feat: NCS-14 implemented insufficient moves rule (#30)

Reviewed-on: #30
Co-authored-by: Janis <janis.e.20@gmx.de>
Co-committed-by: Janis <janis.e.20@gmx.de>

modules/ui/src/main/scala/de/nowchess/ui/gui/ChessBoardView.scala

@@ -1,6 +1,6 @@
 package de.nowchess.ui.gui
 
-import scala.compiletime.uninitialized
+import java.util.concurrent.atomic.AtomicReference
 import scalafx.Includes.*
 import scalafx.application.Platform
 import scalafx.geometry.{Insets, Pos}
@@ -36,13 +36,23 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
     padding = Insets(10)
   }
 
-  private var currentBoard: Board            = engine.board
-  private var currentTurn: Color             = engine.turn
-  private var selectedSquare: Option[Square] = None
-  private val squareViews                    = scala.collection.mutable.Map[(Int, Int), StackPane]()
+  private val currentBoard   = new AtomicReference[Board](engine.board)
+  private val currentTurn    = new AtomicReference[Color](engine.turn)
+  private val selectedSquare = new AtomicReference[Option[Square]](None)
+  private val squareViews    = scala.collection.mutable.Map[(Int, Int), StackPane]()
 
-  private var undoButton: Button = uninitialized
-  private var redoButton: Button = uninitialized
+  private val undoButton: Button = new Button("Undo") {
+    font = Font.font(comicSansFontFamily, 12)
+    onAction = _ => if engine.canUndo then engine.undo()
+    style = "-fx-background-radius: 8; -fx-background-color: #B9DAD1;"
+    disable = !engine.canUndo
+  }
+  private val redoButton: Button = new Button("Redo") {
+    font = Font.font(comicSansFontFamily, 12)
+    onAction = _ => if engine.canRedo then engine.redo()
+    style = "-fx-background-radius: 8; -fx-background-color: #B9C2DA;"
+    disable = !engine.canRedo
+  }
 
   // Initialize UI
   initializeBoard()
@@ -77,23 +87,8 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
         spacing = 10
         alignment = Pos.Center
         children = Seq(
-          {
-            undoButton = new Button("Undo") {
-              font = Font.font(comicSansFontFamily, 12)
-              onAction = _ => if engine.canUndo then engine.undo()
-              style = "-fx-background-radius: 8; -fx-background-color: #B9DAD1;"
-              disable = !engine.canUndo
-            }
-            undoButton
-          }, {
-            redoButton = new Button("Redo") {
-              font = Font.font(comicSansFontFamily, 12)
-              onAction = _ => if engine.canRedo then engine.redo()
-              style = "-fx-background-radius: 8; -fx-background-color: #B9C2DA;"
-              disable = !engine.canRedo
-            }
-            redoButton
-          },
+          undoButton,
+          redoButton,
           new Button("Reset") {
             font = Font.font(comicSansFontFamily, 12)
             onAction = _ => engine.reset()
@@ -160,7 +155,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       squareViews((rank, file)) = square
       boardGrid.add(square, file, 7 - rank) // Flip rank for proper display
 
-    updateBoard(currentBoard, currentTurn)
+    updateBoard(currentBoard.get(), currentTurn.get())
 
   private def createSquare(rank: Int, file: Int): StackPane =
     val isWhite   = (rank + file) % 2 == 0
@@ -183,42 +178,41 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
     square
 
   private def handleSquareClick(rank: Int, file: Int): Unit =
-    if engine.isPendingPromotion then return // Don't allow moves during promotion
+    if !engine.isPendingPromotion then
+      val clickedSquare = Square(File.values(file), Rank.values(rank))
 
-    val clickedSquare = Square(File.values(file), Rank.values(rank))
+      selectedSquare.get() match
+        case None =>
+          // First click - select piece if it belongs to current player
+          currentBoard.get().pieceAt(clickedSquare).foreach { piece =>
+            if piece.color == currentTurn.get() then
+              selectedSquare.set(Some(clickedSquare))
+              highlightSquare(rank, file, PieceSprites.SquareColors.Selected)
 
