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+;;; jao-notmuch-gnus.el --- notmuch-gnus interoperability  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022, 2024  jao
+
+;; Author: jao <mail@jao.io>
+;; Keywords: mail
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;  Helper functions to work in Gnus with mail indexed by notmuch.
+
+;;; Code:
+
+(require 'gnus)
+(require 'ol-gnus)
+(require 'notmuch-show)
+
+;;; Tagging in notmuch from Gnus buffers
+
+(defun jao-notmuch-gnus--notmuch-id (id)
+  (when id (if (string-match "<\\(.+\\)>" id) (match-string 1 id) id)))
+
+(defun jao-notmuch-gnus-message-id (&optional no-show)
+  "Find the id of currently selected message in Gnus or notmuch."
+  (when (and (not no-show) (derived-mode-p 'gnus-summary-mode))
+    (save-window-excursion (gnus-summary-show-article)))
+  (cond (gnus-original-article-buffer
+         (with-current-buffer gnus-original-article-buffer
+           (jao-notmuch-gnus--notmuch-id (message-field-value "message-id"))))
+        ((derived-mode-p 'notmuch-show-mode 'notmuch-tree-mode)
+         (notmuch-show-get-message-id))))
+
+(defun jao-notmuch-gnus-message-tags (id)
+  "Ask notmuch for the tags of a message with the given ID."
+  (let ((cmd (format "notmuch search --output=tags 'id:%s'" id)))
+    (split-string (shell-command-to-string cmd))))
+
+(defun jao-notmuch-gnus-tag-message (&optional id tags no-log)
+  "Interactively add or remove tags to the current message."
+  (interactive)
+  (let* ((id (or id (jao-notmuch-gnus-message-id)))
+         (current (unless tags (jao-notmuch-gnus-message-tags id)))
+         (prompt (format "Change tags %s" (string-join current "/")))
+         (tags (or tags (notmuch-read-tag-changes current prompt))))
+    (notmuch-tag (concat "id:" id) tags)
+    (unless no-log
+      (message "%s -> %s" current (jao-notmuch-gnus-message-tags id)))))
+
+(defun jao-notmuch-gnus-show-tags ()
+  "Display in the echo area the tags of the current message."
+  (interactive)
+  (when-let (id (jao-notmuch-gnus-message-id))
+    (message "%s" (string-join (jao-notmuch-gnus-message-tags id) " "))))
+
+(defun jao-notmuch-gnus-toggle-tags (tags &optional id current)
+  "Toggle the given TAGS list for the current Gnus message."
+  (let* ((id (or id (jao-notmuch-gnus-message-id)))
+         (current (or current (jao-notmuch-gnus-message-tags id)))
+         (tags (mapcar (lambda (x)
+                         (concat (if (member x current) "-" "+") x))
+                       tags)))
+    (notmuch-tag (concat "id:" id) tags)
+    (message "New tags: %s" (jao-notmuch-gnus-message-tags id))))
+
+(defun jao-notmuch-gnus-tag-mark ()
+  "Remove the new tag for an article when it's marked as seen by Gnus."
+  (when-let (id (jao-notmuch-gnus-message-id t))
+    (jao-notmuch-gnus-tag-message id '("-new") t)))
+
+(add-hook 'gnus-mark-article-hook #'jao-notmuch-gnus-tag-mark)
+
+(defun jao-notmuch-gnus--group-tags (group)
+  (when (string-match ".+:\\(.+\\)" group)
+    (split-string (match-string 1 group) "\\.")))
+
+(defun jao-notmuch-gnus-tag-on-move (op headers from to _d)
+  (when-let* ((to-tags (when to (jao-notmuch-gnus--group-tags to)))
+              (id (jao-notmuch-gnus--notmuch-id (mail-header-id headers))))
+    (if (eq op 'delete)
+        (let ((cur (seq-difference (jao-notmuch-gnus--group-tags from) to-tags)))
+          (jao-notmuch-gnus-toggle-tags (append cur to-tags) id cur))
+      (notmuch-tag (concat "id:" id)
+                   (mapcar (lambda (x) (concat "+" x)) to-tags)))))
+
+(defun jao-notmuch-gnus-auto-tag ()
+  (add-hook 'gnus-summary-article-move-hook #'jao-notmuch-gnus-tag-on-move)
+  (add-hook 'gnus-summary-article-expire-hook #'jao-notmuch-gnus-tag-on-move))
+
+;;; Gnus search using notmuch
+
+(add-to-list 'gnus-search-expandable-keys "list")
+
+(cl-defmethod gnus-search-transform-expression ((engine gnus-search-notmuch)
+                                                (expr (head list)))
+  (format "List:%s" (gnus-search-transform-expression engine (cdr expr))))
+
+
+;;; Displaying search results in Gnus
+
+(defvar jao-notmuch-gnus-server "nnml"
+  "Name of the target Gnus server, e.g. nnml+mail.")
+
+(defvar jao-notmuch-gnus-mail-directory (expand-file-name "~/var/mail/gnus")
+  "Directory where Gnus stores its mail.")
+
+(defvar jao-notmuch-gnus-leafnode-directory (expand-file-name "~/var/mail/news")
+  "Directory where leafnode stores its messages as seen by notmuch.")
+
+(defun jao-notmuch-gnus-file-to-group (file &optional maildir newsdir)
+  "Compute the Gnus group name from the given file name.
