class Prime
  @primes = {}
  @next = 2
  def self.include?(n)
    while @next <= n
      @primes[@next] = 0
      @primes[@next*@next] = @next

      while @primes[@next]
        @next += 1
        if @primes[@next]
          m = @primes[@next]
          i = @next + m
          while @primes[i]
            i += m
          end
          @primes[i] = m
        end
      end
    end

    @primes[n] == 0
  end

  def self.next
    @ptr ||= 1
    @ptr += 1
    while not include? @ptr
      @ptr += 1
    end

    return @ptr
  end

  def self.each
    while true
      yield self.next
    end
  end
end

class Fixnum
  def prime?
    Prime.include? self
  end
end