Alpha-Beta move generator can look 2 plays (4 plies) ahead on a 4x4 board and blocks every possible attempt by the player to connect 3.

It should be possible to play on larger boards when the computers 'move' is changed from playing a tile to picking the player's next available color.

src/controller/BoardPanelMouseListener.java

@@ -5,7 +5,8 @@ import java.awt.event.MouseListener;
 import java.awt.event.MouseWheelEvent;
 import java.awt.event.MouseWheelListener;
 
-import model.HumanPlayer;
+import player.HumanPlayer;
+
 
 import view.Tile;
 import view.TileSelectionPanel;

src/controller/TSPMouseListener.java

@@ -3,9 +3,10 @@ package controller;
 import java.awt.event.MouseEvent;
 import java.awt.event.MouseListener;
 
+import player.HumanPlayer;
+
 import view.TileSelectionPanel;
 
-import model.HumanPlayer;
 import model.Board.TileColor;
 
 public class TSPMouseListener implements MouseListener{

src/model/Board.java

@@ -8,12 +8,15 @@ public class Board {
 		BLUE, GREEN, NONE, RED, YELLOW
 	}
 
-	public static final int NUM_COLS = 5;
-	public static final int NUM_ROWS = 5;
+	public static final int NUM_COLS = 4;
+	public static final int NUM_ROWS = 4;
 	public static final int ROW_REMOVAL_SIZE = 3;
 
 	private final TileColor[][] board;
 
+	// a Ply is a play by 1 side. Increments each time setTile() is called.
+	private int numPlies = 0;
+
 	public Board(Board that) {
 		this();
 		for (int i = 0; i < NUM_COLS; i++) {
@@ -22,17 +25,16 @@ public class Board {
 			}
 		}
 	}
-	
+
 	public Board() {
 		board = new TileColor[NUM_ROWS][NUM_COLS];
 		for (int r = 0; r < NUM_ROWS; r++) {
 			for (int c = 0; c < NUM_COLS; c++) {
-				setTile(new CellPointer(r, c), TileColor.NONE);
+				board[r][c] = TileColor.NONE;
 			}
 		}
 	}
 
-	
 	@Override
 	public int hashCode() {
 		final int prime = 31;
@@ -50,7 +52,7 @@ public class Board {
 		if (getClass() != obj.getClass())
 			return false;
 		Board other = (Board) obj;
-		
+
 		for (int r = 0; r < NUM_ROWS; r++) {
 			for (int c = 0; c < NUM_COLS; c++) {
 				if (this.board[r][c] != other.board[r][c]) {
@@ -58,7 +60,7 @@ public class Board {
 				}
 			}
 		}
-		
+
 		return true;
 	}
 
@@ -66,7 +68,31 @@ public class Board {
 		return board[r][c];
 	}
 
-	public void setTile(CellPointer cp, TileColor tile) {
+	public int getTurn() {
+		return numPlies / 2;
+	}
+
+	public boolean isPlayerTurn() {
+		return numPlies % 2 == 0;
+	}
+
+	public boolean isTerminalState() {
+		for (int r = 0; r < Board.NUM_ROWS; r++) {
+			for (int c = 0; c < Board.NUM_COLS; c++) {
+				if (board[r][c] == TileColor.NONE) {
+					return false;
+				}
+			}
+		}
+
+		return true;
+	}
+
+	public boolean playTile(CellPointer cp, TileColor tile) {
+		if (board[cp.r][cp.c] != TileColor.NONE) {
+			return false; //illegal move
+		}
+				
 		board[cp.r][cp.c] = tile;
 
 		if (tile != TileColor.NONE) {
@@ -199,5 +225,8 @@ public class Board {
 				board[cell.r][cell.c] = TileColor.NONE;
 			}
 		}
+
+		numPlies++;
+		return true;
 	}
-}
+}

