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  1. /*
  2.   Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  3.   Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  4.   Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
  5.   Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
  6.  
  7.   Stockfish is free software: you can redistribute it and/or modify
  8.   it under the terms of the GNU General Public License as published by
  9.   the Free Software Foundation, either version 3 of the License, or
  10.   (at your option) any later version.
  11.  
  12.   Stockfish is distributed in the hope that it will be useful,
  13.   but WITHOUT ANY WARRANTY; without even the implied warranty of
  14.   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15.   GNU General Public License for more details.
  16.  
  17.   You should have received a copy of the GNU General Public License
  18.   along with this program.  If not, see <http://www.gnu.org/licenses/>.
  19. */
  20.  
  21. #include <algorithm>
  22. #include <cassert>
  23. #include <cmath>
  24. #include <cstring>   // For std::memset
  25. #include <iostream>
  26. #include <sstream>
  27.  
  28. #include "evaluate.h"
  29. #include "misc.h"
  30. #include "movegen.h"
  31. #include "movepick.h"
  32. #include "position.h"
  33. #include "search.h"
  34. #include "timeman.h"
  35. #include "thread.h"
  36. #include "tt.h"
  37. #include "uci.h"
  38. #include "syzygy/tbprobe.h"
  39.  
  40. namespace Search {
  41.  
  42.   SignalsType Signals;
  43.   LimitsType Limits;
  44. }
  45.  
  46. namespace Tablebases {
  47.  
  48.   int Cardinality;
  49.   bool RootInTB;
  50.   bool UseRule50;
  51.   Depth ProbeDepth;
  52.   Value Score;
  53. }
  54.  
  55. namespace TB = Tablebases;
  56.  
  57. using std::string;
  58. using Eval::evaluate;
  59. using namespace Search;
  60.  
  61. namespace {
  62.  
  63.   // Different node types, used as a template parameter
  64.   enum NodeType { NonPV, PV };
  65.  
  66.   // Razoring and futility margin based on depth
  67.   const int razor_margin[4] = { 483, 570, 603, 554 };
  68.   Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
  69.  
  70.   // Futility and reductions lookup tables, initialized at startup
  71.   int FutilityMoveCounts[2][16]; // [improving][depth]
  72.   int Reductions[2][2][64][64];  // [pv][improving][depth][moveNumber]
  73.  
  74.   template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
  75.     return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
  76.   }
  77.  
  78.   // Skill structure is used to implement strength limit
  79.   struct Skill {
  80.     Skill(int l) : level(l) {}
  81.     bool enabled() const { return level < 20; }
  82.     bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
  83.     Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
  84.     Move pick_best(size_t multiPV);
  85.  
  86.     int level;
  87.     Move best = MOVE_NONE;
  88.   };
  89.  
  90.   // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
  91.   // stable across multiple search iterations, we can quickly return the best move.
  92.   struct EasyMoveManager {
  93.  
  94.     void clear() {
  95.       stableCnt = 0;
  96.       expectedPosKey = 0;
  97.       pv[0] = pv[1] = pv[2] = MOVE_NONE;
  98.     }
  99.  
  100.     Move get(Key key) const {
  101.       return expectedPosKey == key ? pv[2] : MOVE_NONE;
  102.     }
  103.  
  104.     void update(Position& pos, const std::vector<Move>& newPv) {
  105.  
  106.       assert(newPv.size() >= 3);
  107.  
  108.       // Keep track of how many times in a row the 3rd ply remains stable
  109.       stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
  110.  
  111.       if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
  112.       {
  113.           std::copy(newPv.begin(), newPv.begin() + 3, pv);
  114.  
  115.           StateInfo st[2];
  116.           pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0]));
  117.           pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1]));
  118.           expectedPosKey = pos.key();
  119.           pos.undo_move(newPv[1]);
  120.           pos.undo_move(newPv[0]);
  121.       }
  122.     }
  123.  
  124.     int stableCnt;
  125.     Key expectedPosKey;
  126.     Move pv[3];
  127.   };
  128.  
  129.   // Set of rows with half bits set to 1 and half to 0. It is used to allocate
  130.   // the search depths across the threads.
  131.   typedef std::vector<int> Row;
  132.  
  133.   const Row HalfDensity[] = {
  134.     {0, 1},
  135.     {1, 0},
  136.     {0, 0, 1, 1},
  137.     {0, 1, 1, 0},
  138.     {1, 1, 0, 0},
  139.     {1, 0, 0, 1},
  140.     {0, 0, 0, 1, 1, 1},
  141.     {0, 0, 1, 1, 1, 0},
  142.     {0, 1, 1, 1, 0, 0},
  143.     {1, 1, 1, 0, 0, 0},
  144.     {1, 1, 0, 0, 0, 1},
  145.     {1, 0, 0, 0, 1, 1},
  146.     {0, 0, 0, 0, 1, 1, 1, 1},
  147.     {0, 0, 0, 1, 1, 1, 1, 0},
  148.     {0, 0, 1, 1, 1, 1, 0 ,0},
  149.     {0, 1, 1, 1, 1, 0, 0 ,0},
  150.     {1, 1, 1, 1, 0, 0, 0 ,0},
  151.     {1, 1, 1, 0, 0, 0, 0 ,1},
  152.     {1, 1, 0, 0, 0, 0, 1 ,1},
  153.     {1, 0, 0, 0, 0, 1, 1 ,1},
  154.   };
  155.  
  156.   const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;
  157.  
  158.   EasyMoveManager EasyMove;
  159.   Value DrawValue[COLOR_NB];
  160.  
  161.   template <NodeType NT>
  162.   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
  163.  
  164.   template <NodeType NT, bool InCheck>
  165.   Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
  166.  
  167.   Value value_to_tt(Value v, int ply);
  168.   Value value_from_tt(Value v, int ply);
  169.   void update_pv(Move* pv, Move move, Move* childPv);
  170.   void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
  171.   void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
  172.   void check_time();
  173.  
  174. } // namespace
  175.  
  176.  
  177. /// Search::init() is called during startup to initialize various lookup tables
  178.  
  179. void Search::init() {
  180.  
  181.   for (int imp = 0; imp <= 1; ++imp)
  182.       for (int d = 1; d < 64; ++d)
  183.           for (int mc = 1; mc < 64; ++mc)
  184.           {
  185.               double r = log(d) * log(mc) / 2;
  186.               if (r < 0.80)
  187.                 continue;
  188.  
  189.               Reductions[NonPV][imp][d][mc] = int(std::round(r));
  190.               Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
  191.  
  192.               // Increase reduction for non-PV nodes when eval is not improving
  193.               if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
  194.                 Reductions[NonPV][imp][d][mc]++;
  195.           }
  196.  
  197.   for (int d = 0; d < 16; ++d)
  198.   {
  199.       FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
  200.       FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
  201.   }
  202. }
  203.  
  204.  
  205. /// Search::clear() resets search state to zero, to obtain reproducible results
  206.  
  207. void Search::clear() {
  208.  
  209.   TT.clear();
  210.  
  211.   for (Thread* th : Threads)
  212.   {
  213.       th->history.clear();
  214.       th->counterMoves.clear();
  215.       th->fromTo.clear();
  216.       th->counterMoveHistory.clear();
  217.   }
  218.  
  219.   Threads.main()->previousScore = VALUE_INFINITE;
  220. }
  221.  
  222.  
  223. /// Search::perft() is our utility to verify move generation. All the leaf nodes
  224. /// up to the given depth are generated and counted, and the sum is returned.
  225. template<bool Root>
  226. uint64_t Search::perft(Position& pos, Depth depth) {
  227.  
