webmail/lib/recurrence-expansion.ts

/**
 * Client-side recurrence expansion for JSCalendar events.
 *
 * Implements JSCalendar 2.0 (draft-ietf-calext-jscalendarbis-15) §3.3.3.1
 * recurrence rule interpretation algorithm with full byX filtering,
 * implicit byX property addition, and bySetPosition support.
 *
 * Used for servers that do not accept the synthetic ids produced by
 * CalendarEvent/query?expandRecurrences=true in CalendarEvent/set (Stalwart
 * before 0.16.20, see lib/recurrence-instances.ts): raw events (with real,
 * mutable ids) are fetched and recurring series are expanded into individual
 * occurrences in the browser. Occurrences a server already expanded (no
 * recurrence rule of their own) pass through unchanged.
 */

import { parseISO, format, addDays, addWeeks, addMonths, addYears, differenceInCalendarDays } from 'date-fns';
import type { CalendarEvent, CalendarRecurrenceRule, CalendarNDay } from '@/lib/jmap/types';
import { isServerRecurrenceInstance, mergeOverrideParticipants } from '@/lib/recurrence-instances';

const DAY_INDEX: Record<string, number> = { su: 0, mo: 1, tu: 2, we: 3, th: 4, fr: 5, sa: 6 };
const INDEX_TO_DAY: string[] = ['su', 'mo', 'tu', 'we', 'th', 'fr', 'sa'];

/**
 * Given a master event and a date range, return an array of "virtual" occurrence
 * events. Each occurrence carries a synthetic `id` (for dedup in React) and a
 * `masterEventId` field that points back to the real server-side ID so the
 * store can use it for mutations.
 *
 * Non-recurring events are returned as-is. For recurring events the master is
 * **not** returned - only expanded instances within the range.
 */
export function expandRecurringEvents(
  events: CalendarEvent[],
  rangeStart: string,
  rangeEnd: string,
): CalendarEvent[] {
  const start = parseISO(rangeStart);
  const end = parseISO(rangeEnd);
  const result: CalendarEvent[] = [];

  // Collect UIDs of master recurring events so we can skip their
  // server-returned override instances (which have recurrenceId set).
  // The master's recurrenceOverrides already accounts for them.
  const recurringUids = new Set<string>();
  for (const event of events) {
    if (event.recurrenceRules?.length && event.uid && !event.recurrenceId) {
      recurringUids.add(event.uid);
    }
  }

  for (const event of events) {
    // Skip override instances returned by the server - they belong to a
    // master recurring event and are already handled via recurrenceOverrides.
    if (event.recurrenceId && event.uid && recurringUids.has(event.uid)) {
      continue;
    }

    // An occurrence the server already expanded is a single instance even
    // though it carries its series' recurrence rule (lib/recurrence-instances.ts
    // hydrates it with that); never expand it again.
    if (!event.recurrenceRules?.length || isServerRecurrenceInstance(event)) {
      result.push(event);
      continue;
    }

    const occurrences = expandEvent(event, start, end);
    result.push(...occurrences);
  }

  return result;
}

function occurrenceDateKey(master: CalendarEvent, date: Date): string {
  return master.showWithoutTime ? format(date, 'yyyy-MM-dd') : date.toISOString();
}

function expandEvent(
  master: CalendarEvent,
  rangeStart: Date,
  rangeEnd: Date,
): CalendarEvent[] {
  const eventStart = parseISO(master.start);
  if (isNaN(eventStart.getTime())) return [];

  const rules = master.recurrenceRules || [];
  const overrides = master.recurrenceOverrides || {};
  const occurrences: CalendarEvent[] = [];
  const seenDates = new Set<string>();

  // RFC 8984 §4.3.3: occurrences produced by excludedRecurrenceRules
  // (iCalendar EXRULE) are removed from the recurrence set.
  const excludedDates = new Set<string>();
  for (const exRule of master.excludedRecurrenceRules || []) {
    // includeStartDate=false: "the series start is always an occurrence"
    // applies to recurrence rules, not to exclusion rules.
    for (const date of generateDates(eventStart, exRule, rangeStart, rangeEnd, false)) {
      excludedDates.add(occurrenceDateKey(master, date));
    }
  }

  for (const rule of rules) {
    const dates = generateDates(eventStart, rule, rangeStart, rangeEnd);
    for (const date of dates) {
      const dateKey = occurrenceDateKey(master, date);

