function createTodaySync({ storage, getLogin, fetchJson, onRemoteIds, onStatus, createOperationId, createChannel,
  setTimer = globalThis.setTimeout, clearTimer = globalThis.clearTimeout, retryBaseMs = 1000, retryMaxMs = 30000 }) {
  const prefix = 'stackchain.today-sync.v1.';
  const migrationPrefix = 'stackchain.today-sync-migrated.v1.';
  const snapshotPrefix = 'stackchain.today-sync-snapshot.v1.';
  let flushing = null;
  let channel = null;
  let channelKey = '';
  let retryTimer = null;
  let retryAttempt = 0;

  function cancelRetry() {
    if (retryTimer !== null) clearTimer?.(retryTimer);
    retryTimer = null;
  }

  function scheduleRetry(error, ownerKey) {
    if (retryTimer !== null || !pending().length || !ownerKey) return;
    const advised = Number(error?.retryAfter);
    const delayMs = Number.isFinite(advised) && advised >= 0
      ? advised * 1000
      : Math.min(retryMaxMs, retryBaseMs * (2 ** retryAttempt));
    retryAttempt += 1;
    onStatus?.('retrying', { delayMs });
    retryTimer = setTimer?.(async () => {
      retryTimer = null;
      if (key() !== ownerKey) return false;
      return flush();
    }, delayMs);
    retryTimer?.unref?.();
  }

  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 adopt(plan, broadcast = true) {
    if (!Number.isInteger(plan?.revision) || !Array.isArray(plan?.ids)) return false;
    if (plan.revision < savedRevision()) return false;
    try {
      storage?.setItem(snapshotKey(), JSON.stringify({ revision: plan.revision, ids: plan.ids }));
    } catch (_error) {
      // A storage quota failure must not prevent the current tab from using server truth.
    }
    onRemoteIds?.(plan.ids);
    if (broadcast) channel?.postMessage({ revision: plan.revision, ids: plan.ids });
    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-today-' + 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' &&
        ['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();
    if (action === 'remove' && operations.some(operation =>
      operation.action === 'remove' && operation.item_id === itemId
    )) {
      onStatus?.('pending');
      return true;
    }
    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() {
    const ownerKey = key();
    if (!ownerKey) return false;
    ensureChannel();
    try {
      let plan = await fetchJson('api/v1/today');
      let operations = pending();
      let hadConflict = false;
      while (operations.length) {
        if (key() !== ownerKey) return false;
        const operation = operations[0];
        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();
          if (!save(rejected.filter(candidate => candidate.operation_id !== operation.operation_id))) {
            throw new Error('Could not persist rejected Today operation');
          }
          hadConflict = true;
          operations = pending();
          continue;
        }
        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 Today delivery receipt');
        }
        operations = pending();
      }
      adopt(plan);
      onStatus?.(pending().length ? 'pending' : hadConflict ? 'full' : 'saved');
      retryAttempt = 0;
      cancelRetry();
      return !hadConflict;
    } catch (error) {
      if (pending().length) scheduleRetry(error, ownerKey);
      else onStatus?.('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 = createTodaySync;