JS Document Treeification

Yet another Blog about information visualization. The JavaScript working in this page extends the script I wrote for generating table-of-contents, see my according Blog article.

The idea is to not have a separate table-of-contents but to have the document within the TOC, or, in other words, to have a document that is a TOC. This is possible only with expand-controls as used in an UI tree, else the document could not mimic a TOC. Have a look at the treeification example below (it was not only Daphne, there were also Philemon and Baucis :-).

You see just the top-level headings. You can choose which one you are interested in and expand it. When you decide to go into breadth first, you could expand another top-level heading. Or you go into detail and expand the first one further.

The contrary approach is to have the document initially fully expanded. You can skip chapters you are not interested in by collapsing them.

A further idea would be to have just the headers visible, not the chapter text. Wherever there is chapter text, an expand control would be there to make it visible. Haven't implemented this yet.

What is missing for sure are buttons to (1) fully expand and (2) fully collapse the document.

The remainder of this Blog will be about how this was implemented in JS.
And I will layout that as treeification, so please click on the left-side arrows to see text.

Use Old

What I had in mind was to reuse the module introduced in my Blog article about table-of-contents generation, and some folding logic introduced in my article about folding, to generate a "folded document".

Rewrite

The essence of writing is rewriting. This also applies to software!
So the first thing to do was rewrite the TOC-generator to have overridable functions that can be used to change its behaviour. In this case I introduced a new function customizeHeading(level, heading, titleElement) that gets called for every chapter heading found. Remember that the title element could be different from the heading element, e.g. the heading is the parent div, and the title is some nested h3 element. To the title element the expand control must be attached, and the heading element determines the content to be folded by the expand control.

Another adaption was to split the too big parameter list of tableOfContents() into parameters of statical and dynamical character. The staticals went up to the enclosing factory function, while the dynamicals stayed on tableOfContents(), which is now tableOfContents(topElement, headingTags, tocContainer). And I added a statical parameter initiallyFolded, which makes sense only for treeification, not for TOC-generation.

I also introduced a new function globalSettings() that initially receives all defaults and settings used during the document processing. Extensions thus have the chance to know a little about what happens in the TOC generator.

Reuse

To be able to reuse a JS module you must know at least its overridable functions, and what they are doing. Documentation of the base module helps a lot. Here is the start of the new module that builds upon the functionality of the TOC-generator module.

    var that = {};
    
    /**
     * Factory producing content-tree creator instances.
     * @param treeIndentInEm optional, for nested elements, the amount of tree-indentation in em,
     *     defaults to undefined.
     * @param doTocNumbering optional, chapter numbers are prepended to TOC when true,
     *     defaults to true.
     * @param doHeadingNumbering optional, chapter numbers are prepended to headings when true,
     *     defaults to true.
     * @param chapterNumberSeparator optional, separator for chapter numbers,
     *     defaults to ".".
     * @param doTrailingSeparator optional, when true chapter numbers will be
     *     "1.2." instead of "1.2", defaults to false.
     * @param initiallyFolded optional, whether chapters should be initially collapsed,
     *     defaults to false.
     * @return an instance that can treeify a document.
     */
    that.create = function(
              treeIndentInEm,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator,
              initiallyFolded)
    {
      var foldingUtil = folding.create();
      
      var tocUtil = tocCreator.create(
              treeIndentInEm,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator);
      
      var superTableOfContents = tocUtil.tableOfContents;
      
      /** Overwritten to install expand controls into the document tree before terminating. */
      tocUtil.tableOfContents = function(topElement, headingTags, tocContainer) {
        var toReturn = superTableOfContents(topElement, headingTags, tocContainer);
        installExpandControls();
        return toReturn;
      }

      .....

    };
    
    return that;

The create() function is the factory that produces instances you can call tableOfContents() upon. It receives the statical parameters mentioned above. Inside there are two objects that are given to this snippet globally (e.g. through AMD factory arguments). First is folding factory that we need to install expand controls. Second is tocCreator which creates the TOC generator instance we will extend here. Remember that in JS you always need an instance to perform inheritance. The private variable tocUtil receives that instance, and further code uses that instance to extend it with overrides and new functionality. Finally that instance is returned for calling the modified tableOfContents() on it. (That function name does not meet the purpose any more, a convenience function should be introduced that delegates to it.)

The override of tableOfContents() first calls superTableOfContents to execute the document processing. Then, after having received tree information through overrides, it installs expand controls by using the foldingUtil and some new functions that find the content to be folded.

Mind that calling super in JS functional inheritance requires at least two lines of code: storing the super-implementation into some variable, and then calling it from its override.

