Question

Using XSLT, how can I wrap siblings that share the same value for an attribute.

Lets say I need to wrap a one or more <amendment/> with the <chapter/> they belong too.
From this:

<section>
      <heading>some heading text</heading>
      <amendment num='1' chapter='1'>
            <foo/>
      </amendment>
      <amendment num='2' chapter='1'>
            <bar/>
      </amendment>
      <amendment num='3' chapter='2'>
            <baz/>
      </amendment>
      <heading>some heading text</heading>
      <amendment num='4' chapter='3'>
            <baz/>
      </amendment>
</section>

into this:

<section>
      <heading>some heading text</heading>
      <chapter num="1">
            <amendment num='1'>
                  <foo/>
            </amendment>
            <amendment num='2'>
                  <bar/>
            </amendment>
      </chapter>
      <chapter num="2">
            <amendment num='3'>
                  <baz/>
            </amendment>
      </chapter>
      <heading>some heading text</heading>
      <chapter num="3">
            <amendment num='4'>
                  <baz/>
            </amendment>
      </chapter>
</section>

Note 1: Amendments are always listed sorted by chapter in the source XML.

Note 2: Im using PHP5 with XSLT 1.0

Answer 1

This stylesheet:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:key name="kAmendementByChapter" match="amendment" use="@chapter"/>
    <xsl:template match="node()|@*" name="identity">
        <xsl:copy>
            <xsl:apply-templates select="node()|@*"/>
        </xsl:copy>
    </xsl:template>
    <xsl:template match="amendment[count(.|key('kAmendementByChapter',
                                               @chapter)[1])=1]">
        <chapter num="{@chapter}">
            <xsl:apply-templates select="key('kAmendementByChapter',@chapter)"
                                 mode="copy"/>
        </chapter>
    </xsl:template>
    <xsl:template match="amendment"/>
    <xsl:template match="amendment" mode="copy">
        <xsl:call-template name="identity"/>
    </xsl:template>
    <xsl:template match="@chapter"/>
</xsl:stylesheet>

Output:

<section>
    <heading>some heading text</heading>
    <chapter num="1">
        <amendment num="1">
            <foo></foo>
        </amendment>
        <amendment num="2">
            <bar></bar>
        </amendment>
    </chapter>
    <chapter num="2">
        <amendment num="3">
            <baz></baz>
        </amendment>
    </chapter>
    <heading>some heading text</heading>
    <chapter num="3">
        <amendment num="4">
            <baz></baz>
        </amendment>
    </chapter>
</section>

Note: Copy all (indentity rule), grouping on @chapter.