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) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
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.
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.
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/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
46 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as a template parameter
64 enum NodeType { NonPV, PV };
66 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
67 const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
68 const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
70 // Razoring and futility margin based on depth
71 // razor_margin[0] is unused as long as depth >= ONE_PLY in search
72 const int razor_margin[] = { 0, 570, 603, 554 };
73 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
75 // Futility and reductions lookup tables, initialized at startup
76 int FutilityMoveCounts[2][16]; // [improving][depth]
77 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
79 // Threshold used for countermoves based pruning
80 const int CounterMovePruneThreshold = 0;
82 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
83 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
86 // History and stats update bonus, based on depth
87 int stat_bonus(Depth depth) {
88 int d = depth / ONE_PLY ;
89 return d > 17 ? 0 : d * d + 2 * d - 2;
92 // Skill structure is used to implement strength limit
94 Skill(int l) : level(l) {}
95 bool enabled() const { return level < 20; }
96 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
97 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
98 Move pick_best(size_t multiPV);
101 Move best = MOVE_NONE;
104 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is stable
105 // across multiple search iterations, we can quickly return the best move.
106 struct EasyMoveManager {
111 pv[0] = pv[1] = pv[2] = MOVE_NONE;
114 Move get(Key key) const {
115 return expectedPosKey == key ? pv[2] : MOVE_NONE;
118 void update(Position& pos, const std::vector<Move>& newPv) {
120 assert(newPv.size() >= 3);
122 // Keep track of how many times in a row the 3rd ply remains stable
123 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
125 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
127 std::copy(newPv.begin(), newPv.begin() + 3, pv);
130 pos.do_move(newPv[0], st[0]);
131 pos.do_move(newPv[1], st[1]);
132 expectedPosKey = pos.key();
133 pos.undo_move(newPv[1]);
134 pos.undo_move(newPv[0]);
143 EasyMoveManager EasyMove;
144 Value DrawValue[COLOR_NB];
146 template <NodeType NT>
147 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
149 template <NodeType NT, bool InCheck>
150 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
152 Value value_to_tt(Value v, int ply);
153 Value value_from_tt(Value v, int ply);
154 void update_pv(Move* pv, Move move, Move* childPv);
155 void update_cm_stats(Stack* ss, Piece pc, Square s, int bonus);
156 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
162 /// Search::init() is called during startup to initialize various lookup tables
164 void Search::init() {
166 for (int imp = 0; imp <= 1; ++imp)
167 for (int d = 1; d < 64; ++d)
168 for (int mc = 1; mc < 64; ++mc)
170 double r = log(d) * log(mc) / 1.95;
172 Reductions[NonPV][imp][d][mc] = int(std::round(r));
173 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
175 // Increase reduction for non-PV nodes when eval is not improving
176 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
177 Reductions[NonPV][imp][d][mc]++;
180 for (int d = 0; d < 16; ++d)
182 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
183 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
188 /// Search::clear() resets search state to its initial value, to obtain reproducible results
190 void Search::clear() {
194 for (Thread* th : Threads)
196 th->resetCalls = true;
197 th->counterMoves.fill(MOVE_NONE);
200 for (auto& to : th->counterMoveHistory)
204 th->counterMoveHistory[NO_PIECE][0].fill(CounterMovePruneThreshold - 1);
207 Threads.main()->previousScore = VALUE_INFINITE;
211 /// Search::perft() is our utility to verify move generation. All the leaf nodes
212 /// up to the given depth are generated and counted, and the sum is returned.
214 uint64_t Search::perft(Position& pos, Depth depth) {
217 uint64_t cnt, nodes = 0;
218 const bool leaf = (depth == 2 * ONE_PLY);
220 for (const auto& m : MoveList<LEGAL>(pos))
222 if (Root && depth <= ONE_PLY)
227 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
232 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
237 template uint64_t Search::perft<true>(Position&, Depth);
240 /// MainThread::search() is called by the main thread when the program receives
241 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
243 void MainThread::search() {
245 Color us = rootPos.side_to_move();
246 Time.init(Limits, us, rootPos.game_ply());
249 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
250 DrawValue[ us] = VALUE_DRAW - Value(contempt);
251 DrawValue[~us] = VALUE_DRAW + Value(contempt);
253 if (rootMoves.empty())
255 rootMoves.push_back(RootMove(MOVE_NONE));
256 sync_cout << "info depth 0 score "
257 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
262 for (Thread* th : Threads)
264 th->start_searching();
266 Thread::search(); // Let's start searching!
269 // When playing in 'nodes as time' mode, subtract the searched nodes from
270 // the available ones before exiting.