src/model/BoardScorer.java

@@ -0,0 +1,24 @@
+package model;
+
+/**
+ * Scorer for use by various ComPlayer implementations.
+ * 
+ * @author Woody
+ *
+ */
+public class BoardScorer {
+
+	public int getMaxScore(Board board) {
+		return Board.NUM_ROWS * Board.NUM_COLS;
+	}
+
+	public int getScore(Board board) {
+		int score = 0;
+		for (int r = 0; r < Board.NUM_ROWS; r++) {
+			for (int c = 0; c < Board.NUM_COLS; c++) {
+				score += board.getTile(r, c) == Board.TileColor.NONE ? 0 : 1;
+			}
+		}
+		return score;
+	}
+}

src/model/Move.java

@@ -3,9 +3,19 @@ package model;
 import model.Board.TileColor;
 
 public class Move {
+	/**
+	 * Used when genMove() is called but no valid move exists for that player.
+	 */
+	public static final Move NONE = new Move(TileColor.NONE, -1, -1);
+	
 	private final TileColor color;
 	private final CellPointer cp;
 
+	public Move(TileColor color, int row, int column) {
+		cp = new CellPointer(row,column);
+		this.color = color;
+	}
+	
 	public Move(CellPointer cllPntr, TileColor tlClr) {
 		cp = cllPntr;
 		color = tlClr;

src/model/Referee.java

@@ -1,7 +1,13 @@
 package model;
 
+import org.apache.log4j.Logger;
+
+import player.AlphaBetaComPlayer;
+import player.HumanPlayer;
+import player.Player;
 import view.BoardPanel;
 import view.MessagePanel;
+import view.ScorePanel;
 import model.Board.TileColor;
 
 public class Referee implements Runnable {
@@ -10,26 +16,28 @@ public class Referee implements Runnable {
 	public static final String GAME_OVER = "Game over!";
 	public static final String PLAYER_TURN = "Waiting for the player's move.";
 
+	public static final Logger LOGGER = Logger.getLogger(Referee.class
+			.getName());
+
 	private final Board board;
 	private final HumanPlayer humanPlayer = new HumanPlayer();
-	private final Player cc;
+	private final Player computerPlayer;
 
-	private boolean playerTurn;
 	private BoardPanel boardPanel;
-	private int score = 0;
 	private MessagePanel messagePanel;
+	private ScorePanel scorePanel;
 
 	public Referee() {
 		board = new Board();
-		cc = new RandomComPlayer();
-		playerTurn = true;
+		computerPlayer = new AlphaBetaComPlayer();
 	}
 
 	@Override
 	public void run() {
 		int plies = 0;
 		while (!isOver()) {
-			if (playerTurn) {
+			scorePanel.updateScore(getPlayerScore());
+			if (board.isPlayerTurn()) {
 				boolean wasInterrupted = false;
 				while (!humanPlayer.isReady()) {
 					try {
@@ -39,7 +47,8 @@ public class Referee implements Runnable {
 					}
 				}
 				if (wasInterrupted) {
-					System.out.println("Interrupted while waiting for human to move!");
+					System.out
+							.println("Interrupted while waiting for human to move!");
 				} else {
 					Move mv = humanPlayer.getMove(board);
 					if (board.getTile(mv.getCell().r, mv.getCell().c) == TileColor.NONE) {
@@ -49,79 +58,65 @@ public class Referee implements Runnable {
 					}
 				}
 			} else {
-				Move mv = cc.getMove(board);
+				Move mv = computerPlayer.getMove(board);
 				playToken(mv);
 			}
-			
+
 			messagePanel.updateMessage(getMessage());
 			boardPanel.updateIcons();
-			System.out.println("plies: " + plies++);
+			LOGGER.info("plies: " + plies++);
 			try {
 				Thread.sleep(1000L);
 			} catch (InterruptedException ie) {
-				System.out.println("Interrupted while waiting for human view ply!");
+				System.out
+						.println("Interrupted while waiting for human view ply!");
 			}
 		}
 	}
 
 	public Player getComputerPlayer() {
-		return cc;
+		return computerPlayer;
 	}
 
 	public HumanPlayer getHumanPlayer() {
 		return humanPlayer;
 	}
-	
+
 	public String getMessage() {
 		if (isOver()) {
 			return GAME_OVER;
-		} else if (isPlayersTurn()) {
+		} else if (board.isPlayerTurn()) {
 			return PLAYER_TURN;
 		} else {
 			return COM_TURN;
 		}
 	}
-	
-	public boolean isOver() {
-		for (int r = 0; r < Board.NUM_ROWS; r++) {
-			for (int c = 0; c < Board.NUM_COLS; c++) {
-				if (board.getTile(r, c) == TileColor.NONE) {
-					return false;
-				}
-			}
-		}
 
