import { NextRequest } from "next/server";

const DEFAULT_API_BASE = "https://api.openwork.software";
const DEFAULT_AUTH_ORIGIN = "https://app.openwork.software";
const DEFAULT_AUTH_FALLBACK_BASE = "https://den-control-plane-openwork.onrender.com";
const NO_BODY_STATUS = new Set([204, 205, 304]);

const apiBase = normalizeBaseUrl(process.env.DEN_API_BASE ?? DEFAULT_API_BASE);
const authOrigin = normalizeBaseUrl(process.env.DEN_AUTH_ORIGIN ?? DEFAULT_AUTH_ORIGIN);
const authFallbackBase = normalizeBaseUrl(process.env.DEN_AUTH_FALLBACK_BASE ?? DEFAULT_AUTH_FALLBACK_BASE);

type ProxyOptions = {
  routePrefix: string;
  upstreamPathPrefix?: string;
  rewriteAuthLocationsToRequestOrigin?: boolean;
};

function normalizeBaseUrl(value: string): string {
  return value.trim().replace(/\/+$/, "");
}

function normalizePathPrefix(value: string): string {
  return value.replace(/^\/+|\/+$/g, "");
}

function getTargetPath(request: NextRequest, segments: string[], routePrefix: string): string {
  const incoming = new URL(request.url);
  let targetPath = segments.join("/");

  if (!targetPath) {
    const normalizedPrefix = routePrefix.endsWith("/") ? routePrefix : `${routePrefix}/`;
    if (incoming.pathname.startsWith(normalizedPrefix)) {
      targetPath = incoming.pathname.slice(normalizedPrefix.length);
    } else if (incoming.pathname === routePrefix) {
      targetPath = "";
    }
  }

  return targetPath;
}

function buildTargetUrl(
  base: string,
  request: NextRequest,
  targetPath: string,
  upstreamPathPrefix = "",
): string {
  const incoming = new URL(request.url);
  const prefixedPath = [normalizePathPrefix(upstreamPathPrefix), targetPath].filter(Boolean).join("/");
  const upstream = new URL(prefixedPath ? `${base}/${prefixedPath}` : base);
  upstream.search = incoming.search;
  return upstream.toString();
}

function isLikelyHtmlBody(body: ArrayBuffer): boolean {
  if (body.byteLength === 0) {
    return false;
  }

  const preview = new TextDecoder().decode(body.slice(0, 256)).trim().toLowerCase();
  return preview.startsWith("<!doctype") || preview.startsWith("<html") || preview.includes("<body");
}

function shouldFallbackToAuthBase(response: Response, body: ArrayBuffer): boolean {
  if (response.status === 502 || response.status === 503 || response.status === 504) {
    return true;
  }

  if (response.status < 500) {
    return false;
  }

  const contentType = response.headers.get("content-type")?.toLowerCase() ?? "";
  if (contentType.includes("text/html")) {
    return true;
  }

  return isLikelyHtmlBody(body);
}

function buildUpstreamErrorResponse(status: number, error: string): Response {
  return new Response(JSON.stringify({ error }), {
    status,
    headers: {
      "content-type": "application/json",
    },
  });
}

function copySetCookieHeaders(upstreamHeaders: Headers, responseHeaders: Headers): void {
  const getSetCookie = (upstreamHeaders as Headers & { getSetCookie?: () => string[] }).getSetCookie;
  if (typeof getSetCookie === "function") {
    const cookies = getSetCookie.call(upstreamHeaders);
    for (const cookie of cookies) {
      if (cookie) {
        responseHeaders.append("set-cookie", cookie);
      }
    }
    return;
  }

  const cookie = upstreamHeaders.get("set-cookie");
  if (cookie) {
    responseHeaders.append("set-cookie", cookie);
  }
}

function buildHeaders(request: NextRequest, contentType: string | null): Headers {
  const headers = new Headers();
  const copyHeaders = [
    "accept",
    "authorization",
    "cookie",
    "user-agent",
    "x-requested-with",
    "origin",
    "x-forwarded-for",
  ];

  for (const key of copyHeaders) {
    const value = request.headers.get(key);
    if (value) {
      headers.set(key, value);
    }
  }

