Don't run Pandoc at all if neither nocite nor citation keys present

doc/ox-hugo-manual.org

@@ -2074,31 +2074,37 @@ front-matter keys"/ in {{{ox-hugo-test-file}}}.
 :PROPERTIES:
 :EXPORT_FILE_NAME: pandoc-citations
 :END:
-[[https://pandoc.org/][Pandoc]] based citation parsing is enabled by setting the
-~#+hugo_pandoc_citations:~ keyword or ~:EXPORT_HUGO_PANDOC_CITATIONS:~
-subtree property to ~t~.
+The [[https://pandoc.org/][Pandoc]] Citations are prefixed with the *@* character. If the
+citation is ~@foo~, that particular /foo/ reference much be present in
+one of the specified bibliography files.
 
 #+begin_note
 Users need to have the ~pandoc~ executable present in their ~PATH~.
 #+end_note
+**** Enabling
+Pandoc based citation parsing is enabled by setting the
+~#+hugo_pandoc_citations:~ keyword or ~:EXPORT_HUGO_PANDOC_CITATIONS:~
+subtree property to ~t~.
 
-The Pandoc Citations are prefixed with the *@* character. If the
-citation is ~@foo~, that particular /foo/ reference much be present in
-one of the specified bibliography files.
-
+#+begin_note
+If a post has neither [[* Nocite][~nocite~]] meta-data, nor valid citation keys
+(~@foo~), the Pandoc parsing step is skipped *even if* the above
+Pandoc Citations parsing option is enabled.
+#+end_note
+**** Bibliography
 Bibliography files (~example.bib~) are specified using the
 ~#+bibliography:~ keyword or ~:EXPORT_BIBLIOGRAPHY:~ subtree
 property. *It is mandatory to specify at least one bibliography file.*
 
 Multiple comma-separated bibliography files can be
 specified. /Note that the path to these files should be relative to
 the Org file directory./
-
+**** Nocite
 ~nocite~ is a special Pandoc-specific meta-data which can be used to
 add extra citations even when they are not referenced in the post. It
 is set like any other list-type custom front-matter parameter
 (i.e. ~:LIST_PARAM '(ELEMENT1 ELEMENT2)~).
-
+**** Example
 Here is a mini-example using Pandoc Citations:
 #+begin_src org
 ,* Citations Example

ox-hugo-pandoc-cite.el

@@ -151,20 +151,55 @@ LOFFSET is the offset added to the base level of 1 for headings."
       (buffer-substring-no-properties (point-min) (point-max)))))
 
 (defun org-hugo-pandoc-cite--parse-citations-maybe (info)
-  "Check if Pandoc needs to be run to parse citations.
+  "Check if Pandoc needs to be run to parse citations; and run it.
 
 INFO is a plist used as a communication channel."
   ;; (message "pandoc citations keyword: %S"
   ;;          (org-hugo--plist-get-true-p info :hugo-pandoc-citations))
   ;; (message "pandoc citations prop: %S"
   ;;          (org-entry-get nil "EXPORT_HUGO_PANDOC_CITATIONS" :inherit))
-  (let ((orig-outfile (plist-get info :outfile)))
-    (when (and orig-outfile
-               (or (org-entry-get nil "EXPORT_HUGO_PANDOC_CITATIONS" :inherit)
-                   (org-hugo--plist-get-true-p info :hugo-pandoc-citations)))
-      (unless (executable-find "pandoc")
-        (user-error "[ox-hugo] pandoc executable not found in PATH"))
-      (org-hugo-pandoc-cite--parse-citations info orig-outfile))))
+  (let* ((orig-outfile (plist-get info :outfile))
+         (pandoc-enabled (or (org-entry-get nil "EXPORT_HUGO_PANDOC_CITATIONS" :inherit)
+                             (org-hugo--plist-get-true-p info :hugo-pandoc-citations)))
+         (fm (plist-get info :front-matter))
+         (has-nocite (string-match-p "^nocite\\(:\\| =\\) " fm))
+         (orig-outfile-contents (with-temp-buffer
+                                  (insert-file-contents orig-outfile)
+                                  (buffer-substring-no-properties
+                                   (point-min) (point-max))))
+         ;; http://pandoc.org/MANUAL.html#citations
+         ;; Each citation must have a key, composed of `@' + the
+         ;; citation identifier from the database, and may optionally
+         ;; have a prefix, a locator, and a suffix. The citation key
+         ;; must begin with a letter, digit, or _, and may contain
+         ;; alphanumerics, _, and internal punctuation characters
+         ;; (:.#$%&-+?<>~/).
+         ;; A minus sign (-) before the @ will suppress mention of the
+         ;; author in the citation.
+         (valid-citation-key-char-regexp "a-zA-Z0-9_:.#$%&+?<>~/-")
+         (citation-key-regexp (concat "[^" valid-citation-key-char-regexp "]"
+                                      "\\(-?@[a-zA-Z0-9_]"
+                                      "[" valid-citation-key-char-regexp "]+\\)"))
+         (has-@ (string-match-p citation-key-regexp orig-outfile-contents)))
+    (when pandoc-enabled
+      ;; Either the nocite front-matter should be there, or the
+      ;; citation keys should be present in the `orig-outfile'.
+      (if (or has-nocite has-@)
+          (progn
+            (unless (executable-find "pandoc")
+              (user-error "[ox-hugo] pandoc executable not found in PATH"))
+            (org-hugo-pandoc-cite--parse-citations info orig-outfile))
+        ;; Otherwise restore the front-matter format to TOML if set so
+        ;; by the user.
+        (unless (string= fm org-hugo--fm-yaml)
+          (let* ((orig-contents-only
+                  (replace-regexp-in-string
+                   ;; The `orig-contents-only' will always be in YAML.
+                   ;; Delete that first.
+                   "\\`---\n\\(.\\|\n\\)+\n---\n" "" orig-outfile-contents))
+                 (toml-fm-plus-orig-contents (concat fm orig-contents-only)))
+            ;; (message "[ox-hugo-pandoc-cite] orig-contents-only: %S" orig-contents-only)
+            (write-region toml-fm-plus-orig-contents nil orig-outfile)))))))
 
 (defun org-hugo-pandoc-cite--parse-citations (info orig-outfile)
   "Parse Pandoc Citations in ORIG-OUTFILE and update that file.

ox-hugo.el

@@ -978,9 +978,10 @@ This is an internal function."
   (setq org-hugo--section nil)
   (setq org-hugo--bundle nil)
   (advice-remove 'org-babel-exp-code #'org-hugo--org-babel-exp-code)
-  (plist-put info :outfile outfile)
-  (plist-put info :front-matter org-hugo--fm)
-  (org-hugo-pandoc-cite--parse-citations-maybe info)
+  (when outfile
+    (plist-put info :outfile outfile)
+    (plist-put info :front-matter org-hugo--fm)
+    (org-hugo-pandoc-cite--parse-citations-maybe info))
   (setq org-hugo--fm nil)
   (setq org-hugo--fm-yaml nil))