2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
39 volatile SignalsType Signals;
41 std::vector<RootMove> RootMoves;
43 Time::point SearchTime;
44 StateStackPtr SetupStates;
49 using namespace Search;
53 // Different node types, used as template parameter
54 enum NodeType { Root, PV, NonPV };
56 // Dynamic razoring margin based on depth
57 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
59 // Futility lookup tables (initialized at startup) and their access functions
60 int FutilityMoveCounts[2][16]; // [improving][depth]
62 inline Value futility_margin(Depth d) {
63 return Value(200 * d);
66 // Reduction lookup tables (initialized at startup) and their access function
67 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
69 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
70 return (Depth) Reductions[PvNode][i][std::min(int(d), 63)][std::min(mn, 63)];
75 double BestMoveChanges;
76 Value DrawValue[COLOR_NB];
79 MovesStats Countermoves, Followupmoves;
81 template <NodeType NT, bool SpNode>
82 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
84 template <NodeType NT, bool InCheck>
85 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
87 void id_loop(Position& pos);
88 Value value_to_tt(Value v, int ply);
89 Value value_from_tt(Value v, int ply);
90 void update_pv(Move* pv, Move move, Move* childPv);
91 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
92 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta);
95 Skill(int l, size_t rootSize) : level(l),
96 candidates(l < 20 ? std::min(4, (int)rootSize) : 0),
99 if (candidates) // Swap best PV line with the sub-optimal one
100 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
101 RootMoves.end(), best ? best : pick_move()));
104 size_t candidates_size() const { return candidates; }
105 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
116 /// Search::init() is called during startup to initialize various lookup tables
118 void Search::init() {
120 // Init reductions array
121 for (int d = 1; d < 64; ++d)
122 for (int mc = 1; mc < 64; ++mc)
124 double pvRed = 0.00 + log(double(d)) * log(double(mc)) / 3.00;
125 double nonPVRed = 0.33 + log(double(d)) * log(double(mc)) / 2.25;
127 Reductions[1][1][d][mc] = int8_t( pvRed >= 1.0 ? pvRed + 0.5: 0);
128 Reductions[0][1][d][mc] = int8_t(nonPVRed >= 1.0 ? nonPVRed + 0.5: 0);
130 Reductions[1][0][d][mc] = Reductions[1][1][d][mc];
131 Reductions[0][0][d][mc] = Reductions[0][1][d][mc];
133 // Increase reduction when eval is not improving
134 if (Reductions[0][0][d][mc] >= 2)
135 Reductions[0][0][d][mc] += 1;
138 // Init futility move count array
139 for (int d = 0; d < 16; ++d)
141 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
142 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
147 /// Search::perft() is our utility to verify move generation. All the leaf nodes
148 /// up to the given depth are generated and counted and the sum returned.
150 uint64_t Search::perft(Position& pos, Depth depth) {
153 uint64_t cnt, nodes = 0;
155 const bool leaf = (depth == 2 * ONE_PLY);
157 for (MoveList<LEGAL> it(pos); *it; ++it)
159 if (Root && depth <= ONE_PLY)
163 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
164 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
169 sync_cout << UCI::format_move(*it, pos.is_chess960()) << ": " << cnt << sync_endl;
174 template uint64_t Search::perft<true>(Position& pos, Depth depth);
177 /// Search::think() is the external interface to Stockfish's search, and is
178 /// called by the main thread when the program receives the UCI 'go' command. It
179 /// searches from RootPos and at the end prints the "bestmove" to output.
181 void Search::think() {
183 TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
185 int cf = Options["Contempt"] * PawnValueEg / 100; // From centipawns
186 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
187 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(cf);
189 if (RootMoves.empty())
191 RootMoves.push_back(MOVE_NONE);
192 sync_cout << "info depth 0 score "
193 << UCI::format_value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
198 for (size_t i = 0; i < Threads.size(); ++i)
199 Threads[i]->maxPly = 0;
201 Threads.timer->run = true;
202 Threads.timer->notify_one(); // Wake up the recurring timer
204 id_loop(RootPos); // Let's start searching !
206 Threads.timer->run = false;
209 // When we reach the maximum depth, we can arrive here without a raise of
210 // Signals.stop. However, if we are pondering or in an infinite search,
211 // the UCI protocol states that we shouldn't print the best move before the
212 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
213 // until the GUI sends one of those commands (which also raises Signals.stop).