272 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
274 // When we reach the maximum depth, we can arrive here without a raise of
275 // Signals.stop. However, if we are pondering or in an infinite search,
276 // the UCI protocol states that we shouldn't print the best move before the
277 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
278 // until the GUI sends one of those commands (which also raises Signals.stop).
279 if (!Signals.stop && (Limits.ponder || Limits.infinite))
281 Signals.stopOnPonderhit = true;
285 // Stop the threads if not already stopped
288 // Wait until all threads have finished
289 for (Thread* th : Threads)
291 th->wait_for_search_finished();
293 // Check if there are threads with a better score than main thread
294 Thread* bestThread = this;
295 if ( !this->easyMovePlayed
296 && Options["MultiPV"] == 1
298 && !Skill(Options["Skill Level"]).enabled()
299 && rootMoves[0].pv[0] != MOVE_NONE)
301 for (Thread* th : Threads)
303 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
304 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
306 if (scoreDiff > 0 && depthDiff >= 0)
311 previousScore = bestThread->rootMoves[0].score;
313 // Send new PV when needed
314 if (bestThread != this)
315 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
317 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
319 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
320 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
322 std::cout << sync_endl;
326 /// Thread::search() is the main iterative deepening loop. It calls search()
327 /// repeatedly with increasing depth until the allocated thinking time has been
328 /// consumed, the user stops the search, or the maximum search depth is reached.
330 void Thread::search() {
332 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
333 Value bestValue, alpha, beta, delta;
334 Move easyMove = MOVE_NONE;
335 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
337 std::memset(ss-4, 0, 7 * sizeof(Stack));
338 for (int i = 4; i > 0; i--)
339 (ss-i)->history = &this->counterMoveHistory[NO_PIECE][0]; // Use as sentinel
341 bestValue = delta = alpha = -VALUE_INFINITE;
342 beta = VALUE_INFINITE;
343 completedDepth = DEPTH_ZERO;
347 easyMove = EasyMove.get(rootPos.key());
349 mainThread->easyMovePlayed = mainThread->failedLow = false;
350 mainThread->bestMoveChanges = 0;
353 size_t multiPV = Options["MultiPV"];
354 Skill skill(Options["Skill Level"]);
356 // When playing with strength handicap enable MultiPV search that we will
357 // use behind the scenes to retrieve a set of possible moves.
359 multiPV = std::max(multiPV, (size_t)4);
361 multiPV = std::min(multiPV, rootMoves.size());
363 // Iterative deepening loop until requested to stop or the target depth is reached
364 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
366 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
368 // Distribute search depths across the threads
371 int i = (idx - 1) % 20;
372 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
376 // Age out PV variability metric
378 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
380 // Save the last iteration's scores before first PV line is searched and
381 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
382 for (RootMove& rm : rootMoves)
383 rm.previousScore = rm.score;
385 // MultiPV loop. We perform a full root search for each PV line
386 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
388 // Reset aspiration window starting size
389 if (rootDepth >= 5 * ONE_PLY)
392 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
393 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
396 // Start with a small aspiration window and, in the case of a fail
397 // high/low, re-search with a bigger window until we're not failing
401 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
403 // Bring the best move to the front. It is critical that sorting
404 // is done with a stable algorithm because all the values but the
405 // first and eventually the new best one are set to -VALUE_INFINITE
406 // and we want to keep the same order for all the moves except the
407 // new PV that goes to the front. Note that in case of MultiPV
408 // search the already searched PV lines are preserved.
409 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
411 // If search has been stopped, we break immediately. Sorting and
412 // writing PV back to TT is safe because RootMoves is still
413 // valid, although it refers to the previous iteration.
417 // When failing high/low give some update (without cluttering
418 // the UI) before a re-search.
421 && (bestValue <= alpha || bestValue >= beta)
422 && Time.elapsed() > 3000)
423 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
425 // In case of failing low/high increase aspiration window and
426 // re-search, otherwise exit the loop.
