Test Suite Generator

Generate a test suite for the code below.

Work through it in four steps:

1. **Identify what needs testing**
   - List every function, method, or exported value.
   - Note which paths through the code are non-trivial and need coverage.

2. **Write unit tests**
   - Cover the happy path first.
   - Then cover edge cases: empty inputs, boundary values, null/undefined, unexpected types.
   - Each test should have a clear name that describes what it checks.

3. **Suggest mocks and stubs**
   - Identify external dependencies (databases, APIs, file system, timers, random).
   - For each, describe what to mock and why.
   - If a dependency is hard to mock cleanly, flag it as a testing risk.

4. **Flag untestable patterns**
   - Identify any code that is difficult to test: hidden state, tight coupling, missing dependency injection.
   - Suggest the minimal refactor needed to make it testable, if any.

Use the test framework I specify. If I don't specify one, use the most common choice for the language.

Format: one test block per function. Include import/setup boilerplate at the top.

**Language / framework:** [e.g. TypeScript + Vitest, Python + pytest]
**Code to test:**
```
[paste here]
```