1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 #include "media/base/bit_reader.h"
6
7 #include <algorithm>
8
9 namespace media {
10
BitReader(const uint8 * data,off_t size)11 BitReader::BitReader(const uint8* data, off_t size)
12 : data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
13 DCHECK(data_ != NULL && bytes_left_ > 0);
14
15 UpdateCurrByte();
16 }
17
~BitReader()18 BitReader::~BitReader() {}
19
SkipBits(int num_bits)20 bool BitReader::SkipBits(int num_bits) {
21 DCHECK_GE(num_bits, 0);
22 DVLOG_IF(0, num_bits > 100)
23 << "BitReader::SkipBits inefficient for large skips";
24
25 // Skip any bits in the current byte waiting to be processed, then
26 // process full bytes until less than 8 bits remaining.
27 while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
28 num_bits -= num_remaining_bits_in_curr_byte_;
29 num_remaining_bits_in_curr_byte_ = 0;
30 UpdateCurrByte();
31
32 // If there is no more data remaining, only return true if we
33 // skipped all that were requested.
34 if (num_remaining_bits_in_curr_byte_ == 0)
35 return (num_bits == 0);
36 }
37
38 // Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
39 // that the remaining bits we need exist, and adjust them as necessary
40 // for subsequent operations.
41 uint64 not_needed;
42 return ReadBitsInternal(num_bits, ¬_needed);
43 }
44
bits_available() const45 int BitReader::bits_available() const {
46 return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
47 }
48
ReadBitsInternal(int num_bits,uint64 * out)49 bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
50 DCHECK_LE(num_bits, 64);
51
52 *out = 0;
53
54 while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
55 int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
56
57 *out <<= bits_to_take;
58 *out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
59 num_bits -= bits_to_take;
60 num_remaining_bits_in_curr_byte_ -= bits_to_take;
61 curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
62
63 if (num_remaining_bits_in_curr_byte_ == 0)
64 UpdateCurrByte();
65 }
66
67 return num_bits == 0;
68 }
69
UpdateCurrByte()70 void BitReader::UpdateCurrByte() {
71 DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
72
73 if (bytes_left_ == 0)
74 return;
75
76 // Load a new byte and advance pointers.
77 curr_byte_ = *data_;
78 ++data_;
79 --bytes_left_;
80 num_remaining_bits_in_curr_byte_ = 8;
81 }
82
83 } // namespace media
84