package com.rumi

object GameOfLife {

  /**
   * Calculate the next iteration of the Game of Life. The game state is
   * represented by a Set[(Int,Int)] of alive cells. A new set is generated
   * for every generation. Coordinates count from (1,1) to (w,h), while the
   * (1,1) cell is in the top left corner.
   *
   * @param cells The current set of alive cells
   * @param xdim The width dimension of the playing field.
   * @param ydim The height dimension of the playing field.
   * @return The next generation of cells
   */
  def nextIteration(cells: Set[(Int,Int)], xdim: Int, ydim: Int): Set[(Int,Int)] = {
    val allCells = for {
      ox <- 1 to xdim
      oy <- 1 to ydim
    } yield {

      val ac: Int = aliveNeighbourCount(cells, xdim, ydim, ox, oy)

      if (cells.contains((ox, oy))) {            // if this cell is alive
        if (ac < 2) (0, 0)                       // and it has < 2 live neighbours, -> die
        else if (ac == 2 || ac == 3) (ox, oy)    // and it has 2 or 3 live neightbours, -> continue to live
        else (0, 0)                              // more than 3 live neighbours -> die
      }
      else if (ac == 3) {                        // if the cell is dead and it has exactly 3 live neighboutrs
        (ox, oy)                                 // make it alive
      }
      else {
        (0, 0)                                   // everything else -> dead cell
      }

    }

    allCells.filterNot(_ equals (0,0)).toSet
  }

  /**
   * Counts the number of alive cells neighbouring the cell with the given
   * coordinates.
   *
   * @param cells The Set of all alive cells
   * @param xdim The width dimension of the playing field.
   * @param ydim The height dimension of the playing field.
   * @param ox X Coordinate of the cell under test
   * @param oy Y Coordinate of the cell under test
   * @return
   */
  def aliveNeighbourCount(cells: Set[(Int, Int)], xdim: Int, ydim: Int, ox: Int, oy: Int): Int = {
    val aliveCount = (for {
      nx <- Math.max(1, ox - 1) to Math.min(ox + 1, xdim)
      ny <- Math.max(1, oy - 1) to Math.min(oy + 1, ydim)
      if !(nx == ox && ny == oy) && cells.contains((nx, ny))
    } yield 1).sum
    aliveCount
  }

  def main(args: Array[String]): Unit = {

    // The famous "Beacon" starting pattern
    val startCells = Set((2,2), (3,2), (2,3), (5,4), (5,5), (4,5))

    // We create an infinite stream of game iterations but only take 10 of them
    // and print the resulting game states.
    val game = Stream.iterate(startCells){ ni => nextIteration(ni, 6,6) }

    game.take(10).foreach { citer =>
      for {
        x <- 1 to 6
      } {
        for (y <- 1 to 6) {
          if (citer.contains((x,y))) print("X")
          else print(".")
        }
        println()
      }
      println()
    }
    
  }
}