simple method to check a cpf

check_cpf.rb

  def check_cpf(cpf=nil)
    return false if cpf.nil?

    known_invalid_cpfs = %w{12345678909 11111111111 22222222222 33333333333 
                            44444444444 55555555555 66666666666 77777777777 
                            88888888888 99999999999 00000000000 }
    cpf_digits = cpf.scan /[0-9]/
    if cpf_digits.length == 11
      unless known_invalid_cpfs.include?(cpf_digits.join)
        cpf_digits = cpf_digits.map{|x| x.to_i}
        sum = 0
        (0..8).each { |n| sum += (10-n) * cpf_digits[n] }
        sum = sum - (11 * (sum/11))
        expected_tenth_digit = (sum == 0 || sum == 1) ? 0 : 11 - sum
        if expected_tenth_digit == cpf_digits[-2] # is the 10th digit valid?
          sum = 0
          (0..9).each { |n| sum += cpf_digits[n] * (11-n) }
          sum = sum - (11 * (sum/11))
          expected_eleventh_digit = (sum == 0 || sum == 1) ? 0 : 11 - sum
          return true if expected_eleventh_digit == cpf_digits[-1] # is the last digit valid?
        end
      end
    end
    return false
  end