Coding: Credit/Bug-free Implementation

Implement a core data structure or API for a "credit"-related system, with a strong emphasis on being bug-free. The exact prompt is interviewer-defined, but it typically asks you to: Design clear data structures and APIs (e.g., add/spend credits, query balance, time-based expiration, rule-based consumption). Handle edge cases and invalid inputs. Meet expected time complexity (often near O(1) or O(log n)). Produce runnable code and validate it with examples. Note: the original post did not include the full statement; the interview will provide concrete rules.