(Visual FoxPro) XML Path Performance Optimizations

Discusses some important things to know about using Chilkat paths in the Chilkat XML API.

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL loXml
LOCAL lnSuccess
LOCAL lnLicCount
LOCAL s
LOCAL i

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.Xml')
loXml = CreateObject('Chilkat.Xml')

* Let's load XML containing the following:

* <?xml version="1.0" encoding="utf-8"?>
* <xyz>
*     <licenses>
*         <license>
*             <id>1234</id>
*         </license>
*         <license>
*             <id>1234</id>
*         </license>
* ...
* My sample XML contains 64,000 "license" nodes ..
* ...
*         <license>
*             <id>1234</id>
*         </license>
*         <license>
*             <id>1234</id>
*         </license>
*     </licenses>
* </xyz>
* 
lnSuccess = loXml.LoadXmlFile("qa_output/large.xml")
IF (lnSuccess <> 1) THEN
    ? loXml.LastErrorText
    RELEASE loXml
    CANCEL
ENDIF

* Iterating over the individual "license" nodes with this code snippet is
* extremely slow:
lnLicCount = loXml.NumChildrenHavingTag("licenses|license")
? "license count = " + STR(lnLicCount)

i = 0
* If "10" is changed to licCount, then it becomes apparent that this loop gets slower with each iteration.
DO WHILE i < 10
    loXml.I = i
    s = loXml.GetChildContent("licenses|license[i]|id")
    ? STR(i) + ": " + s
    i = i + 1
ENDDO

* The reason it is extremely slow is that the "license[i]" part of the path passed to GetChildContent
* says: find the i'th child of "licenses" having the tag "license".  Chilkat cannot assume that all
* children of an XML node have the same tag.  Therefore it's not possible to directly access the i'th child.
* Internally, Chilkat must start at the 1st child and iterate until it reaches the i'th child having the
* tag "license".

* For example, imagine if the XML was like this:

* <?xml version="1.0" encoding="utf-8"?>
* <xyz>
*     <licenses>
*         <license>
*             <id>1234</id>
*         </license>
*         <somethingElse>
*             <a>abc</a>
*         </somethingElse>
*         <license>
*             <id>1234</id>
*         </license>
* ...

* In the above XML, the 1st "license" is the 1st child of "licenses", but the 2nd "license"
* is the 3rd child of "licenses".

* If you already know that all children have the same tag, there is a shortcut that allows
* for direct access to that child.  Just leave off the tag name, like this:

i = 0
* If "10" is changed to licCount, then we can see the time for each loop is the same, and it's fast.
DO WHILE i < 10
    loXml.I = i
    s = loXml.GetChildContent("licenses|[i]|id")
    ? STR(i) + ": " + s
    i = i + 1
ENDDO

* When we pass just the index "[i]", we're saying: Get the i'th child regardless of tag.
* This is extremely fast because internally we can just access the i'th child directly.

* Another performance improvement is to call NumChildrenAt rather than NumChildrenHavingTag.
* For example:
lnLicCount = loXml.NumChildrenAt("licenses")
? "licCount = " + STR(lnLicCount)

* NumChildrenAt returns the total number of children at the tag path.  If we already know
* all children will have the same tag, we can just get the count

RELEASE loXml