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/*
* Copyright 2015 The WebRTC Project Authors. All rights reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "rtc_base/bit_buffer.h"
#include <algorithm>
#include <limits>
#include "absl/numeric/bits.h"
#include "rtc_base/checks.h"
namespace {
// Returns the highest byte of `val` in a uint8_t.
uint8_t HighestByte(uint64_t val) {
return static_cast<uint8_t>(val >> 56);
}
// Returns the result of writing partial data from `source`, of
// `source_bit_count` size in the highest bits, to `target` at
// `target_bit_offset` from the highest bit.
uint8_t WritePartialByte(uint8_t source,
size_t source_bit_count,
uint8_t target,
size_t target_bit_offset) {
RTC_DCHECK(target_bit_offset < 8);
RTC_DCHECK(source_bit_count < 9);
RTC_DCHECK(source_bit_count <= (8 - target_bit_offset));
// Generate a mask for just the bits we're going to overwrite, so:
uint8_t mask =
// The number of bits we want, in the most significant bits...
static_cast<uint8_t>(0xFF << (8 - source_bit_count))
// ...shifted over to the target offset from the most signficant bit.
>> target_bit_offset;
// We want the target, with the bits we'll overwrite masked off, or'ed with
// the bits from the source we want.
return (target & ~mask) | (source >> target_bit_offset);
}
} // namespace
namespace rtc {
BitBufferWriter::BitBufferWriter(uint8_t* bytes, size_t byte_count)
: writable_bytes_(bytes),
byte_count_(byte_count),
byte_offset_(),
bit_offset_() {
RTC_DCHECK(static_cast<uint64_t>(byte_count_) <=
std::numeric_limits<uint32_t>::max());
}
uint64_t BitBufferWriter::RemainingBitCount() const {
return (static_cast<uint64_t>(byte_count_) - byte_offset_) * 8 - bit_offset_;
}
bool BitBufferWriter::ConsumeBytes(size_t byte_count) {
return ConsumeBits(byte_count * 8);
}
bool BitBufferWriter::ConsumeBits(size_t bit_count) {
if (bit_count > RemainingBitCount()) {
return false;
}
byte_offset_ += (bit_offset_ + bit_count) / 8;
bit_offset_ = (bit_offset_ + bit_count) % 8;
return true;
}
void BitBufferWriter::GetCurrentOffset(size_t* out_byte_offset,
size_t* out_bit_offset) {
RTC_CHECK(out_byte_offset != nullptr);
RTC_CHECK(out_bit_offset != nullptr);
*out_byte_offset = byte_offset_;
*out_bit_offset = bit_offset_;
}
bool BitBufferWriter::Seek(size_t byte_offset, size_t bit_offset) {
if (byte_offset > byte_count_ || bit_offset > 7 ||
(byte_offset == byte_count_ && bit_offset > 0)) {
return false;
}
byte_offset_ = byte_offset;
bit_offset_ = bit_offset;
return true;
}
bool BitBufferWriter::WriteUInt8(uint8_t val) {
return WriteBits(val, sizeof(uint8_t) * 8);
}
bool BitBufferWriter::WriteUInt16(uint16_t val) {
return WriteBits(val, sizeof(uint16_t) * 8);
}
bool BitBufferWriter::WriteUInt32(uint32_t val) {
return WriteBits(val, sizeof(uint32_t) * 8);
}
bool BitBufferWriter::WriteBits(uint64_t val, size_t bit_count) {
if (bit_count > RemainingBitCount()) {
return false;
}
size_t total_bits = bit_count;
// For simplicity, push the bits we want to read from val to the highest bits.
val <<= (sizeof(uint64_t) * 8 - bit_count);
uint8_t* bytes = writable_bytes_ + byte_offset_;
// The first byte is relatively special; the bit offset to write to may put us
// in the middle of the byte, and the total bit count to write may require we
// save the bits at the end of the byte.
size_t remaining_bits_in_current_byte = 8 - bit_offset_;
size_t bits_in_first_byte =
std::min(bit_count, remaining_bits_in_current_byte);
*bytes = WritePartialByte(HighestByte(val), bits_in_first_byte, *bytes,
bit_offset_);
if (bit_count <= remaining_bits_in_current_byte) {
// Nothing left to write, so quit early.
return ConsumeBits(total_bits);
}
// Subtract what we've written from the bit count, shift it off the value, and
// write the remaining full bytes.
val <<= bits_in_first_byte;
bytes++;
bit_count -= bits_in_first_byte;
while (bit_count >= 8) {
*bytes++ = HighestByte(val);
val <<= 8;
bit_count -= 8;
}
// Last byte may also be partial, so write the remaining bits from the top of
// val.
if (bit_count > 0) {
*bytes = WritePartialByte(HighestByte(val), bit_count, *bytes, 0);
}
// All done! Consume the bits we've written.
return ConsumeBits(total_bits);
}
bool BitBufferWriter::WriteNonSymmetric(uint32_t val, uint32_t num_values) {
RTC_DCHECK_LT(val, num_values);
RTC_DCHECK_LE(num_values, uint32_t{1} << 31);
if (num_values == 1) {
// When there is only one possible value, it requires zero bits to store it.
// But WriteBits doesn't support writing zero bits.
return true;
}
size_t count_bits = absl::bit_width(num_values);
uint32_t num_min_bits_values = (uint32_t{1} << count_bits) - num_values;
return val < num_min_bits_values
? WriteBits(val, count_bits - 1)
: WriteBits(val + num_min_bits_values, count_bits);
}
size_t BitBufferWriter::SizeNonSymmetricBits(uint32_t val,
uint32_t num_values) {
RTC_DCHECK_LT(val, num_values);
RTC_DCHECK_LE(num_values, uint32_t{1} << 31);
size_t count_bits = absl::bit_width(num_values);
uint32_t num_min_bits_values = (uint32_t{1} << count_bits) - num_values;
return val < num_min_bits_values ? (count_bits - 1) : count_bits;
}
bool BitBufferWriter::WriteExponentialGolomb(uint32_t val) {
// We don't support reading UINT32_MAX, because it doesn't fit in a uint32_t
// when encoded, so don't support writing it either.
if (val == std::numeric_limits<uint32_t>::max()) {
return false;
}
uint64_t val_to_encode = static_cast<uint64_t>(val) + 1;
// We need to write bit_width(val+1) 0s and then val+1. Since val (as a
// uint64_t) has leading zeros, we can just write the total golomb encoded
// size worth of bits, knowing the value will appear last.
return WriteBits(val_to_encode, absl::bit_width(val_to_encode) * 2 - 1);
}
bool BitBufferWriter::WriteSignedExponentialGolomb(int32_t val) {
if (val == 0) {
return WriteExponentialGolomb(0);
} else if (val > 0) {
uint32_t signed_val = val;
return WriteExponentialGolomb((signed_val * 2) - 1);
} else {
if (val == std::numeric_limits<int32_t>::min())
return false; // Not supported, would cause overflow.
uint32_t signed_val = -val;
return WriteExponentialGolomb(signed_val * 2);
}
}
} // namespace rtc
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