  228.   StateInfo st;
  229.   uint64_t cnt, nodes = 0;
  230.   const bool leaf = (depth == 2 * ONE_PLY);
  231.  
  232.   for (const auto& m : MoveList<LEGAL>(pos))
  233.   {
  234.       if (Root && depth <= ONE_PLY)
  235.           cnt = 1, nodes++;
  236.       else
  237.       {
  238.           pos.do_move(m, st, pos.gives_check(m));
  239.           cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
  240.           nodes += cnt;
  241.           pos.undo_move(m);
  242.       }
  243.       if (Root)
  244.           sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
  245.   }
  246.   return nodes;
  247. }
  248.  
  249. template uint64_t Search::perft<true>(Position&, Depth);
  250.  
  251.  
  252. /// MainThread::search() is called by the main thread when the program receives
  253. /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
  254.  
  255. void MainThread::search() {
  256.  
  257.   Color us = rootPos.side_to_move();
  258.   Time.init(Limits, us, rootPos.game_ply());
  259.  
  260.   int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
  261.   DrawValue[ us] = VALUE_DRAW - Value(contempt);
  262.   DrawValue[~us] = VALUE_DRAW + Value(contempt);
  263.  
  264.   if (rootMoves.empty())
  265.   {
  266.       rootMoves.push_back(RootMove(MOVE_NONE));
  267.       sync_cout << "info depth 0 score "
  268.                 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
  269.                 << sync_endl;
  270.   }
  271.   else
  272.   {
  273.       for (Thread* th : Threads)
  274.           if (th != this)
  275.               th->start_searching();
  276.  
  277.       Thread::search(); // Let's start searching!
  278.   }
  279.  
  280.   // When playing in 'nodes as time' mode, subtract the searched nodes from
  281.   // the available ones before exiting.
  282.   if (Limits.npmsec)
  283.       Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
  284.  
  285.   // When we reach the maximum depth, we can arrive here without a raise of
  286.   // Signals.stop. However, if we are pondering or in an infinite search,
  287.   // the UCI protocol states that we shouldn't print the best move before the
  288.   // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
  289.   // until the GUI sends one of those commands (which also raises Signals.stop).
  290.   if (!Signals.stop && (Limits.ponder || Limits.infinite))
  291.   {
  292.       Signals.stopOnPonderhit = true;
  293.       wait(Signals.stop);
  294.   }
  295.  
  296.   // Stop the threads if not already stopped
  297.   Signals.stop = true;
  298.  
  299.   // Wait until all threads have finished
  300.   for (Thread* th : Threads)
  301.       if (th != this)
  302.           th->wait_for_search_finished();
  303.  
  304.   // Check if there are threads with a better score than main thread
  305.   Thread* bestThread = this;
  306.   if (   !this->easyMovePlayed
  307.       &&  Options["MultiPV"] == 1
  308.       && !Limits.depth
  309.       && !Skill(Options["Skill Level"]).enabled()
  310.       &&  rootMoves[0].pv[0] != MOVE_NONE)
  311.   {
  312.       for (Thread* th : Threads)
  313.           if (   th->completedDepth > bestThread->completedDepth
  314.               && th->rootMoves[0].score > bestThread->rootMoves[0].score)
  315.               bestThread = th;
  316.   }
  317.  
  318.   previousScore = bestThread->rootMoves[0].score;
  319.  
  320.   // Send new PV when needed
  321.   if (bestThread != this)
  322.       sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
  323.  
  324.   sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
  325.  
  326.   if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
  327.       std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
  328.  
  329.   std::cout << sync_endl;
  330. }
  331.  
  332.  
  333. // Thread::search() is the main iterative deepening loop. It calls search()
  334. // repeatedly with increasing depth until the allocated thinking time has been
  335. // consumed, the user stops the search, or the maximum search depth is reached.
  336.  
  337. void Thread::search() {
  338.  
  339.   Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
  340.   Value bestValue, alpha, beta, delta;
  341.   Move easyMove = MOVE_NONE;
  342.   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
  343.  
  344.   std::memset(ss-5, 0, 8 * sizeof(Stack));
  345.  
  346.   bestValue = delta = alpha = -VALUE_INFINITE;
  347.   beta = VALUE_INFINITE;
  348.   completedDepth = DEPTH_ZERO;
  349.  
  350.   if (mainThread)
  351.   {
  352.       easyMove = EasyMove.get(rootPos.key());
  353.       EasyMove.clear();
  354.       mainThread->easyMovePlayed = mainThread->failedLow = false;
  355.       mainThread->bestMoveChanges = 0;
  356.       TT.new_search();
  357.   }
  358.  
  359.   size_t multiPV = Options["MultiPV"];
  360.   Skill skill(Options["Skill Level"]);
  361.  
  362.   // When playing with strength handicap enable MultiPV search that we will
  363.   // use behind the scenes to retrieve a set of possible moves.
  364.   if (skill.enabled())
  365.       multiPV = std::max(multiPV, (size_t)4);
  366.  
  367.   multiPV = std::min(multiPV, rootMoves.size());
  368.  
  369.   // Iterative deepening loop until requested to stop or the target depth is reached
  370.   while (   (rootDepth += ONE_PLY) < DEPTH_MAX
  371.          && !Signals.stop
  372.          && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
  373.   {
  374.       // Set up the new depths for the helper threads skipping on average every
  375.       // 2nd ply (using a half-density matrix).
  376.       if (!mainThread)
  377.       {
  378.           const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
  379.           if (row[(rootDepth / ONE_PLY + rootPos.game_ply()) % row.size()])
  380.              continue;
  381.       }
  382.  
  383.       // Age out PV variability metric
  384.       if (mainThread)
  385.           mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
  386.  
  387.       // Save the last iteration's scores before first PV line is searched and
  388.       // all the move scores except the (new) PV are set to -VALUE_INFINITE.
  389.       for (RootMove& rm : rootMoves)
  390.           rm.previousScore = rm.score;
  391.  
  392.       // MultiPV loop. We perform a full root search for each PV line
  393.       for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
  394.       {
  395.           // Reset aspiration window starting size
  396.           if (rootDepth >= 5 * ONE_PLY)
  397.           {
  398.               delta = Value(18);
  399.               alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
  400.               beta  = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
  401.           }
  402.  
  403.           // Start with a small aspiration window and, in the case of a fail
  404.           // high/low, re-search with a bigger window until we're not failing
  405.           // high/low anymore.
  406.           while (true)
  407.           {
  408.               bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
  409.  
  410.               // Bring the best move to the front. It is critical that sorting
  411.               // is done with a stable algorithm because all the values but the
  412.               // first and eventually the new best one are set to -VALUE_INFINITE
  413.               // and we want to keep the same order for all the moves except the
  414.               // new PV that goes to the front. Note that in case of MultiPV
  415.               // search the already searched PV lines are preserved.
  416.               std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
  417.  
  418.               // If search has been stopped, break immediately. Sorting and
  419.               // writing PV back to TT is safe because RootMoves is still
  420.               // valid, although it refers to the previous iteration.
  421.               if (Signals.stop)
  422.                   break;
  423.  
  424.               // When failing high/low give some update (without cluttering
  425.               // the UI) before a re-search.
  426.               if (   mainThread
  427.                   && multiPV == 1
  428.                   && (bestValue <= alpha || bestValue >= beta)
  429.                   && Time.elapsed() > 3000)
  430.                   sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
  431.  
  432.               // In case of failing low/high increase aspiration window and
  433.               // re-search, otherwise exit the loop.
  434.               if (bestValue <= alpha)
  435.               {
  436.                   beta = (alpha + beta) / 2;
  437.                   alpha = std::max(bestValue - delta, -VALUE_INFINITE);
  438.  
