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checksum.c

/*  Copyright 2001 Rien Croonenborghs, Ben Kibbey, Shaun Jackman

    This file is part of lcab.
    lcab 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.
    lcab 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 lcab; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

#include "checksum.h"

// compute the checksum for the datablock
CHECKSUM compute_checksum(byte *in, word ncbytes, CHECKSUM seed)
{
      int no_ulongs;
      CHECKSUM csum=0;
      byte *stroom;
      CHECKSUM temp;

      no_ulongs = ncbytes / 4;
      csum = seed;
      stroom = in;

      while(no_ulongs-->0)
      {
            temp = ((CHECKSUM) (*stroom++));
            temp |= (((CHECKSUM) (*stroom++)) << 8);
            temp |= (((CHECKSUM) (*stroom++)) << 16);
            temp |= (((CHECKSUM) (*stroom++)) << 24);

            csum ^= temp;
      }

      temp = 0;
      switch(ncbytes%4)
      {
            case 3: temp |= (((CHECKSUM) (*stroom++)) << 16);
            case 2: temp |= (((CHECKSUM) (*stroom++)) << 8);
            case 1: temp |= ((CHECKSUM) (*stroom++));
            default: break;
      }
      
      csum ^= temp;

      return csum;      
}

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