social-app/apps/lib/features/calendar/reminders/reminder_overlap_policy.dart

import '../data/models/schedule_item_model.dart';

class ReminderOverlapGroup {
  final DateTime fireAt;
  final List<ScheduleItemModel> events;

  const ReminderOverlapGroup({required this.fireAt, required this.events});

  bool get isAggregate => events.length > 1;
}

class ReminderOverlapPolicy {
  const ReminderOverlapPolicy();

  List<ReminderOverlapGroup> groupByMinute(
    Iterable<ScheduleItemModel> events, {
    required DateTime now,
  }) {
    final buckets = <String, List<ScheduleItemModel>>{};
    final minuteToFireAt = <String, DateTime>{};

    for (final event in events) {
      final fireAt = resolveFirstFireAt(event, now: now);
      if (fireAt == null) {
        continue;
      }
      final minute = DateTime(
        fireAt.year,
        fireAt.month,
        fireAt.day,
        fireAt.hour,
        fireAt.minute,
      );
      final key = minute.toIso8601String();
      buckets.putIfAbsent(key, () => <ScheduleItemModel>[]).add(event);
      minuteToFireAt[key] = minuteToFireAt[key] ?? fireAt;
    }

    final groups = buckets.entries
        .map(
          (entry) => ReminderOverlapGroup(
            fireAt: minuteToFireAt[entry.key]!,
            events: entry.value,
          ),
        )
        .toList();

    groups.sort((left, right) => left.fireAt.compareTo(right.fireAt));
    return groups;
  }

  DateTime? resolveFirstFireAt(
    ScheduleItemModel event, {
    required DateTime now,
  }) {
    if (event.status != ScheduleStatus.active) {
      return null;
    }
    final reminderMinutes = event.metadata?.reminderMinutes;
    if (reminderMinutes == null) {
      return null;
    }
    final remindAt = event.startAt.subtract(Duration(minutes: reminderMinutes));
    final endAt = event.endAt;

    if (endAt != null && !now.isBefore(endAt)) {
      return null;
    }

    if (now.isBefore(remindAt)) {
      return remindAt;
    }

    if (endAt != null && now.isBefore(endAt)) {
      return now.add(const Duration(seconds: 5));
    }

    return null;
  }
}