  439.                   if (mainThread)
  440.                   {
  441.                       mainThread->failedLow = true;
  442.                       Signals.stopOnPonderhit = false;
  443.                   }
  444.               }
  445.               else if (bestValue >= beta)
  446.               {
  447.                   alpha = (alpha + beta) / 2;
  448.                   beta = std::min(bestValue + delta, VALUE_INFINITE);
  449.               }
  450.               else
  451.                   break;
  452.  
  453.               delta += delta / 4 + 5;
  454.  
  455.               assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
  456.           }
  457.  
  458.           // Sort the PV lines searched so far and update the GUI
  459.           std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
  460.  
  461.           if (!mainThread)
  462.               continue;
  463.  
  464.           if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
  465.               sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
  466.       }
  467.  
  468.       if (!Signals.stop)
  469.           completedDepth = rootDepth;
  470.  
  471.       if (!mainThread)
  472.           continue;
  473.  
  474.       // If skill level is enabled and time is up, pick a sub-optimal best move
  475.       if (skill.enabled() && skill.time_to_pick(rootDepth))
  476.           skill.pick_best(multiPV);
  477.  
  478.       // Have we found a "mate in x"?
  479.       if (   Limits.mate
  480.           && bestValue >= VALUE_MATE_IN_MAX_PLY
  481.           && VALUE_MATE - bestValue <= 2 * Limits.mate)
  482.           Signals.stop = true;
  483.  
  484.       // Do we have time for the next iteration? Can we stop searching now?
  485.       if (Limits.use_time_management())
  486.       {
  487.           if (!Signals.stop && !Signals.stopOnPonderhit)
  488.           {
  489.               // Stop the search if only one legal move is available, or if all
  490.               // of the available time has been used, or if we matched an easyMove
  491.               // from the previous search and just did a fast verification.
  492.               const int F[] = { mainThread->failedLow,
  493.                                 bestValue - mainThread->previousScore };
  494.  
  495.               int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
  496.               double unstablePvFactor = 1 + mainThread->bestMoveChanges;
  497.  
  498.               bool doEasyMove =   rootMoves[0].pv[0] == easyMove
  499.                                && mainThread->bestMoveChanges < 0.03
  500.                                && Time.elapsed() > Time.optimum() * 5 / 42;
  501.  
  502.               if (   rootMoves.size() == 1
  503.                   || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
  504.                   || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
  505.               {
  506.                   // If we are allowed to ponder do not stop the search now but
  507.                   // keep pondering until the GUI sends "ponderhit" or "stop".
  508.                   if (Limits.ponder)
  509.                       Signals.stopOnPonderhit = true;
  510.                   else
  511.                       Signals.stop = true;
  512.               }
  513.           }
  514.  
  515.           if (rootMoves[0].pv.size() >= 3)
  516.               EasyMove.update(rootPos, rootMoves[0].pv);
  517.           else
  518.               EasyMove.clear();
  519.       }
  520.   }
  521.  
  522.   if (!mainThread)
  523.       return;
  524.  
  525.   // Clear any candidate easy move that wasn't stable for the last search
  526.   // iterations; the second condition prevents consecutive fast moves.
  527.   if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
  528.       EasyMove.clear();
  529.  
  530.   // If skill level is enabled, swap best PV line with the sub-optimal one
  531.   if (skill.enabled())
  532.       std::swap(rootMoves[0], *std::find(rootMoves.begin(),
  533.                 rootMoves.end(), skill.best_move(multiPV)));
  534. }
  535.  
  536.  
  537. namespace {
  538.  
  539.   // search<>() is the main search function for both PV and non-PV nodes
  540.  
  541.   template <NodeType NT>
  542.   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
  543.  
  544.     const bool PvNode = NT == PV;
  545.     const bool rootNode = PvNode && (ss-1)->ply == 0;
  546.  
  547.     assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
  548.     assert(PvNode || (alpha == beta - 1));
  549.     assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
  550.     assert(!(PvNode && cutNode));
  551.     assert(depth / ONE_PLY * ONE_PLY == depth);
  552.  
  553.     Move pv[MAX_PLY+1], quietsSearched[64];
  554.     StateInfo st;
  555.     TTEntry* tte;
  556.     Key posKey;
  557.     Move ttMove, move, excludedMove, bestMove;
  558.     Depth extension, newDepth;
  559.     Value bestValue, value, ttValue, eval, nullValue;
  560.     bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
  561.     bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
  562.     Piece moved_piece;
  563.     int moveCount, quietCount;
  564.  
  565.     // Step 1. Initialize node
  566.     Thread* thisThread = pos.this_thread();
  567.     inCheck = pos.checkers();
  568.     moveCount = quietCount =  ss->moveCount = 0;
  569.     ss->history = VALUE_ZERO;
  570.     bestValue = -VALUE_INFINITE;
  571.     ss->ply = (ss-1)->ply + 1;
  572.  
  573.     // Check for the available remaining time
  574.     if (thisThread->resetCalls.load(std::memory_order_relaxed))
  575.     {
  576.         thisThread->resetCalls = false;
  577.         thisThread->callsCnt = 0;
  578.     }
  579.     if (++thisThread->callsCnt > 4096)
  580.     {
  581.         for (Thread* th : Threads)
  582.             th->resetCalls = true;
  583.  
  584.         check_time();
  585.     }
  586.  
  587.     // Used to send selDepth info to GUI
  588.     if (PvNode && thisThread->maxPly < ss->ply)
  589.         thisThread->maxPly = ss->ply;
  590.  
  591.     if (!rootNode)
  592.     {
  593.         // Step 2. Check for aborted search and immediate draw
  594.         if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
  595.             return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
  596.                                                   : DrawValue[pos.side_to_move()];
  597.  
  598.         // Step 3. Mate distance pruning. Even if we mate at the next move our score
  599.         // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
  600.         // a shorter mate was found upward in the tree then there is no need to search
  601.         // because we will never beat the current alpha. Same logic but with reversed
  602.         // signs applies also in the opposite condition of being mated instead of giving
  603.         // mate. In this case return a fail-high score.
  604.         alpha = std::max(mated_in(ss->ply), alpha);
  605.         beta = std::min(mate_in(ss->ply+1), beta);
  606.         if (alpha >= beta)
  607.             return alpha;
  608.     }
  609.  
  610.     assert(0 <= ss->ply && ss->ply < MAX_PLY);
  611.  
  612.     ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
  613.     ss->counterMoves = nullptr;
  614.     (ss+1)->skipEarlyPruning = false;
  615.     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
  616.  
  617.     // Step 4. Transposition table lookup. We don't want the score of a partial
  618.     // search to overwrite a previous full search TT value, so we use a different
  619.     // position key in case of an excluded move.
  620.     excludedMove = ss->excludedMove;
  621.     posKey = pos.key() ^ Key(excludedMove);
  622.     tte = TT.probe(posKey, ttHit);
  623.     ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
  624.     ttMove =  rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
  625.             : ttHit    ? tte->move() : MOVE_NONE;
  626.  
  627.     // At non-PV nodes we check for an early TT cutoff
  628.     if (  !PvNode
  629.         && ttHit
  630.         && tte->depth() >= depth
  631.         && ttValue != VALUE_NONE // Possible in case of TT access race
  632.         && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
  633.                             : (tte->bound() & BOUND_UPPER)))
  634.     {
  635.         // If ttMove is quiet, update killers, history, counter move on TT hit
  636.         if (ttValue >= beta && ttMove)
  637.         {
  638.             int d = depth / ONE_PLY;
  639.  
