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 Libav.
   7  *
   8  * Libav 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  * Libav 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 Libav; 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/common.h"
  35 #include "libavutil/intreadwrite.h"
  36 #include "avcodec.h"
  37 #include "get_bits.h"
  38
  39 typedef struct lag_rac {
  40     AVCodecContext *avctx;
  41     unsigned low;
  42     unsigned range;
  43     unsigned scale;             /**< Number of bits of precision in range. */
  44     unsigned hash_shift;        /**< Number of bits to shift to calculate hash for radix search. */
  45
  46     const uint8_t *bytestream_start;  /**< Start of input bytestream. */
  47     const uint8_t *bytestream;        /**< Current position in input bytestream. */
  48     const uint8_t *bytestream_end;    /**< End position of input bytestream. */
  49
  50     uint32_t prob[258];         /**< Table of cumulative probability for each symbol. */
  51     uint8_t  range_hash[256];   /**< Hash table mapping upper byte to approximate symbol. */
  52 } lag_rac;
  53
  54 void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length);
  55
  56 /* TODO: Optimize */
  57 static inline void lag_rac_refill(lag_rac *l)
  58 {
  59     while (l->range <= 0x800000) {
  60         l->low   <<= 8;
  61         l->range <<= 8;
  62         l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
  63         if (l->bytestream < l->bytestream_end)
  64             l->bytestream++;
  65     }
  66 }
  67
  68 /**
  69  * Decode a single byte from the compressed plane described by *l.
  70  * @param l pointer to lag_rac for the current plane
  71  * @return next byte of decoded data
  72  */
  73 static inline uint8_t lag_get_rac(lag_rac *l)
  74 {
  75     unsigned range_scaled, low_scaled, div;
  76     int val;
  77     uint8_t shift;
  78
  79     lag_rac_refill(l);
  80
  81     range_scaled = l->range >> l->scale;
  82
  83     if (l->low < range_scaled * l->prob[255]) {
  84         /* val = 0 is frequent enough to deserve a shortcut */
  85         if (l->low < range_scaled * l->prob[1]) {
  86             val = 0;
  87         } else {
  88             /* FIXME __builtin_clz is ~20% faster here, but not allowed in generic code. */
  89             shift = 30 - av_log2(range_scaled);
  90             div = ((range_scaled << shift) + (1 << 23) - 1) >> 23;
  91             /* low>>24 ensures that any cases too big for exact FASTDIV are
  92              * under- rather than over-estimated
  93              */
  94             low_scaled = FASTDIV(l->low - (l->low >> 24), div);
  95             shift -= l->hash_shift;
  96             shift &= 31;
  97             low_scaled = (low_scaled << shift) | (low_scaled >> (32 - shift));
  98             /* low_scaled is now a lower bound of low/range_scaled */
  99             val = l->range_hash[(uint8_t) low_scaled];
 100             while (l->low >= range_scaled * l->prob[val + 1])
 101                 val++;
 102         }
 103
 104         l->range = range_scaled * (l->prob[val + 1] - l->prob[val]);
 105     } else {
 106         val = 255;
 107         l->range -= range_scaled * l->prob[255];
 108     }
 109
 110     l->low -= range_scaled * l->prob[val];
 111
 112     return val;
 113 }
 114
 115
 116 #endif /* AVCODEC_LAGARITHRAC_H */