From c52f57293e313b878d72bd5deed38ee5b7560726 Mon Sep 17 00:00:00 2001
From: Arthur Bols <arthur@bols.dev>
Date: Wed, 21 Dec 2022 19:39:54 +0100
Subject: [PATCH] [Board] Implement EnPassant handling

---
 Board.cpp | 20 ++++++++++++++++++--
 Board.hpp | 12 +++++++++++-
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/Board.cpp b/Board.cpp
index b555725..6fc1746 100644
--- a/Board.cpp
+++ b/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;
diff --git a/Board.hpp b/Board.hpp
index 737f4d2..879d6e6 100644
--- a/Board.hpp
+++ b/Board.hpp
@@ -47,7 +47,7 @@ private:
 
     PieceColor mTurn = PieceColor::White;
     CastlingRights mCR = CastlingRights::None;
-    unsigned mEPS = 64;
+    std::optional<Square> mEPS;
 
     enum DefinedBoards : BitBoard {
         AFile = 0x0101010101010101,
@@ -80,6 +80,14 @@ private:
         return (1ULL << i);
     }
 
+    static inline BitBoard genShift(BitBoard x, const int s) {
+        return (s < 0) ? (x >> -s) : (s > 63) ? x : (x << s);
+    }
+
+    static inline BitBoard getRankBB(int r) {
+        return (genShift(1ULL, (r + 1) * 8) - 1) & genShift(~1ULL, r * 8 - 1);
+    }
+
     static inline int toIndex(PieceType t) {
         return static_cast<int>(t);
     }
@@ -118,6 +126,8 @@ private:
     static inline int getLSB(const BitBoard b) {
         return __builtin_ctzll(b);
     }
+
+    void handlePawnDoubleAdvance(const Move &move, BitBoard bb, const Piece &movedPiece);
 };
 
 std::ostream &operator<<(std::ostream &os, const Board &board);