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#include "vpx_ports/mem.h"
12#include "vpx_mem/vpx_mem.h"
13
14#include "vp9/decoder/vp9_reader.h"
15
16// This is meant to be a large, positive constant that can still be efficiently
17// loaded as an immediate (on platforms like ARM, for example).
18// Even relatively modest values like 100 would work fine.
19#define LOTS_OF_BITS 0x40000000
20
21int vp9_reader_init(vp9_reader *r, const uint8_t *buffer, size_t size) {
22  if (size && !buffer) {
23    return 1;
24  } else {
25    r->buffer_end = buffer + size;
26    r->buffer = buffer;
27    r->value = 0;
28    r->count = -8;
29    r->range = 255;
30    vp9_reader_fill(r);
31    return vp9_read_bit(r) != 0;  // marker bit
32  }
33}
34
35void vp9_reader_fill(vp9_reader *r) {
36  const uint8_t *const buffer_end = r->buffer_end;
37  const uint8_t *buffer = r->buffer;
38  BD_VALUE value = r->value;
39  int count = r->count;
40  int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT);
41  int loop_end = 0;
42  const int bits_left = (int)((buffer_end - buffer) * CHAR_BIT);
43  const int x = shift + CHAR_BIT - bits_left;
44
45  if (x >= 0) {
46    count += LOTS_OF_BITS;
47    loop_end = x;
48  }
49
50  if (x < 0 || bits_left) {
51    while (shift >= loop_end) {
52      count += CHAR_BIT;
53      value |= (BD_VALUE)*buffer++ << shift;
54      shift -= CHAR_BIT;
55    }
56  }
57
58  r->buffer = buffer;
59  r->value = value;
60  r->count = count;
61}
62
63const uint8_t *vp9_reader_find_end(vp9_reader *r) {
64  // Find the end of the coded buffer
65  while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
66    r->count -= CHAR_BIT;
67    r->buffer--;
68  }
69  return r->buffer;
70}
71
72int vp9_reader_has_error(vp9_reader *r) {
73  // Check if we have reached the end of the buffer.
74  //
75  // Variable 'count' stores the number of bits in the 'value' buffer, minus
76  // 8. The top byte is part of the algorithm, and the remainder is buffered
77  // to be shifted into it. So if count == 8, the top 16 bits of 'value' are
78  // occupied, 8 for the algorithm and 8 in the buffer.
79  //
80  // When reading a byte from the user's buffer, count is filled with 8 and
81  // one byte is filled into the value buffer. When we reach the end of the
82  // data, count is additionally filled with LOTS_OF_BITS. So when
83  // count == LOTS_OF_BITS - 1, the user's data has been exhausted.
84  //
85  // 1 if we have tried to decode bits after the end of stream was encountered.
86  // 0 No error.
87  return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
88}
89