org-element: Small refactoring

* contrib/lisp/org-element.el (org-element-swap-A-B,
  org-element-drag-backward, org-element-drag-forward): Small
  refactoring.
* testing/lisp/test-org-element.el: Add tests.

contrib/lisp/org-element.el

@@ -3637,27 +3637,25 @@ in-between, if any, are siblings of the element at point."
 (defun org-element-swap-A-B (elem-A elem-B)
   "Swap elements ELEM-A and ELEM-B.
 
-Leave point at the end of ELEM-A.
-
-Assume ELEM-A is before ELEM-B and that they are not nested."
+Leave point at the end of ELEM-A."
   (goto-char (org-element-property :begin elem-A))
-  (let* ((beg-B (org-element-property :begin elem-B))
-	 (end-B-no-blank (save-excursion
-			     (goto-char (org-element-property :end elem-B))
-			     (skip-chars-backward " \r\t\n")
-			     (forward-line)
-			     (point)))
-	 (beg-A (org-element-property :begin elem-A))
-	 (end-A-no-blank (save-excursion
-			   (goto-char (org-element-property :end elem-A))
-			   (skip-chars-backward " \r\t\n")
-			   (forward-line)
-			   (point)))
-	 (body-A (buffer-substring beg-A end-A-no-blank))
-	 (body-B (buffer-substring beg-B end-B-no-blank))
-	 (between-A-B (buffer-substring end-A-no-blank beg-B)))
-    (delete-region beg-A end-B-no-blank)
-    (insert body-B between-A-B body-A)
+  (let* ((beg-A (org-element-property :begin elem-A))
+	 (end-A (save-excursion
+		  (goto-char (org-element-property :end elem-A))
+		  (skip-chars-backward " \r\t\n")
+		  (point-at-eol)))
+	 (beg-B (org-element-property :begin elem-B))
+	 (end-B (save-excursion
+		  (goto-char (org-element-property :end elem-B))
+		  (skip-chars-backward " \r\t\n")
+		  (point-at-eol)))
+	 (body-A (buffer-substring beg-A end-A))
+	 (body-B (delete-and-extract-region beg-B end-B)))
+    (goto-char beg-B)
+    (insert body-A)
+    (goto-char beg-A)
+    (delete-region beg-A end-A)
+    (insert body-B)
     (goto-char (org-element-property :end elem-B))))
 
 (defun org-element-backward ()
@@ -3706,8 +3704,8 @@ Move to the previous element at the same level, when possible."
     (org-element-backward)
     (let ((prev-elem (org-element-at-point)))
       (when (or (org-element-nested-p elem prev-elem)
-		(and (eq (car elem) 'headline)
-		     (not (eq (car prev-elem) 'headline))))
+		(and (eq (org-element-type elem) 'headline)
+		     (not (eq (org-element-type prev-elem) 'headline))))
 	(goto-char pos)
 	(error "Cannot drag element backward"))
       ;; Compute new position of point: it's shifted by PREV-ELEM
@@ -3727,8 +3725,8 @@ Move to the previous element at the same level, when possible."
     (goto-char (org-element-property :end elem))
     (let ((next-elem (org-element-at-point)))
       (when (or (org-element-nested-p elem next-elem)
-		(and (eq (car next-elem) 'headline)
-		     (not (eq (car elem) 'headline))))
+		(and (eq (org-element-type next-elem) 'headline)
+		     (not (eq (org-element-type elem) 'headline))))
 	(goto-char pos)
 	(error "Cannot drag element forward"))
       ;; Compute new position of point: it's shifted by NEXT-ELEM

testing/lisp/test-org-element.el

@@ -431,6 +431,50 @@ Outside."
     (org-element-down)
     (should (looking-at "Paragraph"))))
 
+(ert-deftest test-org-element/drag-backward ()
+  "Test `org-element-drag-backward' specifications."
+  ;; 1. Error when trying to move first element of buffer.
+  (org-test-with-temp-text "Paragraph 1.\n\nParagraph 2."
+    (should-error (org-element-drag-backward)))
+  ;; 2. Error when trying to swap nested elements.
+  (org-test-with-temp-text "#+BEGIN_CENTER\nTest.\n#+END_CENTER"
+    (forward-line)
+    (should-error (org-element-drag-backward)))
+  ;; 3. Error when trying to swap an headline element and
+  ;;    a non-headline element.
+  (org-test-with-temp-text "Test.\n* Head 1"
+    (forward-line)
+    (should-error (org-element-drag-backward)))
+  ;; 4. Otherwise, swap elements, preserving column and blank lines
+  ;;    between elements.
+  (org-test-with-temp-text "Para1\n\n\nParagraph 2\n\nPara3"
+    (search-forward "graph")
+    (org-element-drag-backward)
+    (should (equal (buffer-string) "Paragraph 2\n\n\nPara1\n\nPara3"))
+    (should (looking-at " 2"))))
+
+(ert-deftest test-org-element/drag-forward ()
+  "Test `org-element-drag-forward' specifications."
+  ;; 1. Error when trying to move first element of buffer.
+  (org-test-with-temp-text "Paragraph 1.\n\nParagraph 2."
+    (goto-line 3)
+    (should-error (org-element-drag-forward)))
+  ;; 2. Error when trying to swap nested elements.
+  (org-test-with-temp-text "#+BEGIN_CENTER\nTest.\n#+END_CENTER"
+    (forward-line)
+    (should-error (org-element-drag-forward)))
+  ;; 3. Error when trying to swap a non-headline element and an
+  ;;    headline.
+  (org-test-with-temp-text "Test.\n* Head 1"
+    (should-error (org-element-drag-forward)))
+  ;; 4. Otherwise, swap elements, preserving column and blank lines
+  ;;    between elements.
+  (org-test-with-temp-text "Paragraph 1\n\n\nPara2\n\nPara3"
+    (search-forward "graph")
+    (org-element-drag-forward)
+    (should (equal (buffer-string) "Para2\n\n\nParagraph 1\n\nPara3"))
+    (should (looking-at " 1"))))
+
 
 (provide 'test-org-element)
 ;;; test-org-element.el ends here