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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 "base/sys_byteorder.h"
namespace {
const int kRegWidthInBits = sizeof(uint64_t) * 8;
}
namespace media {
BitReaderCore::ByteStreamProvider::ByteStreamProvider() {
}
BitReaderCore::ByteStreamProvider::~ByteStreamProvider() {
}
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() {
}
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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