-(NSArray *)letters
{
    NSMutableArray *letters = [[NSMutableArray alloc] init];
    for (char myChar = 'a'; myChar <= 'z'; myChar++)
    {
        [letters addObject:[NSString stringWithFormat:@"%c", myChar]];
    }
    for (char myChar = 'A'; myChar <= 'Z'; myChar++)
    {
        [letters addObject:[NSString stringWithFormat:@"%c", myChar]];
    }
    for (NSUInteger i = 0; i < 10; i ++)
    {
        [letters addObject:[NSString stringWithFormat:@"%@", @(i)]];
    }

    [letters addObject:@"#"];
    [letters addObject:@" "];

    return letters;
}
-(void)createSource
{
    _dataSource = [[NSMutableArray alloc] init];

    NSArray *letters = [self letters];

    for (NSUInteger i = 0; i < 50000; i ++)
    {
        RealmContact *aContact = [[RealmContact alloc] init];
        NSMutableString *aContactName = [[NSMutableString alloc] init];
        for (NSUInteger i = 0; i < 10; i ++)
        {
            NSUInteger rand = arc4random_uniform((UInt32)[letters count]);
            [aContactName appendString:letters[rand]];
        }
        [aContact setFirstName:aContactName];
        [_dataSource addObject:aContact];
    }

    NSLog(@"------- Method Author -----");
    NSArray *sortedLetters = nil;
    NSMutableDictionary *nameDic = [NSMutableDictionary new];
    NSDate *date = [NSDate date];
    for (RealmContact *contact in _dataSource)
    {
        if (contact.firstName.length>0)
        {
            NSString *firName= [contact.firstName stringByReplacingOccurrencesOfString:@" " withString:@""];
            NSString *regex = @"^[A-Za-z]+";
            NSPredicate *test = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", regex];
            BOOL result = [test evaluateWithObject:firName];

            if (firName.length>0 && result)
            {
                [nameDic setObject:@"firstletter" forKey:[[firName substringToIndex:1]uppercaseString]];
            }else{
                [nameDic setObject:@"firstletter" forKey:@"#"];
            }
        }
    }
    sortedLetters = [[nameDic allKeys]sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
    NSLog(@"Time1: %@", @([[NSDate date] timeIntervalSinceDate:date]));

    NSLog(@"------- Alternative Method -----");
    NSArray *sortedLetters2 = nil;
    NSMutableSet *set = [[NSMutableSet alloc] init];
    NSCharacterSet *charSet = [NSCharacterSet characterSetWithCharactersInString:@"AZERTYUIOPQSDFGHJKLMWXCVBN"];
    NSDate *date2 = [NSDate date];
    for (RealmContact *contact in _dataSource)
    {
        NSString *firName = [[contact firstName] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
        if ([firName length])
        {
            unichar c = [[firName uppercaseString] characterAtIndex:0];
            if ([charSet characterIsMember:c])
            {
                [set addObject:[NSString stringWithFormat: @"%C", c]];
            }
            else
            {
                [set addObject:@"#"];
            }
        }
    }
    sortedLetters2 = [[set allObjects] sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
    NSLog(@"Time2: %@", @([[NSDate date] timeIntervalSinceDate:date2]));

    NSLog(@"sortedLetters: %@", sortedLetters);
    NSLog(@"sortedLetters2: %@", sortedLetters2);
    NSLog(@"sortedLetters %@ sortedLetters2", [sortedLetters isEqualToArray:sortedLetters2]?@"==":@"!=");
}

/*
------- Method Author -----
Time1: 0.6597279906272888
------- Alternative Method -----
Time2: 0.05883300304412842
*/