How to compare siblings from the same group? - xslt

Question

I have a piece of code that looks like this:
<root>
<applicant>
<id>XYZ</id>
<group>
<start_date>2019-04-01</start_date>
<end_date>2019-04-01</end_date>
</group>
<group>
<start_date>2019-04-02</start_date>
<end_date>2019-04-02</end_date>
</group>
<group>
<start_date>2019-04-03</start_date>
<end_date>2019-04-03</end_date>
</group>
</applicant>
<applicant>
<id>ABC</id>
<group>
<start_date>2019-05-01</start_date>
<end_date>2019-05-01</end_date>
</group>
<group>
<start_date>2019-05-02</start_date>
<end_date>2019-05-02</end_date>
</group>
<group>
<start_date>2019-05-03</start_date>
<end_date>2019-05-03</end_date>
</group>
</applicant>
</root>
and I need to group it by applicant and merge into one node with single start date and end date if date from from following sibling is 1 day different (date difference in days is 1)
so for above code to achieve something like:
<root>
<applicant>
<id>XYZ</id>
<start_date>2019-04-01</start_date>
<end_date>2019-04-03</end_date>
</applicant>
<applicant>
<id>ABC</id>
<start_date>2019-05-01</start_date>
<end_date>2019-05-03</end_date>
</applicant>
</root>
I was thinking about using following-sibling:: or some sort of recurrence.

Answer 1

Assuming you can indeed use XSLT 2.0 or above, you could use xsl:for-each-group here, and group starting with elements whose start_date - 1 doesn't match the end_date of the previous group.
Try this XSLT
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
version="3.0">
<xsl:mode on-no-match="shallow-copy"/>
<xsl:output method="xml" indent="yes" />
<xsl:template match="applicant">
<xsl:copy>
<xsl:copy-of select="id" />
<xsl:for-each-group select="group" group-starting-with="group[not(xs:date(start_date) - xs:dayTimeDuration('P1D') = xs:date(preceding-sibling::group[1]/end_date))]">
<group>
<xsl:copy-of select="start_date, current-group()[last()]/end_date" />
</group>
</xsl:for-each-group>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
(This uses XSLT 3.0. In XSLT 2.0, all you would need to do is replace the xsl:mode with the Identity Template)