From 6afcdaa928c4b7a9711b3df53d4cbb70616975f5 Mon Sep 17 00:00:00 2001
From: Tomasz Sobczyk <tomasz.sobczyk1997@gmail.com>
Date: Sat, 3 Apr 2021 16:15:51 +0200
Subject: [PATCH] Add additional checks for en-passant possiblity when fixing
 the erroneus ep flag from a fen.

---
 src/extra/nnue_data_binpack_format.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/extra/nnue_data_binpack_format.h b/src/extra/nnue_data_binpack_format.h
index dce53b83..b957fae6 100644
--- a/src/extra/nnue_data_binpack_format.h
+++ b/src/extra/nnue_data_binpack_format.h
@@ -6115,6 +6115,26 @@ namespace chess
             return false;
         }
 
+        if (pieceAt(epSquare) != Piece::none())
+        {
+            return false;
+        }
+
+        const auto forward =
+            sideToMove == chess::Color::White
+            ? FlatSquareOffset(0, 1)
+            : FlatSquareOffset(0, -1);
+
+        if (pieceAt(epSquare + forward) != Piece::none())
+        {
+            return false;
+        }
+
+        if (pieceAt(epSquare + -forward) != Piece(PieceType::Pawn, !sideToMove))
+        {
+            return false;
+        }
+
         return isEpPossibleColdPath(epSquare, pawnsAttackingEpSquare, sideToMove);
     }