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[xsl] following-sibling is evil


Subject: [xsl] following-sibling is evil
From: "Costello, Roger L. costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Sun, 6 Jul 2014 09:33:47 -0000

Hi Folks,

I have a list of <Book> elements. Each Book contains Title, Author, and
Genre.

For each Book I wish to determine if there is a following Book with the same
Author and Genre.

I am using Schematron. This XSLT is generated from my Schematron rule:

    <xsl:template match="Book">
        <xsl:variable name="currentAuthor" select="Author" />
        <xsl:variable name="currentGenre" select="Genre" />
        <xsl:if test="following-sibling::Book[Title eq $currentAuthor][Genre
eq $currentGenre][1]">
            <xsl:text>There is another Book with the same Author and
Genre</xsl:text>
        </xsl:if>
    </xsl:template>

Notice the XPath expression:

following-sibling::Book[Title eq $currentAuthor][Genre eq $currentGenre][1]

As I understand it, an XPath processor will evaluate that XPath expression
like so:

Step 1. Gather up all the following sibling Book elements. Let's denote the
resulting set by S1.

Step 2. Filter S1 by eliminating those Books that don't satisfy this
predicate: [Title eq $currentAuthor]. Let's denote the resulting set by S2.

Step 3. Filter S2 by eliminating those Books that don't satisfy this
predicate: [Genre eq $currentGenre]. Let's denote the resulting set by S3.

Step 4. Filter S3 by eliminating all Books except the first.

Do I correctly understand how an XPath processor will evaluate the XPath
expression?

Suppose the list contains 100,000 Books.

Consider evaluating the XPath expression for the first Book in the list. The
XPath processor must collect 99,999 Books and apply the steps to them.

For the second Book the XPath processor must collect 99,998 Books and apply
the steps to them.

For the third Book the XPath processor must collect 99,997 Books and apply the
steps to them.

And so forth.

Yikes!

That XSLT program will take a very long time to execute!

Is there a way to modify my XPath so that the XPath processor stops as soon as
it gets to a Book that matches all the predicates?

/Roger


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