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Re: Emacs 23 character code space

From: Kenichi Handa
Subject: Re: Emacs 23 character code space
Date: Wed, 26 Nov 2008 10:41:01 +0900
User-agent: SEMI/1.14.3 (Ushinoya) FLIM/1.14.2 (Yagi-Nishiguchi) APEL/10.2 Emacs/23.0.60 (i686-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO)

In article <address@hidden>, Eli Zaretskii <address@hidden> writes:

>       A character set is a set of characters, and it assigns a unique code
>     point to each character belonging to the set.  Emacs decodes a
>     specific code point of a specific character set to an Emacs character.

> Does this mean a character set is equivalent to a coding-system,
> meaning that a coding-system is a mapping between a character set and
> the Emacs internal codepoints?

No.  A coding-system is a mapping between a sequence of
characters and a sequence of bytes.  The byte sequence
contains a byte not mapped to a character. For instance,
iso-2022 uses escape sequence, UTF-16 uses surrogate pairs.

>     @defun charset-dimension charset
>     This function returns the dimension of @var{charset}.  Here, dimension
>     means the number of bytes required to represent the highest code point
>     (not an Emacs character code) of a character.  For example, the
>     dimension of @code{iso-8859-1} is one, the dimension of
>     @code{japanese-jisx0208} is two, and the dimension of @code{unicode}
>     is three.
>     @end defun

> I decided not to document this.  I think the concept of charset
> dimension is too obscure to explain, and not really needed for Lisp
> programs, unless they need to define a new charset, or display a
> charset, and those are already done by Emacs infrastructure.  Do you
> see any problems with not documenting this function?

I think no.

>       A translation table has two extra slots.  The first is either
>     @code{nil} or a translation table that performs the reverse
>     translation; the second is the maximum number of characters to look up
>     for translation.

> Could you please elaborate on the second extra slot: when and for what
> purpose would there be a need to look up characters for translation?

To enable sequence-to-char translation.  See the description
of make-translation-table-from-alist.

Kenichi Handa

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