* simple.el (transpose-subr-1): Preserve marker positions

by changing the insertion sequence.

Fixes: debbugs:13122

2 files changed, 20 insertions, 8 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index e89602c8e45..a26fc63fa49 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2012-12-21  Chong Yidong  <cyd@gnu.org>
+
+        * simple.el (transpose-subr-1): Preserve marker positions by
+        changing the insertion sequence (Bug#13122).
+
 2012-12-21  Kelly Dean  <kellydeanch@yahoo.com>  (tiny change)
 
         * simple.el (kill-region): Deactivate mark even for empty regions
diff --git a/lisp/simple.el b/lisp/simple.el
index 8c7e88d04bc..c0ba5477b56 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -5300,14 +5300,21 @@ current object."
       (setq pos1 pos2 pos2 swap)))
   (if (> (cdr pos1) (car pos2)) (error "Don't have two things to transpose"))
   (atomic-change-group
-   (let (word2)
-     ;; FIXME: We first delete the two pieces of text, so markers that
-     ;; used to point to after the text end up pointing to before it :-(
-     (setq word2 (delete-and-extract-region (car pos2) (cdr pos2)))
-     (goto-char (car pos2))
-     (insert (delete-and-extract-region (car pos1) (cdr pos1)))
-     (goto-char (car pos1))
-     (insert word2))))
+    ;; This sequence of insertions attempts to preserve marker
+    ;; positions at the start and end of the transposed objects.
+    (let* ((word (buffer-substring (car pos2) (cdr pos2)))
+           (len1 (- (cdr pos1) (car pos1)))
+           (len2 (length word))
+           (boundary (make-marker)))
+      (set-marker boundary (car pos2))
+      (goto-char (cdr pos1))
+      (insert-before-markers word)
+      (setq word (delete-and-extract-region (car pos1) (+ (car pos1) len1)))
+      (goto-char boundary)
+      (insert word)
+      (goto-char (+ boundary len1))
+      (delete-region (point) (+ (point) len2))
+      (set-marker boundary nil))))
 
 (defun backward-word (&optional arg)
   "Move backward until encountering the beginning of a word.