+ (NSString*)listDaysFromIndexSet:(NSIndexSet*)indexSet
                     outputFormat:(int)numberOfLetters {
    NSArray *dayWords;
    switch (numberOfLetters) {
        case 0:
            dayWords = @[@"1", @"2", @"3", @"4", @"5", @"6", @"7"];
            break;
        case 1:
            dayWords = @[@"Su", @"M", @"Tu", @"W", @"Th", @"F", @"Sa"];
            break;
        case 2:
            dayWords = @[@"Su", @"Mo", @"Tu", @"We", @"Th", @"Fr", @"Sa"];
            break;
        case 3:
            dayWords = @[@"Sun", @"Mon", @"Tue", @"Wed", @"Thu", @"Fri", @"Sat"];
            break;
        case 4:
            dayWords = @[@"Sunday", @"Monday", @"Tuesday", @"Wednesday", @"Thursday", @"Friday", @"Saturday"];
            break;
        default:
            break;
    }
    NSInteger currentIndex = [indexSet firstIndex];
    NSMutableArray *selectedDays = [NSMutableArray array];
    while (currentIndex != NSNotFound) {
        if (currentIndex < dayWords.count) {
            NSString *day = dayWords[currentIndex];
            [selectedDays addObject:day];
        }
        currentIndex = [indexSet indexGreaterThanIndex:currentIndex];
    }
    return [selectedDays componentsJoinedByString:@", "];
}