luhn.rb

Factory.sequence :luhn do |n|
  def luhn?(n)
    odd = false

    0 == n.to_s.scan(/\d/).inject(0) { |a, d|
      i = d.to_i * (odd = !odd ? 2 : 1)
      a + (i > 9 ? i - 9 : i)
    } % 10
  end
  
  @last_luhn ||= 0

  begin
    @last_luhn += 1
  end while !luhn?(@last_luhn)

  @last_luhn
end