164 lines
3.8 KiB
C++
164 lines
3.8 KiB
C++
/* ScummVM - Graphic Adventure Engine
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*
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* ScummVM is the legal property of its developers, whose names
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* are too numerous to list here. Please refer to the COPYRIGHT
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* file distributed with this source distribution.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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*
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*/
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#include "glk/alan2/main.h"
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#include "glk/alan2/decode.h"
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namespace Glk {
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namespace Alan2 {
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/* Bit output */
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static int decodeBuffer; /* Bits to be input */
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static int bitsToGo; /* Bits still in buffer */
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static int garbageBits; /* Bits past EOD */
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static int inputBit() {
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int bit;
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if (!bitsToGo) { /* More bits available ? */
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decodeBuffer = txtfil->readByte(); /* No, so get more */
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if ((uint)decodeBuffer == EOD) {
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garbageBits++;
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if (garbageBits > VALUEBITS - 2)
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syserr("Error in encoded data file.");
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} else
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bitsToGo = 8; /* Another Char, 8 new bits */
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}
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bit = decodeBuffer & 1; /* Get next bit */
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decodeBuffer = decodeBuffer >> 1; /* and remove it */
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bitsToGo--;
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return bit;
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}
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/* Current state of decoding */
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static CodeValue value; /* Currently seen code value */
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static CodeValue low, high; /* Current code region */
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void startDecoding() {
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int i;
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bitsToGo = 0;
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garbageBits = 0;
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value = 0;
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for (i = 0; i < VALUEBITS; i++)
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value = 2 * value + inputBit();
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low = 0;
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high = TOPVALUE;
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}
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int decodeChar() {
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long range;
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int f;
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int symbol;
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range = (long)(high - low) + 1;
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f = (((long)(value - low) + 1) * freq[0] - 1) / range;
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/* Find the symbol */
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for (symbol = 1; (int)freq[symbol] > f; symbol++);
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high = low + range * freq[symbol - 1] / freq[0] - 1;
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low = low + range * freq[symbol] / freq[0];
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for (;;) {
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if (high < HALF)
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;
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else if (low >= HALF) {
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value = value - HALF;
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low = low - HALF;
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high = high - HALF;
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} else if (low >= ONEQUARTER && high < THREEQUARTER) {
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value = value - ONEQUARTER;
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low = low - ONEQUARTER;
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high = high - ONEQUARTER;
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} else
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break;
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/* Scale up the range */
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low = 2 * low;
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high = 2 * high + 1;
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value = 2 * value + inputBit();
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}
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return symbol - 1;
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}
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/* Structure for saved decode info */
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typedef struct DecodeInfo {
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long fpos;
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int buffer;
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int bits;
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CodeValue value;
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CodeValue high;
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CodeValue low;
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} DecodeInfo;
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/*======================================================================
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pushDecode()
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Save so much info about the decoding process so it is possible to
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restore and continue later.
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*/
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void *pushDecode() {
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DecodeInfo *info;
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info = (DecodeInfo *) allocate(sizeof(DecodeInfo));
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info->fpos = txtfil->pos();
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info->buffer = decodeBuffer;
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info->bits = bitsToGo;
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info->value = value;
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info->high = high;
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info->low = low;
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return (info);
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}
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/*======================================================================
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popDecode()
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Restore enough info about the decoding process so it is possible to
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continue after having decoded something else.
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*/
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void popDecode(void *i) {
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DecodeInfo *info = (DecodeInfo *) i;
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fseek(txtfil, info->fpos, 0);
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decodeBuffer = info->buffer;
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bitsToGo = info->bits;
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value = info->value;
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high = info->high;
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low = info->low;
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free(info);
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}
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} // End of namespace Alan2
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} // End of namespace Glk
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