Add BuyXGetOneFreeDiscount

buy_x_get_one_free_discount.rb

@@ -0,0 +1,10 @@
+class BuyXGetOneFreeDiscount
+  def initialize(unit_price:, trigger_quantity: 2)
+    @unit_price = unit_price
+    @trigger_quantity = trigger_quantity.to_f
+  end
+
+  def discount_for_quantity(quantity)
+    return (quantity / @trigger_quantity).floor * @unit_price
+  end
+end

no_discount.rb

@@ -0,0 +1,5 @@
+class NoDiscount
+  def discount_for_quantity(quantity)
+    return 0.00
+  end
+end

product.rb

@@ -1,8 +1,10 @@
+require 'no_discount.rb'
 class Product
-  def initialize(price: 0.00, id:, description: '')
+  def initialize(price: 0.00, id:, discount: nil, description: '')
     @price = price
     @id = id
     @description = description
+    @discount = discount || NoDiscount.new
   end
 
   def price
@@ -13,6 +15,10 @@ class Product
     @id
   end
 
+  def discount
+    @discount
+  end
+
   def description
     @description
   end

product_in_checkout.rb

@@ -17,6 +17,6 @@ class ProductInCheckout
   end
 
   def total
-    @product.price * quantity
+    (@product.price * quantity) - @product.discount.discount_for_quantity(@quantity)
   end
 end

test/buy_x_get_one_free_discount_test.rb

@@ -0,0 +1,25 @@
+require "minitest/autorun"
+
+require 'buy_x_get_one_free_discount.rb'
+
+class BuyXGetOneFreeDiscountTest < Minitest::Test
+  def test_appropriate_discount_is_applied
+    unit_price = 4.00
+    (2..4).each do |trigger_quantity|
+      discount = BuyXGetOneFreeDiscount.new(unit_price: unit_price, trigger_quantity: trigger_quantity)
+
+      # Single discount at and above trigger quantity
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity)
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity + 1)
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity + trigger_quantity - 1)
+
+
+      # No discount if less than trigger quantity
+      assert_equal 0.00, discount.discount_for_quantity(trigger_quantity - 1)
+
+      # Double discount if double trigger quantity
+      assert_equal unit_price * 2, discount.discount_for_quantity(trigger_quantity * 2)
+      assert_equal unit_price * 2, discount.discount_for_quantity((trigger_quantity * 2) + 1)
+    end
+  end
+end

test/checkout_test.rb

@@ -1,13 +1,30 @@
 require "minitest/autorun"
 
 require "checkout.rb"
+require "buy_x_get_one_free_discount.rb"
 
 class CheckoutTest < Minitest::Test
   def setup
     @shop = Shop.new
-    @shop.add_product(Product.new(id: 'PI5',    price: 60.00))
-    @shop.add_product(Product.new(id: 'ZERO2W', price: 15.00))
-    @shop.add_product(Product.new(id: 'PICO2',  price: 4.00))
+    @shop.add_product(
+      Product.new(
+        id: 'PI5',
+        price: 60.00
+      )
+    )
+    @shop.add_product(
+      Product.new(
+        id: 'ZERO2W',
+        price: 15.00,
+        discount: BuyXGetOneFreeDiscount.new(unit_price: 15.00, trigger_quantity: 2)
+      )
+    )
+    @shop.add_product(
+      Product.new(
+        id: 'PICO2',
+        price: 4.00
+      )
+    )
     @checkout = Checkout.new(shop: @shop)
   end
 
@@ -69,21 +86,13 @@ class CheckoutTest < Minitest::Test
   # Discounts
 
   def test_that_bogof_applied
-    skip
-
-    4.times do
-      @checkout.add(:ZERO2W) # Should remove 2 Zeros
-    end
+    @checkout.add(product_id: 'ZERO2W', quantity: 4) # Should remove 2 Zeros
 
     assert_equal 30.00, @checkout.total
   end
 
   def test_that_bogof_applied_with_odd_number
-    skip
-
-    5.times do
-      @checkout.add(:ZERO2W) # Should still remove 2 Zeros, 5th is full price
-    end
+    @checkout.add(product_id: 'ZERO2W', quantity: 5) # Should still remove 2 Zeros, 5th is full price
 
     assert_equal 45.00, @checkout.total
   end