/// <summary>
/// The constructor calculates the score if this is a leaf node. Otherwise it builds the rest
/// of the branch.
/// </summary>
/// <param name="game">The game state at this point in the path.</param>
/// <param name="move">The move taken to get to this game state.</param>
public MinMaxNode(Game game, Tuple <int, int> move = null)
{
this.Move = move;
this._game = game;
// If the game is over, set the scores relative to the X side. When evaluating for O later on,
// we will just negate the scores as needed.
if (this._game.IsXWin)
{
this._xScore = 1.0;
}
else if (this._game.IsOWin)
{
this._xScore = -1.0;
}
else if (this._game.IsTie)
{
this._xScore = 0.0;
}
else
{
// If we got here, it's because the game is not over. Walk the board and try each of the paths.
this._children = new List <MinMaxNode>();
for (int row = 0; row < 3; row++)
{
for (int column = 0; column < 3; column++)
{
if (this._game.Board.IsEmptyAt(row, column))
{
// If we can move here, then copy the game, simulate the move, and add the node to the children.
Game copy = this._game.Copy();
copy.Mark(row, column);
// NOTE: Due to the nature of Tic-Tac-Toe, we know that all boards of the same configuration are
// identical. To optimize the performance, this step caches each board state by its serialized string
// and just reuses identical boards instead of creating new ones and iterating their paths redundantly.
if (MinMaxNode._useCache && MinMaxNode._nodeCache.ContainsKey(copy.Board.Serialize()))
{
this._children.Add(MinMaxNode._nodeCache[copy.Board.Serialize()]);
}
else
{
MinMaxNode node = new MinMaxNode(copy, Tuple.Create(row, column));
this._children.Add(node);
if (MinMaxNode._useCache)
{
MinMaxNode._nodeCache.Add(copy.Board.Serialize(), node);
}
}
}
}
}
}
}
public Tuple <int, int> GetMove(Game game)
{
MinMaxNode minMax = new MinMaxNode(game);
return(minMax.GetMove(game.IsXTurn));
}
