Moving to non-multiply-free entropy coder

libentcode/rangeenc.c

@@ -0,0 +1,145 @@
+#include <stddef.h>
+#include "entenc.h"
+#include "mfrngcod.h"
+
+
+
+/*A multiply-free range encoder.
+  See mfrngdec.c and the references for implementation details
+   \cite{Mar79,MNW98,SM98}.
+
+  @INPROCEEDINGS{Mar79,
+   author="Martin, G.N.N.",
+   title="Range encoding: an algorithm for removing redundancy from a digitised
+    message",
+   booktitle="Video \& Data Recording Conference",
+   year=1979,
+   address="Southampton",
+   month=Jul
+  }
+  @ARTICLE{MNW98,
+   author="Alistair Moffat and Radford Neal and Ian H. Witten",
+   title="Arithmetic Coding Revisited",
+   journal="{ACM} Transactions on Information Systems",
+   year=1998,
+   volume=16,
+   number=3,
+   pages="256--294",
+   month=Jul,
+   URL="http://dev.acm.org/pubs/citations/journals/tois/1998-16-3/p256-moffat/"
+  }
+  @INPROCEEDINGS{SM98,
+   author="Lang Stuiver and Alistair Moffat",
+   title="Piecewise Integer Mapping for Arithmetic Coding",
+   booktitle="Proceedings of the {IEEE} Data Compression Conference",
+   pages="1--10",
+   address="Snowbird, UT",
+   month="Mar./Apr.",
+   year=1998
+  }*/
+
+
+
+/*Outputs a symbol, with a carry bit.
+  If there is a potential to propogate a carry over several symbols, they are
+   buffered until it can be determined whether or not an actual carry will
+   occur.
+  If the counter for the buffered symbols overflows, then the range is
+   truncated to force a carry to occur, towards whichever side maximizes the
+   remaining range.*/
+static void ec_enc_carry_out(ec_enc *_this,int _c){
+  if(_c!=EC_SYM_MAX){
+    /*No further carry propogation possible, flush buffer.*/
+    int carry;
+    carry=_c>>EC_SYM_BITS;
+    /*Don't output a byte on the first write.
+      This compare should be taken care of by branch-prediction thereafter.*/
+    if(_this->rem>=0)ec_byte_write1(_this->buf,_this->rem+carry);
+    if(_this->ext>0){
+      unsigned sym;
+      sym=EC_SYM_MAX+carry&EC_SYM_MAX;
+      do ec_byte_write1(_this->buf,sym);
+      while(--(_this->ext)>0);
+    }
+    _this->rem=_c&EC_SYM_MAX;
+  }
+  else _this->ext++;
+}
+
+static void ec_enc_normalize(ec_enc *_this){
+  /*If the range is too small, output some bits and rescale it.*/
+  while(_this->rng<=EC_CODE_BOT){
+    ec_enc_carry_out(_this,(int)(_this->low>>EC_CODE_SHIFT));
+    /*Move the next-to-high-order symbol into the high-order position.*/
+    _this->low=_this->low<<EC_SYM_BITS&EC_CODE_TOP-1;
+    _this->rng<<=EC_SYM_BITS;
+  }
+}
+
+void ec_enc_init(ec_enc *_this,ec_byte_buffer *_buf){
+  _this->buf=_buf;
+  _this->rem=-1;
+  _this->ext=0;
+  _this->low=0;
+  _this->rng=EC_CODE_TOP;
+}
+
+void ec_encode(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned _ft){
+  unsigned r;
+  unsigned s;
+  r=_this->rng/_ft;
+  if(_fl>0){
+    s=r*(_ft-_fl);
+    _this->low+=_this->rng-s;
+    _this->rng=r*(_fh-_fl);
+  }
+  else _this->rng-=r*(_ft-_fh);
+  ec_enc_normalize(_this);
+}
+
+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){
+    unsigned end;
+    end=EC_CODE_TOP;
+    /*Ensure that the end value is in the range.*/
+    if(end-_this->low>=_this->rng){
+      unsigned 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;
+    }
+  }
+  /*If we have a buffered byte...*/
+  if(_this->rem>=0){
+    unsigned char *p;
+    unsigned char *buf;
+    /*Flush it into the output buffer.*/
+    ec_enc_carry_out(_this,0);
+    /*We may be able to drop some redundant bytes from the end.*/
+    buf=ec_byte_get_buffer(_this->buf);
+    p=buf+ec_byte_bytes(_this->buf)-1;
+    /*Strip trailing zeros.*/
+    while(p>=buf&&!p[0])p--;
+    /*Strip one trailing EC_FOF_RSV1 byte if the buffer ends in a string of
+       consecutive EC_FOF_RSV1 bytes preceded by one (or more) zeros.*/
+    if(p>buf&&p[0]==EC_FOF_RSV1){
+      unsigned char *q;
+      q=p;
+      do q--;
+      while(q>buf&&q[0]==EC_FOF_RSV1);
+      if(!q[0])p--;
+    }
+    ec_byte_writetrunc(_this->buf,p+1-buf);
+  }
+}