/*eslint-env es6:false*/
/*
 * Copyright (c) 2010 Arc90 Inc
 *
 * 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.
 */

/*
 * This code is heavily based on Arc90's readability.js (1.7.1) script
 * available at: http://code.google.com/p/arc90labs-readability
 */

/**
 * Public constructor.
 * @param {Object}       uri     The URI descriptor object.
 * @param {HTMLDocument} doc     The document to parse.
 * @param {Object}       options The options object.
 */
function Readability(uri, doc, options) {
  options = options || {};

  this._uri = uri;
  this._doc = doc;
  this._biggestFrame = false;
  this._articleTitle = null;
  this._articleByline = null;
  this._articleDir = null;

  // Configureable options
  this._debug = !!options.debug;
  this._maxElemsToParse = options.maxElemsToParse || this.DEFAULT_MAX_ELEMS_TO_PARSE;
  this._nbTopCandidates = options.nbTopCandidates || this.DEFAULT_N_TOP_CANDIDATES;
  this._maxPages = options.maxPages || this.DEFAULT_MAX_PAGES;

  // Start with all flags set
  this._flags = this.FLAG_STRIP_UNLIKELYS |
                this.FLAG_WEIGHT_CLASSES |
                this.FLAG_CLEAN_CONDITIONALLY;

  // The list of pages we've parsed in this call of readability,
  // for autopaging. As a key store for easier searching.
  this._parsedPages = {};

  // A list of the ETag headers of pages we've parsed, in case they happen to match,
  // we'll know it's a duplicate.
  this._pageETags = {};

  // Make an AJAX request for each page and append it to the document.
  this._curPageNum = 1;

  var logEl;

  // Control whether log messages are sent to the console
  if (this._debug) {
    logEl = function(e) {
      var rv = e.nodeName + " ";
      if (e.nodeType == e.TEXT_NODE) {
        return rv + '("' + e.textContent + '")';
      }
      var classDesc = e.className && ("." + e.className.replace(/ /g, "."));
      var elDesc = "";
      if (e.id)
        elDesc = "(#" + e.id + classDesc + ")";
      else if (classDesc)
        elDesc = "(" + classDesc + ")";
      return rv + elDesc;
    };
    this.log = function () {
      if (typeof dump !== "undefined") {
        var msg = Array.prototype.map.call(arguments, function(x) {
          return (x && x.nodeName) ? logEl(x) : x;
        }).join(" ");
        dump("Reader: (Readability) " + msg + "\n");
      } else if (typeof console !== "undefined") {
        var args = ["Reader: (Readability) "].concat(arguments);
        console.log.apply(console, args);
      }
    };
  } else {
    this.log = function () {};
  }
}

Readability.prototype = {
  FLAG_STRIP_UNLIKELYS: 0x1,
  FLAG_WEIGHT_CLASSES: 0x2,
  FLAG_CLEAN_CONDITIONALLY: 0x4,

  // Max number of nodes supported by this parser. Default: 0 (no limit)
  DEFAULT_MAX_ELEMS_TO_PARSE: 0,

  // The number of top candidates to consider when analysing how
  // tight the competition is among candidates.
  DEFAULT_N_TOP_CANDIDATES: 5,

  // The maximum number of pages to loop through before we call
  // it quits and just show a link.
  DEFAULT_MAX_PAGES: 5,

  // Element tags to score by default.
  DEFAULT_TAGS_TO_SCORE: "section,h2,h3,h4,h5,h6,p,td,pre".toUpperCase().split(","),

  // All of the regular expressions in use within readability.
  // Defined up here so we don't instantiate them repeatedly in loops.
  REGEXPS: {
    unlikelyCandidates: /banner|breadcrumbs|combx|comment|community|cover-wrap|disqus|extra|foot|header|legends|menu|modal|related|remark|replies|rss|shoutbox|sidebar|skyscraper|social|sponsor|supplemental|ad-break|agegate|pagination|pager|popup|yom-remote/i,
    okMaybeItsACandidate: /and|article|body|column|main|shadow/i,
    positive: /article|body|content|entry|hentry|h-entry|main|page|pagination|post|text|blog|story/i,
    negative: /hidden|^hid$| hid$| hid |^hid |banner|combx|comment|com-|contact|foot|footer|footnote|masthead|media|meta|modal|outbrain|promo|related|scroll|share|shoutbox|sidebar|skyscraper|sponsor|shopping|tags|tool|widget/i,
    extraneous: /print|archive|comment|discuss|e[\-]?mail|share|reply|all|login|sign|single|utility/i,
    byline: /byline|author|dateline|writtenby|p-author/i,
    replaceFonts: /<(\/?)font[^>]*>/gi,
    normalize: /\s{2,}/g,
    videos: /\/\/(www\.)?(dailymotion|youtube|youtube-nocookie|player\.vimeo)\.com/i,
    nextLink: /(next|weiter|continue|>([^\|]|$)|»([^\|]|$))/i,
    prevLink: /(prev|earl|old|new|<|«)/i,
    whitespace: /^\s*$/,
    hasContent: /\S$/,
  },

  DIV_TO_P_ELEMS: [ "A", "BLOCKQUOTE", "DL", "DIV", "IMG", "OL", "P", "PRE", "TABLE", "UL", "SELECT" ],

  ALTER_TO_DIV_EXCEPTIONS: ["DIV", "ARTICLE", "SECTION", "P"],

  /**
   * Run any post-process modifications to article content as necessary.
   *
   * @param Element
   * @return void
  **/
  _postProcessContent: function(articleContent) {
    // Readability cannot open relative uris so we convert them to absolute uris.
    this._fixRelativeUris(articleContent);
  },

  /**
   * Iterates over a NodeList, calls `filterFn` for each node and removes node
   * if function returned `true`.
   *
   * If function is not passed, removes all the nodes in node list.
   *
   * @param NodeList nodeList The nodes to operate on
   * @param Function filterFn the function to use as a filter
   * @return void
   */
  _removeNodes: function(nodeList, filterFn) {
    for (var i = nodeList.length - 1; i >= 0; i--) {
      var node = nodeList[i];
      var parentNode = node.parentNode;
      if (parentNode) {
        if (!filterFn || filterFn.call(this, node, i, nodeList)) {
          parentNode.removeChild(node);
        }
      }
    }
  },

  /**
   * Iterates over a NodeList, and calls _setNodeTag for each node.
   *
   * @param NodeList nodeList The nodes to operate on
   * @param String newTagName the new tag name to use
   * @return void
   */
  _replaceNodeTags: function(nodeList, newTagName) {
    for (var i = nodeList.length - 1; i >= 0; i--) {
      var node = nodeList[i];
      this._setNodeTag(node, newTagName);
    }
  },

  /**
   * Iterate over a NodeList, which doesn't natively fully implement the Array
   * interface.
   *
   * For convenience, the current object context is applied to the provided
   * iterate function.
   *
   * @param  NodeList nodeList The NodeList.
   * @param  Function fn       The iterate function.
   * @return void
   */
  _forEachNode: function(nodeList, fn) {
    Array.prototype.forEach.call(nodeList, fn, this);
  },

  /**
   * Iterate over a NodeList, return true if any of the provided iterate
   * function calls returns true, false otherwise.
   *
   * For convenience, the current object context is applied to the
   * provided iterate function.
   *
   * @param  NodeList nodeList The NodeList.
   * @param  Function fn       The iterate function.
   * @return Boolean
   */
  _someNode: function(nodeList, fn) {
    return Array.prototype.some.call(nodeList, fn, this);
  },

  /**
   * Concat all nodelists passed as arguments.
   *
   * @return ...NodeList
   * @return Array
   */
  _concatNodeLists: function() {
    var slice = Array.prototype.slice;
    var args = slice.call(arguments);
    var nodeLists = args.map(function(list) {
      return slice.call(list);
    });
    return Array.prototype.concat.apply([], nodeLists);
  },

  _getAllNodesWithTag: function(node, tagNames) {
    if (node.querySelectorAll) {
      return node.querySelectorAll(tagNames.join(','));
    }
    return [].concat.apply([], tagNames.map(function(tag) {
      var collection = node.getElementsByTagName(tag);
      return Array.isArray(collection) ? collection : Array.from(collection);
    }));
  },

  /**
   * Converts each <a> and <img> uri in the given element to an absolute URI,
   * ignoring #ref URIs.
   *
   * @param Element
   * @return void
   */
  _fixRelativeUris: function(articleContent) {
    var scheme = this._uri.scheme;
    var prePath = this._uri.prePath;
    var pathBase = this._uri.pathBase;

    function toAbsoluteURI(uri) {
      // If this is already an absolute URI, return it.
      if (/^[a-zA-Z][a-zA-Z0-9\+\-\.]*:/.test(uri))
        return uri;

