Re: charset changes

[Thomas Wolff <towo at towo dot net>]

Andy Koppe schrieb:
> > > - .gb18030 (in zh_CN.gb18030)
> > >       
> > it's supported by Windows XP and later.  Maybe we
> > should add it after 1.7.2?
> >     
> I doubt whether it's possible to support it correctly. GB18030 is a
> monster of an encoding, with 1-byte, 2-byte and 4-byte character codes
> and a huge lookup table to map between GB18030 codepoints and Unicode.
>   
It's kind of a monster but doesn't need a huge table. It's a superset of 
GBK and most of the superset range can be transformed
from/to Unicode algorithmically.
I do handle GB18030 in mined; if you point me to where in newlib this is 
handled, I may try a patch.

While some of the more exotic locales and encodings I had listed can 
faithfully be ignored for a good locale implementation,
GB18030 is somewhat official in China, its support was even declared 
mandatory for any software system to be acceptable
for official purposes or something like that; I don't know whether this 
is still viewed strictly but I guess it's worth handling it.

> The first issue is that MultiByteToWideChar doesn't distinguish
> between incomplete and invalid sequences, which is needed to correctly
> implement mbrtowc. Say we've seen three bytes, and MultiByteToWideChar
> returns zero when looking at those. That could mean that we're still
> missing the fourth byte, in which case we should return -2, or it
> could mean the sequence is already invalid, in which case we should
> return -1.
>
> Hence I suspect Cygwin would need to do its own parsing of GB18030 and
> only hand complete sequences over to MultiByteToWideChar to map them
> to Unicode.
>
> But having done that, there's another problem: four-byte GB18030
> sequences may map both to BMP and non-BMP Unicode codepoints. With
> Cygwin's wchar being 16-bit, this means that two wchars may have to be
> returned for one GB18030 sequence. Yet mbrtowc can only return one
> wchar, and unlike with UTF-8, there's no way to tell before the last
> byte whether two wchars are needed. I don't see a way to address that
> without bending the mbrtowc spec.
>   
This sounds tricky; my encoding support in mined is not based on the 
wchar functions, so it may not be straightforward to
map it into newlib, but I'll see. On the other hand, other systems do 
handle it, too, so there is an open source solution...

Thomas