On 22 Sep 2016, at 12:04, John Brownie <john_brow...@sil.org> wrote:
> I find that I am in need of dealing with versions of strings that may be 
> turned into file names, and thus I need to be comparing normalised forms. 
> However, according to 
> https://developer.apple.com/library/content/qa/qa1173/_index.html, "For 
> example, HFS Plus (Mac OS Extended) uses a variant of Normal Form D in which 
> U+2000 through U+2FFF, U+F900 through U+FAFF, and U+2F800 through U+2FAFF are 
> not decomposed (this avoids problems with round trip conversions from old Mac 
> text encodings)."
> Is there a way to get at this variant? [NSString 
> decomposedStringWithCanonicalMapping] returns "A string made by normalizing 
> the string’s contents using the Unicode Normalization Form D." That seems not 
> to give what I need, but I haven't seen a better option. Is there such a 
> method?

This is rather a hairy problem, actually.  You don’t know that the filesystem 
is actually HFS+.  The filename in question could refer to a file on an NTFS 
volume, or (in future) an APFS volume.  It could also be a network filesystem, 
which could be mounted on a system running a completely different operating 
system with unknown character encoding behaviour; typically UNIX behaviour, for 
instance, is to completely ignore the filename encoding problem and treat 
filenames as a string of bytes (thus you are free to interpret them as UTF-8 if 
you so wish, but the filesystem will regard U+00E9 as different from U+0065 
U+0301, even though the two names will display the same on screen).

Additionally, the version specified in HFS+ was based on a particular version 
of the Unicode standard (I forget which), but importantly *not* the current 
version, which is what the NSString APIs will use.

Perhaps we should start by understanding the context of your question.  Is 
there a specific reason you want to match the filesystem behaviour precisely?

Kind regards,




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