i have implemented a basic AI that plays legal move

src/backend/AutoPlayer.java

@@ -1,17 +1,90 @@
 package backend;
 
+import java.util.ArrayList;
+import java.util.Random;
+
 public class AutoPlayer {
 
+    private final Move moveHelper = new Move();
+    private final Random random = new Random();
 
     /**
-	 * returns the best Move to try on provided board for active player
+     * Returns a random legal move for the current player
-	 * @param board
+     * @param boardObj The board to find moves for
-	 * @return
+     * @return a valid Move or null if no moves available
      */
-	public Move computeBestMove(Board board) {
+    public Move computeBestMove(Board boardObj) {
+        // Create a list of pieces that can move
+        ArrayList<Piece> candidates = new ArrayList<>();
         
-		return null;
+      
+        ArrayList<Piece> allPieces = boardObj.getPieces();
+        int width = boardObj.getWidth();
+        int height = boardObj.getHeight();
+        boolean isWhiteTurn = boardObj.isTurnWhite();
+        
+        
+        Piece[][] boardArray = new Piece[width][height];
+        
+        // Find all pieces with legal moves
+        for (Piece piece : allPieces) {
+            // Only consider pieces of the current player's color
+            if (piece.isWhite() == isWhiteTurn) {
+                
+                clearBoard(boardArray);
+                for (Piece p : allPieces) {
+                    boardArray[p.getX()][p.getY()] = p;
                 }
                 
                 
+                ArrayList<int[]> moves = moveHelper.getValidMoves(
+                    piece, boardArray, width, height, null, boardObj
+                );
+                
+                
+                if (!moves.isEmpty()) {
+                    candidates.add(piece);
+                }
+            }
+        }
+        
+        
+        if (candidates.isEmpty()) return null;
+        
+        // Select a random piece to move
+        Piece selected = candidates.get(random.nextInt(candidates.size()));
+        
+        
+        clearBoard(boardArray);
+        for (Piece p : allPieces) {
+            boardArray[p.getX()][p.getY()] = p;
+        }
+        
+        
+        ArrayList<int[]> moves = moveHelper.getValidMoves(
+            selected, boardArray, width, height, null, boardObj
+        );
+        
+        // Select a random destination for the piece
+        int[] target = moves.get(random.nextInt(moves.size()));
+        
+        
+        Move aiMove = new Move();
+        aiMove.setFromX(selected.getX());
+        aiMove.setFromY(selected.getY());
+        aiMove.setToX(target[0]);
+        aiMove.setToY(target[1]);
+        return aiMove;
+    }
+    
+    /**
+     * Clears the given board array by setting all elements to null
+     */
+    private void clearBoard(Piece[][] boardArray) {
+        for (int x = 0; x < boardArray.length; x++) {
+            for (int y = 0; y < boardArray[x].length; y++) {
+                boardArray[x][y] = null;
+            }
+        }
+    }
 }

src/backend/Board.java

@@ -15,6 +15,7 @@ public class Board {
     private ArrayList<int[]> highlightedSquares = new ArrayList<>();
     private int[] enPassantTarget = null; // [x,y] coordinates of en passant target square
     private Move moveHelper = new Move();
+    private final AutoPlayer ai = new AutoPlayer();
 
     public Board(int colNum, int lineNum) {
         this.width = colNum;
@@ -214,6 +215,12 @@ public class Board {
                 // Clear selection & highlights
                 hasSelectedPiece = false;
                 highlightedSquares.clear();
+                
+                if (!turnWhite) {
+                    Move aiMove = ai.computeBestMove(this);
+                    if (aiMove != null) {
+                        playMove(aiMove);
+                    }
             } 
             // Invalid move: just unselect
             else {
@@ -223,6 +230,8 @@ public class Board {
             }
         }
       
+    }
+
 
     
     /* saving-loading feature */
@@ -342,6 +351,52 @@ public class Board {
     }
     
     public void playMove(Move move) {
-        // TODO
+    	Piece selectedPiece = board[move.getFromX()][move.getFromY()];
+    	if (selectedPiece == null) return;
+
+    	boolean isEnPassant = selectedPiece.getType() == PieceType.Pawn &&
+    		enPassantTarget != null &&
+    		move.getToX() == enPassantTarget[0] &&
+    		move.getToY() == enPassantTarget[1] &&
+    		board[move.getToX()][move.getToY()] == null;
+
+    	previousBoard = cloneBoard(board);
+    	board[move.getToX()][move.getToY()] = selectedPiece;
+    	board[move.getFromX()][move.getFromY()] = null;
+    	selectedPiece.setX(move.getToX());
+    	selectedPiece.setY(move.getToY());
+    	selectedPiece.setMoved(true);
+
+    	if (isEnPassant) {
+    		board[move.getToX()][move.getFromY()] = null;
+    	}
+
+    	if (selectedPiece.getType() == PieceType.King && Math.abs(move.getToX() - move.getFromX()) == 2) {
+    		if (move.getToX() > move.getFromX()) {
+    			Piece rook = board[7][move.getToY()];
+    			board[5][move.getToY()] = rook;
+    			board[7][move.getToY()] = null;
+    			rook.setX(5);
+    			rook.setMoved(true);
+    		} else {
+    			Piece rook = board[0][move.getToY()];
+    			board[3][move.getToY()] = rook;
+    			board[0][move.getToY()] = null;
+    			rook.setX(3);
+    			rook.setMoved(true);
     		}
     	}
+
+    	enPassantTarget = null;
+    	if (selectedPiece.getType() == PieceType.Pawn &&
+    		Math.abs(move.getToY() - move.getFromY()) == 2) {
+    		enPassantTarget = new int[]{move.getToX(), (move.getFromY() + move.getToY()) / 2};
+    	}
+
+    	turnWhite = !turnWhite;
+    	turnNumber++;
+    	hasSelectedPiece = false;
+    	highlightedSquares.clear();
+    }
+
+}

src/backend/Move.java

@@ -143,4 +143,41 @@ public class Move {
     private boolean inBounds(int x, int y, int width, int height) {
         return x >= 0 && x < width && y >= 0 && y < height;
     }
+    private int fromX;
+    private int fromY;
+    private int toX;
+    private int toY;
+
+    public int getFromX() {
+        return fromX;
+    }
+
+    public void setFromX(int fromX) {
+        this.fromX = fromX;
+    }
+
+    public int getFromY() {
+        return fromY;
+    }
+
+    public void setFromY(int fromY) {
+        this.fromY = fromY;
+    }
+
+    public int getToX() {
+        return toX;
+    }
+
+    public void setToX(int toX) {
+        this.toX = toX;
+    }
+
+    public int getToY() {
+        return toY;
+    }
+
+    public void setToY(int toY) {
+        this.toY = toY;
+    }
+
 }