root/dwt/internal/image/PngHuffmanTable.d

/*******************************************************************************
 * Copyright (c) 2000, 2008 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 * Port to the D programming language:
 *     Frank Benoit <benoit@tionex.de>
 *******************************************************************************/
module dwt.internal.image.PngHuffmanTable;

import dwt.internal.image.PngDecodingDataStream;

public class PngHuffmanTable {
    CodeLengthInfo[] codeLengthInfo;
    int[] codeValues;

    static const int MAX_CODE_LENGTH = 15;
    static const int BAD_CODE = 0xFFFFFFF;
    static const int incs[] = [1391376, 463792, 198768, 86961, 33936, 13776, 4592, 1968, 861, 336, 112, 48, 21, 7, 3, 1];

this (int[] lengths) {
    initialize(lengths);
    generateTable(lengths);
}

private void initialize(int[] lengths) {
    codeValues = new int[lengths.length];
    for (int i = 0; i < codeValues.length; i++) {
        codeValues[i] = i;
    }

    // minCodesByLength[n] : The smallest Huffman code of length n + 1.
    // maxCodesByLength[n] : The largest Huffman code of length n + 1.
    // indexesByLength[n] : Index into the values array. First value with a code of length n + 1.
    codeLengthInfo = new CodeLengthInfo[MAX_CODE_LENGTH];
    for (int i = 0; i < MAX_CODE_LENGTH; i++) {
        codeLengthInfo[i] = new CodeLengthInfo();
        codeLengthInfo[i].length = i;
        codeLengthInfo[i].baseIndex = 0;
        codeLengthInfo[i].min = BAD_CODE;
        codeLengthInfo[i].max = -1;
    }
}

private void generateTable(int[] lengths) {
    // Sort the values using shellsort. Primary key is code size. Secondary key is value.
    int codeValuesTemp;
    for (int k = 0; k < 16; k++) {
        for (int h = incs[k], i = h; i < lengths.length; i++) {
            int v = lengths[i];
            codeValuesTemp = codeValues[i];
            int j = i;
            while (j >= h && (lengths[j - h] > v || (lengths[j - h] is v && codeValues[j - h] > codeValuesTemp))) {
                lengths[j] = lengths[j - h];
                codeValues[j] = codeValues[j - h];
                j -= h;
            }
            lengths[j] = v;
            codeValues[j] = codeValuesTemp;
        }
    }

    // These values in these arrays correspond to the elements of the
    // "values" array. The Huffman code for codeValues[N] is codes[N]
    // and the length of the code is lengths[N].
    int[] codes = new int[lengths.length];
    int lastLength = 0;
    int code = 0;
    for (int i = 0; i < lengths.length; i++) {
        while (lastLength !is lengths[i]) {
            lastLength++;
            code <<= 1;
        }
        if (lastLength !is 0) {
            codes[i] = code;
            code++;
        }
    }

    int last = 0;
    for (int i = 0; i < lengths.length; i++) {
        if (last !is lengths[i]) {
            last = lengths[i];
            codeLengthInfo[last - 1].baseIndex = i;
            codeLengthInfo[last - 1].min = codes[i];
        }
        if (last !is 0) codeLengthInfo[last - 1].max = codes[i];
    }
}

int getNextValue(PngDecodingDataStream stream) {
    int code = stream.getNextIdatBit();
    int codelength = 0;

    // Here we are taking advantage of the fact that 1 bits are used as
    // a prefix to the longer codeValues.
    while (codelength < MAX_CODE_LENGTH && code > codeLengthInfo[codelength].max) {
        code = ((code << 1) | stream.getNextIdatBit());
        codelength++;
    }
    if (codelength >= MAX_CODE_LENGTH) stream.error();

    // Now we have a Huffman code of length (codelength + 1) that
    // is somewhere in the range
    // minCodesByLength[codelength]..maxCodesByLength[codelength].
    // This code is the (offset + 1)'th code of (codelength + 1);
    int offset = code - codeLengthInfo[codelength].min;

    // indexesByLength[codelength] is the first code of length (codelength + 1)
    // so now we can look up the value for the Huffman code in the table.
    int index = codeLengthInfo[codelength].baseIndex + offset;
    return codeValues[index];
}

static class CodeLengthInfo {
    int length;
    int max;
    int min;
    int baseIndex;
}

}