427 if (bestValue <= alpha)
429 beta = (alpha + beta) / 2;
430 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
434 mainThread->failedLow = true;
435 Signals.stopOnPonderhit = false;
438 else if (bestValue >= beta)
440 alpha = (alpha + beta) / 2;
441 beta = std::min(bestValue + delta, VALUE_INFINITE);
446 delta += delta / 4 + 5;
448 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
451 // Sort the PV lines searched so far and update the GUI
452 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
457 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
458 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
462 completedDepth = rootDepth;
467 // If skill level is enabled and time is up, pick a sub-optimal best move
468 if (skill.enabled() && skill.time_to_pick(rootDepth))
469 skill.pick_best(multiPV);
471 // Have we found a "mate in x"?
473 && bestValue >= VALUE_MATE_IN_MAX_PLY
474 && VALUE_MATE - bestValue <= 2 * Limits.mate)
477 // Do we have time for the next iteration? Can we stop searching now?
478 if (Limits.use_time_management())
480 if (!Signals.stop && !Signals.stopOnPonderhit)
482 // Stop the search if only one legal move is available, or if all
483 // of the available time has been used, or if we matched an easyMove
484 // from the previous search and just did a fast verification.
485 const int F[] = { mainThread->failedLow,
486 bestValue - mainThread->previousScore };
488 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
489 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
491 bool doEasyMove = rootMoves[0].pv[0] == easyMove
492 && mainThread->bestMoveChanges < 0.03
493 && Time.elapsed() > Time.optimum() * 5 / 44;
495 if ( rootMoves.size() == 1
496 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
497 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
499 // If we are allowed to ponder do not stop the search now but
500 // keep pondering until the GUI sends "ponderhit" or "stop".
502 Signals.stopOnPonderhit = true;
508 if (rootMoves[0].pv.size() >= 3)
509 EasyMove.update(rootPos, rootMoves[0].pv);
518 // Clear any candidate easy move that wasn't stable for the last search
519 // iterations; the second condition prevents consecutive fast moves.
520 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
523 // If skill level is enabled, swap best PV line with the sub-optimal one
525 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
526 rootMoves.end(), skill.best_move(multiPV)));
532 // search<>() is the main search function for both PV and non-PV nodes
534 template <NodeType NT>
535 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
537 const bool PvNode = NT == PV;
538 const bool rootNode = PvNode && (ss-1)->ply == 0;
540 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
541 assert(PvNode || (alpha == beta - 1));
542 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
543 assert(!(PvNode && cutNode));
544 assert(depth / ONE_PLY * ONE_PLY == depth);
546 Move pv[MAX_PLY+1], quietsSearched[64];
550 Move ttMove, move, excludedMove, bestMove;
551 Depth extension, newDepth;
552 Value bestValue, value, ttValue, eval;
553 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
554 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets;
556 int moveCount, quietCount;
558 // Step 1. Initialize node
559 Thread* thisThread = pos.this_thread();
560 inCheck = pos.checkers();
561 moveCount = quietCount = ss->moveCount = 0;
563 bestValue = -VALUE_INFINITE;
564 ss->ply = (ss-1)->ply + 1;
566 // Check for the available remaining time
567 if (thisThread->resetCalls.load(std::memory_order_relaxed))
569 thisThread->resetCalls = false;
571 // At low node count increase the checking rate to about 0.1% of nodes
572 // otherwise use a default value.
573 thisThread->callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024))
577 if (--thisThread->callsCnt <= 0)
579 for (Thread* th : Threads)
580 th->resetCalls = true;
585 // Used to send selDepth info to GUI
586 if (PvNode && thisThread->maxPly < ss->ply)
587 thisThread->maxPly = ss->ply;
591 // Step 2. Check for aborted search and immediate draw
592 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
593 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
594 : DrawValue[pos.side_to_move()];
596 // Step 3. Mate distance pruning. Even if we mate at the next move our score
597 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
598 // a shorter mate was found upward in the tree then there is no need to search
599 // because we will never beat the current alpha. Same logic but with reversed
600 // signs applies also in the opposite condition of being mated instead of giving
601 // mate. In this case return a fail-high score.
602 alpha = std::max(mated_in(ss->ply), alpha);
603 beta = std::min(mate_in(ss->ply+1), beta);
608 assert(0 <= ss->ply && ss->ply < MAX_PLY);
610 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
611 ss->history = &thisThread->counterMoveHistory[NO_PIECE][0];
612 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
613 Square prevSq = to_sq((ss-1)->currentMove);
615 // Step 4. Transposition table lookup. We don't want the score of a partial
616 // search to overwrite a previous full search TT value, so we use a different
617 // position key in case of an excluded move.
