/*
Packets checksum for One Must Fall: Battlegrounds 0.1
by Luigi Auriemma
e-mail: aluigi@autistici.org
web:    aluigi.org

This simple function calculates the checksum of a OMF packet
and returns it as a 32bit number that you must copy at the end
of the packet (the last 4 bytes of each packet).
Remember to pass ALL the size of the packet (checksum included)
because as you can see the last 4 bytes are automatically
skipped into my function

    Copyright 2004,2005,2006 Luigi Auriemma

    This program 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 2 of the License, or
    (at your option) any later version.

    This program 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, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA

    http://www.gnu.org/licenses/gpl.txt

*/

unsigned int omfcksum(unsigned char *buff, int size) {
    unsigned char   eax = 0,
                    *ptr;
    unsigned int   cksum = 0;

    size -= 4;
    ptr = (unsigned char *)&cksum;
    while(size--) {
        ptr[eax] += *buff;
        buff++;
        eax++;
        if(eax == 4) eax = 0;
    }
    return(cksum);
}