Add a HalfKA architecture (a product of K - king, and A - any piece) along with all required infrastructure. HalfKA doesn't discriminate kings compared to HalfKP. Keep old architecture as the default one.

src/nnue/features/a.cpp

@@ -0,0 +1,50 @@
+#include "a.h"
+#include "index_list.h"
+
+// Definition of input feature A of NNUE evaluation function
+namespace Eval::NNUE::Features {
+
+    // Orient a square according to perspective (flip rank for black)
+    inline Square orient(Color perspective, Square s) {
+        return Square(int(s) ^ (bool(perspective) * SQ_A8));
+    }
+
+    // Find the index of the feature quantity from the king position and PieceSquare
+    inline IndexType A::make_index(
+        Color perspective, Square s, Piece pc) {
+        return IndexType(orient(perspective, s) + kpp_board_index[pc][perspective]);
+    }
+
+    // Get a list of indices with a value of 1 among the features
+    void A::append_active_indices(
+        const Position& pos,
+        Color perspective,
+        IndexList* active) {
+
+        Bitboard bb = pos.pieces();
+        while (bb) {
+            Square s = pop_lsb(&bb);
+            active->push_back(make_index(perspective, s, pos.piece_on(s)));
+        }
+    }
+
+    // Get a list of indices whose values ​​have changed from the previous one in the feature quantity
+    void A::append_changed_indices(
+        const Position& pos,
+        Color perspective,
+        IndexList* removed,
+        IndexList* added) {
+
+        const auto& dp = pos.state()->dirtyPiece;
+        for (int i = 0; i < dp.dirty_num; ++i) {
+            Piece pc = dp.piece[i];
+
+            if (dp.from[i] != SQ_NONE)
+              removed->push_back(make_index(perspective, dp.from[i], pc));
+
+            if (dp.to[i] != SQ_NONE)
+              added->push_back(make_index(perspective, dp.to[i], pc));
+        }
+    }
+
+}  // namespace Eval::NNUE::Features