;;; gnus-icalendar.el --- reply to iCalendar meeting requests  -*- lexical-binding:t -*-

;; Copyright (C) 2013-2022 Free Software Foundation, Inc.

;; Author: Jan Tatarik <Jan.Tatarik@gmail.com>
;; Keywords: mail, icalendar, org

;; This file is part of GNU Emacs.

;; GNU Emacs 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.

;; GNU Emacs 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 GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; To install:
;; (require 'gnus-icalendar)
;; (gnus-icalendar-setup)

;; to enable optional iCalendar->Org sync functionality
;; NOTE: both the capture file and the headline(s) inside must already exist
;; (setq gnus-icalendar-org-capture-file "~/org/notes.org")
;; (setq gnus-icalendar-org-capture-headline '("Calendar"))
;; (gnus-icalendar-org-setup)


;;; Code:

(require 'icalendar)
(require 'eieio)
(require 'gmm-utils)
(require 'mm-decode)
(require 'gnus-sum)
(require 'gnus-art)

(eval-when-compile (require 'cl-lib))

(defun gnus-icalendar-find-if (pred seq)
  (catch 'found
    (while seq
      (when (funcall pred (car seq))
        (throw 'found (car seq)))
      (pop seq))))

;;;
;;; ical-event
;;;

(defclass gnus-icalendar-event ()
  ((organizer :initarg :organizer
              :accessor gnus-icalendar-event:organizer
              :initform ""
              :type (or null string))
   (summary :initarg :summary
            :accessor gnus-icalendar-event:summary
            :initform ""
            :type (or null string))
   (description :initarg :description
                :accessor gnus-icalendar-event:description
                :initform ""
                :type (or null string))
   (location :initarg :location
             :accessor gnus-icalendar-event:location
             :initform ""
             :type (or null string))
   (start-time :initarg :start-time
          :accessor gnus-icalendar-event:start-time
          :initform ""
          :type (or null t))
   (end-time :initarg :end-time
        :accessor gnus-icalendar-event:end-time
        :initform ""
        :type (or null t))
   (recur :initarg :recur
          :accessor gnus-icalendar-event:recur
          :initform ""
          :type (or null string))
   (uid :initarg :uid
        :accessor gnus-icalendar-event:uid
        :type string)
   (method :initarg :method
           :accessor gnus-icalendar-event:method
           :initform "PUBLISH"
           :type (or null string))
   (rsvp :initarg :rsvp
         :accessor gnus-icalendar-event:rsvp
         :initform nil
         :type (or null boolean))
   (participation-type :initarg :participation-type
         :accessor gnus-icalendar-event:participation-type
         :initform 'non-participant
         :type (or null t))
   (req-participants :initarg :req-participants
         :accessor gnus-icalendar-event:req-participants
         :initform nil
         :type (or null t))
   (opt-participants :initarg :opt-participants
         :accessor gnus-icalendar-event:opt-participants
         :initform nil
         :type (or null t)))
  "Generic iCalendar Event class.")

(defclass gnus-icalendar-event-request (gnus-icalendar-event)
  nil
  "iCalendar class for REQUEST events.")

(defclass gnus-icalendar-event-cancel (gnus-icalendar-event)
  nil
  "iCalendar class for CANCEL events.")

(defclass gnus-icalendar-event-reply (gnus-icalendar-event)
  nil
  "iCalendar class for REPLY events.")

(cl-defmethod gnus-icalendar-event:recurring-p ((event gnus-icalendar-event))
  "Return t if EVENT is recurring."
  (not (null (gnus-icalendar-event:recur event))))

(cl-defmethod gnus-icalendar-event:recurring-freq ((event gnus-icalendar-event))
  "Return recurring frequency of EVENT."
  (let ((rrule (gnus-icalendar-event:recur event)))
    (string-match "FREQ=\\([[:alpha:]]+\\)" rrule)
    (match-string 1 rrule)))

(cl-defmethod gnus-icalendar-event:recurring-interval ((event gnus-icalendar-event))
  "Return recurring interval of EVENT."
  (let ((rrule (gnus-icalendar-event:recur event))
        (default-interval "1"))

    (if (string-match "INTERVAL=\\([[:digit:]]+\\)" rrule)
        (match-string 1 rrule)
      default-interval)))

(cl-defmethod gnus-icalendar-event:recurring-days ((event gnus-icalendar-event))
  "Return, when available, the week day numbers on which the EVENT recurs."
  (let ((rrule (gnus-icalendar-event:recur event))
        (weekday-map '(("SU" . 0)
                       ("MO" . 1)
                       ("TU" . 2)
                       ("WE" . 3)
                       ("TH" . 4)
                       ("FR" . 5)
                       ("SA" . 6))))
    (when (and rrule (string-match "BYDAY=\\([^;]+\\)" rrule))
      (let ((bydays (split-string (match-string 1 rrule) ",")))
        (seq-map
         (lambda (x) (cdr (assoc x weekday-map)))
         (seq-filter (lambda (x) (string-match "^[A-Z]\\{2\\}$" x)) bydays))))))

(cl-defmethod gnus-icalendar-event:start ((event gnus-icalendar-event))
  (format-time-string "%Y-%m-%d %H:%M" (gnus-icalendar-event:start-time event)))

