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Re: [xsl] following-sibling is evil

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Use a key, and count how many entries there are in the key with that value
combo.

On 6 Jul 2014 10:33, "Costello, Roger L. costello@xxxxxxxxx" <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> 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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