Create new array entries for repeating dates - ios

Question

i am parsing a .ics file and i got a property called rrule. The objects i successfully parsed and added in my array are objects with multiple properties.
The rrule property (which is one of the properties of the object) is a NSString and gives the following information:Rrule:FREQ=WEEKLY;UNTIL=20140322;INTERVAL=1;BYDAY=FR
Now i want to add another object in the array with the same information for every new entry until the "UNTIL DATE" is reached, only the startDate and endDate should change. In the above example for a start on the 28. Feb there would be added objects for the 7.3 and 14.3 and 21.3. The 22.3 is a saturday and after this day there are no other fridays that should be added. The hour, minutes and seconds also should stay just the same.
Anyone who can help me with that ? I managed to parse the information from the .ics file but i struggle with implementing this one so i can really use the information i parsed.
The other properties of an object from the array are called:
NSString *summary, NSString *location, NSDate *startDate, NSDate *endDate, NSString *rrule, NSString *category, BOOL isExam
EDIT:
Ok here is what i have tried based on your answer (sbooth):
NSDictionary *stringToNumber = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt:2],#"MO",
[NSNumber numberWithInt:3],#"TU",
[NSNumber numberWithInt:4],#"WE",
[NSNumber numberWithInt:5],#"TH",
[NSNumber numberWithInt:6],#"FR",
[NSNumber numberWithInt:7],#"SA",
[NSNumber numberWithInt:1],#"SU",
nil];
for(DECalEntry *entry in planEntries) {
if(entry.rrule.length > 1) { //rrule is not empty
NSDateComponents *week = [[NSDateComponents alloc]init];
[week setWeek:1];
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *comp = [[NSDateComponents alloc] init];
NSNumber *number = [stringToNumber objectForKey:[self stringBetweenString:#"BYDAY=" andString:#"" andString:entry.rrule]];
[comp setWeekday:[number intValue]];
NSDate *newStartDate = [gregorian dateByAddingComponents:comp toDate:entry.startDate options:0];
NSDate *newEndDate = [gregorian dateByAddingComponents:comp toDate:entry.endDate options:0];
NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:#"yyyyMMdd"];
NSDate *untilDate = [df dateFromString: [self stringBetweenString:#"UNTIL=" andString:#";" andString:entry.rrule]];
while([newStartDate compare:untilDate] == NSOrderedAscending) {
DECalEntry *newEntry = [[DECalEntry alloc]init];
newEntry.summary = entry.summary;
newEntry.location = entry.location;
newEntry.isExam = entry.isExam;
newEntry.rrule = #"";
newEntry.startDate = newStartDate;
newEntry.endDate = newEndDate;
newEntry.category = entry.category;
[self.helpArray addObject:newEntry];
newStartDate =[gregorian dateByAddingComponents:comp toDate:newStartDate options:0];
newEndDate =[gregorian dateByAddingComponents:comp toDate:newEndDate options:0];
}
}
if([self.helpArray count] > 0) {
for(DECalEntry *entries in self.helpArray) {
[planEntries addObject:entries];
}
}
and the stringBetweenSting method:
-(NSString*)stringBetweenString:(NSString*)start andString:(NSString*)end andString:(NSString *)string{
NSScanner* scanner = [NSScanner scannerWithString:string];
[scanner setCharactersToBeSkipped:nil];
[scanner scanUpToString:start intoString:NULL];
if ([scanner scanString:start intoString:NULL]) {
NSString* result = nil;
if ([scanner scanUpToString:end intoString:&result]) {
return result;
}
}
return nil;
}
EDIT2: Ok i finally got it working. The problem was, that there was one rule with a daily frequence, not weekly. That caused the while loop to never terminate. Now everything is working, thanks again !

Answer 1

You can do this using NSDateComponents and NSDate. The first step is to parse the rule into the appropriate NSDateComponents to be added to the start date. I don't know the full syntax of the rules you're using but for the example you gave the components would consist of one week since the frequency is weekly:
NSDateComponents *week = [[NSDateComponents alloc] init];
[dc setWeek:1];
Since you already know when the interval starts the starting date for calculations should be the first Friday after startDate. To find the first Friday after an arbitrary date using the Gregorian calendar:
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *fri = [[NSDateComponents alloc] init];
[fri setWeekday:6];
NSDate *firstFridayFollowing = [gregorian dateByAddingDateComponents:fri toDate:startDate options:0];
Once you know the first friday you can add weeks to it:
NSDate *oneWeekLater = [gregorian dateByAddingDateComponents:week toDate:firstFridayFollowing options:0];
It is then simple to do that until the next generated date is later than your end date.