BoundedSet.scala

package lascala.sgs.ch5

import scala.collection.immutable.Queue

case class BoundedSet[A](
  data: Queue[A],
  capacity: Int,
) {

  def add(elem: A): BoundedSet[A] = {
    if (elem == null) throw new NullPointerException()
    else {
      val index = data.indexOf(elem)
      val data1 = if (index == -1) data else {
        val (f, b) = data.splitAt(index)
        f ++ b.tail
      }
      val data2 = if (data1.size == capacity) data1.tail else data1
      val data3 = data2.enqueue(elem)
      BoundedSet(data3, capacity)
    }
  }

  def contains(elem: A): Boolean = data.contains(elem)

}

BoundedSetTest.scala

package lascala.sgs.ch5

import scala.collection.immutable.Queue

class BoundedSetTest extends munit.FunSuite {
  test("single element") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    assert(set1.contains(1) == true)
  }

  test("repeated element") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    val set2 = set1.add(1)
    val set3 = set2.add(1)
    val set4 = set3.add(1)
    assert(set4.contains(1) == true)
  }

  test("overflow keeps second") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    val set2 = set1.add(2)
    val set3 = set2.add(3)
    val set4 = set3.add(4)
    assert(set4.contains(2) == true)
  }

  test("overflow removes oldist") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    val set2 = set1.add(2)
    val set3 = set2.add(3)
    val set4 = set3.add(4)
    assert(set4.contains(1) == false)
  }

  test("overflow keeps newest") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    val set2 = set1.add(2)
    val set3 = set2.add(3)
    val set4 = set3.add(4)
    assert(set4.contains(4) == true)
  }

  test("renewal") {
    val set = BoundedSet[Int](Queue.empty, 3)
    val set1 = set.add(1)
    val set2 = set1.add(2)
    val set3 = set2.add(1)
    val set4 = set3.add(3)
    val set5 = set4.add(4)
    assert(set5.contains(1) == true)
  }
}