Add simple normalization before encoding to text

Closes: #13

lib/src/codecs/text/encoder.dart

@@ -17,6 +17,8 @@ class RecurrenceRuleToTextEncoder extends Converter<RecurrenceRule, String> {
 
   @override
   String convert(RecurrenceRule input) {
+    input = _normalize(input);
+
     final frequencyIntervalString =
         l10n.frequencyInterval(input.frequency, input.actualInterval);
     final output = StringBuffer(frequencyIntervalString);
@@ -58,6 +60,35 @@ class RecurrenceRuleToTextEncoder extends Converter<RecurrenceRule, String> {
     return output.toString();
   }
 
+  RecurrenceRule _normalize(RecurrenceRule input) {
+    // Incomplete!
+    input = input.copyWith(clearInterval: input.interval == 1);
+
+    if (input.frequency == Frequency.monthly) {
+      final byEveryWeekDay = {
+        for (final weekDay in DateTime.monday.rangeTo(DateTime.sunday))
+          ByWeekDayEntry(weekDay),
+      };
+      if (!input.hasBySeconds &&
+          !input.hasByMinutes &&
+          !input.hasByHours &&
+          DeepCollectionEquality.unordered()
+              .equals(input.byWeekDays, byEveryWeekDay) &&
+          !input.hasByMonthDays &&
+          !input.hasByYearDays &&
+          !input.hasByWeeks &&
+          !input.hasByMonths &&
+          input.hasBySetPositions) {
+        input = input.copyWith(
+          byWeekDays: {},
+          byMonthDays: input.bySetPositions,
+          bySetPositions: {},
+        );
+      }
+    }
+    return input;
+  }
+
   void _convertDaily(RecurrenceRule input, StringBuffer output) {
     assert(input.byWeekDays.noneHasOccurrence);
 

test/codecs/text/monthly_test.dart

@@ -25,6 +25,13 @@ void main() {
     string: 'RRULE:FREQ=MONTHLY;INTERVAL=4;BYDAY=MO',
   );
 
+  // https://github.com/JonasWanke/rrule/issues/13
+  testText(
+    'Monthly on the last day',
+    string:
+        'RRULE:FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=MO,TU,WE,TH,FR,SA,SU',
+  );
+
   // 0/1 digits in the comment before a text function mean whether each of
   // bySetPositions, byMonths, byMonthDays & byWeekDays (in that order) is
   // included.