1 files changed, 57 insertions, 0 deletions
diff --git a/internal/models/recurrence.go b/internal/models/recurrence.go
new file mode 100644
index 0000000..f42183c
--- /dev/null
+++ b/internal/models/recurrence.go
@@ -0,0 +1,57 @@
+package models
+
+import (
+	"sort"
+	"time"
+)
+
+// ComputeNextOccurrence returns the next occurrence date after due, given a
+// recurrence pattern. weekdays is only consulted when freq == "weekly"; nil
+// or empty means "same weekday as due, every interval weeks." Unknown freq
+// values return due unchanged. interval < 1 is treated as 1.
+//
+// Monthly/yearly rollover uses Go's standard AddDate overflow behavior (a
+// due date of Jan 31 + 1 month becomes Mar 3, not clamped to Feb's last
+// day) -- this is an accepted simplification, and the drift is permanent:
+// each call computes from the previous call's actual result, not an
+// original anchor day, so a drifted date locks onto its new day-of-month
+// going forward. Only anchor days 29-31 are ever affected; every month has
+// at least 28 days, so any anchor day <= 28 never drifts.
+func ComputeNextOccurrence(due time.Time, freq string, interval int, weekdays []int) time.Time {
+	if interval < 1 {
+		interval = 1
+	}
+	switch freq {
+	case "daily":
+		return due.AddDate(0, 0, interval)
+	case "weekly":
+		return nextWeeklyOccurrence(due, interval, weekdays)
+	case "monthly":
+		return due.AddDate(0, interval, 0)
+	case "yearly":
+		return due.AddDate(interval, 0, 0)
+	default:
+		return due
+	}
+}
+
+func nextWeeklyOccurrence(due time.Time, interval int, weekdays []int) time.Time {
+	if len(weekdays) == 0 {
+		return due.AddDate(0, 0, 7*interval)
+	}
+	sorted := append([]int(nil), weekdays...)
+	sort.Ints(sorted)
+	dueWeekday := int(due.Weekday())
+
+	for _, wd := range sorted {
+		if wd > dueWeekday {
+			return due.AddDate(0, 0, wd-dueWeekday)
+		}
+	}
+	// Wrapped past the last active weekday this week: land on the first
+	// active weekday, (interval-1) whole weeks further out than the
+	// immediate next week (interval=1 means "next week", interval=2 means
+	// "skip a week", etc).
+	daysToNextWeekStart := 7 - dueWeekday
+	return due.AddDate(0, 0, daysToNextWeekStart+sorted[0]+7*(interval-1))
+}