-/// MovePicker is a class which is used to pick one pseudo legal move at a time
-/// from the current position. It is initialized with a Position object and a few
-/// moves we have reason to believe are good. The most important method is
-/// MovePicker::get_next_move(), which returns a new pseudo legal move each time
-/// it is called, until there are no moves left, when MOVE_NONE is returned.
-/// In order to improve the efficiency of the alpha beta algorithm, MovePicker
-/// attempts to return the moves which are most likely to get a cut-off first.
+/// The Stats struct stores moves statistics. According to the template parameter
+/// the class can store History, Gains and Refutations statistics. 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. Refutations store the move that refute a previous one.
+/// Entries are stored according only to moving piece and destination square, in
+/// particular two moves with different origin but same destination and same piece
+/// will be considered identical.
+template<bool Gain, typename T>
+struct Stats {
+
+ static const Value Max = Value(2000);
+
+ const T* operator[](Piece p) const { return &table[p][0]; }
+ void clear() { memset(table, 0, sizeof(table)); }
+
+ void update(Piece p, Square to, Move m) { table[p][to] = m; }
+ void update(Piece p, Square to, Value v) {
+
+ if (Gain)
+ table[p][to] = std::max(v, table[p][to] - 1);
+
+ else if (abs(table[p][to] + v) < Max)
+ table[p][to] += v;
+ }
+
+private:
+ T table[PIECE_NB][SQUARE_NB];
+};
+
+typedef Stats<true, Value> Gains;
+typedef Stats<false, Value> History;
+typedef Stats<false, Move> Refutations;
+
+
+/// MovePicker class is used to pick one pseudo legal move at a time from the
+/// current position. The most important method is next_move(), which returns a
+/// new pseudo legal move each time it is called, until there are no moves left,
+/// when MOVE_NONE is returned. In order to improve the efficiency of the alpha
+/// beta algorithm, MovePicker attempts to return the moves which are most likely
+/// to get a cut-off first.