1/* Copyright (c) 2001-2011 Timothy B. Terriberry
2   Copyright (c) 2008-2009 Xiph.Org Foundation */
3/*
4   Redistribution and use in source and binary forms, with or without
5   modification, are permitted provided that the following conditions
6   are met:
7
8   - Redistributions of source code must retain the above copyright
9   notice, this list of conditions and the following disclaimer.
10
11   - Redistributions in binary form must reproduce the above copyright
12   notice, this list of conditions and the following disclaimer in the
13   documentation and/or other materials provided with the distribution.
14
15   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16   ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
18   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
19   OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
20   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
21   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
22   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
23   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
24   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
25   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
26*/
27
28#ifdef HAVE_CONFIG_H
29#include "config.h"
30#endif
31
32#include <stddef.h>
33#include "os_support.h"
34#include "arch.h"
35#include "entdec.h"
36#include "mfrngcod.h"
37
38/*A range decoder.
39  This is an entropy decoder based upon \cite{Mar79}, which is itself a
40   rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.
41  It is very similar to arithmetic encoding, except that encoding is done with
42   digits in any base, instead of with bits, and so it is faster when using
43   larger bases (i.e.: a byte).
44  The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$
45   is the base, longer than the theoretical optimum, but to my knowledge there
46   is no published justification for this claim.
47  This only seems true when using near-infinite precision arithmetic so that
48   the process is carried out with no rounding errors.
49
50  An excellent description of implementation details is available at
51   http://www.arturocampos.com/ac_range.html
52  A recent work \cite{MNW98} which proposes several changes to arithmetic
53   encoding for efficiency actually re-discovers many of the principles
54   behind range encoding, and presents a good theoretical analysis of them.
55
56  End of stream is handled by writing out the smallest number of bits that
57   ensures that the stream will be correctly decoded regardless of the value of
58   any subsequent bits.
59  ec_tell() can be used to determine how many bits were needed to decode
60   all the symbols thus far; other data can be packed in the remaining bits of
61   the input buffer.
62  @PHDTHESIS{Pas76,
63    author="Richard Clark Pasco",
64    title="Source coding algorithms for fast data compression",
65    school="Dept. of Electrical Engineering, Stanford University",
66    address="Stanford, CA",
67    month=May,
68    year=1976
69  }
70  @INPROCEEDINGS{Mar79,
71   author="Martin, G.N.N.",
72   title="Range encoding: an algorithm for removing redundancy from a digitised
73    message",
74   booktitle="Video & Data Recording Conference",
75   year=1979,
76   address="Southampton",
77   month=Jul
78  }
79  @ARTICLE{MNW98,
80   author="Alistair Moffat and Radford Neal and Ian H. Witten",
81   title="Arithmetic Coding Revisited",
82   journal="{ACM} Transactions on Information Systems",
83   year=1998,
84   volume=16,
85   number=3,
86   pages="256--294",
87   month=Jul,
88   URL="http://www.stanford.edu/class/ee398a/handouts/papers/Moffat98ArithmCoding.pdf"
89  }*/
90
91static int ec_read_byte(ec_dec *_this){
92  return _this->offs<_this->storage?_this->buf[_this->offs++]:0;
93}
94
95static int ec_read_byte_from_end(ec_dec *_this){
96  return _this->end_offs<_this->storage?
97   _this->buf[_this->storage-++(_this->end_offs)]:0;
98}
99
100/*Normalizes the contents of val and rng so that rng lies entirely in the
101   high-order symbol.*/
102static void ec_dec_normalize(ec_dec *_this){
103  /*If the range is too small, rescale it and input some bits.*/
104  while(_this->rng<=EC_CODE_BOT){
105    int sym;
106    _this->nbits_total+=EC_SYM_BITS;
107    _this->rng<<=EC_SYM_BITS;
108    /*Use up the remaining bits from our last symbol.*/
109    sym=_this->rem;
110    /*Read the next value from the input.*/
111    _this->rem=ec_read_byte(_this);
112    /*Take the rest of the bits we need from this new symbol.*/
113    sym=(sym<<EC_SYM_BITS|_this->rem)>>(EC_SYM_BITS-EC_CODE_EXTRA);
114    /*And subtract them from val, capped to be less than EC_CODE_TOP.*/
115    _this->val=((_this->val<<EC_SYM_BITS)+(EC_SYM_MAX&~sym))&(EC_CODE_TOP-1);
116  }
117}
118
119void ec_dec_init(ec_dec *_this,unsigned char *_buf,opus_uint32 _storage){
120  _this->buf=_buf;
121  _this->storage=_storage;
122  _this->end_offs=0;
123  _this->end_window=0;
124  _this->nend_bits=0;
125  /*This is the offset from which ec_tell() will subtract partial bits.
