import {
  addDays,
  differenceInCalendarDays,
  parseISO,
  startOfDay,
  subMilliseconds,
} from 'date-fns';
import type { Calendar, CalendarEvent } from '../api/types';
import { colors } from '../theme/tokens';
import type { TranslateFn } from '../stores/locale-store';
import { zonedWallTimeToUtc } from './recurrence-expansion';
import { getEffectiveTimeZone, localDateTimeToInstant, toDisplayDate } from './calendar-timezone';

// ─── Duration ────────────────────────────────────────────
// Parse an ISO 8601 duration ("PT1H30M", "P2D", "PT45M") to milliseconds.
// Mobile's webmail equivalent lives in components/calendar/event-card; we
// inline it here so this lib has no UI deps.
export function parseDuration(iso: string | undefined): number {
  if (!iso) return 0;
  const match = /^P(?:(\d+)D)?(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+(?:\.\d+)?)S)?)?$/.exec(iso);
  if (!match) return 0;
  const [, d, h, m, s] = match;
  const days = d ? parseInt(d, 10) : 0;
  const hours = h ? parseInt(h, 10) : 0;
  const minutes = m ? parseInt(m, 10) : 0;
  const seconds = s ? parseFloat(s) : 0;
  return ((days * 24 + hours) * 3600 + minutes * 60 + seconds) * 1000;
}

// ─── Local-time parsing ──────────────────────────────────
// JSCalendar `start` strings are floating local time ("2026-04-20T09:00:00", no Z).
// Use parseISO so the returned Date represents the wall-clock value in local tz.
export function parseLocalDateTime(iso: string): Date {
  return parseISO(iso);
}

// ─── Event time-range ────────────────────────────────────
/**
 * Start of an event as a *display date* (calendar-timezone): a Date whose
 * local getters read as the wall clock in the calendar's time zone, which is
 * what the grid math, the detail sheet and the editor work with. The real
 * instant while the calendar zone is the device zone; convert a picked time
 * back with `fromDisplayDate`. All-day events are calendar dates and stay as
 * they are. Mirrors the webmail's getEventStartDate.
 */
export function getEventStartDate(
  event: Pick<CalendarEvent, 'start' | 'utcStart' | 'showWithoutTime' | 'timeZone'>,
): Date {
  // Prefer utcStart for timed events but fall back to start if utcStart is
  // missing or unparseable — a malformed utcStart used to surface as an
  // Invalid Date that silently dropped the event from every view (#316).
  if (!event.showWithoutTime && event.utcStart) {
    const utc = parseISO(event.utcStart);
    if (!isNaN(utc.getTime())) return toDisplayDate(utc);
  }
  // Without utcStart, a start in a zone of its own is converted here; a
  // floating one is a wall clock in the calendar's zone already.
  if (!event.showWithoutTime && event.timeZone) {
    const instant = localDateTimeToInstant(event.start, event.timeZone);
    if (instant) return toDisplayDate(instant);
  }
  return parseISO(event.start);
}

// ─── Task due ────────────────────────────────────────────
// A task's `due` is a wall-clock time in its `timeZone` (a CalDAV DUE with a
// TZID keeps it). Resolve it to the instant, as utcStart does for events, so
// a 17:00 Europe/Berlin due shows at the viewer's local time. A floating
// timed due (no zone) is read in the calendar's time zone, the way Stalwart
// computes utcStart for floating events (every query carries that zone), so
// a floating task and a floating event at 17:00 land at the same instant.
// Date-only and all-day dues are calendar dates and stay as they are.
// This is the real instant (reminders fire at it); what the calendar shows
// is getTaskDueDisplayDate.
export function getTaskDueDate(
  task: Pick<CalendarEvent, 'due' | 'timeZone' | 'showWithoutTime'>,
): Date | null {
  if (!task.due) return null;
  const wall = parseISO(task.due);
  if (isNaN(wall.getTime())) return null;
  if (isDateOnlyDue(task)) return wall;
  return localDateTimeToInstant(task.due, task.timeZone || getEffectiveTimeZone())
    ?? zonedWallTimeToUtc(wall, task.timeZone || getEffectiveTimeZone())
    ?? wall;
}

/** A due without a time of day: a date-only value or an all-day task. */
export function isDateOnlyDue(task: Pick<CalendarEvent, 'due' | 'showWithoutTime'>): boolean {
  return !!task.showWithoutTime || /^\d{4}-\d{2}-\d{2}$/.test(task.due ?? '');
}

