/** 银行卡号有效性问题Luhn算法
 *  现行 16 位银联卡现行卡号开头 6 位是 622126～622925 之间的，7 到 15 位是银行自定义的，
 *  可能是发卡分行，发卡网点，发卡序号，第 16 位是校验码。
 *  16 位卡号校验位采用 Luhm 校验方法计算：
 *  1，将未带校验位的 15 位卡号从右依次编号 1 到 15，位于奇数位号上的数字乘以 2
 *  2，将奇位乘积的个十位全部相加，再加上所有偶数位上的数字
 *  3，将加法和加上校验位能被 10 整除。
 */
+ (BOOL)isBankCardNumber:(NSString *)cardNum {
    
    NSString * lastNum = [[cardNum substringFromIndex:(cardNum.length-1)] copy];//取出最后一位
    NSString * forwardNum = [[cardNum substringToIndex:(cardNum.length -1)] copy];//前15或18位
    
    NSMutableArray * forwardArr = [[NSMutableArray alloc] initWithCapacity:0];
    for (int i=0; i<forwardNum.length; i++) {
        NSString * subStr = [forwardNum substringWithRange:NSMakeRange(i, 1)];
        [forwardArr addObject:subStr];
    }
    
    NSMutableArray * forwardDescArr = [[NSMutableArray alloc] initWithCapacity:0];
    for (int i = (int)(forwardArr.count-1); i> -1; i--) {//前15位或者前18位倒序存进数组
        [forwardDescArr addObject:forwardArr[i]];
    }
    
    NSMutableArray * arrOddNum = [[NSMutableArray alloc] initWithCapacity:0];//奇数位*2的积 < 9
    NSMutableArray * arrOddNum2 = [[NSMutableArray alloc] initWithCapacity:0];//奇数位*2的积 > 9
    NSMutableArray * arrEvenNum = [[NSMutableArray alloc] initWithCapacity:0];//偶数位数组
    
    for (int i=0; i< forwardDescArr.count; i++) {
        NSInteger num = [forwardDescArr[i] intValue];
        if (i%2) {//偶数位
            [arrEvenNum addObject:[NSNumber numberWithInteger:num]];
        }else{//奇数位
            if (num * 2 < 9) {
                [arrOddNum addObject:[NSNumber numberWithInteger:num * 2]];
            }else{
                NSInteger decadeNum = (num * 2) / 10;
                NSInteger unitNum = (num * 2) % 10;
                [arrOddNum2 addObject:[NSNumber numberWithInteger:unitNum]];
                [arrOddNum2 addObject:[NSNumber numberWithInteger:decadeNum]];
            }
        }
    }
    
    __block  NSInteger sumOddNumTotal = 0;
    [arrOddNum enumerateObjectsUsingBlock:^(NSNumber * obj, NSUInteger idx, BOOL *stop) {
        sumOddNumTotal += [obj integerValue];
    }];
    
    __block NSInteger sumOddNum2Total = 0;
    [arrOddNum2 enumerateObjectsUsingBlock:^(NSNumber * obj, NSUInteger idx, BOOL *stop) {
        sumOddNum2Total += [obj integerValue];
    }];
    
    __block NSInteger sumEvenNumTotal =0 ;
    [arrEvenNum enumerateObjectsUsingBlock:^(NSNumber * obj, NSUInteger idx, BOOL *stop) {
        sumEvenNumTotal += [obj integerValue];
    }];
    
    NSInteger lastNumber = [lastNum integerValue];
    
    NSInteger luhmTotal = lastNumber + sumEvenNumTotal + sumOddNum2Total + sumOddNumTotal;
    
    return (luhmTotal%10 ==0)?YES:NO;
}