new plus button

lib/utils/shop_calendar.dart

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+/// Month-grid helpers for the shop-log heatmap.
+library;
+
+DateTime dateOnly(DateTime date) => DateTime(date.year, date.month, date.day);
+
+DateTime monthStart(DateTime date) => DateTime(date.year, date.month);
+
+bool isSameDay(DateTime a, DateTime b) =>
+    a.year == b.year && a.month == b.month && a.day == b.day;
+
+bool isSameMonth(DateTime a, DateTime b) =>
+    a.year == b.year && a.month == b.month;
+
+DateTime addMonths(DateTime month, int delta) =>
+    DateTime(month.year, month.month + delta);
+
+/// Monday-first calendar cells for [month], padded with nulls to full weeks.
+List<DateTime?> monthGrid(DateTime month) {
+  final start = DateTime(month.year, month.month, 1);
+  final daysInMonth = DateTime(month.year, month.month + 1, 0).day;
+  final leading = start.weekday - DateTime.monday;
+  final cells = <DateTime?>[
+    ...List<DateTime?>.filled(leading, null),
+    for (var day = 1; day <= daysInMonth; day++)
+      DateTime(month.year, month.month, day),
+  ];
+  while (cells.length % 7 != 0) {
+    cells.add(null);
+  }
+  return cells;
+}
+
+/// How many trips landed on each calendar day.
+Map<DateTime, int> tripCountsByDay(Iterable<DateTime> shoppedAt) {
+  final counts = <DateTime, int>{};
+  for (final date in shoppedAt) {
+    final key = dateOnly(date);
+    counts[key] = (counts[key] ?? 0) + 1;
+  }
+  return counts;
+}
+
+/// GitHub-style 0–4 intensity from a trip count.
+int heatmapLevel(int count) {
+  if (count <= 0) return 0;
+  if (count >= 4) return 4;
+  return count;
+}