import { Logger, type INestApplicationContext } from '@nestjs/common';
import { IoAdapter } from '@nestjs/platform-socket.io';
import { createAdapter } from '@socket.io/redis-adapter';
import { Redis } from 'ioredis';
import type { Server, ServerOptions } from 'socket.io';

/**
 * Socket.io adapter backed by Redis pub/sub (P9 scaling). With the default in-memory
 * adapter, `server.to(room).emit()` only reaches sockets on the SAME instance; behind a
 * load balancer a client on replica B never sees a message emitted by replica A. This
 * adapter fans room emits across every replica through Redis, so realtime works at N>1.
 * Wired only when REDIS_URL is set (single-instance dev keeps the in-memory adapter).
 */
export class RedisIoAdapter extends IoAdapter {
  private readonly logger = new Logger(RedisIoAdapter.name);
  private adapterConstructor?: ReturnType<typeof createAdapter>;
  private clients: Redis[] = [];

  constructor(
    app: INestApplicationContext,
    private readonly url: string,
  ) {
    super(app);
  }

  /** Establish the pub/sub clients + the socket.io Redis adapter factory. */
  async connect(): Promise<void> {
    const pubClient = new Redis(this.url, { maxRetriesPerRequest: null });
    const subClient = pubClient.duplicate();
    this.clients = [pubClient, subClient];
    pubClient.on('error', (err) => this.logger.error(`redis pub error: ${err.message}`));
    subClient.on('error', (err) => this.logger.error(`redis sub error: ${err.message}`));
    this.adapterConstructor = createAdapter(pubClient, subClient);
    this.logger.log('socket.io Redis adapter connected — realtime fans out across replicas');
  }

  createIOServer(port: number, options?: ServerOptions): Server {
    const server = super.createIOServer(port, options) as Server;
    if (this.adapterConstructor) server.adapter(this.adapterConstructor);
    return server;
  }

  async dispose(): Promise<void> {
    await Promise.all(this.clients.map((c) => c.quit().catch(() => undefined)));
  }
}