luhn.rb

require 'minitest/autorun'
require 'pp'

module Luhn
  def self.is_valid?(number)
    each_digit(number).reverse.map.with_index do |e, i|
      r = e.to_i
      r *= 2 if i.odd?
      r -= 9 if r >= 10
      r
    end.inject(&:+) % 10 == 0
  end

  private

  def self.each_digit(number)
    number.to_s.each_char.to_a
  end
end

class TestLuhn < MiniTest::Unit::TestCase
  def test_luhn_valid
    assert Luhn.is_valid?(4_194_560_385_008_504), 'has a valid number'
  end

  def test_luhn_invalid
    assert !Luhn.is_valid?(4_194_560_385_008_505), 'has a valid number'
  end

  def test_luhn_valid2
    assert Luhn.is_valid?(377_681_478_627_336),
           'Check step two: Did you start at the right?'
  end

  def test_luhn_invalid2
    assert !Luhn.is_valid?(377_681_478_627_337),
           'Check step two: Did you start at the right?'
  end
end