+/// The Stats struct stores moves statistics. According to the template parameter
+/// the class can store History, Gains and Countermoves. History records how often
+/// different moves have been successful or unsuccessful during the current search
+/// and is used for reduction and move ordering decisions. Gains records the move's
+/// best evaluation gain from one ply to the next and is used for pruning decisions.
+/// Countermoves store the move that refute a previous one. Entries are stored
+/// using only the moving piece and destination square, hence two moves with
+/// different origin but same destination and piece will be considered identical.
+template<bool Gain, typename T>
+struct Stats {
+
+ static const Value Max = Value(250);
+
+ const T* operator[](Piece pc) const { return table[pc]; }
+ void clear() { std::memset(table, 0, sizeof(table)); }
+
+ void update(Piece pc, Square to, Move m) {
+
+ if (m == table[pc][to].first)
+ return;
+
+ table[pc][to].second = table[pc][to].first;
+ table[pc][to].first = m;
+ }
+
+ void update(Piece pc, Square to, Value v) {
+
+ if (Gain)
+ table[pc][to] = std::max(v, table[pc][to] - 1);
+
+ else if (abs(table[pc][to] + v) < Max)
+ table[pc][to] += v;
+ }
+
+private:
+ T table[PIECE_NB][SQUARE_NB];
+};
+
+typedef Stats< true, Value> GainsStats;
+typedef Stats<false, Value> HistoryStats;
+typedef Stats<false, std::pair<Move, Move> > MovesStats;
+
+