From 8348f5b8c56087f0fb8cdd775a816f63cb57f38f Mon Sep 17 00:00:00 2001
From: Jack Kamm <jackkamm@gmail.com>
Date: Sun, 26 Mar 2023 10:37:47 -0700
Subject: [PATCH 2/2] ox-icalendar: Support repeating timestamps in TODOs

* lisp/ox-icalendar.el (org-icalendar--rrule): New helper function to
generate RRULE.
(org-icalendar--vevent): Use `org-icalendar--rrule' instead of
generating the RRULE directly.
(org-icalendar--vtodo): Generate RRULE for repeating scheduled and
deadline timestamps.
---
 etc/ORG-NEWS         | 13 ++++++++++++
 lisp/ox-icalendar.el | 50 +++++++++++++++++++++++++++++++++-----------
 2 files changed, 51 insertions(+), 12 deletions(-)

diff --git a/etc/ORG-NEWS b/etc/ORG-NEWS
index fb4f82b29..3919b240e 100644
--- a/etc/ORG-NEWS
+++ b/etc/ORG-NEWS
@@ -159,6 +159,19 @@ Running shell blocks with the ~:session~ header freezes Emacs until
 execution completes.  The new ~:async~ header allows users to continue
 editing with Emacs while a ~:session~ block executes.
 
+*** Add support for repeating tasks in iCalendar export
+
+Repeating Scheduled and Deadline timestamps in TODOs are now exported
+as recurring tasks in iCalendar export.
+
+Note that in Org-mode, the repeaters for the Scheduled and Deadline
+timestamps can be different; whereas in iCalendar, the recurrence rule
+applies to both the scheduled start time and the deadline due date.
+
+In case the timestamp repeaters contradict, the correct export
+behavior is not well-defined.  Currently, Org-mode will issue a
+warning and skip the repeaters in this case.
+
 ** Miscellaneous
 *** Remove undocumented ~:target~ header parameter in ~ob-clojure~
 
diff --git a/lisp/ox-icalendar.el b/lisp/ox-icalendar.el
index 63aefcc84..179795ac9 100644
--- a/lisp/ox-icalendar.el
+++ b/lisp/ox-icalendar.el
@@ -726,6 +726,13 @@ (defun org-icalendar-entry (entry contents info)
        ;; Don't forget components from inner entries.
        contents))))
 
+(defun org-icalendar--rrule (unit value)
+  (format "RRULE:FREQ=%s;INTERVAL=%d\n"
+	  (cl-case unit
+	    (hour "HOURLY") (day "DAILY") (week "WEEKLY")
+	    (month "MONTHLY") (year "YEARLY"))
+	  value))
+
 (defun org-icalendar--vevent
     (entry timestamp uid summary location description categories timezone class)
   "Create a VEVENT component.
@@ -752,12 +759,9 @@ (\"PUBLIC\", \"CONFIDENTIAL\", and \"PRIVATE\") are predefined, others
 	     (org-icalendar-convert-timestamp timestamp "DTSTART" nil timezone) "\n"
 	     (org-icalendar-convert-timestamp timestamp "DTEND" t timezone) "\n"
 	     ;; RRULE.
-	     (when (org-element-property :repeater-type timestamp)
-	       (format "RRULE:FREQ=%s;INTERVAL=%d\n"
-		       (cl-case (org-element-property :repeater-unit timestamp)
-			 (hour "HOURLY") (day "DAILY") (week "WEEKLY")
-			 (month "MONTHLY") (year "YEARLY"))
-		       (org-element-property :repeater-value timestamp)))
+             (org-icalendar--rrule
+              (org-element-property :repeater-unit timestamp)
+              (org-element-property :repeater-value timestamp))
 	     "SUMMARY:" summary "\n"
 	     (and (org-string-nw-p location) (format "LOCATION:%s\n" location))
 	     (and (org-string-nw-p class) (format "CLASS:%s\n" class))
@@ -792,7 +796,9 @@ (defun org-icalendar--vtodo
 				   :hour-start (nth 2 now)
 				   :day-start (nth 3 now)
 				   :month-start (nth 4 now)
-				   :year-start (nth 5 now))))))))
+				   :year-start (nth 5 now)))))))
+        (due (and (memq 'todo-due org-icalendar-use-deadline)
+                  (org-element-property :deadline entry))))
     (org-icalendar-fold-string
      (concat "BEGIN:VTODO\n"
 	     "UID:TODO-" uid "\n"
@@ -801,11 +807,31 @@ (defun org-icalendar--vtodo
                (concat (org-icalendar-convert-timestamp
                         start "DTSTART" nil timezone)
                        "\n"))
-	     (and (memq 'todo-due org-icalendar-use-deadline)
-		  (org-element-property :deadline entry)
-		  (concat (org-icalendar-convert-timestamp
-			   (org-element-property :deadline entry) "DUE" nil timezone)
-			  "\n"))
+	     (when due
+	       (concat (org-icalendar-convert-timestamp
+			due "DUE" nil timezone)
+		       "\n"))
+             ;; RRULE
+             (let ((start-repeater-unit (org-element-property
+                                         :repeater-unit start))
+                   (start-repeater-value (org-element-property
+                                          :repeater-value start))
+                   (due-repeater-unit (org-element-property
+                                       :repeater-unit due))
+                   (due-repeater-value (org-element-property
+                                        :repeater-value due)))
+               (when (or start-repeater-value due-repeater-value)
+                 (if (and start due
+                          (not (and (eql start-repeater-unit
+                                         due-repeater-unit)
+                                    (eql start-repeater-value
+                                         due-repeater-value))))
+                     (progn (warn "Scheduled and Deadline repeaters are not equal. Skipping repeater export.")
+                            nil)
+                   (org-icalendar--rrule (or start-repeater-unit
+                                             due-repeater-unit)
+                                         (or start-repeater-value
+                                             due-repeater-value)))))
 	     "SUMMARY:" summary "\n"
 	     (and (org-string-nw-p location) (format "LOCATION:%s\n" location))
 	     (and (org-string-nw-p class) (format "CLASS:%s\n" class))
-- 
2.39.2