text: /**
   * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS-IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  'use strict';
  
  const {readFileSync} = require('fs');
  const url = require('url');
  const {join} = require('path');
  const yaml = require('js-yaml');
  const config = require('@lib/config.js');
  const {setHsts} = require('@lib/utils/cacheHelpers.js');
  const {HEALTH_CHECK_PATH} = require('@lib/routers/healthCheck.js');
  
  const WWW_PREFIX = 'www.';
  
  const REDIRECT_LINKS_DEFINITION = join(__dirname, '../../config/amp-dev-redirects.yaml');
  const redirectLinks = yaml.safeLoad(readFileSync(REDIRECT_LINKS_DEFINITION));
  
  const AVAILABLE_LOCALES = config.getAvailableLocales();
  const LANGUAGE_PATH_PATTERN = /^\/([a-z]{2}(_[a-z]{2})?)\//;
  
  /**
   *
   * @param req
   * @returns The redirect link or undefined if none was found.
   */
  function getRedirectLink(req) {
    let result = redirectLinks[req.path];
    if (!result) {
      const langMatch = req.path.match(LANGUAGE_PATH_PATTERN);
      if (langMatch && AVAILABLE_LOCALES.findIndex(
          (item) => langMatch[1] === item.toLowerCase()) >= 0) {
        const noLangPath = req.path.substr(langMatch[1].length + 1);
        result = redirectLinks[noLangPath];
        if (result) {
          result = '/' + langMatch[1] + result;
        }
      }
    }
    const queryString = url.parse(req.url).query;
    if (result && queryString) {
      result = result + '?' + queryString;
    }
    return result;
  }
  
  /**
   * Implements redirects:
   *
   * - http -> https
   * - www.amp.dev to amp.dev
   * - amp.dev/[shortcut] to deeplink
   */
  module.exports = (req, res, next) => {
    if (req.path === HEALTH_CHECK_PATH) {
      // it's critical that health checks don't redirect for GCE healthchecks to work correctly
      return next();
    }
  
    const redirectTarget = getRedirectLink(req);
    if (redirectTarget) {
      try {
        const targetUrl = new URL(redirectTarget, config.hosts.platform.base);
        res.redirect(targetUrl.toString());
        return;
      } catch (error) {
        console.log('Unable to redirect to ' + redirectTarget, error);
      }
    }
  
    // redirect www.amp.dev to amp.dev
    if (req.get('host').startsWith(WWW_PREFIX)) {
      res.redirect(301, `${req.protocol}://${req.host.substring(WWW_PREFIX.length)}${req.originalUrl}`);
      return;
    }
  
    // the following checks are only relevant for https
    if (config.hosts.platform.scheme !== 'https') {
      return next();
    }
  
    // redirect http to https
    setHsts(res);
    if (req.headers['x-forwarded-proto'] === 'https') {
      return next();
    }
    res.redirect('https://' + req.hostname + req.path);
  };
  