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/* $OpenBSD: iso_cksum.c,v 1.4 2005/10/19 15:44:25 stevesk 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 "ospfd.h"
/* 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; /* counters */
int r0, r1; /* results */
u_int16_t sop;
sop = len - pos - 1; /* pos is an offset (pos 2 is at len 3) */
p += 2;
len -= 2;
while (len--) {
c0 += *p++;
c1 += c0;
if ((len & 0xfff) == 0) {
/* overflow protection */
c0 %= 255;
c1 %= 255;
}
}
r0 = c0 = c0 % 255;
r1 = c1 % 255;
if (pos) {
r0 = ((sop * r0 - r1)) % 255;
if (r0 <= 0)
r0 += 255;
r1 = 510 - r0 - c0;
if (r1 > 255)
r1 -= 255;
}
return (r0 << 8 | r1);
}
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