618 excludedMove = ss->excludedMove;
619 posKey = pos.key() ^ Key(excludedMove);
620 tte = TT.probe(posKey, ttHit);
621 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
622 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
623 : ttHit ? tte->move() : MOVE_NONE;
625 // At non-PV nodes we check for an early TT cutoff
628 && tte->depth() >= depth
629 && ttValue != VALUE_NONE // Possible in case of TT access race
630 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
631 : (tte->bound() & BOUND_UPPER)))
633 // If ttMove is quiet, update move sorting heuristics on TT hit
638 if (!pos.capture_or_promotion(ttMove))
639 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
641 // Extra penalty for a quiet TT move in previous ply when it gets refuted
642 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
643 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
645 // Penalty for a quiet ttMove that fails low
646 else if (!pos.capture_or_promotion(ttMove))
648 int penalty = -stat_bonus(depth);
649 thisThread->history.update(pos.side_to_move(), ttMove, penalty);
650 update_cm_stats(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
656 // Step 4a. Tablebase probe
657 if (!rootNode && TB::Cardinality)
659 int piecesCount = pos.count<ALL_PIECES>();
661 if ( piecesCount <= TB::Cardinality
662 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
663 && pos.rule50_count() == 0
664 && !pos.can_castle(ANY_CASTLING))
667 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
669 if (err != TB::ProbeState::FAIL)
671 thisThread->tbHits++;
673 int drawScore = TB::UseRule50 ? 1 : 0;
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;
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());
688 // Step 5. Evaluate the position statically
691 ss->staticEval = eval = VALUE_NONE;
697 // Never assume anything on values stored in TT
698 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
699 eval = ss->staticEval = evaluate(pos);
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))
708 eval = ss->staticEval =
709 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
710 : -(ss-1)->staticEval + 2 * Eval::Tempo;
712 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
713 ss->staticEval, TT.generation());
716 if (skipEarlyPruning)
719 // Step 6. Razoring (skipped when in check)
721 && depth < 4 * ONE_PLY
722 && eval + razor_margin[depth / ONE_PLY] <= alpha)
724 if (depth <= ONE_PLY)
725 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
727 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
728 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
733 // Step 7. Futility pruning: child node (skipped when in check)
735 && depth < 7 * ONE_PLY
736 && eval - futility_margin(depth) >= beta
737 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
738 && pos.non_pawn_material(pos.side_to_move()))
741 // Step 8. Null move search with verification search (is omitted in PV nodes)
744 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
745 && pos.non_pawn_material(pos.side_to_move()))
748 assert(eval - beta >= 0);
750 // Null move dynamic reduction based on depth and value
751 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
753 ss->currentMove = MOVE_NULL;
754 ss->history = &thisThread->counterMoveHistory[NO_PIECE][0];
756 pos.do_null_move(st);
757 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
758 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
759 pos.undo_null_move();
761 if (nullValue >= beta)
763 // Do not return unproven mate scores
764 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
767 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
770 // Do verification search at high depths
771 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
772 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
779 // Step 9. ProbCut (skipped when in check)
780 // If we have a good enough capture and a reduced search returns a value
781 // much above beta, we can (almost) safely prune the previous move.
783 && depth >= 5 * ONE_PLY
784 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
786 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
788 assert(is_ok((ss-1)->currentMove));
790 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
792 while ((move = mp.next_move()) != MOVE_NONE)
795 ss->currentMove = move;
796 ss->history = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
798 assert(depth >= 5 * ONE_PLY);
799 pos.do_move(move, st);
800 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
807 // Step 10. Internal iterative deepening (skipped when in check)
808 if ( depth >= 6 * ONE_PLY
810 && (PvNode || ss->staticEval + 256 >= beta))
812 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
813 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
815 tte = TT.probe(posKey, ttHit);
816 ttMove = ttHit ? tte->move() : MOVE_NONE;
819 moves_loop: // When in check search starts from here
821 const PieceToHistory& cmh = *(ss-1)->history;
822 const PieceToHistory& fmh = *(ss-2)->history;
823 const PieceToHistory& fm2 = *(ss-4)->history;
825 MovePicker mp(pos, ttMove, depth, ss);
826 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
827 improving = ss->staticEval >= (ss-2)->staticEval
828 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
829 ||(ss-2)->staticEval == VALUE_NONE;
831 singularExtensionNode = !rootNode
832 && depth >= 8 * ONE_PLY
833 && ttMove != MOVE_NONE
834 && ttValue != VALUE_NONE
835 && !excludedMove // Recursive singular search is not allowed
836 && (tte->bound() & BOUND_LOWER)
837 && tte->depth() >= depth - 3 * ONE_PLY;
840 // Step 11. Loop through moves
841 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
842 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
846 if (move == excludedMove)
849 // At root obey the "searchmoves" option and skip moves not listed in Root
850 // Move List. As a consequence any illegal move is also skipped. In MultiPV
851 // mode we also skip PV moves which have been already searched.
