int cd4( char *ccn ) {
    int total = 0;
    int i;
    int co9[]={0,2,4,6,8,1,3,5,7,9};
    int straight[]={1,3,5,7,9,11,13};
    int adjusted[]={0,2,4,6,8,10,12,14};
    for (i=0;i<7;i++) {
        total += ccn[straight[i]]-48;
    }
    for (i=0;i<8;i++) {
        total += co9[ccn[adjusted[i]]-48];
    }
    total *=9;
    return total %10;
}