// Minimal CalDAV client for the application calendar (SOGo).
//
// Two-way sync: reads events in a time window (REPORT calendar-query) and
// creates / updates / deletes events (PUT / DELETE with iCalendar VEVENTs).
// Auth is HTTP Basic over HTTPS using the same mail account as lib/mailer.
//
// Config (DB, editable via /einstellungen — see lib/config.js):
//   CALDAV_URL        full URL of the calendar collection (must end with /)
//   MAIL_USER         login (shared with mail)
//   MAIL_PASSWORD     password (shared with mail)
//   CALDAV_ALARM_MIN  default reminder lead time in minutes (default 60)
//
// Times are handled as UTC instants internally; the UI renders them in
// Europe/Berlin. Wall-clock input from the UI is converted with wallToUtc().

const crypto = require('crypto');
const config = require('./config');

const TZ = 'Europe/Berlin';

// Read fresh on every call so an edit on the /einstellungen page takes effect
// immediately (values live in the DB now, not in process.env/.env).
function cfg() {
  return {
    url: (config.get('CALDAV_URL') || '').trim(),
    user: config.get('MAIL_USER') || '',
    pass: config.get('MAIL_PASSWORD') || '',
    alarmMin: Number(config.get('CALDAV_ALARM_MIN')) || 60,
  };
}

function isConfigured() {
  const c = cfg();
  return !!(c.url && c.user && c.pass);
}

function collectionUrl() {
  let u = cfg().url;
  if (u && !u.endsWith('/')) u += '/';
  return u;
}

function authHeader() {
  const c = cfg();
  return 'Basic ' + Buffer.from(`${c.user}:${c.pass}`).toString('base64');
}

// ---------------------------------------------------------------------------
//  Time helpers
// ---------------------------------------------------------------------------

const pad = (n) => String(n).padStart(2, '0');

// Offset (ms) of a time zone at a given instant: local(tz) - UTC.
function tzOffsetMs(timeZone, date) {
  const utc = new Date(date.toLocaleString('en-US', { timeZone: 'UTC' }));
  const loc = new Date(date.toLocaleString('en-US', { timeZone }));
  return loc.getTime() - utc.getTime();
}

// Interpret (y, mo[1-12], d, h, mi) as wall-clock time in `timeZone` and return
// the corresponding UTC Date (DST-correct, including boundary re-check).
function wallToUtc(y, mo, d, h, mi, timeZone = TZ) {
  const asIfUtc = new Date(Date.UTC(y, mo - 1, d, h, mi, 0));
  const off1 = tzOffsetMs(timeZone, asIfUtc);
  let result = new Date(asIfUtc.getTime() - off1);
  const off2 = tzOffsetMs(timeZone, result);
  if (off2 !== off1) result = new Date(asIfUtc.getTime() - off2);
  return result;
}

// "YYYY-MM-DDTHH:MM" (local Berlin wall time from a form) -> UTC Date.
function localInputToUtc(dateStr, timeStr) {
  const [y, mo, d] = String(dateStr).split('-').map(Number);
  const [h, mi] = String(timeStr || '00:00').split(':').map(Number);
  return wallToUtc(y, mo, d, h || 0, mi || 0, TZ);
}

// UTC Date -> Berliner Wanduhr-Bestandteile { datum: 'YYYY-MM-DD', zeit: 'HH:MM' }
// (für vorausgefüllte Bearbeiten-Formulare).
function utcToWallInputs(date) {
  const p = new Intl.DateTimeFormat('en-CA', {
    timeZone: TZ, year: 'numeric', month: '2-digit', day: '2-digit',
    hour: '2-digit', minute: '2-digit', hour12: false,
  }).formatToParts(date).reduce((a, x) => (a[x.type] = x.value, a), {});
  return { datum: `${p.year}-${p.month}-${p.day}`, zeit: `${p.hour}:${p.minute}` };
}

// UTC Date -> iCalendar UTC stamp "YYYYMMDDTHHMMSSZ".
function toIcsUtc(date) {
  return date.getUTCFullYear() + pad(date.getUTCMonth() + 1) + pad(date.getUTCDate())
    + 'T' + pad(date.getUTCHours()) + pad(date.getUTCMinutes()) + pad(date.getUTCSeconds()) + 'Z';
}