      if (excludedDates.has(dateKey)) continue;
      if (seenDates.has(dateKey)) continue;
      seenDates.add(dateKey);

      const recurrenceId = master.showWithoutTime
        ? format(date, "yyyy-MM-dd'T'00:00:00")
        : format(date, "yyyy-MM-dd'T'HH:mm:ss");

      const override = overrides[recurrenceId] as (Partial<CalendarEvent> & { excluded?: boolean }) | undefined;
      if (override?.excluded) continue;

      occurrences.push(createOccurrence(master, date, recurrenceId, override));
    }
  }

  // Add overrides that define new dates not generated by rules (RDATE equivalent)
  for (const [recurrenceId, rawOverride] of Object.entries(overrides)) {
    const override = rawOverride as Partial<CalendarEvent> & { excluded?: boolean };
    if (override.excluded) continue;
    const overrideDate = parseISO(recurrenceId);
    if (isNaN(overrideDate.getTime())) continue;
    if (overrideDate < rangeStart || overrideDate >= rangeEnd) continue;

    // Overrides take precedence over excludedRecurrenceRules (they re-add
    // a concrete instance), so only dedupe against already-generated dates.
    const dateKey = occurrenceDateKey(master, overrideDate);
    if (seenDates.has(dateKey)) continue;
    seenDates.add(dateKey);

    occurrences.push(createOccurrence(master, overrideDate, recurrenceId, override));
  }

  return occurrences;
}

// Cache Intl formatters per IANA timezone id - constructing them is expensive
// and expansion runs over hundreds of occurrences. `null` marks ids the
// runtime rejected so we don't retry them.
const tzFormatterCache = new Map<string, Intl.DateTimeFormat | null>();

function getTzFormatter(timeZone: string): Intl.DateTimeFormat | null {
  let formatter = tzFormatterCache.get(timeZone);
  if (formatter === undefined) {
    try {
      formatter = new Intl.DateTimeFormat('en-CA', {
        timeZone,
        year: 'numeric',
        month: '2-digit',
        day: '2-digit',
        hour: '2-digit',
        minute: '2-digit',
        second: '2-digit',
        hour12: false,
      });
    } catch {
      formatter = null;
    }
    tzFormatterCache.set(timeZone, formatter);
  }
  return formatter;
}

/** The wall-clock reading of `instant` in the formatter's zone, re-encoded as a UTC timestamp. */
function wallClockAsUtcTimestamp(formatter: Intl.DateTimeFormat, instant: number): number {
  const map: Record<string, string> = {};
  for (const part of formatter.formatToParts(new Date(instant))) {
    if (part.type !== 'literal') map[part.type] = part.value;
  }
  const hour = map.hour === '24' ? 0 : Number(map.hour);
  return Date.UTC(
    Number(map.year), Number(map.month) - 1, Number(map.day),
    hour, Number(map.minute), Number(map.second),
  );
}

/**
 * Interpret the wall-clock fields of `wall` as a local time in `timeZone`
 * and return the corresponding UTC instant. Two fixup iterations converge
 * for all real offsets, including across DST transitions.
 */
function zonedWallTimeToUtc(wall: Date, timeZone: string): Date | null {
  const formatter = getTzFormatter(timeZone);
  if (!formatter) return null;
  const wallAsUtc = Date.UTC(
    wall.getFullYear(), wall.getMonth(), wall.getDate(),
    wall.getHours(), wall.getMinutes(), wall.getSeconds(),
  );
  let guess = wallAsUtc;
  for (let i = 0; i < 2; i++) {
    guess = wallAsUtc - (wallClockAsUtcTimestamp(formatter, guess) - guess);
  }
  return new Date(guess);
}

function createOccurrence(
  master: CalendarEvent,
  date: Date,
  recurrenceId: string,
  override?: Partial<CalendarEvent>,
): CalendarEvent {
  const startStr = master.showWithoutTime
    ? format(date, "yyyy-MM-dd'T'00:00:00")
    : format(date, "yyyy-MM-dd'T'HH:mm:ss");

