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_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
91 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
94 Skill(int l, size_t rootSize) : level(l),
95 candidates(l < 20 ? std::min(4, (int)rootSize) : 0),
98 if (candidates) // Swap best PV line with the sub-optimal one
99 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
100 RootMoves.end(), best ? best : pick_move()));
103 size_t candidates_size() const { return candidates; }
104 bool time_to_pick(int depth) const { return depth == 1 + level; }
115 /// Search::init() is called during startup to initialize various lookup tables
117 void Search::init() {
119 // Init reductions array
120 for (int d = 1; d < 64; ++d)
121 for (int mc = 1; mc < 64; ++mc)
123 double pvRed = 0.00 + log(double(d)) * log(double(mc)) / 3.00;
124 double nonPVRed = 0.33 + log(double(d)) * log(double(mc)) / 2.25;
126 Reductions[1][1][d][mc] = int8_t( pvRed >= 1.0 ? pvRed + 0.5: 0);
127 Reductions[0][1][d][mc] = int8_t(nonPVRed >= 1.0 ? nonPVRed + 0.5: 0);
129 Reductions[1][0][d][mc] = Reductions[1][1][d][mc];
130 Reductions[0][0][d][mc] = Reductions[0][1][d][mc];
132 // Increase reduction when eval is not improving
133 if (Reductions[0][0][d][mc] >= 2)
134 Reductions[0][0][d][mc] += 1;
137 // Init futility move count array
138 for (int d = 0; d < 16; ++d)
140 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
141 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
146 /// Search::perft() is our utility to verify move generation. All the leaf nodes
147 /// up to the given depth are generated and counted and the sum returned.
149 uint64_t Search::perft(Position& pos, Depth depth) {
152 uint64_t cnt, nodes = 0;
154 const bool leaf = (depth == 2 * ONE_PLY);
156 for (MoveList<LEGAL> it(pos); *it; ++it)
158 if (Root && depth <= ONE_PLY)
162 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
163 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
168 sync_cout << UCI::format_move(*it, pos.is_chess960()) << ": " << cnt << sync_endl;
173 template uint64_t Search::perft<true>(Position& pos, Depth depth);
176 /// Search::think() is the external interface to Stockfish's search, and is
177 /// called by the main thread when the program receives the UCI 'go' command. It
178 /// searches from RootPos and at the end prints the "bestmove" to output.
180 void Search::think() {
182 TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
184 int cf = Options["Contempt"] * PawnValueEg / 100; // From centipawns
185 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
186 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(cf);
188 if (RootMoves.empty())
190 RootMoves.push_back(MOVE_NONE);
191 sync_cout << "info depth 0 score "
192 << UCI::format_value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
198 // Reset the threads, still sleeping: will wake up at split time
199 for (size_t i = 0; i < Threads.size(); ++i)
200 Threads[i]->maxPly = 0;
202 Threads.timer->run = true;
203 Threads.timer->notify_one(); // Wake up the recurring timer
205 id_loop(RootPos); // Let's start searching !
207 Threads.timer->run = false; // Stop the timer
211 // When search is stopped this info is not printed
212 sync_cout << "info nodes " << RootPos.nodes_searched()
213 << " time " << Time::now() - SearchTime + 1 << sync_endl;
215 // When we reach the maximum depth, we can arrive here without a raise of
216 // Signals.stop. However, if we are pondering or in an infinite search,
217 // the UCI protocol states that we shouldn't print the best move before the
218 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
219 // until the GUI sends one of those commands (which also raises Signals.stop).
220 if (!Signals.stop && (Limits.ponder || Limits.infinite))
222 Signals.stopOnPonderhit = true;
223 RootPos.this_thread()->wait_for(Signals.stop);
226 // Best move could be MOVE_NONE when searching on a stalemate position
227 sync_cout << "bestmove " << UCI::format_move(RootMoves[0].pv[0], RootPos.is_chess960())
228 << " ponder " << UCI::format_move(RootMoves[0].pv[1], RootPos.is_chess960())
235 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
236 // with increasing depth until the allocated thinking time has been consumed,
237 // user stops the search, or the maximum search depth is reached.
239 void id_loop(Position& pos) {
241 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
243 Value bestValue, alpha, beta, delta;
245 std::memset(ss-2, 0, 5 * sizeof(Stack));
249 bestValue = delta = alpha = -VALUE_INFINITE;
250 beta = VALUE_INFINITE;
255 Countermoves.clear();
256 Followupmoves.clear();
258 size_t multiPV = Options["MultiPV"];
259 Skill skill(Options["Skill Level"], RootMoves.size());
261 // Do we have to play with skill handicap? In this case enable MultiPV search
262 // that we will use behind the scenes to retrieve a set of possible moves.
