FFmpeg: libavcodec/lagarithrac.h Source File

FFmpeg
lagarithrac.h
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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/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  if (!l->range)
111  l->range = 0x80;
112 
113  l->low -= range_scaled * l->prob[val];
114 
115  return val;
116 }
117 
118 
119 #endif /* AVCODEC_LAGARITHRAC_H */
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