/**
 * Auth — backend-first, localStorage-fallback account system.
 *
 *   When the Node backend at /api/* is reachable (production deploy with the
 *   Express server running), Auth talks to it via fetch. In dev or offline,
 *   it transparently falls back to localStorage so the app still works.
 *
 *   The sync surface (Auth.session(), Auth.current(), Auth.isAdmin()) is
 *   preserved by caching the current user / token in localStorage so other
 *   code doesn't have to await anything.
 */

import { backendReachable, getToken, setToken, request, localFallbackAllowed } from './Api.js';

const SERVER_UNREACHABLE = "Can't reach the server right now — please check your connection and try again in a moment.";

const USERS_KEY      = 'echo_users_v1';
const SESSION_KEY    = 'echo_session_v1';
const CACHED_ME_KEY  = 'echo_me_v1';
const ADMIN_EMAILS   = new Set(['admin@echo.app', 'superadmin@echo.app']);

// ============================================================
// LocalStorage fallback (used when backend isn't reachable)
// ============================================================
function hash(s) {
  let h = 5381;
  for (let i = 0; i < s.length; i++) h = ((h << 5) + h + s.charCodeAt(i)) | 0;
  return (h >>> 0).toString(16);
}
function readUsers() { try { return JSON.parse(localStorage.getItem(USERS_KEY) || '{}'); } catch { return {}; } }
function writeUsers(u) { localStorage.setItem(USERS_KEY, JSON.stringify(u)); }

const SEED_ADMIN = { email: 'admin@echo.app', name: 'Administrator', pwd: 'EchoAdm-2n5y59032058' };
(function seedLocalAdmin() {
  const users = readUsers();
  if (users[SEED_ADMIN.email]) return;
  users[SEED_ADMIN.email] = {
    email: SEED_ADMIN.email, name: SEED_ADMIN.name, pwd: hash(SEED_ADMIN.pwd),
    created: Date.now(), stars: 0, mastered: 0, grade: 1, isAdmin: true,
  };
  writeUsers(users);
})();

function cachedMe() {
  try { return JSON.parse(localStorage.getItem(CACHED_ME_KEY) || 'null'); } catch { return null; }
}
function setCachedMe(u) {
  if (u) localStorage.setItem(CACHED_ME_KEY, JSON.stringify(u));
  else   localStorage.removeItem(CACHED_ME_KEY);
}

// ============================================================
// Public API
// ============================================================
export const Auth = {
  /** Sync: is there a session? (Either backend token or local email.) */
  session() { return getToken() || localStorage.getItem(SESSION_KEY); },

  /** Sync: returns the cached current user object. */
  current() {
    const cached = cachedMe();
    if (cached) return cached;
    // Local fallback path
    const e = localStorage.getItem(SESSION_KEY);
    if (!e) return null;
    return readUsers()[e] || null;
  },

  /** Sync: is the current user admin? */
  isAdmin() {
    const me = this.current();
    return !!(me && me.isAdmin);
  },

  async register(email, pwd, name) {
    if (await backendReachable()) {
      const { token, user } = await request('POST', '/api/register', { email, pwd, name });
      setToken(token); setCachedMe(user);
      return user;
    }
    // Backend is the source of truth in production. Only fall back to a
    // device-local account in genuine local/offline dev — otherwise a transient
    // outage would trap the account on this browser and break other devices.
    if (!localFallbackAllowed()) throw new Error(SERVER_UNREACHABLE);
    // Local fallback (dev / offline only)
    email = String(email).trim().toLowerCase();
    if (!/^[^@\s]+@[^@\s]+\.[^@\s]+$/.test(email)) throw new Error('Enter a valid email');
    if (!pwd || pwd.length < 6) throw new Error('Password must be 6+ characters');
    const u = readUsers();
    if (u[email]) throw new Error('Email already registered — try logging in');
    u[email] = { email, name: name || email.split('@')[0], pwd: hash(pwd),
      created: Date.now(), stars: 0, mastered: 0, grade: 1,
      isAdmin: ADMIN_EMAILS.has(email) };
    writeUsers(u);
    localStorage.setItem(SESSION_KEY, email);
    setCachedMe(u[email]);
    return u[email];
  },

  async login(email, pwd) {
    if (await backendReachable()) {
      const { token, user } = await request('POST', '/api/login', { email, pwd });
      setToken(token); setCachedMe(user);
      return user;
    }
    // See register(): never silently authenticate against a device-local store
    // in production, or new devices can never see accounts created elsewhere.
    if (!localFallbackAllowed()) throw new Error(SERVER_UNREACHABLE);
    email = String(email).trim().toLowerCase();
    const u = readUsers();
    if (!u[email]) throw new Error('No account with that email');
    if (u[email].pwd !== hash(pwd)) throw new Error('Wrong password');
    if (ADMIN_EMAILS.has(email) && !u[email].isAdmin) { u[email].isAdmin = true; writeUsers(u); }
    localStorage.setItem(SESSION_KEY, email);
    setCachedMe(u[email]);
    return u[email];
  },

  async logout() {
    try { if (await backendReachable()) await request('POST', '/api/logout'); } catch {}
    setToken(null);
    setCachedMe(null);
    localStorage.removeItem(SESSION_KEY);
  },

