feat: NCS-11 50-move rule (#9)

Summary

  - Implements the FIDE 50-move draw rule: a player may claim a draw if no pawn move or capture has occurred in the last
   50 full moves (100 half-moves)
  - Draw is not automatic — the eligible player must claim it via a TUI menu shown at the start of their turn
  - halfMoveClock: Int is threaded through processMove and gameLoop; resets on pawn move, capture, or en passant;
  increments on all other moves

  Changes

  - GameController.scala: extended MoveResult.Moved and MoveResult.MovedInCheck with newHalfMoveClock: Int; added
  MoveResult.DrawClaimed; added halfMoveClock parameter to processMove and gameLoop; TUI menu shown when clock ≥ 100
  - Main.scala: initial gameLoop call passes halfMoveClock = 0
  - GameControllerTest.scala: updated all existing pattern matches; added 10 new tests covering clock reset, clock
  increment, draw claim, and TUI menu behaviour

  Test plan

  - processMove: 'draw' with halfMoveClock = 100 → DrawClaimed
  - processMove: 'draw' with halfMoveClock = 99 → InvalidFormat
  - Pawn move / capture / en passant → clock resets to 0
  - Quiet piece move → clock increments by 1
  - MovedInCheck carries updated clock
  - TUI menu appears when clock ≥ 100; option 1 claims draw, option 2 continues
  - No TUI menu when clock < 100
  - All 197 tests passing

Co-authored-by: LQ63 <lkhermann@web.de>
Reviewed-on: #9
Co-authored-by: Leon Hermann <lq@blackhole.local>
Co-committed-by: Leon Hermann <lq@blackhole.local>

modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala

@@ -488,3 +488,39 @@ class GameControllerTest extends AnyFunSuite with Matchers:
       PromotionPiece.Knight, Color.White
     )
     result should be (MoveResult.Stalemate)
+
+  // ──── 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")

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

@@ -3,7 +3,7 @@ package de.nowchess.chess.engine
 import scala.collection.mutable
 import de.nowchess.api.board.{Board, Color}
 import de.nowchess.chess.logic.GameHistory
-import de.nowchess.chess.observer.{Observer, GameEvent, MoveExecutedEvent, CheckDetectedEvent, BoardResetEvent, InvalidMoveEvent}
+import de.nowchess.chess.observer.{Observer, GameEvent, MoveExecutedEvent, CheckDetectedEvent, BoardResetEvent, InvalidMoveEvent, FiftyMoveRuleAvailableEvent, DrawClaimedEvent}
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers
 
@@ -302,6 +302,47 @@ class GameEngineTest extends AnyFunSuite with Matchers:
     engine.board shouldBe boardAfterSecondMove
     engine.turn shouldBe Color.White
 
+  // ──── 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
+
   // Mock Observer for testing
   private class MockObserver extends Observer:
     val events = mutable.ListBuffer[GameEvent]()

modules/core/src/test/scala/de/nowchess/chess/logic/GameHistoryTest.scala

@@ -69,3 +69,36 @@ class GameHistoryTest extends AnyFunSuite with Matchers:
     newHistory.moves should have length 1
     newHistory.moves.head.castleSide should be (None)
     newHistory.moves.head.promotionPiece should be (Some(PromotionPiece.Queen))
+
+  // ──── 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

modules/core/src/test/scala/de/nowchess/chess/notation/FenExporterTest.scala

@@ -67,3 +67,22 @@ class FenExporterTest extends AnyFunSuite with Matchers:
     )
     val fen = FenExporter.gameStateToFen(gameState)
     fen shouldBe "rnbqkbnr/pp1ppppp/8/2pP4/8/8/PPPP1PPP/RNBQKBNR w KQkq c6 2 3"
+
+  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)
+    FenParser.parseFen(fen) match
+      case Some(gs) => gs.halfMoveClock shouldBe 42
+      case None     => fail("FEN parsing failed")

modules/core/src/test/scala/de/nowchess/chess/notation/PgnExporterTest.scala

@@ -100,3 +100,15 @@ class PgnExporterTest extends AnyFunSuite with Matchers:
     pgn should include ("e2e4")
     pgn should not include ("=")
   }
+
+  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 *"