/**
 * Modul: Google Calendar Sync
 * Zweck: OAuth 2.0 + bidirektionaler Sync mit Google Calendar API v3
 * Abhängigkeiten: googleapis, server/db.js
 *
 * sync_config-Schlüssel:
 *   google_access_token   - OAuth Access Token
 *   google_refresh_token  - OAuth Refresh Token (langlebig)
 *   google_token_expiry   - ISO-8601-Timestamp bis wann Access Token gültig ist
 *   google_sync_token     - Inkrementeller Sync-Token von Google (events.list)
 *   google_last_sync      - ISO-8601-Timestamp des letzten erfolgreichen Syncs
 */

'use strict';

const { createLogger } = require('../logger');
const log = createLogger('Google');

const { google } = require('googleapis');
const crypto = require('crypto');
const db = require('../db');

const GOOGLE_COLOR = '#4285F4';

// --------------------------------------------------------
// OAuth2-Client (lazy initialisiert)
// --------------------------------------------------------

function createClient() {
  const clientId     = process.env.GOOGLE_CLIENT_ID;
  const clientSecret = process.env.GOOGLE_CLIENT_SECRET;
  const redirectUri  = process.env.GOOGLE_REDIRECT_URI;

  if (!clientId || !clientSecret || !redirectUri) {
    throw new Error('[Google] GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET und GOOGLE_REDIRECT_URI müssen gesetzt sein.');
  }

  return new google.auth.OAuth2(clientId, clientSecret, redirectUri);
}

// --------------------------------------------------------
// sync_config Helfer
// --------------------------------------------------------

function cfgGet(key) {
  const row = db.get().prepare('SELECT value FROM sync_config WHERE key = ?').get(key);
  return row ? row.value : null;
}

function cfgSet(key, value) {
  db.get().prepare(`
    INSERT INTO sync_config (key, value)
    VALUES (?, ?)
    ON CONFLICT(key) DO UPDATE SET value = excluded.value,
                                   updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
  `).run(key, value);
}

function cfgDel(key) {
  db.get().prepare('DELETE FROM sync_config WHERE key = ?').run(key);
}

// --------------------------------------------------------
// Client mit gespeicherten Tokens laden
// --------------------------------------------------------

function loadAuthorizedClient() {
  const accessToken  = cfgGet('google_access_token');
  const refreshToken = cfgGet('google_refresh_token');

  if (!accessToken || !refreshToken) {
    throw new Error('[Google] Nicht konfiguriert - zuerst OAuth durchführen.');
  }

  const client = createClient();
  client.setCredentials({
    access_token:  accessToken,
    refresh_token: refreshToken,
    expiry_date:   cfgGet('google_token_expiry') ? parseInt(cfgGet('google_token_expiry'), 10) : undefined,
  });

  // Token-Refresh automatisch speichern
  client.on('tokens', (tokens) => {
    if (tokens.access_token) cfgSet('google_access_token', tokens.access_token);
    if (tokens.expiry_date)  cfgSet('google_token_expiry', String(tokens.expiry_date));
  });

  return client;
}

// --------------------------------------------------------
// Öffentliche API
// --------------------------------------------------------

/**
 * Generiert die Google OAuth2-URL zum Weiterleiten des Admins.
 * @returns {string} Auth-URL
 */
/**
 * Generiert die Google OAuth2-URL zum Weiterleiten des Admins.
 * Enthalt einen CSRF-sicheren state-Parameter.
 * @param {object} session - Express-Session-Objekt (state wird dort gespeichert)
 * @returns {string} Auth-URL
 */
function getAuthUrl(session) {
  const client = createClient();
  const state = crypto.randomBytes(32).toString('hex');
  if (session) session.googleOAuthState = state;
  return client.generateAuthUrl({
    access_type: 'offline',
    prompt:      'consent',
    scope:       ['https://www.googleapis.com/auth/calendar'],
    state,
  });
}

