Subversion Repositories Games.Chess Giants

/*

  Stockfish, a UCI chess playing engine derived from Glaurung 2.1

  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)

  Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad

  Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad

  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/>.

*/

#include <algorithm>

#include <cassert>

#include <cmath>

#include <cstring>   // For std::memset

#include <iostream>

#include <sstream>

#include "evaluate.h"

#include "misc.h"

#include "movegen.h"

#include "movepick.h"

#include "search.h"

#include "timeman.h"

#include "thread.h"

#include "tt.h"

#include "uci.h"

#include "syzygy/tbprobe.h"

namespace Search {

  SignalsType Signals;

  LimitsType Limits;

  StateStackPtr SetupStates;

}

namespace Tablebases {

  int Cardinality;

  uint64_t Hits;

  bool RootInTB;

  bool UseRule50;

  Depth ProbeDepth;

  Value Score;

}

namespace TB = Tablebases;

using std::string;

using Eval::evaluate;

using namespace Search;

namespace {

  // Different node types, used as a template parameter

  enum NodeType { NonPV, PV };

  // Razoring and futility margin based on depth

  const int razor_margin[4] = { 483, 570, 603, 554 };

  Value futility_margin(Depth d) { return Value(200 * d); }

  // Futility and reductions lookup tables, initialized at startup

  int FutilityMoveCounts[2][16];  // [improving][depth]

  Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]

  template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {

    return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];

  }

  // Skill structure is used to implement strength limit

  struct Skill {

    Skill(int l) : level(l) {}

    bool enabled() const { return level < 20; }

    bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }

    Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }

    Move pick_best(size_t multiPV);

    int level;

    Move best = MOVE_NONE;

  };

  // EasyMoveManager structure is used to detect an 'easy move'. When the PV is

  // stable across multiple search iterations, we can quickly return the best move.

  struct EasyMoveManager {

    void clear() {

      stableCnt = 0;

      expectedPosKey = 0;

      pv[0] = pv[1] = pv[2] = MOVE_NONE;

    }

    Move get(Key key) const {

      return expectedPosKey == key ? pv[2] : MOVE_NONE;

    }

    void update(Position& pos, const std::vector<Move>& newPv) {

      assert(newPv.size() >= 3);

      // Keep track of how many times in a row the 3rd ply remains stable

      stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;

      if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))

      {

          std::copy(newPv.begin(), newPv.begin() + 3, pv);

          StateInfo st[2];

          pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));

          pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));

          expectedPosKey = pos.key();

          pos.undo_move(newPv[1]);

          pos.undo_move(newPv[0]);

      }

    }

    int stableCnt;

    Key expectedPosKey;

    Move pv[3];

  };

  // Set of rows with half bits set to 1 and half to 0. It is used to allocate

  // the search depths across the threads.

  typedef std::vector<int> Row;

  const Row HalfDensity[] = {

    {0, 1},

    {1, 0},

    {0, 0, 1, 1},

    {0, 1, 1, 0},

    {1, 1, 0, 0},

    {1, 0, 0, 1},

    {0, 0, 0, 1, 1, 1},

    {0, 0, 1, 1, 1, 0},

    {0, 1, 1, 1, 0, 0},

    {1, 1, 1, 0, 0, 0},

    {1, 1, 0, 0, 0, 1},

    {1, 0, 0, 0, 1, 1},

    {0, 0, 0, 0, 1, 1, 1, 1},

    {0, 0, 0, 1, 1, 1, 1, 0},

    {0, 0, 1, 1, 1, 1, 0 ,0},

    {0, 1, 1, 1, 1, 0, 0 ,0},

    {1, 1, 1, 1, 0, 0, 0 ,0},

    {1, 1, 1, 0, 0, 0, 0 ,1},

    {1, 1, 0, 0, 0, 0, 1 ,1},

    {1, 0, 0, 0, 0, 1, 1 ,1},

  };

  const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;

  EasyMoveManager EasyMove;

  Value DrawValue[COLOR_NB];

  CounterMoveHistoryStats CounterMoveHistory;

  template <NodeType NT>

  Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);

  template <NodeType NT, bool InCheck>

  Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);

