import { Injectable } from '@nestjs/common';
import { DataSource, QueryRunner, EntityManager } from 'typeorm';

/**
 * Transaction Service - 提供可靠的事务管理
 *
 * 基于 TypeORM 官方推荐的 QueryRunner 模式
 * 参考: https://typeorm.io/docs/advanced-topics/transactions/
 */
@Injectable()
export class TransactionService {
  constructor(private readonly dataSource: DataSource) {}

  /**
   * 在事务中执行操作 (推荐使用)
   *
   * @param operation - 接收 EntityManager 的异步操作函数
   * @returns 操作的返回值
   * @throws 如果操作失败，事务会自动回滚并抛出原始错误
   *
   * @example
   * ```typescript
   * const result = await this.transactionService.runInTransaction(async (manager) => {
   *   await manager.save(PaymentORM, payment);
   *   await manager.update(OrderORM, orderId, { status: 'PAID' });
   *   return payment;
   * });
   * ```
   */
  async runInTransaction<T>(
    operation: (manager: EntityManager) => Promise<T>,
  ): Promise<T> {
    const queryRunner = this.dataSource.createQueryRunner();

    await queryRunner.connect();
    await queryRunner.startTransaction();

    try {
      const result = await operation(queryRunner.manager);
      await queryRunner.commitTransaction();
      return result;
    } catch (error) {
      await queryRunner.rollbackTransaction();
      throw error;
    } finally {
      // 重要：必须释放连接，否则会导致连接池耗尽
      await queryRunner.release();
    }
  }

  /**
   * 使用 QueryRunner 执行事务 (用于需要更多控制的场景)
   *
   * @param operation - 接收 QueryRunner 的异步操作函数
   * @returns 操作的返回值
   *
   * @example
   * ```typescript
   * const result = await this.transactionService.runWithQueryRunner(async (queryRunner) => {
   *   // 可以使用 queryRunner.manager 进行操作
   *   // 也可以使用 queryRunner.query() 执行原始 SQL
   *   await queryRunner.manager.save(entity);
   *   return entity;
   * });
   * ```
   */
  async runWithQueryRunner<T>(
    operation: (queryRunner: QueryRunner) => Promise<T>,
  ): Promise<T> {
    const queryRunner = this.dataSource.createQueryRunner();

    await queryRunner.connect();
    await queryRunner.startTransaction();

    try {
      const result = await operation(queryRunner);
      await queryRunner.commitTransaction();
      return result;
    } catch (error) {
      await queryRunner.rollbackTransaction();
      throw error;
    } finally {
      await queryRunner.release();
    }
  }

  /**
   * 使用乐观锁执行更新 (防止并发覆盖)
   *
   * @param operation - 异步操作函数
   * @param maxRetries - 最大重试次数 (默认 3)
   * @returns 操作的返回值
   * @throws 如果达到最大重试次数仍失败
   *
   * @example
   * ```typescript
   * await this.transactionService.runWithOptimisticLock(async (manager) => {
   *   const payment = await manager.findOne(PaymentORM, { where: { id } });
   *   payment.status = 'COMPLETED';
   *   await manager.save(payment); // 如果 version 不匹配会抛出错误
   * });
   * ```
   */
  async runWithOptimisticLock<T>(
    operation: (manager: EntityManager) => Promise<T>,
    maxRetries: number = 3,
  ): Promise<T> {
    let lastError: Error | null = null;

    for (let attempt = 1; attempt <= maxRetries; attempt++) {
      try {
        return await this.runInTransaction(operation);
      } catch (error: unknown) {
        // TypeORM 乐观锁冲突错误
        if (
          error instanceof Error &&
          (error.name === 'OptimisticLockVersionMismatchError' ||
            error.message.includes('version'))
        ) {
          lastError = error;
          // 短暂延迟后重试
          await this.delay(50 * attempt);
          continue;
        }
        throw error;
      }
    }

    throw new Error(
      `Optimistic lock failed after ${maxRetries} retries: ${lastError?.message}`,
    );
  }

  private delay(ms: number): Promise<void> {
    return new Promise((resolve) => setTimeout(resolve, ms));
  }
}