1 files changed, 86 insertions, 1 deletions

diff --git a/help/devel/calendar/alarm-generation.sgml b/help/devel/calendar/alarm-generation.sgml
index 60bc78343e..77d35258da 100644
--- a/help/devel/calendar/alarm-generation.sgml
+++ b/help/devel/calendar/alarm-generation.sgml
@@ -41,7 +41,92 @@
       <title>Generating trigger instances</title>
 
       <para>
-	
+	Generating absolute triggers is trivial; you just use the date
+	and time they specify.  However, relative triggers are
+	associated to recurrence instances, so in order to generate
+	trigger instances we must generate the corresponding
+	recurrence instances and compute the trigger times based on
+	those.
+      </para>
+
+      <para>
+	Since relative triggers are specified as occurring a certain
+	amount of time before or after each of a calendar component's
+	recurrence instances, we can compute a trigger time by adding
+	or subtracting that amount of time to the corresponding
+	recurrence instance's time.
+      </para>
+
+      <para>
+	Recurrence instances are generated by specifying a range of
+	time and asking the Wombat to generate the instances that
+	occur within that range.  We shall see that the range of time
+	in which instances occur is not necessarily the same range of
+	time in which those instances' alarm triggers occur.
+      </para>
+
+      <para>
+	Consider an alarm that is set to trigger 10 minutes before the
+	start time of an event's occurrence, that is, the trigger has
+	an offset of -10 minutes.  Say this event recurs every hour at
+	5 minutes past the hour:  it would occur at 1:05, 2:05, 3:05,
+	etc.; the corresponding triggers would occur at 12:55, 1:55,
+	2:55, etc.  If we wish to compute the alarm triggers that
+	occur between 4:00 and 6:00 (which would be at 4:55 and 5:55),
+	then we cannot just generate recurrence instances between 4:00
+	and 6:00 because we will miss the 6:05 occurrence which
+	corresponds to the 5:55 trigger.
+      </para>
+
+      <para>
+	The solution is to expand the range of time on both sides to
+	fit the relative triggers that have the largest time periods.
+	If a trigger's offset is negative, like the -10 minutes in the
+	example above, then we must expand the
+	<emphasis>end</emphasis> of the time range: in the case above,
+	the range's ending time of 6:00 must be grown by 10 minutes to
+	6:10 so that the last recurrence instance will be that of
+	6:05; computing the trigger's offset we will get the 5:55
+	trigger, which is what we wanted.  For triggers with positive
+	offsets, like if an alarm were to trigger 20 minutes after an
+	event's occurrence, we must expand the
+	<emphasis>start</emphasis> of the time range in an analogous
+	way, by subtracting the time offset from it.
+      </para>
+
+      <para>
+	Again, absolute triggers need no special computation.  We can
+	just see if the trigger time is within the requested range of
+	time, and if so, we take that trigger occurrence into account
+	for the final result.
+      </para>
+    </sect1>
+
+    <sect1 id="alarm-trigger-generation-code">
+      <title>Alarm trigger generation code</title>
+
+      <para>
+	The main function to generate alarm trigger instances is
+	<function>generate_alarms_for_comp()</function> in
+	<filename>evolution/calendar/pcs/cal-backend-file.c</filename>.
+	This function calls <function>compute_alarm_range()</function>
+	to expand the specified range of time in the way described in
+	the previous section.  It then generates the instances for
+	relative alarm triggers inside the
+	<function>add_alarm_occurrences_cb()</function> callback,
+	which is used by
+	<function>cal_recur_generate_instances()</function> with the
+	expanded range of time.  The callback goes through all of the
+	calendar component's relative alarm triggers and adds the
+	trigger offsets to the occurrence's time; the results are
+	added as <structname>CalAlarmInstance</structname> structures
+	to the final list of trigger instances.  Finally,
+	<function>generate_alarms_for_comp()</function> calls
+	<function>generate_absolute_triggers()</function>, which
+	simply adds the instances for absolute alarm triggers; these
+	are the absolute times that are within the time range that was
+	requested originally.  In the very end, the list of instances
+	is sorted to produce nicer results.
       </para>
     </sect1>
   </chapter>