Interpolate missing years in array

//interpolate missing years:
-(NSArray*)interpolateMissingYears{
    NSMutableArray *years = [[NSMutableArray alloc] initWithCapacity:[self.state count]];
    //create array of years
    for(CoreStateData *state in self.state){
        [years addObject:state.year];
    }
    int yearspan = [[years objectAtIndex:[years count]-1] intValue] - [[years objectAtIndex:0] intValue];
    NSMutableArray *newArray = [[NSMutableArray alloc] initWithCapacity:yearspan];
    NSUInteger len = [years count];
    [newArray addObject:[years objectAtIndex:0]];
    int missingIndex=0;
    int index = 1;
    while (index<len) {
        NSNumber *thisYear = [newArray objectAtIndex:missingIndex];
        missingIndex++;
        NSNumber *nextYear = [years objectAtIndex:index];
        if([nextYear intValue]-[thisYear intValue]==1)
            index++;
            NSNumber *newYear = [NSNumber numberWithInt:[thisYear intValue]+
                                    (([nextYear intValue]-[thisYear intValue])/([nextYear intValue]-[thisYear intValue]))];
        [newArray addObject:newYear];
    }
    return newArray;
}