  Value value_to_tt(Value v, int ply);

  Value value_from_tt(Value v, int ply);

  void update_pv(Move* pv, Move move, Move* childPv);

  void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);

  void check_time();

} // namespace

/// Search::init() is called during startup to initialize various lookup tables

void Search::init() {

  const double K[][2] = {{ 0.799, 2.281 }, { 0.484, 3.023 }};

  for (int pv = 0; pv <= 1; ++pv)

      for (int imp = 0; imp <= 1; ++imp)

          for (int d = 1; d < 64; ++d)

              for (int mc = 1; mc < 64; ++mc)

              {

                  double r = K[pv][0] + log(d) * log(mc) / K[pv][1];

                  if (r >= 1.5)

                      Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;

                  // Increase reduction when eval is not improving

                  if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)

                      Reductions[pv][imp][d][mc] += ONE_PLY;

              }

  for (int d = 0; d < 16; ++d)

  {

      FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));

      FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));

  }

}

/// Search::clear() resets search state to zero, to obtain reproducible results

void Search::clear() {

  TT.clear();

  CounterMoveHistory.clear();

  for (Thread* th : Threads)

  {

      th->history.clear();

      th->counterMoves.clear();

  }

  Threads.main()->previousScore = VALUE_INFINITE;

}

/// Search::perft() is our utility to verify move generation. All the leaf nodes

/// up to the given depth are generated and counted, and the sum is returned.

template<bool Root>

uint64_t Search::perft(Position& pos, Depth depth) {

  StateInfo st;

  uint64_t cnt, nodes = 0;

  CheckInfo ci(pos);

  const bool leaf = (depth == 2 * ONE_PLY);

  for (const auto& m : MoveList<LEGAL>(pos))

  {

      if (Root && depth <= ONE_PLY)

          cnt = 1, nodes++;

      else

      {

          pos.do_move(m, st, pos.gives_check(m, ci));

          cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);

          nodes += cnt;

          pos.undo_move(m);

      }

      if (Root)

          sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;

  }

  return nodes;

}

template uint64_t Search::perft<true>(Position&, Depth);

/// MainThread::search() is called by the main thread when the program receives

/// the UCI 'go' command. It searches from the root position and outputs the "bestmove".

void MainThread::search() {

  Color us = rootPos.side_to_move();

  Time.init(Limits, us, rootPos.game_ply());

  int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns

  DrawValue[ us] = VALUE_DRAW - Value(contempt);

  DrawValue[~us] = VALUE_DRAW + Value(contempt);

  TB::Hits = 0;

  TB::RootInTB = false;

  TB::UseRule50 = Options["Syzygy50MoveRule"];

  TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;

  TB::Cardinality = Options["SyzygyProbeLimit"];

  // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality

  if (TB::Cardinality > TB::MaxCardinality)

  {

      TB::Cardinality = TB::MaxCardinality;

      TB::ProbeDepth = DEPTH_ZERO;

  }

  if (rootMoves.empty())

  {

      rootMoves.push_back(RootMove(MOVE_NONE));

      sync_cout << "info depth 0 score "

                << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)

                << sync_endl;

  }

  else

  {

      if (    TB::Cardinality >=  rootPos.count<ALL_PIECES>(WHITE)

                                + rootPos.count<ALL_PIECES>(BLACK)

          && !rootPos.can_castle(ANY_CASTLING))

      {

          // If the current root position is in the tablebases, then RootMoves

          // contains only moves that preserve the draw or the win.

          TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);

          if (TB::RootInTB)

              TB::Cardinality = 0; // Do not probe tablebases during the search

          else // If DTZ tables are missing, use WDL tables as a fallback

          {

              // Filter out moves that do not preserve the draw or the win.

              TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);

              // Only probe during search if winning

              if (TB::Score <= VALUE_DRAW)

                  TB::Cardinality = 0;

          }

          if (TB::RootInTB)

          {

              TB::Hits = rootMoves.size();

              if (!TB::UseRule50)

                  TB::Score =  TB::Score > VALUE_DRAW ?  VALUE_MATE - MAX_PLY - 1

                             : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1

                                                      :  VALUE_DRAW;

          }

      }

      for (Thread* th : Threads)

      {

          th->maxPly = 0;

          th->rootDepth = DEPTH_ZERO;

          if (th != this)

          {

              th->rootPos = Position(rootPos, th);

              th->rootMoves = rootMoves;

              th->start_searching();

          }

      }

      Thread::search(); // Let's start searching!

