BitUnpacker.icc

Go to the documentation of this file.

/*
  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
*/
 
// $Id$
/**
 * @file CxxUtils/BitUnpacker.icc
 * @author scott snyder <snyder@bnl.gov>
 * @date Nov, 2014
 * @brief Helper to unpack a set of values bitwise from a stream.
 */
 
 
#include "CxxUtils/ones.h"
 
 
namespace CxxUtils {
 
 
/**
 * @brief Constructor.
 * @param nbits Number of bits per item to use in the packed representation.
 * @param stream Input stream object.
 */
template <class STREAM>
inline
BitUnpacker<STREAM>::BitUnpacker (uint8_t nbits, STREAM& stream)
  : m_buf(0),
    m_nbuf(0),
    m_nbits(nbits),
    m_stream (stream),
    m_mask (CxxUtils::ones<uint32_t>(nbits))
{
  assert (m_nbits > 0 && m_nbits <= 32);
}
 
 
/**
 * @brief Unpack one value from the stream.
 */
template <class STREAM>
inline
uint32_t BitUnpacker<STREAM>::unpack()
{
  const uint8_t totbits = 8*sizeof(m_buf);
 
  if (m_nbuf == 0) {
    m_stream >> m_buf;
    m_nbuf = totbits;
  }
 
  uint32_t out = m_buf & m_mask;
  if (m_nbits > m_nbuf) {
    uint8_t nleft = m_nbits - m_nbuf;
    m_stream >> m_buf;
    out |= (m_buf & CxxUtils::ones<uint32_t>(nleft)) << m_nbuf;
    m_buf >>= nleft;
    m_nbuf = totbits - nleft;
  }
  else {
    if (m_nbits >= 32)
      m_buf = 0;
    else
      m_buf >>= m_nbits;
    m_nbuf -= m_nbits;
  }
 
  return out;
}
 
 
/**
 * @brief Constructor.
 * @param stream Input stream object.
 */
template <class STREAM>
inline
BitUnpacker8<STREAM>::BitUnpacker8 (STREAM& stream)
  : m_buf(0),
    m_nbuf(0),
    m_stream (stream)
{
}
 
 
/**
 * @brief Constructor.
 * @param nbits Must be 8.
 * @param stream Input stream object.
 */
template <class STREAM>
inline
BitUnpacker8<STREAM>::BitUnpacker8 (uint8_t /*nbits*/, STREAM& stream)
  : m_buf(0),
    m_nbuf(0),
    m_stream (stream)
{
}
 
 
/**
 * @brief Unpack one value from the stream.
 */
template <class STREAM>
inline
uint32_t BitUnpacker8<STREAM>::unpack() 
{
  if (m_nbuf == 0) {
    m_stream >> m_buf;
    m_nbuf = 4;
  }
 
  uint8_t ret = m_buf & 0xff;
  m_buf >>= 8;
  --m_nbuf;
  return ret;
}
 
 
/**
 * @brief Constructor.
 * @param stream Input stream object.
 */
template <class STREAM>
inline
BitUnpacker16<STREAM>::BitUnpacker16 (STREAM& stream)
  : m_buf(0),
    m_nbuf(0),
    m_stream (stream)
{
}
 
 
/**
 * @brief Constructor.
 * @param nbits Must be 16.
 * @param stream Input stream object.
 */
template <class STREAM>
inline
BitUnpacker16<STREAM>::BitUnpacker16 (uint8_t /*nbits*/, STREAM& stream)
  : m_buf(0),
    m_nbuf(0),
    m_stream (stream)
{
}
 
 
/**
 * @brief Unpack one value from the stream.
 */
template <class STREAM>
inline
uint32_t BitUnpacker16<STREAM>::unpack() 
{
  if (m_nbuf == 0) {
    m_stream >> m_buf;
    m_nbuf = 2;
  }
 
  uint16_t ret = m_buf & 0xffff;
  m_buf >>= 16;
  --m_nbuf;
  return ret;
}
 
 
} // namespace CxxUtils