Support custom GTINs

README.md

@@ -63,6 +63,46 @@ bad.to_all_valid         # => []
 ```
 
 
+Custom GTINs
+------------
+
+If the standard GTINs provided are not enough for your needs, you can implement your own by subclassing `BarcodeValidation::GTIN::Base` or any of its subclasses. If your custom class overlaps with a default class or one of your other custom classes, you can declare `prioritize_before <other class>` to re-order their evaluation order.
+
+An example:
+
+```ruby
+# A custom class that handles any length GTIN as long as it starts with "123".
+# Note that we must still provide a VALID_LENGTH to allow transcoding to other GTINs by zero-padding.
+# Due to this inheriting from Base, it is automatically registered and added to the end of the list of classes to check if it `handles?` an input.
+class MyCustomGTIN < BarcodeValidation::GTIN::Base
+  VALID_LENGTH = 20
+
+  def self.handles?(input)
+    input.start_with?("123") && input.length <= VALID_LENGTH
+  end
+
+  # Custom validity check
+  def valid?
+    self.class.handles?(input) && check_digit.valid?
+  end
+end
+
+# A custom implementation of GTIN13, which addresses a subset of the GTIN13 range.
+class MyCustomGTIN13 < BarcodeValidation::GTIN::GTIN13
+  # Ensure we get a chance to handle GTINs before our parent,
+  # so we can handle the subset we care about and have our parent handle the rest.
+  prioritize_before BarcodeValidation::GTIN::GTIN13
+
+  def self.handles?(input)
+    input.start_with?("123") && super
+  end
+
+  def valid?
+    input.start_with?("123") && super
+  end
+end
+```
+
 
 Development
 -----------

lib/barcodevalidation/gtin.rb

@@ -2,27 +2,64 @@
 
 require "forwardable"
 require_relative "invalid_gtin"
-require_relative "gtin/base"
-require_relative "gtin/check_digit"
-require_relative "gtin/gtin8"
-require_relative "gtin/gtin12"
-require_relative "gtin/gtin13"
-require_relative "gtin/gtin14"
 
 module BarcodeValidation
+  # GTIN is responsible for wrapping input in an appropriate GTIN::Base sub-class.
+  # An important part of this involves managing the prioritized list of GTIN classes we use for handling input.
+  # The methods implemented here are used by GTIN::Base to manage this list and prioritize classes.
   module GTIN
     class << self
       def new(input)
         (class_for_input(input) || BarcodeValidation::InvalidGTIN).new(input)
       end
 
+      # Adds the provided class to the back of the list of prioritized GTIN classes.
+      def append_gtin_class(gtin_class)
+        return if gtin_class?(gtin_class)
+
+        prioritized_gtin_classes.push(gtin_class)
+        nil
+      end
+
+      # Ensure the provided class is removed from the list of prioritized GTIN classes.
+      def remove_gtin_class(gtin_class)
+        prioritized_gtin_classes.delete(gtin_class)
+        nil
+      end
+
+      # Is this a registered prioritized GTIN class?
+      # @return [true, false]
+      def gtin_class?(gtin_class)
+        prioritized_gtin_classes.include?(gtin_class)
+      end
+
+      # @param [Class] high_priority_class The higher priority GTIN class you want to move before the low priority class
+      # @param [Class] low_priority_class The low priority GTIN class that the high priority one is moved before
+      def reprioritize_before(high_priority_class, low_priority_class)
+        low_priority_index = prioritized_gtin_classes.index(low_priority_class)
+        remove_gtin_class(high_priority_class)
+        prioritized_gtin_classes.insert(low_priority_index, high_priority_class)
+        nil
+      end
+
       private
 
+      def prioritized_gtin_classes
+        @prioritized_gtin_classes ||= []
+      end
+
       def class_for_input(input)
-        [GTIN8, GTIN12, GTIN13, GTIN14].find do |klass|
-          input.to_s.size == klass::VALID_LENGTH
-        end
+        input = input.to_s.freeze
+        prioritized_gtin_classes.find { |klass| klass.handles?(input) }
       end
     end
   end
 end
+
+# Load GTIN implementations after we have our registration setup
+require_relative "gtin/base"
+require_relative "gtin/check_digit"
+require_relative "gtin/gtin8"
+require_relative "gtin/gtin12"
+require_relative "gtin/gtin13"
+require_relative "gtin/gtin14"

lib/barcodevalidation/gtin/base.rb

@@ -19,6 +19,34 @@ def initialize(input)
         BarcodeValidation::InvalidGTIN.new(input, error: e)
       end
 
+      # Does this class (potentially) handle a GTIN that matches the input?