(defun gnus-icalendar-event--decode-datefield (event field zone-map)
  (let* ((dtdate (icalendar--get-event-property event field))
         (dtdate-zone (icalendar--find-time-zone
                       (icalendar--get-event-property-attributes
                        event field) zone-map))
         (dtdate-dec (icalendar--decode-isodatetime dtdate nil dtdate-zone)))
    (encode-time dtdate-dec)))

(defun gnus-icalendar-event--find-attendee (ical name-or-email)
  (let* ((event (car (icalendar--all-events ical)))
         (event-props (caddr event)))
    (cl-labels ((attendee-name (att) (plist-get (cadr att) 'CN))
		(attendee-email
		 (att)
		 (replace-regexp-in-string "^.*MAILTO:" "" (caddr att)))
		(attendee-prop-matches-p
		 (prop)
		 (and (eq (car prop) 'ATTENDEE)
		      (or (member (attendee-name prop) name-or-email)
			  (let ((att-email (attendee-email prop)))
			    (gnus-icalendar-find-if
			     (lambda (str-or-fun)
                   (if (functionp str-or-fun)
                       (funcall str-or-fun att-email)
                     (string-match str-or-fun att-email)))
			     name-or-email))))))
      (gnus-icalendar-find-if #'attendee-prop-matches-p event-props))))

(defun gnus-icalendar-event--get-attendee-names (ical)
  (let* ((event (car (icalendar--all-events ical)))
         (attendee-props (seq-filter
                          (lambda (p) (eq (car p) 'ATTENDEE))
                          (caddr event))))

    (cl-labels
	((attendee-role (prop) (plist-get (cadr prop) 'ROLE))
	 (attendee-name
	  (prop)
	  (or (plist-get (cadr prop) 'CN)
	      (replace-regexp-in-string "^.*MAILTO:" "" (caddr prop))))
	 (attendees-by-type (type)
			    (seq-filter
			     (lambda (p) (string= (attendee-role p) type))
			     attendee-props))
	 (attendee-names-by-type
	  (type)
	  (mapcar #'attendee-name (attendees-by-type type))))
      (list
       (attendee-names-by-type "REQ-PARTICIPANT")
       (attendee-names-by-type "OPT-PARTICIPANT")))))

(defun gnus-icalendar-event-from-ical (ical &optional attendee-name-or-email)
  (let* ((event (car (icalendar--all-events ical)))
         (organizer (replace-regexp-in-string
                     "^.*MAILTO:" ""
                     (or (icalendar--get-event-property event 'ORGANIZER) "")))
         (prop-map '((summary . SUMMARY)
                     (description . DESCRIPTION)
                     (location . LOCATION)
                     (recur . RRULE)
                     (uid . UID)))
         (method (caddr (assoc 'METHOD (caddr (car (nreverse ical))))))
         (attendee (when attendee-name-or-email
                     (gnus-icalendar-event--find-attendee
                      ical attendee-name-or-email)))
         (attendee-names (gnus-icalendar-event--get-attendee-names ical))
         (role (plist-get (cadr attendee) 'ROLE))
         (participation-type (pcase role
                               ("REQ-PARTICIPANT" 'required)
                               ("OPT-PARTICIPANT" 'optional)
                               (_                 'non-participant)))
         (zone-map (icalendar--convert-all-timezones ical))
         (args
          (list :method method
                :organizer organizer
                :start-time (gnus-icalendar-event--decode-datefield
                             event 'DTSTART zone-map)
                :end-time (gnus-icalendar-event--decode-datefield
                           event 'DTEND zone-map)
                :rsvp (string= (plist-get (cadr attendee) 'RSVP) "TRUE")
                :participation-type participation-type
                :req-participants (car attendee-names)
                :opt-participants (cadr attendee-names)))
         (event-class
          (cond
           ((string= method "REQUEST") 'gnus-icalendar-event-request)
           ((string= method "CANCEL") 'gnus-icalendar-event-cancel)
           ((string= method "REPLY") 'gnus-icalendar-event-reply)
           (t 'gnus-icalendar-event))))
    (cl-labels
	((map-property
	  (prop)
	  (let ((value (icalendar--get-event-property event prop)))
	    (when value
	      ;; ugly, but cannot get
	      ;;replace-regexp-in-string work with "\\" as
	      ;;REP, plus we should also handle "\\;"
	      (string-replace
	       "\\," ","
	       (string-replace
		"\\n" "\n" (substring-no-properties value))))))
	 (accumulate-args
	  (mapping)
	  (cl-destructuring-bind (slot . ical-property) mapping
	    (setq args (append (list
				(intern (concat ":" (symbol-name slot)))
				(map-property ical-property))
			       args)))))
      (mapc #'accumulate-args prop-map)
      (apply
       #'make-instance
       event-class
       (cl-loop for slot in (eieio-class-slots event-class)
		for keyword = (intern
			       (format ":%s" (eieio-slot-descriptor-name slot)))
		when (plist-member args keyword)
		append (list keyword
                             (if (eq keyword :uid)
                                 ;; The UID has to be a string.
                                 (or (plist-get args keyword) "")
                               (plist-get args keyword))))))))

(defun gnus-icalendar-event-from-buffer (buf &optional attendee-name-or-email)
  "Parse RFC5545 iCalendar in buffer BUF and return an event object.