-		return true;
+	public int getPlayerScore() {
+		return board.getTurn();
 	}
-	
-	private boolean isPlayersTurn() {
-		return playerTurn;
-	}
-	
-	public int getScore() {
-		return score;
-	}
-	
+
 	public TileColor getTile(int r, int c) {
 		return board.getTile(r, c);
 	}
 
+	public boolean isOver() {
+		return board.isTerminalState();
+	}
+
 	public void playToken(Move move) {
-
-		board.setTile(move.getCell(), move.getColor());
-
-		if (playerTurn) {
-			score++;
-		}
-
-		playerTurn = !playerTurn;
+		board.playTile(move.getCell(), move.getColor());
 	}
 
 	public void setBoardPanel(BoardPanel boardPanel) {
 		this.boardPanel = boardPanel;
 	}
-	
+
 	public void setMessagePanel(MessagePanel messagePanel) {
 		this.messagePanel = messagePanel;
 	}
-}
+
+	public void setScorePanel(ScorePanel scorePanel) {
+		this.scorePanel = scorePanel;
+	}
+}

src/player/AlphaBetaComPlayer.java

@@ -0,0 +1,25 @@
+package player;
+
+import model.Board;
+import model.Move;
+import player.generator.AlphaBetaMoveGenerator;
+import player.generator.MoveGenerator;
+
+public class AlphaBetaComPlayer implements Player {
+	private MoveGenerator moveGenerator = new AlphaBetaMoveGenerator();
+
+	@Override
+	public Move getMove(Board board) {
+		return moveGenerator.genMove(board, false);
+	}
+
+	@Override
+	public void denyMove() {
+		throw new UnsupportedOperationException("Not implemented");
+	}
+
+	@Override
+	public boolean isReady() {
+		return true; // always ready to play a random valid move
+	}
+}

src/player/HumanPlayer.java

@@ -1,5 +1,8 @@
-package model;
+package player;
 
+import model.Board;
+import model.CellPointer;
+import model.Move;
 import model.Board.TileColor;
 
 

src/player/MinimaxComPlayer.java

@@ -0,0 +1,5 @@
+package player;
+
+public class MinimaxComPlayer {
+
+}

src/player/MonteCarloComPlayer.java

@@ -1,7 +1,10 @@
-package model;
+package player;
 
 import java.util.Random;
 
+import model.Board;
+import model.CellPointer;
+import model.Move;
 import model.Board.TileColor;
 
 

src/player/Player.java

@@ -1,4 +1,7 @@
-package model;
+package player;
+
+import model.Board;
+import model.Move;
 
 
 public interface Player {

src/player/RandomComPlayer.java

@@ -1,7 +1,10 @@
-package model;
+package player;
 
 import java.util.Random;
 
+import model.Board;
+import model.CellPointer;
+import model.Move;
 import model.Board.TileColor;
 