  /** Pull the latest /me payload and refresh the local cache. No-op if no session. */
  async refreshMe() {
    if (!this.session()) return null;
    if (await backendReachable()) {
      try {
        const me = await request('GET', '/api/me');
        setCachedMe(me);
        return me;
      } catch { return this.current(); }
    }
    return this.current();
  },

  async saveProgress({ stars, mastered, grade }) {
    if (await backendReachable()) {
      try { await request('POST', '/api/me/progress', { stars, mastered, grade }); } catch {}
    }
    const me = cachedMe();
    if (me) { setCachedMe({ ...me, stars, mastered, grade }); }
    const e = localStorage.getItem(SESSION_KEY);
    if (e) { const u = readUsers(); if (u[e]) { u[e] = { ...u[e], stars, mastered, grade }; writeUsers(u); } }
  },

  /** Fetch the per-word progress array for the current user. Returns [] when
   *  not signed in or backend unreachable — caller falls back to local store. */
  async fetchWords() {
    if (!this.session()) return [];
    if (!(await backendReachable())) return [];
    try {
      const { words } = await request('GET', '/api/me/words');
      return Array.isArray(words) ? words : [];
    } catch { return []; }
  },

  /** Upload per-word progress (batched array). No-op when not signed in. */
  async saveWords(words) {
    if (!Array.isArray(words) || !words.length) return;
    if (!this.session()) return;
    if (!(await backendReachable())) return;
    try { await request('POST', '/api/me/words', { words }); } catch {}
  },

  // ----- Sync helpers used by AdminUI local fallback -----
  allUsers() { return Object.values(readUsers()); },
  update(patch) {
    const e = localStorage.getItem(SESSION_KEY);
    if (!e) return;
    const u = readUsers();
    if (!u[e]) return;
    u[e] = { ...u[e], ...patch };
    writeUsers(u);
    setCachedMe(u[e]);
  },
  resetPwd(email, newPwd) {
    email = String(email).trim().toLowerCase();
    const u = readUsers();
    if (!u[email]) throw new Error('No account with that email');
    if (!newPwd || newPwd.length < 6) throw new Error('New password must be 6+ chars');
    u[email].pwd = hash(newPwd);
    writeUsers(u);
    return true;
  },

  // ----- Admin ops — backend-first, local fallback -----
  async adminListUsers() {
    if (await backendReachable()) return request('GET', '/api/admin/users');
    return Object.values(readUsers()).map(u => ({ ...u, online: false }));
  },
  async adminDeleteUser(email) {
    if (await backendReachable()) return request('DELETE', `/api/admin/users/${encodeURIComponent(email)}`);
    email = String(email).toLowerCase();
    const u = readUsers();
    if (email === localStorage.getItem(SESSION_KEY)) throw new Error('Cannot delete the currently logged-in admin');
    if (!u[email]) throw new Error('No such user');
    delete u[email]; writeUsers(u);
  },
  async adminSetAdmin(email, on) {
    if (await backendReachable()) return request('POST', `/api/admin/users/${encodeURIComponent(email)}/admin`, { isAdmin: !!on });
    email = String(email).toLowerCase();
    const u = readUsers();
    if (!u[email]) throw new Error('No such user');
    u[email].isAdmin = !!on; writeUsers(u);
  },
  async adminResetProgress(email) {
    if (await backendReachable()) return request('POST', `/api/admin/users/${encodeURIComponent(email)}/reset`);
    email = String(email).toLowerCase();
    const u = readUsers();
    if (!u[email]) throw new Error('No such user');
    u[email].stars = 0; u[email].mastered = 0; u[email].grade = 1; writeUsers(u);
  },
  async adminCreateUser({ email, pwd, name, isAdmin }) {
    if (await backendReachable()) return request('POST', '/api/admin/users', { email, pwd, name, isAdmin });
    email = String(email).trim().toLowerCase();
    if (!/^[^@\s]+@[^@\s]+\.[^@\s]+$/.test(email)) throw new Error('Enter a valid email');
    if (!pwd || pwd.length < 6) throw new Error('Password must be 6+ characters');
    const u = readUsers();
    if (u[email]) throw new Error('Email already registered');
    u[email] = { email, name: name || email.split('@')[0], pwd: hash(pwd),
      created: Date.now(), stars: 0, mastered: 0, grade: 1, isAdmin: !!isAdmin };
    writeUsers(u);
  },
  async adminExport() {
    if (await backendReachable()) return JSON.stringify(await request('GET', '/api/admin/export'), null, 2);
    return JSON.stringify({ users: readUsers(), exportedAt: Date.now() }, null, 2);
  },
  async adminImport(json) {
    let data;
    try { data = JSON.parse(json); } catch { throw new Error('Invalid JSON'); }
    if (await backendReachable()) {
      // Backend export shape: { users: [...] }; local-fallback shape: { users: {email: u} }
      const arr = Array.isArray(data.users) ? data.users :
                  data.users && typeof data.users === 'object' ? Object.values(data.users) : null;
      if (!arr) throw new Error('Missing users array');
      const r = await request('POST', '/api/admin/import', { users: arr });
      return r.count;
    }
    if (!data || typeof data.users !== 'object') throw new Error('Missing "users" object');
    writeUsers(data.users);
    return Object.keys(data.users).length;
  },
};