/**
 * OAuth-Callback: tauscht Code gegen Tokens, speichert in sync_config.
 * @param {string} code - Code aus dem OAuth-Callback-Query-Parameter
 */
async function handleCallback(code) {
  const client = createClient();
  const { tokens } = await client.getToken(code);

  if (!tokens.refresh_token) {
    throw new Error('[Google] Kein Refresh Token erhalten. Bitte Zugriff in Google-Konto widerrufen und erneut verbinden.');
  }

  cfgSet('google_access_token',  tokens.access_token);
  cfgSet('google_refresh_token', tokens.refresh_token);
  if (tokens.expiry_date) cfgSet('google_token_expiry', String(tokens.expiry_date));

  log.info('OAuth erfolgreich - Tokens gespeichert.');
}

/**
 * Verbindungsstatus zurückgeben.
 * @returns {{ configured: boolean, connected: boolean, lastSync: string|null }}
 */
function getStatus() {
  const configured = !!(process.env.GOOGLE_CLIENT_ID && process.env.GOOGLE_CLIENT_SECRET && process.env.GOOGLE_REDIRECT_URI);
  const connected  = !!(cfgGet('google_access_token') && cfgGet('google_refresh_token'));
  const lastSync   = cfgGet('google_last_sync');
  return { configured, connected, lastSync };
}

/**
 * Tokens und Sync-State löschen (Verbindung trennen).
 */
function disconnect() {
  ['google_access_token', 'google_refresh_token', 'google_token_expiry',
   'google_sync_token', 'google_last_sync'].forEach(cfgDel);
  log.info('Verbindung getrennt.');
}

/**
 * Bidirektionaler Sync.
 * Inbound:  Google → lokale DB (Upsert via external_calendar_id)
 * Outbound: lokale Termine (external_source='local', external_calendar_id IS NULL) → Google
 */
async function sync() {
  const client  = loadAuthorizedClient();
  const calendar = google.calendar({ version: 'v3', auth: client });

  // --------------------------------------------------------
  // Inbound: Google → lokal
  // --------------------------------------------------------
  let syncToken = cfgGet('google_sync_token');
  let pageToken = undefined;
  let newSyncToken = null;

  do {
    let listParams = {
      calendarId:    'primary',
      singleEvents:  true,
      pageToken,
    };

    if (syncToken) {
      listParams.syncToken = syncToken;
    } else {
      // Erstsync: letzte 3 Monate + nächste 12 Monate
      const timeMin = new Date(Date.now() - 90 * 24 * 60 * 60 * 1000).toISOString();
      const timeMax = new Date(Date.now() + 365 * 24 * 60 * 60 * 1000).toISOString();
      listParams.timeMin = timeMin;
      listParams.timeMax = timeMax;
    }

    let response;
    try {
      response = await calendar.events.list(listParams);
    } catch (err) {
      if (err.code === 410) {
        // syncToken abgelaufen → vollständiger Resync
        log.warn('syncToken ungültig - vollständiger Resync.');
        cfgDel('google_sync_token');
        syncToken = null;
        continue;
      }
      throw err;
    }

    const items = response.data.items || [];
    upsertGoogleEvents(items);

    pageToken    = response.data.nextPageToken;
    newSyncToken = response.data.nextSyncToken || newSyncToken;
  } while (pageToken);

  if (newSyncToken) cfgSet('google_sync_token', newSyncToken);

  // --------------------------------------------------------
  // Outbound: lokal → Google
  // --------------------------------------------------------
  const localEvents = db.get().prepare(`
    SELECT * FROM calendar_events
    WHERE external_source = 'local' AND external_calendar_id IS NULL
  `).all();

  for (const event of localEvents) {
    try {
      const gEvent = localEventToGoogle(event);
      const created = await calendar.events.insert({
        calendarId: 'primary',
        requestBody: gEvent,
      });
      db.get().prepare(`
        UPDATE calendar_events SET external_calendar_id = ?, external_source = 'google' WHERE id = ?
      `).run(created.data.id, event.id);
    } catch (err) {
      log.error(`Outbound-Fehler für Event ${event.id}:`, err.message);
    }
  }

  cfgSet('google_last_sync', new Date().toISOString());
  log.info(`Sync abgeschlossen - ${localEvents.length} lokal → Google, Inbound via syncToken.`);
}

