Re: "Coding system"? Eh?

[davidk@lysator.liu.se (David Kågedal)]

Lars Magne Ingebrigtsen <larsi@gnus.org> writes:

> Michael Welsh Duggan <md5i@cs.cmu.edu> writes:
> 
> > No, not really.  A character set is merely a set of characters.
> > latin-1, etc, are often called character sets because they use the
> > same number of characters as extended ASCII, etc.  A coding-system is
> > just that: a coding-system.  The characters could be encoded any which
> > way (including encrypted!).  For example, old-jis uses escapes around
> > sequences of 7-bit characters.  This is an encoding, which you can
> > display using a character set, but not a character set in and of
> > itself.
> 
> All texts consists of characters (from some character set) encoded
> (using some coding system).  iso-8859-1, for instance, represents the
> character LATIN-LETTER-A-WITH-UMLAUT ("ä") with one byte that contains
> the number 0xe4.  The same letter encoded in a different charset (say,
> Unicode) would occupy two bytes.  Other character sets use multiple
> bytes to represent characters, like iso-2022-jp.

Now you are mixing things.  The phrase "encoded in a different charset
(say, Unicode)" is a semantic error.

Unicode defines a character set where LATIT-LETTER-A-WITH-UMLAUT has a
specific number (228 i believe), but Unicode also defines several
character encodings.  There is UCS-2 where all characters occupy two
bytes.  Then there is UTF-8 where most characters can be encoded using
one byte, while 'ä' needs at least two.  Actually, all characters can
be encoded with, say, three bytes in UTF-8.  Unicode also defines
UTF-7 which is so ugly that I won't say anything further about it.
Then ISO-10646, which is in principle a superset of Unicode (but does
not contain any more defined characters) also defines UCS-4, where all
characters are encoded using four bytes, and UTF-16, where all
characters are encoding using two bytes.

Byt the character set is always the same, with numbers ranging from 0
to 65536.

> When one talks about character sets (in, say, MIME) one talks about
> encoded character sets.  Abstract character sets aren't all that
> interesting when fiddling with data.  iso-8859-1, which MULE calls a
> coding system, is something everyone else calls a character set.  The
> same with old-jis and iso-2022-jp.

ISO 8859-1 is both a character set, and an encoding (one-to-one from
charater to byte), I believe.  But I'm not sure how it is defined.

-- 
David Kågedal        <davidk@lysator.liu.se> http://www.lysator.liu.se/~davidk/