properly export events repeating multiple days a week

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/Constants.kt

@@ -82,6 +82,7 @@ val RRULE = "RRULE:"
 val CATEGORIES = "CATEGORIES:"
 val STATUS = "STATUS:"
 val EXDATE = "EXDATE"
+val BYDAY = "BYDAY"
 
 val DISPLAY = "DISPLAY"
 val FREQ = "FREQ"
@@ -96,3 +97,11 @@ val DAILY = "DAILY"
 val WEEKLY = "WEEKLY"
 val MONTHLY = "MONTHLY"
 val YEARLY = "YEARLY"
+
+val MO = "MO"
+val TU = "TU"
+val WE = "WE"
+val TH = "TH"
+val FR = "FR"
+val SA = "SA"
+val SU = "SU"

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/IcsExporter.kt

@@ -64,7 +64,8 @@ class IcsExporter {
         val freq = getFreq(repeatInterval)
         val interval = getInterval(repeatInterval)
         val repeatLimit = getRepeatLimitString(event)
-        val rrule = "$RRULE$FREQ=$freq;$INTERVAL=$interval$repeatLimit"
+        val byDay = getByDay(event)
+        val rrule = "$RRULE$FREQ=$freq;$INTERVAL=$interval$repeatLimit$byDay"
         out.writeLn(rrule)
     }
 
@@ -97,6 +98,34 @@ class IcsExporter {
             ";$UNTIL=${Formatter.getDayCodeFromTS(event.repeatLimit)}"
     }
 
+    private fun getByDay(event: Event): String {
+        return if (event.repeatInterval != WEEK) {
+            ""
+        } else {
+            val days = getByDayString(event.repeatRule)
+            ";$BYDAY=$days"
+        }
+    }
+
+    private fun getByDayString(rule: Int): String {
+        var result = ""
+        if (rule and MONDAY != 0)
+            result += "$MO,"
+        if (rule and TUESDAY != 0)
+            result += "$TU,"
+        if (rule and WEDNESDAY != 0)
+            result += "$WE,"
+        if (rule and THURSDAY != 0)
+            result += "$TH,"
+        if (rule and FRIDAY != 0)
+            result += "$FR,"
+        if (rule and SATURDAY != 0)
+            result += "$SA,"
+        if (rule and SUNDAY != 0)
+            result += "$SU,"
+        return result.trimEnd(',')
+    }
+
     private fun fillReminders(event: Event, out: BufferedWriter) {
         checkReminder(event.reminder1Minutes, out)
         checkReminder(event.reminder2Minutes, out)