1/*
2 *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
3 *
4 *  Use of this source code is governed by a BSD-style license
5 *  that can be found in the LICENSE file in the root of the source
6 *  tree. An additional intellectual property rights grant can be found
7 *  in the file PATENTS.  All contributing project authors may
8 *  be found in the AUTHORS file in the root of the source tree.
9 */
10
11#ifndef VPX_DSP_BITREADER_H_
12#define VPX_DSP_BITREADER_H_
13
14#include <stddef.h>
15#include <limits.h>
16
17#include "./vpx_config.h"
18#include "vpx_ports/mem.h"
19#include "vpx/vp8dx.h"
20#include "vpx/vpx_integer.h"
21#include "vpx_dsp/prob.h"
22
23#ifdef __cplusplus
24extern "C" {
25#endif
26
27typedef size_t BD_VALUE;
28
29#define BD_VALUE_SIZE ((int)sizeof(BD_VALUE) * CHAR_BIT)
30
31// This is meant to be a large, positive constant that can still be efficiently
32// loaded as an immediate (on platforms like ARM, for example).
33// Even relatively modest values like 100 would work fine.
34#define LOTS_OF_BITS 0x40000000
35
36typedef struct {
37  // Be careful when reordering this struct, it may impact the cache negatively.
38  BD_VALUE value;
39  unsigned int range;
40  int count;
41  const uint8_t *buffer_end;
42  const uint8_t *buffer;
43  vpx_decrypt_cb decrypt_cb;
44  void *decrypt_state;
45  uint8_t clear_buffer[sizeof(BD_VALUE) + 1];
46} vpx_reader;
47
48int vpx_reader_init(vpx_reader *r,
49                    const uint8_t *buffer,
50                    size_t size,
51                    vpx_decrypt_cb decrypt_cb,
52                    void *decrypt_state);
53
54void vpx_reader_fill(vpx_reader *r);
55
56const uint8_t *vpx_reader_find_end(vpx_reader *r);
57
58static INLINE int vpx_reader_has_error(vpx_reader *r) {
59  // Check if we have reached the end of the buffer.
60  //
61  // Variable 'count' stores the number of bits in the 'value' buffer, minus
62  // 8. The top byte is part of the algorithm, and the remainder is buffered
63  // to be shifted into it. So if count == 8, the top 16 bits of 'value' are
64  // occupied, 8 for the algorithm and 8 in the buffer.
65  //
66  // When reading a byte from the user's buffer, count is filled with 8 and
67  // one byte is filled into the value buffer. When we reach the end of the
68  // data, count is additionally filled with LOTS_OF_BITS. So when
69  // count == LOTS_OF_BITS - 1, the user's data has been exhausted.
70  //
71  // 1 if we have tried to decode bits after the end of stream was encountered.
72  // 0 No error.
73  return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
74}
75
76static INLINE int vpx_read(vpx_reader *r, int prob) {
77  unsigned int bit = 0;
78  BD_VALUE value;
79  BD_VALUE bigsplit;
80  int count;
81  unsigned int range;
82  unsigned int split = (r->range * prob + (256 - prob)) >> CHAR_BIT;
83
84  if (r->count < 0)
85    vpx_reader_fill(r);
86
87  value = r->value;
88  count = r->count;
89
90  bigsplit = (BD_VALUE)split << (BD_VALUE_SIZE - CHAR_BIT);
91
92  range = split;
93
94  if (value >= bigsplit) {
95    range = r->range - split;
96    value = value - bigsplit;
97    bit = 1;
98  }
99
100  {
101    register unsigned int shift = vpx_norm[range];
102    range <<= shift;
103    value <<= shift;
104    count -= shift;
105  }
106  r->value = value;
107  r->count = count;
108  r->range = range;
109
110  return bit;
111}
112
113static INLINE int vpx_read_bit(vpx_reader *r) {
114  return vpx_read(r, 128);  // vpx_prob_half
115}
116
117static INLINE int vpx_read_literal(vpx_reader *r, int bits) {
118  int literal = 0, bit;
119
120  for (bit = bits - 1; bit >= 0; bit--)
121    literal |= vpx_read_bit(r) << bit;
122
123  return literal;
124}
125
126static INLINE int vpx_read_tree(vpx_reader *r, const vpx_tree_index *tree,
127                                const vpx_prob *probs) {
128  vpx_tree_index i = 0;
129
130  while ((i = tree[i + vpx_read(r, probs[i >> 1])]) > 0)
131    continue;
132
133  return -i;
134}
135
136#ifdef __cplusplus
137}  // extern "C"
138#endif
139
140#endif  // VPX_DSP_BITREADER_H_
141