// Ask the broker, before Node-RED is told anything, whether it takes the login and password the
// mesh delivered — and whether that login may subscribe to every topic, as flows expect.
//
// One MQTT 3.1.1 session: CONNECT (clean, a throwaway client id, so no flow's session is taken
// over), read the CONNACK, optionally SUBSCRIBE once and read the SUBACK, DISCONNECT. No dependency:
// the handful of bytes MQTT needs for this are written here.

import { randomBytes } from "node:crypto";
import { connect } from "node:net";

export interface ProbeResult {
  /** 0 accepted; 4 bad username or password; 5 not authorised. */
  connack: number;
  /** The SUBACK return code for the filter asked about: 0–2 granted, 0x80 refused. */
  suback?: number;
}

export type Probe = (host: string, port: number, username: string, password: string, subscribe?: string) => Promise<ProbeResult>;

function str(v: string): Buffer {
  const b = Buffer.from(v, "utf8");
  const len = Buffer.alloc(2);
  len.writeUInt16BE(b.length);
  return Buffer.concat([len, b]);
}

function packet(type: number, body: Buffer): Buffer {
  let remaining = body.length;
  const lenBytes: number[] = [];
  do {
    let byte = remaining % 128;
    remaining = Math.floor(remaining / 128);
    if (remaining > 0) byte |= 0x80;
    lenBytes.push(byte);
  } while (remaining > 0);
  return Buffer.concat([Buffer.from([type, ...lenBytes]), body]);
}

/** The first complete packet in `buf`: its type byte, its body, and how many bytes it took. */
export function firstPacket(buf: Buffer): { type: number; body: Buffer; used: number } | undefined {
  if (buf.length < 2) return undefined;
  let length = 0;
  let multiplier = 1;
  let i = 1;
  for (;;) {
    if (i >= buf.length) return undefined;
    const byte = buf[i++];
    length += (byte & 0x7f) * multiplier;
    if ((byte & 0x80) === 0) break;
    multiplier *= 128;
    if (i > 4) throw new Error("malformed MQTT remaining length");
  }
  if (buf.length < i + length) return undefined;
  return { type: buf[0], body: buf.subarray(i, i + length), used: i + length };
}

export const probeBroker: Probe = (host, port, username, password, subscribe) => {
  const connectBody = Buffer.concat([
    str("MQTT"),
    Buffer.from([4, 0xc2, 0, 10]), // level 4 (3.1.1); username + password + clean session; keepalive 10s
    str(`mesh-probe-${randomBytes(6).toString("hex")}`),
    str(username),
    str(password),
  ]);
  return new Promise((resolve, reject) => {
    const socket = connect({ host, port });
    let buf = Buffer.alloc(0);
    const result: ProbeResult = { connack: -1 };
    const timer = setTimeout(() => {
      socket.destroy();
      reject(new Error(`no answer from the broker at ${host}:${port} within 10s`));
    }, 10_000);
    const finish = (): void => {
      clearTimeout(timer);
      if (result.connack === 0) socket.end(Buffer.from([0xe0, 0]));
      else socket.destroy();
      resolve(result);
    };
    socket.on("connect", () => socket.write(packet(0x10, connectBody)));
    socket.on("data", (chunk) => {
      buf = Buffer.concat([buf, chunk]);
      for (;;) {
        let p;
        try {
          p = firstPacket(buf);
        } catch (err) {
          clearTimeout(timer);
          socket.destroy();
          reject(err);
          return;
        }
        if (!p) return;
        buf = buf.subarray(p.used);
        const kind = p.type >> 4;
        if (kind === 2) {
          result.connack = p.body[1] ?? -1;
          if (result.connack !== 0 || !subscribe) return finish();
          // SUBSCRIBE, packet id 1, one filter at QoS 0.
          socket.write(packet(0x82, Buffer.concat([Buffer.from([0, 1]), str(subscribe), Buffer.from([0])])));
        } else if (kind === 9) {
          result.suback = p.body[2];
          return finish();
        }
      }
    });
    socket.on("error", (err) => {
      clearTimeout(timer);
      reject(err);
    });
    socket.on("close", () => {
      if (result.connack === -1) {
        clearTimeout(timer);
        reject(new Error(`the broker at ${host}:${port} closed the connection without answering`));
      }
    });
  });
};