entdec.c (8374B)
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 91 static int ec_read_byte(ec_dec *_this){ 92 return _this->offs<_this->storage?_this->buf[_this->offs++]:0; 93 } 94 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; 98 } 99 100 /*Normalizes the contents of val and rng so that rng lies entirely in the 101 high-order symbol.*/ 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){ 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 } 118 119 void 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 } 138 139 unsigned ec_decode(ec_dec *_this,unsigned _ft){ 140 unsigned s; 141 _this->ext=celt_udiv(_this->rng,_ft); 142 s=(unsigned)(_this->val/_this->ext); 143 return _ft-EC_MINI(s+1,_ft); 144 } 145 146 unsigned 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 } 152 153 void 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).*/ 162 int 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 } 176 177 int 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 } 197 198 int ec_dec_icdf16(ec_dec *_this,const opus_uint16 *_icdf,unsigned _ftb){ 199 opus_uint32 r; 200 opus_uint32 d; 201 opus_uint32 s; 202 opus_uint32 t; 203 int ret; 204 s=_this->rng; 205 d=_this->val; 206 r=s>>_ftb; 207 ret=-1; 208 do{ 209 t=s; 210 s=IMUL32(r,_icdf[++ret]); 211 } 212 while(d<s); 213 _this->val=d-s; 214 _this->rng=t-s; 215 ec_dec_normalize(_this); 216 return ret; 217 } 218 219 opus_uint32 ec_dec_uint(ec_dec *_this,opus_uint32 _ft){ 220 unsigned ft; 221 unsigned s; 222 int ftb; 223 /*In order to optimize EC_ILOG(), it is undefined for the value 0.*/ 224 celt_assert(_ft>1); 225 _ft--; 226 ftb=EC_ILOG(_ft); 227 if(ftb>EC_UINT_BITS){ 228 opus_uint32 t; 229 ftb-=EC_UINT_BITS; 230 ft=(unsigned)(_ft>>ftb)+1; 231 s=ec_decode(_this,ft); 232 ec_dec_update(_this,s,s+1,ft); 233 t=(opus_uint32)s<<ftb|ec_dec_bits(_this,ftb); 234 if(t<=_ft)return t; 235 _this->error=1; 236 return _ft; 237 } 238 else{ 239 _ft++; 240 s=ec_decode(_this,(unsigned)_ft); 241 ec_dec_update(_this,s,s+1,(unsigned)_ft); 242 return s; 243 } 244 } 245 246 opus_uint32 ec_dec_bits(ec_dec *_this,unsigned _bits){ 247 ec_window window; 248 int available; 249 opus_uint32 ret; 250 window=_this->end_window; 251 available=_this->nend_bits; 252 if((unsigned)available<_bits){ 253 do{ 254 window|=(ec_window)ec_read_byte_from_end(_this)<<available; 255 available+=EC_SYM_BITS; 256 } 257 while(available<=EC_WINDOW_SIZE-EC_SYM_BITS); 258 } 259 ret=(opus_uint32)window&(((opus_uint32)1<<_bits)-1U); 260 window>>=_bits; 261 available-=_bits; 262 _this->end_window=window; 263 _this->nend_bits=available; 264 _this->nbits_total+=_bits; 265 return ret; 266 }