Transpose output of backlink dynamic block to emphasize backlinks

We are interested in the backlinks so we now put them first.  The backlink
targets (i.e., current item or any of its parents) are now grouped after the
priority of the item containing the backlink.

site-lisp/db-org.el

@@ -1049,7 +1049,7 @@ PARAMS may contain the following values:
   :archives       If non-nil, include archives"
   (let* ((org-ql-match (plist-get params :org-ql-match))
          (archives (plist-get params :archives))
-         headlines)
+         headlines output-lines)
 
     ;; Get all backlinks as list of Org mode IDs.  Each list consists of the ID
     ;; of the headline (current or partent), followed by the IDs linking back to
@@ -1064,20 +1064,42 @@ PARAMS may contain the following values:
                                      (db/org--backlinks-for-id id-at-point org-ql-match archives))))))
                (cl-remove-if #'null)))
 
-    ;; Formatting.
-    (insert (format "| Item | Backlinks | Priority |\n|---|"))
-    (dolist (headline headlines)
-      (when (cdr headline)           ; do not print backlinks if there are none
-        (insert (format "\n| %s |\n|---|" (db/org--format-link-from-org-id (car headline))))
-        (let ((backlink-lines (-> (mapcar #'(lambda (backlink-id)
-                                              (list (db/org--format-link-from-org-id backlink-id)
-                                                    (org-entry-get (org-id-find backlink-id 'marker)
-                                                                   "PRIORITY")))
-                                          (cdr headline))
-                                  (cl-sort #'string< :key #'cl-second))))
-          (dolist (line backlink-lines)
-            (insert (apply #'format "\n| | %s | %s |" line)))
-          (insert "\n|---|"))))
+    ;; Change entries in headlines from the format (headline-id backlink-ids...)
+    ;; to (backlink-id headline-ids ...) for grouping them in the output later
+    (setq headlines
+          (->> headlines
+               (-mapcat #'(lambda (headline)
+                            (mapcar #'(lambda (backlink)
+                                        (cons backlink (car headline)))
+                                    (cdr headline))))
+               ;; Group by backlinks (first entry), returns list of non-empty
+               ;; lists
+               (-group-by #'car)
+               ;; Flatten list, to get a list of (backlink-id headline-ids...)
+               (-map #'(lambda (group)
+                         (cons (car group) (-map #'cdr (cdr group)))))))
+
+    ;; Replace IDs by headlines and add priority for sorting
+    (setq output-lines
+          (->> headlines
+               (-map #'(lambda (line)
+                         (list (db/org--format-link-from-org-id (cl-first line))
+                               (org-entry-get (org-id-find (cl-first line) 'marker)
+                                              "PRIORITY")
+                               (-map #'db/org--format-link-from-org-id (cdr line)))))
+               (-sort #'(lambda (line-1 line-2)
+                          (string< (cl-second line-1) (cl-second line-2))))))
+
+    ;; Format output-lines as Org table
+    (insert (format "| Backlink | Prio | Backlink Target(s) |\n|---|"))
+    (when output-lines
+      (dolist (line output-lines)
+        (insert (format "\n| %s | %s | %s |"
+                        (cl-first line)  ; backlink
+                        (cl-second line) ; priority
+                        (apply #'concat (-interpose ", " (cl-third line))) ; backlink targets
+                        )))
+      (insert "\n|---|"))
     (org-table-align)))
 
 (defun db/org-insert-backlink-block ()