# 50-Move Rule Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Implement the FIDE 50-move rule: track the half-move clock in `GameHistory`, let the player claim a draw by typing `draw` once the clock reaches 100, notify observers when the threshold is crossed, fix PGN to use the `Result` header as its termination marker, and verify FEN round-trips the clock correctly.

**Architecture:** `GameHistory` gains a `halfMoveClock: Int` field updated by `GameController`; `GameEngine` handles the `"draw"` text command and fires two new events (`FiftyMoveRuleAvailableEvent`, `DrawClaimedEvent`); `PgnExporter` derives its termination marker from the `Result` header instead of hardcoding `*`. No changes to `MoveValidator`, `GameRules`, `EnPassantCalculator`, or `CastlingRightsCalculator`.

**Tech Stack:** Scala 3.5.x, ScalaTest `AnyFunSuite with Matchers`, Gradle

---

## File Map

| File | Role |
|------|------|
| `modules/core/src/main/scala/de/nowchess/chess/logic/GameHistory.scala` | Add `halfMoveClock` field; extend `addMove` with clock-reset flags |
| `modules/core/src/main/scala/de/nowchess/chess/notation/PgnExporter.scala` | Use `Result` header as PGN termination marker |
| `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala` | Compute and pass `wasPawnMove`/`wasCapture` flags in `applyNormalMove` and `completePromotion` |
| `modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala` | Add `FiftyMoveRuleAvailableEvent` and `DrawClaimedEvent` |
| `modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala` | Handle `"draw"` input; fire `FiftyMoveRuleAvailableEvent` after eligible moves |
| `modules/core/src/test/scala/de/nowchess/chess/logic/GameHistoryTest.scala` | Clock update rules |
| `modules/core/src/test/scala/de/nowchess/chess/notation/PgnExporterTest.scala` | `1/2-1/2` termination marker |
| `modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala` | Clock values from `applyNormalMove` / `completePromotion` |
| `modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala` | `"draw"` command and `FiftyMoveRuleAvailableEvent` |
| `modules/core/src/test/scala/de/nowchess/chess/notation/FenExporterTest.scala` | FEN round-trip for non-zero `halfMoveClock` |

---

## Task 1: `GameHistory` — half-move clock field and `addMove` flags

**Files:**
- Modify: `modules/core/src/main/scala/de/nowchess/chess/logic/GameHistory.scala`
- Modify: `modules/core/src/test/scala/de/nowchess/chess/logic/GameHistoryTest.scala`

---

- [ ] **Step 1: Write the failing tests**

Add to `GameHistoryTest.scala` (after the existing tests):

```scala
  // ──── half-move clock ────────────────────────────────────────────────

  test("halfMoveClock starts at 0"):
    GameHistory.empty.halfMoveClock shouldBe 0

  test("halfMoveClock increments on a non-pawn non-capture move"):
    val h = GameHistory.empty.addMove(sq(File.G, Rank.R1), sq(File.F, Rank.R3))
    h.halfMoveClock shouldBe 1

  test("halfMoveClock resets to 0 on a pawn move"):
    val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4), wasPawnMove = true)
    h.halfMoveClock shouldBe 0

  test("halfMoveClock resets to 0 on a capture"):
    val h = GameHistory.empty.addMove(sq(File.E, Rank.R5), sq(File.D, Rank.R6), wasCapture = true)
    h.halfMoveClock shouldBe 0

  test("halfMoveClock resets to 0 when both wasPawnMove and wasCapture are true"):
    val h = GameHistory.empty.addMove(sq(File.E, Rank.R5), sq(File.D, Rank.R6), wasPawnMove = true, wasCapture = true)
    h.halfMoveClock shouldBe 0

  test("halfMoveClock carries across multiple moves"):
    val h = GameHistory.empty
      .addMove(sq(File.G, Rank.R1), sq(File.F, Rank.R3))          // +1 → 1
      .addMove(sq(File.G, Rank.R8), sq(File.F, Rank.R6))          // +1 → 2
      .addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4), wasPawnMove = true)  // reset → 0
      .addMove(sq(File.B, Rank.R1), sq(File.C, Rank.R3))          // +1 → 1
    h.halfMoveClock shouldBe 1

  test("GameHistory can be initialised with a non-zero halfMoveClock"):
    val h = GameHistory(halfMoveClock = 42)
    h.halfMoveClock shouldBe 42
```

- [ ] **Step 2: Run tests to verify they fail**

```bash
./gradlew :modules:core:test --tests "de.nowchess.chess.logic.GameHistoryTest" 2>&1 | tail -20
```

Expected: compile error — `halfMoveClock` not a field of `GameHistory`, `wasPawnMove`/`wasCapture` not params of `addMove`.

