Write tests

Cover behavior, not implementation details.

Steps

1. Identify the unit's contract: inputs, outputs, and side effects.
2. Enumerate cases: happy path, boundaries, invalid input, error handling, and any state-dependent behavior.
3. Name each test for the scenario it checks (e.g. "returns 0 for an empty list").
4. Arrange–Act–Assert; one logical assertion per test where practical.
5. Mock only true external dependencies (network, clock, filesystem) — never the code under test.

Output

A complete test file in the project's framework, with the riskiest cases first.