feat: NCS-40 Rework Draw System (#34)

Reviewed-on: #34
Reviewed-by: Shahd Lala <shosho996@blackhole.local>
Co-authored-by: Janis <janis.e.20@gmx.de>
Co-committed-by: Janis <janis.e.20@gmx.de>

modules/api/CHANGELOG.md

@@ -42,3 +42,12 @@
 * NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
 * NCS-21 Write Scripts to automate certain tasks ([#15](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/15)) ([8051871](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/80518719d536a087d339fe02530825dc07f8b388))
 * NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+##  (2026-04-19)
+
+### Features
+
+* NCS-13 Implement Threefold Repetition ([#31](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/31)) ([767d305](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/767d3051a76c266050b6335774d66e2db2273c16))
+* NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
+* NCS-21 Write Scripts to automate certain tasks ([#15](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/15)) ([8051871](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/80518719d536a087d339fe02530825dc07f8b388))
+* NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+* NCS-41 Bot Platform ([#33](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/33)) ([dceab08](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dceab0875e6d15f7d3958633cf5dd5b29a851b1d))

modules/api/versions.env

@@ -1,3 +1,3 @@
 MAJOR=0
-MINOR=6
+MINOR=7
 PATCH=0

modules/core/CHANGELOG.md

@@ -286,3 +286,30 @@
 * correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
 * update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
 * update move validation to check for king safety ([#13](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/13)) ([e5e20c5](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/e5e20c566e368b12ca1dc59680c34e9112bf6762))
+##  (2026-04-19)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-10 Implement Pawn Promotion ([#12](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/12)) ([13bfc16](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/13bfc16cfe25db78ec607db523ca6d993c13430c))
+* NCS-11 50-move rule ([#9](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/9)) ([412ed98](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/412ed986a95703a3b282276540153480ceed229d))
+* NCS-13 Implement Threefold Repetition ([#31](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/31)) ([767d305](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/767d3051a76c266050b6335774d66e2db2273c16))
+* NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-17 Implement basic ScalaFX UI ([#14](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/14)) ([3ff8031](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/3ff80318b4f16c59733a46498581a5c27f048287))
+* NCS-21 Write Scripts to automate certain tasks ([#15](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/15)) ([8051871](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/80518719d536a087d339fe02530825dc07f8b388))
+* NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+* NCS-41 Bot Platform ([#33](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/33)) ([dceab08](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dceab0875e6d15f7d3958633cf5dd5b29a851b1d))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+* update move validation to check for king safety ([#13](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/13)) ([e5e20c5](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/e5e20c566e368b12ca1dc59680c34e9112bf6762))

modules/core/build.gradle.kts

@@ -63,3 +63,8 @@ tasks.test {
 tasks.reportScoverage {
     dependsOn(tasks.test)
 }
+
+tasks.jar {
+    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+}
+

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -31,7 +31,9 @@ class GameEngine(
     else initialContext
   @SuppressWarnings(Array("DisableSyntax.var"))
   private var currentContext: GameContext = contextWithInitialBoard
-  private val invoker                     = new CommandInvoker()
+  @SuppressWarnings(Array("DisableSyntax.var"))
+  private var pendingDrawOffer: Option[Color] = None
+  private val invoker                         = new CommandInvoker()
 
   private implicit val ec: ExecutionContext = ExecutionContext.global
 
@@ -77,12 +79,12 @@ class GameEngine(
           notifyObservers(
             InvalidMoveEvent(
               currentContext,
-              "Draw cannot be claimed: neither the 50-move rule nor threefold repetition has been triggered.",
+              InvalidMoveReason.DrawCannotBeClaimed,
             ),
           )
 
       case "" =>
-        notifyObservers(InvalidMoveEvent(currentContext, "Please enter a valid move or command."))
+        notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.EmptyInput))
 
