系統設計

1type BuyResult = "queued" | "sold-out" | "duplicate" | "throttled";
2 
3class FlashSale {
4  private buyers = new Set<string>();
5  private hits = new Map<string, number>();
6  queue: string[] = [];
7  orders = new Set<string>();
8 
9  constructor(private stock: number, private perUserLimit = 3) {}
10 
11  buy(user: string): BuyResult {
12    const hits = (this.hits.get(user) ?? 0) + 1;
13    this.hits.set(user, hits);
14    if (hits > this.perUserLimit) return "throttled";
15    if (this.buyers.has(user)) return "duplicate";
16    this.stock -= 1;
17    if (this.stock < 0) {
18      this.stock += 1;
19      return "sold-out";
20    }
21    this.buyers.add(user);
22    this.queue.push(user);
23    return "queued";
24  }
25 
26  // Wrong with more than one server: both can read 1 and both sell it.
27  buyNaive(user: string): BuyResult {
28    const left = this.stock;
29    if (left <= 0) return "sold-out";
30    this.stock = left - 1;
31    this.queue.push(user);
32    return "queued";
33  }
34 
35  drain(): number {
36    let written = 0;
37    while (this.queue.length > 0) {
38      const user = this.queue.shift()!;
39      if (this.orders.has(user)) continue;
40      this.orders.add(user);
41      written++;
42    }
43    return written;
44  }
45 
46  release(user: string): boolean {
47    if (!this.orders.delete(user)) return false;
48    this.buyers.delete(user);
49    this.stock += 1;
50    return true;
51  }
52}