  640.             if (!pos.capture_or_promotion(ttMove))
  641.             {
  642.                 Value bonus = Value(d * d + 2 * d - 2);
  643.                 update_stats(pos, ss, ttMove, nullptr, 0, bonus);
  644.             }
  645.  
  646.             // Extra penalty for a quiet TT move in previous ply when it gets refuted
  647.             if ((ss-1)->moveCount == 1 && !pos.captured_piece())
  648.             {
  649.                 Value penalty = Value(d * d + 4 * d + 1);
  650.                 Square prevSq = to_sq((ss-1)->currentMove);
  651.                 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
  652.             }
  653.         }
  654.         return ttValue;
  655.     }
  656.  
  657.     // Step 4a. Tablebase probe
  658.     if (!rootNode && TB::Cardinality)
  659.     {
  660.         int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
  661.  
  662.         if (    piecesCnt <= TB::Cardinality
  663.             && (piecesCnt <  TB::Cardinality || depth >= TB::ProbeDepth)
  664.             &&  pos.rule50_count() == 0
  665.             && !pos.can_castle(ANY_CASTLING))
  666.         {
  667.             int found, v = Tablebases::probe_wdl(pos, &found);
  668.  
  669.             if (found)
  670.             {
  671.                 thisThread->tbHits++;
  672.  
  673.                 int drawScore = TB::UseRule50 ? 1 : 0;
  674.  
  675.                 value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
  676.                        : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
  677.                                         :  VALUE_DRAW + 2 * v * drawScore;
  678.  
  679.                 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
  680.                           std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
  681.                           MOVE_NONE, VALUE_NONE, TT.generation());
  682.  
  683.                 return value;
  684.             }
  685.         }
  686.     }
  687.  
  688.     // Step 5. Evaluate the position statically
  689.     if (inCheck)
  690.     {
  691.         ss->staticEval = eval = VALUE_NONE;
  692.         goto moves_loop;
  693.     }
  694.  
  695.     else if (ttHit)
  696.     {
  697.         // Never assume anything on values stored in TT
  698.         if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
  699.             eval = ss->staticEval = evaluate(pos);
  700.  
  701.         // Can ttValue be used as a better position evaluation?
  702.         if (ttValue != VALUE_NONE)
  703.             if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
  704.                 eval = ttValue;
  705.     }
  706.     else
  707.     {
  708.         eval = ss->staticEval =
  709.         (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
  710.                                          : -(ss-1)->staticEval + 2 * Eval::Tempo;
  711.  
  712.         tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
  713.                   ss->staticEval, TT.generation());
  714.     }
  715.  
  716.     if (ss->skipEarlyPruning)
  717.         goto moves_loop;
  718.  
  719.     // Step 6. Razoring (skipped when in check)
  720.     if (   !PvNode
  721.         &&  depth < 4 * ONE_PLY
  722.         &&  ttMove == MOVE_NONE
  723.         &&  eval + razor_margin[depth / ONE_PLY] <= alpha)
  724.     {
  725.         if (depth <= ONE_PLY)
  726.             return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
  727.  
  728.         Value ralpha = alpha - razor_margin[depth / ONE_PLY];
  729.         Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
  730.         if (v <= ralpha)
  731.             return v;
  732.     }
  733.  
  734.     // Step 7. Futility pruning: child node (skipped when in check)
  735.     if (   !rootNode
  736.         &&  depth < 7 * ONE_PLY
  737.         &&  eval - futility_margin(depth) >= beta
  738.         &&  eval < VALUE_KNOWN_WIN  // Do not return unproven wins
  739.         &&  pos.non_pawn_material(pos.side_to_move()))
  740.         return eval;
  741.  
  742.     // Step 8. Null move search with verification search (is omitted in PV nodes)
  743.     if (   !PvNode
  744.         &&  eval >= beta
  745.         && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
  746.         &&  pos.non_pawn_material(pos.side_to_move()))
  747.     {
  748.         ss->currentMove = MOVE_NULL;
  749.         ss->counterMoves = nullptr;
  750.  
  751.         assert(eval - beta >= 0);
  752.  
  753.         // Null move dynamic reduction based on depth and value
  754.         Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
  755.  
  756.         pos.do_null_move(st);
  757.         (ss+1)->skipEarlyPruning = true;
  758.         nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
  759.                                       : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
  760.         (ss+1)->skipEarlyPruning = false;
  761.         pos.undo_null_move();
  762.  
  763.         if (nullValue >= beta)
  764.         {
  765.             // Do not return unproven mate scores
  766.             if (nullValue >= VALUE_MATE_IN_MAX_PLY)
  767.                 nullValue = beta;
  768.  
  769.             if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
  770.                 return nullValue;
  771.  
  772.             // Do verification search at high depths
  773.             ss->skipEarlyPruning = true;
  774.             Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
  775.                                         :  search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
  776.             ss->skipEarlyPruning = false;
  777.  
  778.             if (v >= beta)
  779.                 return nullValue;
  780.         }
  781.     }
  782.  
  783.     // Step 9. ProbCut (skipped when in check)
  784.     // If we have a good enough capture and a reduced search returns a value
  785.     // much above beta, we can (almost) safely prune the previous move.
  786.     if (   !PvNode
  787.         &&  depth >= 5 * ONE_PLY
  788.         &&  abs(beta) < VALUE_MATE_IN_MAX_PLY)
  789.     {
  790.         Value rbeta = std::min(beta + 200, VALUE_INFINITE);
  791.         Depth rdepth = depth - 4 * ONE_PLY;
  792.  
  793.         assert(rdepth >= ONE_PLY);
  794.         assert((ss-1)->currentMove != MOVE_NONE);
  795.         assert((ss-1)->currentMove != MOVE_NULL);
  796.  
  797.         MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
  798.  
  799.         while ((move = mp.next_move()) != MOVE_NONE)
  800.             if (pos.legal(move))
  801.             {
  802.                 ss->currentMove = move;
  803.                 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
  804.                 pos.do_move(move, st, pos.gives_check(move));
  805.                 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
  806.                 pos.undo_move(move);
  807.                 if (value >= rbeta)
  808.                     return value;
  809.             }
  810.     }
  811.  
  812.     // Step 10. Internal iterative deepening (skipped when in check)
  813.     if (    depth >= 6 * ONE_PLY
  814.         && !ttMove
  815.         && (PvNode || ss->staticEval + 256 >= beta))
  816.     {
  817.         Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
  818.         ss->skipEarlyPruning = true;
  819.         search<NT>(pos, ss, alpha, beta, d, cutNode);
  820.         ss->skipEarlyPruning = false;
  821.  
  822.         tte = TT.probe(posKey, ttHit);
  823.         ttMove = ttHit ? tte->move() : MOVE_NONE;
  824.     }
  825.  
  826. moves_loop: // When in check search starts from here
  827.  
  828.     const CounterMoveStats* cmh  = (ss-1)->counterMoves;
  829.     const CounterMoveStats* fmh  = (ss-2)->counterMoves;
  830.     const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
  831.  
  832.     MovePicker mp(pos, ttMove, depth, ss);
  833.     value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
  834.     improving =   ss->staticEval >= (ss-2)->staticEval
  835.             /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
  836.                ||(ss-2)->staticEval == VALUE_NONE;
  837.  
  838.     singularExtensionNode =   !rootNode
  839.                            &&  depth >= 8 * ONE_PLY
  840.                            &&  ttMove != MOVE_NONE
  841.                            &&  ttValue != VALUE_NONE
  842.                            && !excludedMove // Recursive singular search is not allowed
  843.                            && (tte->bound() & BOUND_LOWER)
  844.                            &&  tte->depth() >= depth - 3 * ONE_PLY;
  845.  