Return a gnus-icalendar-event object representing the first event
contained in the invitation.  Return nil for calendars without an
event entry.

ATTENDEE-NAME-OR-EMAIL is a list of strings that will be matched
against the event's attendee names and emails.  Invitation rsvp
status will be retrieved from the first matching attendee record."
  (let ((ical (with-current-buffer (icalendar--get-unfolded-buffer (get-buffer buf))
                (goto-char (point-min))
                (icalendar--read-element nil nil))))

    (when ical
      (gnus-icalendar-event-from-ical ical attendee-name-or-email))))

;;;
;;; gnus-icalendar-event-reply
;;;

(defun gnus-icalendar-event--build-reply-event-body (ical-request status identities)
  (let ((summary-status (capitalize (symbol-name status)))
        (attendee-status (upcase (symbol-name status)))
        reply-event-lines)
    (cl-labels
	((update-summary
	  (line)
	  (if (string-match "^[^:]+:" line)
	      (replace-match (format "\\&%s: " summary-status) t nil line)
	    line))
	 (update-dtstamp ()
			 (format-time-string "DTSTAMP:%Y%m%dT%H%M%SZ" nil t))
	 (attendee-matches-identity
	  (line)
	  (gnus-icalendar-find-if (lambda (name) (string-match-p name line))
				  identities))
	 (update-attendee-status
	  (line)
	  (when (and (attendee-matches-identity line)
		     (string-match "\\(PARTSTAT=\\)[^;]+" line))
	    (replace-match (format "\\1%s" attendee-status) t nil line)))
	 (process-event-line
	  (line)
	  (when (string-match "^\\([^;:]+\\)" line)
	    (let* ((key (match-string 0 line))
		   ;; NOTE: not all of the below fields are mandatory,
		   ;; but they are often present in other clients'
		   ;; replies. Can be helpful for debugging, too.
		   (new-line
		    (cond
		     ((string= key "ATTENDEE") (update-attendee-status line))
		     ((string= key "SUMMARY") (update-summary line))
		     ((string= key "DTSTAMP") (update-dtstamp))
		     ((member key '("ORGANIZER" "DTSTART" "DTEND"
				    "LOCATION" "DURATION" "SEQUENCE"
				    "RECURRENCE-ID" "UID"))
                      line)
		     (t nil))))
	      (when new-line
		(push new-line reply-event-lines))))))

      (mapc #'process-event-line (split-string ical-request "\n"))

      (unless (gnus-icalendar-find-if (lambda (x) (string-match "^ATTENDEE" x))
				      reply-event-lines)
        (lwarn 'gnus-icalendar :warning
               "Could not find an event attendee matching given identity"))

      (mapconcat #'identity `("BEGIN:VEVENT"
                              ,@(nreverse reply-event-lines)
                              "END:VEVENT")
                 "\n"))))

(defun gnus-icalendar-event-reply-from-buffer (buf status identities)
  "Build a calendar event reply for request contained in BUF.
The reply will have STATUS (`accepted', `tentative' or  `declined').
The reply will be composed for attendees matching any entry
on the IDENTITIES list."
  (cl-labels
      ((extract-block
	(blockname)
	(save-excursion
	  (let ((block-start-re (format "^BEGIN:%s" blockname))
		(block-end-re (format "^END:%s" blockname))
		start)
	    (when (re-search-forward block-start-re nil t)
	      (setq start (line-beginning-position))
	      (re-search-forward block-end-re)
	      (buffer-substring-no-properties start (line-end-position)))))))
    (let (zone event)
      (with-current-buffer (icalendar--get-unfolded-buffer (get-buffer buf))
        (goto-char (point-min))
        (setq zone (extract-block "VTIMEZONE")
              event (extract-block "VEVENT")))

      (when event
        (let ((contents (list "BEGIN:VCALENDAR"
                              "METHOD:REPLY"
                              "PRODID:Gnus"
                              "VERSION:2.0"
                              zone
                              (gnus-icalendar-event--build-reply-event-body event status identities)
                              "END:VCALENDAR")))

          (mapconcat #'identity (delq nil contents) "\n"))))))

;;;
;;; gnus-icalendar-org
;;
;; TODO: this is an optional feature, and it's only available with org-mode
;; 7+, so will need to properly handle emacsen with no/outdated org-mode

(require 'org)
(require 'org-capture)

(defgroup gnus-icalendar-org nil
  "Settings for Calendar Event gnus/org integration."
  :version "24.4"
  :group 'gnus-icalendar
  :prefix "gnus-icalendar-org-")

(defcustom gnus-icalendar-org-capture-file nil
  "Target Org file for storing captured calendar events."
  :type '(choice (const nil) file))

(defcustom gnus-icalendar-org-capture-headline nil
  "Target outline in `gnus-icalendar-org-capture-file' for storing captured events."
  :type '(repeat string))

(defcustom gnus-icalendar-org-template-name "used by gnus-icalendar-org"
  "Org-mode template name."
  :type '(string))

(defcustom gnus-icalendar-org-template-key "#"
  "Org-mode template hotkey."
  :type '(string))

(defvar gnus-icalendar-org-enabled-p nil)


