(feat): simplify format of `INFO` for find-ref and add `FILTER` (#567)

* (feat): simplify format of `INFO` for find-ref

The format of the `INFO` argument of `org-roam-find` has been simplified, thanks
to the split of `type` and `ref` in the db permitted by #547.

The non-interactive behaviour of `org-roam-find-ref` has also been isolated
into `org-roam--find-ref`.  Even if it makes us repeat ourselves a bit, it
avoids having to nil the interactive argument just to get the non-interactive
us, which I think is better syntax.

* (feat): implement get-ref-path-completions filtering

org-roam.el

@@ -598,34 +598,64 @@ See `org-roam--get-ref-path-completions' for details."
   :type 'boolean
   :group 'org-roam)
 
-(defun org-roam--get-ref-path-completions (&optional interactive)
+(defun org-roam--get-ref-path-completions (&optional interactive filter)
   "Return a list of cons pairs for refs to absolute path of Org-roam files.
-When INTERACTIVE `org-roam-include-type-in-ref-path-completions'
-are non-nil, format the car of the completion-candidates as
-'type:ref'."
+When `org-roam-include-type-in-ref-path-completions' and
+INTERACTIVE are non-nil, format the car of the
+completion-candidates as 'type:ref'.
+FILTER can either be a string or a function:
+- If it is a string, it should be the type of refs to include as
+candidates (e.g. \"cite\" ,\"website\" ,etc.)
+- If it is a function, it should be the name of a function that
+takes three arguments: the type, the ref, and the file of the
+current candidate.  It should return t if that candidate is to be
+included as a candidate."
   (let ((rows (org-roam-db-query [:select [type ref file] :from refs]))
         (include-type (and interactive
-                           org-roam-include-type-in-ref-path-completions)))
-    (mapcar (lambda (row)
-              (cl-destructuring-bind (type ref file) row
-                (cons (if include-type
-                          (format "%s:%s" type ref)
-                        ref)
-                      file)))
-            rows)))
-
-(defun org-roam-find-ref (arg &optional info)
+                           org-roam-include-type-in-ref-path-completions))
+        candidates)
+    (dolist (row rows (nreverse candidates))
+      (cl-destructuring-bind (type ref file) row
+        (when (pcase filter
+                ('nil t)
+                ((pred stringp) (string= type filter))
+                ((pred functionp) (funcall filter type ref file))
+                (wrong-type (signal 'wrong-type-argument
+                                    `((stringp functionp)
+                                      ,wrong-type))))
+          (let ((candidate (cons (if include-type
+                                     (format "%s:%s" type ref)
+                                   ref)
+                                 file)))
+            (push candidate candidates)))))))
+
+(defun org-roam--find-ref (ref)
+  "Find and open and Org-roam file from REF if it exists.
+REF should be the value of '#+ROAM_KEY:' without any
+type-information (e.g. 'cite:').
+Return nil if the file does not exist."
+  (when-let* ((completions (org-roam--get-ref-path-completions))
+              (file (cdr (assoc ref completions))))
+    (find-file file)))
+
+(defun org-roam-find-ref (arg &optional filter)
   "Find and open an Org-roam file from a ref.
-INFO is an alist containing additional information.
 ARG is used to forward interactive calls to
-`org-roam--get-ref-path-completions'"
+`org-roam--get-ref-path-completions'
+FILTER can either be a string or a function:
+- If it is a string, it should be the type of refs to include as
+candidates (e.g. \"cite\" ,\"website\" ,etc.)
+- If it is a function, it should be the name of a function that
+takes three arguments: the type, the ref, and the file of the
+current candidate.  It should return t if that candidate is to be
+included as a candidate."
   (interactive "p")
-  (let* ((completions (org-roam--get-ref-path-completions arg))
-         (ref (or (cdr (assoc 'ref info))
-                  (org-roam-completion--completing-read "Ref: "
-                                                        completions
-                                                        :require-match t))))
-    (find-file (cdr (assoc ref completions)))))
+  (let* ((completions (org-roam--get-ref-path-completions arg filter))
+         (ref (org-roam-completion--completing-read "Ref: "
+                                                    completions
+                                                    :require-match t))
+         (file (cdr (assoc ref completions))))
+    (find-file file)))
 
 (defun org-roam--get-roam-buffers ()
   "Return a list of buffers that are Org-roam files."