random NSString in objective-c

rand-nsstring.m

NSString *rand_str() {
    NSString *alphabet = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789";
    u_int32_t len = arc4random_uniform(20) + 5;
    NSMutableString *s = [NSMutableString stringWithCapacity:len];
    for (NSUInteger i = 0U; i < len; i++) {
        u_int32_t r = arc4random() % [alphabet length];
        unichar c = [alphabet characterAtIndex:r];
        [s appendFormat:@"%C", c];
    }
    return s;
}