/*
  Copyright (C) 2001-2023 Free Software Foundation, Inc.

  This file is part of GNU Inetutils.

  GNU Inetutils is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or (at
  your option) any later version.

  GNU Inetutils is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see `http://www.gnu.org/licenses/'. */

#include <config.h>

#include <sys/types.h>
#include <unistd.h>

unsigned short
icmp_cksum (unsigned char *addr, int len)
{
  register int sum = 0;
  unsigned short answer = 0;
  unsigned short *wp;

  for (wp = (unsigned short *) addr; len > 1; wp++, len -= 2)
    sum += *wp;

  /* Take in an odd byte if present */
  if (len == 1)
    {
      *(unsigned char *) &answer = *(unsigned char *) wp;
      sum += answer;
    }

  sum = (sum >> 16) + (sum & 0xffff);	/* add high 16 to low 16 */
  sum += (sum >> 16);		/* add carry */
  answer = ~sum;		/* truncate to 16 bits */
  return answer;
}