(cl-defmethod gnus-icalendar-event:org-repeat ((event gnus-icalendar-event))
  "Return `org-mode' timestamp repeater string for recurring EVENT.
Return nil for non-recurring EVENT."
  (when (gnus-icalendar-event:recurring-p event)
    (let* ((freq-map '(("HOURLY" . "h")
                       ("DAILY" . "d")
                       ("WEEKLY" . "w")
                       ("MONTHLY" . "m")
                       ("YEARLY" . "y")))
           (org-freq (cdr (assoc (gnus-icalendar-event:recurring-freq event) freq-map))))

      (when org-freq
        (format "+%s%s" (gnus-icalendar-event:recurring-interval event) org-freq)))))

(defun gnus-icalendar--find-day (start-date end-date day)
  (let ((time-1-day 86400))
    (if (= (decoded-time-weekday (decode-time start-date))
           day)
        (list start-date end-date)
      (gnus-icalendar--find-day (time-add start-date time-1-day)
				(time-add end-date time-1-day)
				day))))

(defun gnus-icalendar-event--org-timestamp (start end org-repeat)
  (let* ((start-date (format-time-string "%Y-%m-%d" start))
         (start-time (format-time-string "%H:%M" start))
         (start-at-midnight (string= start-time "00:00"))
         (end-date (format-time-string "%Y-%m-%d" end))
         (end-time (format-time-string "%H:%M" end))
         (end-at-midnight (string= end-time "00:00"))
         (start-end-date-diff
	  (time-to-number-of-days
	   (time-subtract (org-time-string-to-time end-date)
			  (org-time-string-to-time start-date))))
         (repeat (if org-repeat (concat " " org-repeat) ""))
	 (time-1-day 86400))

    ;; NOTE: special care is needed with appointments ending at midnight
    ;; (typically all-day events): the end time has to be changed to 23:59 to
    ;; prevent org agenda showing the event on one additional day
    (cond
     ;; start/end midnight
     ;; A 0:0 - A+1 0:0 -> A
     ;; A 0:0 - A+n 0:0 -> A - A+n-1
     ((and start-at-midnight end-at-midnight) (if (> start-end-date-diff 1)
                                                  (let ((end-ts (format-time-string "%Y-%m-%d" (time-subtract end time-1-day))))
                                                    (format "<%s>--<%s>" start-date end-ts))
                                                (format "<%s%s>" start-date repeat)))
     ;; end midnight
     ;; A .:. - A+1 0:0 -> A .:.-23:59
     ;; A .:. - A+n 0:0 -> A .:. - A_n-1
     (end-at-midnight (if (= start-end-date-diff 1)
                          (format "<%s %s-23:59%s>" start-date start-time repeat)
                        (let ((end-ts (format-time-string "%Y-%m-%d" (time-subtract end time-1-day))))
                          (format "<%s %s>--<%s>" start-date start-time end-ts))))
     ;; start midnight
     ;; A 0:0 - A .:. -> A 0:0-.:. (default 1)
     ;; A 0:0 - A+n .:. -> A - A+n .:.
     ((and start-at-midnight
           (cl-plusp start-end-date-diff)) (format "<%s>--<%s %s>" start-date end-date end-time))
     ;; default
     ;; A .:. - A .:. -> A .:.-.:.
     ;; A .:. - B .:.
     ((zerop start-end-date-diff) (format "<%s %s-%s%s>" start-date start-time end-time repeat))
     (t (format "<%s %s>--<%s %s>" start-date start-time end-date end-time))))
  )

(cl-defmethod gnus-icalendar-event:org-timestamp ((event gnus-icalendar-event))
  "Build `org-mode' timestamp from EVENT start/end dates and recurrence info."
  ;; if org-repeat +1d or +1w and byday: generate one timestamp per
  ;; byday, starting at start-date. Change +1d to +7d.
  (let ((start (gnus-icalendar-event:start-time event))
        (end (gnus-icalendar-event:end-time event))
        (org-repeat (gnus-icalendar-event:org-repeat event))
        (recurring-days (gnus-icalendar-event:recurring-days event)))
    (if (and (or (string= org-repeat "+1d")
                 (string= org-repeat "+1w"))
             recurring-days)
      (let ((repeat "+1w")
            (dates (seq-sort-by
		    'car
		    'time-less-p
		    (seq-map (lambda (x)
			       (gnus-icalendar--find-day start end x))
			     recurring-days))))
        (mapconcat (lambda (x)
		     (gnus-icalendar-event--org-timestamp (car x) (cadr x)
							  repeat)) dates "\n"))
      (gnus-icalendar-event--org-timestamp start end org-repeat))))

(defun gnus-icalendar--format-summary-line (summary &optional location)
  (if location
      (format "%s (%s)" summary location)
    (format "%s" summary)))


(defun gnus-icalendar--format-participant-list (participants)
  (mapconcat #'identity participants ", "))