 

src/player/generator/AlphaBetaMoveGenerator.java

@@ -0,0 +1,132 @@
+package player.generator;
+
+import java.util.Arrays;
+import java.util.List;
+
+import model.Board;
+import model.BoardScorer;
+import model.Move;
+
+import org.apache.log4j.Logger;
+
+public class AlphaBetaMoveGenerator implements MoveGenerator {
+	private static final Logger LOGGER = Logger
+			.getLogger(AlphaBetaMoveGenerator.class.getName());
+	private static final int DEFAULT_RECURSIVE_PLAYS = 2;
+
+	private final BoardScorer scorer = new BoardScorer();
+	private final ValidMoveGenerator validMoveGenerator = new ValidMoveGenerator();
+
+	private Move bestPick = Move.NONE;
+
+	@Override
+	public Move genMove(Board board, boolean asHuman) {
+
+		int alpha = Integer.MIN_VALUE;
+		int beta = Integer.MAX_VALUE;
+
+		if (!asHuman) {
+			getMaxValue(board, asHuman, DEFAULT_RECURSIVE_PLAYS * 2, alpha,
+					beta);
+		} else {
+			getMinValue(board, asHuman, DEFAULT_RECURSIVE_PLAYS * 2, alpha,
+					beta);
+		}
+
+		return bestPick;
+	}
+
+	private int getMaxValue(Board board, boolean asHuman, int recursionLevel,
+			int alpha, int beta) {
+		if (terminalTest(recursionLevel)) {
+			return getUtility(board);
+		}
+
+		List<Move> validMoves = validMoveGenerator.genMoves(board, asHuman,
+				MoveGenerator.ALL_MOVES);
+
+		int value = Integer.MIN_VALUE;
+
+		for (Move nextMove : validMoves) {
+			Board nextBoard = new Board(board);
+
+			if (!nextBoard.playTile(nextMove.getCell(), nextMove.getColor())) {
+				throw new RuntimeException(
+						"Illegal move attempted during search!");
+			}
+
+			int minValue = getMinValue(nextBoard, !asHuman, recursionLevel - 1,
+					alpha, beta);
+
+			if (minValue > value) {
+				value = minValue;
+				if (recursionLevel == DEFAULT_RECURSIVE_PLAYS * 2) {
+					bestPick = nextMove;
+				}
+			}
+
+			if (value >= beta) {
+				return value;
+			}
+			alpha = Math.max(alpha, value);
+		}
+
+		return value;
+	}
+
+	private int getMinValue(Board board, boolean asHuman, int recursionLevel,
+			int alpha, int beta) {
+		if (terminalTest(recursionLevel)) {
+			return getUtility(board);
+		}
+
+		List<Move> validMoves = validMoveGenerator.genMoves(board, asHuman,
+				MoveGenerator.ALL_MOVES);
+
+		int value = Integer.MAX_VALUE;
+
+		for (Move nextMove : validMoves) {
+			Board nextBoard = new Board(board);
+
+			if (!nextBoard.playTile(nextMove.getCell(), nextMove.getColor())) {
+				throw new RuntimeException(
+						"Illegal move attempted during search!");
+			}
+
+			int maxValue = getMaxValue(nextBoard, !asHuman, recursionLevel - 1,
+					alpha, beta);
+
+			if (maxValue < value) {
+				value = maxValue;
+				if (recursionLevel == 2 * DEFAULT_RECURSIVE_PLAYS) {
+					bestPick = nextMove;
+				}
+			}
+
+			if (value <= alpha) {
+				return value;
+			}
+			beta = Math.min(beta, value);
+		}
+
+		return value;
+	}
+
+	private boolean terminalTest(int recursionLevel) {
+		return recursionLevel < 1;
+	}
+
+	private int getUtility(Board board) {
+		return scorer.getScore(board);
+	}
+
+	/**
+	 * AlphaBetaMoveGenerator2 does not support this method.
+	 */
+	@Override
+	public List<Move> genMoves(Board board, boolean asHuman, int nMoves) {
+		Move[] doNothing = new Move[] { Move.NONE };
+		LOGGER.info("Minimax genMoves() stub returning []");
+		return Arrays.asList(doNothing);
+	}
+}

src/player/generator/GameScore.java

@@ -0,0 +1,26 @@
+package player.generator;
+
+public class GameScore {
+	boolean terminal;
+	int maxScore;
+	int score;
+	
+	public GameScore(boolean terminal, int maxScore, int score) {
+		super();
+		this.terminal = terminal;
+		this.maxScore = maxScore;
+		this.score = score;
+	}
+
+	public boolean isTerminal() {
+		return terminal;
+	}
+
+	public int getMaxScore() {
+		return maxScore;
+	}
+
+	public int getScore() {
+		return score;
+	}
+}

