oc: Refactor affixes extraction

* lisp/oc.el (org-cite-main-affixes): New function.
* lisp/oc-natbib.el (org-cite-natbib--build-optional-arguments): Use
new function.
* testing/lisp/test-oc.el (test-org-cite/main-affixes): New test.

lisp/oc-natbib.el

@@ -119,11 +119,7 @@ If \"natbib\" package is already required in the document, e.g., through
 (defun org-cite-natbib--build-optional-arguments (citation info)
   "Build optional arguments for citation command.
 CITATION is the citation object.  INFO is the export state, as a property list."
-  (let* ((origin (pcase (org-cite-get-references citation)
-                   (`(,reference) reference)
-                   (_ citation)))
-         (suffix (org-element-property :suffix origin))
-         (prefix (org-element-property :prefix origin)))
+  (pcase-let ((`(,prefix . ,suffix) (org-cite-main-affixes citation)))
     (concat (and prefix (format "[%s]" (org-trim (org-export-data prefix info))))
             (cond
              (suffix (format "[%s]" (org-trim (org-export-data suffix info))))

lisp/oc.el

@@ -638,6 +638,24 @@ in the current buffer.  Positions include leading \"@\" character."
       (re-search-forward org-element-citation-key-re end t)
       (cons (match-beginning 0) (match-end 0)))))
 
+(defun org-cite-main-affixes (citation)
+  "Return main affixes for CITATION object.
+
+Some export back-ends only support a single pair of affixes per
+citation, even if it contains multiple keys.  This function
+decides what affixes are the most appropriate.
+
+Return a pair (PREFIX . SUFFIX) where PREFIX and SUFFIX are
+parsed data."
+  (let ((source
+         ;; When there are multiple references, use global affixes.
+         ;; Otherwise, local affixes have priority.
+         (pcase (org-cite-get-references citation)
+           (`(,reference) reference)
+           (_ citation))))
+    (cons (org-element-property :prefix source)
+          (org-element-property :suffix source))))
+
 (defun org-cite-supported-styles (&optional processors)
   "List of supported citation styles and variants.
 

testing/lisp/test-oc.el

@@ -251,6 +251,45 @@
                (car boundaries)
                (cdr boundaries)))))))
 
+(ert-deftest test-org-cite/main-affixes ()
+  "Test`org-cite-main-affixes'."
+  (should
+   (equal '(nil . nil)
+          (org-test-with-temp-text "[cite:@key]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(nil . nil)
+          (org-test-with-temp-text "[cite:@key1;@key2]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(("pre ") . nil)
+          (org-test-with-temp-text "[cite:pre @key]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(("pre ") . (" post"))
+          (org-test-with-temp-text "[cite:pre @key post]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(("pre ") . nil)
+          (org-test-with-temp-text "[cite:global pre;pre @key]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(nil . (" post"))
+          (org-test-with-temp-text "[cite:@key post;global post]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(("global pre") . ("global post"))
+          (org-test-with-temp-text "[cite:global pre;@key1;@key2;global post]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(("global pre") . nil)
+          (org-test-with-temp-text "[cite:global pre;pre1 @key1;pre2 @key2]"
+            (org-cite-main-affixes (org-element-context)))))
+  (should
+   (equal '(nil . ("global post"))
+          (org-test-with-temp-text "[cite:@key1 post1;@key2 post2; global post]"
+            (org-cite-main-affixes (org-element-context))))))
+
 (ert-deftest test-org-cite/supported-styles ()
   "Test `org-cite-supported-styles'."
   ;; Default behavior is to use export processors.