API Rate Limiter Implementation

Your task is to design and implement an API rate limiter. The rate limiter should enforce a limit on the number of requests each user can make within a fixed time window (e.g., 100 requests per 60 seconds). Your solution should:

• Track and limit requests per user in real-time.
• Handle concurrent requests effectively, ensuring thread safety (if applicable in your chosen language).
• Remove or expire old request records as the time window advances.
• Include unit tests that verify the correctness of your implementation under normal conditions as well as edge cases (e.g., burst traffic, exact window transitions).

You may choose your preferred programming language and data structures. Clearly document any assumptions made during your design. Consider different approaches (for example, sliding window vs. fixed window) and justify your choice if applicable.