ox: Implement `org-export-get-reference'

* lisp/ox.el (org-export-get-reference): New function.

* etc/ORG-NEWS: Signal new function.

etc/ORG-NEWS

@@ -279,6 +279,7 @@ in the output, depending on the parameters.
 *** Extend ~org-export-first-sibling-p~ and ~org-export-last-sibling-p~
 These functions now support any element or object, not only headlines.
 *** New function: ~org-export-table-row-in-header-p~
+*** New function: ~org-export-get-reference~
 *** New function: ~org-element-lineage~
 This function deprecates ~org-export-get-genealogy~.  It also provides
 more features.  See docstring for details.

lisp/ox.el

@@ -4165,9 +4165,31 @@ has type \"radio\"."
 
 ;;;; For References
 ;;
+;; `org-export-get-reference' associate a unique reference for any
+;; object or element.
+;;
 ;; `org-export-get-ordinal' associates a sequence number to any object
 ;; or element.
 
+(defun org-export-get-reference (datum info)
+  "Return a unique reference for DATUM, as a string.
+DATUM is either an element or an object.  INFO is the current
+export state, as a plist.  Returned reference consists of
+alphanumeric characters only."
+  (let ((type (org-element-type datum))
+	(cache (or (plist-get info :internal-references)
+		   (let ((h (make-hash-table :test #'eq)))
+		     (plist-put info :internal-references h)
+		     h))))
+    (or (gethash datum cache)
+	(puthash datum
+		 (format "org%s%d"
+			 (if type
+			     (replace-regexp-in-string "-" "" (symbol-name type))
+			   "secondarystring")
+			 (incf (gethash type cache 0)))
+		 cache))))
+
 (defun org-export-get-ordinal (element info &optional types predicate)
   "Return ordinal number of an element or object.