  846.     // Step 11. Loop through moves
  847.     // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
  848.     while ((move = mp.next_move()) != MOVE_NONE)
  849.     {
  850.       assert(is_ok(move));
  851.  
  852.       if (move == excludedMove)
  853.           continue;
  854.  
  855.       // At root obey the "searchmoves" option and skip moves not listed in Root
  856.       // Move List. As a consequence any illegal move is also skipped. In MultiPV
  857.       // mode we also skip PV moves which have been already searched.
  858.       if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
  859.                                   thisThread->rootMoves.end(), move))
  860.           continue;
  861.  
  862.       ss->moveCount = ++moveCount;
  863.  
  864.       if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
  865.           sync_cout << "info depth " << depth / ONE_PLY
  866.                     << " currmove " << UCI::move(move, pos.is_chess960())
  867.                     << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
  868.  
  869.       if (PvNode)
  870.           (ss+1)->pv = nullptr;
  871.  
  872.       extension = DEPTH_ZERO;
  873.       captureOrPromotion = pos.capture_or_promotion(move);
  874.       moved_piece = pos.moved_piece(move);
  875.  
  876.       givesCheck =  type_of(move) == NORMAL && !pos.discovered_check_candidates()
  877.                   ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
  878.                   : pos.gives_check(move);
  879.  
  880.       moveCountPruning =   depth < 16 * ONE_PLY
  881.                         && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
  882.  
  883.       // Step 12. Extend checks
  884.       if (    givesCheck
  885.           && !moveCountPruning
  886.           &&  pos.see_ge(move, VALUE_ZERO))
  887.           extension = ONE_PLY;
  888.  
  889.       // Singular extension search. If all moves but one fail low on a search of
  890.       // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
  891.       // is singular and should be extended. To verify this we do a reduced search
  892.       // on all the other moves but the ttMove and if the result is lower than
  893.       // ttValue minus a margin then we extend the ttMove.
  894.       if (    singularExtensionNode
  895.           &&  move == ttMove
  896.           && !extension
  897.           &&  pos.legal(move))
  898.       {
  899.           Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
  900.           Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
  901.           ss->excludedMove = move;
  902.           ss->skipEarlyPruning = true;
  903.           value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode);
  904.           ss->skipEarlyPruning = false;
  905.           ss->excludedMove = MOVE_NONE;
  906.  
  907.           if (value < rBeta)
  908.               extension = ONE_PLY;
  909.       }
  910.  
  911.       // Update the current move (this must be done after singular extension search)
  912.       newDepth = depth - ONE_PLY + extension;
  913.  
  914.       // Step 13. Pruning at shallow depth
  915.       if (  !rootNode
  916.           && bestValue > VALUE_MATED_IN_MAX_PLY)
  917.       {
  918.           if (   !captureOrPromotion
  919.               && !givesCheck
  920.               && !pos.advanced_pawn_push(move))
  921.           {
  922.               // Move count based pruning
  923.               if (moveCountPruning)
  924.                   continue;
  925.  
  926.               // Reduced depth of the next LMR search
  927.               int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
  928.  
  929.               // Countermoves based pruning
  930.               if (   lmrDepth < 3
  931.                   && (!cmh  || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
  932.                   && (!fmh  || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
  933.                   && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
  934.                   continue;
  935.  
  936.               // Futility pruning: parent node
  937.               if (   lmrDepth < 7
  938.                   && !inCheck
  939.                   && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
  940.                   continue;
  941.  
  942.               // Prune moves with negative SEE
  943.               if (   lmrDepth < 8
  944.                   && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
  945.                   continue;
  946.           }
  947.           else if (   depth < 7 * ONE_PLY
  948.                    && !extension
  949.                    && !pos.see_ge(move, Value(-35 * depth / ONE_PLY * depth / ONE_PLY)))
  950.                   continue;
  951.       }
  952.  
  953.       // Speculative prefetch as early as possible
  954.       prefetch(TT.first_entry(pos.key_after(move)));
  955.  
  956.       // Check for legality just before making the move
  957.       if (!rootNode && !pos.legal(move))
  958.       {
  959.           ss->moveCount = --moveCount;
  960.           continue;
  961.       }
  962.  
  963.       ss->currentMove = move;
  964.       ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
  965.  
  966.       // Step 14. Make the move
  967.       pos.do_move(move, st, givesCheck);
  968.  
  969.       // Step 15. Reduced depth search (LMR). If the move fails high it will be
  970.       // re-searched at full depth.
  971.       if (    depth >= 3 * ONE_PLY
  972.           &&  moveCount > 1
  973.           && (!captureOrPromotion || moveCountPruning))
  974.       {
  975.           Depth r = reduction<PvNode>(improving, depth, moveCount);
  976.  
  977.           if (captureOrPromotion)
  978.               r -= r ? ONE_PLY : DEPTH_ZERO;
  979.           else
  980.           {
  981.               // Increase reduction for cut nodes
  982.               if (cutNode)
  983.                   r += 2 * ONE_PLY;
  984.  
  985.               // Decrease reduction for moves that escape a capture. Filter out
  986.               // castling moves, because they are coded as "king captures rook" and
  987.               // hence break make_move(). Also use see() instead of see_sign(),
  988.               // because the destination square is empty.
  989.               else if (   type_of(move) == NORMAL
  990.                        && type_of(pos.piece_on(to_sq(move))) != PAWN
  991.                        && !pos.see_ge(make_move(to_sq(move), from_sq(move)),  VALUE_ZERO))
  992.                   r -= 2 * ONE_PLY;
  993.  
  994.               ss->history = thisThread->history[moved_piece][to_sq(move)]
  995.                            +    (cmh  ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
  996.                            +    (fmh  ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
  997.                            +    (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
  998.                            +    thisThread->fromTo.get(~pos.side_to_move(), move)
  999.                            -    8000; // Correction factor
  1000.  
  1001.               // Decrease/increase reduction by comparing opponent's stat score
  1002.               if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
  1003.                   r -= ONE_PLY;
  1004.  
  1005.               else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
  1006.                   r += ONE_PLY;
  1007.  
  1008.               // Decrease/increase reduction for moves with a good/bad history
  1009.               r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
  1010.           }
  1011.  
  1012.           Depth d = std::max(newDepth - r, ONE_PLY);
  1013.  
  1014.           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
  1015.  
  1016.           doFullDepthSearch = (value > alpha && d != newDepth);
  1017.       }
  1018.       else
  1019.           doFullDepthSearch = !PvNode || moveCount > 1;
  1020.  
  1021.       // Step 16. Full depth search when LMR is skipped or fails high
  1022.       if (doFullDepthSearch)
  1023.           value = newDepth <   ONE_PLY ?
  1024.                             givesCheck ? -qsearch<NonPV,  true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
  1025.                                        : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
  1026.                                        : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
  1027.  
  1028.       // For PV nodes only, do a full PV search on the first move or after a fail
  1029.       // high (in the latter case search only if value < beta), otherwise let the
  1030.       // parent node fail low with value <= alpha and try another move.
  1031.       if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
  1032.       {
  1033.           (ss+1)->pv = pv;
  1034.           (ss+1)->pv[0] = MOVE_NONE;
  1035.  
  1036.           value = newDepth <   ONE_PLY ?
  1037.                             givesCheck ? -qsearch<PV,  true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
  1038.                                        : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
  1039.                                        : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
  1040.       }
  1041.  
  1042.       // Step 17. Undo move
  1043.       pos.undo_move(move);
  1044.  
  1045.       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
  1046.  
  1047.       // Step 18. Check for a new best move
  1048.       // Finished searching the move. If a stop occurred, the return value of
  1049.       // the search cannot be trusted, and we return immediately without
  1050.       // updating best move, PV and TT.