// UTC Date -> iCalendar DATE "YYYYMMDD" in Europe/Berlin (for all-day events).
function toIcsDate(date) {
  const parts = new Intl.DateTimeFormat('en-CA', { timeZone: TZ, year: 'numeric', month: '2-digit', day: '2-digit' })
    .formatToParts(date).reduce((a, p) => (a[p.type] = p.value, a), {});
  return `${parts.year}${parts.month}${parts.day}`;
}

// Parse an iCalendar DTSTART/DTEND value (+params) to { date: Date, allDay }.
function icsToDate(value, params) {
  const v = String(value).trim();
  const tzid = (params && params.TZID) || null;
  if (/^\d{8}$/.test(v)) {
    const y = +v.slice(0, 4), mo = +v.slice(4, 6), d = +v.slice(6, 8);
    return { date: wallToUtc(y, mo, d, 0, 0, tzid || TZ), allDay: true };
  }
  const m = v.match(/^(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})(Z)?$/);
  if (!m) return { date: null, allDay: false };
  const [, y, mo, d, h, mi, s, z] = m;
  if (z) return { date: new Date(Date.UTC(+y, +mo - 1, +d, +h, +mi, +s)), allDay: false };
  // TZID or floating -> treat as wall time in that zone (Berlin fallback).
  return { date: wallToUtc(+y, +mo, +d, +h, +mi, tzid || TZ), allDay: false };
}

// ---------------------------------------------------------------------------
//  iCalendar generation
// ---------------------------------------------------------------------------

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

function unescapeText(s) {
  return String(s == null ? '' : s)
    .replace(/\\n/gi, '\n')
    .replace(/\\,/g, ',')
    .replace(/\\;/g, ';')
    .replace(/\\\\/g, '\\');
}

// Some clients store SUMMARY/LOCATION/DESCRIPTION with HTML entities baked in
// (e.g. "Vorstellungsgespräch – dot.haus"). iCal text is plain, so
// decode them back to real characters before we hand the value on.
function decodeHtmlEntities(s) {
  return String(s == null ? '' : s)
    .replace(/&#(\d+);/g, (_, n) => String.fromCodePoint(parseInt(n, 10)))
    .replace(/&#x([0-9a-fA-F]+);/g, (_, n) => String.fromCodePoint(parseInt(n, 16)))
    .replace(/"/g, '"')
    .replace(/'/g, "'")
    .replace(/&lt;/g, '<')
    .replace(/&gt;/g, '>')
    .replace(/&nbsp;/g, ' ')
    .replace(/&amp;/g, '&');
}

// Fold a content line to <=75 octets per RFC 5545 (CRLF + single space).
function fold(line) {
  if (Buffer.byteLength(line, 'utf8') <= 75) return line;
  const out = [];
  let cur = '';
  for (const ch of line) {
    if (Buffer.byteLength(cur + ch, 'utf8') > 73) { out.push(cur); cur = ' ' + ch; }
    else cur += ch;
  }
  if (cur) out.push(cur);
  return out.join('\r\n');
}

