feat: NCS-13 Implement Threefold Repetition (#31)

Reviewed-on: #31

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

@@ -17,8 +17,13 @@ class GameEngine(
     val initialContext: GameContext = GameContext.initial,
     val ruleSet: RuleSet = DefaultRules,
 ) extends Observable:
+  // Ensure that initialBoard is set correctly for threefold repetition detection
+  private val contextWithInitialBoard = if initialContext.moves.isEmpty && initialContext.board != initialContext.initialBoard then
+    initialContext.copy(initialBoard = initialContext.board)
+  else
+    initialContext
   @SuppressWarnings(Array("DisableSyntax.var"))
-  private var currentContext: GameContext = initialContext
+  private var currentContext: GameContext = contextWithInitialBoard
   private val invoker                     = new CommandInvoker()
 
   /** Pending promotion: the Move that triggered it (from/to only, moveType filled in later). */
@@ -63,11 +68,15 @@ class GameEngine(
           currentContext = currentContext.withResult(Some(GameResult.Draw(DrawReason.FiftyMoveRule)))
           invoker.clear()
           notifyObservers(DrawEvent(currentContext, DrawReason.FiftyMoveRule))
+        else if ruleSet.isThreefoldRepetition(currentContext) then
+          currentContext = currentContext.withResult(Some(GameResult.Draw(DrawReason.ThreefoldRepetition)))
+          invoker.clear()
+          notifyObservers(DrawEvent(currentContext, DrawReason.ThreefoldRepetition))
         else
           notifyObservers(
             InvalidMoveEvent(
               currentContext,
-              "Draw cannot be claimed: the 50-move rule has not been triggered.",
+              "Draw cannot be claimed: neither the 50-move rule nor threefold repetition has been triggered.",
             ),
           )
 
@@ -154,7 +163,7 @@ class GameEngine(
     invoker.clear()
 
     if ctx.moves.isEmpty then
-      currentContext = ctx
+      currentContext = ctx.copy(initialBoard = ctx.board)
       Right(())
     else replayMoves(ctx.moves, savedContext)
 
@@ -182,7 +191,11 @@ class GameEngine(
 
   /** Load an arbitrary board position, clearing all history and undo/redo state. */
   def loadPosition(newContext: GameContext): Unit = synchronized {
-    currentContext = newContext
+    val contextWithInitialBoard = if newContext.moves.isEmpty then
+      newContext.copy(initialBoard = newContext.board)
+    else
+      newContext
+    currentContext = contextWithInitialBoard
     pendingPromotion = None
     invoker.clear()
     notifyObservers(BoardResetEvent(currentContext))
@@ -237,6 +250,7 @@ class GameEngine(
     else if ruleSet.isCheck(currentContext) then notifyObservers(CheckDetectedEvent(currentContext))
 
     if currentContext.halfMoveClock >= 100 then notifyObservers(FiftyMoveRuleAvailableEvent(currentContext))
+    if ruleSet.isThreefoldRepetition(currentContext) then notifyObservers(ThreefoldRepetitionAvailableEvent(currentContext))
 
   private def translateMoveToNotation(move: Move, boardBefore: Board): String =
     move.moveType match

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

@@ -56,6 +56,11 @@ case class FiftyMoveRuleAvailableEvent(
     context: GameContext,
 ) extends GameEvent
 
+/** Fired after any move where the same position occurs for the third time — threefold repetition is now claimable. */
+case class ThreefoldRepetitionAvailableEvent(
+    context: GameContext,
+) extends GameEvent
+
 /** Fired when a move is undone, carrying PGN notation of the reversed move. */
 case class MoveUndoneEvent(
     context: GameContext,

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

@@ -98,6 +98,7 @@ class GameEngineIntegrationTest extends AnyFunSuite with Matchers:
       def isStalemate(context: GameContext): Boolean               = false
       def isInsufficientMaterial(context: GameContext): Boolean    = false
       def isFiftyMoveRule(context: GameContext): Boolean           = false
+      def isThreefoldRepetition(context: GameContext): Boolean     = false
       def applyMove(context: GameContext)(move: Move): GameContext = DefaultRules.applyMove(context)(move)
 
     val engine = new GameEngine(ruleSet = permissiveRules)
@@ -119,6 +120,7 @@ class GameEngineIntegrationTest extends AnyFunSuite with Matchers:
       def isStalemate(context: GameContext): Boolean                       = false
       def isInsufficientMaterial(context: GameContext): Boolean            = false
       def isFiftyMoveRule(context: GameContext): Boolean                   = false
+      def isThreefoldRepetition(context: GameContext): Boolean             = false
       def applyMove(context: GameContext)(move: Move): GameContext         = context
 
     val engine = new GameEngine(ruleSet = noLegalMoves)

