refactor(core): NCS-22 update GameEngine to use GameContext and integrate Rule module

modules/rule/src/main/scala/de/nowchess/rules/RuleSet.scala

@@ -31,3 +31,9 @@ trait RuleSet:
 
   /** True if halfMoveClock >= 100 (50-move rule). */
   def isFiftyMoveRule(context: GameContext): Boolean
+
+  /** Apply a legal move to produce the next game context.
+   *  Handles all special move types: castling, en passant, promotion.
+   *  Updates castling rights, en passant square, half-move clock, turn, and move history.
+   */
+  def applyMove(context: GameContext, move: Move): GameContext

modules/rule/src/main/scala/de/nowchess/rules/StandardRules.scala

@@ -1,7 +1,7 @@
 package de.nowchess.rules
 
 import de.nowchess.api.game.GameContext
-import de.nowchess.api.board.{Board, Color, Square, PieceType, Piece}
+import de.nowchess.api.board.{Board, CastlingRights, Color, File, Rank, Square, PieceType, Piece}
 import de.nowchess.api.move.{Move, MoveType, PromotionPiece}
 import scala.annotation.tailrec
 
@@ -274,6 +274,95 @@ object StandardRules extends RuleSet:
     val nextContext = context.withBoard(nextBoard)
     isCheck(nextContext)
 
+  // ── Move application ───────────────────────────────────────────────
+
+  override def applyMove(context: GameContext, move: Move): GameContext =
+    val color = context.turn
+    val board  = context.board
+
+    val newBoard = move.moveType match
+      case MoveType.CastleKingside  => applyCastle(board, color, kingside = true)
+      case MoveType.CastleQueenside => applyCastle(board, color, kingside = false)
+      case MoveType.EnPassant       => applyEnPassant(board, move, color)
+      case MoveType.Promotion(pp)   => applyPromotion(board, move, color, pp)
+      case MoveType.Normal          => board.applyMove(move)
+
+    val newCastlingRights = updateCastlingRights(context.castlingRights, board, move, color)
+    val newEnPassantSquare = computeEnPassantSquare(board, move, color)
+    val isCapture = board.pieceAt(move.to).isDefined || move.moveType == MoveType.EnPassant
+    val isPawnMove = board.pieceAt(move.from).exists(_.pieceType == PieceType.Pawn)
+    val newClock = if isPawnMove || isCapture then 0 else context.halfMoveClock + 1
+
+    context
+      .withBoard(newBoard)
+      .withTurn(color.opposite)
+      .withCastlingRights(newCastlingRights)
+      .withEnPassantSquare(newEnPassantSquare)
+      .withHalfMoveClock(newClock)
+      .withMove(move)
+
+  private def applyCastle(board: Board, color: Color, kingside: Boolean): Board =
+    val rank = if color == Color.White then Rank.R1 else Rank.R8
+    val (kingFrom, kingTo, rookFrom, rookTo) =
+      if kingside then
+        (Square(File.E, rank), Square(File.G, rank), Square(File.H, rank), Square(File.F, rank))
+      else
+        (Square(File.E, rank), Square(File.C, rank), Square(File.A, rank), Square(File.D, rank))
+    val king = board.pieceAt(kingFrom).getOrElse(Piece(color, PieceType.King))
+    val rook = board.pieceAt(rookFrom).getOrElse(Piece(color, PieceType.Rook))
+    board
+      .removed(kingFrom).removed(rookFrom)
+      .updated(kingTo, king)
+      .updated(rookTo, rook)
+
+  private def applyEnPassant(board: Board, move: Move, color: Color): Board =
+    val capturedRank = move.from.rank  // the captured pawn is on the same rank as the moving pawn
+    val capturedSquare = Square(move.to.file, capturedRank)
+    board.applyMove(move).removed(capturedSquare)
+
+  private def applyPromotion(board: Board, move: Move, color: Color, pp: PromotionPiece): Board =
+    val promotedType = pp match
+      case PromotionPiece.Queen  => PieceType.Queen
+      case PromotionPiece.Rook   => PieceType.Rook
+      case PromotionPiece.Bishop => PieceType.Bishop
+      case PromotionPiece.Knight => PieceType.Knight
+    board.removed(move.from).updated(move.to, Piece(color, promotedType))
+
+  private def updateCastlingRights(rights: CastlingRights, board: Board, move: Move, color: Color): CastlingRights =
+    val piece = board.pieceAt(move.from)
+    val isKingMove = piece.exists(_.pieceType == PieceType.King)
+    val isRookMove = piece.exists(_.pieceType == PieceType.Rook)
+
+    // Helper to check if a square is a rook's starting square
+    val whiteKingsideRook  = Square(File.H, Rank.R1)
+    val whiteQueensideRook = Square(File.A, Rank.R1)
+    val blackKingsideRook  = Square(File.H, Rank.R8)
+    val blackQueensideRook = Square(File.A, Rank.R8)
+
+    var r = rights
+    if isKingMove then r = r.revokeColor(color)
+    else if isRookMove then
+      if move.from == whiteKingsideRook  then r = r.revokeKingSide(Color.White)
+      if move.from == whiteQueensideRook then r = r.revokeQueenSide(Color.White)
+      if move.from == blackKingsideRook  then r = r.revokeKingSide(Color.Black)
+      if move.from == blackQueensideRook then r = r.revokeQueenSide(Color.Black)
+    // Also revoke if a rook is captured
+    if move.to == whiteKingsideRook  then r = r.revokeKingSide(Color.White)
+    if move.to == whiteQueensideRook then r = r.revokeQueenSide(Color.White)
+    if move.to == blackKingsideRook  then r = r.revokeKingSide(Color.Black)
+    if move.to == blackQueensideRook then r = r.revokeQueenSide(Color.Black)
+    r
+
+  private def computeEnPassantSquare(board: Board, move: Move, color: Color): Option[Square] =
+    val piece = board.pieceAt(move.from)
+    val isDoublePawnPush = piece.exists(_.pieceType == PieceType.Pawn) &&
+      math.abs(move.to.rank.ordinal - move.from.rank.ordinal) == 2
+    if isDoublePawnPush then
+      // EP square is the square the pawn passed through
+      val epRankOrd = (move.from.rank.ordinal + move.to.rank.ordinal) / 2
+      Some(Square(move.from.file, Rank.values(epRankOrd)))
+    else None
+
   // ── Insufficient material ──────────────────────────────────────────
 
   private def insufficientMaterial(board: Board): Boolean =