/**
 * node:http ↔ WHATWG fetch bridge for the /api transport (host side of the
 * web carrier; the fetch-shaped handler itself is transport-agnostic).
 */

import type { IncomingMessage, ServerResponse } from 'node:http'

/**
 * Bridge one node:http request to the fetch-shaped handler (client close
 * aborts; SSE bodies stream out chunk by chunk).
 * @param req - incoming node:http request (fully read before dispatch).
 * @param res - node:http response the bridge writes and owns to completion.
 * @param apiHandler - fetch-shaped API carrier the request is dispatched to.
 */
export async function bridge(req: IncomingMessage, res: ServerResponse, apiHandler: { fetch: typeof fetch }): Promise<void> {
  const abort = new AbortController()
  // Client-disconnect detection MUST hang off the response, not the request:
  // since Node 16, IncomingMessage 'close' fires as soon as the request body is
  // fully consumed (immediately for a bodyless GET), which would abort every SSE
  // stream right after open. ServerResponse 'close' fires on connection teardown;
  // writableEnded distinguishes a normal end() from the client going away.
  res.on('close', () => {
    if (!res.writableEnded) abort.abort()
  })
  const chunks: Buffer[] = []
  for await (const chunk of req) chunks.push(chunk as Buffer)
  /* v8 ignore next 3 -- `??` arms: node:http always sets url/method on server
  requests; the fields are only optional on the client-side IncomingMessage type */
  const request = new Request(new URL(req.url ?? '/', 'http://dsh.internal'), {
    method: req.method ?? 'GET',
    headers: Object.fromEntries(Object.entries(req.headers).filter(([, v]) => typeof v === 'string') as [string, string][]),
    ...chunks.length > 0 ? { body: Buffer.concat(chunks) } : {},
    signal: abort.signal,
  })
  const response = await apiHandler.fetch(request)
  res.writeHead(response.status, Object.fromEntries(response.headers.entries()))
  if (response.body === null) {
    res.end()
    return
  }
  for await (const chunk of response.body) {
    // Backpressure: a false return means the socket buffer is full — wait for drain
    // instead of buffering unboundedly (slow/suspended SSE consumers). 'close' also
    // resolves so a mid-wait disconnect can't park this loop forever; the close
    // handler above aborts the handler stream, which then ends the iteration.
    if (!res.write(chunk)) {
      await new Promise<void>((resolve) => {
        const done = (): void => {
          res.off('drain', done)
          res.off('close', done)
          resolve()
        }
        res.once('drain', done)
        res.once('close', done)
      })
    }
  }
  res.end()
}