org: Improved consistency + tests

* lisp/org.el (org-get-valid-level): The function now returns 1 for
  parameters level=0, change=1 and `org-odd-levels-only'=t.  Before
  it returned 3.

lisp/org.el

@@ -8262,13 +8262,14 @@ time to headlines when structure editing, based on the value of
   (if org-odd-levels-only 2 1))
 
 (defun org-get-valid-level (level &optional change)
-  "Rectify a level change under the influence of `org-odd-levels-only'
-LEVEL is a current level, CHANGE is by how much the level should be
-modified.  Even if CHANGE is nil, LEVEL may be returned modified because
-even level numbers will become the next higher odd number."
+  "Rectify a level change under the influence of `org-odd-levels-only'.
+LEVEL is a current level, CHANGE is by how much the level should
+be modified.  Even if CHANGE is nil, LEVEL may be returned
+modified because even level numbers will become the next higher
+odd number.  Returns values greater than 0."
   (if org-odd-levels-only
       (cond ((or (not change) (= 0 change)) (1+ (* 2 (/ level 2))))
-	    ((> change 0) (1+ (* 2 (/ (+ level (* 2 change)) 2))))
+	    ((> change 0) (1+ (* 2 (/ (+ (1- level) (* 2 change)) 2))))
 	    ((< change 0) (max 1 (1+ (* 2 (/ (+ level (* 2 change)) 2))))))
     (max 1 (+ level (or change 0)))))
 

testing/lisp/test-org.el

@@ -3786,6 +3786,35 @@ Text.
       (forward-line)
       (org-get-indentation)))))
 
+(ert-deftest test-org/org-get-valid-level ()
+  "Test function `org-get-valid-level' specifications."
+  (let ((org-odd-levels-only nil))
+    (should (equal 1 (org-get-valid-level 0 0)))
+    (should (equal 1 (org-get-valid-level 0 1)))
+    (should (equal 2 (org-get-valid-level 0 2)))
+    (should (equal 3 (org-get-valid-level 0 3)))
+    (should (equal 1 (org-get-valid-level 1 0)))
+    (should (equal 2 (org-get-valid-level 1 1)))
+    (should (equal 23 (org-get-valid-level 1 22)))
+    (should (equal 1 (org-get-valid-level 1 -1)))
+    (should (equal 1 (org-get-valid-level 2 -1))))
+  (let ((org-odd-levels-only t))
+    (should (equal 1 (org-get-valid-level 0 0)))
+    (should (equal 1 (org-get-valid-level 0 1)))
+    (should (equal 3 (org-get-valid-level 0 2)))
+    (should (equal 5 (org-get-valid-level 0 3)))
+    (should (equal 1 (org-get-valid-level 1 0)))
+    (should (equal 3 (org-get-valid-level 1 1)))
+    (should (equal 3 (org-get-valid-level 2 1)))
+    (should (equal 5 (org-get-valid-level 3 1)))
+    (should (equal 5 (org-get-valid-level 4 1)))
+    (should (equal 43 (org-get-valid-level 1 21)))
+    (should (equal 1 (org-get-valid-level 1 -1)))
+    (should (equal 1 (org-get-valid-level 2 -1)))
+    (should (equal 1 (org-get-valid-level 3 -1)))
+    (should (equal 3 (org-get-valid-level 4 -1)))
+    (should (equal 3 (org-get-valid-level 5 -1)))))
+
 
 ;;; Planning