// Implementation of mimimax with alpha-beta pruning.
private int minimaxAB(Node n, bool min, int a,
int b)
{
int alpha = a;
int beta = b;
if (n.isLeaf ()) {
return n.value (this._player);
}
Node child;
if (min) {
while (((child = n.getChild ()) != null) && !(alpha > beta)) {
int val = minimaxAB (child, false, alpha, beta);
if (val < beta) {
beta = val;
}
}
return beta;
} else {
while (((child = n.getChild ()) != null) && !(alpha > beta)) {
int val = minimaxAB (child, true, alpha, beta);
if (val > alpha) {
alpha = val;
}
}
return alpha;
}
}
/**
* Gets the next move of this player.
*
* @param board
* <code>Board</code> representation of the current game state.
* @param prev
* <code>Move</code> representing the previous move in the the
* game.
* @param color
* <code>int</code> representing the pieces this
* <code>Player</code> is playing with. One of
* <code>TicTacToe.NOUGHTS</code> or
* <code>TicTacToe.CROSSES</code>
* @return <code>Move</code> Next move of this player.
*/
public Move getMove(int[,] board, Move prev, int color)
{
this._player = color;
Node root = new Node (board, prev);
int max = int.MinValue;
Node child;
Node bestNode = null;
while ((child = root.getChild ()) != null) {
int val = minimaxAB (child, true, int.MinValue,
int.MaxValue);
if (val >= max) {
max = val;
bestNode = child;
}
}
return bestNode.move ();
}
/**
* Returns all the children of this node found so far. You probably won't
* want to call this method until you're sure you're done with getChild().
*
* @return <code>Node[]</code> of all known children for this node.
* @see getChild()
*/
public Node[] children()
{
Node[] temp = new Node[this.childNum + 1];
for (int i = 0; i < this.childNum + 1; i++) {
temp [i] = this._children [i];
}
return temp;
}
/**
* Gets the "next" child for this node, or <code>null</code> if all
* children of this node have already been found.
*
* @return <code>Node</code> Representation of child (one move on) in the
* game tree.
*/
public Node getChild()
{
bool firstColumn = true;
for (int x = this._x; x < 3; x++) {
int starter = 0;
if (firstColumn) {
starter = this._y;
}
for (int y = starter; y < 3; y++) {
Move m = new Move (x, y, this.player ());
if (Board.isEmpty (this._board, m)) {
int[,] newPos = Board.copy (this._board);
Board.placePiece (newPos, m);
Node child = new Node (newPos, m);
this._children [this.childNum++] = child;
if (y >= (Board.SIZE - 1)) {
this._x = (int)(x + 1);
this._y = 0;
} else {
this._x = x;
this._y = (int)(y + 1);
}
this._leaf = false;
this._leafdef = true;
return child;
}
}
firstColumn = false;
}
this._leaf = true;
this._leafdef = true;
return null;
}
