系統設計

1class VisibilityQueue {
2  private ready: number[] = [];                  // waiting, oldest first
3  private head = 0;
4  // Received but not yet acknowledged: id -> deadline. Deadlines are
5  // receive time + a fixed timeout, so the oldest is always first.
6  private inFlight = new Map<number, number>();
7  private done = new Set<number>();
8 
9  constructor(private visibilitySec: number) {}
10 
11  send(id: number): void {
12    this.ready.push(id);
13  }
14 
15  receive(now: number): number | null {
16    for (const [id, deadline] of this.inFlight) {
17      if (deadline > now) break;
18      this.inFlight.delete(id);
19      this.ready.push(id);
20    }
21    while (this.head < this.ready.length) {
22      const id = this.ready[this.head++];
23      if (this.done.has(id)) continue;
24      this.inFlight.set(id, now + this.visibilitySec);
25      return id;
26    }
27    return null;
28  }
29 
30  ack(id: number): void {
31    this.done.add(id);
32    this.inFlight.delete(id);
33  }
34}