214 if (!Signals.stop && (Limits.ponder || Limits.infinite))
216 Signals.stopOnPonderhit = true;
217 RootPos.this_thread()->wait_for(Signals.stop);
220 sync_cout << "bestmove " << UCI::format_move(RootMoves[0].pv[0], RootPos.is_chess960())
221 << " ponder " << UCI::format_move(RootMoves[0].pv[1], RootPos.is_chess960())
228 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
229 // with increasing depth until the allocated thinking time has been consumed,
230 // user stops the search, or the maximum search depth is reached.
232 void id_loop(Position& pos) {
234 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
236 Value bestValue, alpha, beta, delta;
238 std::memset(ss-2, 0, 5 * sizeof(Stack));
242 bestValue = delta = alpha = -VALUE_INFINITE;
243 beta = VALUE_INFINITE;
248 Countermoves.clear();
249 Followupmoves.clear();
251 size_t multiPV = Options["MultiPV"];
252 Skill skill(Options["Skill Level"], RootMoves.size());
254 // Do we have to play with skill handicap? In this case enable MultiPV search
255 // that we will use behind the scenes to retrieve a set of possible moves.
256 multiPV = std::max(multiPV, skill.candidates_size());
258 // Iterative deepening loop until requested to stop or target depth reached
259 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
261 // Age out PV variability metric
262 BestMoveChanges *= 0.5;
264 // Save the last iteration's scores before first PV line is searched and
265 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
266 for (size_t i = 0; i < RootMoves.size(); ++i)
267 RootMoves[i].prevScore = RootMoves[i].score;
269 // MultiPV loop. We perform a full root search for each PV line
270 for (PVIdx = 0; PVIdx < std::min(multiPV, RootMoves.size()) && !Signals.stop; ++PVIdx)
272 // Reset aspiration window starting size
273 if (depth >= 5 * ONE_PLY)
276 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
277 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
280 // Start with a small aspiration window and, in the case of a fail
281 // high/low, re-search with a bigger window until we're not failing
285 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
287 // Bring the best move to the front. It is critical that sorting
288 // is done with a stable algorithm because all the values but the
289 // first and eventually the new best one are set to -VALUE_INFINITE
290 // and we want to keep the same order for all the moves except the
291 // new PV that goes to the front. Note that in case of MultiPV
292 // search the already searched PV lines are preserved.
293 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
295 // Write PV back to transposition table in case the relevant
296 // entries have been overwritten during the search.
297 for (size_t i = 0; i <= PVIdx; ++i)
298 RootMoves[i].insert_pv_in_tt(pos);
300 // If search has been stopped break immediately. Sorting and
301 // writing PV back to TT is safe because RootMoves is still
302 // valid, although it refers to previous iteration.
306 // When failing high/low give some update (without cluttering
307 // the UI) before a re-search.
308 if ( (bestValue <= alpha || bestValue >= beta)
309 && Time::now() - SearchTime > 3000)
310 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
312 // In case of failing low/high increase aspiration window and
313 // re-search, otherwise exit the loop.
314 if (bestValue <= alpha)
316 beta = (alpha + beta) / 2;
317 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
319 Signals.failedLowAtRoot = true;
320 Signals.stopOnPonderhit = false;
322 else if (bestValue >= beta)
324 alpha = (alpha + beta) / 2;
325 beta = std::min(bestValue + delta, VALUE_INFINITE);
332 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
335 // Sort the PV lines searched so far and update the GUI
336 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
339 sync_cout << "info nodes " << RootPos.nodes_searched()
340 << " time " << Time::now() - SearchTime << sync_endl;
342 else if ( PVIdx + 1 == std::min(multiPV, RootMoves.size())
343 || Time::now() - SearchTime > 3000)
344 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
347 // If skill levels are enabled and time is up, pick a sub-optimal best move
348 if (skill.candidates_size() && skill.time_to_pick(depth))
351 // Have we found a "mate in x"?
353 && bestValue >= VALUE_MATE_IN_MAX_PLY
354 && VALUE_MATE - bestValue <= 2 * Limits.mate)
357 // Do we have time for the next iteration? Can we stop searching now?
358 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
360 // Take some extra time if the best move has changed
361 if (depth > 4 * ONE_PLY && multiPV == 1)
362 TimeMgr.pv_instability(BestMoveChanges);
364 // Stop the search if only one legal move is available or all
365 // of the available time has been used.
366 if ( RootMoves.size() == 1
367 || Time::now() - SearchTime > TimeMgr.available_time())
369 // If we are allowed to ponder do not stop the search now but
370 // keep pondering until the GUI sends "ponderhit" or "stop".