  if (contentType) {
    headers.set("content-type", contentType);
  }

  if (!headers.has("accept")) {
    headers.set("accept", "application/json");
  }

  if (!headers.has("origin")) {
    headers.set("origin", authOrigin);
  }

  const incoming = new URL(request.url);
  headers.set("x-forwarded-host", request.headers.get("host") ?? incoming.host);
  headers.set("x-forwarded-proto", incoming.protocol.replace(/:$/, ""));

  return headers;
}

async function fetchUpstream(
  request: NextRequest,
  targetUrl: string,
  contentType: string | null,
  body: Uint8Array | null,
): Promise<{ response: Response; body: ArrayBuffer }> {
  const init: RequestInit = {
    method: request.method,
    headers: buildHeaders(request, contentType),
    redirect: "manual",
  };

  if (body && request.method !== "GET" && request.method !== "HEAD") {
    init.body = body;
  }

  const response = await fetch(targetUrl, init);
  const responseBody = await response.arrayBuffer();
  return { response, body: responseBody };
}

function rewriteLocationHeader(location: string, request: NextRequest): string {
  let parsedLocation: URL;
  try {
    parsedLocation = new URL(location);
  } catch {
    return location;
  }

  const requestOrigin = new URL(request.url).origin;
  const rewriteableOrigins = [apiBase, authFallbackBase]
    .map((value) => {
      try {
        return new URL(value).origin;
      } catch {
        return null;
      }
    })
    .filter((value): value is string => Boolean(value));

  if (!rewriteableOrigins.includes(parsedLocation.origin) || !parsedLocation.pathname.startsWith("/api/auth/")) {
    return location;
  }

  return `${requestOrigin}${parsedLocation.pathname}${parsedLocation.search}${parsedLocation.hash}`;
}

export async function proxyUpstream(
  request: NextRequest,
  segments: string[] = [],
  options: ProxyOptions,
): Promise<Response> {
  const targetPath = getTargetPath(request, segments, options.routePrefix);
  const primaryTargetUrl = buildTargetUrl(apiBase, request, targetPath, options.upstreamPathPrefix);
  const fallbackTargetUrl = buildTargetUrl(authFallbackBase, request, targetPath, options.upstreamPathPrefix);
  const contentType = request.headers.get("content-type");
  const requestBody = request.method !== "GET" && request.method !== "HEAD"
    ? new Uint8Array(await request.arrayBuffer())
    : null;

  let upstream: Response | null = null;
  let body: ArrayBuffer | null = null;

  try {
    const primary = await fetchUpstream(request, primaryTargetUrl, contentType, requestBody);
    upstream = primary.response;
    body = primary.body;
  } catch {
    if (apiBase !== authFallbackBase) {
      try {
        const fallback = await fetchUpstream(request, fallbackTargetUrl, contentType, requestBody);
        upstream = fallback.response;
        body = fallback.body;
      } catch {}
    }
  }

  if (!upstream || !body) {
    return buildUpstreamErrorResponse(502, "Upstream request failed.");
  }

  if (apiBase !== authFallbackBase && shouldFallbackToAuthBase(upstream, body)) {
    try {
      const fallback = await fetchUpstream(request, fallbackTargetUrl, contentType, requestBody);
      upstream = fallback.response;
      body = fallback.body;
    } catch {}
  }

  const responseContentType = upstream.headers.get("content-type")?.toLowerCase() ?? "";
  if (upstream.status >= 500 && (responseContentType.includes("text/html") || isLikelyHtmlBody(body))) {
    return buildUpstreamErrorResponse(upstream.status, "Upstream service unavailable.");
  }

  const responseHeaders = new Headers();
  const passThroughHeaders = ["content-type", "location", "cache-control"];

  for (const key of passThroughHeaders) {
    const value = upstream.headers.get(key);
    if (!value) {
      continue;
    }

    if (key === "location" && options.rewriteAuthLocationsToRequestOrigin) {
      responseHeaders.set(key, rewriteLocationHeader(value, request));
      continue;
    }

    responseHeaders.set(key, value);
  }

  copySetCookieHeaders(upstream.headers, responseHeaders);

  const shouldDropBody = request.method === "HEAD" || NO_BODY_STATUS.has(upstream.status);

  return new Response(shouldDropBody ? null : body, {
    status: upstream.status,
    headers: responseHeaders,
  });
}