Handle negative prefix arguments to mark-paragraph correctly

* textmodes/paragraph.el (mark-paragraph): Handle negative
arguments correctly (bug#18847).  This makes `M- M-h M-h' do the
correct thing with expanding the region (like other marking
commands) backwards.  Also fix problem at the end of the buffer,
where the numbers of paragraphs left in the buffer is less than
ARG, then paragraphs would also be marked *before* the current
paragraph.  Also clarify the doc string.

1 files changed, 40 insertions, 23 deletions

diff --git a/lisp/textmodes/paragraphs.el b/lisp/textmodes/paragraphs.el
index e22e3f48994..b0975291428 100644
--- a/lisp/textmodes/paragraphs.el
+++ b/lisp/textmodes/paragraphs.el
@@ -371,33 +371,50 @@ See `forward-paragraph' for more information."
 
 (defun mark-paragraph (&optional arg allow-extend)
   "Put point at beginning of this paragraph, mark at end.
-The paragraph marked is the one that contains point or follows point.
+The paragraph marked is the one that contains point or follows
+point.
 
-With argument ARG, puts mark at end of a following paragraph, so that
-the number of paragraphs marked equals ARG.
+With argument ARG, puts mark at the end of this or a following
+paragraph, so that the number of paragraphs marked equals ARG.
 
-If ARG is negative, point is put at end of this paragraph, mark is put
-at beginning of this or a previous paragraph.
+If ARG is negative, point is put at the end of this paragraph,
+mark is put at the beginning of this or a previous paragraph.
 
 Interactively (or if ALLOW-EXTEND is non-nil), if this command is
-repeated or (in Transient Mark mode) if the mark is active,
-it marks the next ARG paragraphs after the ones already marked."
-  (interactive "p\np")
-  (unless arg (setq arg 1))
-  (when (zerop arg)
-    (error "Cannot mark zero paragraphs"))
-  (cond ((and allow-extend
-	      (or (and (eq last-command this-command) (mark t))
-		  (and transient-mark-mode mark-active)))
-	 (set-mark
-	  (save-excursion
-	    (goto-char (mark))
-	    (forward-paragraph arg)
-	    (point))))
-	(t
-	 (forward-paragraph arg)
-	 (push-mark nil t t)
-	 (backward-paragraph arg))))
+repeated or (in Transient Mark mode) if the mark is active, it
+marks the next ARG paragraphs after the region already marked.
+This also means when activating the mark immediately before using
+this command, the current paragraph is only marked from point."
+  (interactive "P\np")
+  (let ((numeric-arg (prefix-numeric-value arg)))
+    (cond ((zerop numeric-arg))
+	  ((and allow-extend
+		(or (and (eq last-command this-command) mark-active)
+		    (region-active-p)))
+	   (if arg
+	       (setq arg numeric-arg)
+	     (if (< (mark) (point))
+		 (setq arg -1)
+	       (setq arg 1)))
+	   (set-mark
+	    (save-excursion
+	      (goto-char (mark))
+	      (forward-paragraph arg)
+	      (point))))
+	  ;; don't activate the mark when at eob
+	  ((and (eobp) (> numeric-arg 0)))
+	  (t
+	   (unless (save-excursion
+		     (forward-line 0)
+		     (looking-at  paragraph-start))
+	     (backward-paragraph (cond ((> numeric-arg 0) 1)
+                                       ((< numeric-arg 0) -1)
+                                       (t 0))))
+	   (push-mark
+	    (save-excursion
+	      (forward-paragraph numeric-arg)
+	      (point))
+            t t)))))
 
 (defun kill-paragraph (arg)
   "Kill forward to end of paragraph.