372 Signals.stopOnPonderhit = true;
381 // search<>() is the main search function for both PV and non-PV nodes and for
382 // normal and SplitPoint nodes. When called just after a split point the search
383 // is simpler because we have already probed the hash table, done a null move
384 // search, and searched the first move before splitting, so we don't have to
385 // repeat all this work again. We also don't need to store anything to the hash
386 // table here: This is taken care of after we return from the split point.
388 template <NodeType NT, bool SpNode>
389 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
391 const bool RootNode = NT == Root;
392 const bool PvNode = NT == PV || NT == Root;
394 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
395 assert(PvNode || (alpha == beta - 1));
396 assert(depth > DEPTH_ZERO);
398 Move pv[MAX_PLY+1], quietsSearched[64];
401 SplitPoint* splitPoint;
403 Move ttMove, move, excludedMove, bestMove;
404 Depth ext, newDepth, predictedDepth;
405 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
406 bool inCheck, givesCheck, singularExtensionNode, improving;
407 bool captureOrPromotion, dangerous, doFullDepthSearch;
408 int moveCount, quietCount;
410 // Step 1. Initialize node
411 Thread* thisThread = pos.this_thread();
412 inCheck = pos.checkers();
416 splitPoint = ss->splitPoint;
417 bestMove = splitPoint->bestMove;
418 bestValue = splitPoint->bestValue;
420 ttMove = excludedMove = MOVE_NONE;
421 ttValue = VALUE_NONE;
423 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
428 moveCount = quietCount = 0;
429 bestValue = -VALUE_INFINITE;
430 ss->ply = (ss-1)->ply + 1;
432 // Used to send selDepth info to GUI
433 if (PvNode && thisThread->maxPly < ss->ply)
434 thisThread->maxPly = ss->ply;
438 // Step 2. Check for aborted search and immediate draw
439 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
440 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
442 // Step 3. Mate distance pruning. Even if we mate at the next move our score
443 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
444 // a shorter mate was found upward in the tree then there is no need to search
445 // because we will never beat the current alpha. Same logic but with reversed
446 // signs applies also in the opposite condition of being mated instead of giving
447 // mate. In this case return a fail-high score.
448 alpha = std::max(mated_in(ss->ply), alpha);
449 beta = std::min(mate_in(ss->ply+1), beta);
454 assert(0 <= ss->ply && ss->ply < MAX_PLY);
456 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
457 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
458 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
460 // Step 4. Transposition table lookup
461 // We don't want the score of a partial search to overwrite a previous full search
462 // TT value, so we use a different position key in case of an excluded move.
463 excludedMove = ss->excludedMove;
464 posKey = excludedMove ? pos.exclusion_key() : pos.key();
465 tte = TT.probe(posKey);
466 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
467 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
469 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
472 && tte->depth() >= depth
473 && ttValue != VALUE_NONE // Only in case of TT access race
474 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
475 : (tte->bound() & BOUND_UPPER)))
477 ss->currentMove = ttMove; // Can be MOVE_NONE
479 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
480 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
481 update_stats(pos, ss, ttMove, depth, NULL, 0);
486 // Step 5. Evaluate the position statically and update parent's gain statistics
489 ss->staticEval = eval = VALUE_NONE;
495 // Never assume anything on values stored in TT
496 if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
497 eval = ss->staticEval = evaluate(pos);
499 // Can ttValue be used as a better position evaluation?
500 if (ttValue != VALUE_NONE)
501 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
506 eval = ss->staticEval =
507 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
509 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
512 if ( !pos.captured_piece_type()
513 && ss->staticEval != VALUE_NONE
514 && (ss-1)->staticEval != VALUE_NONE
515 && (move = (ss-1)->currentMove) != MOVE_NULL
517 && type_of(move) == NORMAL)
519 Square to = to_sq(move);
520 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
523 // Step 6. Razoring (skipped when in check)
525 && depth < 4 * ONE_PLY
526 && eval + razor_margin(depth) <= alpha
527 && ttMove == MOVE_NONE
528 && !pos.pawn_on_7th(pos.side_to_move()))
530 if ( depth <= ONE_PLY
531 && eval + razor_margin(3 * ONE_PLY) <= alpha)
532 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
534 Value ralpha = alpha - razor_margin(depth);
535 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
540 // Step 7. Futility pruning: child node (skipped when in check)
543 && depth < 7 * ONE_PLY
544 && eval - futility_margin(depth) >= beta
545 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
546 && pos.non_pawn_material(pos.side_to_move()))
547 return eval - futility_margin(depth);
549 // Step 8. Null move search with verification search (is omitted in PV nodes)
552 && depth >= 2 * ONE_PLY
554 && pos.non_pawn_material(pos.side_to_move()))
556 ss->currentMove = MOVE_NULL;
558 assert(eval - beta >= 0);
560 // Null move dynamic reduction based on depth and value
561 Depth R = (3 + depth / 4 + std::min(int(eval - beta) / PawnValueMg, 3)) * ONE_PLY;
563 pos.do_null_move(st);
564 (ss+1)->skipNullMove = true;
565 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
566 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
567 (ss+1)->skipNullMove = false;
568 pos.undo_null_move();
570 if (nullValue >= beta)
572 // Do not return unproven mate scores
573 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
576 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
579 // Do verification search at high depths
580 ss->skipNullMove = true;
581 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
582 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
583 ss->skipNullMove = false;
590 // Step 9. ProbCut (skipped when in check)
591 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
592 // and a reduced search returns a value much above beta, we can (almost) safely
593 // prune the previous move.