/** A task's due as a display date (see getEventStartDate), for the tasks list and editor. */
export function getTaskDueDisplayDate(
  task: Pick<CalendarEvent, 'due' | 'timeZone' | 'showWithoutTime'>,
): Date | null {
  const due = getTaskDueDate(task);
  if (!due || isDateOnlyDue(task)) return due;
  return toDisplayDate(due);
}

/**
 * A JSCalendar LocalDateTime moved by whole calendar days, wall clock kept:
 * the same time of day in the same zone (duplicating an event one day
 * later). Returned unchanged when it doesn't parse.
 */
export function addDaysToLocalDateTime(value: string, days: number): string {
  const m = /^(\d{4})-(\d{2})-(\d{2})(.*)$/.exec(value);
  if (!m) return value;
  const d = new Date(Date.UTC(Number(m[1]), Number(m[2]) - 1, Number(m[3]) + days));
  const pad = (n: number, len = 2) => String(n).padStart(len, '0');
  return `${pad(d.getUTCFullYear(), 4)}-${pad(d.getUTCMonth() + 1)}-${pad(d.getUTCDate())}${m[4]}`;
}

/** End of an event as a display date - see getEventStartDate. */
export function getEventEndDate(event: CalendarEvent): Date {
  // A timed event without a duration lasts zero seconds (RFC 8984's default
  // PT0S). Stalwart stores a DTEND equal to DTSTART as no duration and gives
  // the event a utcEnd of 23:59:59 on its start day, so it filled the rest
  // of the day in the grid (#837).
  if (!event.showWithoutTime && event.utcEnd && event.duration) {
    const utc = parseISO(event.utcEnd);
    if (!isNaN(utc.getTime())) return toDisplayDate(utc);
  }
  const start = getEventStartDate(event);
  if (!event.duration) return start;
  return new Date(start.getTime() + parseDuration(event.duration));
}

export function getEventDisplayEndDate(event: CalendarEvent): Date {
  const end = getEventEndDate(event);
  const start = getEventStartDate(event);
  if (!event.showWithoutTime || end.getTime() <= start.getTime()) {
    return end;
  }
  return subMilliseconds(end, 1);
}

// ─── Time-of-day formatting ──────────────────────────────
// Maps the user's 12h/24h preference to a date-fns pattern, matching webmail
// (event-card.tsx: `timeFormat === "12h" ? "h:mm a" : "HH:mm"`). Components
// thread the `calendarTimeFormat` setting through so the calendar renders
// times in the same format the webmail does.
export type TimeFormat = '12h' | '24h';

export function timePattern(timeFormat: TimeFormat | undefined): string {
  return timeFormat === '12h' ? 'h:mm a' : 'HH:mm';
}

export interface EventTimeRange {
  start: Date;
  end: Date;
  allDay: boolean;
}

export function eventTimeRange(event: CalendarEvent): EventTimeRange {
  return {
    start: getEventStartDate(event),
    end: getEventEndDate(event),
    allDay: !!event.showWithoutTime,
  };
}

// ─── Day overlap ─────────────────────────────────────────
export function eventsOnDay(events: CalendarEvent[], day: Date): CalendarEvent[] {
  const dayStart = startOfDay(day);
  const nextDayStart = addDays(dayStart, 1);
  return events.filter((event) => {
    const start = getEventStartDate(event);
    const end = getEventDisplayEndDate(event);
    return end > dayStart && start < nextDayStart;
  });
}

// ─── Day index (O(1) per-day lookup) ─────────────────────
// A month view touches 42 day cells. Calling `eventsOnDay` on each cell costs
// O(days × events) with two parseISO calls per event per day. For ~200 events
// that's ~16k parseISO calls per month render, redone on every swipe.
// `buildEventDayIndex` parses each event's start/end once, then walks only the
// days that event actually touches - typically 1 day. Result is a map from
// local-date key (yyyy-MM-dd) to the events on that day.
export type EventDayIndex = Map<string, CalendarEvent[]>;

export function dayKey(day: Date): string {
  const y = day.getFullYear();
  const m = (day.getMonth() + 1).toString().padStart(2, '0');
  const d = day.getDate().toString().padStart(2, '0');
  return `${y}-${m}-${d}`;
}

