Tutorial :XSLT: a variation on the pagination problem


I must transform some XML data into a paginated list of fields. Here is an example.

Input XML:

<?xml version="1.0" encoding="UTF-8"?>  <data>      <books>          <book title="t0"/>          <book title="t1"/>          <book title="t2"/>          <book title="t3"/>          <book title="t4"/>      </books>      <library name="my library"/>  </data>  

Desired output:

<?xml version="1.0" encoding="UTF-8"?>  <pages>      <page number="1">          <field name="library_name" value="my library"/>          <field name="book_1" value="t0"/>          <field name="book_2" value="t1"/>      </page>      <page number="2">          <field name="book_1" value="t2"/>          <field name="book_2" value="t3"/>      </page>      <page number="3">          <field name="book_1" value="t4"/>      </page>  </pages>  

In the above example I assume that I want at most 2 fields named book_n (with n ranging between 1 and 2) per page. Tags <page> must have an attribute number. Finally, the field named library_name must appear only the first <page>.

Here is my current solution using XSLT:

<?xml version="1.0" encoding="UTF-8"?>  <xsl:stylesheet       xmlns:xsl="http://www.w3.org/1999/XSL/Transform"      version="2.0"      exclude-result-prefixes="trx xs">        <xsl:output method="xml" indent="yes" omit-xml-declaration="no" />        <xsl:variable name="max" select="2"/>        <xsl:template match="//books">            <xsl:for-each-group select="book" group-ending-with="*[position() mod $max = 0]">                <xsl:variable name="pageNum" select="position()"/>                <page number="{$pageNum}">                    <xsl:for-each select="current-group()">                      <xsl:variable name="idx" select="if (position() mod $max = 0) then $max else position() mod $max"/>                      <field value="{@title}">                          <xsl:attribute name="name">book_<xsl:value-of select="$idx"/>                          </xsl:attribute>                      </field>                                  </xsl:for-each>                    <xsl:if test="$pageNum = 1">                      <xsl:call-template name="templateFor_library"/>                              </xsl:if>                    </page>            </xsl:for-each-group>            </xsl:template>        <xsl:template name="templateFor_library">          <xsl:for-each select="//library">              <field name="library_name" value="{@name}" />          </xsl:for-each>              </xsl:template>    </xsl:stylesheet>   

Is there a better/simpler way to perform this transformation?


Yes there is.

<xsl:param name="pagesize" select="2" />    <xsl:template match="data">    <pages>      <xsl:apply-templates mode="page" select="        books/book[position() mod $pagesize = 1]      " />    </pages>  </xsl:template>    <xsl:template match="book" mode="page">    <page number="{position()}">      <xsl:apply-templates select="        . | following-sibling::book[position() &lt; $pagesize]      " />    </page>  </xsl:template>    <xsl:template match="book">    <field name="book_{position()}" value="{@title}" />  </xsl:template>  

EDIT #1: The above is XSLT 1.0 compliant. You can still modify it to use XSLT 2.0's <xsl:for-each-group>, if you want to. Personally, I find separate templates more appealing than one big fat nested for-each construct, YMMV.

EDIT #2: As per request in the comments. To make something appear for the first page only, modify one template:

<xsl:template match="book" mode="page">    <page number="{position()}">      <xsl:if test="position() = 1">        <xsl:attribute name="library_name`>          <xsl:value-of select="ancestor::data/library/@name" />        </xsl:attribute>      </xsl:if>      <xsl:apply-templates select="        . | following-sibling::book[position() &lt; $pagesize]      " />    </page>  </xsl:template>  

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