"Fossies" - the Fresh Open Source Software Archive

Member "stockfish-11-linux/src/search.cpp" (18 Jan 2020, 68412 Bytes) of package /linux/privat/stockfish-11-linux.zip:


As a special service "Fossies" has tried to format the requested source page into HTML format using (guessed) C and C++ source code syntax highlighting (style: standard) with prefixed line numbers and code folding option. Alternatively you can here view or download the uninterpreted source code file. See also the last Fossies "Diffs" side-by-side code changes report for "search.cpp": 9-linux_vs_10-linux.

    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) 2015-2020 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 "thread.h"
   35 #include "timeman.h"
   36 #include "tt.h"
   37 #include "uci.h"
   38 #include "syzygy/tbprobe.h"
   39 
   40 namespace Search {
   41 
   42   LimitsType Limits;
   43 }
   44 
   45 namespace Tablebases {
   46 
   47   int Cardinality;
   48   bool RootInTB;
   49   bool UseRule50;
   50   Depth ProbeDepth;
   51 }
   52 
   53 namespace TB = Tablebases;
   54 
   55 using std::string;
   56 using Eval::evaluate;
   57 using namespace Search;
   58 
   59 namespace {
   60 
   61   // Different node types, used as a template parameter
   62   enum NodeType { NonPV, PV };
   63 
   64   constexpr uint64_t ttHitAverageWindow     = 4096;
   65   constexpr uint64_t ttHitAverageResolution = 1024;
   66 
   67   // Razor and futility margins
   68   constexpr int RazorMargin = 531;
   69   Value futility_margin(Depth d, bool improving) {
   70     return Value(217 * (d - improving));
   71   }
   72 
   73   // Reductions lookup table, initialized at startup
   74   int Reductions[MAX_MOVES]; // [depth or moveNumber]
   75 
   76   Depth reduction(bool i, Depth d, int mn) {
   77     int r = Reductions[d] * Reductions[mn];
   78     return (r + 511) / 1024 + (!i && r > 1007);
   79   }
   80 
   81   constexpr int futility_move_count(bool improving, Depth depth) {
   82     return (5 + depth * depth) * (1 + improving) / 2 - 1;
   83   }
   84 
   85   // History and stats update bonus, based on depth
   86   int stat_bonus(Depth d) {
   87     return d > 15 ? -8 : 19 * d * d + 155 * d - 132;
   88   }
   89 
   90   // Add a small random component to draw evaluations to avoid 3fold-blindness
   91   Value value_draw(Thread* thisThread) {
   92     return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
   93   }
   94 
   95   // Skill structure is used to implement strength limit
   96   struct Skill {
   97     explicit Skill(int l) : level(l) {}
   98     bool enabled() const { return level < 20; }
   99     bool time_to_pick(Depth depth) const { return depth == 1 + level; }
  100     Move pick_best(size_t multiPV);
  101 
  102     int level;
  103     Move best = MOVE_NONE;
  104   };
  105 
  106   // Breadcrumbs are used to mark nodes as being searched by a given thread
  107   struct Breadcrumb {
  108     std::atomic<Thread*> thread;
  109     std::atomic<Key> key;
  110   };
  111   std::array<Breadcrumb, 1024> breadcrumbs;
  112 
  113   // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
  114   // node for potential reductions. A free node will be marked upon entering the moves
  115   // loop by the constructor, and unmarked upon leaving that loop by the destructor.
  116   struct ThreadHolding {
  117     explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
  118        location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
  119        otherThread = false;
  120        owning = false;
  121        if (location)
  122        {
  123           // See if another already marked this location, if not, mark it ourselves
  124           Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
  125           if (tmp == nullptr)
  126           {
  127               (*location).thread.store(thisThread, std::memory_order_relaxed);
  128               (*location).key.store(posKey, std::memory_order_relaxed);
  129               owning = true;
  130           }
  131           else if (   tmp != thisThread
  132                    && (*location).key.load(std::memory_order_relaxed) == posKey)
  133               otherThread = true;
  134        }
  135     }
  136 
  137     ~ThreadHolding() {
  138        if (owning) // Free the marked location
  139            (*location).thread.store(nullptr, std::memory_order_relaxed);
  140     }
  141 
  142     bool marked() { return otherThread; }
  143 
  144     private:
  145     Breadcrumb* location;
  146     bool otherThread, owning;
  147   };
  148 
  149   template <NodeType NT>
  150   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
  151 
  152   template <NodeType NT>
  153   Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
  154 
  155   Value value_to_tt(Value v, int ply);
  156   Value value_from_tt(Value v, int ply, int r50c);
  157   void update_pv(Move* pv, Move move, Move* childPv);
  158   void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
  159   void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
  160   void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
  161                         Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
  162 
  163   // perft() is our utility to verify move generation. All the leaf nodes up
  164   // to the given depth are generated and counted, and the sum is returned.
  165   template<bool Root>
  166   uint64_t perft(Position& pos, Depth depth) {
  167 
  168     StateInfo st;
  169     uint64_t cnt, nodes = 0;
  170     const bool leaf = (depth == 2);
  171 
  172     for (const auto& m : MoveList<LEGAL>(pos))
  173     {
  174         if (Root && depth <= 1)
  175             cnt = 1, nodes++;
  176         else
  177         {
  178             pos.do_move(m, st);
  179             cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
  180             nodes += cnt;
  181             pos.undo_move(m);
  182         }
  183         if (Root)
  184             sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
  185     }
  186     return nodes;
  187   }
  188 
  189 } // namespace
  190 
  191 
  192 /// Search::init() is called at startup to initialize various lookup tables
  193 
  194 void Search::init() {
  195 
  196   for (int i = 1; i < MAX_MOVES; ++i)
  197       Reductions[i] = int((24.8 + std::log(Threads.size()) / 2) * std::log(i));
  198 }
  199 
  200 
  201 /// Search::clear() resets search state to its initial value
  202 
  203 void Search::clear() {
  204 
  205   Threads.main()->wait_for_search_finished();
  206 
  207   Time.availableNodes = 0;
  208   TT.clear();
  209   Threads.clear();
  210   Tablebases::init(Options["SyzygyPath"]); // Free mapped files
  211 }
  212 
  213 
  214 /// MainThread::search() is started when the program receives the UCI 'go'
  215 /// command. It searches from the root position and outputs the "bestmove".
  216 
  217 void MainThread::search() {
  218 
  219   if (Limits.perft)
  220   {
  221       nodes = perft<true>(rootPos, Limits.perft);
  222       sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
  223       return;
  224   }
  225 
  226   Color us = rootPos.side_to_move();
  227   Time.init(Limits, us, rootPos.game_ply());
  228   TT.new_search();
  229 
  230   if (rootMoves.empty())
  231   {
  232       rootMoves.emplace_back(MOVE_NONE);
  233       sync_cout << "info depth 0 score "
  234                 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
  235                 << sync_endl;
  236   }
  237   else
  238   {
  239       for (Thread* th : Threads)
  240       {
  241           th->bestMoveChanges = 0;
  242           if (th != this)
  243               th->start_searching();
  244       }
  245 
  246       Thread::search(); // Let's start searching!
  247   }
  248 
  249   // When we reach the maximum depth, we can arrive here without a raise of
  250   // Threads.stop. However, if we are pondering or in an infinite search,
  251   // the UCI protocol states that we shouldn't print the best move before the
  252   // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
  253   // until the GUI sends one of those commands.
  254 
  255   while (!Threads.stop && (ponder || Limits.infinite))
  256   {} // Busy wait for a stop or a ponder reset
  257 
  258   // Stop the threads if not already stopped (also raise the stop if
  259   // "ponderhit" just reset Threads.ponder).
  260   Threads.stop = true;
  261 
  262   // Wait until all threads have finished
  263   for (Thread* th : Threads)
  264       if (th != this)
  265           th->wait_for_search_finished();
  266 
  267   // When playing in 'nodes as time' mode, subtract the searched nodes from
  268   // the available ones before exiting.