  1051.       if (Signals.stop.load(std::memory_order_relaxed))
  1052.           return VALUE_ZERO;
  1053.  
  1054.       if (rootNode)
  1055.       {
  1056.           RootMove& rm = *std::find(thisThread->rootMoves.begin(),
  1057.                                     thisThread->rootMoves.end(), move);
  1058.  
  1059.           // PV move or new best move ?
  1060.           if (moveCount == 1 || value > alpha)
  1061.           {
  1062.               rm.score = value;
  1063.               rm.pv.resize(1);
  1064.  
  1065.               assert((ss+1)->pv);
  1066.  
  1067.               for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
  1068.                   rm.pv.push_back(*m);
  1069.  
  1070.               // We record how often the best move has been changed in each
  1071.               // iteration. This information is used for time management: When
  1072.               // the best move changes frequently, we allocate some more time.
  1073.               if (moveCount > 1 && thisThread == Threads.main())
  1074.                   ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
  1075.           }
  1076.           else
  1077.               // All other moves but the PV are set to the lowest value: this is
  1078.               // not a problem when sorting because the sort is stable and the
  1079.               // move position in the list is preserved - just the PV is pushed up.
  1080.               rm.score = -VALUE_INFINITE;
  1081.       }
  1082.  
  1083.       if (value > bestValue)
  1084.       {
  1085.           bestValue = value;
  1086.  
  1087.           if (value > alpha)
  1088.           {
  1089.               // If there is an easy move for this position, clear it if unstable
  1090.               if (    PvNode
  1091.                   &&  thisThread == Threads.main()
  1092.                   &&  EasyMove.get(pos.key())
  1093.                   && (move != EasyMove.get(pos.key()) || moveCount > 1))
  1094.                   EasyMove.clear();
  1095.  
  1096.               bestMove = move;
  1097.  
  1098.               if (PvNode && !rootNode) // Update pv even in fail-high case
  1099.                   update_pv(ss->pv, move, (ss+1)->pv);
  1100.  
  1101.               if (PvNode && value < beta) // Update alpha! Always alpha < beta
  1102.                   alpha = value;
  1103.               else
  1104.               {
  1105.                   assert(value >= beta); // Fail high
  1106.                   break;
  1107.               }
  1108.           }
  1109.       }
  1110.  
  1111.       if (!captureOrPromotion && move != bestMove && quietCount < 64)
  1112.           quietsSearched[quietCount++] = move;
  1113.     }
  1114.  
  1115.     // The following condition would detect a stop only after move loop has been
  1116.     // completed. But in this case bestValue is valid because we have fully
  1117.     // searched our subtree, and we can anyhow save the result in TT.
  1118.     /*
  1119.        if (Signals.stop)
  1120.         return VALUE_DRAW;
  1121.     */
  1122.  
  1123.     // Step 20. Check for mate and stalemate
  1124.     // All legal moves have been searched and if there are no legal moves, it
  1125.     // must be a mate or a stalemate. If we are in a singular extension search then
  1126.     // return a fail low score.
  1127.  
  1128.     assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
  1129.  
  1130.     if (!moveCount)
  1131.         bestValue = excludedMove ? alpha
  1132.                    :     inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
  1133.     else if (bestMove)
  1134.     {
  1135.         int d = depth / ONE_PLY;
  1136.  
  1137.         // Quiet best move: update killers, history and countermoves
  1138.         if (!pos.capture_or_promotion(bestMove))
  1139.         {
  1140.             Value bonus = Value(d * d + 2 * d - 2);
  1141.             update_stats(pos, ss, bestMove, quietsSearched, quietCount, bonus);
  1142.         }
  1143.  
  1144.         // Extra penalty for a quiet TT move in previous ply when it gets refuted
  1145.         if ((ss-1)->moveCount == 1 && !pos.captured_piece())
  1146.         {
  1147.             Value penalty = Value(d * d + 4 * d + 1);
  1148.             Square prevSq = to_sq((ss-1)->currentMove);
  1149.             update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
  1150.         }
  1151.     }
  1152.     // Bonus for prior countermove that caused the fail low
  1153.     else if (    depth >= 3 * ONE_PLY
  1154.              && !pos.captured_piece()
  1155.              && is_ok((ss-1)->currentMove))
  1156.     {
  1157.         int d = depth / ONE_PLY;
  1158.         Value bonus = Value(d * d + 2 * d - 2);
  1159.         Square prevSq = to_sq((ss-1)->currentMove);
  1160.         update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, bonus);
  1161.     }
  1162.  
  1163.     tte->save(posKey, value_to_tt(bestValue, ss->ply),
  1164.               bestValue >= beta ? BOUND_LOWER :
  1165.               PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
  1166.               depth, bestMove, ss->staticEval, TT.generation());
  1167.  
  1168.     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
  1169.  
  1170.     return bestValue;
  1171.   }
  1172.  
  1173.  
  1174.   // qsearch() is the quiescence search function, which is called by the main
  1175.   // search function when the remaining depth is zero (or, to be more precise,
  1176.   // less than ONE_PLY).
  1177.  
  1178.   template <NodeType NT, bool InCheck>
  1179.   Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
  1180.  
  1181.     const bool PvNode = NT == PV;
  1182.  
  1183.     assert(InCheck == !!pos.checkers());
  1184.     assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
  1185.     assert(PvNode || (alpha == beta - 1));
  1186.     assert(depth <= DEPTH_ZERO);
  1187.     assert(depth / ONE_PLY * ONE_PLY == depth);
  1188.  
  1189.     Move pv[MAX_PLY+1];
  1190.     StateInfo st;
  1191.     TTEntry* tte;
  1192.     Key posKey;
  1193.     Move ttMove, move, bestMove;
  1194.     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
  1195.     bool ttHit, givesCheck, evasionPrunable;
  1196.     Depth ttDepth;
  1197.  
  1198.     if (PvNode)
  1199.     {
  1200.         oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
  1201.         (ss+1)->pv = pv;
  1202.         ss->pv[0] = MOVE_NONE;
  1203.     }
  1204.  
  1205.     ss->currentMove = bestMove = MOVE_NONE;
  1206.     ss->ply = (ss-1)->ply + 1;
  1207.  
  1208.     // Check for an instant draw or if the maximum ply has been reached
  1209.     if (pos.is_draw() || ss->ply >= MAX_PLY)
  1210.         return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
  1211.                                               : DrawValue[pos.side_to_move()];
  1212.  
  1213.     assert(0 <= ss->ply && ss->ply < MAX_PLY);
  1214.  
  1215.     // Decide whether or not to include checks: this fixes also the type of
  1216.     // TT entry depth that we are going to use. Note that in qsearch we use
  1217.     // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
  1218.     ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
  1219.                                                   : DEPTH_QS_NO_CHECKS;
  1220.  
  1221.     // Transposition table lookup
  1222.     posKey = pos.key();
  1223.     tte = TT.probe(posKey, ttHit);
  1224.     ttMove = ttHit ? tte->move() : MOVE_NONE;
  1225.     ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
  1226.  
  1227.     if (  !PvNode
  1228.         && ttHit
  1229.         && tte->depth() >= ttDepth
  1230.         && ttValue != VALUE_NONE // Only in case of TT access race
  1231.         && (ttValue >= beta ? (tte->bound() &  BOUND_LOWER)
  1232.                             : (tte->bound() &  BOUND_UPPER)))
  1233.         return ttValue;
  1234.  
  1235.     // Evaluate the position statically
  1236.     if (InCheck)
  1237.     {
  1238.         ss->staticEval = VALUE_NONE;
  1239.         bestValue = futilityBase = -VALUE_INFINITE;
  1240.     }
  1241.     else
  1242.     {
  1243.         if (ttHit)
  1244.         {
  1245.             // Never assume anything on values stored in TT
  1246.             if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
  1247.                 ss->staticEval = bestValue = evaluate(pos);
  1248.  
