User Purchase Platform

Design a system that processes a stream of user purchase events and answers online analytics queries such as total spend or purchase count per user over time ranges and top‑K purchasers. The core challenge is supporting high‑frequency updates and efficient range/top‑K queries, so expect to use time‑indexed aggregation and dynamic ranking structures (e.g., sliding‑window aggregation, Fenwick/segment trees, heaps or ordered maps) to meet tight time/space constraints.