  269   if (Limits.npmsec)
  270       Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
  271 
  272   Thread* bestThread = this;
  273 
  274   // Check if there are threads with a better score than main thread
  275   if (    Options["MultiPV"] == 1
  276       && !Limits.depth
  277       && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
  278       &&  rootMoves[0].pv[0] != MOVE_NONE)
  279   {
  280       std::map<Move, int64_t> votes;
  281       Value minScore = this->rootMoves[0].score;
  282 
  283       // Find out minimum score
  284       for (Thread* th: Threads)
  285           minScore = std::min(minScore, th->rootMoves[0].score);
  286 
  287       // Vote according to score and depth, and select the best thread
  288       for (Thread* th : Threads)
  289       {
  290           votes[th->rootMoves[0].pv[0]] +=
  291               (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
  292 
  293           if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
  294           {
  295               // Make sure we pick the shortest mate
  296               if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
  297                   bestThread = th;
  298           }
  299           else if (   th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
  300                    || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
  301               bestThread = th;
  302       }
  303   }
  304 
  305   previousScore = bestThread->rootMoves[0].score;
  306 
  307   // Send again PV info if we have a new best thread
  308   if (bestThread != this)
  309       sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
  310 
  311   sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
  312 
  313   if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
  314       std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
  315 
  316   std::cout << sync_endl;
  317 }
  318 
  319 
  320 /// Thread::search() is the main iterative deepening loop. It calls search()
  321 /// repeatedly with increasing depth until the allocated thinking time has been
  322 /// consumed, the user stops the search, or the maximum search depth is reached.
  323 
  324 void Thread::search() {
  325 
  326   // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
  327   // The former is needed to allow update_continuation_histories(ss-1, ...),
  328   // which accesses its argument at ss-6, also near the root.
  329   // The latter is needed for statScores and killer initialization.
  330   Stack stack[MAX_PLY+10], *ss = stack+7;
  331   Move  pv[MAX_PLY+1];
  332   Value bestValue, alpha, beta, delta;
  333   Move  lastBestMove = MOVE_NONE;
  334   Depth lastBestMoveDepth = 0;
  335   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
  336   double timeReduction = 1, totBestMoveChanges = 0;
  337   Color us = rootPos.side_to_move();
  338   int iterIdx = 0;
  339 
  340   std::memset(ss-7, 0, 10 * sizeof(Stack));
  341   for (int i = 7; i > 0; i--)
  342       (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
  343 
  344   ss->pv = pv;
  345 
  346   bestValue = delta = alpha = -VALUE_INFINITE;
  347   beta = VALUE_INFINITE;
  348 
  349   if (mainThread)
  350   {
  351       if (mainThread->previousScore == VALUE_INFINITE)
  352           for (int i=0; i<4; ++i)
  353               mainThread->iterValue[i] = VALUE_ZERO;
  354       else
  355           for (int i=0; i<4; ++i)
  356               mainThread->iterValue[i] = mainThread->previousScore;
  357   }
  358 
  359   size_t multiPV = Options["MultiPV"];
  360 
  361   // Pick integer skill levels, but non-deterministically round up or down
  362   // such that the average integer skill corresponds to the input floating point one.
  363   // UCI_Elo is converted to a suitable fractional skill level, using anchoring
  364   // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
  365   // for match (TC 60+0.6) results spanning a wide range of k values.
  366   PRNG rng(now());
  367   double floatLevel = Options["UCI_LimitStrength"] ?
  368                         clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
  369                         double(Options["Skill Level"]);
  370   int intLevel = int(floatLevel) +
  371                  ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024  ? 1 : 0);
  372   Skill skill(intLevel);
  373 
  374   // When playing with strength handicap enable MultiPV search that we will
  375   // use behind the scenes to retrieve a set of possible moves.
  376   if (skill.enabled())
  377       multiPV = std::max(multiPV, (size_t)4);
  378 
  379   multiPV = std::min(multiPV, rootMoves.size());
  380   ttHitAverage = ttHitAverageWindow * ttHitAverageResolution / 2;
  381 
  382   int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
  383 
  384   // In analysis mode, adjust contempt in accordance with user preference
  385   if (Limits.infinite || Options["UCI_AnalyseMode"])
  386       ct =  Options["Analysis Contempt"] == "Off"  ? 0
  387           : Options["Analysis Contempt"] == "Both" ? ct
  388           : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
  389           : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
  390           : ct;
  391 
  392   // Evaluation score is from the white point of view
  393   contempt = (us == WHITE ?  make_score(ct, ct / 2)
  394                           : -make_score(ct, ct / 2));
  395 
  396   int searchAgainCounter = 0;
  397 
  398   // Iterative deepening loop until requested to stop or the target depth is reached
  399   while (   ++rootDepth < MAX_PLY
  400          && !Threads.stop
  401          && !(Limits.depth && mainThread && rootDepth > Limits.depth))
  402   {
  403       // Age out PV variability metric
  404       if (mainThread)
  405           totBestMoveChanges /= 2;
  406 
  407       // Save the last iteration's scores before first PV line is searched and
  408       // all the move scores except the (new) PV are set to -VALUE_INFINITE.
  409       for (RootMove& rm : rootMoves)
  410           rm.previousScore = rm.score;
  411 
  412       size_t pvFirst = 0;
  413       pvLast = 0;
  414 
  415       if (!Threads.increaseDepth)
  416          searchAgainCounter++;
  417 
  418       // MultiPV loop. We perform a full root search for each PV line
  419       for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
  420       {
  421           if (pvIdx == pvLast)
  422           {
  423               pvFirst = pvLast;
  424               for (pvLast++; pvLast < rootMoves.size(); pvLast++)
  425                   if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
  426                       break;
  427           }
  428 
  429           // Reset UCI info selDepth for each depth and each PV line
  430           selDepth = 0;
  431 
  432           // Reset aspiration window starting size
  433           if (rootDepth >= 4)
  434           {
  435               Value previousScore = rootMoves[pvIdx].previousScore;
  436               delta = Value(21 + abs(previousScore) / 256);
  437               alpha = std::max(previousScore - delta,-VALUE_INFINITE);
  438               beta  = std::min(previousScore + delta, VALUE_INFINITE);
  439 
  440               // Adjust contempt based on root move's previousScore (dynamic contempt)
  441               int dct = ct + (102 - ct / 2) * previousScore / (abs(previousScore) + 157);
  442 
  443               contempt = (us == WHITE ?  make_score(dct, dct / 2)
  444                                       : -make_score(dct, dct / 2));
  445           }
  446 
  447           // Start with a small aspiration window and, in the case of a fail
  448           // high/low, re-search with a bigger window until we don't fail
  449           // high/low anymore.
  450           int failedHighCnt = 0;
  451           while (true)
  452           {
  453               Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
  454               bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
  455 
  456               // Bring the best move to the front. It is critical that sorting
  457               // is done with a stable algorithm because all the values but the
  458               // first and eventually the new best one are set to -VALUE_INFINITE
  459               // and we want to keep the same order for all the moves except the
  460               // new PV that goes to the front. Note that in case of MultiPV
  461               // search the already searched PV lines are preserved.
  462               std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
  463 
  464               // If search has been stopped, we break immediately. Sorting is
  465               // safe because RootMoves is still valid, although it refers to
  466               // the previous iteration.
  467               if (Threads.stop)
  468                   break;
  469 
  470               // When failing high/low give some update (without cluttering
  471               // the UI) before a re-search.
  472               if (   mainThread
  473                   && multiPV == 1
  474                   && (bestValue <= alpha || bestValue >= beta)
  475                   && Time.elapsed() > 3000)
  476                   sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
  477 
  478               // In case of failing low/high increase aspiration window and
  479               // re-search, otherwise exit the loop.
  480               if (bestValue <= alpha)
  481               {
  482                   beta = (alpha + beta) / 2;
  483                   alpha = std::max(bestValue - delta, -VALUE_INFINITE);
  484 
  485                   failedHighCnt = 0;
  486                   if (mainThread)
  487                       mainThread->stopOnPonderhit = false;
  488               }
  489               else if (bestValue >= beta)
  490               {
  491                   beta = std::min(bestValue + delta, VALUE_INFINITE);
  492                   ++failedHighCnt;
  493               }
  494               else
  495               {
  496                   ++rootMoves[pvIdx].bestMoveCount;
  497                   break;
  498               }
  499 
  500               delta += delta / 4 + 5;
  501 
  502               assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
  503           }
  504 
  505           // Sort the PV lines searched so far and update the GUI
  506           std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
  507 
  508           if (    mainThread
  509               && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
  510               sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
  511       }
  512 
  513       if (!Threads.stop)
  514           completedDepth = rootDepth;
  515 
  516       if (rootMoves[0].pv[0] != lastBestMove) {
  517          lastBestMove = rootMoves[0].pv[0];
  518          lastBestMoveDepth = rootDepth;
  519       }
  520 
  521       // Have we found a "mate in x"?