  // Compute utcStart/utcEnd for this occurrence so that getEventStartDate()
  // and getEventEndDate() (which prefer utcStart/utcEnd for timed events)
  // return the correct dates instead of the master's original UTC times.
  let utcStart: string | undefined;
  let utcEnd: string | undefined;
  if (!master.showWithoutTime) {
    let durationMs: number | null = null;
    if (master.utcStart && master.utcEnd) {
      const ms = parseISO(master.utcEnd).getTime() - parseISO(master.utcStart).getTime();
      if (!isNaN(ms)) durationMs = ms;
    }

    // Convert the occurrence's wall time using the event's own timezone, so
    // occurrences on the other side of a DST transition in that zone keep
    // their wall time. Reusing the master's fixed UTC offset (the fallback
    // below) would shift them by the DST delta.
    const zoned = master.timeZone ? zonedWallTimeToUtc(date, master.timeZone) : null;
    if (zoned) {
      utcStart = zoned.toISOString();
      if (durationMs !== null) {
        utcEnd = new Date(zoned.getTime() + durationMs).toISOString();
      }
    } else if (master.utcStart && master.start) {
      // Floating events (or an unrecognized timezone id): keep the master's
      // offset, which by definition doesn't vary.
      const masterLocal = parseISO(master.start);
      const masterUtc = parseISO(master.utcStart);
      const offsetMs = masterUtc.getTime() - masterLocal.getTime();
      utcStart = new Date(date.getTime() + offsetMs).toISOString();
      if (durationMs !== null) {
        utcEnd = new Date(date.getTime() + offsetMs + durationMs).toISOString();
      }
    }
  }

  return {
    ...master,
    ...(override || {}),
    ...(override?.participants ? { participants: mergeOverrideParticipants(master.participants, override.participants) } : {}),
    id: `${master.id}:${recurrenceId}`,
    originalId: master.originalId || master.id,
    uid: master.uid,
    calendarIds: master.calendarIds,
    start: (override?.start) || startStr,
    ...(utcStart && !override?.utcStart ? { utcStart } : {}),
    ...(utcEnd && !override?.utcEnd ? { utcEnd } : {}),
    recurrenceId,
    recurrenceRules: master.recurrenceRules,
    recurrenceOverrides: master.recurrenceOverrides,
    excludedRecurrenceRules: master.excludedRecurrenceRules,
  };
}

// ---------------------------------------------------------------------------
// §3.3.3.1 - Implicit byX property addition
// ---------------------------------------------------------------------------
function addImplicitByX(
  rule: CalendarRecurrenceRule,
  eventStart: Date,
): CalendarRecurrenceRule {
  const r = { ...rule };
  const freq = r.frequency;

  // bySecond
  if (freq !== 'secondly' && !r.bySecond?.length) {
    r.bySecond = [eventStart.getSeconds()];
  }
  // byMinute
  if (freq !== 'secondly' && freq !== 'minutely' && !r.byMinute?.length) {
    r.byMinute = [eventStart.getMinutes()];
  }
  // byHour
  if (freq !== 'secondly' && freq !== 'minutely' && freq !== 'hourly' && !r.byHour?.length) {
    r.byHour = [eventStart.getHours()];
  }
  // weekly: implicit byDay
  if (freq === 'weekly' && !r.byDay?.length) {
    r.byDay = [{ day: INDEX_TO_DAY[eventStart.getDay()] }];
  }
  // monthly: implicit byMonthDay
  if (freq === 'monthly' && !r.byDay?.length && !r.byMonthDay?.length) {
    r.byMonthDay = [eventStart.getDate()];
  }
  // yearly: implicit byMonth / byMonthDay / byDay
  if (freq === 'yearly' && !r.byYearDay?.length) {
    if (!r.byMonth?.length && !r.byWeekNo?.length && (r.byMonthDay?.length || !r.byDay?.length)) {
      r.byMonth = [String(eventStart.getMonth() + 1)];
    }
    if (!r.byMonthDay?.length && !r.byWeekNo?.length && !r.byDay?.length) {
      r.byMonthDay = [eventStart.getDate()];
    }
    if (r.byWeekNo?.length && !r.byMonthDay?.length && !r.byDay?.length) {
      r.byDay = [{ day: INDEX_TO_DAY[eventStart.getDay()] }];
    }
  }

