193 lines
6.1 KiB
C
193 lines
6.1 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 "entenc.h"
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#include "mfrngcod.h"
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/*A range encoder.
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See rangedec.c and the references for implementation details
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\cite{Mar79,MNW98}.
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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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/*Outputs a symbol, with a carry bit.
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If there is a potential to propagate a carry over several symbols, they are
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buffered until it can be determined whether or not an actual carry will
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occur.
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If the counter for the buffered symbols overflows, then the stream becomes
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undecodable.
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This gives a theoretical limit of a few billion symbols in a single packet on
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32-bit systems.
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The alternative is to truncate the range in order to force a carry, but
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requires similar carry tracking in the decoder, needlessly slowing it down.*/
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static void ec_enc_carry_out(ec_enc *_this,int _c){
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if(_c!=EC_SYM_MAX){
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/*No further carry propagation possible, flush buffer.*/
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int carry;
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carry=_c>>EC_SYM_BITS;
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/*Don't output a byte on the first write.
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This compare should be taken care of by branch-prediction thereafter.*/
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if(_this->rem>=0)ec_byte_write1(_this->buf,_this->rem+carry);
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if(_this->ext>0){
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unsigned sym;
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sym=EC_SYM_MAX+carry&EC_SYM_MAX;
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do ec_byte_write1(_this->buf,sym);
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while(--(_this->ext)>0);
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}
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_this->rem=_c&EC_SYM_MAX;
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}
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else _this->ext++;
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}
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static inline void ec_enc_normalize(ec_enc *_this){
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/*If the range is too small, output some bits and rescale it.*/
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while(_this->rng<=EC_CODE_BOT){
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ec_enc_carry_out(_this,(int)(_this->low>>EC_CODE_SHIFT));
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/*Move the next-to-high-order symbol into the high-order position.*/
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_this->low=_this->low<<EC_SYM_BITS&EC_CODE_TOP-1;
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_this->rng<<=EC_SYM_BITS;
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}
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}
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void ec_enc_init(ec_enc *_this,ec_byte_buffer *_buf){
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_this->buf=_buf;
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_this->rem=-1;
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_this->ext=0;
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_this->low=0;
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_this->rng=EC_CODE_TOP;
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}
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void ec_encode(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned _ft){
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ec_uint32 r;
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r=_this->rng/_ft;
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if(_fl>0){
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_this->low+=_this->rng-IMUL32(r,(_ft-_fl));
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_this->rng=IMUL32(r,(_fh-_fl));
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}
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else _this->rng-=IMUL32(r,(_ft-_fh));
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ec_enc_normalize(_this);
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}
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void ec_encode_bin(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned bits){
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ec_uint32 r, ft;
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r=_this->rng>>bits;
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ft = (ec_uint32)1<<bits;
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if(_fl>0){
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_this->low+=_this->rng-IMUL32(r,(ft-_fl));
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_this->rng=IMUL32(r,(_fh-_fl));
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}
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else _this->rng-=IMUL32(r,(ft-_fh));
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ec_enc_normalize(_this);
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}
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long ec_enc_tell(ec_enc *_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)+(_this->rem>=0)+_this->ext)*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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void ec_enc_done(ec_enc *_this){
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/*We compute the integer in the current interval that has the largest number
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of trailing zeros, and write that to the stream.
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This is guaranteed to yield the smallest possible encoding.*/
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if(_this->low){
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ec_uint32 end;
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end=EC_CODE_TOP;
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/*Ensure that the end value is in the range.*/
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if(end-_this->low>=_this->rng){
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ec_uint32 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=(((_this->low+msk)&~msk)|msk+1);
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}
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while(end-_this->low>=_this->rng);
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}
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/*The remaining output is the next free end.*/
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while(end){
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ec_enc_carry_out(_this,end>>EC_CODE_SHIFT);
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end=end<<EC_SYM_BITS&EC_CODE_TOP-1;
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}
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}
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/*If we have a buffered byte flush it into the output buffer.*/
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if(_this->rem>0||_this->ext>0){
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ec_enc_carry_out(_this,0);
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_this->rem=-1;
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}
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}
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