NSString substringWithRange crash

I have this code:

- (void)splitAndSendString:(NSString *)string
             withAlignment:(UITextAlignment)alignment
         verticalExpansion:(NSUInteger)verticalExpansion
       horizontalExpansion:(NSUInteger)horizontalExpansion
                     inRed:(BOOL)isRed
          leftBumperString:(NSString *)leftString
         rightBumperString:(NSString *)rightString
                lineLength:(NSUInteger)lineLength
{
    NSInteger charactersLeftAfterString = lineLength - [string length] -
            [leftString length] - [rightString length];

   // if either of the bumpers is nil, then replace it with @""
   //so that we don't get "null" printed
   if (leftString == nil)  { leftString = @"";     }
   if (rightString == nil) { rightString = @"";    }

   if (charactersLeftAfterString < 0) {
        NSInteger charactersAvailableForString =
           [string length] + charactersLeftAfterString;
        [self sendString:[NSString stringWithFormat:@开发者_运维技巧"%@%@%@", leftString, [string substringWithRange:NSMakeRange(0, charactersAvailableForString)], rightString] withAlignment:UITextAlignmentLeft verticalExpansion:verticalExpansion horizontalExpansion:horizontalExpansion inRed:isRed];
   }
   //(a lot of other code here that isn't relevant)
}

It's crashing on the sendString method, because of a bad NSRange. I don't understand how this is possible given charactersLeftAfterString is necessarily less than 0 given the condition of the if statement, so charactersAvailableForString is always less than the string length, so how can the range be longer than the string length?

Have you used the debugger and checked the value of charactersAvailableForString?

... or printed it?

... or looked at the value when it crashes?

If your string is empty, i.e [string length] = 0

[string substringWithRange:NSMakeRange(0, charactersAvailableForString)]

will always give a NSRange exception every time, no matter what as

charactersAvailableForString < 0

Also similar problems can arise if [string length] = 5 and charactersLeftAfterString = -12. charactersAvailableForString = 5-12 = -7 [string substringWithRange:NSMakeRange(0, charactersAvailableForString)] will give NsRange Exception.