/**
 * Browser auto-backup in IndexedDB (spec §8, plan B4). Only the latest project text is kept.
 * Every call can fail (private window, blocked storage); callers catch and show the failure.
 */
const DB_NAME = 'pierzainer';
const STORE = 'backup';
const KEY = 'current';

export interface Backup {
  text: string;
  at: string;
}

function openDb(): Promise<IDBDatabase> {
  return new Promise((resolve, reject) => {
    const req = indexedDB.open(DB_NAME, 1);
    req.onupgradeneeded = () => req.result.createObjectStore(STORE);
    req.onsuccess = () => resolve(req.result);
    req.onerror = () => reject(req.error);
  });
}

function run<T>(mode: IDBTransactionMode, action: (store: IDBObjectStore) => IDBRequest<T>): Promise<T> {
  return openDb().then(
    (db) =>
      new Promise<T>((resolve, reject) => {
        const tx = db.transaction(STORE, mode);
        const req = action(tx.objectStore(STORE));
        tx.oncomplete = () => {
          db.close();
          resolve(req.result);
        };
        tx.onerror = () => {
          db.close();
          reject(tx.error);
        };
      }),
  );
}

export function writeBackup(text: string): Promise<unknown> {
  const value: Backup = { text, at: new Date().toISOString() };
  return run('readwrite', (store) => store.put(value, KEY));
}

export async function readBackup(): Promise<Backup | null> {
  const value = await run<Backup | undefined>('readonly', (store) => store.get(KEY));
  return value ?? null;
}

export function clearBackup(): Promise<unknown> {
  return run('readwrite', (store) => store.delete(KEY));
}