Conditional Streaming Accumulator in XSLT 3 - xslt-3.0

Question

With input XML like this:
<?xml version="1.0" encoding="UTF-8"?>
<Root>
<Entry>
<Amount>2088</Amount>
<DebitCredit>C</DebitCredit>
</Entry>
<Entry>
<Amount>9074</Amount>
<DebitCredit>D</DebitCredit>
</Entry>
...
</Root>
I'd like to create a streamable transformation with separate accumulators for debits & credits, but when I attempted to create an accumulator for one type or the other similar to this
<xsl:accumulator name="debitcount" initial-value="0" streamable="yes">
<xsl:accumulator-rule phase="end"
match="Entry[DebitCredit eq 'D']"
select="$value + 1"/>
</xsl:accumulator>
I'm finding that apparently the sweep of any patterns in the match, select, or sequence constructor must be motionless. I'm able to access attribute values for the current element but not child elements or anything ahead of the current element.
I'm wondering if what I'm trying to do is even possible with accumulators in streaming mode - I'm pretty sure I can accomplish what I'm aiming for here with iterator params, but it seems like a pretty big limitation if I'm understanding the docs correctly.

Answer 1

In my understanding, the only way with a streaming accumulator to store the content of an element is to match on the text child node so e.g.
<xsl:accumulator name="current-amount" as="xs:decimal?" initial-value="()" streamable="yes">
<xsl:accumulator-rule match="Entry/Amount/text()" select="xs:decimal(.)"/>
</xsl:accumulator>
Then you should be able to add up those values in similar rules like
<xsl:accumulator name="credit" as="xs:decimal" initial-value="0" streamable="yes">
<xsl:accumulator-rule match="Entry/DebitCredit/text()" select="if (. = 'C') then $value + accumulator-before('current-amount') else $value"/>
</xsl:accumulator>
<xsl:accumulator name="debit" as="xs:decimal" initial-value="0" streamable="yes">
<xsl:accumulator-rule match="Entry/DebitCredit/text()" select="if (. = 'D') then $value - accumulator-before('current-amount') else $value"/>
</xsl:accumulator>
The same approach (i.e. matching on the text child node) should do if you only want to count the number of DebitCredit/text()[. = 'C'] nodes.
It is a bit of pain if you are used to normal XSLT/XPath where I certainly advice people to never explicitly select text nodes unless they have to deal with mixed contents but I have found that uses streaming forces you to change your XSLT/XPath coding approach quite a lot.
Here is an example, with the input being
<Root>
<Entry>
<Amount>100</Amount>
<DebitCredit>C</DebitCredit>
</Entry>
<Entry>
<Amount>50</Amount>
<DebitCredit>D</DebitCredit>
</Entry>
<Entry>
<Amount>10</Amount>
<DebitCredit>C</DebitCredit>
</Entry>
<Entry>
<Amount>20</Amount>
<DebitCredit>D</DebitCredit>
</Entry>
</Root>
Saxon 9.8.0.8 EE with the stylesheet
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:math="http://www.w3.org/2005/xpath-functions/math"
exclude-result-prefixes="xs math"
version="3.0">
<xsl:output method="text"/>
<xsl:mode on-no-match="shallow-skip" streamable="yes" use-accumulators="#all"/>
<xsl:accumulator name="current-amount" as="xs:decimal?" initial-value="()" streamable="yes">
<xsl:accumulator-rule match="Entry/Amount/text()" select="xs:decimal(.)"/>
</xsl:accumulator>
<xsl:accumulator name="credit" as="xs:decimal" initial-value="0" streamable="yes">
<xsl:accumulator-rule match="Entry/DebitCredit/text()" select="if (. = 'C') then $value + accumulator-before('current-amount') else $value"/>
</xsl:accumulator>
<xsl:accumulator name="debit" as="xs:decimal" initial-value="0" streamable="yes">
<xsl:accumulator-rule match="Entry/DebitCredit/text()" select="if (. = 'D') then $value - accumulator-before('current-amount') else $value"/>
</xsl:accumulator>
<xsl:accumulator name="debitcount" initial-value="0" streamable="yes">
<xsl:accumulator-rule
match="Entry/DebitCredit/text()[. = 'D']"
select="$value + 1"/>
</xsl:accumulator>
<xsl:template match="/*" expand-text="yes">
<xsl:apply-templates/>
Debit count {accumulator-after('debitcount')}
Sum of credits {accumulator-after('credit')},
Sum of debits {accumulator-after('debit')}
</xsl:template>
</xsl:stylesheet>
outputs
Debit count 2
Sum of credits 110,
Sum of debits -70