export function notificationText(event, service, config) {
  const en = config.locale === 'en';
  const name = service.name[en ? 'en' : 'ru'];
  const scope = event.scope === 'scenario' ? (en ? 'User journey' : 'Пользовательский сценарий') : (en ? 'Availability' : 'Доступность');
  const recovered = event.kind === 'recovery';
  const title = recovered ? (en ? 'Recovered' : 'Восстановлено') : (en ? 'Outage confirmed' : 'Сбой подтверждён');
  const date = new Intl.DateTimeFormat(en ? 'en' : 'ru', { dateStyle: 'short', timeStyle: 'short', timeZone: config.timezone }).format(event.created_at);
  const duration = recovered && event.resolved_at ? Math.max(1, Math.round((event.resolved_at - event.started_at) / 60000)) : null;
  return [`RideBus Status · ${title}`, name, scope, date,
    ...(duration === null ? [] : [`${en ? 'Confirmed incident duration' : 'Длительность подтверждённого инцидента'}: ${duration} ${en ? 'min' : 'мин'}`]),
    ...(config.publicUrl ? [config.publicUrl] : []),
  ].join('\n').slice(0, 3500);
}

export async function sendTelegram(config, text, { fetchImpl = fetch } = {}) {
  try {
    const response = await fetchImpl(`https://api.telegram.org/bot${config.token}/sendMessage`, {
      method: 'POST', headers: { 'Content-Type': 'application/json' }, redirect: 'error',
      body: JSON.stringify({ chat_id: config.chatId, text, link_preview_options: { is_disabled: true } }),
      signal: AbortSignal.timeout(5000),
    });
    let body;
    try { body = await response.json(); } catch { body = null; }
    if (response.ok && body?.ok === true) return { ok: true };
    const code = body?.error_code || response.status;
    const retryAfter = Number(body?.parameters?.retry_after);
    return { ok: false, retryable: code === 429 || code >= 500, reason: code === 429 ? 'rate_limited' : code === 401 || code === 403 ? 'access_denied' : 'telegram_error', retryAfterSeconds: code === 429 && Number.isFinite(retryAfter) ? Math.max(1, Math.min(3600, retryAfter)) : null };
  } catch { return { ok: false, retryable: true, reason: 'network', retryAfterSeconds: null }; }
}

export function createNotifier(store, config, { send = sendTelegram, now = Date.now } = {}) {
  let active = null, stopped = true, timer;
  async function flush() {
    if (!config.telegram.enabled) return;
    if (active) return active;
    active = (async () => {
      for (const event of store.dueNotifications(now())) {
        const service = config.services.find(service => service.id === event.service_id);
        const signature = event.scope === 'scenario' ? service?.scenario?.signature : service?.signature;
        if (!service || signature !== event.signature || now() - event.created_at > 86400000) {
          store.notificationResult(event.id, { status: 'discarded', reason: 'obsolete' }, now()); continue;
        }
        const result = await send(config.telegram, notificationText(event, service, config.telegram));
        if (result.ok) store.notificationResult(event.id, { status: 'sent' }, now());
        else {
          const retry = result.retryable && event.attempts < 7;
          const delaySeconds = result.retryAfterSeconds || Math.min(3600, 30 * 2 ** event.attempts);
          store.notificationResult(event.id, { status: retry ? 'pending' : 'failed', retryAt: now() + delaySeconds * 1000, reason: result.reason }, now());
          // The database holds this incident's recovery until its outage is sent.
        }
      }
    })();
    try { await active; } finally { active = null; }
  }
  async function loop() {
    try { await flush(); } catch { console.error('Notification delivery failed; pending events remain in the database.'); }
    if (!stopped) timer = setTimeout(loop, 15000);
  }
  return { flush,
    start() { if (!stopped || !config.telegram.enabled) return; stopped = false; void loop(); },
    async stop() { stopped = true; clearTimeout(timer); if (active) await active; },
  };
}