XSLT Muenchian grouping create key inside group

My goal is to group the nodes first by <RowBreak>, then in each <RowBreak> group, I want to group by <ColumnBreak>.

Here is my XML.

<?xml version="1.0" encoding="utf-8" ?>
<Tree>
  <Item>
    <Label>Item 1</Label>
  </Item>
  <Item>
    <Label>Item 2</Label>
  </Item>
  <ColumnBreak />
  <Item>
    <Label>Item 3</Label>
  </Item>
  <Item>
    <Label>Item 4</Label>
  </Item>
  <Item>
    <Label>Item 5</Label>
  </Item>
  <RowBreak />
  <Item>
    <Label>Item 6</Label>
  </Item>
  <Item>
    <Label>Item 7</Label>
  </Item>
  <ColumnBreak />
  <Item>
    <Label>Item 8</Label>
  </Item>
  <RowBreak />
  <Item>
    <Label>Item 9</Label>
  </Item>
  <Item>
    <Label>Item 10</Label>
  </Item>
</Tree>

The output should be:

Item 1  Item 3
Item 2  Item 4
        Item 5

Item 6  Item 8
Item 7

Item 9
Item 10

My current XSLT is like this:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
  <xsl:output method="html" indent="yes"/>
  <xsl:key name="rowGroups" match="Tree/*[not(self::RowBreak)]" use="count(preceding-sibling::RowBreak)" />

  <xsl:template match="Tree">
    <xsl:variable name="rowGroupings" select="*[not(self::RowBreak)][generate-id() = generate-id(key('rowGroups', count(preceding-sibling::RowBreak))[1])]" />
    <xsl:variable name="position" select="position()" />
    <table>
      <xsl:for-each select="$rowGroupings">
        <xsl:variable name="rowId" select="generate-id()"/>
        <xsl:variable name="colGroupings" select="*[not(self::ColumnBreak)][generate-id()=$rowId][1]" />
        <tr>
            <xsl:for-each select="$colGroupings">
              <!--Do logic here to group by ColumnBreak-->
            </xsl:for-each>
        </tr>
      </xsl:for-each>
    </table>
  </xsl:template>
</xsl:stylesheet>

However, I'm getting problem with extracting the <ColumnBreak> groups in every <RowBreak> (see colGroupings variable). I want to create a <key> for every <RowBreak> in the loop (similar to rowGroups), but as per my understanding of <xsl:key> element, it has to be declared top-level and the match should work on actual nodes, not on variables.

Solution

This is what I would do as the first pass:

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:strip-space elements="*"/>

<xsl:template match="/Tree">
    <cells>
        <xsl:apply-templates select="Item[1]" mode="sibling">
            <xsl:with-param name="row" select="1"/>
            <xsl:with-param name="col" select="1"/>
        </xsl:apply-templates>  
    </cells>
</xsl:template>

<xsl:template match="Item" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <cell row="{$row}" col="{$col}">
        <xsl:value-of select="Label"/>
    </cell>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row"/>
        <xsl:with-param name="col" select="$col"/>
    </xsl:apply-templates>  
</xsl:template>

<xsl:template match="ColumnBreak" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row"/>
        <xsl:with-param name="col" select="$col + 1"/>
    </xsl:apply-templates>  
</xsl:template>

<xsl:template match="RowBreak" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row + 1"/>
        <xsl:with-param name="col" select="1"/>
    </xsl:apply-templates>  
</xsl:template>

</xsl:stylesheet>

Applied to your example input, this would produce:

Result

<?xml version="1.0" encoding="UTF-8"?>
<cells>
  <cell row="1" col="1">Item 1</cell>
  <cell row="1" col="1">Item 2</cell>
  <cell row="1" col="2">Item 3</cell>
  <cell row="1" col="2">Item 4</cell>
  <cell row="1" col="2">Item 5</cell>
  <cell row="2" col="1">Item 6</cell>
  <cell row="2" col="1">Item 7</cell>
  <cell row="2" col="2">Item 8</cell>
  <cell row="3" col="1">Item 9</cell>
  <cell row="3" col="1">Item 10</cell>
</cells>

which is something that can be actually worked with.

Added:

Here's a complete stylesheet that processes the input in two passes:

the first pass uses sibling recursion as described above;
the second pass does Muenchian grouping by rows and by columns in order to produce the table described by the original document:

XSLT 1.0 (with EXSLT node-set() extension function)

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:exsl="http://exslt.org/common"
extension-element-prefixes="exsl">
<xsl:strip-space elements="*"/>
<xsl:output method="xml" version="1.0" encoding="utf-8" indent="yes"/>

<xsl:key name="cell-by-row" match="cell" use="@row" />
<xsl:key name="cell-by-col" match="cell" use="concat(@row, '|', @col)" />

<xsl:template match="/Tree">
    <!-- first-pass -->
    <xsl:variable name="cells">
        <xsl:apply-templates select="Item[1]" mode="sibling">
            <xsl:with-param name="row" select="1"/>
            <xsl:with-param name="col" select="1"/>
        </xsl:apply-templates>  
    </xsl:variable>
    <!-- output -->
    <table border = "1">
        <!-- for each distinct row -->
        <xsl:for-each select="exsl:node-set($cells)/cell[count(. | key('cell-by-row', @row)[1]) = 1]">
            <tr>
                <!-- for each distinct cell in the current row -->
                <xsl:for-each select="key('cell-by-row', @row)[count(. | key('cell-by-col', concat(@row, '|', @col))[1]) = 1]">
                    <td>
                        <!-- get the values in the current cell -->
                        <xsl:for-each select="key('cell-by-col', concat(@row, '|', @col))">
                            <xsl:value-of select="."/>
                            <br/>
                        </xsl:for-each>
                    </td>
                </xsl:for-each>
            </tr>
        </xsl:for-each>
    </table>
</xsl:template>

<xsl:template match="Item" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <cell row="{$row}" col="{$col}">
        <xsl:value-of select="Label"/>
    </cell>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row"/>
        <xsl:with-param name="col" select="$col"/>
    </xsl:apply-templates>  
</xsl:template>

<xsl:template match="ColumnBreak" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row"/>
        <xsl:with-param name="col" select="$col + 1"/>
    </xsl:apply-templates>  
</xsl:template>

<xsl:template match="RowBreak" mode="sibling">
    <xsl:param name="row"/>
    <xsl:param name="col"/>
    <xsl:apply-templates select="following-sibling::*[1]" mode="sibling">
        <xsl:with-param name="row" select="$row + 1"/>
        <xsl:with-param name="col" select="1"/>
    </xsl:apply-templates>  
</xsl:template>

</xsl:stylesheet>

Result

<?xml version="1.0" encoding="utf-8"?>
<table border="1">
  <tr>
    <td>Item 1<br/>Item 2<br/></td>
    <td>Item 3<br/>Item 4<br/>Item 5<br/></td>
  </tr>
  <tr>
    <td>Item 6<br/>Item 7<br/></td>
    <td>Item 8<br/></td>
  </tr>
  <tr>
    <td>Item 9<br/>Item 10<br/></td>
  </tr>
</table>

Rendered