Re: `looking-back' strange warning

[Tassilo Horn <tsdh@gnu.org>]

Drew Adams <drew.adams@oracle.com> writes:

>> > My two cents: Even if you specify an optimal limit, `looking-back' can
>> > still be unnecessarily slow.  For example,
>> >    (looking-back "xy" 2)
>> > is much slower than
>> >     (and (> (point) 2)
>> >          (save-excursion (goto-char (- (point) 2))
>> >                          (looking-at "xy")))
>> > where "xy" stands for any plain string.
>> 
>> Definitely.  And this is in fact a typical case of the
>> misuse: the string to check is often a literal, so its
>> length is known.
>> 
>> This is the first thing to mention about `looking-back',
>> in terms of performance: avoid it altogether, if you can.
>
> BTW/FWIW -
> I use this function in such cases, which can be pretty common:
>
> (defun chars-before (chars)
>   "Return non-nil if the literal string CHARS is right before point.
> This is more efficient that `looking-back' for this use case."
>   (let* ((len  (length chars))
>          (idx  (1- len))
>          (pt   (point)))
>     (catch 'chars-before
>       (dolist (char  (append chars ()))
>         (unless (condition-case nil
>                     (eq char (char-before (- pt idx)))
>                   (error nil))          ; e.g. `bobp'
>           (throw 'chars-before nil))
>         (setq idx  (1- idx)))
>       t)))

This version is about 6 times faster in the t case and a still a bit
faster in the nil case where the char before point is already different
(which is the best case for your function).

(defun chars-before (chars)
  "Return non-nil if the literal string CHARS is right before point.
This is more efficient that `looking-back' for this use case."
  (let ((beg (- (point) (length chars))))
    (unless (< beg 0)
      (save-excursion
	(goto-char beg)
	(looking-at (regexp-quote chars))))))

Bye,
Tassilo