Future-Aware Cache Eviction
Reported by candidates from Benchling's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
The mistake that sinks a first attempt at Benchling's Future-Aware Cache Eviction is rescanning the rest of the request list on every miss. Candidates reported this OA in August 2025, and the setup looks like a gentle cache problem until you see 200000 requests. This is Belady's optimal eviction: you know the future, so you evict the key used farthest away. Keys never requested again count as infinitely far, and ties go to the lexicographically larger key. If you blank on the data structure choice during the live OA, StealthCoder can sit invisibly on your screen as a safety net. Better to walk in knowing the trick.
The problem
Process the known requests through a cache of fixed capacity. On a miss when full, evict the cached key whose next request is farthest in the future; a key never requested again is farthest. Break equal next-use ties by evicting the lexicographically larger key. Return one entry per request: the evicted key, or "-" when nothing was evicted. Function futureAwareEvictions(requests: String[], capacity: int) → String[] Examples Example 1 requests = ["a","a","b","c","b","c"] capacity = 2 return = ["-","-","-","a","-","-"] a is never needed again when c arrives, so it is evicted. Constraints 1 <= requests.length <= 200000. 1 <= capacity <= 1000. Keys contain lowercase letters and digits.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is precomputing next-use indices. Walk the requests once from the right and store, for each position i, the index of the next occurrence of the same key, or infinity if none. Then simulate left to right. Keep the cache as a map from key to its next-use index, and update that value on every hit and every insert. On a miss with a full cache, pick the key with the max next-use, breaking ties by the larger string. Capacity is at most 1000, so a linear scan of the cache per miss is fine: 200000 times 1000 is workable. A heap with lazy deletion also works, but it's more bug-prone. The classic pitfall is forgetting to refresh next-use on a hit, so stale values cause wrong evictions. Another is the tie-break direction. If you blank on the bookkeeping live, StealthCoder is your hedge, but the logic here is short.
The honest play: practice the pattern, and have StealthCoder ready for the one you didn't see coming.
You can drill Future-Aware Cache Eviction cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built for the candidate who saw this exact problem leak two days before his OA and wondered if anyone had a play.
Get StealthCoderRelated leaked OAs
You've seen the question.
Make sure you actually pass Benchling's OA.
Benchling reuses patterns across OAs. Built for the candidate who saw this exact problem leak two days before his OA and wondered if anyone had a play. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Future-Aware Cache Eviction FAQ
What's the actual trick in Future-Aware Cache Eviction?+
Precompute each request's next occurrence index with one backward pass using a hash map. Then simulate forward, storing each cached key's next-use index. On a full-cache miss, evict the key with the largest next-use. Never-again keys get infinity. That removes any per-miss rescanning of the future.
How do I handle ties between keys never requested again?+
Give all of them the same infinite next-use value. When comparing candidates, if next-use is equal, choose the lexicographically larger key. A simple comparison on the pair (nextUse, key) and taking the max handles both rules in one step.
Is a linear scan of the cache fast enough?+
Capacity is capped at 1000 and requests at 200000, so scanning the cache on each miss is about 200 million simple operations worst case. That's usually acceptable. If you want safer performance, use a heap with lazy deletion, but mind stale entries and the tie-break.
What output do I return on hits and on misses with space?+
Return one entry per request. If nothing was evicted, which covers hits and misses while the cache still has room, output "-". Only a miss on a full cache produces the evicted key string. Example 1 shows this: only the fourth request evicts a.
How do I prepare for this in 48 hours?+
Write the backward next-occurrence pass and the forward simulation from memory twice. Test on the sample plus cases with repeated hits, keys never reused, and tied keys. Check that hits update next-use. Those three cases catch most first-attempt bugs.