How to traverse and sum XML document with many-to-many relationship using XSLT 1.0?

I need help to process following XML document using XSL 1.0, and I don't have the words to describe what the actual problem is.

I have a collection of items collectionA I'd like to process. collectionA is related to collectionB from which I'd like to get the sum of all related num values. The relationship is a many-to-many relationship, defined by the collectionC items, that connects the A and B items.

In any other object model based programming language I would use a loop. Problem is that I don't see how to both loop AND to collect the sum of all values in xslt 1.0...?

<root>
 <collectionA>
   <id>1</id>
 </collectionA>
 <collectionA>
  <id>2</id>
 </collectionA>

 <collectionB>
  <id>1</id>
  <num>11</num>
 </collectionB>
 <collectionB>
  <id>2</id>
  <num>22</num>
 </collectionB>
 <collectionB>
  <id>3</id>
  <num>33</num>
 </collectionB>
 <collectionB>
  <id>4</id>
  <num>44</num>
 </collectionB>

 <collectionC>
  <collectionAid>1</collectionAid>
  <collectionBid>1</collectionBid>
  <collectionBid>2</collectionBid>
 </collectionC>
 <collectionC>
  <collectionAid>2</collectionAid>
  <collectionBid>3</collectionBid>
  <collectionBid>4</collectionBid>
 </collectionC>
</root>

Expected result is:

<root>
 <collectionA>
  <id>1</id>
  <sum>33</sum>
 </collectionA>
 <collectionA>
  <id>2</id>
  <sum>77</sum>
 </collectionA>
</root>

Anyone!?

Solution

I don't see how to both loop AND to collect the sum of all values

You do not need to do either one of these. All you need is a pair of keys to resolve the cross-references. Then use them to select the related nodes and sum their values:

XSLT 1.0

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

<xsl:key name="b" match="collectionB" use="id" />
<xsl:key name="c" match="collectionC" use="collectionAid" />

<xsl:template match="/root">
    <xsl:copy>
        <xsl:for-each select="collectionA">
            <xsl:copy>
                <xsl:copy-of select="id"/>
                <sum>
                    <xsl:value-of select="sum(key('b', key('c', id)/collectionBid)/num)"/>
                </sum>
            </xsl:copy>
        </xsl:for-each>
    </xsl:copy>
</xsl:template>

</xsl:stylesheet>