Override

Some functions should not work like they do when treeifying the document. First there should not be a TOC, because the document itself will be like a TOC. Then we do not need hyperlinks from the TOC to the chapter heading. Finally we want to receive tree information from the document traversal, and we override the new function customizeHeading(level, heading, titleElement) for that. Additionally we receive the ground-tree-level and the fact whether structure analysis is done by h1 - h6 elements in globalSettings().

      var tree = [];

      var levelByH1To6;
      var groundLevel;
            
      /** Overwritten to store global settings. */
      tocUtil.globalSettings = function(headingsParam, topElementParam, groundLevelParam, headingTagsParam, levelByH1To6Param) {
        levelByH1To6 = levelByH1To6Param;
        groundLevel = groundLevelParam;
      };
      
      /** Overwritten to NOT insert TOC into document. */
      tocUtil.createDefaultTocContainer = function() {
        return document.createElement("div");
      };

      /** Overwritten to do nothing, nothing gets linked here. */
      tocUtil.setHyperlinkIdToHeading = function() {
        return undefined;
      };

      /** Overwritten to create just an LI item for chapter number counting. */
      tocUtil.createTocItem = function() {
        return tocUtil.createTocListItem();
      };
      
      /** Overwritten to build a tree from super's iterations through heading elements. */
      tocUtil.customizeHeading = function(level, heading, titleElement) {
        tree.push({
          level: level,
          heading: heading,
          titleElement: titleElement
        });
      };

For storing tree information we hold the private instance-bound array tree. Each tree node, ordered depth-first, will be in there as an object with heading, title and tree-level. After super's document iteration we will use this array to install folding, and it must stay untouched during the whole page lifetime, because it will be used any time an expand control is clicked to retrieve child elements to be shown or hidden.

To Create New

Add

So what further do we have to do here? Install the folding controls.

      var installExpandControls = function() {
        for (var i = 0; i < tree.length; i++) {
          var t = tree[i];
          installExpandControl(t.level, t.heading, t.titleElement);
        }
      };

Unfortunately it is quite complex to find the correct elements to show and hide on expand-control clicks. When you read my Blog about treetables you might remember that on "collapse" we make invisible all elements below the control, but on "expand" we must not make visible elements that are managed by other expand controls. Else the expansion state of child items will be lost.
To make it even harder we must distinguish between the chapter hierarchy and the HTML element hierarchy, which may be identical, but could be different. In case of h1 - h6 structuring a structural child might be a HTML sibling. And that kind of structuring is more popular than nesting div or section elements into each other.

So let's solve the easy problems first.

Structural Children

Following function shows how the tree array is used to find a list of structural children, whereby that list contains children of any depth, meaning the whole sub-tree of the given heading, recursively.

      var structuralChildren = function(heading) {
        var structuralChildren = [];
        var level;
        
        for (var i = 0; i < tree.length; i++) {
          var h = tree[i].heading;
          var l = tree[i].level;
          
          if (level !== undefined) {
            if (l > level)
              structuralChildren.push(tree[i]);
            else if (l <= level)
              return structuralChildren;
          }
          else if (h === heading) {
            level = l;
          }
        }
        return structuralChildren;
      };

And this one finds the next structural sibling of a heading.

      var nextStructuralSibling = function(heading) {
        var level;
        for (var i = 0; i < tree.length; i++) {
          var h = tree[i].heading;
          var l = tree[i].level;
          if (level !== undefined && l <= level)
            return h;
          else if (h === heading)
            level = l;
        }
        return undefined;
      };

Mind that is legal to return undefined, because the last element won't have a next sibling.
These two functions give us all we need to know about structural elements.

Elements to Toggle

When an expand control is closed, we need to hide (1) direct children except the title element and any parent of it, and (2) in case the structural children are not HTML children, all siblings elements until the next structural sibling. The same is for opening an expand control, with the difference that we must retain the expansion states of children.

      var directChildrenWithout = function(titleElement, heading) {
        var directChildren = [];
        for (var i = 0; i < heading.children.length; i++) {
          var child = heading.children[i];
          if ( ! domUtil.isElementOrParentOf(child, titleElement, heading) )
            directChildren.push(child);
        }
        return directChildren;
      };

This finds direct HTML children (not the whole sub-tree recursively), excluding the given title element and any of its parents.
The next function finds HTML sibling elements until a given stop-sibling.

      var siblingsUntil = function(heading, stopElement) {
        var siblings = [];
        var inBranch = false;
        var parent = heading.parentNode;
        for (var i = 0; i < parent.children.length && parent.children[i] !== stopElement; i++) {
          var sibling = parent.children[i];
          if (inBranch)
            siblings.push(sibling);
          else if (sibling === heading)
            inBranch = true;
        }
        return siblings;
      };

