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crc.c

/**********************************************************************
 *
 * Filename:    crc.c
 * 
 * Description: Slow and fast implementations of the CRC standards.
 *
 * Notes:       The parameters for each supported CRC standard are
 *                      defined in the header file crc.h.  The implementations
 *                      here should stand up to further additions to that list.
 *
 * 
 * Copyright (c) 2000 by Michael Barr.  This software is placed into
 * the public domain and may be used for any purpose.  However, this
 * notice must not be changed or removed and no warranty is either
 * expressed or implied by its publication or distribution.
 **********************************************************************/
 
#ifdef      HAVE_CONFIG_H
#include <config.h>
#endif/*HAVE_CONFIG_H*/

#include <os.h>

#include "crc.h"


/*
 * Derive parameters from the standard-specific parameters in crc.h.
 */
#define WIDTH    (8 * sizeof(crc))
#define TOPBIT   (1 << (WIDTH - 1))

#if REFLECT_DATA
#undef  REFLECT_DATA
#define REFLECT_DATA(X)             ((unsigned char) reflect((X), 8))
#else
#undef  REFLECT_DATA
#define REFLECT_DATA(X)             (X)
#endif

#if REFLECT_REMAINDER
#undef  REFLECT_REMAINDER
#define REFLECT_REMAINDER(X)  ((crc) reflect((X), WIDTH))
#else
#undef  REFLECT_REMAINDER
#define REFLECT_REMAINDER(X)  (X)
#endif


/*********************************************************************
 *
 * Function:    reflect()
 * 
 * Description: Reorder the bits of a binary sequence, by reflecting
 *                      them about the middle position.
 *
 * Notes:         No checking is done that nBits <= 32.
 *
 * Returns:       The reflection of the original data.
 *
 *********************************************************************/
static unsigned long
reflect(unsigned long data, unsigned char nBits)
{
      unsigned long  reflection = 0x00000000;
      unsigned char  bit;

      /*
       * Reflect the data about the center bit.
       */
      for (bit = 0; bit < nBits; ++bit)
      {
            /*
             * If the LSB bit is set, set the reflection of it.
             */
            if (data & 0x01)
            {
                  reflection |= (1 << ((nBits - 1) - bit));
            }

            data = (data >> 1);
      }

      return (reflection);

}     /* reflect() */


/*********************************************************************
 *
 * Function:    crcSlow()
 * 
 * Description: Compute the CRC of a given message.
 *
 * Notes:         
 *
 * Returns:       The CRC of the message.
 *
 *********************************************************************/
crc
crcSlow(unsigned char const message[], int nBytes)
{
    crc            remainder = INITIAL_REMAINDER;
      int            byte;
      unsigned char  bit;


    /*
     * Perform modulo-2 division, a byte at a time.
     */
    for (byte = 0; byte < nBytes; ++byte)
    {
        /*
         * Bring the next byte into the remainder.
         */
        remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8));

        /*
         * Perform modulo-2 division, a bit at a time.
         */
        for (bit = 8; bit > 0; --bit)
        {
            /*
             * Try to divide the current data bit.
             */
            if (remainder & TOPBIT)
            {
                remainder = (remainder << 1) ^ POLYNOMIAL;
            }
            else
            {
                remainder = (remainder << 1);
            }
        }
    }

    /*
     * The final remainder is the CRC result.
     */
    return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);

}   /* crcSlow() */


crc  crcTable[256];


/*********************************************************************
 *
 * Function:    crcInit()
 * 
 * Description: Populate the partial CRC lookup table.
 *
 * Notes:         This function must be rerun any time the CRC standard
 *                      is changed.  If desired, it can be run "offline" and
 *                      the table results stored in an embedded system's ROM.
 *
 * Returns:       None defined.
 *
 *********************************************************************/
void
crcInit(void)
{
    crc                    remainder;
      int                  dividend;
      unsigned char  bit;


    /*
     * Compute the remainder of each possible dividend.
     */
    for (dividend = 0; dividend < 256; ++dividend)
    {
        /*
         * Start with the dividend followed by zeros.
         */
        remainder = dividend << (WIDTH - 8);

        /*
         * Perform modulo-2 division, a bit at a time.
         */
        for (bit = 8; bit > 0; --bit)
        {
            /*
             * Try to divide the current data bit.
             */               
            if (remainder & TOPBIT)
            {
                remainder = (remainder << 1) ^ POLYNOMIAL;
            }
            else
            {
                remainder = (remainder << 1);
            }
        }

        /*
         * Store the result into the table.
         */
        crcTable[dividend] = remainder;
    }

}   /* crcInit() */


/*********************************************************************
 *
 * Function:    crcFast()
 * 
 * Description: Compute the CRC of a given message.
 *
 * Notes:         crcInit() must be called first.
 *
 * Returns:       The CRC of the message.
 *
 *********************************************************************/
crc
crcFast(unsigned char const message[], int nBytes)
{
    crc                remainder = INITIAL_REMAINDER;
    unsigned char  data;
      int            byte;


    /*
     * Divide the message by the polynomial, a byte at a time.
     */
    for (byte = 0; byte < nBytes; ++byte)
    {
        data = REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8));
            remainder = crcTable[data] ^ (remainder << 8);
    }

    /*
     * The final remainder is the CRC.
     */
    return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);

}   /* crcFast() */

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