263 multiPV = std::max(multiPV, skill.candidates_size());
265 // Iterative deepening loop until requested to stop or target depth reached
266 while (++depth < MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
268 // Age out PV variability metric
269 BestMoveChanges *= 0.5;
271 // Save the last iteration's scores before first PV line is searched and
272 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
273 for (size_t i = 0; i < RootMoves.size(); ++i)
274 RootMoves[i].prevScore = RootMoves[i].score;
276 // MultiPV loop. We perform a full root search for each PV line
277 for (PVIdx = 0; PVIdx < std::min(multiPV, RootMoves.size()) && !Signals.stop; ++PVIdx)
279 // Reset aspiration window starting size
283 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
284 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
287 // Start with a small aspiration window and, in the case of a fail
288 // high/low, re-search with a bigger window until we're not failing
292 bestValue = search<Root, false>(pos, ss, alpha, beta, depth * ONE_PLY, false);
294 // Bring the best move to the front. It is critical that sorting
295 // is done with a stable algorithm because all the values but the
296 // first and eventually the new best one are set to -VALUE_INFINITE
297 // and we want to keep the same order for all the moves except the
298 // new PV that goes to the front. Note that in case of MultiPV
299 // search the already searched PV lines are preserved.
300 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
302 // Write PV back to transposition table in case the relevant
303 // entries have been overwritten during the search.
304 for (size_t i = 0; i <= PVIdx; ++i)
305 RootMoves[i].insert_pv_in_tt(pos);
307 // If search has been stopped break immediately. Sorting and
308 // writing PV back to TT is safe because RootMoves is still
309 // valid, although it refers to previous iteration.
313 // When failing high/low give some update (without cluttering
314 // the UI) before a re-search.
315 if ( (bestValue <= alpha || bestValue >= beta)
316 && Time::now() - SearchTime > 3000)
317 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
319 // In case of failing low/high increase aspiration window and
320 // re-search, otherwise exit the loop.
321 if (bestValue <= alpha)
323 beta = (alpha + beta) / 2;
324 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
326 Signals.failedLowAtRoot = true;
327 Signals.stopOnPonderhit = false;
329 else if (bestValue >= beta)
331 alpha = (alpha + beta) / 2;
332 beta = std::min(bestValue + delta, VALUE_INFINITE);
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);
346 && ( PVIdx + 1 == std::min(multiPV, RootMoves.size())
347 || Time::now() - SearchTime > 3000))
348 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
351 // If skill levels are enabled and time is up, pick a sub-optimal best move
352 if (skill.candidates_size() && skill.time_to_pick(depth))
355 // Have we found a "mate in x"?
357 && bestValue >= VALUE_MATE_IN_MAX_PLY
358 && VALUE_MATE - bestValue <= 2 * Limits.mate)
361 // Do we have time for the next iteration? Can we stop searching now?
362 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
364 // Take some extra time if the best move has changed
365 if (depth > 4 && multiPV == 1)
366 TimeMgr.pv_instability(BestMoveChanges);
368 // Stop the search if only one legal move is available or all
369 // of the available time has been used.
370 if ( RootMoves.size() == 1
371 || Time::now() - SearchTime > TimeMgr.available_time())
373 // If we are allowed to ponder do not stop the search now but
374 // keep pondering until the GUI sends "ponderhit" or "stop".
376 Signals.stopOnPonderhit = true;
385 // search<>() is the main search function for both PV and non-PV nodes and for
386 // normal and SplitPoint nodes. When called just after a split point the search
387 // is simpler because we have already probed the hash table, done a null move
388 // search, and searched the first move before splitting, so we don't have to
389 // repeat all this work again. We also don't need to store anything to the hash
390 // table here: This is taken care of after we return from the split point.