Putting It Together

      var installExpandControl = function(level, heading, titleElement) {
        var elementsToHide = getElementsToHide(heading, titleElement);
        var childHeadings = structuralChildren(heading);
        
        var content = function(displayed) {
          if (displayed)
            return getElementsToShow(elementsToHide, childHeadings);
          else
            return elementsToHide;
        };

        foldingUtil.connect(
                titleElement,
                content,
                initiallyFolded === undefined ? false : initiallyFolded,
                false);
        
        if ( ! levelByH1To6 && treeIndentInEm !== undefined)
          heading.style["padding-left"] = ""+((level - groundLevel) * treeIndentInEm)+"em";
      };

This installs the expand controls, and indents the heading when possible and required. How folding works is beyond the scope of this article. In short, it prepends an arrow to the given titleElement, and connects the click callback of that arrow to a function that sets given content visible or invisible. These content elements can be given as single element, as array of elements, or as a function that returns element(s). Here a function is used, that in the case of "setting displayed" returns what getElementsToShow(elementsToHide, childHeadings) delivers, and in case of "setting not displayed" returns the array of elements to hide.

As a precondition for that, the function reads the list of elements to hide from getElementsToHide(heading, titleElement). Parts of that list will also be used in case of "setting displayed".

      var getElementsToHide = function(heading, titleElement) {
        var directChildren = directChildrenWithout(titleElement, heading);
        var stopElement = nextStructuralSibling(heading);
        var siblings = siblingsUntil(heading, stopElement);
        
        var elementsToHide = [];
        for (var i = 0; i < directChildren.length; i++)
          elementsToHide.push(directChildren[i]);
        for (var i = 0; i < siblings.length; i++)
          elementsToHide.push(siblings[i]);
        
        return elementsToHide;
      };

This collects the direct children of the heading element, excluding the title, and all siblings until the next structural sibling.

Mind that no arrays are passed around. Any returned array is built from scratch, several arrays are joined into a new array. This is the safer way to program with lists or arrays.

      var getElementsToShow = function(elementsToHide, childHeadings) {
        var collapsedChildHeadings = [];
        for (var i = 0; i < childHeadings.length; i++) {
          var childHeading = childHeadings[i];
          if ( ! foldingUtil.isExpanded(childHeading.titleElement) )
            collapsedChildHeadings.push(childHeading);
        }
        
        var elementsToShow = [];
        for (var j = 0; j < elementsToHide.length; j++) {
          var elementToHide = elementsToHide[j];
          var isBelowCollapsed = false;
          
          for (var k = 0; k < collapsedChildHeadings.length && isBelowCollapsed === false; k++) {
            var collapsedHeading = collapsedChildHeadings[k];
            var elementsBelowCollapsed = getElementsToHide(collapsedHeading.heading, collapsedHeading.titleElement);
            if (contains(elementsBelowCollapsed, elementToHide))
              isBelowCollapsed = true;
          }
          
          if (isBelowCollapsed === false)
            elementsToShow.push(elementToHide);
        }
        
        return elementsToShow;
      };

      var contains = function(array, element) {
        for (var i = 0; i < array.length; i++)
          if (array[i] === element)
            return true;
        return false;
      };

This is the solution for the complexity when opening an expand control. It retains the expansion states of child elements.

The parameter elementsToHide contains all elements that would be hidden when closing the control. This is filtered by means of the second parameter that contains all child headings (recursively) that also can expand or collapse content.

First those child headings are retrieved that are currently collapsed. Then the new array elementsToShow is created. It receives all elements from elementsToHide that are not below any of the collapsed child headings.

Use it

What has not been shown here are the folding and domUtil modules. You can find these in the folded full source on bottom of this page.

Here is a use case that calls the new module:

<script type="text/javascript">
  "use strict";

  var treeCreatorInstance = treeCreator.create(
      1,          /* indentation */
      undefined,  /* whether TOC items should have numbering, true/false */
      false,      /* whether chapter headings should have numbering, true/false */
      undefined,  /* chapter number separator different from "." */
      undefined,  /* whether chapter numbers should have a trailing separator, true/false */
      true        /* whether document tree should be initially collapsed, true/false */
  );
  var blogDiv = document.getElementById("blog");
  treeCreatorInstance.tableOfContents(blogDiv);
  
</script>

For me there were several reasons why I wanted to reuse the TOC-generator module:

optional chapter numbering
ready-made document structuring engine, providing overrides
the default "guess document structure" implementation that will be sufficient for most cases

There are a lot of new ideas now to continue this work. One thing is making the initiallyFolded parameter more flexible. It should be configurable for any chapter individually whether it is collapsed or not.