      // Scheme-rooted relative URI.
      if (uri.substr(0, 2) == "//")
        return scheme + "://" + uri.substr(2);

      // Prepath-rooted relative URI.
      if (uri[0] == "/")
        return prePath + uri;

      // Dotslash relative URI.
      if (uri.indexOf("./") === 0)
        return pathBase + uri.slice(2);

      // Ignore hash URIs:
      if (uri[0] == "#")
        return uri;

      // Standard relative URI; add entire path. pathBase already includes a
      // trailing "/".
      return pathBase + uri;
    }

    var links = articleContent.getElementsByTagName("a");
    this._forEachNode(links, function(link) {
      var href = link.getAttribute("href");
      if (href) {
        // Replace links with javascript: URIs with text content, since
        // they won't work after scripts have been removed from the page.
        if (href.indexOf("javascript:") === 0) {
          var text = this._doc.createTextNode(link.textContent);
          link.parentNode.replaceChild(text, link);
        } else {
          link.setAttribute("href", toAbsoluteURI(href));
        }
      }
    });

    var imgs = articleContent.getElementsByTagName("img");
    this._forEachNode(imgs, function(img) {
      var src = img.getAttribute("src");
      if (src) {
        img.setAttribute("src", toAbsoluteURI(src));
      }
    });
  },

  /**
   * Get the article title as an H1.
   *
   * @return void
   **/
  _getArticleTitle: function() {
    var doc = this._doc;
    var curTitle = "";
    var origTitle = "";

    try {
      curTitle = origTitle = doc.title;

      // If they had an element with id "title" in their HTML
      if (typeof curTitle !== "string")
        curTitle = origTitle = this._getInnerText(doc.getElementsByTagName('title')[0]);
    } catch (e) {/* ignore exceptions setting the title. */}

    if (curTitle.match(/ [\|\-] /)) {
      curTitle = origTitle.replace(/(.*)[\|\-] .*/gi, '$1');

      if (curTitle.split(' ').length < 3)
        curTitle = origTitle.replace(/[^\|\-]*[\|\-](.*)/gi, '$1');
    } else if (curTitle.indexOf(': ') !== -1) {
      // Check if we have an heading containing this exact string, so we
      // could assume it's the full title.
      var headings = this._concatNodeLists(
        doc.getElementsByTagName('h1'),
        doc.getElementsByTagName('h2')
      );
      var match = this._someNode(headings, function(heading) {
        return heading.textContent === curTitle;
      });

      // If we don't, let's extract the title out of the original title string.
      if (!match) {
        curTitle = origTitle.substring(origTitle.lastIndexOf(':') + 1);

        // If the title is now too short, try the first colon instead:
        if (curTitle.split(' ').length < 3)
          curTitle = origTitle.substring(origTitle.indexOf(':') + 1);
      }
    } else if (curTitle.length > 150 || curTitle.length < 15) {
      var hOnes = doc.getElementsByTagName('h1');

      if (hOnes.length === 1)
        curTitle = this._getInnerText(hOnes[0]);
    }

    curTitle = curTitle.trim();

    if (curTitle.split(' ').length <= 4)
      curTitle = origTitle;

    return curTitle;
  },

  /**
   * Prepare the HTML document for readability to scrape it.
   * This includes things like stripping javascript, CSS, and handling terrible markup.
   *
   * @return void
   **/
  _prepDocument: function() {
    var doc = this._doc;

    // Remove all style tags in head
    this._removeNodes(doc.getElementsByTagName("style"));

    if (doc.body) {
      this._replaceBrs(doc.body);
    }

    this._replaceNodeTags(doc.getElementsByTagName("font"), "SPAN");
  },

  /**
   * Finds the next element, starting from the given node, and ignoring
   * whitespace in between. If the given node is an element, the same node is
   * returned.
   */
  _nextElement: function (node) {
    var next = node;
    while (next
        && (next.nodeType != Node.ELEMENT_NODE)
        && this.REGEXPS.whitespace.test(next.textContent)) {
      next = next.nextSibling;
    }
    return next;
  },

  /**
   * Replaces 2 or more successive <br> elements with a single <p>.
   * Whitespace between <br> elements are ignored. For example:
   *   <div>foo<br>bar<br> <br><br>abc</div>
   * will become:
   *   <div>foo<br>bar<p>abc</p></div>
   */
  _replaceBrs: function (elem) {
    this._forEachNode(this._getAllNodesWithTag(elem, ["br"]), function(br) {
      var next = br.nextSibling;

      // Whether 2 or more <br> elements have been found and replaced with a
      // <p> block.
      var replaced = false;

      // If we find a <br> chain, remove the <br>s until we hit another element
      // or non-whitespace. This leaves behind the first <br> in the chain
      // (which will be replaced with a <p> later).
      while ((next = this._nextElement(next)) && (next.tagName == "BR")) {
        replaced = true;
        var brSibling = next.nextSibling;
        next.parentNode.removeChild(next);
        next = brSibling;
      }

      // If we removed a <br> chain, replace the remaining <br> with a <p>. Add
      // all sibling nodes as children of the <p> until we hit another <br>
      // chain.
      if (replaced) {
        var p = this._doc.createElement("p");
        br.parentNode.replaceChild(p, br);

        next = p.nextSibling;
        while (next) {
          // If we've hit another <br><br>, we're done adding children to this <p>.
          if (next.tagName == "BR") {
            var nextElem = this._nextElement(next);
            if (nextElem && nextElem.tagName == "BR")
              break;
          }

          // Otherwise, make this node a child of the new <p>.
          var sibling = next.nextSibling;
          p.appendChild(next);
          next = sibling;
        }
      }
    });
  },

  _setNodeTag: function (node, tag) {
    this.log("_setNodeTag", node, tag);
    if (node.__JSDOMParser__) {
      node.localName = tag.toLowerCase();
      node.tagName = tag.toUpperCase();
      return node;
    }

    var replacement = node.ownerDocument.createElement(tag);
    while (node.firstChild) {
      replacement.appendChild(node.firstChild);
    }
    node.parentNode.replaceChild(replacement, node);
    if (node.readability)
      replacement.readability = node.readability;

    for (var i = 0; i < node.attributes.length; i++) {
      replacement.setAttribute(node.attributes[i].name, node.attributes[i].value);
    }
    return replacement;
  },

  /**
   * Prepare the article node for display. Clean out any inline styles,
   * iframes, forms, strip extraneous <p> tags, etc.
   *
   * @param Element
   * @return void
   **/
  _prepArticle: function(articleContent) {
    this._cleanStyles(articleContent);

    // Check for data tables before we continue, to avoid removing items in
    // those tables, which will often be isolated even though they're
    // visually linked to other content-ful elements (text, images, etc.).
    this._markDataTables(articleContent);

    // Clean out junk from the article content
    this._cleanConditionally(articleContent, "form");
    this._cleanConditionally(articleContent, "fieldset");
    this._clean(articleContent, "object");
    this._clean(articleContent, "embed");
    this._clean(articleContent, "h1");
    this._clean(articleContent, "footer");

    // Clean out elements have "share" in their id/class combinations from final top candidates,
    // which means we don't remove the top candidates even they have "share".
    this._forEachNode(articleContent.children, function(topCandidate) {
      this._cleanMatchedNodes(topCandidate, /share/);
    });

    // If there is only one h2 and its text content substantially equals article title,
    // they are probably using it as a header and not a subheader,
    // so remove it since we already extract the title separately.
    var h2 = articleContent.getElementsByTagName('h2');
    if (h2.length === 1) {
      var lengthSimilarRate = (h2[0].textContent.length - this._articleTitle.length) / this._articleTitle.length;
      if (Math.abs(lengthSimilarRate) < 0.5 &&
          (lengthSimilarRate > 0 ? h2[0].textContent.includes(this._articleTitle) :
                                   this._articleTitle.includes(h2[0].textContent))) {
        this._clean(articleContent, "h2");
      }
    }

    this._clean(articleContent, "iframe");
    this._clean(articleContent, "input");
    this._clean(articleContent, "textarea");
    this._clean(articleContent, "select");
    this._clean(articleContent, "button");
    this._cleanHeaders(articleContent);

    // Do these last as the previous stuff may have removed junk
    // that will affect these
    this._cleanConditionally(articleContent, "table");
    this._cleanConditionally(articleContent, "ul");
    this._cleanConditionally(articleContent, "div");

