>From f8e3318cab3b3975040efc37225c79c272c4dabe Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh@janestreet.com>
Date: Wed, 13 Mar 2024 10:57:44 -0400
Subject: [PATCH] Use regexp-opt in dired-omit-regexp

In my benchmarking, for large dired buffers, using regexp-opt provides
around a 3x speedup in omitting.

regexp-opt takes around 5 milliseconds, so to avoid slowing down
omitting in small dired buffers we cache the return value.

Since omitting is now 3x faster, increase dired-omit-size-limit by 3x.

* lisp/dired-x.el (dired-omit--extension-regexp-cache): Add.
(dired-omit-regexp): Use regexp-opt.
(dired-omit-size-limit): Increase, since omitting is now faster.
---
 lisp/dired-x.el | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/lisp/dired-x.el b/lisp/dired-x.el
index 5a494569989..aa4320a393e 100644
--- a/lisp/dired-x.el
+++ b/lisp/dired-x.el
@@ -77,12 +77,12 @@ dired-vm-read-only-folders
 		 (other :tag "non-writable only" if-file-read-only))
   :group 'dired-x)
 
-(defcustom dired-omit-size-limit 100000
+(defcustom dired-omit-size-limit 300000
   "Maximum size for the \"omitting\" feature.
 If nil, there is no maximum size."
   :type '(choice (const :tag "no maximum" nil) integer)
   :group 'dired-x
-  :version "29.1")
+  :version "30.1")
 
 (defcustom dired-omit-case-fold 'filesystem
   "Determine whether \"omitting\" patterns are case-sensitive.
@@ -504,14 +504,23 @@ dired-omit-expunge
                                       (re-search-forward dired-re-mark nil t))))
         count)))
 
+(defvar dired-omit--extension-regexp-cache
+  nil
+  "A cache of `regexp-opt' applied to `dired-omit-extensions'.
+
+This is a cons whose car is a list of strings and whose cdr is a
+regexp produced by `regexp-opt'.")
+
 (defun dired-omit-regexp ()
+  (unless (equal dired-omit-extensions (car dired-omit--extension-regexp-cache))
+    (setq dired-omit--extension-regexp-cache
+          (cons dired-omit-extensions (regexp-opt dired-omit-extensions))))
   (concat (if dired-omit-files (concat "\\(" dired-omit-files "\\)") "")
           (if (and dired-omit-files dired-omit-extensions) "\\|" "")
           (if dired-omit-extensions
               (concat ".";; a non-extension part should exist
-                      "\\("
-                      (mapconcat 'regexp-quote dired-omit-extensions "\\|")
-                      "\\)$")
+                      (cdr dired-omit--extension-regexp-cache)
+                      "$")
             "")))
 
 ;; Returns t if any work was done, nil otherwise.
-- 
2.39.3