  "use strict";


  var tocCreator = function()
  {
    var that = {};
    
    /**
     * Factory producing TOC creator instances.
     * @param treeIndentInEm optional, for nested elements, the amount of tree-indentation in em,
     *     defaults to 1.
     * @param doTocNumbering optional, chapter numbers are prepended to TOC when true,
     *     defaults to true.
     * @param doHeadingNumbering optional, chapter numbers are prepended to headings when true,
     *     defaults to true.
     * @param chapterNumberSeparator optional, separator for chapter numbers,
     *     defaults to ".".
     * @param doTrailingSeparator optional, when true chapter numbers will be
     *     "1.2." instead of "1.2", defaults to false.
     * @return an instance that can create dynamic table-of-contents.
     */
    that.create = function(
              treeIndentInEm,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator)
    {
      
      var tocUtil = {};
    
      /**
       * Reads headings, numbers them, extracts them to a linked table-of-contents
       * and inserts that TOC below a single h1 heading, or as first child of given top-element.
       * @param topElement optional, the HTML element to search for headings,
       *     defaults to document body.
       * @param headingTags optional, the CSS selector to use for search of structuring elements,
       *     defaults to "h2, h3, h4, h5, h6".
       * @param tocContainer optional, the container where to insert the created TOC,
       *     defaults to topElement below a single H1, or on top.
       * @return the given or created container of the table-of-contents.
       */
      tocUtil.tableOfContents = function(topElement, headingTags, tocContainer) {
        topElement = topElement || document.body;
        headingTags = headingTags || guessHeadingTags(topElement);
        doTocNumbering = (doTocNumbering === undefined) ? true : doTocNumbering;
        doHeadingNumbering = (doHeadingNumbering === undefined) ? true : doHeadingNumbering;
        chapterNumberSeparator = chapterNumberSeparator || ".";
        doTrailingSeparator = (doTrailingSeparator === undefined) ? false : doTrailingSeparator;

        var headings = tocUtil.findHeadingsInOrder(topElement, headingTags);
        if ( ! headings || ! headings.length )
          return;

        tocContainer = tocContainer || tocUtil.createDefaultTocContainer(topElement);

        var levelByH1To6 = /h[1-6]/i.test(headingTags);
        var groundLevel= tocUtil.getLevel(headings[0], topElement, headingTags, levelByH1To6);
        tocUtil.globalSettings(headings, topElement, groundLevel, headingTags, levelByH1To6);

        var stack = [];
        stack.push({
          level: groundLevel,
          chapterList: appendTocList(tocContainer)
        });

        for (var i = 0; i < headings.length; i++) {
          var heading = headings[i];
          var currentList = peek(stack);
          var currentLevel = currentList.level;
          var headingLevel = tocUtil.getLevel(heading, topElement, headingTags, levelByH1To6);

          if (headingLevel === currentLevel + 1) {
            stack.push({
              level: headingLevel,
              chapterList: appendTocList(getLastListElement(currentList.chapterList))
            });
          }
          else if (headingLevel < currentLevel) {
            for (; headingLevel < currentLevel; currentLevel--)
              stack.pop();
          }
          else if (headingLevel > currentLevel + 1) {
            throw "Inconsistent document structure, incrementing level from "+currentLevel+" to "+headingLevel;
          }

          appendTocItem(
                  stack,
                  levelByH1To6,
                  heading,
                  doTocNumbering,
                  doHeadingNumbering,
                  chapterNumberSeparator,
                  doTrailingSeparator);
        }

        return tocContainer;
      };

      /**
       * Publish global settings (that won't change) to sub-objects.
       * This implementation does nothing, it is to be overridden.
       */
      tocUtil.globalSettings = function(headings, topElement, groundLevel, headingTags, levelByH1To6) {
      };
      
      var guessHeadingTags = function(topElement) {
        var defaultHeadingTags = "h2, h3, h4, h5, h6";
        var defaultHeadings = tocUtil.findHeadingsInOrder(topElement, defaultHeadingTags);
        var firstTagName;
        for (var i = 0; i < defaultHeadings.length; i++) {
          if ( ! firstTagName )
            firstTagName = defaultHeadings[i].tagName;
          else
            if (firstTagName !== defaultHeadings[i].tagName)
              return defaultHeadingTags; /* found at least two different heading tags */
        }

        if (hasMoreThanOne(topElement, "div"))
          return "div";
        if (hasMoreThanOne(topElement, "section"))
          return "section";
        if (hasMoreThanOne(topElement, "article"))
          return "article";

        return defaultHeadingTags;
      };

      var hasMoreThanOne = function(topElement, cssSelector) {
        var headings = tocUtil.findHeadingsInOrder(topElement, cssSelector);
        return headings && headings.length > 1;
      };

