major refactor

lib/components/calculator/calculator.dart

@@ -25,9 +25,6 @@ class StationAutocomplete extends StatefulWidget {
 class _StationAutocompleteState extends State<StationAutocomplete> {
   late final TextEditingController _controller;
 
-  // Simulated list of over 10,000 stations
-  final List<String> stations = List.generate(10000, (i) => 'Station $i');
-
   @override
   void initState() {
     super.initState();
@@ -50,15 +47,7 @@ class _StationAutocompleteState extends State<StationAutocomplete> {
         if (textEditingValue.text.isEmpty) {
           return const Iterable<String>.empty();
         }
-        final query = textEditingValue.text.toLowerCase();
-        final matches = widget.allStations
-            .map((s) => s.name)
-            .where((name) => name.toLowerCase().contains(query))
-            .toList();
-
-        matches.sort((a, b) => a.length.compareTo(b.length));
-
-        return matches.take(10);
+        return _findTopMatches(textEditingValue.text);
       },
       onSelected: (String selection) {
         _controller.text = selection;
@@ -73,19 +62,12 @@ class _StationAutocompleteState extends State<StationAutocomplete> {
               focusNode: focusNode,
               textInputAction: TextInputAction.done,
               onSubmitted: (_) {
-                final query = textEditingController.text.toLowerCase();
-                final matches = widget.allStations
-                    .map((s) => s.name)
-                    .where((name) => name.toLowerCase().contains(query))
-                    .toList();
-
-                if (matches.isNotEmpty) {
-                  matches.sort((a, b) => a.length.compareTo(b.length));
-                  final firstMatch = matches.first;
-                  _controller.text = firstMatch;
-                  widget.onChanged(firstMatch);
-                  focusNode.unfocus(); // optionally close keyboard
-                }
+                final matches = _findTopMatches(textEditingController.text);
+                final firstMatch = matches.isEmpty ? null : matches.first;
+                if (firstMatch == null) return;
+                _controller.text = firstMatch;
+                widget.onChanged(firstMatch);
+                focusNode.unfocus(); // optionally close keyboard
               },
               decoration: const InputDecoration(
                 labelText: 'Select station',
@@ -95,6 +77,42 @@ class _StationAutocompleteState extends State<StationAutocomplete> {
           },
     );
   }
+
+  Iterable<String> _findTopMatches(String rawQuery) {
+    final query = rawQuery.trim().toLowerCase();
+    if (query.isEmpty) return const <String>[];
+
+    // Keep a bounded, sorted list (by shortest name, then alpha) without
+    // sorting the entire match set.
+    final best = <String>[];
+    for (final station in widget.allStations) {
+      final name = station.name;
+      if (name.isEmpty) continue;
+      if (!name.toLowerCase().contains(query)) continue;
+
+      _insertCandidate(best, name, max: 10);
+    }
+    return best;
+  }
+
+  void _insertCandidate(List<String> best, String candidate, {required int max}) {
+    final existingIndex = best.indexOf(candidate);
+    if (existingIndex >= 0) return;
+
+    int insertAt = 0;
+    while (insertAt < best.length &&
+        _candidateCompare(best[insertAt], candidate) <= 0) {
+      insertAt++;
+    }
+    best.insert(insertAt, candidate);
+    if (best.length > max) best.removeLast();
+  }
+
+  int _candidateCompare(String a, String b) {
+    final byLength = a.length.compareTo(b.length);
+    if (byLength != 0) return byLength;
+    return a.compareTo(b);
+  }
 }
 
 class RouteCalculator extends StatefulWidget {
@@ -146,20 +164,28 @@ class _RouteCalculatorState extends State<RouteCalculator> {
       _routeResult = null;
     });
     final api = context.read<ApiService>(); // context is valid here
-    final res = await api.post('/route/distance2', {
-      'route': stations.where((s) => s.trim().isNotEmpty).toList(),
-    });
+    try {
+      final res = await api.post('/route/distance2', {
+        'route': stations.where((s) => s.trim().isNotEmpty).toList(),
+      });
 
-    if (res['error'] == false) {
-      setState(() {
-        _routeResult = RouteResult.fromJson(res);
-      });
-      final distance = (_routeResult?.distance ?? 0);
-      widget.onDistanceComputed?.call(distance);
-    } else {
-      setState(() {
-        _errorMessage = RouteError.fromJson(res["error_obj"][0]).msg;
-      });
+      if (res is Map && res['error'] == false) {
+        setState(() {
+          _routeResult = RouteResult.fromJson(Map<String, dynamic>.from(res));
+        });
+        final distance = (_routeResult?.distance ?? 0);
+        widget.onDistanceComputed?.call(distance);
+      } else if (res is Map && res['error_obj'] is List && res['error_obj'].isNotEmpty) {
+        setState(() {
+          _errorMessage = RouteError.fromJson(
+            Map<String, dynamic>.from(res['error_obj'][0] as Map),
+          ).msg;
+        });
+      } else {
+        setState(() => _errorMessage = 'Failed to calculate route.');
+      }
+    } catch (e) {
+      setState(() => _errorMessage = 'Failed to calculate route: $e');
     }
   }