/*
 * This is the common Service Worker API library to support the Spec Proxy product.
 * If you are a user, you likely are looking for a platform-specific library like
 * spec-proxy-cloudflare-worker or spec-proxy-fastly-worker.
 */
import { parse as parseCookies, serialize as serializeSetCookie } from "cookie";

// path prefix to always route traffic to Spec Proxy
const SPEC_PATH_PREFIX = "/spec_traffic";
// header that controls request forwarding for Spec Proxy
const SPEC_HEADER_FORWARD_ORIGIN = "x-spec-forward-origin";
// cookie key for the Spec ID
const SPEC_COOKIE_ID = "x-spec-id";
// cookie for the mirror mode key
const SPEC_HEADER_CUSTOMER_KEY = "x-spec-customer-authorization";
// the Set-Cookie header
const HEADER_SET_COOKIE = "set-cookie";
// the Cookie header
const HEADER_COOKIE = "cookie";
// standard X-Forwarded-For header
const HEADER_HOST = "host";
// standard X-Forwarded-For header
const HEADER_X_FORWARDED_FOR = "x-forwarded-for";

// Note: type RequestInfo = string | Request;
export type PlatformRequest = new (
  url: RequestInfo,
  info?: RequestInit,
  original?: Request,
  config?: SpecConfiguration,
) => Request;

// Spec Proxy configuration object
export interface SpecConfiguration {
  /**
   * When true, disable Spec Proxy, this library and all functionality is disabled
   */
  disableSpecProxy?: boolean;
  /**
   * When true, the request returned by this function is modified
   * to make a request to Spec Proxy, which will result in Spec Proxy making
   * the request to the customer origin itself
   */
  inlineMode?: boolean;
  /**
   * A number between 0 and 100 that identifies the percentage of IP traffic
   * the SpecTrust platform should process.
   */
  percentageOfIPs?: number;
  /**
   * An key provided by spec, which validates traffic as originating from
   * the customer when in mirror mode.
   */
  customerKey?: string;
  /**
   * Disables routing traffic prefixed with /spec_traffic to Spec
   */
  disableSpecTraffic?: boolean;
}

/**
 * This is one of two lower-level entrypoints into the Spec Proxy framework.
 * This function takes the Service Worker `event` object that's provided as an
 * argument to the 'fetch' event and a configuration object that describes how
 * the library should behave.
 *
 * Clients of this library should use the returned request as if it were the request
 * originally provided by your Edge Worker.
 *
 * The returned request should always be processed with `processSpecProxyResponse`
 * before being returned.
 *
 * Use this lower-level API if you need to do additional work between the
 * request and response, such as integrate additional libraries in your worker
 * function.
 *
 * ```javascript
 * import {
 *   specProxyProcessRequest,
 *   specProxyProcessResponse,
 * } from '@specprotected/spec-proxy-service-worker';
 *
 * addEventListener('fetch', event => {
 *   // if we don't catch the exception, fail open to original traffic path
 *   event.passThroughOnException();
 *
 *   event.respondWith(handleEvent(event));
 * })
 *
 * const specConfig = { inlineMode: false };
 *
 * async function handleEvent(event) {
 *   let request = specProxyProcessRequest(event, specConfig);
 *
 *   let response = await fetch(request);
 *
 *   // do more work, if required
 *
 *   // call the Spec Proxy response method to enact potential response changes
 *   return specProxyProcessResponse(request, response, specConfig);
 * }
 * ```
 *
 * @param event - the FetchEvent created by the Fetch API
 * @param config - configuration object to adjust Spec Proxy behavior
 * @returns the modified Request object
 */
export function specProxyProcessRequest(
  event: FetchEvent,
  config: SpecConfiguration = {},
  requestConstructor: PlatformRequest = Request,
): Request {
  // first check to see if we shouldn't process anything at all
  if (config.disableSpecProxy) {
    return event.request;
  }

  // check if we should handle this http request
  if (
    !shouldHandleRequest(
      new URL(event.request.url),
      event.request.headers,
      config,
    )
  ) {
    return event.request;
  }

  let request = event.request;
  let specUrl = new URL(request.url);
  // save it in case we're in Inline mode
  // Note: this was just `.host` before, but for local development sometimes
  // ports will be included, using hostname avoids setting overwriting port values
  let originalHost = specUrl.hostname;

