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,vpx_decrypt_cb decrypt_cb,void * decrypt_state)21 int vp9_reader_init(vp9_reader *r,
22 const uint8_t *buffer,
23 size_t size,
24 vpx_decrypt_cb decrypt_cb,
25 void *decrypt_state) {
26 if (size && !buffer) {
27 return 1;
28 } else {
29 r->buffer_end = buffer + size;
30 r->buffer = buffer;
31 r->value = 0;
32 r->count = -8;
33 r->range = 255;
34 r->decrypt_cb = decrypt_cb;
35 r->decrypt_state = decrypt_state;
36 vp9_reader_fill(r);
37 return vp9_read_bit(r) != 0; // marker bit
38 }
39 }
40
vp9_reader_fill(vp9_reader * r)41 void vp9_reader_fill(vp9_reader *r) {
42 const uint8_t *const buffer_end = r->buffer_end;
43 const uint8_t *buffer = r->buffer;
44 const uint8_t *buffer_start = buffer;
45 BD_VALUE value = r->value;
46 int count = r->count;
47 int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT);
48 int loop_end = 0;
49 const size_t bytes_left = buffer_end - buffer;
50 const size_t bits_left = bytes_left * CHAR_BIT;
51 const int x = (int)(shift + CHAR_BIT - bits_left);
52
53 if (r->decrypt_cb) {
54 size_t n = MIN(sizeof(r->clear_buffer), bytes_left);
55 r->decrypt_cb(r->decrypt_state, buffer, r->clear_buffer, (int)n);
56 buffer = r->clear_buffer;
57 buffer_start = r->clear_buffer;
58 }
59
60 if (x >= 0) {
61 count += LOTS_OF_BITS;
62 loop_end = x;
63 }
64
65 if (x < 0 || bits_left) {
66 while (shift >= loop_end) {
67 count += CHAR_BIT;
68 value |= (BD_VALUE)*buffer++ << shift;
69 shift -= CHAR_BIT;
70 }
71 }
72
73 // NOTE: Variable 'buffer' may not relate to 'r->buffer' after decryption,
74 // so we increase 'r->buffer' by the amount that 'buffer' moved, rather than
75 // assign 'buffer' to 'r->buffer'.
76 r->buffer += buffer - buffer_start;
77 r->value = value;
78 r->count = count;
79 }
80
vp9_reader_find_end(vp9_reader * r)81 const uint8_t *vp9_reader_find_end(vp9_reader *r) {
82 // Find the end of the coded buffer
83 while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
84 r->count -= CHAR_BIT;
85 r->buffer--;
86 }
87 return r->buffer;
88 }
89
vp9_reader_has_error(vp9_reader * r)90 int vp9_reader_has_error(vp9_reader *r) {
91 // Check if we have reached the end of the buffer.
92 //
93 // Variable 'count' stores the number of bits in the 'value' buffer, minus
94 // 8. The top byte is part of the algorithm, and the remainder is buffered
95 // to be shifted into it. So if count == 8, the top 16 bits of 'value' are
96 // occupied, 8 for the algorithm and 8 in the buffer.
97 //
98 // When reading a byte from the user's buffer, count is filled with 8 and
99 // one byte is filled into the value buffer. When we reach the end of the
100 // data, count is additionally filled with LOTS_OF_BITS. So when
101 // count == LOTS_OF_BITS - 1, the user's data has been exhausted.
102 //
103 // 1 if we have tried to decode bits after the end of stream was encountered.
104 // 0 No error.
105 return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
106 }
107