text: /**
   * Copyright 2018 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 HINT_TAGS = new Set(['dns-prefetch', 'preconnect', 'prefetch', 'preload', 'prerender']);
  
  /**
   * Removes duplicate browser resource hint link header directives.
   *
   * To avoid wasted requests for preloaded resources strip references to duplicate
   * items.
   */
  class PruneDuplicateResourceHints {
    transform(tree) {
      const preloaded = new Map();
      const html = tree.root.firstChildByTag('html');
      if (!html) {
        return;
      }
      const head = html.firstChildByTag('head');
      if (!head) {
        return;
      }
      const childNodes = [];
      for (let node = head.firstChild; node !== null; node = node.nextSibling) {
        if (this._notPruneableHintLink(node)) {
          childNodes.push(node);
        } else if (!this._alreadyLoaded(node, preloaded)) {
          this._markPreloaded(node, preloaded);
          childNodes.push(node);
        }
      }
      // replace the child node list
      head.childNodes = childNodes;
    }
  
    _notPruneableHintLink(node) {
      if (node.tagName !== 'link') {
        return true;
      }
      if (!node.attribs) {
        return true;
      }
      if (!node.attribs.rel) {
        return true;
      }
      if (!node.attribs.href) {
        return true;
      }
      if (node.attribs.rel === 'preload' && !node.attribs.as) {
        return true;
      }
      return !HINT_TAGS.has(node.attribs.rel);
    }
  
    _alreadyLoaded(link, preloaded) {
      const rel = link.attribs.rel;
      const href = link.attribs.href;
      if (!preloaded.has(href)) {
        return false;
      }
      const relations = preloaded.get(href);
      return relations.has(rel);
    }
  
    _markPreloaded(link, preloaded) {
      const rel = link.attribs.rel;
      const href = link.attribs.href;
      let relations = preloaded.get(href);
      if (!relations) {
        relations = new Set();
        preloaded.set(href, relations);
      }
      relations.add(rel);
    }
  }
  
  module.exports = new PruneDuplicateResourceHints();
  