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/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
44 Time::point SearchTime;
45 StateStackPtr SetupStates;
50 using namespace Search;
54 // Different node types, used as template parameter
55 enum NodeType { Root, PV, NonPV };
57 // Dynamic razoring margin based on depth
58 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
60 // Futility lookup tables (initialized at startup) and their access functions
61 int FutilityMoveCounts[2][32]; // [improving][depth]
63 inline Value futility_margin(Depth d) {
64 return Value(200 * d);
67 // Reduction lookup tables (initialized at startup) and their access function
68 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
70 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
71 return (Depth) Reductions[PvNode][i][std::min(int(d), 63)][std::min(mn, 63)];
76 double BestMoveChanges;
77 Value DrawValue[COLOR_NB];
80 MovesStats Countermoves, Followupmoves;
82 template <NodeType NT, bool SpNode>
83 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
85 template <NodeType NT, bool InCheck>
86 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
88 void id_loop(Position& pos);
89 Value value_to_tt(Value v, int ply);
90 Value value_from_tt(Value v, int ply);
91 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
92 string uci_pv(const Position& pos, int 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(int depth) const { return depth == 1 + level; }
116 /// Search::init() is called during startup to initialize various lookup tables
118 void Search::init() {
120 int d; // depth (ONE_PLY == 2)
121 int hd; // half depth (ONE_PLY == 1)
124 // Init reductions array
125 for (hd = 1; hd < 64; ++hd) for (mc = 1; mc < 64; ++mc)
127 double pvRed = 0.00 + log(double(hd)) * log(double(mc)) / 3.00;
128 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
130 Reductions[1][1][hd][mc] = int8_t( pvRed >= 1.0 ? pvRed + 0.5: 0);
131 Reductions[0][1][hd][mc] = int8_t(nonPVRed >= 1.0 ? nonPVRed + 0.5: 0);
133 Reductions[1][0][hd][mc] = Reductions[1][1][hd][mc];
134 Reductions[0][0][hd][mc] = Reductions[0][1][hd][mc];
136 if (Reductions[0][0][hd][mc] >= 2)
137 Reductions[0][0][hd][mc] += 1;
140 // Init futility move count array
141 for (d = 0; d < 32; ++d)
143 FutilityMoveCounts[0][d] = int(2.4 + 0.222 * pow(d * 2 + 0.00, 1.8));
144 FutilityMoveCounts[1][d] = int(3.0 + 0.300 * pow(d * 2 + 0.98, 1.8));
149 /// Search::perft() is our utility to verify move generation. All the leaf nodes
150 /// up to the given depth are generated and counted and the sum returned.
152 uint64_t Search::perft(Position& pos, Depth depth) {
155 uint64_t cnt, nodes = 0;
157 const bool leaf = (depth == 2 * ONE_PLY);
159 for (MoveList<LEGAL> it(pos); *it; ++it)
161 if (Root && depth <= ONE_PLY)
165 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
166 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
171 sync_cout << move_to_uci(*it, pos.is_chess960()) << ": " << cnt << sync_endl;
176 template uint64_t Search::perft<true>(Position& pos, Depth depth);
179 /// Search::think() is the external interface to Stockfish's search, and is
180 /// called by the main thread when the program receives the UCI 'go' command. It
181 /// searches from RootPos and at the end prints the "bestmove" to output.
183 void Search::think() {
185 TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
187 int cf = Options["Contempt"] * PawnValueEg / 100; // From centipawns
188 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
189 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(cf);
191 if (RootMoves.empty())
193 RootMoves.push_back(MOVE_NONE);
194 sync_cout << "info depth 0 score "
195 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
201 // Reset the threads, still sleeping: will wake up at split time
202 for (size_t i = 0; i < Threads.size(); ++i)
203 Threads[i]->maxPly = 0;
205 Threads.timer->run = true;
206 Threads.timer->notify_one(); // Wake up the recurring timer
208 id_loop(RootPos); // Let's start searching !
210 Threads.timer->run = false; // Stop the timer
214 // When search is stopped this info is not printed
215 sync_cout << "info nodes " << RootPos.nodes_searched()
216 << " time " << Time::now() - SearchTime + 1 << sync_endl;
218 // When we reach the maximum depth, we can arrive here without a raise of
219 // Signals.stop. However, if we are pondering or in an infinite search,
220 // the UCI protocol states that we shouldn't print the best move before the
221 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
222 // until the GUI sends one of those commands (which also raises Signals.stop).