export function buildEventDayIndex(events: CalendarEvent[]): EventDayIndex {
  const idx: EventDayIndex = new Map();
  for (const event of events) {
    const start = getEventStartDate(event);
    const end = getEventDisplayEndDate(event);
    let day = startOfDay(start);
    // Cap at 366 days: matches the original semantics (end > dayStart) - we
    // include day while day < end. Safety bound guards against malformed
    // multi-year events that would otherwise blow up the loop.
    let safety = 0;
    while (day < end && safety < 366) {
      const key = dayKey(day);
      const arr = idx.get(key);
      if (arr) arr.push(event);
      else idx.set(key, [event]);
      day = addDays(day, 1);
      safety++;
    }
  }
  // Sort each day's bucket: all-day events first, then timed events by start.
  // Without this, events from different calendars come out in arrival order
  // (roughly grouped by calendar) rather than chronologically.
  for (const arr of idx.values()) {
    arr.sort(compareEventsForDay);
  }
  return idx;
}

function compareEventsForDay(a: CalendarEvent, b: CalendarEvent): number {
  const aAllDay = !!a.showWithoutTime;
  const bAllDay = !!b.showWithoutTime;
  if (aAllDay !== bAllDay) return aAllDay ? -1 : 1;
  const aStart = getEventStartDate(a).getTime();
  const bStart = getEventStartDate(b).getTime();
  if (aStart !== bStart) return aStart - bStart;
  // Tie-breaker on end (shorter events first), then title for stability.
  const aEnd = getEventEndDate(a).getTime();
  const bEnd = getEventEndDate(b).getTime();
  if (aEnd !== bEnd) return aEnd - bEnd;
  return (a.title ?? '').localeCompare(b.title ?? '');
}

export function eventsOnDayFromIndex(
  index: EventDayIndex,
  day: Date,
): CalendarEvent[] {
  return index.get(dayKey(day)) ?? [];
}

export function getEventDayBounds(event: CalendarEvent): { startDay: Date; endDay: Date } {
  return {
    startDay: startOfDay(getEventStartDate(event)),
    endDay: startOfDay(getEventDisplayEndDate(event)),
  };
}

// ─── All-day duration helpers ────────────────────────────
export function normalizeAllDayDuration(duration: string | undefined): string | undefined {
  if (!duration) return undefined;
  const totalMs = parseDuration(duration);
  const totalDays = Math.max(1, Math.ceil(totalMs / (24 * 60 * 60 * 1000)));
  return `P${totalDays}D`;
}

export function buildAllDayDuration(start: Date, inclusiveEnd: Date): string {
  const dayCount = Math.max(
    1,
    differenceInCalendarDays(startOfDay(inclusiveEnd), startOfDay(start)) + 1,
  );
  return `P${dayCount}D`;
}

// ─── Per-day timed bounds (for week/day grids) ───────────
export interface TimedDayBounds {
  startMinutes: number;
  endMinutes: number;
  continuesBefore: boolean;
  continuesAfter: boolean;
}

function wallClockMinutes(date: Date): number {
  return (
    date.getHours() * 60 +
    date.getMinutes() +
    date.getSeconds() / 60 +
    date.getMilliseconds() / 60000
  );
}

export function getTimedEventBoundsForDay(
  event: CalendarEvent,
  day: Date,
): TimedDayBounds | null {
  if (event.showWithoutTime) return null;

  const eventStart = getEventStartDate(event);
  const eventEnd = getEventEndDate(event);
  const dayStart = startOfDay(day);
  const nextDayStart = addDays(dayStart, 1);

  if (eventEnd <= dayStart || eventStart >= nextDayStart) return null;

  // Wall-clock minutes, not elapsed time since midnight: on a DST change day
  // the day has 23 or 25 hours, and elapsed time would shift every event
  // after the change by an hour.
  const startMinutes =
    eventStart > dayStart ? Math.floor(wallClockMinutes(eventStart)) : 0;
  const endMinutes =
    eventEnd < nextDayStart
      ? Math.max(startMinutes, Math.ceil(wallClockMinutes(eventEnd)))
      : 1440;

  return {
    startMinutes,
    endMinutes,
    continuesBefore: eventStart < dayStart,
    continuesAfter: eventEnd > nextDayStart,
  };
}

