import { createHash } from "node:crypto";
import { createServer } from "node:http";
import { createServer as createNetServer } from "node:net";

export async function freePort() {
  const server = createNetServer();
  await new Promise((resolve, reject) => { server.once("error", reject); server.listen(0, "127.0.0.1", resolve); });
  const port = server.address().port;
  await new Promise(resolve => server.close(resolve));
  return port;
}

// Accepts the WebSocket upgrade handshake so the frontend's real decoder-connect path
// (src/decoder_runtime.js) reaches `connected: true` without a physical AMMC decoder.
// sendPassing() pushes a PASSING frame for scenarios that need the app to see a transponder.
export async function startFakeDecoder() {
  const port = await freePort();
  const server = createServer();
  // The upgraded sockets are intentionally left open (no decoder frames needed), so a
  // plain server.close() would wait forever for them to end; track and destroy them
  // explicitly instead. closeAllConnections() does not reach upgraded sockets.
  const sockets = new Set();
  let totalConnections = 0;
  server.on("upgrade", (req, socket) => {
    totalConnections += 1;
    sockets.add(socket);
    socket.on("close", () => sockets.delete(socket));
    const key = req.headers["sec-websocket-key"];
    if (!key) { socket.destroy(); return; }
    const accept = createHash("sha1").update(key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11").digest("base64");
    socket.write(["HTTP/1.1 101 Switching Protocols", "Upgrade: websocket", "Connection: Upgrade", `Sec-WebSocket-Accept: ${accept}`, "", ""].join("\r\n"));
  });
  await new Promise((resolve, reject) => { server.once("error", reject); server.listen(port, "127.0.0.1", resolve); });
  return {
    port,
    activeConnections: () => sockets.size,
    totalConnections: () => totalConnections,
    // Drops every currently-open socket with a raw TCP reset (not a clean WS close
    // handshake) to simulate a decoder/network interruption, while the server keeps
    // listening so the frontend's own reconnect logic has somewhere to reconnect to.
    dropConnections: () => { for (const socket of sockets) socket.destroy(); },
    // Sends an AMMC-style PASSING message as an unmasked server->client WebSocket text frame.
    // `at` (ms) sets the decoder's own passing time, for exact lap times in a test.
    sendPassing: (transponder, at = Date.now()) => {
      const payload = Buffer.from(JSON.stringify({ msg: "PASSING", transponder: String(transponder), rtc_time: new Date(at).toISOString() }));
      const header = payload.length < 126
        ? Buffer.from([0x81, payload.length])
        : Buffer.from([0x81, 126, payload.length >> 8, payload.length & 0xff]);
      for (const socket of sockets) socket.write(Buffer.concat([header, payload]));
    },
    close: () => new Promise(resolve => {
      for (const socket of sockets) socket.destroy();
      server.close(() => resolve());
    }),
  };
}