      /**
       * Reads all headings to extract and number, in document order:
       * querySelectorAll() does it, see
       * http://www.w3.org/TR/selectors-api/#queryselectorall.
       * @param topElement the HTML element to search for headings.
       * @param headingTags the CSS selector to use for search.
       * @return something that has a length and can be iterated with a for loop.
       */
      tocUtil.findHeadingsInOrder = function(topElement, headingTags) {
        return topElement.querySelectorAll(headingTags);
      };

      /**
       * Searches for a h1, places TOC as div below it when found, else
       * places TOC as first child of given HTML element or body.
       * @return what is to be used as container for the table of contents.
       */
      tocUtil.createDefaultTocContainer = function(topElement) {
        var toc = document.createElement("div");
        var headings1 = topElement.getElementsByTagName("h1");
        var heading1 = (headings1 && headings1.length === 1) ? headings1[0] : undefined;
        if (heading1)
          heading1.parentElement.insertBefore(toc, heading1.nextSibling);
        else
          topElement.insertBefore(toc, topElement.children[0]);
        return toc;
      };

      tocUtil.getLevel = function(heading, topElement, headingTags, levelByH1To6) {
        return levelByH1To6 ?
            tocUtil.getHeadingLevel(heading) :
            tocUtil.getNestingLevel(heading, topElement, headingTags);
      };

      /** @return the given heading's number, e.g. "3" from "h3". */
      tocUtil.getHeadingLevel = function(heading) {
        return parseInt(heading.tagName.substring(1));
      };

      /** @return the 0-n level of given element, relative to given topElement. */
      tocUtil.getNestingLevel = function(element, topElement, nestingTags) {
        var parent = element.parentNode;
        var i = 0;
        while (parent !== topElement) {
          if (parent.tagName && new RegExp(parent.tagName, "i").test(nestingTags))
            i++; /* when tag is in nesting-tags, count this as level */
          parent = parent.parentNode;
        }
        return i;
      };

      var peek = function(stack) {
        return stack[stack.length - 1];
      };

      var getLastListElement = function(list) {
        return list.children[list.children.length - 1];
      };

      /**
       * This implementation returns the input number unchanged.
       * @return a chapter number converted to arbitrary numbering system (Roman, letter, ...).
       */
      tocUtil.convertChapterNumber = function(number) {
        return number;
      };

      var createNextChapterNumber = function(stack, chapterNumberSeparator) {
        var number = "";
        for (var i = 0; i < stack.length; i++) {
          var currentList = stack[i];
          var currentNumber = currentList.chapterList.children.length;
          if (i === stack.length - 1) /* end of nestings, at rightmost chapter number */
            currentNumber++; /* create NEXT number */
          currentNumber = tocUtil.convertChapterNumber(currentNumber);
          number = number+(number === "" ? "" : chapterNumberSeparator)+currentNumber;
        }
        return number;
      };

      /**
       * This implementation creates an UL element without list-style-type (no bullets).
       * @return a list that will hold a chapter headings.
       */
      tocUtil.createTocList = function() {
        var list = document.createElement("ul");
        list.style.cssText = 
                "list-style-type: none; "+
                (treeIndentInEm !== undefined ? "padding-left: "+treeIndentInEm+"em;" : "");
        return list;
      };

      var appendTocList = function(parent) {
        var list = tocUtil.createTocList();
        parent.appendChild(list);
        return list;
      };

      /**
       * This implementation creates a LI element.
       * @return an TOC item element that will receive a chapter heading link.
       */
      tocUtil.createTocListItem = function() {
        return document.createElement("li");
      };

      /**
       * This implementation creates and links an A element.
       * @return a TOC link element that will receive a chapter heading text.
       */
      tocUtil.createHyperlink = function(headingHtml, headingId) {
        var hyperLink = document.createElement("a");
        hyperLink.innerHTML = headingHtml;
        hyperLink.setAttribute("href", "#"+headingId);
        return hyperLink;
      };

      var getTextContent = function(nestingElement) {
        var title = (nestingElement.childNodes[0] && nestingElement.childNodes[0].nodeName === "#text") ?
            nestingElement.childNodes[0].textContent :
            undefined;
        return (title && (title = title.trim())) ? title : undefined;
      };

