OK, it came to my attention today that hash functions in spark, like xxhash64, actually always seed with 42: https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/hash.scala#L655
This is an issue if you want the hash of some value in Spark to match the hash you compute with xxhash64 somewhere else, and, AFAICT most any other impl will start with seed=0. I'm guessing there wasn't a *great* reason for this, just seemed like 42 was a nice default seed. And we can't change it now without maybe subtly changing program behaviors. And, I am guessing it's messy to let the function now take a seed argument, esp. in SQL. So I'm left with, I guess we should doc that? I can do it if so. And just a cautionary tale I guess, for hash function users.
