Case class hashing

Language Design
1

Currently the hashing of case classes are delegated to scala.util.hashing.MurmurHash3.productHash.
This function takes into account the productPrefix only when the productArity is 0.
This produces the following problem:

scala> Left(1).##
res1: Int = 818387364

scala> Right(1).##
res2: Int = 818387364

Should productPrefix always be mixed into the hashCode by using productPrefix.## as the seed?

1 Like
2

Yes, that looks like a bug. Dotty-compiled case classes do take Product prefix into account.

scala> case class Left(x: Int) 
defined class Left
scala> case class Right(x: Int) 
defined class Right
scala> Left(1).## 
val res3: Int = -1674650338
scala> Right(1).## 
val res4: Int = 1333177388
scala>
3

Thanks Martin. I’ll start an issue on scala/bug. This relates to a Hash typeclass I’m working on for cats.

1 Like