392 template <NodeType NT, bool SpNode>
393 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
395 const bool RootNode = NT == Root;
396 const bool PvNode = NT == PV || NT == Root;
398 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
399 assert(PvNode || (alpha == beta - 1));
400 assert(depth > DEPTH_ZERO);
402 Move quietsSearched[64];
405 SplitPoint* splitPoint;
407 Move ttMove, move, excludedMove, bestMove;
408 Depth ext, newDepth, predictedDepth;
409 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
410 bool inCheck, givesCheck, singularExtensionNode, improving;
411 bool captureOrPromotion, dangerous, doFullDepthSearch;
412 int moveCount, quietCount;
414 // Step 1. Initialize node
415 Thread* thisThread = pos.this_thread();
416 inCheck = pos.checkers();
420 splitPoint = ss->splitPoint;
421 bestMove = splitPoint->bestMove;
422 bestValue = splitPoint->bestValue;
424 ttMove = excludedMove = MOVE_NONE;
425 ttValue = VALUE_NONE;
427 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
432 moveCount = quietCount = 0;
433 bestValue = -VALUE_INFINITE;
434 ss->ply = (ss-1)->ply + 1;
436 // Used to send selDepth info to GUI
437 if (PvNode && thisThread->maxPly < ss->ply)
438 thisThread->maxPly = ss->ply;
442 // Step 2. Check for aborted search and immediate draw
443 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
444 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
446 // Step 3. Mate distance pruning. Even if we mate at the next move our score
447 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
448 // a shorter mate was found upward in the tree then there is no need to search
449 // because we will never beat the current alpha. Same logic but with reversed
450 // signs applies also in the opposite condition of being mated instead of giving
451 // mate. In this case return a fail-high score.
452 alpha = std::max(mated_in(ss->ply), alpha);
453 beta = std::min(mate_in(ss->ply+1), beta);
458 assert(0 <= ss->ply && ss->ply < MAX_PLY);
460 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
461 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
462 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
464 // Step 4. Transposition table lookup
465 // We don't want the score of a partial search to overwrite a previous full search
466 // TT value, so we use a different position key in case of an excluded move.
467 excludedMove = ss->excludedMove;
468 posKey = excludedMove ? pos.exclusion_key() : pos.key();
469 tte = TT.probe(posKey);
470 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
471 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
473 // At PV nodes we check for exact scores, whilst at non-PV nodes we check for
474 // a fail high/low. The biggest advantage to probing at PV nodes is to have a
475 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
476 // we should also update RootMoveList to avoid bogus output.
479 && tte->depth() >= depth
480 && ttValue != VALUE_NONE // Only in case of TT access race
481 && ( PvNode ? tte->bound() == BOUND_EXACT
482 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
483 : (tte->bound() & BOUND_UPPER)))
485 ss->currentMove = ttMove; // Can be MOVE_NONE
487 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
488 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
489 update_stats(pos, ss, ttMove, depth, NULL, 0);
494 // Step 5. Evaluate the position statically and update parent's gain statistics
497 ss->staticEval = eval = VALUE_NONE;
503 // Never assume anything on values stored in TT
504 if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
505 eval = ss->staticEval = evaluate(pos);
507 // Can ttValue be used as a better position evaluation?
508 if (ttValue != VALUE_NONE)
509 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
514 eval = ss->staticEval =
515 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
517 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
520 if ( !pos.captured_piece_type()
521 && ss->staticEval != VALUE_NONE
522 && (ss-1)->staticEval != VALUE_NONE
523 && (move = (ss-1)->currentMove) != MOVE_NULL
525 && type_of(move) == NORMAL)
527 Square to = to_sq(move);
528 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
531 // Step 6. Razoring (skipped when in check)
533 && depth < 4 * ONE_PLY
534 && eval + razor_margin(depth) <= alpha
535 && ttMove == MOVE_NONE
536 && !pos.pawn_on_7th(pos.side_to_move()))
538 if ( depth <= ONE_PLY
539 && eval + razor_margin(3 * ONE_PLY) <= alpha)
540 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
542 Value ralpha = alpha - razor_margin(depth);
543 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
548 // Step 7. Futility pruning: child node (skipped when in check)
551 && depth < 7 * ONE_PLY
552 && eval - futility_margin(depth) >= beta
553 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
554 && pos.non_pawn_material(pos.side_to_move()))
555 return eval - futility_margin(depth);
557 // Step 8. Null move search with verification search (is omitted in PV nodes)
560 && depth >= 2 * ONE_PLY
562 && pos.non_pawn_material(pos.side_to_move()))
564 ss->currentMove = MOVE_NULL;
566 assert(eval - beta >= 0);
568 // Null move dynamic reduction based on depth and value
569 Depth R = (3 + depth / 4 + std::min(int(eval - beta) / PawnValueMg, 3)) * ONE_PLY;
571 pos.do_null_move(st);
572 (ss+1)->skipNullMove = true;
573 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
574 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
575 (ss+1)->skipNullMove = false;
576 pos.undo_null_move();
578 if (nullValue >= beta)
580 // Do not return unproven mate scores
581 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
584 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
587 // Do verification search at high depths
588 ss->skipNullMove = true;
589 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
590 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
591 ss->skipNullMove = false;
598 // Step 9. ProbCut (skipped when in check)
599 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
600 // and a reduced search returns a value much above beta, we can (almost) safely
601 // prune the previous move.
