/* updcrc(3), crc(1) - calculate crc polynomials
 *
 * Calculate, intelligently, the CRC of a dataset incrementally given a 
 * buffer full at a time.
 * 
 * Usage:
 * 	newcrc = updcrc( oldcrc, bufadr, buflen )
 * 		unsigned int oldcrc, buflen;
 * 		char *bufadr;
 *
 * Compiling with -DTEST creates a program to print the CRC of stdin to stdout.
 * Compile with -DMAKETAB to print values for crctab to stdout.  If you change
 *	the CRC polynomial parameters, be sure to do this and change
 *	crctab's initial value.
 *
 * Notes:
 *  Regards the data stream as an integer whose MSB is the MSB of the first
 *  byte recieved.  This number is 'divided' (using xor instead of subtraction)
 *  by the crc-polynomial P.
 *  XMODEM does things a little differently, essentially treating the LSB of
 * the first data byte as the MSB of the integer. Define SWAPPED to make
 * things behave in this manner.
 *
 * Author:	Mark G. Mendel, 7/86
 *		UUCP: ihnp4!umn-cs!hyper!mark, GEnie: mgm
 */

/* The CRC polynomial.
 * These 4 values define the crc-polynomial.
 * If you change them, you must change crctab[]'s initial value to what is
 * printed by initcrctab() [see 'compile with -DMAKETAB' above].
 */
    /* Value used by:	    		CITT	XMODEM	ARC  	*/
#define	P	 0xA001	 /* the poly:	0x1021	0x1021	A001	*/
#define INIT_CRC 0L	 /* init value:	-1	0	0	*/
#define SWAPPED		 /* bit order:	undef	defined	defined */
#define W	16	 /* bits in CRC:16	16	16	*/

    /* data type that holds a W-bit unsigned integer */
#if W <= 16
#  define WTYPE	unsigned short
#else
#  define WTYPE   unsigned long
#endif

    /* the number of bits per char: don't change it. */
#define B	8

static WTYPE crctab[1<<B] = {
     0x0000,  0xc0c1,  0xc181,  0x0140,  0xc301,  0x3c0,   0x0280,  0xc241,
     0xc601,  0x06c0,  0x0780,  0xc741,  0x0500,  0xc5c1,  0xc481,  0x0440,
     0xcc01,  0x0cc0,  0x0d80,  0xcd41,  0x0f00,  0xcfc1,  0xce81,  0x0e40,
     0x0a00,  0xcac1,  0xcb81,  0x0b40,  0xc901,  0x09c0,  0x0880,  0xc841,
     0xd801,  0x18c0,  0x1980,  0xd941,  0x1b00,  0xdbc1,  0xda81,  0x1a40,
     0x1e00,  0xdec1,  0xdf81,  0x1f40,  0xdd01,  0x1dc0,  0x1c80,  0xdc41,
     0x1400,  0xd4c1,  0xd581,  0x1540,  0xd701,  0x17c0,  0x1680,  0xd641,
     0xd201,  0x12c0,  0x1380,  0xd341,  0x1100,  0xd1c1,  0xd081,  0x1040,
     0xf001,  0x30c0,  0x3180,  0xf141,  0x3300,  0xf3c1,  0xf281,  0x3240,
     0x3600,  0xf6c1,  0xf781,  0x3740,  0xf501,  0x35c0,  0x3480,  0xf441,
     0x3c00,  0xfcc1,  0xfd81,  0x3d40,  0xff01,  0x3fc0,  0x3e80,  0xfe41,
     0xfa01,  0x3ac0,  0x3b80,  0xfb41,  0x3900,  0xf9c1,  0xf881,  0x3840,
     0x2800,  0xe8c1,  0xe981,  0x2940,  0xeb01,  0x2bc0,  0x2a80,  0xea41,
     0xee01,  0x2ec0,  0x2f80,  0xef41,  0x2d00,  0xedc1,  0xec81,  0x2c40,
     0xe401,  0x24c0,  0x2580,  0xe541,  0x2700,  0xe7c1,  0xe681,  0x2640,
     0x2200,  0xe2c1,  0xe381,  0x2340,  0xe101,  0x21c0,  0x2080,  0xe041,
     0xa001,  0x60c0,  0x6180,  0xa141,  0x6300,  0xa3c1,  0xa281,  0x6240,
     0x6600,  0xa6c1,  0xa781,  0x6740,  0xa501,  0x65c0,  0x6480,  0xa441,
     0x6c00,  0xacc1,  0xad81,  0x6d40,  0xaf01,  0x6fc0,  0x6e80,  0xae41,
     0xaa01,  0x6ac0,  0x6b80,  0xab41,  0x6900,  0xa9c1,  0xa881,  0x6840,
     0x7800,  0xb8c1,  0xb981,  0x7940,  0xbb01,  0x7bc0,  0x7a80,  0xba41,
     0xbe01,  0x7ec0,  0x7f80,  0xbf41,  0x7d00,  0xbdc1,  0xbc81,  0x7c40,
     0xb401,  0x74c0,  0x7580,  0xb541,  0x7700,  0xb7c1,  0xb681,  0x7640,
     0x7200,  0xb2c1,  0xb381,  0x7340,  0xb101,  0x71c0,  0x7080,  0xb041,
     0x5000,  0x90c1,  0x9181,  0x5140,  0x9301,  0x53c0,  0x5280,  0x9241,
     0x9601,  0x56c0,  0x5780,  0x9741,  0x5500,  0x95c1,  0x9481,  0x5440,
     0x9c01,  0x5cc0,  0x5d80,  0x9d41,  0x5f00,  0x9fc1,  0x9e81,  0x5e40,
     0x5a00,  0x9ac1,  0x9b81,  0x5b40,  0x9901,  0x59c0,  0x5880,  0x9841,
     0x8801,  0x48c0,  0x4980,  0x8941,  0x4b00,  0x8bc1,  0x8a81,  0x4a40,
     0x4e00,  0x8ec1,  0x8f81,  0x4f40,  0x8d01,  0x4dc0,  0x4c80,  0x8c41,
     0x4400,  0x84c1,  0x8581,  0x4540,  0x8701,  0x47c0,  0x4680,  0x8641,
     0x8201,  0x42c0,  0x4380,  0x8341,  0x4100,  0x81c1,  0x8081,  0x4040,
};


WTYPE updcrc(WTYPE icrc, unsigned char *icp, int icnt ) {
     register WTYPE crc = icrc;
     register unsigned char *cp = icp;
     register int cnt = icnt;

     while( cnt-- )
          crc = (crc>>B) ^ crctab[(crc & ((1<<B)-1)) ^ *cp++];

     return( crc );
}