improved performance and battery usage on older devices

lib/utils/recurrence_occurrences.dart

@@ -0,0 +1,75 @@
+import 'package:rrule/rrule.dart';
+
+/// Memoised RRULE expansion.
+///
+/// `RecurrenceRule.getInstances` always starts at the series anchor, so asking
+/// for one week of a daily event created two years ago walks ~750 occurrences
+/// (tens of ms each time on older phones) — and the calendar asks once per
+/// week and event, the home widget twice per publish. Here each series keeps
+/// its parsed rule, the occurrences found so far and the live iterator, so
+/// later queries only extend the expansion instead of restarting it.
+class RecurrenceOccurrences {
+  RecurrenceOccurrences._();
+
+  static const int _maxSeries = 64;
+  static final Map<(String, DateTime), _Series> _cache = {};
+
+  /// UTC occurrences of [rule] anchored at [anchorUtc] in
+  /// `[fromUtc, toUtc)`, in ascending order. Throws like
+  /// [RecurrenceRule.fromString] for an invalid rule.
+  static List<DateTime> between(
+    String rule,
+    DateTime anchorUtc,
+    DateTime fromUtc,
+    DateTime toUtc,
+  ) {
+    final key = (rule, anchorUtc);
+    var series = _cache.remove(key);
+    series ??= _Series(RecurrenceRule.fromString(rule), anchorUtc);
+    // Re-insert to keep the map in least-recently-used order.
+    _cache[key] = series;
+    if (_cache.length > _maxSeries) _cache.remove(_cache.keys.first);
+    return series.between(fromUtc, toUtc);
+  }
+
+  /// Drops all memoised series (tests).
+  static void clear() => _cache.clear();
+}
+
+class _Series {
+  final Iterator<DateTime> _iterator;
+  final List<DateTime> _found = [];
+  bool _exhausted = false;
+
+  _Series(RecurrenceRule rule, DateTime anchorUtc)
+    : _iterator = rule.getInstances(start: anchorUtc).iterator;
+
+  List<DateTime> between(DateTime fromUtc, DateTime toUtc) {
+    while (!_exhausted && (_found.isEmpty || _found.last.isBefore(toUtc))) {
+      if (_iterator.moveNext()) {
+        _found.add(_iterator.current);
+      } else {
+        _exhausted = true;
+      }
+    }
+    final start = _lowerBound(fromUtc);
+    return [
+      for (var i = start; i < _found.length && _found[i].isBefore(toUtc); i++)
+        _found[i],
+    ];
+  }
+
+  int _lowerBound(DateTime value) {
+    var low = 0;
+    var high = _found.length;
+    while (low < high) {
+      final mid = (low + high) >> 1;
+      if (_found[mid].isBefore(value)) {
+        low = mid + 1;
+      } else {
+        high = mid;
+      }
+    }
+    return low;
+  }
+}