media/filters/h264_bit_reader.cc

   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 #include "base/logging.h"
   6 #include "media/filters/h264_bit_reader.h"
   7
   8 namespace media {
   9
  10 H264BitReader::H264BitReader()
  11     : data_(NULL),
  12       bytes_left_(0),
  13       curr_byte_(0),
  14       num_remaining_bits_in_curr_byte_(0),
  15       prev_two_bytes_(0),
  16       emulation_prevention_bytes_(0) {}
  17
  18 H264BitReader::~H264BitReader() {}
  19
  20 bool H264BitReader::Initialize(const uint8* data, off_t size) {
  21   DCHECK(data);
  22
  23   if (size < 1)
  24     return false;
  25
  26   data_ = data;
  27   bytes_left_ = size;
  28   num_remaining_bits_in_curr_byte_ = 0;
  29   // Initially set to 0xffff to accept all initial two-byte sequences.
  30   prev_two_bytes_ = 0xffff;
  31   emulation_prevention_bytes_ = 0;
  32
  33   return true;
  34 }
  35
  36 bool H264BitReader::UpdateCurrByte() {
  37   if (bytes_left_ < 1)
  38     return false;
  39
  40   // Emulation prevention three-byte detection.
  41   // If a sequence of 0x000003 is found, skip (ignore) the last byte (0x03).
  42   if (*data_ == 0x03 && (prev_two_bytes_ & 0xffff) == 0) {
  43     // Detected 0x000003, skip last byte.
  44     ++data_;
  45     --bytes_left_;
  46     ++emulation_prevention_bytes_;
  47     // Need another full three bytes before we can detect the sequence again.
  48     prev_two_bytes_ = 0xffff;
  49
  50     if (bytes_left_ < 1)
  51       return false;
  52   }
  53
  54   // Load a new byte and advance pointers.
  55   curr_byte_ = *data_++ & 0xff;
  56   --bytes_left_;
  57   num_remaining_bits_in_curr_byte_ = 8;
  58
  59   prev_two_bytes_ = (prev_two_bytes_ << 8) | curr_byte_;
  60
  61   return true;
  62 }
  63
  64 // Read |num_bits| (1 to 31 inclusive) from the stream and return them
  65 // in |out|, with first bit in the stream as MSB in |out| at position
  66 // (|num_bits| - 1).
  67 bool H264BitReader::ReadBits(int num_bits, int* out) {
  68   int bits_left = num_bits;
  69   *out = 0;
  70   DCHECK(num_bits <= 31);
  71
  72   while (num_remaining_bits_in_curr_byte_ < bits_left) {
  73     // Take all that's left in current byte, shift to make space for the rest.
  74     *out |= (curr_byte_ << (bits_left - num_remaining_bits_in_curr_byte_));
  75     bits_left -= num_remaining_bits_in_curr_byte_;
  76
  77     if (!UpdateCurrByte())
  78       return false;
  79   }
  80
  81   *out |= (curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_left));
  82   *out &= ((1 << num_bits) - 1);
  83   num_remaining_bits_in_curr_byte_ -= bits_left;
  84
  85   return true;
  86 }
  87
  88 off_t H264BitReader::NumBitsLeft() {
  89   return (num_remaining_bits_in_curr_byte_ + bytes_left_ * 8);
  90 }
  91
  92 bool H264BitReader::HasMoreRBSPData() {
  93   // Make sure we have more bits, if we are at 0 bits in current byte
  94   // and updating current byte fails, we don't have more data anyway.
  95   if (num_remaining_bits_in_curr_byte_ == 0 && !UpdateCurrByte())
  96     return false;
  97
  98   // On last byte?
  99   if (bytes_left_)
 100     return true;
 101
 102   // Last byte, look for stop bit;
 103   // We have more RBSP data if the last non-zero bit we find is not the
 104   // first available bit.
 105   return (curr_byte_ &
 106           ((1 << (num_remaining_bits_in_curr_byte_ - 1)) - 1)) != 0;
 107 }
 108
 109 size_t H264BitReader::NumEmulationPreventionBytesRead() {
 110   return emulation_prevention_bytes_;
 111 }
 112
 113 }  // namespace media