XSLT Cookbook

Determining Set Equality by Value

Problem

You need to determine if the nodes in one node set are equal (by value) to the nodes in another node set (ignoring order).

Solution

This problem is slightly more subtle than it appears on the surface. Consider an obvious solution that works in many cases:

<xsl:template name="vset:equal-text-values">
  <xsl:param name="nodes1" select="/.."/>
  <xsl:param name="nodes2" select="/.."/>
  <xsl:choose>
   <!--Empty node-sets have equal values -->
    <xsl:when test="not($nodes1) and not($nodes2)">
      <xsl:value-of select="true(  )"/>
      </xsl:when>
    <!--Node sets of unequal sizes can not have equal values -->
    <xsl:when test="count($nodes1) != count($nodes2)"/>
    <!--If an element of nodes1 is present in nodes2 then the node sets 
     have equal values if the node sets without the common element have equal 
     values -->
    <xsl:when test="$nodes1[1] = $nodes2">
      <xsl:call-template name="vset:equal-text-values">
          <xsl:with-param name="nodes1" select="$nodes1[position(  )>1]"/>
          <xsl:with-param name="nodes2" 
                         select="$nodes2[not(. = $nodes1[1])]"/>
      </xsl:call-template>
    </xsl:when>
    <xsl:otherwise/>
  </xsl:choose>
</xsl:template>

We have chosen a name for this equality test to emphasize the context in which it should be applied. That is when value equality indicate string-value equality. Clearly, this template will not give the correct result if equality is based on attributes or criteria that are more complex. However, this template has a more subtle problem. It tacitly assumes that the ...