Add support for org-id

For a discussion, read issue 8 on the GitHub mirror:
<https://github.com/protesilaos/denote/issues/8>.

denote-link.el

@@ -196,6 +196,9 @@ and/or the documentation string of `display-buffer'."
 (defconst denote-link--format-org "[[denote:%s][%s]]"
   "Format of Org link to note.")
 
+(defconst denote-link--format-org-with-id "[[id:%s][%s]]"
+  "Format of Org link to note for `denote-use-org-id'.")
+
 (defconst denote-link--format-markdown "[%2$s](denote:%1$s)"
   "Format of Markdown link to note.")
 
@@ -203,7 +206,7 @@ and/or the documentation string of `display-buffer'."
   "Format of identifier-only link to note.")
 
 (defconst denote-link--regexp-org
-  (concat "\\[\\[" "denote:"  "\\(?1:" denote--id-regexp "\\)" "]" "\\[.*?]]"))
+  (concat "\\[\\[" "\\(denote\\|[Ii][Dd]\\):"  "\\(?1:" denote--id-regexp "\\)" "]" "\\[.*?]]"))
 
 (defconst denote-link--regexp-markdown
   (concat "\\[.*?]" "(denote:"  "\\(?1:" denote--id-regexp "\\)" ")"))
@@ -213,9 +216,12 @@ and/or the documentation string of `display-buffer'."
 
 (defun denote-link--file-type-format (file)
   "Return link format based on FILE format."
+  (let ((org-format (if denote-use-org-id
+                        denote-link--format-org-with-id
+                      denote-link--format-org)))
     (pcase (file-name-extension file)
       ("md" denote-link--format-markdown)
-    (_ denote-link--format-org))) ; Includes backup files.  Maybe we can remove them?
+      (_ org-format)))) ; Includes backup files.  Maybe we can remove them?
 
 (defun denote-link--file-type-regexp (file)
   "Return link regexp based on FILE format."

denote-retrieve.el

@@ -39,7 +39,7 @@ The match that needs to be extracted is explicityly marked as
 group 1.")
 
 (defconst denote-retrieve--identifier-regexp
-  "^.?.?\\b\\(?:identifier\\|ID\\)\\s-*[:=]\\s-*[\"']?\\(?1:[0-9T]+\\)[\"']?"
+  "^.?.?\\b\\(?:identifier\\|[Ii][Dd]\\)\\s-*[:=]\\s-*[\"']?\\(?1:[0-9T]+\\)[\"']?"
   "Regular expression for identifier key and value.
 The match that needs to be extracted is explicityly marked as
 group 1.")

denote.el

@@ -211,6 +211,25 @@ is suspended: we use whatever the user wants."
           (string :tag "Custom format for `format-time-string'"))
   :group 'denote)
 
+(defcustom denote-use-org-id nil
+  "When non-nil use the ID property and link type in Org files.
+To use the ID property, a PROPERTIES drawer is added to the top
+of newly created notes.  Furthermore, newly created links will
+use the standard 'id:' hyperlink type instead of the custom
+'denote:' type.
+
+In practical terms, the ID ensures maximum compatibility with the
+Org ecosystem.
+
+When the value is nil, Denote uses a generic front matter for new
+notes and links rely on the custom 'denote:' type (which should
+behave the same as the standard 'file:' type).
+
+Other files types beside Org always use the 'denote:' links and
+keep their generic front matter."
+  :type 'boolean
+  :group 'denote)
+
 ;;;; Main variables
 
 (defconst denote--id-format "%Y%m%dT%H%M%S"
@@ -246,6 +265,8 @@ We consider those characters illigal for our purposes.")
         `(metadata (category . ,category))
       (complete-with-action action candidates string pred))))
 
+(defvar org-id-extra-files)
+
 (defun denote-directory ()
   "Return path of variable `denote-directory' as a proper directory."
   (let* ((val (or (buffer-local-value 'denote-directory (current-buffer))
@@ -253,6 +274,8 @@ We consider those characters illigal for our purposes.")
          (path (if (or (eq val 'default-directory) (eq val 'local)) default-directory val)))
     (unless (file-directory-p path)
       (make-directory path t))
+    (when (and denote-use-org-id (require 'org-id nil t))
+      (setq org-id-extra-files (directory-files path nil "\.org$")))
     (file-name-as-directory path)))
 
 (defun denote--extract (regexp str &optional group)
@@ -508,6 +531,16 @@ and do not use any empty line before it.
 These help ensure consistency and might prove useful if we need
 to operate on the front matter as a whole.")
 
+(defvar denote-org-with-id-front-matter
+  ":PROPERTIES:
+:ID:          %4$s
+:END:
+#+title:      %1$s
+#+date:       %2$s
+#+filetags:   %3$s
+\n"
+  "Like `denote-org-front-matter' but for `denote-use-org-id'.")
+
 (defun denote--file-meta-header (title date keywords id &optional filetype)
   "Front matter for new notes.
 
@@ -517,12 +550,13 @@ TITLE, DATE, KEYWORDS, FILENAME, ID are all strings which are
 Optional FILETYPE is one of the values of `denote-file-type',
 else that variable is used."
   (let ((kw-space (denote--file-meta-keywords keywords))
-        (kw-toml (denote--file-meta-keywords keywords 'toml)))
+        (kw-toml (denote--file-meta-keywords keywords 'toml))
+        (org-front (if denote-use-org-id denote-org-with-id-front-matter denote-org-front-matter)))
     (pcase (or filetype denote-file-type)
       ('markdown-toml (format denote-toml-front-matter title date kw-toml id))
       ('markdown-yaml (format denote-yaml-front-matter title date kw-space id))
       ('text (format denote-text-front-matter title date kw-space id denote-text-front-matter-delimiter))
-      (_ (format denote-org-front-matter title date kw-space id)))))
+      (_ (format org-front title date kw-space id)))))
 
 (defun denote--path (title keywords &optional dir id)
   "Return path to new file with TITLE and KEYWORDS.