Re: [xml] EXSLT random() function

[Daniel Veillard <veillard redhat com>]

On Tue, Feb 19, 2008 at 12:06:38PM -0600, Chuck Bearden wrote:
> Should I expect the results of the EXSLT random() function to be 
> different for each invocation?  I transform an XML file with an identity 
> transform, changing it only by adding an attribute @tyche with the value 
> of the EXSLT random function.  For a given input file and a given 
> stylesheet, the output of all transformations are identical.  Is this 
> expected?  My hope was to use random() to make a random selection of 
> elements from a document, such that each time I made the selection I 
> would get a different selection.
>
> I'm posting sample input, stylesheet, and output files at
> <http://www.owlnet.rice.edu/~cbearden/exslt_random/>
> 'randomize.xsl' is the stylesheet.  'modules.xml' is the input. 'one' 
> and 'two' are the output from two invocations of the transformation as 
> 'xsltproc randomize.xsl modules.xml'.
>
> $ xsltproc --version
> Using libxml 20627, libxslt 10120 and libexslt 813
> xsltproc was compiled against libxml 20627, libxslt 10120 and libexslt 813
> libxslt 10120 was compiled against libxml 20627
> libexslt 813 was compiled against libxml 20627

 Use the source, it's available !

static double
exsltMathRandom (void) {
    double ret;
    int num;

    num = rand();
    ret = (double)num / (double)RAND_MAX;
    return(ret);
}

and man rand() states

--------------------
       The  rand()  function  returns  a  pseudo-random  integer between 0 and
       RAND_MAX.

       The srand() function sets its argument as the seed for a  new  sequence
       of  pseudo-random  integers  to be returned by rand().  These sequences
       are repeatable by calling srand() with the same seed value.

       If no seed value is provided,  the  rand()  function  is  automatically
       seeded with a value of 1.
--------------------

I don't see any srand() call in that module.
So yes I would expect the behaviour of exslt:random to be predictable and
reproductible.

  http://www.exslt.org/math/functions/random/index.html

just states 
  'The math:random function returns a random number from 0 to 1.'

so apparently libexslt implementation might just be conformant, but
as usual we takes patches ... But I don't see how you would seed in
a truely random fashion while keeping the code portable.

I would actually think that if you link to libxslt, and use it to
process multiple document with that stylesheet in the same process, you
would get the expect effect, just that a single xsltproc instance will
always return the same on the same system.

Daniel

-- 
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine  http://rpmfind.net/