// 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);
}
}
