[PATCH] ox-icalendar.el: create alarm at event time

[Mikhail Skorzhinskii <mskorzhinskiy@eml.cc>]

* lisp/ox-icalendar.el (org-icalendar-force-alarm): option to set alarm
even if alarm time is set to zero.
* lisp/ox-icalendar.el (org-icalendar--valarm): create VALARM at the
event start if the alarm time is set to zero and
`org-icalendar-force-alarm' is set to true.
---
 lisp/ox-icalendar.el | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/lisp/ox-icalendar.el b/lisp/ox-icalendar.el
index 0a56e08e5..15b5e3e37 100644
--- a/lisp/ox-icalendar.el
+++ b/lisp/ox-icalendar.el
@@ -66,6 +66,15 @@ for timed events.  If non-zero, alarms are created.
   :version "24.1"
   :type 'integer)
 
+(defcustom org-icalendar-force-alarm nil
+  "Non-nil means alarm will be created even if is set to zero.
+
+This overrides default behaviour where zero means no alarm. With
+this set to non-nil and alarm set to zero, alarm will be created
+and will fire at the event start."
+  :group 'org-export-icalendar
+  :type 'bool)
+
 (defcustom org-icalendar-combined-name "OrgMode"
   "Calendar name for the combined iCalendar representing all agenda
files."
   :group 'org-export-icalendar
@@ -803,8 +812,11 @@ Return VALARM component as a string, or nil if it
isn't allowed."
   (let ((alarm-time
         (let ((warntime
                (org-element-property :APPT_WARNTIME entry)))
-          (if warntime (string-to-number warntime) 0))))
-    (and (or (> alarm-time 0) (> org-icalendar-alarm-time 0))
+          (if warntime (string-to-number warntime) nil))))
+    (and (or (and alarm-time
+                 (> alarm-time 0))
+            (> org-icalendar-alarm-time 0)
+            org-icalendar-force-alarm)
         (org-element-property :hour-start timestamp)
         (format "BEGIN:VALARM
 ACTION:DISPLAY
@@ -812,7 +824,13 @@ DESCRIPTION:%s
 TRIGGER:-P0DT0H%dM0S
 END:VALARM\n"
                 summary
-                (if (zerop alarm-time) org-icalendar-alarm-time alarm-
time)))))
+                (if org-icalendar-force-alarm
+                    (if alarm-time
+                        alarm-time
+                      org-icalendar-alarm-time)
+                  (if (zerop alarm-time)
+                      org-icalendar-alarm-time
+                    alarm-time))))))
 
 
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