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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/main.h"
#include "glk/alan2/decode.h"
namespace Glk {
namespace Alan2 {
/* Bit output */
static int decodeBuffer; /* Bits to be input */
static int bitsToGo; /* Bits still in buffer */
static int garbageBits; /* Bits past EOD */
static int inputBit() {
int bit;
if (!bitsToGo) { /* More bits available ? */
decodeBuffer = txtfil->readByte(); /* No, so get more */
if ((uint)decodeBuffer == EOD) {
garbageBits++;
if (garbageBits > VALUEBITS - 2)
syserr("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;
}
/* Current state of decoding */
static CodeValue value; /* Currently seen code value */
static CodeValue low, high; /* Current code region */
void startDecoding() {
int i;
bitsToGo = 0;
garbageBits = 0;
value = 0;
for (i = 0; i < VALUEBITS; i++)
value = 2 * value + inputBit();
low = 0;
high = TOPVALUE;
}
int decodeChar() {
long range;
int f;
int symbol;
range = (long)(high - low) + 1;
f = (((long)(value - low) + 1) * freq[0] - 1) / range;
/* Find the symbol */
for (symbol = 1; (int)freq[symbol] > f; symbol++);
high = low + range * freq[symbol - 1] / freq[0] - 1;
low = low + range * freq[symbol] / freq[0];
for (;;) {
if (high < HALF)
;
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;
}
/* Structure for saved decode info */
typedef struct DecodeInfo {
long fpos;
int buffer;
int bits;
CodeValue value;
CodeValue high;
CodeValue low;
} DecodeInfo;
/*======================================================================
pushDecode()
Save so much info about the decoding process so it is possible to
restore and continue later.
*/
void *pushDecode() {
DecodeInfo *info;
info = (DecodeInfo *) allocate(sizeof(DecodeInfo));
info->fpos = txtfil->pos();
info->buffer = decodeBuffer;
info->bits = bitsToGo;
info->value = value;
info->high = high;
info->low = low;
return (info);
}
/*======================================================================
popDecode()
Restore enough info about the decoding process so it is possible to
continue after having decoded something else.
*/
void popDecode(void *i) {
DecodeInfo *info = (DecodeInfo *) i;
fseek(txtfil, info->fpos, 0);
decodeBuffer = info->buffer;
bitsToGo = info->bits;
value = info->value;
high = info->high;
low = info->low;
free(info);
}
} // End of namespace Alan2
} // End of namespace Glk
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