Implements a quick and dirty way to define start position by its fen

src/main/java/com/fathzer/jchess/settings/Settings.java

@@ -1,11 +1,13 @@
 package com.fathzer.jchess.settings;
 
 import java.util.Random;
+import java.util.function.Function;
 import java.util.function.Supplier;
 
 import com.fathzer.games.Color;
 import com.fathzer.jchess.Board;
 import com.fathzer.jchess.Move;
+import com.fathzer.jchess.fen.FENUtils;
 import com.fathzer.jchess.GameBuilders;
 
 import lombok.AllArgsConstructor;
@@ -20,6 +22,8 @@ public class Settings {
 	private static final Random RANDOM_GENERATOR = new Random();
 
 	private Variant variant = Variant.STANDARD;
+	//TODO fen should be obtained by Settings panel 
+	private String fen = buildFEN();
 	private boolean tabletMode = true;
 	private boolean showPossibleMoves = true;
 	private boolean touchMove = false;
@@ -29,18 +33,30 @@ public class Settings {
 	private ColorSetting player1Color = ColorSetting.RANDOM;
 	private PlayerSettings player2 = new PlayerSettings();
 
+	private static final String buildFEN() {
+		var fen = System.getProperty("fen");
+		final String result = fen==null ? fen : fen.replace('_', ' ');
+		if (result!=null) {
+			System.out.println("Fen of start position was retrieved in 'fen' system property: "+result);
+		}
+		return result;
+	}
 
 	@AllArgsConstructor
 	public enum Variant {
-		STANDARD(GameBuilders.STANDARD), CHESS960(GameBuilders.CHESS960);
-		private final Supplier<Board<Move>> rules;
+		STANDARD(toFunction(GameBuilders.STANDARD)), CHESS960(toFunction(GameBuilders.CHESS960));
+		private final Function<String, Board<Move>> rules;
 
 		/** Gets a Supplier that can create a new game.
 		 * <br>Typically, it is able to create the representation of a new game (for example, a chess board at the beginning of the game)
 		 */
-		public Supplier<Board<Move>> getRules() {
+		public Function<String, Board<Move>> getRules() {
 			return rules;
 		}
+
+		private static Function<String, Board<Move>> toFunction(Supplier<Board<Move>> s) {
+			return fen -> fen==null ? s.get() : FENUtils.from(fen);
+		}
 	}
 
 	public enum ColorSetting {

src/main/java/com/fathzer/jchess/swing/ChessBoardPanel.java

@@ -7,7 +7,6 @@
 import java.io.UncheckedIOException;
 import java.util.Arrays;
 import java.util.List;
-import java.util.function.Supplier;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -35,7 +34,6 @@ public class ChessBoardPanel extends JPanel implements MouseListener {
 
 	private final transient com.fathzer.jchess.Dimension dimension;
     private final transient SpriteMover sprites;
-	private transient Supplier<Board<Move>> rules;
 	private int selected;
 	private boolean reverted;
     private transient Board<Move> board;
@@ -89,12 +87,6 @@ public void setBoard(Board<Move> board) {
     	this.repaint();
     }
 
-    public void setChessRules(Supplier<Board<Move>> rules) {
-    	this.rules = rules;
-    	this.updatePossibleMoves();
-    	this.repaint();
-    }
-
     public void setCellColors(Color whiteColor, Color blackColor) {
 		this.whiteColor = whiteColor;
         this.blackColor = blackColor;
@@ -122,7 +114,7 @@ public void setManualMoveEnabled(boolean enabled) {
 
     private void updatePossibleMoves() {
     	this.targets = new int[0];
-    	if (board!=null && rules!=null) {
+    	if (board!=null) {
     		this.moveList = board.getLegalMoves();
     	}
     }

src/main/java/com/fathzer/jchess/swing/GameSession.java

@@ -97,7 +97,7 @@ private void doRevenge() {
 	}
 
 	private void initGame() {
-		this.game = new Game(settings.getVariant().getRules().get(), buildClock());
+		this.game = new Game(settings.getVariant().getRules().apply(settings.getFen()), buildClock());
 		this.game.setStartClockAfterFirstMove(settings.isStartClockAfterFirstMove());
 		panel.setPlayer1Color(player1Color);
 		panel.setClock(game.getClock());
@@ -348,7 +348,6 @@ public void setSettings(Settings settings) {
 			throw new IllegalStateException("Can't change the game settings during the game");
 		}
 		this.settings = settings;
-		panel.getBoard().setChessRules(settings.getVariant().getRules());
 		panel.setPlayer1Human(settings.getPlayer1().getEngine()==null);
 		panel.setPlayer2Human(settings.getPlayer2().getEngine()==null);
 		panel.getBoard().setShowPossibleMoves(settings.isShowPossibleMoves());