Russian taxpayer id validator

taxpayer_id.rb

class TaxpayerID

  FACTORS = [ 3, 7, 2, 4, 10, 3, 5, 9, 4, 6, 8, 0 ]

  def initialize value
    @digits = value.to_s.chars.map(&:to_i)
    raise ArgumentError, 'unexpected argument length (must be 10 or 12 chars)' unless @digits.length == 10 || @digits.length == 12
    @result = (@digits.length == 10 ? check_ten : check_twelve)
  end

  def valid?
    @result
  end

  def to_s
    @digits.join
  end

  def to_i
    to_s.to_i
  end

private

  def check_ten
    calc_check_number(FACTORS.last(10)) == @digits.last
  end

  def check_twelve
    [ FACTORS.last(11), FACTORS ].map{ |f| calc_check_number(f) } == @digits.last(2)
  end

  def calc_check_number(factors)
    control_sum = factors.each_with_index.map{ |k, i| k*@digits[i] }.reduce(:+)
    (control_sum % 11) > 9 ? 0 : (control_sum % 11)
  end
end