source: MondoRescue/branches/3.2/mindi-busybox/libbb/inet_cksum.c

/*
 * Checksum routine for Internet Protocol family headers (C Version)
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */

#include "libbb.h"

uint16_t FAST_FUNC inet_cksum(uint16_t *addr, int nleft)
{
    /*
     * Our algorithm is simple, using a 32 bit accumulator,
     * we add sequential 16 bit words to it, and at the end, fold
     * back all the carry bits from the top 16 bits into the lower
     * 16 bits.
     */
    unsigned sum = 0;
    while (nleft > 1) {
        sum += *addr++;
        nleft -= 2;
    }

    /* Mop up an odd byte, if necessary */
    if (nleft == 1) {
        if (BB_LITTLE_ENDIAN)
            sum += *(uint8_t*)addr;
        else
            sum += *(uint8_t*)addr << 8;
    }

    /* Add back carry outs from top 16 bits to low 16 bits */
    sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
    sum += (sum >> 16);                     /* add carry */

    return (uint16_t)~sum;
}