1 /* Copyright (c) 2001-2011 Timothy B. Terriberry
2 Copyright (c) 2008-2009 Xiph.Org Foundation */
4 Redistribution and use in source and binary forms, with or without
5 modification, are permitted provided that the following conditions
8 - Redistributions of source code must retain the above copyright
9 notice, this list of conditions and the following disclaimer.
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.
15 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16 ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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18 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR
19 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
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25 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
33 #include "os_support.h"
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.
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.
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
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
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",
71 author="Martin, G.N.N.",
72 title="Range encoding: an algorithm for removing redundancy from a digitised
74 booktitle="Video & Data Recording Conference",
76 address="Southampton",
80 author="Alistair Moffat and Radford Neal and Ian H. Witten",
81 title="Arithmetic Coding Revisited",
82 journal="{ACM} Transactions on Information Systems",
88 URL="http://www.stanford.edu/class/ee398/handouts/papers/Moffat98ArithmCoding.pdf"
91 static int ec_read_byte(ec_dec *_this){
92 return _this->offs<_this->storage?_this->buf[_this->offs++]:0;
95 static 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;
100 /*Normalizes the contents of val and rng so that rng lies entirely in the
102 static 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){
106 _this->nbits_total+=EC_SYM_BITS;
107 _this->rng<<=EC_SYM_BITS;
108 /*Use up the remaining bits from our last symbol.*/
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);
119 void ec_dec_init(ec_dec *_this,unsigned char *_buf,opus_uint32 _storage){
121 _this->storage=_storage;
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;
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));
135 /*Normalize the interval.*/
136 ec_dec_normalize(_this);
139 unsigned ec_decode(ec_dec *_this,unsigned _ft){
141 _this->ext=_this->rng/_ft;
142 s=(unsigned)(_this->val/_this->ext);
143 return _ft-EC_MINI(s+1,_ft);
146 unsigned ec_decode_bin(ec_dec *_this,unsigned _bits){
148 _this->ext=_this->rng>>_bits;
149 s=(unsigned)(_this->val/_this->ext);
150 return (1U<<_bits)-EC_MINI(s+1U,1U<<_bits);
153 void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,unsigned _ft){
155 s=IMUL32(_this->ext,_ft-_fh);
157 _this->rng=_fl>0?IMUL32(_this->ext,_fh-_fl):_this->rng-s;
158 ec_dec_normalize(_this);
161 /*The probability of having a "one" is 1/(1<<_logp).*/
162 int ec_dec_bit_logp(ec_dec *_this,unsigned _logp){
171 if(!ret)_this->val=d-s;
172 _this->rng=ret?s:r-s;
173 ec_dec_normalize(_this);
177 int ec_dec_icdf(ec_dec *_this,const unsigned char *_icdf,unsigned _ftb){
189 s=IMUL32(r,_icdf[++ret]);
194 ec_dec_normalize(_this);
198 opus_uint32 ec_dec_uint(ec_dec *_this,opus_uint32 _ft){
202 /*In order to optimize EC_ILOG(), it is undefined for the value 0.*/
206 if(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);
219 s=ec_decode(_this,(unsigned)_ft);
220 ec_dec_update(_this,s,s+1,(unsigned)_ft);
225 opus_uint32 ec_dec_bits(ec_dec *_this,unsigned _bits){
229 window=_this->end_window;
230 available=_this->nend_bits;
231 if((unsigned)available<_bits){
233 window|=(ec_window)ec_read_byte_from_end(_this)<<available;
234 available+=EC_SYM_BITS;
236 while(available<=EC_WINDOW_SIZE-EC_SYM_BITS);
238 ret=(opus_uint32)window&(((opus_uint32)1<<_bits)-1U);
241 _this->end_window=window;
242 _this->nend_bits=available;
243 _this->nbits_total+=_bits;