// ─── Timed events that fill the day ──────────────────────
// Events shown in the timed grid that effectively span 00:00–24:00 on the
// given day get promoted to the all-day strip in the week/day views (matches
// webmail behavior - keeps the timed grid usable when an event covers the day).
export function isTimedEventFullDayOnDate(event: CalendarEvent, day: Date): boolean {
  const bounds = getTimedEventBoundsForDay(event, day);
  return bounds?.startMinutes === 0 && bounds?.endMinutes === 1440;
}

// ─── Multi-day all-day segment packing (week strip) ──────
// Webmail draws multi-day all-day events as a single bar across the days they
// span. RN was previously rendering one chip per day, which broke continuity.
export interface CalendarWeekSegment {
  event: CalendarEvent;
  startIndex: number;     // first day index in weekDays this segment covers
  span: number;           // number of days
  row: number;            // assigned row after packing
  continuesBefore: boolean;
  continuesAfter: boolean;
}

export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWeekSegment[] {
  rawSegments.sort((left, right) => {
    if (left.startIndex !== right.startIndex) return left.startIndex - right.startIndex;
    if (left.span !== right.span) return right.span - left.span;
    if (left.event.showWithoutTime !== right.event.showWithoutTime) {
      return left.event.showWithoutTime ? -1 : 1;
    }
    const timeDiff = getEventStartDate(left.event).getTime() - getEventStartDate(right.event).getTime();
    if (timeDiff !== 0) return timeDiff;
    return (left.event.title || '').localeCompare(right.event.title || '');
  });

  const rowEndIndices: number[] = [];
  return rawSegments.map((segment) => {
    const segmentEndIndex = segment.startIndex + segment.span - 1;
    let row = rowEndIndices.findIndex((endIndex) => endIndex < segment.startIndex);
    if (row === -1) {
      row = rowEndIndices.length;
      rowEndIndices.push(segmentEndIndex);
    } else {
      rowEndIndices[row] = segmentEndIndex;
    }
    return { ...segment, row };
  });
}

export function buildWeekSegmentsRaw(
  events: CalendarEvent[],
  weekDays: Date[],
): CalendarWeekSegment[] {
  if (weekDays.length === 0) return [];

  const weekStart = startOfDay(weekDays[0]);
  const weekEnd = startOfDay(weekDays[weekDays.length - 1]);

  return events.flatMap((event) => {
    const { startDay, endDay } = getEventDayBounds(event);
    if (endDay < weekStart || startDay > weekEnd) return [];

    const segmentStart = startDay < weekStart ? weekStart : startDay;
    const segmentEnd = endDay > weekEnd ? weekEnd : endDay;
    const startIndex = differenceInCalendarDays(segmentStart, weekStart);
    const span = differenceInCalendarDays(segmentEnd, segmentStart) + 1;

    return [{
      event,
      startIndex,
      span,
      row: -1,
      continuesBefore: startDay < weekStart,
      continuesAfter: endDay > weekEnd,
    } satisfies CalendarWeekSegment];
  });
}

// Timed events that fill whole days of `weekDays` (consecutive days), as
// segments. Each event is only tested on the days it spans, not on every
// day: the freely scrolling week and day grids hand in months of days (#759).
export function buildTimedFullDayWeekSegments(
  events: CalendarEvent[],
  weekDays: Date[],
): CalendarWeekSegment[] {
  if (weekDays.length === 0) return [];

  const firstDay = startOfDay(weekDays[0]);
  const lastIndex = weekDays.length - 1;
  const segments: CalendarWeekSegment[] = [];
  const pushSegment = (event: CalendarEvent, startIndex: number, endIndex: number) => {
    segments.push({
      event,
      startIndex,
      span: endIndex - startIndex + 1,
      row: -1,
      continuesBefore: isTimedEventFullDayOnDate(event, addDays(weekDays[startIndex], -1)),
      continuesAfter: isTimedEventFullDayOnDate(event, addDays(weekDays[endIndex], 1)),
    });
  };

  for (const event of events) {
    if (event.showWithoutTime) continue;
    const from = Math.max(0, differenceInCalendarDays(getEventStartDate(event), firstDay));
    const to = Math.min(lastIndex, differenceInCalendarDays(getEventEndDate(event), firstDay));
    if (!(from <= to)) continue;
    let runStart = -1;
    for (let index = from; index <= to; index++) {
      if (isTimedEventFullDayOnDate(event, weekDays[index])) {
        if (runStart < 0) runStart = index;
      } else if (runStart >= 0) {
        pushSegment(event, runStart, index - 1);
        runStart = -1;
      }
    }
    if (runStart >= 0) pushSegment(event, runStart, to);
  }