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

@@ -221,3 +221,75 @@ class GameEngineOutcomesTest extends AnyFunSuite with Matchers:
     evt.isDefined shouldBe true
     evt.get.reason shouldBe DrawReason.InsufficientMaterial
     engine.context.result shouldBe Some(GameResult.Draw(DrawReason.InsufficientMaterial))
+
+  // ── Threefold Repetition ──────────────────────────────────────────
+
+  test("draw command rejected when neither 50-move rule nor threefold repetition available"):
+    val engine   = EngineTestHelpers.makeEngine()
+    val observer = new EngineTestHelpers.MockObserver()
+    engine.subscribe(observer)
+
+    engine.processUserInput("e2e4")
+    observer.clear()
+
+    engine.processUserInput("draw")
+
+    observer.hasEvent[InvalidMoveEvent] shouldBe true
+
+  test("threefold repetition fires ThreefoldRepetitionAvailableEvent after 8-move shuffle"):
+    val engine   = EngineTestHelpers.makeEngine()
+    val observer = new EngineTestHelpers.MockObserver()
+    engine.subscribe(observer)
+
+    // Both knights shuffle home: initial position occurs 3 times on move 8 (Ng8)
+    engine.processUserInput("g1f3")
+    engine.processUserInput("g8f6")
+    engine.processUserInput("f3g1")
+    engine.processUserInput("f6g8")
+    engine.processUserInput("g1f3")
+    engine.processUserInput("g8f6")
+    engine.processUserInput("f3g1")
+    observer.clear()
+
+    engine.processUserInput("f6g8")  // 3rd occurrence of initial position
+
+    observer.hasEvent[ThreefoldRepetitionAvailableEvent] shouldBe true
+    engine.context.result shouldBe None  // claimable, not automatic
+
+  test("draw claim via threefold repetition ends game with DrawEvent"):
+    val engine   = EngineTestHelpers.makeEngine()
+    val observer = new EngineTestHelpers.MockObserver()
+    engine.subscribe(observer)
+
+    engine.processUserInput("g1f3")
+    engine.processUserInput("g8f6")
+    engine.processUserInput("f3g1")
+    engine.processUserInput("f6g8")
+    engine.processUserInput("g1f3")
+    engine.processUserInput("g8f6")
+    engine.processUserInput("f3g1")
+    engine.processUserInput("f6g8")  // threefold now available
+
+    observer.clear()
+    engine.processUserInput("draw")
+
+    val evt = observer.getEvent[DrawEvent]
+    evt.isDefined shouldBe true
+    evt.get.reason shouldBe DrawReason.ThreefoldRepetition
+    engine.context.result shouldBe Some(GameResult.Draw(DrawReason.ThreefoldRepetition))
+
+  test("loadPosition with non-empty moves preserves context as-is"):
+    val engine   = EngineTestHelpers.makeEngine()
+    val observer = new EngineTestHelpers.MockObserver()
+    engine.subscribe(observer)
+
+    // Build a context that already has a move in its history
+    val move = de.nowchess.api.move.Move(
+      de.nowchess.api.board.Square(de.nowchess.api.board.File.E, de.nowchess.api.board.Rank.R2),
+      de.nowchess.api.board.Square(de.nowchess.api.board.File.E, de.nowchess.api.board.Rank.R4),
+    )
+    val ctxWithMove = de.nowchess.api.game.GameContext.initial.withMove(move)
+
+    engine.loadPosition(ctxWithMove)
+
+    observer.hasEvent[BoardResetEvent] shouldBe true

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

@@ -173,6 +173,8 @@ class GameEnginePromotionTest extends AnyFunSuite with Matchers:
         DefaultRules.isInsufficientMaterial(context)
       def isFiftyMoveRule(context: GameContext): Boolean =
         DefaultRules.isFiftyMoveRule(context)
+      def isThreefoldRepetition(context: GameContext): Boolean =
+        DefaultRules.isThreefoldRepetition(context)
       def applyMove(context: GameContext)(move: Move): GameContext =
         DefaultRules.applyMove(context)(move)