Where's the bug in this tree traversal code?

Question

There's a bug in Traverse() that's causing it to iterate nodes more than once.

Bugged Code

public IEnumerable<HtmlNode> Traverse()
{
    foreach (var node in _context)
    {
        yield return node;
        foreach (var child in Children().Traverse())
            yield return child;
    }
}

public SharpQuery Children()
{
    return new SharpQuery(_context.SelectMany(n => n.ChildNodes).Where(n => n.NodeType == HtmlNodeType.Element), this);
}

public SharpQuery(IEnumerable<HtmlNode> nodes, SharpQuery previous = null)
{
    if (nodes == null) throw new ArgumentNullException("nodes");
    _previous = previous;
    _context = new List<HtmlNode>(nodes);
}

Test Code

    static void Main(string[] args)
    {
        var sq = new SharpQuery(@"
<a>
    <b>
        <c/>
        <d/>
        <e/>
        <f>
            <g/>
            <h/>
            <i/>
        </f>
    </b>
</a>");
        var nodes = sq.Traverse();
        Console.WriteLine("{0} nodes: {1}", nodes.Count(), string.Join(",", nodes.Select(n => n.Name)));
        Console.ReadLine();

Output

19 nodes: #document,a,b,c,g,h,i,d,g,h,i,e,g,h,i,f,g,h,i

Expected Output

Each letter a-i printed once.

Can't seem to figure out where's it going wrong... node.ChildNodes does return just direct children, right? (from HtmlAgilityPack)

---

Full class (condensed) if you want to try and run it yourself.

public class SQLite
{
    private readonly List<HtmlNode> _context = new List<HtmlNode>();
    private readonly SQLite _previous = null;

    public SQLite(string html)
    {
        var doc = new HtmlDocument();
        doc.LoadHtml(html);
        _context.Add(doc.DocumentNode);
    }

    public SQLite(IEnumerable<HtmlNode> nodes, SQLite previous = null)
    {
        if (nodes == null) throw new ArgumentNullException("nodes");
        _previous = previous;
        _context = new List<HtmlNode>(nodes);
    }

    public IEnumerable<HtmlNode> Traverse()
    {
        foreach (var node in _context)
        {
            yield return node;
            foreach (var child in Children().Traverse())
                yield return child;
        }
    }

    public SQLite Children()
    {
        return new SQLite(_context.SelectMany(n => n.ChildNodes).Where(n => n.NodeType == HtmlNodeType.Element), this);
    }
}

Answer 1

First, note that everything goes according to plan as long as we're iterating over elements that don't have any sibling. As soon as we hit element <c>, things start going haywire. It's also interesting that <c>, <d> and <e> all think they contain <f>'s children.

Let's take a closer look at your call to SelectMany():

_context.SelectMany(n => n.ChildNodes)

That iterates through the items in _context and accumulates the child nodes of each item. Let's have a look at _context. Everything should be okay, since it's a List of length 1. Or is it?

I suspect your SharpQuery(string) constructor stores sibling elements inside the same list. In that case, _context may not be of length 1 anymore and, remember, SelectMany() accumulates the child nodes of each item in the list.

For example, if _context is a list containing <c>, <d>, <e> and <f>, only <f> has children, and SelectMany() is called once for each element, it will accumulate and return the children of <f> four times.

I think that's your bug.

EDIT: Since you've posted the full class, I don't have to guess anymore. Look at the sequence of operations when you iterate over <b> (iterators replaced by lists for better comprehension):

1. Call Children() on <b>, which returns <c>, <d>, <e> and <f>,
2. Call Traverse() on the resulting list:
   1. Call Children() on <c>, but _context actually contains <c>, <d>, <e> and <f>, not only <c>, so that returns <g>, <h> and <i>,
   2. Call Children() on <d>, same thing since _context is the same for both <c> and <d> (and <e>, and <f>),
   3. Lather, rinse, repeat.

Answer 2

Children() is broken, it selects children of siblings as well. I'd rewrite to something like this:

public IEnumerable<HtmlNode> Traverse(IEnumerable<HtmlNode> nodes)
{
    foreach (var node in nodes)
    {
        yield return node;
        foreach (var child in Traverse(ChildNodes(node)))
            yield return child;
    }
}

private IEnumerable<HtmlNode> ChildNodes(HtmlNode node)
{
    return node.ChildNodes.Where(n => n.NodeType == HtmlNodeType.Element);
}


public SharpQuery(IEnumerable<HtmlNode> nodes, SharpQuery previous = null)
{
    if (nodes == null) throw new ArgumentNullException("nodes");
    _previous = previous; // Is this necessary?
    _context = new List<HtmlNode>(nodes);
}