/*      $OpenBSD: iso_cksum.c,v 1.1 2007/10/08 10:44:50 norby Exp $ */

/*
 * Copyright (c) 2005 Claudio Jeker <claudio@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>

#include "ospf6d.h"

/* implementation of Fletcher Checksum -- see RFC 1008 for more info */

/* pos needs to be 0 for verify and 2 <= pos < len for calculation */
u_int16_t
iso_cksum(void *buf, u_int16_t len, u_int16_t pos)
{
        u_int8_t        *p = buf;
        int              c0 = 0, c1 = 0, x;     /* counters */
        u_int16_t        sop;

        sop = len - pos - 1;    /* pos is an offset (pos 2 is 3rd element) */
        p += 2;                 /* jump over age field */
        len -= 2;
        while (len--) {
                c0 += *p++;
                c1 += c0;
                if ((len & 0xfff) == 0) {
                        /* overflow protection */
                        c0 %= 255;
                        c1 %= 255;
                }
        }

        if (pos) {
                x = ((sop * c0 - c1)) % 255;
                if (x <= 0)
                        x += 255;
                c1 = 510 - c0 - x;
                if (c1 > 255)
                        c1 -= 255;
                c0 = x;
        }

        return (c0 << 8 | c1);
}