    // Remove extra paragraphs
    this._removeNodes(articleContent.getElementsByTagName('p'), function (paragraph) {
      var imgCount = paragraph.getElementsByTagName('img').length;
      var embedCount = paragraph.getElementsByTagName('embed').length;
      var objectCount = paragraph.getElementsByTagName('object').length;
      // At this point, nasty iframes have been removed, only remain embedded video ones.
      var iframeCount = paragraph.getElementsByTagName('iframe').length;
      var totalCount = imgCount + embedCount + objectCount + iframeCount;

      return totalCount === 0 && !this._getInnerText(paragraph, false);
    });

    this._forEachNode(this._getAllNodesWithTag(articleContent, ["br"]), function(br) {
      var next = this._nextElement(br.nextSibling);
      if (next && next.tagName == "P")
        br.parentNode.removeChild(br);
    });
  },

  /**
   * Initialize a node with the readability object. Also checks the
   * className/id for special names to add to its score.
   *
   * @param Element
   * @return void
  **/
  _initializeNode: function(node) {
    node.readability = {"contentScore": 0};

    switch (node.tagName) {
      case 'DIV':
        node.readability.contentScore += 5;
        break;

      case 'PRE':
      case 'TD':
      case 'BLOCKQUOTE':
        node.readability.contentScore += 3;
        break;

      case 'ADDRESS':
      case 'OL':
      case 'UL':
      case 'DL':
      case 'DD':
      case 'DT':
      case 'LI':
      case 'FORM':
        node.readability.contentScore -= 3;
        break;

      case 'H1':
      case 'H2':
      case 'H3':
      case 'H4':
      case 'H5':
      case 'H6':
      case 'TH':
        node.readability.contentScore -= 5;
        break;
    }

    node.readability.contentScore += this._getClassWeight(node);
  },

  _removeAndGetNext: function(node) {
    var nextNode = this._getNextNode(node, true);
    node.parentNode.removeChild(node);
    return nextNode;
  },

  /**
   * Traverse the DOM from node to node, starting at the node passed in.
   * Pass true for the second parameter to indicate this node itself
   * (and its kids) are going away, and we want the next node over.
   *
   * Calling this in a loop will traverse the DOM depth-first.
   */
  _getNextNode: function(node, ignoreSelfAndKids) {
    // First check for kids if those aren't being ignored
    if (!ignoreSelfAndKids && node.firstElementChild) {
      return node.firstElementChild;
    }
    // Then for siblings...
    if (node.nextElementSibling) {
      return node.nextElementSibling;
    }
    // And finally, move up the parent chain *and* find a sibling
    // (because this is depth-first traversal, we will have already
    // seen the parent nodes themselves).
    do {
      node = node.parentNode;
    } while (node && !node.nextElementSibling);
    return node && node.nextElementSibling;
  },

  /**
   * Like _getNextNode, but for DOM implementations with no
   * firstElementChild/nextElementSibling functionality...
   */
  _getNextNodeNoElementProperties: function(node, ignoreSelfAndKids) {
    function nextSiblingEl(n) {
      do {
        n = n.nextSibling;
      } while (n && n.nodeType !== n.ELEMENT_NODE);
      return n;
    }
    // First check for kids if those aren't being ignored
    if (!ignoreSelfAndKids && node.children[0]) {
      return node.children[0];
    }
    // Then for siblings...
    var next = nextSiblingEl(node);
    if (next) {
      return next;
    }
    // And finally, move up the parent chain *and* find a sibling
    // (because this is depth-first traversal, we will have already
    // seen the parent nodes themselves).
    do {
      node = node.parentNode;
      if (node)
        next = nextSiblingEl(node);
    } while (node && !next);
    return node && next;
  },

  _checkByline: function(node, matchString) {
    if (this._articleByline) {
      return false;
    }

    if (node.getAttribute !== undefined) {
      var rel = node.getAttribute("rel");
    }

    if ((rel === "author" || this.REGEXPS.byline.test(matchString)) && this._isValidByline(node.textContent)) {
      this._articleByline = node.textContent.trim();
      return true;
    }

    return false;
  },

  _getNodeAncestors: function(node, maxDepth) {
    maxDepth = maxDepth || 0;
    var i = 0, ancestors = [];
    while (node.parentNode) {
      ancestors.push(node.parentNode);
      if (maxDepth && ++i === maxDepth)
        break;
      node = node.parentNode;
    }
    return ancestors;
  },

  /***
   * grabArticle - Using a variety of metrics (content score, classname, element types), find the content that is
   *         most likely to be the stuff a user wants to read. Then return it wrapped up in a div.
   *
   * @param page a document to run upon. Needs to be a full document, complete with body.
   * @return Element
  **/
  _grabArticle: function (page) {
    this.log("**** grabArticle ****");
    var doc = this._doc;
    var isPaging = (page !== null ? true: false);
    page = page ? page : this._doc.body;

    // We can't grab an article if we don't have a page!
    if (!page) {
      this.log("No body found in document. Abort.");
      return null;
    }

    var pageCacheHtml = page.innerHTML;

    while (true) {
      var stripUnlikelyCandidates = this._flagIsActive(this.FLAG_STRIP_UNLIKELYS);

      // First, node prepping. Trash nodes that look cruddy (like ones with the
      // class name "comment", etc), and turn divs into P tags where they have been
      // used inappropriately (as in, where they contain no other block level elements.)
      var elementsToScore = [];
      var node = this._doc.documentElement;

      while (node) {
        var matchString = node.className + " " + node.id;

        // Check to see if this node is a byline, and remove it if it is.
        if (this._checkByline(node, matchString)) {
          node = this._removeAndGetNext(node);
          continue;
        }

        // Remove unlikely candidates
        if (stripUnlikelyCandidates) {
          if (this.REGEXPS.unlikelyCandidates.test(matchString) &&
              !this.REGEXPS.okMaybeItsACandidate.test(matchString) &&
              node.tagName !== "BODY" &&
              node.tagName !== "A") {
            this.log("Removing unlikely candidate - " + matchString);
            node = this._removeAndGetNext(node);
            continue;
          }
        }

        // Remove empty DIV, SECTION, and HEADER nodes
        if ((node.tagName === "DIV" || node.tagName === "SECTION" || node.tagName === "HEADER" ||
             node.tagName === "H1" || node.tagName === "H2" || node.tagName === "H3" ||
             node.tagName === "H4" || node.tagName === "H5" || node.tagName === "H6") &&
            this._isEmptyElement(node)) {
          node = this._removeAndGetNext(node);
          continue;
        }

        if (this.DEFAULT_TAGS_TO_SCORE.indexOf(node.tagName) !== -1) {
          elementsToScore.push(node);
        }

        // Turn all divs that don't have children block level elements into p's
        if (node.tagName === "DIV") {
          // Sites like http://mobile.slate.com encloses each paragraph with a DIV
          // element. DIVs with only a P element inside and no text content can be
          // safely converted into plain P elements to avoid confusing the scoring
          // algorithm with DIVs with are, in practice, paragraphs.
          if (this._hasSinglePInsideElement(node)) {
            var newNode = node.children[0];
            node.parentNode.replaceChild(newNode, node);
            node = newNode;
          } else if (!this._hasChildBlockElement(node)) {
            node = this._setNodeTag(node, "P");
            elementsToScore.push(node);
          } else {
            // EXPERIMENTAL
            this._forEachNode(node.childNodes, function(childNode) {
              if (childNode.nodeType === Node.TEXT_NODE && childNode.textContent.trim().length > 0) {
                var p = doc.createElement('p');
                p.textContent = childNode.textContent;
                p.style.display = 'inline';
                p.className = 'readability-styled';
                node.replaceChild(p, childNode);
              }
            });
          }
        }
        node = this._getNextNode(node);
      }

      /**
       * Loop through all paragraphs, and assign a score to them based on how content-y they look.
       * Then add their score to their parent node.
       *
       * A score is determined by things like number of commas, class names, etc. Maybe eventually link density.
      **/
      var candidates = [];
      this._forEachNode(elementsToScore, function(elementToScore) {
        if (!elementToScore.parentNode || typeof(elementToScore.parentNode.tagName) === 'undefined')
          return;

        // If this paragraph is less than 25 characters, don't even count it.
        var innerText = this._getInnerText(elementToScore);
        if (innerText.length < 25)
          return;

        // Exclude nodes with no ancestor.
        var ancestors = this._getNodeAncestors(elementToScore, 3);
        if (ancestors.length === 0)
          return;

        var contentScore = 0;

        // Add a point for the paragraph itself as a base.
        contentScore += 1;