// Build a full VCALENDAR/VEVENT document.
// evt: { uid, summary, location, description, start(Date), end(Date), allDay, alarmMin }
function buildICS(evt) {
  const uid = evt.uid || (crypto.randomUUID() + '@bewerbungs-tracker');
  const lines = [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    'PRODID:-//NextJobs//Kalender//DE',
    'CALSCALE:GREGORIAN',
    'BEGIN:VEVENT',
    'UID:' + uid,
    'DTSTAMP:' + toIcsUtc(new Date()),
  ];
  if (evt.allDay) {
    lines.push('DTSTART;VALUE=DATE:' + toIcsDate(evt.start));
    const endDate = evt.end || new Date(evt.start.getTime() + 24 * 3600 * 1000);
    lines.push('DTEND;VALUE=DATE:' + toIcsDate(endDate));
  } else {
    lines.push('DTSTART:' + toIcsUtc(evt.start));
    const end = evt.end || new Date(evt.start.getTime() + 60 * 60 * 1000);
    lines.push('DTEND:' + toIcsUtc(end));
  }
  lines.push('SUMMARY:' + escapeText(evt.summary || 'Termin'));
  if (evt.location) lines.push('LOCATION:' + escapeText(evt.location));
  if (evt.description) lines.push('DESCRIPTION:' + escapeText(evt.description));
  const alarm = Number(evt.alarmMin);
  if (!evt.allDay && alarm > 0) {
    lines.push('BEGIN:VALARM', 'ACTION:DISPLAY', 'DESCRIPTION:' + escapeText(evt.summary || 'Termin'),
      'TRIGGER:-PT' + Math.round(alarm) + 'M', 'END:VALARM');
  }
  lines.push('END:VEVENT', 'END:VCALENDAR');
  return { uid, body: lines.map(fold).join('\r\n') + '\r\n' };
}

// ---------------------------------------------------------------------------
//  iCalendar parsing (single VEVENT out of a VCALENDAR)
// ---------------------------------------------------------------------------

function unfold(ics) {
  const raw = String(ics).split(/\r\n|\n|\r/);
  const out = [];
  for (const line of raw) {
    if ((line.startsWith(' ') || line.startsWith('\t')) && out.length) out[out.length - 1] += line.slice(1);
    else out.push(line);
  }
  return out;
}

function parseVEvent(ics) {
  const lines = unfold(ics);
  let inEvent = false, nested = 0;
  const ev = {};
  for (const line of lines) {
    if (line === 'BEGIN:VEVENT') { inEvent = true; continue; }
    if (line === 'END:VEVENT') break;
    if (!inEvent) continue;
    // Skip nested components (e.g. VALARM) so their fields don't leak in.
    if (line.startsWith('BEGIN:')) { nested++; continue; }
    if (line.startsWith('END:')) { if (nested > 0) nested--; continue; }
    if (nested > 0) continue;
    const idx = line.indexOf(':');
    if (idx === -1) continue;
    const namePart = line.slice(0, idx);
    const value = line.slice(idx + 1);
    const [name, ...paramParts] = namePart.split(';');
    const params = {};
    paramParts.forEach((p) => { const [k, v] = p.split('='); if (k) params[k.toUpperCase()] = v; });
    const key = name.toUpperCase();
    if (key === 'UID') ev.uid = value.trim();
    else if (key === 'SUMMARY') ev.summary = decodeHtmlEntities(unescapeText(value));
    else if (key === 'LOCATION') ev.location = decodeHtmlEntities(unescapeText(value));
    else if (key === 'DESCRIPTION') ev.description = decodeHtmlEntities(unescapeText(value));
    else if (key === 'DTSTART') { const r = icsToDate(value, params); ev.start = r.date; ev.allDay = r.allDay; }
    else if (key === 'DTEND') { const r = icsToDate(value, params); ev.end = r.date; }
  }
  return ev.uid || ev.start ? ev : null;
}

// ---------------------------------------------------------------------------
//  HTTP / CalDAV
// ---------------------------------------------------------------------------

async function request(method, url, { headers = {}, body } = {}) {
  const res = await fetch(url, {
    method,
    headers: { Authorization: authHeader(), ...headers },
    body,
  });
  const text = await res.text().catch(() => '');
  if (res.status === 401) throw new Error('CalDAV-Anmeldung fehlgeschlagen (401) — Zugangsdaten prüfen.');
  return { status: res.status, ok: res.ok, headers: res.headers, text };
}

function decodeXml(s) {
  return String(s).replace(/&lt;/g, '<').replace(/&gt;/g, '>')
    .replace(/&quot;/g, '"').replace(/&apos;/g, "'").replace(/&#13;/g, '\r').replace(/&amp;/g, '&');
}

function tagContent(xml, tag) {
  const m = xml.match(new RegExp('<[^>]*' + tag + '[^>]*>([\\s\\S]*?)</[^>]*' + tag + '>', 'i'));
  return m ? m[1] : null;
}

