luhn.rb

module Luhn
  extend self

  def valid_cc?(num)
    sum(num) % 10 == 0
  end

  def string_with_check_number(string)
    sum = sum(string, true)
    "#{string}#{check_digit(sum)}"
  end

  private

  def check_digit(sum)
    10 - sum % 10
  end

  def sum(num, includes_check_number = false)
    digits = split(num)

    digits.reverse.each_slice(2).inject(0) do |sum, d|
      d1, d2 = d[0].to_i, d[1].to_i

      if includes_check_number
        # swap
        d1, d2 = d2, d1
      end

      d2 = d2 * 2
      d2 -= 9 if d2 > 9
      sum += d1 + d2
    end
  end

  def split(num)
    num.to_s.split('').map(&:to_i)
  end
end

########################################################################################

# unit tests

require 'test/unit'

class TestLuhn < Test::Unit::TestCase
  # http://www.paypalobjects.com/en_US/vhelp/paypalmanager_help/credit_card_numbers.htm
  def test_valid_cc?
    assert_equal(true, Luhn.valid_cc?(5105105105105100))
    assert_equal(true, Luhn.valid_cc?(4111111111111111))
    assert_equal(true, Luhn.valid_cc?(4222222222222))
    assert_equal(true, Luhn.valid_cc?(378282246310005))
    assert_equal(false, Luhn.valid_cc?(42222222222))
  end

  def test_string_with_check_number
    assert_equal('79927398713', Luhn.string_with_check_number('7992739871'))
  end
end