function positiveInteger(value, fallback) {
  const parsed = Number(value);
  return Number.isInteger(parsed) && parsed > 0 ? parsed : fallback;
}

export function createShadowObservationDispatcher({
  observe,
  onCompleted = async () => {},
  onFailed = async () => {},
  onSkipped = async () => {},
  maxConcurrent = 2,
  maxQueued = 100,
  schedule = setImmediate,
  now = Date.now,
  logger = console,
} = {}) {
  const enabled = typeof observe === 'function';
  const concurrency = positiveInteger(maxConcurrent, 2);
  const queueLimit = positiveInteger(maxQueued, 100);
  const queue = [];
  let active = 0;
  let drainScheduled = false;

  function reportCallbackFailure(label, error) {
    logger.warn(
      `[orchestrator-shadow] ${label} callback failed:`,
      error instanceof Error ? error.message : error,
    );
  }

  function scheduleDrain() {
    if (!enabled || drainScheduled || !queue.length || active >= concurrency) return;
    drainScheduled = true;
    schedule(drain);
  }

  function runObservation(entry) {
    active += 1;
    const context = {
      enqueuedAt: entry.enqueuedAt,
      startedAt: now(),
    };
    void Promise.resolve()
      .then(() => observe(entry.input))
      .then((result) => onCompleted(entry.input, result, context))
      .catch((error) => onFailed(entry.input, error, context))
      .catch((error) => reportCallbackFailure('failure', error))
      .finally(() => {
        active -= 1;
        scheduleDrain();
      });
  }

  function drain() {
    drainScheduled = false;
    while (active < concurrency && queue.length) {
      runObservation(queue.shift());
    }
  }

  return {
    dispatch(input) {
      if (!enabled) return false;
      if (queue.length >= queueLimit) {
        const context = {
          enqueuedAt: now(),
          reason: 'shadow_queue_full',
        };
        void Promise.resolve()
          .then(() => onSkipped(input, context))
          .catch((error) => reportCallbackFailure('skip', error));
        return false;
      }
      queue.push({ input, enqueuedAt: now() });
      scheduleDrain();
      return true;
    },

    status() {
      return {
        enabled,
        active,
        queued: queue.length,
        maxConcurrent: concurrency,
        maxQueued: queueLimit,
      };
    },
  };
}

export const shadowDispatcherInternals = {
  positiveInteger,
};