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SubjectRe: Fwd: NLS mappings for iso-8859-* encodings
At 00:07 08/05/02, Petr Vandrovec wrote:
>On 8 May 02 at 0:08, Urban Widmark wrote:
> > On Tue, 7 May 2002, Petr Vandrovec wrote:
> > But if you have checked that you are not mapping two values to the same
> > thing (which would break the back-and-forth translation that smbfs does) I
> > don't see how that patch can harm anything.
>
>Yes, I checked it. After changing iso* all singlebyte encodings except
>cp874 contain unique mapping for all byte values (cp874 is unique, but
>some values are unmappable).

Wrong. The NLS tables do not guarantee unique mapping. So all fs which do
"back-and-forth" translation are broken, the only encoding which really
works is UTF-8.

We found out the hard way in ntfs. An example: take CP936 (GB2312).

Take a Unicode character between 0x4e00 and 0x9fa5, i.e. the CJK Ideograph
range (yes we found examples using these characters on various (chinese?)
websites).

Convert to gb2312 using NLS and then back to Unicode and you end up with a
Unicode character in the range 0xF900-0xFA2D, i.e. the CJK Compatibility
Ideographs.

Concrete example we ran into with ntfs, Unicode character 0x884C (a CJK
Ideograph). Translates to gb2312 character sequence 0xD0, 0xD0, and then
back to Unicode character 0xFA08 (a CJK Compatibility Ideograph).

I double checked the translation manually and I also checked the original
translation tables on the microsoft website and this is indeed what
happens. If you looks at the translation table there are several Unicode
characters mapping to the gb2312 character sequence 0xD0, 0xD0, but
obviously this only maps back to a single Unicode character.

Also if you lookup the Unicode character database 2.1 (I checked rev 2.1.8)
from the Unicode Consortium it specifies this as correct:

[snip]
4E00;<CJK Ideograph, First>;Lo;0;L;;;;;N;;;;;
9FA5;<CJK Ideograph, Last>;Lo;0;L;;;;;N;;;;;
[snip]
FA08;CJK COMPATIBILITY IDEOGRAPH-FA08;Lo;0;L;884C;;;;N;;;;;
[snip]

This means that Unicode itself is not a one-to-one mapping. Apparently
multiple characters have the same meanings... )))-:

I never imagined I would find something so braindamaged in Unicode but
there you go!

Basically this means, at least for NTFS, but I think it is the same for all
file systems, that on directory lookups, either we have to search the
directory by just looking at EVERY directory entry and converting each to
the current NLS and comparing for identity to the name being searched for
or we have to use UTF-8 as that guarantees to preserve back-and-forth
mappings one-to-one (I believe).

Doing a directory lookup where the whole directory is scanned linearly is
incredibly slow and the overhead of having to convert every single
directory entry to compare it every time a lookup() happens is very large,
so I don't want to implement that on NTFS, so if anyone complains to me
about their character translation not working properly they will just hear
use UTF-8 and it will work.

But you will have to find UTF-8 fonts and user space support code in order
to see the correct output displayed. Otherwise you just see random
characters in your filenames... But at least It Works(TM).

Best regards,

Anton


--
"I've not lost my mind. It's backed up on tape somewhere." - Unknown
--
Anton Altaparmakov <aia21 at cantab.net> (replace at with @)
Linux NTFS Maintainer / IRC: #ntfs on irc.openprojects.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

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