Rebuild the Checkout class, no discounts yet

Checkout now stores its items using a quantity and a link back to the
original product instance. It has a reference to the store to make sure
that product IDs added are valid

checkout.rb

@@ -1,62 +1,43 @@
+require 'product_in_checkout.rb'
+
 class Checkout
   class InvalidItem < StandardError; end
 
-  AVAILABLE_ITEMS = {
-    PI5: 60.00,
-    ZERO2W: 15.00,
-    PICO2: 4.00
-  }
-
-  def initialize
+  def initialize(shop:)
+    @shop = shop
     @items = []
   end
 
-  def add(item)
-    raise InvalidItem unless valid_item?(item)
-    @items.push item
-  end
+  def add(product_id:, quantity: 1)
+    product = @shop.product_by_id(product_id)
 
-  def remove(item)
-    if index = @items.index(item)
-      @items.delete_at(index)
-    end
+    raise InvalidItem unless product
 
-    @items
+    product_in_checkout = new_or_existing_product_in_checkout(product)
+    product_in_checkout.add_quantity(quantity)
+
+    @items.push product_in_checkout
   end
 
   def items
     @items
   end
 
-  def total
-    pre_discount_total - current_discount
+  def item_quantities
+    @items.map { |item| [item.product_id, item.quantity] }.to_h
   end
 
   def clear
     @items = []
   end
 
+  def total
+    @items.reduce(0.00) { |running_total, item| running_total + item.total }
+  end
+
   private
 
-  def valid_item?(item)
-    return AVAILABLE_ITEMS.keys.include?(item)
-  end
-
-  def current_discount
-    # BOGOF ZERO2W
-    discount = (@items.count(:ZERO2W) / 2.0).floor * AVAILABLE_ITEMS[:ZERO2W]
-
-    # Bulk PICO2
-    if(items.count(:PICO2) >= 3)
-      discount += items.count(:PICO2) * 0.50
-    end
-
-    return discount
-  end
-
-  def pre_discount_total
-    @items.reduce(0.00) do |running_total, item|
-      running_total + AVAILABLE_ITEMS[item]
-    end
+  def new_or_existing_product_in_checkout(product)
+    @items.find { |item| item.product_id == product.id } || ProductInCheckout.new(product: product)
   end
 end