  }

  // When playing in 'nodes as time' mode, subtract the searched nodes from

  // the available ones before exiting.

  if (Limits.npmsec)

      Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();

  // When we reach the maximum depth, we can arrive here without a raise of

  // Signals.stop. However, if we are pondering or in an infinite search,

  // the UCI protocol states that we shouldn't print the best move before the

  // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here

  // until the GUI sends one of those commands (which also raises Signals.stop).

  if (!Signals.stop && (Limits.ponder || Limits.infinite))

  {

      Signals.stopOnPonderhit = true;

      wait(Signals.stop);

  }

  // Stop the threads if not already stopped

  Signals.stop = true;

  // Wait until all threads have finished

  for (Thread* th : Threads)

      if (th != this)

          th->wait_for_search_finished();

  // Check if there are threads with a better score than main thread

  Thread* bestThread = this;

  if (   !this->easyMovePlayed

      &&  Options["MultiPV"] == 1

      && !Skill(Options["Skill Level"]).enabled())

  {

      for (Thread* th : Threads)

          if (   th->completedDepth > bestThread->completedDepth

              && th->rootMoves[0].score > bestThread->rootMoves[0].score)

              bestThread = th;

  }

  previousScore = bestThread->rootMoves[0].score;

  // Send new PV when needed

  if (bestThread != this)

      sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;

  sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());

  if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))

      std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());

  std::cout << sync_endl;

}

// Thread::search() is the main iterative deepening loop. It calls search()

// repeatedly with increasing depth until the allocated thinking time has been

// consumed, the user stops the search, or the maximum search depth is reached.

void Thread::search() {

  Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)

  Value bestValue, alpha, beta, delta;

  Move easyMove = MOVE_NONE;

  MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);

  std::memset(ss-2, 0, 5 * sizeof(Stack));

  bestValue = delta = alpha = -VALUE_INFINITE;

  beta = VALUE_INFINITE;

  completedDepth = DEPTH_ZERO;

  if (mainThread)

  {

      easyMove = EasyMove.get(rootPos.key());

      EasyMove.clear();

      mainThread->easyMovePlayed = mainThread->failedLow = false;

      mainThread->bestMoveChanges = 0;

      TT.new_search();

  }

  size_t multiPV = Options["MultiPV"];

  Skill skill(Options["Skill Level"]);

  // When playing with strength handicap enable MultiPV search that we will

  // use behind the scenes to retrieve a set of possible moves.

  if (skill.enabled())

      multiPV = std::max(multiPV, (size_t)4);

  multiPV = std::min(multiPV, rootMoves.size());

  // Iterative deepening loop until requested to stop or the target depth is reached.

  while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))

  {

      // Set up the new depths for the helper threads skipping on average every

      // 2nd ply (using a half-density matrix).

      if (!mainThread)

      {

          const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];

          if (row[(rootDepth + rootPos.game_ply()) % row.size()])

             continue;

      }

      // Age out PV variability metric

      if (mainThread)

          mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;

      // Save the last iteration's scores before first PV line is searched and

      // all the move scores except the (new) PV are set to -VALUE_INFINITE.

      for (RootMove& rm : rootMoves)

          rm.previousScore = rm.score;

      // MultiPV loop. We perform a full root search for each PV line

      for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)

      {

          // Reset aspiration window starting size

          if (rootDepth >= 5 * ONE_PLY)

          {

              delta = Value(18);

              alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);

              beta  = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);

          }

          // Start with a small aspiration window and, in the case of a fail

          // high/low, re-search with a bigger window until we're not failing

          // high/low anymore.

          while (true)

          {

              bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);

