bug#30343: make transpose-regions interactive

[Charles A. Roelli]

> Date: Sat, 10 Feb 2018 12:58:23 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > @@ -183,7 +185,7 @@ Transpose
> >  (@code{transpose-lines}) exchanges lines.  They work like @kbd{M-t}
> >  except as regards what units of text they transpose.
> >  
> > -  A numeric argument to a transpose command serves as a repeat count: it
> > +  A numeric argument to most transpose commands serves as a repeat count: 
> > it
> > [...]
> > +@findex transpose-regions
> > +  @kbd{M-x transpose-regions} transposes the text between point and
> > +mark with the text between the first two elements of the mark ring.
> 
> I would say "... with the text between the last two marks pushed to
> the mark ring."  I would also add a cross-reference here to where
> set-mark-command is described.
> 
> > +Unlike the other transpose commands, it does not behave differently
> > +when given a prefix argument.  This command is best used for
> > +transposing multiple units of text (words, sentences, paragraphs) in
> > +one go.
> 
> Would it make sense to make the new command interpret the prefix
> argument the same way as the other transpose commands?  E.g., why not
> use pairs of marks further back in the mark ring?
> 
> > -DEFUN ("transpose-regions", Ftranspose_regions, Stranspose_regions, 4, 5, 
> > 0,
> > +DEFUN ("transpose-regions", Ftranspose_regions, Stranspose_regions, 4, 5,
> > +       "(if (< (length mark-ring) 2)\
> > +            (error \"Mark ring must contain at least two elements\")\
> > +          (list (point) (mark) (car mark-ring) (cadr mark-ring)))",
> 
> The error message could be made more clear, e.g. by saying that the
> other region should be marked first.  Talking about the size of the
> mark ring is too technical, IMO.
> 
> > +Interactively, STARTR1 and ENDR1 are point and mark.  STARTR2 and
> > +ENDR2 are the first and second markers in the mark ring.
> > +LEAVE-MARKERS is nil.  */)
> 
> "First and second" is ambiguous, since you don't tell from which end
> they are counted.  Also, these 3 sentences all talk about interactive
> invocation, so they should probably be a single sentence separated
> with semi-colons instead.

Thanks for the review.  Could you please look at the following second
iteration?  All your points should be addressed.

diff --git a/doc/emacs/fixit.texi b/doc/emacs/fixit.texi
index 7cacac4..eb783d1 100644
--- a/doc/emacs/fixit.texi
+++ b/doc/emacs/fixit.texi
@@ -149,6 +149,8 @@ Transpose
 Transpose two balanced expressions (@code{transpose-sexps}).
 @item C-x C-t
 Transpose two lines (@code{transpose-lines}).
+@item M-x transpose-regions
+Transpose two regions.
 @end table
 
 @kindex C-t
@@ -204,6 +206,15 @@ Transpose
 transpose the character (or word or expression or line) ending after
 point with the one ending after the mark.
 
+@findex transpose-regions
+  @kbd{M-x transpose-regions} transposes the text between point and
+mark with the text between the last two marks pushed to the mark ring
+(@pxref{Setting Mark}).  With a numeric prefix argument, it transposes
+the text between point and mark with the text between two successive
+marks that many entries back in the mark ring.  This command is best
+used for transposing multiple characters (or words or sentences or
+paragraphs) in one go.
+
 @node Fixing Case
 @section Case Conversion
 
diff --git a/src/editfns.c b/src/editfns.c
index 96bb271..9183bd1 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -5112,7 +5112,16 @@ transpose_markers (ptrdiff_t start1, ptrdiff_t end1,
     }
 }
 
-DEFUN ("transpose-regions", Ftranspose_regions, Stranspose_regions, 4, 5, 0,
+DEFUN ("transpose-regions", Ftranspose_regions, Stranspose_regions, 4, 5,
+       "(if (< (length mark-ring) 2)\
+           (error \"Other region must be marked before transposing two 
regions\")\
+         (let* ((num (if current-prefix-arg\
+                        (prefix-numeric-value current-prefix-arg)\
+                       0))\
+                (ring-length (length mark-ring))\
+                (eltnum (mod num ring-length))\
+                (eltnum2 (mod (1+ num) ring-length)))\
+           (list (point) (mark) (elt mark-ring eltnum) (elt mark-ring 
eltnum2))))",
        doc: /* Transpose region STARTR1 to ENDR1 with STARTR2 to ENDR2.
 The regions should not be overlapping, because the size of the buffer is
 never changed in a transposition.
@@ -5120,7 +5129,14 @@ never changed in a transposition.
 Optional fifth arg LEAVE-MARKERS, if non-nil, means don't update
 any markers that happen to be located in the regions.
 
-Transposing beyond buffer boundaries is an error.  */)
+Transposing beyond buffer boundaries is an error.
+
+Interactively, STARTR1 and ENDR1 are point and mark; STARTR2 and ENDR2
+are the last two marks pushed to the mark ring; LEAVE-MARKERS is nil.
+If a prefix argument N is given, STARTR2 and ENDR2 are the two
+successive marks N entries backwards in the mark ring.  A negative
+prefix argument instead counts forwards from the oldest mark in the
+mark ring.  */)
   (Lisp_Object startr1, Lisp_Object endr1, Lisp_Object startr2, Lisp_Object 
endr2, Lisp_Object leave_markers)
 {
   register ptrdiff_t start1, end1, start2, end2;