function createLaterSync({ storage, getLogin, fetchJson, onRemoteRecords, onStatus, createOperationId, createChannel }) {
  const prefix = 'stackchain.later-sync.v1.';
  const migrationPrefix = 'stackchain.later-sync-migrated.v1.';
  const snapshotPrefix = 'stackchain.later-sync-snapshot.v1.';
  let flushing = null;
  let channel = null;
  let channelKey = '';

  function key() {
    const login = String(getLogin?.() || '').trim().toLowerCase();
    return login ? prefix + encodeURIComponent(login) : '';
  }

  function snapshotKey() {
    const storageKey = key();
    return storageKey ? snapshotPrefix + storageKey.slice(prefix.length) : '';
  }

  function savedRevision() {
    try {
      const snapshot = JSON.parse(storage?.getItem(snapshotKey()) || 'null');
      return Number.isInteger(snapshot?.revision) ? snapshot.revision : -1;
    } catch (_error) {
      return -1;
    }
  }

  function validRecords(records) {
    return records && typeof records === 'object' && !Array.isArray(records);
  }

  function adopt(plan, broadcast = true) {
    if (!Number.isInteger(plan?.revision) || !validRecords(plan?.records)) return false;
    if (plan.revision < savedRevision()) return false;
    const snapshot = { revision: plan.revision, records: plan.records };
    try {
      storage?.setItem(snapshotKey(), JSON.stringify(snapshot));
    } catch (_error) {
      // Server truth remains usable in this tab when storage is unavailable.
    }
    onRemoteRecords?.(plan.records);
    if (broadcast) channel?.postMessage(snapshot);
    return true;
  }

  function ensureChannel() {
    const storageKey = key();
    if (!storageKey || channelKey === storageKey) return;
    channel?.close?.();
    const factory = createChannel || (globalThis.window?.BroadcastChannel
      ? name => new globalThis.window.BroadcastChannel(name)
      : null);
    channelKey = storageKey;
    channel = factory?.('stackchain-later-' + storageKey.slice(prefix.length)) || null;
    channel?.addEventListener?.('message', event => {
      if (key() === storageKey) adopt(event.data, false);
    });
  }

  function pending() {
    const storageKey = key();
    if (!storageKey || !storage) return [];
    try {
      const value = JSON.parse(storage.getItem(storageKey) || '[]');
      return Array.isArray(value) ? value.filter(operation =>
        operation && typeof operation.operation_id === 'string' &&
        ['defer', 'restore'].includes(operation.action) &&
        typeof operation.item_id === 'string' &&
        (operation.action === 'restore' || typeof operation.wake_at === 'string')
      ) : [];
    } catch (_error) {
      return [];
    }
  }

  function save(operations) {
    const storageKey = key();
    if (!storageKey || !storage) return false;
    try {
      if (operations.length) storage.setItem(storageKey, JSON.stringify(operations));
      else storage.removeItem(storageKey);
      return true;
    } catch (_error) {
      return false;
    }
  }

  function operationId() {
    if (createOperationId) return createOperationId();
    if (globalThis.crypto?.randomUUID) return globalThis.crypto.randomUUID();
    return Date.now().toString(36) + '-' + Math.random().toString(36).slice(2);
  }

  function enqueue(action, itemId, wakeAt = null) {
    if (!['defer', 'restore'].includes(action) || !itemId ||
        (action === 'defer' && typeof wakeAt !== 'string')) return false;
    const operations = pending().filter(operation => operation.item_id !== itemId);
    operations.push({ operation_id: operationId(), action, item_id: itemId, wake_at: wakeAt });
    const saved = save(operations);
    onStatus?.(saved ? 'pending' : 'error');
    return saved;
  }

  function migrate(records) {
    const storageKey = key();
    if (!storageKey || !storage) return false;
    const marker = migrationPrefix + storageKey.slice(prefix.length);
    try {
      if (storage.getItem(marker)) return false;
      Object.entries(records || {}).forEach(([itemId, wakeAt]) =>
        enqueue('defer', itemId, wakeAt)
      );
      storage.setItem(marker, '1');
      return true;
    } catch (_error) {
      return false;
    }
  }

  async function run() {
    if (!key()) return false;
    ensureChannel();
    try {
      let plan = await fetchJson('api/v1/later');
      let operations = pending();
      while (operations.length) {
        const operation = operations[0];
        plan = await fetchJson('api/v1/later', {
          method: 'PATCH',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify(operation),
        });
        const remaining = pending();
        const delivered = remaining.findIndex(candidate => candidate.operation_id === operation.operation_id);
        if (delivered >= 0 && !save(remaining.filter((_, index) => index !== delivered))) {
          throw new Error('Could not persist Later delivery receipt');
        }
        operations = pending();
      }
      adopt(plan);
      onStatus?.(pending().length ? 'pending' : 'saved');
      return true;
    } catch (_error) {
      onStatus?.(pending().length ? 'pending' : 'error');
      return false;
    }
  }

  function flush() {
    if (!flushing) flushing = run().finally(() => { flushing = null; });
    return flushing;
  }

  function startLifecycle({ window: windowObject, document: documentObject }) {
    windowObject?.addEventListener?.('online', flush);
    documentObject?.addEventListener?.('visibilitychange', () =>
      documentObject.hidden ? false : flush()
    );
  }

  return { enqueue, migrate, flush, pending, startLifecycle };
}

if (typeof module !== 'undefined' && module.exports) module.exports = createLaterSync;