Sounds a nice idea independent of any speed benefits. I toyed with the idea
of parsing XSLT match pattterns backwards at one stage, but didn't pursue
it. My brain is wired for left-to-right reading...

As a matter of interest, is there any problem with decoding UTF-8 when
reading backwards?

Shame that the spec doesn't require ">" to be escaped, I imagine this causes
a fair problem with backtracking, for example how do you cope with

...... <a><b/><c/></a>  -->

which might or might not be part of a comment?

IIRC we were able to read files backwards on ICL VME. That's history, but
many of its features have reappeared in Windows 25 years later ...

Michael Kay
http://www.saxonica.com/

 

> -----Original Message-----
> From: Sean McGrath [mailto:sean.mcgrath@p...] 
> Sent: 01 April 2006 08:22
> To: xml-dev@l...
> Subject: [xml-dev] Exploiting multi-core CPUs during XML parsing
> 
> I have sketched out an algorithm for fast XML WF parsing 
> utilising two 
> threads that each start at opposite ends of the octet-stream 
> and meet in 
> the middle. The algorithm hinges on the fact that start- and end-tags 
> are balanced. i.e. as one thread reads forward looking for foo 
> start-tag, the other thread is reading backwards looking for 
> foo end-tag.
> 
> This also has the nice side effect of giving you accurate 
> error messages 
> quickly. i.e. as soon as a mismatched tag is found, it can be 
> reported. 
> This is particularly useful with recursive element types.
> 
> Modern machines (even desktop boxes) will increasingly have 
> multi-core 
> CPUs as standard and I think it behooves us to find ways of 
> exploiting 
> that in the XML world.
> 
> But to get the most out of the algorithm, you really need a 
> filesystem 
> that supports reading byte-streams backwards.
> 
> Does anyone know of such a filesystem?
> 
> thanks,
> Sean
> 
> 
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