#include <stdio.h>

const unsigned short cnCRC_16 = 0x8005;      /*  CRC-16 = X16 + X15 + X2 + X0 */
const unsigned short cnCRC_CCITT = 0x1021;   /*  CRC-CCITT = X16 + X12 + X5 + X0 */
/*  CRC-32 = X32 + X26 + X23 + X22 + X16 + X11 + X10 + X8 + X7 + X5 + X4 + X2 + X1 + X0 */
const unsigned long cnCRC_32 = 0x04C10DB7;   

unsigned long Table_CRC[256];    /*  CRC 表 */

FILE * fp;

void BuildTable16( unsigned short aPoly )
{
    unsigned short i, j;
    unsigned short nData;
    unsigned short nAccum;
    
    for ( i = 0; i < 256; i++ )
    {
        fprintf(fp,"nData = %0x, \t",i);
        nData = ( unsigned short )( i << 8 );
        nAccum = 0;

        for ( j = 0; j < 8; j++ )
        {
            if (( nData ^ nAccum ) & 0x8000 )  nAccum = ( nAccum << 1 ) ^ aPoly;
            else                               nAccum <<= 1;
            nData <<= 1;
        }
        Table_CRC[i] = ( unsigned long )nAccum;
        fprintf(fp,"Table_CRC[%d] = %0x\n",i,Table_CRC[i]);
    }
}

/*  計算 16 位 CRC 值，CRC-16 或 CRC-CCITT */

unsigned short CRC_16( unsigned char * aData, unsigned long aSize )
{
    unsigned long i;
    unsigned short nAccum = 0;

/*   BuildTable16( cnCRC_16 );    // CRC_16 */
    BuildTable16( cnCRC_CCITT ); /*  CRC_CCITT */

    fprintf(fp,"\nCalculate CRC Code by Lookup Table: \n\n");

    for ( i = 0; i < aSize; i++ ){
        fprintf(fp,"Input data = %0x, \t", *aData);
        nAccum = ( nAccum << 8 ) ^ ( unsigned short )Table_CRC[( nAccum >> 8 ) ^ *aData++];
        fprintf(fp,"CRC = %0x\n",nAccum);
    }

    return nAccum;
}

void main()
{

    unsigned char aData[] = {0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01};

    unsigned long aSize = 8;

    fp = fopen("table_crc.dat","wt");

    CRC_16(aData, aSize );

}