feat: add recurrence grid expansion

src/lib/recur.js

@@ -0,0 +1,54 @@
+/**
+ * Recurrence grid expansion - port of ical_editor.py's occurrence generation.
+ * occurrences are computed on-demand from exdates, never stored on the model.
+ */
+
+/** Interval in days for a recurring event. */
+export function intervalDays(ev) {
+  if (!ev.rrule) return 0
+  const { freq, interval } = ev.rrule
+  const step = interval || 1
+  return freq === 'WEEKLY' ? 7 * step : step
+}
+
+function toISO(date) {
+  const y = date.getFullYear()
+  const m = String(date.getMonth() + 1).padStart(2, '0')
+  const d = String(date.getDate()).padStart(2, '0')
+  return `${y}-${m}-${d}`
+}
+
+/**
+ * Expand a recurring event into an occurrence grid.
+ * Returns [{ date: 'YYYY-MM-DD', skipped: boolean }].
+ */
+export function expandOccurrences(ev) {
+  // Non-recurring: single occurrence
+  if (!ev.rrule) {
+    return [{ date: ev.dtstartDate, skipped: false }]
+  }
+
+  const step = intervalDays(ev)
+  if (step <= 0) {
+    return [{ date: ev.dtstartDate, skipped: false }]
+  }
+
+  const start = new Date(ev.dtstartDate + 'T00:00:00')
+  const until = ev.rrule.untilDate
+    ? new Date(ev.rrule.untilDate + 'T00:00:00')
+    : new Date(start.getTime() + 365 * 24 * 3600 * 1000) // default +1 year
+
+  const exdateSet = new Set(ev.exdates || [])
+  const occs = []
+  const cur = new Date(start)
+  let count = 0
+
+  while (cur <= until && count < 500) {
+    const iso = toISO(cur)
+    occs.push({ date: iso, skipped: exdateSet.has(iso) })
+    cur.setDate(cur.getDate() + step)
+    count++
+  }
+
+  return occs
+}