mikemccand commented on code in PR #12716:
URL: https://github.com/apache/lucene/pull/12716#discussion_r1370148505
##########
lucene/CHANGES.txt:
##########
@@ -190,6 +190,8 @@ Improvements
* GITHUB#12705, GITHUB#12705: Improve handling of NullPointerException and
IllegalStateException
in MMapDirectory's IndexInputs. (Uwe Schindler, Michael Sokolov)
+* GITHUB#12716: Improve hash mixing in FST's double-barrel LRU hash. (Shubham
Chaudhary)
Review Comment:
Maybe also add something more end-user friendly like `, making FST
compilation faster and more RAM-efficient` or so?
##########
lucene/core/src/java/org/apache/lucene/util/fst/NodeHash.java:
##########
@@ -182,8 +183,9 @@ private long hash(FSTCompiler.UnCompiledNode<T> node) {
h += 17;
}
}
-
- return h;
+ // Here we multiply the hash with the gold constant BitMixer#PHI_C64
+ // This makes a real difference in the evenness of the value distribution
+ return h * BitMixer.PHI_C64;
Review Comment:
Could you also add a link that @dweiss provided in [this
comment](https://github.com/apache/lucene/issues/12704#issuecomment-1774156970)?
I always freak out a bit when I see magical constants and I don't understand
their magic. [That
link](https://softwareengineering.stackexchange.com/questions/402542/where-do-magic-hashing-constants-like-0x9e3779b9-and-0x9e3779b1-come-from)
goes into a nice little complex history lesson...). Or perhaps put the link
on the `BitMixer.PHI_C64` as a comment?
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