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-/* Copyright (c) 2001-2011 Timothy B. Terriberry
- Copyright (c) 2008-2009 Xiph.Org Foundation */
-/*
- Redistribution and use in source and binary forms, with or without
- modification, are permitted provided that the following conditions
- are met:
-
- - Redistributions of source code must retain the above copyright
- notice, this list of conditions and the following disclaimer.
-
- - Redistributions in binary form must reproduce the above copyright
- notice, this list of conditions and the following disclaimer in the
- documentation and/or other materials provided with the distribution.
-
- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
- OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
- EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
- PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
- PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
- LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
- NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
- SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-*/
-
-#ifdef HAVE_CONFIG_H
-#include "config.h"
-#endif
-
-#include <stddef.h>
-#include "os_support.h"
-#include "arch.h"
-#include "entdec.h"
-#include "mfrngcod.h"
-
-/*A range decoder.
- This is an entropy decoder based upon \cite{Mar79}, which is itself a
- rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.
- It is very similar to arithmetic encoding, except that encoding is done with
- digits in any base, instead of with bits, and so it is faster when using
- larger bases (i.e.: a byte).
- The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$
- is the base, longer than the theoretical optimum, but to my knowledge there
- is no published justification for this claim.
- This only seems true when using near-infinite precision arithmetic so that
- the process is carried out with no rounding errors.
-
- An excellent description of implementation details is available at
- http://www.arturocampos.com/ac_range.html
- A recent work \cite{MNW98} which proposes several changes to arithmetic
- encoding for efficiency actually re-discovers many of the principles
- behind range encoding, and presents a good theoretical analysis of them.
-
- End of stream is handled by writing out the smallest number of bits that
- ensures that the stream will be correctly decoded regardless of the value of
- any subsequent bits.
- ec_tell() can be used to determine how many bits were needed to decode
- all the symbols thus far; other data can be packed in the remaining bits of
- the input buffer.
- @PHDTHESIS{Pas76,
- author="Richard Clark Pasco",
- title="Source coding algorithms for fast data compression",
- school="Dept. of Electrical Engineering, Stanford University",
- address="Stanford, CA",
- month=May,
- year=1976
- }
- @INPROCEEDINGS{Mar79,
- author="Martin, G.N.N.",
- title="Range encoding: an algorithm for removing redundancy from a digitised
- message",
- booktitle="Video & Data Recording Conference",
- year=1979,
- address="Southampton",
- month=Jul
- }
- @ARTICLE{MNW98,
- author="Alistair Moffat and Radford Neal and Ian H. Witten",
- title="Arithmetic Coding Revisited",
- journal="{ACM} Transactions on Information Systems",
- year=1998,
- volume=16,
- number=3,
- pages="256--294",
- month=Jul,
- URL="http://www.stanford.edu/class/ee398a/handouts/papers/Moffat98ArithmCoding.pdf"
- }*/
-
-static int ec_read_byte(ec_dec *_this){
- return _this->offs<_this->storage?_this->buf[_this->offs++]:0;
-}
-
-static int ec_read_byte_from_end(ec_dec *_this){
- return _this->end_offs<_this->storage?
- _this->buf[_this->storage-++(_this->end_offs)]:0;
-}
-
-/*Normalizes the contents of val and rng so that rng lies entirely in the
- high-order symbol.*/
-static void ec_dec_normalize(ec_dec *_this){
- /*If the range is too small, rescale it and input some bits.*/
- while(_this->rng<=EC_CODE_BOT){
- int sym;
- _this->nbits_total+=EC_SYM_BITS;
- _this->rng<<=EC_SYM_BITS;
- /*Use up the remaining bits from our last symbol.*/
- sym=_this->rem;
- /*Read the next value from the input.*/
- _this->rem=ec_read_byte(_this);
- /*Take the rest of the bits we need from this new symbol.*/
- sym=(sym<<EC_SYM_BITS|_this->rem)>>(EC_SYM_BITS-EC_CODE_EXTRA);
- /*And subtract them from val, capped to be less than EC_CODE_TOP.*/
- _this->val=((_this->val<<EC_SYM_BITS)+(EC_SYM_MAX&~sym))&(EC_CODE_TOP-1);
- }
-}
-
-void ec_dec_init(ec_dec *_this,unsigned char *_buf,opus_uint32 _storage){
- _this->buf=_buf;
- _this->storage=_storage;
- _this->end_offs=0;
- _this->end_window=0;
- _this->nend_bits=0;
- /*This is the offset from which ec_tell() will subtract partial bits.