  1249.             // Can ttValue be used as a better position evaluation?
  1250.             if (ttValue != VALUE_NONE)
  1251.                 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
  1252.                     bestValue = ttValue;
  1253.         }
  1254.         else
  1255.             ss->staticEval = bestValue =
  1256.             (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
  1257.                                              : -(ss-1)->staticEval + 2 * Eval::Tempo;
  1258.  
  1259.         // Stand pat. Return immediately if static value is at least beta
  1260.         if (bestValue >= beta)
  1261.         {
  1262.             if (!ttHit)
  1263.                 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
  1264.                           DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
  1265.  
  1266.             return bestValue;
  1267.         }
  1268.  
  1269.         if (PvNode && bestValue > alpha)
  1270.             alpha = bestValue;
  1271.  
  1272.         futilityBase = bestValue + 128;
  1273.     }
  1274.  
  1275.     // Initialize a MovePicker object for the current position, and prepare
  1276.     // to search the moves. Because the depth is <= 0 here, only captures,
  1277.     // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
  1278.     // be generated.
  1279.     MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
  1280.  
  1281.     // Loop through the moves until no moves remain or a beta cutoff occurs
  1282.     while ((move = mp.next_move()) != MOVE_NONE)
  1283.     {
  1284.       assert(is_ok(move));
  1285.  
  1286.       givesCheck =  type_of(move) == NORMAL && !pos.discovered_check_candidates()
  1287.                   ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
  1288.                   : pos.gives_check(move);
  1289.  
  1290.       // Futility pruning
  1291.       if (   !InCheck
  1292.           && !givesCheck
  1293.           &&  futilityBase > -VALUE_KNOWN_WIN
  1294.           && !pos.advanced_pawn_push(move))
  1295.       {
  1296.           assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
  1297.  
  1298.           futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
  1299.  
  1300.           if (futilityValue <= alpha)
  1301.           {
  1302.               bestValue = std::max(bestValue, futilityValue);
  1303.               continue;
  1304.           }
  1305.  
  1306.           if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
  1307.           {
  1308.               bestValue = std::max(bestValue, futilityBase);
  1309.               continue;
  1310.           }
  1311.       }
  1312.  
  1313.       // Detect non-capture evasions that are candidates to be pruned
  1314.       evasionPrunable =    InCheck
  1315.                        &&  bestValue > VALUE_MATED_IN_MAX_PLY
  1316.                        && !pos.capture(move);
  1317.  
  1318.       // Don't search moves with negative SEE values
  1319.       if (  (!InCheck || evasionPrunable)
  1320.           &&  type_of(move) != PROMOTION
  1321.           &&  !pos.see_ge(move, VALUE_ZERO))
  1322.           continue;
  1323.  
  1324.       // Speculative prefetch as early as possible
  1325.       prefetch(TT.first_entry(pos.key_after(move)));
  1326.  
  1327.       // Check for legality just before making the move
  1328.       if (!pos.legal(move))
  1329.           continue;
  1330.  
  1331.       ss->currentMove = move;
  1332.  
  1333.       // Make and search the move
  1334.       pos.do_move(move, st, givesCheck);
  1335.       value = givesCheck ? -qsearch<NT,  true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
  1336.                          : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
  1337.       pos.undo_move(move);
  1338.  
  1339.       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
  1340.  
  1341.       // Check for a new best move
  1342.       if (value > bestValue)
  1343.       {
  1344.           bestValue = value;
  1345.  
  1346.           if (value > alpha)
  1347.           {
  1348.               if (PvNode) // Update pv even in fail-high case
  1349.                   update_pv(ss->pv, move, (ss+1)->pv);
  1350.  
  1351.               if (PvNode && value < beta) // Update alpha here!
  1352.               {
  1353.                   alpha = value;
  1354.                   bestMove = move;
  1355.               }
  1356.               else // Fail high
  1357.               {
  1358.                   tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
  1359.                             ttDepth, move, ss->staticEval, TT.generation());
  1360.  
  1361.                   return value;
  1362.               }
  1363.           }
  1364.        }
  1365.     }
  1366.  
  1367.     // All legal moves have been searched. A special case: If we're in check
  1368.     // and no legal moves were found, it is checkmate.
  1369.     if (InCheck && bestValue == -VALUE_INFINITE)
  1370.         return mated_in(ss->ply); // Plies to mate from the root
  1371.  
  1372.     tte->save(posKey, value_to_tt(bestValue, ss->ply),
  1373.               PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
  1374.               ttDepth, bestMove, ss->staticEval, TT.generation());
  1375.  
  1376.     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
  1377.  
  1378.     return bestValue;
  1379.   }
  1380.  
  1381.  
  1382.   // value_to_tt() adjusts a mate score from "plies to mate from the root" to
  1383.   // "plies to mate from the current position". Non-mate scores are unchanged.
  1384.   // The function is called before storing a value in the transposition table.
  1385.  
  1386.   Value value_to_tt(Value v, int ply) {
  1387.  
  1388.     assert(v != VALUE_NONE);
  1389.  
  1390.     return  v >= VALUE_MATE_IN_MAX_PLY  ? v + ply
  1391.           : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
  1392.   }
  1393.  
  1394.  
  1395.   // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
  1396.   // from the transposition table (which refers to the plies to mate/be mated
  1397.   // from current position) to "plies to mate/be mated from the root".
  1398.  
  1399.   Value value_from_tt(Value v, int ply) {
  1400.  
  1401.     return  v == VALUE_NONE             ? VALUE_NONE
  1402.           : v >= VALUE_MATE_IN_MAX_PLY  ? v - ply
  1403.           : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
  1404.   }
  1405.  
  1406.  
  1407.   // update_pv() adds current move and appends child pv[]
  1408.  
  1409.   void update_pv(Move* pv, Move move, Move* childPv) {
  1410.  
  1411.     for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
  1412.         *pv++ = *childPv++;
  1413.     *pv = MOVE_NONE;
  1414.   }
  1415.  
  1416.  
  1417.   // update_cm_stats() updates countermove and follow-up move history
  1418.  
  1419.   void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
  1420.  
  1421.     CounterMoveStats* cmh  = (ss-1)->counterMoves;
  1422.     CounterMoveStats* fmh1 = (ss-2)->counterMoves;
  1423.     CounterMoveStats* fmh2 = (ss-4)->counterMoves;
  1424.  
  1425.     if (cmh)
  1426.         cmh->update(pc, s, bonus);
  1427.  
  1428.     if (fmh1)
  1429.         fmh1->update(pc, s, bonus);
  1430.  
  1431.     if (fmh2)
  1432.         fmh2->update(pc, s, bonus);
  1433.   }
  1434.  
  1435.  
  1436.   // update_stats() updates killers, history, countermove and countermove plus
  1437.   // follow-up move history when a new quiet best move is found.
  1438.  
  1439.   void update_stats(const Position& pos, Stack* ss, Move move,
  1440.                     Move* quiets, int quietsCnt, Value bonus) {
  1441.  
  1442.     if (ss->killers[0] != move)
  1443.     {
  1444.         ss->killers[1] = ss->killers[0];
  1445.         ss->killers[0] = move;
  1446.     }
  1447.  
  1448.     Color c = pos.side_to_move();
  1449.     Thread* thisThread = pos.this_thread();
  1450.     thisThread->fromTo.update(c, move, bonus);
  1451.     thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
  1452.     update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
  1453.  
