/**
 * SSE Hub — manages connected clients and broadcasts events.
 * Product backends create an SSEHub instance and push events to it;
 * the hub fans out to all connected SSE clients.
 */

import type { ServerResponse } from 'node:http';

export interface SSEMessage {
  event?: string;
  data: string;
  id?: string;
  retry?: number;
}

interface ConnectedClient {
  id: string;
  userId?: string;
  res: ServerResponse;
  connectedAt: string;
}

export class SSEHub {
  private clients = new Map<string, ConnectedClient>();
  private clientCounter = 0;

  /**
   * Add a new SSE client connection.
   * Sets up the SSE headers and returns a client ID.
   */
  addClient(res: ServerResponse, userId?: string): string {
    const id = `sse_${++this.clientCounter}_${Date.now()}`;

    res.writeHead(200, {
      'Content-Type': 'text/event-stream',
      'Cache-Control': 'no-cache',
      Connection: 'keep-alive',
      'X-Accel-Buffering': 'no',
    });

    // Send initial connection event
    res.write(`event: connected\ndata: ${JSON.stringify({ clientId: id })}\n\n`);

    const client: ConnectedClient = {
      id,
      userId,
      res,
      connectedAt: new Date().toISOString(),
    };

    this.clients.set(id, client);

    // Clean up on close
    res.on('close', () => {
      this.clients.delete(id);
    });

    return id;
  }

  /**
   * Broadcast an SSE message to all connected clients.
   */
  broadcast(message: SSEMessage): number {
    let sent = 0;
    const formatted = formatSSE(message);

    for (const [id, client] of this.clients) {
      try {
        client.res.write(formatted);
        sent++;
      } catch {
        this.clients.delete(id);
      }
    }

    return sent;
  }

  /**
   * Send an SSE message to a specific user's connections.
   */
  sendToUser(userId: string, message: SSEMessage): number {
    let sent = 0;
    const formatted = formatSSE(message);

    for (const [id, client] of this.clients) {
      if (client.userId === userId) {
        try {
          client.res.write(formatted);
          sent++;
        } catch {
          this.clients.delete(id);
        }
      }
    }

    return sent;
  }

  /**
   * Send a heartbeat (comment) to all clients to keep connections alive.
   */
  heartbeat(): void {
    for (const [id, client] of this.clients) {
      try {
        client.res.write(': heartbeat\n\n');
      } catch {
        this.clients.delete(id);
      }
    }
  }

  /**
   * Get count of connected clients.
   */
  get clientCount(): number {
    return this.clients.size;
  }

  /**
   * Disconnect all clients.
   */
  disconnectAll(): void {
    for (const [, client] of this.clients) {
      try {
        client.res.end();
      } catch {
        /* already closed */
      }
    }
    this.clients.clear();
  }
}

function formatSSE(message: SSEMessage): string {
  let output = '';
  if (message.id) output += `id: ${message.id}\n`;
  if (message.event) output += `event: ${message.event}\n`;
  if (message.retry) output += `retry: ${message.retry}\n`;
  output += `data: ${message.data}\n\n`;
  return output;
}