stockfish/src/search.h

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2026 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 SEARCH_H_INCLUDED
#define SEARCH_H_INCLUDED

#include <algorithm>
#include <array>
#include <atomic>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <functional>
#include <map>
#include <memory>
#include <string>
#include <string_view>
#include <vector>

#include "history.h"
#include "misc.h"
#include "nnue/network.h"
#include "nnue/nnue_accumulator.h"
#include "numa.h"
#include "position.h"
#include "score.h"
#include "syzygy/tbprobe.h"
#include "timeman.h"
#include "types.h"

namespace Stockfish {

// Different node types, used as a template parameter
enum NodeType {
    NonPV,
    PV,
    Root
};

class TranspositionTable;
class ThreadPool;
class OptionsMap;

namespace Search {

// Stack struct keeps track of the information we need to remember from nodes
// shallower and deeper in the tree during the search. Each search thread has
// its own array of Stack objects, indexed by the current ply.
struct Stack {
    Move*                       pv;
    PieceToHistory*             continuationHistory;
    CorrectionHistory<PieceTo>* continuationCorrectionHistory;
    int                         ply;
    Move                        currentMove;
    Move                        excludedMove;
    Value                       staticEval;
    int                         statScore;
    int                         moveCount;
    bool                        inCheck;
    bool                        ttPv;
    bool                        ttHit;
    bool                        followPV;
    int                         cutoffCnt;
    int                         reduction;
};


// RootMove struct is used for moves at the root of the tree. For each root move
// we store a score and a PV (really a refutation in the case of moves which
// fail low). Score is normally set at -VALUE_INFINITE for all non-pv moves.
struct RootMove {

    explicit RootMove(Move m) :
        pv(1, m) {}
    bool extract_ponder_from_tt(const TranspositionTable& tt, Position& pos);
    bool operator==(const Move& m) const { return pv[0] == m; }
    // Sort in descending order
    bool operator<(const RootMove& m) const {
        return m.score != score ? m.score < score : m.previousScore < previousScore;
    }

    uint64_t          effort           = 0;
    Value             score            = -VALUE_INFINITE;
    Value             previousScore    = -VALUE_INFINITE;
    Value             averageScore     = -VALUE_INFINITE;
    Value             meanSquaredScore = -VALUE_INFINITE * VALUE_INFINITE;
    Value             uciScore         = -VALUE_INFINITE;
    bool              scoreLowerbound  = false;
    bool              scoreUpperbound  = false;
    int               selDepth         = 0;
    int               tbRank           = 0;
    Value             tbScore;
    std::vector<Move> pv;
};

using RootMoves = std::vector<RootMove>;


// LimitsType struct stores information sent by the caller about the analysis required.
struct LimitsType {

    // Init explicitly due to broken value-initialization of non POD in MSVC
    LimitsType() {
        time[WHITE] = time[BLACK] = inc[WHITE] = inc[BLACK] = npmsec = movetime = TimePoint(0);
        movestogo = depth = mate = perft = infinite = 0;
        nodes                                       = 0;
        ponderMode                                  = false;
    }

    bool use_time_management() const { return time[WHITE] || time[BLACK]; }

    std::vector<std::string> searchmoves;
    TimePoint                time[COLOR_NB], inc[COLOR_NB], npmsec, movetime, startTime;
    int                      movestogo, depth, mate, perft, infinite;
    uint64_t                 nodes;
    bool                     ponderMode;
};


// The UCI stores the uci options, thread pool, and transposition table.
// This struct is used to easily forward data to the Search::Worker class.
struct SharedState {
    SharedState(const OptionsMap&                                         optionsMap,
                ThreadPool&                                               threadPool,
                TranspositionTable&                                       transpositionTable,
                std::map<NumaIndex, SharedHistories>&                     sharedHists,
                const LazyNumaReplicatedSystemWide<Eval::NNUE::Networks>& nets) :
        options(optionsMap),
        threads(threadPool),
        tt(transpositionTable),
        sharedHistories(sharedHists),
        networks(nets) {}

    const OptionsMap&                                         options;
    ThreadPool&                                               threads;
    TranspositionTable&                                       tt;
    std::map<NumaIndex, SharedHistories>&                     sharedHistories;
    const LazyNumaReplicatedSystemWide<Eval::NNUE::Networks>& networks;
};

class Worker;

// Null Object Pattern, implement a common interface for the SearchManagers.
// A Null Object will be given to non-mainthread workers.
class ISearchManager {
   public:
    virtual ~ISearchManager() {}
    virtual void check_time(Search::Worker&) = 0;
};

struct InfoShort {
    int   depth;
    Score score;
};

struct InfoFull: InfoShort {
    int              selDepth;
    size_t           multiPV;
    std::string_view wdl;
    std::string_view bound;
    size_t           timeMs;
    size_t           nodes;
    size_t           nps;
    size_t           tbHits;
    std::string_view pv;
    int              hashfull;
};

struct InfoIteration {
    int              depth;
    std::string_view currmove;
    size_t           currmovenumber;
};

// Skill structure is used to implement strength limit. If we have a UCI_Elo,
// we convert it to an appropriate skill level, anchored to the Stash engine.
// This method is based on a fit of the Elo results for games played between
// Stockfish at various skill levels and various versions of the Stash engine.
// Skill 0 .. 19 now covers CCRL Blitz Elo from 1320 to 3190, approximately
// Reference: https://github.com/vondele/Stockfish/commit/a08b8d4e9711c2
struct Skill {
    // Lowest and highest Elo ratings used in the skill level calculation
    constexpr static int LowestElo  = 1320;
    constexpr static int HighestElo = 3190;

