Design a Music Player with Queue, Loop, and Shuffle

Design the objects and interfaces for a music player with `play`, `pause`, and `next` operations. Add a play queue, playlist looping, and shuffle. Explain the state transitions and how the player coordinates these features with an audio backend whose operations can be stubbed. ### Constraints & Assumptions - This is an object-oriented design and communication exercise. Actual audio decoding and device output are behind a stubbed interface. - Define queue precedence, end-of-playlist behavior, loop mode, and shuffle semantics explicitly; the source lists the features without fixing those policies. - Track identity must not depend on the title alone. A playlist may contain repeated tracks. - Begin with one local user and serialized commands. Network streaming and multi-device synchronization are outside the baseline. ### Clarifying Questions to Ask - Does a manually queued track play before the next playlist entry? - Does loop mean repeat the current track, repeat the playlist, or both? - Should shuffle preserve the current track and visit each remaining playlist occurrence once before repeating? - Is `play` while paused a resume, and what should `next` do while paused? ```hint Separate selection from playback The component that chooses the next track should not need to decode audio. The component that plays audio should not decide queue or shuffle policy. ``` ### What a Strong Answer Covers - Track/playlist identity, player state, queue state, and an audio-backend interface. - Explicit behavior for play, pause, next, completion callbacks, and empty inputs. - Queue, looping, and shuffle policies that do not contradict one another. - Testability through backend and randomness injection. - Safe handling of stale completion callbacks and failed track loads. ### Follow-up Questions - How would toggling shuffle midway through a playlist affect already-played and queued tracks? - How would you test the player without playing real audio or relying on nondeterministic random results?