126    The final value after the ec_dec_normalize() call will be the same as in
127     the encoder, but we have to compensate for the bits that are added there.*/
128  _this->nbits_total=EC_CODE_BITS+1
129   -((EC_CODE_BITS-EC_CODE_EXTRA)/EC_SYM_BITS)*EC_SYM_BITS;
130  _this->offs=0;
131  _this->rng=1U<<EC_CODE_EXTRA;
132  _this->rem=ec_read_byte(_this);
133  _this->val=_this->rng-1-(_this->rem>>(EC_SYM_BITS-EC_CODE_EXTRA));
134  _this->error=0;
135  /*Normalize the interval.*/
136  ec_dec_normalize(_this);
137}
138
139unsigned ec_decode(ec_dec *_this,unsigned _ft){
140  unsigned s;
141  _this->ext=_this->rng/_ft;
142  s=(unsigned)(_this->val/_this->ext);
143  return _ft-EC_MINI(s+1,_ft);
144}
145
146unsigned ec_decode_bin(ec_dec *_this,unsigned _bits){
147   unsigned s;
148   _this->ext=_this->rng>>_bits;
149   s=(unsigned)(_this->val/_this->ext);
150   return (1U<<_bits)-EC_MINI(s+1U,1U<<_bits);
151}
152
153void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,unsigned _ft){
154  opus_uint32 s;
155  s=IMUL32(_this->ext,_ft-_fh);
156  _this->val-=s;
157  _this->rng=_fl>0?IMUL32(_this->ext,_fh-_fl):_this->rng-s;
158  ec_dec_normalize(_this);
159}
160
161/*The probability of having a "one" is 1/(1<<_logp).*/
162int ec_dec_bit_logp(ec_dec *_this,unsigned _logp){
163  opus_uint32 r;
164  opus_uint32 d;
165  opus_uint32 s;
166  int         ret;
167  r=_this->rng;
168  d=_this->val;
169  s=r>>_logp;
170  ret=d<s;
171  if(!ret)_this->val=d-s;
172  _this->rng=ret?s:r-s;
173  ec_dec_normalize(_this);
174  return ret;
175}
176
177int ec_dec_icdf(ec_dec *_this,const unsigned char *_icdf,unsigned _ftb){
178  opus_uint32 r;
179  opus_uint32 d;
180  opus_uint32 s;
181  opus_uint32 t;
182  int         ret;
183  s=_this->rng;
184  d=_this->val;
185  r=s>>_ftb;
186  ret=-1;
187  do{
188    t=s;
189    s=IMUL32(r,_icdf[++ret]);
190  }
191  while(d<s);
192  _this->val=d-s;
193  _this->rng=t-s;
194  ec_dec_normalize(_this);
195  return ret;
196}
197
198opus_uint32 ec_dec_uint(ec_dec *_this,opus_uint32 _ft){
199  unsigned ft;
200  unsigned s;
201  int      ftb;
202  /*In order to optimize EC_ILOG(), it is undefined for the value 0.*/
203  celt_assert(_ft>1);
204  _ft--;
205  ftb=EC_ILOG(_ft);
206  if(ftb>EC_UINT_BITS){
207    opus_uint32 t;
208    ftb-=EC_UINT_BITS;
209    ft=(unsigned)(_ft>>ftb)+1;
210    s=ec_decode(_this,ft);
211    ec_dec_update(_this,s,s+1,ft);
212    t=(opus_uint32)s<<ftb|ec_dec_bits(_this,ftb);
213    if(t<=_ft)return t;
214    _this->error=1;
215    return _ft;
216  }
217  else{
218    _ft++;
219    s=ec_decode(_this,(unsigned)_ft);
220    ec_dec_update(_this,s,s+1,(unsigned)_ft);
221    return s;
222  }
223}
224
225opus_uint32 ec_dec_bits(ec_dec *_this,unsigned _bits){
226  ec_window   window;
227  int         available;
228  opus_uint32 ret;
229  window=_this->end_window;
230  available=_this->nend_bits;
231  if((unsigned)available<_bits){
232    do{
233      window|=(ec_window)ec_read_byte_from_end(_this)<<available;
234      available+=EC_SYM_BITS;
235    }
236    while(available<=EC_WINDOW_SIZE-EC_SYM_BITS);
237  }
238  ret=(opus_uint32)window&(((opus_uint32)1<<_bits)-1U);
239  window>>=_bits;
240  available-=_bits;
241  _this->end_window=window;
242  _this->nend_bits=available;
243  _this->nbits_total+=_bits;
244  return ret;
245}
246