import 'dart:async';
import 'dart:collection';

import 'package:flutter/foundation.dart';

class SCRoomEffectScheduler {
  static const Duration _deferredDrainInterval = Duration(milliseconds: 60);

  static final SCRoomEffectScheduler _instance =
      SCRoomEffectScheduler._internal();

  factory SCRoomEffectScheduler() => _instance;

  SCRoomEffectScheduler._internal();

  final Queue<_DeferredRoomEffectTask> _deferredTasks =
      Queue<_DeferredRoomEffectTask>();

  Timer? _deferredDrainTimer;
  int _pendingHighCostTaskCount = 0;
  int _nextTaskId = 0;

  bool get hasActiveHighCostEffects => _pendingHighCostTaskCount > 0;

  void registerHighCostTaskQueued({String? debugLabel}) {
    _pendingHighCostTaskCount += 1;
    _cancelDeferredDrain();
    _log(
      'register_high_cost task=${debugLabel ?? "unknown"} '
      'pending=$_pendingHighCostTaskCount deferred=${_deferredTasks.length}',
    );
  }

  void completeHighCostTask({String? debugLabel}) {
    if (_pendingHighCostTaskCount > 0) {
      _pendingHighCostTaskCount -= 1;
    }
    _log(
      'complete_high_cost task=${debugLabel ?? "unknown"} '
      'pending=$_pendingHighCostTaskCount deferred=${_deferredTasks.length}',
    );
    _scheduleDeferredDrainIfNeeded();
  }

  void clearHighCostTasks({String reason = 'unknown'}) {
    if (_pendingHighCostTaskCount == 0) {
      _scheduleDeferredDrainIfNeeded();
      return;
    }
    _pendingHighCostTaskCount = 0;
    _log(
      'clear_high_cost reason=$reason '
      'pending=$_pendingHighCostTaskCount deferred=${_deferredTasks.length}',
    );
    _scheduleDeferredDrainIfNeeded();
  }

  void scheduleDeferredEffect({
    required String debugLabel,
    required VoidCallback action,
  }) {
    if (!hasActiveHighCostEffects) {
      action();
      return;
    }

    final task = _DeferredRoomEffectTask(
      id: _nextTaskId++,
      debugLabel: debugLabel,
      action: action,
    );
    _deferredTasks.add(task);
    _log(
      'defer task=${task.debugLabel} id=${task.id} '
      'pending=$_pendingHighCostTaskCount deferred=${_deferredTasks.length}',
    );
  }

  void clearDeferredTasks({String reason = 'unknown'}) {
    if (_deferredTasks.isEmpty) {
      _cancelDeferredDrain();
      return;
    }
    _deferredTasks.clear();
    _cancelDeferredDrain();
    _log('clear_deferred reason=$reason');
  }

  void _scheduleDeferredDrainIfNeeded() {
    if (hasActiveHighCostEffects ||
        _deferredTasks.isEmpty ||
        _deferredDrainTimer != null) {
      return;
    }
    _deferredDrainTimer = Timer(_deferredDrainInterval, _drainNextDeferredTask);
  }

  void _drainNextDeferredTask() {
    _deferredDrainTimer = null;
    if (hasActiveHighCostEffects || _deferredTasks.isEmpty) {
      return;
    }

    final task = _deferredTasks.removeFirst();
    _log(
      'drain_deferred task=${task.debugLabel} id=${task.id} '
      'remaining=${_deferredTasks.length}',
    );
    try {
      task.action();
    } catch (error, stackTrace) {
      debugPrint(
        '[RoomEffectScheduler] deferred task failed: $error\n$stackTrace',
      );
    }

    _scheduleDeferredDrainIfNeeded();
  }

  void _cancelDeferredDrain() {
    _deferredDrainTimer?.cancel();
    _deferredDrainTimer = null;
  }

  void _log(String message) {
    debugPrint('[RoomEffectScheduler] $message');
  }
}

class _DeferredRoomEffectTask {
  const _DeferredRoomEffectTask({
    required this.id,
    required this.debugLabel,
    required this.action,
  });

  final int id;
  final String debugLabel;
  final VoidCallback action;
}