grouping with complex "selection" - grouping

Question

This is the source XML:
<root>
<!-- a and b have the same date entries, c is different -->
<variant name="a">
<booking>
<date from="2017-01-01" to="2017-01-02" />
<date from="2017-01-04" to="2017-01-06" />
</booking>
</variant>
<variant name="b">
<booking>
<date from="2017-01-01" to="2017-01-02" />
<date from="2017-01-04" to="2017-01-06" />
</booking>
</variant>
<variant name="c">
<booking>
<date from="2017-04-06" to="2017-04-07" />
<date from="2017-04-07" to="2017-04-09" />
</booking>
</variant>
</root>
I'd like to group the three variants so that each variants with same #from and #to in each date should be grouped together.
My attempt is:
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output indent="yes"></xsl:output>
<xsl:template match="root">
<variants>
<xsl:for-each-group select="for $i in variant return $i" group-by="booking/date/#from">
<group>
<xsl:attribute name="cgk" select="current-grouping-key()"/>
<xsl:copy-of select="current-group()"></xsl:copy-of>
</group>
</xsl:for-each-group>
</variants>
</xsl:template>
</xsl:stylesheet>
But this gives too many groups. (How) is this possible to achieve?

Answer 1

Using a composite key and XSLT 3.0 you could use
<xsl:template match="root">
<variants>
<xsl:for-each-group select="variant" group-by="booking/date/(#from, #to)" composite="yes">
<group key="{current-grouping-key()}">
<xsl:copy-of select="current-group()"/>
</group>
</xsl:for-each-group>
</variants>
</xsl:template>
which should group any variant elements together which have the same descendant date element sequence.
XSLT 3.0 is supported by Saxon 9.8 (any edition) or 9.7 (PE and EE) or a 2017 release of Altova XMLSpy/Raptor.
Using XSLT 2.0 you could concatenate all those date values with string-join():
<xsl:template match="root">
<variants>
<xsl:for-each-group select="variant" group-by="string-join(booking/date/(#from, #to), '|')">
<group key="{current-grouping-key()}">
<xsl:copy-of select="current-group()"/>
</group>
</xsl:for-each-group>
</variants>
</xsl:template>
Like the XSLT 3.0 solution, it only groups variant with the same sequence of date descendants, I am not sure whether that suffices or whether you might want to sort any date descendants first before computing the grouping key. In the XSLT 3 case you could do that easily with
<xsl:for-each-group select="variant" group-by="sort(booking/date, (), function($d) { xs:date($d/#from), xs:date($d/#to) })!(#from, #to)" composite="yes">
inline (although that leaves 9.8 HE behind as it does not support function expressions/higher order functions, so there you would need to move the sorting to your own user-defined xsl:function and in there use xsl:perform-sort).