Can solve the N-queens - but can't count!

Subject: Can solve the N-queens - but can't count!
From: "Oren Ben-Kiki" <oren@xxxxxxxxxxxxx>
Date: Mon, 14 Jun 1999 20:33:58 +0200
I've written an XSL stylesheet which solves the N queens problem (place N
chess queens on an NxN square board so that no queen threatens another),
just to see whether it can be done. I attach the stylesheet - it is "only"
165 lines long. The input is an XML document of the form:


And the output is an HTML document containing all valid chess boards (each a
separate HTML table).

This seems a silly thing to do, but I found it to be educational, since it
uncovered an inherent problem in the XSL no-side-effects-ever approach. I
can't count the resulting solutions! Actually, that's not exactly correct; I
could do it if I was willing to accept exponentially large template "stack"

Granted that solving the N-queens problem isn't what XSL was designed for,
but this problem is real. As things stand, one can only count things if they
(i) can appear inside a 'count' expression or (ii) if you are willing to
accept a nested template call for each increase in the count (without
exiting the nested template as long as you need the count!).

Another limitation, one which I managed to work around, was not being able
to apply match pattern to tree-valued variables. Instead I used string
operations, which is much less elegant.

This stylesheet should be interesting for anyone doing XSL benchmarking, or
trying to perform all sorts of optimizations on XSL stylesheets.

Have fun,

    Oren Ben-Kiki

Attachment: queens.xsl
Description: Binary data

Current Thread