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- // Copyright 2014 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #include "media/video/h264_bit_reader.h"
- #include "base/check.h"
- namespace media {
- H264BitReader::H264BitReader()
- : data_(nullptr),
- bytes_left_(0),
- curr_byte_(0),
- num_remaining_bits_in_curr_byte_(0),
- prev_two_bytes_(0),
- emulation_prevention_bytes_(0) {}
- H264BitReader::~H264BitReader() = default;
- bool H264BitReader::Initialize(const uint8_t* data, off_t size) {
- DCHECK(data);
- if (size < 1)
- return false;
- data_ = data;
- bytes_left_ = size;
- num_remaining_bits_in_curr_byte_ = 0;
- // Initially set to 0xffff to accept all initial two-byte sequences.
- prev_two_bytes_ = 0xffff;
- emulation_prevention_bytes_ = 0;
- return true;
- }
- bool H264BitReader::UpdateCurrByte() {
- if (bytes_left_ < 1)
- return false;
- // Emulation prevention three-byte detection.
- // If a sequence of 0x000003 is found, skip (ignore) the last byte (0x03).
- if (*data_ == 0x03 && (prev_two_bytes_ & 0xffff) == 0) {
- // Detected 0x000003, skip last byte.
- ++data_;
- --bytes_left_;
- ++emulation_prevention_bytes_;
- // Need another full three bytes before we can detect the sequence again.
- prev_two_bytes_ = 0xffff;
- if (bytes_left_ < 1)
- return false;
- }
- // Load a new byte and advance pointers.
- curr_byte_ = *data_++ & 0xff;
- --bytes_left_;
- num_remaining_bits_in_curr_byte_ = 8;
- prev_two_bytes_ = ((prev_two_bytes_ & 0xff) << 8) | curr_byte_;
- return true;
- }
- // Read |num_bits| (1 to 31 inclusive) from the stream and return them
- // in |out|, with first bit in the stream as MSB in |out| at position
- // (|num_bits| - 1).
- bool H264BitReader::ReadBits(int num_bits, int* out) {
- int bits_left = num_bits;
- *out = 0;
- DCHECK(num_bits <= 31);
- while (num_remaining_bits_in_curr_byte_ < bits_left) {
- // Take all that's left in current byte, shift to make space for the rest.
- *out |= (curr_byte_ << (bits_left - num_remaining_bits_in_curr_byte_));
- bits_left -= num_remaining_bits_in_curr_byte_;
- if (!UpdateCurrByte())
- return false;
- }
- *out |= (curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_left));
- *out &= ((1u << num_bits) - 1u);
- num_remaining_bits_in_curr_byte_ -= bits_left;
- return true;
- }
- off_t H264BitReader::NumBitsLeft() {
- return (num_remaining_bits_in_curr_byte_ + bytes_left_ * 8);
- }
- bool H264BitReader::HasMoreRBSPData() {
- // Make sure we have more bits, if we are at 0 bits in current byte and
- // updating current byte fails, we don't have more data anyway.
- if (num_remaining_bits_in_curr_byte_ == 0 && !UpdateCurrByte())
- return false;
- // If there is no more RBSP data, then |curr_byte_| contains the stop bit and
- // zero padding. Check to see if there is other data instead.
- // (We don't actually check for the stop bit itself, instead treating the
- // invalid case of all trailing zeros identically).
- if ((curr_byte_ & ((1 << (num_remaining_bits_in_curr_byte_ - 1)) - 1)) != 0)
- return true;
- // While the spec disallows it (7.4.1: "The last byte of the NAL unit shall
- // not be equal to 0x00"), some streams have trailing null bytes anyway. We
- // don't handle emulation prevention sequences because HasMoreRBSPData() is
- // not used when parsing slices (where cabac_zero_word elements are legal).
- for (off_t i = 0; i < bytes_left_; i++) {
- if (data_[i] != 0)
- return true;
- }
- bytes_left_ = 0;
- return false;
- }
- size_t H264BitReader::NumEmulationPreventionBytesRead() {
- return emulation_prevention_bytes_;
- }
- } // namespace media
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