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