XOM API for Processing XML with Java

[Elliotte Harold <elharo AT metalab.unc.edu>]

Wolfgang Hoschek wrote:
> I see. However, it seems you can reformulate the check as follows
>
>             else if (this.getParent() != null && child.getChildCount() > 
> 0 && isCycle(child, this.getParent())) {
>                 throw new CycleException(
>                   "Cannot add an ancestor as a child");
>             }


I'm not sure that's a big help because if the parent is null thats 
caught in the second pass through the loop in insertionAllowed:

   private static boolean isCycle(Node child, ParentNode parent) {

        while (parent != null) {
            if (parent == child) return true;
            parent = parent.getParent();
        }
        return false;

    }


Possibly I could eliminate the first pass through the loop because it's 
redundant. insertionAllowed has already checked that the parent is not 
the child:

        else if (child.isElement()) {
            if (child == this) {
                throw new CycleException(
                  "Cannot add a node to itself");
            }
            else if (child.getChildCount() > 0 && isCycle(child, this)) {
                throw new CycleException(
                  "Cannot add an ancestor as a child");
            }
            return;
        }


That is, I could rewrite isCycle like this:

        parent = parent.getParent();
        while (parent != null) {
            if (parent == child) return true;
            parent = parent.getParent();
        }
        return false;

If I do this I should probably just pull the complete cycle check into 
the same method for clarity rather than having half of it in 
insertionAllowed and half in isCycle(). I doubt the savings in execution 
time is noticeable.

By the way, thinking about this I suspect the unproven hypothesis I 
added to ParentNode recently ("it is significantly faster to  construct 
documents from the top down than the bottom up.  That is, one should add 
an element to its parent before adding its children. This short-circuits 
some possibly expensive checks.") is in fact false. I don't think it 
makes a difference.

--
Elliotte Rusty Harold  elharo AT metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!
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