  return segments;
}

// ─── Overlapping timed-event layout (cluster-based packing) ──
// Webmail rewrote this to track clusters: a new cluster starts when an event
// begins after every prior event has ended. Within a cluster every event gets
// the same `totalColumns`; across clusters, events get to use the full width
// they actually need. The previous global packing forced an event with no
// neighbors at 17:00 to share columns with a 9am cluster - visible squish.
export interface TimedEventLayout {
  event: CalendarEvent;
  column: number;
  totalColumns: number;
  startMinutes: number;
  endMinutes: number;
  continuesBefore: boolean;
  continuesAfter: boolean;
}

export function layoutOverlappingEvents(
  events: CalendarEvent[],
  day: Date,
): TimedEventLayout[] {
  const layoutInputs = events.flatMap((event) => {
    const bounds = getTimedEventBoundsForDay(event, day);
    return bounds ? [{ event, ...bounds }] : [];
  });

  const sorted = layoutInputs.sort((a, b) => {
    const diff = a.startMinutes - b.startMinutes;
    if (diff !== 0) return diff;
    return (b.endMinutes - b.startMinutes) - (a.endMinutes - a.startMinutes);
  });

  const result: TimedEventLayout[] = [];
  let columns: { end: number }[][] = [];
  let clusterStart = 0;
  let clusterMaxEnd = 0;

  const flushCluster = () => {
    const total = columns.length;
    for (let i = clusterStart; i < result.length; i++) {
      result[i].totalColumns = total;
    }
  };

  for (const item of sorted) {
    if (columns.length > 0 && item.startMinutes >= clusterMaxEnd) {
      flushCluster();
      clusterStart = result.length;
      columns = [];
      clusterMaxEnd = 0;
    }

    let placed = false;
    for (let col = 0; col < columns.length; col++) {
      if (columns[col].every((e) => e.end <= item.startMinutes)) {
        columns[col].push({ end: item.endMinutes });
        result.push({ ...item, column: col, totalColumns: 0 });
        placed = true;
        break;
      }
    }
    if (!placed) {
      columns.push([{ end: item.endMinutes }]);
      result.push({ ...item, column: columns.length - 1, totalColumns: 0 });
    }
    clusterMaxEnd = Math.max(clusterMaxEnd, item.endMinutes);
  }

  flushCluster();
  return result;
}

// ─── Calendar color resolution ───────────────────────────
// Falls back to a deterministic palette slice when `color` is missing,
// mirroring webmail's behavior.
const CALENDAR_PALETTE = [
  colors.calendar.blue,
  colors.calendar.green,
  colors.calendar.purple,
  colors.calendar.orange,
  colors.calendar.red,
  colors.calendar.pink,
  colors.calendar.teal,
  colors.calendar.indigo,
] as const;

export const CALENDAR_COLOR_PALETTE = CALENDAR_PALETTE;

/**
 * The name of a palette color ("Blue") for screen readers; any other color
 * as it is written.
 */
export function calendarColorName(color: string, t: TranslateFn): string {
  const index = CALENDAR_PALETTE.findIndex((c) => c.toLowerCase() === color.toLowerCase());
  if (index < 0) return color;
  // In palette order.
  const names = [
    t('email_viewer.color_tag.blue', 'Blue'),
    t('email_viewer.color_tag.green', 'Green'),
    t('email_viewer.color_tag.purple', 'Purple'),
    t('email_viewer.color_tag.orange', 'Orange'),
    t('email_viewer.color_tag.red', 'Red'),
    t('email_viewer.color_tag.pink', 'Pink'),
    t('calendar.colors.teal', 'Teal'),
    t('calendar.colors.indigo', 'Indigo'),
  ];
  return names[index];
}

function hashString(value: string): number {
  let hash = 0;
  for (let i = 0; i < value.length; i++) {
    hash = (hash * 31 + value.charCodeAt(i)) | 0;
  }
  return Math.abs(hash);
}

export function getCalendarColor(calendar: Pick<Calendar, 'id' | 'color'> | undefined): string {
  if (!calendar) return CALENDAR_PALETTE[0];
  if (calendar.color) return calendar.color;
  return CALENDAR_PALETTE[hashString(calendar.id) % CALENDAR_PALETTE.length];
}