  return r;
}

// ---------------------------------------------------------------------------
// §3.3.3.1 - Generate occurrence dates
// ---------------------------------------------------------------------------
function generateDates(
  eventStart: Date,
  rawRule: CalendarRecurrenceRule,
  rangeStart: Date,
  rangeEnd: Date,
  includeStartDate = true,
): Date[] {
  const rule = addImplicitByX(rawRule, eventStart);
  const dates: Date[] = [];
  const interval = rule.interval || 1;
  const countLimit = rule.count || Infinity;
  const until = rule.until ? parseISO(rule.until) : null;
  let totalCount = 0;
  // Without a `count` limit we can jump straight to the period containing
  // the visible range. With one, every occurrence since the series start
  // must be generated so it counts against `count`.
  let current = rule.count
    ? new Date(eventStart)
    : fastForwardToRange(eventStart, rule.frequency, interval, rangeStart);

  const maxIterations = 2000;
  // Cap on *emitted* (in-range) dates. This must not count occurrences
  // before rangeStart, otherwise a series started long ago (e.g. a daily
  // event from two years back) exhausts the budget before reaching the
  // visible range and silently renders nothing.
  const maxOccurrences = 500;
  let iterations = 0;

  while (iterations++ < maxIterations) {
    if (totalCount >= countLimit || dates.length >= maxOccurrences) break;
    if (until && current > until) break;
    // Stop once no candidate in this or a later period can fall before
    // rangeEnd. Monthly/yearly candidates may precede the period anchor
    // within the same month/year, so floor the anchor before comparing.
    if (rule.frequency === 'monthly') {
      if (new Date(current.getFullYear(), current.getMonth(), 1) >= rangeEnd) break;
    } else if (rule.frequency === 'yearly') {
      if (new Date(current.getFullYear(), 0, 1) >= rangeEnd) break;
    } else if (current >= rangeEnd) {
      break;
    }

    // Generate candidates for the current period, then filter
    const candidates = generateCandidatesForPeriod(current, rule, eventStart);

    // Apply bySetPosition if present
    const filtered = rule.bySetPosition?.length
      ? applyBySetPosition(candidates, rule.bySetPosition)
      : candidates;

    for (const d of filtered) {
      if (until && d > until) break;
      if (totalCount >= countLimit || dates.length >= maxOccurrences) break;

      // Spec rule 4: eliminate dates before event start
      if (d < eventStart) continue;

      totalCount++;
      if (d >= rangeStart && d < rangeEnd) {
        dates.push(d);
      }
      if (d >= rangeEnd) break;
    }

    if (totalCount >= countLimit || dates.length >= maxOccurrences) break;

    current = advancePeriod(current, rule.frequency, interval, rule.firstDayOfWeek || 'mo');
    if (current <= eventStart && iterations === 1) {
      // Safety: ensure we don't go backward
      current = advancePeriod(eventStart, rule.frequency, interval, rule.firstDayOfWeek || 'mo');
    }
  }

  // Spec rule 1: the initial start date-time is ALWAYS the first occurrence
  if (includeStartDate && dates.length > 0 && dates[0].getTime() !== eventStart.getTime()) {
    if (eventStart >= rangeStart && eventStart < rangeEnd) {
      // Check it's not already in the list
      if (!dates.some(d => d.getTime() === eventStart.getTime())) {
        dates.unshift(eventStart);
      }
    }
  }

  return dates;
}

// ---------------------------------------------------------------------------
// Generate all candidates within one period, filtered by byX properties
// ---------------------------------------------------------------------------
function generateCandidatesForPeriod(
  periodStart: Date,
  rule: CalendarRecurrenceRule,
  eventStart: Date,
): Date[] {
  const freq = rule.frequency;
  let candidates: Date[];

  // Step 1: Generate base candidates based on frequency + expansion byX
  switch (freq) {
    case 'yearly':
      candidates = expandYearly(periodStart, rule, eventStart);
      break;
    case 'monthly':
      candidates = expandMonthly(periodStart, rule, eventStart);
      break;
    case 'weekly':
      candidates = expandWeekly(periodStart, rule, eventStart);
      break;
    case 'daily':
      // The start's wall-clock time on this day. The period anchor is
      // advanced day by day and keeps whatever time a DST gap pushed it to
      // (02:30 -> 03:30, or midnight -> 01:00 for an all-day event), so using
      // it made every later day fail the implicit byHour and the series
      // stopped at the gap.
      candidates = [new Date(
        periodStart.getFullYear(), periodStart.getMonth(), periodStart.getDate(),
        eventStart.getHours(), eventStart.getMinutes(), eventStart.getSeconds(), eventStart.getMilliseconds(),
      )];
      break;
    case 'hourly':
    case 'minutely':
    case 'secondly':
      candidates = [new Date(periodStart)];
      break;
    default:
      candidates = [new Date(periodStart)];
  }

