/// <summary>
/// Expand the tree out to our maximum depth, only expanding leaf nodes with the best-possible score
/// </summary>
/// <param name="maxDepth"></param>
public void Expand(int maxDepth)
{
NumLeaves = 0;
ConsideredBoardStates = new List <TakBoard>();
List <TakMove> nextMoves = PlayerAI.EnumerateMoves(MyColor, Root.InitialState);
foreach (TakMove m in nextMoves)
{
MoveNode n = new MoveNode(Root.InitialState, m);
if (!ConsideredBoardStates.Contains(n.ResultingState.Board))
{
ConsideredBoardStates.Add(n.ResultingState.Board);
n.Depth = 1;
n.Parent = null;
n.Score = n.ResultingState.Evaluate(MyColor);
Root.Children.Add(n);
}
}
// recursively expand the tree
MaxDepth = maxDepth;
foreach (MoveNode n in Root.Children)
{
n.Score = ABPrune(n, (double)int.MinValue - 1, (double)int.MaxValue + 1, true);
}
Root.Children.Sort();
PrintDebug(string.Format("Explored {0} board states", ConsideredBoardStates.Count));
PrintDebug(string.Format("Explored {0} leaves", NumLeaves));
}
/// <summary>
/// Expand a node using AB pruning
/// </summary>
/// <param name="root"></param>
/// <returns></returns>
private double ABPrune(MoveNode root, double alpha, double beta, bool isMaxing)
{
// don't re-expand if we hit an automatic win or loss, or if we're at the depth limit
if (root.Depth >= MaxDepth || root.ResultingState.GameOver)
{
NumLeaves++;
root.Score = root.ResultingState.Evaluate(MyColor);
return(root.Score);
}
else
{
List <TakMove> nextMoves = PlayerAI.EnumerateMoves(isMaxing ? MyColor : TheirColor, root.ResultingState);
double v;
if (isMaxing)
{
v = (double)int.MinValue - 1;
foreach (TakMove m in nextMoves)
{
MoveNode n = new MoveNode(root.ResultingState, m);
n.Depth = root.Depth + 1;
root.Children.Add(n);
n.Parent = root;
if (!ConsideredBoardStates.Contains(n.ResultingState.Board))
{
//Console.WriteLine("Considering\n{0}", n.ResultingState.Board);
ConsideredBoardStates.Add(n.ResultingState.Board);
v = Math.Max(v, ABPrune(n, alpha, beta, false));
alpha = Math.Max(alpha, v);
if (beta <= alpha)
{
break;
}
}
}
}
else
{
v = (double)int.MaxValue + 1;
foreach (TakMove m in nextMoves)
{
MoveNode n = new MoveNode(root.ResultingState, m);
n.Depth = root.Depth + 1;
root.Children.Add(n);
n.Parent = root;
if (!ConsideredBoardStates.Contains(n.ResultingState.Board))
{
//Console.WriteLine("Considering\n{0}", n.ResultingState.Board);
ConsideredBoardStates.Add(n.ResultingState.Board);
v = Math.Max(v, ABPrune(n, alpha, beta, true));
beta = Math.Min(beta, v);
if (beta <= alpha)
{
break;
}
}
}
}
root.Children.Sort();
if (isMaxing)
{
root.Children.Reverse();
}
return(v);
}
}
