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Change end-of-stream handling in the range coder.

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Instead of trying to maximize the number of trailing zeros (minimize the number
 of bits encoded), we try to maximize the number of trailing bits that can
 contain arbitrary data.
Note that this requires ec_enc_tell() and ec_dec_tell() to reserve an extra
 bit, since depending on the exact final codeword, as little as half the final
 range might be available for storing arbitrary data.
This is the first step needed to start packing literal bits outside the range
 coder (for speed and robustness purposes).
This commit is contained in:
Timothy B. Terriberry 2008-10-19 14:41:38 -04:00 • committed by Jean-Marc Valin
parent 164a229644
commit 8d940a664e
4 changed files with 58 additions and 193 deletions
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47
libcelt/rangeenc.c
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@ -143,11 +143,8 @@ long ec_enc_tell(ec_enc *_this,int _b){
nbits=(ec_byte_bytes(_this->buf)+(_this->rem>=0)+_this->ext)*EC_SYM_BITS;
/*To handle the non-integral number of bits still left in the encoder state,
we compute the number of bits of low that must be encoded to ensure that
the value is inside the range for any possible subsequent bits.
Note that this is subtly different than the actual value we would end the
stream with, which tries to make as many of the trailing bits zeros as
possible.*/
nbits+=EC_CODE_BITS;
the value is inside the range for any possible subsequent bits.*/
nbits+=EC_CODE_BITS+1;
nbits<<=_b;
l=EC_ILOG(_this->rng);
r=_this->rng>>l-16;
@ -162,30 +159,26 @@ long ec_enc_tell(ec_enc *_this,int _b){
}
void ec_enc_done(ec_enc *_this){
/*We compute the integer in the current interval that has the largest number
of trailing zeros, and write that to the stream.
This is guaranteed to yield the smallest possible encoding.*/
if(_this->low){
ec_uint32 end;
end=EC_CODE_TOP;
/*Ensure that the end value is in the range.*/
if(end-_this->low>=_this->rng){
ec_uint32 msk;
msk=EC_CODE_TOP-1;
do{
msk>>=1;
end=_this->low+msk&~msk|msk+1;
}
while(end-_this->low>=_this->rng);
}
/*The remaining output is the next free end.*/
while(end){
ec_enc_carry_out(_this,end>>EC_CODE_SHIFT);
end=end<<EC_SYM_BITS&EC_CODE_TOP-1;
}
ec_uint32 msk;
ec_uint32 end;
int l;
/*We output the minimum number of bits that ensures that the symbols encoded
thus far will be decoded correctly regardless of the bits that follow.*/
l=EC_CODE_BITS-EC_ILOG(_this->rng);
msk=EC_CODE_TOP-1>>l;
end=_this->low+msk&~msk;
if((end|msk)>=_this->low+_this->rng){
l++;
msk>>=1;
end=_this->low+msk&~msk;
}
while(l>0){
ec_enc_carry_out(_this,(int)(end>>EC_CODE_SHIFT));
end=end<<EC_SYM_BITS&EC_CODE_TOP-1;
l-=EC_SYM_BITS;
}
/*If we have a buffered byte flush it into the output buffer.*/
if(_this->rem>0||_this->ext>0){
if(_this->rem>=0||_this->ext>0){
ec_enc_carry_out(_this,0);
_this->rem=-1;
}