Re: get-byte

[Stefan Monnier]

> Yes, we can use encode-char to implement get-byte as this:

> (defun get-byte (&optional pos string)
>   (let ((multibyte (if string (multibyte-string-p string)
>                    enable-multibyte-characters))
>       (ch (if string (aref string (or pos 0)) 
>             (char-after (or pos (point))))))
>     (if (< ch #x80)
>       ch
>       (if multibyte
>         (or (encode-char ch 'eight-bit)
>             (error "Not an ASCII nor an 8-bit character: %d" ch))
>       ch))))

> But it's 5 to 10 times slower than the C version.

I'm not opposed to `get-byte' being implemented in C.  I just think it
should be implementable as

  (defun get-byte (&optional pos string)
    (let ((ch (if string (aref string (or pos 0))
                (char-after pos))))
      (or (encode-char ch 'binary)
          (error "Not an ASCII nor an 8-bit character: %d" ch))))

Given that, in most cases where you'd use get-byte you could replace it
with either (encode-char (char-after POS) 'binary)
or (encode-char (aref STRING POS) 'binary).  It may still be
significantly slower than a direct C implementation of get-byte, but it
is the right functionality to provide (i.e. get-byte is only there for
optimization purposes) and in some cases get-byte is not an option
(e.g. in cases such as (mapcar (lambda (c) (... (encode-char c 'binary)
..)) <string>)).

>> > I wrote it in C because, I think it must run very fast in
>> > the situaiont when this function is called.
>> Currently I don't see it being used.  Where is it going to be used?
> At everywhere you want to play with binary data that is
> stored in a multibyte buffer/string.  By grepping
> multibyte-char-to-unibyte, I found these places;
> quoted-printable-encode-region, ctext-post-read-conversion.

I see.

> It seems that arc-mode should also use it unless it is
> re-written to use buffer-swap-text as tar-mode.

This one needs to be converted to use buffer-swap-text indeed.


        Stefan