static int getCheckSum(str _ean)
{
    int         i;
    int         v;
    int         s;
    int         q;
    int         r;
    int         m;
    ;

    if( strLen(_ean) != 7  &&   // EAN 8
        strLen(_ean) != 11 &&   // UPC-A
        strLen(_ean) != 12 &&   // EAN 13
        strLen(_ean) != 13 &&   // EAN 14, ITF 14
        strLen(_ean) != 17 )    // SSCC
    {
        throw error("EAN not valid.");
    }

    for (i=strLen(_ean); i>0; i--)
    {
        v = str2int(substr(_ean, i, 1));

        m = ( m == 3 ? 1 : 3 );

        v = v * m;

        s += v;
    }

    q = s mod 10;
    if(q == 0)
    {
        r = 0;
    }
    else
    {
        r = 10 - q;
    }

    return r;
}