595 && depth >= 5 * ONE_PLY
597 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
599 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
600 Depth rdepth = depth - 4 * ONE_PLY;
602 assert(rdepth >= ONE_PLY);
603 assert((ss-1)->currentMove != MOVE_NONE);
604 assert((ss-1)->currentMove != MOVE_NULL);
606 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
609 while ((move = mp.next_move<false>()) != MOVE_NONE)
610 if (pos.legal(move, ci.pinned))
612 ss->currentMove = move;
613 pos.do_move(move, st, ci, pos.gives_check(move, ci));
614 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
621 // Step 10. Internal iterative deepening (skipped when in check)
622 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
624 && (PvNode || ss->staticEval + 256 >= beta))
626 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
627 ss->skipNullMove = true;
628 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
629 ss->skipNullMove = false;
631 tte = TT.probe(posKey);
632 ttMove = tte ? tte->move() : MOVE_NONE;
635 moves_loop: // When in check and at SpNode search starts from here
637 Square prevMoveSq = to_sq((ss-1)->currentMove);
638 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
639 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
641 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
642 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
643 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
645 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
647 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
648 improving = ss->staticEval >= (ss-2)->staticEval
649 || ss->staticEval == VALUE_NONE
650 ||(ss-2)->staticEval == VALUE_NONE;
652 singularExtensionNode = !RootNode
654 && depth >= 8 * ONE_PLY
655 && ttMove != MOVE_NONE
656 /* && ttValue != VALUE_NONE Already implicit in the next condition */
657 && abs(ttValue) < VALUE_KNOWN_WIN
658 && !excludedMove // Recursive singular search is not allowed
659 && (tte->bound() & BOUND_LOWER)
660 && tte->depth() >= depth - 3 * ONE_PLY;
662 // Step 11. Loop through moves
663 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
664 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
668 if (move == excludedMove)
671 // At root obey the "searchmoves" option and skip moves not listed in Root
672 // Move List. As a consequence any illegal move is also skipped. In MultiPV
673 // mode we also skip PV moves which have been already searched.
674 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
679 // Shared counter cannot be decremented later if the move turns out to be illegal
680 if (!pos.legal(move, ci.pinned))
683 moveCount = ++splitPoint->moveCount;
684 splitPoint->mutex.unlock();
691 Signals.firstRootMove = (moveCount == 1);
693 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
694 sync_cout << "info depth " << depth
695 << " currmove " << UCI::format_move(move, pos.is_chess960())
696 << " currmovenumber " << moveCount + PVIdx << sync_endl;
703 captureOrPromotion = pos.capture_or_promotion(move);
705 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
706 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
707 : pos.gives_check(move, ci);
709 dangerous = givesCheck
710 || type_of(move) != NORMAL
711 || pos.advanced_pawn_push(move);
713 // Step 12. Extend checks
714 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
717 // Singular extension search. If all moves but one fail low on a search of
718 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
719 // is singular and should be extended. To verify this we do a reduced search
720 // on all the other moves but the ttMove and if the result is lower than
721 // ttValue minus a margin then we extend the ttMove.