;; TODO: make the template customizable
(cl-defmethod gnus-icalendar-event->org-entry ((event gnus-icalendar-event) reply-status)
  "Return string with new `org-mode' entry describing EVENT."
  (with-temp-buffer
    (org-mode)
    (with-slots (organizer summary description location
                           recur uid) event
      (let* ((reply (if reply-status (capitalize (symbol-name reply-status))
                      "Not replied yet"))
             (props `(("ICAL_EVENT" . "t")
                      ("ID" . ,uid)
                      ("ORGANIZER" . ,(gnus-icalendar-event:organizer event))
                      ("LOCATION" . ,(gnus-icalendar-event:location event))
                      ("PARTICIPATION_TYPE" . ,(symbol-name (gnus-icalendar-event:participation-type event)))
                      ("REQ_PARTICIPANTS" . ,(gnus-icalendar--format-participant-list (gnus-icalendar-event:req-participants event)))
                      ("OPT_PARTICIPANTS" . ,(gnus-icalendar--format-participant-list (gnus-icalendar-event:opt-participants event)))
                      ("RRULE" . ,(gnus-icalendar-event:recur event))
                      ("REPLY" . ,reply))))

        (insert (format "* %s\n\n"
                        (gnus-icalendar--format-summary-line summary location)))
        (mapc (lambda (prop)
                (org-entry-put (point) (car prop) (cdr prop)))
              props))

      (save-restriction
        (narrow-to-region (point) (point))
        (insert (gnus-icalendar-event:org-timestamp event)
                "\n\n"
                (or description "No description"))
        (indent-region (point-min) (point-max) 2)
        (fill-region (point-min) (point-max)))

      (buffer-string))))

(defun gnus-icalendar--deactivate-org-timestamp (ts)
  (replace-regexp-in-string "[<>]"
                            (lambda (m) (cond ((string= m "<") "[")
                                              ((string= m ">") "]")))
                            ts))

(defun gnus-icalendar-find-org-event-file (event &optional org-file)
  "Return the name of the file containing EVENT org entry.
Return nil when not found.

All org agenda files are searched for the EVENT entry.  When
the optional ORG-FILE argument is specified, only that one file
is searched."
  (let ((uid (gnus-icalendar-event:uid event))
        (files (or org-file (org-agenda-files t 'ifmode))))
    (cl-labels
        ((find-event-in
	  (file)
	  (org-check-agenda-file file)
	  (with-current-buffer (find-file-noselect file)
	    (let ((event-pos (org-find-entry-with-id uid)))
	      (when (and event-pos
			 (string= (cdr (assoc "ICAL_EVENT"
					      (org-entry-properties event-pos)))
				  "t"))
		(throw 'found file))))))
      (gnus-icalendar-find-if #'find-event-in files))))


(defun gnus-icalendar--show-org-event (event &optional org-file)
  (let ((file (gnus-icalendar-find-org-event-file event org-file)))
    (when file
      (switch-to-buffer (find-file file))
      (goto-char (org-find-entry-with-id (gnus-icalendar-event:uid event)))
      (org-show-entry))))


(defun gnus-icalendar--update-org-event (event reply-status &optional org-file)
  (let ((file (gnus-icalendar-find-org-event-file event org-file)))
    (when file
      (with-current-buffer (find-file-noselect file)
        (with-slots (uid summary description organizer location recur
                         participation-type req-participants opt-participants) event
          (let ((event-pos (org-find-entry-with-id uid)))
            (when event-pos
              (goto-char event-pos)

              ;; update the headline, keep todo, priority and tags, if any
              (save-excursion
                (let* ((priority (org-entry-get (point) "PRIORITY"))
                       (headline (delq nil (list
                                            (org-entry-get (point) "TODO")
                                            (when priority (format "[#%s]" priority))
                                            (gnus-icalendar--format-summary-line summary location)
                                            (org-entry-get (point) "TAGS")))))

                  (re-search-forward "^\\*+ " (line-end-position))
                  (delete-region (point) (line-end-position))
                  (insert (mapconcat #'identity headline " "))))

              ;; update props and description
              (let ((entry-end (org-entry-end-position))
                    (entry-outline-level (org-outline-level)))

                ;; delete body of the entry, leave org drawers intact
                (save-restriction
                  (org-narrow-to-element)
                  (goto-char entry-end)
                  (re-search-backward "^[\t ]*:END:")
                  (forward-line)
                  (delete-region (point) entry-end))

                ;; put new event description in the entry body
                (when description
                  (save-restriction
                    (narrow-to-region (point) (point))
                    (insert "\n"
                            (gnus-icalendar-event:org-timestamp event)
                            "\n\n"
                            (replace-regexp-in-string "[\n]+$" "\n" description)
                            "\n")
                    (indent-region (point-min) (point-max) (1+ entry-outline-level))
                    (fill-region (point-min) (point-max))))

                ;; update entry properties
                (cl-labels
                    ((update-org-entry
		      (position property value)
		      (if (or (null value)
			      (string= value ""))
			  (org-entry-delete position property)
			(org-entry-put position property value))))

                  (update-org-entry event-pos "ORGANIZER" organizer)
                  (update-org-entry event-pos "LOCATION" location)
                  (update-org-entry event-pos "PARTICIPATION_TYPE"
				    (symbol-name participation-type))
                  (update-org-entry event-pos "REQ_PARTICIPANTS"
				    (gnus-icalendar--format-participant-list
				     req-participants))
                  (update-org-entry event-pos "OPT_PARTICIPANTS"
				    (gnus-icalendar--format-participant-list
				     opt-participants))
                  (update-org-entry event-pos "RRULE" recur)
                  (update-org-entry
		   event-pos "REPLY"
		   (if reply-status (capitalize (symbol-name reply-status))
		     "Not replied yet")))
                (save-buffer)))))))))