223 if (!Signals.stop && (Limits.ponder || Limits.infinite))
225 Signals.stopOnPonderhit = true;
226 RootPos.this_thread()->wait_for(Signals.stop);
229 // Best move could be MOVE_NONE when searching on a stalemate position
230 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
231 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
238 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
239 // with increasing depth until the allocated thinking time has been consumed,
240 // user stops the search, or the maximum search depth is reached.
242 void id_loop(Position& pos) {
244 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
246 Value bestValue, alpha, beta, delta;
248 std::memset(ss-2, 0, 5 * sizeof(Stack));
252 bestValue = delta = alpha = -VALUE_INFINITE;
253 beta = VALUE_INFINITE;
258 Countermoves.clear();
259 Followupmoves.clear();
261 size_t multiPV = Options["MultiPV"];
262 Skill skill(Options["Skill Level"], RootMoves.size());
264 // Do we have to play with skill handicap? In this case enable MultiPV search
265 // that we will use behind the scenes to retrieve a set of possible moves.
266 multiPV = std::max(multiPV, skill.candidates_size());
268 // Iterative deepening loop until requested to stop or target depth reached
269 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
271 // Age out PV variability metric
272 BestMoveChanges *= 0.5;
274 // Save the last iteration's scores before first PV line is searched and
275 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
276 for (size_t i = 0; i < RootMoves.size(); ++i)
277 RootMoves[i].prevScore = RootMoves[i].score;
279 // MultiPV loop. We perform a full root search for each PV line
280 for (PVIdx = 0; PVIdx < std::min(multiPV, RootMoves.size()) && !Signals.stop; ++PVIdx)
282 // Reset aspiration window starting size
286 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
287 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
290 // Start with a small aspiration window and, in the case of a fail
291 // high/low, re-search with a bigger window until we're not failing
295 bestValue = search<Root, false>(pos, ss, alpha, beta, depth * ONE_PLY, false);
297 // Bring the best move to the front. It is critical that sorting
298 // is done with a stable algorithm because all the values but the
299 // first and eventually the new best one are set to -VALUE_INFINITE
300 // and we want to keep the same order for all the moves except the
301 // new PV that goes to the front. Note that in case of MultiPV
302 // search the already searched PV lines are preserved.
303 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
305 // Write PV back to transposition table in case the relevant
306 // entries have been overwritten during the search.
307 for (size_t i = 0; i <= PVIdx; ++i)
308 RootMoves[i].insert_pv_in_tt(pos);
310 // If search has been stopped break immediately. Sorting and
311 // writing PV back to TT is safe because RootMoves is still
312 // valid, although it refers to previous iteration.
316 // When failing high/low give some update (without cluttering
317 // the UI) before a re-search.
318 if ( (bestValue <= alpha || bestValue >= beta)
319 && Time::now() - SearchTime > 3000)
320 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
322 // In case of failing low/high increase aspiration window and
323 // re-search, otherwise exit the loop.
324 if (bestValue <= alpha)
326 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
328 Signals.failedLowAtRoot = true;
329 Signals.stopOnPonderhit = false;
331 else if (bestValue >= beta)
332 beta = std::min(bestValue + delta, VALUE_INFINITE);
337 delta += 3 * delta / 8;
339 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
342 // Sort the PV lines searched so far and update the GUI
343 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
345 if (PVIdx + 1 == std::min(multiPV, RootMoves.size()) || Time::now() - SearchTime > 3000)
346 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
349 // If skill levels are enabled and time is up, pick a sub-optimal best move
350 if (skill.candidates_size() && skill.time_to_pick(depth))
353 // Have we found a "mate in x"?
355 && bestValue >= VALUE_MATE_IN_MAX_PLY
356 && VALUE_MATE - bestValue <= 2 * Limits.mate)
359 // Do we have time for the next iteration? Can we stop searching now?
360 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
362 // Take some extra time if the best move has changed
363 if (depth > 4 && multiPV == 1)
364 TimeMgr.pv_instability(BestMoveChanges);
366 // Stop the search if only one legal move is available or all
367 // of the available time has been used.
368 if ( RootMoves.size() == 1
369 || Time::now() - SearchTime > TimeMgr.available_time())
371 // If we are allowed to ponder do not stop the search now but
372 // keep pondering until the GUI sends "ponderhit" or "stop".
