Improve hash mixing in FST's double-barrel LRU hash

[mikemccand]

Description

Spinoff from this cool comment, thanks to hashing guru @bruno-roustant:

Instead, we should multiply with the gold constant BitMixer#PHI_C64 (make it public).
This really makes a difference in the evenness of the value distribution. This is one of the secrets of the HPPC hashing. By applying this, we get multiple advantages:

  * lookup should be improved (less hash collision)
  * we can try to rehash at 3/4 occupancy because the performance should not be impacted until this point.
  * in case of hash collision, we can lookup linearly with a pos = pos + 1 instead of quadratic probe (lines 95 and 327); this may avoid some mem cache miss.
  * 
(same for the other hash method)

This is a simple change, we just need to test on some real FST building cases to confirm good mixing "in practice". The new IndexToFST tool in luceneutil is helpful for this.

[bruno-roustant]

@dweiss will probably say more than me about the awesome BitMixer#PHI_C64 constant!

[dweiss]

I borrowed that constant in BitMixer from Sebastiano Vigna, I believe. Here is a nice overview of its origin/ rationale:

https://softwareengineering.stackexchange.com/questions/402542/where-do-magic-hashing-constants-like-0x9e3779b9-and-0x9e3779b1-come-from

I can only confirm that a good hash redistribution function, along with linear probing, give very good results in most hash/index redistribution problems I've seen.

[shubhamvishu]

I opened a PR to make use of this constant : #12716

Also, I was thinking if this constant could be utilised in other hash function implementations as well in the codebase?

[dweiss]

If you'd like to do so, I'd suggest moving such a "scattering remix" utility to a separate class and reusing it elsewhere, much like here: https://github.com/carrotsearch/hppc/blob/master/hppc/src/main/java/com/carrotsearch/hppc/BitMixer.java#L96-L99

Makes it easier to change the remixing strategy everywhere at once.