gistfile1.builder

  def self.luhn_valid?(number)
    return false unless number.to_s =~ /\A\d+\z/
    digits = number.scan(/\d/).map(&:to_i).reverse
    digits.each_with_index do |digit, index|
      if index.odd?
        digits[index] = (digit >= 5 ? digit * 2 - 9 : digit * 2)
      end
    end
    digits.sum % 10 == 0
  end