Instructions

Your task is to extend the existing code with new functionality. The existing code simulates a shopping cart from the corner shop. The current shopping cart has a limited list of available products:

Name	Price	SKU
Potatoes	10.00	STAPOT
Rice	30.00	STARIC
Coffee	14.99	STACOF
Newspaper	2.99	MEDNEW

And provides three public methods:

• ShoppingCart#add(sku): Takes a stock-keeping unit (SKU) and adds it to the cart.
• ShoppingCart#total_amount: The value of the items in the cart as a number with two decimal places (e.g. 3.99).
• ShoppingCart#items_list: A list of the names of the items in the cart, ordered alphabetically, with no duplicates, separated by a comma and a space.

For example:

shopping_cart.add(:STAPOT)
shopping_cart.add(:STARIC)

shopping_cart.total_amount
// Returns: 40.00

shopping_cart.items_list
// Returns: "Potatoes, Rice"

In order to keep customers happy, the shop has added discount coupons. This means that the code extension should be able to read and apply discount coupons to the current items. The supported coupons are:

Type	SKU	Rule
Fixed price discount	FIX_TEN	Discounts 10.00 off the total amount of the cart
Product discount	HALF_STAPOT	Reduces all STAPOT items to half price

Your task is to define a new ShoppingCart#scan_coupon method which takes a SKU and adds it to the cart. The SKU is a Symbol comprised of the first three uppercase letters of the discount type + the SKU of the relevant product. In case the coupon type is a fixed price discount, the second part of the SKU is the discounted amount in uppercase letters. Coupons are unique and can be applied only once.

Please bear in mind that the total_amount must not be a negative number, the minimum value for the total_amount is 0.

Example:

shopping_cart = ShoppingCart.new
shopping_cart.add(:STAPOT)
shopping_cart.add(:MEDNEW)
shopping_cart.add(:MEDNEW)
shopping_cart.scan_coupon(:FIX_TEN)

shopping_cart.total_amount
// Returns: 5.98

Debugging

When a test fails, a message is displayed describing what went wrong and for which input. You can also use the fact that any console output will be shown too. You can write to the console using:

debug("Debug message")