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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.
*/
#ifndef RTC_BASE_BIT_BUFFER_H_
#define RTC_BASE_BIT_BUFFER_H_
#include <stddef.h> // For size_t.
#include <stdint.h> // For integer types.
#include "rtc_base/constructor_magic.h"
namespace rtc {
// A class, similar to ByteBuffer, that can parse bit-sized data out of a set of
// bytes. Has a similar API to ByteBuffer, plus methods for reading bit-sized
// and exponential golomb encoded data. For a writable version, use
// BitBufferWriter. Unlike ByteBuffer, this class doesn't make a copy of the
// source bytes, so it can be used on read-only data.
// Sizes/counts specify bits/bytes, for clarity.
// Byte order is assumed big-endian/network.
class BitBuffer {
public:
BitBuffer(const uint8_t* bytes, size_t byte_count);
// Gets the current offset, in bytes/bits, from the start of the buffer. The
// bit offset is the offset into the current byte, in the range [0,7].
void GetCurrentOffset(size_t* out_byte_offset, size_t* out_bit_offset);
// The remaining bits in the byte buffer.
uint64_t RemainingBitCount() const;
// Reads byte-sized values from the buffer. Returns false if there isn't
// enough data left for the specified type.
bool ReadUInt8(uint8_t* val);
bool ReadUInt16(uint16_t* val);
bool ReadUInt32(uint32_t* val);
// Reads bit-sized values from the buffer. Returns false if there isn't enough
// data left for the specified bit count.
bool ReadBits(uint32_t* val, size_t bit_count);
// Peeks bit-sized values from the buffer. Returns false if there isn't enough
// data left for the specified number of bits. Doesn't move the current
// offset.
bool PeekBits(uint32_t* val, size_t bit_count);
// Reads value in range [0, num_values - 1].
// This encoding is similar to ReadBits(val, Ceil(Log2(num_values)),
// but reduces wastage incurred when encoding non-power of two value ranges
// Non symmetric values are encoded as:
// 1) n = countbits(num_values)
// 2) k = (1 << n) - num_values
// Value v in range [0, k - 1] is encoded in (n-1) bits.
// Value v in range [k, num_values - 1] is encoded as (v+k) in n bits.
// https://aomediacodec.github.io/av1-spec/#nsn
// Returns false if there isn't enough data left.
bool ReadNonSymmetric(uint32_t* val, uint32_t num_values);
// Reads the exponential golomb encoded value at the current offset.
// Exponential golomb values are encoded as:
// 1) x = source val + 1
// 2) In binary, write [countbits(x) - 1] 0s, then x
// To decode, we count the number of leading 0 bits, read that many + 1 bits,
// and increment the result by 1.
// Returns false if there isn't enough data left for the specified type, or if
// the value wouldn't fit in a uint32_t.
bool ReadExponentialGolomb(uint32_t* val);
// Reads signed exponential golomb values at the current offset. Signed
// exponential golomb values are just the unsigned values mapped to the
// sequence 0, 1, -1, 2, -2, etc. in order.
bool ReadSignedExponentialGolomb(int32_t* val);
// Moves current position |byte_count| bytes forward. Returns false if
// there aren't enough bytes left in the buffer.
bool ConsumeBytes(size_t byte_count);
// Moves current position |bit_count| bits forward. Returns false if
// there aren't enough bits left in the buffer.
bool ConsumeBits(size_t bit_count);
// Sets the current offset to the provied byte/bit offsets. The bit
// offset is from the given byte, in the range [0,7].
bool Seek(size_t byte_offset, size_t bit_offset);
protected:
const uint8_t* const bytes_;
// The total size of |bytes_|.
size_t byte_count_;
// The current offset, in bytes, from the start of |bytes_|.
size_t byte_offset_;
// The current offset, in bits, into the current byte.
size_t bit_offset_;
RTC_DISALLOW_COPY_AND_ASSIGN(BitBuffer);
};
// A BitBuffer API for write operations. Supports symmetric write APIs to the
// reading APIs of BitBuffer. Note that the read/write offset is shared with the
// BitBuffer API, so both reading and writing will consume bytes/bits.
class BitBufferWriter : public BitBuffer {
public:
// Constructs a bit buffer for the writable buffer of |bytes|.
BitBufferWriter(uint8_t* bytes, size_t byte_count);
// Writes byte-sized values from the buffer. Returns false if there isn't
// enough data left for the specified type.
bool WriteUInt8(uint8_t val);
bool WriteUInt16(uint16_t val);
bool WriteUInt32(uint32_t val);
// Writes bit-sized values to the buffer. Returns false if there isn't enough
// room left for the specified number of bits.
bool WriteBits(uint64_t val, size_t bit_count);
// Writes value in range [0, num_values - 1]
// See ReadNonSymmetric documentation for the format,
// Call SizeNonSymmetricBits to get number of bits needed to store the value.
// Returns false if there isn't enough room left for the value.
bool WriteNonSymmetric(uint32_t val, uint32_t num_values);
// Returns number of bits required to store |val| with NonSymmetric encoding.
static size_t SizeNonSymmetricBits(uint32_t val, uint32_t num_values);
// Writes the exponential golomb encoded version of the supplied value.
// Returns false if there isn't enough room left for the value.
bool WriteExponentialGolomb(uint32_t val);
// Writes the signed exponential golomb version of the supplied value.
// Signed exponential golomb values are just the unsigned values mapped to the
// sequence 0, 1, -1, 2, -2, etc. in order.
bool WriteSignedExponentialGolomb(int32_t val);
private:
// The buffer, as a writable array.
uint8_t* const writable_bytes_;
RTC_DISALLOW_COPY_AND_ASSIGN(BitBufferWriter);
};
} // namespace rtc
#endif // RTC_BASE_BIT_BUFFER_H_
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