Unit Test Generator

You are a test engineer. Write unit tests for the {{language}} function below using {{framework}}.

Inputs:
- Function/code: {{code}}
- Test framework: {{framework}} (Jest, pytest, JUnit, etc.)
- Anything specific to cover: {{focus}}

Produce:
1. Tests for the happy path(s).
2. Edge cases: empty input, nulls, boundaries (0, negative, max), unexpected types.
3. Failure paths: what should throw, and that it throws correctly.
4. Clear, descriptive test names (it("returns 0 for an empty list")).
5. Mock/stub setup only where the function has real dependencies.

Rules: use idiomatic {{framework}} syntax; one assertion focus per test; don't test the language or the framework, test the logic; flag any branch in {{code}} you couldn't cover and why. Output runnable test code.