227 lines
7.8 KiB
C
227 lines
7.8 KiB
C
/* (C) 2001-2008 Timothy B. Terriberry
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(C) 2008 Jean-Marc Valin */
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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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- Neither the name of the Xiph.org Foundation nor the names of its
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contributors may be used to endorse or promote products derived from
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this software without specific prior written permission.
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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 "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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@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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/*Gets the next byte of input.
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After all the bytes in the current packet have been consumed, and the extra
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end code returned if needed, this function will continue to return zero each
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time it is called.
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Return: The next byte of input.*/
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static int ec_dec_in(ec_dec *_this){
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int ret;
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ret=ec_byte_read1(_this->buf);
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if(ret<0){
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ret=0;
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/*Needed to keep oc_dec_tell() operating correctly.*/
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ec_byte_adv1(_this->buf);
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}
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return ret;
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}
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/*Normalizes the contents of dif and rng so that rng lies entirely in the
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high-order symbol.*/
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static inline 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->rng<<=EC_SYM_BITS;
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/*Use up the remaining bits from our last symbol.*/
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sym=_this->rem<<EC_CODE_EXTRA&EC_SYM_MAX;
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/*Read the next value from the input.*/
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_this->rem=ec_dec_in(_this);
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/*Take the rest of the bits we need from this new symbol.*/
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sym|=_this->rem>>(EC_SYM_BITS-EC_CODE_EXTRA);
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_this->dif=(_this->dif<<EC_SYM_BITS)-sym&(EC_CODE_MASK);
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/*dif can never be larger than EC_CODE_TOP.
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This is equivalent to the slightly more readable:
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if(_this->dif>EC_CODE_TOP)_this->dif-=EC_CODE_TOP;*/
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_this->dif^=_this->dif&_this->dif-1&EC_CODE_TOP;
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}
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}
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void ec_dec_init(ec_dec *_this,ec_byte_buffer *_buf){
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_this->buf=_buf;
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_this->rem=ec_dec_in(_this);
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_this->rng=1U<<EC_CODE_EXTRA;
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_this->dif=_this->rng-(_this->rem>>(EC_SYM_BITS-EC_CODE_EXTRA));
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/*Normalize the interval.*/
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ec_dec_normalize(_this);
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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->nrm=_this->rng/_ft;
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s=(unsigned)((_this->dif-1)/_this->nrm);
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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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ec_uint32 ft;
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ft = (ec_uint32)1<<bits;
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_this->nrm=_this->rng>>bits;
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s=(unsigned)((_this->dif-1)/_this->nrm);
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return ft-EC_MINI(s+1,ft);
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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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ec_uint32 s;
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s=IMUL32(_this->nrm,(_ft-_fh));
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_this->dif-=s;
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_this->rng=_fl>0?IMUL32(_this->nrm,(_fh-_fl)):_this->rng-s;
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ec_dec_normalize(_this);
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}
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long ec_dec_tell(ec_dec *_this,int _b){
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ec_uint32 r;
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int l;
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long nbits;
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nbits=(ec_byte_bytes(_this->buf)-(EC_CODE_BITS+EC_SYM_BITS-1)/EC_SYM_BITS)*
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EC_SYM_BITS;
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/*To handle the non-integral number of bits still left in the encoder state,
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we compute the number of bits of low that must be encoded to ensure that
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the value is inside the range for any possible subsequent bits.
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Note that this is subtly different than the actual value we would end the
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stream with, which tries to make as many of the trailing bits zeros as
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possible.*/
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nbits+=EC_CODE_BITS;
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nbits<<=_b;
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l=EC_ILOG(_this->rng);
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r=_this->rng>>(l-16);
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while(_b-->0){
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int b;
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r=r*r>>15;
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b=(int)(r>>16);
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l=l<<1|b;
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r>>=b;
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}
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return nbits-l;
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}
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#if 0
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int ec_dec_done(ec_dec *_this){
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unsigned low;
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int ret;
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/*Check to make sure we've used all the input bytes.
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This ensures that no more ones would ever be inserted into the decoder.*/
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if(_this->buf->ptr-ec_byte_get_buffer(_this->buf)<=
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ec_byte_bytes(_this->buf)){
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return 0;
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}
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/*We compute the smallest finitely odd fraction that fits inside the current
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range, and write that to the stream.
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This is guaranteed to yield the smallest possible encoding.*/
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/*TODO: Fix this line, as it is wrong.
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It doesn't seem worth being able to make this check to do an extra
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subtraction for every symbol decoded.*/
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low=/*What we want: _this->top-_this->rng; What we have:*/_this->dif
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if(low){
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unsigned end;
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end=EC_CODE_TOP;
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/*Ensure that the next free end is in the range.*/
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if(end-low>=_this->rng){
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unsigned msk;
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msk=EC_CODE_TOP-1;
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do{
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msk>>=1;
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end=(low+msk)&~msk|msk+1;
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}
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while(end-low>=_this->rng);
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}
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/*The remaining input should have been the next free end.*/
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return end-low!=_this->dif;
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}
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return 1;
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}
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#endif
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