Guava library's approach is pretty naive: Hashing.combineUnordered It just adds each byte together, I don't think it is better than the one you proposed.
BTW, I don't understand why we need c? Isn't a*17+b good enough to avoid the corner case? On 2020/07/15 20:33:55, Haisheng Yuan <hy...@apache.org> wrote: > > a) compute xor of the hashes > > b) compute sum of the hashes > > c) compute product of the hashes (with 0 replaced by 1 to avoid 0*n=0) > > Then combine the three values somehow. For instance: (a*17+b)*17+c > > That is really good one. Better than pure XOR. > I also found this in scala: > https://github.com/scala/scala/blob/2.11.x/src/library/scala/util/hashing/MurmurHash3.scala#L88 > > > On 2020/07/15 20:16:13, Vladimir Sitnikov <sitnikov.vladi...@gmail.com> > wrote: > > > things might be bad if we want do dedup RexNode children using > > Set<RexNode> > > > > I guess the following might work better than XOR: > > > > a) compute xor of the hashes > > b) compute sum of the hashes > > c) compute product of the hashes (with 0 replaced by 1 to avoid 0*n=0) > > Then combine the three values somehow. For instance: (a*17+b)*17+c > > > > That would result in a hash code that tolerates item reordering, it is > > better than XOR, and it should be fast. > > > > Unfortunately, we can't use the improved function in Pair since Map.Entry > > specifies the exact computation formula :-/ > > > > Vladimir > > >
