/* compute crc's */
/* crc-16 is based on the polynomial x^16+x^15+x^2+1 */
/*  The data is assumed to be fed in from least to most significant bit */
/* crc-ccitt is based on the polynomial x^16+x^12+x^5+1 */
/*  The data is fed in from most to least significant bit */
/* The prescription for determining the mask to use for a given polynomial
        is as follows:
                1.  Represent the polynomial by a 17-bit number
                2.  Assume that the most and least significant bits are 1
                3.  Place the right 16 bits into an integer
                4.  Bit reverse if serial LSB's are sent first
*/
/* Usage : crc2 [filename] */

#include        <stdio.h>
#include        <stdlib.h>
#include        <string.h>

#define         M16     0xA001          /* crc-16 mask */
#define         MTT     0x1021          /* crc-ccitt mask */

/* function declarations */
unsigned short int updcrc(unsigned short int,int,unsigned short int);
unsigned short int updcrcr(unsigned short int,int,unsigned short int);
void perr(char *);

/* variables */
char filename[100];
unsigned short int crc16,crctt;
int ch;
unsigned long num;
FILE *fp;

/* driver */
main(int argc,char **argv)
{
        if(argc>2) perr("Usage:  crc2 [filename]");
        if(argc==2) strcpy(filename,argv[1]);
        else
        {
                printf("\nEnter filename:  "); gets(filename);
        }
        if((fp=fopen(filename,"rb"))==NULL) perr("Can't open file");
        num=0L; crc16=crctt=0;
        while((ch=fgetc(fp))!=EOF)
        {
                num++;
                crc16=updcrcr(crc16,ch,M16);
                crctt=updcrc(crctt,ch,MTT);
        }
        fclose(fp);
        printf("\nNumber of bytes = %lu\nCRC16 = %04X\nCRCTT = %04X\n",
                (unsigned long) num, (unsigned long) crc16, (unsigned long) crctt);
}

/* update crc */
unsigned short int updcrc(unsigned short int crc,int c,unsigned short int mask)
{
        int i;
        c<<=8;
        for(i=0;i<8;i++)
        {
                if((crc ^ c) & 0x8000) crc=(crc<<1)^mask;
                else crc<<=1;
                c<<=1;
        }
        return crc;
}

/* update crc reverse */
unsigned short int updcrcr(unsigned short int crc,int c,unsigned short int mask)
{
        int i;
        for(i=0;i<8;i++)
        {
                if((crc ^ c) & 1) crc=(crc>>1)^mask;
                else crc>>=1;
                c>>=1;
        }
        return crc;
}

/* error abort */
void perr(char *s)
{
        printf("\n%s",s); exit(1);
}