add filter panel to calculator

lib/services/data_service/data_service_core.dart

@@ -72,9 +72,14 @@ class DataService extends ChangeNotifier {
   bool get isEventFieldsLoading => _isEventFieldsLoading;
 
   // Station Data
-  List<Station>? _cachedStations;
-  DateTime? _stationsFetchedAt;
-  Future<List<Station>>? _stationsInFlight;
+  final Map<String, List<Station>> _stationCache = {};
+  final Map<String, Future<List<Station>>?> _stationInFlightByKey = {};
+  List<String> _stationNetworks = [];
+  Map<String, List<String>> _stationCountryNetworks = {};
+  DateTime? _stationFiltersFetchedAt;
+  List<String> get stationNetworks => _stationNetworks;
+  Map<String, List<String>> get stationCountryNetworks =>
+      _stationCountryNetworks;
 
   List<String> stations = [""];
 
@@ -365,37 +370,75 @@ class DataService extends ChangeNotifier {
         0;
   }
 
-  Future<List<Station>> fetchStations() async {
+  Future<void> fetchStationFilters() async {
     final now = DateTime.now();
-
-    // If cache exists and is less than 30 minutes old, return it
-    if (_cachedStations != null &&
-        _stationsFetchedAt != null &&
-        now.difference(_stationsFetchedAt!) < Duration(minutes: 30)) {
-      return _cachedStations!;
+    if (_stationFiltersFetchedAt != null &&
+        now.difference(_stationFiltersFetchedAt!) < const Duration(minutes: 30) &&
+        _stationNetworks.isNotEmpty) {
+      return;
     }
+    try {
+      final response = await api.get('/stations/filter');
+      if (response is List && response.isNotEmpty && response.first is Map) {
+        final map = Map<String, dynamic>.from(response.first as Map);
+        final networks = (map['networks'] as List? ?? const [])
+            .whereType<String>()
+            .toList();
+        final countryNetworksRaw =
+            map['country_networks'] as Map? ?? const <String, dynamic>{};
+        final countryNetworks = <String, List<String>>{};
+        countryNetworksRaw.forEach((key, value) {
+          if (value is List) {
+            countryNetworks[key] = value.whereType<String>().toList();
+          }
+        });
+        _stationNetworks = networks;
+        _stationCountryNetworks = countryNetworks;
+        _stationFiltersFetchedAt = now;
+      }
+    } catch (e) {
+      debugPrint('Failed to fetch station filters: $e');
+    }
+  }
 
-    if (_stationsInFlight != null) return _stationsInFlight!;
+  String _stationKey(List<String> countries, List<String> networks) {
+    final c = countries..sort();
+    final n = networks..sort();
+    return 'c:${c.join('|')};n:${n.join('|')}';
+  }
 
-    _stationsInFlight = () async {
+  Future<List<Station>> fetchStations({
+    List<String> countries = const [],
+    List<String> networks = const [],
+  }) async {
+    final key = _stationKey(List.from(countries), List.from(networks));
+
+    if (_stationCache.containsKey(key)) return _stationCache[key]!;
+    final inflight = _stationInFlightByKey[key];
+    if (inflight != null) return inflight;
+
+    final future = () async {
       try {
-        final response = await api.get('/location');
+        final response = await api.post('/location', {
+          'countries_filter': countries,
+          'network_filter': networks,
+        });
         if (response is! List) return const <Station>[];
         final parsed = response
             .whereType<Map>()
             .map((e) => Station.fromJson(Map<String, dynamic>.from(e)))
             .toList();
-        _cachedStations = parsed;
-        _stationsFetchedAt = now;
+        _stationCache[key] = parsed;
         return parsed;
       } catch (e) {
         debugPrint('Failed to fetch stations: $e');
         return const <Station>[];
       } finally {
-        _stationsInFlight = null;
+        _stationInFlightByKey.remove(key);
       }
     }();
 
-    return _stationsInFlight!;
+    _stationInFlightByKey[key] = future;
+    return future;
   }
 }