Add `string-pad'

* doc/lispref/strings.texi (Creating Strings): Document it.
* lisp/emacs-lisp/shortdoc.el (string): Add example.

* lisp/emacs-lisp/subr-x.el (string-pad): New function.

doc/lispref/strings.texi

@@ -419,6 +419,15 @@ Split @var{string} into a list of strings on newline boundaries.  If
 @var{omit-nulls}, remove empty lines from the results.
 @end defun
 
+@defun string-pad string length &optional padding
+Pad @var{string} to the be of @var{length} using @var{padding} as the
+padding character (defaulting to the space character).  If
+@var{string} is shorter than @var{length}, no padding is done.  If
+@var{length} is positive, the padding is done to the end of the
+string, and if it's negative, to the start of the string (using the
+absolute value).
+@end defun
+
 @node Modifying Strings
 @section Modifying Strings
 @cindex modifying strings

etc/NEWS

@@ -1443,7 +1443,7 @@ that makes it a valid button.
 +++
 *** A number of new string manipulation functions have been added.
 'string-clean-whitespace', 'string-fill', 'string-limit',
-'string-limit' and 'slice-string'.
+'string-limit', 'string-pad' and 'slice-string'.
 
 +++
 *** New variable 'current-minibuffer-command'.

lisp/emacs-lisp/shortdoc.el

@@ -131,6 +131,10 @@ There can be any number of :example/:result elements."
   (mapconcat
    :eval (mapconcat (lambda (a) (concat "[" a "]"))
                     '("foo" "bar" "zot") " "))
+  (string-pad
+   :eval (string-pad "foo" 5)
+   :eval (string-pad "foobar" 5)
+   :eval (string-pad "foo" -5 ?-))
   (mapcar
    :eval (mapcar #'identity "123"))
   (format

lisp/emacs-lisp/subr-x.el

@@ -317,6 +317,26 @@ The boundaries that match REGEXP are not omitted from the results."
       (push (substring string start-substring) result)
       (nreverse result))))
 
+(defun string-pad (string length &optional padding)
+  "Pad STRING to LENGTH using PADDING.
+If PADDING is nil, the space character is used.  If not nil, it
+should be a character.
+
+If STRING is longer than the absolute value of LENGTH, no padding
+is done.
+
+If LENGTH is positive, the padding is done to the end of the
+string, and if it's negative, padding is done to the start of the
+string."
+  (if (> (length string) (abs length))
+      string
+    (let ((pad-length (- (abs length) (length string))))
+      (concat (and (< length 0)
+                   (make-string pad-length (or padding ?\s)))
+              string
+              (and (> length 0)
+                   (make-string pad-length (or padding ?\s)))))))
+
 (defun replace-region-contents (beg end replace-fn
                                     &optional max-secs max-costs)
   "Replace the region between BEG and END using REPLACE-FN.

test/lisp/emacs-lisp/subr-x-tests.el

@@ -608,5 +608,11 @@
   (should (equal (slice-string "-foo-bar-" "-") '("-foo" "-bar" "-")))
   (should (equal (slice-string "ooo" "lala") '("ooo"))))
 
+(ert-deftest subr-string-pad ()
+  (should (equal (string-pad "foo" 5) "foo  "))
+  (should (equal (string-pad "foo" 5 ?-) "foo--"))
+  (should (equal (string-pad "foo" -5 ?-) "--foo"))
+  (should (equal (string-pad "foo" 2 ?-) "foo")))
+
 (provide 'subr-x-tests)
 ;;; subr-x-tests.el ends here