src/features/report/report.mapper.ts

@@ -25,14 +25,57 @@ const toLabel = (start: string, end: string, type: "weekly" | "monthly") => {
   })}`;
 };
 
+const getWeekOfMonth = (date: Date) => {
+  const firstDay = new Date(date.getFullYear(), date.getMonth(), 1);
+  return Math.ceil((date.getDate() + firstDay.getDay()) / 7);
+};
+
+const findCompareBucket = (
+  current: ReportBucket,
+  buckets: ReportBucket[],
+  type: "weekly" | "monthly"
+): ReportBucket | undefined => {
+  const start = new Date(current.start);
+
+  if (type === "monthly") {
+    const targetYear = start.getFullYear() - 1;
+    const targetMonth = start.getMonth();
+
+    return buckets.find(b => {
+      const d = new Date(b.start);
+      return (
+        d.getFullYear() === targetYear &&
+        d.getMonth() === targetMonth
+      );
+    });
+  }
+
+  if (type === "weekly") {
+    const weekIndex = getWeekOfMonth(start); // you must define this
+    const target = new Date(start);
+    target.setMonth(target.getMonth() - 1);
+
+    return buckets.find(b => {
+      const d = new Date(b.start);
+      return (
+        d.getFullYear() === target.getFullYear() &&
+        d.getMonth() === target.getMonth() &&
+        getWeekOfMonth(d) === weekIndex
+      );
+    });
+  }
+
+  return undefined;
+};
+
 const toPoints = (
   buckets: ReportBucket[],
   type: "weekly" | "monthly",
   flow: "expenses" | "incomes"
 ): ChartDataPoint[] => {
-  return buckets.map((b, i) => {
+  return buckets.map((b) => {
     const amount = sumBucket(b, flow);
-    const prev = buckets[i - 1];
+    const prev = findCompareBucket(b, buckets, type);
 
     return {
       id: toLabel(b.start, b.end, type),