- [ ] **Step 3: Implement the changes in `GameHistory.scala`**

Replace the entire file with:

```scala
package de.nowchess.chess.logic

import de.nowchess.api.board.Square
import de.nowchess.api.move.PromotionPiece

/** A single move recorded in the game history. Distinct from api.move.Move which represents user intent. */
case class HistoryMove(
  from: Square,
  to: Square,
  castleSide: Option[CastleSide],
  promotionPiece: Option[PromotionPiece] = None
)

/** Complete game history: ordered list of moves plus the half-move clock for the 50-move rule.
 *
 *  @param moves         moves played so far, oldest first
 *  @param halfMoveClock plies since the last pawn move or capture (FIDE 50-move rule counter)
 */
case class GameHistory(moves: List[HistoryMove] = List.empty, halfMoveClock: Int = 0):

  /** Add a raw HistoryMove record. Clock increments by 1.
   *  Use the coordinate overload when you know whether the move is a pawn move or capture.
   */
  def addMove(move: HistoryMove): GameHistory =
    GameHistory(moves :+ move, halfMoveClock + 1)

  /** Add a move by coordinates.
   *
   *  @param wasPawnMove true when the moving piece is a pawn — resets the clock to 0
   *  @param wasCapture  true when a piece was captured (including en passant) — resets the clock to 0
   *
   *  If neither flag is set the clock increments by 1.
   */
  def addMove(
    from: Square,
    to: Square,
    castleSide: Option[CastleSide] = None,
    promotionPiece: Option[PromotionPiece] = None,
    wasPawnMove: Boolean = false,
    wasCapture: Boolean = false
  ): GameHistory =
    val newClock = if wasPawnMove || wasCapture then 0 else halfMoveClock + 1
    GameHistory(moves :+ HistoryMove(from, to, castleSide, promotionPiece), newClock)

object GameHistory:
  val empty: GameHistory = GameHistory()
```

- [ ] **Step 4: Run tests**

```bash
./gradlew :modules:core:test 2>&1 | tail -10
```

Expected: BUILD SUCCESSFUL — all existing tests pass (the new `halfMoveClock` field defaults to 0 so no existing construction sites break), new clock tests pass.

- [ ] **Step 5: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/logic/GameHistory.scala \
        modules/core/src/test/scala/de/nowchess/chess/logic/GameHistoryTest.scala
git commit -m "feat: NCS-11 add halfMoveClock to GameHistory with addMove reset flags"
```

---

## Task 2: `PgnExporter` — use `Result` header as termination marker

**Files:**
- Modify: `modules/core/src/main/scala/de/nowchess/chess/notation/PgnExporter.scala`
- Modify: `modules/core/src/test/scala/de/nowchess/chess/notation/PgnExporterTest.scala`

---

- [ ] **Step 1: Write failing tests**

Add to `PgnExporterTest.scala` (after the existing tests):

```scala
  test("exportGame uses Result header as termination marker"):
    val history = GameHistory()
      .addMove(HistoryMove(Square(File.E, Rank.R2), Square(File.E, Rank.R4), None))
    val pgn = PgnExporter.exportGame(Map("Result" -> "1/2-1/2"), history)
    pgn should endWith("1/2-1/2")

  test("exportGame with no Result header still uses * as default"):
    val history = GameHistory()
      .addMove(HistoryMove(Square(File.E, Rank.R2), Square(File.E, Rank.R4), None))
    val pgn = PgnExporter.exportGame(Map.empty, history)
    pgn shouldBe "1. e2e4 *"
```

- [ ] **Step 2: Run tests to verify they fail**

```bash
./gradlew :modules:core:test --tests "de.nowchess.chess.notation.PgnExporterTest" 2>&1 | tail -20
```

Expected: FAIL — `exportGame` with `"Result" -> "1/2-1/2"` still produces `*`.

- [ ] **Step 3: Implement the fix in `PgnExporter.scala`**

Find the line that builds the move text — currently:

```scala
moveLines.mkString(" ") + " *"
```

Replace it with:

```scala
val termination = headers.getOrElse("Result", "*")
moveLines.mkString(" ") + s" $termination"
```

The full updated `exportGame` method:

```scala
  def exportGame(headers: Map[String, String], history: GameHistory): String =
    val headerLines = headers.map { case (key, value) =>
      s"""[$key "$value"]"""
    }.mkString("\n")

    val moveText = if history.moves.isEmpty then ""
    else
      val groupedMoves = history.moves.zipWithIndex.groupBy(_._2 / 2)
      val moveLines = for (moveNumber, movePairs) <- groupedMoves.toList.sortBy(_._1) yield
        val moveNum = moveNumber + 1
        val whiteMoveStr = movePairs.find(_._2 % 2 == 0).map(p => moveToAlgebraic(p._1)).getOrElse("")
        val blackMoveStr = movePairs.find(_._2 % 2 == 1).map(p => moveToAlgebraic(p._1)).getOrElse("")
        if blackMoveStr.isEmpty then s"$moveNum. $whiteMoveStr"
        else s"$moveNum. $whiteMoveStr $blackMoveStr"

      val termination = headers.getOrElse("Result", "*")
      moveLines.mkString(" ") + s" $termination"

    if headerLines.isEmpty then moveText
    else if moveText.isEmpty then headerLines
    else s"$headerLines\n\n$moveText"
```

- [ ] **Step 4: Run tests**

```bash
./gradlew :modules:core:test --tests "de.nowchess.chess.notation.PgnExporterTest" 2>&1 | tail -10
```

Expected: BUILD SUCCESSFUL, all PGN exporter tests pass.

- [ ] **Step 5: Run full test suite**

```bash
./gradlew :modules:core:test 2>&1 | tail -10
```

Expected: BUILD SUCCESSFUL.

- [ ] **Step 6: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/notation/PgnExporter.scala \
        modules/core/src/test/scala/de/nowchess/chess/notation/PgnExporterTest.scala
git commit -m "feat: NCS-11 derive PGN termination marker from Result header"
```

---

## Task 3: `GameController` — pass clock flags in `applyNormalMove` and `completePromotion`

**Files:**
- Modify: `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala`
- Modify: `modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala`

---

- [ ] **Step 1: Write failing tests**

Add to `GameControllerTest.scala` (after the existing tests):

```scala
  // ──── half-move clock propagation ────────────────────────────────────

  test("processMove: non-pawn non-capture increments halfMoveClock"):
    // g1f3 is a knight move — not a pawn, not a capture
    processMove(Board.initial, GameHistory.empty, Color.White, "g1f3") match
      case MoveResult.Moved(_, newHistory, _, _) =>
        newHistory.halfMoveClock shouldBe 1
      case other => fail(s"Expected Moved, got $other")

  test("processMove: pawn move resets halfMoveClock to 0"):
    processMove(Board.initial, GameHistory.empty, Color.White, "e2e4") match
      case MoveResult.Moved(_, newHistory, _, _) =>
        newHistory.halfMoveClock shouldBe 0
      case other => fail(s"Expected Moved, got $other")

  test("processMove: capture resets halfMoveClock to 0"):
    // White pawn on e5, Black pawn on d6 — exd6 is a capture
    val board = Board(Map(
      sq(File.E, Rank.R5) -> Piece.WhitePawn,
      sq(File.D, Rank.R6) -> Piece.BlackPawn,
      sq(File.E, Rank.R1) -> Piece.WhiteKing,
      sq(File.E, Rank.R8) -> Piece.BlackKing
    ))
    val history = GameHistory(halfMoveClock = 10)
    processMove(board, history, Color.White, "e5d6") match
      case MoveResult.Moved(_, newHistory, _, _) =>
        newHistory.halfMoveClock shouldBe 0
      case other => fail(s"Expected Moved, got $other")

  test("processMove: clock carries from previous history on non-pawn non-capture"):
    val history = GameHistory(halfMoveClock = 5)
    processMove(Board.initial, history, Color.White, "g1f3") match
      case MoveResult.Moved(_, newHistory, _, _) =>
        newHistory.halfMoveClock shouldBe 6
      case other => fail(s"Expected Moved, got $other")
```

- [ ] **Step 2: Run tests to verify they fail**

```bash
./gradlew :modules:core:test --tests "de.nowchess.chess.controller.GameControllerTest" 2>&1 | tail -20
```

Expected: FAIL — clock tests show 1 where 0 is expected (pawn/capture not resetting) and 0 where 1 is expected (knight not incrementing from initial empty history).

