1 files changed, 61 insertions, 1 deletions
diff --git a/test/lisp/emacs-lisp/regexp-opt-tests.el b/test/lisp/emacs-lisp/regexp-opt-tests.el
index 927de8c6a5f..3658964faac 100644
--- a/test/lisp/emacs-lisp/regexp-opt-tests.el
+++ b/test/lisp/emacs-lisp/regexp-opt-tests.el
@@ -1,4 +1,4 @@
-;;; regexp-opt-tests.el --- Tests for regexp-opt.el
+;;; regexp-opt-tests.el --- Tests for regexp-opt.el  -*- lexical-binding: t -*-
 
 ;; Copyright (C) 2013-2019 Free Software Foundation, Inc.
 
@@ -25,6 +25,66 @@
 
 (require 'regexp-opt)
 
+(defun regexp-opt-test--permutation (n list)
+  "The Nth permutation of LIST, 0 ≤ N < (length LIST)!."
+  (let ((len (length list))
+        (perm-list nil))
+    (dotimes (i len)
+      (let* ((d (- len i))
+             (k (mod n d)))
+        (push (nth k list) perm-list)
+        (setq list (append (butlast list (- (length list) k))
+                           (nthcdr (1+ k) list)))
+        (setq n (/ n d))))
+    (nreverse perm-list)))
+
+(defun regexp-opt-test--factorial (n)
+  "N!"
+  (apply #'* (number-sequence 1 n)))
+
+(defun regexp-opt-test--permutations (list)
+  "All permutations of LIST."
+  (mapcar (lambda (i) (regexp-opt-test--permutation i list))
+          (number-sequence 0 (1- (regexp-opt-test--factorial (length list))))))
+
+(defun regexp-opt-test--match-all (words re)
+  (mapcar (lambda (w) (and (string-match re w)
+                           (match-string 0 w)))
+          words))
+
+(defun regexp-opt-test--check-perm (perm)
+  (let* ((ref-re (mapconcat #'regexp-quote perm "\\|"))
+         (opt-re (regexp-opt perm nil t))
+         (ref (regexp-opt-test--match-all perm ref-re))
+         (opt (regexp-opt-test--match-all perm opt-re)))
+    (equal opt ref)))
+
+(defun regexp-opt-test--explain-perm (perm)
+  (let* ((ref-re (mapconcat #'regexp-quote perm "\\|"))
+         (opt-re (regexp-opt perm nil t))
+         (ref (regexp-opt-test--match-all perm ref-re))
+         (opt (regexp-opt-test--match-all perm opt-re)))
+    (concat "\n"
+            (format "Naïve regexp:     %s\n" ref-re)
+            (format "Optimised regexp: %s\n" opt-re)
+            (format "Got:      %s\n" opt)
+            (format "Expected: %s\n" ref))))
+
+(put 'regexp-opt-test--check-perm 'ert-explainer 'regexp-opt-test--explain-perm)
+
+(ert-deftest regexp-opt-keep-order ()
+  "Check that KEEP-ORDER works."
+  (dolist (perm (regexp-opt-test--permutations '("abc" "bca" "cab")))
+    (should (regexp-opt-test--check-perm perm)))
+  (dolist (perm (regexp-opt-test--permutations '("abc" "ab" "bca" "bc")))
+    (should (regexp-opt-test--check-perm perm)))
+  (dolist (perm (regexp-opt-test--permutations '("abxy" "cdxy")))
+    (should (regexp-opt-test--check-perm perm)))
+  (dolist (perm (regexp-opt-test--permutations '("afgx" "bfgx" "afgy" "bfgy")))
+    (should (regexp-opt-test--check-perm perm)))
+  (dolist (perm (regexp-opt-test--permutations '("a" "ab" "ac" "abc")))
+    (should (regexp-opt-test--check-perm perm))))
+
 (ert-deftest regexp-opt-charset ()
   (should (equal (regexp-opt-charset '(?a ?b ?a)) "[ab]"))
   (should (equal (regexp-opt-charset '(?D ?d ?B ?a ?b ?C ?7 ?a ?c ?A))