#ifdef HAVE_CONFIG_H

#include "config.h"

#endif

#include "arch.h"

#include "entdec.h"

#include "mfrngcod.h"

/*A range decoder.

  This is an entropy decoder based upon \cite{Mar79}, which is itself a

   rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.

  It is very similar to arithmetic encoding, except that encoding is done with

   digits in any base, instead of with bits, and so it is faster when using

   larger bases (i.e.: a byte).

  The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$

   is the base, longer than the theoretical optimum, but to my knowledge there

   is no published justification for this claim.

  This only seems true when using near-infinite precision arithmetic so that

   the process is carried out with no rounding errors.

  IBM (the author's employer) never sought to patent the idea, and to my

   knowledge the algorithm is unencumbered by any patents, though its

   performance is very competitive with proprietary arithmetic coding.

  The two are based on very similar ideas, however.

  An excellent description of implementation details is available at

   http://www.arturocampos.com/ac_range.html

  A recent work \cite{MNW98} which proposes several changes to arithmetic

   encoding for efficiency actually re-discovers many of the principles

   behind range encoding, and presents a good theoretical analysis of them.

  This coder handles the end of the stream in a slightly more graceful fashion

   than most arithmetic or range coders.

  Once the final symbol has been encoded, the coder selects the code word with

   the shortest number of bits that still falls within the final interval.

  This method is not novel.

  Here, by the length of the code word, we refer to the number of bits until

   its final 1.

  Any trailing zeros may be discarded, since the encoder, once it runs out of

   input, will pad its buffer with zeros.

  But this means that no encoded stream would ever have any zero bytes at the

   end.

  Since there are some coded representations we cannot produce, it implies that

   there is still some redundancy in the stream.

  In this case, we can pick a special byte value, RSV1, and should the stream

   end in a sequence of zeros, followed by the RSV1 byte, we can code the

   zeros, and discard the RSV1 byte.

  The decoder, knowing that the encoder would never produce a sequence of zeros

   at the end, would then know to add in the RSV1 byte if it observed it.

  Now, the encoder would never produce a stream that ended in a sequence of

   zeros followed by a RSV1 byte.

  So, if the stream ends in a non-empty sequence of zeros, followed by any

   positive number of RSV1 bytes, the last RSV1 byte is discarded.

  The decoder, if it encounters a stream that ends in non-empty sequence of

   zeros followed by any non-negative number of RSV1 bytes, adds an additional

   RSV1 byte to the stream.

  With this strategy, every possible sequence of input bytes is transformed to

   one that could actually be produced by the encoder.

  The only question is what non-zero value to use for RSV1.

  We select 0x80, since it has the nice property of producing the shortest

   possible byte streams when using our strategy for selecting a number within

   the final interval to encode.

  Clearly if the shortest possible code word that falls within the interval has

   its last one bit as the most significant bit of the final byte, and the

   previous bytes were a non-empty sequence of zeros followed by a non-negative

   number of 0x80 bytes, then the last byte would be discarded.

  If the shortest code word is not so formed, then no other code word in the

   interval would result in any more bytes being discarded.

  Any longer code word would have an additional one bit somewhere, and so would

   require at a minimum that that byte would be coded.

  If the shortest code word has a 1 before the final one that is preventing the

   stream from ending in a non-empty sequence of zeros followed by a

   non-negative number of 0x80's, then there is no code word of the same length

   which contains that bit as a zero.

  If there were, then we could simply leave that bit a 1, and drop all the bits

   after it without leaving the interval, thus producing a shorter code word.

  In this case, RSV1 can only drop 1 bit off the final stream.

  Other choices could lead to savings of up to 8 bits for particular streams,

   but this would produce the odd situation that a stream with more non-zero

   bits is actually encoded in fewer bytes.

  @PHDTHESIS{Pas76,

    author="Richard Clark Pasco",

    title="Source coding algorithms for fast data compression",

    school="Dept. of Electrical Engineering, Stanford University",

    address="Stanford, CA",

    month=May,

    year=1976

  }

  @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"

  }*/

/*Gets the next byte of input.

