/*      $OpenBSD: in_cksum.c,v 1.3 2022/12/28 21:30:19 jmc Exp $        */

/*
 * Copyright (c) 1988, 1992, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)in_cksum.c  8.1 (Berkeley) 6/10/93
 */

#include <sys/types.h>

#include "interface.h"

/*
 * Given the host-byte-order value of the checksum field in a packet
 * header, and the network-byte-order computed checksum of the data
 * that the checksum covers (including the checksum itself), compute
 * what the checksum field *should* have been.
 */
u_int16_t
in_cksum_shouldbe(u_int16_t sum, u_int16_t computed_sum)
{
        u_int32_t shouldbe;

        /*
         * The value that should have gone into the checksum field
         * is the negative of the value gotten by summing up everything
         * *but* the checksum field.
         *
         * We can compute that by subtracting the value of the checksum
         * field from the sum of all the data in the packet, and then
         * computing the negative of that value.
         *
         * "sum" is the value of the checksum field, and "computed_sum"
         * is the negative of the sum of all the data in the packets,
         * so that's -(-computed_sum - sum), or (sum + computed_sum).
         *
         * All the arithmetic in question is one's complement, so the
         * addition must include an end-around carry; we do this by
         * doing the arithmetic in 32 bits (with no sign-extension),
         * and then adding the upper 16 bits of the sum, which contain
         * the carry, to the lower 16 bits of the sum, and then do it
         * again in case *that* sum produced a carry.
         *
         * As RFC 1071 notes, the checksum can be computed without
         * byte-swapping the 16-bit words; summing 16-bit words
         * on a big-endian machine gives a big-endian checksum, which
         * can be directly stuffed into the big-endian checksum fields
         * in protocol headers, and summing words on a little-endian
         * machine gives a little-endian checksum, which must be
         * byte-swapped before being stuffed into a big-endian checksum
         * field.
         *
         * "computed_sum" is a network-byte-order value, so we must put
         * it in host byte order before subtracting it from the
         * host-byte-order value from the header; the adjusted checksum
         * will be in host byte order, which is what we'll return.
         */
        shouldbe = sum;
        shouldbe += ntohs(computed_sum);
        shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
        shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
        return shouldbe;
}

uint32_t
in_cksum_add(const void *buf, size_t len, uint32_t sum)
{
        const uint16_t *words = buf;

        while (len > 1) {
                sum += *words++;
                len -= sizeof(*words);
        }

        if (len == 1) {
                uint8_t byte = *(const uint8_t *)words;
                sum += htons(byte << 8);
        }

        return (sum);
}

uint16_t
in_cksum_fini(uint32_t sum)
{
        sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
        sum += (sum >> 16);                     /* add carry */

        return (~sum);
}

/*
 * compute an IP header checksum.
 * don't modify the packet.
 */
uint16_t
in_cksum(const void *addr, size_t len, uint32_t sum)
{
        return (in_cksum_fini(in_cksum_add(addr, len, sum)));
}