Scala Parallelization

Coursera - Parallel Programming

Week3 - Data Parallelism

Homework

import scala.concurrent.duration.Duration
import scala.concurrent.{Await, Future}

import scala.concurrent.ExecutionContext.Implicits.global


trait GenIterable[A]

trait Iterable[A] extends GenIterable[A]

trait ParIterable[A] extends GenIterable[A]

trait Iterator[A] {
  def next(): A

  def hasNext: Boolean

  def foldLeft[B](z: B)(f: (B, A) => B): B = {
    var s = z
    while (hasNext) s = f(s, next())
    s
  }
}

trait Splitter[A] extends Iterator[A] {
  def split: Seq[Splitter[A]]

  def remaining: Int

  def fold(z: A)(f: (A, A) => A): A = {
    if (remaining < 1000) foldLeft(z)(f)
    else {
      val children = for {
        child <- split
      } yield Future {
        child.fold(z)(f)
      }
      Await.result(Future.sequence(children), Duration.Inf).foldLeft(z)(f)
    }
  }
}

trait Builder[A, Repr] {
  def +=(elem: A): Builder[A, Repr]

  def result: Repr
}

trait Combiner[A, Repr] extends Builder[A, Repr] {
  def combine(that: Combiner[A, Repr]): Combiner[A, Repr]
}

trait SplittableLike[A, Repr] extends IterableLike[A, Repr] {
  def splitter: Splitter[A] // on every parallel collection
}

trait IterableLike[A, Repr <: Iterable[A]] extends Buildable[A, Repr] {
  def iterator: Iterator[A] // on every collection

  def foldLeft[B](z: B)(f: (B, A) => B): B = {
    iterator.foldLeft(z)(f)
  }

  def filter(p: A => Boolean): Repr = {
    val b = newBuilder
    for (x <- this) if (p(x)) b += x
    b.result
  }
}

trait ParIterableLike[A, Repr <: ParIterable[A]] extends SplittableLike[A, Repr]
  with Combinable[A, Repr] {

  override def filter(p: A => Boolean): Repr = {

    def helper(p: A => Boolean)(sp: Splitter[A]): Combiner[A, Repr] = {
      if (sp.remaining < 1000) {
        val c = newCombiner
        while (sp.hasNext) {
          val n = sp.next()
          if (p(n)) c += n
        }
        c
      } else {
        val children = for {
          child <- sp.split
        } yield Future {
          helper(p)(child)
        }
        Await.result(Future.sequence(children), Duration.Inf).reduceLeft(_ combine _)
      }
    }
    helper(p)(splitter).result
  }
}

trait Buildable[A, Repr] {
  def newBuilder: Builder[A, Repr] // on every collection
}

trait Combinable[A, Repr] extends Buildable[A, Repr] {
  def newCombiner: Combiner[A, Repr] // on every parallel collection
}

trait MySeqLike[T, Repr <: MySeq[T]] extends IterableLike[T, Repr] {

}

trait MyParSeqLike[T, Repr <: MyParSeq[T]] extends ParIterableLike[T, Repr] {
  def fold(z: T)(f: (T, T) => T): T = {
    splitter.fold(z)(f)
  }
}

class MySeq[T] extends MySeqLike[T, Seq[T]] {
  override def iterator: Iterator[T] = ???

  override def newBuilder: Builder[T, Seq[T]] = ???
}

class MyParSeq[T] extends MyParSeqLike[T, MyParSeq[T]] {
  override def splitter: Splitter[T] = ???

  override def iterator: Iterator[T] = ???

  override def newCombiner: Combiner[T, MyParSeq[T]] = ???

  override def newBuilder: Builder[T, MyParSeq[T]] = ???
}