Bzip2HuffmanStageEncoder.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

bzip2bitwriter, bzip2huffmanstageencoder, bzip2movetofronttable, huffman_encode_max_code_length, huffman_group_run_length, huffman_high_symbol_cost, util

The Bzip2HuffmanStageEncoder.java Java example source code

/*
 * 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 io.netty.buffer.ByteBuf;

import java.util.Arrays;

import static io.netty.handler.codec.compression.Bzip2Constants.*;

/**
 * An encoder for the Bzip2 Huffman encoding stage.
 */
final class Bzip2HuffmanStageEncoder {
    /**
     * Used in initial Huffman table generation.
     */
    private static final int HUFFMAN_HIGH_SYMBOL_COST = 15;

    /**
     * The {@link Bzip2BitWriter} to which the Huffman tables and data is written.
     */
    private final Bzip2BitWriter writer;

    /**
     * The output of the Move To Front Transform and Run Length Encoding[2] stages.
     */
    private final char[] mtfBlock;

    /**
     * The actual number of values contained in the {@link #mtfBlock} array.
     */
    private final int mtfLength;

    /**
     * The number of unique values in the {@link #mtfBlock} array.
     */
    private final int mtfAlphabetSize;

    /**
     * The global frequencies of values within the {@link #mtfBlock} array.
     */
    private final int[] mtfSymbolFrequencies;

    /**
     * The Canonical Huffman code lengths for each table.
     */
    private final int[][] huffmanCodeLengths;

    /**
     * Merged code symbols for each table. The value at each position is ((code length << 24) | code).
     */
    private final int[][] huffmanMergedCodeSymbols;

    /**
     * The selectors for each segment.
     */
    private final byte[] selectors;

    /**
     * @param writer The {@link Bzip2BitWriter} which provides bit-level writes
     * @param mtfBlock The MTF block data
     * @param mtfLength The actual length of the MTF block
     * @param mtfAlphabetSize The size of the MTF block's alphabet
     * @param mtfSymbolFrequencies The frequencies the MTF block's symbols
     */
    Bzip2HuffmanStageEncoder(final Bzip2BitWriter writer, final char[] mtfBlock,
                             final int mtfLength, final int mtfAlphabetSize, final int[] mtfSymbolFrequencies) {
        this.writer = writer;
        this.mtfBlock = mtfBlock;
        this.mtfLength = mtfLength;
        this.mtfAlphabetSize = mtfAlphabetSize;
        this.mtfSymbolFrequencies = mtfSymbolFrequencies;

        final int totalTables = selectTableCount(mtfLength);

        huffmanCodeLengths = new int[totalTables][mtfAlphabetSize];
        huffmanMergedCodeSymbols = new int[totalTables][mtfAlphabetSize];
        selectors = new byte[(mtfLength + HUFFMAN_GROUP_RUN_LENGTH - 1) / HUFFMAN_GROUP_RUN_LENGTH];
    }

    /**
     * Selects an appropriate table count for a given MTF length.
     * @param mtfLength The length to select a table count for
     * @return The selected table count
     */
    private static int selectTableCount(final int mtfLength) {
        if (mtfLength >= 2400) {
            return 6;
        }
        if (mtfLength >= 1200) {
            return 5;
        }
        if (mtfLength >= 600) {
            return 4;
        }
        if (mtfLength >= 200) {
            return 3;
        }
        return 2;
    }

    /**
     * Generate a Huffman code length table for a given list of symbol frequencies.
     * @param alphabetSize The total number of symbols
     * @param symbolFrequencies The frequencies of the symbols
     * @param codeLengths The array to which the generated code lengths should be written
     */
    private static void generateHuffmanCodeLengths(final int alphabetSize,
                                                   final int[] symbolFrequencies, final int[] codeLengths) {

        final int[] mergedFrequenciesAndIndices = new int[alphabetSize];
        final int[] sortedFrequencies = new int[alphabetSize];

        // The Huffman allocator needs its input symbol frequencies to be sorted, but we need to
        // return code lengths in the same order as the corresponding frequencies are passed in.