        // Add points for any commas within this paragraph.
        contentScore += innerText.split(',').length;

        // For every 100 characters in this paragraph, add another point. Up to 3 points.
        contentScore += Math.min(Math.floor(innerText.length / 100), 3);

        // Initialize and score ancestors.
        this._forEachNode(ancestors, function(ancestor, level) {
          if (!ancestor.tagName)
            return;

          if (typeof(ancestor.readability) === 'undefined') {
            this._initializeNode(ancestor);
            candidates.push(ancestor);
          }

          // Node score divider:
          // - parent:             1 (no division)
          // - grandparent:        2
          // - great grandparent+: ancestor level * 3
          if (level === 0)
            var scoreDivider = 1;
          else if (level === 1)
            scoreDivider = 2;
          else
            scoreDivider = level * 3;
          ancestor.readability.contentScore += contentScore / scoreDivider;
        });
      });

      // After we've calculated scores, loop through all of the possible
      // candidate nodes we found and find the one with the highest score.
      var topCandidates = [];
      for (var c = 0, cl = candidates.length; c < cl; c += 1) {
        var candidate = candidates[c];

        // Scale the final candidates score based on link density. Good content
        // should have a relatively small link density (5% or less) and be mostly
        // unaffected by this operation.
        var candidateScore = candidate.readability.contentScore * (1 - this._getLinkDensity(candidate));
        candidate.readability.contentScore = candidateScore;

        this.log('Candidate:', candidate, "with score " + candidateScore);

        for (var t = 0; t < this._nbTopCandidates; t++) {
          var aTopCandidate = topCandidates[t];

          if (!aTopCandidate || candidateScore > aTopCandidate.readability.contentScore) {
            topCandidates.splice(t, 0, candidate);
            if (topCandidates.length > this._nbTopCandidates)
              topCandidates.pop();
            break;
          }
        }
      }

      var topCandidate = topCandidates[0] || null;
      var neededToCreateTopCandidate = false;
      var parentOfTopCandidate;

      // If we still have no top candidate, just use the body as a last resort.
      // We also have to copy the body node so it is something we can modify.
      if (topCandidate === null || topCandidate.tagName === "BODY") {
        // Move all of the page's children into topCandidate
        topCandidate = doc.createElement("DIV");
        neededToCreateTopCandidate = true;
        // Move everything (not just elements, also text nodes etc.) into the container
        // so we even include text directly in the body:
        var kids = page.childNodes;
        while (kids.length) {
          this.log("Moving child out:", kids[0]);
          topCandidate.appendChild(kids[0]);
        }

        page.appendChild(topCandidate);

        this._initializeNode(topCandidate);
      } else if (topCandidate) {
        // Find a better top candidate node if it contains (at least three) nodes which belong to `topCandidates` array
        // and whose scores are quite closed with current `topCandidate` node.
        var alternativeCandidateAncestors = [];
        for (var i = 1; i < topCandidates.length; i++) {
          if (topCandidates[i].readability.contentScore / topCandidate.readability.contentScore >= 0.75) {
            alternativeCandidateAncestors.push(this._getNodeAncestors(topCandidates[i]));
          }
        }
        var MINIMUM_TOPCANDIDATES = 3;
        if (alternativeCandidateAncestors.length >= MINIMUM_TOPCANDIDATES) {
          parentOfTopCandidate = topCandidate.parentNode;
          while (parentOfTopCandidate.tagName !== "BODY") {
            var listsContainingThisAncestor = 0;
            for (var ancestorIndex = 0; ancestorIndex < alternativeCandidateAncestors.length && listsContainingThisAncestor < MINIMUM_TOPCANDIDATES; ancestorIndex++) {
              listsContainingThisAncestor += Number(alternativeCandidateAncestors[ancestorIndex].includes(parentOfTopCandidate));
            }
            if (listsContainingThisAncestor >= MINIMUM_TOPCANDIDATES) {
              topCandidate = parentOfTopCandidate;
              break;
            }
            parentOfTopCandidate = parentOfTopCandidate.parentNode;
          }
        }
        if (!topCandidate.readability) {
          this._initializeNode(topCandidate);
        }

        // Because of our bonus system, parents of candidates might have scores
        // themselves. They get half of the node. There won't be nodes with higher
        // scores than our topCandidate, but if we see the score going *up* in the first
        // few steps up the tree, that's a decent sign that there might be more content
        // lurking in other places that we want to unify in. The sibling stuff
        // below does some of that - but only if we've looked high enough up the DOM
        // tree.
        parentOfTopCandidate = topCandidate.parentNode;
        var lastScore = topCandidate.readability.contentScore;
        // The scores shouldn't get too low.
        var scoreThreshold = lastScore / 3;
        while (parentOfTopCandidate.tagName !== "BODY") {
          if (!parentOfTopCandidate.readability) {
            parentOfTopCandidate = parentOfTopCandidate.parentNode;
            continue;
          }
          var parentScore = parentOfTopCandidate.readability.contentScore;
          if (parentScore < scoreThreshold)
            break;
          if (parentScore > lastScore) {
            // Alright! We found a better parent to use.
            topCandidate = parentOfTopCandidate;
            break;
          }
          lastScore = parentOfTopCandidate.readability.contentScore;
          parentOfTopCandidate = parentOfTopCandidate.parentNode;
        }

        // If the top candidate is the only child, use parent instead. This will help sibling
        // joining logic when adjacent content is actually located in parent's sibling node.
        parentOfTopCandidate = topCandidate.parentNode;
        while (parentOfTopCandidate.tagName != "BODY" && parentOfTopCandidate.children.length == 1) {
          topCandidate = parentOfTopCandidate;
          parentOfTopCandidate = topCandidate.parentNode;
        }
        if (!topCandidate.readability) {
          this._initializeNode(topCandidate);
        }
      }

      // Now that we have the top candidate, look through its siblings for content
      // that might also be related. Things like preambles, content split by ads
      // that we removed, etc.
      var articleContent = doc.createElement("DIV");
      if (isPaging)
        articleContent.id = "readability-content";

      var siblingScoreThreshold = Math.max(10, topCandidate.readability.contentScore * 0.2);
      // Keep potential top candidate's parent node to try to get text direction of it later.
      parentOfTopCandidate = topCandidate.parentNode;
      var siblings = parentOfTopCandidate.children;

      for (var s = 0, sl = siblings.length; s < sl; s++) {
        var sibling = siblings[s];
        var append = false;

        this.log("Looking at sibling node:", sibling, sibling.readability ? ("with score " + sibling.readability.contentScore) : '');
        this.log("Sibling has score", sibling.readability ? sibling.readability.contentScore : 'Unknown');

        if (sibling === topCandidate) {
          append = true;
        } else {
          var contentBonus = 0;

          // Give a bonus if sibling nodes and top candidates have the example same classname
          if (sibling.className === topCandidate.className && topCandidate.className !== "")
            contentBonus += topCandidate.readability.contentScore * 0.2;

          if (sibling.readability &&
              ((sibling.readability.contentScore + contentBonus) >= siblingScoreThreshold)) {
            append = true;
          } else if (sibling.nodeName === "P") {
            var linkDensity = this._getLinkDensity(sibling);
            var nodeContent = this._getInnerText(sibling);
            var nodeLength = nodeContent.length;

            if (nodeLength > 80 && linkDensity < 0.25) {
              append = true;
            } else if (nodeLength < 80 && nodeLength > 0 && linkDensity === 0 &&
                       nodeContent.search(/\.( |$)/) !== -1) {
              append = true;
            }
          }
        }

        if (append) {
          this.log("Appending node:", sibling);

          if (this.ALTER_TO_DIV_EXCEPTIONS.indexOf(sibling.nodeName) === -1) {
            // We have a node that isn't a common block level element, like a form or td tag.
            // Turn it into a div so it doesn't get filtered out later by accident.
            this.log("Altering sibling:", sibling, 'to div.');

            sibling = this._setNodeTag(sibling, "DIV");
          }

          articleContent.appendChild(sibling);
          // siblings is a reference to the children array, and
          // sibling is removed from the array when we call appendChild().
          // As a result, we must revisit this index since the nodes
          // have been shifted.
          s -= 1;
          sl -= 1;
        }
      }

      if (this._debug)
        this.log("Article content pre-prep: " + articleContent.innerHTML);
      // So we have all of the content that we need. Now we clean it up for presentation.
      this._prepArticle(articleContent);
      if (this._debug)
        this.log("Article content post-prep: " + articleContent.innerHTML);

      if (this._curPageNum === 1) {
        if (neededToCreateTopCandidate) {
          // We already created a fake div thing, and there wouldn't have been any siblings left
          // for the previous loop, so there's no point trying to create a new div, and then
          // move all the children over. Just assign IDs and class names here. No need to append
          // because that already happened anyway.
          topCandidate.id = "readability-page-1";
          topCandidate.className = "page";
        } else {
          var div = doc.createElement("DIV");
          div.id = "readability-page-1";
          div.className = "page";
          var children = articleContent.childNodes;
          while (children.length) {
            div.appendChild(children[0]);
          }
          articleContent.appendChild(div);
        }
      }

      if (this._debug)
        this.log("Article content after paging: " + articleContent.innerHTML);

