LuhnValidator.rb

class Bank::LuhnValidator
  def self.valid_credit_card_number?(card_number)
    valid = true
    # Input requirements dictate that the card_number
    # be numeric and that it be upto 19 characters
    valid &= card_number.is_a?(Integer) &&
              (1..19).cover?(card_number.to_s.size)

    valid &= luhn_valid?(card_number)

    valid
  end

  def self.luhn_valid?(card_number)
    digits = card_number.to_s.split('').map(&:to_i)

    digits.reverse!

    i = 0
    luhn_sum = digits.inject(0) do |sum, curr_digit|
      curr_digit *= 2 if i.odd?
      sum += sum_digits(curr_digit)

      i += 1

      sum
    end


    (luhn_sum % 10).zero?
  end

  private

  def self.sum_digits(num)
    num.to_s.split('').map(&:to_i).inject {|sum, x| sum + x }
  end
end