(defun gnus-icalendar--cancel-org-event (event &optional org-file)
  (let ((file (gnus-icalendar-find-org-event-file event org-file)))
    (when file
      (with-current-buffer (find-file-noselect file)
        (let ((event-pos (org-find-entry-with-id (gnus-icalendar-event:uid event))))
          (when event-pos
            (let ((ts (org-entry-get event-pos "DT")))
              (when ts
                (org-entry-put event-pos "DT" (gnus-icalendar--deactivate-org-timestamp ts))
                (save-buffer)))))))))


(defun gnus-icalendar--get-org-event-reply-status (event &optional org-file)
  (let ((file (gnus-icalendar-find-org-event-file event org-file)))
    (when file
      (save-excursion
        (with-current-buffer (find-file-noselect file)
          (let ((event-pos (org-find-entry-with-id (gnus-icalendar-event:uid event))))
            (org-entry-get event-pos "REPLY")))))))


(defun gnus-icalendar-insinuate-org-templates ()
  (unless (gnus-icalendar-find-if (lambda (x) (string= (cadr x) gnus-icalendar-org-template-name))
                      org-capture-templates)
    (setq org-capture-templates
          (append `((,gnus-icalendar-org-template-key
                     ,gnus-icalendar-org-template-name
                     entry
                     (file+olp ,gnus-icalendar-org-capture-file ,@gnus-icalendar-org-capture-headline)
                     "%i"
                     :immediate-finish t))
                  org-capture-templates))

    ;; hide the template from interactive template selection list
    ;; (org-capture)
    ;; NOTE: doesn't work when capturing from string
    ;; (when (boundp 'org-capture-templates-contexts)
    ;;   (push `(,gnus-icalendar-org-template-key "" ((in-mode . "gnus-article-mode")))
    ;;         org-capture-templates-contexts))
    ))

(defun gnus-icalendar:org-event-save (event reply-status)
  (with-temp-buffer
    (org-capture-string (gnus-icalendar-event->org-entry event reply-status)
                        gnus-icalendar-org-template-key)))

(defun gnus-icalendar-show-org-agenda (event)
  (let* ((time-delta (time-subtract (gnus-icalendar-event:end-time event)
                                    (gnus-icalendar-event:start-time event)))
         (duration-days (1+ (floor (time-convert time-delta 'integer) 86400))))
    (org-agenda-list nil (gnus-icalendar-event:start event) duration-days)))

(cl-defmethod gnus-icalendar-event:sync-to-org ((event gnus-icalendar-event-request) reply-status)
  (if (gnus-icalendar-find-org-event-file event)
      (gnus-icalendar--update-org-event event reply-status)
    (gnus-icalendar:org-event-save event reply-status)))

(cl-defmethod gnus-icalendar-event:sync-to-org ((event gnus-icalendar-event-cancel) _reply-status)
  (when (gnus-icalendar-find-org-event-file event)
    (gnus-icalendar--cancel-org-event event)))

(defun gnus-icalendar-org-setup ()
  (if (and gnus-icalendar-org-capture-file gnus-icalendar-org-capture-headline)
      (progn
        (gnus-icalendar-insinuate-org-templates)
        (setq gnus-icalendar-org-enabled-p t))
    (message "Cannot enable Calendar->Org: missing capture file, headline")))

;;;
;;; gnus-icalendar
;;;

(defgroup gnus-icalendar nil
  "Settings for inline display of iCalendar invitations."
  :version "24.4"
  :group 'gnus-article
  :prefix "gnus-icalendar-")

(defcustom gnus-icalendar-reply-bufname "*CAL*"
  "Buffer used for building iCalendar invitation reply."
  :type '(string))

(defcustom gnus-icalendar-additional-identities nil
  "We need to know your identity to make replies to calendar requests work.

Gnus will only offer you the Accept/Tentative/Decline buttons for
calendar events if any of your identities matches at least one
RSVP participant.

Your identity is guessed automatically from the variables
`user-full-name', `user-mail-address',
`gnus-ignored-from-addresses' and `message-alternative-emails'.

If you need even more aliases you can define them here.  It really
only makes sense to define names or email addresses."

  :type '(repeat string))

(defvar-local gnus-icalendar-reply-status nil)

(defvar-local gnus-icalendar-event nil)

(defvar-local gnus-icalendar-handle nil)

(defun gnus-icalendar-identities ()
  "Return list of regexp-quoted names and email addresses belonging to the user.

These will be used to retrieve the RSVP information from ical events."
  (apply #'append
         (mapcar
	  (lambda (x) (if (listp x) x (list x)))
	  (list user-full-name (regexp-quote user-mail-address)
		;; NOTE: these can be lists
		gnus-ignored-from-addresses ; String or function.
		message-alternative-emails  ;  String or function.
		(mapcar #'regexp-quote gnus-icalendar-additional-identities)))))

;; TODO: make the template customizable
(cl-defmethod gnus-icalendar-event->gnus-calendar ((event gnus-icalendar-event) &optional reply-status)
  "Format an overview of EVENT details."
  (cl-labels
      ((format-header (x)
		      (format "%-12s%s"
			      (propertize (concat (car x) ":") 'face 'bold)
			      (cadr x))))

