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Add a generic CDF decoding routine.

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This decodes a value encoded with ec_encode_bin() without using any
 divisions.
It is only meant for small alphabets.
If a symbol can take on a large number of possible values, a binary
 search would be better.

This patch also converts spread_decision to use it, since it is
 faster and introduces less rounding error to encode a single
 decision for the entire value than to encode it a bit at a time.
This commit is contained in:
Timothy B. Terriberry 2010-12-17 10:49:00 -08:00 • committed by Jean-Marc Valin
parent 3fed34ae00
commit a0b664df3d
3 changed files with 36 additions and 13 deletions
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22
libcelt/rangedec.c
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@ -203,6 +203,28 @@ int ec_dec_bit_prob(ec_dec *_this,unsigned _prob){
return val;
}
int ec_dec_cdf(ec_dec *_this,const int *_cdf,unsigned _ftb){
ec_uint32 r;
ec_uint32 d;
ec_uint32 s;
ec_uint32 t;
int val;
r=_this->rng>>_ftb;
d=_this->dif;
_cdf++;
val=0;
t=0;
s=IMUL32(r,(1<<_ftb)-_cdf[0]);
while(d<=s){
t=s;
s=IMUL32(r,(1<<_ftb)-_cdf[++val]);
}
_this->dif=d-s;
_this->rng=(val?t:_this->rng)-s;
ec_dec_normalize(_this);
return val;
}
ec_uint32 ec_dec_tell(ec_dec *_this,int _b){
ec_uint32 r;
int l;