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.