A few questiosn about encoding

[wxjmfauth at gmail.com]

Le jeudi 20 juin 2013 13:43:28 UTC+2, MRAB a écrit :
> On 20/06/2013 07:26, Steven D'Aprano wrote:
> 
> > On Wed, 19 Jun 2013 18:46:59 -0700, Rick Johnson wrote:
> 
> >
> 
> >> On Thursday, June 13, 2013 2:11:08 AM UTC-5, Steven D'Aprano wrote:
> 
> >>
> 
> >>> Gah! That's twice I've screwed that up. Sorry about that!
> 
> >>
> 
> >> Yeah, and your difficulty explaining the Unicode implementation reminds
> 
> >> me of a passage from the Python zen:
> 
> >>
> 
> >>  "If the implementation is hard to explain, it's a bad idea."
> 
> >
> 
> > The *implementation* is easy to explain. It's the names of the encodings
> 
> > which I get tangled up in.
> 
> >
> 
> You're off by one below!
> 
> >
> 
> > ASCII: Supports exactly 127 code points, each of which takes up exactly 7
> 
> > bits. Each code point represents a character.
> 
> >
> 
> 128 codepoints.
> 
> 
> 
> > Latin-1, Latin-2, MacRoman, MacGreek, ISO-8859-7, Big5, Windows-1251, and
> 
> > about a gazillion other legacy charsets, all of which are mutually
> 
> > incompatible: supports anything from 127 to 65535 different code points,
> 
> > usually under 256.
> 
> >
> 
> 128 to 65536 codepoints.
> 
> 
> 
> > UCS-2: Supports exactly 65535 code points, each of which takes up exactly
> 
> > two bytes. That's fewer than required, so it is obsoleted by:
> 
> >
> 
> 65536 codepoints.
> 
> 
> 
> etc.
> 
> 
> 
> > UTF-16: Supports all 1114111 code points in the Unicode charset, using a
> 
> > variable-width system where the most popular characters use exactly two-
> 
> > bytes and the remaining ones use a pair of characters.
> 
> >
> 
> > UCS-4: Supports exactly 4294967295 code points, each of which takes up
> 
> > exactly four bytes. That is more than needed for the Unicode charset, so
> 
> > this is obsoleted by:
> 
> >
> 
> > UTF-32: Supports all 1114111 code points, using exactly four bytes each.
> 
> > Code points outside of the range 0 through 1114111 inclusive are an error.
> 
> >
> 
> > UTF-8: Supports all 1114111 code points, using a variable-width system
> 
> > where popular ASCII characters require 1 byte, and others use 2, 3 or 4
> 
> > bytes as needed.
> 
> >
> 
> >
> 
> > Ignoring the legacy charsets, only UTF-16 is a terribly complicated
> 
> > implementation, due to the surrogate pairs. But even that is not too bad.
> 
> > The real complication comes from the interactions between systems which
> 
> > use different encodings, and that's nothing to do with Unicode.
> 
> >
> 
> >

And all these coding schemes have something in common,
they work all with a unique set of code points, more
precisely a unique set of encoded code points (not
the set of implemented code points (byte)).

Just what the flexible string representation is not
doing, it artificially devides unicode in subsets and try
to handle eache subset differently.

On this other side, that is because it is impossible to
work properly with multiple sets of encoded code points
that all these coding schemes exist today. There are simply
no other way.

Even "exotic" schemes like "CID-fonts" used in pdf
are based on that scheme.

jmf