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
vp9_reader_init(vp9_reader * r,const uint8_t * buffer,size_t size)21 int 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
vp9_reader_fill(vp9_reader * r)35 void 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
vp9_reader_find_end(vp9_reader * r)63 const 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
vp9_reader_has_error(vp9_reader * r)72 int 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