/* compute crc's */
/* crc-ccitt is based on the polynomial x^16+x^12+x^5+1 */
/* 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
*/
/* Usage : ccitt [filename] */

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

//#define     MTT 0x10000000100011020     /* crc-ccitt mask */
#define     MTT 0x100011021     /* crc-ccitt mask */
#define     CLK_TCK CLOCKS_PER_SEC

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

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

/* driver */
int main(argc,argv)
int argc; char **argv;
{
 clock_t start,end;
    if(argc>2) perr("Usage:  ccitt [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; crctt=0;
    start=clock();

    while((ch=fgetc(fp))!=EOF)
    {
        //if (ch == '\n')
        //  continue;
        num++;
        //int i; for (i=0;i<10000;i++){
        crctt=updcrc(crctt,ch,MTT);
        //}
    }
    end=clock();
    fclose(fp);
    printf("\nNumber of bytes = %lu\n CRCTT = %04llX\n",  num,crctt);
    printf("Czas trwania: %f\n",(double)(end-start)/CLK_TCK);
    getchar();
    return 0;
}

/* update crc */
unsigned long long int updcrc(crc,c,mask)
unsigned long long int crc,mask; int c;
{
    int i;
    c<<=56;
    for(i=0;i<8;i++)
    {
        if((crc ^ c) & 0x8000000000000000) crc=(crc<<1)^mask;
        else crc<<=1;
        c<<=1;
    }
    return crc;
}

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