Welcome to our session on eliminating duplicated code and improving maintainability by extracting methods and refactoring magic numbers. Writing clean and maintainable code is a key element in modern software development, with refactoring playing a vital role in achieving this. This lesson is dedicated to identifying duplicate code sections and restructuring them to enhance readability and maintainability.

In this course, we use Ruby, a dynamic, reflective, object-oriented, and general-purpose programming language, along with RSpec, a Behavior Driven Development (BDD) framework for Ruby. These tools aid us in writing reliable tests and handling code changes smoothly.

This lesson continues from an existing ShoppingCart example to focus on the 'Refactor' aspect of the 'Red, Green, Refactor' cycle. Let's dive into identifying and restructuring duplicated code to improve our codebase.

Before focusing on specific "smells," we should first understand what "code smells" signify. Code smells suggest potential issues in the code, such as poorly structured code or potential defects, even though they aren't bugs themselves. Common examples include duplicate code, long methods, and magic numbers. These issues often hinder code readability and maintainability, leading to significant problems over time.

Refactoring patterns offer structured techniques to tackle these code smells and improve the overall quality of the codebase while preventing behavioral changes. By using these patterns, we can systematically transform problematic sections into cleaner, more efficient code. In this lesson, we'll combat duplications by applying refactoring patterns, like extracting methods and refactoring magic numbers, to enhance our application code's clarity and adaptability. We can rely on our tests to ensure that we are not breaking any existing functionality!

When similar or identical code segments are replicated across a codebase, it leads to a "Code Duplication" issue or smell. This can result in maintenance difficulties and promote bugs. Consider, for instance, when constant values are spread across a codebase without a clear label. This complicates updates or adjustments, as changes need to be applied in multiple places, increasing the risk of errors.

Maintaining the DRY (Don't Repeat Yourself) principle ensures that every piece of knowledge has a singular, unambiguous representation within the system. This enhances maintainability and understanding while reducing debugging efforts. Remember, refactoring to eliminate code duplication will be part of our work routine in TDD, providing minimal disturbance to existing functionalities.

We'll look at how to extract common logic blocks as stand-alone methods using our ShoppingCart class. This example assumes we have repetitive logic that calculates the total cost for each item while considering potential discounts. We're starting with a set of passing tests that clearly demonstrate duplication in their implementation.

During refactoring, we fit calculate_item_cost into the ShoppingCart class so that all methods needing this logic can easily employ it. Ensuring that functionality remains the same while the code quality is enhanced is the essence of TDD.

The repetitive logic can be noticed within calculate_total and calculate_total_with_student_discount methods.

We can extract a method called calculate_subtotal that can be used by calculate_total and calculate_total_with_student_discount to get rid of the redundancy!

Now let's see "Magic Numbers." These are exact numbers that make an appearance in your code. Magic Numbers can cause maintenance headaches when these values need to be updated in the future, especially if they appear at different locations.

Here is an example:

The solution to magic numbers is simple: make them well-defined constants that can be reused throughout the codebase.

In this lesson, we explored the importance of refactoring in eliminating code duplication and magic numbers. By sticking to the TDD cycle — Red (write a failing test), Green (make it pass with bare minimum code), and Refactor (clean the code without changing behavior) — we ensure our code is both functional and maintainable.

Now, as you get ready for the upcoming practice exercises, concentrate on leveraging these skills to improve your ability to manage intricate codebases. Apply the concepts of DRY and TDD consistently to keep your code robust and adaptable. Keep practicing and mastering these principles, and you'll soon find yourself capable of developing highly efficient and scalable applications.