Use Estonian reference number format instead of ISO 11649

app/models/billing/reference_no.rb

@@ -0,0 +1,10 @@
+module Billing
+  class ReferenceNo
+    REGEXP = /\A\d{2,20}\z/
+
+    def self.generate
+      base = Base.generate
+      "#{base}#{base.check_digit}"
+    end
+  end
+end

app/models/billing/reference_no/base.rb

@@ -0,0 +1,48 @@
+module Billing
+  class ReferenceNo
+    class Base
+      def self.generate
+        new(SecureRandom.random_number(1..1_000_000))
+      end
+
+      def initialize(base)
+        @base = base.to_s
+      end
+
+      def check_digit
+        amount = amount_after_multiplication
+        next_number_ending_with_zero(amount) - amount
+      end
+
+      def to_s
+        base
+      end
+
+      private
+
+      attr_reader :base
+
+      def next_number_ending_with_zero(number)
+        next_number = number
+
+        loop do
+          next_number = next_number.next
+          return next_number if next_number.to_s.end_with?('0')
+        end
+      end
+
+      def amount_after_multiplication
+        multipliers = [7, 3, 1]
+        enumerator = multipliers.cycle
+        amount = 0
+
+        base.reverse.each_char do |char|
+          digit = char.to_i
+          amount += (digit * enumerator.next)
+        end
+
+        amount
+      end
+    end
+  end
+end

app/models/registrar.rb

@@ -14,9 +14,10 @@ class Registrar < ActiveRecord::Base
   delegate :balance, to: :cash_account, allow_nil: true
 
   validates :name, :reg_no, :country_code, :email, :code, presence: true
-  validates :name, :reference_no, :code, uniqueness: true
+  validates :name, :code, uniqueness: true
   validates :accounting_customer_code, presence: true
   validates :language, presence: true
+  validates :reference_no, format: Billing::ReferenceNo::REGEXP
   validate :forbid_special_code
 
   validates :vat_rate, presence: true, if: 'foreign_vat_payer? && vat_no.blank?'
@@ -29,7 +30,6 @@ class Registrar < ActiveRecord::Base
 
   attribute :vat_rate, ::Type::VATRate.new
   after_initialize :set_defaults
-  before_validation :generate_iso_11649_reference_no
 
   validates :email, :billing_email,
     email_format: { message: :invalid },
@@ -167,27 +167,6 @@ class Registrar < ActiveRecord::Base
     errors.add(:code, :forbidden) if code == 'CID'
   end
 
-  def generate_iso_11649_reference_no
-    return if reference_no.present?
-
-    loop do
-      base = nil
-      loop do
-        base = SecureRandom.random_number.to_s.last(8)
-        break if base.to_i != 0 && base.length == 8
-      end
-
-      control_base = (base + '2715' + '00').to_i
-      reminder = control_base % 97
-      check_digits = 98 - reminder
-
-      check_digits = check_digits < 10 ? "0#{check_digits}" : check_digits.to_s
-
-      self.reference_no = "RF#{check_digits}#{base}"
-      break unless self.class.exists?(reference_no: reference_no)
-    end
-  end
-
   def home_vat_payer?
     country == Registry.instance.legal_address_country
   end