crc.cpp

Jpeg编解码器的源代码

//
//  Title:  Cyclic Reducancy Check Template Class Implementation
//
//  Copyright 1999, Colosseum Builders, Inc.
//  All rights reserved.
//
//  Colosseum Builders, Inc. makes no warranty, expressed or implied
//  with regards to this software. It is provided as is.
//
//  Author:  John M. Miano ([e-mail])
//
//  Date:    March 15, 1999
//
//  Version: 1
//
//  Description:
//
//    This is a template class for implementing various forms of the
//    CRC function.
//
//  Revision History:
//
//

#include <climits>
#include "crc.h"
#include "checks.h"

namespace
{
const UBYTE4 bits [CHAR_BIT * 4] =
 { 0x00000001UL, 0x00000002UL, 0x00000004UL, 0x00000008UL,
   0x00000010UL, 0x00000020UL, 0x00000040UL, 0x00000080UL,
   0x00000100UL, 0x00000200UL, 0x00000400UL, 0x00000800UL,
   0x00001000UL, 0x00002000UL, 0x00004000UL, 0x00008000UL,
   0x00010000UL, 0x00020000UL, 0x00040000UL, 0x00080000UL,
   0x00100000UL, 0x00200000UL, 0x00400000UL, 0x00800000UL,
   0x01000000UL, 0x02000000UL, 0x04000000UL, 0x08000000UL,
   0x10000000UL, 0x20000000UL, 0x40000000UL, 0x80000000UL,
 } ;


//
//  Description:
//
//    This function returns the reversed bit patter from its input.
//    For example, 1010 becomes 0101.
//
//  Parameters:
//
//    value: The value to reverse
//    bitcount: The number of bits to reverse.
//
BYTE4 Reverse (UBYTE4 value, unsigned int bitcount)
{
  unsigned long result = 0 ;

  for (unsigned int jj = 0 ; jj < bitcount ; ++ jj)
  {
    if ((value & bits [jj]) != 0)
      result |= bits [bitcount - jj - 1] ;
  }
  return result ;
}

//
//  Description:
//
//    This function creates a CRC table entry for a non-reversed
//    CRC function.
//
//  Parameters:
//
//      polynomial: The CRC Polynomial.
//      entryindex: The index of the CRC table entry.
//      bitcount:  The number of bits per CRC table entry.
//
//  Return Value:
//
//      The value for the specified CRC table entry.
//
UBYTE4 ForwardTableEntry (UBYTE4 polynomial,
                          unsigned int entryindex,
                          unsigned int bitcount)
{

  UBYTE4 result = entryindex << (bitcount - CHAR_BIT) ;
  for (unsigned int ii = 0 ; ii < CHAR_BIT ; ++ ii)
  {
    if ((result & bits [bitcount - 1]) == 0)
      result <<= 1 ;
    else
      result = (result << 1) ^ polynomial ;
  }
  unsigned long mask = ((1UL << (bitcount - 1)) - 1UL)
                     | (1UL << (bitcount - 1)) ;
  result &= mask ;
  return result ;
}

//
//  Description:
//
//    This function creates a CRC table entry for a reversed
//    CRC function.
//
//  Parameters:
//
//      polynomial: The CRC Polynomial.
//      entryindex: The index of the CRC table entry.
//      bitcount:  The number of bits per CRC table entry.
//
//  Return Value:
//
//      The value for the specified CRC table entry.
//
UBYTE4 ReverseTableEntry (UBYTE4 polynomial,
                          unsigned int entryindex,
                          unsigned int bitcount)
{
  UBYTE4 result = entryindex ;
  for (unsigned int ii = 0 ; ii < CHAR_BIT ; ++ ii)
  {
    if ((result & 1) == 0)
      result >>= 1 ;
    else
      result = (result >> 1) ^ Reverse (polynomial, bitcount) ;
  }
  unsigned long mask = ((1UL << (bitcount - 1)) - 1)
                     | (1UL << (bitcount - 1)) ;
  result &=  mask ;
  return result ;
}

} // END unnamed namespace

namespace Colosseum
{
//
// Definition of the Crc class's static member variable.
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
const Crc < BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK > :: CrcTable 
          Crc < BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK > :: crc_table ;

//
//  Description:
//
//    Default Constructor
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::Crc (void)
{
  CHECKARGUMENT (BITCOUNT <= 32)

  reset () ;
  return ;
}
//
//  Description:
//
//    Copy Constructor
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::Crc (const Crc &source)
: crc_register (source.crc_register)
{
  return ;
}

//
//  Description:
//
//    Assignment Operator
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>
  &Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::operator=(const Crc &source)
{
  crc_register = source.crc_register ;
  return *this ;
}
//
//  Description:
//
//    This function resets the CRC register to it initial state.  Call
//    this member function after you have calculated a CRC value and
//    want to calculate another.
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
void Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::reset ()
{
  crc_register = INITIAL ;
  return ;
}

//
//  Description:
//
//    This function retrieves the value of the CRC register. Some forms of the
//    CRC use an XOR operation
//
//  Return Value:
//
//    The current CRC value.
//
export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
UBYTE4 Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::value () const
{
  UBYTE4 result = crc_register ^ FINALMASK ;
  // The initial value is 1 << BITCOUNT - 1. The convolutions
  // are to prevent an integer overflow.
  static const UBYTE4 mask = ((1UL << (BITCOUNT - 1)) - 1UL)
                             | (1UL << (BITCOUNT - 1)) ;
  result &= mask ;
  return result ;
}

//
//  Description:
//
//    This function updates the value of the CRC register based upon
//    the contents of a buffer.
//
//  Parameters:
//
//    buffer: The input buffer
//    length: The length of the input buffer.
//

export template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
void Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::update (const char *buffer, unsigned int length)
{
  // The process for updating depends upon whether or not we are using
  // the reversed CRC form.
  if (REVERSE)
  {
    for (unsigned int ii = 0 ; ii < length ; ++ ii)
    {
      crc_register = crc_table.values [(crc_register ^ buffer [ii]) & 0xFF]
                   ^ (crc_register >> 8) ;
    }
  }
  else
  {
    for (unsigned int ii = 0 ; ii < length ; ++ ii)
    {
      UBYTE4 index = ((crc_register >> (BITCOUNT - CHAR_BIT)) ^ buffer [ii]) ;
      crc_register = crc_table.values [index & 0xFF]
                   ^ (crc_register << CHAR_BIT) ;
    }
  }
  return ;
}

//
//  Description:
//
//    CrcTable Class Constructor
//
//    The CRC table is implemented as a separate class to that
//    CRC tables can be static members, initialized at program
//    startup.
//
//
template <unsigned int BITCOUNT,
          unsigned long POLYNOMIAL,
          bool REVERSE,
          unsigned long INITIAL,
          unsigned long FINALMASK>
Crc<BITCOUNT,POLYNOMIAL,REVERSE,INITIAL,FINALMASK>::CrcTable::CrcTable ()
{
  if (REVERSE)
  {
    for (unsigned int ii = 0 ; ii < MAXBYTEVALUES ; ++ ii)
    {
      values [ii] = ReverseTableEntry (POLYNOMIAL, ii, BITCOUNT) ;
    }
  }
  else
  {
    for (unsigned int ii = 0 ; ii < MAXBYTEVALUES ; ++ ii)
    {
      values [ii] = ForwardTableEntry (POLYNOMIAL, ii, BITCOUNT) ;
    }
  }
  return ;
}

}