- [ ] **Step 3: Implement the fix in `GameController.scala`**

Update `applyNormalMove` and `completePromotion`. Replace the entire file with:

```scala
package de.nowchess.chess.controller

import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
import de.nowchess.api.move.PromotionPiece
import de.nowchess.chess.logic.*

// ---------------------------------------------------------------------------
// Result ADT returned by the pure processMove function
// ---------------------------------------------------------------------------

sealed trait MoveResult
object MoveResult:
  case object Quit                                                                       extends MoveResult
  case class  InvalidFormat(raw: String)                                                extends MoveResult
  case object NoPiece                                                                   extends MoveResult
  case object WrongColor                                                                extends MoveResult
  case object IllegalMove                                                               extends MoveResult
  case class  PromotionRequired(
    from: Square,
    to: Square,
    boardBefore: Board,
    historyBefore: GameHistory,
    captured: Option[Piece],
    turn: Color
  ) extends MoveResult
  case class  Moved(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], newTurn: Color)      extends MoveResult
  case class  MovedInCheck(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], newTurn: Color) extends MoveResult
  case class  Checkmate(winner: Color)                                                  extends MoveResult
  case object Stalemate                                                                 extends MoveResult

// ---------------------------------------------------------------------------
// Controller
// ---------------------------------------------------------------------------

object GameController:

  /** Pure function: interprets one raw input line against the current game context.
   *  Has no I/O side effects — all output must be handled by the caller.
   */
  def processMove(board: Board, history: GameHistory, turn: Color, raw: String): MoveResult =
    raw.trim match
      case "quit" | "q" => MoveResult.Quit
      case trimmed =>
        Parser.parseMove(trimmed) match
          case None              => MoveResult.InvalidFormat(trimmed)
          case Some((from, to))  => validateAndApply(board, history, turn, from, to)

  /** Apply a previously detected promotion move with the chosen piece.
   *  Called after processMove returned PromotionRequired.
   */
  def completePromotion(
    board: Board,
    history: GameHistory,
    from: Square,
    to: Square,
    piece: PromotionPiece,
    turn: Color
  ): MoveResult =
    val (boardAfterMove, captured) = board.withMove(from, to)
    val promotedPieceType = piece match
      case PromotionPiece.Queen  => PieceType.Queen
      case PromotionPiece.Rook   => PieceType.Rook
      case PromotionPiece.Bishop => PieceType.Bishop
      case PromotionPiece.Knight => PieceType.Knight
    val newBoard   = boardAfterMove.updated(to, Piece(turn, promotedPieceType))
    // Promotion is always a pawn move → clock resets
    val newHistory = history.addMove(from, to, None, Some(piece), wasPawnMove = true)
    toMoveResult(newBoard, newHistory, captured, turn)

  // ---------------------------------------------------------------------------
  // Private helpers
  // ---------------------------------------------------------------------------

  private def validateAndApply(board: Board, history: GameHistory, turn: Color, from: Square, to: Square): MoveResult =
    board.pieceAt(from) match
      case None                              => MoveResult.NoPiece
      case Some(piece) if piece.color != turn => MoveResult.WrongColor
      case Some(_) =>
        if !MoveValidator.isLegal(board, history, from, to) then MoveResult.IllegalMove
        else if MoveValidator.isPromotionMove(board, from, to) then
          MoveResult.PromotionRequired(from, to, board, history, board.pieceAt(to), turn)
        else applyNormalMove(board, history, turn, from, to)

  private def applyNormalMove(board: Board, history: GameHistory, turn: Color, from: Square, to: Square): MoveResult =
    val castleOpt            = Option.when(MoveValidator.isCastle(board, from, to))(MoveValidator.castleSide(from, to))
    val isEP                 = EnPassantCalculator.isEnPassant(board, history, from, to)
    val (newBoard, captured) = castleOpt match
      case Some(side) => (board.withCastle(turn, side), None)
      case None =>
        val (b, cap) = board.withMove(from, to)
        if isEP then
          val capturedSq = EnPassantCalculator.capturedPawnSquare(to, turn)
          (b.removed(capturedSq), board.pieceAt(capturedSq))
        else (b, cap)
    val wasPawnMove = board.pieceAt(from).exists(_.pieceType == PieceType.Pawn)
    val wasCapture  = captured.isDefined
    val newHistory  = history.addMove(from, to, castleOpt, wasPawnMove = wasPawnMove, wasCapture = wasCapture)
    toMoveResult(newBoard, newHistory, captured, turn)

  private def toMoveResult(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], turn: Color): MoveResult =
    GameRules.gameStatus(newBoard, newHistory, turn.opposite) match
      case PositionStatus.Normal  => MoveResult.Moved(newBoard, newHistory, captured, turn.opposite)
      case PositionStatus.InCheck => MoveResult.MovedInCheck(newBoard, newHistory, captured, turn.opposite)
      case PositionStatus.Mated   => MoveResult.Checkmate(turn)
      case PositionStatus.Drawn   => MoveResult.Stalemate
```

