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/* 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 2
* 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, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*
*/
#include "glk/alan2/decode.h"
namespace Glk {
namespace Alan2 {
int Decode::inputBit() {
int bit;
if (!_bitsToGo) { // More bits available ?
_decodeBuffer = _txtFile->readByte(); // No, so get more
if (_txtFile->eos()) {
_garbageBits++;
if (_garbageBits > VALUEBITS - 2)
error("Error in encoded data file.");
} else
_bitsToGo = 8; // Another Char, 8 new bits
}
bit = _decodeBuffer & 1; // Get next bit
_decodeBuffer = _decodeBuffer >> 1; // and remove it
_bitsToGo--;
return bit;
}
void Decode::startDecoding() {
_bitsToGo = 0;
_garbageBits = 0;
_value = 0;
for (int i = 0; i < VALUEBITS; i++)
_value = 2 * _value + inputBit();
_low = 0;
_high = TOPVALUE;
}
int Decode::decodeChar() {
const long range = (long)(_high - _low) + 1;
const uint f = (((long)(_value - _low) + 1) * _freq[0] - 1) / range;
int symbol;
// Find the symbol
for (symbol = 1; _freq[symbol] > f; symbol++);
_high = _low + range * _freq[symbol - 1] / _freq[0] - 1;
_low = _low + range * _freq[symbol] / _freq[0];
for (;;) {
if (_high < HALF) {
// Do nothing
} else if (_low >= HALF) {
_value = _value - HALF;
_low = _low - HALF;
_high = _high - HALF;
} else if (_low >= ONEQUARTER && _high < THREEQUARTER) {
_value = _value - ONEQUARTER;
_low = _low - ONEQUARTER;
_high = _high - ONEQUARTER;
} else
break;
// Scale up the range
_low = 2 * _low;
_high = 2 * _high + 1;
_value = 2 * _value + inputBit();
}
return symbol - 1;
}
DecodeInfo *Decode::pushDecode() {
DecodeInfo *info = new DecodeInfo();
info->fpos = _txtFile->pos();
info->buffer = _decodeBuffer;
info->bits = _bitsToGo;
info->value = _value;
info->high = _high;
info->low = _low;
return info;
}
void Decode::popDecode (DecodeInfo *info) {
_txtFile->seek(info->fpos, SEEK_CUR);
_decodeBuffer = info->buffer;
_bitsToGo = info->bits;
_value = info->value;
_high = info->high;
_low = info->low;
delete info;
}
} // End of namespace Alan2
} // End of namespace Glk
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