OO Design: Continuous Time Intervals from Two Timestamp Signal Streams

Phone screen with a real Waymo business framing: given two kinds of timestamp-based signals, design a class that returns the continuous time intervals during which a condition holds. The interviewer cares about how you analyze the problem and pin down details, and explicitly wants you to ask about anything ambiguous before coding. Requirements Two distinct streams of timestamp-based signals are provided. Design a class / API that ingests both streams and returns the continuous time intervals over which a stated condition is satisfied. The exact condition is deliberately under-specified — clarifying it (and the edge semantics) with the interviewer is part of the round. Notes Clarify first. The interviewer values problem analysis and locking down details: are the two signal types combined with AND / OR? Are timestamps discrete events or interval boundaries? Are intervals half-open [start, end)? Do back-to-back qualifying intervals merge? Settle these before writing code — candidates are told explicitly to ask whenever something is unclear. Modeling. Treat each signal as a sequence of (timestamp, state-change) events, merge the two streams into one time-ordered event timeline, and sweep a running condition state across it, emitting an interval each time the combined condition flips from false→true→false. This is a sweep over event boundaries rather than a scan of fixed buckets. Class boundary. Separate the ingestion / storage of each signal stream from the interval-evaluation logic so the condition can be changed without rewriting the merge. State the class responsibilities up front — the design discussion is graded. Edge cases to raise. Overlapping or coincident timestamps from the two streams, a condition true at the very start or still true at the end of the observed window, and zero-length intervals.