// pucData - pointer to the byte stream of the .d2s file
// iSize - number of bytes in the stream ( filesize )
DWORD Checksum( unsigned char *pucData, int iSize )
{
    // delete old checksum at offset 0x0C
    *((unsigned int*)(pucData+12)) = 0;

    // init new checksum with 0
    unsigned int uiCS = 0;

    // this is the whole checksum calculation
    for ( int i = 0; i < iSize; ++i )
        uiCS = (uiCS<<1) + pucData[i] + ( uiCS & 0x80000000 ? 1 : 0 );

    // write new checksum to stream
    *((unsigned int*)(pucData+12)) = uiCS;
    return uiCS;
}