// --------------------------------------------------------
// Helfer: Google-Event in lokale DB upserten
// --------------------------------------------------------

function upsertGoogleEvents(items) {
  const upsert = db.get().prepare(`
    INSERT INTO calendar_events
      (title, description, start_datetime, end_datetime, all_day,
       location, color, external_calendar_id, external_source, recurrence_rule, created_by)
    VALUES (?, ?, ?, ?, ?, ?, ?, ?, 'google', ?, 1)
    ON CONFLICT(external_calendar_id) DO UPDATE SET
      title          = excluded.title,
      description    = excluded.description,
      start_datetime = excluded.start_datetime,
      end_datetime   = excluded.end_datetime,
      all_day        = excluded.all_day,
      location       = excluded.location,
      recurrence_rule = excluded.recurrence_rule
  `);

  const del = db.get().prepare(`
    DELETE FROM calendar_events WHERE external_calendar_id = ? AND external_source = 'google'
  `);

  // Erst external_calendar_id UNIQUE index anlegen falls noch nicht vorhanden
  // (Migration 2 legt idx_calendar_external_id an, aber kein UNIQUE constraint)
  // Wir nutzen stattdessen manuelles Upsert mit SELECT + INSERT/UPDATE
  const insertOrUpdate = db.transaction((item) => {
    if (item.status === 'cancelled') {
      del.run(item.id);
      return;
    }

    const allDay      = !!(item.start?.date && !item.start?.dateTime);
    const startDt     = allDay ? item.start.date : (item.start?.dateTime || item.start?.date);
    const endDt       = allDay ? (item.end?.date || null) : (item.end?.dateTime || item.end?.date || null);
    const title       = item.summary || '(kein Titel)';
    const description = item.description || null;
    const location    = item.location    || null;
    const rrule       = item.recurrence  ? item.recurrence[0] : null;

    const existing = db.get().prepare(
      'SELECT id FROM calendar_events WHERE external_calendar_id = ? AND external_source = ?'
    ).get(item.id, 'google');

    if (existing) {
      db.get().prepare(`
        UPDATE calendar_events
        SET title = ?, description = ?, start_datetime = ?, end_datetime = ?,
            all_day = ?, location = ?, recurrence_rule = ?
        WHERE id = ?
      `).run(title, description, startDt, endDt, allDay ? 1 : 0, location, rrule, existing.id);
    } else {
      db.get().prepare(`
        INSERT INTO calendar_events
          (title, description, start_datetime, end_datetime, all_day,
           location, color, external_calendar_id, external_source, recurrence_rule, created_by)
        VALUES (?, ?, ?, ?, ?, ?, ?, ?, 'google', ?, 1)
      `).run(title, description, startDt, endDt, allDay ? 1 : 0, location, GOOGLE_COLOR, item.id, rrule);
    }
  });

  for (const item of items) {
    try {
      insertOrUpdate(item);
    } catch (err) {
      log.error(`Upsert-Fehler für Event ${item.id}:`, err.message);
    }
  }
}

// --------------------------------------------------------
// Helfer: lokales Event → Google Calendar Event Format
// --------------------------------------------------------

function localEventToGoogle(event) {
  const allDay = !!event.all_day;
  const gEvent = {
    summary:     event.title,
    description: event.description || undefined,
    location:    event.location    || undefined,
  };

  if (allDay) {
    gEvent.start = { date: event.start_datetime.slice(0, 10) };
    gEvent.end   = { date: event.end_datetime ? event.end_datetime.slice(0, 10) : event.start_datetime.slice(0, 10) };
  } else {
    gEvent.start = { dateTime: event.start_datetime, timeZone: 'Europe/Berlin' };
    gEvent.end   = { dateTime: event.end_datetime   || event.start_datetime, timeZone: 'Europe/Berlin' };
  }

  if (event.recurrence_rule) {
    gEvent.recurrence = [event.recurrence_rule];
  }

  return gEvent;
}

module.exports = { getAuthUrl, handleCallback, getStatus, disconnect, sync };