media/libvpx/vp9/decoder/vp9_dboolhuff.c@ac0c01689b40
media/libvpx/vp9/decoder/vp9_dboolhuff.c
Thu, 15 Jan 2015 15:59:08 +0100
- author
- Michael Schloh von Bennewitz <michael@schloh.com>
- date
- Thu, 15 Jan 2015 15:59:08 +0100
- branch
- TOR_BUG_9701
- changeset 10
- ac0c01689b40
- permissions
- -rw-r--r--
Implement a real Private Browsing Mode condition by changing the API/ABI;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.
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 */
11 #include "vpx_ports/mem.h"
12 #include "vpx_mem/vpx_mem.h"
14 #include "vp9/decoder/vp9_dboolhuff.h"
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
22 int vp9_reader_init(vp9_reader *r, const uint8_t *buffer, size_t size) {
23 int marker_bit;
25 r->buffer_end = buffer + size;
26 r->buffer = buffer;
27 r->value = 0;
28 r->count = -8;
29 r->range = 255;
31 if (size && !buffer)
32 return 1;
34 vp9_reader_fill(r);
35 marker_bit = vp9_read_bit(r);
36 return marker_bit != 0;
37 }
39 void vp9_reader_fill(vp9_reader *r) {
40 const uint8_t *const buffer_end = r->buffer_end;
41 const uint8_t *buffer = r->buffer;
42 VP9_BD_VALUE value = r->value;
43 int count = r->count;
44 int shift = BD_VALUE_SIZE - 8 - (count + 8);
45 int loop_end = 0;
46 const int bits_left = (int)((buffer_end - buffer)*CHAR_BIT);
47 const int x = shift + CHAR_BIT - bits_left;
49 if (x >= 0) {
50 count += LOTS_OF_BITS;
51 loop_end = x;
52 }
54 if (x < 0 || bits_left) {
55 while (shift >= loop_end) {
56 count += CHAR_BIT;
57 value |= (VP9_BD_VALUE)*buffer++ << shift;
58 shift -= CHAR_BIT;
59 }
60 }
62 r->buffer = buffer;
63 r->value = value;
64 r->count = count;
65 }
67 const uint8_t *vp9_reader_find_end(vp9_reader *r) {
68 // Find the end of the coded buffer
69 while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
70 r->count -= CHAR_BIT;
71 r->buffer--;
72 }
73 return r->buffer;
74 }
76 int vp9_reader_has_error(vp9_reader *r) {
77 // Check if we have reached the end of the buffer.
78 //
79 // Variable 'count' stores the number of bits in the 'value' buffer, minus
80 // 8. The top byte is part of the algorithm, and the remainder is buffered
81 // to be shifted into it. So if count == 8, the top 16 bits of 'value' are
82 // occupied, 8 for the algorithm and 8 in the buffer.
83 //
84 // When reading a byte from the user's buffer, count is filled with 8 and
85 // one byte is filled into the value buffer. When we reach the end of the
86 // data, count is additionally filled with LOTS_OF_BITS. So when
87 // count == LOTS_OF_BITS - 1, the user's data has been exhausted.
88 //
89 // 1 if we have tried to decode bits after the end of stream was encountered.
90 // 0 No error.
91 return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
92 }
