Convert a string like "aabcccccbba" to one like "2a1b5c2b1a" more efficiently than this one: https://gist.github.com/jakehawken/482315e0a7d07fc15b1a

LetterCounterWithOneMutableString.m

- (void)letterCounter:(NSString *)originalString
{
  NSMutableString *outputString = [NSMutableString new];
  NSInteger stringIndex = 0;
  
  while (stringIndex < originalString.length)
  {
    unichar currentChar = [originalString characterAtIndex:stringIndex];
    NSInteger characterCount = 1;
    for (NSInteger i = stringIndex + 1; i < originalString.length; i++)
    {
      if ([originalString characterAtIndex:i] == currentChar)
      {
        characterCount++;
      }
      else
      {
        break;
      }
    }
    [outputString appendString:[NSString stringWithFormat:@"%ld%c", characterCount, currentChar]];
    stringIndex += characterCount;
  }
  
  printf("%s\n", [outputString UTF8String]);
}