1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
|
// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "media/base/bit_reader.h"
#include <algorithm>
namespace media {
BitReader::BitReader(const uint8* data, off_t size)
: data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
DCHECK(data_ != NULL && bytes_left_ > 0);
UpdateCurrByte();
}
BitReader::~BitReader() {}
bool BitReader::SkipBits(int num_bits) {
DCHECK_GE(num_bits, 0);
DLOG_IF(INFO, num_bits > 100)
<< "BitReader::SkipBits inefficient for large skips";
// Skip any bits in the current byte waiting to be processed, then
// process full bytes until less than 8 bits remaining.
while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
num_bits -= num_remaining_bits_in_curr_byte_;
num_remaining_bits_in_curr_byte_ = 0;
UpdateCurrByte();
// If there is no more data remaining, only return true if we
// skipped all that were requested.
if (num_remaining_bits_in_curr_byte_ == 0)
return (num_bits == 0);
}
// Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
// that the remaining bits we need exist, and adjust them as necessary
// for subsequent operations.
uint64 not_needed;
return ReadBitsInternal(num_bits, ¬_needed);
}
int BitReader::bits_available() const {
return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
}
bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
DCHECK_LE(num_bits, 64);
*out = 0;
while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
*out <<= bits_to_take;
*out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
num_bits -= bits_to_take;
num_remaining_bits_in_curr_byte_ -= bits_to_take;
curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
if (num_remaining_bits_in_curr_byte_ == 0)
UpdateCurrByte();
}
return num_bits == 0;
}
void BitReader::UpdateCurrByte() {
DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
if (bytes_left_ == 0)
return;
// Load a new byte and advance pointers.
curr_byte_ = *data_;
++data_;
--bytes_left_;
num_remaining_bits_in_curr_byte_ = 8;
}
} // namespace media
|