603 && depth >= 5 * ONE_PLY
605 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
607 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
608 Depth rdepth = depth - 4 * ONE_PLY;
610 assert(rdepth >= ONE_PLY);
611 assert((ss-1)->currentMove != MOVE_NONE);
612 assert((ss-1)->currentMove != MOVE_NULL);
614 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
617 while ((move = mp.next_move<false>()) != MOVE_NONE)
618 if (pos.legal(move, ci.pinned))
620 ss->currentMove = move;
621 pos.do_move(move, st, ci, pos.gives_check(move, ci));
622 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
629 // Step 10. Internal iterative deepening (skipped when in check)
630 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
632 && (PvNode || ss->staticEval + 256 >= beta))
634 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
635 ss->skipNullMove = true;
636 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
637 ss->skipNullMove = false;
639 tte = TT.probe(posKey);
640 ttMove = tte ? tte->move() : MOVE_NONE;
643 moves_loop: // When in check and at SpNode search starts from here
645 Square prevMoveSq = to_sq((ss-1)->currentMove);
646 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
647 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
649 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
650 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
651 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
653 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
655 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
656 improving = ss->staticEval >= (ss-2)->staticEval
657 || ss->staticEval == VALUE_NONE
658 ||(ss-2)->staticEval == VALUE_NONE;
660 singularExtensionNode = !RootNode
662 && depth >= 8 * ONE_PLY
663 && ttMove != MOVE_NONE
664 /* && ttValue != VALUE_NONE Already implicit in the next condition */
665 && abs(ttValue) < VALUE_KNOWN_WIN
666 && !excludedMove // Recursive singular search is not allowed
667 && (tte->bound() & BOUND_LOWER)
668 && tte->depth() >= depth - 3 * ONE_PLY;
670 // Step 11. Loop through moves
671 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
672 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
676 if (move == excludedMove)
679 // At root obey the "searchmoves" option and skip moves not listed in Root
680 // Move List. As a consequence any illegal move is also skipped. In MultiPV
681 // mode we also skip PV moves which have been already searched.
682 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
687 // Shared counter cannot be decremented later if the move turns out to be illegal
688 if (!pos.legal(move, ci.pinned))
691 moveCount = ++splitPoint->moveCount;
692 splitPoint->mutex.unlock();
699 Signals.firstRootMove = (moveCount == 1);
701 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
702 sync_cout << "info depth " << depth
703 << " currmove " << UCI::format_move(move, pos.is_chess960())
704 << " currmovenumber " << moveCount + PVIdx << sync_endl;
708 captureOrPromotion = pos.capture_or_promotion(move);
710 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
711 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
712 : pos.gives_check(move, ci);
714 dangerous = givesCheck
715 || type_of(move) != NORMAL
716 || pos.advanced_pawn_push(move);
718 // Step 12. Extend checks
719 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
722 // Singular extension search. If all moves but one fail low on a search of
723 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
724 // is singular and should be extended. To verify this we do a reduced search
725 // on all the other moves but the ttMove and if the result is lower than
726 // ttValue minus a margin then we extend the ttMove.
727 if ( singularExtensionNode
730 && pos.legal(move, ci.pinned))
732 Value rBeta = ttValue - int(2 * depth);
733 ss->excludedMove = move;
734 ss->skipNullMove = true;
735 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
736 ss->skipNullMove = false;
737 ss->excludedMove = MOVE_NONE;
743 // Update the current move (this must be done after singular extension search)
744 newDepth = depth - ONE_PLY + ext;
746 // Step 13. Pruning at shallow depth (exclude PV nodes)
748 && !captureOrPromotion
751 && bestValue > VALUE_MATED_IN_MAX_PLY)
753 // Move count based pruning
754 if ( depth < 16 * ONE_PLY
755 && moveCount >= FutilityMoveCounts[improving][depth])
758 splitPoint->mutex.lock();
763 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
765 // Futility pruning: parent node
766 if (predictedDepth < 7 * ONE_PLY)
768 futilityValue = ss->staticEval + futility_margin(predictedDepth)
769 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
771 if (futilityValue <= alpha)
773 bestValue = std::max(bestValue, futilityValue);
777 splitPoint->mutex.lock();
778 if (bestValue > splitPoint->bestValue)
779 splitPoint->bestValue = bestValue;
785 // Prune moves with negative SEE at low depths
786 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
789 splitPoint->mutex.lock();
795 // Speculative prefetch as early as possible
796 prefetch((char*)TT.first_entry(pos.key_after(move)));
798 // Check for legality just before making the move
799 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
805 ss->currentMove = move;
806 if (!SpNode && !captureOrPromotion && quietCount < 64)
807 quietsSearched[quietCount++] = move;
809 // Step 14. Make the move
810 pos.do_move(move, st, ci, givesCheck);
812 // Step 15. Reduced depth search (LMR). If the move fails high it will be
813 // re-searched at full depth.