// Change tag of the collection — cheap way to detect any change.
async function getCtag() {
  if (!isConfigured()) return null;
  const r = await request('PROPFIND', collectionUrl(), {
    headers: { Depth: '0', 'Content-Type': 'application/xml; charset=utf-8' },
    body: '<?xml version="1.0"?><d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/"><d:prop><cs:getctag/></d:prop></d:propfind>',
  });
  return tagContent(r.text, 'getctag');
}

// List events overlapping [from, to] (Date objects). Returns parsed events with
// href/etag so they can be updated/deleted later.
async function listEvents({ from, to }) {
  if (!isConfigured()) return [];
  const body =
    '<?xml version="1.0"?><c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">' +
    '<d:prop><d:getetag/><c:calendar-data/></d:prop>' +
    '<c:filter><c:comp-filter name="VCALENDAR"><c:comp-filter name="VEVENT">' +
    `<c:time-range start="${toIcsUtc(from)}" end="${toIcsUtc(to)}"/>` +
    '</c:comp-filter></c:comp-filter></c:filter></c:calendar-query>';
  const r = await request('REPORT', collectionUrl(), {
    headers: { Depth: '1', 'Content-Type': 'application/xml; charset=utf-8' },
    body,
  });
  const events = [];
  const responses = r.text.match(/<[^>]*:?response[^>]*>[\s\S]*?<\/[^>]*:?response>/gi) || [];
  for (const resp of responses) {
    const href = tagContent(resp, 'href');
    const etag = tagContent(resp, 'getetag');
    const calData = tagContent(resp, 'calendar-data');
    if (!calData) continue;
    const ev = parseVEvent(decodeXml(calData));
    if (!ev || !ev.start) continue;
    ev.href = href ? new URL(decodeXml(href).trim(), collectionUrl()).toString() : null;
    ev.etag = etag ? etag.replace(/^["']|["']$/g, '').trim() : null;
    events.push(ev);
  }
  return events;
}

async function createEvent(evt) {
  if (!isConfigured()) throw new Error('CalDAV ist nicht konfiguriert.');
  const built = buildICS(evt);
  const url = collectionUrl() + encodeURIComponent(built.uid) + '.ics';
  const r = await request('PUT', url, {
    headers: { 'Content-Type': 'text/calendar; charset=utf-8', 'If-None-Match': '*' },
    body: built.body,
  });
  if (!r.ok) throw new Error(`Termin konnte nicht angelegt werden (HTTP ${r.status}).`);
  return { uid: built.uid, href: url, etag: (r.headers.get('etag') || '').replace(/^["']|["']$/g, '') || null };
}

async function updateEvent({ href, etag, ...evt }) {
  if (!isConfigured()) throw new Error('CalDAV ist nicht konfiguriert.');
  const built = buildICS(evt);
  const headers = { 'Content-Type': 'text/calendar; charset=utf-8' };
  if (etag) headers['If-Match'] = `"${etag}"`;
  const r = await request('PUT', href, { headers, body: built.body });
  if (!r.ok) throw new Error(`Termin konnte nicht aktualisiert werden (HTTP ${r.status}).`);
  return { uid: evt.uid || built.uid, href, etag: (r.headers.get('etag') || '').replace(/^["']|["']$/g, '') || null };
}

async function deleteEvent({ href, etag }) {
  if (!isConfigured() || !href) return;
  const headers = {};
  if (etag) headers['If-Match'] = `"${etag}"`;
  const r = await request('DELETE', href, { headers });
  // 404/412 are non-fatal: the event is already gone or changed remotely.
  if (!r.ok && r.status !== 404 && r.status !== 412) {
    throw new Error(`Termin konnte nicht gelöscht werden (HTTP ${r.status}).`);
  }
}

module.exports = {
  TZ,
  isConfigured,
  collectionUrl,
  wallToUtc,
  localInputToUtc,
  utcToWallInputs,
  buildICS,
  parseVEvent,
  getCtag,
  listEvents,
  createEvent,
  updateEvent,
  deleteEvent,
  // exported for tests
  _internals: { toIcsUtc, toIcsDate, icsToDate, escapeText, unescapeText, fold, unfold },
};