      /**
       * @return this is called when the document is structured by
       *    nested elements, and it is expected to return the heading-element
       *    for given nested container, e.g. the heading in a section.
       *    This implementation returns the first child element that
       *    is not a single child.
       */
      tocUtil.getTitleElementFromNesting = function(nestedElement) {
        while (nestedElement.children.length === 1)
          nestedElement = nestedElement.children[0]; /* in case there are wrappers */
        
        var titleElement = nestedElement.children[0] ? nestedElement.children[0] : nestedElement;
        while (titleElement.children.length === 1)
          titleElement = titleElement.children[0]; /* in case there are wrappers */
        
        return titleElement;
      };

      tocUtil.setHyperlinkIdToHeading = function(nextChapterNumber, titleElement) {
        var headingId = "h"+nextChapterNumber.replace(/\./g, '_');
        titleElement.setAttribute("id", headingId);
        return headingId;
      };

      tocUtil.prependToElement = function(levelByH1To6, toPrepend, titleText, titleElement) {
        if (levelByH1To6) /* assuming H[1-6] has no children */
          titleElement.innerHTML = toPrepend+"&nbsp;"+titleText;
        else
          titleElement.insertBefore(document.createTextNode(toPrepend+" "), titleElement.childNodes[0]);
      };

      tocUtil.createTocItem = function(
              doTocNumbering,
              nextChapterNumber,
              headingText,
              headingId)
      {
        var tocItem = tocUtil.createTocListItem();
        if (doTocNumbering) {
          var chapterNumber = document.createElement("span");
          chapterNumber.innerHTML = nextChapterNumber+"&nbsp;";
          tocItem.appendChild(chapterNumber);
        }

        var hyperLink = tocUtil.createHyperlink(headingText, headingId);
        tocItem.appendChild(hyperLink);

        return tocItem;
      };

      var appendTocItem = function(
              stack,
              levelByH1To6,
              heading,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator)
      {
        var nextChapterNumber = createNextChapterNumber(stack, chapterNumberSeparator);
        if (doTrailingSeparator)
          nextChapterNumber = nextChapterNumber + chapterNumberSeparator;

        var headingText = levelByH1To6 ? heading.innerHTML : getTextContent(heading);
        var titleElement = (levelByH1To6 || headingText) ? heading : tocUtil.getTitleElementFromNesting(heading);
        if ( ! headingText )
          headingText = titleElement.innerHTML;

        if (doHeadingNumbering)
          tocUtil.prependToElement(levelByH1To6, nextChapterNumber, headingText, titleElement);

        var currentList = peek(stack);
        tocUtil.customizeHeading(currentList.level, heading, titleElement);

        var headingId = tocUtil.setHyperlinkIdToHeading(nextChapterNumber, titleElement);
        var tocItem = tocUtil.createTocItem(
                doTocNumbering,
                nextChapterNumber,
                headingText,
                headingId);

        currentList.chapterList.appendChild(tocItem);
      };

      /*
       * Called once for each chapter heading. Does nothing. To be overridden.
       * @param level the tree level of heading.
       * @param heading the current heading/nesting element to customize.
       * @param titleElement the element holding the chapter title, mostly identical with heading.
       */
      tocUtil.customizeHeading = function(level, heading, titleElement) {
      };


      return tocUtil;
    };
    
    
    return that;
    
  }();
  
  
  
  
  var domUtil = function()
  {
    var setDisplayed = function(element, displayed) {
      if (displayed === true && ! isDisplayed(element)) {
        var displayStyle = element.oldDisplayStyle ? element.oldDisplayStyle : "";
        element.style.display = displayStyle;
      }
      else if ( ! displayed && isDisplayed(element)) {
        element.oldDisplayStyle = element.style.display;
        element.style.display = "none";
      }
    };

    var isDisplayed = function(element) {
      return element.style.display !== "none";
    };

    var isElementOrParentOf = function(toSearch, startElement, topMostElement) {
      var parent = startElement;
      while (parent && parent !== topMostElement) {
        if (parent === toSearch)
          return true;
        parent = parent.parentNode;
      }
      return false;
    };
    
    
    return {
      isDisplayed: isDisplayed,
      setDisplayed: setDisplayed,
      isElementOrParentOf: isElementOrParentOf,
    };

  }();
  
  
  
  
  var folding = function()
  { 
    var that = {};

    that.blackTriangleRight = "\u25B6";
    that.blackTriangleDown = "\u25BC";
    that.whiteTriangleRight = "\u25B7";
    that.whiteTriangleDown = "\u25BD";
    that.plus = "+";
    that.minus = "\u2012";
    that.blackTriangleRightSmall = "\u25B8";
    that.blackTriangleDownSmall = "\u25BE";
    that.whiteTriangleRightSmall = "\u25B9";
    that.whiteTriangleDownSmall = "\u25BF";

    that.create = function(expandSymbol, collapseSymbol, styling)
    {
      var foldingUtil = {};

      var symbolExpand = expandSymbol || that.whiteTriangleRight;
      var symbolCollapse = collapseSymbol || that.whiteTriangleDown;

