Reimplement checkout item removal. All existing tests now pass

checkout.rb

@@ -13,10 +13,21 @@ class Checkout
 
     raise InvalidItem unless product
 
-    product_in_checkout = new_or_existing_product_in_checkout(product)
+    new_or_existing_product_in_checkout(product).add_quantity(quantity)
-    product_in_checkout.add_quantity(quantity)
+  end
 
-    @items.push product_in_checkout
+  def remove(product_id:, quantity: 1)
+    product = @shop.product_by_id(product_id)
+
+    raise InvalidItem unless product
+
+    product_in_checkout = existing_product_in_checkout(product)
+
+    return false unless product_in_checkout
+
+    product_in_checkout.remove_quantity(quantity)
+
+    trim_empty_items
   end
 
   def items
@@ -37,7 +48,21 @@ class Checkout
 
   private
 
+  def existing_product_in_checkout(product)
+    @items.find { |item| item.product_id == product.id }
+  end
+
   def new_or_existing_product_in_checkout(product)
-    @items.find { |item| item.product_id == product.id } || ProductInCheckout.new(product: product)
+    product_in_checkout = existing_product_in_checkout(product)
+    unless product_in_checkout
+      product_in_checkout = ProductInCheckout.new(product: product)
+      @items.push(product_in_checkout)
+    end
+
+    product_in_checkout
+  end
+
+  def trim_empty_items
+    @items = @items.filter { |item| item.quantity > 0 }
   end
 end

product_in_checkout.rb

@@ -16,6 +16,10 @@ class ProductInCheckout
     @quantity += quantity
   end
 
+  def remove_quantity(quantity)
+    @quantity = [@quantity - quantity, 0].max
+  end
+
   def total
     (@product.price * quantity) - @product.discount.discount_for_quantity(@quantity)
   end

test/checkout_test.rb

@@ -127,72 +127,54 @@ class CheckoutTest < Minitest::Test
   # Item removal
 
   def test_that_removing_an_item_not_in_the_checkout_removes_nothing
-    skip
+    @checkout.add(product_id: 'PICO2')
+    @checkout.remove(product_id: 'PI5')
 
-    @checkout.add(:PICO2)
+    assert_equal({ 'PICO2' => 1 }, @checkout.item_quantities)
-    @checkout.remove(:PI5)
-
-    assert_equal [:PICO2], @checkout.items
   end
 
   def test_that_items_are_removed_one_by_one
-    skip
+    @checkout.add(product_id: 'ZERO2W', quantity: 4)
+    @checkout.add(product_id: 'PICO2', quantity: 3)
 
-    4.times do
+    assert_equal({ 'ZERO2W' => 4, 'PICO2' => 3 }.sort, @checkout.item_quantities.sort)
-      @checkout.add(:ZERO2W)
-    end
 
-    3.times do
+    @checkout.remove(product_id: 'ZERO2W')
-      @checkout.add(:PICO2)
-    end
 
-    assert_equal [:ZERO2W, :ZERO2W, :ZERO2W, :ZERO2W, :PICO2, :PICO2, :PICO2].sort, @checkout.items.sort
+    assert_equal({ 'ZERO2W' => 3, 'PICO2' => 3 }.sort, @checkout.item_quantities.sort)
 
-    @checkout.remove(:ZERO2W)
+    @checkout.remove(product_id: 'PICO2')
 
-    assert_equal [:ZERO2W, :ZERO2W, :ZERO2W, :PICO2, :PICO2, :PICO2].sort, @checkout.items.sort
+    assert_equal({ 'ZERO2W' => 3, 'PICO2' => 2 }.sort, @checkout.item_quantities.sort)
 
-    @checkout.remove(:PICO2)
 
-    assert_equal [:ZERO2W, :ZERO2W, :ZERO2W, :PICO2, :PICO2].sort, @checkout.items.sort
+    @checkout.remove(product_id: 'PICO2', quantity: 2)
 
-    2.times do
+    assert_equal({ 'ZERO2W' => 3 }.sort, @checkout.item_quantities.sort)
-      @checkout.remove(:PICO2)
-    end
-
-    assert_equal [:ZERO2W, :ZERO2W, :ZERO2W].sort, @checkout.items.sort
   end
 
   def test_that_total_is_correctly_recalculated_when_items_are_added
-    skip
 
-    2.times do
+    @checkout.add(product_id: 'PICO2', quantity: 2)
-      @checkout.add(:PICO2)
-    end
 
     assert_equal 8.00, @checkout.total
 
-    @checkout.add(:PICO2) # 3 in total now, so discount applies
+    @checkout.add(product_id: 'PICO2', quantity: 1) # 3 in total now, so discount applies
 
     assert_equal 10.50, @checkout.total
 
-    4.times do
+    @checkout.add(product_id: 'ZERO2W', quantity: 4)
-      @checkout.add(:ZERO2W) # 30.00 with BOGOF
-    end
 
     assert_equal 40.50, @checkout.total
   end
 
   def test_that_total_is_correctly_recalculated_when_items_are_removed
-    skip
 
-    3.times do
+    @checkout.add(product_id: 'PICO2', quantity: 3)
-      @checkout.add(:PICO2) # Bulk discount applies
-    end
 
     assert_equal 10.50, @checkout.total
 
-    @checkout.remove(:PICO2) # Bulk discount no longer applies
+    @checkout.remove(product_id: 'PICO2', quantity: 1) # Bulk discount no longer applies
 
     assert_equal 8.00, @checkout.total
   end