       case moveInput =>
         Parser.parseMove(moveInput) match
@@ -90,7 +92,7 @@ class GameEngine(
             notifyObservers(
               InvalidMoveEvent(
                 currentContext,
-                s"Invalid move format '$moveInput'. Use coordinate notation, e.g. e2e4.",
+                InvalidMoveReason.InvalidMoveFormat,
               ),
             )
           case Some((from, to, promotionPiece: Option[PromotionPiece])) =>
@@ -100,26 +102,26 @@ class GameEngine(
   private def handleParsedMove(from: Square, to: Square, promotionPiece: Option[PromotionPiece]): Unit =
     currentContext.board.pieceAt(from) match
       case None =>
-        notifyObservers(InvalidMoveEvent(currentContext, "No piece on that square."))
+        notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NoSourcePiece))
       case Some(piece) if piece.color != currentContext.turn =>
-        notifyObservers(InvalidMoveEvent(currentContext, "That is not your piece."))
+        notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NotYourPiece))
       case Some(piece) =>
         val legal = ruleSet.legalMoves(currentContext)(from)
         // Find all legal moves going to `to`
         val candidates = legal.filter(_.to == to)
         candidates match
           case Nil =>
-            notifyObservers(InvalidMoveEvent(currentContext, "Illegal move."))
+            notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.IllegalMove))
           case _ if isPromotionMove(piece, to) =>
             if promotionPiece.isEmpty then
               notifyObservers(
-                InvalidMoveEvent(currentContext, "Promotion piece required: append q, r, b, or n to the move."),
+                InvalidMoveEvent(currentContext, InvalidMoveReason.PromotionPieceRequired),
               )
             else
               candidates.find(_.moveType == MoveType.Promotion(promotionPiece.get)) match
                 case None =>
                   notifyObservers(
-                    InvalidMoveEvent(currentContext, "Error completing promotion: no matching legal move."),
+                    InvalidMoveEvent(currentContext, InvalidMoveReason.PromotionPieceInvalid),
                   )
                 case Some(move) => executeMove(move)
           case move :: _ =>
@@ -137,6 +139,62 @@ class GameEngine(
   /** Redo the last undone move. */
   def redo(): Unit = synchronized(performRedo())
 
+  /** Resign from the game. The opponent wins. */
+  def resign(color: Color): Unit = synchronized {
+    if currentContext.result.isDefined then
+      notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.GameAlreadyOver))
+    else
+      currentContext = currentContext.withResult(Some(GameResult.Win(color.opposite)))
+      pendingDrawOffer = None
+      invoker.clear()
+      notifyObservers(ResignEvent(currentContext, color))
+  }
+
+  /** Offer a draw. */
+  def offerDraw(color: Color): Unit = synchronized {
+    if currentContext.result.isDefined then
+      notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.GameAlreadyOver))
+    else
+      pendingDrawOffer match
+        case Some(_) =>
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.DrawOfferPending))
+        case None =>
+          pendingDrawOffer = Some(color)
+          notifyObservers(DrawOfferEvent(currentContext, color))
+  }
+
+  /** Accept a pending draw offer. */
+  def acceptDraw(color: Color): Unit = synchronized {
+    if currentContext.result.isDefined then
+      notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.GameAlreadyOver))
+    else
+      pendingDrawOffer match
+        case None =>
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NoDrawOfferToAccept))
+        case Some(offerer) if offerer == color =>
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.CannotAcceptOwnDrawOffer))
+        case Some(_) =>
+          currentContext = currentContext.withResult(Some(GameResult.Draw(DrawReason.Agreement)))
+          pendingDrawOffer = None
+          invoker.clear()
+          notifyObservers(DrawEvent(currentContext, DrawReason.Agreement))
+  }
+
+  /** Decline a pending draw offer. */
+  def declineDraw(color: Color): Unit = synchronized {
+    if currentContext.result.isDefined then
+      notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.GameAlreadyOver))
+    else
+      pendingDrawOffer match
+        case None =>
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NoDrawOfferToDecline))
+        case Some(offerer) if offerer == color =>
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.CannotDeclineOwnDrawOffer))
+        case Some(_) =>
+          pendingDrawOffer = None
+          notifyObservers(DrawOfferDeclinedEvent(currentContext, color))
+  }
+
   /** Load a game using the provided importer. If the imported context has moves, they are replayed through the command
     * system. Otherwise, the position is set directly. Notifies observers with PgnLoadedEvent on success.
     */
@@ -145,6 +203,7 @@ class GameEngine(
       case Left(err) => Left(err)
       case Right(ctx) =>
         replayGame(ctx).map { _ =>
+          pendingDrawOffer = None
           notifyObservers(PgnLoadedEvent(currentContext))
         }
   }
@@ -186,6 +245,7 @@ class GameEngine(
       if newContext.moves.isEmpty then newContext.copy(initialBoard = newContext.board)
       else newContext
     currentContext = contextWithInitialBoard
+    pendingDrawOffer = None
     invoker.clear()
     notifyObservers(BoardResetEvent(currentContext))
   }
@@ -193,6 +253,7 @@ class GameEngine(
   /** Reset the board to initial position. */
   def reset(): Unit = synchronized {
     currentContext = GameContext.initial
+    pendingDrawOffer = None
     invoker.clear()
     notifyObservers(BoardResetEvent(currentContext))
   }
@@ -323,9 +384,9 @@ class GameEngine(
           legal.find(m => m.to == to && m.moveType == move.moveType) match
             case Some(legalMove) => executeMove(legalMove)
             case None =>
-              notifyObservers(InvalidMoveEvent(currentContext, s"Bot move ${from}${to} is illegal"))
+              notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.BotMoveIllegal))
         case _ =>
-          notifyObservers(InvalidMoveEvent(currentContext, "Bot move has invalid source square"))
+          notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.BotMoveInvalidSource))
     }
 
