Write unit tests that cover the edge cases

Write unit tests for the code below using [Test framework]. First list the cases worth testing, grouped as: - normal use; - edge cases, such as empty input, limits and boundaries; - invalid input and errors. Then write the tests in one code block. Name each test after the behavior it checks. Mock only what the code reaches outside itself, such as the network, the clock or a database. If the code has a bug, write a test that exposes it and point out the bug; don't change the code. Code: