Circuit breaker wrapper for flaky third-party APIs

                  type State = 'closed' | 'open' | 'half_open';

export function circuitBreaker<T>(fn: () => Promise<T>, opts: { failThreshold: number; coolDownMs: number }) {
  let state: State = 'closed';
  let failures = 0;
  let openedAt = 0;

  return async () => {
    const now = Date.now();
    if (state === 'open') {
      if (now - openedAt > opts.coolDownMs) state = 'half_open';
      else throw new Error('CIRCUIT_OPEN');
    }

    try {
      const result = await fn();
      failures = 0;
      state = 'closed';
      return result;
    } catch (e) {
      failures += 1;
      if (failures >= opts.failThreshold) {
        state = 'open';
        openedAt = Date.now();
      }
      throw e;
    }
  };
}