import { Inject } from '@nestjs/common';
import { OnEvent } from '@nestjs/event-emitter';
import {
  WebSocketGateway,
  WebSocketServer,
  type OnGatewayConnection,
  type OnGatewayDisconnect,
  type OnGatewayInit,
} from '@nestjs/websockets';
import type { Server, Socket } from 'socket.io';
// eslint-disable-next-line @typescript-eslint/consistent-type-imports -- NestJS DI requires value imports
import { TokenService, type JwtPayload } from '@modules/auth';
// eslint-disable-next-line @typescript-eslint/consistent-type-imports -- NestJS DI requires value imports
import { LoggerService, RedisService } from '@modules/shared';
import type { NotificationSentEvent } from '../../domain/events/notification-sent.event';
import {
  NOTIFICATION_REPOSITORY,
  type INotificationRepository,
} from '../../domain/repositories/notification.repository';

/** Redis key for the per-user unread notification counter. */
const UNREAD_COUNT_KEY = (userId: string) => `notifications:unread:${userId}`;

/** TTL for the cached unread count (1 hour). */
const UNREAD_COUNT_TTL = 3600;

@WebSocketGateway({
  namespace: '/notifications',
  cors: {
    origin: (process.env['CORS_ORIGINS'] ?? 'http://localhost:3000')
      .split(',')
      .map((o) => o.trim()),
    credentials: true,
  },
})
export class NotificationsGateway
  implements OnGatewayInit, OnGatewayConnection, OnGatewayDisconnect
{
  @WebSocketServer()
  server!: Server;

  /** Track connected sockets per user for multi-device support. */
  private readonly userSockets = new Map<string, Set<string>>();

  constructor(
    private readonly tokenService: TokenService,
    private readonly logger: LoggerService,
    private readonly redisService: RedisService,
    @Inject(NOTIFICATION_REPOSITORY)
    private readonly notificationRepo: INotificationRepository,
  ) {}

  afterInit(): void {
    this.logger.log('NotificationsGateway initialized', 'NotificationsGateway');
  }

  /* ────────────────────────────────────────────
   * Connection lifecycle
   * ──────────────────────────────────────────── */

  async handleConnection(client: Socket): Promise<void> {
    try {
      const payload = this.extractAndVerifyToken(client);
      if (!payload) {
        client.disconnect(true);
        return;
      }

      // Attach identity to the socket for later use
      client.data['userId'] = payload.sub;
      client.data['role'] = payload.role;

      // Join the user's private room
      await client.join(`user:${payload.sub}`);

      // Track socket for bookkeeping
      if (!this.userSockets.has(payload.sub)) {
        this.userSockets.set(payload.sub, new Set());
      }
      this.userSockets.get(payload.sub)!.add(client.id);

      // Push the current unread count on connect
      const unreadCount = await this.getUnreadCount(payload.sub);
      client.emit('notification:unread-count', { unreadCount });

      this.logger.debug(
        `WS connected: user=${payload.sub} socket=${client.id}`,
        'NotificationsGateway',
      );
    } catch (error) {
      this.logger.error(
        `WS connection error: ${error instanceof Error ? error.message : error}`,
        error instanceof Error ? error.stack : undefined,
        'NotificationsGateway',
      );
      client.disconnect(true);
    }
  }

  handleDisconnect(client: Socket): void {
    const userId = client.data['userId'] as string | undefined;
    if (userId) {
      const sockets = this.userSockets.get(userId);
      if (sockets) {
        sockets.delete(client.id);
        if (sockets.size === 0) {
          this.userSockets.delete(userId);
        }
      }
    }
    this.logger.debug(
      `WS disconnected: user=${userId ?? 'unknown'} socket=${client.id}`,
      'NotificationsGateway',
    );
  }

  /* ────────────────────────────────────────────
   * Domain event handlers
   * ──────────────────────────────────────────── */

  /**
   * Listens to the `notification.sent` domain event emitted by
   * {@link SendNotificationHandler} after a notification is persisted & sent.
   *
   * Pushes `notification:new` to the user's room and bumps the
   * cached unread counter.
   */
  @OnEvent('notification.sent', { async: true })
  async handleNotificationSent(event: NotificationSentEvent): Promise<void> {
    try {
      this.server.to(`user:${event.userId}`).emit('notification:new', {
        id: event.aggregateId,
        templateKey: event.templateKey,
        channel: event.channel,
        occurredAt: event.occurredAt.toISOString(),
      });

