Design a **low-level key-value store** library (like an embedded storage engine) under an interview constraint: you may use **only map/dictionary-like data structures** (plus primitive types / arrays if needed) for your in-memory indexing. ## Core API - `put(key, value)` - `get(key) -> value | null` - `delete(key)` ## Required properties - Expected `O(1)` average time for `get/put/delete` - Safe under concurrent access (multiple threads) - Optional but good to discuss: durability (crash recovery), compaction, TTL/expiration ## Deliverable Explain your design choices (data model, concurrency strategy, persistence approach if any), and how you would test it.