722 if ( singularExtensionNode
725 && pos.legal(move, ci.pinned))
727 Value rBeta = ttValue - int(2 * depth);
728 ss->excludedMove = move;
729 ss->skipNullMove = true;
730 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
731 ss->skipNullMove = false;
732 ss->excludedMove = MOVE_NONE;
738 // Update the current move (this must be done after singular extension search)
739 newDepth = depth - ONE_PLY + ext;
741 // Step 13. Pruning at shallow depth (exclude PV nodes)
743 && !captureOrPromotion
746 && bestValue > VALUE_MATED_IN_MAX_PLY)
748 // Move count based pruning
749 if ( depth < 16 * ONE_PLY
750 && moveCount >= FutilityMoveCounts[improving][depth])
753 splitPoint->mutex.lock();
758 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
760 // Futility pruning: parent node
761 if (predictedDepth < 7 * ONE_PLY)
763 futilityValue = ss->staticEval + futility_margin(predictedDepth)
764 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
766 if (futilityValue <= alpha)
768 bestValue = std::max(bestValue, futilityValue);
772 splitPoint->mutex.lock();
773 if (bestValue > splitPoint->bestValue)
774 splitPoint->bestValue = bestValue;
780 // Prune moves with negative SEE at low depths
781 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
784 splitPoint->mutex.lock();
790 // Speculative prefetch as early as possible
791 prefetch((char*)TT.first_entry(pos.key_after(move)));
793 // Check for legality just before making the move
794 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
800 ss->currentMove = move;
801 if (!SpNode && !captureOrPromotion && quietCount < 64)
802 quietsSearched[quietCount++] = move;
804 // Step 14. Make the move
805 pos.do_move(move, st, ci, givesCheck);
807 // Step 15. Reduced depth search (LMR). If the move fails high it will be
808 // re-searched at full depth.
809 if ( depth >= 3 * ONE_PLY
811 && !captureOrPromotion
812 && move != ss->killers[0]
813 && move != ss->killers[1])
815 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
817 if ( (!PvNode && cutNode)
818 || History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
819 ss->reduction += ONE_PLY;
821 if (move == countermoves[0] || move == countermoves[1])
822 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
824 // Decrease reduction for moves that escape a capture
826 && type_of(move) == NORMAL
827 && type_of(pos.piece_on(to_sq(move))) != PAWN
828 && pos.see(make_move(to_sq(move), from_sq(move))) < 0)
829 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
831 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
833 alpha = splitPoint->alpha;
835 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
837 // Re-search at intermediate depth if reduction is very high
838 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
840 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
841 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
844 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
845 ss->reduction = DEPTH_ZERO;
848 doFullDepthSearch = !PvNode || moveCount > 1;
850 // Step 16. Full depth search, when LMR is skipped or fails high
851 if (doFullDepthSearch)
854 alpha = splitPoint->alpha;
856 value = newDepth < ONE_PLY ?
857 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
858 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
859 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
862 // For PV nodes only, do a full PV search on the first move or after a fail
863 // high (in the latter case search only if value < beta), otherwise let the
864 // parent node fail low with value <= alpha and to try another move.
865 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
868 (ss+1)->pv[0] = MOVE_NONE;
870 value = newDepth < ONE_PLY ?
871 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
872 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
873 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
876 // Step 17. Undo move
879 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
881 // Step 18. Check for new best move
884 splitPoint->mutex.lock();
885 bestValue = splitPoint->bestValue;
886 alpha = splitPoint->alpha;
889 // Finished searching the move. If a stop or a cutoff occurred, the return
890 // value of the search cannot be trusted, and we return immediately without
891 // updating best move, PV and TT.
892 if (Signals.stop || thisThread->cutoff_occurred())
897 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
899 // PV move or new best move ?
900 if (moveCount == 1 || value > alpha)
907 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
910 // We record how often the best move has been changed in each
911 // iteration. This information is used for time management: When
912 // the best move changes frequently, we allocate some more time.
917 // All other moves but the PV are set to the lowest value: this is
918 // not a problem when sorting because the sort is stable and the
919 // move position in the list is preserved - just the PV is pushed up.
920 rm.score = -VALUE_INFINITE;
923 if (value > bestValue)
925 bestValue = SpNode ? splitPoint->bestValue = value : value;
929 bestMove = SpNode ? splitPoint->bestMove = move : move;
931 if (PvNode && !RootNode) // Update pv even in fail-high case
932 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
934 if (PvNode && value < beta) // Update alpha! Always alpha < beta
935 alpha = SpNode ? splitPoint->alpha = value : value;
938 assert(value >= beta); // Fail high
941 splitPoint->cutoff = true;
948 // Step 19. Check for splitting the search
950 && Threads.size() >= 2
951 && depth >= Threads.minimumSplitDepth
952 && ( !thisThread->activeSplitPoint
953 || !thisThread->activeSplitPoint->allSlavesSearching)
954 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
956 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
958 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
959 depth, moveCount, &mp, NT, cutNode);
961 if (Signals.stop || thisThread->cutoff_occurred())
964 if (bestValue >= beta)
972 // Following condition would detect a stop or a cutoff set only after move
973 // loop has been completed. But in this case bestValue is valid because we
974 // have fully searched our subtree, and we can anyhow save the result in TT.
