系統設計

1const EPOCH = 1577836800000n;            // 2020-01-01, in Unix ms
2const MACHINE_BITS = 10n, SEQUENCE_BITS = 12n;
3const MAX_SEQUENCE = (1n << SEQUENCE_BITS) - 1n;
4 
5class Snowflake {
6  private lastMs = -1n;
7  private sequence = 0n;
8  constructor(private machine: bigint) {}
9 
10  // nowMs: this machine's clock, in Unix milliseconds. Null: this
11  // millisecond's sequence numbers are used up; ask again in the next one.
12  nextId(nowMs: number): bigint | null {
13    const now = BigInt(nowMs);
14    if (now < this.lastMs) {
15      throw new Error("clock moved backwards; refusing to reuse time");
16    }
17    if (now === this.lastMs) {
18      if (this.sequence === MAX_SEQUENCE) return null;
19      this.sequence += 1n;
20    } else {
21      this.sequence = 0n;
22    }
23    this.lastMs = now;
24    return ((now - EPOCH) << (MACHINE_BITS + SEQUENCE_BITS))
25      | (this.machine << SEQUENCE_BITS)
26      | this.sequence;
27  }
28}
29 
30function decode(id: bigint): { ms: number; machine: number; sequence: number } {
31  return {
32    ms: Number((id >> (MACHINE_BITS + SEQUENCE_BITS)) + EPOCH),
33    machine: Number((id >> SEQUENCE_BITS) & ((1n << MACHINE_BITS) - 1n)),
34    sequence: Number(id & MAX_SEQUENCE),
35  };
36}