Use Estonian reference number format instead of ISO 11649

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