Optimize check and checkmate calculation. Do it only when needed

src/board/include/tartan/board/board.hpp

@@ -857,7 +857,8 @@ class Board : private BoardT {
 	 * @brief Turn index
 	 *
 	 * Underlying varriable gets inclremented 
-	 * on Turn::apply() and decremented on Turn::undo().
+	 * on Turn::apply(). 
+	 * Acts like a timestamp fo current Board state.
 	 *
 	 * @return last Piece::Turn index
 	 * @sa b_turnIndex

src/board/turn.cpp

@@ -1,3 +1,4 @@
+#include <iostream>
 #include <tartan/board.hpp>
 #include <tartan/board/exceptions.hpp>
 
@@ -26,7 +27,6 @@ void Turn::apply(int) {
 
 void Turn::undo() {
 	Board& cb = *t_piece->p_board;
-	cb.b_turnIndex--;
 
 	t_piece->move(t_from);
 

src/chess/include/tartan/chess/chess.hpp

@@ -460,7 +460,13 @@ class King : public Piece {
 	 */
 	bool castled() const { return k_castled; };
 private:
+	bool calculateCheck() const;
+	bool calculateCheckmate() const;
 	bool k_castled = false;
+	mutable bool k_check = false;
+	mutable bool k_checkmate = false;
+	mutable std::size_t k_checkTurnIndex = -1;
+	mutable std::size_t k_checkmateTurnIndex = -1;
 };
 
 }

src/chess/pieces/king/king.cpp

@@ -72,6 +72,14 @@ TurnMap King::moveMap(int mode) const {
 }
 
 bool King::check() const {
+	if (k_checkTurnIndex != board()->turnIndex()) {
+		k_check = calculateCheck();
+		k_checkTurnIndex = board()->turnIndex();
+	}
+	return k_check;
+}
+
+bool King::calculateCheck() const {
 	for (auto& row : p_board->board()) {
 		for (auto& p : row) {
 			if (!p or p->color() == p_color)
@@ -97,7 +105,16 @@ bool King::check() const {
 	return false;
 }
 
+
 bool King::checkmate() const {
+	if (k_checkmateTurnIndex != board()->turnIndex()) {
+		k_checkmate = calculateCheckmate();
+		k_checkmateTurnIndex = board()->turnIndex();
+	}
+	return k_checkmate;
+}
+
+bool King::calculateCheckmate() const {
 	if (!check() or p_board->possibleMoves(this).possible())
 		return false;