/*
    From comp.lang.c Wed Nov  7 16:07:00 1990
    From: ron@scocan.sco.COM (Ron Irvine)
    Subject: CCITT in C
    Date: 5 Nov 90 22:31:57 GMT
    Originator: ron@capri

    There are two popular 16 bit crc calculations.  The one I
    used is the crc16 used for DDCMP and Bisync communications.
    Its polynomial is: x^16 + x^15 + x^2 + 1, with an initial
    value of 0. I call it crc16().  The other common crc is used
    in ADCCP, HDLC and SDLC; its polynomial is: x^16 + x^12 + x^5 + 1,
    with an initial value of -1.  I call it ccitt(). If you are
    using the crc to hash you may want to play with both or invent
    your own.
 */

/*	Computing a POLY number from the crc equation.
 *	Crc's are usually expressed as an polynomial expression such as:
 *
 *		x^16 + x^12 + x^5 + 1
 *
 *	Since the highest order term is to the power 16 this is a
 *	16 bit crc.  The POLY number is determined by setting bits
 *	corresponding to the power terms in the polynomial equation
 *	in an integer.  To do this we number the bits of the integer
 *	with the most significant bit = bit 0. CAUTION: this is the
 *	opposite to the some bit numbering schemes.  This is due to
 *	the least significant bit first convention of crc calculations.
 *	The above equation becomes:
 *
 *	  1   0   0   0   0   1   0   0   0   0   0   0   1   0   0   0
 *	|___.___.___.___|___.___.___.___|___.___.___.___|___.___.___.___|
 *	  0   1   2   3   4   5   6   7   8   9   10  11  12  13  14  15
 *	msb                                                          lsb
 *
 *	Note: the "1" term is equivalent to "x^0" and the "x^16"
 *	term is ignored (it does determine the length to be 16 bits).
 *	Thus the POLY number is 0x8408 (ccitt crc).
 */

/* CCITT - ADCCP, HDLC, SDLC (16 bit)
 *	x^16 + x^12 + x^5 + 1
 */

#define	INIT_CCITT	-1


/* nibble table for CCITT crc */
static unsigned int ccitt_h[] = {
0x0000, 0x1081, 0x2102, 0x3183, 0x4204, 0x5285, 0x6306, 0x7387,
0x8408, 0x9489, 0xa50a, 0xb58b, 0xc60c, 0xd68d, 0xe70e, 0xf78f, };

static unsigned int ccitt_l[] = {
0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7, };

unsigned short checksum_ccitt(unsigned char * in, int length)
{
	register unsigned int	n, crc;

	crc = INIT_CCITT;
	while (length-- > 0) {
		n = *in++ ^ crc;
		crc = ccitt_l[n&0x0f] ^ ccitt_h[(n>>4)&0x0f] ^ (crc>>8);
	};
	return((unsigned short)crc);
}