976 if (Signals.stop || thisThread->cutoff_occurred())
980 // Step 20. Check for mate and stalemate
981 // All legal moves have been searched and if there are no legal moves, it
982 // must be mate or stalemate. If we are in a singular extension search then
983 // return a fail low score.
985 bestValue = excludedMove ? alpha
986 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
988 // Quiet best move: update killers, history, countermoves and followupmoves
989 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
990 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
992 TT.store(posKey, value_to_tt(bestValue, ss->ply),
993 bestValue >= beta ? BOUND_LOWER :
994 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
995 depth, bestMove, ss->staticEval);
997 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1003 // qsearch() is the quiescence search function, which is called by the main
1004 // search function when the remaining depth is zero (or, to be more precise,
1005 // less than ONE_PLY).
1007 template <NodeType NT, bool InCheck>
1008 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1010 const bool PvNode = NT == PV;
1012 assert(NT == PV || NT == NonPV);
1013 assert(InCheck == !!pos.checkers());
1014 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1015 assert(PvNode || (alpha == beta - 1));
1016 assert(depth <= DEPTH_ZERO);
1022 Move ttMove, move, bestMove;
1023 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1024 bool givesCheck, evasionPrunable;
1029 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1031 ss->pv[0] = MOVE_NONE;
1034 ss->currentMove = bestMove = MOVE_NONE;
1035 ss->ply = (ss-1)->ply + 1;
1037 // Check for an instant draw or if the maximum ply has been reached
1038 if (pos.is_draw() || ss->ply >= MAX_PLY)
1039 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1041 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1043 // Decide whether or not to include checks: this fixes also the type of
1044 // TT entry depth that we are going to use. Note that in qsearch we use
1045 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1046 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1047 : DEPTH_QS_NO_CHECKS;
1049 // Transposition table lookup
1051 tte = TT.probe(posKey);
1052 ttMove = tte ? tte->move() : MOVE_NONE;
1053 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1057 && tte->depth() >= ttDepth
1058 && ttValue != VALUE_NONE // Only in case of TT access race
1059 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1060 : (tte->bound() & BOUND_UPPER)))
1062 ss->currentMove = ttMove; // Can be MOVE_NONE
1066 // Evaluate the position statically
1069 ss->staticEval = VALUE_NONE;
1070 bestValue = futilityBase = -VALUE_INFINITE;
1076 // Never assume anything on values stored in TT
1077 if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
1078 ss->staticEval = bestValue = evaluate(pos);
1080 // Can ttValue be used as a better position evaluation?
1081 if (ttValue != VALUE_NONE)
1082 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1083 bestValue = ttValue;
1086 ss->staticEval = bestValue =
1087 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1089 // Stand pat. Return immediately if static value is at least beta
1090 if (bestValue >= beta)
1093 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1094 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1099 if (PvNode && bestValue > alpha)
1102 futilityBase = bestValue + 128;
1105 // Initialize a MovePicker object for the current position, and prepare
1106 // to search the moves. Because the depth is <= 0 here, only captures,
1107 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1109 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1112 // Loop through the moves until no moves remain or a beta cutoff occurs
1113 while ((move = mp.next_move<false>()) != MOVE_NONE)
1115 assert(is_ok(move));
1117 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1118 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1119 : pos.gives_check(move, ci);
1125 && futilityBase > -VALUE_KNOWN_WIN
1126 && !pos.advanced_pawn_push(move))
1128 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1130 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1132 if (futilityValue < beta)
1134 bestValue = std::max(bestValue, futilityValue);
1138 if (futilityBase < beta && pos.see(move) <= VALUE_ZERO)
1140 bestValue = std::max(bestValue, futilityBase);
1145 // Detect non-capture evasions that are candidates to be pruned
1146 evasionPrunable = InCheck
1147 && bestValue > VALUE_MATED_IN_MAX_PLY
1148 && !pos.capture(move)
1149 && !pos.can_castle(pos.side_to_move());
1151 // Don't search moves with negative SEE values
1153 && (!InCheck || evasionPrunable)
1154 && type_of(move) != PROMOTION
1155 && pos.see_sign(move) < VALUE_ZERO)
1158 // Speculative prefetch as early as possible
1159 prefetch((char*)TT.first_entry(pos.key_after(move)));
1161 // Check for legality just before making the move
1162 if (!pos.legal(move, ci.pinned))
1165 ss->currentMove = move;
1167 // Make and search the move
1168 pos.do_move(move, st, ci, givesCheck);
1169 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1170 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1171 pos.undo_move(move);
1173 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1175 // Check for new best move
1176 if (value > bestValue)
1182 if (PvNode) // Update pv even in fail-high case
1183 update_pv(ss->pv, move, (ss+1)->pv);
1185 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1192 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1193 ttDepth, move, ss->staticEval);
1201 // All legal moves have been searched. A special case: If we're in check
1202 // and no legal moves were found, it is checkmate.
