截断 FNV hash

Question

I'm using 32-bit FNV-1a hashing, but now I want to reserve one of the bits to hold useful information about the input key. That is, I want to use only 31 of the 32 bits for hash and 1 bit for something else.

Assuming FNV is well distributed for my application, is it safe to assume that dropping 1 bit this will increase collision rate by 32/31, as opposed to something dramatic?

The algo recommends XOR the discarded MSB with the LSB, but for 1-bit, that seems pointless. As such, would it matter which bit is discarded (MSB or LSB)? And if not, would it matter if the LSB MSB were discard after hashing each byte (ie using a even numbered "prime") or after 32-bit hashing the entire byte-array first.

Answer 1

Removing a single bit from a 32-bit hash code will have a larger effect than a 32/31 increase in the collision rate. To see why, note that there are 2 ³² possible 32-bit hashes and 2 ³¹ possible 31-bit hashes, meaning that removing a bit from the hash cuts the numbers of possible hashes down by a factor of two - a pretty significant reduction in the number of possible hashes. This brings about roughly a doubling of the probability that you see a hash collision across your hashes.

If you have a sufficiently small number of hashes that collisions are rare, then cutting out a single bit is unlikely to change much. But if collisions were already an issue, dropping a bit will roughly double the chance you see them.