Fix #71: Implement BYSETPOS in RecurrenceEngineV2

core/events/RecurrenceEngineV2.js

@@ -309,9 +309,41 @@ export class RecurrenceEngineV2 {
       next.setMonth(next.getMonth() + rule.interval);
       this.setToNthWeekdayOfMonth(next, byDay.weekday, nthOccurrence);
     } else if (rule.bySetPos && rule.bySetPos.length > 0) {
-      // BYSETPOS for selecting from set
+      // BYSETPOS selects from the set of candidates generated by other BY* rules
       next.setMonth(next.getMonth() + rule.interval);
-      // Complex BYSETPOS logic would go here
+      next.setDate(1);
+
+      const dayMap = { SU: 0, MO: 1, TU: 2, WE: 3, TH: 4, FR: 5, SA: 6 };
+      const candidates = [];
+
+      if (rule.byDay && rule.byDay.length > 0) {
+        // Generate all matching weekday occurrences in the month
+        const lastDay = new Date(next.getFullYear(), next.getMonth() + 1, 0).getDate();
+        for (let d = 1; d <= lastDay; d++) {
+          const date = new Date(next.getFullYear(), next.getMonth(), d);
+          const dayOfWeek = date.getDay();
+          for (const byDay of rule.byDay) {
+            const dayStr = typeof byDay === 'string' ? byDay.replace(/^-?\d+/, '') : byDay.weekday;
+            if (dayMap[dayStr] === dayOfWeek) {
+              candidates.push(d);
+              break;
+            }
+          }
+        }
+      }
+
+      // Apply BYSETPOS indices (1-based, negative from end)
+      const selectedDays = [];
+      for (const pos of rule.bySetPos) {
+        const index = pos > 0 ? pos - 1 : candidates.length + pos;
+        if (index >= 0 && index < candidates.length) {
+          selectedDays.push(candidates[index]);
+        }
+      }
+
+      if (selectedDays.length > 0) {
+        next.setDate(selectedDays[0]);
+      }
     } else {
       // Same day of next month
       const currentDay = next.getDate();