      // Now that we've gone through the full algorithm, check to see if
      // we got any meaningful content. If we didn't, we may need to re-run
      // grabArticle with different flags set. This gives us a higher likelihood of
      // finding the content, and the sieve approach gives us a higher likelihood of
      // finding the -right- content.
      if (this._getInnerText(articleContent, true).length < 500) {
        page.innerHTML = pageCacheHtml;

        if (this._flagIsActive(this.FLAG_STRIP_UNLIKELYS)) {
          this._removeFlag(this.FLAG_STRIP_UNLIKELYS);
        } else if (this._flagIsActive(this.FLAG_WEIGHT_CLASSES)) {
          this._removeFlag(this.FLAG_WEIGHT_CLASSES);
        } else if (this._flagIsActive(this.FLAG_CLEAN_CONDITIONALLY)) {
          this._removeFlag(this.FLAG_CLEAN_CONDITIONALLY);
        } else {
          return null;
        }
      } else {
        // Find out text direction from ancestors of final top candidate.
        var ancestors = [parentOfTopCandidate, topCandidate].concat(this._getNodeAncestors(parentOfTopCandidate));
        this._someNode(ancestors, function(ancestor) {
          if (!ancestor.tagName)
            return false;
          var articleDir = ancestor.getAttribute("dir");
          if (articleDir) {
            this._articleDir = articleDir;
            return true;
          }
          return false;
        });
        return articleContent;
      }
    }
  },

  /**
   * Check whether the input string could be a byline.
   * This verifies that the input is a string, and that the length
   * is less than 100 chars.
   *
   * @param possibleByline {string} - a string to check whether its a byline.
   * @return Boolean - whether the input string is a byline.
   */
  _isValidByline: function(byline) {
    if (typeof byline == 'string' || byline instanceof String) {
      byline = byline.trim();
      return (byline.length > 0) && (byline.length < 100);
    }
    return false;
  },

  /**
   * Attempts to get excerpt and byline metadata for the article.
   *
   * @return Object with optional "excerpt" and "byline" properties
   */
  _getArticleMetadata: function() {
    var metadata = {};
    var values = {};
    var metaElements = this._doc.getElementsByTagName("meta");

    // Match "description", or Twitter's "twitter:description" (Cards)
    // in name attribute.
    var namePattern = /^\s*((twitter)\s*:\s*)?(description|title)\s*$/gi;

    // Match Facebook's Open Graph title & description properties.
    var propertyPattern = /^\s*og\s*:\s*(description|title)\s*$/gi;

    // Find description tags.
    this._forEachNode(metaElements, function(element) {
      var elementName = element.getAttribute("name");
      var elementProperty = element.getAttribute("property");

      if ([elementName, elementProperty].indexOf("author") !== -1) {
        metadata.byline = element.getAttribute("content");
        return;
      }

      var name = null;
      if (namePattern.test(elementName)) {
        name = elementName;
      } else if (propertyPattern.test(elementProperty)) {
        name = elementProperty;
      }

      if (name) {
        var content = element.getAttribute("content");
        if (content) {
          // Convert to lowercase and remove any whitespace
          // so we can match below.
          name = name.toLowerCase().replace(/\s/g, '');
          values[name] = content.trim();
        }
      }
    });

    if ("description" in values) {
      metadata.excerpt = values["description"];
    } else if ("og:description" in values) {
      // Use facebook open graph description.
      metadata.excerpt = values["og:description"];
    } else if ("twitter:description" in values) {
      // Use twitter cards description.
      metadata.excerpt = values["twitter:description"];
    }

    metadata.title = this._getArticleTitle();
    if (!metadata.title) {
      if ("og:title" in values) {
        // Use facebook open graph title.
        metadata.title = values["og:title"];
      } else if ("twitter:title" in values) {
        // Use twitter cards title.
        metadata.title = values["twitter:title"];
      }
    }

    return metadata;
  },

  /**
   * Removes script tags from the document.
   *
   * @param Element
  **/
  _removeScripts: function(doc) {
    this._removeNodes(doc.getElementsByTagName('script'), function(scriptNode) {
      scriptNode.nodeValue = "";
      scriptNode.removeAttribute('src');
      return true;
    });
    this._removeNodes(doc.getElementsByTagName('noscript'));
  },

  /**
   * Check if this node has only whitespace and a single P element
   * Returns false if the DIV node contains non-empty text nodes
   * or if it contains no P or more than 1 element.
   *
   * @param Element
  **/
  _hasSinglePInsideElement: function(element) {
    // There should be exactly 1 element child which is a P:
    if (element.children.length != 1 || element.children[0].tagName !== "P") {
      return false;
    }

    // And there should be no text nodes with real content
    return !this._someNode(element.childNodes, function(node) {
      return node.nodeType === Node.TEXT_NODE &&
             this.REGEXPS.hasContent.test(node.textContent);
    });
  },

  _isEmptyElement: function(node) {
    return node.nodeType === Node.ELEMENT_NODE &&
      node.children.length == 0 &&
      node.textContent.trim().length == 0;
  },

  /**
   * Determine whether element has any children block level elements.
   *
   * @param Element
   */
  _hasChildBlockElement: function (element) {
    return this._someNode(element.childNodes, function(node) {
      return this.DIV_TO_P_ELEMS.indexOf(node.tagName) !== -1 ||
             this._hasChildBlockElement(node);
    });
  },

  /**
   * Get the inner text of a node - cross browser compatibly.
   * This also strips out any excess whitespace to be found.
   *
   * @param Element
   * @param Boolean normalizeSpaces (default: true)
   * @return string
  **/
  _getInnerText: function(e, normalizeSpaces) {
    normalizeSpaces = (typeof normalizeSpaces === 'undefined') ? true : normalizeSpaces;
    var textContent = e.textContent.trim();

    if (normalizeSpaces) {
      return textContent.replace(this.REGEXPS.normalize, " ");
    }
    return textContent;
  },

  /**
   * Get the number of times a string s appears in the node e.
   *
   * @param Element
   * @param string - what to split on. Default is ","
   * @return number (integer)
  **/
  _getCharCount: function(e, s) {
    s = s || ",";
    return this._getInnerText(e).split(s).length - 1;
  },

  /**
   * Remove the style attribute on every e and under.
   * TODO: Test if getElementsByTagName(*) is faster.
   *
   * @param Element
   * @return void
  **/
  _cleanStyles: function(e) {
    e = e || this._doc;
    if (!e)
      return;
    var cur = e.firstChild;

    // Remove any root styles, if we're able.
    if (typeof e.removeAttribute === 'function' && e.className !== 'readability-styled')
      e.removeAttribute('style');

    // Go until there are no more child nodes
    while (cur !== null) {
      if (cur.nodeType === cur.ELEMENT_NODE) {
        // Remove style attribute(s) :
        if (cur.className !== "readability-styled")
          cur.removeAttribute("style");

        this._cleanStyles(cur);
      }

      cur = cur.nextSibling;
    }
  },

  /**
   * Get the density of links as a percentage of the content
   * This is the amount of text that is inside a link divided by the total text in the node.
   *
   * @param Element
   * @return number (float)
  **/
  _getLinkDensity: function(element) {
    var textLength = this._getInnerText(element).length;
    if (textLength === 0)
      return 0;

    var linkLength = 0;

    // XXX implement _reduceNodeList?
    this._forEachNode(element.getElementsByTagName("a"), function(linkNode) {
      linkLength += this._getInnerText(linkNode).length;
    });

    return linkLength / textLength;
  },

  /**
   * Find a cleaned up version of the current URL, to use for comparing links for possible next-pageyness.
   *
   * @author Dan Lacy
   * @return string the base url
  **/
  _findBaseUrl: function() {
    var uri = this._uri;
    var noUrlParams = uri.path.split("?")[0];
    var urlSlashes = noUrlParams.split("/").reverse();
    var cleanedSegments = [];
    var possibleType = "";

    for (var i = 0, slashLen = urlSlashes.length; i < slashLen; i += 1) {
      var segment = urlSlashes[i];

