[XeTeX] not enough \XeTeXcharclass registers

[David Carlisle]

Thanks for the test sources,

It all seems to work for me (texlive 2015/cygwin 64 build), but..

I do wonder if this change is going in the right direction.

The main problem with the char classes is not the overall number, in fact
since the important thing as far as specifying code is the boundary between
different classes rather than the classes themselves, there are now around
300 million such boundaries that could be specified, which seems more than
enough!

The main problem is that each character can only be in one class which
means that it is very hard to use these for any generic code. If you have
already classified characters by (say) line breaking properties and then
another package wants to classify by unicode block, or by default writing
direction, then the only way to handle that is to enumerate all the
intersecting properties and assign a a unique character class to each
intersection, this leads to a combinatorial explosion in the number of
boundary tokens that need to be specified. Where you may have had a single
specification for the boundary between LTR and RTL if you also want to
classify each unicode block you need  separate classes for LTR and RTL
characters in each block and then need to specify the same boundary tokens
for all the possible changes of LTR in one block followed by RTL in another.

That limitation of course has always been there, but increasing the number
of classes available highlights it more strongly.

Would it be impossibly difficult to extend the concept so that a character
takes a list of character classes so that you can classify characters in
more than one way without needing impossibly many character classes to do
that?

Sorry for sounding ungrateful for the extension:-)

David
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