Re: "Coding system"? Eh?

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

François Pinard <pinard@iro.umontreal.ca> writes:

> davidk@lysator.liu.se (David Kågedal) écrit:
> 
> > 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.
> 
> You mean, all Unicode characters.  ISO 10646 might need more then three,
> as UTF-8 is also available for ISO 10646.

True.  I was talking about Unicode.

> > Unicode also defines UTF-7 which is so ugly that I won't say anything
> > further about it.
> 
> Does Unicode now defines UTF-7?  It originated from the IETF, and UTF-7
> is specifically for MIME contexts, which Unicode does not address.

I might be wrong about the origin of UTF-7.  But it's still ugly.

> > Then ISO-10646, which is in principle a superset of Unicode (but does
> > not contain any more defined characters) [...]
> 
> Some convergence happened, indeed, but the details are a bit more complex.
> 
> > also defines UCS-4, where all characters are encoded using four bytes,
> > and UTF-16, where all characters are encoding using two bytes.
> 
> I do not remember that ISO 10646 introduced UTF-16, I thought it was a
> Unicode invention, but once again, I'm no specialist and may easily be
> wrong.  ISO 10646 redefined the BMP so there is room for UTF-16 coding,
> so ISO 10646 is aware and compatible with Unicode on this.  By the way,
> UTF-16 encodes characters using either two or four bytes.

The difference between UTF-16 and UCS-2 is that it can encode some of
the charaters outside the Unicode range (BMP).  So I guess Unicode has
no need for UTF-16.

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