export type Timer = Readonly<{ readonly id: number; readonly at: number; readonly callback: () => void }>;
export class VirtualClock {
  private current = 0;
  private nextId = 0;
  private timers: Timer[] = [];
  now(): number {
    return this.current;
  }
  sleep(ms: number, callback: () => void): number {
    if (!Number.isFinite(ms) || ms < 0) throw new RangeError("virtual sleep requires a finite non-negative duration");
    const timer = { id: this.nextId++, at: this.current + ms, callback };
    this.timers.push(timer);
    this.timers.sort((a, b) => a.at - b.at || a.id - b.id);
    return timer.id;
  }
  cancel(id: number): void {
    this.timers = this.timers.filter((timer) => timer.id !== id);
  }
  advance(ms: number): void {
    if (!Number.isFinite(ms) || ms < 0) throw new RangeError("virtual clock can only advance forwards");
    this.current += ms;
    this.flush();
  }
  advanceToNextTimer(): boolean {
    const timer = this.timers[0];
    if (!timer) return false;
    this.current = Math.max(this.current, timer.at);
    this.flush();
    return true;
  }
  runUntilIdle(limit = 10_000): void {
    if (!Number.isInteger(limit) || limit < 1) throw new RangeError("virtual-time limit must be a positive integer");
    let steps = 0;
    while (this.timers.length) {
      const timer = this.timers[0]!;
      this.current = Math.max(this.current, timer.at);
      while (this.timers[0]?.at <= this.current) {
        if (++steps > limit)
          throw Object.assign(new Error("VIRTUAL_TIME_BUDGET_EXHAUSTED: virtual-time callback budget exhausted"), {
            code: "VIRTUAL_TIME_BUDGET_EXHAUSTED",
            details: { limit, now: this.current },
          });
        this.timers.shift()?.callback();
      }
    }
  }
  pending(): readonly Timer[] {
    return [...this.timers];
  }
  clear(): void {
    this.timers = [];
  }
  private flush(): void {
    let steps = 0;
    while (this.timers[0]?.at <= this.current) {
      if (++steps > 10_000)
        throw Object.assign(new Error("virtual-time callback budget exhausted"), {
          code: "VIRTUAL_TIME_BUDGET_EXHAUSTED",
        });
      this.timers.shift()?.callback();
    }
  }
}