      // Increment cached unread count
      await this.incrementUnreadCount(event.userId);

      // Also emit updated count
      const unreadCount = await this.getUnreadCount(event.userId);
      this.server
        .to(`user:${event.userId}`)
        .emit('notification:unread-count', { unreadCount });
    } catch (error) {
      this.logger.error(
        `Failed to emit WS notification for user ${event.userId}: ${
          error instanceof Error ? error.message : error
        }`,
        error instanceof Error ? error.stack : undefined,
        'NotificationsGateway',
      );
    }
  }

  /* ────────────────────────────────────────────
   * Public helpers — used by the controller
   * ──────────────────────────────────────────── */

  /**
   * Emit an updated unread count to a user after they mark
   * notifications as read (called from the controller).
   */
  async emitUnreadCount(userId: string): Promise<void> {
    const unreadCount = await this.getUnreadCount(userId);
    this.server
      .to(`user:${userId}`)
      .emit('notification:unread-count', { unreadCount });
  }

  /**
   * Invalidate the cached unread count (called after mark-as-read).
   */
  async invalidateUnreadCount(userId: string): Promise<void> {
    if (this.redisService.isAvailable()) {
      await this.redisService.del(UNREAD_COUNT_KEY(userId));
    }
  }

  /* ────────────────────────────────────────────
   * Private helpers
   * ──────────────────────────────────────────── */

  /**
   * Extract JWT from the socket handshake and verify it.
   *
   * Supports three sources (in priority order):
   * 1. `handshake.auth.token` — Socket.IO `auth` option (recommended)
   * 2. `handshake.headers.authorization` — HTTP upgrade header
   * 3. `handshake.query.token` — query string (least secure)
   */
  private extractAndVerifyToken(client: Socket): JwtPayload | null {
    const raw: unknown =
      client.handshake.auth?.['token'] ??
      client.handshake.headers?.['authorization'] ??
      client.handshake.query?.['token'];

    if (!raw || typeof raw !== 'string') {
      this.logger.warn(
        `WS auth failed: no token provided (socket=${client.id})`,
        'NotificationsGateway',
      );
      return null;
    }

    const token = raw.startsWith('Bearer ') ? raw.slice(7) : raw;
    const payload = this.tokenService.verifyAccessToken(token);
    if (!payload) {
      this.logger.warn(
        `WS auth failed: invalid token (socket=${client.id})`,
        'NotificationsGateway',
      );
    }
    return payload;
  }

  /**
   * Read the unread count from Redis (cache-aside pattern).
   * Falls back to the database when Redis is unavailable or cache misses.
   */
  private async getUnreadCount(userId: string): Promise<number> {
    if (this.redisService.isAvailable()) {
      try {
        const cached = await this.redisService.get(UNREAD_COUNT_KEY(userId));
        if (cached !== null) {
          return Number(cached);
        }
      } catch {
        // Redis unavailable — fall through to DB
      }
    }

    const count = await this.notificationRepo.countUnreadByUserId(userId);

    // Warm the cache
    if (this.redisService.isAvailable()) {
      try {
        await this.redisService.set(
          UNREAD_COUNT_KEY(userId),
          String(count),
          UNREAD_COUNT_TTL,
        );
      } catch {
        // Non-critical — continue without cache
      }
    }

    return count;
  }

  /**
   * Increment the cached unread counter in Redis (if available).
   * The counter is lazily initialised from the DB on the next read if
   * the key does not exist.
   */
  private async incrementUnreadCount(userId: string): Promise<void> {
    if (!this.redisService.isAvailable()) return;

    try {
      const client = this.redisService.getClient();
      const key = UNREAD_COUNT_KEY(userId);
      const exists = await client.exists(key);
      if (exists) {
        await client.incr(key);
      }
      // If key doesn't exist, getUnreadCount will populate it on next read
    } catch {
      // Non-critical
    }
  }
}