/**
 * The fields of a calendar edit that differ from the calendar, and only
 * those. Stalwart 1.0 accepts a colour only as #rgb, #rrggbb or a CSS name,
 * so re-sending one another client wrote - Apple Calendar's 8-digit hex -
 * failed every rename. The colour is compared with the one the editor
 * started from (getCalendarColor).
 */
export function changedCalendarFields(
  calendar: Pick<Calendar, 'id' | 'name' | 'color' | 'description'>,
  values: { name: string; color: string; description: string },
): { name?: string; color?: string; description?: string | null } {
  const changes: { name?: string; color?: string; description?: string | null } = {};
  if (values.name !== calendar.name) changes.name = values.name;
  if (values.color.toLowerCase() !== getCalendarColor(calendar).toLowerCase()) changes.color = values.color;
  if (values.description !== (calendar.description ?? '')) changes.description = values.description || null;
  return changes;
}

export function getEventColor(
  event: Pick<CalendarEvent, 'calendarIds' | 'color'>,
  calendars: Calendar[],
): string {
  const calendarId = Object.keys(event.calendarIds || {})[0];
  const cal = calendars.find((c) => c.id === calendarId);
  // A local color override on a shared calendar wins over per-event colors,
  // so the whole shared calendar paints uniformly in the viewer's chosen hue.
  if (cal?.colorIsLocalOverride && cal.color) return cal.color;
  if (event.color) return event.color;
  return getCalendarColor(cal);
}

// ─── Per-viewer colors for shared calendars (#345) ───────
// Shared calendars are recolored locally only: the viewer usually can't write
// the owner's calendar, and doing so would recolor it for everyone. Overrides
// live in the settings store keyed by sharedCalendarColorKey().

/**
 * Stable key for a shared calendar's local color override. Built from the
 * owning JMAP account + the calendar id so it stays unique across accounts.
 */
export function sharedCalendarColorKey(
  cal: Pick<Calendar, 'id' | 'accountId' | 'originalId'>,
): string {
  // Keyed on the raw server id (like webmail) so the key doesn't depend on
  // the store's `${accountId}:${id}` namespacing format.
  return `${cal.accountId ?? ''}|${cal.originalId ?? cal.id}`;
}

/**
 * Pick a random palette color not present in `usedColors`. Once every palette
 * entry is taken, fall back to a random palette color (collisions are
 * unavoidable past CALENDAR_PALETTE.length calendars).
 */
export function pickUnusedCalendarColor(usedColors: Iterable<string>): string {
  const used = new Set<string>();
  for (const c of usedColors) {
    if (c) used.add(c.toLowerCase());
  }
  const available = CALENDAR_PALETTE.filter((c) => !used.has(c.toLowerCase()));
  const pool = available.length > 0 ? available : CALENDAR_PALETTE;
  return pool[Math.floor(Math.random() * pool.length)];
}

/**
 * Apply each shared calendar's local color override (per-viewer recolor).
 * The override replaces the calendar's color and wins over per-event colors
 * via the `colorIsLocalOverride` flag (see getEventColor). Personal calendars
 * are passed through untouched.
 */
export function applySharedCalendarColors(
  calendars: Calendar[],
  overrides: Record<string, string>,
): Calendar[] {
  return calendars.map((cal) => {
    if (!cal.isShared) return cal;
    const override = overrides[sharedCalendarColorKey(cal)];
    if (!override) return cal;
    return { ...cal, color: override, colorIsLocalOverride: true };
  });
}

export function getPrimaryCalendarId(
  event: Pick<CalendarEvent, 'calendarIds'>,
): string | undefined {
  return Object.keys(event.calendarIds || {})[0];
}

/**
 * The event as the editor last had it: the stored event with the unsaved
 * edits on top, a field cleared in the editor (sent as `null`) removed. Used
 * to reopen the editor when the recurrence-scope question is cancelled.
 */
export function reopenedEdit(event: CalendarEvent, updates: Partial<CalendarEvent>): CalendarEvent {
  const merged: Record<string, unknown> = { ...event };
  for (const [key, value] of Object.entries(updates)) {
    if (value === null) delete merged[key];
    else if (value !== undefined) merged[key] = value;
  }
  return merged as unknown as CalendarEvent;
}