  // write to spec proxy
  specUrl.hostname = originalHost + ".spec-internal.com";

  // For all requests destined for spec proxy, add the customer key to the
  // headers if provided.
  let specHeaders = new Headers(request.headers);
  if (config.customerKey) {
    specHeaders.set(SPEC_HEADER_CUSTOMER_KEY, config.customerKey);
  }

  // use this flag to signal that Spec Proxy is responsible for forwarding the
  // request on to the customer's servers.
  if (
    config.inlineMode ||
    // or if we're sending some traffic to /spec_traffic
    (!config.disableSpecTraffic &&
      specUrl.pathname.startsWith(SPEC_PATH_PREFIX))
  ) {
    // Note: /spec_traffic ignores forwarding directives
    specHeaders.set(SPEC_HEADER_FORWARD_ORIGIN, originalHost);
    return new requestConstructor(
      specUrl.toString(),
      {
        body: request.body,
        headers: specHeaders,
        method: request.method,
        redirect: request.redirect,
      },
      request,
      config,
    );
  } else {
    // Otherwise, we'll mirror the request. We will register this request promise
    // to be awaited in the background by the Service Worker API and in order to
    // do that without reading the body, we need to duplicate the body stream.

    // clone the ReadableStream, if it exists.
    let teedBody: [ReadableStream | null, ReadableStream | null] = [null, null];
    if (request.body !== null && request.body !== undefined) {
      teedBody = request.body.tee();
    }

    // build a request with the new url and the new ReadableStream
    const specRequest = new requestConstructor(
      specUrl.toString(),
      {
        body: teedBody[0],
        headers: specHeaders,
        method: request.method,
        redirect: request.redirect,
      },
      request,
      config,
    );

    // background the request to spec proxy, but ask service worker to wait
    // until it's finished even though we don't await it
    event.waitUntil(fetch(specRequest));

    // create the new request with the adjusted host if redirectHeader is set
    // and use the new teed ReadableStream body
    return new requestConstructor(
      request.url,
      {
        body: teedBody[1],
        headers: request.headers,
        method: request.method,
        redirect: request.redirect,
      },
      request,
      config,
    );
  }
}

/**
 * Process the Response as it returns to the originator of the Request. This
 * function generally adds any details the SpecTrust platform requires to identify
 * site visitors, such as the Spec Cookie.
 *
 * Note: this function does not process the Body of a Response, so won't require
 * awaiting while reading the body stream, which enables efficient processing.
 *
 * @param request - the request object that was sent to customer servers
 * @param response - the response object that was returned from customer servers
 * @param config - the configuration object that defines how this library should behave
 * @returns - the modified response object
 */
export function specProxyProcessResponse(
  request: Request,
  response: Response,
  config: SpecConfiguration = {},
): Response {
  if (
    config.disableSpecProxy ||
    config.inlineMode === true ||
    !shouldHandleRequest(new URL(request.url), request.headers, config)
  ) {
    return response;
  }

  let cookies: Record<string, string | undefined> = {};
  if (request.headers) {
    cookies = parseCookies(request.headers.get(HEADER_COOKIE) || "");
  }

  // Note: falsy check because we set our cookie on undefined or "" values
  if (!cookies[SPEC_COOKIE_ID]) {
    let specId = crypto.randomUUID();
    let setCookie = serializeSetCookie(SPEC_COOKIE_ID, specId, {
      // Note: 10 years long, essentially a "very long time"
      maxAge: 320000000,
      // the || will turn the (empty string | null) into undefined
      domain: extractTopLevelDomain(request.headers) || undefined,
      // valid for all paths
      path: "/",
      sameSite: "none",
      secure: true,
    });

