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/* $OpenBSD: iso_cksum.c,v 1.5 2006/02/03 12:51:12 claudio Exp $ */ |
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/* |
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* Copyright (c) 2005 Claudio Jeker <claudio@openbsd.org> |
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* |
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* Permission to use, copy, modify, and distribute this software for any |
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* purpose with or without fee is hereby granted, provided that the above |
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* copyright notice and this permission notice appear in all copies. |
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* |
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES |
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF |
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR |
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES |
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN |
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF |
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. |
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*/ |
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#include <sys/types.h> |
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#include "ospfd.h" |
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/* implementation of Fletcher Checksum -- see RFC 1008 for more info */ |
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/* pos needs to be 0 for verify and 2 <= pos < len for calculation */ |
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u_int16_t |
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iso_cksum(void *buf, u_int16_t len, u_int16_t pos) |
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{ |
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u_int8_t *p = buf; |
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int c0 = 0, c1 = 0, x; /* counters */ |
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u_int16_t sop; |
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sop = len - pos - 1; /* pos is an offset (pos 2 is 3rd element) */ |
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p += 2; /* jump over age field */ |
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len -= 2; |
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while (len--) { |
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c0 += *p++; |
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c1 += c0; |
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if ((len & 0xfff) == 0) { |
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/* overflow protection */ |
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c0 %= 255; |
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c1 %= 255; |
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} |
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} |
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if (pos) { |
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x = ((sop * c0 - c1)) % 255; |
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if (x <= 0) |
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x += 255; |
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c1 = 510 - c0 - x; |
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if (c1 > 255) |
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c1 -= 255; |
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c0 = x; |
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} |
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return (c0 << 8 | c1); |
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} |