  1454.     if ((ss-1)->counterMoves)
  1455.     {
  1456.         Square prevSq = to_sq((ss-1)->currentMove);
  1457.         thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
  1458.     }
  1459.  
  1460.     // Decrease all the other played quiet moves
  1461.     for (int i = 0; i < quietsCnt; ++i)
  1462.     {
  1463.         thisThread->fromTo.update(c, quiets[i], -bonus);
  1464.         thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
  1465.         update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
  1466.     }
  1467.   }
  1468.  
  1469.  
  1470.   // When playing with strength handicap, choose best move among a set of RootMoves
  1471.   // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
  1472.  
  1473.   Move Skill::pick_best(size_t multiPV) {
  1474.  
  1475.     const RootMoves& rootMoves = Threads.main()->rootMoves;
  1476.     static PRNG rng(now()); // PRNG sequence should be non-deterministic
  1477.  
  1478.     // RootMoves are already sorted by score in descending order
  1479.     Value topScore = rootMoves[0].score;
  1480.     int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
  1481.     int weakness = 120 - 2 * level;
  1482.     int maxScore = -VALUE_INFINITE;
  1483.  
  1484.     // Choose best move. For each move score we add two terms, both dependent on
  1485.     // weakness. One is deterministic and bigger for weaker levels, and one is
  1486.     // random. Then we choose the move with the resulting highest score.
  1487.     for (size_t i = 0; i < multiPV; ++i)
  1488.     {
  1489.         // This is our magic formula
  1490.         int push = (  weakness * int(topScore - rootMoves[i].score)
  1491.                     + delta * (rng.rand<unsigned>() % weakness)) / 128;
  1492.  
  1493.         if (rootMoves[i].score + push > maxScore)
  1494.         {
  1495.             maxScore = rootMoves[i].score + push;
  1496.             best = rootMoves[i].pv[0];
  1497.         }
  1498.     }
  1499.  
  1500.     return best;
  1501.   }
  1502.  
  1503.  
  1504.   // check_time() is used to print debug info and, more importantly, to detect
  1505.   // when we are out of available time and thus stop the search.
  1506.  
  1507.   void check_time() {
  1508.  
  1509.     static TimePoint lastInfoTime = now();
  1510.  
  1511.     int elapsed = Time.elapsed();
  1512.     TimePoint tick = Limits.startTime + elapsed;
  1513.  
  1514.     if (tick - lastInfoTime >= 1000)
  1515.     {
  1516.         lastInfoTime = tick;
  1517.         dbg_print();
  1518.     }
  1519.  
  1520.     // An engine may not stop pondering until told so by the GUI
  1521.     if (Limits.ponder)
  1522.         return;
  1523.  
  1524.     if (   (Limits.use_time_management() && elapsed > Time.maximum() - 10)
  1525.         || (Limits.movetime && elapsed >= Limits.movetime)
  1526.         || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
  1527.             Signals.stop = true;
  1528.   }
  1529.  
  1530. } // namespace
  1531.  
  1532.  
  1533. /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
  1534. /// that all (if any) unsearched PV lines are sent using a previous search score.
  1535.  
  1536. string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
  1537.  
  1538.   std::stringstream ss;
  1539.   int elapsed = Time.elapsed() + 1;
  1540.   const RootMoves& rootMoves = pos.this_thread()->rootMoves;
  1541.   size_t PVIdx = pos.this_thread()->PVIdx;
  1542.   size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
  1543.   uint64_t nodesSearched = Threads.nodes_searched();
  1544.   uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
  1545.  
  1546.   for (size_t i = 0; i < multiPV; ++i)
  1547.   {
  1548.       bool updated = (i <= PVIdx);
  1549.  
  1550.       if (depth == ONE_PLY && !updated)
  1551.           continue;
  1552.  
  1553.       Depth d = updated ? depth : depth - ONE_PLY;
  1554.       Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
  1555.  
  1556.       bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
  1557.       v = tb ? TB::Score : v;
  1558.  
  1559.       if (ss.rdbuf()->in_avail()) // Not at first line
  1560.           ss << "\n";
  1561.  
  1562.       ss << "info"
  1563.          << " depth "    << d / ONE_PLY
  1564.          << " seldepth " << pos.this_thread()->maxPly
  1565.          << " multipv "  << i + 1
  1566.          << " score "    << UCI::value(v);
  1567.  
  1568.       if (!tb && i == PVIdx)
  1569.           ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
  1570.  
  1571.       ss << " nodes "    << nodesSearched
  1572.          << " nps "      << nodesSearched * 1000 / elapsed;
  1573.  
  1574.       if (elapsed > 1000) // Earlier makes little sense
  1575.           ss << " hashfull " << TT.hashfull();
  1576.  
  1577.       ss << " tbhits "   << tbHits
  1578.          << " time "     << elapsed
  1579.          << " pv";
  1580.  
  1581.       for (Move m : rootMoves[i].pv)
  1582.           ss << " " << UCI::move(m, pos.is_chess960());
  1583.   }
  1584.  
  1585.   return ss.str();
  1586. }
  1587.  
  1588.  
  1589. /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
  1590. /// before exiting the search, for instance, in case we stop the search during a
  1591. /// fail high at root. We try hard to have a ponder move to return to the GUI,
  1592. /// otherwise in case of 'ponder on' we have nothing to think on.
  1593.  
  1594. bool RootMove::extract_ponder_from_tt(Position& pos) {
  1595.  
  1596.     StateInfo st;
  1597.     bool ttHit;
  1598.  
  1599.     assert(pv.size() == 1);
  1600.  
  1601.     if (!pv[0])
  1602.         return false;
  1603.  
  1604.     pos.do_move(pv[0], st, pos.gives_check(pv[0]));
  1605.     TTEntry* tte = TT.probe(pos.key(), ttHit);
  1606.  
  1607.     if (ttHit)
  1608.     {
  1609.         Move m = tte->move(); // Local copy to be SMP safe
  1610.         if (MoveList<LEGAL>(pos).contains(m))
  1611.             pv.push_back(m);
  1612.     }
  1613.  
  1614.     pos.undo_move(pv[0]);
  1615.     return pv.size() > 1;
  1616. }
  1617.  
  1618. void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
  1619.  
  1620.     RootInTB = false;
  1621.     UseRule50 = Options["Syzygy50MoveRule"];
  1622.     ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
  1623.     Cardinality = Options["SyzygyProbeLimit"];
  1624.  
  1625.     // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
  1626.     if (Cardinality > MaxCardinality)
  1627.     {
  1628.         Cardinality = MaxCardinality;
  1629.         ProbeDepth = DEPTH_ZERO;
  1630.     }
  1631.  
  1632.     if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
  1633.         return;
  1634.  
  1635.     // If the current root position is in the tablebases, then RootMoves
  1636.     // contains only moves that preserve the draw or the win.
  1637.     RootInTB = root_probe(pos, rootMoves, TB::Score);
  1638.  
  1639.     if (RootInTB)
  1640.         Cardinality = 0; // Do not probe tablebases during the search
  1641.  
  1642.     else // If DTZ tables are missing, use WDL tables as a fallback
  1643.     {
  1644.         // Filter out moves that do not preserve the draw or the win.
  1645.         RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
  1646.  
  1647.         // Only probe during search if winning
  1648.         if (RootInTB && TB::Score <= VALUE_DRAW)
  1649.             Cardinality = 0;
  1650.     }
  1651.  
  1652.     if (RootInTB && !UseRule50)
  1653.         TB::Score =  TB::Score > VALUE_DRAW ?  VALUE_MATE - MAX_PLY - 1
  1654.                    : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
  1655.                                             :  VALUE_DRAW;
  1656. }
  1657.