C job interview - casting and comparing

Question

If he is really dissatisfied with this approach (which is essentially a brain fart, since you aren't comparing megabytes or gigabytes of data, so one shan't really be worrying about "efficiency" and "speed" in this case), just tell him that you trust the quality and speed of the standard library:

int isEqual(MAC* addr1, MAC* addr2)
{
    return memcmp(&addr1->data, &addr2->data, sizeof(addr1->data)) == 0;
}

If your interviewer demands that you produce undefined behavior, I would probably look for a job elsewhere.

The correct initial approach would be to store the MAC address in something like a uint64_t, at least in-memory. Then comparisons would be trivial, and implementable efficiently.

Cowboy time:

typedef struct macA {
   char data[6];
} MAC;

typedef struct sometimes_works {
   long some;
   short more;
} cast_it

typedef union cowboy
{
    MAC real;
    cast_it hack;
} cowboy_cast;

int isEqual(MAC* addr1, MAC* addr2)
{
     assert(sizeof(MAC) == sizeof(cowboy_cast));  // Can't be bigger
     assert(sizeof(MAC) == sizeof(cast_it));      // Can't be smaller

     if ( ( ((cowboy_cast *)addr1)->hack.some == ((cowboy_cast *)addr2)->hack.some )
       && ( ((cowboy_cast *)addr1)->hack.more == ((cowboy_cast *)addr2)->hack.more ) )
             return (0 == 0);

     return (0 == 42);
}

There is nothing wrong with an efficient implementation, for all you know this has been determined to be hot code that is called many many times. And in any case, its okay for interview questions to have odd constraints.

Logical AND is a priori a branching instruction due to short-circuit evaluation even if it doesn't compile this way, so lets avoid it, we don't need it. Nor do we need to convert our return value to a true bool (true or false, not 0 or anything that's not zero).

Here is a fast solution on 32-bit: XOR will capture the differences, OR will record difference in both parts, and NOT will negate the condition into EQUALS, not UNEQUAL. The LHS and RHS are independent computations, so a superscalar processor can do this in parallel.

int isEqual(MAC* addr1, MAC* addr2)
{
    return ~((*(int*)addr2 ^ *(int*)addr1) | (int)(((short*)addr2)[2] ^ ((short*)addr1)[2]));
}

EDIT
The purpose of the above code was to show that this could be done efficiently without branching. Comments have pointed out this C++ classifies this as undefined behavior. While true, VS handles this fine. Without changing the interviewer's struct definition and function signature, in order to avoid undefined behavior an extra copy must be made. So the non-undefined behavior way without branching but with an extra copy would be as follows:

int isEqual(MAC* addr1, MAC* addr2)
{
    struct IntShort
    {
        int   i;
        short s;
    };

    union MACU
    {
        MAC addr;
        IntShort is;
    };

    MACU u1;
    MACU u2;

    u1.addr = *addr1; // extra copy
    u2.addr = *addr2; // extra copy

    return ~((u1.is.i ^ u2.is.i) | (int)(u1.is.s ^ u2.is.s)); // still no branching
}

This would work on most systems,and be faster than your solution.

int isEqual(MAC* addr1, MAC* addr2)
{
    return ((int32*)addr1)[0] == ((int32*)addr2)[0] &&  ((int16*)addr1)[2] == ((int16*)addr2)[2];
}

would inline nicely too, could be handy at the center of loop on a system where you can check the details are viable.