function createTodaySessionSync({
  fetchJson, getDeviceId, timer, onRemote = () => {}, onTransferred = () => {}, onStatus = () => {},
  setInterval = globalThis.setInterval, clearInterval = globalThis.clearInterval,
}) {
  let current = null;
  let ownedRevision = 0;
  let pollTimer = null;
  const endpoint = 'api/v1/today/session';
  const deviceId = () => String(getDeviceId?.() || '').trim();

  function adopt(session) {
    if (!session || !Number.isInteger(session.revision)) return null;
    const previousOwned = current?.device_id === deviceId() && current?.running;
    current = session;
    if (session.device_id === deviceId()) {
      ownedRevision = session.revision;
      onRemote(null);
    } else if (session.running && session.identity) {
      if (previousOwned) {
        timer?.pause?.();
        onTransferred(session);
      }
      onRemote(session);
    } else {
      onRemote(null);
    }
    return session;
  }

  async function refresh() {
    try {
      const session = await fetchJson(endpoint);
      onStatus('online');
      return adopt(session);
    } catch (error) {
      onStatus('offline', error);
      return null;
    }
  }

  async function claim() {
    if (!current?.running || !current.identity || current.device_id === deviceId() || !deviceId()) return null;
    try {
      const session = await fetchJson(endpoint, {
        method:'PATCH', headers:{'Content-Type':'application/json'},
        body:JSON.stringify({
          base_revision:current.revision, device_id:deviceId(), identity:current.identity,
          elapsed_ms:current.elapsed_ms, running:true,
        }),
      });
      adopt(session);
      timer?.adopt?.(session.identity, session.elapsed_ms, session.running);
      return session;
    } catch (error) {
      onStatus('conflict', error);
      await refresh();
      return null;
    }
  }

  async function publish(snapshot = timer?.snapshot?.()) {
    if (!snapshot?.identity || !deviceId()) return null;
    try {
      const session = await fetchJson(endpoint, {
        method:'PATCH', headers:{'Content-Type':'application/json'},
        body:JSON.stringify({
          base_revision:ownedRevision, device_id:deviceId(), identity:snapshot.identity,
          elapsed_ms:Math.max(0, Math.floor(Number(snapshot.elapsed_ms) || 0)),
          running:Boolean(snapshot.running),
        }),
      });
      adopt(session);
      return session;
    } catch (error) {
      onStatus('offline', error);
      return null;
    }
  }

  const pulse = () => current?.device_id === deviceId() && current?.running
    ? publish()
    : refresh();

  return {
    refresh, claim, publish,
    session:() => current,
    start(intervalMs = 15000) {
      if (pollTimer !== null) return false;
      pulse();
      pollTimer = setInterval?.(pulse, intervalMs);
      pollTimer?.unref?.();
      return true;
    },
    stop() {
      if (pollTimer === null) return false;
      clearInterval?.(pollTimer);
      pollTimer = null;
      return true;
    },
  };
}

function attachTodaySessionHandoff({
  fetchJson, storage, timer, qs, items, identity, selectToday, startItem, announce, renderTimer,
}) {
  const deviceKey = 'stackchain.today-session-device.v1';
  const getDeviceId = () => {
    let value = storage.getItem(deviceKey);
    if (!value) {
      value = globalThis.crypto?.randomUUID?.() || Math.random().toString(36).slice(2);
      storage.setItem(deviceKey, value);
    }
    return value;
  };
  let offered = null;
  const sync = createTodaySessionSync({
    fetchJson, getDeviceId, timer,
    onRemote:session => {
      offered = session;
      const handoff = qs('#today-session-handoff');
      handoff.hidden = !session;
      if (!session) return;
      const item = items().find(entry => identity(entry) === session.identity);
      const minutes = Math.max(0, Math.floor(Number(session.elapsed_ms || 0) / 60000));
      qs('#today-session-handoff-summary').textContent =
        `${item?.title || 'Current Today item'} · ${minutes} min elapsed`;
    },
    onTransferred:() => {
      announce('Today continued on another device. Timer paused here.');
      renderTimer();
    },
  });
  qs('#continue-today-session').addEventListener('click', async () => {
    const target = offered;
    const claimed = await sync.claim();
    if (!claimed || !target) return;
    selectToday();
    const item = items().find(entry => identity(entry) === claimed.identity);
    if (item) startItem(item);
    else announce('Today session moved here; refresh work to open its item.');
    renderTimer();
  });
  sync.start(5000);
  return sync;
}

if (typeof module !== 'undefined' && module.exports) module.exports = {
  createTodaySessionSync, attachTodaySessionHandoff,
};