Counting distinct items in XSLT - xslt

Question

I have XML like this:
<assessment>
<variables>
<variable>
<attributes>
<variable_name value="FRED"/>
</attributes>
</variable>
</variables>
<variables>
<variable>
<attributes>
<variable_name value="MORTIMER"/>
</attributes>
</variable>
</variables>
<variables>
<variable>
<attributes>
<variable_name value="FRED"/>
</attributes>
</variable>
</variables>
</assessment>
I know that with this XSLT:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
<xsl:output method="html"/>
<xsl:template match="assessment">
<xsl:for-each select=".//variables/variable/attributes/variable_name">
<xsl:value-of select="#value"/>
<br/>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
I can output the following:
FRED
MORTIMER
FRED
But what I really want to output is this:
FRED: 2
MORTIMER: 1
That is, I want to list the distinct elements and how many times each occurs. Note that I want the elements to appear in the order of their first appearance (which might rule out some solutions that use sorting).
How do I do this?

Answer 1

This transformation:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text"/>
<xsl:key name="kValueByVal" match="variable_name/#value"
use="."/>
<xsl:template match="/">
<xsl:for-each select="
/*/*/variable/attributes/variable_name/#value
[generate-id()
=
generate-id(key('kValueByVal', .)[1])
]
">
<xsl:value-of select=
"concat(., ' ', count(key('kValueByVal', .)), '
')"/>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
when applied on the provided XML document, produces the wanted, correct result:
FRED 2
MORTIMER 1