From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.help Subject: Re: Performance of `re-search-backward' vs `re-search-forward' Date: Mon, 12 Apr 2021 16:59:32 -0400 Message-ID: References: <87czv0awpq.fsf@mbork.pl> <8735vvb8c5.fsf@mbork.pl> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="19958"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) Cc: help-gnu-emacs@gnu.org To: Marcin Borkowski Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Mon Apr 12 23:01:14 2021 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lW3gH-00054T-Ng for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 12 Apr 2021 23:01:13 +0200 Original-Received: from localhost ([::1]:48294 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lW3gG-0004rk-NV for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 12 Apr 2021 17:01:12 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:49300) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lW3eo-0004rE-1M for help-gnu-emacs@gnu.org; Mon, 12 Apr 2021 16:59:42 -0400 Original-Received: from mailscanner.iro.umontreal.ca ([132.204.25.50]:10613) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lW3el-0005St-OL for help-gnu-emacs@gnu.org; Mon, 12 Apr 2021 16:59:41 -0400 Original-Received: from pmg2.iro.umontreal.ca (localhost.localdomain [127.0.0.1]) by pmg2.iro.umontreal.ca (Proxmox) with ESMTP id 401D680820; Mon, 12 Apr 2021 16:59:36 -0400 (EDT) Original-Received: from mail01.iro.umontreal.ca (unknown [172.31.2.1]) by pmg2.iro.umontreal.ca (Proxmox) with ESMTP id 4698880B76; Mon, 12 Apr 2021 16:59:34 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=iro.umontreal.ca; s=mail; t=1618261174; bh=+/kyRPINlbokmqwzG13BE1fwNpxiAh2+4tJLNkfcF8w=; h=From:To:Cc:Subject:References:Date:In-Reply-To:From; b=NNKXUd9m52k5C2iEuNMqqxbhQB1Xl/Fqni4LFrqTkmb6uNrvrNBCCOh+4gacPUiB0 ggv/65mDQ/zHSF+5/jXs02RSPwTiPmttk1fCYRs8bhrxKYMM54jrLd1tavcNrjRGS7 ywm27RBwo3yoVOYk6aNEpmeriM/MKSlDXH4abxwLdcpx1+eX7Oii2pTazG3vjqsC3W ziofjkq91bM82Dssn1CA7ps6LQDHTRUx+1s+LHkgnxfIC+7QhNJ4kJoBH3r10O4XpI uJmwq+VkAz2c8H8jMOK94deyl8eDtrkGsr2A+xTo11SeHNB/ihl6q56eDdRjBISkX2 yc7/dowKbKF4g== Original-Received: from alfajor (104-222-126-84.cpe.teksavvy.com [104.222.126.84]) by mail01.iro.umontreal.ca (Postfix) with ESMTPSA id 17537120679; Mon, 12 Apr 2021 16:59:34 -0400 (EDT) In-Reply-To: <8735vvb8c5.fsf@mbork.pl> (Marcin Borkowski's message of "Mon, 12 Apr 2021 21:39:54 +0200") Received-SPF: pass client-ip=132.204.25.50; envelope-from=monnier@iro.umontreal.ca; helo=mailscanner.iro.umontreal.ca X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Original-Sender: "help-gnu-emacs" Xref: news.gmane.io gmane.emacs.help:128977 Archived-At: >>> I seem to (very vaguely) remember reading that `re-search-backward' is >>> significantly slower than `re-search-forward'. However, I can't find >>> anything about it in the docstring (nor in the Elisp reference) now. Am >>> I even correct? >> I haven't looked at the code recently so my memory might be off, but >> I can't think of any reason why it should be noticeably slower, no. > So both basically move character by character and check if the regex > matches there (more or less)? Yes, they're more or less loops that move char-by-char and call `looking-at` each time. The `looking-at` is always doing a "forward match" even if the outer loop is search backward, which is why the performance difference should be negligible even if there might be a difference in the performance of the outer loop. >> Maybe you're confusing it with `looking-back` which is much slower than >> `looking-at`? > Yes, that was it! Thanks! Right: `looking-back` is actually doing a `re-search-backward` so it has an outer loop which calls `looking-at` each time, hence it's O(n) times slower than `looking-at`. Stefan