-    selectedSquare match
-      case None =>
-        // First click - select piece if it belongs to current player
-        currentBoard.pieceAt(clickedSquare).foreach { piece =>
-          if piece.color == currentTurn then
-            selectedSquare = Some(clickedSquare)
-            highlightSquare(rank, file, PieceSprites.SquareColors.Selected)
+              val legalDests = engine.ruleSet
+                .legalMoves(engine.context)(clickedSquare)
+                .collect { case move if move.from == clickedSquare => move.to }
+              legalDests.foreach { sq =>
+                highlightSquare(sq.rank.ordinal, sq.file.ordinal, PieceSprites.SquareColors.ValidMove)
+              }
+          }
 
-            val legalDests = engine.ruleSet
-              .legalMoves(engine.context)(clickedSquare)
-              .collect { case move if move.from == clickedSquare => move.to }
-            legalDests.foreach { sq =>
-              highlightSquare(sq.rank.ordinal, sq.file.ordinal, PieceSprites.SquareColors.ValidMove)
-            }
-        }
-
-      case Some(fromSquare) =>
-        // Second click - attempt move
-        if clickedSquare == fromSquare then
-          // Deselect
-          selectedSquare = None
-          updateBoard(currentBoard, currentTurn)
-        else
-          // Try to move
-          val moveStr = s"${fromSquare}$clickedSquare"
-          engine.processUserInput(moveStr)
-          selectedSquare = None
+        case Some(fromSquare) =>
+          // Second click - attempt move
+          if clickedSquare == fromSquare then
+            // Deselect
+            selectedSquare.set(None)
+            updateBoard(currentBoard.get(), currentTurn.get())
+          else
+            // Try to move
+            val moveStr = s"${fromSquare}$clickedSquare"
+            engine.processUserInput(moveStr)
+            selectedSquare.set(None)
 
   def updateBoard(board: Board, turn: Color): Unit =
-    currentBoard = board
-    currentTurn = turn
-    selectedSquare = None
+    currentBoard.set(board)
+    currentTurn.set(turn)
+    selectedSquare.set(None)
 
     // Update all squares
     for
@@ -240,9 +234,9 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
         val square      = Square(File.values(file), Rank.values(rank))
         val pieceOption = board.pieceAt(square)
 
-        val children = pieceOption match
+        val children: Seq[scalafx.scene.Node] = pieceOption match
           case Some(piece) =>
-            Seq(bgRect, PieceSprites.loadPieceImage(piece, squareSize * 0.8))
+            Seq(bgRect) ++ PieceSprites.loadPieceImage(piece, squareSize * 0.8).toSeq
           case None =>
             Seq(bgRect)
 
@@ -252,8 +246,8 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
     updateUndoRedoButtons()
 
   def updateUndoRedoButtons(): Unit =
-    if undoButton != null then undoButton.disable = !engine.canUndo
-    if redoButton != null then redoButton.disable = !engine.canRedo
+    undoButton.disable = !engine.canUndo
+    redoButton.disable = !engine.canRedo
 
   private def highlightSquare(rank: Int, file: Int, color: String): Unit =
     squareViews.get((rank, file)).foreach { stackPane =>
@@ -266,13 +260,13 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       }
 
       val square      = Square(File.values(file), Rank.values(rank))
-      val pieceOption = currentBoard.pieceAt(square)
+      val pieceOption = currentBoard.get().pieceAt(square)
 
-      stackPane.children = pieceOption match
+      stackPane.children = (pieceOption match
         case Some(piece) =>
-          Seq(bgRect, PieceSprites.loadPieceImage(piece, squareSize * 0.8))
+          Seq(bgRect) ++ PieceSprites.loadPieceImage(piece, squareSize * 0.8).toSeq
         case None =>
-          Seq(bgRect)
+          Seq(bgRect)): Seq[scalafx.scene.Node]
     }
 
   def showMessage(msg: String): Unit =
@@ -315,8 +309,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       extensionFilters.add(new ExtensionFilter("All files", "*.*"))
     }
 
