Continue Developing the calculateDiscount Function

Welcome to your second lesson, which delves deeper into Test Driven Development (TDD) practices using Rust. In this unit, we will extend the functionality of our calculate_discount function by implementing five new requirements.

Building on the previous unit, this course emphasizes practical experience by sequentially providing requirements through tests. Your mission is to implement code that satisfies each test, mirroring a real-world TDD scenario. Consider this guide your pair programmer, offering test-driven prompts to enhance your expertise incrementally.

As a reminder, the Red-Green-Refactor cycle is crucial in TDD, steering your development process:

• Red: Begin by writing a failing test to define what to do next.
• Green: Develop just enough code to pass the test, concentrating on meeting the functionality.
• Refactor: Enhance the code for clarity and efficiency without changing its behavior.

• Description: The function should not accept negative discount percentages and must return an error in such cases.
• Test Case:
  #[cfg(test)]
  mod tests {
      use project::*;
  
      #[test]
      fn test_calculate_discount_negative_discount_returns_error() {
          // Arrange
          let original_price = 100.0;
          let discount_percentage = -10.0;
  
          // Act
          let result = calculate_discount(original_price, discount_percentage);
  
          // Assert
          assert!(result.is_err());
          assert_eq!(result.unwrap_err(), "discount cannot be negative");
      }
  }

• Description: The function should ensure that discounted prices don’t dip below the minimum limit due to very small original prices.
• Test Case:
  #[cfg(test)]
  mod tests {
      use project::*;
  
      #[test]
      fn test_calculate_discount_handles_very_small_prices_correctly() {
          // Arrange
          let original_price = 0.001;
          let discount_percentage = 1.0;
          let expected_discounted_price = 0.01;
  
          // Act
          let result = calculate_discount(original_price, discount_percentage).unwrap();
  
          // Assert
          assert!((result - expected_discounted_price).abs() < 0.01);
      }
  }