1203 if (InCheck && bestValue == -VALUE_INFINITE)
1204 return mated_in(ss->ply); // Plies to mate from the root
1206 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1207 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1208 ttDepth, bestMove, ss->staticEval);
1210 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1216 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1217 // "plies to mate from the current position". Non-mate scores are unchanged.
1218 // The function is called before storing a value in the transposition table.
1220 Value value_to_tt(Value v, int ply) {
1222 assert(v != VALUE_NONE);
1224 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1225 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1229 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1230 // from the transposition table (which refers to the plies to mate/be mated
1231 // from current position) to "plies to mate/be mated from the root".
1233 Value value_from_tt(Value v, int ply) {
1235 return v == VALUE_NONE ? VALUE_NONE
1236 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1237 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1241 // update_pv() adds current move and appends child pv[]
1243 void update_pv(Move* pv, Move move, Move* childPv) {
1245 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1250 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1253 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1255 if (ss->killers[0] != move)
1257 ss->killers[1] = ss->killers[0];
1258 ss->killers[0] = move;
1261 // Increase history value of the cut-off move and decrease all the other
1262 // played quiet moves.
1263 Value bonus = Value(int(depth) * int(depth));
1264 History.update(pos.moved_piece(move), to_sq(move), bonus);
1265 for (int i = 0; i < quietsCnt; ++i)
1268 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1271 if (is_ok((ss-1)->currentMove))
1273 Square prevMoveSq = to_sq((ss-1)->currentMove);
1274 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1277 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1279 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1280 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1285 // When playing with a strength handicap, choose best move among the first 'candidates'
1286 // RootMoves using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1288 Move Skill::pick_move() {
1292 // PRNG sequence should be not deterministic
1293 for (int i = Time::now() % 50; i > 0; --i)
1294 rk.rand<unsigned>();
1296 // RootMoves are already sorted by score in descending order
1297 int variance = std::min(RootMoves[0].score - RootMoves[candidates - 1].score, PawnValueMg);
1298 int weakness = 120 - 2 * level;
1299 int max_s = -VALUE_INFINITE;
1302 // Choose best move. For each move score we add two terms both dependent on
1303 // weakness. One deterministic and bigger for weaker moves, and one random,
1304 // then we choose the move with the resulting highest score.
1305 for (size_t i = 0; i < candidates; ++i)
1307 int s = RootMoves[i].score;
1309 // Don't allow crazy blunders even at very low skills
1310 if (i > 0 && RootMoves[i - 1].score > s + 2 * PawnValueMg)
1313 // This is our magic formula
1314 s += ( weakness * int(RootMoves[0].score - s)
1315 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1320 best = RootMoves[i].pv[0];
1327 // uci_pv() formats PV information according to the UCI protocol. UCI
1328 // requires that all (if any) unsearched PV lines are sent using a previous
1331 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1333 std::stringstream ss;
1334 Time::point elapsed = Time::now() - SearchTime + 1;
1335 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1338 for (size_t i = 0; i < Threads.size(); ++i)
1339 if (Threads[i]->maxPly > selDepth)
1340 selDepth = Threads[i]->maxPly;
1342 for (size_t i = 0; i < uciPVSize; ++i)
1344 bool updated = (i <= PVIdx);
1346 if (depth == 1 && !updated)
1349 Depth d = updated ? depth : depth - ONE_PLY;
1350 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1352 if (ss.rdbuf()->in_avail()) // Not at first line
1355 ss << "info depth " << d / ONE_PLY
1356 << " seldepth " << selDepth
1357 << " multipv " << i + 1
1358 << " score " << (i == PVIdx ? UCI::format_value(v, alpha, beta) : UCI::format_value(v))
1359 << " nodes " << pos.nodes_searched()
1360 << " nps " << pos.nodes_searched() * 1000 / elapsed
1361 << " time " << elapsed
1364 for (size_t j = 0; j < RootMoves[i].pv.size(); ++j)
1365 ss << " " << UCI::format_move(RootMoves[i].pv[j], pos.is_chess960());
1374 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1375 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1376 /// first, even if the old TT entries have been overwritten.
