Re: Why no Ada.Wide_Directories?

[Adam Beneschan <adam@irvine.com>]

On Oct 18, 10:27 am, "J-P. Rosen" <ro...@adalog.fr> wrote:
> Le 18/10/2011 17:34, Adam Beneschan a crit :
>
>
>
> > On Oct 18, 12:55 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> > wrote:
> >> On Mon, 17 Oct 2011 18:10:35 -0700 (PDT), Adam Beneschan wrote:
> >>> I have a feeling you're fundamentally confused about what UTF-8 is, as
> >>> compared to "Latin-1".  Latin-1 is a character mapping.  It defines,
> >>> for all integers in the range 0..255, what character that integer
> >>> represents (e.g. 77 represents 'M', etc.).  Unicode is a character
> >>> mapping that defines characters for a much larger integer range.
>
> >> No, Unicode is a standard describes character mappings. Both UTF-8 and
> >> Latin-1 are encodings. Latin-1 as an encoding has a property that there is
> >> 1-1 octet to code point correspondence, at the cost that some (most) of
> >> code points cannot be represented by the encoding. UTF-8 lacks this
> >> property, but is capable to represent all code points.
>
> > Sigh... I guess you're right about the term "Latin-1".  It appears to
> > be *both* a character mapping *and* an encoding, based on a bit of
> > Wikipedia research.  The problem for me is this: what does that make
> > Latin-2, Latin-3, KOI8-R, etc.?  Those seem to describe the same
> > encoding mechanism as Latin-1 (each code represented as one 8-bit
> > byte), but with different meanings for the codes in the 16#A0#..16#FF#
> > range.  So the same encoding scheme seems to have multiple different
> > names.  That's very confusing to me.
>
> Not 100% sure, but I think  here is the picture.
> 1) Code points are always 31 bits (or maybe 30).
> 2) Below is the lower left corner of BMP (use fixed fonts!):
>
> |
> |____________________
> |         |         |
> | Latin 1 | Latin 2 |
> |_________|_________|_______
>
> The lower halves of Latin-1 and Latin-2 are identical, i.e. the same
> characters have two different code-points, differing by 256.
>
> When you use Latin-1 with 8 bit bytes, you can view this as an encoding
> with the 24 upper bits being 16#00_00_00#. When you use Latin-2 with 8
> bit bytes, you can view this as an encoding with the 24 upper bits being
> 16#00_00_01#.
>
> So in a sense, Latin-1 and Latin-2 are both character sets, and when
> represented on only 8 bits, an encoding.
>
> Does this make sense?

No, I don't think so.  In Latin-2 (ISO/IEC-8859-2), the code points
16#00#..16#A0# have the same meanings as in Latin-1 and Unicode.  Past
that, though, the correspondence is all over the place.  Thus, 16#A1#
in Latin-2 corresponds to 16#0104# in the Unicode BMP; 16#A2# ->
16#02D8#, 16#A3# -> 16#0141#, 16#A5# -> 16#013D#, etc.

                     -- Adam