sub cd_AM_HOP {  # iterator solution

    my ($n_digits,  # n of digits for which to generate luhn checksum
        ) = @_;

    my $order = $n_digits & 1;
    my @straight = grep +($_ & $order), 0 .. $n_digits-1;  # dd '---', \@straight;
    my @adjusted = grep !($_ & $order), 0 .. $n_digits-1;  # dd '===', \@adjusted;

    my @check_digit = map { (10 - $_ % 10) % 10 } 0 .. 9 * $n_digits;

    return sub {

        my $ccn = shift;  # digits for which to generate checksum

        my $total = 0;

        for (@straight) {
            $total += substr($ccn, $_, 1);
            }

        for (@adjusted) {
            $total += $x2_cast_out_9[ substr($ccn, $_, 1) ];
            }

      # return $check_digit[ $total ];  # return check digit
        $total *= 9;
        return chop $total;  # return check digit

        };  # end sub iterator

    }