1378 void RootMove::insert_pv_in_tt(Position& pos) {
1380 StateInfo state[MAX_PLY], *st = state;
1384 for ( ; idx < pv.size(); ++idx)
1386 tte = TT.probe(pos.key());
1388 if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
1389 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
1391 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1393 pos.do_move(pv[idx], *st++);
1396 while (idx) pos.undo_move(pv[--idx]);
1400 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1402 void Thread::idle_loop() {
1404 // Pointer 'this_sp' is not null only if we are called from split(), and not
1405 // at the thread creation. This means we are the split point's master.
1406 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1408 assert(!this_sp || (this_sp->masterThread == this && searching));
1412 // If this thread has been assigned work, launch a search
1415 Threads.mutex.lock();
1417 assert(activeSplitPoint);
1418 SplitPoint* sp = activeSplitPoint;
1420 Threads.mutex.unlock();
1422 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1423 Position pos(*sp->pos, this);
1425 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1426 ss->splitPoint = sp;
1430 assert(activePosition == NULL);
1432 activePosition = &pos;
1434 if (sp->nodeType == NonPV)
1435 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1437 else if (sp->nodeType == PV)
1438 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1440 else if (sp->nodeType == Root)
1441 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1449 activePosition = NULL;
1450 sp->slavesMask.reset(idx);
1451 sp->allSlavesSearching = false;
1452 sp->nodes += pos.nodes_searched();
1454 // Wake up the master thread so to allow it to return from the idle
1455 // loop in case we are the last slave of the split point.
1456 if ( this != sp->masterThread
1457 && sp->slavesMask.none())
1459 assert(!sp->masterThread->searching);
1460 sp->masterThread->notify_one();
1463 // After releasing the lock we can't access any SplitPoint related data
1464 // in a safe way because it could have been released under our feet by
1468 // Try to late join to another split point if none of its slaves has
1469 // already finished.
1470 if (Threads.size() > 2)
1471 for (size_t i = 0; i < Threads.size(); ++i)
1473 const int size = Threads[i]->splitPointsSize; // Local copy
1474 sp = size ? &Threads[i]->splitPoints[size - 1] : NULL;
1477 && sp->allSlavesSearching
1478 && available_to(Threads[i]))
1480 // Recheck the conditions under lock protection
1481 Threads.mutex.lock();
1484 if ( sp->allSlavesSearching
1485 && available_to(Threads[i]))
1487 sp->slavesMask.set(idx);
1488 activeSplitPoint = sp;
1493 Threads.mutex.unlock();
1495 break; // Just a single attempt
1500 // Grab the lock to avoid races with Thread::notify_one()
1503 // If we are master and all slaves have finished then exit idle_loop
1504 if (this_sp && this_sp->slavesMask.none())
1511 // If we are not searching, wait for a condition to be signaled instead of
1512 // wasting CPU time polling for work.
1513 if (!searching && !exit)
1514 sleepCondition.wait(mutex);
1521 /// check_time() is called by the timer thread when the timer triggers. It is
1522 /// used to print debug info and, more importantly, to detect when we are out of
1523 /// available time and thus stop the search.
1527 static Time::point lastInfoTime = Time::now();
1528 Time::point elapsed = Time::now() - SearchTime;
1530 if (Time::now() - lastInfoTime >= 1000)
1532 lastInfoTime = Time::now();
1536 if (Limits.use_time_management() && !Limits.ponder)
1538 bool stillAtFirstMove = Signals.firstRootMove
1539 && !Signals.failedLowAtRoot
1540 && elapsed > TimeMgr.available_time() * 75 / 100;
1542 if ( stillAtFirstMove
1543 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1544 Signals.stop = true;
1546 else if (Limits.movetime && elapsed >= Limits.movetime)
1547 Signals.stop = true;
1549 else if (Limits.nodes)
1551 Threads.mutex.lock();
1553 int64_t nodes = RootPos.nodes_searched();
1555 // Loop across all split points and sum accumulated SplitPoint nodes plus
1556 // all the currently active positions nodes.
1557 for (size_t i = 0; i < Threads.size(); ++i)
1558 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1560 SplitPoint& sp = Threads[i]->splitPoints[j];
1566 for (size_t idx = 0; idx < Threads.size(); ++idx)
1567 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1568 nodes += Threads[idx]->activePosition->nodes_searched();
1573 Threads.mutex.unlock();
1575 if (nodes >= Limits.nodes)
1576 Signals.stop = true;