验证银行卡是否合法

#pragma mark 判断银行卡号是否合法
-(BOOL)isBankCard:(NSString *)cardNumber{
    if(cardNumber.length==0){
        return NO;
    }
    NSString *digitsOnly = @"";
    char c;
    for (int i = 0; i < cardNumber.length; i++){
        c = [cardNumber characterAtIndex:i];
        if (isdigit(c)){
            digitsOnly =[digitsOnly stringByAppendingFormat:@"%c",c];
        }
    }
    int sum = 0;
    int digit = 0;
    int addend = 0;
    BOOL timesTwo = false;
    for (NSInteger i = digitsOnly.length - 1; i >= 0; i--){
        digit = [digitsOnly characterAtIndex:i] - '0';
        if (timesTwo){
            addend = digit * 2;
            if (addend > 9) {
                addend -= 9;
            }
        }
        else {
            addend = digit;
        }
        sum += addend;
        timesTwo = !timesTwo;
    }
    int modulus = sum % 10;
    return modulus == 0;
}