Count-Based

// Count-based circuit breaker: trip when the failure rate over the last N calls
// crosses the threshold. The simplest sliding window there is.
class CountBasedBreaker {
  private state: "CLOSED" | "OPEN" | "HALF" = "CLOSED";
  private window: boolean[] = [];        // true = ok, false = fail
  private trials: boolean[] = [];
  private openUntil = 0;

  constructor(
    private size = 10,
    private threshold = 0.5,             // 50%
    private cooldownMs = 5000,
    private trialCount = 3,
    private minCalls = 5,                // don't trip on tiny samples
  ) {}

  async call<T>(work: () => Promise<T>): Promise<T> {
    if (this.state === "OPEN") {
      if (Date.now() >= this.openUntil) { this.state = "HALF"; this.trials = []; }
      else throw new Error("circuit open");
    }
    try {
      const result = await work();
      this.record(true);
      return result;
    } catch (err) {
      this.record(false);
      throw err;
    }
  }

  private record(ok: boolean) {
    if (this.state === "HALF") {
      this.trials.push(ok);
      if (this.trials.length >= this.trialCount) {
        const fails = this.trials.filter(x => !x).length;
        if (fails / this.trials.length >= this.threshold) this.trip();
        else this.close();
      }
      return;
    }
    this.window.push(ok);
    if (this.window.length > this.size) this.window.shift();
    if (this.window.length >= this.minCalls) {
      const fails = this.window.filter(x => !x).length;
      if (fails / this.window.length >= this.threshold) this.trip();
    }
  }

  private trip()  { this.state = "OPEN"; this.openUntil = Date.now() + this.cooldownMs; this.trials = []; }
  private close() { this.state = "CLOSED"; this.window = []; this.trials = []; }
}