+Example:
+
+  IN: /home/jao/var/mail/jao/foo/cur/1259184569.M4818P3384.localhost,W=6921:2,S
+  OUT: nnml:jao.foo
+
+  IN: /home/jao/.emacs.d/gnus/Mail/jao.trove/32, /home/jao/.emacs.d/gnus/Mail/
+  OUT: nnml:jao.trove
+
+  IN: /home/jao/var/mail/gmane/foo/bar/100
+  OUT: nntp:gmane.foo.bar
+
+  IN: /home/jao/var/mail/bigml/cur/1259176906.M17483P24679.localhost,W=2488:2,S
+  OUT:nnimap:bigml/inbox"
+  (let* ((maildir (or maildir jao-notmuch-gnus-mail-directory))
+         (newsdir (or newsdir jao-notmuch-gnus-leafnode-directory))
+         (g (directory-file-name (file-name-directory file)))
+         (g (replace-regexp-in-string (file-name-as-directory maildir) "" g))
+         (g (replace-regexp-in-string (file-name-as-directory newsdir) "" g))
+         (nntp (string-match-p "^\\(gmane\\|gwene\\)/" g))
+         (g (cond (nntp (concat "nntp:" g))
+                  ((file-name-directory g)
+                   (replace-regexp-in-string "^\\([^/]+\\)/"
+                                             (concat jao-notmuch-gnus-server
+                                                     ":\\1/")
+                                             (file-name-directory g) t))
+                  (t (concat jao-notmuch-gnus-server ":" g))))
+         (g (replace-regexp-in-string "/" "." g))
+         (g (replace-regexp-in-string "[/.]$" "" g)))
+    (cond ((string-match ":$" g) (concat g "inbox"))
+          (nntp g)
+          (t (replace-regexp-in-string ":\\." ":" g)))))
+
+(defun jao-notmuch-gnus-id-to-file (id)
+  (when id
+    (let ((cmd (format "notmuch search --output=files %s" id)))
+      (string-trim (shell-command-to-string cmd)))))
+
+(defun jao-notmuch-gnus-goto-message (&optional msg-id filename)
+  "Open a summary buffer containing the current notmuch article."
+  (interactive)
+  (let* ((filename (or filename
+                       (jao-notmuch-gnus-id-to-file msg-id)
+                       (notmuch-show-get-filename)))
+         (group (when filename (jao-notmuch-gnus-file-to-group filename)))
+         (msg-id (or msg-id (notmuch-show-get-message-id)))
+         (msg-id (when msg-id (replace-regexp-in-string "^id:" "" msg-id))))
+    (if (and group msg-id)
+        (org-gnus-follow-link group msg-id)
+      (message "Couldn't get relevant infos for switching to Gnus."))))
+
+(defun jao-notmuch-gnus-engine (prefix config)
+  (let ((prefix (file-name-as-directory (expand-file-name prefix "~")))
+        (config (expand-file-name config gnus-home-directory)))
+    `(gnus-search-engine gnus-search-notmuch
+                         (remove-prefix ,prefix)
+                         (config-file ,config))))
+
+;;; Org links
+(defun jao-notmuch-gnus--fname (id)
+  (let ((cmd (format "notmuch search --output=files id:%s" id)))
+    (car (split-string (shell-command-to-string cmd)))))
+
+(defun jao-notmuch-gnus-org-follow (id)
+  (when-let* ((fname (jao-notmuch-gnus--fname id))
+              (group (jao-notmuch-gnus-file-to-group fname)))
+    (org-gnus-follow-link group id)))
+
+(defun jao-notmuch-gnus-org-store ()
+  (when-let (d (or (when (derived-mode-p 'notmuch-show-mode 'notmuch-tree-mode)
+                     (cons (notmuch-show-get-message-id)
+                           (notmuch-show-get-subject)))
+                    (when (derived-mode-p 'gnus-summary-mode 'gnus-article-mode)
+                      (cons (jao-notmuch-gnus-message-id)
+                            (gnus-summary-article-subject)))))
+    (org-link-store-props :type "mail"
+                          :link (concat "mail:" (car d))
+                          :description (concat "Mail: " (cdr d)))))
+
+(org-link-set-parameters "mail"
+                         :follow #'jao-notmuch-gnus-org-follow
+                         :store #'jao-notmuch-gnus-org-store)
+
+(org-link-set-parameters "gnus" :store #'ignore)
+(org-link-set-parameters "notmuch" :store #'ignore)
+
+;;; consult-notmuch
+
+(with-eval-after-load "consult-notmuch"
+  (defun jao-notmuch-gnus--open-candidate (candidate)
+    "Open a notmuch-search completion candidate email in Gnus."
+    (message "candidate: %S" candidate)
+    (jao-notmuch-gnus-goto-message (consult-notmuch--candidate-id candidate)))
+
+  (defun jao-gnus-consult-notmuch ()
+    "Run a consult-notmuch query that opens candidates in Gnus."
+    (interactive)
+    (jao-notmuch-gnus--open-candidate (consult-notmuch--search)))
+
+  (consult-customize jao-gnus-consult-notmuch :preview-key 'any))
+
+;;; .
+(provide 'jao-notmuch-gnus)
+;;; jao-notmuch-gnus.el ends here