814 if ( depth >= 3 * ONE_PLY
816 && !captureOrPromotion
817 && move != ss->killers[0]
818 && move != ss->killers[1])
820 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
822 if ( (!PvNode && cutNode)
823 || History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
824 ss->reduction += ONE_PLY;
826 if (move == countermoves[0] || move == countermoves[1])
827 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
829 // Decrease reduction for moves that escape a capture
831 && type_of(move) == NORMAL
832 && type_of(pos.piece_on(to_sq(move))) != PAWN
833 && pos.see(make_move(to_sq(move), from_sq(move))) < 0)
834 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
836 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
838 alpha = splitPoint->alpha;
840 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
842 // Re-search at intermediate depth if reduction is very high
843 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
845 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
846 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
849 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
850 ss->reduction = DEPTH_ZERO;
853 doFullDepthSearch = !PvNode || moveCount > 1;
855 // Step 16. Full depth search, when LMR is skipped or fails high
856 if (doFullDepthSearch)
859 alpha = splitPoint->alpha;
861 value = newDepth < ONE_PLY ?
862 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
863 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
864 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
867 // For PV nodes only, do a full PV search on the first move or after a fail
868 // high (in the latter case search only if value < beta), otherwise let the
869 // parent node fail low with value <= alpha and to try another move.
870 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
871 value = newDepth < ONE_PLY ?
872 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
873 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
874 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
875 // Step 17. Undo move
878 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
880 // Step 18. Check for new best move
883 splitPoint->mutex.lock();
884 bestValue = splitPoint->bestValue;
885 alpha = splitPoint->alpha;
888 // Finished searching the move. If a stop or a cutoff occurred, the return
889 // value of the search cannot be trusted, and we return immediately without
890 // updating best move, PV and TT.
891 if (Signals.stop || thisThread->cutoff_occurred())
896 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
898 // PV move or new best move ?
899 if (moveCount == 1 || value > alpha)
902 rm.extract_pv_from_tt(pos);
904 // We record how often the best move has been changed in each
905 // iteration. This information is used for time management: When
906 // the best move changes frequently, we allocate some more time.
911 // All other moves but the PV are set to the lowest value: this is
912 // not a problem when sorting because the sort is stable and the
913 // move position in the list is preserved - just the PV is pushed up.
914 rm.score = -VALUE_INFINITE;
917 if (value > bestValue)
919 bestValue = SpNode ? splitPoint->bestValue = value : value;
923 bestMove = SpNode ? splitPoint->bestMove = move : move;
925 if (PvNode && value < beta) // Update alpha! Always alpha < beta
926 alpha = SpNode ? splitPoint->alpha = value : value;
929 assert(value >= beta); // Fail high
932 splitPoint->cutoff = true;
939 // Step 19. Check for splitting the search
941 && Threads.size() >= 2
942 && depth >= Threads.minimumSplitDepth
943 && ( !thisThread->activeSplitPoint
944 || !thisThread->activeSplitPoint->allSlavesSearching)
945 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
947 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
949 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
950 depth, moveCount, &mp, NT, cutNode);
952 if (Signals.stop || thisThread->cutoff_occurred())
955 if (bestValue >= beta)
963 // Following condition would detect a stop or a cutoff set only after move
964 // loop has been completed. But in this case bestValue is valid because we
965 // have fully searched our subtree, and we can anyhow save the result in TT.
967 if (Signals.stop || thisThread->cutoff_occurred())
971 // Step 20. Check for mate and stalemate
972 // All legal moves have been searched and if there are no legal moves, it
973 // must be mate or stalemate. If we are in a singular extension search then
974 // return a fail low score.
976 bestValue = excludedMove ? alpha
977 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
979 // Quiet best move: update killers, history, countermoves and followupmoves
980 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
981 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
983 TT.store(posKey, value_to_tt(bestValue, ss->ply),
984 bestValue >= beta ? BOUND_LOWER :
985 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
986 depth, bestMove, ss->staticEval);
988 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
994 // qsearch() is the quiescence search function, which is called by the main
995 // search function when the remaining depth is zero (or, to be more precise,
996 // less than ONE_PLY).