374 Signals.stopOnPonderhit = true;
383 // search<>() is the main search function for both PV and non-PV nodes and for
384 // normal and SplitPoint nodes. When called just after a split point the search
385 // is simpler because we have already probed the hash table, done a null move
386 // search, and searched the first move before splitting, so we don't have to
387 // repeat all this work again. We also don't need to store anything to the hash
388 // table here: This is taken care of after we return from the split point.
390 template <NodeType NT, bool SpNode>
391 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
393 const bool RootNode = NT == Root;
394 const bool PvNode = NT == PV || NT == Root;
396 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
397 assert(PvNode || (alpha == beta - 1));
398 assert(depth > DEPTH_ZERO);
400 Move quietsSearched[64];
403 SplitPoint* splitPoint;
405 Move ttMove, move, excludedMove, bestMove;
406 Depth ext, newDepth, predictedDepth;
407 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
408 bool inCheck, givesCheck, pvMove, singularExtensionNode, improving;
409 bool captureOrPromotion, dangerous, doFullDepthSearch;
410 int moveCount, quietCount;
412 // Step 1. Initialize node
413 Thread* thisThread = pos.this_thread();
414 inCheck = pos.checkers();
418 splitPoint = ss->splitPoint;
419 bestMove = splitPoint->bestMove;
420 bestValue = splitPoint->bestValue;
422 ttMove = excludedMove = MOVE_NONE;
423 ttValue = VALUE_NONE;
425 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
430 moveCount = quietCount = 0;
431 bestValue = -VALUE_INFINITE;
432 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
433 ss->ply = (ss-1)->ply + 1;
434 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
435 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
437 // Used to send selDepth info to GUI
438 if (PvNode && thisThread->maxPly < ss->ply)
439 thisThread->maxPly = ss->ply;
443 // Step 2. Check for aborted search and immediate draw
444 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
445 return ss->ply > MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
447 // Step 3. Mate distance pruning. Even if we mate at the next move our score
448 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
449 // a shorter mate was found upward in the tree then there is no need to search
450 // because we will never beat the current alpha. Same logic but with reversed
451 // signs applies also in the opposite condition of being mated instead of giving
452 // mate. In this case return a fail-high score.
453 alpha = std::max(mated_in(ss->ply), alpha);
454 beta = std::min(mate_in(ss->ply+1), beta);
459 // Step 4. Transposition table lookup
460 // We don't want the score of a partial search to overwrite a previous full search
461 // TT value, so we use a different position key in case of an excluded move.
462 excludedMove = ss->excludedMove;
463 posKey = excludedMove ? pos.exclusion_key() : pos.key();
464 tte = TT.probe(posKey);
465 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
466 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
468 // At PV nodes we check for exact scores, whilst at non-PV nodes we check for
469 // a fail high/low. The biggest advantage to probing at PV nodes is to have a
470 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
471 // we should also update RootMoveList to avoid bogus output.
474 && tte->depth() >= depth
475 && ttValue != VALUE_NONE // Only in case of TT access race
476 && ( PvNode ? tte->bound() == BOUND_EXACT
477 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
478 : (tte->bound() & BOUND_UPPER)))
480 ss->currentMove = ttMove; // Can be MOVE_NONE
482 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
483 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
484 update_stats(pos, ss, ttMove, depth, NULL, 0);
489 // Step 5. Evaluate the position statically and update parent's gain statistics
492 ss->staticEval = eval = VALUE_NONE;
498 // Never assume anything on values stored in TT
499 if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
500 eval = ss->staticEval = evaluate(pos);
502 // Can ttValue be used as a better position evaluation?
