Temporal Event Processing And Interval Algorithms
Asked of: Software Engineer
Last updated
What's being tested
This tests interval algorithms and timestamp-ordered event processing: sorting ranges, merging boundaries, replaying events deterministically, and preserving accounting invariants under out-of-order input. Interviewers are probing whether you can turn ambiguous time semantics into precise code with clear complexity and edge-case handling.
Patterns & templates
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Sort-then-scan interval merge — sort by
start, maintain current[lo, hi];O(n log n)time,O(n)output space. -
Endpoint semantics — decide whether
[a,b]and[b,c]merge; closed intervals usually merge onnext.start <= cur.end. -
Zero-length intervals — handle
[t,t]explicitly; they may represent instantaneous events, empty ranges, or valid closed intervals. -
Event replay ledger — store events sorted by
(timestamp, tie_breaker, sequence_id)and recompute balances deterministically after inserts. -
Tie-breaking rules — define add-before-charge or charge-before-add at equal timestamps; encode it directly in the sort key.
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Expiring credits — model credits as lots with
amount,created_at,expires_at; consume using FIFO/earliest-expiry via queue or heap. -
Complexity tradeoff — full replay is simple but
O(n)per insert; balanced trees or indexed timelines improve repeated historical queries.
Common pitfalls
Pitfall: Treating touching intervals inconsistently; state whether
end == startmeans overlap before writing merge logic.
Pitfall: Sorting only by timestamp in ledgers; equal-time adds and charges need deterministic tie-breaking to avoid flaky balances.
Pitfall: Mutating current balance incrementally for out-of-order events without replay or correction; historical inserts can change all future states.
Practice these
The practice cards below cover the canonical variants — solve all of them and time yourself.
Practice questions
- Implement credit ledger with out-of-order timestampsOpenAI · Software Engineer · Technical Screen · hard
- Implement expiring credit ledgerOpenAI · Software Engineer · Technical Screen · Medium
- Merge overlapping intervalsOpenAI · Software Engineer · Technical Screen · Medium
- Merge overlapping time intervals efficientlyOpenAI · Software Engineer · Technical Screen · Medium
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