    Skill(int skill_level, int uci_elo) {
        if (uci_elo)
        {
            double e = double(uci_elo - LowestElo) / (HighestElo - LowestElo);
            level = std::clamp((((37.2473 * e - 40.8525) * e + 22.2943) * e - 0.311438), 0.0, 19.0);
        }
        else
            level = double(skill_level);
    }
    bool enabled() const { return level < 20.0; }
    bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
    Move pick_best(const RootMoves&, size_t multiPV);

    double level;
    Move   best = Move::none();
};

// SearchManager manages the search from the main thread. It is responsible for
// keeping track of the time, and storing data strictly related to the main thread.
class SearchManager: public ISearchManager {
   public:
    using UpdateShort    = std::function<void(const InfoShort&)>;
    using UpdateFull     = std::function<void(const InfoFull&)>;
    using UpdateIter     = std::function<void(const InfoIteration&)>;
    using UpdateBestmove = std::function<void(std::string_view, std::string_view)>;

    struct UpdateContext {
        UpdateShort    onUpdateNoMoves;
        UpdateFull     onUpdateFull;
        UpdateIter     onIter;
        UpdateBestmove onBestmove;
    };


    SearchManager(const UpdateContext& updateContext) :
        updates(updateContext) {}

    void check_time(Search::Worker& worker) override;

    void pv(Search::Worker&           worker,
            const ThreadPool&         threads,
            const TranspositionTable& tt,
            Depth                     depth);

    Stockfish::TimeManagement tm;
    double                    originalTimeAdjust;
    int                       callsCnt;
    std::atomic_bool          ponder;

    std::array<Value, 4> iterValue;
    double               previousTimeReduction;
    Value                bestPreviousScore;
    Value                bestPreviousAverageScore;
    bool                 stopOnPonderhit;

    size_t id;

    const UpdateContext& updates;
};

class NullSearchManager: public ISearchManager {
   public:
    void check_time(Search::Worker&) override {}
};

// Search::Worker is the class that does the actual search.
// It is instantiated once per thread, and it is responsible for keeping track
// of the search history, and storing data required for the search.
class Worker {
   public:
    Worker(SharedState&,
           std::unique_ptr<ISearchManager>,
           size_t,
           size_t,
           size_t,
           NumaReplicatedAccessToken);

    // Called at instantiation to initialize reductions tables.
    // Reset histories, usually before a new game.
    void clear();

    // Called when the program receives the UCI 'go' command.
    // It searches from the root position and outputs the "bestmove".
    void start_searching();

    bool is_mainthread() const { return threadIdx == 0; }

    void ensure_network_replicated();

    // Public because they need to be updatable by the stats
    ButterflyHistory mainHistory;
    LowPlyHistory    lowPlyHistory;

    CapturePieceToHistory           captureHistory;
    ContinuationHistory             continuationHistory[2][2];
    CorrectionHistory<Continuation> continuationCorrectionHistory;

    TTMoveHistory    ttMoveHistory;
    SharedHistories& sharedHistory;

   private:
    bool iterative_deepening();

    void do_move(Position& pos, const Move move, StateInfo& st, Stack* const ss);
    void
    do_move(Position& pos, const Move move, StateInfo& st, const bool givesCheck, Stack* const ss);
    void do_null_move(Position& pos, StateInfo& st, Stack* const ss);
    void undo_move(Position& pos, const Move move);
    void undo_null_move(Position& pos);

    // This is the main search function, for both PV and non-PV nodes
    template<NodeType nodeType>
    Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);

    // Quiescence search function, which is called by the main search
    template<NodeType nodeType>
    Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta);

    int reduction(bool i, Depth d, int mn, int delta) const;

    // Pointer to the search manager, only allowed to be called by the main thread
    SearchManager* main_manager() const {
        assert(threadIdx == 0);
        return static_cast<SearchManager*>(manager.get());
    }

    TimePoint elapsed() const;
    TimePoint elapsed_time() const;

    Value evaluate(const Position&);

    LimitsType limits;

    size_t                pvIdx, pvLast;
    std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;
    int                   selDepth, nmpMinPly;

    Value optimism[COLOR_NB];

    Position  rootPos;
    StateInfo rootState;
    RootMoves rootMoves;
    Depth     rootDepth, completedDepth;
    Value     rootDelta;

    std::vector<Move> lastIterationPV;

    size_t                    threadIdx, numaThreadIdx, numaTotal;
    NumaReplicatedAccessToken numaAccessToken;

    // Reductions lookup table initialized at startup
    std::array<int, MAX_MOVES> reductions;  // [depth or moveNumber]

    // The main thread has a SearchManager, the others have a NullSearchManager
    std::unique_ptr<ISearchManager> manager;

    Tablebases::Config tbConfig;

    const OptionsMap&                                         options;
    ThreadPool&                                               threads;
    TranspositionTable&                                       tt;
    const LazyNumaReplicatedSystemWide<Eval::NNUE::Networks>& networks;

    // Used by NNUE
    Eval::NNUE::AccumulatorStack  accumulatorStack;
    Eval::NNUE::AccumulatorCaches refreshTable;

    friend class Stockfish::ThreadPool;
    friend class SearchManager;
};

struct ConthistBonus {
    int index;
    int weight;
};


}  // namespace Search

}  // namespace Stockfish

#endif  // #ifndef SEARCH_H_INCLUDED