gistfile1.builder

def luhn?(number)
  digits = number.scan(/\d/).map { |d| d.to_i }.reverse
  digits.each_with_index do |digit, index|
    if index % 2 == 1
      digits[index] = (digit >= 5 ? digit * 2 - 9 : digit * 2)
    end
  end
  digits.inject(0) { |a,b| a + b } % 10 == 0
end