improved performance and battery usage on older devices

lib/view/pages/timetable/data/calendar_logic.dart

@@ -1,7 +1,7 @@
-import 'package:rrule/rrule.dart';
 import 'package:syncfusion_flutter_calendar/calendar.dart';
 
 import '../../../../extensions/date_time.dart';
+import '../../../../utils/recurrence_occurrences.dart';
 import 'arbitrary_appointment.dart';
 import 'calendar_layout.dart';
 import 'lesson_period_schedule.dart';
@@ -103,7 +103,6 @@ partitionAppointmentsForWeek(
       continue;
     }
     try {
-      final parsed = RecurrenceRule.fromString(rule);
       final anchorUtc = a.startTime.toUtc();
       final duration = a.endTime.difference(a.startTime);
       // Day-keyed set of exception dates so occurrences scheduled for one
@@ -112,9 +111,12 @@ partitionAppointmentsForWeek(
       final exceptionDayKeys = (a.recurrenceExceptionDates ?? const <DateTime>[])
           .map((d) => '${d.year}-${d.month}-${d.day}')
           .toSet();
-      for (final occUtc in parsed.getInstances(start: anchorUtc)) {
-        if (!occUtc.isBefore(weekEndUtc)) break;
-        if (occUtc.isBefore(weekStartUtc)) continue;
+      for (final occUtc in RecurrenceOccurrences.between(
+        rule,
+        anchorUtc,
+        weekStartUtc,
+        weekEndUtc,
+      )) {
         final occLocal = occUtc.toLocal();
         if (exceptionDayKeys.contains(
           '${occLocal.year}-${occLocal.month}-${occLocal.day}',