const DB_NAME = 'dww11-account-switch-v3';
let opening;
export function openDB() {
  if (!opening) opening = new Promise((resolve, reject) => {
    const req = indexedDB.open(DB_NAME, 1);
    req.onupgradeneeded = () => {
      req.result.createObjectStore('accounts', { keyPath: 'username' });
      req.result.createObjectStore('meta');
    };
    req.onsuccess = () => {
      const db = req.result;
      db.onversionchange = () => { db.close(); opening = null; };
      db.onclose = () => { opening = null; };
      resolve(db);
    };
    req.onerror = () => { opening = null; reject(new Error('无法打开 IndexedDB：' + req.error?.message)); };
    req.onblocked = () => { opening = null; reject(new Error('数据库升级被阻塞，请关闭扩展管理页后重试。')); };
  });
  return opening;
}

// 所有状态变更跨 accounts/meta 使用同一事务。回调必须同步，避免 await 使事务自动提交。
export async function transact(reducer, write = true) {
  const db = await openDB();
  return new Promise((resolve, reject) => {
    const tx = db.transaction(['accounts', 'meta'], write ? 'readwrite' : 'readonly', write ? { durability: 'strict' } : undefined);
    const accountsStore = tx.objectStore('accounts'), metaStore = tx.objectStore('meta');
    const a = accountsStore.getAll(), m = metaStore.get('state');
    let output, failure, ready = 0;
    const loaded = () => {
      if (++ready !== 2) return;
      try {
        const state = m.result || { schedule: null, flow: null };
        output = reducer(a.result, state, accountsStore);
        if (write) metaStore.put(state, 'state');
      } catch (e) { failure = e; tx.abort(); }
    };
    a.onsuccess = loaded; m.onsuccess = loaded;
    tx.oncomplete = () => resolve(output);
    tx.onabort = () => reject(failure || new Error('数据库事务未提交：' + (tx.error?.message || '已中止')));
    tx.onerror = () => {}; // 由 onabort 统一处理，不在提交之前返回成功。
  });
}