Class HPackHuffman

java/org/apache/coyote/http2/HPackHuffman.java:31–701

Utility class for Huffman encoding and decoding used in HPACK compression for HTTP/2 headers.

 * Utility class for Huffman encoding and decoding used in HPACK compression for HTTP/2 headers.
 */
public class HPackHuffman {

    /**
     * Private constructor to prevent instantiation.
     */
    private HPackHuffman() {
    }

    /**
     * String manager for error messages.
     */
    protected static final StringManager sm = StringManager.getManager(HPackHuffman.class);

    private static final HuffmanCode[] HUFFMAN_CODES;

    /**
     * array based tree representation of a huffman code.
     * <p/>
     * the high two bytes corresponds to the tree node if the bit is set, and the low two bytes for if it is clear if
     * the high bit is set it is a terminal node, otherwise it contains the next node position.
     */
    private static final int[] DECODING_TABLE;

    private static final int LOW_TERMINAL_BIT = (0b10000000) << 8;
    private static final int HIGH_TERMINAL_BIT = (0b10000000) << 24;
    private static final int LOW_MASK = 0b0111111111111111;


    static {

        HuffmanCode[] codes = new HuffmanCode[257];

        codes[0] = new HuffmanCode(0x1ff8, 13);
        codes[1] = new HuffmanCode(0x7fffd8, 23);
        codes[2] = new HuffmanCode(0xfffffe2, 28);
        codes[3] = new HuffmanCode(0xfffffe3, 28);
        codes[4] = new HuffmanCode(0xfffffe4, 28);
        codes[5] = new HuffmanCode(0xfffffe5, 28);
        codes[6] = new HuffmanCode(0xfffffe6, 28);
        codes[7] = new HuffmanCode(0xfffffe7, 28);
        codes[8] = new HuffmanCode(0xfffffe8, 28);
        codes[9] = new HuffmanCode(0xffffea, 24);
        codes[10] = new HuffmanCode(0x3ffffffc, 30);
        codes[11] = new HuffmanCode(0xfffffe9, 28);
        codes[12] = new HuffmanCode(0xfffffea, 28);
        codes[13] = new HuffmanCode(0x3ffffffd, 30);
        codes[14] = new HuffmanCode(0xfffffeb, 28);
        codes[15] = new HuffmanCode(0xfffffec, 28);
        codes[16] = new HuffmanCode(0xfffffed, 28);
        codes[17] = new HuffmanCode(0xfffffee, 28);
        codes[18] = new HuffmanCode(0xfffffef, 28);
        codes[19] = new HuffmanCode(0xffffff0, 28);
        codes[20] = new HuffmanCode(0xffffff1, 28);
        codes[21] = new HuffmanCode(0xffffff2, 28);
        codes[22] = new HuffmanCode(0x3ffffffe, 30);
        codes[23] = new HuffmanCode(0xffffff3, 28);
        codes[24] = new HuffmanCode(0xffffff4, 28);
        codes[25] = new HuffmanCode(0xffffff5, 28);