Implement a simple service with tests

You're given a short written specification for a small service class (no external dependencies). Implement the class using clear control flow (primarily if/else) and robust input validation. The exercise has three parts: ( 1) implement the core logic; ( 2) extend it with an additional requirement based on clarified assumptions; ( 3) write unit tests that cover happy paths, edge cases, and error handling for both parts. State all assumptions explicitly and provide method signatures and representative test cases.