    // reconstruct Response to avoid immutability
    response = new Response(response.body, response);
    response.headers.append(HEADER_SET_COOKIE, setCookie);
  }

  return response;
}

/**
 * This is the simplest method in the Spec Proxy Service Worker library.
 * If the edge worker you're writing only requires integration Spec Proxy and
 * no other additional libraries, this is the easiest way to use Spec Proxy.
 *
 * It only requires one method, so we'll show an example:
 *
 * ```javascript
 * import { specProxyProcess } from '@spectrust/server-worker-proxy';
 *
 * addEventListener('fetch', event => {
 *   // if we don't catch the exception, fail open to original traffic path
 *   event.passThroughOnException();
 *
 *   event.respondWith(specProxyProcess(event, {
 *     // set to false to mirror traffic to Spec Proxy instead of redirecting it
 *     inlineMode: true
 *   }));
 * })
 * ```
 *
 * @param event - the FetchEvent originating from the Fetch API
 * @param config - configuration object to control Spec Proxy behavior
 * @returns - a Promise to resolve to the modified response from the customer server
 */
export async function specProxyProcess(
  event: FetchEvent,
  config: SpecConfiguration,
  requestConstructor: PlatformRequest,
): Promise<Response> {
  let request = specProxyProcessRequest(event, config, requestConstructor);
  let response = await fetch(request);
  return specProxyProcessResponse(request, response, config);
}

/**
 * Function that determines if Spec Proxy should handle the incoming request.
 * This involves observing the configuration object and resolving whether or not
 * we should process this Request under the given configuration values.
 *
 * @param headers - Header map from the originating request
 * @param config - configuration object to control Spec Proxy behavior
 * @returns - true if we should process the request
 */
function shouldHandleRequest(
  url: URL,
  headers: Headers,
  config: SpecConfiguration,
): boolean {
  // If we're directing /spec_traffic to Spec, we should always do that!
  if (!config.disableSpecTraffic && url.pathname.startsWith(SPEC_PATH_PREFIX)) {
    return true;
  }

  // if we're not filtering out a percentage of IPs, or the filter is 100%
  // we should always handle traffic.
  if (config.percentageOfIPs === undefined || config.percentageOfIPs >= 100) {
    return true;
  }
  // early abort if it's impossible to match
  else if (config.percentageOfIPs <= 0) {
    return false;
  }

  // split up the ip address into octets, convert them to integers, and then sum them.
  // default the string to 99 so if, for some reason, there's a problem the traffic
  // doesn't go through unless it's at 100%. Note: 99 because there's 100 numbers in
  // [0, 99]!
  let ip_octet_sum = (headers.get(HEADER_X_FORWARDED_FOR) || "99")
    .split(".")
    .map((octet) => parseInt(octet))
    .reduce((acc, n) => acc + n);

  // if we don't know what number this is...don't assume anything
  if (isNaN(ip_octet_sum)) {
    ip_octet_sum = 99;
  }

  // not `<=` because it's a percentage, e.g. "allow 1%" would allow
  // IP octect sums that result in `0`, which is 1 slice in the range [0, 99]
  return ip_octet_sum % 100 < config.percentageOfIPs;
}

/**
 * Extract the top-level (apex) domain from the Host header.
 * This will exclude the `.spec-internal.com` domain if it is present.
 * In the event that we do not match on the Host header for any reason,
 * the value of the header itself is returned.
 *
 * @param headers - Header map from the originating request
 * @returns - top-level domain if the Host header was present, otherwise null
 */
function extractTopLevelDomain(headers: Headers): string | null {
  let host = headers.get(HEADER_HOST);
  if (host) {
    // regex attempts to match as much as it can, lazily, then a sequence of non-"." characters,
    // a ".", then more non-"." to comprise the apex domain. if .spec-internal.com is present,
    // the final group will attempt to match it, removing it from the apex domain.
    const domain_extract =
      /^(.*?\.)?(?<domain>[^.]+\.(com|co|org|edu|net|int|gov|mil|uk|co\.uk|ac\.uk|gov\.uk|ltd\.uk|me\.uk|net\.uk|nhs\.uk|org\.uk|plc\.uk|police\.uk))(\.spec-internal\.com)?$/;
    let matches = domain_extract.exec(host);
    if (matches?.groups?.domain) {
      host = matches.groups.domain;
    }
  }

  return host;
}