let database: Promise<IDBDatabase> | undefined;

function open(): Promise<IDBDatabase> {
  return database ??= new Promise((resolve, reject) => {
    if (typeof indexedDB === "undefined") {
      return reject(new Error("Your browser cannot save image drafts"));
    }
    const request = indexedDB.open("berry.drafts", 1);
    request.onupgradeneeded = () => request.result.createObjectStore("images");
    request.onerror = () => reject(request.error);
    request.onblocked = () => reject(new Error("Close other berry tabs to enable image drafts"));
    request.onsuccess = () => {
      request.result.onversionchange = () => request.result.close();
      resolve(request.result);
    };
  });
}

/** Browser-local drafts, sharing a store with the original image drafts. */
export async function storedDraft<T>(key: string, value?: readonly T[]): Promise<T[]> {
  const db = await open();
  return await new Promise((resolve, reject) => {
    const transaction = db.transaction("images", value === undefined ? "readonly" : "readwrite");
    const store = transaction.objectStore("images");
    const request = value === undefined
      ? store.get(key)
      : value.length === 0
      ? store.delete(key)
      : store.put(value, key);
    transaction.oncomplete = () => resolve(value === undefined ? request.result ?? [] : []);
    transaction.onabort = () =>
      reject(transaction.error ?? new Error("Image draft storage failed"));
  });
}