first commit

lib/utils/bill_grouping.dart

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+import '../models/bill.dart';
+
+/// 按日期分组的账单
+class BillDayGroup {
+  final String date; // yyyy-MM-dd
+  final List<Bill> bills;
+  final double expenseTotal; // type==1 的合计
+  final double incomeTotal; // type==2 的合计
+
+  const BillDayGroup({
+    required this.date,
+    required this.bills,
+    required this.expenseTotal,
+    required this.incomeTotal,
+  });
+}
+
+/// 按 billDate 分组，并分别累计当日支出/收入。
+///
+/// 后端已按 `billDate DESC, createTime DESC` 排序，Dart 的 Map 保持插入顺序，
+/// 因此直接顺序遍历累积即可，不要再次排序（否则会打乱同日内的时间顺序）。
+List<BillDayGroup> groupBillsByDate(List<Bill> bills) {
+  final grouped = <String, List<Bill>>{};
+  for (final bill in bills) {
+    grouped.putIfAbsent(bill.billDate, () => []).add(bill);
+  }
+
+  return grouped.entries.map((entry) {
+    var expense = 0.0;
+    var income = 0.0;
+    for (final bill in entry.value) {
+      if (bill.isExpense) {
+        expense += bill.absAmount;
+      } else {
+        income += bill.absAmount;
+      }
+    }
+    return BillDayGroup(
+      date: entry.key,
+      bills: entry.value,
+      expenseTotal: expense,
+      incomeTotal: income,
+    );
+  }).toList();
+}