stockfish/src/history.h

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2024 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 HISTORY_H_INCLUDED
#define HISTORY_H_INCLUDED

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

#include "position.h"

namespace Stockfish {

constexpr int PAWN_HISTORY_SIZE        = 512;    // has to be a power of 2
constexpr int CORRECTION_HISTORY_SIZE  = 32768;  // has to be a power of 2
constexpr int CORRECTION_HISTORY_LIMIT = 1024;
constexpr int LOW_PLY_HISTORY_SIZE     = 4;

static_assert((PAWN_HISTORY_SIZE & (PAWN_HISTORY_SIZE - 1)) == 0,
              "PAWN_HISTORY_SIZE has to be a power of 2");

static_assert((CORRECTION_HISTORY_SIZE & (CORRECTION_HISTORY_SIZE - 1)) == 0,
              "CORRECTION_HISTORY_SIZE has to be a power of 2");

enum PawnHistoryType {
    Normal,
    Correction
};

template<PawnHistoryType T = Normal>
inline int pawn_structure_index(const Position& pos) {
    return pos.pawn_key() & ((T == Normal ? PAWN_HISTORY_SIZE : CORRECTION_HISTORY_SIZE) - 1);
}

inline int major_piece_index(const Position& pos) {
    return pos.major_piece_key() & (CORRECTION_HISTORY_SIZE - 1);
}

inline int minor_piece_index(const Position& pos) {
    return pos.minor_piece_key() & (CORRECTION_HISTORY_SIZE - 1);
}

template<Color c>
inline int non_pawn_index(const Position& pos) {
    return pos.non_pawn_key(c) & (CORRECTION_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) {
        static_assert(D <= std::numeric_limits<T>::max(), "D overflows T");

        // Make sure that bonus is in range [-D, D]
        int clampedBonus = std::clamp(bonus, -D, D);
        entry += clampedBonus - entry * std::abs(clampedBonus) / D;

        assert(std::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 https://www.chessprogramming.org/Butterfly_Boards (~11 elo)
using ButterflyHistory = Stats<int16_t, 7183, COLOR_NB, int(SQUARE_NB) * int(SQUARE_NB)>;

// LowPlyHistory is adressed by play and move's from and to squares, used
// to improve move ordering near the root
using LowPlyHistory = Stats<int16_t, 7183, LOW_PLY_HISTORY_SIZE, int(SQUARE_NB) * int(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, 30000, 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>;

// PawnHistory 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>;

// Correction histories record differences between the static evaluation of
// positions and their search score. It is used to improve the static evaluation
// used by some search heuristics.
// see https://www.chessprogramming.org/Static_Evaluation_Correction_History
enum CorrHistType {
    Pawn,          // By color and pawn structure
    Major,         // By color and positions of major pieces (Queen, Rook) and King
    Minor,         // By color and positions of minor pieces (Knight, Bishop) and King
    NonPawn,       // By color and non-pawn material positions
    PieceTo,       // By [piece][to] move
    Continuation,  // Combined history of move pairs
};

template<CorrHistType _>
struct CorrHistTypedef {
    using type = Stats<int16_t, CORRECTION_HISTORY_LIMIT, COLOR_NB, CORRECTION_HISTORY_SIZE>;
};

template<>
struct CorrHistTypedef<PieceTo> {
    using type = Stats<int16_t, CORRECTION_HISTORY_LIMIT, PIECE_NB, SQUARE_NB>;
};

template<>
struct CorrHistTypedef<Continuation> {
    using type = Stats<CorrHistTypedef<PieceTo>::type, NOT_USED, PIECE_NB, SQUARE_NB>;
};

template<CorrHistType T>
using CorrectionHistory = typename CorrHistTypedef<T>::type;

}  // namespace Stockfish

#endif  // #ifndef HISTORY_H_INCLUDED