              // Bring the best move to the front. It is critical that sorting

              // is done with a stable algorithm because all the values but the

              // first and eventually the new best one are set to -VALUE_INFINITE

              // and we want to keep the same order for all the moves except the

              // new PV that goes to the front. Note that in case of MultiPV

              // search the already searched PV lines are preserved.

              std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());

              // Write PV back to the transposition table in case the relevant

              // entries have been overwritten during the search.

              for (size_t i = 0; i <= PVIdx; ++i)

                  rootMoves[i].insert_pv_in_tt(rootPos);

              // If search has been stopped, break immediately. Sorting and

              // writing PV back to TT is safe because RootMoves is still

              // valid, although it refers to the previous iteration.

              if (Signals.stop)

                  break;

              // When failing high/low give some update (without cluttering

              // the UI) before a re-search.

              if (   mainThread

                  && multiPV == 1

                  && (bestValue <= alpha || bestValue >= beta)

                  && Time.elapsed() > 3000)

                  sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;

              // In case of failing low/high increase aspiration window and

              // re-search, otherwise exit the loop.

              if (bestValue <= alpha)

              {

                  beta = (alpha + beta) / 2;

                  alpha = std::max(bestValue - delta, -VALUE_INFINITE);

                  if (mainThread)

                  {

                      mainThread->failedLow = true;

                      Signals.stopOnPonderhit = false;

                  }

              }

              else if (bestValue >= beta)

              {

                  alpha = (alpha + beta) / 2;

                  beta = std::min(bestValue + delta, VALUE_INFINITE);

              }

              else

                  break;

              delta += delta / 4 + 5;

              assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);

          }

          // Sort the PV lines searched so far and update the GUI

          std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);

          if (!mainThread)

              break;

          if (Signals.stop)

              sync_cout << "info nodes " << Threads.nodes_searched()

                        << " time " << Time.elapsed() << sync_endl;

          else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)

              sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;

      }

      if (!Signals.stop)

          completedDepth = rootDepth;

      if (!mainThread)

          continue;

      // If skill level is enabled and time is up, pick a sub-optimal best move

      if (skill.enabled() && skill.time_to_pick(rootDepth))

          skill.pick_best(multiPV);

      // Have we found a "mate in x"?

      if (   Limits.mate

          && bestValue >= VALUE_MATE_IN_MAX_PLY

          && VALUE_MATE - bestValue <= 2 * Limits.mate)

          Signals.stop = true;

      // Do we have time for the next iteration? Can we stop searching now?

      if (Limits.use_time_management())

      {

          if (!Signals.stop && !Signals.stopOnPonderhit)

          {

              // Stop the search if only one legal move is available, or if all

              // of the available time has been used, or if we matched an easyMove

              // from the previous search and just did a fast verification.

              const bool F[] = { !mainThread->failedLow,

                                 bestValue >= mainThread->previousScore };

              int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];

              double unstablePvFactor = 1 + mainThread->bestMoveChanges;

              bool doEasyMove =   rootMoves[0].pv[0] == easyMove

                               && mainThread->bestMoveChanges < 0.03

                               && Time.elapsed() > Time.optimum() * 25 / 204;

              if (   rootMoves.size() == 1

                  || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 634

                  || (mainThread->easyMovePlayed = doEasyMove))

              {

                  // If we are allowed to ponder do not stop the search now but

                  // keep pondering until the GUI sends "ponderhit" or "stop".

                  if (Limits.ponder)

                      Signals.stopOnPonderhit = true;

                  else

                      Signals.stop = true;

              }

          }

          if (rootMoves[0].pv.size() >= 3)

              EasyMove.update(rootPos, rootMoves[0].pv);

          else

              EasyMove.clear();

      }

  }

  if (!mainThread)

      return;

  // Clear any candidate easy move that wasn't stable for the last search

  // iterations; the second condition prevents consecutive fast moves.

  if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)

      EasyMove.clear();

  // If skill level is enabled, swap best PV line with the sub-optimal one

  if (skill.enabled())

      std::swap(rootMoves[0], *std::find(rootMoves.begin(),

                rootMoves.end(), skill.best_move(multiPV)));