- The final value after the ec_dec_normalize() call will be the same as in
- the encoder, but we have to compensate for the bits that are added there.*/
- _this->nbits_total=EC_CODE_BITS+1
- -((EC_CODE_BITS-EC_CODE_EXTRA)/EC_SYM_BITS)*EC_SYM_BITS;
- _this->offs=0;
- _this->rng=1U<<EC_CODE_EXTRA;
- _this->rem=ec_read_byte(_this);
- _this->val=_this->rng-1-(_this->rem>>(EC_SYM_BITS-EC_CODE_EXTRA));
- _this->error=0;
- /*Normalize the interval.*/
- ec_dec_normalize(_this);
-}
-
-unsigned ec_decode(ec_dec *_this,unsigned _ft){
- unsigned s;
- _this->ext=_this->rng/_ft;
- s=(unsigned)(_this->val/_this->ext);
- return _ft-EC_MINI(s+1,_ft);
-}
-
-unsigned ec_decode_bin(ec_dec *_this,unsigned _bits){
- unsigned s;
- _this->ext=_this->rng>>_bits;
- s=(unsigned)(_this->val/_this->ext);
- return (1U<<_bits)-EC_MINI(s+1U,1U<<_bits);
-}
-
-void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,unsigned _ft){
- opus_uint32 s;
- s=IMUL32(_this->ext,_ft-_fh);
- _this->val-=s;
- _this->rng=_fl>0?IMUL32(_this->ext,_fh-_fl):_this->rng-s;
- ec_dec_normalize(_this);
-}
-
-/*The probability of having a "one" is 1/(1<<_logp).*/
-int ec_dec_bit_logp(ec_dec *_this,unsigned _logp){
- opus_uint32 r;
- opus_uint32 d;
- opus_uint32 s;
- int ret;
- r=_this->rng;
- d=_this->val;
- s=r>>_logp;
- ret=d<s;
- if(!ret)_this->val=d-s;
- _this->rng=ret?s:r-s;
- ec_dec_normalize(_this);
- return ret;
-}
-
-int ec_dec_icdf(ec_dec *_this,const unsigned char *_icdf,unsigned _ftb){
- opus_uint32 r;
- opus_uint32 d;
- opus_uint32 s;
- opus_uint32 t;
- int ret;
- s=_this->rng;
- d=_this->val;
- r=s>>_ftb;
- ret=-1;
- do{
- t=s;
- s=IMUL32(r,_icdf[++ret]);
- }
- while(d<s);
- _this->val=d-s;
- _this->rng=t-s;
- ec_dec_normalize(_this);
- return ret;
-}
-
-opus_uint32 ec_dec_uint(ec_dec *_this,opus_uint32 _ft){
- unsigned ft;
- unsigned s;
- int ftb;
- /*In order to optimize EC_ILOG(), it is undefined for the value 0.*/
- celt_assert(_ft>1);
- _ft--;
- ftb=EC_ILOG(_ft);
- if(ftb>EC_UINT_BITS){
- opus_uint32 t;
- ftb-=EC_UINT_BITS;
- ft=(unsigned)(_ft>>ftb)+1;
- s=ec_decode(_this,ft);
- ec_dec_update(_this,s,s+1,ft);
- t=(opus_uint32)s<<ftb|ec_dec_bits(_this,ftb);
- if(t<=_ft)return t;
- _this->error=1;
- return _ft;
- }
- else{
- _ft++;
- s=ec_decode(_this,(unsigned)_ft);
- ec_dec_update(_this,s,s+1,(unsigned)_ft);
- return s;
- }
-}
-
-opus_uint32 ec_dec_bits(ec_dec *_this,unsigned _bits){
- ec_window window;
- int available;
- opus_uint32 ret;
- window=_this->end_window;
- available=_this->nend_bits;
- if((unsigned)available<_bits){
- do{
- window|=(ec_window)ec_read_byte_from_end(_this)<<available;
- available+=EC_SYM_BITS;
- }
- while(available<=EC_WINDOW_SIZE-EC_SYM_BITS);
- }
- ret=(opus_uint32)window&(((opus_uint32)1<<_bits)-1U);
- window>>=_bits;
- available-=_bits;
- _this->end_window=window;
- _this->nend_bits=available;
- _this->nbits_total+=_bits;
- return ret;
-}
generated by cgit (git 2.34.1) at 2024-11-15 17:52:46 +0000