luhn.rb

class Fixnum
  def digits
    to_s.split('').map(&:to_i)
  end

  def luhn?
    luhn_checksum % 10 == 0
  end

  def luhn_checksum
    odd_digits = []
    even_digits = []
    digits.reverse.each_with_index do |o, i|
      odd_digits << o if (i+1).odd?
      even_digits << o if (i+1).even?
    end
    total = odd_digits.inject(&:+)
    even_digits.each do |o|
      total += (2 * o).digits.inject(&:+)
    end
    total
  end

  def luhn
    self * 10 + ((self * 10).luhn_checksum * 9).digits.last
  end
end