      foldingUtil.connect = function(titleElement, content, initiallyFolded, canClickWholeTitle) {
        var expandControl = prependExpandControlContainer(titleElement);
        expandControl.innerHTML = (initiallyFolded ? symbolExpand : symbolCollapse);

        var notifier = canClickWholeTitle ? titleElement : expandControl;
        notifier.addEventListener("click", function() {
          clickListener(expandControl, content);
        });

        if (canClickWholeTitle)
          notifier.style.cursor = "pointer";

        if (initiallyFolded)
          setDisplayed(content, false);

        return notifier;
      };

      foldingUtil.isExpanded = function(titleElement) {
        var expandControl = getExpandControl(titleElement);
        return expandControl ? (expandControl.innerHTML === symbolCollapse) : true;
      };


      var prependExpandControlContainer = function(titleElement) {
        var expandControl = getExpandControl(titleElement);

        if ( ! expandControl ) {
          expandControl = document.createElement("div");
          expandControl.style.display = "inline-block";
          expandControl.style.width = "1em";
          expandControl.style.height = "1em";
          expandControl.style.cursor = "pointer";
          expandControl.style.cssText += "; text-align: center; margin-right: 0.5em;";

          if (styling)
             styling(expandControl);

          titleElement.insertBefore(expandControl, titleElement.childNodes[0]);
        }

        return expandControl;
      };

      var getExpandControl = function(titleElement) {
        var expandControl = titleElement.children[0];
        if (expandControl) {
          var titleString = expandControl.innerHTML;
          if (titleString !== symbolCollapse && titleString !== symbolExpand)
            expandControl = undefined;
        }
        return expandControl;
      };

      var clickListener = function(expandControl, content) {
        var displayed = isDisplayed(content);
        setDisplayed(content, displayed === false);
        expandControl.innerHTML = displayed ? symbolExpand : symbolCollapse;
      };

      var setDisplayed = function(content, displayed) {
        if (content.style)
          domUtil.setDisplayed(content, displayed);
        else if (content instanceof Array)
          for (var i = 0; i < content.length; i++)
            setDisplayed(content[i], displayed);
        else
          setDisplayed(content(displayed), displayed);
      };

      var isDisplayed = function(content) {
        if (content.style)
          return domUtil.isDisplayed(content);
        else if (content instanceof Array)
          return content.length > 0 && isDisplayed(content[0]);
        else
          return isDisplayed(content(true));
      };


      return foldingUtil;
      
    };


    return that;

  }();




  var treeCreator = function()
  {
    var that = {};
    
    /**
     * Factory producing content-tree creator instances.
     * @param treeIndentInEm optional, for nested elements, the amount of tree-indentation in em,
     *     defaults to undefined.
     * @param doTocNumbering optional, chapter numbers are prepended to TOC when true,
     *     defaults to true.
     * @param doHeadingNumbering optional, chapter numbers are prepended to headings when true,
     *     defaults to true.
     * @param chapterNumberSeparator optional, separator for chapter numbers,
     *     defaults to ".".
     * @param doTrailingSeparator optional, when true chapter numbers will be
     *     "1.2." instead of "1.2", defaults to false.
     * @param initiallyFolded optional, whether chapters should be initially collapsed,
     *     defaults to false.
     * @return an instance that can treeify a document.
     */
    that.create = function(
              treeIndentInEm,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator,
              initiallyFolded)
    {
      var foldingUtil = folding.create();
      
      var tocUtil = tocCreator.create(
              treeIndentInEm,
              doTocNumbering,
              doHeadingNumbering,
              chapterNumberSeparator,
              doTrailingSeparator);
      
      var superTableOfContents = tocUtil.tableOfContents;
      
      var tree = [];

      var levelByH1To6;
      var groundLevel;
      
      /** Overwritten to install expand controls into the document tree before terminating. */
      tocUtil.tableOfContents = function(topElement, headingTags, tocContainer) {
        var toReturn = superTableOfContents(topElement, headingTags, tocContainer);
        installExpandControls();
        return toReturn;
      }
      
      /** Overwritten to store global settings. */
      tocUtil.globalSettings = function(headingsParam, topElementParam, groundLevelParam, headingTagsParam, levelByH1To6Param) {
        levelByH1To6 = levelByH1To6Param;
        groundLevel = groundLevelParam;
      };
      
      /** Overwritten to NOT insert TOC into document. */
      tocUtil.createDefaultTocContainer = function() {
        return document.createElement("div");
      };

      /** Overwritten to do nothing, nothing gets linked here. */
      tocUtil.setHyperlinkIdToHeading = function() {
        return undefined;
      };

      /** Overwritten to create just an LI item for chapter number counting. */
      tocUtil.createTocItem = function() {
        return tocUtil.createTocListItem();
      };
      
