/// <summary>
/// Performs the tree search.
/// </summary>
/// <param name="rootState"></param>
/// <param name="maxIterations"></param>
/// <param name="randomizeState"></param>
/// <returns></returns>
internal static TreeNode Search(PlayingState rootState, int maxIterations = int.MaxValue, bool randomizeState = true)
{
var rootNode = new TreeNode();
HandRandomizer randomizer = null;
if ((randomizeState) && (rootState.NeedRandomize()))
randomizer = new HandRandomizer(rootState);
for (var i = 0; i < maxIterations; i++)
{
var node = rootNode;
// 1. Determinize.
var state = rootState.CloneAndRandomize(randomizer);
// 2. Select.
var validMoves = state.GetValidMoves();
while ((validMoves != 0) && (node.GetUntriedMoves(validMoves) == 0)) // The node is fully expanded and non-terminal.
{
node = node.UcbSelectChild(validMoves);
state.DoMove(node.Move);
validMoves = state.GetValidMoves();
}
// 3. Expand.
var untriedMoves = node.GetUntriedMoves(validMoves);
if (untriedMoves != 0)
{
var randomMove = GetRandomMove(untriedMoves, state);
var activeIndex = state.ActiveIndex;
state.DoMove(randomMove);
node = node.AddChild(randomMove, activeIndex);
}
// 4. Simulate.
validMoves = state.GetValidMoves();
while (validMoves != 0)
{
var randomMove = GetRandomMove(validMoves, state);
state.DoMove(randomMove);
validMoves = state.GetValidMoves();
}
// 5. Backpropagate.
while (node != null)
{
node.UpdateStatistics(state);
node = node.Parent;
}
}
return rootNode.GetBestChild();
}
/// <summary>
/// Adds a child node to this node.
/// </summary>
/// <param name="move"></param>
/// <param name="activeIndex"></param>
/// <returns>A new child node.</returns>
internal TreeNode AddChild(Move move, int activeIndex)
{
var child = new TreeNode
{
Move = move,
ActiveIndex = activeIndex,
Parent = this
};
if (FirstChild == null)
{
FirstChild = child;
LastChild = child;
}
else
{
LastChild.NextSibling = child;
LastChild = child;
}
return child;
}