1 import scala.util._

     2 

     3 def print_board(n: Int)(steps: List[(Int, Int)]): Unit = synchronized {

     4   for (i <- 0 until n) {

     5     for (j <- 0 until n) {

     6       print(f"${steps.indexOf((i, j))}%3.0f ")

     7     }

     8     println

     9   } 

    10   //readLine()

    11   System.exit(0)

    12 }

    13 

    14 def add_pair(x: (Int, Int))(y: (Int, Int)) = 

    15   (x._1 + y._1, x._2 + y._2)

    16 

    17 def is_legal(n: Int)(x: (Int, Int)) = 

    18   0 <= x._1 && 0 <= x._2 && x._1 < n && x._2 < n

    19 

    20 def moves(n: Int)(x: (Int, Int)): List[(Int, Int)] = {

    21   List((1, 2),(2, 1),(2, -1),(1, -2),

    22        (-1, -2),(-2, -1),(-2, 1),(-1, 2)).map(add_pair(x)).filter(is_legal(n))

    23 }

    24 

    25 def ordered_moves(n: Int)(steps: List[(Int, Int)])(x : (Int, Int)): List[(Int, Int)] = 

    26   moves(n)(x).sortBy((x: (Int, Int)) => moves(n)(x).filterNot(steps.contains(_)).length)

    27 

    28 moves(8)(1,3)

    29 ordered_moves(8)(Nil)(1,3)

    30 ordered_moves(8)(List((2, 4), (2, 6)))(1,3)

    31 

    32 // non-circle tour parallel

    33 def tour(n: Int)(steps: List[(Int, Int)]): Unit = {

    34   if (steps.length ==  n * n) // && moves(n)(steps.head).contains(steps.last))

    35     print_board(n)(steps)

    36   else 

    37     for (x <- ordered_moves(n)(steps)(steps.head); if (!steps.contains(x))) tour(n)(x :: steps)

    38 }

    39 

    40 val n = 21

    41 println(s"circle tour parallel fast: n = $n")

    42 

    43 val starts = for (i <- 0 until n; j <- 0 until n) yield (i, j)

    44 

    45 starts.par.foreach((x:(Int, Int)) => tour(n)(List(x)))