  522       if (   Limits.mate
  523           && bestValue >= VALUE_MATE_IN_MAX_PLY
  524           && VALUE_MATE - bestValue <= 2 * Limits.mate)
  525           Threads.stop = true;
  526 
  527       if (!mainThread)
  528           continue;
  529 
  530       // If skill level is enabled and time is up, pick a sub-optimal best move
  531       if (skill.enabled() && skill.time_to_pick(rootDepth))
  532           skill.pick_best(multiPV);
  533 
  534       // Do we have time for the next iteration? Can we stop searching now?
  535       if (    Limits.use_time_management()
  536           && !Threads.stop
  537           && !mainThread->stopOnPonderhit)
  538       {
  539           double fallingEval = (332 +  6 * (mainThread->previousScore - bestValue)
  540                                     +  6 * (mainThread->iterValue[iterIdx]  - bestValue)) / 704.0;
  541           fallingEval = clamp(fallingEval, 0.5, 1.5);
  542 
  543           // If the bestMove is stable over several iterations, reduce time accordingly
  544           timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.94 : 0.91;
  545           double reduction = (1.41 + mainThread->previousTimeReduction) / (2.27 * timeReduction);
  546 
  547           // Use part of the gained time from a previous stable move for the current move
  548           for (Thread* th : Threads)
  549           {
  550               totBestMoveChanges += th->bestMoveChanges;
  551               th->bestMoveChanges = 0;
  552           }
  553           double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
  554 
  555           // Stop the search if we have only one legal move, or if available time elapsed
  556           if (   rootMoves.size() == 1
  557               || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
  558           {
  559               // If we are allowed to ponder do not stop the search now but
  560               // keep pondering until the GUI sends "ponderhit" or "stop".
  561               if (mainThread->ponder)
  562                   mainThread->stopOnPonderhit = true;
  563               else
  564                   Threads.stop = true;
  565           }
  566           else if (   Threads.increaseDepth
  567                    && !mainThread->ponder
  568                    && Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability * 0.6)
  569                    Threads.increaseDepth = false;
  570           else
  571                    Threads.increaseDepth = true;
  572       }
  573 
  574       mainThread->iterValue[iterIdx] = bestValue;
  575       iterIdx = (iterIdx + 1) & 3;
  576   }
  577 
  578   if (!mainThread)
  579       return;
  580 
  581   mainThread->previousTimeReduction = timeReduction;
  582 
  583   // If skill level is enabled, swap best PV line with the sub-optimal one
  584   if (skill.enabled())
  585       std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
  586                 skill.best ? skill.best : skill.pick_best(multiPV)));
  587 }
  588 
  589 
  590 namespace {
  591 
  592   // search<>() is the main search function for both PV and non-PV nodes
  593 
  594   template <NodeType NT>
  595   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
  596 
  597     constexpr bool PvNode = NT == PV;
  598     const bool rootNode = PvNode && ss->ply == 0;
  599 
  600     // Check if we have an upcoming move which draws by repetition, or
  601     // if the opponent had an alternative move earlier to this position.
  602     if (   pos.rule50_count() >= 3
  603         && alpha < VALUE_DRAW
  604         && !rootNode
  605         && pos.has_game_cycle(ss->ply))
  606     {
  607         alpha = value_draw(pos.this_thread());
  608         if (alpha >= beta)
  609             return alpha;
  610     }
  611 
  612     // Dive into quiescence search when the depth reaches zero
  613     if (depth <= 0)
  614         return qsearch<NT>(pos, ss, alpha, beta);
  615 
  616     assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
  617     assert(PvNode || (alpha == beta - 1));
  618     assert(0 < depth && depth < MAX_PLY);
  619     assert(!(PvNode && cutNode));
  620 
  621     Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
  622     StateInfo st;
  623     TTEntry* tte;
  624     Key posKey;
  625     Move ttMove, move, excludedMove, bestMove;
  626     Depth extension, newDepth;
  627     Value bestValue, value, ttValue, eval, maxValue;
  628     bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
  629     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
  630     Piece movedPiece;
  631     int moveCount, captureCount, quietCount;
  632 
  633     // Step 1. Initialize node
  634     Thread* thisThread = pos.this_thread();
  635     inCheck = pos.checkers();
  636     priorCapture = pos.captured_piece();
  637     Color us = pos.side_to_move();
  638     moveCount = captureCount = quietCount = ss->moveCount = 0;
  639     bestValue = -VALUE_INFINITE;
  640     maxValue = VALUE_INFINITE;
  641 
  642     // Check for the available remaining time
  643     if (thisThread == Threads.main())
  644         static_cast<MainThread*>(thisThread)->check_time();
  645 
  646     // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
  647     if (PvNode && thisThread->selDepth < ss->ply + 1)
  648         thisThread->selDepth = ss->ply + 1;
  649 
  650     if (!rootNode)
  651     {
  652         // Step 2. Check for aborted search and immediate draw
  653         if (   Threads.stop.load(std::memory_order_relaxed)
  654             || pos.is_draw(ss->ply)
  655             || ss->ply >= MAX_PLY)
  656             return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
  657                                                     : value_draw(pos.this_thread());
  658 
  659         // Step 3. Mate distance pruning. Even if we mate at the next move our score
  660         // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
  661         // a shorter mate was found upward in the tree then there is no need to search
  662         // because we will never beat the current alpha. Same logic but with reversed
  663         // signs applies also in the opposite condition of being mated instead of giving
  664         // mate. In this case return a fail-high score.
  665         alpha = std::max(mated_in(ss->ply), alpha);
  666         beta = std::min(mate_in(ss->ply+1), beta);
  667         if (alpha >= beta)
  668             return alpha;
  669     }
  670 
  671     assert(0 <= ss->ply && ss->ply < MAX_PLY);
  672 
  673     (ss+1)->ply = ss->ply + 1;
  674     (ss+1)->excludedMove = bestMove = MOVE_NONE;
  675     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
  676     Square prevSq = to_sq((ss-1)->currentMove);
  677 
  678     // Initialize statScore to zero for the grandchildren of the current position.
  679     // So statScore is shared between all grandchildren and only the first grandchild
  680     // starts with statScore = 0. Later grandchildren start with the last calculated
  681     // statScore of the previous grandchild. This influences the reduction rules in
  682     // LMR which are based on the statScore of parent position.
  683     if (rootNode)
  684         (ss+4)->statScore = 0;
  685     else
  686         (ss+2)->statScore = 0;
  687 
  688     // Step 4. Transposition table lookup. We don't want the score of a partial
  689     // search to overwrite a previous full search TT value, so we use a different
  690     // position key in case of an excluded move.
