CUIT - CUIL

valid_cuit.rb

class Object
  def valid_cuit?
    number = self.to_s.gsub(/\D/, '').split('')

    return if number.size != 11

    numbers = %w(5 4 3 2 7 6 5 4 3 2)
    result  = 0;

    9.times.each { |i| result += number[i].to_i * numbers[i].to_i }

    result = result % 11
    result = 11 - result

    result = 0 if result == 11
    result = 9 if result == 10

    result == number[10].to_i
  end
end

20311251407.valid_cuit? # => true
20311251405.valid_cuit? # => false