      // Split off and save anything that looks like a file type.
      if (segment.indexOf(".") !== -1) {
        possibleType = segment.split(".")[1];

        // If the type isn't alpha-only, it's probably not actually a file extension.
        if (!possibleType.match(/[^a-zA-Z]/))
          segment = segment.split(".")[0];
      }

      // If our first or second segment has anything looking like a page number, remove it.
      if (segment.match(/((_|-)?p[a-z]*|(_|-))[0-9]{1,2}$/i) && ((i === 1) || (i === 0)))
        segment = segment.replace(/((_|-)?p[a-z]*|(_|-))[0-9]{1,2}$/i, "");

      var del = false;

      // If this is purely a number, and it's the first or second segment,
      // it's probably a page number. Remove it.
      if (i < 2 && segment.match(/^\d{1,2}$/))
        del = true;

      // If this is the first segment and it's just "index", remove it.
      if (i === 0 && segment.toLowerCase() === "index")
        del = true;

      // If our first or second segment is smaller than 3 characters,
      // and the first segment was purely alphas, remove it.
      if (i < 2 && segment.length < 3 && !urlSlashes[0].match(/[a-z]/i))
        del = true;

      // If it's not marked for deletion, push it to cleanedSegments.
      if (!del)
        cleanedSegments.push(segment);
    }

    // This is our final, cleaned, base article URL.
    return uri.scheme + "://" + uri.host + cleanedSegments.reverse().join("/");
  },

  /**
   * Look for any paging links that may occur within the document.
   *
   * @param body
   * @return object (array)
  **/
  _findNextPageLink: function(elem) {
    var uri = this._uri;
    var possiblePages = {};
    var allLinks = elem.getElementsByTagName('a');
    var articleBaseUrl = this._findBaseUrl();

    // Loop through all links, looking for hints that they may be next-page links.
    // Things like having "page" in their textContent, className or id, or being a child
    // of a node with a page-y className or id.
    //
    // Also possible: levenshtein distance? longest common subsequence?
    //
    // After we do that, assign each page a score, and
    for (var i = 0, il = allLinks.length; i < il; i += 1) {
      var link = allLinks[i];
      var linkHref = allLinks[i].href.replace(/#.*$/, '').replace(/\/$/, '');

      // If we've already seen this page, ignore it.
      if (linkHref === "" ||
        linkHref === articleBaseUrl ||
        linkHref === uri.spec ||
        linkHref in this._parsedPages) {
        continue;
      }

      // If it's on a different domain, skip it.
      if (uri.host !== linkHref.split(/\/+/g)[1])
        continue;

      var linkText = this._getInnerText(link);

      // If the linkText looks like it's not the next page, skip it.
      if (linkText.match(this.REGEXPS.extraneous) || linkText.length > 25)
        continue;

      // If the leftovers of the URL after removing the base URL don't contain
      // any digits, it's certainly not a next page link.
      var linkHrefLeftover = linkHref.replace(articleBaseUrl, '');
      if (!linkHrefLeftover.match(/\d/))
        continue;

      if (!(linkHref in possiblePages)) {
        possiblePages[linkHref] = {"score": 0, "linkText": linkText, "href": linkHref};
      } else {
        possiblePages[linkHref].linkText += ' | ' + linkText;
      }

      var linkObj = possiblePages[linkHref];

      // If the articleBaseUrl isn't part of this URL, penalize this link. It could
      // still be the link, but the odds are lower.
      // Example: http://www.actionscript.org/resources/articles/745/1/JavaScript-and-VBScript-Injection-in-ActionScript-3/Page1.html
      if (linkHref.indexOf(articleBaseUrl) !== 0)
        linkObj.score -= 25;

      var linkData = linkText + ' ' + link.className + ' ' + link.id;
      if (linkData.match(this.REGEXPS.nextLink))
        linkObj.score += 50;

      if (linkData.match(/pag(e|ing|inat)/i))
        linkObj.score += 25;

      if (linkData.match(/(first|last)/i)) {
        // -65 is enough to negate any bonuses gotten from a > or » in the text,
        // If we already matched on "next", last is probably fine.
        // If we didn't, then it's bad. Penalize.
        if (!linkObj.linkText.match(this.REGEXPS.nextLink))
          linkObj.score -= 65;
      }

      if (linkData.match(this.REGEXPS.negative) || linkData.match(this.REGEXPS.extraneous))
        linkObj.score -= 50;

      if (linkData.match(this.REGEXPS.prevLink))
        linkObj.score -= 200;

      // If a parentNode contains page or paging or paginat
      var parentNode = link.parentNode;
      var positiveNodeMatch = false;
      var negativeNodeMatch = false;

      while (parentNode) {
        var parentNodeClassAndId = parentNode.className + ' ' + parentNode.id;

        if (!positiveNodeMatch && parentNodeClassAndId && parentNodeClassAndId.match(/pag(e|ing|inat)/i)) {
          positiveNodeMatch = true;
          linkObj.score += 25;
        }

        if (!negativeNodeMatch && parentNodeClassAndId && parentNodeClassAndId.match(this.REGEXPS.negative)) {
          // If this is just something like "footer", give it a negative.
          // If it's something like "body-and-footer", leave it be.
          if (!parentNodeClassAndId.match(this.REGEXPS.positive)) {
            linkObj.score -= 25;
            negativeNodeMatch = true;
          }
        }

        parentNode = parentNode.parentNode;
      }

      // If the URL looks like it has paging in it, add to the score.
      // Things like /page/2/, /pagenum/2, ?p=3, ?page=11, ?pagination=34
      if (linkHref.match(/p(a|g|ag)?(e|ing|ination)?(=|\/)[0-9]{1,2}/i) || linkHref.match(/(page|paging)/i))
        linkObj.score += 25;

      // If the URL contains negative values, give a slight decrease.
      if (linkHref.match(this.REGEXPS.extraneous))
        linkObj.score -= 15;

      /**
       * Minor punishment to anything that doesn't match our current URL.
       * NOTE: I'm finding this to cause more harm than good where something is exactly 50 points.
       *     Dan, can you show me a counterexample where this is necessary?
       * if (linkHref.indexOf(window.location.href) !== 0) {
       *  linkObj.score -= 1;
       * }
      **/

      // If the link text can be parsed as a number, give it a minor bonus, with a slight
      // bias towards lower numbered pages. This is so that pages that might not have 'next'
      // in their text can still get scored, and sorted properly by score.
      var linkTextAsNumber = parseInt(linkText, 10);
      if (linkTextAsNumber) {
        // Punish 1 since we're either already there, or it's probably
        // before what we want anyways.
        if (linkTextAsNumber === 1) {
          linkObj.score -= 10;
        } else {
          linkObj.score += Math.max(0, 10 - linkTextAsNumber);
        }
      }
    }

    // Loop thrugh all of our possible pages from above and find our top
    // candidate for the next page URL. Require at least a score of 50, which
    // is a relatively high confidence that this page is the next link.
    var topPage = null;
    for (var page in possiblePages) {
      if (possiblePages.hasOwnProperty(page)) {
        if (possiblePages[page].score >= 50 &&
          (!topPage || topPage.score < possiblePages[page].score))
          topPage = possiblePages[page];
      }
    }

    var nextHref = null;
    if (topPage) {
      nextHref = topPage.href.replace(/\/$/, '');

      this.log('NEXT PAGE IS ' + nextHref);
      this._parsedPages[nextHref] = true;
    }
    return nextHref;
  },

  _successfulRequest: function(request) {
    return (request.status >= 200 && request.status < 300) ||
        request.status === 304 ||
         (request.status === 0 && request.responseText);
  },

  _ajax: function(url, options) {
    var request = new XMLHttpRequest();

    function respondToReadyState(readyState) {
      if (request.readyState === 4) {
        if (this._successfulRequest(request)) {
          if (options.success)
            options.success(request);
        } else if (options.error) {
          options.error(request);
        }
      }
    }

    if (typeof options === 'undefined')
      options = {};

    request.onreadystatechange = respondToReadyState;

    request.open('get', url, true);
    request.setRequestHeader('Accept', 'text/html');

    try {
      request.send(options.postBody);
    } catch (e) {
      if (options.error)
        options.error();
    }

    return request;
  },

  _appendNextPage: function(nextPageLink) {
    var doc = this._doc;
    this._curPageNum += 1;

    var articlePage = doc.createElement("DIV");
    articlePage.id = 'readability-page-' + this._curPageNum;
    articlePage.className = 'page';
    articlePage.innerHTML = '<p class="page-separator" title="Page ' + this._curPageNum + '">&sect;</p>';

    doc.getElementById("readability-content").appendChild(articlePage);

    if (this._curPageNum > this._maxPages) {
      var nextPageMarkup = "<div style='text-align: center'><a href='" + nextPageLink + "'>View Next Page</a></div>";
      articlePage.innerHTML = articlePage.innerHTML + nextPageMarkup;
      return;
    }