503 if (ttValue != VALUE_NONE)
504 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
509 eval = ss->staticEval =
510 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
512 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
515 if ( !pos.captured_piece_type()
516 && ss->staticEval != VALUE_NONE
517 && (ss-1)->staticEval != VALUE_NONE
518 && (move = (ss-1)->currentMove) != MOVE_NULL
520 && type_of(move) == NORMAL)
522 Square to = to_sq(move);
523 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
526 // Step 6. Razoring (skipped when in check)
528 && depth < 4 * ONE_PLY
529 && eval + razor_margin(depth) <= alpha
530 && ttMove == MOVE_NONE
531 && !pos.pawn_on_7th(pos.side_to_move()))
533 if ( depth <= ONE_PLY
534 && eval + razor_margin(3 * ONE_PLY) <= alpha)
535 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
537 Value ralpha = alpha - razor_margin(depth);
538 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
543 // Step 7. Futility pruning: child node (skipped when in check)
546 && depth < 7 * ONE_PLY
547 && eval - futility_margin(depth) >= beta
548 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
549 && pos.non_pawn_material(pos.side_to_move()))
550 return eval - futility_margin(depth);
552 // Step 8. Null move search with verification search (is omitted in PV nodes)
555 && depth >= 2 * ONE_PLY
557 && pos.non_pawn_material(pos.side_to_move()))
559 ss->currentMove = MOVE_NULL;
561 assert(eval - beta >= 0);
563 // Null move dynamic reduction based on depth and value
564 Depth R = (3 + depth / 4 + std::min(int(eval - beta) / PawnValueMg, 3)) * ONE_PLY;
566 pos.do_null_move(st);
567 (ss+1)->skipNullMove = true;
568 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
569 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
570 (ss+1)->skipNullMove = false;
571 pos.undo_null_move();
573 if (nullValue >= beta)
575 // Do not return unproven mate scores
576 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
579 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
582 // Do verification search at high depths
583 ss->skipNullMove = true;
584 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
585 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
586 ss->skipNullMove = false;
593 // Step 9. ProbCut (skipped when in check)
594 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
595 // and a reduced search returns a value much above beta, we can (almost) safely
596 // prune the previous move.
598 && depth >= 5 * ONE_PLY
600 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
602 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
603 Depth rdepth = depth - 4 * ONE_PLY;
605 assert(rdepth >= ONE_PLY);
606 assert((ss-1)->currentMove != MOVE_NONE);
607 assert((ss-1)->currentMove != MOVE_NULL);
609 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
612 while ((move = mp.next_move<false>()) != MOVE_NONE)
613 if (pos.legal(move, ci.pinned))
615 ss->currentMove = move;
616 pos.do_move(move, st, ci, pos.gives_check(move, ci));
617 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
624 // Step 10. Internal iterative deepening (skipped when in check)
625 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
627 && (PvNode || ss->staticEval + 256 >= beta))
629 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
630 ss->skipNullMove = true;
631 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
632 ss->skipNullMove = false;
634 tte = TT.probe(posKey);
635 ttMove = tte ? tte->move() : MOVE_NONE;
638 moves_loop: // When in check and at SpNode search starts from here
640 Square prevMoveSq = to_sq((ss-1)->currentMove);
641 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
642 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
644 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
645 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
646 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
648 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
650 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
651 improving = ss->staticEval >= (ss-2)->staticEval
652 || ss->staticEval == VALUE_NONE
653 ||(ss-2)->staticEval == VALUE_NONE;
655 singularExtensionNode = !RootNode
657 && depth >= 8 * ONE_PLY
658 && ttMove != MOVE_NONE
659 /* && ttValue != VALUE_NONE Already implicit in the next condition */
660 && abs(ttValue) < VALUE_KNOWN_WIN
661 && !excludedMove // Recursive singular search is not allowed
662 && (tte->bound() & BOUND_LOWER)
663 && tte->depth() >= depth - 3 * ONE_PLY;
665 // Step 11. Loop through moves
666 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
667 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
671 if (move == excludedMove)
674 // At root obey the "searchmoves" option and skip moves not listed in Root
675 // Move List. As a consequence any illegal move is also skipped. In MultiPV
676 // mode we also skip PV moves which have been already searched.
677 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
682 // Shared counter cannot be decremented later if the move turns out to be illegal
683 if (!pos.legal(move, ci.pinned))
686 moveCount = ++splitPoint->moveCount;
687 splitPoint->mutex.unlock();
694 Signals.firstRootMove = (moveCount == 1);
696 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
697 sync_cout << "info depth " << depth
698 << " currmove " << move_to_uci(move, pos.is_chess960())
699 << " 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 /* && move != ttMove Already implicit in the next condition */
747 && bestValue > VALUE_MATED_IN_MAX_PLY)
749 // Move count based pruning
750 if ( depth < 16 * ONE_PLY
751 && moveCount >= FutilityMoveCounts[improving][depth])
754 splitPoint->mutex.lock();
759 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
761 // Futility pruning: parent node
762 if (predictedDepth < 7 * ONE_PLY)
764 futilityValue = ss->staticEval + futility_margin(predictedDepth)
765 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
767 if (futilityValue <= alpha)
769 bestValue = std::max(bestValue, futilityValue);
773 splitPoint->mutex.lock();
774 if (bestValue > splitPoint->bestValue)
775 splitPoint->bestValue = bestValue;
781 // Prune moves with negative SEE at low depths
782 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
785 splitPoint->mutex.lock();
791 // Speculative prefetch
792 prefetch((char*)TT.first_entry(pos.hash_after_move(move)));
794 // Check for legality just before making the move
795 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
801 pvMove = PvNode && moveCount == 1;
802 ss->currentMove = move;
803 if (!SpNode && !captureOrPromotion && quietCount < 64)
804 quietsSearched[quietCount++] = move;
806 // Step 14. Make the move
807 pos.do_move(move, st, ci, givesCheck);
809 // Step 15. Reduced depth search (LMR). If the move fails high it will be
810 // re-searched at full depth.
