Functional Programming in Scala, chapter 6, created at London Scala User Group Coding Dojo, Feb 21, 2013

gistfile1.scala

import sun.reflect.generics.reflectiveObjects.NotImplementedException

object X extends App {

  trait RNG {
    def nextInt: (Int, RNG)
  }

  object RNG {
    def simple(seed: Long): RNG = new RNG {
      def nextInt = {
        val seed2 = (seed*0x5DEECE66DL + 0xBL) &
          ((1L << 48) - 1)
        ((seed2 >>> 16).asInstanceOf[Int],
          simple(seed2))
      }
    }
  }

  def positiveInt(rng: RNG): (Int, RNG) = {
    val (n, newRng) = rng.nextInt
    val pos = if (n == Int.MinValue) 0 else n.abs
    (pos, newRng)
  }

  def positiveInt2(rng: RNG): (Int, RNG) = rng.nextInt match {
    case (Int.MinValue, nextRng) => (0, nextRng)
    case (n, nextRng) => (n.abs, nextRng)
  }

  def ints(count: Int)(rng: RNG): (List[Int], RNG) =
    if (count <= 0) (Nil, rng)
    else rng.nextInt match {
      case (n, nextRng) =>
         val (tail, nextRng2) = ints(count - 1)(nextRng)
        (n :: tail, nextRng2)
    }

  type Rand[+A] = RNG => (A, RNG)

  def map[A,B](s: Rand[A])(f: A => B): Rand[B] =
    rng => {
      val (a, rng2) = s(rng)
      (f(a), rng2)
    }

  def positiveMax(n: Int): Rand[Int] = map(_.nextInt) { r => (r % n).abs }

  val double: Rand[Double] = map(_.nextInt) { r => r.toDouble / Int.MaxValue }

  def map2[A,B,C](ra: Rand[A], rb: Rand[B])(f: (A, B) => C): Rand[C] =
    rng => {
      val (a, rng2) = ra(rng)
      val (b, rng3) = rb(rng2)
      (f(a, b), rng3)
    }

  val intDouble: (RNG) => ((Int, Double), RNG) = map2(_.nextInt, double) { (_, _) }

  val doubleInt = map2(double, _.nextInt) { (_, _) }

  def sequence[A](fs: List[Rand[A]]): Rand[List[A]] = fs match {
    case Nil => { rng => (Nil, rng) }
    case head :: tail => { rng =>
      val (a, interRng) = head(rng)
      val (rest, nextRng) = sequence(tail)(interRng)
      (a :: rest, nextRng)
    }
  }


  def RngMock(nums: List[Int]): RNG = new RNG {
    def nextInt = nums match {
      case Nil => throw new NotImplementedException
      case head :: tail => (head, RngMock(tail))
    }
  }

  println(RNG.simple(4).nextInt)

  println((Int.MinValue - 1).abs)
  println(Int.MinValue.abs)

  println(ints(10)(RngMock(List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))))
  println(positiveMax(100000)(RngMock(List(100010))))

  println(sequence(List[Rand[Int]](_.nextInt, _.nextInt, _.nextInt))(RngMock(List(1, 4, 8))))
}