    // Now that we've built the article page DOM element, get the page content
    // asynchronously and load the cleaned content into the div we created for it.
    (function(pageUrl, thisPage) {
      this._ajax(pageUrl, {
        success: function(r) {

          // First, check to see if we have a matching ETag in headers - if we do, this is a duplicate page.
          var eTag = r.getResponseHeader('ETag');
          if (eTag) {
            if (eTag in this._pageETags) {
              this.log("Exact duplicate page found via ETag. Aborting.");
              articlePage.style.display = 'none';
              return;
            }
            this._pageETags[eTag] = 1;
          }

          // TODO: this ends up doubling up page numbers on NYTimes articles. Need to generically parse those away.
          var page = doc.createElement("DIV");

          // Do some preprocessing to our HTML to make it ready for appending.
          // - Remove any script tags. Swap and reswap newlines with a unicode
          //   character because multiline regex doesn't work in javascript.
          // - Turn any noscript tags into divs so that we can parse them. This
          //   allows us to find any next page links hidden via javascript.
          // - Turn all double br's into p's - was handled by prepDocument in the original view.
          //   Maybe in the future abstract out prepDocument to work for both the original document
          //   and AJAX-added pages.
          var responseHtml = r.responseText.replace(/\n/g, '\uffff').replace(/<script.*?>.*?<\/script>/gi, '');
          responseHtml = responseHtml.replace(/\n/g, '\uffff').replace(/<script.*?>.*?<\/script>/gi, '');
          responseHtml = responseHtml.replace(/\uffff/g, '\n').replace(/<(\/?)noscript/gi, '<$1div');
          responseHtml = responseHtml.replace(this.REGEXPS.replaceFonts, '<$1span>');

          page.innerHTML = responseHtml;
          this._replaceBrs(page);

          // Reset all flags for the next page, as they will search through it and
          // disable as necessary at the end of grabArticle.
          this._flags = 0x1 | 0x2 | 0x4;

          var secondNextPageLink = this._findNextPageLink(page);

          // NOTE: if we end up supporting _appendNextPage(), we'll need to
          // change this call to be async
          var content = this._grabArticle(page);

          if (!content) {
            this.log("No content found in page to append. Aborting.");
            return;
          }

          // Anti-duplicate mechanism. Essentially, get the first paragraph of our new page.
          // Compare it against all of the the previous document's we've gotten. If the previous
          // document contains exactly the innerHTML of this first paragraph, it's probably a duplicate.
          var firstP = content.getElementsByTagName("P").length ? content.getElementsByTagName("P")[0] : null;
          if (firstP && firstP.innerHTML.length > 100) {
            for (var i = 1; i <= this._curPageNum; i += 1) {
              var rPage = doc.getElementById('readability-page-' + i);
              if (rPage && rPage.innerHTML.indexOf(firstP.innerHTML) !== -1) {
                this.log('Duplicate of page ' + i + ' - skipping.');
                articlePage.style.display = 'none';
                this._parsedPages[pageUrl] = true;
                return;
              }
            }
          }

          this._removeScripts(content);

          thisPage.innerHTML = thisPage.innerHTML + content.innerHTML;

          // After the page has rendered, post process the content. This delay is necessary because,
          // in webkit at least, offsetWidth is not set in time to determine image width. We have to
          // wait a little bit for reflow to finish before we can fix floating images.
          setTimeout((function() {
            this._postProcessContent(thisPage);
          }).bind(this), 500);


          if (secondNextPageLink)
            this._appendNextPage(secondNextPageLink);
        }
      });
    }).bind(this)(nextPageLink, articlePage);
  },

  /**
   * Get an elements class/id weight. Uses regular expressions to tell if this
   * element looks good or bad.
   *
   * @param Element
   * @return number (Integer)
  **/
  _getClassWeight: function(e) {
    if (!this._flagIsActive(this.FLAG_WEIGHT_CLASSES))
      return 0;

    var weight = 0;

    // Look for a special classname
    if (typeof(e.className) === 'string' && e.className !== '') {
      if (this.REGEXPS.negative.test(e.className))
        weight -= 25;

      if (this.REGEXPS.positive.test(e.className))
        weight += 25;
    }

    // Look for a special ID
    if (typeof(e.id) === 'string' && e.id !== '') {
      if (this.REGEXPS.negative.test(e.id))
        weight -= 25;

      if (this.REGEXPS.positive.test(e.id))
        weight += 25;
    }

    return weight;
  },

  /**
   * Clean a node of all elements of type "tag".
   * (Unless it's a youtube/vimeo video. People love movies.)
   *
   * @param Element
   * @param string tag to clean
   * @return void
   **/
  _clean: function(e, tag) {
    var isEmbed = ["object", "embed", "iframe"].indexOf(tag) !== -1;

    this._removeNodes(e.getElementsByTagName(tag), function(element) {
      // Allow youtube and vimeo videos through as people usually want to see those.
      if (isEmbed) {
        var attributeValues = [].map.call(element.attributes, function(attr) {
          return attr.value;
        }).join("|");

        // First, check the elements attributes to see if any of them contain youtube or vimeo
        if (this.REGEXPS.videos.test(attributeValues))
          return false;

        // Then check the elements inside this element for the same.
        if (this.REGEXPS.videos.test(element.innerHTML))
          return false;
      }

      return true;
    });
  },

  /**
   * Check if a given node has one of its ancestor tag name matching the
   * provided one.
   * @param  HTMLElement node
   * @param  String      tagName
   * @param  Number      maxDepth
   * @param  Function    filterFn a filter to invoke to determine whether this node 'counts'
   * @return Boolean
   */
  _hasAncestorTag: function(node, tagName, maxDepth, filterFn) {
    maxDepth = maxDepth || 3;
    tagName = tagName.toUpperCase();
    var depth = 0;
    while (node.parentNode) {
      if (maxDepth > 0 && depth > maxDepth)
        return false;
      if (node.parentNode.tagName === tagName && (!filterFn || filterFn(node.parentNode)))
        return true;
      node = node.parentNode;
      depth++;
    }
    return false;
  },

  /**
   * Return an object indicating how many rows and columns this table has.
   */
  _getRowAndColumnCount: function(table) {
    var rows = 0;
    var columns = 0;
    var trs = table.getElementsByTagName("tr");
    for (var i = 0; i < trs.length; i++) {
      var rowspan = trs[i].getAttribute("rowspan") || 0;
      if (rowspan) {
        rowspan = parseInt(rowspan, 10);
      }
      rows += (rowspan || 1);

      // Now look for column-related info
      var columnsInThisRow = 0;
      var cells = trs[i].getElementsByTagName("td");
      for (var j = 0; j < cells.length; j++) {
        var colspan = cells[j].getAttribute("colspan") || 0;
        if (colspan) {
          colspan = parseInt(colspan, 10);
        }
        columnsInThisRow += (colspan || 1);
      }
      columns = Math.max(columns, columnsInThisRow);
    }
    return {rows: rows, columns: columns};
  },

  /**
   * Look for 'data' (as opposed to 'layout') tables, for which we use
   * similar checks as
   * https://dxr.mozilla.org/mozilla-central/rev/71224049c0b52ab190564d3ea0eab089a159a4cf/accessible/html/HTMLTableAccessible.cpp#920
   */
  _markDataTables: function(root) {
    var tables = root.getElementsByTagName("table");
    for (var i = 0; i < tables.length; i++) {
      var table = tables[i];
      var role = table.getAttribute("role");
      if (role == "presentation") {
        table._readabilityDataTable = false;
        continue;
      }
      var datatable = table.getAttribute("datatable");
      if (datatable == "0") {
        table._readabilityDataTable = false;
        continue;
      }
      var summary = table.getAttribute("summary");
      if (summary) {
        table._readabilityDataTable = true;
        continue;
      }

      var caption = table.getElementsByTagName("caption")[0];
      if (caption && caption.childNodes.length > 0) {
        table._readabilityDataTable = true;
        continue;
      }

