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      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
    114