blob: 45c3d75c00c53c1b0b459b12df93fdab9885820a [file] [log] [blame]
// 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