852 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
853 thisThread->rootMoves.end(), move))
856 ss->moveCount = ++moveCount;
858 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
859 sync_cout << "info depth " << depth / ONE_PLY
860 << " currmove " << UCI::move(move, pos.is_chess960())
861 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
864 (ss+1)->pv = nullptr;
866 extension = DEPTH_ZERO;
867 captureOrPromotion = pos.capture_or_promotion(move);
868 moved_piece = pos.moved_piece(move);
870 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
871 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
872 : pos.gives_check(move);
874 moveCountPruning = depth < 16 * ONE_PLY
875 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
877 // Step 12. Singular and Gives Check Extensions
879 // Singular extension search. If all moves but one fail low on a search of
880 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
881 // is singular and should be extended. To verify this we do a reduced search
882 // on all the other moves but the ttMove and if the result is lower than
883 // ttValue minus a margin then we will extend the ttMove.
884 if ( singularExtensionNode
888 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
889 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
890 ss->excludedMove = move;
891 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
892 ss->excludedMove = MOVE_NONE;
902 // Calculate new depth for this move
903 newDepth = depth - ONE_PLY + extension;
905 // Step 13. Pruning at shallow depth
907 && pos.non_pawn_material(pos.side_to_move())
908 && bestValue > VALUE_MATED_IN_MAX_PLY)
910 if ( !captureOrPromotion
912 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
914 // Move count based pruning
915 if (moveCountPruning)
921 // Reduced depth of the next LMR search
922 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
924 // Countermoves based pruning
926 && (cmh[moved_piece][to_sq(move)] < CounterMovePruneThreshold)
927 && (fmh[moved_piece][to_sq(move)] < CounterMovePruneThreshold))
930 // Futility pruning: parent node
933 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
936 // Prune moves with negative SEE
938 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
941 else if ( depth < 7 * ONE_PLY
943 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
947 // Speculative prefetch as early as possible
948 prefetch(TT.first_entry(pos.key_after(move)));
950 // Check for legality just before making the move
951 if (!rootNode && !pos.legal(move))
953 ss->moveCount = --moveCount;
957 // Update the current move (this must be done after singular extension search)
958 ss->currentMove = move;
959 ss->history = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
961 // Step 14. Make the move
962 pos.do_move(move, st, givesCheck);
964 // Step 15. Reduced depth search (LMR). If the move fails high it will be
965 // re-searched at full depth.
966 if ( depth >= 3 * ONE_PLY
968 && (!captureOrPromotion || moveCountPruning))
970 Depth r = reduction<PvNode>(improving, depth, moveCount);
972 if (captureOrPromotion)
973 r -= r ? ONE_PLY : DEPTH_ZERO;
976 // Increase reduction for cut nodes
980 // Decrease reduction for moves that escape a capture. Filter out
981 // castling moves, because they are coded as "king captures rook" and
982 // hence break make_move().
983 else if ( type_of(move) == NORMAL
984 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
987 ss->statScore = cmh[moved_piece][to_sq(move)]
988 + fmh[moved_piece][to_sq(move)]
989 + fm2[moved_piece][to_sq(move)]
990 + thisThread->history[~pos.side_to_move()][from_to(move)]
991 - 4000; // Correction factor
993 // Decrease/increase reduction by comparing opponent's stat score
994 if (ss->statScore > 0 && (ss-1)->statScore < 0)
997 else if (ss->statScore < 0 && (ss-1)->statScore > 0)
1000 // Decrease/increase reduction for moves with a good/bad history
1001 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1004 Depth d = std::max(newDepth - r, ONE_PLY);
1006 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1008 doFullDepthSearch = (value > alpha && d != newDepth);
1011 doFullDepthSearch = !PvNode || moveCount > 1;
1013 // Step 16. Full depth search when LMR is skipped or fails high
1014 if (doFullDepthSearch)
1015 value = newDepth < ONE_PLY ?
