系統設計

1type Device = { device: string; gateway: string };
2type Message = { conv: string; seq: number; from: string; text: string };
3 
4class Messenger {
5  private sessions = new Map<string, Device[]>(); // user -> connected devices
6  private seqs = new Map<string, number>();       // conversation -> last seq
7  private log = new Map<string, Message[]>();     // conversation -> messages
8  delivered: Array<[string, string, number]> = []; // [gateway, device, seq]
9  pushed: Array<[string, number]> = [];            // [user, seq]
10 
11  connect(user: string, device: string, gateway: string): void {
12    const list = this.sessions.get(user) ?? [];
13    list.push({ device, gateway });
14    this.sessions.set(user, list);
15  }
16 
17  disconnect(user: string, device: string): void {
18    const list = (this.sessions.get(user) ?? []).filter((d) => d.device !== device);
19    this.sessions.set(user, list);
20  }
21 
22  send(from: string, conv: string, members: string[], text: string): Message {
23    const seq = (this.seqs.get(conv) ?? 0) + 1;
24    this.seqs.set(conv, seq);
25    const message = { conv, seq, from, text };
26    const log = this.log.get(conv) ?? [];
27    log.push(message);
28    this.log.set(conv, log);
29    for (const user of members) {
30      if (user === from) continue;
31      const devices = this.sessions.get(user) ?? [];
32      if (devices.length === 0) {
33        this.pushed.push([user, seq]);
34        continue;
35      }
36      for (const d of devices) {
37        this.delivered.push([d.gateway, d.device, seq]);
38      }
39    }
40    return message;
41  }
42 
43  sync(conv: string, afterSeq: number): Message[] {
44    return (this.log.get(conv) ?? []).filter((m) => m.seq > afterSeq);
45  }
46}