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

/// Service to manage per-trip budget envelopes (multi-devise) locally.
///
/// Stores envelopes in `SharedPreferences` under `budget_<tripId>` so they
/// remain available offline. Integration with expense data can later update
/// the [spent] field.
class BudgetService {
  /// Loads budget categories for the trip.
  Future<List<BudgetCategory>> loadBudgets(String tripId) async {
    final prefs = await SharedPreferences.getInstance();
    final raw = prefs.getString(_key(tripId));
    if (raw == null) return const [];
    try {
      return BudgetCategory.decodeList(raw);
    } catch (_) {
      await prefs.remove(_key(tripId));
      return const [];
    }
  }

  /// Persists full list.
  Future<void> saveBudgets(
    String tripId,
    List<BudgetCategory> categories,
  ) async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString(_key(tripId), BudgetCategory.encodeList(categories));
  }

  /// Adds an envelope.
  Future<List<BudgetCategory>> addBudget(
    String tripId,
    BudgetCategory category,
  ) async {
    final current = await loadBudgets(tripId);
    final updated = [...current, category];
    await saveBudgets(tripId, updated);
    return updated;
  }

  /// Deletes an envelope.
  Future<List<BudgetCategory>> deleteBudget(
    String tripId,
    String categoryId,
  ) async {
    final current = await loadBudgets(tripId);
    final updated = current.where((c) => c.id != categoryId).toList();
    await saveBudgets(tripId, updated);
    return updated;
  }

  /// Updates spent amount for a category (used later by expense sync).
  Future<List<BudgetCategory>> updateSpent(
    String tripId,
    String categoryId,
    double spent,
  ) async {
    final current = await loadBudgets(tripId);
    final updated = current
        .map((c) {
          if (c.id != categoryId) return c;
          return c.copyWith(spent: spent);
        })
        .toList(growable: false);
    await saveBudgets(tripId, updated);
    return updated;
  }

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