  691     excludedMove = ss->excludedMove;
  692     posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
  693     tte = TT.probe(posKey, ttHit);
  694     ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
  695     ttMove =  rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
  696             : ttHit    ? tte->move() : MOVE_NONE;
  697     ttPv = PvNode || (ttHit && tte->is_pv());
  698     // thisThread->ttHitAverage can be used to approximate the running average of ttHit
  699     thisThread->ttHitAverage =   (ttHitAverageWindow - 1) * thisThread->ttHitAverage / ttHitAverageWindow
  700                                 + ttHitAverageResolution * ttHit;
  701 
  702     // At non-PV nodes we check for an early TT cutoff
  703     if (  !PvNode
  704         && ttHit
  705         && tte->depth() >= depth
  706         && ttValue != VALUE_NONE // Possible in case of TT access race
  707         && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
  708                             : (tte->bound() & BOUND_UPPER)))
  709     {
  710         // If ttMove is quiet, update move sorting heuristics on TT hit
  711         if (ttMove)
  712         {
  713             if (ttValue >= beta)
  714             {
  715                 if (!pos.capture_or_promotion(ttMove))
  716                     update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
  717 
  718                 // Extra penalty for early quiet moves of the previous ply
  719                 if ((ss-1)->moveCount <= 2 && !priorCapture)
  720                     update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
  721             }
  722             // Penalty for a quiet ttMove that fails low
  723             else if (!pos.capture_or_promotion(ttMove))
  724             {
  725                 int penalty = -stat_bonus(depth);
  726                 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
  727                 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
  728             }
  729         }
  730 
  731         if (pos.rule50_count() < 90)
  732             return ttValue;
  733     }
  734 
  735     // Step 5. Tablebases probe
  736     if (!rootNode && TB::Cardinality)
  737     {
  738         int piecesCount = pos.count<ALL_PIECES>();
  739 
  740         if (    piecesCount <= TB::Cardinality
  741             && (piecesCount <  TB::Cardinality || depth >= TB::ProbeDepth)
  742             &&  pos.rule50_count() == 0
  743             && !pos.can_castle(ANY_CASTLING))
  744         {
  745             TB::ProbeState err;
  746             TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
  747 
  748             // Force check of time on the next occasion
  749             if (thisThread == Threads.main())
  750                 static_cast<MainThread*>(thisThread)->callsCnt = 0;
  751 
  752             if (err != TB::ProbeState::FAIL)
  753             {
  754                 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
  755 
  756                 int drawScore = TB::UseRule50 ? 1 : 0;
  757 
  758                 value =  wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
  759                        : wdl >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply - 1
  760                                           :  VALUE_DRAW + 2 * wdl * drawScore;
  761 
  762                 Bound b =  wdl < -drawScore ? BOUND_UPPER
  763                          : wdl >  drawScore ? BOUND_LOWER : BOUND_EXACT;
  764 
  765                 if (    b == BOUND_EXACT
  766                     || (b == BOUND_LOWER ? value >= beta : value <= alpha))
  767                 {
  768                     tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
  769                               std::min(MAX_PLY - 1, depth + 6),
  770                               MOVE_NONE, VALUE_NONE);
  771 
  772                     return value;
  773                 }
  774 
  775                 if (PvNode)
  776                 {
  777                     if (b == BOUND_LOWER)
  778                         bestValue = value, alpha = std::max(alpha, bestValue);
  779                     else
  780                         maxValue = value;
  781                 }
  782             }
  783         }
  784     }
  785 
  786     // Step 6. Static evaluation of the position
  787     if (inCheck)
  788     {
  789         ss->staticEval = eval = VALUE_NONE;
  790         improving = false;
  791         goto moves_loop;  // Skip early pruning when in check
  792     }
  793     else if (ttHit)
  794     {
  795         // Never assume anything about values stored in TT
  796         ss->staticEval = eval = tte->eval();
  797         if (eval == VALUE_NONE)
  798             ss->staticEval = eval = evaluate(pos);
  799 
  800         if (eval == VALUE_DRAW)
  801             eval = value_draw(thisThread);
  802 
  803         // Can ttValue be used as a better position evaluation?
  804         if (    ttValue != VALUE_NONE
  805             && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
  806             eval = ttValue;
  807     }
  808     else
  809     {
  810         if ((ss-1)->currentMove != MOVE_NULL)
  811         {
  812             int bonus = -(ss-1)->statScore / 512;
  813 
  814             ss->staticEval = eval = evaluate(pos) + bonus;
  815         }
  816         else
  817             ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
  818 
  819         tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
  820     }
  821 
  822     // Step 7. Razoring (~1 Elo)
  823     if (   !rootNode // The required rootNode PV handling is not available in qsearch
  824         &&  depth < 2
  825         &&  eval <= alpha - RazorMargin)
  826         return qsearch<NT>(pos, ss, alpha, beta);
  827 
  828     improving =  (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval >= (ss-4)->staticEval
  829               || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval >= (ss-2)->staticEval;
  830 
  831     // Step 8. Futility pruning: child node (~50 Elo)
  832     if (   !PvNode
  833         &&  depth < 6
  834         &&  eval - futility_margin(depth, improving) >= beta
  835         &&  eval < VALUE_KNOWN_WIN) // Do not return unproven wins
  836         return eval;
  837 
  838     // Step 9. Null move search with verification search (~40 Elo)
  839     if (   !PvNode
  840         && (ss-1)->currentMove != MOVE_NULL
  841         && (ss-1)->statScore < 23397
  842         &&  eval >= beta
  843         &&  eval >= ss->staticEval
  844         &&  ss->staticEval >= beta - 32 * depth + 292 - improving * 30
  845         && !excludedMove
  846         &&  pos.non_pawn_material(us)
  847         && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
  848     {
  849         assert(eval - beta >= 0);
  850 
  851         // Null move dynamic reduction based on depth and value
  852         Depth R = (854 + 68 * depth) / 258 + std::min(int(eval - beta) / 192, 3);
  853 
  854         ss->currentMove = MOVE_NULL;
  855         ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
  856 
  857         pos.do_null_move(st);
  858 
  859         Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
  860 
  861         pos.undo_null_move();
  862 
  863         if (nullValue >= beta)
  864         {
  865             // Do not return unproven mate scores
  866             if (nullValue >= VALUE_MATE_IN_MAX_PLY)
  867                 nullValue = beta;
  868 
  869             if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
  870                 return nullValue;
  871 
  872             assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
  873 
  874             // Do verification search at high depths, with null move pruning disabled
  875             // for us, until ply exceeds nmpMinPly.
  876             thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
  877             thisThread->nmpColor = us;
  878 
  879             Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
  880 
  881             thisThread->nmpMinPly = 0;
  882 
  883             if (v >= beta)
  884                 return nullValue;
  885         }
  886     }
  887 
  888     // Step 10. ProbCut (~10 Elo)
  889     // If we have a good enough capture and a reduced search returns a value
  890     // much above beta, we can (almost) safely prune the previous move.
  891     if (   !PvNode
  892         &&  depth >= 5
  893         &&  abs(beta) < VALUE_MATE_IN_MAX_PLY)
  894     {
  895         Value raisedBeta = std::min(beta + 189 - 45 * improving, VALUE_INFINITE);
  896         MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
  897         int probCutCount = 0;
  898 
  899         while (  (move = mp.next_move()) != MOVE_NONE
  900                && probCutCount < 2 + 2 * cutNode)
  901             if (move != excludedMove && pos.legal(move))
  902             {
  903                 assert(pos.capture_or_promotion(move));
  904                 assert(depth >= 5);
  905 
  906                 captureOrPromotion = true;
  907                 probCutCount++;
  908 
  909                 ss->currentMove = move;
  910                 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
  911                                                                           [captureOrPromotion]
  912                                                                           [pos.moved_piece(move)]
  913                                                                           [to_sq(move)];
  914 
  915                 pos.do_move(move, st);
  916 
  917                 // Perform a preliminary qsearch to verify that the move holds
  918                 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
  919 
  920                 // If the qsearch held, perform the regular search
  921                 if (value >= raisedBeta)
  922                     value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
  923 
  924                 pos.undo_move(move);
  925 
  926                 if (value >= raisedBeta)
  927                     return value;
  928             }
  929     }
  930 
  931     // Step 11. Internal iterative deepening (~1 Elo)
  932     if (depth >= 7 && !ttMove)
  933     {
  934         search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
  935 
  936         tte = TT.probe(posKey, ttHit);
  937         ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
  938         ttMove = ttHit ? tte->move() : MOVE_NONE;
  939     }
  940 
  941 moves_loop: // When in check, search starts from here
  942 
  943     const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
  944                                           nullptr                   , (ss-4)->continuationHistory,
  945                                           nullptr                   , (ss-6)->continuationHistory };
  946 
  947     Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
  948 
  949     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
  950                                       &thisThread->captureHistory,
  951                                       contHist,
  952                                       countermove,
  953                                       ss->killers);
  954 
  955     value = bestValue;
  956     singularLMR = moveCountPruning = false;
  957     ttCapture = ttMove && pos.capture_or_promotion(ttMove);
  958 
  959     // Mark this node as being searched
  960     ThreadHolding th(thisThread, posKey, ss->ply);
  961 
  962     // Step 12. Loop through all pseudo-legal moves until no moves remain
  963     // or a beta cutoff occurs.
