Add seq-random-elt to seq.el

* lisp/emacs-lisp/seq.el (seq-random-elt): Add function to return a
  random element from it's sequence parameter.

* test/lisp/emacs-lisp/seq-tests.el (test-seq-random-elt-take-all
  test-seq-random-elt-return-nil): Test the new function

* doc/lispref/sequences.texi: Document the new function

doc/lispref/sequences.texi

@@ -1037,6 +1037,26 @@ followed by a variable name to be bound to the rest of
 @end example
 @end defmac
 
+@defun seq-random-elt sequence
+  This function returns an element of @var{sequence} taken at random. If @var{sequence} is nil, the function returns nil.
+
+@example
+@group
+(seq-random-elt [1 2 3 4])
+@result{} 3
+(seq-random-elt [1 2 3 4])
+@result{} 2
+(seq-random-elt [1 2 3 4])
+@result{} 4
+(seq-random-elt [1 2 3 4])
+@result{} 2
+(seq-random-elt [1 2 3 4])
+@result{} 1
+@end group
+@end example
+
+  If @var{sequence} is empty, the function signals an error.
+@end defun
 
 @node Arrays
 @section Arrays

lisp/emacs-lisp/seq.el

@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: sequences
-;; Version: 2.18
+;; Version: 2.19
 ;; Package: seq
 
 ;; Maintainer: emacs-devel@gnu.org
@@ -476,6 +476,13 @@ SEQUENCE must be a sequence of numbers or markers."
   "Return element of SEQUENCE at the index N.
 If no element is found, return nil."
   (ignore-errors (seq-elt sequence n)))
+
+(cl-defgeneric seq-random-elt (sequence)
+  "Return a random element from SEQUENCE.
+Return nil if SEQUENCE is nil."
+  (if (seq-empty-p sequence)
+      (error "Sequence cannot be empty")
+    (seq-elt sequence (random (seq-length sequence)))))
 
 
 ;;; Optimized implementations for lists

test/lisp/emacs-lisp/seq-tests.el

@@ -28,6 +28,7 @@
 
 (require 'ert)
 (require 'seq)
+(require 'map)
 
 (defmacro with-test-sequences (spec &rest body)
   "Successively bind VAR to a list, vector, and string built from SEQ.
@@ -371,5 +372,21 @@ Evaluate BODY for each created sequence.
     (should (equal (seq-sort-by #'seq-length #'> seq)
                    ["xxx" "xx" "x"]))))
 
+(ert-deftest test-seq-random-elt-take-all ()
+  (let ((seq '(a b c d e))
+        (count '()))
+    (should (= 0 (map-length count)))
+    (dotimes (_ 1000)
+      (let ((random-elt (seq-random-elt seq)))
+        (map-put count
+                 random-elt
+                 (map-elt count random-elt 0))))
+    (should (= 5 (map-length count)))))
+
+(ert-deftest test-seq-random-elt-signal-on-empty ()
+  (should-error (seq-random-elt nil))
+  (should-error (seq-random-elt []))
+  (should-error (seq-random-elt "")))
+
 (provide 'seq-tests)
 ;;; seq-tests.el ends here