/* 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 int updcrc(unsigned int,int,unsigned int);unsigned int updcrcr(unsigned int,int,unsigned int);void perr(char *);/* variables */char filename[100];unsigned int crc16,crctt;int ch;unsigned long num;FILE *fp;/* driver */main(argc,argv)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",		num,crc16,crctt);}/* update crc */unsigned int updcrc(crc,c,mask)unsigned int crc,mask; int c;{	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 int updcrcr(crc,c,mask)unsigned int crc,mask; int c;{	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(s)char *s;{	printf("\n%s",s); exit(1);}