  964     while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
  965     {
  966       assert(is_ok(move));
  967 
  968       if (move == excludedMove)
  969           continue;
  970 
  971       // At root obey the "searchmoves" option and skip moves not listed in Root
  972       // Move List. As a consequence any illegal move is also skipped. In MultiPV
  973       // mode we also skip PV moves which have been already searched and those
  974       // of lower "TB rank" if we are in a TB root position.
  975       if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
  976                                   thisThread->rootMoves.begin() + thisThread->pvLast, move))
  977           continue;
  978 
  979       ss->moveCount = ++moveCount;
  980 
  981       if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
  982           sync_cout << "info depth " << depth
  983                     << " currmove " << UCI::move(move, pos.is_chess960())
  984                     << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
  985       if (PvNode)
  986           (ss+1)->pv = nullptr;
  987 
  988       extension = 0;
  989       captureOrPromotion = pos.capture_or_promotion(move);
  990       movedPiece = pos.moved_piece(move);
  991       givesCheck = pos.gives_check(move);
  992 
  993       // Calculate new depth for this move
  994       newDepth = depth - 1;
  995 
  996       // Step 13. Pruning at shallow depth (~200 Elo)
  997       if (  !rootNode
  998           && pos.non_pawn_material(us)
  999           && bestValue > VALUE_MATED_IN_MAX_PLY)
 1000       {
 1001           // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
 1002           moveCountPruning = moveCount >= futility_move_count(improving, depth);
 1003 
 1004           if (   !captureOrPromotion
 1005               && !givesCheck)
 1006           {
 1007               // Reduced depth of the next LMR search
 1008               int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
 1009 
 1010               // Countermoves based pruning (~20 Elo)
 1011               if (   lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
 1012                   && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
 1013                   && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
 1014                   continue;
 1015 
 1016               // Futility pruning: parent node (~5 Elo)
 1017               if (   lmrDepth < 6
 1018                   && !inCheck
 1019                   && ss->staticEval + 235 + 172 * lmrDepth <= alpha
 1020                   &&  thisThread->mainHistory[us][from_to(move)]
 1021                     + (*contHist[0])[movedPiece][to_sq(move)]
 1022                     + (*contHist[1])[movedPiece][to_sq(move)]
 1023                     + (*contHist[3])[movedPiece][to_sq(move)] < 25000)
 1024                   continue;
 1025 
 1026               // Prune moves with negative SEE (~20 Elo)
 1027               if (!pos.see_ge(move, Value(-(32 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
 1028                   continue;
 1029           }
 1030           else if (!pos.see_ge(move, Value(-194) * depth)) // (~25 Elo)
 1031                   continue;
 1032       }
 1033 
 1034       // Step 14. Extensions (~75 Elo)
 1035 
 1036       // Singular extension search (~70 Elo). If all moves but one fail low on a
 1037       // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
 1038       // then that move is singular and should be extended. To verify this we do
 1039       // a reduced search on all the other moves but the ttMove and if the
 1040       // result is lower than ttValue minus a margin then we will extend the ttMove.
 1041       if (    depth >= 6
 1042           &&  move == ttMove
 1043           && !rootNode
 1044           && !excludedMove // Avoid recursive singular search
 1045        /* &&  ttValue != VALUE_NONE Already implicit in the next condition */
 1046           &&  abs(ttValue) < VALUE_KNOWN_WIN
 1047           && (tte->bound() & BOUND_LOWER)
 1048           &&  tte->depth() >= depth - 3
 1049           &&  pos.legal(move))
 1050       {
 1051           Value singularBeta = ttValue - 2 * depth;
 1052           Depth halfDepth = depth / 2;
 1053           ss->excludedMove = move;
 1054           value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
 1055           ss->excludedMove = MOVE_NONE;
 1056 
 1057           if (value < singularBeta)
 1058           {
 1059               extension = 1;
 1060               singularLMR = true;
 1061           }
 1062 
 1063           // Multi-cut pruning
 1064           // Our ttMove is assumed to fail high, and now we failed high also on a reduced
 1065           // search without the ttMove. So we assume this expected Cut-node is not singular,
 1066           // that multiple moves fail high, and we can prune the whole subtree by returning
 1067           // a soft bound.
 1068           else if (singularBeta >= beta)
 1069               return singularBeta;
 1070       }
 1071 
 1072       // Check extension (~2 Elo)
 1073       else if (    givesCheck
 1074                && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
 1075           extension = 1;
 1076 
 1077       // Passed pawn extension
 1078       else if (   move == ss->killers[0]
 1079                && pos.advanced_pawn_push(move)
 1080                && pos.pawn_passed(us, to_sq(move)))
 1081           extension = 1;
 1082 
 1083       // Last captures extension
 1084       else if (   PieceValue[EG][pos.captured_piece()] > PawnValueEg
 1085                && pos.non_pawn_material() <= 2 * RookValueMg)
 1086           extension = 1;
 1087 
 1088       // Castling extension
 1089       if (type_of(move) == CASTLING)
 1090           extension = 1;
 1091 
 1092       // Add extension to new depth
 1093       newDepth += extension;
 1094 
 1095       // Speculative prefetch as early as possible
 1096       prefetch(TT.first_entry(pos.key_after(move)));
 1097 
 1098       // Check for legality just before making the move
 1099       if (!rootNode && !pos.legal(move))
 1100       {
 1101           ss->moveCount = --moveCount;
 1102           continue;
 1103       }
 1104 
 1105       // Update the current move (this must be done after singular extension search)
 1106       ss->currentMove = move;
 1107       ss->continuationHistory = &thisThread->continuationHistory[inCheck]
 1108                                                                 [captureOrPromotion]
 1109                                                                 [movedPiece]
 1110                                                                 [to_sq(move)];
 1111 
 1112       // Step 15. Make the move
 1113       pos.do_move(move, st, givesCheck);
 1114 
 1115       // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
 1116       // re-searched at full depth.
 1117       if (    depth >= 3
 1118           &&  moveCount > 1 + rootNode + (rootNode && bestValue < alpha)
 1119           && (!rootNode || thisThread->best_move_count(move) == 0)
 1120           && (  !captureOrPromotion
 1121               || moveCountPruning
 1122               || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
 1123               || cutNode
 1124               || thisThread->ttHitAverage < 375 * ttHitAverageResolution * ttHitAverageWindow / 1024))
 1125       {
 1126           Depth r = reduction(improving, depth, moveCount);
 1127 
 1128           // Decrease reduction if the ttHit running average is large
 1129           if (thisThread->ttHitAverage > 500 * ttHitAverageResolution * ttHitAverageWindow / 1024)
 1130               r--;
 1131 
 1132           // Reduction if other threads are searching this position.
