/**
 * AriaEngine Storage Backend — 存储后端抽象层
 * @module engine/aria/store/backend
 *
 * 封装底层浏览器存储 API（IndexedDB / OPFS / Memory 回退），
 * 供 Buffer Pool 的 PageIO 和 WAL 的 WALStore 使用。
 */

import { DatabaseError } from '../../../constants';

// ---------------------------------------------------------------------------
//  StorageBackend 接口
// ---------------------------------------------------------------------------

export interface IStorageBackend {
  /** 打开存储 */
  open(name: string): Promise<void>;
  /** 关闭存储 */
  close(): Promise<void>;
  /** 是否已打开 */
  isOpen(): boolean;
  /** 读取数据块 */
  read(key: string): Promise<ArrayBuffer | null>;
  /** 写入数据块 */
  write(key: string, data: ArrayBuffer): Promise<void>;
  /** 删除数据块 */
  delete(key: string): Promise<void>;
  /** 列出所有 key */
  listKeys(): Promise<string[]>;
  /** 检查 key 是否存在 */
  exists(key: string): Promise<boolean>;
  /** 清空所有数据 */
  clear(): Promise<void>;
}

// =======================================================================
//  IndexedDB Backend
// =======================================================================

export class IndexedDBBackend implements IStorageBackend {
  private db: IDBDatabase | null = null;
  private dbName = '';
  private storeName = 'data';

  async open(name: string): Promise<void> {
    this.dbName = `aria-${name}`;
    return new Promise((resolve, reject) => {
      const request = indexedDB.open(this.dbName, 1);
      request.onupgradeneeded = () => {
        const db = request.result;
        if (!db.objectStoreNames.contains(this.storeName)) {
          db.createObjectStore(this.storeName);
        }
      };
      request.onsuccess = () => {
        this.db = request.result;
        resolve();
      };
      request.onerror = () => reject(new DatabaseError('Failed to open AriaEngine IndexedDB', 'ARIA_IDB_OPEN_ERROR'));
    });
  }

  async close(): Promise<void> {
    if (this.db) {
      this.db.close();
      this.db = null;
    }
  }

  isOpen(): boolean {
    return this.db !== null;
  }

  async read(key: string): Promise<ArrayBuffer | null> {
    const db = this.ensureDB();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(this.storeName, 'readonly');
      const req = tx.objectStore(this.storeName).get(key);
      req.onsuccess = () => resolve(req.result ?? null);
      req.onerror = () => reject(new DatabaseError('Failed to read from AriaEngine store', 'ARIA_READ_ERROR'));
    });
  }

  async write(key: string, data: ArrayBuffer): Promise<void> {
    const db = this.ensureDB();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(this.storeName, 'readwrite');
      tx.objectStore(this.storeName).put(data, key);
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(new DatabaseError('Failed to write to AriaEngine store', 'ARIA_WRITE_ERROR'));
    });
  }

  async delete(key: string): Promise<void> {
    const db = this.ensureDB();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(this.storeName, 'readwrite');
      tx.objectStore(this.storeName).delete(key);
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(new DatabaseError('Failed to delete from AriaEngine store', 'ARIA_DELETE_ERROR'));
    });
  }

  async listKeys(): Promise<string[]> {
    const db = this.ensureDB();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(this.storeName, 'readonly');
      const req = tx.objectStore(this.storeName).getAllKeys();
      req.onsuccess = () => resolve((req.result ?? []) as string[]);
      req.onerror = () => reject(new DatabaseError('Failed to list keys', 'ARIA_LIST_ERROR'));
    });
  }

  async exists(key: string): Promise<boolean> {
    const result = await this.read(key);
    return result !== null;
  }

  async clear(): Promise<void> {
    const db = this.ensureDB();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(this.storeName, 'readwrite');
      tx.objectStore(this.storeName).clear();
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(new DatabaseError('Failed to clear AriaEngine store', 'ARIA_CLEAR_ERROR'));
    });
  }

  private ensureDB(): IDBDatabase {
    if (!this.db) throw new DatabaseError('AriaEngine storage not opened', 'ARIA_DB_NOT_OPEN');
    return this.db;
  }
}

// =======================================================================
//  Memory Backend（回退 / 测试用）
// =======================================================================

export class MemoryBackend implements IStorageBackend {
  private store: Map<string, ArrayBuffer> = new Map();
  private opened = false;

  async open(_name: string): Promise<void> {
    this.opened = true;
  }

  async close(): Promise<void> {
    this.store.clear();
    this.opened = false;
  }

  isOpen(): boolean {
    return this.opened;
  }

  async read(key: string): Promise<ArrayBuffer | null> {
    return this.store.get(key) ?? null;
  }

  async write(key: string, data: ArrayBuffer): Promise<void> {
    this.store.set(key, data);
  }

  async delete(key: string): Promise<void> {
    this.store.delete(key);
  }

  async listKeys(): Promise<string[]> {
    return Array.from(this.store.keys());
  }

  async exists(key: string): Promise<boolean> {
    return this.store.has(key);
  }

  async clear(): Promise<void> {
    this.store.clear();
  }
}