FFmpeg
 All Data Structures Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups Pages
rangecoder.c
Go to the documentation of this file.
1 /*
2  * Range coder
3  * Copyright (c) 2004 Michael Niedermayer <michaelni@gmx.at>
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 /**
23  * @file
24  * Range coder.
25  * based upon
26  * "Range encoding: an algorithm for removing redundancy from a digitised
27  * message.
28  * G. N. N. Martin Presented in March 1979 to the Video &
29  * Data Recording Conference,
30  * IBM UK Scientific Center held in Southampton July 24-27 1979."
31  *
32  */
33 
34 #include <string.h>
35 
36 #include "libavutil/attributes.h"
37 #include "libavutil/avassert.h"
38 #include "avcodec.h"
39 #include "rangecoder.h"
40 #include "bytestream.h"
41 
43 {
44  c->bytestream_start =
45  c->bytestream = buf;
46  c->bytestream_end = buf + buf_size;
47  c->low = 0;
48  c->range = 0xFF00;
49  c->outstanding_count = 0;
50  c->outstanding_byte = -1;
51 }
52 
54  int buf_size)
55 {
56  /* cast to avoid compiler warning */
57  ff_init_range_encoder(c, (uint8_t *)buf, buf_size);
58 
59  c->low = bytestream_get_be16((const uint8_t **)&c->bytestream);
60 }
61 
62 void ff_build_rac_states(RangeCoder *c, int factor, int max_p)
63 {
64  const int64_t one = 1LL << 32;
65  int64_t p;
66  int last_p8, p8, i;
67 
68  memset(c->zero_state, 0, sizeof(c->zero_state));
69  memset(c->one_state, 0, sizeof(c->one_state));
70 
71  last_p8 = 0;
72  p = one / 2;
73  for (i = 0; i < 128; i++) {
74  p8 = (256 * p + one / 2) >> 32; // FIXME: try without the one
75  if (p8 <= last_p8)
76  p8 = last_p8 + 1;
77  if (last_p8 && last_p8 < 256 && p8 <= max_p)
78  c->one_state[last_p8] = p8;
79 
80  p += ((one - p) * factor + one / 2) >> 32;
81  last_p8 = p8;
82  }
83 
84  for (i = 256 - max_p; i <= max_p; i++) {
85  if (c->one_state[i])
86  continue;
87 
88  p = (i * one + 128) >> 8;
89  p += ((one - p) * factor + one / 2) >> 32;
90  p8 = (256 * p + one / 2) >> 32; // FIXME: try without the one
91  if (p8 <= i)
92  p8 = i + 1;
93  if (p8 > max_p)
94  p8 = max_p;
95  c->one_state[i] = p8;
96  }
97 
98  for (i = 1; i < 255; i++)
99  c->zero_state[i] = 256 - c->one_state[256 - i];
100 }
101 
102 /* Return the number of bytes written. */
104 {
105  c->range = 0xFF;
106  c->low += 0xFF;
107  renorm_encoder(c);
108  c->range = 0xFF;
109  renorm_encoder(c);
110 
111  av_assert1(c->low == 0);
112  av_assert1(c->range >= 0x100);
113 
114  return c->bytestream - c->bytestream_start;
115 }
116 
117 #ifdef TEST
118 #define SIZE 10240
119 
120 #include "libavutil/lfg.h"
121 #include "libavutil/log.h"
122 
123 static uint8_t b[9 * SIZE];
124 static uint8_t r[9 * SIZE];
125 
126 int main(void)
127 {
128  RangeCoder c;
129  int i;
130  uint8_t state[10];
131  AVLFG prng;
132 
133  av_lfg_init(&prng, 1);
134 
136  ff_build_rac_states(&c, 0.05 * (1LL << 32), 128 + 64 + 32 + 16);
137 
138  memset(state, 128, sizeof(state));
139 
140  for (i = 0; i < SIZE; i++)
141  r[i] = av_lfg_get(&prng) % 7;
142 
143  for (i = 0; i < SIZE; i++)
144  put_rac(&c, state, r[i] & 1);
145 
146  ff_rac_terminate(&c);
147 
148  ff_init_range_decoder(&c, b, SIZE);
149 
150  memset(state, 128, sizeof(state));
151 
152  for (i = 0; i < SIZE; i++)
153  if ((r[i] & 1) != get_rac(&c, state)) {
154  av_log(NULL, AV_LOG_ERROR, "rac failure at %d\n", i);
155  return 1;
156  }
157 
158  return 0;
159 }
160 #endif /* TEST */