 1133           if (th.marked())
 1134               r++;
 1135 
 1136           // Decrease reduction if position is or has been on the PV (~10 Elo)
 1137           if (ttPv)
 1138               r -= 2;
 1139 
 1140           // Decrease reduction if opponent's move count is high (~5 Elo)
 1141           if ((ss-1)->moveCount > 14)
 1142               r--;
 1143 
 1144           // Decrease reduction if ttMove has been singularly extended (~3 Elo)
 1145           if (singularLMR)
 1146               r -= 2;
 1147 
 1148           if (!captureOrPromotion)
 1149           {
 1150               // Increase reduction if ttMove is a capture (~5 Elo)
 1151               if (ttCapture)
 1152                   r++;
 1153 
 1154               // Increase reduction for cut nodes (~10 Elo)
 1155               if (cutNode)
 1156                   r += 2;
 1157 
 1158               // Decrease reduction for moves that escape a capture. Filter out
 1159               // castling moves, because they are coded as "king captures rook" and
 1160               // hence break make_move(). (~2 Elo)
 1161               else if (    type_of(move) == NORMAL
 1162                        && !pos.see_ge(reverse_move(move)))
 1163                   r -= 2;
 1164 
 1165               ss->statScore =  thisThread->mainHistory[us][from_to(move)]
 1166                              + (*contHist[0])[movedPiece][to_sq(move)]
 1167                              + (*contHist[1])[movedPiece][to_sq(move)]
 1168                              + (*contHist[3])[movedPiece][to_sq(move)]
 1169                              - 4926;
 1170 
 1171               // Reset statScore to zero if negative and most stats shows >= 0
 1172               if (    ss->statScore < 0
 1173                   && (*contHist[0])[movedPiece][to_sq(move)] >= 0
 1174                   && (*contHist[1])[movedPiece][to_sq(move)] >= 0
 1175                   && thisThread->mainHistory[us][from_to(move)] >= 0)
 1176                   ss->statScore = 0;
 1177 
 1178               // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
 1179               if (ss->statScore >= -102 && (ss-1)->statScore < -114)
 1180                   r--;
 1181 
 1182               else if ((ss-1)->statScore >= -116 && ss->statScore < -154)
 1183                   r++;
 1184 
 1185               // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
 1186               r -= ss->statScore / 16384;
 1187           }
 1188 
 1189           // Increase reduction for captures/promotions if late move and at low depth
 1190           else if (depth < 8 && moveCount > 2)
 1191               r++;
 1192 
 1193           Depth d = clamp(newDepth - r, 1, newDepth);
 1194 
 1195           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 1196 
 1197           doFullDepthSearch = (value > alpha && d != newDepth), didLMR = true;
 1198       }
 1199       else
 1200           doFullDepthSearch = !PvNode || moveCount > 1, didLMR = false;
 1201 
 1202       // Step 17. Full depth search when LMR is skipped or fails high
 1203       if (doFullDepthSearch)
 1204       {
 1205           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
 1206 
 1207           if (didLMR && !captureOrPromotion)
 1208           {
 1209               int bonus = value > alpha ?  stat_bonus(newDepth)
 1210                                         : -stat_bonus(newDepth);
 1211 
 1212               if (move == ss->killers[0])
 1213                   bonus += bonus / 4;
 1214 
 1215               update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
 1216           }
 1217       }
 1218 
 1219       // For PV nodes only, do a full PV search on the first move or after a fail
 1220       // high (in the latter case search only if value < beta), otherwise let the
 1221       // parent node fail low with value <= alpha and try another move.
 1222       if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
 1223       {
 1224           (ss+1)->pv = pv;
 1225           (ss+1)->pv[0] = MOVE_NONE;
 1226 
 1227           value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
 1228       }
 1229 
 1230       // Step 18. Undo move
 1231       pos.undo_move(move);
 1232 
 1233       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
 1234 
 1235       // Step 19. Check for a new best move
 1236       // Finished searching the move. If a stop occurred, the return value of
 1237       // the search cannot be trusted, and we return immediately without
 1238       // updating best move, PV and TT.
 1239       if (Threads.stop.load(std::memory_order_relaxed))
 1240           return VALUE_ZERO;
 1241 
 1242       if (rootNode)
 1243       {
 1244           RootMove& rm = *std::find(thisThread->rootMoves.begin(),
 1245                                     thisThread->rootMoves.end(), move);
 1246 
 1247           // PV move or new best move?
 1248           if (moveCount == 1 || value > alpha)
 1249           {
 1250               rm.score = value;
 1251               rm.selDepth = thisThread->selDepth;
 1252               rm.pv.resize(1);
 1253 
 1254               assert((ss+1)->pv);
 1255 
 1256               for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
 1257                   rm.pv.push_back(*m);
 1258 
 1259               // We record how often the best move has been changed in each
 1260               // iteration. This information is used for time management: When
 1261               // the best move changes frequently, we allocate some more time.
 1262               if (moveCount > 1)
 1263                   ++thisThread->bestMoveChanges;
 1264           }
 1265           else
 1266               // All other moves but the PV are set to the lowest value: this
 1267               // is not a problem when sorting because the sort is stable and the
 1268               // move position in the list is preserved - just the PV is pushed up.
 1269               rm.score = -VALUE_INFINITE;
 1270       }
 1271 
 1272       if (value > bestValue)
 1273       {
 1274           bestValue = value;
 1275 
 1276           if (value > alpha)
 1277           {
 1278               bestMove = move;
 1279 
 1280               if (PvNode && !rootNode) // Update pv even in fail-high case
 1281                   update_pv(ss->pv, move, (ss+1)->pv);
 1282 
 1283               if (PvNode && value < beta) // Update alpha! Always alpha < beta
 1284                   alpha = value;
 1285               else
 1286               {
 1287                   assert(value >= beta); // Fail high
 1288                   ss->statScore = 0;
 1289                   break;
 1290               }
 1291           }
 1292       }
 1293 
 1294       if (move != bestMove)
 1295       {
 1296           if (captureOrPromotion && captureCount < 32)
 1297               capturesSearched[captureCount++] = move;
 1298 
 1299           else if (!captureOrPromotion && quietCount < 64)
 1300               quietsSearched[quietCount++] = move;
 1301       }
 1302     }
 1303 
 1304     // The following condition would detect a stop only after move loop has been
 1305     // completed. But in this case bestValue is valid because we have fully
 1306     // searched our subtree, and we can anyhow save the result in TT.
 1307     /*
 1308        if (Threads.stop)
 1309         return VALUE_DRAW;
 1310     */
 1311 
 1312     // Step 20. Check for mate and stalemate
 1313     // All legal moves have been searched and if there are no legal moves, it
 1314     // must be a mate or a stalemate. If we are in a singular extension search then
 1315     // return a fail low score.
 1316 
 1317     assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
 1318 
 1319     if (!moveCount)
 1320         bestValue = excludedMove ? alpha
 1321                    :     inCheck ? mated_in(ss->ply) : VALUE_DRAW;
 1322 
 1323     else if (bestMove)
 1324         update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
 1325                          quietsSearched, quietCount, capturesSearched, captureCount, depth);
 1326 
 1327     // Bonus for prior countermove that caused the fail low
 1328     else if (   (depth >= 3 || PvNode)
 1329              && !priorCapture)
 1330         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
 1331 
 1332     if (PvNode)
 1333         bestValue = std::min(bestValue, maxValue);
 1334 
 1335     if (!excludedMove)
 1336         tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
 1337                   bestValue >= beta ? BOUND_LOWER :
 1338                   PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
 1339                   depth, bestMove, ss->staticEval);
 1340 
 1341     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 1342 
 1343     return bestValue;
 1344   }
 1345 
 1346 
 1347   // qsearch() is the quiescence search function, which is called by the main search
 1348   // function with zero depth, or recursively with further decreasing depth per call.
 1349   template <NodeType NT>
 1350   Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
 1351 
 1352     constexpr bool PvNode = NT == PV;
 1353 
 1354     assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
 1355     assert(PvNode || (alpha == beta - 1));
 1356     assert(depth <= 0);
 1357 
 1358     Move pv[MAX_PLY+1];
 1359     StateInfo st;
 1360     TTEntry* tte;
 1361     Key posKey;
 1362     Move ttMove, move, bestMove;
 1363     Depth ttDepth;
 1364     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
 1365     bool ttHit, pvHit, inCheck, givesCheck, captureOrPromotion, evasionPrunable;
 1366     int moveCount;
 1367 
 1368     if (PvNode)
 1369     {
 1370         oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
 1371         (ss+1)->pv = pv;
 1372         ss->pv[0] = MOVE_NONE;
 1373     }
 1374 
 1375     Thread* thisThread = pos.this_thread();
 1376     (ss+1)->ply = ss->ply + 1;
 1377     bestMove = MOVE_NONE;
 1378     inCheck = pos.checkers();
 1379     moveCount = 0;
 1380 
 1381     // Check for an immediate draw or maximum ply reached
 1382     if (   pos.is_draw(ss->ply)
 1383         || ss->ply >= MAX_PLY)
 1384         return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
 1385 
 1386     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 1387 
 1388     // Decide whether or not to include checks: this fixes also the type of
 1389     // TT entry depth that we are going to use. Note that in qsearch we use
 1390     // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
 1391     ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
 1392                                                   : DEPTH_QS_NO_CHECKS;
 1393     // Transposition table lookup
 1394     posKey = pos.key();
 1395     tte = TT.probe(posKey, ttHit);
 1396     ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
 1397     ttMove = ttHit ? tte->move() : MOVE_NONE;
 1398     pvHit = ttHit && tte->is_pv();
 1399 
 1400     if (  !PvNode
 1401         && ttHit
 1402         && tte->depth() >= ttDepth
 1403         && ttValue != VALUE_NONE // Only in case of TT access race
 1404         && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
 1405                             : (tte->bound() & BOUND_UPPER)))
 1406         return ttValue;
 1407 
 1408     // Evaluate the position statically
 1409     if (inCheck)
 1410     {
 1411         ss->staticEval = VALUE_NONE;
 1412         bestValue = futilityBase = -VALUE_INFINITE;
 1413     }
 1414     else
 1415     {
 1416         if (ttHit)
 1417         {
 1418             // Never assume anything about values stored in TT
 1419             if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
 1420                 ss->staticEval = bestValue = evaluate(pos);
 1421 
 1422             // Can ttValue be used as a better position evaluation?