998 template <NodeType NT, bool InCheck>
999 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1001 const bool PvNode = NT == PV;
1003 assert(NT == PV || NT == NonPV);
1004 assert(InCheck == !!pos.checkers());
1005 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1006 assert(PvNode || (alpha == beta - 1));
1007 assert(depth <= DEPTH_ZERO);
1012 Move ttMove, move, bestMove;
1013 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1014 bool givesCheck, evasionPrunable;
1017 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1021 ss->currentMove = bestMove = MOVE_NONE;
1022 ss->ply = (ss-1)->ply + 1;
1024 // Check for an instant draw or if the maximum ply has been reached
1025 if (pos.is_draw() || ss->ply >= MAX_PLY)
1026 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1028 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1030 // Decide whether or not to include checks: this fixes also the type of
1031 // TT entry depth that we are going to use. Note that in qsearch we use
1032 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1033 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1034 : DEPTH_QS_NO_CHECKS;
1036 // Transposition table lookup
1038 tte = TT.probe(posKey);
1039 ttMove = tte ? tte->move() : MOVE_NONE;
1040 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1043 && tte->depth() >= ttDepth
1044 && ttValue != VALUE_NONE // Only in case of TT access race
1045 && ( PvNode ? tte->bound() == BOUND_EXACT
1046 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1047 : (tte->bound() & BOUND_UPPER)))
1049 ss->currentMove = ttMove; // Can be MOVE_NONE
1053 // Evaluate the position statically
1056 ss->staticEval = VALUE_NONE;
1057 bestValue = futilityBase = -VALUE_INFINITE;
1063 // Never assume anything on values stored in TT
1064 if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
1065 ss->staticEval = bestValue = evaluate(pos);
1067 // Can ttValue be used as a better position evaluation?
1068 if (ttValue != VALUE_NONE)
1069 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1070 bestValue = ttValue;
1073 ss->staticEval = bestValue =
1074 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1076 // Stand pat. Return immediately if static value is at least beta
1077 if (bestValue >= beta)
1080 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1081 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1086 if (PvNode && bestValue > alpha)
1089 futilityBase = bestValue + 128;
1092 // Initialize a MovePicker object for the current position, and prepare
1093 // to search the moves. Because the depth is <= 0 here, only captures,
1094 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1096 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1099 // Loop through the moves until no moves remain or a beta cutoff occurs
1100 while ((move = mp.next_move<false>()) != MOVE_NONE)
1102 assert(is_ok(move));
1104 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1105 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1106 : pos.gives_check(move, ci);
1112 && futilityBase > -VALUE_KNOWN_WIN
1113 && !pos.advanced_pawn_push(move))
1115 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1117 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1119 if (futilityValue < beta)
1121 bestValue = std::max(bestValue, futilityValue);
1125 if (futilityBase < beta && pos.see(move) <= VALUE_ZERO)
1127 bestValue = std::max(bestValue, futilityBase);
1132 // Detect non-capture evasions that are candidates to be pruned
1133 evasionPrunable = InCheck
1134 && bestValue > VALUE_MATED_IN_MAX_PLY
1135 && !pos.capture(move)
1136 && !pos.can_castle(pos.side_to_move());
1138 // Don't search moves with negative SEE values
1140 && (!InCheck || evasionPrunable)
1141 && type_of(move) != PROMOTION
1142 && pos.see_sign(move) < VALUE_ZERO)
1145 // Speculative prefetch as early as possible
1146 prefetch((char*)TT.first_entry(pos.key_after(move)));
1148 // Check for legality just before making the move
1149 if (!pos.legal(move, ci.pinned))
1152 ss->currentMove = move;
1154 // Make and search the move
1155 pos.do_move(move, st, ci, givesCheck);
1156 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1157 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1158 pos.undo_move(move);
1160 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1162 // Check for new best move
1163 if (value > bestValue)
1169 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1176 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1177 ttDepth, move, ss->staticEval);
1185 // All legal moves have been searched. A special case: If we're in check
1186 // and no legal moves were found, it is checkmate.
1187 if (InCheck && bestValue == -VALUE_INFINITE)
1188 return mated_in(ss->ply); // Plies to mate from the root
1190 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1191 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1192 ttDepth, bestMove, ss->staticEval);
1194 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1200 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1201 // "plies to mate from the current position". Non-mate scores are unchanged.
1202 // The function is called before storing a value in the transposition table.
