1// Copyright 2014 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// This file contains an implementation of an H264 Annex-B video stream parser.
6// Note: ported from Chromium commit head: 77be7ae
7
8#ifndef H264_BIT_READER_H_
9#define H264_BIT_READER_H_
10
11#include <stddef.h>
12#include <stdint.h>
13#include <sys/types.h>
14
15#include "base/macros.h"
16
17namespace media {
18
19// A class to provide bit-granularity reading of H.264 streams.
20// This is not a generic bit reader class, as it takes into account
21// H.264 stream-specific constraints, such as skipping emulation-prevention
22// bytes and stop bits. See spec for more details.
23class H264BitReader {
24 public:
25  H264BitReader();
26  ~H264BitReader();
27
28  // Initialize the reader to start reading at |data|, |size| being size
29  // of |data| in bytes.
30  // Return false on insufficient size of stream..
31  // TODO(posciak,fischman): consider replacing Initialize() with
32  // heap-allocating and creating bit readers on demand instead.
33  bool Initialize(const uint8_t* data, off_t size);
34
35  // Read |num_bits| next bits from stream and return in |*out|, first bit
36  // from the stream starting at |num_bits| position in |*out|.
37  // |num_bits| may be 1-32, inclusive.
38  // Return false if the given number of bits cannot be read (not enough
39  // bits in the stream), true otherwise.
40  bool ReadBits(int num_bits, int* out);
41
42  // Return the number of bits left in the stream.
43  off_t NumBitsLeft();
44
45  // See the definition of more_rbsp_data() in spec.
46  bool HasMoreRBSPData();
47
48  // Return the number of emulation prevention bytes already read.
49  size_t NumEmulationPreventionBytesRead();
50
51 private:
52  // Advance to the next byte, loading it into curr_byte_.
53  // Return false on end of stream.
54  bool UpdateCurrByte();
55
56  // Pointer to the next unread (not in curr_byte_) byte in the stream.
57  const uint8_t* data_;
58
59  // Bytes left in the stream (without the curr_byte_).
60  off_t bytes_left_;
61
62  // Contents of the current byte; first unread bit starting at position
63  // 8 - num_remaining_bits_in_curr_byte_ from MSB.
64  int curr_byte_;
65
66  // Number of bits remaining in curr_byte_
67  int num_remaining_bits_in_curr_byte_;
68
69  // Used in emulation prevention three byte detection (see spec).
70  // Initially set to 0xffff to accept all initial two-byte sequences.
71  int prev_two_bytes_;
72
73  // Number of emulation preventation bytes (0x000003) we met.
74  size_t emulation_prevention_bytes_;
75
76  DISALLOW_COPY_AND_ASSIGN(H264BitReader);
77};
78
79}  // namespace media
80
81#endif  // H264_BIT_READER_H_
82