// Part 3 about finding a single tour using the Warnsdorf Rule

//=============================================================

type Pos = (Int, Int)

type Path = List[Pos]

def print_board(dim: Int, path: Path): Unit = {

  println

  for (i <- 0 until dim) {

    for (j <- 0 until dim) {

      print(f"${path.reverse.indexOf((i, j))}%3.0f ")

    }

    println

  } 

}

def add_pair(x: Pos)(y: Pos): Pos = 

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

def is_legal(dim: Int, path: Path)(x: Pos): Boolean = 

  0 <= x._1 && 0 <= x._2 && x._1 < dim && x._2 < dim && !path.contains(x)

def moves(x: Pos): List[Pos] = 

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

       (-1, -2),(-2, -1),(-2,  1),(-1,  2)).map(add_pair(x))

def legal_moves(dim: Int, path: Path, x: Pos): List[Pos] = 

  moves(x).filter(is_legal(dim, path))

def ordered_moves(dim: Int, path: Path, x: Pos): List[Pos] = 

  legal_moves(dim, path, x).sortBy((x) => legal_moves(dim, path, x).length)

import scala.annotation.tailrec

/*

@tailrec

def first(xs: List[Pos], f: Pos => Option[Path]): Option[Path] = xs match {

  case Nil => None

  case x::xs => {

    val result = f(x)

    if (result.isDefined) result else first(xs, f)

  }

}

*/

def first[A, B](xs: List[A], f: A => Option[B]): Option[B] =

  xs.flatMap(f(_)).headOption

def first_closed_tour_heuristics(dim: Int, path: Path): Option[Path] = {

  if (path.length == dim * dim && moves(path.head).contains(path.last)) Some(path)

  else

    first(ordered_moves(dim, path, path.head), (x: Pos) => first_closed_tour_heuristics(dim, x::path))

}

/*

for (dim <- 1 to 6) {

  val t = first_closed_tour_heuristics(dim, List((dim / 2, dim / 2)))

  println(s"${dim} x ${dim} closed: " + (if (t == None) "" else { print_board(dim, t.get) ; "" }))

}

*/

//@tailrec

def first_tour_heuristics(dim: Int, path: Path): Option[Path] = {

  @tailrec

  def aux(dim: Int, path: Path, moves: List[Position]): Option[Path 

  if (path.length == dim * dim) Some(path)

  else

    moves match {

      case Nil => None

      case x::xs => {

        val r = first_tour_heuristics(dim, x::path)

        if (r.isDefined) Some(r) else aux(dim, path, xs)

    }    

  aux(dim, path, ordered_moves(dim, path, path.head)) 

}

/*

def first_tour_heuristics(dim: Int, path: Path): Option[Path] = {

  if (path.length == dim * dim) Some(path)

  else

    for (p <- ordered_moves(dim, path, path.head))

      val r = first_tour_heuristics(dim, x::path)

    //first(ordered_moves(dim, path, path.head), (x: Pos) => first_tour_heuristics(dim, x::path))

    ordered_moves(dim, path, path.head).view.flatMap((x: Pos) => first_tour_heuristics(dim, x::path)).headOption

}

*/ 

/*

for (dim <- 1 to 50) {

  val t = first_tour_heuristics(dim, List((dim / 2, dim / 2)))

  println(s"${dim} x ${dim}: " + (if (t == None) "" else { print_board(dim, t.get) ; "" }))

}

*/

print_board(50, first_tour_heuristics(50, (25,25)::Nil).get)