// iCalendar-Erzeugung (RFC 5545) ohne Abhängigkeiten.

const CRLF = '\r\n';

export function escapeText(s) {
  return String(s ?? '')
    .replace(/\\/g, '\\\\')
    .replace(/;/g, '\\;')
    .replace(/,/g, '\\,')
    .replace(/\r?\n/g, '\\n');
}

/** Zeilen länger als 75 Oktette werden mit CRLF + Leerzeichen gefaltet. */
export function foldLine(line) {
  const bytes = Buffer.from(line, 'utf8');
  if (bytes.length <= 75) return line;
  const parts = [];
  let start = 0;
  let limit = 75;
  while (start < bytes.length) {
    let end = Math.min(start + limit, bytes.length);
    // Nicht mitten in einer UTF-8-Sequenz trennen
    while (end < bytes.length && (bytes[end] & 0xc0) === 0x80) end -= 1;
    parts.push((parts.length ? ' ' : '') + bytes.subarray(start, end).toString('utf8'));
    start = end;
    limit = 74;
  }
  return parts.join(CRLF);
}

const VTIMEZONE_BERLIN = [
  'BEGIN:VTIMEZONE',
  'TZID:Europe/Berlin',
  'BEGIN:DAYLIGHT',
  'TZOFFSETFROM:+0100',
  'TZOFFSETTO:+0200',
  'TZNAME:CEST',
  'DTSTART:19700329T020000',
  'RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU',
  'END:DAYLIGHT',
  'BEGIN:STANDARD',
  'TZOFFSETFROM:+0200',
  'TZOFFSETTO:+0100',
  'TZNAME:CET',
  'DTSTART:19701025T030000',
  'RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU',
  'END:STANDARD',
  'END:VTIMEZONE',
];

function stamp(d = new Date()) {
  return d.toISOString().replace(/[-:]/g, '').replace(/\.\d{3}/, '');
}

function addMinutes(hhmm, minutes) {
  const [h, m] = hhmm.split(':').map(Number);
  const total = h * 60 + m + minutes;
  const day = Math.floor(total / (24 * 60));
  const rest = total % (24 * 60);
  return { day, time: `${String(Math.floor(rest / 60)).padStart(2, '0')}${String(rest % 60).padStart(2, '0')}00` };
}

/**
 * Ein VEVENT als Array von (ungefalteten) Zeilen.
 * time = 'HH:MM' oder leer für ganztägig.
 */
export function buildEventLines({ uid, date, summary, description = '', time = '', durationMinutes = 15, attendees = [], organizer = null, sequence = 0, now = new Date() }) {
  const compact = date.replace(/-/g, '');
  const lines = ['BEGIN:VEVENT', `UID:${uid}`, `DTSTAMP:${stamp(now)}`, `SEQUENCE:${sequence}`];
  if (time) {
    const start = addMinutes(time, 0).time;
    const end = addMinutes(time, durationMinutes);
    const endDate = end.day ? addDaysCompact(date, end.day) : compact;
    lines.push(`DTSTART;TZID=Europe/Berlin:${compact}T${start}`);
    lines.push(`DTEND;TZID=Europe/Berlin:${endDate}T${end.time}`);
  } else {
    lines.push(`DTSTART;VALUE=DATE:${compact}`);
    lines.push(`DTEND;VALUE=DATE:${addDaysCompact(date, 1)}`);
  }
  lines.push(`SUMMARY:${escapeText(summary)}`);
  if (description) lines.push(`DESCRIPTION:${escapeText(description)}`);
  // Teilnehmer nur mit ORGANIZER – ohne ihn verschickt Nextcloud keine Einladungen und iCal-Clients meckern
  if (attendees.length > 0 && organizer?.email) {
    lines.push(`ORGANIZER;CN=${escapeParam(organizer.name || organizer.email)}:mailto:${organizer.email}`);
    for (const a of attendees) {
      lines.push(`ATTENDEE;CN=${escapeParam(a.name)};ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=FALSE:mailto:${a.email}`);
    }
  }
  lines.push('TRANSP:OPAQUE', 'END:VEVENT');
  return lines;
}

function escapeParam(s) {
  return '"' + String(s ?? '').replace(/["\r\n]/g, '') + '"';
}

function addDaysCompact(iso, n) {
  const d = new Date(iso + 'T00:00:00Z');
  d.setUTCDate(d.getUTCDate() + n);
  return d.toISOString().slice(0, 10).replace(/-/g, '');
}

/**
 * Vollständiger Kalender aus einer Liste von Event-Zeilenarrays.
 * method nur für Dateien setzen (z. B. PUBLISH) – CalDAV-Objekte dürfen laut RFC 4791 keine METHOD tragen,
 * Nextcloud lehnt einen PUT damit ab.
 */
export function buildCalendar(eventLineArrays, { name = 'Morning Meeting', method = '' } = {}) {
  const lines = [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    'PRODID:-//Rotatory//Morning Meeting//DE',
    'CALSCALE:GREGORIAN',
    ...(method ? [`METHOD:${method}`] : []),
    `X-WR-CALNAME:${escapeText(name)}`,
    'X-WR-TIMEZONE:Europe/Berlin',
    ...VTIMEZONE_BERLIN,
    ...eventLineArrays.flat(),
    'END:VCALENDAR',
  ];
  return lines.map(foldLine).join(CRLF) + CRLF;
}

export function eventUid(date, host = 'rotatory.local') {
  return `rotatory-${date}@${host}`;
}