Re: Why no Ada.Wide_Directories?

["J-P. Rosen" <rosen@adalog.fr>]

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?
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           J-P. Rosen (rosen@adalog.fr)
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