  // Step 2: Filter candidates by all applicable byX constraints. A daily
  // candidate is judged by the time it was built for: on the day of a DST
  // gap that time does not exist and the occurrence moves past the gap
  // (RFC 5545 3.3.5), which is still this occurrence.
  candidates = candidates.filter(d => matchesByX(d, rule, freq === 'daily' ? eventStart : undefined));

  candidates.sort((a, b) => a.getTime() - b.getTime());
  return candidates;
}

// ---------------------------------------------------------------------------
// Yearly expansion
// ---------------------------------------------------------------------------
function expandYearly(
  periodStart: Date,
  rule: CalendarRecurrenceRule,
  eventStart: Date,
): Date[] {
  const year = periodStart.getFullYear();
  const h = eventStart.getHours();
  const m = eventStart.getMinutes();
  const s = eventStart.getSeconds();
  let dates: Date[] = [];

  // Determine which months to iterate
  const months = rule.byMonth?.length
    ? rule.byMonth.map(ms => parseInt(ms.replace('L', ''), 10) - 1)
    : [eventStart.getMonth()];

  if (rule.byWeekNo?.length) {
    // Expand by ISO week numbers
    for (const wn of rule.byWeekNo) {
      const weekDates = datesInISOWeek(year, wn, rule.firstDayOfWeek || 'mo');
      dates.push(...weekDates.map(d => { d.setHours(h, m, s, 0); return d; }));
    }
  } else if (rule.byYearDay?.length) {
    // Expand by day-of-year
    for (const yd of rule.byYearDay) {
      const d = dayOfYear(year, yd);
      if (d) { d.setHours(h, m, s, 0); dates.push(d); }
    }
  } else if (rule.byDay?.length && rule.byMonthDay?.length) {
    // Both byDay and byMonthDay: expand byMonthDay in each month, then byDay filters later
    for (const mo of months) {
      for (const md of rule.byMonthDay) {
        const d = resolveMonthDay(year, mo, md);
        if (d) { d.setHours(h, m, s, 0); dates.push(d); }
      }
    }
  } else if (rule.byDay?.length) {
    // byDay with nthOfPeriod in yearly context = nth weekday of year or month
    for (const mo of months) {
      const expanded = expandByDayInMonth(year, mo, rule.byDay, h, m, s);
      dates.push(...expanded);
    }
  } else if (rule.byMonthDay?.length) {
    for (const mo of months) {
      for (const md of rule.byMonthDay) {
        const d = resolveMonthDay(year, mo, md);
        if (d) { d.setHours(h, m, s, 0); dates.push(d); }
      }
    }
  } else {
    // Simple yearly: same date each year
    for (const mo of months) {
      const d = new Date(year, mo, eventStart.getDate(), h, m, s, 0);
      if (d.getMonth() === mo) dates.push(d);
    }
  }

  return dates;
}

// ---------------------------------------------------------------------------
// Monthly expansion
// ---------------------------------------------------------------------------
function expandMonthly(
  periodStart: Date,
  rule: CalendarRecurrenceRule,
  eventStart: Date,
): Date[] {
  const year = periodStart.getFullYear();
  const month = periodStart.getMonth();
  const h = eventStart.getHours();
  const m = eventStart.getMinutes();
  const s = eventStart.getSeconds();
  const dates: Date[] = [];

  if (rule.byDay?.length) {
    const expanded = expandByDayInMonth(year, month, rule.byDay, h, m, s);
    dates.push(...expanded);
  } else if (rule.byMonthDay?.length) {
    for (const md of rule.byMonthDay) {
      const d = resolveMonthDay(year, month, md);
      if (d) { d.setHours(h, m, s, 0); dates.push(d); }
    }
  } else {
    // Implicit byMonthDay already added, but as fallback:
    const d = new Date(year, month, eventStart.getDate(), h, m, s, 0);
    if (d.getMonth() === month) dates.push(d);
  }

