/*
     This file is part of GNUnet.
     (C) 2010 Christian Grothoff

     GNUnet 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, or (at your
     option) any later version.

     GNUnet 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 GNUnet; see the file COPYING.  If not, write to the
     Free Software Foundation, Inc., 59 Temple Place - Suite 330,
     Boston, MA 02111-1307, USA.
*/

/**
 * @file vpn/gnunet-vpn-checksum.c
 * @brief
 * @author Philipp Toelke
 */

#include "gnunet-vpn-checksum.h"

uint32_t calculate_checksum_update(uint32_t sum, uint16_t *hdr, short len) {
    for(; len >= 2; len -= 2)
      sum += *(hdr++);
    if (len == 1)
      sum += *((unsigned char*)hdr);
    return sum;
}

uint16_t calculate_checksum_end(uint32_t sum) {
    while (sum >> 16)
      sum = (sum >> 16) + (sum & 0xFFFF);

    return ~sum;
}

/**
 * Calculate the checksum of an IPv4-Header
 */
uint16_t
calculate_ip_checksum(uint16_t* hdr, short len) {
    uint32_t sum = calculate_checksum_update(0, hdr, len);
    return calculate_checksum_end(sum);
}