huffman.h

/* ScummVM - Graphic Adventure Engine
 *
 * ScummVM is the legal property of its developers, whose names
 * are too numerous to list here. Please refer to the COPYRIGHT
 * file distributed with this source distribution.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

// Based on xoreos' Huffman code

#ifndef COMMON_HUFFMAN_H
#define COMMON_HUFFMAN_H

#include "common/array.h"
#include "common/list.h"
#include "common/types.h"

namespace Common {

inline uint32 REVERSEBITS(uint32 x) {
    x = (((x & ~0x55555555) >> 1) | ((x & 0x55555555) << 1));
    x = (((x & ~0x33333333) >> 2) | ((x & 0x33333333) << 2));
    x = (((x & ~0x0F0F0F0F) >> 4) | ((x & 0x0F0F0F0F) << 4));
    x = (((x & ~0x00FF00FF) >> 8) | ((x & 0x00FF00FF) << 8));

    return((x >> 16) | (x << 16));
}

template<class BITSTREAM>
class Huffman {
public:
    Huffman(uint8 maxLength, uint32 codeCount, const uint32 *codes, const uint8 *lengths, const uint32 *symbols = nullptr);

    uint32 getSymbol(BITSTREAM &bits) const;

private:
    struct Symbol {
        uint32 code;
        uint32 symbol;

        Symbol(uint32 c, uint32 s) : code(c), symbol(s) {}
    };

    typedef List<Symbol> CodeList;
    typedef Array<CodeList> CodeLists;

    CodeLists _codes;

    struct PrefixEntry {
        uint32 symbol;
        uint8  length;

        PrefixEntry() : length(0xFF) {}
    };

    static const uint8 _prefixTableBits = 8;
    PrefixEntry _prefixTable[1 << _prefixTableBits];
};

template <class BITSTREAM>
Huffman<BITSTREAM>::Huffman(uint8 maxLength, uint32 codeCount, const uint32 *codes, const uint8 *lengths, const uint32 *symbols) {
    assert(codeCount > 0);

    assert(codes);
    assert(lengths);

    if (maxLength == 0)
        for (uint32 i = 0; i < codeCount; i++)
            maxLength = MAX(maxLength, lengths[i]);

    assert(maxLength <= 32);

    // Codes that do not fit in the prefix table are stored in the _codes array.
    _codes.resize(MAX(maxLength - _prefixTableBits, 0));

    for (uint i = 0; i < codeCount; i++) {
        uint8 length = lengths[i];

        // The symbol. If none was specified, assume it is identical to the code index.
        uint32 symbol = symbols ? symbols[i] : i;

        if (length <= _prefixTableBits) {
            // Short codes go in the prefix lookup table. Set all the entries in the table
            // with an index starting with the code to the symbol value.
            uint32 startIndex;
            if (BITSTREAM::isMSB2LSB()) {
                startIndex = codes[i] << (_prefixTableBits - length);
            } else {
                startIndex = REVERSEBITS(codes[i]) >> (32 - _prefixTableBits);
            }

            uint32 endIndex = startIndex | ((1 << (_prefixTableBits - length)) - 1);

            for (uint32 j = startIndex; j <= endIndex; j++) {
                uint32 index = BITSTREAM::isMSB2LSB() ? j : REVERSEBITS(j) >> (32 - _prefixTableBits);
                _prefixTable[index].symbol = symbol;
                _prefixTable[index].length = length;
            }
        } else {
            // Put the code and symbol into the correct list for the length.
            _codes[lengths[i] - 1 - _prefixTableBits].push_back(Symbol(codes[i], symbol));
        }
    }
}

template <class BITSTREAM>
uint32 Huffman<BITSTREAM>::getSymbol(BITSTREAM &bits) const {
    uint32 code = bits.peekBits(_prefixTableBits);

    uint8 length = _prefixTable[code].length;

    if (length != 0xFF) {
        bits.skip(length);
        return _prefixTable[code].symbol;
    } else {
        bits.skip(_prefixTableBits);

        for (uint32 i = 0; i < _codes.size(); i++) {
            bits.addBit(code, i + _prefixTableBits);

            for (typename CodeList::const_iterator cCode = _codes[i].begin(); cCode != _codes[i].end(); ++cCode)
                if (code == cCode->code)
                    return cCode->symbol;
        }
    }

    error("Unknown Huffman code");
    return 0;
}

} // End of namespace Common

#endif // COMMON_HUFFMAN_H