File content as of revision 4:721ae30c92f8:

 
#include "crc2.h"


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

#if (REFLECT_DATA == TRUE)
#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 == TRUE)
#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()
{
    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() */