How to partition dates with XSLT - xslt

Question

I have a group of dates and I'd like to create partitions with a criterion such as "exactly 7 days apart" For example this is my source xml:
<root>
<entry date="2019-05-12" />
<entry date="2019-05-19" />
<entry date="2019-05-26" />
<entry date="2019-06-16" />
<entry date="2019-06-23" />
</root>
The result should be like this:
<root>
<group>
<val>12.5.</val>
<val>19.5.</val>
<val>26.5.</val>
</group>
<group>
<val>16.6.</val>
<val>23.6.</val>
</group>
</root>
since the first three and the last two dates are all on a Sunday without a gap.
What I have so far is this:
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:sd="urn:someprefix"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="#all"
>
<xsl:output indent="yes"/>
<xsl:template match="root">
<root>
<xsl:copy-of select="sd:partition(distinct-values(for $i in entry/#date return $i cast as xs:date))"/>
</root>
</xsl:template>
<xsl:function name="sd:partition">
<xsl:param name="dates" as="xs:date*"/>
<xsl:for-each-group select="$dates" group-adjacent="format-date(., '[F]')">
<group>
<xsl:for-each select="current-group()">
<val>
<xsl:value-of select="format-date(.,'[D].[M].')"/>
</val>
</xsl:for-each>
</group>
</xsl:for-each-group>
</xsl:function>
</xsl:stylesheet>
Which only generates one group.
How can I ask for the previous element to be 7 days apart? I know of duration (xs:dayTimeDuration('P1D')), but I don't know how to compare it to a previous value.
I use Saxon 9.8 HE.

Answer 1

I think you can also do it using group-adjacent:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="#all"
expand-text="yes"
version="3.0">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="root">
<xsl:copy>
<xsl:for-each-group select="entry/#date/xs:date(.)"
group-adjacent=". - (position() - 1) * xs:dayTimeDuration('P7D')">
<group>
<xsl:apply-templates select="current-group()"/>
</group>
</xsl:for-each-group>
</xsl:copy>
</xsl:template>
<xsl:template match=".[. instance of xs:date]">
<val>{format-date(.,'[D].[M].')}</val>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/ncdD7mM

Answer 2

To do your grouping, you really need to know the difference in days with the previous element, then you can group starting with dates where the difference is not 7 days. So, you can declare a variable where you build up some new XML with the dates and differences, and then use that to group.
Try this function in your XSLT instead.
<xsl:function name="sd:partition">
<xsl:param name="dates" as="xs:date*"/>
<xsl:variable name="datesWithDiff" as="element()*">
<xsl:for-each select="$dates">
<xsl:variable name="pos" select="position()" />
<date diff="{(. - $dates[$pos - 1]) div xs:dayTimeDuration('P1D')}">
<xsl:value-of select="." />
</date>
</xsl:for-each>
</xsl:variable>
<xsl:for-each-group select="$datesWithDiff" group-starting-with="date[#diff = '' or xs:int(#diff) gt 7]">
<group>
<xsl:for-each select="current-group()">
<val>
<xsl:value-of select="format-date(.,'[D].[M].')"/>
</val>
</xsl:for-each>
</group>
</xsl:for-each-group>
</xsl:function>