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ref: 8c23a3a0fd2dae1407a56e994485b0f9c57910cc
parent: a0b664df3dd7cdaf01d1c30af928bb24c675211e
author: Timothy B. Terriberry <[email protected]>
date: Fri Dec 17 09:32:00 EST 2010 

Subtract one from dif in the range decoder.

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.


--- a/libcelt/entdec.h
+++ b/libcelt/entdec.h
@@ -47,7 +47,8 @@
    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;
--- a/libcelt/rangedec.c
+++ b/libcelt/rangedec.c
@@ -126,11 +126,11 @@
     _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 @@
   _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 */
@@ -151,7 +151,7 @@
 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);
 }
 
@@ -158,7 +158,7 @@
 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 @@
   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 @@
   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]);
   }