stockfish/src/movepick.h

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)

  Stockfish is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  Stockfish is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef MOVEPICK_H_INCLUDED
#define MOVEPICK_H_INCLUDED

#include <array>
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <limits>
#include <type_traits>  // IWYU pragma: keep

#include "movegen.h"
#include "types.h"
#include "position.h"

namespace Stockfish {

constexpr int PAWN_HISTORY_SIZE = 512;

inline int pawn_structure(const Position& pos) { return pos.pawn_key() & (PAWN_HISTORY_SIZE - 1); }

// StatsEntry stores the stat table value. It is usually a number but could
// be a move or even a nested history. We use a class instead of a naked value
// to directly call history update operator<<() on the entry so to use stats
// tables at caller sites as simple multi-dim arrays.
template<typename T, int D>
class StatsEntry {

    T entry;

   public:
    void operator=(const T& v) { entry = v; }
    T*   operator&() { return &entry; }
    T*   operator->() { return &entry; }
    operator const T&() const { return entry; }

    void operator<<(int bonus) {
        assert(abs(bonus) <= D);  // Ensure range is [-D, D]
        static_assert(D <= std::numeric_limits<T>::max(), "D overflows T");

        entry += (bonus * D - entry * abs(bonus)) / (D * 5 / 4);

        assert(abs(entry) <= D);
    }
};

// Stats is a generic N-dimensional array used to store various statistics.
// The first template parameter T is the base type of the array, and the second
// template parameter D limits the range of updates in [-D, D] when we update
// values with the << operator, while the last parameters (Size and Sizes)
// encode the dimensions of the array.
template<typename T, int D, int Size, int... Sizes>
struct Stats: public std::array<Stats<T, D, Sizes...>, Size> {
    using stats = Stats<T, D, Size, Sizes...>;

    void fill(const T& v) {

        // For standard-layout 'this' points to the first struct member
        assert(std::is_standard_layout_v<stats>);

        using entry = StatsEntry<T, D>;
        entry* p    = reinterpret_cast<entry*>(this);
        std::fill(p, p + sizeof(*this) / sizeof(entry), v);
    }
};

template<typename T, int D, int Size>
struct Stats<T, D, Size>: public std::array<StatsEntry<T, D>, Size> {};

// In stats table, D=0 means that the template parameter is not used
enum StatsParams {
    NOT_USED = 0
};
enum StatsType {
    NoCaptures,
    Captures
};

// ButterflyHistory records how often quiet moves have been successful or
// unsuccessful during the current search, and is used for reduction and move
// ordering decisions. It uses 2 tables (one for each color) indexed by
// the move's from and to squares, see www.chessprogramming.org/Butterfly_Boards
// (~11 elo)
using ButterflyHistory = Stats<int16_t, 7183, COLOR_NB, int(SQUARE_NB) * int(SQUARE_NB)>;

// CounterMoveHistory stores counter moves indexed by [piece][to] of the previous
// move, see www.chessprogramming.org/Countermove_Heuristic
using CounterMoveHistory = Stats<Move, NOT_USED, PIECE_NB, SQUARE_NB>;

// CapturePieceToHistory is addressed by a move's [piece][to][captured piece type]
using CapturePieceToHistory = Stats<int16_t, 10692, PIECE_NB, SQUARE_NB, PIECE_TYPE_NB>;

// PieceToHistory is like ButterflyHistory but is addressed by a move's [piece][to]
using PieceToHistory = Stats<int16_t, 29952, PIECE_NB, SQUARE_NB>;

// ContinuationHistory is the combined history of a given pair of moves, usually
// the current one given a previous one. The nested history table is based on
// PieceToHistory instead of ButterflyBoards.
// (~63 elo)
using ContinuationHistory = Stats<PieceToHistory, NOT_USED, PIECE_NB, SQUARE_NB>;

// PawnStructureHistory is addressed by the pawn structure and a move's [piece][to]
using PawnHistory = Stats<int16_t, 8192, PAWN_HISTORY_SIZE, PIECE_NB, SQUARE_NB>;

// 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.
class MovePicker {

    enum PickType {
        Next,
        Best
    };

   public:
    MovePicker(const MovePicker&)            = delete;
    MovePicker& operator=(const MovePicker&) = delete;
    MovePicker(const Position&,
               Move,
               Depth,
               const ButterflyHistory*,
               const CapturePieceToHistory*,
               const PieceToHistory**,
               const PawnHistory&,
               Move,
               const Move*);
    MovePicker(const Position&,
               Move,
               Depth,
               const ButterflyHistory*,
               const CapturePieceToHistory*,
               const PieceToHistory**,
               const PawnHistory&,
               Square);
    MovePicker(const Position&, Move, Value, const CapturePieceToHistory*, const PawnHistory&);
    Move next_move(bool skipQuiets = false);

   private:
    template<PickType T, typename Pred>
    Move select(Pred);
    template<GenType>
    void     score();
    ExtMove* begin() { return cur; }
    ExtMove* end() { return endMoves; }

    const Position&              pos;
    const ButterflyHistory*      mainHistory;
    const CapturePieceToHistory* captureHistory;
    const PieceToHistory**       continuationHistory;
    const PawnHistory&           pawnHistory;
    Move                         ttMove;
    ExtMove                      refutations[3], *cur, *endMoves, *endBadCaptures;
    int                          stage;
    Square                       recaptureSquare;
    Value                        threshold;
    Depth                        depth;
    ExtMove                      moves[MAX_MOVES];
};

}  // namespace Stockfish

#endif  // #ifndef MOVEPICK_H_INCLUDED