    (with-slots (organizer summary description location recur uid
                           method rsvp participation-type)
        event
      (let ((headers `(("Summary" ,summary)
		       ("Location" ,(or location ""))
		       ("Time" ,(gnus-icalendar-event:org-timestamp event))
		       ("Organizer" ,organizer)
		       ("Attendance" ,(if (eq participation-type 'non-participant)
					  "You are not listed as an attendee"
					(capitalize (symbol-name participation-type))))
		       ("Method" ,method))))

	(when (and (not (gnus-icalendar-event-reply-p event)) rsvp)
	  (setq headers (append headers
				`(("Status" ,(or reply-status "Not replied yet"))))))

	(concat
	 (mapconcat #'format-header headers "\n")
	 "\n\n"
	 description)))))

(defmacro gnus-icalendar-with-decoded-handle (handle &rest body)
  "Execute BODY in buffer containing the decoded contents of HANDLE."
  (let ((charset (make-symbol "charset")))
    `(let ((,charset (cdr (assoc 'charset (mm-handle-type ,handle)))))
       (with-temp-buffer
         (mm-insert-part ,handle)
         (when (and ,charset (string= (downcase ,charset) "utf-8"))
           (decode-coding-region (point-min) (point-max) 'utf-8))
         ,@body))))


(defun gnus-icalendar-event-from-handle (handle &optional attendee-name-or-email)
  (gnus-icalendar-with-decoded-handle handle
                       (gnus-icalendar-event-from-buffer (current-buffer) attendee-name-or-email)))

(defun gnus-icalendar-insert-button (text callback data)
  ;; FIXME: the gnus-mime-button-map keymap does not make sense for this kind
  ;; of button.
  (let ((start (point)))
    (add-text-properties
     start
     (progn
       (insert "[ " text " ]")
       (point))
     `(gnus-callback
       ,callback
       keymap ,gnus-mime-button-map
       face ,gnus-article-button-face
       follow-link t
       category t
       button t
       gnus-data ,data))))

(defun gnus-icalendar-send-buffer-by-mail (buffer-name subject)
  (let ((message-signature nil))
    (with-current-buffer gnus-summary-buffer
      (gnus-summary-reply)
      (message-goto-body)
      (mml-insert-multipart "alternative")
      (mml-insert-empty-tag 'part 'type "text/plain")
      (mml-attach-buffer buffer-name "text/calendar; method=REPLY; charset=UTF-8")
      (message-goto-subject)
      (delete-region (line-beginning-position) (line-end-position))
      (insert "Subject: " subject)
      (message-send-and-exit))))

(defun gnus-icalendar-reply (data)
  (let* ((handle (car data))
         (status (cadr data))
         (event (caddr data))
         (reply (gnus-icalendar-with-decoded-handle handle
                  (gnus-icalendar-event-reply-from-buffer
                   (current-buffer) status (gnus-icalendar-identities)))))

    (when reply
      (cl-labels
	  ((fold-icalendar-buffer
	    ()
	    (goto-char (point-min))
	    (while (re-search-forward "^\\(.\\{72\\}\\)\\(.+\\)$" nil t)
	      (replace-match "\\1\n \\2")
	      (goto-char (line-beginning-position)))))
        (let ((subject (concat (capitalize (symbol-name status))
                               ": " (gnus-icalendar-event:summary event))))

          (with-current-buffer (gnus-get-buffer-create gnus-icalendar-reply-bufname)
            (delete-region (point-min) (point-max))
            (insert reply)
            (fold-icalendar-buffer)
            (gnus-icalendar-send-buffer-by-mail (buffer-name) subject))

          ;; Back in article buffer
          (setq-local gnus-icalendar-reply-status status)
          (when gnus-icalendar-org-enabled-p
            (gnus-icalendar--update-org-event event status)
            ;; refresh article buffer to update the reply status
            (with-current-buffer gnus-summary-buffer
              (gnus-summary-show-article))))))))

(defun gnus-icalendar-sync-event-to-org (event)
  (gnus-icalendar-event:sync-to-org event gnus-icalendar-reply-status))

(cl-defmethod gnus-icalendar-event:inline-reply-buttons ((event gnus-icalendar-event) handle)
  (when (gnus-icalendar-event:rsvp event)
    `(("Accept" gnus-icalendar-reply (,handle accepted ,event))
      ("Tentative" gnus-icalendar-reply (,handle tentative ,event))
      ("Decline" gnus-icalendar-reply (,handle declined ,event)))))

(cl-defmethod gnus-icalendar-event:inline-reply-buttons ((_event gnus-icalendar-event-reply) _handle)
  "No buttons for REPLY events."
  nil)

(cl-defmethod gnus-icalendar-event:inline-reply-status ((event gnus-icalendar-event))
  (or (when gnus-icalendar-org-enabled-p
        (gnus-icalendar--get-org-event-reply-status event))
      "Not replied yet"))

(cl-defmethod gnus-icalendar-event:inline-reply-status ((_event gnus-icalendar-event-reply))
  "No reply status for REPLY events."
  nil)


(cl-defmethod gnus-icalendar-event:inline-org-buttons ((event gnus-icalendar-event))
  (let* ((org-entry-exists-p (gnus-icalendar-find-org-event-file event))
         (export-button-text (if org-entry-exists-p "Update Org Entry" "Export to Org")))

