01-intersectionWrong.scala

//    PDF: https://moodle.epfl.ch/pluginfile.php/3175938/mod_folder/content/0/week03-4-Scala-Parallel-Collections.pdf
//  Video: https://mediaspace.epfl.ch/playlist/dedicated/31866/0_icv10qux/0_h4jt9i7k
// Slides: 27

package lecture3

import scala.collection.parallel.CollectionConverters.IterableIsParallelizable
import scala.collection.parallel.immutable.ParSet

// Note: we don't use `GenSet` because it is deprecated since Scala 2.13.

def parIntersectionWrong(a: ParSet[Int], b: ParSet[Int]) =
  val result = collection.mutable.Set[Int]()
  for x <- a do
    if b.contains(x) then
      result += x
  result

@main def intersectionWrong =
  val a = (0 until 10000).toSet
  val b = (0 until 10000 by 4).toSet
  val seqRes = a.intersect(b)
  val parRes = parIntersectionWrong(a.par.toSet, b.par.toSet)
  assert(seqRes.size == 2500)
  assert(parRes.size != 2500)