  return dates;
}

// ---------------------------------------------------------------------------
// Weekly expansion
// ---------------------------------------------------------------------------
function expandWeekly(
  periodStart: Date,
  rule: CalendarRecurrenceRule,
  eventStart: Date,
): Date[] {
  const dates: Date[] = [];
  const baseDay = periodStart.getDay();

  const byDay = rule.byDay?.length ? rule.byDay : [{ day: INDEX_TO_DAY[eventStart.getDay()] }];

  for (const { day } of byDay) {
    const targetDay = DAY_INDEX[day];
    if (targetDay === undefined) continue;
    let diff = targetDay - baseDay;
    if (diff < 0) diff += 7;
    const d = addDays(periodStart, diff);
    d.setHours(eventStart.getHours(), eventStart.getMinutes(), eventStart.getSeconds(), 0);
    dates.push(d);
  }

  return dates.sort((a, b) => a.getTime() - b.getTime());
}

// ---------------------------------------------------------------------------
// byX matching (Step 2 of §3.3.3.1)
// ---------------------------------------------------------------------------
function matchesByX(date: Date, rule: CalendarRecurrenceRule, timeOf: Date = date): boolean {
  if (rule.byMonth?.length) {
    const month = String(date.getMonth() + 1);
    if (!rule.byMonth.some(m => m.replace('L', '') === month)) return false;
  }
  if (rule.byWeekNo?.length) {
    const wn = getISOWeekNumber(date);
    const weeksInYear = getISOWeeksInYear(date.getFullYear());
    if (!rule.byWeekNo.some(w => (w > 0 ? w : weeksInYear + 1 + w) === wn)) return false;
  }
  if (rule.byYearDay?.length) {
    const yd = getDayOfYear(date);
    const daysInYear = isLeapYear(date.getFullYear()) ? 366 : 365;
    if (!rule.byYearDay.some(d => (d > 0 ? d : daysInYear + 1 + d) === yd)) return false;
  }
  if (rule.byMonthDay?.length) {
    const md = date.getDate();
    const daysInMonth = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
    if (!rule.byMonthDay.some(d => (d > 0 ? d : daysInMonth + 1 + d) === md)) return false;
  }
  if (rule.byDay?.length) {
    const dayName = INDEX_TO_DAY[date.getDay()];
    const freq = rule.frequency;
    if (!rule.byDay.some(nd => {
      if (nd.day !== dayName) return false;
      if (nd.nthOfPeriod == null) return true;
      if (freq === 'monthly') {
        return nd.nthOfPeriod === nthWeekdayInMonth(date, nd.nthOfPeriod);
      }
      if (freq === 'yearly') {
        // When byMonth is present, nthOfPeriod scopes to the month (iCalendar semantics)
        if (rule.byMonth?.length) {
          return nd.nthOfPeriod === nthWeekdayInMonth(date, nd.nthOfPeriod);
        }
        return nd.nthOfPeriod === nthWeekdayInYear(date, nd.nthOfPeriod);
      }
      return true;
    })) return false;
  }
  if (rule.byHour?.length) {
    if (!rule.byHour.includes(timeOf.getHours())) return false;
  }
  if (rule.byMinute?.length) {
    if (!rule.byMinute.includes(timeOf.getMinutes())) return false;
  }
  if (rule.bySecond?.length) {
    if (!rule.bySecond.includes(timeOf.getSeconds())) return false;
  }
  return true;
}

// ---------------------------------------------------------------------------
// bySetPosition (Step 3 of §3.3.3.1)
// ---------------------------------------------------------------------------
function applyBySetPosition(dates: Date[], positions: number[]): Date[] {
  if (!dates.length) return dates;
  const result: Date[] = [];
  const len = dates.length;
  for (const pos of positions) {
    const idx = pos > 0 ? pos - 1 : len + pos;
    if (idx >= 0 && idx < len) {
      result.push(dates[idx]);
    }
  }
  return result.sort((a, b) => a.getTime() - b.getTime());
}

