Merging pairs of nodes based on attribute, new to template matching - xslt

Question

Say I have the following XML:
<root>
<tokens>
<token ID="t1">blah</token>
<token ID="t2">blabla</token>
<token ID="t3">shovel</token>
</tokens>
<relatedStuff>
<group gID="s1">
<references tokID="t1"/>
<references tokID="t2"/>
</group>
<group gID="s2">
<references tokID="t3"/>
</group>
</relatedStuff>
</root>
Now, considering that a for-each loop for every token would be pretty inefficient and a bad idea, how would one go about using template matching, to transform this xml into the following?
<s id="everything_merged">
<tok id="t1" gID="s1" >blah</tok>
<tok id="t2" gID="s1" >blabla</tok>
<tok id="t3" gID="s2" >shovel</tok>
</s>
All I want from <s> is the "gID", the gID corresponding to the token in the <tokens>.
<xsl:for-each select="b:root/a:tokens/a:token">
<!-- and here some template matching -->
<xsl:attribute name="gID">
<xsl:value-of select="--correspondingNode's--#gID"/>
</xsl:attribute>
</xsl:for-each>
I'm pretty fuzzy on this sort of thing, so thank you very much for any help!

Answer 1

The following stylesheet:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<s id="everything_merged">
<xsl:apply-templates select="/root/tokens/token" />
</s>
</xsl:template>
<xsl:template match="token">
<tok id="{#ID}" gID="{/root/relatedStuff/group[
references[#tokID=current()/#ID]]/#gID}">
<xsl:apply-templates />
</tok>
</xsl:template>
</xsl:stylesheet>
Applied to this input (corrected for well-formedness):
<root>
<tokens>
<token ID="t1">blah</token>
<token ID="t2">blabla</token>
<token ID="t3">shovel</token>
</tokens>
<relatedStuff>
<group gID="s1">
<references tokID="t1" />
<references tokID="t2" />
</group>
<group gID="s2">
<references tokID="t3" />
</group>
</relatedStuff>
</root>
Produces:
<s id="everything_merged">
<tok id="t1" gID="s1">blah</tok>
<tok id="t2" gID="s1">blabla</tok>
<tok id="t3" gID="s2">shovel</tok>
</s>

Answer 2

A solution using keys and pure "push-style:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="kgIDfromTokId" match="#gID"
use="../*/#tokID"/>
<xsl:template match="tokens">
<s id="everything_merged">
<xsl:apply-templates/>
</s>
</xsl:template>
<xsl:template match="token">
<tok id="{#ID}" gID="{key('kgIDfromTokId', #ID)}">
<xsl:apply-templates/>
</tok>
</xsl:template>
</xsl:stylesheet>
when applied on the provided XML document:
<root>
<tokens>
<token ID="t1">blah</token>
<token ID="t2">blabla</token>
<token ID="t3">shovel</token>
</tokens>
<relatedStuff>
<group gID="s1">
<references tokID="t1" />
<references tokID="t2" />
</group>
<group gID="s2">
<references tokID="t3" />
</group>
</relatedStuff>
</root>
the wanted, correct result is produced:
<s id="everything_merged">
<tok id="t1" gID="s1">blah</tok>
<tok id="t2" gID="s1">blabla</tok>
<tok id="t3" gID="s2">shovel</tok>
</s>