1204 Value value_to_tt(Value v, int ply) {
1206 assert(v != VALUE_NONE);
1208 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1209 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1213 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1214 // from the transposition table (which refers to the plies to mate/be mated
1215 // from current position) to "plies to mate/be mated from the root".
1217 Value value_from_tt(Value v, int ply) {
1219 return v == VALUE_NONE ? VALUE_NONE
1220 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1221 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1225 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1228 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1230 if (ss->killers[0] != move)
1232 ss->killers[1] = ss->killers[0];
1233 ss->killers[0] = move;
1236 // Increase history value of the cut-off move and decrease all the other
1237 // played quiet moves.
1238 Value bonus = Value(4 * int(depth) * int(depth));
1239 History.update(pos.moved_piece(move), to_sq(move), bonus);
1240 for (int i = 0; i < quietsCnt; ++i)
1243 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1246 if (is_ok((ss-1)->currentMove))
1248 Square prevMoveSq = to_sq((ss-1)->currentMove);
1249 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1252 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1254 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1255 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1260 // When playing with a strength handicap, choose best move among the first 'candidates'
1261 // RootMoves using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1263 Move Skill::pick_move() {
1267 // PRNG sequence should be not deterministic
1268 for (int i = Time::now() % 50; i > 0; --i)
1269 rk.rand<unsigned>();
1271 // RootMoves are already sorted by score in descending order
1272 int variance = std::min(RootMoves[0].score - RootMoves[candidates - 1].score, PawnValueMg);
1273 int weakness = 120 - 2 * level;
1274 int max_s = -VALUE_INFINITE;
1277 // Choose best move. For each move score we add two terms both dependent on
1278 // weakness. One deterministic and bigger for weaker moves, and one random,
1279 // then we choose the move with the resulting highest score.
1280 for (size_t i = 0; i < candidates; ++i)
1282 int s = RootMoves[i].score;
1284 // Don't allow crazy blunders even at very low skills
1285 if (i > 0 && RootMoves[i - 1].score > s + 2 * PawnValueMg)
1288 // This is our magic formula
1289 s += ( weakness * int(RootMoves[0].score - s)
1290 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1295 best = RootMoves[i].pv[0];
1302 // uci_pv() formats PV information according to the UCI protocol. UCI
1303 // requires that all (if any) unsearched PV lines are sent using a previous
1306 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1308 std::stringstream ss;
1309 Time::point elapsed = Time::now() - SearchTime + 1;
1310 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1313 for (size_t i = 0; i < Threads.size(); ++i)
1314 if (Threads[i]->maxPly > selDepth)
1315 selDepth = Threads[i]->maxPly;
1317 for (size_t i = 0; i < uciPVSize; ++i)
1319 bool updated = (i <= PVIdx);
1321 if (depth == 1 && !updated)
1324 int d = updated ? depth : depth - 1;
1325 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1327 if (ss.rdbuf()->in_avail()) // Not at first line
1330 ss << "info depth " << d
1331 << " seldepth " << selDepth
1332 << " multipv " << i + 1
1333 << " score " << (i == PVIdx ? UCI::format_value(v, alpha, beta) : UCI::format_value(v))
1334 << " nodes " << pos.nodes_searched()
1335 << " nps " << pos.nodes_searched() * 1000 / elapsed
1336 << " time " << elapsed
1339 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; ++j)
1340 ss << " " << UCI::format_move(RootMoves[i].pv[j], pos.is_chess960());
1349 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1350 /// We also consider both failing high nodes and BOUND_EXACT nodes here to
1351 /// ensure that we have a ponder move even when we fail high at root. This
1352 /// results in a long PV to print that is important for position analysis.
1354 void RootMove::extract_pv_from_tt(Position& pos) {
1356 StateInfo state[MAX_PLY], *st = state;
1358 int ply = 1; // At root ply is 1...
1359 Move m = pv[0]; // ...instead pv[] array starts from 0
1360 Value expectedScore = score;
1367 assert(MoveList<LEGAL>(pos).contains(pv[ply - 1]));
1369 pos.do_move(pv[ply++ - 1], *st++);
1370 tte = TT.probe(pos.key());
1371 expectedScore = -expectedScore;
1374 && expectedScore == value_from_tt(tte->value(), ply)
1375 && pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
1376 && pos.legal(m, pos.pinned_pieces(pos.side_to_move()))
1378 && (!pos.is_draw() || ply <= 2));
1380 pv.push_back(MOVE_NONE); // Must be zero-terminating
1382 while (--ply) pos.undo_move(pv[ply - 1]);
1386 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1387 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1388 /// first, even if the old TT entries have been overwritten.
