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/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 */
lag_rac::low
unsigned low
Definition: lagarithrac.h:39
lag_rac::scale
unsigned scale
Number of bits of precision in range.
Definition: lagarithrac.h:41
lag_rac::hash_shift
unsigned hash_shift
Number of bits to shift to calculate hash for radix search.
Definition: lagarithrac.h:42
lag_rac::overread
int overread
Definition: lagarithrac.h:48
lag_get_rac
static uint8_t lag_get_rac(lag_rac *l)
Decode a single byte from the compressed plane described by *l.
Definition: lagarithrac.h:76
lag_rac::prob
uint32_t prob[258]
Table of cumulative probability for each symbol.
Definition: lagarithrac.h:51
lag_rac::range_hash
uint8_t range_hash[1024]
Hash table mapping upper byte to approximate symbol.
Definition: lagarithrac.h:52
GetBitContext
Definition: get_bits.h:108
val
static double val(void *priv, double ch)
Definition: aeval.c:78
intreadwrite.h
get_bits.h
lag_rac
Definition: lagarithrac.h:37
ff_lag_rac_init
void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length)
Definition: lagarithrac.c:33
lag_rac::range
unsigned range
Definition: lagarithrac.h:40
lag_rac_refill
static void lag_rac_refill(lag_rac *l)
Definition: lagarithrac.h:58
lag_rac::bytestream_start
const uint8_t * bytestream_start
Start of input bytestream.
Definition: lagarithrac.h:44
lag_rac::bytestream
const uint8_t * bytestream
Current position in input bytestream.
Definition: lagarithrac.h:45
lag_rac::bytestream_end
const uint8_t * bytestream_end
End position of input bytestream.
Definition: lagarithrac.h:46
lag_rac::logctx
void * logctx
Definition: lagarithrac.h:38
AV_RB16
uint64_t_TMPL AV_WL64 unsigned int_TMPL AV_WL32 unsigned int_TMPL AV_WL24 unsigned int_TMPL AV_WL16 uint64_t_TMPL AV_WB64 unsigned int_TMPL AV_WB32 unsigned int_TMPL AV_WB24 unsigned int_TMPL AV_RB16
Definition: bytestream.h:98