Chinese ID card number checksum

chinese_id.rb

module ChineseId

  WEIGHT = [7, 9, 10, 5, 8, 4, 2, 1, 6, 3, 7, 9, 10, 5, 8, 4, 2]
  CODE = ["1", "0", "X", "9", "8", "7", "6", "5", "4", "3", "2"]

  #S = Sum(Ai * Wi)
  #Y = mod(S, 11)
  #C = Cy
  def self.checksum id
    
    if "String" == id.class.name
      number = id.split(//).map{|c| c.to_i}
    else
      number = id
    end
    sum = 0
    number[0..16].zip(WEIGHT) do |x,y|
      sum += x * y
    end
    y = sum % 11
    CODE[y]
  end

end