/// <summary>
/// Builds the tree
/// </summary>
/// <returns>tree</returns>
static MinimaxTree <Configuration> BuildTree()
{
// build root node
binContents.Clear();
binContents.Add(3); //3,2;2,1;1,0
binContents.Add(2); //2,2;2,2;0,0
binContents.Add(1); //1,1;0,0;0,0
Configuration rootConfiguration =
new Configuration(binContents);
// build complete tree
MinimaxTree <Configuration> tree =
new MinimaxTree <Configuration>(rootConfiguration);
LinkedList <MinimaxTreeNode <Configuration> > nodeList =
new LinkedList <MinimaxTreeNode <Configuration> >();
nodeList.AddLast(tree.Root);
while (nodeList.Count > 0)
{
MinimaxTreeNode <Configuration> currentNode =
nodeList.First.Value;
nodeList.RemoveFirst();
List <Configuration> children =
GetNextConfigurations(currentNode.Value);
foreach (Configuration child in children)
{
MinimaxTreeNode <Configuration> childNode =
new MinimaxTreeNode <Configuration>(
child, currentNode);
tree.AddNode(childNode);
nodeList.AddLast(childNode);
}
}
return(tree);
}
/// <summary>
/// Removes the given node as a child this node
/// </summary>
/// <param name="child">child to remove</param>
/// <returns>true if the child was removed, false otherwise</returns>
public bool RemoveChild(MinimaxTreeNode <T> child)
{
// only remove children in list
if (children.Contains(child))
{
child.Parent = null;
return(children.Remove(child));
}
else
{
return(false);
}
}
/// <summary>
/// Assigns minimax scores to the tree nodes
/// </summary>
/// <param name="tree">tree to mark with scores</param>
/// <param name="maximizing">whether or not we're maximizing</param>
static void Minimax(MinimaxTreeNode <Configuration> tree,
bool maximizing)
{
// recurse on children
IList <MinimaxTreeNode <Configuration> > children = tree.Children;
if (children.Count > 0)
{
foreach (MinimaxTreeNode <Configuration> child in children)
{
// toggle maximizing as we move down
Minimax(child, !maximizing);
}
// set default node minimax score
if (maximizing)
{
tree.MinimaxScore = int.MinValue;
}
else
{
tree.MinimaxScore = int.MaxValue;
}
// find maximum or minimum value in children
foreach (MinimaxTreeNode <Configuration> child in children)
{
if (maximizing)
{
// check for higher minimax score
if (child.MinimaxScore > tree.MinimaxScore)
{
tree.MinimaxScore = child.MinimaxScore;
}
}
else
{
// minimizing, check for lower minimax score
if (child.MinimaxScore < tree.MinimaxScore)
{
tree.MinimaxScore = child.MinimaxScore;
}
}
}
}
else
{
// leaf nodes are the base case
AssignMinimaxScore(tree, maximizing);
}
}
/// <summary>
/// Clears all the nodes from the tree
/// </summary>
void Clear()
{
// remove all the children from each node
// so nodes can be garbage collected
foreach (MinimaxTreeNode <T> node in nodes)
{
node.Parent = null;
node.RemoveAllChildren();
}
// now remove all the nodes from the tree and set root to null
for (int i = nodes.Count - 1; i >= 0; i--)
{
nodes.RemoveAt(i);
}
root = null;
}
/// <summary>
/// Assigns a minimax score to the given node
/// </summary>
/// <param name="node">node to mark with score</param>
static void AssignMinimaxScore(
MinimaxTreeNode <Configuration> node,
bool maximizing)
{
// for the lecture, only score end-of-game configurations
if (node.Value.Empty)
{
if (maximizing)
{
// player 2 took the last teddy
node.MinimaxScore = 1;
}
else
{
// player 1 took the last teddy
node.MinimaxScore = 0;
}
}
}
/// <summary>
/// Removes the given node from the tree. If the node isn't
/// found in the tree, the method returns false.
///
/// Note that the subtree with the node to remove as its
/// root is pruned from the tree
/// </summary>
/// <param name="removeNode">node to remove</param>
/// <returns>true if the node is removed, false otherwise</returns>
public bool RemoveNode(MinimaxTreeNode <T> removeNode)
{
if (removeNode == null)
{
return(false);
}
else if (removeNode == root)
{
// removing the root clears the tree
Clear();
return(true);
}
else
{
// remove as child of parent
bool success = removeNode.Parent.RemoveChild(removeNode);
if (!success)
{
return(false);
}
// remove node from tree
success = nodes.Remove(removeNode);
if (!success)
{
return(false);
}
// check for branch node
if (removeNode.Children.Count > 0)
{
// recursively prune subtree
IList <MinimaxTreeNode <T> > children = removeNode.Children;
for (int i = children.Count - 1; i >= 0; i--)
{
RemoveNode(children[i]);
}
}
return(true);
}
}
/// <summary>
/// Adds the given node as a child this node
/// </summary>
/// <param name="child">child to add</param>
/// <returns>true if the child was added, false otherwise</returns>
public bool AddChild(MinimaxTreeNode <T> child)
{
// don't add duplicate children
if (children.Contains(child))
{
return(false);
}
else if (child == this)
{
// don't add self as child
return(false);
}
else
{
// add as child and add self as parent
children.Add(child);
child.Parent = this;
return(true);
}
}
/// <summary>
/// Adds the given node to the tree. If the given node is
/// null the method returns false. If the parent node is null
/// or isn't in the tree the method returns false. If the given
/// node is already a child of the parent node the method returns
/// false
/// </summary>
/// <param name="node">node to add</param>
/// <returns>true if the node is added, false otherwise</returns>
public bool AddNode(MinimaxTreeNode <T> node)
{
if (node == null ||
node.Parent == null ||
!nodes.Contains(node.Parent))
{
return(false);
}
else if (node.Parent.Children.Contains(node))
{
// node already a child of parent
return(false);
}
else
{
// add child as tree node and as a child to parent
nodes.Add(node);
return(node.Parent.AddChild(node));
}
}
/// <summary>
/// Executes minimax search
/// </summary>
/// <param name="args">command-line arguments</param>
static void Main(string[] args)
{
// build and mark the tree with minimax scores
MinimaxTree <Configuration> tree = BuildTree();
Minimax(tree.Root, true);
// find child node with maximum score
IList <MinimaxTreeNode <Configuration> > children =
tree.Root.Children;
MinimaxTreeNode <Configuration> maxChildNode = children[0];
for (int i = 1; i < children.Count; i++)
{
if (children[i].MinimaxScore > maxChildNode.MinimaxScore)
{
maxChildNode = children[i];
}
}
Console.WriteLine("Best move is to configuration " +
maxChildNode.Value);
Console.WriteLine();
}
/// <summary>
/// Constructor
/// </summary>
/// <param name="value">value for the node</param>
/// <param name="parent">parent for the node</param>
public MinimaxTreeNode(T value, MinimaxTreeNode <T> parent)
{
this.value = value;
this.parent = parent;
children = new List <MinimaxTreeNode <T> >();
}
/// <summary>
/// Constructor
/// </summary>
/// <param name="value">value of the root node</param>
public MinimaxTree(T value)
{
root = new MinimaxTreeNode <T>(value, null);
nodes.Add(root);
}