Generate a list of distinct elements from a Scalacheck generator

distinct-list.scala

import scala.collection.mutable.ListBuffer
import org.scalacheck._, Arbitrary._, Prop._
  
// a specs2 example showing that it works  
def e1 =
  forAll(distinctListOf[Int]) { ls =>
    ls.distinct === ls
  }.set(minTestsOk = 10000, maxSize = 1000)
  
// distinct list of elements from a given arbitrary
def distinctListOf[T : Arbitrary] =
  distinctListOfGen(arbitrary[T])
  
// distinct list of elements from a given generator
// with a maximum number of elements to discard
def distinctListOfGen[T](gen: Gen[T], maxDiscarded: Int = 1000): Gen[List[T]] = {
  val seen: ListBuffer[T] = new ListBuffer[T]
  var discarded = 0
  
  Gen.sized { size =>
    if (size == seen.size) seen.toList
    else {
      while (seen.size <= size && discarded < maxDiscarded)
        gen.sample match {
          case Some(t) if !seen.contains(t) =>
            seen.+=:(t)
          case _ => discarded += 1
        }
      seen.toList
    }
  }
}