[Board] Implement EnPassant handling

Board.cpp

@@ -53,11 +53,11 @@ CastlingRights Board::castlingRights() const {
 void Board::setEnPassantSquare(const Square::Optional &square) {
     if (!square.has_value())
         return;
-    mEPS = square->index();
+    mEPS = square;
 }
 
 Square::Optional Board::enPassantSquare() const {
-    return Square::fromIndex(mEPS);
+    return mEPS;
 }
 
 void Board::makeMove(const Move &move) {
@@ -108,10 +108,26 @@ void Board::makeMove(const Move &move) {
         }
     }
 
+    // en passant
+    handlePawnDoubleAdvance(move, toBB, movedPiece);
+
 
     // change turn
     mTurn = !mTurn;
 }
+void Board::handlePawnDoubleAdvance(const Move &move, BitBoard bb, const Piece &movedPiece) {
+    if (movedPiece.type() == PieceType::Pawn) {
+        auto fromR = move.from().rank();
+        auto toR = move.to().rank();
+        auto diff = abs(static_cast<int>(fromR) - static_cast<int>(toR));
+        if (diff == 2 && (mPieceBBs[toIndex(PieceType::Pawn)] & (bb << 1 | bb >> 1) & getRankBB(static_cast<int>(toR)))) {
+            mEPS = Square::fromCoordinates(move.to().file(), std::max(fromR, toR) - 1);
+            return;
+        }
+    }
+
+    mEPS = std::nullopt;
+}
 
 void Board::pseudoLegalMoves(MoveVec &moves) const {
     (void) moves;