-    val selectedFile = fileChooser.showSaveDialog(stage)
-    if selectedFile != null then
+    Option(fileChooser.showSaveDialog(stage)).foreach { selectedFile =>
       val result = FileSystemGameService.saveGameToFile(
         engine.context,
         selectedFile.toPath,
@@ -325,6 +318,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       result match
         case Right(_)  => showMessage(s"✓ Game saved to: ${selectedFile.getName}")
         case Left(err) => showMessage(s"⚠️ Error saving file: $err")
+    }
 
   private def doJsonImport(): Unit =
     val fileChooser = new FileChooser {
@@ -333,8 +327,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       extensionFilters.add(new ExtensionFilter("All files", "*.*"))
     }
 
-    val selectedFile = fileChooser.showOpenDialog(stage)
-    if selectedFile != null then
+    Option(fileChooser.showOpenDialog(stage)).foreach { selectedFile =>
       val result = FileSystemGameService.loadGameFromFile(
         selectedFile.toPath,
         JsonParser,
@@ -345,6 +338,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
           showMessage(s"✓ Game loaded from: ${selectedFile.getName}")
         case Left(err) =>
           showMessage(s"⚠️ Error: $err")
+    }
 
   private def doExport(exporter: GameContextExport, formatName: String): Unit = {
     val exported = exporter.exportGameContext(engine.context)
@@ -368,7 +362,7 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
     area.setPrefRowCount(4)
     val alert = new javafx.scene.control.Alert(javafx.scene.control.Alert.AlertType.INFORMATION)
     alert.setTitle(title)
-    alert.setHeaderText(null)
+    alert.setHeaderText("")
     alert.getDialogPane.setContent(area)
     alert.getDialogPane.setPrefWidth(500)
     alert.initOwner(stage.delegate)
@@ -386,8 +380,8 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
       javafx.scene.control.ButtonType.CANCEL,
     )
     dialog.setResultConverter { bt =>
-      if bt == javafx.scene.control.ButtonType.OK then area.getText else null
+      if bt == javafx.scene.control.ButtonType.OK then area.getText else ""
     }
     dialog.initOwner(stage.delegate)
     val result = dialog.showAndWait()
-    if result.isPresent && result.get != null && result.get.nonEmpty then Some(result.get) else None
+    if result.isPresent && result.get.nonEmpty then Some(result.get) else None

modules/ui/src/main/scala/de/nowchess/ui/gui/ChessGUI.scala

@@ -31,8 +31,7 @@ class ChessGUIApp extends JFXApplication:
       root = boardView
       // Load CSS if available
       try {
-        val cssUrl = getClass.getResource("/styles.css")
-        if cssUrl != null then stylesheets.add(cssUrl.toExternalForm)
+        Option(getClass.getResource("/styles.css")).foreach(url => stylesheets.add(url.toExternalForm))
       } catch {
         case _: Exception => // CSS is optional
       }
@@ -46,12 +45,12 @@ class ChessGUIApp extends JFXApplication:
 
 /** Launcher object that holds the engine reference and launches GUI in separate thread. */
 object ChessGUILauncher:
-  @volatile private var engine: GameEngine = scala.compiletime.uninitialized
+  private val engineRef = new java.util.concurrent.atomic.AtomicReference[GameEngine]()
 
-  def getEngine: GameEngine = engine
+  def getEngine: GameEngine = engineRef.get()
 
   def launch(eng: GameEngine): Unit =
-    engine = eng
+    engineRef.set(eng)
     val guiThread = new Thread(() => JFXApplication.launch(classOf[ChessGUIApp]))
     guiThread.setDaemon(false)
     guiThread.setName("ScalaFX-GUI-Thread")

modules/ui/src/main/scala/de/nowchess/ui/gui/GUIObserver.scala

@@ -5,6 +5,7 @@ import scalafx.scene.control.Alert
 import scalafx.scene.control.Alert.AlertType
 import de.nowchess.chess.observer.{GameEvent, Observer, *}
 import de.nowchess.api.board.Board
+import de.nowchess.api.game.DrawReason
 
