Prime::A2Generator < Prime::PseudoPrimeGenerator

prime.a2.rb

# original: http://log.ttsky.net/article/19515687.html
require 'prime'

class Prime

  # An implementation of +Prime::PseudoPrimeGenerator+.
  class A2Generator < PseudoPrimeGenerator
    def initialize
      @h = {}
      @n = nil
      @d = nil
      super
    end

    def succ
      if @n.nil?
        @n = 1
        return 2
      elsif @d.nil?
        @d = 4
        return 3
      end
      n = @n
      d = @d
      while true
        m = n += d
        if (b = @h[n])
          k = ((m + b) % 3 == 0) ? 2 : 4
          k = 6 - k while @h[m += b*k]
          @h[m] = b
          @h.delete(n)
        else
          k = 4
          k = 6 - k while @h[m += n*k]
          @d = 6 - d
          return @n = @h[m] = n
        end
        d = 6 - d
      end
    end
    alias next succ

    def rewind
      initialize
    end
  end
end

if $0 == __FILE__
  max = ARGV.size == 1 ? ARGV[0].to_i : 100
  pr = 0
  Prime.each(max, Prime::A2Generator.new){|num|pr=num}
  puts "The max prime number (<=#{max}) is #{pr}."
  pr = Prime.each(nil, Prime::A2Generator.new).take(max)[-1]
  puts "The #{max}th prime number is #{pr}."
end