Circular Primes

circular_primes.rb

# Ruby solution to http://www.beatmycode.com/challenge/5/show

class Integer # in real apps should use refinements instead of monkey patching
  def prime?
    return false if self < 2
    ! 2.upto(self**0.5).any? { |n| self % n == 0 }
  end

  def rotations
    n, ss = to_s.length, to_s * 2
    0.upto(n-1).map { |offset| ss[offset, n].to_i }
  end

  def circular_prime?
    s = to_s
    return false if self > 5 && s =~ /[02458]/
    return false if self > 3 && s.each_char.map(&:to_i).inject(:+) % 3 == 0
    self.rotations.all?(&:prime?)
  end
end

module CircularPrimes
  def self.upto(upper_limit)
    1.upto(upper_limit).select(&:circular_prime?)
  end
end

if __FILE__ == $0
  max_n = ARGV[0]&.to_i || 1_000_000
  v = CircularPrimes.upto(max_n)
  puts "There are #{v.count} circular primes between 1 and #{max_n}"
end