// Use this function to schedule a task for a root. There's only one task per
// root; if a task was already scheduled, we'll check to make sure the priority
// of the existing task is the same as the priority of the next level that the
// root has work on. This function is called on every update, and right before
// exiting a task.
function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  const existingCallbackNode = root.callbackNode;

  // Check if any lanes are being starved by other work. If so, mark them as
  // expired so we know to work on those next.
  markStarvedLanesAsExpired(root, currentTime);

  // Determine the next lanes to work on, and their priority.
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );

  if (nextLanes === NoLanes) {
    // Special case: There's nothing to work on.
    if (existingCallbackNode !== null) {
      cancelCallback(existingCallbackNode);
    }
    root.callbackNode = null;
    root.callbackPriority = NoLane;
    return;
  }

  // We use the highest priority lane to represent the priority of the callback.
  const newCallbackPriority = getHighestPriorityLane(nextLanes);

  // Check if there's an existing task. We may be able to reuse it.
  const existingCallbackPriority = root.callbackPriority;
  if (
    existingCallbackPriority === newCallbackPriority &&
    // Special case related to `act`. If the currently scheduled task is a
    // Scheduler task, rather than an `act` task, cancel it and re-scheduled
    // on the `act` queue.
    !(
      __DEV__ &&
      ReactCurrentActQueue.current !== null &&
      existingCallbackNode !== fakeActCallbackNode
    )
  ) {
    if (__DEV__) {
      // If we're going to re-use an existing task, it needs to exist.
      // Assume that discrete update microtasks are non-cancellable and null.
      // TODO: Temporary until we confirm this warning is not fired.
      if (
        existingCallbackNode == null &&
        existingCallbackPriority !== SyncLane
      ) {
        console.error(
          'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
        );
      }
    }
    // The priority hasn't changed. We can reuse the existing task. Exit.
    return;
  }

  if (existingCallbackNode != null) {
    // Cancel the existing callback. We'll schedule a new one below.
    cancelCallback(existingCallbackNode);
  }

  // Schedule a new callback.
  let newCallbackNode;
  if (newCallbackPriority === SyncLane) {
    // Special case: Sync React callbacks are scheduled on a special
    // internal queue
    if (root.tag === LegacyRoot) {
      if (__DEV__ && ReactCurrentActQueue.isBatchingLegacy !== null) {
        ReactCurrentActQueue.didScheduleLegacyUpdate = true;
      }
      scheduleLegacySyncCallback(performSyncWorkOnRoot.bind(null, root));
    } else {
      scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
    }
    if (supportsMicrotasks) {
      // Flush the queue in a microtask.
      if (__DEV__ && ReactCurrentActQueue.current !== null) {
        // Inside `act`, use our internal `act` queue so that these get flushed
        // at the end of the current scope even when using the sync version
        // of `act`.
        ReactCurrentActQueue.current.push(flushSyncCallbacks);
      } else {
        scheduleMicrotask(() => {
          // In Safari, appending an iframe forces microtasks to run.
          // https://github.com/facebook/react/issues/22459
          // We don't support running callbacks in the middle of render
          // or commit so we need to check against that.
          if (
            (executionContext & (RenderContext | CommitContext)) ===
            NoContext
          ) {
            // Note that this would still prematurely flush the callbacks
            // if this happens outside render or commit phase (e.g. in an event).
            flushSyncCallbacks();
          }
        });
      }
    } else {
      // Flush the queue in an Immediate task.
      scheduleCallback(ImmediateSchedulerPriority, flushSyncCallbacks);
    }
    newCallbackNode = null;
  } else {
    let schedulerPriorityLevel;
    switch (lanesToEventPriority(nextLanes)) {
      case DiscreteEventPriority:
        schedulerPriorityLevel = ImmediateSchedulerPriority;
        break;
      case ContinuousEventPriority:
        schedulerPriorityLevel = UserBlockingSchedulerPriority;
        break;
      case DefaultEventPriority:
        schedulerPriorityLevel = NormalSchedulerPriority;
        break;
      case IdleEventPriority:
        schedulerPriorityLevel = IdleSchedulerPriority;
        break;
      default:
        schedulerPriorityLevel = NormalSchedulerPriority;
        break;
    }
    newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performConcurrentWorkOnRoot.bind(null, root),
    );
  }

  root.callbackPriority = newCallbackPriority;
  root.callbackNode = newCallbackNode;
}

export function scheduleSyncCallback(callback: SchedulerCallback) {
  // Push this callback into an internal queue. We'll flush these either in
  // the next tick, or earlier if something calls `flushSyncCallbackQueue`.
  if (syncQueue === null) {
    syncQueue = [callback];
  } else {
    // Push onto existing queue. Don't need to schedule a callback because
    // we already scheduled one when we created the queue.
    syncQueue.push(callback);
  }
}

export function queueJob(job: SchedulerJob) {
  // the dedupe search uses the startIndex argument of Array.includes()
  // by default the search index includes the current job that is being run
  // so it cannot recursively trigger itself again.
  // if the job is a watch() callback, the search will start with a +1 index to
  // allow it recursively trigger itself - it is the user's responsibility to
  // ensure it doesn't end up in an infinite loop.
  if (
    (!queue.length ||
      !queue.includes(
        job,
        isFlushing && job.allowRecurse ? flushIndex + 1 : flushIndex
      )) &&
    job !== currentPreFlushParentJob
  ) {
    if (job.id == null) {
      queue.push(job)
    } else {
      queue.splice(findInsertionIndex(job.id), 0, job)
    }
    queueFlush()
  }
}

const resolvedPromise = /*#__PURE__*/ Promise.resolve() as Promise<any>
function queueFlush() {
  if (!isFlushing && !isFlushPending) {
    isFlushPending = true
    currentFlushPromise = resolvedPromise.then(flushJobs)
  }
}

export function nextTick<T = void>(
  this: T,
  fn?: (this: T) => void
): Promise<void> {
  const p = currentFlushPromise || resolvedPromise
  return fn ? p.then(this ? fn.bind(this) : fn) : p
}