How to do reverse memcmp?

Question

How can I do reverse memory comparison? As in, I give the ends of two sequences and I want the pointer to be decremented towards the beginning, not incremented towards the end.

Answer 1

There's no built-in function in the C standard library to do it. Here's a simple way to roll your own:

int memrcmp(const void *s1, const void *s2, size_t n)
{
    if(n == 0)
        return 0;

    // Grab pointers to the end and walk backwards
    const unsigned char *p1 = (const unsigned char*)s1 + n - 1;
    const unsigned char *p2 = (const unsigned char*)s2 + n - 1;

    while(n > 0)
    {
        // If the current characters differ, return an appropriately signed
        // value; otherwise, keep searching backwards
        if(*p1 != *p2)
            return *p1 - *p2;
        p1--;
        p2--;
        n--;
    }

    return 0;
}

If you something high-performance, you should compare 4-byte words at a time instead of individual bytes, since memory latency will be the bottleneck; however, that solution is significantly more complex and not really worth it.

Answer 2

Much like in a post (C memcpy in reverse) originally linked by Vlad Lazarenko, here is a solution based on that, that I haven't yet tested but should get you started.

int reverse_memcmp(const void *s1, const void *s2, size_t n)
{
    unsigned char *a, *b;
    a = s1;
    b = s2;
    size_t i = 0;

    // subtracting i from last position and comparing
    for (i = 0; i < n; i++) {
        if (a[n-1-i] != b[n-1-i]) {
            // return differences between different byte, strcmp()-style
            return (a[n-1-i] - b[n-1-i]);
        }
    }

    return 0;
}

Answer 3

All you need to do is specify your two ends and the size that you'd like to compare, as well as a step size. Please note especially that the step size may be the most important part for getting the expected results. It will greatly ease the implementation if you restrict the sizes. For the size of a char you could do something like:

int compare (void *one, void *two, size_t size)
  {
  char *one_char = (char *)one;
  char *two_char = (char *)two;
  size_t i;

  for (i = 0; i < size; i++)
    {
    if (*(one_char - i) != *(two_char - i))
       return(NOT_EQUAL);
    }  

  return(EQUAL);
  }

Answer 4

shorter code (C code doesn't need force pointer type cast):

int reverse_memcmp(const void *end1, const void *end2, size_t n) {
    const unsigned char *a = end1, *b = end2;
    for (; n; --n)
        if (*--a != *--b) return *a - *b;
    return 0;
}