//01 function alphabeta(node, depth, α, β, maximizingPlayer)
//02 if depth = 0 or node is a terminal node
//03 return the heuristic value of node
//04 if maximizingPlayer
//05 v := -∞
//06 for each child of node
//07 v := max(v, alphabeta(child, depth – 1, α, β, FALSE))
//08 α := max(α, v)
//09 if β ≤ α
//10 break (* β cut-off *)
//11 return v
//12 else
//13 v := ∞
//14 for each child of node
//15 v := min(v, alphabeta(child, depth – 1, α, β, TRUE))
//16 β := min(β, v)
//17 if β ≤ α
//18 break (* α cut-off *)
//19 return v
//
//Initial - alphabeta(origin, depth, -∞, +∞, TRUE)
public GameTree MinimaxAlphaBeta(GameTree node, int depth, double alpha, double beta, bool maximizingPlayer)
{
bool terminal = node.Children.Count() == 0;
if (depth == 0 || terminal)
{
node.Score = Evaluation.Evaluate(node.State, PlayerNumber, OpponentNumber);
return(node);
}
GameTree gameTreeScore = new GameTree();
if (maximizingPlayer)
{
gameTreeScore.Score.Score = double.MinValue;
foreach (GameTree child in node.Children)
{
GameTree childScore = MinimaxAlphaBeta(child, depth - 1, alpha, beta, false);
if (childScore.Score.Score >= gameTreeScore.Score.Score)
{
gameTreeScore = childScore;
}
alpha = Math.Max(alpha, gameTreeScore.Score.Score);
if (beta <= alpha)
{
break;
}
}
return(gameTreeScore);
}
else
{
gameTreeScore.Score.Score = double.MaxValue;
foreach (GameTree child in node.Children)
{
GameTree childScore = MinimaxAlphaBeta(child, depth - 1, alpha, beta, true);
if (childScore.Score.Score <= gameTreeScore.Score.Score)
{
gameTreeScore = childScore;
}
beta = Math.Min(beta, gameTreeScore.Score.Score);
if (beta <= alpha)
{
break;
}
}
return(gameTreeScore);
}
}
string compute()
{
//
GameState currentState = States[States.Count() - 1];
//
int dof = currentState.GetDOF(PlayerNumber);
if (dof > 1)
{
//
Timer.Reset();
Timer.Start();
//
string opponentDebug = "";
int opponent = currentState.GetOpponent(PlayerNumber, PlayerCount, ref opponentDebug);
debug(opponentDebug.Trim());
int depth = 3;
//SpanTree(null, current, playerNumber, opponentNumber, 3 (me them me), 0 (current))
GameTree root = GameTree.SpanTree(null, currentState, PlayerNumber, opponent, depth, 0);
//alphabeta(origin, depth, -∞, +∞, TRUE)
TronMinimax tronMinimax = new TronMinimax(PlayerNumber, opponent);
GameTree next = tronMinimax.MinimaxAlphaBeta(root, depth, double.MinValue, double.MaxValue, true);
//
string vis = currentState.GetVisual(next.Score.TerritoryPlayer, next.Score.TerritoryOpponent);
debug("Score - " + next.Score.Score);
//
Timer.Stop();
long time = Timer.ElapsedMilliseconds;
debug("Time elapsed - " + time + "ms");
debug(vis);
//
return(next.Label.Trim('>').Split('>')[0]);
}
else
{
Point target = currentState.GetAnyPoint(PlayerNumber);
Direction dir = GameState.GetDirection(currentState.Players[PlayerNumber], target);
return(dir.ToString());
}
}
//Initial - SpanTree(null, current, playerNumber, opponentNumber, 3 (me them me), 0 (current))
public static GameTree SpanTree(GameTree ancestor, GameState state, int playerNumber, int opponentNumber, int depth, int ply)
{
GameTree gameTree = new GameTree();
gameTree.Ancestor = ancestor;
gameTree.State = state;
gameTree.Label = ancestor == null ? "" : (ancestor.Label + ">" + GameState.GetDirection(
ancestor.State.Players[(ply % 2 == 1) ? playerNumber : opponentNumber],
state.Players[(ply % 2 == 1) ? playerNumber : opponentNumber]).ToString());
gameTree.Ply = ply;
if (depth > 0)
{
int currentPly = ply + 1;
int currentPlayer = (currentPly % 2 == 1) ? playerNumber : opponentNumber;
List <GameState> childStates = state.GetChildren(currentPlayer);
foreach (GameState childState in childStates)
{
gameTree.Children.Add(SpanTree(gameTree, childState, playerNumber, opponentNumber, depth - 1, currentPly));
}
}
return(gameTree);
}
