Validate Danish organization numbers (CVR). Uses https://github.com/barsoom/attr_extras.

danish_organization_number.rb

# Danish "CVR-nummer".
#
# A 7 digit serial number followed by a mod-11 check digit:
# https://erhvervsstyrelsen.dk/modulus-11-kontrol
#
# Search for real CVRs, to see examples: https://datacvr.virk.dk/data/
# E.g. 35408002, 30715063.

class DanishOrganizationNumber
  MOD_11_WEIGHTS = [ 2, 7, 6, 5, 4, 3, 2 ]

  pattr_initialize :string

  def valid?
    return false unless string.match(/\A[\s\d-]+\z/)
    return false unless digits.length == 8
    return false unless valid_check_digit?

    true
  end

  private

  def valid_check_digit?
    serial = digits[0, 7]
    expected_check_digit = digits[-1]

    weighted_serials = serial.zip(MOD_11_WEIGHTS).map { |digit, weight| digit * weight }
    mod = weighted_serials.sum % 11
    actual_check_digit = mod == 0 ? 0 : 11 - mod

    actual_check_digit == expected_check_digit
  end

  def digits
    @digits ||= string.to_s.gsub(/\D/, "").chars.map(&:to_i)
  end
end

danish_organization_number_spec.rb

require "spec_helper"
require "danish_organization_number"

describe DanishOrganizationNumber, "#valid?" do
  it "is true for an 8-digit string with valid check digit" do
    expect_valid "35408002"
  end

  it "accounts for the special case of a 0 check digit" do
    expect_valid "25472020"
  end

  it "allows '-' separators" do
    expect_valid "35-40-80-02"
  end

  it "allows whitespace separators" do
    expect_valid " 35 40 80 02"
  end

  it "does not allow extra text" do
    expect_invalid "foo 35408002 bar"
  end

  it "rejects too-short strings" do
    expect_invalid "123"
  end

  it "rejects strings with invalid check digit" do
    expect_invalid "35408003"
  end

  # Let's validate some real examples to make sure we got it right.
  # From: https://datacvr.virk.dk/data/
  %w[ 37272884 25555619 21459895 35408002 ].each do |number|
    it "accepts real-life number #{number}" do
      expect_valid number
    end
  end

  private

  def expect_valid(number)
    expect(DanishOrganizationNumber.new(number).valid?).to be_true
  end

  def expect_invalid(number)
    expect(DanishOrganizationNumber.new(number).valid?).to be_false
  end
end