prime? method (in fact a lambda) implemented in Crystal

prime.cr

macro break_if_enough divisible, divisor, to_return = nil
  q, r = {{divisible}}.divmod {{divisor}}
  {% if to_return %}
    return false if r == 0
    return true if q <= {{divisor}}
  {% else %}
    break false if r == 0
    break true if q <= {{divisor}}
  {% end %}
end

prime? = -> {
  # this array permanently stores table of primes up to the sqrt() of the nunmber you are checking
  primes = [2, 3, 5] of Int32

  candidate = 5
  step = 4

  -> (n : Int64) {
    # return false if n < 2

    primes.each do |prime|
      break_if_enough n, prime, :return
    end

    loop do
      loop do
        candidate += step = 6 - step
        break if primes.each do |prime|
          break_if_enough candidate, prime
        end
      end
      primes << candidate
      break_if_enough n, candidate
    end
  }

}.call