KillerTable.java

/*
    CuckooChess - A java chess program.
    Copyright (C) 2011  Peter Österlund, peterosterlund2@gmail.com

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package chess;

/**
 * Implement a table of killer moves for the killer heuristic.
 * @author petero
 */
public class KillerTable {
    /** There is one KTEntry for each ply in the search tree. */
    static final class KTEntry {
        public KTEntry() {
            move0 = move1 = 0;
        }
        int move0;
        int move1;
    }
    KTEntry[] ktList;

    /** Create an empty killer table. */
    public KillerTable() {
        ktList = new KTEntry[200];
        for (int i = 0; i < ktList.length; i++)
            ktList[i] = new KTEntry();
    }

    /** Add a killer move to the table. Moves are replaced on an LRU basis. */
    final public void addKiller(int ply, Move m) {
        if (ply >= ktList.length)
            return;
        int move = (short)(m.from + (m.to << 6) + (m.promoteTo << 12));
        KTEntry ent = ktList[ply];
        if (move != ent.move0) {
            ent.move1 = ent.move0;
            ent.move0 = move;
        }
    }

    /**
     * Get a score for move m based on hits in the killer table.
     * The score is 4 for primary   hit at ply.
     * The score is 3 for secondary hit at ply.
     * The score is 2 for primary   hit at ply - 2.
     * The score is 1 for secondary hit at ply - 2.
     * The score is 0 otherwise.
     */
    final public int getKillerScore(int ply, Move m) {
        int move = (short)(m.from + (m.to << 6) + (m.promoteTo << 12));
        if (ply < ktList.length) {
            KTEntry ent = ktList[ply];
            if (move == ent.move0) {
                return 4;
            } else if (move == ent.move1) {
                return 3;
            }
        }
        if ((ply - 2 >= 0) && (ply - 2 < ktList.length)) {
            KTEntry ent = ktList[ply - 2];
            if (move == ent.move0) {
                return 2;
            } else if (move == ent.move1) {
                return 1;
            }
        }
        return 0;
    }
}