      // If the table has a descendant with any of these tags, consider a data table:
      var dataTableDescendants = ["col", "colgroup", "tfoot", "thead", "th"];
      var descendantExists = function(tag) {
        return !!table.getElementsByTagName(tag)[0];
      };
      if (dataTableDescendants.some(descendantExists)) {
        this.log("Data table because found data-y descendant");
        table._readabilityDataTable = true;
        continue;
      }

      // Nested tables indicate a layout table:
      if (table.getElementsByTagName("table")[0]) {
        table._readabilityDataTable = false;
        continue;
      }

      var sizeInfo = this._getRowAndColumnCount(table);
      if (sizeInfo.rows >= 10 || sizeInfo.columns > 4) {
        table._readabilityDataTable = true;
        continue;
      }
      // Now just go by size entirely:
      table._readabilityDataTable = sizeInfo.rows * sizeInfo.columns > 10;
    }
  },

  /**
   * Clean an element of all tags of type "tag" if they look fishy.
   * "Fishy" is an algorithm based on content length, classnames, link density, number of images & embeds, etc.
   *
   * @return void
   **/
  _cleanConditionally: function(e, tag) {
    if (!this._flagIsActive(this.FLAG_CLEAN_CONDITIONALLY))
      return;

    var isList = tag === "ul" || tag === "ol";

    // Gather counts for other typical elements embedded within.
    // Traverse backwards so we can remove nodes at the same time
    // without effecting the traversal.
    //
    // TODO: Consider taking into account original contentScore here.
    this._removeNodes(e.getElementsByTagName(tag), function(node) {
      // First check if we're in a data table, in which case don't remove us.
      var isDataTable = function(t) {
        return t._readabilityDataTable;
      };

      if (this._hasAncestorTag(node, "table", -1, isDataTable)) {
        return false;
      }

      var weight = this._getClassWeight(node);
      var contentScore = 0;

      this.log("Cleaning Conditionally", node);

      if (weight + contentScore < 0) {
        return true;
      }

      if (this._getCharCount(node, ',') < 10) {
        // If there are not very many commas, and the number of
        // non-paragraph elements is more than paragraphs or other
        // ominous signs, remove the element.
        var p = node.getElementsByTagName("p").length;
        var img = node.getElementsByTagName("img").length;
        var li = node.getElementsByTagName("li").length - 100;
        var input = node.getElementsByTagName("input").length;

        var embedCount = 0;
        var embeds = node.getElementsByTagName("embed");
        for (var ei = 0, il = embeds.length; ei < il; ei += 1) {
          if (!this.REGEXPS.videos.test(embeds[ei].src))
            embedCount += 1;
        }

        var linkDensity = this._getLinkDensity(node);
        var contentLength = this._getInnerText(node).length;

        var haveToRemove =
          (img > 1 && p / img < 0.5 && !this._hasAncestorTag(node, "figure")) ||
          (!isList && li > p) ||
          (input > Math.floor(p/3)) ||
          (!isList && contentLength < 25 && (img === 0 || img > 2) && !this._hasAncestorTag(node, "figure")) ||
          (!isList && weight < 25 && linkDensity > 0.2) ||
          (weight >= 25 && linkDensity > 0.5) ||
          ((embedCount === 1 && contentLength < 75) || embedCount > 1);
        return haveToRemove;
      }
      return false;
    });
  },

  /**
   * Clean out elements whose id/class combinations match specific string.
   *
   * @param Element
   * @param RegExp match id/class combination.
   * @return void
   **/
  _cleanMatchedNodes: function(e, regex) {
    var endOfSearchMarkerNode = this._getNextNode(e, true);
    var next = this._getNextNode(e);
    while (next && next != endOfSearchMarkerNode) {
      if (regex.test(next.className + " " + next.id)) {
        next = this._removeAndGetNext(next);
      } else {
        next = this._getNextNode(next);
      }
    }
  },

  /**
   * Clean out spurious headers from an Element. Checks things like classnames and link density.
   *
   * @param Element
   * @return void
  **/
  _cleanHeaders: function(e) {
    for (var headerIndex = 1; headerIndex < 3; headerIndex += 1) {
      this._removeNodes(e.getElementsByTagName('h' + headerIndex), function (header) {
        return this._getClassWeight(header) < 0;
      });
    }
  },

  _flagIsActive: function(flag) {
    return (this._flags & flag) > 0;
  },

  _addFlag: function(flag) {
    this._flags = this._flags | flag;
  },

  _removeFlag: function(flag) {
    this._flags = this._flags & ~flag;
  },

  /**
   * Decides whether or not the document is reader-able without parsing the whole thing.
   *
   * @return boolean Whether or not we suspect parse() will suceeed at returning an article object.
   */
  isProbablyReaderable: function(helperIsVisible) {
    var nodes = this._getAllNodesWithTag(this._doc, ["p", "pre"]);

    // Get <div> nodes which have <br> node(s) and append them into the `nodes` variable.
    // Some articles' DOM structures might look like
    // <div>
    //   Sentences<br>
    //   <br>
    //   Sentences<br>
    // </div>
    var brNodes = this._getAllNodesWithTag(this._doc, ["div > br"]);
    if (brNodes.length) {
      var set = new Set();
      [].forEach.call(brNodes, function(node) {
        set.add(node.parentNode);
      });
      nodes = [].concat.apply(Array.from(set), nodes);
    }

    // FIXME we should have a fallback for helperIsVisible, but this is
    // problematic because of jsdom's elem.style handling - see
    // https://github.com/mozilla/readability/pull/186 for context.

    var score = 0;
    // This is a little cheeky, we use the accumulator 'score' to decide what to return from
    // this callback:
    return this._someNode(nodes, function(node) {
      if (helperIsVisible && !helperIsVisible(node))
        return false;
      var matchString = node.className + " " + node.id;

      if (this.REGEXPS.unlikelyCandidates.test(matchString) &&
          !this.REGEXPS.okMaybeItsACandidate.test(matchString)) {
        return false;
      }

      if (node.matches && node.matches("li p")) {
        return false;
      }

      var textContentLength = node.textContent.trim().length;
      if (textContentLength < 140) {
        return false;
      }

      score += Math.sqrt(textContentLength - 140);

      if (score > 20) {
        return true;
      }
      return false;
    });
  },

  /**
   * Runs readability.
   *
   * Workflow:
   *  1. Prep the document by removing script tags, css, etc.
   *  2. Build readability's DOM tree.
   *  3. Grab the article content from the current dom tree.
   *  4. Replace the current DOM tree with the new one.
   *  5. Read peacefully.
   *
   * @return void
   **/
  parse: function () {
    // Avoid parsing too large documents, as per configuration option
    if (this._maxElemsToParse > 0) {
      var numTags = this._doc.getElementsByTagName("*").length;
      if (numTags > this._maxElemsToParse) {
        throw new Error("Aborting parsing document; " + numTags + " elements found");
      }
    }

    if (typeof this._doc.documentElement.firstElementChild === "undefined") {
      this._getNextNode = this._getNextNodeNoElementProperties;
    }
    // Remove script tags from the document.
    this._removeScripts(this._doc);

    // FIXME: Disabled multi-page article support for now as it
    // needs more work on infrastructure.

    // Make sure this document is added to the list of parsed pages first,
    // so we don't double up on the first page.
    // this._parsedPages[uri.spec.replace(/\/$/, '')] = true;

    // Pull out any possible next page link first.
    // var nextPageLink = this._findNextPageLink(doc.body);

    this._prepDocument();

    var metadata = this._getArticleMetadata();
    this._articleTitle = metadata.title;

    var articleContent = this._grabArticle();
    if (!articleContent)
      return null;

    this.log("Grabbed: " + articleContent.innerHTML);

    this._postProcessContent(articleContent);

    // if (nextPageLink) {
    //   // Append any additional pages after a small timeout so that people
    //   // can start reading without having to wait for this to finish processing.
    //   setTimeout((function() {
    //     this._appendNextPage(nextPageLink);
    //   }).bind(this), 500);
    // }

    // If we haven't found an excerpt in the article's metadata, use the article's
    // first paragraph as the excerpt. This is used for displaying a preview of
    // the article's content.
    if (!metadata.excerpt) {
      var paragraphs = articleContent.getElementsByTagName("p");
      if (paragraphs.length > 0) {
        metadata.excerpt = paragraphs[0].textContent.trim();
      }
    }

    var textContent = articleContent.textContent;
    return {
      uri: this._uri,
      title: this._articleTitle,
      byline: metadata.byline || this._articleByline,
      dir: this._articleDir,
      content: articleContent.innerHTML,
      textContent: textContent,
      length: textContent.length,
      excerpt: metadata.excerpt,
    };
  }
};

if (typeof module === "object") {
  module.exports = Readability;
}