public Move GetBestMove()
{
Move bestMove = Move.Invalid;
int bestScore = int.MinValue;
System.Diagnostics.Debug.Assert(Children.Count > 0, "Invoke 'Expand' before calling this method");
foreach (var child in Children)
{
if (child.Score > bestScore)
{
bestMove = child.Move;
bestScore = child.Score;
}
else if (child.Score == bestScore)
{
// TODO: Handle the case where the expanded tree doesn't contain a winner
// TODO: #1 prefer to connect with other pieces
// TODO: #2 prefer to create line with open spaces on both ends
// TODO: #3 prefer a move that will win if opponent doesn't make best play
// TOOD: for now prefer middle columns (more likely to fulfill #2)
int midpoint = Game.Width / 2;
int adjustedBestScore = midpoint - Math.Abs(bestMove.X - midpoint);
int adjustedChildScore = midpoint - Math.Abs(child.Move.X - midpoint);
if (adjustedChildScore > adjustedBestScore)
{
bestMove = child.Move;
}
}
}
System.Diagnostics.Debug.Assert(bestMove != Move.Invalid);
return(bestMove);
}
public static void MakeMove(C4Game game)
{
GameTree tree = new GameTree(game);
tree.Expand(8);
Move bestMove = tree.GetBestMove();
game.RecordMove(bestMove);
}
private void Expand(int depthLimit, Node node, State state, int depth)
{
System.Diagnostics.Debug.Assert(depthLimit > 0);
if (node != null)
{
var winner = Game.CheckWinner(state);
// Check to end recursion
if (winner != null)
{
if (winner == Game.CurrentPlayer)
{
node.Score = int.MaxValue - depth; // win
}
else if (winner == Player.Invalid)
{
node.Score = 0; // draw
}
else
{
node.Score = int.MinValue + depth; // lose
}
return;
}
else if (depth == depthLimit)
{
node.Score = 0;
return;
}
}
var children = node != null ? node.Children : Children;
var playerAtDepth = (Game.CurrentPlayer + depth) % Game.PlayerCount;
for (int i = 0; i < Game.Width; ++i)
{
for (int j = Game.Height - 1; j >= 0; --j)
{
var move = new Move(i, j);
if (state[move] == Player.Invalid)
{
var child = new Node(move);
children.Add(child);
state[move] = playerAtDepth;
Expand(depthLimit, child, state, depth + 1);
state[move] = Player.Invalid;
// there's only one move per column
break;
}
}
}
System.Diagnostics.Debug.Assert(children.Count > 0, "there should have been an available move");
// Score the node
if (node != null)
{
var depthPlayerIsMe = playerAtDepth == Game.CurrentPlayer;
int score = depthPlayerIsMe ? int.MinValue : int.MaxValue;
foreach (var child in node.Children)
{
if (depthPlayerIsMe)
{
score = Math.Max(score, child.Score);
}
else
{
score = Math.Min(score, child.Score);
}
}
node.Score = score;
node.Children = null; // free the memory, we're done with it
}
}
public Node(Move move)
{
Move = move;
}