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