From 61203a9ec4d497f10f54286dcc7350b40010b799 Mon Sep 17 00:00:00 2001
From: LQ63 <lkhermann@web.de>
Date: Sun, 29 Mar 2026 15:40:39 +0200
Subject: [PATCH] feat: NCS-9 apply en passant board mutation in GameController

Implement explicit removal of the captured pawn in en passant captures.
The pawn is located at a different square than the destination (same file
but one rank closer to the player). Use EnPassantCalculator to compute
the capture square and remove it from the board after withMove.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 .../chess/controller/GameController.scala     |  8 ++++-
 .../chess/controller/GameControllerTest.scala | 36 +++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala b/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
index acb7d17..4717430 100644
--- a/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
+++ b/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
@@ -51,9 +51,15 @@ object GameController:
                   val castleOpt = if MoveValidator.isCastle(board, from, to)
                                   then Some(MoveValidator.castleSide(from, to))
                                   else None
+                  val isEP = EnPassantCalculator.isEnPassant(board, history, from, to)
                   val (newBoard, captured) = castleOpt match
                     case Some(side) => (board.withCastle(turn, side), None)
-                    case None       => board.withMove(from, to)
+                    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 newHistory = history.addMove(from, to, castleOpt)
                   GameRules.gameStatus(newBoard, newHistory, turn.opposite) match
                     case PositionStatus.Normal  => MoveResult.Moved(newBoard, newHistory, captured, turn.opposite)
diff --git a/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala b/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
index 5e1a71e..8124005 100644
--- a/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
+++ b/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
@@ -365,3 +365,39 @@ class GameControllerTest extends AnyFunSuite with Matchers:
       case MoveResult.MovedInCheck(_, newHistory, _, _) =>
         castlingRights(newHistory, Color.White).queenSide shouldBe false
       case other => fail(s"Expected Moved or MovedInCheck, got $other")
+
+  // ──── en passant ────────────────────────────────────────────────────────
+
+  test("en passant capture removes the captured pawn from the board"):
+    // Setup: white pawn e5, black pawn just double-pushed to d5 (ep target = d6)
+    val b = Board(Map(
+      Square(File.E, Rank.R5) -> Piece.WhitePawn,
+      Square(File.D, Rank.R5) -> Piece.BlackPawn,
+      Square(File.E, Rank.R1) -> Piece.WhiteKing,
+      Square(File.E, Rank.R8) -> Piece.BlackKing
+    ))
+    val h = GameHistory.empty.addMove(Square(File.D, Rank.R7), Square(File.D, Rank.R5))
+    val result = GameController.processMove(b, h, Color.White, "e5d6")
+    result match
+      case MoveResult.Moved(newBoard, _, captured, _) =>
+        newBoard.pieceAt(Square(File.D, Rank.R5)) shouldBe None  // captured pawn removed
+        newBoard.pieceAt(Square(File.D, Rank.R6)) shouldBe Some(Piece.WhitePawn)  // capturing pawn placed
+        captured shouldBe Some(Piece.BlackPawn)
+      case other => fail(s"Expected Moved but got $other")
+
+  test("en passant capture by black removes the captured white pawn"):
+    // Setup: black pawn d4, white pawn just double-pushed to e4 (ep target = e3)
+    val b = Board(Map(
+      Square(File.D, Rank.R4) -> Piece.BlackPawn,
+      Square(File.E, Rank.R4) -> Piece.WhitePawn,
+      Square(File.E, Rank.R8) -> Piece.BlackKing,
+      Square(File.E, Rank.R1) -> Piece.WhiteKing
+    ))
+    val h = GameHistory.empty.addMove(Square(File.E, Rank.R2), Square(File.E, Rank.R4))
+    val result = GameController.processMove(b, h, Color.Black, "d4e3")
+    result match
+      case MoveResult.Moved(newBoard, _, captured, _) =>
+        newBoard.pieceAt(Square(File.E, Rank.R4)) shouldBe None  // captured pawn removed
+        newBoard.pieceAt(Square(File.E, Rank.R3)) shouldBe Some(Piece.BlackPawn)  // capturing pawn placed
+        captured shouldBe Some(Piece.WhitePawn)
+      case other => fail(s"Expected Moved but got $other")