// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.6

part of dart.async;

typedef void _AsyncCallback();

class _AsyncCallbackEntry {
  final _AsyncCallback callback;
  _AsyncCallbackEntry next;
  _AsyncCallbackEntry(this.callback);
}

/** Head of single linked list of pending callbacks. */
_AsyncCallbackEntry _nextCallback;
/** Tail of single linked list of pending callbacks. */
_AsyncCallbackEntry _lastCallback;
/**
 * Tail of priority callbacks added by the currently executing callback.
 *
 * Priority callbacks are put at the beginning of the
 * callback queue, so that if one callback schedules more than one
 * priority callback, they are still enqueued in scheduling order.
 */
_AsyncCallbackEntry _lastPriorityCallback;
/**
 * Whether we are currently inside the callback loop.
 *
 * If we are inside the loop, we never need to schedule the loop,
 * even if adding a first element.
 */
bool _isInCallbackLoop = false;

void _microtaskLoop() {
  while (_nextCallback != null) {
    _lastPriorityCallback = null;
    _AsyncCallbackEntry entry = _nextCallback;
    _nextCallback = entry.next;
    if (_nextCallback == null) _lastCallback = null;
    (entry.callback)();
  }
}

void _startMicrotaskLoop() {
  _isInCallbackLoop = true;
  try {
    // Moved to separate function because try-finally prevents
    // good optimization.
    _microtaskLoop();
  } finally {
    _lastPriorityCallback = null;
    _isInCallbackLoop = false;
    if (_nextCallback != null) {
      _AsyncRun._scheduleImmediate(_startMicrotaskLoop);
    }
  }
}

/**
 * Schedules a callback to be called as a microtask.
 *
 * The microtask is called after all other currently scheduled
 * microtasks, but as part of the current system event.
 */
void _scheduleAsyncCallback(_AsyncCallback callback) {
  _AsyncCallbackEntry newEntry = new _AsyncCallbackEntry(callback);
  if (_nextCallback == null) {
    _nextCallback = _lastCallback = newEntry;
    if (!_isInCallbackLoop) {
      _AsyncRun._scheduleImmediate(_startMicrotaskLoop);
    }
  } else {
    _lastCallback.next = newEntry;
    _lastCallback = newEntry;
  }
}

/**
 * Schedules a callback to be called before all other currently scheduled ones.
 *
 * This callback takes priority over existing scheduled callbacks.
 * It is only used internally to give higher priority to error reporting.
 *
 * Is always run in the root zone.
 */
void _schedulePriorityAsyncCallback(_AsyncCallback callback) {
  if (_nextCallback == null) {
    _scheduleAsyncCallback(callback);
    _lastPriorityCallback = _lastCallback;
    return;
  }
  _AsyncCallbackEntry entry = new _AsyncCallbackEntry(callback);
  if (_lastPriorityCallback == null) {
    entry.next = _nextCallback;
    _nextCallback = _lastPriorityCallback = entry;
  } else {
    entry.next = _lastPriorityCallback.next;
    _lastPriorityCallback.next = entry;
    _lastPriorityCallback = entry;
    if (entry.next == null) {
      _lastCallback = entry;
    }
  }
}

/**
 * Runs a function asynchronously.
 *
 * Callbacks registered through this function are always executed in order and
 * are guaranteed to run before other asynchronous events (like [Timer] events,
 * or DOM events).
 *
 * **Warning:** it is possible to starve the DOM by registering asynchronous
 * callbacks through this method. For example the following program runs
 * the callbacks without ever giving the Timer callback a chance to execute:
 *
 *     main() {
 *       Timer.run(() { print("executed"); });  // Will never be executed.
 *       foo() {
 *         scheduleMicrotask(foo);  // Schedules [foo] in front of other events.
 *       }
 *       foo();
 *     }
 *
 * ## Other resources
 *
 * * [The Event Loop and Dart](https://www.dartlang.org/articles/event-loop/):
 * Learn how Dart handles the event queue and microtask queue, so you can write
 * better asynchronous code with fewer surprises.
 */
void scheduleMicrotask(void callback()) {
  _Zone currentZone = Zone.current;
  if (identical(_rootZone, currentZone)) {
    // No need to bind the callback. We know that the root's scheduleMicrotask
    // will be invoked in the root zone.
    _rootScheduleMicrotask(null, null, _rootZone, callback);
    return;
  }
  _ZoneFunction implementation = currentZone._scheduleMicrotask;
  if (identical(_rootZone, implementation.zone) &&
      _rootZone.inSameErrorZone(currentZone)) {
    _rootScheduleMicrotask(
        null, null, currentZone, currentZone.registerCallback(callback));
    return;
  }
  Zone.current.scheduleMicrotask(Zone.current.bindCallbackGuarded(callback));
}

class _AsyncRun {
  /** Schedule the given callback before any other event in the event-loop. */
  external static void _scheduleImmediate(void callback());
}