Added EqLinkedHashSet, an identity-based linked hash set

Author: mucciolo

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

@@ -0,0 +1,46 @@
+package dotty.tools.dotc.util
+
+import scala.collection.mutable.ArrayBuffer
+
+class EqLinkedHashSet[T](
+  initialCapacity: Int = 8, capacityMultiple: Int = 2
+) extends MutableSet[T] {
+
+  private val map: MutableMap[T, Unit] = new EqHashMap(initialCapacity, capacityMultiple)
+  private val linkingArray: ArrayBuffer[T] = new ArrayBuffer(initialCapacity)
+
+  override def +=(x: T): Unit =
+    map.update(x, ())
+    if map.size != linkingArray.size then linkingArray += x
+
+  override def put(x: T): T =
+    this += x
+    x
+
+  override def -=(x: T): Unit =
+    map -= x
+    if map.size != linkingArray.size then linkingArray -= x
+
+  override def clear(): Unit =
+    map.clear()
+    linkingArray.clear()
+
+  override def lookup(x: T): T | Null = if map.contains(x) then x else null
+
+  override def size: Int = map.size
+
+  override def iterator: Iterator[T] = linkingArray.iterator
+
+}
+
+object EqLinkedHashSet {
+  def apply[T](x: T): EqLinkedHashSet[T] =
+    val set = new EqLinkedHashSet[T]
+    set += x
+    set
+
+  def apply[T](x: T, y: T): EqLinkedHashSet[T] =
+    val set = EqLinkedHashSet(x)
+    set += y
+    set
+}