/// <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>
/// 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);
}
}
