Welcome to your fourth unit of this course dedicated to practicing Test Driven Development (TDD). We're going to continue building our ShoppingCart system by adding even more features using Go and Testify.

In this course, emphasis is placed on hands-on practice, where you'll receive requirements through tests, one at a time. Your task is to write tests AND implement code that makes each test pass, simulating a real-world TDD environment.

Remember to use the core concepts of the Red-Green-Refactor cycle while completing these coding exercises. I'm still here to help! Just ask.

• Description: Trying to remove an item that is not present in the cart should return an error, indicating that the item was not found.
• Details:
  • Implement the removal through a RemoveItem(id int) error function.
  • Ensure the function returns an error with an appropriate message if the item is not found in the cart.
• Examples: Attempting to remove an item with Id: 999 should return an error with the message "Item not found".

• Description: Applying a percentage discount should adjust the total price of the items in the cart accordingly.
• Details:
  • Use the ApplyDiscount(percentage float64) float64 method to apply a percentage discount to the total.
  • Ensure GetTotal() returns the adjusted price after applying the discount.
• Examples: Applying a 10% discount to a total price of 100 should result in a new total of 90.

• Description: Automatically apply a 10% discount when the total price of items in the cart exceeds $150.
• Details:
  • If the total exceeds 150, the 10% discount should be automatically applied.
  • The GetTotal() method should return the discounted total when the discount is applicable.
• Examples: Adding a "Book" with a price of 200 should result in a total of 180 after applying the bulk discount.

• Description: The cart should have the functionality to remove all items, resetting both the item count and the total price to zero.
• Details:
  • Implement a Clear() method to remove all items from the cart.
  • Ensure GetItemCount() returns 0 after clearing.
  • Verify GetTotal() is 0 after clearing.
• Examples: If there are multiple items in the cart, calling Clear should leave the count as 0 and the total as 0.

• Description: When the quantity of an item in the cart is changed, both the item count and total price should accurately reflect the new quantity.
• Details:
  • Allow item quantities to be updated using an UpdateQuantity(id int, quantity int) error function.
  • Ensure GetItemCount() returns the correct total item count after the quantity is updated.
  • Ensure GetTotal() returns the correct total price after the quantity is updated.
• Examples: Updating the quantity of a "Book" with a price of 10 from 2 to 3 should result in a count of 3 and a total of 30.

In this unit, you explored the design of test cases for a ShoppingCart struct, focusing on essential features like handling an empty cart, adding items, managing item quantities, and removing items. Now it's your turn to ensure that the described functionality is implemented by writing comprehensive test cases and ensuring that the tests pass with the least amount of code needed.

As you undertake these exercises, remember to engage in the Red-Green-Refactor cycle. Be sure to practice writing tests first, and do not write implementation code unless the test asks for it.

Red! Green! Refactor!