feat: NCS-9 add EnPassantCalculator with target derivation and capture logic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
2026-03-29 15:26:44 +02:00
parent 51f1b08686
commit ccbd31fe32
2 changed files with 133 additions and 0 deletions
@@ -0,0 +1,32 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.*
+
+object EnPassantCalculator:
+
+  /** Returns the en passant target square if the last move was a double pawn push.
+   *  The target is the square the pawn passed through (e.g. e2→e4 yields e3).
+   */
+  def enPassantTarget(board: Board, history: GameHistory): Option[Square] =
+    history.moves.lastOption.flatMap: move =>
+      val rankDiff = move.to.rank.ordinal - move.from.rank.ordinal
+      val isDoublePush = math.abs(rankDiff) == 2
+      val isPawn = board.pieceAt(move.to).exists(_.pieceType == PieceType.Pawn)
+      if isDoublePush && isPawn then
+        val midRankIdx = move.from.rank.ordinal + rankDiff / 2
+        Some(Square(move.to.file, Rank.values(midRankIdx)))
+      else None
+
+  /** True if moving from→to is an en passant capture. */
+  def isEnPassant(board: Board, history: GameHistory, from: Square, to: Square): Boolean =
+    board.pieceAt(from).exists(_.pieceType == PieceType.Pawn) &&
+    enPassantTarget(board, history).contains(to) &&
+    math.abs(to.file.ordinal - from.file.ordinal) == 1
+
+  /** Returns the square of the pawn to remove when an en passant capture lands on `to`.
+   *  White captures upward → captured pawn is one rank below `to`.
+   *  Black captures downward → captured pawn is one rank above `to`.
+   */
+  def capturedPawnSquare(to: Square, color: Color): Square =
+    val capturedRankIdx = to.rank.ordinal + (if color == Color.White then -1 else 1)
+    Square(to.file, Rank.values(capturedRankIdx))
@@ -0,0 +1,101 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.*
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class EnPassantCalculatorTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+  private def board(entries: (Square, Piece)*): Board = Board(entries.toMap)
+
+  // ──── enPassantTarget ────────────────────────────────────────────────
+
+  test("enPassantTarget returns None for empty history"):
+    val b = board(sq(File.E, Rank.R4) -> Piece.WhitePawn)
+    EnPassantCalculator.enPassantTarget(b, GameHistory.empty) shouldBe None
+
+  test("enPassantTarget returns None when last move was a single pawn push"):
+    val b = board(sq(File.E, Rank.R3) -> Piece.WhitePawn)
+    val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R3))
+    EnPassantCalculator.enPassantTarget(b, h) shouldBe None
+
+  test("enPassantTarget returns None when last move was not a pawn"):
+    val b = board(sq(File.E, Rank.R4) -> Piece.WhiteRook)
+    val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    EnPassantCalculator.enPassantTarget(b, h) shouldBe None
+
+  test("enPassantTarget returns e3 after white pawn double push e2-e4"):
+    val b = board(sq(File.E, Rank.R4) -> Piece.WhitePawn)
+    val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    EnPassantCalculator.enPassantTarget(b, h) shouldBe Some(sq(File.E, Rank.R3))
+
+  test("enPassantTarget returns e6 after black pawn double push e7-e5"):
+    val b = board(sq(File.E, Rank.R5) -> Piece.BlackPawn)
+    val h = GameHistory.empty.addMove(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    EnPassantCalculator.enPassantTarget(b, h) shouldBe Some(sq(File.E, Rank.R6))
+
+  test("enPassantTarget returns d3 after white pawn double push d2-d4"):
+    val b = board(sq(File.D, Rank.R4) -> Piece.WhitePawn)
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    EnPassantCalculator.enPassantTarget(b, h) shouldBe Some(sq(File.D, Rank.R3))
+
+  // ──── capturedPawnSquare ─────────────────────────────────────────────
+
+  test("capturedPawnSquare for white capturing on e6 returns e5"):
+    EnPassantCalculator.capturedPawnSquare(sq(File.E, Rank.R6), Color.White) shouldBe sq(File.E, Rank.R5)
+
+  test("capturedPawnSquare for black capturing on e3 returns e4"):
+    EnPassantCalculator.capturedPawnSquare(sq(File.E, Rank.R3), Color.Black) shouldBe sq(File.E, Rank.R4)
+
+  test("capturedPawnSquare for white capturing on d6 returns d5"):
+    EnPassantCalculator.capturedPawnSquare(sq(File.D, Rank.R6), Color.White) shouldBe sq(File.D, Rank.R5)
+
+  // ──── isEnPassant ────────────────────────────────────────────────────
+
+  test("isEnPassant returns true for valid white en passant capture"):
+    // White pawn on e5, black pawn just double-pushed to d5 (ep target = d6)
+    val b = board(
+      sq(File.E, Rank.R5) -> Piece.WhitePawn,
+      sq(File.D, Rank.R5) -> Piece.BlackPawn
+    )
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
+    EnPassantCalculator.isEnPassant(b, h, sq(File.E, Rank.R5), sq(File.D, Rank.R6)) shouldBe true
+
+  test("isEnPassant returns true for valid black en passant capture"):
+    // Black pawn on d4, white pawn just double-pushed to e4 (ep target = e3)
+    val b = board(
+      sq(File.D, Rank.R4) -> Piece.BlackPawn,
+      sq(File.E, Rank.R4) -> Piece.WhitePawn
+    )
+    val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    EnPassantCalculator.isEnPassant(b, h, sq(File.D, Rank.R4), sq(File.E, Rank.R3)) shouldBe true
+
+  test("isEnPassant returns false when no en passant target in history"):
+    val b = board(
+      sq(File.E, Rank.R5) -> Piece.WhitePawn,
+      sq(File.D, Rank.R5) -> Piece.BlackPawn
+    )
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R6), sq(File.D, Rank.R5))  // single push
+    EnPassantCalculator.isEnPassant(b, h, sq(File.E, Rank.R5), sq(File.D, Rank.R6)) shouldBe false
+
+  test("isEnPassant returns false when piece at from is not a pawn"):
+    val b = board(
+      sq(File.E, Rank.R5) -> Piece.WhiteRook,
+      sq(File.D, Rank.R5) -> Piece.BlackPawn
+    )
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
+    EnPassantCalculator.isEnPassant(b, h, sq(File.E, Rank.R5), sq(File.D, Rank.R6)) shouldBe false
+
+  test("isEnPassant returns false when to does not match ep target"):
+    val b = board(
+      sq(File.E, Rank.R5) -> Piece.WhitePawn,
+      sq(File.D, Rank.R5) -> Piece.BlackPawn
+    )
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
+    EnPassantCalculator.isEnPassant(b, h, sq(File.E, Rank.R5), sq(File.E, Rank.R6)) shouldBe false
+
+  test("isEnPassant returns false when from square is empty"):
+    val b = board(sq(File.D, Rank.R5) -> Piece.BlackPawn)
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
+    EnPassantCalculator.isEnPassant(b, h, sq(File.E, Rank.R5), sq(File.D, Rank.R6)) shouldBe false