Bzip2HuffmanStageEncoder Class — netty Architecture

Source Code

codec-compression/src/main/java/io/netty/handler/codec/compression/Bzip2HuffmanStageEncoder.java lines 28–374

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) {