// Implementation of hook method which performs the move on the actual
// Tic Tac Toe game board
protected override AbstractMCTSNode <Pos, CellState> DoMakeMove(Pos move)
{
// Get a copy of the game board
Board boardCopy = board.Copy();
// Perform the move
boardCopy.DoMove(move);
// Return a new node representing the new state of the board
return(new TicTacToeMCTSNode(boardCopy, move));
}
// Perform a playout / simulation using the given strategy
public override CellState Playout(Func <IList <Pos>, Pos> strategy)
{
// Do the playout on a copy of the board
Board boardCopy = board.Copy();
// Keep playing using the given strategy until an endgame is reached
while (!boardCopy.Status().HasValue)
{
// Use the given strategy to obtain a move
Pos move = strategy(ValidMovesFromBoard(boardCopy));
// Perform move
boardCopy.DoMove(move);
}
return(boardCopy.Status().Value);
}
// Perform a move
public void Move(Pos pos)
{
gameBoard.DoMove(pos);
}
// Process given board with ABNegamax
private (float score, Pos move) Negamax(Board board, int depth)
{
// Increment number of evaluations (recursive ABNegamax calls)
numEvals++;
// Check what's the status of this board
if (board.Status().HasValue)
{
// GAME OVER! Who won?
if (board.Status().Value == CellState.Undecided)
{
// It's a tie, return 0
return(0, Board.NoMove);
}
else if (board.Status().Value == board.Turn)
{
// Current player wins, return max heuristic value
return(heuristic.WinScore, Board.NoMove);
}
else
{
// The other player won, return min heuristic value
return(-heuristic.WinScore, Board.NoMove);
}
}
else if (depth == maxDepth)
{
// We reached the max depth, return the heuristic value for this
// board
return(heuristic.Evaluate(board, board.Turn), Board.NoMove);
}
else
{
// Game is not over and we haven't reached maximum depth, so let's
// recursively call Negamax on all possible moves
// Declare best move, which for now is no move at all
(float score, Pos move)bestMove =
(float.NegativeInfinity, Board.NoMove);
// Try to play on all board positions
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
// Get the current board position
Pos pos = new Pos(i, j);
// Only consider making a move at this position if it's
// not already occupied
if (board.GetStateAt(pos) == CellState.Undecided)
{
// Score for current move
float score;
// Make a virtual move at this position
board.DoMove(pos);
// Get score for this move
score = -Negamax(board, depth + 1).score;
// Undo the move we just evaluated
board.UndoMove();
// Is this the best move so far?
if (score > bestMove.score)
{
// If so, keep it
bestMove = (score, pos);
}
}
}
}
// Return the best move found among all the tested moves
return(bestMove);
}
}