// MasterHttpRelay exit node for Cloudflare Workers.
// Deploy as HTTP endpoint and set PSK to a strong secret.

const PSK = "CHANGE_ME_TO_A_STRONG_SECRET";

const STRIP_HEADERS = new Set([
  "host",
  "connection",
  "content-length",
  "transfer-encoding",
  "proxy-connection",
  "proxy-authorization",
  "x-forwarded-for",
  "x-forwarded-host",
  "x-forwarded-proto",
  "x-forwarded-port",
  "x-real-ip",
  "forwarded",
  "via",
  // Internal relay hop header — must not propagate to the final target.
  "x-mhr-hop",
  // Workers cannot decompress gzip/br/deflate — stripping accept-encoding
  // forces targets to reply with plain bodies the Worker can forward as-is.
  "accept-encoding",
]);

function decodeBase64ToBytes(input) {
  const bin = atob(input);
  const out = new Uint8Array(bin.length);
  for (let i = 0; i < bin.length; i++) out[i] = bin.charCodeAt(i);
  return out;
}

function encodeBytesToBase64(bytes) {
  let bin = "";
  for (let i = 0; i < bytes.length; i++) bin += String.fromCharCode(bytes[i]);
  return btoa(bin);
}

function sanitizeHeaders(h) {
  const out = {};
  if (!h || typeof h !== "object") return out;
  for (const [k, v] of Object.entries(h)) {
    if (!k) continue;
    if (STRIP_HEADERS.has(k.toLowerCase())) continue;
    out[k] = String(v ?? "");
  }
  return out;
}

export default {
  async fetch(req) {
    try {
      // Cloudflare dashboard and browsers commonly test a Worker with GET.
      // Return a friendly health response so users don't misread it as failure.
      if (req.method === "GET") {
        return Response.json(
          {
            ok: true,
            status: "healthy",
            message: "Everything is OK. Worker is deployed and reachable.",
            usage: "Send POST with relay payload for actual proxy requests.",
          },
          { status: 200 }
        );
      }

      if (req.method !== "POST") {
        return Response.json(
          {
            e: "method_not_allowed",
            message: "Use POST for relay requests. GET is only a health check.",
          },
          { status: 405 }
        );
      }

      const body = await req.json();
      if (!body || typeof body !== "object") {
        return Response.json({ e: "bad_json" }, { status: 400 });
      }

      if (!PSK) {
        return Response.json({ e: "server_psk_missing" }, { status: 500 });
      }

      const k = String(body.k ?? "");
      const u = String(body.u ?? "");
      const m = String(body.m ?? "GET").toUpperCase();
      const h = sanitizeHeaders(body.h);
      const b64 = body.b;

      if (k !== PSK) return Response.json({ e: "unauthorized" }, { status: 401 });
      if (!/^https?:\/\//i.test(u)) return Response.json({ e: "bad_url" }, { status: 400 });

      // ── Loop detection ────────────────────────────────────────────────────
      // Case 1 — self-loop: target URL resolves back to this Worker.
      //   Happens when a user sets exit_node_url to the Worker URL itself.
      try {
        const targetHost = new URL(u).hostname.toLowerCase();
        const workerHost = new URL(req.url).hostname.toLowerCase();
        if (targetHost === workerHost) {
          return Response.json(
            { e: "loop_detected", detail: "target URL resolves to this Worker" },
            { status: 508 }
          );
        }
      } catch (_) {
        // Malformed URL already caught by the regex above; ignore parse errors.
      }
      // Case 2 — GAS→Worker→GAS loop: the incoming request was relayed from
      //   a Google Apps Script instance (x-mhr-hop header set), and the
      //   target is another Apps Script script URL.
      //   Without this guard, a misconfigured chain would bounce between
      //   Apps Script and Cloudflare until quota is exhausted.
      const hopHeader = req.headers.get("x-mhr-hop");
      if (hopHeader && /\/macros\/s\//i.test(u)) {
        return Response.json(
          { e: "loop_detected", detail: "GAS→Worker→GAS relay loop" },
          { status: 508 }
        );
      }

      let payload;
      if (typeof b64 === "string" && b64.length > 0) payload = decodeBase64ToBytes(b64);
      const requestBody = payload ? Uint8Array.from(payload) : undefined;

      const resp = await fetch(u, {
        method: m,
        headers: h,
        body: requestBody,
        redirect: "manual",
      });

      const data = new Uint8Array(await resp.arrayBuffer());
      const respHeaders = {};
      resp.headers.forEach((value, key) => {
        respHeaders[key] = value;
      });

      return Response.json({
        s: resp.status,
        h: respHeaders,
        b: encodeBytesToBase64(data),
      });
    } catch (err) {
      const message = err instanceof Error ? err.message : String(err);
      return Response.json({ e: message }, { status: 500 });
    }
  },
};