Enhance doNullMove for en passant and add related tests (#45)

* Update doNullMove to handle en passant state

* Add tests for move, null move, and undo move

Reason for the bug: uninitialized diffs in entry
---------

Co-authored-by: GitHub Actions <actions@github.com>

Author: winapiadmin

position.h

@@ -182,6 +182,13 @@ template <typename PieceC = EnginePiece, typename = std::enable_if_t<is_piece_en
 
     inline void doNullMove() {
         history.push_back(current_state);
+        current_state.incr_sqs[0] = current_state.incr_sqs[1] = current_state.incr_sqs[2] = current_state.incr_sqs[3] = SQ_NONE;
+        current_state.incr_pc[0] = current_state.incr_pc[1] = current_state.incr_pc[2] = current_state.incr_pc[3] =
+            PieceC::NO_PIECE;
+        if (current_state.epIncluded)
+            current_state.hash ^= zobrist::RandomEP[file_of(ep_square())];
+        current_state.epIncluded = false;
+        current_state.enPassant = SQ_NONE;
         current_state.turn = ~current_state.turn;
         current_state.hash ^= zobrist::RandomTurn;
         current_state.fullMoveNumber += (current_state.turn == WHITE);

tests.cpp

@@ -402,6 +402,29 @@ TEST_CASE("Experienced bugs in this repo") {
         REQUIRE(p.zobrist() == p.hash());
         REQUIRE(p.hash() == 4177524090105507023);
     }
+    {
+        Position p;
+        p.doMove<false>(Move(SQ_A2, SQ_A3));
+        REQUIRE(p.zobrist() == p.hash());
+        p.doMove<false>(Move(SQ_B8, SQ_A6));
+        REQUIRE(p.zobrist() == p.hash());
+        p.doMove<false>(Move(SQ_F2, SQ_F3));
+        REQUIRE(p.zobrist() == p.hash());
+        p.doMove<false>(Move(SQ_F7, SQ_F5));
+        REQUIRE(p.zobrist() == p.hash());
+        p.doNullMove();
+        REQUIRE(p.zobrist() == p.hash());
+        p.undoMove();
+        REQUIRE(p.zobrist() == p.hash());
+        p.undoMove();
+        REQUIRE(p.zobrist() == p.hash());
+        p.undoMove();
+        REQUIRE(p.zobrist() == p.hash());
+        p.undoMove();
+        REQUIRE(p.zobrist() == p.hash());
+        p.undoMove();
+        REQUIRE(p.zobrist() == p.hash());
+    }
     {
         Position p;
         REQUIRE(p.getCastlingPath(WHITE, true) == 0x60);