Convert a string like "aabcccccbba" to one like "2a1b5c2b1a"

LetterCounter.m

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