text: /* ===========================================================
   * sw.js
   * ===========================================================
   * Copyright 2016 @huxpro
   * Licensed under Apache 2.0 
   * Register service worker.
   * ========================================================== */
  
  const PRECACHE = 'precache-v1';
  const RUNTIME = 'runtime';
  const HOSTNAME_WHITELIST = [
    self.location.hostname,
    "huangxuan.me",
    "yanshuo.io",
    "cdnjs.cloudflare.com"
  ]
  
  
  // The Util Function to hack URLs of intercepted requests
  const getFixedUrl = (req) => {
    var now = Date.now();
    url = new URL(req.url)
  
    // 1. fixed http URL
    // Just keep syncing with location.protocol 
    // fetch(httpURL) belongs to active mixed content. 
    // And fetch(httpRequest) is not supported yet.
    url.protocol = self.location.protocol
  
    // 2. add query for caching-busting.
    // Github Pages served with Cache-Control: max-age=600
    // max-age on mutable content is error-prone, with SW life of bugs can even extend.
    // Until cache mode of Fetch API landed, we have to workaround cache-busting with query string.
    // Cache-Control-Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=453190
    url.search += (url.search ? '&' : '?') + 'cache-bust=' + now;
    return url.href
  }
  
  // The Util Function to detect and polyfill req.mode="navigate"
  // request.mode of 'navigate' is unfortunately not supported in Chrome
  // versions older than 49, so we need to include a less precise fallback,
  // which checks for a GET request with an Accept: text/html header.
  const isNavigationReq = (req) => (req.mode === 'navigate' || (req.method === 'GET' && req.headers.get('accept').includes('text/html')))
  
  // The Util Function to detect if a req is end with extension
  // Accordin to Fetch API spec <https://fetch.spec.whatwg.org/#concept-request-destination>
  // Any HTML's navigation has consistently mode="navigate" type="" and destination="document" 
  // including requesting an img (or any static resources) from URL Bar directly.
  // So It ends up with that regExp is still the king of URL routing ;)
  // P.S. An url.pathname has no '.' can not indicate it ends with extension (e.g. /api/version/1.2/)
  const endWithExtension = (req) => Boolean(new URL(req.url).pathname.match(/\.\w+$/))
  
  // Redirect in SW manually fixed github pages arbitray 404s on things?blah 
  // what we want:
  //    repo?blah -> !(gh 404) -> sw 302 -> repo/?blah 
  //    .ext?blah -> !(sw 302 -> .ext/?blah -> gh 404) -> .ext?blah 
  // If It's a navigation req and it's url.pathname isn't end with '/' or '.ext'
  // it should be a dir/repo request and need to be fixed (a.k.a be redirected)
  // Tracking https://twitter.com/Huxpro/status/798816417097224193
  const shouldRedirect = (req) => (isNavigationReq(req) && new URL(req.url).pathname.substr(-1) !== "/" && !endWithExtension(req))
  
  // The Util Function to get redirect URL
  // `${url}/` would mis-add "/" in the end of query, so we use URL object.
  // P.P.S. Always trust url.pathname instead of the whole url string.
  const getRedirectUrl = (req) => {
    url = new URL(req.url)
    url.pathname += "/"
    return url.href
  }
  
  /**
   *  @Lifecycle Install
   *  Precache anything static to this version of your app.
   *  e.g. App Shell, 404, JS/CSS dependencies...
   *
   *  waitUntil() : installing ====> installed
   *  skipWaiting() : waiting(installed) ====> activating
   */
  self.addEventListener('install', e => {
    e.waitUntil(
      caches.open(PRECACHE).then(cache => {
        return cache.add('offline.html')
        .then(self.skipWaiting())
        .catch(err => console.log(err))
      })
    )
  });
  
  
  /**
   *  @Lifecycle Activate
   *  New one activated when old isnt being used.
   *
   *  waitUntil(): activating ====> activated
   */
  self.addEventListener('activate',  event => {
    console.log('service worker activated.')
    event.waitUntil(self.clients.claim());
  });
  
  
  /**
   *  @Functional Fetch
   *  All network requests are being intercepted here.
   * 
   *  void respondWith(Promise<Response> r);
   */
  self.addEventListener('fetch', event => {
    // logs for debugging
    console.log(`fetch ${event.request.url}`)
    //console.log(` - type: ${event.request.type}; destination: ${event.request.destination}`)
    //console.log(` - mode: ${event.request.mode}, accept: ${event.request.headers.get('accept')}`)
  
    // Skip some of cross-origin requests, like those for Google Analytics.
    if (HOSTNAME_WHITELIST.indexOf(new URL(event.request.url).hostname) > -1) {
      
      // Redirect in SW manually fixed github pages 404s on repo?blah 
      if(shouldRedirect(event.request)){
        event.respondWith(Response.redirect(getRedirectUrl(event.request)))
        return;
      }
  
      // Stale-while-revalidate 
      // similar to HTTP's stale-while-revalidate: https://www.mnot.net/blog/2007/12/12/stale
      // Upgrade from Jake's to Surma's: https://gist.github.com/surma/eb441223daaedf880801ad80006389f1
      const cached = caches.match(event.request);
      const fixedUrl = getFixedUrl(event.request);
      const fetched = fetch(fixedUrl, {cache: "no-store"});
      const fetchedCopy = fetched.then(resp => resp.clone());
  
      // Call respondWith() with whatever we get first.
      // If the fetch fails (e.g disconnected), wait for the cache.
      // If there’s nothing in cache, wait for the fetch. 
      // If neither yields a response, return offline pages.
      event.respondWith(
        Promise.race([fetched.catch(_ => cached), cached])
          .then(resp => resp || fetched)
          .catch(_ => caches.match('offline.html'))
      );
      
  
      // Update the cache with the version we fetched (only for ok status)
      event.waitUntil(
        Promise.all([fetchedCopy, caches.open(RUNTIME)])
          .then(([response, cache]) => response.ok && cache.put(event.request, response))
          .catch(_ => {/* eat any errors */})
      );
    }
  });
  