* lisp/character-fold.el: Remove special case-folding support

(character-fold-to-regexp): Remove special code for
case-folding.  Char-fold search still respects the
`case-fold-search' variable (i.e., f matches F).  This only
removes the code that was added to ensure that f also matched
all chars that F matched.  For instance, after this commit, f
no longer matches 𝔽.

This was necessary because the logic created a regexp with
2^(length of the string) redundant paths.  So, when a very
long string "almost" matched, Emacs took a very long time to
figure out that it didn't.  This became particularly relevant
because isearch's lazy-highlight does a search bounded by (1-
match-end) (which, in most circumstances, is a search that
almost matches).  A recipe for this can be found in bug#22090.

2 files changed, 25 insertions, 18 deletions

diff --git a/lisp/character-fold.el b/lisp/character-fold.el
index fb28bae7281..1e49fe2f0e5 100644
--- a/lisp/character-fold.el
+++ b/lisp/character-fold.el
@@ -157,8 +157,6 @@ FROM is for internal use.  It specifies an index in the STRING
 from which to start."
   (let* ((spaces 0)
          (multi-char-table (char-table-extra-slot character-fold-table 0))
-         (lower-case-table (current-case-table))
-         (upper-case-table (char-table-extra-slot lower-case-table 0))
          (i (or from 0))
          (end (length string))
          (out nil))
@@ -178,21 +176,9 @@ from which to start."
              (setq spaces 0))
            (let ((regexp (or (aref character-fold-table c)
                              (regexp-quote (string c))))
-                 (alist nil))
-             ;; Long string.  The regexp would probably be too long.
-             (unless (> end 50)
-               (setq alist (aref multi-char-table c))
-               (when case-fold-search
-                 (let ((other-c (aref lower-case-table c)))
-                   (when (or (not other-c)
-                             (eq other-c c))
-                     (setq other-c (aref upper-case-table c)))
-                   (when other-c
-                     (setq alist (append alist (aref multi-char-table other-c)))
-                     (setq regexp (concat "\\(?:" regexp "\\|"
-                                          (or (aref character-fold-table other-c)
-                                              (regexp-quote (string other-c)))
-                                          "\\)"))))))
+                 ;; Long string.  The regexp would probably be too long.
+                 (alist (unless (> end 50)
+                          (aref multi-char-table c))))
              (push (let ((matched-entries nil)
                          (max-length 0))
                      (dolist (entry alist)
@@ -229,7 +215,7 @@ from which to start."
       (push (character-fold--make-space-string spaces) out))
     (let ((regexp (apply #'concat (nreverse out))))
       ;; Limited by `MAX_BUF_SIZE' in `regex.c'.
-      (if (> (length regexp) 10000)
+      (if (> (length regexp) 5000)
           (regexp-quote string)
         regexp))))
 
diff --git a/test/automated/character-fold-tests.el b/test/automated/character-fold-tests.el
index 40735e5df7f..4e8761e6f7b 100644
--- a/test/automated/character-fold-tests.el
+++ b/test/automated/character-fold-tests.el
@@ -98,6 +98,27 @@
     ;; (character-fold--test-match-exactly "a12" "xxyy")
     ))
 
+(ert-deftest character-fold--speed-test ()
+  (dolist (string (append '("tty-set-up-initial-frame-face"
+                            "tty-set-up-initial-frame-face-frame-faceframe-faceframe-faceframe-face")
+                          (mapcar #'character-fold--random-word '(10 50 100
+                                                                     50 100))))
+    (message "Testing %s" string)
+    ;; Make sure we didn't just fallback on the trivial search.
+    (should-not (string= (regexp-quote string)
+                         (character-fold-to-regexp string)))
+    (with-temp-buffer
+      (save-excursion (insert string))
+      (let ((time (time-to-seconds (current-time))))
+        ;; Our initial implementation of case-folding in char-folding
+        ;; created a lot of redundant paths in the regexp. Because of
+        ;; that, if a really long string "almost" matches, the regexp
+        ;; engine took a long time to realise that it doesn't match.
+        (should-not (character-fold-search-forward (concat string "c") nil 'noerror))
+        ;; Ensure it took less than a second.
+        (should (< (- (time-to-seconds (current-time))
+                      time)
+                   1))))))
 
 (provide 'character-fold-tests)
 ;;; character-fold-tests.el ends here