Euler21.scala

package euler

object Euler21 extends App {

  def properDivisors(n: Int): Seq[Int] =
    (1 to n / 2).filter(n % _ == 0)

  val dn = for {n <- 0 until 10000} yield properDivisors(n).sum

  private def isAmicable(n: Int, i: Int): Boolean =
    n < 10000 && dn(n) == i && dn(n) != n

  val r = (for {
    (n, i) <- dn.zipWithIndex if isAmicable(n, i)
  } yield n).sum

  println(r)
  assert(r == 31626)

}