 1423             if (    ttValue != VALUE_NONE
 1424                 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
 1425                 bestValue = ttValue;
 1426         }
 1427         else
 1428             ss->staticEval = bestValue =
 1429             (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
 1430                                              : -(ss-1)->staticEval + 2 * Eval::Tempo;
 1431 
 1432         // Stand pat. Return immediately if static value is at least beta
 1433         if (bestValue >= beta)
 1434         {
 1435             if (!ttHit)
 1436                 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
 1437                           DEPTH_NONE, MOVE_NONE, ss->staticEval);
 1438 
 1439             return bestValue;
 1440         }
 1441 
 1442         if (PvNode && bestValue > alpha)
 1443             alpha = bestValue;
 1444 
 1445         futilityBase = bestValue + 154;
 1446     }
 1447 
 1448     const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
 1449                                           nullptr                   , (ss-4)->continuationHistory,
 1450                                           nullptr                   , (ss-6)->continuationHistory };
 1451 
 1452     // Initialize a MovePicker object for the current position, and prepare
 1453     // to search the moves. Because the depth is <= 0 here, only captures,
 1454     // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
 1455     // be generated.
 1456     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
 1457                                       &thisThread->captureHistory,
 1458                                       contHist,
 1459                                       to_sq((ss-1)->currentMove));
 1460 
 1461     // Loop through the moves until no moves remain or a beta cutoff occurs
 1462     while ((move = mp.next_move()) != MOVE_NONE)
 1463     {
 1464       assert(is_ok(move));
 1465 
 1466       givesCheck = pos.gives_check(move);
 1467       captureOrPromotion = pos.capture_or_promotion(move);
 1468 
 1469       moveCount++;
 1470 
 1471       // Futility pruning
 1472       if (   !inCheck
 1473           && !givesCheck
 1474           &&  futilityBase > -VALUE_KNOWN_WIN
 1475           && !pos.advanced_pawn_push(move))
 1476       {
 1477           assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
 1478 
 1479           futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
 1480 
 1481           if (futilityValue <= alpha)
 1482           {
 1483               bestValue = std::max(bestValue, futilityValue);
 1484               continue;
 1485           }
 1486 
 1487           if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
 1488           {
 1489               bestValue = std::max(bestValue, futilityBase);
 1490               continue;
 1491           }
 1492       }
 1493 
 1494       // Detect non-capture evasions that are candidates to be pruned
 1495       evasionPrunable =    inCheck
 1496                        &&  (depth != 0 || moveCount > 2)
 1497                        &&  bestValue > VALUE_MATED_IN_MAX_PLY
 1498                        && !pos.capture(move);
 1499 
 1500       // Don't search moves with negative SEE values
 1501       if (  (!inCheck || evasionPrunable) && !pos.see_ge(move))
 1502           continue;
 1503 
 1504       // Speculative prefetch as early as possible
 1505       prefetch(TT.first_entry(pos.key_after(move)));
 1506 
 1507       // Check for legality just before making the move
 1508       if (!pos.legal(move))
 1509       {
 1510           moveCount--;
 1511           continue;
 1512       }
 1513 
 1514       ss->currentMove = move;
 1515       ss->continuationHistory = &thisThread->continuationHistory[inCheck]
 1516                                                                 [captureOrPromotion]
 1517                                                                 [pos.moved_piece(move)]
 1518                                                                 [to_sq(move)];
 1519 
 1520       // Make and search the move
 1521       pos.do_move(move, st, givesCheck);
 1522       value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
 1523       pos.undo_move(move);
 1524 
 1525       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
 1526 
 1527       // Check for a new best move
 1528       if (value > bestValue)
 1529       {
 1530           bestValue = value;
 1531 
 1532           if (value > alpha)
 1533           {
 1534               bestMove = move;
 1535 
 1536               if (PvNode) // Update pv even in fail-high case
 1537                   update_pv(ss->pv, move, (ss+1)->pv);
 1538 
 1539               if (PvNode && value < beta) // Update alpha here!
 1540                   alpha = value;
 1541               else
 1542                   break; // Fail high
 1543           }
 1544        }
 1545     }
 1546 
 1547     // All legal moves have been searched. A special case: If we're in check
 1548     // and no legal moves were found, it is checkmate.
 1549     if (inCheck && bestValue == -VALUE_INFINITE)
 1550         return mated_in(ss->ply); // Plies to mate from the root
 1551 
 1552     tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
 1553               bestValue >= beta ? BOUND_LOWER :
 1554               PvNode && bestValue > oldAlpha  ? BOUND_EXACT : BOUND_UPPER,
 1555               ttDepth, bestMove, ss->staticEval);
 1556 
 1557     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 1558 
 1559     return bestValue;
 1560   }
 1561 
 1562 
 1563   // value_to_tt() adjusts a mate score from "plies to mate from the root" to
 1564   // "plies to mate from the current position". Non-mate scores are unchanged.
 1565   // The function is called before storing a value in the transposition table.
 1566 
 1567   Value value_to_tt(Value v, int ply) {
 1568 
 1569     assert(v != VALUE_NONE);
 1570 
 1571     return  v >= VALUE_MATE_IN_MAX_PLY  ? v + ply
 1572           : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
 1573   }
 1574 
 1575 
 1576   // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
 1577   // from the transposition table (which refers to the plies to mate/be mated
 1578   // from current position) to "plies to mate/be mated from the root".
 1579 
 1580   Value value_from_tt(Value v, int ply, int r50c) {
 1581 
 1582     return  v == VALUE_NONE             ? VALUE_NONE
 1583           : v >= VALUE_MATE_IN_MAX_PLY  ? VALUE_MATE - v > 99 - r50c ? VALUE_MATE_IN_MAX_PLY  : v - ply
 1584           : v <= VALUE_MATED_IN_MAX_PLY ? VALUE_MATE + v > 99 - r50c ? VALUE_MATED_IN_MAX_PLY : v + ply : v;
 1585   }
 1586 
 1587 
 1588   // update_pv() adds current move and appends child pv[]
 1589 
 1590   void update_pv(Move* pv, Move move, Move* childPv) {
 1591 
 1592     for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
 1593         *pv++ = *childPv++;
 1594     *pv = MOVE_NONE;
 1595   }
 1596 
 1597 
 1598   // update_all_stats() updates stats at the end of search() when a bestMove is found
 1599 
 1600   void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
 1601                         Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
 1602 
 1603     int bonus1, bonus2;
 1604     Color us = pos.side_to_move();
 1605     Thread* thisThread = pos.this_thread();
 1606     CapturePieceToHistory& captureHistory = thisThread->captureHistory;
 1607     Piece moved_piece = pos.moved_piece(bestMove);
 1608     PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
 1609 
 1610     bonus1 = stat_bonus(depth + 1);
 1611     bonus2 = bestValue > beta + PawnValueMg ? bonus1               // larger bonus
 1612                                             : stat_bonus(depth);   // smaller bonus
 1613 
 1614     if (!pos.capture_or_promotion(bestMove))
 1615     {
 1616         update_quiet_stats(pos, ss, bestMove, bonus2);
 1617 
 1618         // Decrease all the non-best quiet moves
 1619         for (int i = 0; i < quietCount; ++i)
 1620         {
 1621             thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
 1622             update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
 1623         }
 1624     }
 1625     else
 1626         captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
 1627 
 1628     // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
 1629     if (   ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
 1630         && !pos.captured_piece())
 1631             update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
 1632 
 1633     // Decrease all the non-best capture moves
 1634     for (int i = 0; i < captureCount; ++i)
 1635     {
 1636         moved_piece = pos.moved_piece(capturesSearched[i]);
 1637         captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
 1638         captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
 1639     }
 1640   }
 1641 
 1642 
 1643   // update_continuation_histories() updates histories of the move pairs formed
 1644   // by moves at ply -1, -2, and -4 with current move.
