GameOfLife.scala

object Cell {

  def willLive(isAlive: Boolean, neighbourCount: Int): Boolean =
    if (isAlive) neighbourCount == 2 || neighbourCount == 3
    else neighbourCount == 3
}

case class GridPosition(x: Int, y: Int)

object Game {

  type CellCollection = Set[GridPosition]

  def evolve(current: CellCollection): CellCollection = {

    def neighbourhood(pos: GridPosition): CellCollection =
      (for (dx <- Seq(-1, 0, 1); dy <- Seq(-1, 0, 1)) yield GridPosition(pos.x + dx, pos.y + dy)).toSet

    def neighboursCount(pos: GridPosition) =
      (neighbourhood(pos) - pos)
        .foldLeft(0) { case (sum, pos) => if (current.contains(pos)) sum + 1 else sum }

    current
      .flatMap(neighbourhood)
      .filter { pos => Cell.willLive(isAlive = current.contains(pos), neighboursCount(pos)) }
  }
}