/* 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     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 = %04X\n",	num,crctt);
	printf("Czas trwania: %f\n",(double)(end-start)/CLK_TCK);
	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) & 0x80000000000000003) crc=(crc<<1)^mask;
		else crc<<=1;
		c<<=1;
	}
	return crc;
}

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