Remove EvalList

This patch removes the EvalList structure from the Position object and generally simplifies the interface between do_move() and the NNUE code.

The NNUE evaluation function first calculates the "accumulator". The accumulator consists of two halves: one for white's perspective, one for black's perspective.

If the "friendly king" has moved or the accumulator for the parent position is not available, the accumulator for this half has to be calculated from scratch. To do this, the NNUE node needs to know the positions and types of all non-king pieces and the position of the friendly king. This information can easily be obtained from the Position object.

If the "friendly king" has not moved, its half of the accumulator can be calculated by incrementally updating the accumulator for the previous position. For this, the NNUE code needs to know which pieces have been added to which squares and which pieces have been removed from which squares. In principle this information can be derived from the Position object and StateInfo struct (in the same way as undo_move() does this). However, it is probably a bit faster to prepare this information in do_move(), so I have kept the DirtyPiece struct. Since the DirtyPiece struct now stores the squares rather than "PieceSquare" indices, there are now at most three "dirty pieces" (previously two). A promotion move that captures a piece removes the capturing pawn and the captured piece from the board (to SQ_NONE) and moves the promoted piece to the promotion square (from SQ_NONE).

An STC test has confirmed a small speedup:

https://tests.stockfishchess.org/tests/view/5f43f06b5089a564a10d850a
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 87704 W: 9763 L: 9500 D: 68441
Ptnml(0-2): 426, 6950, 28845, 7197, 434

closes https://github.com/official-stockfish/Stockfish/pull/3068

No functional change

src/nnue/features/feature_set.h

@@ -68,8 +68,7 @@ namespace Eval::NNUE::Features {
         reset[perspective] = false;
         switch (trigger) {
           case TriggerEvent::kFriendKingMoved:
-            reset[perspective] =
-                dp.pieceId[0] == PIECE_ID_KING + perspective;
+            reset[perspective] = dp.piece[0] == make_piece(perspective, KING);
             break;
           default:
             assert(false);

src/nnue/features/half_kp.cpp

@@ -23,25 +23,17 @@
 
 namespace Eval::NNUE::Features {
 
-  // Find the index of the feature quantity from the king position and PieceSquare
-  template <Side AssociatedKing>
-  inline IndexType HalfKP<AssociatedKing>::MakeIndex(Square sq_k, PieceSquare p) {
-    return static_cast<IndexType>(PS_END) * static_cast<IndexType>(sq_k) + p;
+  // Orient a square according to perspective (rotates by 180 for black)
+  inline Square orient(Color perspective, Square s) {
+    return Square(int(s) ^ (bool(perspective) * 63));
   }
 
-  // Get pieces information
+  // Find the index of the feature quantity from the king position and PieceSquare
   template <Side AssociatedKing>
-  inline void HalfKP<AssociatedKing>::GetPieces(
-      const Position& pos, Color perspective,
-      PieceSquare** pieces, Square* sq_target_k) {
+  inline IndexType HalfKP<AssociatedKing>::MakeIndex(
+      Color perspective, Square s, Piece pc, Square ksq) {
 
-    *pieces = (perspective == BLACK) ?
-        pos.eval_list()->piece_list_fb() :
-        pos.eval_list()->piece_list_fw();
-    const PieceId target = (AssociatedKing == Side::kFriend) ?
-        static_cast<PieceId>(PIECE_ID_KING + perspective) :
-        static_cast<PieceId>(PIECE_ID_KING + ~perspective);
-    *sq_target_k = static_cast<Square>(((*pieces)[target] - PS_W_KING) % SQUARE_NB);
+    return IndexType(orient(perspective, s) + kpp_board_index[pc][perspective] + PS_END * ksq);
   }
 
   // Get a list of indices for active features
@@ -49,16 +41,11 @@ namespace Eval::NNUE::Features {
   void HalfKP<AssociatedKing>::AppendActiveIndices(
       const Position& pos, Color perspective, IndexList* active) {
 
-    // Do nothing if array size is small to avoid compiler warning
-    if (RawFeatures::kMaxActiveDimensions < kMaxActiveDimensions) return;
-
-    PieceSquare* pieces;
-    Square sq_target_k;
-    GetPieces(pos, perspective, &pieces, &sq_target_k);
-    for (PieceId i = PIECE_ID_ZERO; i < PIECE_ID_KING; ++i) {
-      if (pieces[i] != PS_NONE) {
-        active->push_back(MakeIndex(sq_target_k, pieces[i]));
-      }
+    Square ksq = orient(perspective, pos.square<KING>(perspective));
+    Bitboard bb = pos.pieces() & ~pos.pieces(KING);
+    while (bb) {
+      Square s = pop_lsb(&bb);
+      active->push_back(MakeIndex(perspective, s, pos.piece_on(s), ksq));
     }
   }
 
@@ -68,22 +55,15 @@ namespace Eval::NNUE::Features {
       const Position& pos, Color perspective,
       IndexList* removed, IndexList* added) {
 
-    PieceSquare* pieces;
-    Square sq_target_k;
-    GetPieces(pos, perspective, &pieces, &sq_target_k);
+    Square ksq = orient(perspective, pos.square<KING>(perspective));
     const auto& dp = pos.state()->dirtyPiece;
     for (int i = 0; i < dp.dirty_num; ++i) {
-      if (dp.pieceId[i] >= PIECE_ID_KING) continue;
-      const auto old_p = static_cast<PieceSquare>(
-          dp.old_piece[i].from[perspective]);
-      if (old_p != PS_NONE) {
-        removed->push_back(MakeIndex(sq_target_k, old_p));
-      }
-      const auto new_p = static_cast<PieceSquare>(
-          dp.new_piece[i].from[perspective]);
-      if (new_p != PS_NONE) {
-        added->push_back(MakeIndex(sq_target_k, new_p));
-      }
+      Piece pc = dp.piece[i];
+      if (type_of(pc) == KING) continue;
+      if (dp.from[i] != SQ_NONE)
+        removed->push_back(MakeIndex(perspective, dp.from[i], pc, ksq));
+      if (dp.to[i] != SQ_NONE)
+        added->push_back(MakeIndex(perspective, dp.to[i], pc, ksq));
     }
   }
 

src/nnue/features/half_kp.h

@@ -41,7 +41,7 @@ namespace Eval::NNUE::Features {
     static constexpr IndexType kDimensions =
         static_cast<IndexType>(SQUARE_NB) * static_cast<IndexType>(PS_END);
     // Maximum number of simultaneously active features
-    static constexpr IndexType kMaxActiveDimensions = PIECE_ID_KING;
+    static constexpr IndexType kMaxActiveDimensions = 30; // Kings don't count
     // Trigger for full calculation instead of difference calculation
     static constexpr TriggerEvent kRefreshTrigger = TriggerEvent::kFriendKingMoved;
 
@@ -53,13 +53,9 @@ namespace Eval::NNUE::Features {
     static void AppendChangedIndices(const Position& pos, Color perspective,
                                      IndexList* removed, IndexList* added);
 
-    // Index of a feature for a given king position and another piece on some square
-    static IndexType MakeIndex(Square sq_k, PieceSquare p);
-
    private:
-    // Get pieces information
-    static void GetPieces(const Position& pos, Color perspective,
-                          PieceSquare** pieces, Square* sq_target_k);
+    // Index of a feature for a given king position and another piece on some square
+    static IndexType MakeIndex(Color perspective, Square s, Piece pc, Square sq_k);
   };
 
 }  // namespace Eval::NNUE::Features