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libavcodec/lagarithrac.h

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00001 /*
00002  * Lagarith range decoder
00003  * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
00004  * Copyright (c) 2009 David Conrad
00005  *
00006  * This file is part of FFmpeg.
00007  *
00008  * FFmpeg is free software; you can redistribute it and/or
00009  * modify it under the terms of the GNU Lesser General Public
00010  * License as published by the Free Software Foundation; either
00011  * version 2.1 of the License, or (at your option) any later version.
00012  *
00013  * FFmpeg is distributed in the hope that it will be useful,
00014  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00015  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00016  * Lesser General Public License for more details.
00017  *
00018  * You should have received a copy of the GNU Lesser General Public
00019  * License along with FFmpeg; if not, write to the Free Software
00020  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
00021  */
00022 
00030 #ifndef AVCODEC_LAGARITHRAC_H
00031 #define AVCODEC_LAGARITHRAC_H
00032 
00033 #include <stdint.h>
00034 #include "libavutil/common.h"
00035 #include "libavutil/intreadwrite.h"
00036 #include "avcodec.h"
00037 #include "get_bits.h"
00038 
00039 typedef struct lag_rac {
00040     AVCodecContext *avctx;
00041     unsigned low;
00042     unsigned range;
00043     unsigned scale;             
00044     unsigned hash_shift;        
00046     const uint8_t *bytestream_start;  
00047     const uint8_t *bytestream;        
00048     const uint8_t *bytestream_end;    
00050     uint32_t prob[258];         
00051     uint8_t  range_hash[256];   
00052 } lag_rac;
00053 
00054 void lag_rac_init(lag_rac *l, GetBitContext *gb, int length);
00055 
00056 /* TODO: Optimize */
00057 static inline void lag_rac_refill(lag_rac *l)
00058 {
00059     while (l->range <= 0x800000) {
00060         l->low   <<= 8;
00061         l->range <<= 8;
00062         l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
00063         if (l->bytestream < l->bytestream_end)
00064             l->bytestream++;
00065     }
00066 }
00067 
00073 static inline uint8_t lag_get_rac(lag_rac *l)
00074 {
00075     unsigned range_scaled, low_scaled, div;
00076     int val;
00077     uint8_t shift;
00078 
00079     lag_rac_refill(l);
00080 
00081     range_scaled = l->range >> l->scale;
00082 
00083     if (l->low < range_scaled * l->prob[255]) {
00084         /* val = 0 is frequent enough to deserve a shortcut */
00085         if (l->low < range_scaled * l->prob[1]) {
00086             val = 0;
00087         } else {
00088             /* FIXME __builtin_clz is ~20% faster here, but not allowed in generic code. */
00089             shift = 30 - av_log2(range_scaled);
00090             div = ((range_scaled << shift) + (1 << 23) - 1) >> 23;
00091             /* low>>24 ensures that any cases too big for exact FASTDIV are
00092              * under- rather than over-estimated
00093              */
00094             low_scaled = FASTDIV(l->low - (l->low >> 24), div);
00095             shift -= l->hash_shift;
00096             shift &= 31;
00097             low_scaled = (low_scaled << shift) | (low_scaled >> (32 - shift));
00098             /* low_scaled is now a lower bound of low/range_scaled */
00099             val = l->range_hash[(uint8_t) low_scaled];
00100             while (l->low >= range_scaled * l->prob[val + 1])
00101                 val++;
00102         }
00103 
00104         l->range = range_scaled * (l->prob[val + 1] - l->prob[val]);
00105     } else {
00106         val = 255;
00107         l->range -= range_scaled * l->prob[255];
00108     }
00109 
00110     l->low -= range_scaled * l->prob[val];
00111 
00112     return val;
00113 }
00114 
00115 
00116 #endif /* AVCODEC_LAGARITHRAC_H */

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