Question

I am confused with what value should I provide for the len parameter in the MurmurHash3_x86_128() function (and there are similar functions also). The murmurhash3 code can be found here.

If your answer is that it is the length of the key then please explain the code at line no.154.

Answer 1

It is mostly certainly the length of the data to hash in bytes, the code at line 154:

const int nblocks = len / 16;

divides this length into 128bit blocks (or 16 bytes each), this is to promote faster hashing, as MurmurHash3_x86_128() processes and mixes 16 bytes (4 x uint32) per iteration.

you’ll notice on line 205:

switch(len & 15)

this catches any trialing bytes, if the size is not a multiple of 16, and the switch cases combine them into the final hash.