811 if ( depth >= 3 * ONE_PLY
813 && !captureOrPromotion
815 && move != ss->killers[0]
816 && move != ss->killers[1])
818 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
820 if ( (!PvNode && cutNode)
821 || History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
822 ss->reduction += ONE_PLY;
824 if (move == countermoves[0] || move == countermoves[1])
825 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
827 // Decrease reduction for moves that escape a capture
829 && type_of(move) == NORMAL
830 && type_of(pos.piece_on(to_sq(move))) != PAWN
831 && pos.see(make_move(to_sq(move), from_sq(move))) < 0)
832 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
834 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
836 alpha = splitPoint->alpha;
838 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
840 // Re-search at intermediate depth if reduction is very high
841 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
843 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
844 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
847 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
848 ss->reduction = DEPTH_ZERO;
851 doFullDepthSearch = !pvMove;
853 // Step 16. Full depth search, when LMR is skipped or fails high
854 if (doFullDepthSearch)
857 alpha = splitPoint->alpha;
859 value = newDepth < ONE_PLY ?
860 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
861 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
862 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
865 // For PV nodes only, do a full PV search on the first move or after a fail
866 // high (in the latter case search only if value < beta), otherwise let the
867 // parent node fail low with value <= alpha and to try another move.
868 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
869 value = newDepth < ONE_PLY ?
870 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
871 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
872 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
873 // Step 17. Undo move
876 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
878 // Step 18. Check for new best move
881 splitPoint->mutex.lock();
882 bestValue = splitPoint->bestValue;
883 alpha = splitPoint->alpha;
886 // Finished searching the move. If a stop or a cutoff occurred, the return
887 // value of the search cannot be trusted, and we return immediately without
888 // updating best move, PV and TT.
889 if (Signals.stop || thisThread->cutoff_occurred())
894 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
896 // PV move or new best move ?
897 if (pvMove || value > alpha)
900 rm.extract_pv_from_tt(pos);
902 // We record how often the best move has been changed in each
903 // iteration. This information is used for time management: When
904 // the best move changes frequently, we allocate some more time.
909 // All other moves but the PV are set to the lowest value: this is
910 // not a problem when sorting because the sort is stable and the
911 // move position in the list is preserved - just the PV is pushed up.
912 rm.score = -VALUE_INFINITE;
915 if (value > bestValue)
917 bestValue = SpNode ? splitPoint->bestValue = value : value;
921 bestMove = SpNode ? splitPoint->bestMove = move : move;
923 if (PvNode && value < beta) // Update alpha! Always alpha < beta
924 alpha = SpNode ? splitPoint->alpha = value : value;
927 assert(value >= beta); // Fail high
930 splitPoint->cutoff = true;
937 // Step 19. Check for splitting the search
939 && Threads.size() >= 2
940 && depth >= Threads.minimumSplitDepth
941 && ( !thisThread->activeSplitPoint
942 || !thisThread->activeSplitPoint->allSlavesSearching)
943 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
945 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
947 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
948 depth, moveCount, &mp, NT, cutNode);
950 if (Signals.stop || thisThread->cutoff_occurred())
953 if (bestValue >= beta)
961 // Following condition would detect a stop or a cutoff set only after move
962 // loop has been completed. But in this case bestValue is valid because we
963 // have fully searched our subtree, and we can anyhow save the result in TT.
965 if (Signals.stop || thisThread->cutoff_occurred())
969 // Step 20. Check for mate and stalemate
970 // All legal moves have been searched and if there are no legal moves, it
971 // must be mate or stalemate. If we are in a singular extension search then
972 // return a fail low score.
