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/*
libdemac - A Monkey's Audio decoder
$Id$
Copyright (C) Dave Chapman 2007
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 St, Fifth Floor, Boston, MA 02110, USA
*/
/*
Range decoder adapted from rangecod.c included in:
http://www.compressconsult.com/rangecoder/rngcod13.zip
rangecod.c range encoding
(c) Michael Schindler
1997, 1998, 1999, 2000
http://www.compressconsult.com/
michael@compressconsult.com
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.
The encoding functions were removed, and functions turned into "static
inline" functions and moved to a .h file. Some minor cosmetic changes
were made (e.g. turning pre-processor symbols into upper-case,
removing the rc parameter from each function (and the RNGC macro)).
*/
/* BITSTREAM READING FUNCTIONS */
/* We deal with the input data one byte at a time - to ensure
functionality on CPUs of any endianness regardless of any requirements
for aligned reads.
*/
static unsigned char* bytebuffer IBSS_ATTR;
static int bytebufferoffset IBSS_ATTR;
static inline void skip_byte(void)
{
if (bytebufferoffset) {
bytebufferoffset--;
} else {
bytebufferoffset = 3;
bytebuffer += 4;
}
}
static inline int read_byte(void)
{
int ch = bytebuffer[bytebufferoffset];
skip_byte();
return ch;
}
/* RANGE DECODING FUNCTIONS */
/* SIZE OF RANGE ENCODING CODE VALUES. */
#define CODE_BITS 32
#define TOP_VALUE ((unsigned int)1 << (CODE_BITS-1))
#define SHIFT_BITS (CODE_BITS - 9)
#define EXTRA_BITS ((CODE_BITS-2) % 8 + 1)
#define BOTTOM_VALUE (TOP_VALUE >> 8)
struct rangecoder_t
{
uint32_t low; /* low end of interval */
uint32_t range; /* length of interval */
uint32_t help; /* bytes_to_follow resp. intermediate value */
unsigned int buffer; /* buffer for input/output */
};
static struct rangecoder_t rc IBSS_ATTR;
/* Start the decoder */
static inline void range_start_decoding(void)
{
rc.buffer = read_byte();
rc.low = rc.buffer >> (8 - EXTRA_BITS);
rc.range = (uint32_t) 1 << EXTRA_BITS;
}
static inline void range_dec_normalize(void)
{
while (rc.range <= BOTTOM_VALUE)
{
rc.buffer = (rc.buffer << 8) | read_byte();
rc.low = (rc.low << 8) | ((rc.buffer >> 1) & 0xff);
rc.range <<= 8;
}
}
/* Calculate culmulative frequency for next symbol. Does NO update!*/
/* tot_f is the total frequency */
/* or: totf is (code_value)1<<shift */
/* returns the culmulative frequency */
static inline int range_decode_culfreq(int tot_f)
{ int tmp;
range_dec_normalize();
rc.help = rc.range / tot_f;
tmp = rc.low / rc.help;
return tmp;
}
static inline int range_decode_culshift(int shift)
{
int tmp;
range_dec_normalize();
rc.help = rc.range>>shift;
tmp = rc.low/rc.help;
return tmp;
}
/* Update decoding state */
/* sy_f is the interval length (frequency of the symbol) */
/* lt_f is the lower end (frequency sum of < symbols) */
static inline void range_decode_update(int sy_f, int lt_f)
{ int tmp;
tmp = rc.help * lt_f;
rc.low -= tmp;
rc.range = rc.help * sy_f;
}
/* Decode a byte/short without modelling */
static inline unsigned char decode_byte(void)
{ int tmp = range_decode_culshift(8);
range_decode_update( 1,tmp);
return tmp;
}
static inline int short range_decode_short(void)
{ int tmp = range_decode_culshift(16);
range_decode_update( 1,tmp);
return tmp;
}
/* Decode n bits (n <= 16) without modelling - based on range_decode_short */
static inline int range_decode_bits(int n)
{ int tmp = range_decode_culshift(n);
range_decode_update( 1,tmp);
return tmp;
}
/* Finish decoding */
static inline void range_done_decoding(void)
{ range_dec_normalize(); /* normalize to use up all bytes */
}
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