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+;;; denote-capf.el --- completion-at-point for denote -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;; `denote-capf' provides `completion-at-point' functions for Denote
+;; note titles and keywords.  Enable it in Org buffers with:
+;;
+;;   (add-hook 'org-mode-hook #'denote-capf-setup)
+;;
+;; Or call `M-x denote-capf-setup' manually.  Use `M-x
+;; denote-capf-teardown' to disable it in the current buffer, and
+;; `M-x denote-capf-refresh' to rebuild the cache.
+
+;;; Code:
+
+(require 'denote)
+(require 'subr-x)
+(require 'seq)
+(require 'thingatpt)
+
+(declare-function org-in-src-block-p "org")
+(defvar corfu-auto)
+
+(defgroup denote-capf nil
+  "Completion-at-point functions for Denote links and keywords."
+  :group 'denote)
+
+(defcustom denote-capf-min-prefix 3
+  "Minimum length of the word at point before offering note completions."
+  :type 'integer :group 'denote-capf)
+
+(defcustom denote-capf-include-encrypted nil
+  "When non-nil, include encrypted notes in the completion candidates."
+  :type 'boolean :group 'denote-capf)
+(defcustom denote-capf-directories nil
+  "Directories whose org files get denote completion-at-point.
+When nil, `denote-capf-setup' enables completion in every org
+buffer.  When set, only files living under one of these
+directories are touched, leaving every other org buffer alone."
+  :type '(repeat directory) :group 'denote-capf)
+
+(defvar denote-capf--titles nil)
+(defvar denote-capf--title->id nil)
+(defvar denote-capf--timer nil)
+
+(defun denote-capf--build ()
+  "Build the list of denote note titles and their identifier mapping."
+  (let ((titles '())
+        (table (make-hash-table :test 'equal)))
+    (dolist (file (denote-directory-files nil nil t (unless denote-capf-include-encrypted "\\.\\(gpg\\|age\\)\\'")))
+      (let ((id (denote-retrieve-filename-identifier file))
+            (title (denote-retrieve-title-or-filename file 'org)))
+        (when (and id title (not (string-empty-p title)))
+          (unless (gethash title table) (push title titles))
+          (puthash title id table))))
+    (setq denote-capf--titles (nreverse titles)
+          denote-capf--title->id table)))
+
+(defun denote-capf--ensure ()
+  "Build the completion cache if it is not already populated."
+  (unless denote-capf--title->id (denote-capf--build)))
+
+(defun denote-capf--schedule ()
+  "Schedule a rebuild of the completion cache during idle time."
+  (when (timerp denote-capf--timer) (cancel-timer denote-capf--timer))
+  (setq denote-capf--timer (run-with-idle-timer 0.5 nil #'denote-capf--build)))
+
+(defun denote-capf-refresh ()
+  "Rebuild the denote completion cache.
+When called interactively, show how many notes were cached."
+  (interactive)
+  (denote-capf--build)
+  (when (called-interactively-p 'any)
+    (message "Denote completion cache refreshed (%d notes)"
+             (length denote-capf--titles))))
+
+(defun denote-capf--invalidate ()
+  "Clear the completion cache and schedule a rebuild."
+  (setq denote-capf--title->id nil denote-capf--titles nil)
+  (denote-capf--schedule))
+
+(add-hook 'denote-after-new-note-hook #'denote-capf--invalidate)
+(add-hook 'denote-after-rename-file-hook #'denote-capf--invalidate)
+
+(defun denote-capf--in-link-p ()
+  "Return non-nil when point is inside an Org link."
+  (save-excursion
+    (let ((pos (point)) (bol (line-beginning-position)) (depth 0))
+      (goto-char bol)
+      (while (< (point) pos)
+        (cond ((looking-at "\\[\\[") (setq depth (1+ depth)) (forward-char 2))
+              ((looking-at "\\]\\]") (when (> depth 0) (setq depth (1- depth))) (forward-char 2))
+              (t (forward-char 1))))
+      (> depth 0))))
+
+(defun denote-capf--link-exit (str status)
+  "Exit function for `denote-capf-link'.
