// 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 ();
}