1016 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1017 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1018 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1020 // For PV nodes only, do a full PV search on the first move or after a fail
1021 // high (in the latter case search only if value < beta), otherwise let the
1022 // parent node fail low with value <= alpha and try another move.
1023 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1026 (ss+1)->pv[0] = MOVE_NONE;
1028 value = newDepth < ONE_PLY ?
1029 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1030 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1031 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1034 // Step 17. Undo move
1035 pos.undo_move(move);
1037 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1039 // Step 18. Check for a new best move
1040 // Finished searching the move. If a stop occurred, the return value of
1041 // the search cannot be trusted, and we return immediately without
1042 // updating best move, PV and TT.
1043 if (Signals.stop.load(std::memory_order_relaxed))
1048 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1049 thisThread->rootMoves.end(), move);
1051 // PV move or new best move ?
1052 if (moveCount == 1 || value > alpha)
1059 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1060 rm.pv.push_back(*m);
1062 // We record how often the best move has been changed in each
1063 // iteration. This information is used for time management: When
1064 // the best move changes frequently, we allocate some more time.
1065 if (moveCount > 1 && thisThread == Threads.main())
1066 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1069 // All other moves but the PV are set to the lowest value: this is
1070 // not a problem when sorting because the sort is stable and the
1071 // move position in the list is preserved - just the PV is pushed up.
1072 rm.score = -VALUE_INFINITE;
1075 if (value > bestValue)
1083 if (PvNode && !rootNode) // Update pv even in fail-high case
1084 update_pv(ss->pv, move, (ss+1)->pv);
1086 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1090 assert(value >= beta); // Fail high
1096 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1097 quietsSearched[quietCount++] = move;
1100 // The following condition would detect a stop only after move loop has been
1101 // completed. But in this case bestValue is valid because we have fully
1102 // searched our subtree, and we can anyhow save the result in TT.
1108 // Step 20. Check for mate and stalemate
1109 // All legal moves have been searched and if there are no legal moves, it
1110 // must be a mate or a stalemate. If we are in a singular extension search then
1111 // return a fail low score.
1113 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1116 bestValue = excludedMove ? alpha
1117 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1120 // Quiet best move: update move sorting heuristics
1121 if (!pos.capture_or_promotion(bestMove))
1122 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1124 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1125 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1126 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1128 // Bonus for prior countermove that caused the fail low
1129 else if ( depth >= 3 * ONE_PLY
1130 && !pos.captured_piece()
1131 && is_ok((ss-1)->currentMove))
1132 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1135 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1136 bestValue >= beta ? BOUND_LOWER :
1137 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1138 depth, bestMove, ss->staticEval, TT.generation());
1140 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1146 // qsearch() is the quiescence search function, which is called by the main
1147 // search function with depth zero, or recursively with depth less than ONE_PLY.
1149 template <NodeType NT, bool InCheck>
1150 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1152 const bool PvNode = NT == PV;
1154 assert(InCheck == !!pos.checkers());
1155 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1156 assert(PvNode || (alpha == beta - 1));
1157 assert(depth <= DEPTH_ZERO);
1158 assert(depth / ONE_PLY * ONE_PLY == depth);
1164 Move ttMove, move, bestMove;
1165 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1166 bool ttHit, givesCheck, evasionPrunable;
1172 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1174 ss->pv[0] = MOVE_NONE;
1177 ss->currentMove = bestMove = MOVE_NONE;
1178 ss->ply = (ss-1)->ply + 1;
1181 // Check for an instant draw or if the maximum ply has been reached
1182 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1183 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1184 : DrawValue[pos.side_to_move()];
1186 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1188 // Decide whether or not to include checks: this fixes also the type of
1189 // TT entry depth that we are going to use. Note that in qsearch we use
1190 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1191 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1192 : DEPTH_QS_NO_CHECKS;
1194 // Transposition table lookup
1196 tte = TT.probe(posKey, ttHit);
1197 ttMove = ttHit ? tte->move() : MOVE_NONE;
1198 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1202 && tte->depth() >= ttDepth
1203 && ttValue != VALUE_NONE // Only in case of TT access race
1204 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1205 : (tte->bound() & BOUND_UPPER)))
1208 // Evaluate the position statically
1211 ss->staticEval = VALUE_NONE;
1212 bestValue = futilityBase = -VALUE_INFINITE;
1218 // Never assume anything on values stored in TT
1219 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1220 ss->staticEval = bestValue = evaluate(pos);
1222 // Can ttValue be used as a better position evaluation?
