UniqueSeq.scala

import scala.collection.generic.CanBuildFrom
import scala.collection.mutable.ArrayBuffer
import scala.collection.{IndexedSeqLike, mutable}

/**
  * An ordered collection type that ensures it always contains unique elements.
  */
final class UniqueSeq[+A] private(_input: IndexedSeq[A])
  extends IndexedSeq[A]
    with IndexedSeqLike[A, UniqueSeq[A]] {

  val underlying: IndexedSeq[A] = _input.distinct

  def length: Int = underlying.length
  def apply(index: Int): A = underlying(index)

  override protected[this] def newBuilder: mutable.Builder[A, UniqueSeq[A]] = {
    UniqueSeq.newBuilder
  }
}

object UniqueSeq {
  val empty: UniqueSeq[Nothing] = {
    new UniqueSeq(IndexedSeq.empty)
  }

  def apply[A](elements: A*): UniqueSeq[A] = {
    new UniqueSeq[A](elements.toIndexedSeq)
  }

  def newBuilder[A]: mutable.Builder[A, UniqueSeq[A]] = {
    new ArrayBuffer[A].mapResult(new UniqueSeq[A](_))
  }

  implicit def canBuildFrom[A]: CanBuildFrom[UniqueSeq[_], A, UniqueSeq[A]] = {
    new CanBuildFrom[UniqueSeq[_], A, UniqueSeq[A]] {
      override def apply(from: UniqueSeq[_]): mutable.Builder[A, UniqueSeq[A]] = newBuilder[A]
      override def apply(): mutable.Builder[A, UniqueSeq[A]] = newBuilder[A]
    }
  }
}