974 bestValue = excludedMove ? alpha
975 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
977 // Quiet best move: update killers, history, countermoves and followupmoves
978 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
979 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
981 TT.store(posKey, value_to_tt(bestValue, ss->ply),
982 bestValue >= beta ? BOUND_LOWER :
983 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
984 depth, bestMove, ss->staticEval);
986 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
992 // qsearch() is the quiescence search function, which is called by the main
993 // search function when the remaining depth is zero (or, to be more precise,
994 // less than ONE_PLY).
996 template <NodeType NT, bool InCheck>
997 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
999 const bool PvNode = NT == PV;
1001 assert(NT == PV || NT == NonPV);
1002 assert(InCheck == !!pos.checkers());
1003 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1004 assert(PvNode || (alpha == beta - 1));
1005 assert(depth <= DEPTH_ZERO);
1010 Move ttMove, move, bestMove;
1011 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1012 bool givesCheck, evasionPrunable;
1015 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1019 ss->currentMove = bestMove = MOVE_NONE;
1020 ss->ply = (ss-1)->ply + 1;
1022 // Check for an instant draw or if the maximum ply has been reached
1023 if (pos.is_draw() || ss->ply > MAX_PLY)
1024 return ss->ply > MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1026 // Decide whether or not to include checks: this fixes also the type of
1027 // TT entry depth that we are going to use. Note that in qsearch we use
1028 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1029 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1030 : DEPTH_QS_NO_CHECKS;
1032 // Transposition table lookup
1034 tte = TT.probe(posKey);
1035 ttMove = tte ? tte->move() : MOVE_NONE;
1036 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1039 && tte->depth() >= ttDepth
1040 && ttValue != VALUE_NONE // Only in case of TT access race
1041 && ( PvNode ? tte->bound() == BOUND_EXACT
1042 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1043 : (tte->bound() & BOUND_UPPER)))
1045 ss->currentMove = ttMove; // Can be MOVE_NONE
1049 // Evaluate the position statically
1052 ss->staticEval = VALUE_NONE;
1053 bestValue = futilityBase = -VALUE_INFINITE;
1059 // Never assume anything on values stored in TT
1060 if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
1061 ss->staticEval = bestValue = evaluate(pos);
1063 // Can ttValue be used as a better position evaluation?
1064 if (ttValue != VALUE_NONE)
1065 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1066 bestValue = ttValue;
1069 ss->staticEval = bestValue =
1070 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1072 // Stand pat. Return immediately if static value is at least beta
1073 if (bestValue >= beta)
1076 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1077 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1082 if (PvNode && bestValue > alpha)
1085 futilityBase = bestValue + 128;
1088 // Initialize a MovePicker object for the current position, and prepare
1089 // to search the moves. Because the depth is <= 0 here, only captures,
1090 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1092 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1095 // Loop through the moves until no moves remain or a beta cutoff occurs
1096 while ((move = mp.next_move<false>()) != MOVE_NONE)
1098 assert(is_ok(move));
1100 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1101 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1102 : pos.gives_check(move, ci);
1109 && futilityBase > -VALUE_KNOWN_WIN
1110 && !pos.advanced_pawn_push(move))
1112 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1114 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1116 if (futilityValue < beta)
1118 bestValue = std::max(bestValue, futilityValue);
1122 if (futilityBase < beta && pos.see(move) <= VALUE_ZERO)
1124 bestValue = std::max(bestValue, futilityBase);
1129 // Detect non-capture evasions that are candidates to be pruned
1130 evasionPrunable = InCheck
1131 && bestValue > VALUE_MATED_IN_MAX_PLY
1132 && !pos.capture(move)
1133 && !pos.can_castle(pos.side_to_move());
1135 // Don't search moves with negative SEE values
1137 && (!InCheck || evasionPrunable)
1139 && type_of(move) != PROMOTION
1140 && pos.see_sign(move) < VALUE_ZERO)
1143 // Speculative prefetch
1144 prefetch((char*)TT.first_entry(pos.hash_after_move(move)));
1146 // Check for legality just before making the move
1147 if (!pos.legal(move, ci.pinned))
1150 ss->currentMove = move;
1152 // Make and search the move
1153 pos.do_move(move, st, ci, givesCheck);
1154 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1155 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1156 pos.undo_move(move);
1158 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1160 // Check for new best move
1161 if (value > bestValue)
1167 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1174 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1175 ttDepth, move, ss->staticEval);
1183 // All legal moves have been searched. A special case: If we're in check
1184 // and no legal moves were found, it is checkmate.