}

namespace {

  // search<>() is the main search function for both PV and non-PV nodes

  template <NodeType NT>

  Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {

    const bool PvNode = NT == PV;

    const bool rootNode = PvNode && (ss-1)->ply == 0;

    assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);

    assert(PvNode || (alpha == beta - 1));

    assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);

    Move pv[MAX_PLY+1], quietsSearched[64];

    StateInfo st;

    TTEntry* tte;

    Key posKey;

    Move ttMove, move, excludedMove, bestMove;

    Depth extension, newDepth, predictedDepth;

    Value bestValue, value, ttValue, eval, nullValue, futilityValue;

    bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;

    bool captureOrPromotion, doFullDepthSearch;

    int moveCount, quietCount;

    // Step 1. Initialize node

    Thread* thisThread = pos.this_thread();

    inCheck = pos.checkers();

    moveCount = quietCount =  ss->moveCount = 0;

    bestValue = -VALUE_INFINITE;

    ss->ply = (ss-1)->ply + 1;

    // Check for the available remaining time

    if (thisThread->resetCalls.load(std::memory_order_relaxed))

    {

        thisThread->resetCalls = false;

        thisThread->callsCnt = 0;

    }

    if (++thisThread->callsCnt > 4096)

    {

        for (Thread* th : Threads)

            th->resetCalls = true;

        check_time();

    }

    // Used to send selDepth info to GUI

    if (PvNode && thisThread->maxPly < ss->ply)

        thisThread->maxPly = ss->ply;

    if (!rootNode)

    {

        // Step 2. Check for aborted search and immediate draw

        if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)

            return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)

                                                  : DrawValue[pos.side_to_move()];

        // Step 3. Mate distance pruning. Even if we mate at the next move our score

        // would be at best mate_in(ss->ply+1), but if alpha is already bigger because

        // a shorter mate was found upward in the tree then there is no need to search

        // because we will never beat the current alpha. Same logic but with reversed

        // signs applies also in the opposite condition of being mated instead of giving

        // mate. In this case return a fail-high score.

        alpha = std::max(mated_in(ss->ply), alpha);

        beta = std::min(mate_in(ss->ply+1), beta);

        if (alpha >= beta)

            return alpha;

    }

    assert(0 <= ss->ply && ss->ply < MAX_PLY);

    ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;

    (ss+1)->skipEarlyPruning = false;

    (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;

    // Step 4. Transposition table lookup. We don't want the score of a partial

    // search to overwrite a previous full search TT value, so we use a different

    // position key in case of an excluded move.

    excludedMove = ss->excludedMove;

    posKey = excludedMove ? pos.exclusion_key() : pos.key();

    tte = TT.probe(posKey, ttHit);

    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;

    ttMove =  rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]

            : ttHit    ? tte->move() : MOVE_NONE;

    // At non-PV nodes we check for an early TT cutoff

    if (  !PvNode

        && ttHit

        && tte->depth() >= depth

        && ttValue != VALUE_NONE // Possible in case of TT access race

        && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)

                            : (tte->bound() & BOUND_UPPER)))

    {

        ss->currentMove = ttMove; // Can be MOVE_NONE

        // If ttMove is quiet, update killers, history, counter move on TT hit

        if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))

            update_stats(pos, ss, ttMove, depth, nullptr, 0);

        return ttValue;

    }

    // Step 4a. Tablebase probe

    if (!rootNode && TB::Cardinality)

    {

        int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);

        if (    piecesCnt <= TB::Cardinality

            && (piecesCnt <  TB::Cardinality || depth >= TB::ProbeDepth)

            &&  pos.rule50_count() == 0

            && !pos.can_castle(ANY_CASTLING))

        {

            int found, v = Tablebases::probe_wdl(pos, &found);

            if (found)

            {

                TB::Hits++;

                int drawScore = TB::UseRule50 ? 1 : 0;

                value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply

                       : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply

                                        :  VALUE_DRAW + 2 * v * drawScore;

                tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,

                          std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),

                          MOVE_NONE, VALUE_NONE, TT.generation());

                return value;

            }

        }

    }

    // Step 5. Evaluate the position statically

    if (inCheck)

    {

        ss->staticEval = eval = VALUE_NONE;