job-queue-delayed-and-priority-extension.md

Job Queue: Delayed Jobs & Priority Ordering

Difficulty: Medium. Core topic: scheduling, priority queue.

The extend-without-breaking round. A working FIFO queue needs two new powers — jobs that only become dispatchable at a future time, and a priority order among the jobs that are ready — without disturbing the plain FIFO behavior that already passes.

Scenario

You are given a small, correct in-memory FIFO job queue with enqueue(job), dequeue(now) -> job, and size(). Your task is to extend it with delayed jobs and priority ordering, all inside the single stubbed dequeue method per language, while the existing FIFO tests keep passing.

Jobs may carry a run_at time and an integer priority (default 0). A delayed job is only dispatchable once now >= run_at; dequeue(now) must skip jobs that are not yet due and signal empty when nothing is currently dispatchable — but a not-yet-due job still counts toward size(). Among the due jobs, a higher priority number wins, ties break by insertion order, and a high-priority job that is not yet due must never jump ahead of a due lower-priority one. Time is injected via now everywhere, so the bundled tests run with no wall clock and no sleeping.

Requirements

• dequeue(now) never returns a job whose run_at is still in the future.
• Signal empty (raise/throw) when no due job exists, even if future jobs remain.
• Among due jobs, return the highest priority first (higher number is higher priority).
• Break priority ties by insertion order — stable FIFO.
• Not-yet-due jobs still count toward size().
• Preserve the original FIFO behavior for plain jobs.

Edge cases to handle

• A high-priority job that is not yet due sitting behind a due low-priority job
• Several due jobs at the same priority requiring stable FIFO order
• now exactly equal to a job's run_at
• A queue holding only future jobs — dispatchable-empty but non-zero size

What interviewers look for

Whether due-ness is checked before priority so scheduling can never be undercut by a ranking, whether ties stay stable rather than depending on container quirks, and whether the empty signal distinguishes "nothing dispatchable now" from "truly nothing left". Clean answers keep the injected now as the single source of time and leave the original FIFO path intact.

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Practice this in a real repo with a failing test suite (free) → https://gronex.org/problems/job-queue-delayed-and-priority-extension