Support additional cases

1. When 4 products are added (one is free) and 5th one is added
2. When 7 producs are added (one is free) and 8th is added as free

Author: lukaszreszke

ecommerce/pricing/lib/pricing/discounts.rb

@@ -64,8 +64,10 @@ def exists?
     class ThreePlusOneGratis
       def apply(product_quantities, product_id, base_price)
         product = product_quantities.find { |product_quantity| product_quantity[:product_id] == product_id }
-        return base_price if product.nil?
-        product[:quantity] == 3 ? 0 : base_price
+        return [false, price: base_price] if product.nil?
+        quantity = product[:quantity] + 1
+        return [false, base_price] if quantity < 4
+        return [true, 0] if quantity % 4 == 0
       end
     end
   end

ecommerce/pricing/lib/pricing/offer.rb

@@ -14,8 +14,10 @@ def initialize(id)
 
     def add_item(product_id, base_price, promotion = nil)
       if promotion
-        price = promotion.apply(@list.quantities, product_id, base_price)
-      else
+        promotion_applies, price = promotion.apply(@list.quantities, product_id, base_price)
+      end
+
+      unless promotion_applies
         price = @discounts.inject(Discounts::NoPercentageDiscount.new, :add).apply(base_price)
       end
 
@@ -27,8 +29,8 @@ def add_item(product_id, base_price, promotion = nil)
         base_total_value: @list.base_sum + base_price,
         total_value: @list.actual_sum + price,
       }
-
-      data[:applied_promotion] = promotion.class.name if promotion
+      
+      data[:applied_promotion] = promotion.class.name if promotion_applies
 
       apply PriceItemAdded.new(data:)
     end

ecommerce/pricing/test/three_plus_one_test.rb

@@ -285,5 +285,115 @@ def test_given_3_plus_one__when_10_percent_discount_for_offer__then_offer_price_
         )
       ) { set_percentage_discount(order_id, 10) }
     end
+
+    def test_given_three_plus_one_promotion_when_five_items_are_added_then_only_one_item_is_free
+      product_id = SecureRandom.uuid
+      set_price(product_id, 20)
+      order_id = SecureRandom.uuid
+      stream = "Pricing::Offer$#{order_id}"
+
+      3.times { add_item(order_id, product_id, promotion: true) }
+
+      assert_events(
+        stream,
+        PriceItemAdded.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            base_price: 20,
+            price: 0,
+            base_total_value: 80,
+            total_value: 60,
+            applied_promotion: Pricing::Discounts::ThreePlusOneGratis.to_s
+          }
+        ),
+        OrderTotalValueCalculated.new(
+          data: {
+            order_id: order_id,
+            total_amount: 80,
+            discounted_amount: 60
+          }
+        ),
+        PriceItemValueCalculated.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            quantity: 4,
+            amount: 80,
+            discounted_amount: 60,
+          }
+        )
+      ) { add_item(order_id, product_id, promotion: true) }
+
+      assert_events(
+        stream,
+        PriceItemAdded.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            base_price: 20,
+            price: 20,
+            base_total_value: 100,
+            total_value: 80,
+          }
+        ),
+        OrderTotalValueCalculated.new(
+          data: {
+            order_id: order_id,
+            total_amount: 100,
+            discounted_amount: 80
+          }
+        ),
+        PriceItemValueCalculated.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            quantity: 5,
+            amount: 100,
+            discounted_amount: 80,
+          }
+        )
+      ) { add_item(order_id, product_id, promotion: true) }
+    end
+
+    def test_given_three_plus_one_promotion_when_eight_items_are_added_then_two_items_are_free
+      product_id = SecureRandom.uuid
+      set_price(product_id, 20)
+      order_id = SecureRandom.uuid
+      stream = "Pricing::Offer$#{order_id}"
+
+      7.times { add_item(order_id, product_id, promotion: true) }
+
+      assert_events(
+        stream,
+        PriceItemAdded.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            base_price: 20,
+            price: 0,
+            base_total_value: 160,
+            total_value: 120,
+            applied_promotion: Pricing::Discounts::ThreePlusOneGratis.to_s
+          }
+        ),
+        OrderTotalValueCalculated.new(
+          data: {
+            order_id: order_id,
+            total_amount: 160,
+            discounted_amount: 120
+          }
+        ),
+        PriceItemValueCalculated.new(
+          data: {
+            order_id: order_id,
+            product_id: product_id,
+            quantity: 8,
+            amount: 160,
+            discounted_amount: 120,
+          }
+        )
+      ) { add_item(order_id, product_id, promotion: true) }
+    end
   end
 end