add castling + fix promote undo

app/src/main/java/chess/model/ChessBoard.java

@@ -58,7 +58,7 @@ public class ChessBoard {
 	}
 
 	public void undoLastMove() {
-		assert this.lastVirtualMove != null: "Can't undo at the beginning!";
+		assert this.lastVirtualMove != null : "Can't undo at the beginning!";
 
 		undoMove(this.lastVirtualMove, this.lastEjectedPiece);
 	}
@@ -181,6 +181,45 @@ public class ChessBoard {
 		return result;
 	}
 
+	private boolean canCastle(Color color, int rookX, Direction kingDirection) {
+		if (isKingInCheck(color))
+			return false;
+
+		int colorLine = color == Color.White ? 7 : 0;
+
+		Coordinate kingCoords = new Coordinate(4, colorLine);
+		Coordinate rookCoords = new Coordinate(rookX, colorLine);
+		Piece king = pieceAt(kingCoords);
+		Piece rook = pieceAt(rookCoords);
+
+		if (king == null || rook == null || king.hasMoved() || rook.hasMoved())
+			return false;
+
+		for (int step = 1; step <= 2; step++) {
+			Coordinate dest = Coordinate.fromIndex(kingCoords.toIndex() + step * kingDirection.getIndexOffset());
+			Piece obstacle = pieceAt(dest);
+			if (obstacle != null)
+				return false;
+
+			applyMove(new Move(kingCoords, dest));
+			if (isKingInCheck(color)) {
+				undoLastMove();
+				return false;
+			}
+			undoLastMove();
+		}
+
+		return true;
+	}
+
+	public boolean canSmallCastle(Color color) {
+		return canCastle(color, 7, Direction.Right);
+	}
+
+	public boolean canBigCastle(Color color) {
+		return canCastle(color, 0, Direction.Left);
+	}
+
 	public Move getLastMove() {
 		return this.lastMove;
 	}