Welcome to your first lesson in this course dedicated to practicing Test Driven Development (TDD) using Swift and XCTest. Test Driven Development is an effective approach that prioritizes writing tests before coding. This guides you to develop your application around testing, ensuring that each component functions correctly as intended. In this lesson, you'll learn the fundamentals of TDD and the Red-Green-Refactor cycle and understand their roles in creating consistent and maintainable code.

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 implement code that makes each test pass, simulating a real-world TDD environment. As the course guide, it's akin to being your pair programmer, providing test-driven prompts to hone your skills as you progress.

As a reminder, the Red-Green-Refactor cycle is central to TDD, guiding your development process:

• Red: Start by writing a failing test to define the next step.
• Green: Implement just enough code to pass the test, focusing on functionality.
• Refactor: Optimize the code for clarity and efficiency without changing its behavior.

In this unit, we will outline the test cases that guide the implementation of the calculateDiscount function by employing the Red-Green-Refactor cycle of Test Driven Development (TDD). Each test case serves as a specific requirement, starting with writing a failing test ("Red") to define a behavior, followed by implementing minimal code changes to pass the test ("Green"), and finalizing with code optimization ("Refactor"). We will cover essential scenarios, such as applying correct percentage-based discounts, handling zero discount cases, managing decimal discount precision, and ensuring valid input handling to guarantee robustness and correctness.