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Core
Caching and Eviction
A cache is bounded memory in front of expensive work, so the real design question is what to throw away when it fills. LRU evicts the least recently used entry and is the default; it is built from a hash map plus a doubly linked list to get O(1) get and put. TTL adds time-based expiry. Picking and implementing the right eviction policy is one of the most common practical FDE coding screens.
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