// ---------------------------------------------------------------------------
// Advance to the start of the next period
// ---------------------------------------------------------------------------
function advancePeriod(
  date: Date,
  frequency: CalendarRecurrenceRule['frequency'],
  interval: number,
  _firstDayOfWeek: string,
): Date {
  switch (frequency) {
    case 'daily': return addDays(date, interval);
    case 'weekly': return addWeeks(date, interval);
    case 'monthly': return addMonths(date, interval);
    case 'yearly': return addYears(date, interval);
    case 'hourly': return new Date(date.getTime() + interval * 3600000);
    case 'minutely': return new Date(date.getTime() + interval * 60000);
    case 'secondly': return new Date(date.getTime() + interval * 1000);
    default: return addDays(date, interval);
  }
}

// ---------------------------------------------------------------------------
// Jump to the period just before the visible range in O(1), so long-running
// series don't burn the iteration budget on years of out-of-range periods.
// Only valid for rules without `count` (counted rules must enumerate every
// occurrence from the series start).
// ---------------------------------------------------------------------------
function fastForwardToRange(
  eventStart: Date,
  frequency: CalendarRecurrenceRule['frequency'],
  interval: number,
  rangeStart: Date,
): Date {
  if (eventStart >= rangeStart) return new Date(eventStart);

  let periods: number;
  switch (frequency) {
    case 'secondly':
      periods = Math.floor((rangeStart.getTime() - eventStart.getTime()) / (1000 * interval));
      break;
    case 'minutely':
      periods = Math.floor((rangeStart.getTime() - eventStart.getTime()) / (60000 * interval));
      break;
    case 'hourly':
      periods = Math.floor((rangeStart.getTime() - eventStart.getTime()) / (3600000 * interval));
      break;
    case 'daily':
      periods = Math.floor(differenceInCalendarDays(rangeStart, eventStart) / interval);
      break;
    case 'weekly':
      periods = Math.floor(differenceInCalendarDays(rangeStart, eventStart) / (7 * interval));
      break;
    case 'monthly': {
      const months = (rangeStart.getFullYear() - eventStart.getFullYear()) * 12
        + (rangeStart.getMonth() - eventStart.getMonth());
      periods = Math.floor(months / interval);
      break;
    }
    case 'yearly':
      periods = Math.floor((rangeStart.getFullYear() - eventStart.getFullYear()) / interval);
      break;
    default:
      return new Date(eventStart);
  }

  // Land one full period early so candidates inside the boundary period
  // (which can precede the period anchor) are still generated.
  periods = Math.max(0, periods - 1);
  if (periods === 0) return new Date(eventStart);

  switch (frequency) {
    case 'secondly': return new Date(eventStart.getTime() + periods * interval * 1000);
    case 'minutely': return new Date(eventStart.getTime() + periods * interval * 60000);
    case 'hourly': return new Date(eventStart.getTime() + periods * interval * 3600000);
    case 'daily': return addDays(eventStart, periods * interval);
    case 'weekly': return addWeeks(eventStart, periods * interval);
    case 'monthly': return addMonths(eventStart, periods * interval);
    case 'yearly': return addYears(eventStart, periods * interval);
    default: return new Date(eventStart);
  }
}

// ---------------------------------------------------------------------------
// Helper: expand byDay within a specific month (monthly or yearly context)
// ---------------------------------------------------------------------------
function expandByDayInMonth(
  year: number,
  month: number,
  byDay: CalendarNDay[],
  h: number, m: number, s: number,
): Date[] {
  const dates: Date[] = [];

  for (const { day, nthOfPeriod } of byDay) {
    const targetDay = DAY_INDEX[day];
    if (targetDay === undefined) continue;

    if (nthOfPeriod != null && nthOfPeriod !== 0) {
      const d = nthWeekdayOfMonth(year, month, targetDay, nthOfPeriod);
      if (d) { d.setHours(h, m, s, 0); dates.push(d); }
    } else {
      // Every occurrence of this weekday in the month
      let d = new Date(year, month, 1);
      while (d.getDay() !== targetDay) d = addDays(d, 1);
      while (d.getMonth() === month) {
        const occ = new Date(d);
        occ.setHours(h, m, s, 0);
        dates.push(occ);
        d = addDays(d, 7);
      }
    }
  }

  return dates;
}

// ---------------------------------------------------------------------------
// Helper: find nth weekday of month
// ---------------------------------------------------------------------------
function nthWeekdayOfMonth(
  year: number,
  month: number,
  weekday: number,
  nth: number,
): Date | null {
  if (nth > 0) {
    let d = new Date(year, month, 1);
    while (d.getDay() !== weekday) d = addDays(d, 1);
    d = addDays(d, (nth - 1) * 7);
    return d.getMonth() === month ? d : null;
  } else {
    let d = new Date(year, month + 1, 0); // last day of month
    while (d.getDay() !== weekday) d = addDays(d, -1);
    if (nth < -1) d = addDays(d, (nth + 1) * 7);
    return d.getMonth() === month ? d : null;
  }
}