1390 void RootMove::insert_pv_in_tt(Position& pos) {
1392 StateInfo state[MAX_PLY], *st = state;
1394 int idx = 0; // Ply starts from 1, we need to start from 0
1397 tte = TT.probe(pos.key());
1399 if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
1400 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
1402 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1404 pos.do_move(pv[idx++], *st++);
1406 } while (pv[idx] != MOVE_NONE);
1408 while (idx) pos.undo_move(pv[--idx]);
1412 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1414 void Thread::idle_loop() {
1416 // Pointer 'this_sp' is not null only if we are called from split(), and not
1417 // at the thread creation. This means we are the split point's master.
1418 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1420 assert(!this_sp || (this_sp->masterThread == this && searching));
1424 // If this thread has been assigned work, launch a search
1427 Threads.mutex.lock();
1429 assert(activeSplitPoint);
1430 SplitPoint* sp = activeSplitPoint;
1432 Threads.mutex.unlock();
1434 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1435 Position pos(*sp->pos, this);
1437 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1438 ss->splitPoint = sp;
1442 assert(activePosition == NULL);
1444 activePosition = &pos;
1446 if (sp->nodeType == NonPV)
1447 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1449 else if (sp->nodeType == PV)
1450 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1452 else if (sp->nodeType == Root)
1453 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1461 activePosition = NULL;
1462 sp->slavesMask.reset(idx);
1463 sp->allSlavesSearching = false;
1464 sp->nodes += pos.nodes_searched();
1466 // Wake up the master thread so to allow it to return from the idle
1467 // loop in case we are the last slave of the split point.
1468 if ( this != sp->masterThread
1469 && sp->slavesMask.none())
1471 assert(!sp->masterThread->searching);
1472 sp->masterThread->notify_one();
1475 // After releasing the lock we can't access any SplitPoint related data
1476 // in a safe way because it could have been released under our feet by
1480 // Try to late join to another split point if none of its slaves has
1481 // already finished.
1482 if (Threads.size() > 2)
1483 for (size_t i = 0; i < Threads.size(); ++i)
1485 const int size = Threads[i]->splitPointsSize; // Local copy
1486 sp = size ? &Threads[i]->splitPoints[size - 1] : NULL;
1489 && sp->allSlavesSearching
1490 && available_to(Threads[i]))
1492 // Recheck the conditions under lock protection
1493 Threads.mutex.lock();
1496 if ( sp->allSlavesSearching
1497 && available_to(Threads[i]))
1499 sp->slavesMask.set(idx);
1500 activeSplitPoint = sp;
1505 Threads.mutex.unlock();
1507 break; // Just a single attempt
1512 // Grab the lock to avoid races with Thread::notify_one()
1515 // If we are master and all slaves have finished then exit idle_loop
1516 if (this_sp && this_sp->slavesMask.none())
1523 // If we are not searching, wait for a condition to be signaled instead of
1524 // wasting CPU time polling for work.
1525 if (!searching && !exit)
1526 sleepCondition.wait(mutex);
1533 /// check_time() is called by the timer thread when the timer triggers. It is
1534 /// used to print debug info and, more importantly, to detect when we are out of
1535 /// available time and thus stop the search.
1539 static Time::point lastInfoTime = Time::now();
1540 Time::point elapsed = Time::now() - SearchTime;
1542 if (Time::now() - lastInfoTime >= 1000)
1544 lastInfoTime = Time::now();
1548 if (Limits.use_time_management() && !Limits.ponder)
1550 bool stillAtFirstMove = Signals.firstRootMove
1551 && !Signals.failedLowAtRoot
1552 && elapsed > TimeMgr.available_time() * 75 / 100;
1554 if ( stillAtFirstMove
1555 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1556 Signals.stop = true;
1558 else if (Limits.movetime && elapsed >= Limits.movetime)
1559 Signals.stop = true;
1561 else if (Limits.nodes)
1563 Threads.mutex.lock();
1565 int64_t nodes = RootPos.nodes_searched();
1567 // Loop across all split points and sum accumulated SplitPoint nodes plus
1568 // all the currently active positions nodes.
1569 for (size_t i = 0; i < Threads.size(); ++i)
1570 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1572 SplitPoint& sp = Threads[i]->splitPoints[j];
1578 for (size_t idx = 0; idx < Threads.size(); ++idx)
1579 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1580 nodes += Threads[idx]->activePosition->nodes_searched();
1585 Threads.mutex.unlock();
1587 if (nodes >= Limits.nodes)
1588 Signals.stop = true;