   private def handleBotNoMove(): Unit =
@@ -344,7 +405,7 @@ class GameEngine(
           moveCmd.previousContext.foreach(currentContext = _)
           invoker.undo()
           notifyObservers(MoveUndoneEvent(currentContext, moveCmd.notation))
-    else notifyObservers(InvalidMoveEvent(currentContext, "Nothing to undo."))
+    else notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NothingToUndo))
 
   private def performRedo(): Unit =
     if invoker.canRedo then
@@ -364,4 +425,4 @@ class GameEngine(
                 capturedDesc,
               ),
             )
-    else notifyObservers(InvalidMoveEvent(currentContext, "Nothing to redo."))
+    else notifyObservers(InvalidMoveEvent(currentContext, InvalidMoveReason.NothingToRedo))

modules/core/src/main/scala/de/nowchess/chess/observer/InvalidMoveReason.scala

@@ -0,0 +1,21 @@
+package de.nowchess.chess.observer
+
+enum InvalidMoveReason:
+  case GameAlreadyOver
+  case NoSourcePiece
+  case NotYourPiece
+  case IllegalMove
+  case PromotionPieceRequired
+  case PromotionPieceInvalid
+  case InvalidMoveFormat
+  case EmptyInput
+  case DrawCannotBeClaimed
+  case NothingToUndo
+  case NothingToRedo
+  case BotMoveIllegal
+  case BotMoveInvalidSource
+  case DrawOfferPending
+  case NoDrawOfferToAccept
+  case CannotAcceptOwnDrawOffer
+  case NoDrawOfferToDecline
+  case CannotDeclineOwnDrawOffer

modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala

@@ -36,7 +36,7 @@ case class DrawEvent(
 /** Fired when a move is invalid. */
 case class InvalidMoveEvent(
     context: GameContext,
-    reason: String,
+    reason: InvalidMoveReason,
 ) extends GameEvent
 
 /** Fired when the board is reset. */
@@ -74,6 +74,24 @@ case class PgnLoadedEvent(
     context: GameContext,
 ) extends GameEvent
 
+/** Fired when a player resigns. The opponent wins. */
+case class ResignEvent(
+    context: GameContext,
+    resignedColor: Color,
+) extends GameEvent
+
+/** Fired when a player offers a draw. Waiting for opponent to accept or decline. */
+case class DrawOfferEvent(
+    context: GameContext,
+    offeredBy: Color,
+) extends GameEvent
+
+/** Fired when the opponent declines a draw offer. */
+case class DrawOfferDeclinedEvent(
+    context: GameContext,
+    declinedBy: Color,
+) extends GameEvent
+
 /** Observer trait: implement to receive game state updates. */
 trait Observer:
   def onGameEvent(event: GameEvent): Unit

modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineDrawOfferTest.scala

@@ -0,0 +1,241 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.Color
+import de.nowchess.api.game.{DrawReason, GameResult}
+import de.nowchess.chess.observer.{
+  DrawEvent,
+  DrawOfferDeclinedEvent,
+  DrawOfferEvent,
+  GameEvent,
+  InvalidMoveEvent,
+  InvalidMoveReason,
+  Observer,
+}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class GameEngineDrawOfferTest extends AnyFunSuite with Matchers:
+
+  test("White offers draw"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: DrawOfferEvent =>
+        event.offeredBy shouldBe Color.White
+      case other =>
+        fail(s"Expected DrawOfferEvent, but got $other")
+
+  test("Black accepts White's draw offer"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.acceptDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: DrawEvent =>
+        event.reason shouldBe DrawReason.Agreement
+        event.context.result shouldBe Some(GameResult.Draw(DrawReason.Agreement))
+      case other =>
+        fail(s"Expected DrawEvent, but got $other")
+
+  test("Black declines White's draw offer"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.declineDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: DrawOfferDeclinedEvent =>
+        event.declinedBy shouldBe Color.Black
+      case other =>
+        fail(s"Expected DrawOfferDeclinedEvent, but got $other")
+
+  test("Black offers draw"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: DrawOfferEvent =>
+        event.offeredBy shouldBe Color.Black
+      case other =>
+        fail(s"Expected DrawOfferEvent, but got $other")
+
+  test("White accepts Black's draw offer"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.Black)
+    observer.events.clear()
+    engine.acceptDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: DrawEvent =>
+        event.reason shouldBe DrawReason.Agreement
+        event.context.result shouldBe Some(GameResult.Draw(DrawReason.Agreement))
+      case other =>
+        fail(s"Expected DrawEvent, but got $other")
+
+  test("Cannot accept draw when no offer pending"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.acceptDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.NoDrawOfferToAccept
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Cannot decline draw when no offer pending"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.declineDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.NoDrawOfferToDecline
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Cannot offer draw when game is already over"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    // End the game with checkmate
+    engine.processUserInput("f2f3")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("g2g4")
+    observer.events.clear()
+    engine.processUserInput("d8h4")
+
+    // Try to offer draw
+    observer.events.clear()
+    engine.offerDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.GameAlreadyOver
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Cannot accept your own draw offer"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.acceptDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.CannotAcceptOwnDrawOffer
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Cannot decline your own draw offer"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.declineDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.CannotDeclineOwnDrawOffer
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Cannot make second draw offer when one is already pending"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.offerDraw(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.DrawOfferPending
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Draw offer is cleared when game ends by resignation (accept)"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.resign(Color.Black)
+
+    // Try to accept the now-cleared draw offer
+    observer.events.clear()
+    engine.acceptDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.GameAlreadyOver
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+  test("Draw offer is cleared when game ends by resignation (decline)"):
+    val engine   = new GameEngine()
+    val observer = new DrawOfferMockObserver()
+    engine.subscribe(observer)
+
+    engine.offerDraw(Color.White)
+    observer.events.clear()
+    engine.resign(Color.Black)
+
+    // Try to accept the now-cleared draw offer
+    observer.events.clear()
+    engine.declineDraw(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.GameAlreadyOver
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+private class DrawOfferMockObserver extends Observer:
+  val events = mutable.ListBuffer[GameEvent]()
+
+  override def onGameEvent(event: GameEvent): Unit =
+    events += event

modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineIntegrationTest.scala

@@ -3,7 +3,7 @@ package de.nowchess.chess.engine
 import de.nowchess.api.board.{Board, Color, File, PieceType, Rank, Square}
 import de.nowchess.api.game.GameContext
 import de.nowchess.api.move.{Move, MoveType, PromotionPiece}
-import de.nowchess.chess.observer.{GameEvent, InvalidMoveEvent, MoveRedoneEvent, Observer}
+import de.nowchess.chess.observer.{GameEvent, InvalidMoveEvent, InvalidMoveReason, MoveRedoneEvent, Observer}
 import de.nowchess.io.GameContextImport
 import de.nowchess.rules.RuleSet
 import de.nowchess.rules.sets.DefaultRules
@@ -50,8 +50,8 @@ class GameEngineIntegrationTest extends AnyFunSuite with Matchers:
     engine.processUserInput("e2e5")
 
     events.exists {
-      case InvalidMoveEvent(_, reason) => reason.contains("Illegal move")
-      case _                           => false
+      case InvalidMoveEvent(_, InvalidMoveReason.IllegalMove) => true
+      case _                                                  => false
     } shouldBe true
 
   test("loadGame returns Left when importer fails"):

modules/core/src/test/scala/de/nowchess/chess/engine/GameEnginePromotionTest.scala

