Unit Test Writer

Write unit tests for this function:

```[LANGUAGE]
[PASTE FUNCTION]
```

Using: [Jest / Pytest / JUnit / other]

Generate tests for:
1. Happy path (normal inputs, expected outputs)
2. Edge cases (empty, null, boundary values)
3. Error cases (invalid inputs, exceptions)
4. Performance (if applicable)

Use descriptive test names: "should [expected behavior] when [condition]"