Understanding a C function for C# translation

Question

I am side-checking some code that was translated from C to C#. I have a question on the original C:

...
#define getblock(p, i) (p[i])
...
void MurmurHash3_x86_32 ( const void * key, int len,
                          uint32_t seed, void * out )
{
  const uint8_t * data = (const uint8_t*)key;
  const int nblocks = len / 4;
  int i;

  uint32_t h1 = seed;

  uint32_t c1 = 0xcc9e2d51;
  uint32_t c2 = 0x1b873593;

  const uint32_t * blocks = (const uint32_t *)(data + nblocks*4);

  for(i = -nblocks; i; i++)
  {
    uint32_t k1 = getblock(blocks,i);
...

The part for(i = -nblocks; i; i++) ... is this looping through the data backwards? I've never seen data referred to with a negative index.

Answer 1

No it's not looping through the data backwards. It starts at the beginning of data, and indexes up.

As you can see, here the pointer "blocks" is advanced past "data" already. It points "nblocks" past the beginning of data.

const uint32_t * blocks = (const uint32_t *)(data + nblocks*4);

So, you need a negative index to get to the beginning of data (-nblocks). The start of data is precisely at "blocks[-nblocks]". The "for" loop simply starts there, and counts up.

for(i = -nblocks; i; i++)

Answer 2

The blocks variable is initialized ahead of data by nblocks (assuming sizeof(uint32_t) == 4). The for loop then starts from the beginning of data up to the end pointed to by blocks, so negative indices are used. So, it is not looping through the data backwards, but forwards.

Answer 3

In fact it's an algorithm used hashing (https://en.wikipedia.org/wiki/MurmurHash), your source might be that one https://github.com/JeffBezanson/libsupport/blob/master/MurmurHash3.c ;)