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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/base/bit_reader_core.h"
- #include <stdint.h>
- #include <cstring>
- #include "base/sys_byteorder.h"
- namespace {
- const int kRegWidthInBits = sizeof(uint64_t) * 8;
- }
- namespace media {
- BitReaderCore::ByteStreamProvider::ByteStreamProvider() = default;
- BitReaderCore::ByteStreamProvider::~ByteStreamProvider() = default;
- BitReaderCore::BitReaderCore(ByteStreamProvider* byte_stream_provider)
- : byte_stream_provider_(byte_stream_provider),
- bits_read_(0),
- nbits_(0),
- reg_(0),
- nbits_next_(0),
- reg_next_(0) {
- }
- BitReaderCore::~BitReaderCore() = default;
- bool BitReaderCore::ReadFlag(bool* flag) {
- if (nbits_ == 0 && !Refill(1))
- return false;
- *flag = (reg_ & (UINT64_C(1) << (kRegWidthInBits - 1))) != 0;
- reg_ <<= 1;
- nbits_--;
- bits_read_++;
- return true;
- }
- int BitReaderCore::PeekBitsMsbAligned(int num_bits, uint64_t* out) {
- // Try to have at least |num_bits| in the bit register.
- if (nbits_ < num_bits)
- Refill(num_bits);
- *out = reg_;
- return nbits_;
- }
- bool BitReaderCore::SkipBitsSmall(int num_bits) {
- DCHECK_GE(num_bits, 0);
- uint64_t dummy;
- while (num_bits >= kRegWidthInBits) {
- if (!ReadBitsInternal(kRegWidthInBits, &dummy))
- return false;
- num_bits -= kRegWidthInBits;
- }
- return ReadBitsInternal(num_bits, &dummy);
- }
- bool BitReaderCore::SkipBits(int num_bits) {
- DCHECK_GE(num_bits, 0);
- const int remaining_bits = nbits_ + nbits_next_;
- if (remaining_bits >= num_bits)
- return SkipBitsSmall(num_bits);
- // Skip first the remaining available bits.
- num_bits -= remaining_bits;
- bits_read_ += remaining_bits;
- nbits_ = 0;
- reg_ = 0;
- nbits_next_ = 0;
- reg_next_ = 0;
- // Next, skip an integer number of bytes.
- const int nbytes = num_bits / 8;
- if (nbytes > 0) {
- const uint8_t* byte_stream_window;
- const int window_size =
- byte_stream_provider_->GetBytes(nbytes, &byte_stream_window);
- DCHECK_GE(window_size, 0);
- DCHECK_LE(window_size, nbytes);
- if (window_size < nbytes) {
- // Note that some bytes were consumed.
- bits_read_ += 8 * window_size;
- return false;
- }
- num_bits -= 8 * nbytes;
- bits_read_ += 8 * nbytes;
- }
- // Skip the remaining bits.
- return SkipBitsSmall(num_bits);
- }
- int BitReaderCore::bits_read() const {
- return bits_read_;
- }
- bool BitReaderCore::ReadBitsInternal(int num_bits, uint64_t* out) {
- DCHECK_GE(num_bits, 0);
- if (num_bits == 0) {
- *out = 0;
- return true;
- }
- if (num_bits > nbits_ && !Refill(num_bits)) {
- // Any subsequent ReadBits should fail:
- // empty the current bit register for that purpose.
- nbits_ = 0;
- reg_ = 0;
- *out = 0;
- return false;
- }
- bits_read_ += num_bits;
- if (num_bits == kRegWidthInBits) {
- // Special case needed since for example for a 64 bit integer "a"
- // "a << 64" is not defined by the C/C++ standard.
- *out = reg_;
- reg_ = 0;
- nbits_ = 0;
- return true;
- }
- *out = reg_ >> (kRegWidthInBits - num_bits);
- reg_ <<= num_bits;
- nbits_ -= num_bits;
- return true;
- }
- bool BitReaderCore::Refill(int min_nbits) {
- DCHECK_LE(min_nbits, kRegWidthInBits);
- // Transfer from the next to the current register.
- RefillCurrentRegister();
- if (min_nbits <= nbits_)
- return true;
- DCHECK_EQ(nbits_next_, 0);
- DCHECK_EQ(reg_next_, 0u);
- // Max number of bytes to refill.
- int max_nbytes = sizeof(reg_next_);
- // Refill.
- const uint8_t* byte_stream_window;
- int window_size =
- byte_stream_provider_->GetBytes(max_nbytes, &byte_stream_window);
- DCHECK_GE(window_size, 0);
- DCHECK_LE(window_size, max_nbytes);
- if (window_size == 0)
- return false;
- reg_next_ = 0;
- memcpy(®_next_, byte_stream_window, window_size);
- reg_next_ = base::NetToHost64(reg_next_);
- nbits_next_ = window_size * 8;
- // Transfer from the next to the current register.
- RefillCurrentRegister();
- return (nbits_ >= min_nbits);
- }
- void BitReaderCore::RefillCurrentRegister() {
- // No refill possible if the destination register is full
- // or the source register is empty.
- if (nbits_ == kRegWidthInBits || nbits_next_ == 0)
- return;
- reg_ |= (reg_next_ >> nbits_);
- int free_nbits = kRegWidthInBits - nbits_;
- if (free_nbits >= nbits_next_) {
- nbits_ += nbits_next_;
- reg_next_ = 0;
- nbits_next_ = 0;
- return;
- }
- nbits_ += free_nbits;
- reg_next_ <<= free_nbits;
- nbits_next_ -= free_nbits;
- }
- } // namespace media
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