1223 if (ttValue != VALUE_NONE)
1224 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1225 bestValue = ttValue;
1228 ss->staticEval = bestValue =
1229 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1230 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1232 // Stand pat. Return immediately if static value is at least beta
1233 if (bestValue >= beta)
1236 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1237 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1242 if (PvNode && bestValue > alpha)
1245 futilityBase = bestValue + 128;
1248 // Initialize a MovePicker object for the current position, and prepare
1249 // to search the moves. Because the depth is <= 0 here, only captures,
1250 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1252 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1254 // Loop through the moves until no moves remain or a beta cutoff occurs
1255 while ((move = mp.next_move()) != MOVE_NONE)
1257 assert(is_ok(move));
1259 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1260 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1261 : pos.gives_check(move);
1268 && futilityBase > -VALUE_KNOWN_WIN
1269 && !pos.advanced_pawn_push(move))
1271 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1273 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1275 if (futilityValue <= alpha)
1277 bestValue = std::max(bestValue, futilityValue);
1281 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1283 bestValue = std::max(bestValue, futilityBase);
1288 // Detect non-capture evasions that are candidates to be pruned
1289 evasionPrunable = InCheck
1290 && (depth != DEPTH_ZERO || moveCount > 2)
1291 && bestValue > VALUE_MATED_IN_MAX_PLY
1292 && !pos.capture(move);
1294 // Don't search moves with negative SEE values
1295 if ( (!InCheck || evasionPrunable)
1296 && type_of(move) != PROMOTION
1297 && !pos.see_ge(move))
1300 // Speculative prefetch as early as possible
1301 prefetch(TT.first_entry(pos.key_after(move)));
1303 // Check for legality just before making the move
1304 if (!pos.legal(move))
1310 ss->currentMove = move;
1312 // Make and search the move
1313 pos.do_move(move, st, givesCheck);
1314 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1315 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1316 pos.undo_move(move);
1318 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1320 // Check for a new best move
1321 if (value > bestValue)
1327 if (PvNode) // Update pv even in fail-high case
1328 update_pv(ss->pv, move, (ss+1)->pv);
1330 if (PvNode && value < beta) // Update alpha here!
1337 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1338 ttDepth, move, ss->staticEval, TT.generation());
1346 // All legal moves have been searched. A special case: If we're in check
1347 // and no legal moves were found, it is checkmate.
1348 if (InCheck && bestValue == -VALUE_INFINITE)
1349 return mated_in(ss->ply); // Plies to mate from the root
1351 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1352 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1353 ttDepth, bestMove, ss->staticEval, TT.generation());
1355 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1361 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1362 // "plies to mate from the current position". Non-mate scores are unchanged.
1363 // The function is called before storing a value in the transposition table.
1365 Value value_to_tt(Value v, int ply) {
1367 assert(v != VALUE_NONE);
1369 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1370 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1374 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1375 // from the transposition table (which refers to the plies to mate/be mated
1376 // from current position) to "plies to mate/be mated from the root".
1378 Value value_from_tt(Value v, int ply) {
1380 return v == VALUE_NONE ? VALUE_NONE
1381 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1382 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1386 // update_pv() adds current move and appends child pv[]
1388 void update_pv(Move* pv, Move move, Move* childPv) {
1390 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1396 // update_cm_stats() updates countermove and follow-up move history
1398 void update_cm_stats(Stack* ss, Piece pc, Square s, int bonus) {
1400 for (int i : {1, 2, 4})
1401 if (is_ok((ss-i)->currentMove))
1402 (ss-i)->history->update(pc, s, bonus);
1406 // update_stats() updates move sorting heuristics when a new quiet best move is found
1408 void update_stats(const Position& pos, Stack* ss, Move move,
1409 Move* quiets, int quietsCnt, int bonus) {
1411 if (ss->killers[0] != move)
1413 ss->killers[1] = ss->killers[0];
1414 ss->killers[0] = move;
1417 Color c = pos.side_to_move();
1418 Thread* thisThread = pos.this_thread();
1419 thisThread->history.update(c, move, bonus);
1420 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1422 if (is_ok((ss-1)->currentMove))
1424 Square prevSq = to_sq((ss-1)->currentMove);
1425 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq]=move;
1428 // Decrease all the other played quiet moves
1429 for (int i = 0; i < quietsCnt; ++i)
1431 thisThread->history.update(c, quiets[i], -bonus);
1432 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1437 // When playing with strength handicap, choose best move among a set of RootMoves
1438 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1440 Move Skill::pick_best(size_t multiPV) {
1442 const RootMoves& rootMoves = Threads.main()->rootMoves;
1443 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1445 // RootMoves are already sorted by score in descending order
1446 Value topScore = rootMoves[0].score;
1447 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1448 int weakness = 120 - 2 * level;
1449 int maxScore = -VALUE_INFINITE;
1451 // Choose best move. For each move score we add two terms, both dependent on
1452 // weakness. One is deterministic and bigger for weaker levels, and one is
1453 // random. Then we choose the move with the resulting highest score.
