Unit Tests by Equivalence Class and Boundary

Act as a senior test engineer. Write unit tests for this code by systematically choosing inputs, not by guessing a few happy paths.

Language and test framework: {{LANGUAGE}} / {{TEST_FRAMEWORK}}
Code under test: {{CODE}}
What it's supposed to do, if different from what the code says: {{INTENDED_BEHAVIOR}}

Do this:
1. List the input equivalence classes (groups of inputs the code should treat the same way), including invalid ones.
2. For each class, identify the boundaries: just below, at and just above each limit, plus empty, single-item and maximum-size cases.
3. Present a test table: case name, input, expected output or error, and which class and boundary it covers.
4. Write the tests in my framework: one behavior per test, descriptive names, arrange-act-assert layout, and parameterized tests where the framework supports them.
5. Flag any case where the code's actual behavior differs from the intended behavior. Write those tests to the intended behavior and mark them as expected to fail now.

Don't mock what you don't need to. Don't test private implementation details; test behavior through the public interface. If expected outputs require calculation, show the calculation so I can check it.