255 lines
8.0 KiB
C
255 lines
8.0 KiB
C
/* Copyright (c) 2001-2011 Timothy B. Terriberry
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Copyright (c) 2008-2009 Xiph.Org Foundation */
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/*
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions
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are met:
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- Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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- Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in the
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documentation and/or other materials provided with the distribution.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR
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CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
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NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
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SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#include <stddef.h>
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#include "os_support.h"
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#include "arch.h"
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#include "entdec.h"
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#include "mfrngcod.h"
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/*A range decoder.
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This is an entropy decoder based upon \cite{Mar79}, which is itself a
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rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.
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It is very similar to arithmetic encoding, except that encoding is done with
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digits in any base, instead of with bits, and so it is faster when using
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larger bases (i.e.: a byte).
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The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$
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is the base, longer than the theoretical optimum, but to my knowledge there
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is no published justification for this claim.
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This only seems true when using near-infinite precision arithmetic so that
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the process is carried out with no rounding errors.
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IBM (the author's employer) never sought to patent the idea, and to my
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knowledge the algorithm is unencumbered by any patents, though its
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performance is very competitive with proprietary arithmetic coding.
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The two are based on very similar ideas, however.
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An excellent description of implementation details is available at
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http://www.arturocampos.com/ac_range.html
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A recent work \cite{MNW98} which proposes several changes to arithmetic
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encoding for efficiency actually re-discovers many of the principles
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behind range encoding, and presents a good theoretical analysis of them.
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End of stream is handled by writing out the smallest number of bits that
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ensures that the stream will be correctly decoded regardless of the value of
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any subsequent bits.
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ec_tell() can be used to determine how many bits were needed to decode
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all the symbols thus far; other data can be packed in the remaining bits of
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the input buffer.
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@PHDTHESIS{Pas76,
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author="Richard Clark Pasco",
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title="Source coding algorithms for fast data compression",
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school="Dept. of Electrical Engineering, Stanford University",
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address="Stanford, CA",
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month=May,
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year=1976
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}
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@INPROCEEDINGS{Mar79,
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author="Martin, G.N.N.",
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title="Range encoding: an algorithm for removing redundancy from a digitised
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message",
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booktitle="Video & Data Recording Conference",
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year=1979,
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address="Southampton",
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month=Jul
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}
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@ARTICLE{MNW98,
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author="Alistair Moffat and Radford Neal and Ian H. Witten",
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title="Arithmetic Coding Revisited",
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journal="{ACM} Transactions on Information Systems",
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year=1998,
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volume=16,
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number=3,
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pages="256--294",
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month=Jul,
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URL="http://www.stanford.edu/class/ee398/handouts/papers/Moffat98ArithmCoding.pdf"
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}*/
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static int ec_read_byte(ec_dec *_this){
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return _this->offs<_this->storage?_this->buf[_this->offs++]:0;
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}
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static int ec_read_byte_from_end(ec_dec *_this){
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return _this->end_offs<_this->storage?
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_this->buf[_this->storage-++(_this->end_offs)]:0;
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}
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/*Normalizes the contents of val and rng so that rng lies entirely in the
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high-order symbol.*/
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static void ec_dec_normalize(ec_dec *_this){
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/*If the range is too small, rescale it and input some bits.*/
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while(_this->rng<=EC_CODE_BOT){
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int sym;
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_this->nbits_total+=EC_SYM_BITS;
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_this->rng<<=EC_SYM_BITS;
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/*Use up the remaining bits from our last symbol.*/
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sym=_this->rem;
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/*Read the next value from the input.*/
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_this->rem=ec_read_byte(_this);
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/*Take the rest of the bits we need from this new symbol.*/
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sym=(sym<<EC_SYM_BITS|_this->rem)>>EC_SYM_BITS-EC_CODE_EXTRA;
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/*And subtract them from val, capped to be less than EC_CODE_TOP.*/
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_this->val=(_this->val<<EC_SYM_BITS)+(EC_SYM_MAX&~sym)&EC_CODE_TOP-1;
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}
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}
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void ec_dec_init(ec_dec *_this,unsigned char *_buf,celt_uint32 _storage){
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_this->buf=_buf;
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_this->storage=_storage;
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_this->end_offs=0;
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_this->end_window=0;
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_this->nend_bits=0;
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_this->offs=0;
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_this->rng=1U<<EC_CODE_EXTRA;
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_this->rem=ec_read_byte(_this);
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_this->val=_this->rng-1-(_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA);
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_this->error=0;
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/*Normalize the interval.*/
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ec_dec_normalize(_this);
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/*This is the offset from which ec_tell() will subtract partial bits.