src/player/generator/MoveCandidate.java

@@ -0,0 +1,13 @@
+package player.generator;
+
+import model.Move;
+
+public class MoveCandidate {
+	public final Move move;
+	public final GameScore score;
+
+	public MoveCandidate(Move move, GameScore score) {
+		this.move = move;
+		this.score = score;
+	}
+}

src/player/generator/MoveGenerator.java

@@ -0,0 +1,13 @@
+package player.generator;
+
+import java.util.List;
+
+import model.Board;
+import model.Move;
+
+public interface MoveGenerator {
+	public static final int ALL_MOVES = 0;
+	
+	public Move genMove(Board board, boolean asHuman);
+	public List<Move> genMoves(Board board, boolean asHuman, int nMoves);
+}

src/player/generator/ValidMoveGenerator.java

@@ -0,0 +1,38 @@
+package player.generator;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import model.Board;
+import model.Board.TileColor;
+import model.Move;
+
+import org.apache.log4j.Logger;
+
+public class ValidMoveGenerator implements MoveGenerator {
+	private static final Logger LOGGER = Logger.getLogger(ValidMoveGenerator.class.getName());
+	
+	@Override
+	public Move genMove(Board board, boolean asHuman) {
+		LOGGER.info("ValidMoveGenerator genMove() stub returning NONE");
+		return Move.NONE;
+	}
+
+	@Override
+	public List<Move> genMoves(Board board, boolean asHuman, int nMoves) {
+		
+		List<Move> validMoves = new ArrayList<Move>();
+
+		for (int i = 0; i < Board.NUM_ROWS; i++) {
+			for (int j = 0; j < Board.NUM_COLS; j++) {
+				if (board.getTile(i, j) == TileColor.NONE) {
+				for (TileColor color : TileColor.values()) {
+					validMoves.add(new Move(color, i, j));
+				}
+				}
+			}
+		}
+
+		return validMoves;
+	}
+}

src/view/MainFrame.java

@@ -35,8 +35,9 @@ public class MainFrame extends JFrame {
 
 	private void init() {
 		ScorePanel sp = new ScorePanel(referee);
-		TileSelectionPanel tp = new TileSelectionPanel(referee.getHumanPlayer());
+		referee.setScorePanel(sp);
 		
+		TileSelectionPanel tp = new TileSelectionPanel(referee.getHumanPlayer());
 		BoardPanel bp = new BoardPanel(referee,tp);
 		referee.setBoardPanel(bp);
 		
@@ -82,7 +83,7 @@ public class MainFrame extends JFrame {
 		add(vWrapper,BorderLayout.CENTER);
 		
 		//To ensure correct size, pre-populate the score and message panels with text.
-		sp.updateScore();
+		sp.updateScore(0);
 		mp.updateMessage("Loading new game...");
 		
 		pack();

src/view/ScorePanel.java

@@ -11,17 +11,15 @@ public class ScorePanel extends JPanel {
 	private static final long serialVersionUID = 1L;
 
 	private final JLabel message = new JLabel();
-	private final Referee referee;
-
+	
 	public ScorePanel(Referee ref) {
-		referee = ref;
 		add(message);
 	}
 
-	public void updateScore() {
+	public void updateScore(final int score) {
 		SwingUtilities.invokeLater(new Runnable() {
 			public void run() {
-				message.setText("Score: " + referee.getScore());
+				message.setText("Score: " + score);
 			}
 		});
 	}

src/view/TileSelectionPanel.java

@@ -9,9 +9,10 @@ import javax.swing.JLabel;
 import javax.swing.JPanel;
 import javax.swing.SwingUtilities;
 
+import player.HumanPlayer;
+
 import controller.TSPMouseListener;
 
-import model.HumanPlayer;
 import model.Board.TileColor;
 
 public class TileSelectionPanel extends JPanel {