1185 if (InCheck && bestValue == -VALUE_INFINITE)
1186 return mated_in(ss->ply); // Plies to mate from the root
1188 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1189 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1190 ttDepth, bestMove, ss->staticEval);
1192 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1198 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1199 // "plies to mate from the current position". Non-mate scores are unchanged.
1200 // The function is called before storing a value in the transposition table.
1202 Value value_to_tt(Value v, int ply) {
1204 assert(v != VALUE_NONE);
1206 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1207 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1211 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1212 // from the transposition table (which refers to the plies to mate/be mated
1213 // from current position) to "plies to mate/be mated from the root".
1215 Value value_from_tt(Value v, int ply) {
1217 return v == VALUE_NONE ? VALUE_NONE
1218 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1219 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1223 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1226 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1228 if (ss->killers[0] != move)
1230 ss->killers[1] = ss->killers[0];
1231 ss->killers[0] = move;
1234 // Increase history value of the cut-off move and decrease all the other
1235 // played quiet moves.
1236 Value bonus = Value(4 * int(depth) * int(depth));
1237 History.update(pos.moved_piece(move), to_sq(move), bonus);
1238 for (int i = 0; i < quietsCnt; ++i)
1241 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1244 if (is_ok((ss-1)->currentMove))
1246 Square prevMoveSq = to_sq((ss-1)->currentMove);
1247 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1250 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1252 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1253 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1258 // When playing with a strength handicap, choose best move among the first 'candidates'
1259 // RootMoves using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1261 Move Skill::pick_move() {
1265 // PRNG sequence should be not deterministic
1266 for (int i = Time::now() % 50; i > 0; --i)
1267 rk.rand<unsigned>();
1269 // RootMoves are already sorted by score in descending order
1270 int variance = std::min(RootMoves[0].score - RootMoves[candidates - 1].score, PawnValueMg);
1271 int weakness = 120 - 2 * level;
1272 int max_s = -VALUE_INFINITE;
1275 // Choose best move. For each move score we add two terms both dependent on
1276 // weakness. One deterministic and bigger for weaker moves, and one random,
1277 // then we choose the move with the resulting highest score.
1278 for (size_t i = 0; i < candidates; ++i)
1280 int s = RootMoves[i].score;
1282 // Don't allow crazy blunders even at very low skills
1283 if (i > 0 && RootMoves[i - 1].score > s + 2 * PawnValueMg)
1286 // This is our magic formula
1287 s += ( weakness * int(RootMoves[0].score - s)
1288 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1293 best = RootMoves[i].pv[0];
1300 // uci_pv() formats PV information according to the UCI protocol. UCI
1301 // requires that all (if any) unsearched PV lines are sent using a previous
1304 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1306 std::stringstream ss;
1307 Time::point elapsed = Time::now() - SearchTime + 1;
1308 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1311 for (size_t i = 0; i < Threads.size(); ++i)
1312 if (Threads[i]->maxPly > selDepth)
1313 selDepth = Threads[i]->maxPly;
1315 for (size_t i = 0; i < uciPVSize; ++i)
1317 bool updated = (i <= PVIdx);
1319 if (depth == 1 && !updated)
1322 int d = updated ? depth : depth - 1;
1323 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1325 if (ss.rdbuf()->in_avail()) // Not at first line
1328 ss << "info depth " << d
1329 << " seldepth " << selDepth
1330 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1331 << " nodes " << pos.nodes_searched()
1332 << " nps " << pos.nodes_searched() * 1000 / elapsed
1333 << " time " << elapsed
1334 << " multipv " << i + 1
1337 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; ++j)
1338 ss << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1347 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1348 /// We also consider both failing high nodes and BOUND_EXACT nodes here to
1349 /// ensure that we have a ponder move even when we fail high at root. This
1350 /// results in a long PV to print that is important for position analysis.
1352 void RootMove::extract_pv_from_tt(Position& pos) {
1354 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1356 int ply = 1; // At root ply is 1...