 /** GUI Observer that implements the Observer pattern. Receives game events from GameEngine and updates the ScalaFX UI.
   * All UI updates must be done on the JavaFX Application Thread.
@@ -29,9 +30,14 @@ class GUIObserver(private val boardView: ChessBoardView) extends Observer:
           boardView.updateBoard(e.context.board, e.context.turn)
           showAlert(AlertType.Information, "Game Over", s"Checkmate! ${e.winner.label} wins.")
 
-        case e: StalemateEvent =>
+        case e: DrawEvent =>
           boardView.updateBoard(e.context.board, e.context.turn)
-          showAlert(AlertType.Information, "Game Over", "Stalemate! The game is a draw.")
+          val msg = e.reason match
+            case DrawReason.Stalemate            => "Stalemate! The game is a draw."
+            case DrawReason.InsufficientMaterial => "Draw by insufficient material."
+            case DrawReason.FiftyMoveRule        => "Draw claimed under the 50-move rule."
+            case DrawReason.Agreement            => "Draw by agreement."
+          showAlert(AlertType.Information, "Game Over", msg)
 
         case e: InvalidMoveEvent =>
           boardView.showMessage(s"⚠️  ${e.reason}")
@@ -43,12 +49,8 @@ class GUIObserver(private val boardView: ChessBoardView) extends Observer:
         case e: PromotionRequiredEvent =>
           boardView.showPromotionDialog(e.from, e.to)
 
-        case e: DrawClaimedEvent =>
-          boardView.updateBoard(e.context.board, e.context.turn)
-          showAlert(AlertType.Information, "Draw Claimed", "Draw claimed! The game is a draw.")
-
         case e: FiftyMoveRuleAvailableEvent =>
-          boardView.showMessage("50-move rule available! The game is a draw.")
+          boardView.showMessage("50-move rule is now available — type 'draw' to claim.")
 
         case e: MoveUndoneEvent =>
           boardView.updateBoard(e.context.board, e.context.turn)

modules/ui/src/main/scala/de/nowchess/ui/gui/PieceSprites.scala

@@ -6,26 +6,24 @@ import de.nowchess.api.board.{Color, Piece, PieceType}
 /** Utility object for loading chess piece sprites. */
 object PieceSprites:
 
-  private val spriteCache = scala.collection.mutable.Map[String, Image]()
+  private val spriteCache = scala.collection.mutable.Map[String, Option[Image]]()
 
   /** Load a piece sprite image from resources. Sprites are cached for performance.
     */
-  def loadPieceImage(piece: Piece, size: Double = 60.0): ImageView =
-    val key   = s"${piece.color.label.toLowerCase}_${piece.pieceType.label.toLowerCase}"
-    val image = spriteCache.getOrElseUpdate(key, loadImage(key))
-
-    new ImageView(image) {
-      fitWidth = size
-      fitHeight = size
-      preserveRatio = true
-      smooth = true
+  def loadPieceImage(piece: Piece, size: Double = 60.0): Option[ImageView] =
+    val key = s"${piece.color.label.toLowerCase}_${piece.pieceType.label.toLowerCase}"
+    spriteCache.getOrElseUpdate(key, loadImage(key)).map { image =>
+      new ImageView(image) {
+        fitWidth = size
+        fitHeight = size
+        preserveRatio = true
+        smooth = true
+      }
     }
 
-  private def loadImage(key: String): Image =
-    val path   = s"/sprites/pieces/$key.png"
-    val stream = getClass.getResourceAsStream(path)
-    if stream == null then throw new RuntimeException(s"Could not load sprite: $path")
-    new Image(stream)
+  private def loadImage(key: String): Option[Image] =
+    val path = s"/sprites/pieces/$key.png"
+    Option(getClass.getResourceAsStream(path)).map(new Image(_))
 