+      # Subclasses can choose to implement their own logic. The default is to look at +VALID_LENGTH+ and use that to match the length of the input the class handles.
+      def self.handles?(input)
+        return false unless self.const_defined?(:VALID_LENGTH)
+
+        input.length == self::VALID_LENGTH
+      end
+
+      # Upon inheritance, register the subclass so users of the library can dynamically add more GTINs in their own code.
+      def self.inherited(subclass)
+        BarcodeValidation::GTIN.append_gtin_class(subclass)
+      end
+
+      # Ensure this class is earlier in the GTIN classes list than +other_gtin_class+ and thus will get asked earlier if it handles a GTIN.
+      def self.prioritize_before(other_gtin_class)
+        raise ArgumentError, "The class you want to prioritize before is not a registered prioritized GTIN class." unless GTIN.gtin_class?(other_gtin_class)
+
+        GTIN.reprioritize_before(self, other_gtin_class)
+      end
+
+      # This class is abstract and should not be included in the list of GTIN classes that actually implement a GTIN.
+      def self.abstract_class
+        BarcodeValidation::GTIN.remove_gtin_class(self)
+      end
+
+      # GTIN::Base is an abstract class. See GTIN8/12/13/14 for implementations of actual GTINs.
+      abstract_class
+
       def valid?
         valid_length == length && check_digit.valid?
       end

spec/lib/barcodevalidation/gtin_spec.rb

@@ -111,4 +111,75 @@
       end
     end
   end
+
+  describe "custom classes" do
+    after(:each) do
+      # Remove references to classes we may have generated during a test to ensure we don't pollute the list of classes for other tests
+      classes_to_cleanup.each do |class_sym|
+        next unless Object.const_defined?(class_sym)
+
+        class_obj = Object.const_get(class_sym)
+        BarcodeValidation::GTIN.remove_gtin_class(class_obj)
+        Object.send(:remove_const, class_sym)
+      end
+    end
+
+    describe "inheritance" do
+      let(:classes_to_cleanup) { [:MyCustomGTIN] }
+
+      it "appends itself to BarcodeValidation::GTIN.prioritized_gtin_classes when inheriting from GTIN::Base" do
+        class MyCustomGTIN < BarcodeValidation::GTIN::Base
+        end
+
+        expect(BarcodeValidation::GTIN.gtin_class?(MyCustomGTIN)).to eq(true)
+      end
+
+      it "appends itself to BarcodeValidation::GTIN.prioritized_gtin_classes when inheriting from a GTIN::Base descendant" do
+        class MyCustomGTIN < BarcodeValidation::GTIN::GTIN12
+        end
+
+        expect(BarcodeValidation::GTIN.gtin_class?(MyCustomGTIN)).to eq(true)
+      end
+    end
+
+    describe "prioritize_before" do
+      let(:classes_to_cleanup) { [:MyCustomGTIN13] }
+
+      it "causes this class to be evaluated before the other one when checking which one handles a GTIN input" do
+        class MyCustomGTIN13 < BarcodeValidation::GTIN::GTIN13
+          prioritize_before BarcodeValidation::GTIN::GTIN13
+
+          def self.handles?(input)
+            input.start_with?("123") && super
+          end
+        end
+
+        expect(BarcodeValidation::GTIN.new("1234567890123").class).to eq(MyCustomGTIN13)
+        expect(BarcodeValidation::GTIN.new("1004567890123").class).to eq(BarcodeValidation::GTIN::GTIN13)
+      end
+    end
+
+    describe "abstract_class" do
+      let(:classes_to_cleanup) { %i[MyCustomAbstractGTIN MyCustomParentGTIN MyCustomChildGTIN] }
+
+      it "removes itself from BarcodeValidation::GTIN.prioritized_gtin_classes" do
+        class MyCustomAbstractGTIN < BarcodeValidation::GTIN::Base
+          abstract_class
+        end
+
+        expect(BarcodeValidation::GTIN.gtin_class?(MyCustomAbstractGTIN)).to eq(false)
+      end
+
+      it "is not inherited by children" do
+        class MyCustomParentGTIN < BarcodeValidation::GTIN::Base
+          abstract_class
+        end
+
+        class MyCustomChildGTIN < MyCustomParentGTIN
+        end
+
+        expect(BarcodeValidation::GTIN.gtin_class?(MyCustomChildGTIN)).to eq(true)
+      end
+    end
+  end
 end