From: Tassilo Horn <tsdh@gnu.org>
To: Drew Adams <drew.adams@oracle.com>
Cc: Michael Heerdegen <michael_heerdegen@web.de>, help-gnu-emacs@gnu.org
Subject: Re: `looking-back' strange warning
Date: Fri, 02 Oct 2015 22:59:41 +0200 [thread overview]
Message-ID: <87lhblrnpe.fsf@gnu.org> (raw)
In-Reply-To: <190e27a5-9099-45ce-a8db-a314b9e32b35@default> (Drew Adams's message of "Fri, 2 Oct 2015 13:28:15 -0700 (PDT)")
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
next prev parent reply other threads:[~2015-10-02 20:59 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-30 8:25 `looking-back' strange warning Andreas Röhler
2015-10-01 6:20 ` Michael Heerdegen
2015-10-01 6:25 ` Tassilo Horn
2015-10-01 6:50 ` Dmitry Gutov
2015-10-01 7:49 ` Andreas Röhler
2015-10-01 8:20 ` Tassilo Horn
2015-10-01 8:46 ` Andreas Röhler
2015-10-01 9:29 ` Tassilo Horn
2015-10-02 0:10 ` Stefan Monnier
2015-10-02 15:21 ` Tassilo Horn
2015-10-01 16:00 ` Drew Adams
2015-10-01 17:56 ` Dmitry Gutov
2015-10-01 18:24 ` Andreas Röhler
2015-10-01 18:59 ` Dmitry Gutov
2015-10-01 19:55 ` Andreas Röhler
2015-10-01 20:26 ` Dmitry Gutov
2015-10-02 0:15 ` Stefan Monnier
2015-10-02 1:18 ` Michael Heerdegen
2015-10-02 5:13 ` Drew Adams
2015-10-02 20:28 ` Drew Adams
2015-10-02 20:59 ` Tassilo Horn [this message]
2015-10-02 21:35 ` Drew Adams
2015-10-03 6:26 ` Tassilo Horn
2015-10-03 6:45 ` Andreas Röhler
2015-10-03 15:20 ` Drew Adams
2015-10-03 15:17 ` Drew Adams
2015-10-02 1:19 ` Michael Heerdegen
2015-10-02 5:12 ` Drew Adams
2015-10-02 6:16 ` Andreas Röhler
2015-10-02 6:36 ` Michael Heerdegen
2015-10-02 12:46 ` Stefan Monnier
2015-10-02 15:02 ` Drew Adams
2015-10-02 19:08 ` Tom Tromey
2015-10-02 19:33 ` Stefan Monnier
2015-10-03 18:11 ` Stefan Monnier
[not found] ` <mailman.176.1443721449.16064.help-gnu-emacs@gnu.org>
2015-10-01 18:50 ` Barry Margolin
[not found] ` <<barmar-D40496.14502501102015@88-209-239-213.giganet.hu>
2015-10-01 23:42 ` Drew Adams
2015-10-02 0:12 ` Stefan Monnier
2015-10-02 5:10 ` Drew Adams
2015-10-02 12:48 ` Stefan Monnier
2015-10-02 15:03 ` Drew Adams
2015-10-02 15:21 ` Stefan Monnier
2015-10-02 15:24 ` Drew Adams
2015-10-02 19:28 ` Stefan Monnier
2015-10-01 12:35 ` Artur Malabarba
[not found] <<560B9C7F.2060301@easy-emacs.de>
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