import 'package:shared_preferences/shared_preferences.dart';
import 'package:travel_mate/models/checklist_item.dart';

/// Service responsible for storing and retrieving per-trip checklists.
///
/// Persistence relies on `SharedPreferences` with one JSON string per trip
/// key. All methods are resilient to corrupted payloads and return empty
/// lists rather than throwing to keep the UI responsive.
class TripChecklistService {
  /// Loads the checklist items for the given [tripId].
  ///
  /// Returns an empty list if no data exists or if deserialization fails.
  Future<List<ChecklistItem>> loadChecklist(String tripId) async {
    final prefs = await SharedPreferences.getInstance();
    final raw = prefs.getString(_key(tripId));

    if (raw == null) {
      return const [];
    }

    try {
      return ChecklistItem.decodeList(raw);
    } catch (_) {
      // Corrupted payload: clear it to avoid persisting errors.
      await prefs.remove(_key(tripId));
      return const [];
    }
  }

  /// Persists the provided [items] list for [tripId].
  ///
  /// This method overrides the previously stored list; use helpers like
  /// [addItem], [toggleItem], or [deleteItem] for incremental updates.
  Future<void> saveChecklist(String tripId, List<ChecklistItem> items) async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString(_key(tripId), ChecklistItem.encodeList(items));
  }

  /// Adds a new [item] to the checklist for [tripId].
  ///
  /// Items are appended in creation order; the updated list is persisted.
  Future<List<ChecklistItem>> addItem(String tripId, ChecklistItem item) async {
    final current = await loadChecklist(tripId);
    final updated = [...current, item];
    await saveChecklist(tripId, updated);
    return updated;
  }

  /// Toggles the completion state of the item matching [itemId].
  ///
  /// Returns the updated list. If the item is not found, the list is
  /// returned unchanged to keep UI state consistent.
  Future<List<ChecklistItem>> toggleItem(String tripId, String itemId) async {
    final current = await loadChecklist(tripId);
    final updated = current
        .map((item) {
          if (item.id != itemId) {
            return item;
          }
          return item.copyWith(isDone: !item.isDone);
        })
        .toList(growable: false);

    await saveChecklist(tripId, updated);
    return updated;
  }

  /// Deletes the item matching [itemId] and persists the change.
  Future<List<ChecklistItem>> deleteItem(String tripId, String itemId) async {
    final current = await loadChecklist(tripId);
    final updated = current.where((item) => item.id != itemId).toList();
    await saveChecklist(tripId, updated);
    return updated;
  }

  String _key(String tripId) => 'checklist_$tripId';
}