Kolakoski prototype

kolakoski.scala

val integers = Stream.from(0)


object E1 {
  val evenIntegers = integers.filter(_ % 2 == 0)
}
object E2 {
  val evenIntegers = integers.map(_ * 2)
}
object E3 {
  val evenIntegers = Stream.iterate(0)(_ + 2)
}
object E4 {
  val evenIntegers: Stream[Int] = 0 #:: evenIntegers.map(_ + 2)
}
val firstEvenIntegers = 0 to 30 by 2
assert(E1.evenIntegers startsWith firstEvenIntegers)
assert(E2.evenIntegers startsWith firstEvenIntegers)
assert(E3.evenIntegers startsWith firstEvenIntegers)
assert(E4.evenIntegers startsWith firstEvenIntegers)


object Primes {
  def isPrime(n: Int) = n > 1 && (2 to n / 2).forall(n % _ != 0)

  val integers = Stream.from(0)
  val primes = integers.filter(isPrime)

  println(integers)  // Stream(0, 1, 2, ?)
  println(primes)    // Stream(2, ?)

  println(primes(4)) // 11
  println(integers)  // Stream(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ?)
  println(primes)    // Stream(2, 3, 5, 7, 11, ?)
}
Primes

val onesAndTwos = Stream.continually(List(1, 2)).flatten

def rest(kolakoski: Stream[Int]): Stream[Int] = {
  kolakoski.drop(2) zip onesAndTwos flatMap { case (k, n) ⇒ Seq.fill(k)(n) }
}

val kolakoski: Stream[Int] = 1 #:: 2 #:: 2 #:: rest(kolakoski)

val expectedKolakoski = List(1, 2, 2, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1)

expectedKolakoski.take(23)       .mkString("")
kolakoski        .take(23).toList.mkString("")

assert(kolakoski startsWith expectedKolakoski)