1357 Move m = pv[0]; // ...instead pv[] array starts from 0
1358 Value expectedScore = score;
1365 assert(MoveList<LEGAL>(pos).contains(pv[ply - 1]));
1367 pos.do_move(pv[ply++ - 1], *st++);
1368 tte = TT.probe(pos.key());
1369 expectedScore = -expectedScore;
1372 && expectedScore == value_from_tt(tte->value(), ply)
1373 && pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
1374 && pos.legal(m, pos.pinned_pieces(pos.side_to_move()))
1376 && (!pos.is_draw() || ply <= 2));
1378 pv.push_back(MOVE_NONE); // Must be zero-terminating
1380 while (--ply) pos.undo_move(pv[ply - 1]);
1384 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1385 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1386 /// first, even if the old TT entries have been overwritten.
1388 void RootMove::insert_pv_in_tt(Position& pos) {
1390 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1392 int idx = 0; // Ply starts from 1, we need to start from 0
1395 tte = TT.probe(pos.key());
1397 if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
1398 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
1400 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1402 pos.do_move(pv[idx++], *st++);
1404 } while (pv[idx] != MOVE_NONE);
1406 while (idx) pos.undo_move(pv[--idx]);
1410 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1412 void Thread::idle_loop() {
1414 // Pointer 'this_sp' is not null only if we are called from split(), and not
1415 // at the thread creation. This means we are the split point's master.
1416 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1418 assert(!this_sp || (this_sp->masterThread == this && searching));
1422 // If this thread has been assigned work, launch a search
1425 Threads.mutex.lock();
1427 assert(activeSplitPoint);
1428 SplitPoint* sp = activeSplitPoint;
1430 Threads.mutex.unlock();
1432 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
1433 Position pos(*sp->pos, this);
1435 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1436 ss->splitPoint = sp;
1440 assert(activePosition == NULL);
1442 activePosition = &pos;
1444 if (sp->nodeType == NonPV)
1445 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1447 else if (sp->nodeType == PV)
1448 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1450 else if (sp->nodeType == Root)
1451 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1459 activePosition = NULL;
1460 sp->slavesMask.reset(idx);
1461 sp->allSlavesSearching = false;
1462 sp->nodes += pos.nodes_searched();
1464 // Wake up the master thread so to allow it to return from the idle
1465 // loop in case we are the last slave of the split point.
1466 if ( this != sp->masterThread
1467 && sp->slavesMask.none())
1469 assert(!sp->masterThread->searching);
1470 sp->masterThread->notify_one();
1473 // After releasing the lock we can't access any SplitPoint related data
1474 // in a safe way because it could have been released under our feet by
1478 // Try to late join to another split point if none of its slaves has
1479 // already finished.
1480 if (Threads.size() > 2)
1481 for (size_t i = 0; i < Threads.size(); ++i)
1483 const int size = Threads[i]->splitPointsSize; // Local copy
1484 sp = size ? &Threads[i]->splitPoints[size - 1] : NULL;
1487 && sp->allSlavesSearching
1488 && available_to(Threads[i]))
1490 // Recheck the conditions under lock protection
1491 Threads.mutex.lock();
1494 if ( sp->allSlavesSearching
1495 && available_to(Threads[i]))
1497 sp->slavesMask.set(idx);
1498 activeSplitPoint = sp;
1503 Threads.mutex.unlock();
1505 break; // Just a single attempt
1510 // Grab the lock to avoid races with Thread::notify_one()
1513 // If we are master and all slaves have finished then exit idle_loop
1514 if (this_sp && this_sp->slavesMask.none())
1521 // If we are not searching, wait for a condition to be signaled instead of
1522 // wasting CPU time polling for work.
1523 if (!searching && !exit)
1524 sleepCondition.wait(mutex);
1531 /// check_time() is called by the timer thread when the timer triggers. It is
1532 /// used to print debug info and, more importantly, to detect when we are out of
1533 /// available time and thus stop the search.
1537 static Time::point lastInfoTime = Time::now();
1538 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1540 if (Time::now() - lastInfoTime >= 1000)
1542 lastInfoTime = Time::now();
1551 Threads.mutex.lock();
1553 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();
1576 Time::point elapsed = Time::now() - SearchTime;
1577 bool stillAtFirstMove = Signals.firstRootMove
1578 && !Signals.failedLowAtRoot
1579 && elapsed > TimeMgr.available_time() * 75 / 100;
1581 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution
1582 || stillAtFirstMove;
1584 if ( (Limits.use_time_management() && noMoreTime)
1585 || (Limits.movetime && elapsed >= Limits.movetime)
1586 || (Limits.nodes && nodes >= Limits.nodes))
1587 Signals.stop = true;