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diff --git a/lisp/compare-w.el b/lisp/compare-w.el
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+;; Compare text between windows for Emacs.
+;; Copyright (C) 1986, 1989 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 1, or (at your option)
+;; any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs; see the file COPYING.  If not, write to
+;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+
+(provide 'compare-w)
+
+(defvar compare-windows-whitespace " \t\n"
+  "*String of characters considered whitespace for \\[compare-windows].
+Changes in whitespace are optionally ignored.
+
+The value of `compare-windows-whitespace' may instead be a function; this
+function is called in each buffer, with point at the current scanning point.
+The function's job is to categorize any whitespace around (including before)
+point; it should also advance past any whitespace.
+
+The function is passed one argument, the point where `compare-windows'
+was originally called; it should not consider any text before that point.
+If the function returns the same value for both buffers, then the
+whitespace is considered to match, and is skipped.")
+
+(defvar compare-ignore-case nil
+  "*Non-nil means \\[compare-windows] ignores case differences.")
+
+;;;###autoload
+(defun compare-windows (ignore-whitespace)
+  "Compare text in current window with text in next window.
+Compares the text starting at point in each window,
+moving over text in each one as far as they match.
+
+A prefix arg means ignore changes in whitespace.
+The variable `compare-windows-whitespace' controls how whitespace is skipped.
+If `compare-ignore-case' is non-nil, changes in case are also ignored."
+  (interactive "P")
+  (let* (p1 p2 maxp1 maxp2 b1 b2 w2
+	    success size
+	    (opoint1 (point))
+	    opoint2
+	    (skip-whitespace (if ignore-whitespace
+				 compare-windows-whitespace))
+	    (skip-whitespace-regexp (concat "[" skip-whitespace "]+")))
+    (setq p1 (point) b1 (current-buffer))
+    (setq w2 (next-window (selected-window)))
+    (if (eq w2 (selected-window))
+	(error "No other window"))
+    (setq p2 (window-point w2)
+	  b2 (window-buffer w2))
+    (setq opoint2 p2)
+    (setq maxp1 (point-max))
+    (save-excursion
+      (set-buffer b2)
+      (setq maxp2 (point-max)))
+
+    (setq success t)
+    (while success
+      (setq success nil)
+      ;; if interrupted, show how far we've gotten
+      (goto-char p1)
+      (set-window-point w2 p2)
+
+      ;; If both buffers have whitespace next to point,
+      ;; optionally skip over it.
+
+      (and skip-whitespace
+	   (save-excursion
+	     (let (p1a p2a w1 w2 result1 result2)
+	       (if (stringp skip-whitespace)
+		   (progn
+		     (if (not (eobp))
+			 (skip-chars-backward skip-whitespace opoint1))
+		     (and (looking-at skip-whitespace-regexp)
+			  (setq p1a (match-end 0) result1 t)))
+		 (setq result1 (funcall skip-whitespace opoint1))
+		 (setq p1a (point)))
+	       (set-buffer b2)
+	       (goto-char p2)
+	       (if (stringp skip-whitespace)
+		   (progn
+		     (if (not (eobp))
+			 (skip-chars-backward skip-whitespace opoint2))
+		     (and (looking-at skip-whitespace-regexp)
+			  (setq p2a (match-end 0) result2 t)))
+		 (setq result2 (funcall skip-whitespace opoint2))
+		 (setq p2a (point)))
+	       (and result1 result2 (eq result1 result2)
+		    (setq p1 p1a
+			  p2 p2a)))))
+
+      ;; Try advancing comparing 1000 chars at a time.
+      ;; When that fails, go 500 chars at a time, and so on.
+      (let ((size 1000)
+	    success-1)
+	(while (> size 0)
+	  (setq success-1 t)
+	  (while success-1
+	    (setq size (min size (- maxp1 p1) (- maxp2 p2)))
+	    (save-excursion
+	      (set-buffer b2)
+	      (setq s2 (buffer-substring p2 (+ size p2))))
+	    (setq success-1
+		  (and (> size 0)
+		       (if compare-ignore-case
+			   (let ((case-fold-search t))
+			     (save-excursion
+			       (search-forward s2 (+ p1 size) t)))
+			 (equal (buffer-substring p1 (+ size p1)) s2))))
+	    (if success-1
+		(setq p1 (+ p1 size) p2 (+ p2 size)
+		      success t)))
+	  (setq size (/ size 2)))))
+
+    (goto-char p1)
+    (set-window-point w2 p2)
+    (if (= (point) opoint1)
+	(ding))))