function createTodaySync({ storage, getLogin, fetchJson, onRemoteIds, onStatus, createOperationId }) {
  const prefix = 'stackchain.today-sync.v1.';
  const migrationPrefix = 'stackchain.today-sync-migrated.v1.';
  let flushing = null;

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

  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' &&
        ['add', 'remove', 'move'].includes(operation.action) &&
        typeof operation.item_id === '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, direction = null) {
    const operations = pending();
    operations.push({ operation_id: operationId(), action, item_id: itemId, direction });
    const saved = save(operations);
    onStatus?.(saved ? 'pending' : 'error');
    return saved;
  }

  function migrate(ids) {
    const storageKey = key();
    if (!storageKey || !storage) return false;
    const marker = migrationPrefix + storageKey.slice(prefix.length);
    try {
      if (storage.getItem(marker)) return false;
      for (const id of ids || []) enqueue('add', id);
      storage.setItem(marker, '1');
      return true;
    } catch (_error) {
      return false;
    }
  }

  async function run() {
    if (!key()) return false;
    try {
      let plan = await fetchJson('api/v1/today');
      const operations = pending();
      for (const operation of operations) {
        try {
          plan = await fetchJson('api/v1/today', {
            method: 'PATCH',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify(operation),
          });
        } catch (error) {
          if (error?.status !== 409) throw error;
          const rejected = pending();
          save(rejected.filter(candidate => candidate.operation_id !== operation.operation_id));
          onRemoteIds?.(Array.isArray(plan.ids) ? plan.ids : []);
          onStatus?.('full');
          return false;
        }
        const remaining = pending();
        if (remaining[0]?.operation_id === operation.operation_id) save(remaining.slice(1));
      }
      onRemoteIds?.(Array.isArray(plan.ids) ? plan.ids : []);
      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;
  }

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

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