   /** Get square colors for the board using theme. */
   object SquareColors:

modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala

@@ -1,7 +1,9 @@
 package de.nowchess.ui.terminal
 
+import java.util.concurrent.atomic.AtomicBoolean
 import scala.io.StdIn
 import de.nowchess.api.move.PromotionPiece
+import de.nowchess.api.game.DrawReason
 import de.nowchess.chess.engine.GameEngine
 import de.nowchess.chess.observer.*
 import de.nowchess.ui.utils.Renderer
@@ -10,8 +12,8 @@ import de.nowchess.ui.utils.Renderer
   * I/O and user interaction in the terminal.
   */
 class TerminalUI(engine: GameEngine) extends Observer:
-  private var running           = true
-  private var awaitingPromotion = false
+  private val running           = new AtomicBoolean(true)
+  private val awaitingPromotion = new AtomicBoolean(false)
 
   /** Called by GameEngine whenever a game event occurs. */
   override def onGameEvent(event: GameEvent): Unit =
@@ -43,8 +45,13 @@ class TerminalUI(engine: GameEngine) extends Observer:
         println()
         print(Renderer.render(e.context.board))
 
-      case e: StalemateEvent =>
-        println("Stalemate! The game is a draw.")
+      case e: DrawEvent =>
+        val msg = e.reason match
+          case DrawReason.Stalemate            => "Stalemate! The game is a draw."
+          case DrawReason.InsufficientMaterial => "Draw by insufficient material."
+          case DrawReason.FiftyMoveRule        => "Draw claimed under the 50-move rule."
+          case DrawReason.Agreement            => "Draw by agreement."
+        println(msg)
         println()
         print(Renderer.render(e.context.board))
 
@@ -59,13 +66,9 @@ class TerminalUI(engine: GameEngine) extends Observer:
 
       case _: PromotionRequiredEvent =>
         println("Promote to: q=Queen, r=Rook, b=Bishop, n=Knight")
-        synchronized { awaitingPromotion = true }
-      case _: DrawClaimedEvent =>
-        println("Draw claimed! The game is a draw.")
-        println()
-        print(Renderer.render(engine.board))
+        awaitingPromotion.set(true)
       case _: FiftyMoveRuleAvailableEvent =>
-        println("50-move rule available! The game is a draw.")
+        println("50-move rule is now available — type 'draw' to claim.")
 
       case e: PgnLoadedEvent =>
         println("PGN loaded successfully.")
@@ -84,22 +87,22 @@ class TerminalUI(engine: GameEngine) extends Observer:
     printPrompt(engine.turn)
 
     // Game loop
-    while running do
+    while running.get() do
       val input = Option(StdIn.readLine()).getOrElse("quit").trim
       synchronized {
-        if awaitingPromotion then
+        if awaitingPromotion.get() then
           input.toLowerCase match
-            case "q" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Queen)
-            case "r" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Rook)
-            case "b" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Bishop)
-            case "n" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Knight)
+            case "q" => awaitingPromotion.set(false); engine.completePromotion(PromotionPiece.Queen)
+            case "r" => awaitingPromotion.set(false); engine.completePromotion(PromotionPiece.Rook)
+            case "b" => awaitingPromotion.set(false); engine.completePromotion(PromotionPiece.Bishop)
+            case "n" => awaitingPromotion.set(false); engine.completePromotion(PromotionPiece.Knight)
             case _ =>
               println("Invalid choice. Enter q, r, b, or n.")
               println("Promote to: q=Queen, r=Rook, b=Bishop, n=Knight")
         else
           input.toLowerCase match
             case "quit" | "q" =>
-              running = false
+              running.set(false)
               println("Game over. Goodbye!")
             case "" =>
               printPrompt(engine.turn)