import 'dart:async';
import 'package:flutter/material.dart';

class SleepTimer {
  final VoidCallback onElapsed;
  final DateTime Function() now;

  late final ValueNotifier<Duration?> remaining = ValueNotifier(null);
  Timer? _timer;
  DateTime? _deadline;
  bool _firedThisSession = false;

  SleepTimer({
    required this.onElapsed,
    DateTime Function()? now,
  }) : now = now ?? DateTime.now;

  void start(Duration duration) {
    if (duration <= Duration.zero) {
      cancel();
      return;
    }

    cancel(); // Replace any running timer
    _firedThisSession = false;
    // ponytail: Wall-Clock-Deadline, selbstkorrigierend gegen OS-Throttling.
    // Nachteil: manuelle Systemuhr-Änderung feuert früh/spät — akzeptiert.
    _deadline = now().add(duration);
    _updateRemaining();

    // Start periodic updates (1Hz for UI)
    _timer = Timer.periodic(const Duration(seconds: 1), (_) => tick());
  }

  void tick() {
    _updateRemaining();
    _checkElapsed();
  }

  void _checkElapsed() {
    if (_deadline == null || _firedThisSession) return;

    if (!now().isBefore(_deadline!)) {
      _firedThisSession = true;
      _deadline = null;
      _timer?.cancel();
      _timer = null;
      remaining.value = null;
      onElapsed();
    }
  }

  void _updateRemaining() {
    if (_deadline == null) {
      remaining.value = null;
      return;
    }

    final diff = _deadline!.difference(now());
    if (diff.isNegative) {
      remaining.value = null;
    } else {
      remaining.value = diff;
    }
  }

  void cancel() {
    _timer?.cancel();
    _timer = null;
    _deadline = null;
    _firedThisSession = false;
    remaining.value = null;
  }
}