@@ -4,7 +4,16 @@ import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square
 import de.nowchess.api.game.{DrawReason, GameContext}
 import de.nowchess.api.move.{Move, MoveType, PromotionPiece}
 import de.nowchess.io.fen.FenParser
-import de.nowchess.chess.observer.*
+import de.nowchess.chess.observer.{
+  CheckDetectedEvent,
+  CheckmateEvent,
+  DrawEvent,
+  GameEvent,
+  InvalidMoveEvent,
+  InvalidMoveReason,
+  MoveExecutedEvent,
+  Observer,
+}
 import de.nowchess.rules.RuleSet
 import de.nowchess.rules.sets.DefaultRules
 import org.scalatest.funsuite.AnyFunSuite
@@ -30,8 +39,8 @@ class GameEnginePromotionTest extends AnyFunSuite with Matchers:
     engine.processUserInput("e7e8")
 
     events.exists {
-      case InvalidMoveEvent(_, reason) => reason.contains("Promotion piece required")
-      case _                           => false
+      case InvalidMoveEvent(_, InvalidMoveReason.PromotionPieceRequired) => true
+      case _                                                             => false
     } should be(true)
   }
 
@@ -198,5 +207,5 @@ class GameEnginePromotionTest extends AnyFunSuite with Matchers:
       case _                   => false
     } should be(true)
     val invalidEvt = events.collect { case e: InvalidMoveEvent => e }.last
-    invalidEvt.reason should include("Error completing promotion")
+    invalidEvt.reason shouldBe InvalidMoveReason.PromotionPieceInvalid
   }

modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineResignTest.scala

@@ -0,0 +1,70 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.Color
+import de.nowchess.api.game.GameResult
+import de.nowchess.chess.observer.{GameEvent, InvalidMoveEvent, InvalidMoveReason, Observer, ResignEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class GameEngineResignTest extends AnyFunSuite with Matchers:
+
+  test("White resigns"):
+    val engine   = new GameEngine()
+    val observer = new ResignMockObserver()
+    engine.subscribe(observer)
+
+    engine.resign(Color.White)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: ResignEvent =>
+        event.resignedColor shouldBe Color.White
+        event.context.result shouldBe Some(GameResult.Win(Color.Black))
+      case other =>
+        fail(s"Expected ResignEvent, but got $other")
+
+  test("Black resigns"):
+    val engine   = new GameEngine()
+    val observer = new ResignMockObserver()
+    engine.subscribe(observer)
+
+    engine.resign(Color.Black)
+
+    observer.events should have length 1
+    observer.events.head match
+      case event: ResignEvent =>
+        event.resignedColor shouldBe Color.Black
+        event.context.result shouldBe Some(GameResult.Win(Color.White))
+      case other =>
+        fail(s"Expected ResignEvent, but got $other")
+
+  test("Cannot resign when game is already over"):
+    val engine   = new GameEngine()
+    val observer = new ResignMockObserver()
+    engine.subscribe(observer)
+
+    // End the game with checkmate
+    engine.processUserInput("f2f3")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("g2g4")
+    observer.events.clear()
+    engine.processUserInput("d8h4")
+
+    // Try to resign
+    observer.events.clear()
+    engine.resign(Color.White)
+
+    // Should get InvalidMoveEvent with GameAlreadyOver reason
+    observer.events.length shouldBe 1
+    observer.events.head match
+      case event: InvalidMoveEvent =>
+        event.reason shouldBe InvalidMoveReason.GameAlreadyOver
+      case other =>
+        fail(s"Expected InvalidMoveEvent, but got $other")
+
+private class ResignMockObserver extends Observer:
+  val events = mutable.ListBuffer[GameEvent]()
+
+  override def onGameEvent(event: GameEvent): Unit =
+    events += event

