Re: [xsl] Question on duplicate node elimination

[Michael Kay <mike@xxxxxxxxxxxx>]

> But how could the algorithm step of "duplicate elimination" be done?
> How can the duplicates be determined and removed, correctly?
>
>    

What makes you think it would be difficult?

Of course, a processor needs some way to decide whether two nodes are 
identical/distinct. Given such a mechanism, it's not difficult to come 
up with algorithms that eliminate duplicate nodes.

In practice, when XPath 1.0 is used as part of XSLT 1.0, the XPath 
requirement to eliminate duplicates can always be combined with the XSLT 
requirement to deliver the node-set sorted in document order. So the 
natural way to eliminate duplicates is as part of the sorting process.

For performance, the most important technique is static analysis to 
identify those path expressions where the sort (and duplicate 
elimination) are unnecessary. For example, this is the case for the 
expression /a/b/c if it is evaluated either (a) using nested loops, or 
(b) by scanning the entire source document looking for nodes that match 
this pattern. For the expression //x//y, a sort is necessary if the 
evaluation uses nested loops, but not if it uses a whole-document scan 
and pattern matching. Remember that the evaluation techniques used 
internally may be very different from the descriptions you find in 
explanations of the semantics of the language.

The way you have phrased the question suggests that you might be 
worrying about how exslt:node-set() affects the process. Simple answer - 
it doesn't.

Michael Kay
Saxonica