func is_circular_prime(n) {
    n.is_prime || return false

    var circular = n.digits
    circular.min < circular.tail && return false

    for k in (1 ..^ circular.len) {
        with (circular.rotate(k).digits2num) {|p|
            (p.is_prime && (p >= n)) || return false
        }
    }

    return true
}

say "The first 19 circular primes are:"
say 19.by(is_circular_prime)

say "\nThe next 4 circular primes, in repunit format, are:"
{|n| (10**n - 1)/9 -> is_prob_prime }.first(4, 4..Inf).each {|n|
    say "R(#{n})"
}

say "\nRepunit testing:"
[5003, 9887, 15073, 25031, 35317, 49081].each {|n|
    var now = Time.micro
    say ("R(#{n}) -> ", is_prob_prime((10**n - 1)/9) ? 'probably prime' : 'composite',
        " (took: #{'%.3f' % Time.micro-now} sec)")
}