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#pragma once
#include "std/cstdint.hpp"
#include "std/limits.hpp"
#include "base/assert.hpp"
#include "base/logging.hpp"
namespace
{
uint8_t const kByteMask = 0xFF;
} // namespace
static_assert(CHAR_BIT == 8, "");
template <typename TWriter>
class BitWriter
{
public:
BitWriter(TWriter & writer) : m_writer(writer), m_buf(0), m_bitsWritten(0) {}
~BitWriter()
{
try
{
Flush();
}
catch (...)
{
LOG(LWARNING, ("Caught an exception when flushing BitWriter."));
}
}
// Returns the number of bits that have been sent to BitWriter,
// including those that are in m_buf and are possibly
// not flushed yet.
uint64_t BitsWritten() const { return m_bitsWritten; }
// Writes n bits starting with the least significant bit.
// They are written one byte at a time so endianness is of no concern.
// All the other bits except for the first n must be set to zero.
void Write(uint8_t bits, uint32_t n)
{
if (n == 0)
return;
ASSERT_LESS_OR_EQUAL(n, CHAR_BIT, ());
uint32_t bufferedBits = m_bitsWritten % CHAR_BIT;
m_bitsWritten += n;
if (n + bufferedBits > CHAR_BIT)
{
uint8_t b = (bits << bufferedBits) | m_buf;
m_writer.Write(&b, 1);
m_buf = bits >> (CHAR_BIT - bufferedBits);
}
else
{
if (bufferedBits > 0)
{
bits = (bits << bufferedBits) | m_buf;
n += bufferedBits;
}
if (n == CHAR_BIT)
{
m_writer.Write(&bits, 1);
bits = 0;
}
m_buf = bits;
}
}
private:
// Writes up to CHAR_BIT-1 last bits if they have not been written yet
// and pads them with zeros.
// This method cannot be made public because once a byte has been flushed there is no going back.
void Flush()
{
if (m_bitsWritten % CHAR_BIT != 0)
m_writer.Write(&m_buf, 1);
}
TWriter & m_writer;
uint8_t m_buf;
uint64_t m_bitsWritten;
};
template <typename TSource>
class BitReader
{
public:
BitReader(TSource & src) : m_src(src), m_bitsRead(0), m_bufferedBits(0), m_buf(0) {}
// Returns the total number of bits read from this BitReader.
uint64_t BitsRead() const { return m_bitsRead; }
// Reads n bits and returns them as the least significant bits of an 8-bit number.
// The underlying m_src is supposed to be byte-aligned (which is the
// case when it reads from the place that was written to using BitWriter).
// Read may use one lookahead byte.
uint8_t Read(uint32_t n)
{
if (n == 0)
return 0;
ASSERT_LESS_OR_EQUAL(n, CHAR_BIT, ());
m_bitsRead += n;
uint8_t result = 0;
if (n <= m_bufferedBits)
{
result = m_buf & (kByteMask >> (CHAR_BIT - n));
m_bufferedBits -= n;
m_buf >>= n;
}
else
{
uint8_t nextByte;
m_src.Read(&nextByte, 1);
uint32_t low = n - m_bufferedBits;
result = ((nextByte & (kByteMask >> (CHAR_BIT - low))) << m_bufferedBits) | m_buf;
m_buf = nextByte >> low;
m_bufferedBits = CHAR_BIT - low;
}
return result;
}
private:
TSource & m_src;
uint64_t m_bitsRead;
uint32_t m_bufferedBits;
uint8_t m_buf;
};
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