Rails TCKN validator

tckn_validator.rb

class TcknValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless validate_tckn(value)
      record.errors.add(attribute, options[:message] || :invalid)
    end
  end

  protected
    def validate_tckn(tckn)
      digits = tckn[0..8].each_char.map(&:to_i).each_with_index

      first, last =
        digits.reduce([0, 0]) do |memo, (digit, idx)|
          add = digit * (idx.even? ? 7 : -1)
          [
            # Multiply all even digits with 7 and odd digits with -1 and sum
            (memo.first + add) % 10,
            # Sum all the digits and add the first sum. We add the first sum
            #  by accumulating it on to this number as well.
            (memo.last + digit + add) % 10
          ]
        end

      tckn[9] == first.to_s && tckn[10] == last.to_s
    end
end