193 lines
5.8 KiB
C
193 lines
5.8 KiB
C
#include <stddef.h>
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#include "entenc.h"
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#include "mfrngcod.h"
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/*A multiply-free range encoder.
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See mfrngdec.c and the references for implementation details
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\cite{Mar79,MNW98,SM98}.
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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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@INPROCEEDINGS{SM98,
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author="Lang Stuiver and Alistair Moffat",
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title="Piecewise Integer Mapping for Arithmetic Coding",
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booktitle="Proceedings of the {IEEE} Data Compression Conference",
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pages="1--10",
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address="Snowbird, UT",
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month="Mar./Apr.",
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year=1998
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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 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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unsigned r;
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unsigned s;
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unsigned d;
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int nrm;
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/*Step 1: we want ft in the range of [rng/2,rng).
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The high-order bits of the rng and ft are computed via a logarithm.
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This could also be done on some architectures with some custom assembly,
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which would provide even more speed.*/
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nrm=EC_ILOG(_this->rng)-EC_ILOG(_ft);
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/*Having the same high order bit may be too much.
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We may need to shift one less to ensure that ft is actually in the proper
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range.*/
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_ft<<=nrm;
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d=_ft>_this->rng;
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_ft>>=d;
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nrm-=d;
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/*We then scale everything by the computed power of 2.*/
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_fl<<=nrm;
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_fh<<=nrm;
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/*Step 2: compute the two values of the partition function.
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d is the splitting point of the interval [0,ft).*/
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d=_this->rng-_ft;
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r=_fh+EC_MINI(_fh,d);
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s=_fl+EC_MINI(_fl,d);
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/*Step 3: Update the end-point and range of the interval.*/
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_this->low+=s;
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_this->rng=r-s;
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/*Step 4: Normalize the interval.*/
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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<<3;
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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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unsigned 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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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=_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...*/
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if(_this->rem>=0){
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unsigned char *p;
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unsigned char *buf;
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/*Flush it into the output buffer.*/
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ec_enc_carry_out(_this,0);
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_this->rem=-1;
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/*We may be able to drop some redundant bytes from the end.*/
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buf=ec_byte_get_buffer(_this->buf);
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p=buf+ec_byte_bytes(_this->buf)-1;
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/*Strip trailing zeros.*/
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while(p>=buf&&!p[0])p--;
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/*Strip one trailing EC_FOF_RSV1 byte if the buffer ends in a string of
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consecutive EC_FOF_RSV1 bytes preceded by one (or more) zeros.*/
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if(p>buf&&p[0]==EC_FOF_RSV1){
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unsigned char *q;
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q=p;
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do q--;
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while(q>buf&&q[0]==EC_FOF_RSV1);
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if(!q[0])p--;
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}
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ec_byte_writetrunc(_this->buf,p+1-buf);
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}
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}
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