Process hashcode bits in util.HashMap and HashSet

The code was taken from scala.collection.mutable.AnyRefMap

Author: odersky

compiler/src/dotty/tools/dotc/util/HashMap.scala

@@ -11,7 +11,12 @@ extends GenericHashMap[Key, Value](initialCapacity, capacityMultiple):
   /** Hashcode is left-shifted by 1, so lowest bit is not lost
    *  when taking the index.
    */
-  final def hash(x: Key): Int = x.hashCode << 1
+  final def hash(key: Key): Int =
+    val h = key.hashCode
+    // Part of the MurmurHash3 32 bit finalizer
+    val i = (h ^ (h >>> 16)) * 0x85EBCA6B
+    val j = (i ^ (i >>> 13)) & 0x7FFFFFFF
+    (if j==0 then 0x41081989 else j) << 1
 
   final def isEqual(x: Key, y: Key): Boolean = x.equals(y)
 

compiler/src/dotty/tools/dotc/util/HashSet.scala

@@ -48,8 +48,13 @@ class HashSet[T](initialCapacity: Int = 8, capacityMultiple: Int = 2) extends Mu
 
   protected def isDense = limit < DenseLimit
 
-  /** Hashcode, by defualt `x.hashCode`, can be overridden */
-  protected def hash(x: T): Int = x.hashCode
+  /** Hashcode, by default a processed `x.hashCode`, can be overridden */
+  protected def hash(key: T): Int =
+    val h = key.hashCode
+    // Part of the MurmurHash3 32 bit finalizer
+    val i = (h ^ (h >>> 16)) * 0x85EBCA6B
+    val j = (i ^ (i >>> 13)) & 0x7FFFFFFF
+    if j==0 then 0x41081989 else j
 
   /** Hashcode, by default `equals`, can be overridden */
   protected def isEqual(x: T, y: T): Boolean = x.equals(y)