 1645 
 1646   void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
 1647 
 1648     for (int i : {1, 2, 4, 6})
 1649         if (is_ok((ss-i)->currentMove))
 1650             (*(ss-i)->continuationHistory)[pc][to] << bonus;
 1651   }
 1652 
 1653 
 1654   // update_quiet_stats() updates move sorting heuristics
 1655 
 1656   void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
 1657 
 1658     if (ss->killers[0] != move)
 1659     {
 1660         ss->killers[1] = ss->killers[0];
 1661         ss->killers[0] = move;
 1662     }
 1663 
 1664     Color us = pos.side_to_move();
 1665     Thread* thisThread = pos.this_thread();
 1666     thisThread->mainHistory[us][from_to(move)] << bonus;
 1667     update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
 1668 
 1669     if (type_of(pos.moved_piece(move)) != PAWN)
 1670         thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
 1671 
 1672     if (is_ok((ss-1)->currentMove))
 1673     {
 1674         Square prevSq = to_sq((ss-1)->currentMove);
 1675         thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
 1676     }
 1677   }
 1678 
 1679   // When playing with strength handicap, choose best move among a set of RootMoves
 1680   // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
 1681 
 1682   Move Skill::pick_best(size_t multiPV) {
 1683 
 1684     const RootMoves& rootMoves = Threads.main()->rootMoves;
 1685     static PRNG rng(now()); // PRNG sequence should be non-deterministic
 1686 
 1687     // RootMoves are already sorted by score in descending order
 1688     Value topScore = rootMoves[0].score;
 1689     int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
 1690     int weakness = 120 - 2 * level;
 1691     int maxScore = -VALUE_INFINITE;
 1692 
 1693     // Choose best move. For each move score we add two terms, both dependent on
 1694     // weakness. One is deterministic and bigger for weaker levels, and one is
 1695     // random. Then we choose the move with the resulting highest score.
 1696     for (size_t i = 0; i < multiPV; ++i)
 1697     {
 1698         // This is our magic formula
 1699         int push = (  weakness * int(topScore - rootMoves[i].score)
 1700                     + delta * (rng.rand<unsigned>() % weakness)) / 128;
 1701 
 1702         if (rootMoves[i].score + push >= maxScore)
 1703         {
 1704             maxScore = rootMoves[i].score + push;
 1705             best = rootMoves[i].pv[0];
 1706         }
 1707     }
 1708 
 1709     return best;
 1710   }
 1711 
 1712 } // namespace
 1713 
 1714 /// MainThread::check_time() is used to print debug info and, more importantly,
 1715 /// to detect when we are out of available time and thus stop the search.
 1716 
 1717 void MainThread::check_time() {
 1718 
 1719   if (--callsCnt > 0)
 1720       return;
 1721 
 1722   // When using nodes, ensure checking rate is not lower than 0.1% of nodes
 1723   callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
 1724 
 1725   static TimePoint lastInfoTime = now();
 1726 
 1727   TimePoint elapsed = Time.elapsed();
 1728   TimePoint tick = Limits.startTime + elapsed;
 1729 
 1730   if (tick - lastInfoTime >= 1000)
 1731   {
 1732       lastInfoTime = tick;
 1733       dbg_print();
 1734   }
 1735 
 1736   // We should not stop pondering until told so by the GUI
 1737   if (ponder)
 1738       return;
 1739 
 1740   if (   (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
 1741       || (Limits.movetime && elapsed >= Limits.movetime)
 1742       || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
 1743       Threads.stop = true;
 1744 }
 1745 
 1746 
 1747 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
 1748 /// that all (if any) unsearched PV lines are sent using a previous search score.
 1749 
 1750 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
 1751 
 1752   std::stringstream ss;
 1753   TimePoint elapsed = Time.elapsed() + 1;
 1754   const RootMoves& rootMoves = pos.this_thread()->rootMoves;
 1755   size_t pvIdx = pos.this_thread()->pvIdx;
 1756   size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
 1757   uint64_t nodesSearched = Threads.nodes_searched();
 1758   uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
 1759 
 1760   for (size_t i = 0; i < multiPV; ++i)
 1761   {
 1762       bool updated = rootMoves[i].score != -VALUE_INFINITE;
 1763 
 1764       if (depth == 1 && !updated)
 1765           continue;
 1766 
 1767       Depth d = updated ? depth : depth - 1;
 1768       Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
 1769 
 1770       bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
 1771       v = tb ? rootMoves[i].tbScore : v;
 1772 
 1773       if (ss.rdbuf()->in_avail()) // Not at first line
 1774           ss << "\n";
 1775 
 1776       ss << "info"
 1777          << " depth "    << d
 1778          << " seldepth " << rootMoves[i].selDepth
 1779          << " multipv "  << i + 1
 1780          << " score "    << UCI::value(v);
 1781 
 1782       if (!tb && i == pvIdx)
 1783           ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
 1784 
 1785       ss << " nodes "    << nodesSearched
 1786          << " nps "      << nodesSearched * 1000 / elapsed;
 1787 
 1788       if (elapsed > 1000) // Earlier makes little sense
 1789           ss << " hashfull " << TT.hashfull();
 1790 
 1791       ss << " tbhits "   << tbHits
 1792          << " time "     << elapsed
 1793          << " pv";
 1794 
 1795       for (Move m : rootMoves[i].pv)
 1796           ss << " " << UCI::move(m, pos.is_chess960());
 1797   }
 1798 
 1799   return ss.str();
 1800 }
 1801 
 1802 
 1803 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
 1804 /// before exiting the search, for instance, in case we stop the search during a
 1805 /// fail high at root. We try hard to have a ponder move to return to the GUI,
 1806 /// otherwise in case of 'ponder on' we have nothing to think on.
 1807 
 1808 bool RootMove::extract_ponder_from_tt(Position& pos) {
 1809 
 1810     StateInfo st;
 1811     bool ttHit;
 1812 
 1813     assert(pv.size() == 1);
 1814 
 1815     if (pv[0] == MOVE_NONE)
 1816         return false;
 1817 
 1818     pos.do_move(pv[0], st);
 1819     TTEntry* tte = TT.probe(pos.key(), ttHit);
 1820 
 1821     if (ttHit)
 1822     {
 1823         Move m = tte->move(); // Local copy to be SMP safe
 1824         if (MoveList<LEGAL>(pos).contains(m))
 1825             pv.push_back(m);
 1826     }
 1827 
 1828     pos.undo_move(pv[0]);
 1829     return pv.size() > 1;
 1830 }
 1831 
 1832 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
 1833 
 1834     RootInTB = false;
 1835     UseRule50 = bool(Options["Syzygy50MoveRule"]);
 1836     ProbeDepth = int(Options["SyzygyProbeDepth"]);
 1837     Cardinality = int(Options["SyzygyProbeLimit"]);
 1838     bool dtz_available = true;
 1839 
 1840     // Tables with fewer pieces than SyzygyProbeLimit are searched with
 1841     // ProbeDepth == DEPTH_ZERO
 1842     if (Cardinality > MaxCardinality)
 1843     {
 1844         Cardinality = MaxCardinality;
 1845         ProbeDepth = 0;
 1846     }
 1847 
 1848     if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
 1849     {
 1850         // Rank moves using DTZ tables
 1851         RootInTB = root_probe(pos, rootMoves);
 1852 
 1853         if (!RootInTB)
 1854         {
 1855             // DTZ tables are missing; try to rank moves using WDL tables
 1856             dtz_available = false;
 1857             RootInTB = root_probe_wdl(pos, rootMoves);
 1858         }
 1859     }
 1860 
 1861     if (RootInTB)
 1862     {
 1863         // Sort moves according to TB rank
 1864         std::sort(rootMoves.begin(), rootMoves.end(),
 1865                   [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
 1866 
 1867         // Probe during search only if DTZ is not available and we are winning
 1868         if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
 1869             Cardinality = 0;
 1870     }
 1871     else
 1872     {
 1873         // Clean up if root_probe() and root_probe_wdl() have failed
 1874         for (auto& m : rootMoves)
 1875             m.tbRank = 0;
 1876     }
 1877 }