+Replace the completed title with a denote link when STATUS is one
+of finished, exact or sole."
+  (when (memq status '(finished exact sole))
+    (when-let* ((id (gethash str denote-capf--title->id)))
+      (delete-region (- (point) (length str)) (point))
+      (insert (format "[[denote:%s][%s]]" id str)))))
+
+(defun denote-capf-link ()
+  "Completion-at-point function for denote note links."
+  (when (and (derived-mode-p 'org-mode)
+             (not (org-in-src-block-p))
+             (not (save-excursion (beginning-of-line) (looking-at-p "[ \t]*#\\+")))
+             (not (denote-capf--in-link-p)))
+    (when-let* ((bounds (bounds-of-thing-at-point 'word)))
+      (when (>= (- (cdr bounds) (car bounds)) denote-capf-min-prefix)
+        (denote-capf--ensure)
+        (list (car bounds) (cdr bounds) denote-capf--titles
+              :exclusive 'no
+              :company-kind (lambda (_) 'reference)
+              :annotation-function (lambda (_) " ✦ note")
+              :exit-function #'denote-capf--link-exit)))))
+
+(defun denote-capf-keywords ()
+  "Completion-at-point function for denote keywords on #+filetags lines."
+  (when (and (derived-mode-p 'org-mode)
+             (save-excursion (beginning-of-line)
+                             (looking-at-p "[ \t]*#\\+filetags:")))
+    (let ((end (point))
+          (start (save-excursion (skip-chars-backward "A-Za-z0-9_-") (point))))
+      (when (< start end)
+        (list start end (denote-keywords)
+              :exclusive 'no
+              :company-kind (lambda (_) 'keyword)
+              :annotation-function (lambda (_) " ✦ keyword"))))))
+
+(defun denote-capf--enabled-here-p ()
+  "Return non-nil when denote completion should run in this buffer.
+Honors `denote-capf-directories'."
+  (or (null denote-capf-directories)
+      (and buffer-file-name
+           (seq-some (lambda (dir) (file-in-directory-p buffer-file-name dir))
+                     denote-capf-directories))))
+
+(defun denote-capf--add-capfs ()
+  "Add denote capfs to `completion-at-point-functions' locally.
+Does nothing if they are already present."
+  (unless (memq #'denote-capf-keywords completion-at-point-functions)
+    (add-hook 'completion-at-point-functions #'denote-capf-keywords nil t))
+  (unless (memq #'denote-capf-link completion-at-point-functions)
+    (add-hook 'completion-at-point-functions #'denote-capf-link nil t)))
+
+;;;###autoload
+(defun denote-capf-setup ()
+  "Set up denote completion-at-point in the current buffer.
+When called interactively, print a status message.  The setup is
+only performed when `denote-capf--enabled-here-p' returns non-nil."
+  (interactive)
+  (if (denote-capf--enabled-here-p)
+      (progn
+        (setq-local corfu-auto nil)
+        (denote-capf--add-capfs)
+        (unless denote-capf--title->id (denote-capf--schedule))
+        (when (called-interactively-p 'any)
+          (message "Denote completion-at-point enabled in this buffer")))
+    (when (called-interactively-p 'any)
+      (message "Denote completion-at-point is not enabled here"))))
+
+;;;###autoload
+(defun denote-capf-teardown ()
+  "Remove denote completion-at-point from the current buffer.
+When called interactively, print a status message."
+  (interactive)
+  (remove-hook 'completion-at-point-functions #'denote-capf-keywords t)
+  (remove-hook 'completion-at-point-functions #'denote-capf-link t)
+  (when (called-interactively-p 'any)
+    (message "Denote completion-at-point disabled in this buffer")))
+
+(provide 'denote-capf)
+;;; denote-capf.el ends here