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Subtract one from dif in the range decoder.

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It turns out to be more convenient to store dif=low+rng-code-1
 instead of dif=low+rng-code.
This gets rid of a decrement in the normal decode path, replaces a
 decrement and an "and" in the normalization loop with a single
 add, and makes it clear that the new ec_dec_cdf() will not result
 in an infinite loop.
This does not change the bitstream.
This commit is contained in:
Timothy B. Terriberry 2010-12-17 14:32:00 -08:00 • committed by Jean-Marc Valin
parent a0b664df3d
commit 8c23a3a0fd
2 changed files with 11 additions and 10 deletions
Download patch file Download diff file Expand all files Collapse all files

3
libcelt/entdec.h
View file

@ -47,7 +47,8 @@ struct ec_dec{
int rem;
/*The number of values in the current range.*/
ec_uint32 rng;
/*The difference between the top of the current range and the input value.*/
/*The difference between the top of the current range and the input value,
minus one.*/
ec_uint32 dif;
/*Normalization factor.*/
ec_uint32 nrm;

18
libcelt/rangedec.c
View file

@ -126,11 +126,11 @@ static inline void ec_dec_normalize(ec_dec *_this){
_this->rem=ec_dec_in(_this);
/*Take the rest of the bits we need from this new symbol.*/
sym|=_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA;
_this->dif=(_this->dif<<EC_SYM_BITS)-sym&EC_CODE_MASK;
/*dif can never be larger than EC_CODE_TOP.
_this->dif=(_this->dif<<EC_SYM_BITS)-sym+((1<<EC_SYM_BITS)-1)&EC_CODE_MASK;
/*dif must be smaller than EC_CODE_TOP.
This is equivalent to the slightly more readable:
if(_this->dif>EC_CODE_TOP)_this->dif-=EC_CODE_TOP;*/
_this->dif^=_this->dif&_this->dif-1&EC_CODE_TOP;
if(_this->dif>=EC_CODE_TOP)_this->dif-=EC_CODE_TOP;*/
_this->dif^=_this->dif&EC_CODE_TOP;
}
}
@ -138,7 +138,7 @@ void ec_dec_init(ec_dec *_this,ec_byte_buffer *_buf){
_this->buf=_buf;
_this->rem=ec_dec_in(_this);
_this->rng=1U<<EC_CODE_EXTRA;
_this->dif=_this->rng-(_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA);
_this->dif=_this->rng-1-(_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA);
/*Normalize the interval.*/
ec_dec_normalize(_this);
_this->end_byte=0; /* Required for platforms that have chars > 8 bits */
@ -151,14 +151,14 @@ void ec_dec_init(ec_dec *_this,ec_byte_buffer *_buf){
unsigned ec_decode(ec_dec *_this,unsigned _ft){
unsigned s;
_this->nrm=_this->rng/_ft;
s=(unsigned)((_this->dif-1)/_this->nrm);
s=(unsigned)(_this->dif/_this->nrm);
return _ft-EC_MINI(s+1,_ft);
}
unsigned ec_decode_bin(ec_dec *_this,unsigned _bits){
unsigned s;
_this->nrm=_this->rng>>_bits;
s=(unsigned)((_this->dif-1)/_this->nrm);
s=(unsigned)(_this->dif/_this->nrm);
return (1<<_bits)-EC_MINI(s+1,1<<_bits);
}
@ -196,7 +196,7 @@ int ec_dec_bit_prob(ec_dec *_this,unsigned _prob){
r=_this->rng;
d=_this->dif;
s=(r>>16)*_prob;
val=d<=s;
val=d<s;
if(!val)_this->dif=d-s;
_this->rng=val?s:r-s;
ec_dec_normalize(_this);
@ -215,7 +215,7 @@ int ec_dec_cdf(ec_dec *_this,const int *_cdf,unsigned _ftb){
val=0;
t=0;
s=IMUL32(r,(1<<_ftb)-_cdf[0]);
while(d<=s){
while(d<s){
t=s;
s=IMUL32(r,(1<<_ftb)-_cdf[++val]);
}