      /** Overwritten to build a tree from super's iterations through heading elements. */
      tocUtil.customizeHeading = function(level, heading, titleElement) {
        tree.push({
          level: level,
          heading: heading,
          titleElement: titleElement
        });
      };


      var directChildrenWithout = function(titleElement, heading) {
        var directChildren = [];
        for (var i = 0; i < heading.children.length; i++) {
          var child = heading.children[i];
          if ( ! domUtil.isElementOrParentOf(child, titleElement, heading) )
            directChildren.push(child);
        }
        return directChildren;
      };

      var siblingsUntil = function(heading, stopElement) {
        var siblings = [];
        var inBranch = false;
        var parent = heading.parentNode;
        for (var i = 0; i < parent.children.length && parent.children[i] !== stopElement; i++) {
          var sibling = parent.children[i];
          if (inBranch)
            siblings.push(sibling);
          else if (sibling === heading)
            inBranch = true;
        }
        return siblings;
      };

      var nextStructuralSibling = function(heading) {
        var level;
        for (var i = 0; i < tree.length; i++) {
          var h = tree[i].heading;
          var l = tree[i].level;
          if (level !== undefined && l <= level)
            return h;
          else if (h === heading)
            level = l;
        }
        return undefined;
      };

      var structuralChildren = function(heading) {
        var structuralChildren = [];
        var level;
        
        for (var i = 0; i < tree.length; i++) {
          var h = tree[i].heading;
          var l = tree[i].level;
          
          if (level !== undefined) {
            if (l > level)
              structuralChildren.push(tree[i]);
            else if (l <= level)
              return structuralChildren;
          }
          else if (h === heading) {
            level = l;
          }
        }
        return structuralChildren;
      };

      var getElementsToHide = function(heading, titleElement) {
        var directChildren = directChildrenWithout(titleElement, heading);
        var stopElement = nextStructuralSibling(heading);
        var siblings = siblingsUntil(heading, stopElement);
        
        var elementsToHide = [];
        for (var i = 0; i < directChildren.length; i++)
          elementsToHide.push(directChildren[i]);
        for (var i = 0; i < siblings.length; i++)
          elementsToHide.push(siblings[i]);
        
        return elementsToHide;
      };
      
      var getElementsToShow = function(elementsToHide, childHeadings) {
        var collapsedChildHeadings = [];
        for (var i = 0; i < childHeadings.length; i++) {
          var childHeading = childHeadings[i];
          if ( ! foldingUtil.isExpanded(childHeading.titleElement) )
            collapsedChildHeadings.push(childHeading);
        }
        
        var elementsToShow = [];
        for (var j = 0; j < elementsToHide.length; j++) {
          var elementToHide = elementsToHide[j];
          var isBelowCollapsed = false;
          
          for (var k = 0; k < collapsedChildHeadings.length && isBelowCollapsed === false; k++) {
            var collapsedHeading = collapsedChildHeadings[k];
            var elementsBelowCollapsed = getElementsToHide(collapsedHeading.heading, collapsedHeading.titleElement);
            if (contains(elementsBelowCollapsed, elementToHide))
              isBelowCollapsed = true;
          }
          
          if (isBelowCollapsed === false)
            elementsToShow.push(elementToHide);
        }
        
        return elementsToShow;
      };

      var contains = function(array, element) {
        for (var i = 0; i < array.length; i++)
          if (array[i] === element)
            return true;
        return false;
      };
      
      
      var installExpandControls = function() {
        for (var i = 0; i < tree.length; i++) {
          var t = tree[i];
          installExpandControl(t.level, t.heading, t.titleElement);
        }
      };
      
      /**
       * Installs expand controls on document headings.
       * An expand control toggles the visibility of direct
       * children of the given heading element (excluding the titleElement
       * holding the expand control, and none of that title's parents),
       * and all its siblings down to next heading element of same
       * or smaller level.
       */
      var installExpandControl = function(level, heading, titleElement) {
        var elementsToHide = getElementsToHide(heading, titleElement);
        var childHeadings = structuralChildren(heading);
        
        var content = function(displayed) {
          if (displayed)
            return getElementsToShow(elementsToHide, childHeadings);
          else
            return elementsToHide;
        };

        var folded;
        if (initiallyFolded === undefined)
          folded = false;
        else if (initiallyFolded === true || initiallyFolded === false)
          folded = initiallyFolded;
        else
          folded = initiallyFolded(heading, level);
        
        foldingUtil.connect(titleElement, content, folded, false);
        
        if ( ! levelByH1To6 && treeIndentInEm !== undefined)
          heading.style["padding-left"] = ""+((level - groundLevel) * treeIndentInEm)+"em";
      };


      return tocUtil;
    };
    
    
    return that;
    
  }();