XSLT 1.0 Greedy knapsack grouping methods? - xslt

Question

I have an XML dataset (provided from SharePoint 2007 to a DVWP) structured something like:
<Rows>
<Row ID="1" Spanoffset="0" Span="55" Spantail="55"/>
<Row ID="2" Spanoffset="30" Span="31" Spantail="61"/>
<Row ID="3" Spanoffset="61" Span="20" Spantail="81"/>
<Row ID="4" Spanoffset="82" Span="30" Spantail="112"/>
</Rows>
Say each row represents a bar that starts at #Spanoffset and is #Span wide, #Spantail is there so I don't have to calculate it if I need it. I am trying to pack the rows together efficently so that rows that wont overlap get grouped together. The dataset is pre-sorted by #Spanoffset. This is essentially a knapsack problem as each Row could fit in multiple possible groups. What I want to do is a simple greedy solution, and know how I could code it in say c# or java, but since I cannot mark nodes as visited (well I can, but I lose it when I come back up the recursion tree) and I cannot seem to remove nodes as I visit them, I am at a loss for how to approach this.
For example the above data would look something like this:
<div style="clear:both">
<div style="width: 110px; margin-left: 0px; float:left;">1</div>
<div style="width: 40px; margin-left: 12px; float:left;">3</div>
<div style="width: 60px; margin-left: 2px; float:left;">4</div>
</div>
<div style="clear:both">
<div style="width: 62px; margin-left: 60px; float:left;">2</div>
</div>
I haven't been bothering trying to get the floats to work right as I have yet to be able to get the Row nodes to appear only one time each, in the right order. Once I get them there I am fairly certain I can get the formatting to work out.
The best XSLT I have come up with so far has been:
<xsl:template match="row">
<xsl:variable name="tail" select="#Spantail"/>
<div style="width:{2*#Span}px;
left:{2*(#Spanoffset)}px;">
<xsl:value-of select="#ID"/>
</div>
<xsl:apply-templates select="(following-sibling::row)[#Spanoffset>=$tail][1]"/>
</xsl:template>
Which generates
<div style="width: 110px;left: 0px">1</div>
<div style="width: 40px; left: 122px">3</div>
<div style="width: 60px; left: 164px">4</div>
<div style="width: 62px; left: 60px">2</div>
<div style="width: 40px; left: 122px">3</div>
<div style="width: 60px; left: 164px">4</div>
<div style="width: 40px; left: 122px">3</div>
<div style="width: 60px; left: 164px">4</div>
<div style="width: 60px; left: 164px">4</div>
So my problems are 2 (that I see) and I think they are intertwined.
1) How to fix/re-factor my template(s) to only emit each row once.
and
2) How to wrap the groupings in container <div> elements.
Been banging my head against this for 2 days, any one able to help?
Edit:
Well, after some sleep, I have the wrapping container by adding a boolean parameter to my template, and using some CDATA tags to emit <div> tags when its true. The boolean defaults to true, and when I call the nested apply-templates I set it to false, hence wrapping up the groups in containers. I still cant see a way of marking <Row>s as visited yet.

Answer 1

I think it's very tricky with just core XSLT, but it's easier with node-set(), a XSLT extension function:
<xsl:template name="add-row">
<xsl:param name="row"/>
<xsl:param name="prev-group" />
<xsl:if test="$row and not($row/#ID = $prev-group/Row/#ID)">
<xsl:copy-of select="$row" />
<xsl:call-template name="add-row">
<xsl:with-param name="row" select="$row/following-sibling::Row[#Spanoffset > $row/#Spantail][1]" />
<xsl:with-param name="prev-group" select="$prev-group" />
</xsl:call-template>
</xsl:if>
</xsl:template>
<xsl:template name="add-group">
<xsl:param name="first-row" />
<xsl:param name="prev-group" select="exsl:node-set(/)" />
<xsl:if test="$first-row">
<xsl:variable name="group">
<xsl:call-template name="add-row">
<xsl:with-param name="row" select="$first-row" />
<xsl:with-param name="prev-group" select="$prev-group" />
</xsl:call-template>
</xsl:variable>
<div clear="both">
<xsl:for-each select="exsl:node-set($group)/Row">
<div style="width: {2*#Span}px; left: {2*#Spanoffset}px"><xsl:value-of select="#ID"/></div>
</xsl:for-each>
</div>
<xsl:call-template name="add-group">
<xsl:with-param name="first-row" select="$first-row/following-sibling::Row[#Spanoffset < preceding-sibling::Row/#Spantail][1]" />
<xsl:with-param name="prev-group" select="exsl:node-set($group)" />
</xsl:call-template>
</xsl:if>
</xsl:template>
<xsl:template match="Rows">
<xsl:call-template name="add-group">
<xsl:with-param name="first-row" select="Row[1]" />
</xsl:call-template>
</xsl:template>
Don't forget to declare extension prefix and namespace in your stylesheet tag:
<xsl:stylesheet
version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
extension-element-prefixes="exsl"
xmlns:exsl="http://exslt.org/common">
http://exslt.org/common is a valid namespace for Java XSLT processors, such as Xalan or Saxon; if you're using MSXML, use urn:schemas-microsoft-com:xslt instead.