        // The symbol frequency and index are merged into a single array of
        // integers - frequency in the high 23 bits, index in the low 9 bits.
        //     2^23 = 8,388,608 which is higher than the maximum possible frequency for one symbol in a block
        //     2^9 = 512 which is higher than the maximum possible alphabet size (== 258)
        // Sorting this array simultaneously sorts the frequencies and
        // leaves a lookup that can be used to cheaply invert the sort.
        for (int i = 0; i < alphabetSize; i++) {
            mergedFrequenciesAndIndices[i] = (symbolFrequencies[i] << 9) | i;
        }
        Arrays.sort(mergedFrequenciesAndIndices);
        for (int i = 0; i < alphabetSize; i++) {
            sortedFrequencies[i] = mergedFrequenciesAndIndices[i] >>> 9;
        }

        // Allocate code lengths - the allocation is in place,
        // so the code lengths will be in the sortedFrequencies array afterwards
        Bzip2HuffmanAllocator.allocateHuffmanCodeLengths(sortedFrequencies, HUFFMAN_ENCODE_MAX_CODE_LENGTH);

        // Reverse the sort to place the code lengths in the same order as the symbols whose frequencies were passed in
        for (int i = 0; i < alphabetSize; i++) {
            codeLengths[mergedFrequenciesAndIndices[i] & 0x1ff] = sortedFrequencies[i];
        }
    }

    /**
     * Generate initial Huffman code length tables, giving each table a different low cost section
     * of the alphabet that is roughly equal in overall cumulative frequency. Note that the initial
     * tables are invalid for actual Huffman code generation, and only serve as the seed for later
     * iterative optimisation in {@link #optimiseSelectorsAndHuffmanTables(boolean)}.
     */
    private void generateHuffmanOptimisationSeeds() {
        final int[][] huffmanCodeLengths = this.huffmanCodeLengths;
        final int[] mtfSymbolFrequencies = this.mtfSymbolFrequencies;
        final int mtfAlphabetSize = this.mtfAlphabetSize;

        final int totalTables = huffmanCodeLengths.length;

        int remainingLength = mtfLength;
        int lowCostEnd = -1;

        for (int i = 0; i < totalTables; i++) {

            final int targetCumulativeFrequency = remainingLength / (totalTables - i);
            final int lowCostStart = lowCostEnd + 1;
            int actualCumulativeFrequency = 0;

            while (actualCumulativeFrequency < targetCumulativeFrequency && lowCostEnd < mtfAlphabetSize - 1) {
                actualCumulativeFrequency += mtfSymbolFrequencies[++lowCostEnd];
            }

            if (lowCostEnd > lowCostStart && i != 0 && i != totalTables - 1 && (totalTables - i & 1) == 0) {
                actualCumulativeFrequency -= mtfSymbolFrequencies[lowCostEnd--];
            }

            final int[] tableCodeLengths = huffmanCodeLengths[i];
            for (int j = 0; j < mtfAlphabetSize; j++) {
                if (j < lowCostStart || j > lowCostEnd) {
                    tableCodeLengths[j] = HUFFMAN_HIGH_SYMBOL_COST;
                }
            }

            remainingLength -= actualCumulativeFrequency;
        }
    }

    /**
     * Co-optimise the selector list and the alternative Huffman table code lengths. This method is
     * called repeatedly in the hope that the total encoded size of the selectors, the Huffman code
     * lengths and the block data encoded with them will converge towards a minimum.<br>
     * If the data is highly incompressible, it is possible that the total encoded size will
     * instead diverge (increase) slightly.<br>
     * @param storeSelectors If {@code true}, write out the (final) chosen selectors
     */
    private void optimiseSelectorsAndHuffmanTables(final boolean storeSelectors) {
        final char[] mtfBlock = this.mtfBlock;
        final byte[] selectors = this.selectors;
        final int[][] huffmanCodeLengths = this.huffmanCodeLengths;
        final int mtfLength = this.mtfLength;
        final int mtfAlphabetSize = this.mtfAlphabetSize;

        final int totalTables = huffmanCodeLengths.length;
        final int[][] tableFrequencies = new int[totalTables][mtfAlphabetSize];

        int selectorIndex = 0;

        // Find the best table for each group of 50 block bytes based on the current Huffman code lengths
        for (int groupStart = 0; groupStart < mtfLength;) {

            final int groupEnd = Math.min(groupStart + HUFFMAN_GROUP_RUN_LENGTH, mtfLength) - 1;

            // Calculate the cost of this group when encoded by each table
            short[] cost = new short[totalTables];
            for (int i = groupStart; i <= groupEnd; i++) {
                final int value = mtfBlock[i];
                for (int j = 0; j < totalTables; j++) {
                    cost[j] += huffmanCodeLengths[j][value];
                }
            }

            // Find the table with the least cost for this group
            byte bestTable = 0;
            int bestCost = cost[0];
            for (byte i = 1 ; i < totalTables; i++) {
                final int tableCost = cost[i];
                if (tableCost < bestCost) {
                    bestCost = tableCost;
                    bestTable = i;
                }
            }

            // Accumulate symbol frequencies for the table chosen for this block
            final int[] bestGroupFrequencies = tableFrequencies[bestTable];
            for (int i = groupStart; i <= groupEnd; i++) {
                bestGroupFrequencies[mtfBlock[i]]++;
            }

