From e1873371279fea434cec415e75c0f6584899a808 Mon Sep 17 00:00:00 2001
From: timeTable2 dev <dev@local>
Date: Mon, 13 Jul 2026 18:32:46 +0800
Subject: [PATCH] feat: add recurrence grid expansion

---
 src/lib/recur.js   | 54 ++++++++++++++++++++++++++++++++++++
 test/recur.test.js | 69 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 123 insertions(+)
 create mode 100644 src/lib/recur.js
 create mode 100644 test/recur.test.js

diff --git a/src/lib/recur.js b/src/lib/recur.js
new file mode 100644
index 0000000..7e3c3ae
--- /dev/null
+++ b/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
+}
diff --git a/test/recur.test.js b/test/recur.test.js
new file mode 100644
index 0000000..886082b
--- /dev/null
+++ b/test/recur.test.js
@@ -0,0 +1,69 @@
+import { describe, it, expect } from 'vitest'
+import { expandOccurrences, intervalDays } from '../src/lib/recur.js'
+
+describe('intervalDays', () => {
+  it('returns 7 for WEEKLY interval=1', () => {
+    const ev = { rrule: { freq: 'WEEKLY', interval: 1 } }
+    expect(intervalDays(ev)).toBe(7)
+  })
+
+  it('returns 14 for WEEKLY interval=2', () => {
+    const ev = { rrule: { freq: 'WEEKLY', interval: 2 } }
+    expect(intervalDays(ev)).toBe(14)
+  })
+
+  it('returns interval for DAILY', () => {
+    const ev = { rrule: { freq: 'DAILY', interval: 3 } }
+    expect(intervalDays(ev)).toBe(3)
+  })
+})
+
+describe('expandOccurrences', () => {
+  it('returns single occurrence for non-recurring event', () => {
+    const ev = { dtstartDate: '2026-07-27', rrule: null, exdates: [] }
+    const occs = expandOccurrences(ev)
+    expect(occs).toHaveLength(1)
+    expect(occs[0]).toEqual({ date: '2026-07-27', skipped: false })
+  })
+
+  it('expands weekly event from Jul 27 to Oct 19', () => {
+    const ev = {
+      dtstartDate: '2026-07-27',
+      rrule: { freq: 'WEEKLY', interval: 1, byday: 'MO', untilDate: '2026-10-19' },
+      exdates: ['2026-09-21'],
+    }
+    const occs = expandOccurrences(ev)
+    // Jul 27 + 12 weeks = Oct 19 -> 13 occurrences
+    expect(occs).toHaveLength(13)
+    expect(occs[0].date).toBe('2026-07-27')
+    expect(occs[12].date).toBe('2026-10-19')
+    // Sep 21 is skipped
+    const sep21 = occs.find(o => o.date === '2026-09-21')
+    expect(sep21.skipped).toBe(true)
+    // Others not skipped
+    expect(occs[0].skipped).toBe(false)
+  })
+
+  it('respects exdates set', () => {
+    const ev = {
+      dtstartDate: '2026-07-27',
+      rrule: { freq: 'WEEKLY', interval: 1, byday: 'MO', untilDate: '2026-08-24' },
+      exdates: ['2026-08-03', '2026-08-17'],
+    }
+    const occs = expandOccurrences(ev)
+    expect(occs).toHaveLength(5) // Jul27, Aug3, Aug10, Aug17, Aug24
+    expect(occs[1].skipped).toBe(true)  // Aug 3
+    expect(occs[2].skipped).toBe(false) // Aug 10
+    expect(occs[3].skipped).toBe(true)  // Aug 17
+  })
+
+  it('caps at 500 occurrences', () => {
+    const ev = {
+      dtstartDate: '2020-01-01',
+      rrule: { freq: 'DAILY', interval: 1, untilDate: '2030-12-31' },
+      exdates: [],
+    }
+    const occs = expandOccurrences(ev)
+    expect(occs.length).toBeLessThanOrEqual(500)
+  })
+})