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This must be after the initial ec_dec_normalize(), or you will have to
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compensate for the bits that are read there.*/
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_this->nbits_total=EC_CODE_BITS+1;
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}
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unsigned ec_decode(ec_dec *_this,unsigned _ft){
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unsigned s;
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_this->ext=_this->rng/_ft;
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s=(unsigned)(_this->val/_this->ext);
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return _ft-EC_MINI(s+1,_ft);
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}
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unsigned ec_decode_bin(ec_dec *_this,unsigned _bits){
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unsigned s;
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_this->ext=_this->rng>>_bits;
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s=(unsigned)(_this->val/_this->ext);
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return (1<<_bits)-EC_MINI(s+1,1<<_bits);
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}
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void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,unsigned _ft){
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celt_uint32 s;
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s=IMUL32(_this->ext,_ft-_fh);
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_this->val-=s;
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_this->rng=_fl>0?IMUL32(_this->ext,_fh-_fl):_this->rng-s;
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ec_dec_normalize(_this);
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}
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/*The probability of having a "one" is 1/(1<<_logp).*/
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int ec_dec_bit_logp(ec_dec *_this,unsigned _logp){
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celt_uint32 r;
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celt_uint32 d;
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celt_uint32 s;
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int ret;
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r=_this->rng;
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d=_this->val;
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s=r>>_logp;
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ret=d<s;
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if(!ret)_this->val=d-s;
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_this->rng=ret?s:r-s;
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ec_dec_normalize(_this);
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return ret;
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}
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int ec_dec_icdf(ec_dec *_this,const unsigned char *_icdf,unsigned _ftb){
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celt_uint32 r;
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celt_uint32 d;
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celt_uint32 s;
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celt_uint32 t;
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int ret;
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s=_this->rng;
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d=_this->val;
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r=s>>_ftb;
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ret=-1;
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do{
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t=s;
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s=IMUL32(r,_icdf[++ret]);
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}
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while(d<s);
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_this->val=d-s;
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_this->rng=t-s;
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ec_dec_normalize(_this);
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return ret;
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}
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celt_uint32 ec_dec_uint(ec_dec *_this,celt_uint32 _ft){
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unsigned ft;
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unsigned s;
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int ftb;
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/*In order to optimize EC_ILOG(), it is undefined for the value 0.*/
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celt_assert(_ft>1);
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_ft--;
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ftb=EC_ILOG(_ft);
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if(ftb>EC_UINT_BITS){
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celt_uint32 t;
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ftb-=EC_UINT_BITS;
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ft=(unsigned)(_ft>>ftb)+1;
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s=ec_decode(_this,ft);
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ec_dec_update(_this,s,s+1,ft);
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t=(celt_uint32)s<<ftb|ec_dec_bits(_this,ftb);
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if(t<=_ft)return t;
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_this->error=1;
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return _ft;
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}
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else{
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_ft++;
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s=ec_decode(_this,(unsigned)_ft);
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ec_dec_update(_this,s,s+1,(unsigned)_ft);
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return s;
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}
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}
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celt_uint32 ec_dec_bits(ec_dec *_this,unsigned _bits){
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ec_window window;
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int available;
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celt_uint32 ret;
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window=_this->end_window;
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available=_this->nend_bits;
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if(available<_bits){
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do{
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window|=(ec_window)ec_read_byte_from_end(_this)<<available;
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available+=EC_SYM_BITS;
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}
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while(available<=EC_WINDOW_SIZE-EC_SYM_BITS);
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}
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ret=(celt_uint32)window&((celt_uint32)1<<_bits)-1;
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window>>=_bits;
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available-=_bits;
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_this->end_window=window;
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_this->nend_bits=available;
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_this->nbits_total+=_bits;
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return ret;
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}
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