Write unit tests for a function

Write unit tests for the function below using [testing framework]. Cover:
- Typical inputs and expected outputs
- Edge cases such as empty values, boundaries and very large inputs
- Invalid input and error handling

Use clear test names that describe the behaviour being tested, keep each test focused on one thing, and add a short comment where the expected result is not obvious.

Function:
[paste function here]

Replace the values in [square brackets] with your own details before using the prompt.