XSLT stylesheet: merging elements into one - xslt

Question

What is an XSLT 2.0 stylesheet that will transform
<paramList>
<param name="y" out="true"/>
<param name="y" in="true"/>
<param name="z" out="true"/>
<param name="x" in="true"/>
</paramList>
into
<paramList>
<param name="x" in="true" />
<param name="y" in="true" out="true"/>
<param name="z" out="true"/>
</paramList>
In the result, "in, only" parameters precede "in & out" parameters, which, in turn, precede "out, only" parameters. Also, the two "y" elements have been combined into one.

Answer 1

<?xml version="1.0" ?>
<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/paramList">
<xsl:copy>
<xsl:for-each-group select="param" group-by="#name">
<xsl:sort select="current-group()/#in" order="descending"/>
<xsl:sort select="current-group()/#out"/>
<param name="{current-grouping-key()}">
<xsl:for-each select="current-group()/#*">
<xsl:copy/>
</xsl:for-each>
</param>
</xsl:for-each-group>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>

Answer 2

A small improvement:
In both #Nick-Jones ' and #Obalix 's solutions, it is shorter to write:
<xsl:copy-of select="current-group()/#*"/>
or
<xsl:copy-of select="key('paramsByName', #name)/#*"/>
than, respectively:
<xsl:for-each select="current-group()/#*">
<xsl:copy/>
</xsl:for-each>
or
<xsl:for-each select="key('paramsByName', #name)">
<xsl:copy-of select="#*"/>
</xsl:for-each>

Answer 3

Just in case someone needs to do it in XSLT 1:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl"
>
<xsl:output method="xml" indent="yes"/>
<xsl:key name="paramsByName" match="param" use="#name"/>
<xsl:template match="/paramList">
<xsl:copy>
<xsl:for-each select="param[count(. | key('paramsByName', #name)[1]) = 1]">
<xsl:sort select="#name"/>
<xsl:copy>
<xsl:for-each select="key('paramsByName', #name)">
<xsl:copy-of select="#*"/>
</xsl:for-each>
</xsl:copy>
</xsl:for-each>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
This uses Munchian grouping as XSLT 1 does not have a grouping construct.
Edit:
It is obviously also possible to just copy the in and out attributes in this case the following style sheet does the job (also following Dimetre's suggestions:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl"
>
<xsl:output method="xml" indent="yes"/>
<xsl:key name="paramsByName" match="param" use="#name"/>
<xsl:template match="/paramList">
<xsl:copy>
<xsl:for-each select="param[count(. | key('paramsByName', #name)[1]) = 1]">
<xsl:sort select="#name"/>
<xsl:copy-of select="key('paramsByName', #name)/#*[local-name() = 'in' or local-name() = 'out']"/>
</xsl:for-each>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>

Answer 4

Obalix's answer may not work in the case there are multiple paramList elements in the input document. I assume this may interest the poster if the document describe a software interface where there are multiple procedures with a paramList for each.
Here is a sample input:
<root>
<func name="one">
<paramList>
<param name="y" out="true"/>
<param name="y" in="true"/>
<param name="z" out="true"/>
<param name="x" in="true"/>
</paramList>
</func>
<func name="two">
<paramList>
<param name="z" in="true"/>
</paramList>
</func>
</root>
Here is my proposed stylesheet, built on Obalix's answer. The trick is to use a local key id containing the ID of the paramListelement.
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl"
>
<xsl:output method="xml" indent="yes"/>
<xsl:key name="paramsByName" match="param" use="concat(generate-id(..), '/', #name)"/>
<xsl:template match="paramList">
<xsl:copy>
<xsl:variable name="id" select="generate-id(.)"/>
<xsl:for-each select="param[count(. | key('paramsByName', concat($id, '/', #name))[1]) = 1]">
<xsl:copy>
<xsl:copy-of select="key('paramsByName', concat($id, '/', #name))/#*"/>
</xsl:copy>
</xsl:for-each>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>