diff --git a/dired-duplicates.el b/dired-duplicates.el
index d62af02..4af37a3 100644
--- a/dired-duplicates.el
+++ b/dired-duplicates.el
@@ -54,7 +54,6 @@
 (defcustom dired-duplicates-separate-results
   t
   "Boolean value indicating whether to separate results with new-lines."
-  :group 'dired-duplicates
   :tag "Separate results"
   :type 'boolean)
 
@@ -64,7 +63,6 @@
 
 The checksums will be used for comparison of files of the same
 size."
-  :group 'dired-duplicates
   :tag "Checksum executable"
   :type 'string)
 
@@ -73,10 +71,9 @@ size."
   "The comparison function used for sorting grouped results.
 
 The sorting can be in ascending (<) or descending (>) order."
-  :group 'dired-duplicates
   :tag "Ascending or descending file size sort order"
-  :type '(choice (const :tag "Ascending" :value <)
-                 (const :tag "Descending" :value >)))
+  :type '(choice (const :tag "Ascending" <)
+                 (const :tag "Descending" >)))
 
 (defcustom dired-duplicates-file-filter-functions
   nil
@@ -84,14 +81,12 @@ The sorting can be in ascending (<) or descending (>) order."
 
 A filter function must accept as its single argument the file and
 return boolean t if the file matches a criteria, otherwise nil."
-  :group 'dired-duplicates
   :tag "File filter functions"
   :type 'hook)
 
 (defcustom dired-duplicates-search-directories-recursively
   t
   "Search directories recursively."
-  :group 'dired-duplicates
   :tag "Search directories recursively"
   :type 'boolean)
 
@@ -103,21 +98,22 @@ return boolean t if the file matches a criteria, otherwise nil."
 
 The executable used is defined by `dired-duplicates-checksum-exec'."
   (let* ((default-directory (file-name-directory (expand-file-name file)))
-         (exec (executable-find dired-duplicates-checksum-exec t)))
+         (exec (executable-find dired-duplicates-checksum-exec t))
+	 (file (expand-file-name (file-local-name file))))
     (unless exec
-      (user-error "Checksum program %s not found in exec-path" exec))
-    (car (split-string
-          (shell-command-to-string
-           (concat exec " \"" (expand-file-name (file-local-name file)) "\""))
-          nil
-          t))))
+      (user-error "Checksum program %s not found in `exec-path'" exec))
+    (with-temp-buffer
+      (unless (zerop (call-process exec nil t nil file))
+	(error "Failed to start checksum program %s" exec))
+      (goto-char (point-min))
+      (if (looking-at "\\`[[:alnum:]]+")
+	  (match-string 0)
+	(error "Unexpected output from checksum program %s" exec)))))
 
 (defun dired-duplicates--apply-file-filter-functions (files)
   "Apply file filter functions to FILES, returning the resulting list."
-  (if (and dired-duplicates-file-filter-functions files)
-      (dolist (filter-func dired-duplicates-file-filter-functions files)
-        (setf files (cl-delete-if-not filter-func files)))
-    files))
+  (dolist (filter-func dired-duplicates-file-filter-functions files)
+    (setf files (cl-delete-if-not filter-func files))))
 
 (defun dired-duplicates--find-and-filter-files (directories)
   "Search below DIRECTORIES for duplicate files.
@@ -140,14 +136,14 @@ duplicate files as values."
                     (append (gethash size same-size-table) (list f)))
            finally
            (cl-loop for same-size-files being the hash-values in same-size-table
-                    if (> (length same-size-files) 1) do
+                    if (> (length same-size-files) 1) do ;see length>
                     (cl-loop for f in same-size-files
                              for checksum = (dired-duplicates-checksum-file f)
                              do (setf (gethash checksum checksum-table)
                                       (append (gethash checksum checksum-table) (list f)))))
            (cl-loop for same-files being the hash-value in checksum-table using (hash-key checksum)
                     do
-                    (if (> (length same-files) 1)
+                    (if (cdr same-files) ;(> (length same-files) 1) in O(1)
                         (setf (gethash checksum checksum-table)
                               (cons (file-attribute-size (file-attributes (car same-files)))
                                     (sort same-files #'string<)))
@@ -180,7 +176,6 @@ Currently, this simply adds a new-line after each results group."
                for len in lengths
                do
                (forward-line len)
-               ;; (forward-line len)
                (let ((inhibit-read-only t))
                  (beginning-of-line)
                  (unless (= (point) (point-max))
@@ -225,7 +220,7 @@ This is the same as `dired-do-flagged-delete', but calls
 
 (defvar dired-duplicates-map
   (let ((map (make-sparse-keymap)))
-    ;; workaround for Emacs bug #57565
+    ;; workaround for Emacs bug#57565
     (when (< emacs-major-version 29)
       (define-key map (kbd "x") 'dired-duplicates--do-flagged-delete)
       (define-key map (kbd "D") 'dired-duplicates--do-delete))
@@ -248,7 +243,7 @@ The results will be shown in a Dired buffer."
                                                default-directory)))
   (unless directories
     (user-error "Specify one or more directories to search in"))
-  (let* ((directories (if (listp directories) directories (list directories)))
+  (let* ((directories (if (listp directories) directories (list directories))) ;see `ensure-list'
          (truncated-dirs (truncate-string-to-width (string-join directories ", ") 40 0 nil t)))
     (message "Finding duplicate files in %s..." truncated-dirs)
     (if-let ((default-directory "/")
@@ -257,7 +252,7 @@ The results will be shown in a Dired buffer."
           (message "Finding duplicate files in %s completed." truncated-dirs)
           (dired (cons "/" (flatten-list results)))
           (set-keymap-parent dired-duplicates-map (current-local-map))
-          (setf (current-local-map) dired-duplicates-map)
+	  (use-local-map dired-duplicates-map)
           (setq-local dired-duplicates-directories directories)
           (setq-local revert-buffer-function 'dired-duplicates-dired-revert)
           (dired-duplicates--post-process-dired-buffer results))