import { PassThrough } from 'node:stream'
import type { HttpContext } from '@adonisjs/core/http'

export interface SseChannel {
  /**
   * Sends one event to the browser. Ignored once the connection is closed.
   */
  send(event: string, data: unknown): void

  /**
   * Registers a callback invoked once, when the browser disconnects or the
   * channel is closed by the server.
   */
  onClose(callback: () => void): void

  close(): void
}

/**
 * Turns the response into a Server-Sent Events stream (used with
 * EventSource on the Angular side for the live metrics, logs and jobs).
 */
export function openSse({ response }: HttpContext): SseChannel {
  const stream = new PassThrough()
  const closeCallbacks: (() => void)[] = []
  let closed = false

  response.header('Content-Type', 'text/event-stream; charset=utf-8')
  response.header('Cache-Control', 'no-cache, no-transform')
  response.header('Connection', 'keep-alive')
  response.header('X-Accel-Buffering', 'no')
  response.stream(stream)

  /**
   * Comments keep the connection open through Caddy and let the browser
   * know quickly that the stream is established.
   */
  stream.write('retry: 5000\n\n')
  const heartbeat = setInterval(() => stream.write(': ping\n\n'), 25_000)

  const close = () => {
    if (closed) return
    closed = true
    clearInterval(heartbeat)
    for (const callback of closeCallbacks) callback()
    stream.end()
  }

  /**
   * "close" on the response (not the request) is emitted when the browser
   * goes away.
   */
  response.response.on('close', close)

  return {
    send(event, data) {
      if (closed) return
      stream.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`)
    },
    onClose(callback) {
      closeCallbacks.push(callback)
    },
    close,
  }
}