libavcodec/lagarithrac.h

   1 /*
   2  * Lagarith range decoder
   3  * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
   4  * Copyright (c) 2009 David Conrad
   5  *
   6  * This file is part of FFmpeg.
   7  *
   8  * FFmpeg is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * FFmpeg is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with FFmpeg; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 /**
  24  * @file
  25  * Lagarith range decoder
  26  * @author Nathan Caldwell
  27  * @author David Conrad
  28  */
  29
  30 #ifndef AVCODEC_LAGARITHRAC_H
  31 #define AVCODEC_LAGARITHRAC_H
  32
  33 #include <stdint.h>
  34 #include "libavutil/intreadwrite.h"
  35 #include "get_bits.h"
  36
  37 typedef struct lag_rac {
  38     void    *logctx;
  39     unsigned low;
  40     unsigned range;
  41     unsigned scale;             /**< Number of bits of precision in range. */
  42     unsigned hash_shift;        /**< Number of bits to shift to calculate hash for radix search. */
  43
  44     const uint8_t *bytestream_start;  /**< Start of input bytestream. */
  45     const uint8_t *bytestream;        /**< Current position in input bytestream. */
  46     const uint8_t *bytestream_end;    /**< End position of input bytestream. */
  47
  48     int overread;
  49 #define MAX_OVERREAD 4
  50
  51     uint32_t prob[258];         /**< Table of cumulative probability for each symbol. */
  52     uint8_t  range_hash[1024];   /**< Hash table mapping upper byte to approximate symbol. */
  53 } lag_rac;
  54
  55 void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length);
  56
  57 /* TODO: Optimize */
  58 static inline void lag_rac_refill(lag_rac *l)
  59 {
  60     while (l->range <= 0x800000) {
  61         l->low   <<= 8;
  62         l->range <<= 8;
  63         l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
  64         if (l->bytestream < l->bytestream_end)
  65             l->bytestream++;
  66         else
  67             l->overread++;
  68     }
  69 }
  70
  71 /**
  72  * Decode a single byte from the compressed plane described by *l.
  73  * @param l pointer to lag_rac for the current plane
  74  * @return next byte of decoded data
  75  */
  76 static inline uint8_t lag_get_rac(lag_rac *l)
  77 {
  78     unsigned range_scaled, low_scaled;
  79     int val;
  80
  81     lag_rac_refill(l);
  82
  83     range_scaled = l->range >> l->scale;
  84
  85     if (l->low < range_scaled * l->prob[255]) {
  86         /* val = 0 is frequent enough to deserve a shortcut */
  87         if (l->low < range_scaled * l->prob[1]) {
  88             val = 0;
  89         } else {
  90             low_scaled = l->low / (range_scaled<<(l->hash_shift));
  91
  92             val = l->range_hash[low_scaled];
  93             while (l->low >= range_scaled * l->prob[val + 1])
  94                 val++;
  95         }
  96
  97         l->range = range_scaled * (l->prob[val + 1] - l->prob[val]);
  98     } else {
  99         val = 255;
 100         l->range -= range_scaled * l->prob[255];
 101     }
 102
 103     if (!l->range)
 104         l->range = 0x80;
 105
 106     l->low -= range_scaled * l->prob[val];
 107
 108     return val;
 109 }
 110
 111
 112 #endif /* AVCODEC_LAGARITHRAC_H */