            // Store a selector indicating the table chosen for this block
            if (storeSelectors) {
                selectors[selectorIndex++] = bestTable;
            }
            groupStart = groupEnd + 1;
        }

        // Generate new Huffman code lengths based on the frequencies for each table accumulated in this iteration
        for (int i = 0; i < totalTables; i++) {
            generateHuffmanCodeLengths(mtfAlphabetSize, tableFrequencies[i], huffmanCodeLengths[i]);
        }
    }

    /**
     * Assigns Canonical Huffman codes based on the calculated lengths.
     */
    private void assignHuffmanCodeSymbols() {
        final int[][] huffmanMergedCodeSymbols = this.huffmanMergedCodeSymbols;
        final int[][] huffmanCodeLengths = this.huffmanCodeLengths;
        final int mtfAlphabetSize = this.mtfAlphabetSize;

        final int totalTables = huffmanCodeLengths.length;

        for (int i = 0; i < totalTables; i++) {
            final int[] tableLengths = huffmanCodeLengths[i];

            int minimumLength = 32;
            int maximumLength = 0;
            for (int j = 0; j < mtfAlphabetSize; j++) {
                final int length = tableLengths[j];
                if (length > maximumLength) {
                    maximumLength = length;
                }
                if (length < minimumLength) {
                    minimumLength = length;
                }
            }

            int code = 0;
            for (int j = minimumLength; j <= maximumLength; j++) {
                for (int k = 0; k < mtfAlphabetSize; k++) {
                    if ((huffmanCodeLengths[i][k] & 0xff) == j) {
                        huffmanMergedCodeSymbols[i][k] = (j << 24) | code;
                        code++;
                    }
                }
                code <<= 1;
            }
        }
    }

    /**
     * Write out the selector list and Huffman tables.
     */
    private void writeSelectorsAndHuffmanTables(ByteBuf out) {
        final Bzip2BitWriter writer = this.writer;
        final byte[] selectors = this.selectors;
        final int totalSelectors = selectors.length;
        final int[][] huffmanCodeLengths = this.huffmanCodeLengths;
        final int totalTables = huffmanCodeLengths.length;
        final int mtfAlphabetSize = this.mtfAlphabetSize;

        writer.writeBits(out, 3, totalTables);
        writer.writeBits(out, 15, totalSelectors);

        // Write the selectors
        Bzip2MoveToFrontTable selectorMTF = new Bzip2MoveToFrontTable();
        for (byte selector : selectors) {
            writer.writeUnary(out, selectorMTF.valueToFront(selector));
        }

        // Write the Huffman tables
        for (final int[] tableLengths : huffmanCodeLengths) {
            int currentLength = tableLengths[0];

            writer.writeBits(out, 5, currentLength);

            for (int j = 0; j < mtfAlphabetSize; j++) {
                final int codeLength = tableLengths[j];
                final int value = currentLength < codeLength ? 2 : 3;
                int delta = Math.abs(codeLength - currentLength);
                while (delta-- > 0) {
                    writer.writeBits(out, 2, value);
                }
                writer.writeBoolean(out, false);
                currentLength = codeLength;
            }
        }
    }

    /**
     * Writes out the encoded block data.
     */
    private void writeBlockData(ByteBuf out) {
        final Bzip2BitWriter writer = this.writer;
        final int[][] huffmanMergedCodeSymbols = this.huffmanMergedCodeSymbols;
        final byte[] selectors = this.selectors;
        final char[] mtf = mtfBlock;
        final int mtfLength = this.mtfLength;

        int selectorIndex = 0;
        for (int mtfIndex = 0; mtfIndex < mtfLength;) {
            final int groupEnd = Math.min(mtfIndex + HUFFMAN_GROUP_RUN_LENGTH, mtfLength) - 1;
            final int[] tableMergedCodeSymbols = huffmanMergedCodeSymbols[selectors[selectorIndex++]];

            while (mtfIndex <= groupEnd) {
                final int mergedCodeSymbol = tableMergedCodeSymbols[mtf[mtfIndex++]];
                writer.writeBits(out, mergedCodeSymbol >>> 24, mergedCodeSymbol);
            }
        }
    }

    /**
     * Encodes and writes the block data.
     */
    void encode(ByteBuf out) {
        // Create optimised selector list and Huffman tables
        generateHuffmanOptimisationSeeds();
        for (int i = 3; i >= 0; i--) {
            optimiseSelectorsAndHuffmanTables(i == 0);
        }
        assignHuffmanCodeSymbols();

        // Write out the tables and the block data encoded with them
        writeSelectorsAndHuffmanTables(out);
        writeBlockData(out);
    }
}

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