libcelt/rangeenc.c

/* Copyright (c) 2001-2008 Timothy B. Terriberry
   Copyright (c) 2008-2009 Xiph.Org Foundation */
/*
   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:

   - Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.

   - Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

   - Neither the name of the Xiph.org Foundation nor the names of its
   contributors may be used to endorse or promote products derived from
   this software without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR
   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "arch.h"
#include "entenc.h"
#include "mfrngcod.h"



/*A range encoder.
  See rangedec.c and the references for implementation details
   \cite{Mar79,MNW98}.

  @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://www.stanford.edu/class/ee398/handouts/papers/Moffat98ArithmCoding.pdf"
  }*/



/*Outputs a symbol, with a carry bit.
  If there is a potential to propagate 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 stream becomes
   undecodable.
  This gives a theoretical limit of a few billion symbols in a single packet on
   32-bit systems.
  The alternative is to truncate the range in order to force a carry, but
   requires similar carry tracking in the decoder, needlessly slowing it down.*/
static void ec_enc_carry_out(ec_enc *_this,int _c){
  if(_c!=EC_SYM_MAX){
    /*No further carry propagation 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)_this->error|=ec_byte_write(_this->buf,_this->rem+carry);
    if(_this->ext>0){
      unsigned sym;
      sym=EC_SYM_MAX+carry&EC_SYM_MAX;
      do _this->error|=ec_byte_write(_this->buf,sym);
      while(--(_this->ext)>0);
    }
    _this->rem=_c&EC_SYM_MAX;
  }
  else _this->ext++;
}

static inline 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;
    _this->nbits_total+=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;
  _this->end_window=0;
  _this->nend_bits=0;
  /*This is the offset from which ec_enc_tell() will subtract partial bits.*/
  _this->nbits_total=EC_CODE_BITS+1;
  _this->error=0;
}

void ec_encode(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned _ft){
  ec_uint32 r;
  r=_this->rng/_ft;
  if(_fl>0){
    _this->low+=_this->rng-IMUL32(r,(_ft-_fl));
    _this->rng=IMUL32(r,(_fh-_fl));
  }
  else _this->rng-=IMUL32(r,(_ft-_fh));
  ec_enc_normalize(_this);
}

void ec_encode_bin(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned _bits){
  ec_uint32 r;
  r=_this->rng>>_bits;
  if(_fl>0){
    _this->low+=_this->rng-IMUL32(r,((1<<_bits)-_fl));
    _this->rng=IMUL32(r,(_fh-_fl));
  }
  else _this->rng-=IMUL32(r,((1<<_bits)-_fh));
  ec_enc_normalize(_this);
}

/*The probability of having a "one" is given in 1/65536.*/
void ec_enc_bit_prob(ec_enc *_this,int _val,unsigned _prob){
  ec_uint32 r;
  ec_uint32 s;
  ec_uint32 l;
  r=_this->rng;
  l=_this->low;
  s=(r>>16)*_prob;
  r-=s;
  if(_val)_this->low=l+r;
  _this->rng=_val?s:r;
  ec_enc_normalize(_this);
}

/*The probability of having a "one" is 1/(1<<_logp).*/
void ec_enc_bit_logp(ec_enc *_this,int _val,unsigned _logp){
  ec_uint32 r;
  ec_uint32 s;
  ec_uint32 l;
  r=_this->rng;
  l=_this->low;
  s=r>>_logp;
  r-=s;
  if(_val)_this->low=l+r;
  _this->rng=_val?s:r;
  ec_enc_normalize(_this);
}

void ec_enc_icdf(ec_enc *_this,int _s,const unsigned char *_icdf,unsigned _ftb){
  ec_uint32 r;
  r=_this->rng>>_ftb;
  if(_s>0){
    _this->low+=_this->rng-IMUL32(r,_icdf[_s-1]);
    _this->rng=IMUL32(r,_icdf[_s-1]-_icdf[_s]);
  }
  else _this->rng-=IMUL32(r,_icdf[_s]);
  ec_enc_normalize(_this);
}

void ec_enc_bits(ec_enc *_this,ec_uint32 _fl,unsigned _bits){
  ec_window window;
  int       used;
  window=_this->end_window;
  used=_this->nend_bits;
  if(used+_bits>EC_WINDOW_SIZE){
    do{
      _this->error|=
       ec_byte_write_at_end(_this->buf,(unsigned)window&EC_SYM_MAX);
      window>>=EC_SYM_BITS;
      used-=EC_SYM_BITS;
    }
    while(used>=EC_SYM_BITS);
  }
  window|=(ec_window)_fl<<used;
  used+=_bits;
  _this->end_window=window;
  _this->nend_bits=used;
  _this->nbits_total+=_bits;
}

ec_uint32 ec_enc_tell(ec_enc *_this,int _b){
  ec_uint32 nbits;
  ec_uint32 r;
  int       l;
  /*To handle the non-integral number of bits still left in the decoder state,
     we compute the worst-case number of bits of low that must be encoded to
     ensure that the value is inside the range for any possible subsequent
     bits.
    The computation here is independent of low itself (the decoder does not
     even track that value), even though the real number of bits used after
     ec_enc_done() may be 1 smaller if rng is a power of two and the
     corresponding trailing bits of low are all zeros.
    If we did try to track that special case, then coding a value with a
     probability of 1/(1<<n) might sometimes appear to use more than n bits.
    This may help explain the surprising result that a newly initialized
     encoder claims to have used 1 bit.*/
  nbits=_this->nbits_total<<_b;
  l=EC_ILOG(_this->rng);
  r=_this->rng>>l-16;
  while(_b-->0){
    int b;
    r=r*r>>15;
    b=(int)(r>>16);
    l=l<<1|b;
    r>>=b;
  }
  return nbits-l;
}

void ec_enc_done(ec_enc *_this){
  ec_window window;
  int       used;
  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)ec_enc_carry_out(_this,0);
  /*If we have buffered extra bits, flush them as well.*/
  window=_this->end_window;
  used=_this->nend_bits;
  while(used>=EC_SYM_BITS){
    _this->error|=
     ec_byte_write_at_end(_this->buf,(unsigned)window&EC_SYM_MAX);
    window>>=EC_SYM_BITS;
    used-=EC_SYM_BITS;
  }
  /*Clear any excess space and add any remaining extra bits to the last byte.*/
  if(!_this->error)_this->error=ec_byte_write_done(_this->buf,-l,window,used);
}