Objective-C中的NSString中的Shuffling Letters

我已经编写了这个@L_618_0@来调整NSString的内容,它似乎工作,但它偶尔会崩溃.这可能是一种迂回的方式,但我把字符放入一个数组,随机交换数组中的元素,然后将数组转回一个字符串.

我不确定我在做什么不安全会导致它崩溃.我以为我可能正在设置finalLetteRSString = result,但我也尝试了finalLetteRSString = [NSString StringWithString：result]并且也崩溃了.我感到困惑的原因是因为它不会每次都崩溃.我只是按下随机按钮,有时它会崩溃.我应该看的任何地方？

/* This function shuffles the letters in the String finalLetteRSString */

-(IBACtion)shuffleLetters:(id)sender{
    int length = [finalLetteRSString length];
    NSMutableArray * letters = [NSMutableArray arrayWithCapacity:length]; 
    NSLog(@"final letters: %@",finalLetteRSString);
    for(int i = 0; i < length; i++){
        char ch = [finalLetteRSString characterATindex:i];
        NSLog(@"%c",ch);
        NSString * cur = [NSString StringWithFormat:@"%c",ch];
        [letters insertObject:cur aTindex:i];
    }

    NSLog(@"LETTERS:: %@",letters);

    for(int i = length - 1; i >= 0; i--){
        int j = arc4random() % (i + 1);
        //NSLog(@"%d %d",i,j);
        //swap at positions i and j
        NSString * str_i = [letters objectATindex:i];
        [letters replaceObjectATindex:i withObject:[letters objectATindex:j]];
        [letters replaceObjectATindex:j withObject:str_i];      
    }
    NSLog(@"NEW SHUFFLED LETTERS %@",letters);

    NSString * result = @"";
    for(int i = 0; i < length; i++){
        result = [result StringByAppendingString:[letters objectATindex:i]];
    }

    NSLog(@"Final String: %@",result);
    finalLetteRSString = result;
    finalLetters.text = finalLetteRSString;
}

NSUInteger length = [finalLetteRSString length];

if (!length) return; // nothing to shuffle    

unichar *buffer = calloc(length,sizeof (unichar));

[finalLetteRSString getCharacters:buffer range:NsmakeRange(0,length)];

for(int i = length - 1; i >= 0; i--){
    int j = arc4random() % (i + 1);
    //NSLog(@"%d %d",j);
    //swap at positions i and j
    unichar c = buffer[i];
    buffer[i] = buffer[j];
    buffer[j] = c;
}

NSString *result = [NSString StringWithCharacters:buffer length:length];
free(buffer);

// caution,autoreleased. Allocate explicitly above or retain below to
// keep the String.
finalLetteRSString = result;