Prob44_2.scala

object Prob44_2 {
  def pentagonal(n: Int) = n*(3*n - 1)/2
  private[this] val pentagonals = Iterator.from(0).map(pentagonal).takeWhile(_ < (Int.MaxValue >> 2)).toArray

  def isPenta(n: Int) = {
    val dbl = (1.0 + math.sqrt(1 + 24*n))/6.0
    val int = dbl.toInt
    pentagonal(int) == n || pentagonal(int + 1) == n
  }

  def check(j: Int, k: Int): Boolean = {
    val pj = pentagonals(j)
    val pk = pentagonals(k)
    isPenta(pj + pk) && isPenta((pj - pk).abs)
  }

  def w(){
    val result = collection.mutable.ArrayBuffer[(Int, Int)]()
    var i = 1
    while(i <= 10000){
      var j = i + 1
      while(j <= 10001){
        if(check(i, j)){
          result += ((i, j))
        }
        j += 1 
      }
      i += 1
    }
    println(result.head)
  }

  def f(){
    val result = for {
      i <- 1 to 10000
      j <- (i + 1) to 10001
      if check(i, j)
    } yield (i, j)
    println(result.head)
  }

  def time[A](f: => A): A = {
    val start = System.nanoTime
    val r = f
    val end = System.nanoTime
    println(((end - start) / 1000000))
    r
  }

  def main(args: Array[String]) { 
    print("while ")
    time(w())
    print("for ")
    time(f())
  }
}