- [ ] **Step 4: Run tests**

```bash
./gradlew :modules:core:test 2>&1 | tail -10
```

Expected: BUILD SUCCESSFUL.

- [ ] **Step 5: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala \
        modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
git commit -m "feat: NCS-11 propagate half-move clock flags through GameController"
```

---

## Task 4: `Observer` events + `GameEngine` draw command

**Files:**
- Modify: `modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala`
- Modify: `modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala`
- Modify: `modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala`
- Modify: `modules/core/src/test/scala/de/nowchess/chess/notation/FenExporterTest.scala`

---

- [ ] **Step 1: Write failing tests**

Add the following to `GameEngineTest.scala`. The mock observer class is already present at the bottom of that file — add these tests before it:

```scala
  // ──── 50-move rule ───────────────────────────────────────────────────

  test("GameEngine: 'draw' rejected when halfMoveClock < 100"):
    val engine = new GameEngine()
    val observer = new MockObserver()
    engine.subscribe(observer)
    engine.processUserInput("draw")
    observer.events.size shouldBe 1
    observer.events.head shouldBe a[InvalidMoveEvent]

  test("GameEngine: 'draw' accepted and fires DrawClaimedEvent when halfMoveClock >= 100"):
    val engine = new GameEngine(initialHistory = GameHistory(halfMoveClock = 100))
    val observer = new MockObserver()
    engine.subscribe(observer)
    engine.processUserInput("draw")
    observer.events.size shouldBe 1
    observer.events.head shouldBe a[DrawClaimedEvent]

  test("GameEngine: state resets to initial after draw claimed"):
    val engine = new GameEngine(initialHistory = GameHistory(halfMoveClock = 100))
    engine.processUserInput("draw")
    engine.board shouldBe Board.initial
    engine.history shouldBe GameHistory.empty
    engine.turn shouldBe Color.White

  test("GameEngine: FiftyMoveRuleAvailableEvent fired when move brings clock to 100"):
    // Start at clock 99; a knight move (non-pawn, non-capture) increments to 100
    val engine = new GameEngine(initialHistory = GameHistory(halfMoveClock = 99))
    val observer = new MockObserver()
    engine.subscribe(observer)
    engine.processUserInput("g1f3")  // knight move on initial board
    // Should receive MoveExecutedEvent AND FiftyMoveRuleAvailableEvent
    observer.events.exists(_.isInstanceOf[FiftyMoveRuleAvailableEvent]) shouldBe true

  test("GameEngine: FiftyMoveRuleAvailableEvent not fired when clock is below 100 after move"):
    val engine = new GameEngine(initialHistory = GameHistory(halfMoveClock = 5))
    val observer = new MockObserver()
    engine.subscribe(observer)
    engine.processUserInput("g1f3")
    observer.events.exists(_.isInstanceOf[FiftyMoveRuleAvailableEvent]) shouldBe false
```

Also add the import to `GameEngineTest.scala`'s import block:

```scala
import de.nowchess.chess.observer.{Observer, GameEvent, MoveExecutedEvent, CheckDetectedEvent, BoardResetEvent, InvalidMoveEvent, FiftyMoveRuleAvailableEvent, DrawClaimedEvent}
```

Add the following to `FenExporterTest.scala`:

