/// <summary>
/// Expands a selected node, if possible.
/// </summary>
/// <param name="id">The selected node.</param>
/// <param name="tree">The game tree object.</param>
/// <returns></returns>
private static int Expand(int id, GameTree tree)
{
// Get the node object that corresponds with the input ID.
GameTreeNode node = tree.GetNode(id);
// If the node has at least one unplayed child, is not a dead end, and is not a goal state.
if (node.Unplayed.Count > 0 && !node.DeadEnd && !node.IsGoal)
{
// Choose an unplayed child at random.
GameTreeEdge outgoing = node.Unplayed.PickRandom <GameTreeEdge>();
// Remove the child from the list of unplayed edges.
node.Unplayed.Remove(outgoing);
// Create the node object and store the child's ID.
int child = tree.CreateNode(node.Domain, node.Problem, outgoing).ID;
// Save the parent node to disk.
tree.SetNode(node);
// Return the child ID.
return(child);
}
// Return the leaf node.
return(node.ID);
}
/// <summary>
/// Given a number of plays and a game tree object, performs MCTS.
/// </summary>
/// <param name="plays">The number of MCTS plays to perform.</param>
/// <param name="tree">The game tree object on which to perform the search.</param>
public static void Search(int plays, GameTree tree)
{
// Loop for the number of input play throughs.
for (int i = 0; i < plays; i++)
{
// Select a leaf node.
int select = Select(0, tree);
// Expand the leaf node.
int expand = Expand(select, tree);
// Simulate a roll out from the leaf node.
bool result = Simulate(expand, tree);
// Propagate the result of the roll out back up the tree.
Propagate(result, expand, tree);
}
}
/// <summary>
/// Selects a leaf node to expand.
/// </summary>
/// <param name="id">The ID of the current node.</param>
/// <param name="tree">The game tree object.</param>
/// <returns></returns>
private static int Select(int id, GameTree tree)
{
// Get the node object that corresponds to the current ID.
GameTreeNode node = tree.GetNode(id);
// If the node has not been played before, has at least one outgoing edge, is not a dead end, and is not a goal state...
if (node.Unplayed.Count == 0 && node.Outgoing.Count > 0 && !node.DeadEnd && !node.IsGoal)
{
// Create a variable to store the child with the highest interval.
int highestChild = -1;
// If it's not the player's turn...
if (!tree.TurnOrder[node.Depth % tree.TurnOrder.Count].Equals(tree.Player))
{
// Set the highest child to be the first child.
highestChild = node.Outgoing[0].Child;
// Loop through every remaining child.
for (int i = 1; i < node.Outgoing.Count; i++)
{
// If the current child has a higher interval than the stored child.
if (tree.GetInterval(node.Outgoing[i].Child) > tree.GetInterval(highestChild))
{
// Store the current child.
highestChild = node.Outgoing[i].Child;
}
}
}
// If it's the player's turn, chose an outgoing edge at random.
else
{
highestChild = node.Outgoing.PickRandom <GameTreeEdge>().Child;
}
// Recursively call this method with the selected child.
return(Select(highestChild, tree));
}
// Return the leaf node.
return(node.ID);
}
/// <summary>
/// Propagates a roll out's result back through the tree.
/// </summary>
/// <param name="result">Whether the roll out resulted in a win or loss.</param>
/// <param name="node">The current node.</param>
/// <param name="tree">The game tree object.</param>
private static void Propagate(bool result, int node, GameTree tree)
{
// Get the current node's parent ID.
int parent = tree.GetParent(node);
// Loop until we hit the root node's null parent link.
while (parent != -1)
{
// Get the parent's node object from the game tree.
GameTreeNode parentNode = tree.GetNode(parent);
// Add the win/loss to the parent's node object.
parentNode.AddResult(result);
// Save the parent's node object to disk.
tree.SetNode(parentNode);
// Set the parent ID to the grandparent ID.
parent = tree.GetParent(parent);
}
}
/// <summary>
/// Given a node, simulate a roll out.
/// </summary>
/// <param name="id">The ID of the selected node.</param>
/// <param name="tree">The game tree object.</param>
/// <returns></returns>
private static bool Simulate(int id, GameTree tree)
{
// Ask the tree to simulate a roll out and return the result.
return(tree.Simulate(id));
}