Rails BSN (burgerservicenummer) validator

bsn.rb

module BSN
  # https://nl.wikipedia.org/wiki/Burgerservicenummer#11-proef
  def valid?(val)
    num = val.to_s.scan(/\d/).join.to_i
    return false if num == 0

    digits = ("%09d" % num).split(//).map(&:to_i).reverse
    return false unless digits.size == 9

    digits[0] = -digits[0]
    digits.reduce([]){|m,n| m <<  (n * (m.size + 1))}.reduce{|m,n| m + n} % 11 == 0
  end
  module_function :valid?

  if defined?(::ActiveRecord)
    module ActiveRecordValidations
      def validates_bsn_elfproef_of(*attr_names)
        attr_names = attr_names.dup
        options = attr_names.extract_options!

        validate do |record|
          attr_names.each do |attr|
            next if options[:if] && !options[:if][record]

            val = record.public_send(attr)
            next if options[:allow_blank] && val.blank?

            unless BSN.valid?(val)
              record.errors.add(attr, I18n.t('activerecord.errors.messages.bsn_malformed'))
            end
          end
        end
      end
    end

    class ::ActiveRecord::Base
      extend BSN::ActiveRecordValidations
    end
  end
end

if $0 == __FILE__
  require 'test/unit'

  class BSNTest < Test::Unit::TestCase
    def test_valid?
      assert_equal true, BSN.valid?('111222333')
      assert_equal true, BSN.valid?('123456782')
      assert_equal false, BSN.valid?('123456781')
    end
  end
end