const DateTime = { tz: '', hour12: false, ready: false, listeners: [] };

const DT_FALLBACK_TZ = 'Europe/Moscow';
const DT_CACHE = new Map();

function dtValidTz(tz) {
  if (!tz || typeof tz !== 'string') return false;
  try { new Intl.DateTimeFormat('ru-RU', { timeZone: tz }); return true; } catch { return false; }
}

function dtBrowserTz() {
  try { return Intl.DateTimeFormat().resolvedOptions().timeZone || DT_FALLBACK_TZ; } catch { return DT_FALLBACK_TZ; }
}

function dtFmt(opts) {
  const key = DateTime.tz + '|' + (DateTime.hour12 ? '12' : '24') + '|' + JSON.stringify(opts);
  let f = DT_CACHE.get(key);
  if (!f) {
    f = new Intl.DateTimeFormat('ru-RU', Object.assign({ timeZone: DateTime.tz, hour12: DateTime.hour12 }, opts));
    DT_CACHE.set(key, f);
  }
  return f;
}

function dtDate(v) {
  if (!v) return null;
  const d = v instanceof Date ? v : new Date(v);
  return Number.isNaN(d.getTime()) ? null : d;
}

function fmtDt(v, opts) {
  const d = dtDate(v);
  return d ? dtFmt(opts).format(d) : '';
}

function dtFmtParts() {
  return { day: '2-digit', month: '2-digit', year: 'numeric', hour: '2-digit', minute: '2-digit', second: '2-digit' };
}

function fmtFull(v) { return fmtDt(v, dtFmtParts()); }
function fmtDateTime(v) { return fmtDt(v, { day: '2-digit', month: '2-digit', hour: '2-digit', minute: '2-digit' }); }
function fmtDateTimeY(v) { return fmtDt(v, { day: '2-digit', month: '2-digit', year: 'numeric', hour: '2-digit', minute: '2-digit' }); }
function fmtDateFull(v) { return fmtDt(v, { day: '2-digit', month: '2-digit', year: 'numeric' }); }
function fmtDayMonth(v) { return fmtDt(v, { day: '2-digit', month: '2-digit' }); }
function fmtDateShort(v) { return fmtDt(v, { day: '2-digit', month: '2-digit', year: '2-digit' }); }
function fmtDateLong(v) { return fmtDt(v, { day: 'numeric', month: 'long', year: 'numeric' }); }
function fmtTimeOnly(v) { return fmtDt(v, { hour: '2-digit', minute: '2-digit' }); }

function dtIsoMatch(v) {
  const m = /^(\d{4})-(\d{2})-(\d{2})/.exec(String(v == null ? '' : v));
  return m ? { y: m[1], m: m[2], d: m[3] } : null;
}

function dtPad(n) { return String(n).padStart(2, '0'); }

function fmtDateOnlyIso(v) {
  const p = dtIsoMatch(v);
  return p ? `${p.d}.${p.m}.${p.y}` : '';
}

function fmtDayMonthIso(v) {
  const p = dtIsoMatch(v);
  return p ? `${p.d}.${p.m}` : '';
}

function fmtShortIso(v) {
  const p = dtIsoMatch(v);
  return p ? `${p.d}.${p.m}.${p.y.slice(2)}` : '';
}

function fmtHmStr(v) {
  const m = /^(\d{1,2}):(\d{2})/.exec(String(v == null ? '' : v));
  if (!m) return '';
  const h = Number(m[1]);
  const mm = m[2];
  if (!DateTime.hour12) return `${dtPad(h)}:${mm}`;
  const ap = h < 12 ? 'AM' : 'PM';
  const h12 = h % 12 === 0 ? 12 : h % 12;
  return `${h12}:${mm} ${ap}`;
}

function fmtDateOnlyLongIso(v) {
  const p = dtIsoMatch(v);
  if (!p) return '';
  const months = ['января','февраля','марта','апреля','мая','июня','июля','августа','сентября','октября','ноября','декабря'];
  return `${Number(p.d)} ${months[Number(p.m) - 1] || ''} ${p.y}`;
}

function dtZoneDate(d) {
  try { return new Intl.DateTimeFormat('en-CA', { timeZone: DateTime.tz, year: 'numeric', month: '2-digit', day: '2-digit' }).format(d); } catch { return ''; }
}

function todayIso() { return dtZoneDate(new Date()); }

function todayDow() {
  const names = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
  try { return names.indexOf(new Intl.DateTimeFormat('en-US', { timeZone: DateTime.tz, weekday: 'short' }).format(new Date())); } catch { return new Date().getDay(); }
}

function nowHm() {
  try { return new Intl.DateTimeFormat('en-GB', { timeZone: DateTime.tz, hour: '2-digit', minute: '2-digit', hour12: false }).format(new Date()); } catch { return ''; }
}

function nowYear() {
  try { return new Intl.DateTimeFormat('en-CA', { timeZone: DateTime.tz, year: 'numeric' }).format(new Date()); } catch { return String(new Date().getFullYear()); }
}

function isoAddDays(iso, n) {
  const p = dtIsoMatch(iso);
  if (!p) return '';
  const d = new Date(Date.UTC(Number(p.y), Number(p.m) - 1, Number(p.d)));
  d.setUTCDate(d.getUTCDate() + n);
  return d.toISOString().slice(0, 10);
}

function dtApply(s) {
  if (!s || typeof s !== 'object') return;
  if (dtValidTz(s.timezone)) DateTime.tz = s.timezone;
  DateTime.hour12 = s.time_format === '12h';
  DateTime.listeners.forEach(fn => { try { fn(DateTime); } catch {} });
}

async function initDateTime() {
  DateTime.tz = dtBrowserTz();
  DateTime.hour12 = false;
  try {
    const res = await fetch('/api/public-settings');
    if (res.ok) dtApply(await res.json());
  } catch {}
  DateTime.ready = true;
  return DateTime;
}

function onDateTimeChange(fn) {
  if (typeof fn === 'function') DateTime.listeners.push(fn);
}