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Coordinate Video Reading and Rendering at 25 Frames per Second

NuroSoftware Engineer · Technical Screen
HardSoftware EngineerTechnical ScreenSoftware Engineering Fundamentals

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Design a 25fps video player with serialized unsafe APIs, bounded producer-consumer buffering, deadline pacing, and explicit timing feasibility limits.

You are given a Frame type and a VideoPlayer with two operations: Read() returns the next frame or an end-of-video indication, and Render(frame) displays a frame. Design on_play_click() to play the complete video from first frame to last at a target of exactly 25 frames per second.

Neither Read() nor Render() is thread-safe. Explain how you serialize access safely, use producer/consumer coordination, and schedule rendering without losing or reordering frames.

Constraints

Preserve the 25-frame-per-second requirement, equivalent to 40 milliseconds between presentation starts. For a conservative baseline, assume that Read and Render share unsafe player state and must never overlap, even with each other. The API durations, UI-thread affinity, buffering limit, and behavior under missed deadlines are unspecified and must be clarified. Do not promise strict real-time playback under arbitrary delays.

Clarifying Questions

  • What are the worst-case Read and Render durations, and can reads be prefetched before playback begins?
  • Must Render execute on the UI thread, and which clock and scheduler are available?
  • How much buffering is allowed, and what happens if a frame is not ready at its deadline?
  • What should repeated play clicks, cancellation, or end of video do?

Hints

Hint 1

Avoid waiting while holding the player lock: A bounded queue and the non-thread-safe player APIs protect different state. Holding the API lock while waiting for queue space can prevent the consumer from making progress.

Follow-up questions

  • Can the requirement be met if Render alone sometimes takes longer than 40 milliseconds?
  • How would you distinguish a temporarily empty buffer from the end of the video?