1454 for (size_t i = 0; i < multiPV; ++i)
1456 // This is our magic formula
1457 int push = ( weakness * int(topScore - rootMoves[i].score)
1458 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1460 if (rootMoves[i].score + push > maxScore)
1462 maxScore = rootMoves[i].score + push;
1463 best = rootMoves[i].pv[0];
1471 // check_time() is used to print debug info and, more importantly, to detect
1472 // when we are out of available time and thus stop the search.
1476 static TimePoint lastInfoTime = now();
1478 int elapsed = Time.elapsed();
1479 TimePoint tick = Limits.startTime + elapsed;
1481 if (tick - lastInfoTime >= 1000)
1483 lastInfoTime = tick;
1487 // An engine may not stop pondering until told so by the GUI
1491 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1492 || (Limits.movetime && elapsed >= Limits.movetime)
1493 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1494 Signals.stop = true;
1500 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1501 /// that all (if any) unsearched PV lines are sent using a previous search score.
1503 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1505 std::stringstream ss;
1506 int elapsed = Time.elapsed() + 1;
1507 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1508 size_t PVIdx = pos.this_thread()->PVIdx;
1509 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1510 uint64_t nodesSearched = Threads.nodes_searched();
1511 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1513 for (size_t i = 0; i < multiPV; ++i)
1515 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1517 if (depth == ONE_PLY && !updated)
1520 Depth d = updated ? depth : depth - ONE_PLY;
1521 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1523 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1524 v = tb ? TB::Score : v;
1526 if (ss.rdbuf()->in_avail()) // Not at first line
1530 << " depth " << d / ONE_PLY
1531 << " seldepth " << pos.this_thread()->maxPly
1532 << " multipv " << i + 1
1533 << " score " << UCI::value(v);
1535 if (!tb && i == PVIdx)
1536 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1538 ss << " nodes " << nodesSearched
1539 << " nps " << nodesSearched * 1000 / elapsed;
1541 if (elapsed > 1000) // Earlier makes little sense
1542 ss << " hashfull " << TT.hashfull();
1544 ss << " tbhits " << tbHits
1545 << " time " << elapsed
1548 for (Move m : rootMoves[i].pv)
1549 ss << " " << UCI::move(m, pos.is_chess960());
1556 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1557 /// before exiting the search, for instance, in case we stop the search during a
1558 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1559 /// otherwise in case of 'ponder on' we have nothing to think on.
1561 bool RootMove::extract_ponder_from_tt(Position& pos) {
1566 assert(pv.size() == 1);
1571 pos.do_move(pv[0], st);
1572 TTEntry* tte = TT.probe(pos.key(), ttHit);
1576 Move m = tte->move(); // Local copy to be SMP safe
1577 if (MoveList<LEGAL>(pos).contains(m))
1581 pos.undo_move(pv[0]);
1582 return pv.size() > 1;
1585 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1588 UseRule50 = Options["Syzygy50MoveRule"];
1589 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1590 Cardinality = Options["SyzygyProbeLimit"];
1592 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1593 if (Cardinality > MaxCardinality)
1595 Cardinality = MaxCardinality;
1596 ProbeDepth = DEPTH_ZERO;
1599 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1602 // If the current root position is in the tablebases, then RootMoves
1603 // contains only moves that preserve the draw or the win.
1604 RootInTB = root_probe(pos, rootMoves, TB::Score);
1607 Cardinality = 0; // Do not probe tablebases during the search
1609 else // If DTZ tables are missing, use WDL tables as a fallback
1611 // Filter out moves that do not preserve the draw or the win.
1612 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1614 // Only probe during search if winning
1615 if (RootInTB && TB::Score <= VALUE_DRAW)
1619 if (RootInTB && !UseRule50)
1620 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1621 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1