sub analyze {
    my $number = shift;
    my $length = length $number;
    my $duplicates = 0;
    my %seen;

    while (length $number >= 4) {
        $seen{substr($number, -4)}++ and $duplicates++;
        substr($number, -1, 1, '');
    }

    my $missing = 10000 - keys %seen;
    return ($length, $missing, $duplicates)
}