  After all the bytes in the current packet have been consumed, and the extra

   end code returned if needed, this function will continue to return zero each

   time it is called.

  Return: The next byte of input.*/

static int ec_dec_in(ec_dec *_this){

  int ret;

  ret=ec_byte_read1(_this->buf);

  if(ret<0){

    ret=0;

    /*Needed to keep oc_dec_tell() operating correctly.*/

    ec_byte_adv1(_this->buf);

  }

  return ret;

}

/*Normalizes the contents of dif and rng so that rng lies entirely in the

   high-order symbol.*/

static inline void ec_dec_normalize(ec_dec *_this){

  /*If the range is too small, rescale it and input some bits.*/

  while(_this->rng<=EC_CODE_BOT){

    int sym;

    _this->rng<<=EC_SYM_BITS;

    /*Use up the remaining bits from our last symbol.*/

    sym=_this->rem<

    /*Read the next value from the input.*/

    _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<

    /*dif can never be larger 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;

  }

}

void ec_dec_init(ec_dec *_this,ec_byte_buffer *_buf){

  _this->buf=_buf;

  _this->rem=ec_dec_in(_this);

  _this->rng=1U<

  _this->dif=_this->rng-(_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA);

  /*Normalize the interval.*/

  ec_dec_normalize(_this);

}

unsigned ec_decode(ec_dec *_this,unsigned _ft){

  unsigned s;

  _this->nrm=_this->rng/_ft;

  s=(unsigned)((_this->dif-1)/_this->nrm);

  return _ft-EC_MINI(s+1,_ft);

}

unsigned ec_decode_bin(ec_dec *_this,unsigned bits){

   unsigned s;

   ec_uint32 ft;

   ft = (ec_uint32)1<

   _this->nrm=_this->rng>>bits;

   s=(unsigned)((_this->dif-1)/_this->nrm);

   return ft-EC_MINI(s+1,ft);

}

void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,unsigned _ft){

  ec_uint32 s;

  s=IMUL32(_this->nrm,(_ft-_fh));

  _this->dif-=s;

  _this->rng=_fl>0?IMUL32(_this->nrm,(_fh-_fl)):_this->rng-s;

  ec_dec_normalize(_this);

}

long ec_dec_tell(ec_dec *_this,int _b){

  ec_uint32 r;

  int       l;

  long      nbits;

  nbits=(ec_byte_bytes(_this->buf)-(EC_CODE_BITS+EC_SYM_BITS-1)/EC_SYM_BITS)*

   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;

  nbits<<=_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;

}

#if 0

int ec_dec_done(ec_dec *_this){

  unsigned low;

  int      ret;

  /*Check to make sure we've used all the input bytes.

    This ensures that no more ones would ever be inserted into the decoder.*/

  if(_this->buf->ptr-ec_byte_get_buffer(_this->buf)<=

   ec_byte_bytes(_this->buf)){

    return 0;

  }

  /*We compute the smallest finitely odd fraction that fits inside the current

     range, and write that to the stream.

    This is guaranteed to yield the smallest possible encoding.*/

  /*TODO: Fix this line, as it is wrong.

    It doesn't seem worth being able to make this check to do an extra

     subtraction for every symbol decoded.*/

  low=/*What we want: _this->top-_this->rng; What we have:*/_this->dif

  if(low){

    unsigned end;

    end=EC_CODE_TOP;

    /*Ensure that the next free end is in the range.*/

    if(end-low>=_this->rng){

      unsigned msk;

      msk=EC_CODE_TOP-1;

      do{

        msk>>=1;

        end=(low+msk)&~msk|msk+1;

      }

      while(end-low>=_this->rng);

    }

    /*The remaining input should have been the next free end.*/

    return end-low!=_this->dif;

  }

  return 1;

}

#endif