// ---------------------------------------------------------------------------
// Helper: check if date is the nth (or nth-last) weekday in its month
// ---------------------------------------------------------------------------
function nthWeekdayInMonth(date: Date, nth: number): number {
  if (nth > 0) {
    // Count from start: which occurrence of this weekday is it?
    return Math.floor((date.getDate() - 1) / 7) + 1;
  } else {
    // Count from end
    const daysInMonth = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
    return -(Math.floor((daysInMonth - date.getDate()) / 7) + 1);
  }
}

// ---------------------------------------------------------------------------
// Helper: check if date is the nth weekday in its year
// ---------------------------------------------------------------------------
function nthWeekdayInYear(date: Date, nth: number): number {
  const yd = getDayOfYear(date);
  const weekday = date.getDay();
  if (nth > 0) {
    // Find first occurrence of this weekday in the year
    const jan1 = new Date(date.getFullYear(), 0, 1);
    let first = jan1;
    while (first.getDay() !== weekday) first = addDays(first, 1);
    const firstYd = getDayOfYear(first);
    return Math.floor((yd - firstYd) / 7) + 1;
  } else {
    const dec31 = new Date(date.getFullYear(), 11, 31);
    let last = dec31;
    while (last.getDay() !== weekday) last = addDays(last, -1);
    const lastYd = getDayOfYear(last);
    return -(Math.floor((lastYd - yd) / 7) + 1);
  }
}

// ---------------------------------------------------------------------------
// Helper: resolve negative/positive byMonthDay to a real date
// ---------------------------------------------------------------------------
function resolveMonthDay(year: number, month: number, day: number): Date | null {
  const daysInMonth = new Date(year, month + 1, 0).getDate();
  const actualDay = day > 0 ? day : daysInMonth + 1 + day;
  if (actualDay < 1 || actualDay > daysInMonth) return null;
  return new Date(year, month, actualDay);
}

// ---------------------------------------------------------------------------
// Helper: day of year (1-indexed)
// ---------------------------------------------------------------------------
function getDayOfYear(date: Date): number {
  const start = new Date(date.getFullYear(), 0, 0);
  const diff = date.getTime() - start.getTime();
  return Math.floor(diff / 86400000);
}

// ---------------------------------------------------------------------------
// Helper: day-of-year to Date
// ---------------------------------------------------------------------------
function dayOfYear(year: number, yd: number): Date | null {
  const daysInYear = isLeapYear(year) ? 366 : 365;
  const actual = yd > 0 ? yd : daysInYear + 1 + yd;
  if (actual < 1 || actual > daysInYear) return null;
  const d = new Date(year, 0, actual);
  return d;
}

function isLeapYear(year: number): boolean {
  return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
}

// ---------------------------------------------------------------------------
// ISO week helpers
// ---------------------------------------------------------------------------
function getISOWeekNumber(date: Date): number {
  const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d.getTime() - yearStart.getTime()) / 86400000 + 1) / 7);
}

function getISOWeeksInYear(year: number): number {
  const dec28 = new Date(Date.UTC(year, 11, 28));
  return getISOWeekNumber(dec28);
}

function datesInISOWeek(year: number, weekNo: number, _firstDayOfWeek: string): Date[] {
  const weeksInYear = getISOWeeksInYear(year);
  const actual = weekNo > 0 ? weekNo : weeksInYear + 1 + weekNo;
  if (actual < 1 || actual > weeksInYear) return [];

  // Find Monday of ISO week 1
  const jan4 = new Date(year, 0, 4);
  const dayOfWeek = jan4.getDay() || 7; // Monday=1 ... Sunday=7
  const week1Monday = addDays(jan4, 1 - dayOfWeek);
  const targetMonday = addDays(week1Monday, (actual - 1) * 7);

  const dates: Date[] = [];
  for (let i = 0; i < 7; i++) {
    dates.push(addDays(targetMonday, i));
  }
  return dates;
}