decode-coding-string and consing

decode-coding-string and consing

[Jesper Harder]

,----[ C-h f decode-coding-string RET ]
| decode-coding-string is a built-in function.
| (decode-coding-string STRING CODING-SYSTEM &optional NOCOPY)
| 
| Decode STRING which is encoded in CODING-SYSTEM, and return the result.
| Optional arg NOCOPY non-nil means it is OK to return STRING itself
| if the decoding operation is trivial.
`----

Under what conditions is the decoding operation "trivial"?

I was hoping that something like:

  (decode-coding-string "abc" 'us-ascii t) or
  (decode-coding-string "abc" 'latin-1 t)

wouldn't cons a new string; but unfortunaley it does.

Re: decode-coding-string and consing

[Eli Zaretskii]

> Newsgroups: gnu.emacs.help
> From: Jesper Harder <harder@myrealbox.com>
> Date: Mon, 10 Feb 2003 17:47:53 +0100
> 
> ,----[ C-h f decode-coding-string RET ]
> | decode-coding-string is a built-in function.
> | (decode-coding-string STRING CODING-SYSTEM &optional NOCOPY)
> | 
> | Decode STRING which is encoded in CODING-SYSTEM, and return the result.
> | Optional arg NOCOPY non-nil means it is OK to return STRING itself
> | if the decoding operation is trivial.
> `----
> 
> Under what conditions is the decoding operation "trivial"?

If CODING-SYSTEM is nil or such that no encoding is necessary at all.

Re: decode-coding-string and consing

[Jesper Harder]

"Eli Zaretskii" <eliz@is.elta.co.il> writes:

>> ,----[ C-h f decode-coding-string RET ]
>> | decode-coding-string is a built-in function.
>> | (decode-coding-string STRING CODING-SYSTEM &optional NOCOPY)
>> | 
>> | Decode STRING which is encoded in CODING-SYSTEM, and return the result.
>> | Optional arg NOCOPY non-nil means it is OK to return STRING itself
>> | if the decoding operation is trivial.
>> `----
>> 
>> Under what conditions is the decoding operation "trivial"?
>
> If CODING-SYSTEM is nil or such that no encoding is necessary at all.

But I don't understand why any encoding is needed at all in:

    (decode-coding-string "abc" 'us-ascii t)

The input string and the output string are identical.  Why is it
necessary to cons a new string here?

Re: decode-coding-string and consing

[Eli Zaretskii]

On Mon, 10 Feb 2003, Jesper Harder wrote:

> >> Under what conditions is the decoding operation "trivial"?
> >
> > If CODING-SYSTEM is nil or such that no encoding is necessary at all.
> 
> But I don't understand why any encoding is needed at all in:
> 
>     (decode-coding-string "abc" 'us-ascii t)

Why did you use us-ascii at all here?  It's not really a coding system.
Why not nil or raw-text?

Anyway, is there some real problem behind this discussion?

Re: decode-coding-string and consing

[Jesper Harder]

Eli Zaretskii <eliz@is.elta.co.il> writes:

> On Mon, 10 Feb 2003, Jesper Harder wrote:
>
>> > If CODING-SYSTEM is nil or such that no encoding is necessary at
>> > all.
>> 
>> But I don't understand why any encoding is needed at all in:
>> 
>>     (decode-coding-string "abc" 'us-ascii t)
>
> Why did you use us-ascii at all here?

It's just as an example where it's obvious that no encoding is needed.
What I'm really interested in is:

     (decode-coding-string "abc" 'latin-1 t)

i.e. decoding a pure ASCII string as latin-1 (or another strict superset
of ASCII).

This pseudo-code for what I'd like to do efficiently:

,----
| (defun my-decode-coding-string (string coding-system nocopy)
|   (if (and nocopy
| 	     (STRING has no 8bit chars)
| 	     (CODING-SYSTEM is a superset of ascii))
|       string
|     (decode-coding-string string coding-system nocopy)))
`----

> Anyway, is there some real problem behind this discussion?

Yes.  Gnus conses excessively.  

If `decode-coding-string' was a bit more clever about not consing a
string when it's not strictly necessary, we could cons ~ 2*N less
strings when constructing a summary buffer (N = number of articles).

It's also an issue when constructing a server buffer, where N is usually
large -- i.e. ~ 50.000-100.000 unnecessary strings are consed.