```scala
  test("halfMoveClock round-trips through FEN export and import"):
    import de.nowchess.chess.logic.GameHistory
    import de.nowchess.chess.notation.FenParser
    val history = GameHistory(halfMoveClock = 42)
    val gameState = GameState(
      piecePlacement  = FenExporter.boardToFen(de.nowchess.api.board.Board.initial),
      activeColor     = Color.White,
      castlingWhite   = CastlingRights.Both,
      castlingBlack   = CastlingRights.Both,
      enPassantTarget = None,
      halfMoveClock   = history.halfMoveClock,
      fullMoveNumber  = 1,
      status          = GameStatus.InProgress
    )
    val fen    = FenExporter.gameStateToFen(gameState)
    val parsed = FenParser.parseFen(fen).get
    parsed.halfMoveClock shouldBe 42
```

- [ ] **Step 2: Run tests to verify they fail**

```bash
./gradlew :modules:core:test --tests "de.nowchess.chess.engine.GameEngineTest" 2>&1 | tail -20
```

Expected: compile error — `DrawClaimedEvent` and `FiftyMoveRuleAvailableEvent` not yet defined.

- [ ] **Step 3: Add new events to `Observer.scala`**

Add the two new event cases to `Observer.scala`, after `BoardResetEvent`:

```scala
/** Fired after any move where the half-move clock reaches 100 — the 50-move rule is now claimable. */
case class FiftyMoveRuleAvailableEvent(
  board: Board,
  history: GameHistory,
  turn: Color
) extends GameEvent

/** Fired when a player successfully claims a draw under the 50-move rule. */
case class DrawClaimedEvent(
  board: Board,
  history: GameHistory,
  turn: Color
) extends GameEvent
```

- [ ] **Step 4: Update `GameEngine.scala` — add `"draw"` case and `FiftyMoveRuleAvailableEvent` notification**

In `processUserInput`, add the `"draw"` case immediately before the `case moveInput =>` fallthrough (after `case "redo" =>`):

```scala
      case "draw" =>
        if currentHistory.halfMoveClock >= 100 then
          currentBoard   = Board.initial
          currentHistory = GameHistory.empty
          currentTurn    = Color.White
          invoker.clear()
          notifyObservers(DrawClaimedEvent(currentBoard, currentHistory, currentTurn))
        else
          notifyObservers(InvalidMoveEvent(
            currentBoard, currentHistory, currentTurn,
            "Draw cannot be claimed: the 50-move rule has not been triggered."
          ))
```

In the same method, in both the `MoveResult.Moved` and `MoveResult.MovedInCheck` handling branches, add a `FiftyMoveRuleAvailableEvent` check **after** the existing `notifyObservers` call for that branch.

The `Moved` branch currently reads:

```scala
              case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
                invoker.execute(updatedCmd)
                updateGameState(newBoard, newHistory, newTurn)
                emitMoveEvent(from.toString, to.toString, captured, newTurn)
```

Replace it with:

```scala
              case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
                invoker.execute(updatedCmd)
                updateGameState(newBoard, newHistory, newTurn)
                emitMoveEvent(from.toString, to.toString, captured, newTurn)
                if newHistory.halfMoveClock >= 100 then
                  notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
```

The `MovedInCheck` branch currently reads:

```scala
              case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
                invoker.execute(updatedCmd)
                updateGameState(newBoard, newHistory, newTurn)
                emitMoveEvent(from.toString, to.toString, captured, newTurn)
                notifyObservers(CheckDetectedEvent(currentBoard, currentHistory, currentTurn))
```

Replace it with:

```scala
              case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
                invoker.execute(updatedCmd)
                updateGameState(newBoard, newHistory, newTurn)
                emitMoveEvent(from.toString, to.toString, captured, newTurn)
                notifyObservers(CheckDetectedEvent(currentBoard, currentHistory, currentTurn))
                if newHistory.halfMoveClock >= 100 then
                  notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
```

Also add `FiftyMoveRuleAvailableEvent` and `DrawClaimedEvent` to the import at the top of `GameEngine.scala`:

```scala
import de.nowchess.chess.observer.*
```

(Already a wildcard import — no change needed there.)

- [ ] **Step 5: Run tests**

```bash
./gradlew :modules:core:test 2>&1 | tail -10
```

Expected: BUILD SUCCESSFUL, all tests pass.

- [ ] **Step 6: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala \
        modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala \
        modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala \
        modules/core/src/test/scala/de/nowchess/chess/notation/FenExporterTest.scala
git commit -m "feat: NCS-11 implement 50-move rule draw claim and observer events"
```