Re: Coding systems vietnamese-vscii and vietnamese-tcvn

[Ulrich Mueller <ulm@gentoo.org>]

>>>>> On Fri, 21 Jul 2023, Eli Zaretskii wrote:

>> From: Ulrich Mueller <ulm@gentoo.org>
>> Cc: emacs-devel@gnu.org
>> Date: Fri, 21 Jul 2023 14:52:55 +0200
>> 
>> >>>>> On Fri, 21 Jul 2023, Eli Zaretskii wrote:
>> 
>> > Why is the fact that we have two separate coding systems a problem in
>> > this case?
>> 
>> Presumably it works fine as-is, but is there any benefit of having such
>> duplication?

> I cannot see any clear benefits either way.

> Maybe we need to ask Vietnamese users.  The comments in
> lisp/language/vietnamese.el say that vscii is deprecated.

I have asked a Vietnamese speaker. Paraphrasing their answer: VSCII is
the encoding described in standard TCVN 5712:1993. The terms can be used
interchangeably; there is no reason to have separate character tables.

So the statement in the Vietnamese language info that "VSCII is
deprecated in favor of TCVN-5712" doesn't appear to be correct; the two
terms are synonyms for the same encoding.

I also looked up the original standard. TCVN 5712:1993 defines two
encodings which it names VN1 (aka VSCII-1) and VN2 (aka VSCII-2).

VSCII-2 (VN2) defines these code points:
- 0x00 to 0x7f are identical to ASCII,
- 0x80 to 0x9f are the C1 controls,
- 0xa0 to 0xff contain 96 non-ASCII characters.

VSCII-1 (VN1) is different from VSCII-2 in that it also replaces code
points 0x01-0x02, 0x04-0x06, 0x11-0x17, and 0x80-0x9f by 44 additional
non-ASCII characters, for a total of 140 non-ASCII characters.

There is also an updated standard TCVN 5712:1999 which mentions only one
encoding identical to VSCII-1, i.e. VSCII-2 is no longer part of this
later version of the standard.

So I suggest to define only coding system vietnamese-vscii, and make
vietnamese-tcvn an alias of it. If this a acceptable, I can prepare a
patch for lisp/language/vietnamese.el.