Efficient mapping of two large lists

Question

I've been tasked with writing some XSLT 2.0 to translate an XML document to another XML document. I'm relatively new to XSLT but I have learn alot during the days I've do this. During this time I have had to map simple values, i.e. 002 -> TH etc. This has been fine for small lists of less than 10 values, I used xsl:choose. However I need to map over 300 values from one list to another and vice versa. Each list has a value and textual description. The two list values do not always directly map, so I may have to compare textual descriptions and use default values if necessary.

I have two solutions to the problem:

1. Use xsl:choose: This I think could be slow and possible hard to update if either of the lists changes.

2. Have a XML document with the relationship between each list item. I would use an XPath expressions to retrieve an associated value: This is my preferred solution because I believe it will be more maintainable and easier to update. Although I'm not sure it is efficient.

What solution should I use, one of my suggestion, or is there a better way to map these values?

Answer 1

Here is an XSLT 2.0 solution.

Source XML file:

<input>
  <data>001</data>
  <data>002</data>
  <data>005</data>
</input>

"Mapping" xml file:

<map>
  <default>?-?-?</default>
    <input value="001">RZ</input>
    <input value="002">TH</input>
    <input value="003">SC</input>
</map>

XSLT transformation:

<xsl:stylesheet version="2.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output omit-xml-declaration="yes" indent="yes"/>

  <xsl:param name="pmapFile" 
       select="'C:/temp/deleteMap.xml'" />

  <xsl:variable name="vMap" 
       select="document($pmapFile)" />

  <xsl:variable name="vDefault" 
       select="$vMap/*/default/text()" />

  <xsl:key name="kInputByVal" match="input" 
   use="@value" />

  <xsl:template match="/*">
    <output>
      <xsl:apply-templates/>
    </output>
  </xsl:template>

  <xsl:template match="data">
    <data>
        <xsl:sequence select= 
         "(key('kInputByVal', ., $vMap)[1]/text(),
           $vDefault
           )[1]
         "/>
    </data> 
  </xsl:template>
</xsl:stylesheet>

Output:

<output>
  <data>RZ</data>
  <data>TH</data>
  <data>?-?-?</data>
</output>

Do note the following:

1. The use of the document() function to access the "mapping" xml document, which is stored in a separate XML file.

2. The use of <xsl:key/> and the XSLT 2.0 key() function to determine and access each corresponding output value. The third argument specifies the xml document that must be accessed and indexed.

Answer 2

Here is a way to do what you intend, using an <xsl:key> and otherwise following your method two.

The sample input file (data.xml):

<?xml version="1.0" encoding="utf-8"?>
<input>
  <data>001</data>
  <data>002</data>
  <data>005</data>
</input>

The sample map file (map.xml):

<?xml version="1.0" encoding="utf-8"?>
<map default="??">
  <entry key="001">RZ</entry>
  <entry key="002">TH</entry>
  <entry key="003">SC</entry>
</map>

The sample XSL stylesheet, explanation follows:

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

  <xsl:param name="map-file" select="string('map.xml')" />
  <xsl:variable name="map-doc" select="document($map-file)" />
  <xsl:variable name="default-value" select="$map-doc/map/@default" />
  <xsl:key name="map" match="/map/entry" use="@key" />

  <xsl:template match="/input">
    <output>
      <xsl:apply-templates select="data" />
    </output>
  </xsl:template>

  <xsl:template match="data">
    <xsl:variable name="raw-value" select="." />
    <xsl:variable name="mapped-value">
      <xsl:for-each select="$map-doc">
        <xsl:value-of select="key('map', $raw-value)" />
      </xsl:for-each>
    </xsl:variable>
    <data>
      <xsl:choose>
        <xsl:when test="$mapped-value = ''">
          <xsl:value-of select="$default-value" />
        </xsl:when>
        <xsl:otherwise>
          <xsl:value-of select="$mapped-value" />
        </xsl:otherwise>
      </xsl:choose>
    </data>
  </xsl:template>
</xsl:stylesheet>

What this does is:

• use document() to open map.xml, saving the resulting node-set to a variable
• save the default value for further reference
• prepare an <xsl:key> to work against the "map" node set
• use <xsl:for-each> not as a loop, but as a means to switch the execution context before calling the key() function - otherwise key() would work against the "data" document and return nothing
• find the corresponding node with the key() function, save it in a variable
• check the variable value on output - if it is empty, use the default value
• repeat (through <xsl:apply-templates>)

The credit for the neat <xsl:for-each> trick goes to Jeni Tennison, who described the technique on the XSL mailing list. Be sure to read the thread.

Output of running the stylesheet against data.xml:

<?xml version="1.0" encoding="utf-8"?>
<output>
  <data>RZ</data>
  <data>TH</data>
  <data>??</data>
</output>

All of this is XSLT 1.0. I'm convinced a better/more elegant version exists that makes use of the advantages XSLT 2.0 offers, but unfortunately I'm not overly familiar with XSLT 2.0. Maybe someone else posts a better solution.

---

EDIT

Through Dimitre Novatchev's hint in the comments, I was able to create a a considerably shorter and more preferable stylesheet:

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

  <xsl:param name="map-file" select="string('map.xml')" />
  <xsl:variable name="map-doc" select="document($map-file)" />
  <xsl:variable name="default" select="$map-doc/map/default[1]" />
  <xsl:key name="map" match="/map/entry" use="@key" />

  <xsl:template match="/input">
    <output>
      <xsl:apply-templates select="data" />
    </output>
  </xsl:template>

  <xsl:template match="data">
    <xsl:variable name="raw-value" select="." />
    <data>
      <xsl:for-each select="$map-doc">
        <xsl:value-of select="(key('map', $raw-value)|$default)[1]" />
      </xsl:for-each>
    </data>
  </xsl:template>
</xsl:stylesheet>

However, this one requires a slightly different map file to work in XSLT 1.0:

<?xml version="1.0" encoding="utf-8"?>
<map>
  <entry key="001">RZ</entry>
  <entry key="002">TH</entry>
  <entry key="003">SC</entry>
  <!-- default entry must be last in document -->
  <default>??</default>
</map>