FlatArrayFactory with Boxed/Unboxed representation type-class

FlatArrayFactory.scala

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

trait Representation[Unboxed, Boxed] {
  def box(x: Unboxed): Boxed
  def unbox(x: Boxed): Unboxed
}

abstract class FlatArrayFactory[@specialized Unboxed: ClassManifest, Boxed] {

  class FlatArray(elems: Array[Unboxed])(implicit repr: Representation[Unboxed, Boxed])
      extends IndexedSeq[Boxed] with IndexedSeqLike[Boxed, FlatArray] {
    override protected[this] def newBuilder: Builder[Boxed, FlatArray] = FlatArray.newBuilder
    def apply(idx: Int): Boxed = repr.box(elems(idx))
    def update(idx: Int, elem: Boxed): Unit = elems(idx) = repr.unbox(elem)
    def length = elems.length
    override def foreach[U](f: Boxed => U): Unit = elems foreach (elem => f(repr.box(elem)))
    override def stringPrefix = "FlatArray"
  }

  object FlatArray {
    def fromSeq(elems: Seq[Boxed])(implicit repr: Representation[Unboxed, Boxed]): FlatArray = {
      val xs: Array[Unboxed] = new Array[Unboxed](elems.length)
      var i = 0
      for (elem <- elems) {
        xs(i) = repr.unbox(elem)
        i += 1
      }
      new FlatArray(xs)
    }

    def apply(elems: Boxed*)(implicit repr: Representation[Unboxed, Boxed]) = fromSeq(elems)

    def newBuilder(implicit repr: Representation[Unboxed, Boxed]): Builder[Boxed, FlatArray] =
      new ArrayBuffer mapResult fromSeq

    implicit def canBuildFrom(implicit repr: Representation[Unboxed, Boxed]) =
      new CanBuildFrom[FlatArray, Boxed, FlatArray] {
        def apply(): Builder[Boxed, FlatArray] = newBuilder
        def apply(from: FlatArray): Builder[Boxed, FlatArray] = newBuilder
      }
  }

}