    (delq nil (list
               `("Show Agenda" gnus-icalendar-show-org-agenda ,event)
               (when (gnus-icalendar-event-request-p event)
                 `(,export-button-text gnus-icalendar-sync-event-to-org ,event))
               (when org-entry-exists-p
                 `("Show Org Entry" gnus-icalendar--show-org-event ,event))))))


(cl-defmethod gnus-icalendar-event:inline-org-buttons ((event gnus-icalendar-event-cancel))
  (let ((org-entry-exists-p (gnus-icalendar-find-org-event-file event)))

    (delq nil (list
               `("Show Agenda" gnus-icalendar-show-org-agenda ,event)
               (when org-entry-exists-p
                 `("Update Org Entry" gnus-icalendar-sync-event-to-org ,event))
               (when org-entry-exists-p
                 `("Show Org Entry" gnus-icalendar--show-org-event ,event))))))

;;;###autoload
(defun gnus-icalendar-mm-inline (handle)
  (let ((event (gnus-icalendar-event-from-handle handle (gnus-icalendar-identities))))

    (setq gnus-icalendar-reply-status nil)

    (when event
      (cl-labels
	  ((insert-button-group
	    (buttons)
	    (when buttons
	      (mapc (lambda (x)
		      (apply #'gnus-icalendar-insert-button x)
		      (insert "    "))
		    buttons)
	      (insert "\n\n"))))

        (insert-button-group
	 (gnus-icalendar-event:inline-reply-buttons event handle))

        (when gnus-icalendar-org-enabled-p
          (insert-button-group (gnus-icalendar-event:inline-org-buttons event)))

        (setq gnus-icalendar-event event
              gnus-icalendar-handle handle)

        (insert (gnus-icalendar-event->gnus-calendar
                 event
                 (gnus-icalendar-event:inline-reply-status event)))))))

(defun gnus-icalendar-save-part (handle)
  (let (event)
    (when (and (equal (car (mm-handle-type handle)) "text/calendar")
               (setq event (gnus-icalendar-event-from-handle handle (gnus-icalendar-identities))))

      (gnus-icalendar-event:sync-to-org event))))


(defun gnus-icalendar-save-event ()
  "Save the Calendar event in the text/calendar part under point."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (gnus-article-check-buffer)
  (let ((data (get-text-property (point) 'gnus-data)))
    (when data
      (gnus-icalendar-save-part data))))

(defun gnus-icalendar-reply-accept ()
  "Accept invitation in the current article."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (with-current-buffer gnus-article-buffer
    (gnus-icalendar-reply (list gnus-icalendar-handle 'accepted gnus-icalendar-event))
    (setq-local gnus-icalendar-reply-status 'accepted)))

(defun gnus-icalendar-reply-tentative ()
  "Send tentative response to invitation in the current article."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (with-current-buffer gnus-article-buffer
    (gnus-icalendar-reply (list gnus-icalendar-handle 'tentative gnus-icalendar-event))
    (setq-local gnus-icalendar-reply-status 'tentative)))

(defun gnus-icalendar-reply-decline ()
  "Decline invitation in the current article."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (with-current-buffer gnus-article-buffer
    (gnus-icalendar-reply (list gnus-icalendar-handle 'declined gnus-icalendar-event))
    (setq-local gnus-icalendar-reply-status 'declined)))

(defun gnus-icalendar-event-export ()
  "Export calendar event to `org-mode', or update existing agenda entry."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (with-current-buffer gnus-article-buffer
    (gnus-icalendar-sync-event-to-org gnus-icalendar-event))
  ;; refresh article buffer in case the reply had been sent before initial org
  ;; export
  (with-current-buffer gnus-summary-buffer
    (gnus-summary-show-article)))

(defun gnus-icalendar-event-show ()
  "Display `org-mode' agenda entry related to the calendar event."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (gnus-icalendar--show-org-event
   (with-current-buffer gnus-article-buffer
     gnus-icalendar-event)))

(defun gnus-icalendar-event-check-agenda ()
  "Display `org-mode' agenda for days between event start and end dates."
  (interactive nil gnus-article-mode gnus-summary-mode)
  (gnus-icalendar-show-org-agenda
   (with-current-buffer gnus-article-buffer gnus-icalendar-event)))

(defvar gnus-mime-action-alist)         ; gnus-art

(defun gnus-icalendar-setup ()
  ;; FIXME: Get rid of this!
  ;; The three add-to-list are now redundant (good), but I think the rest
  ;; is still not automatically setup.
  (add-to-list 'mm-inlined-types "text/calendar")
  (add-to-list 'mm-automatic-display "text/calendar")
  (add-to-list 'mm-inline-media-tests '("text/calendar" gnus-icalendar-mm-inline identity))

  (gnus-define-keys (gnus-summary-calendar-map "i" gnus-summary-mode-map)
    "a" gnus-icalendar-reply-accept
    "t" gnus-icalendar-reply-tentative
    "d" gnus-icalendar-reply-decline
    "c" gnus-icalendar-event-check-agenda
    "e" gnus-icalendar-event-export
    "s" gnus-icalendar-event-show)

  (require 'gnus-art)
  (add-to-list 'gnus-mime-action-alist
               (cons "save calendar event" #'gnus-icalendar-save-event)
               t))

(provide 'gnus-icalendar)

;;; gnus-icalendar.el ends here
