Error Percentage (Hystrix)

// Hystrix-style breaker: trip only when (a) request volume crosses a minimum
// AND (b) error rate crosses the threshold. The volume gate is the whole point.
class ErrorPercentageBreaker {
  private state: "CLOSED" | "OPEN" | "HALF" = "CLOSED";
  private calls: Array<{ t: number; ok: boolean }> = [];
  private openUntil = 0;

  constructor(
    private windowMs = 10_000,
    private minVolume = 20,           // the gate
    private errorThreshold = 0.5,
    private cooldownMs = 5_000,
  ) {}

  async call<T>(work: () => Promise<T>): Promise<T> {
    if (this.state === "OPEN" && Date.now() >= this.openUntil) this.state = "HALF";
    if (this.state === "OPEN") throw new Error("circuit open");

    try {
      const result = await work();
      this.onResult(true);
      return result;
    } catch (err) {
      this.onResult(false);
      throw err;
    }
  }

  private onResult(ok: boolean) {
    const now = Date.now();

    if (this.state === "HALF") {
      // Single trial call decides everything.
      if (ok) { this.state = "CLOSED"; this.calls = []; }
      else    { this.state = "OPEN"; this.openUntil = now + this.cooldownMs; }
      return;
    }

    this.calls.push({ t: now, ok });
    this.prune(now);

    const total = this.calls.length;
    if (total < this.minVolume) return;           // GATE 1: volume

    const fails = this.calls.filter(c => !c.ok).length;
    if (fails / total < this.errorThreshold) return;  // GATE 2: error %

    this.state = "OPEN";
    this.openUntil = now + this.cooldownMs;
  }

  private prune(now: number) {
    const lo = now - this.windowMs;
    while (this.calls.length && this.calls[0].t < lo) this.calls.shift();
  }
}