modules/core/versions.env

@@ -1,3 +1,3 @@
 MAJOR=0
-MINOR=14
+MINOR=15
 PATCH=0

modules/io/CHANGELOG.md

@@ -56,3 +56,13 @@
 * NCS-29 JSON - Cherry Picked ([#28](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/28)) ([dbcafd2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dbcafd286993e0604a6fa286c5543581a149439e))
 * NCS-30 FEN Parser using ParserCombinators ([#21](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/21)) ([b4bc72f](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b4bc72f7e49f94d6e1bc805c68680e5fe8ef8e36))
 * NCS-31 FastParse FEN ([#22](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/22)) ([7a045d3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7a045d31d757bbc5aa6f4bad2664ebe8b8519cac))
+##  (2026-04-19)
+
+### Features
+
+* NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
+* NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+* NCS-29 JSON - Cherry Picked ([#28](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/28)) ([dbcafd2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dbcafd286993e0604a6fa286c5543581a149439e))
+* NCS-30 FEN Parser using ParserCombinators ([#21](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/21)) ([b4bc72f](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b4bc72f7e49f94d6e1bc805c68680e5fe8ef8e36))
+* NCS-31 FastParse FEN ([#22](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/22)) ([7a045d3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7a045d31d757bbc5aa6f4bad2664ebe8b8519cac))
+* NCS-41 Bot Platform ([#33](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/33)) ([dceab08](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dceab0875e6d15f7d3958633cf5dd5b29a851b1d))

modules/io/versions.env

@@ -1,3 +1,3 @@
 MAJOR=0
-MINOR=8
+MINOR=9
 PATCH=0

modules/rule/CHANGELOG.md

@@ -55,3 +55,15 @@
 ### Bug Fixes
 
 * NCS-32 Queenside Castle doesn't care about pieces in the way ([#23](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/23)) ([fe8e3c0](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fe8e3c05397f433bfa34d1999e9738c82790adf7))
+##  (2026-04-19)
+
+### Features
+
+* NCS-13 Implement Threefold Repetition ([#31](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/31)) ([767d305](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/767d3051a76c266050b6335774d66e2db2273c16))
+* NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
+* NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+* NCS-41 Bot Platform ([#33](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/33)) ([dceab08](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dceab0875e6d15f7d3958633cf5dd5b29a851b1d))
+
+### Bug Fixes
+
+* NCS-32 Queenside Castle doesn't care about pieces in the way ([#23](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/23)) ([fe8e3c0](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fe8e3c05397f433bfa34d1999e9738c82790adf7))

modules/rule/versions.env

@@ -1,3 +1,3 @@
 MAJOR=0
-MINOR=3
+MINOR=4
 PATCH=0

modules/ui/CHANGELOG.md

@@ -91,3 +91,16 @@
 * NCS-21 Write Scripts to automate certain tasks ([#15](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/15)) ([8051871](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/80518719d536a087d339fe02530825dc07f8b388))
 * NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
 * NCS-29 JSON - Cherry Picked ([#28](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/28)) ([dbcafd2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dbcafd286993e0604a6fa286c5543581a149439e))
+##  (2026-04-19)
+
+### Features
+
+* NCS-10 Implement Pawn Promotion ([#12](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/12)) ([13bfc16](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/13bfc16cfe25db78ec607db523ca6d993c13430c))
+* NCS-13 Implement Threefold Repetition ([#31](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/31)) ([767d305](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/767d3051a76c266050b6335774d66e2db2273c16))
+* NCS-14 implemented insufficient moves rule ([#30](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/30)) ([b0399a4](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/b0399a4e489950083066c9538df9a84dcc7a4613))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-17 Implement basic ScalaFX UI ([#14](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/14)) ([3ff8031](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/3ff80318b4f16c59733a46498581a5c27f048287))
+* NCS-21 Write Scripts to automate certain tasks ([#15](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/15)) ([8051871](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/80518719d536a087d339fe02530825dc07f8b388))
+* NCS-25 Add linters to keep quality up ([#27](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/27)) ([fd4e67d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/fd4e67d4f782a7e955822d90cb909d0a81676fb2))
+* NCS-29 JSON - Cherry Picked ([#28](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/28)) ([dbcafd2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dbcafd286993e0604a6fa286c5543581a149439e))
+* NCS-41 Bot Platform ([#33](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/33)) ([dceab08](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/dceab0875e6d15f7d3958633cf5dd5b29a851b1d))

modules/ui/versions.env

@@ -1,3 +1,3 @@
 MAJOR=0
-MINOR=11
+MINOR=12
 PATCH=0