2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
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/>.
23 #include <cstring> // For std::memset
36 #include "syzygy/tbprobe.h"
40 volatile SignalsType Signals;
42 RootMoveVector RootMoves;
44 Time::point SearchTime;
45 StateStackPtr SetupStates;
48 namespace Tablebases {
58 namespace TB = Tablebases;
62 using namespace Search;
66 // Different node types, used as template parameter
67 enum NodeType { Root, PV, NonPV };
69 // Razoring and futility margin based on depth
70 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
71 inline Value futility_margin(Depth d) { return Value(200 * d); }
73 // Futility and reductions lookup tables, initialized at startup
74 int FutilityMoveCounts[2][16]; // [improving][depth]
75 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
77 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
78 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
81 // Skill struct is used to implement strength limiting
83 Skill(int l) : level(l) {}
84 bool enabled() const { return level < 20; }
85 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
86 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
87 Move pick_best(size_t multiPV);
90 Move best = MOVE_NONE;
95 double BestMoveChanges;
96 Value DrawValue[COLOR_NB];
99 MovesStats Countermoves, Followupmoves;
101 template <NodeType NT, bool SpNode>
102 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
104 template <NodeType NT, bool InCheck>
105 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
107 void id_loop(Position& pos);
108 Value value_to_tt(Value v, int ply);
109 Value value_from_tt(Value v, int ply);
110 void update_pv(Move* pv, Move move, Move* childPv);
111 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
116 /// Search::init() is called during startup to initialize various lookup tables
118 void Search::init() {
120 const double K[][2] = {{ 0.83, 2.25 }, { 0.50, 3.00 }};
122 for (int pv = 0; pv <= 1; ++pv)
123 for (int imp = 0; imp <= 1; ++imp)
124 for (int d = 1; d < 64; ++d)
125 for (int mc = 1; mc < 64; ++mc)
127 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
130 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
132 // Increase reduction when eval is not improving
133 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
134 Reductions[pv][imp][d][mc] += ONE_PLY;
137 for (int d = 0; d < 16; ++d)
139 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
140 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
145 /// Search::perft() is our utility to verify move generation. All the leaf nodes
146 /// up to the given depth are generated and counted and the sum returned.
148 uint64_t Search::perft(Position& pos, Depth depth) {
151 uint64_t cnt, nodes = 0;
153 const bool leaf = (depth == 2 * ONE_PLY);
155 for (const ExtMove& ms : MoveList<LEGAL>(pos))
157 if (Root && depth <= ONE_PLY)
161 pos.do_move(ms.move, st, ci, pos.gives_check(ms.move, ci));
162 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
164 pos.undo_move(ms.move);
167 sync_cout << UCI::move(ms.move, pos.is_chess960()) << ": " << cnt << sync_endl;
172 template uint64_t Search::perft<true>(Position& pos, Depth depth);
175 /// Search::think() is the external interface to Stockfish's search, and is
176 /// called by the main thread when the program receives the UCI 'go' command. It
177 /// searches from RootPos and at the end prints the "bestmove" to output.
179 void Search::think() {
181 TimeMgr.init(Limits, RootPos.side_to_move(), RootPos.game_ply());
183 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
184 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
185 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
188 TB::RootInTB = false;
189 TB::UseRule50 = Options["Syzygy50MoveRule"];
190 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
191 TB::Cardinality = Options["SyzygyProbeLimit"];
193 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
194 if (TB::Cardinality > TB::MaxCardinality)
196 TB::Cardinality = TB::MaxCardinality;
197 TB::ProbeDepth = DEPTH_ZERO;
200 if (RootMoves.empty())
202 RootMoves.push_back(MOVE_NONE);
203 sync_cout << "info depth 0 score "
204 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
209 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
210 + RootPos.count<ALL_PIECES>(BLACK))
212 // If the current root position is in the tablebases then RootMoves
213 // contains only moves that preserve the draw or win.
214 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
217 TB::Cardinality = 0; // Do not probe tablebases during the search
219 else // If DTZ tables are missing, use WDL tables as a fallback
221 // Filter out moves that do not preserve a draw or win
222 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
224 // Only probe during search if winning
225 if (TB::Score <= VALUE_DRAW)
231 TB::Hits = RootMoves.size();
234 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
235 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
240 for (Thread* th : Threads)
243 Threads.timer->run = true;
244 Threads.timer->notify_one(); // Wake up the recurring timer
246 id_loop(RootPos); // Let's start searching !
248 Threads.timer->run = false;
251 // When we reach the maximum depth, we can arrive here without a raise of
252 // Signals.stop. However, if we are pondering or in an infinite search,
253 // the UCI protocol states that we shouldn't print the best move before the
254 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
255 // until the GUI sends one of those commands (which also raises Signals.stop).
256 if (!Signals.stop && (Limits.ponder || Limits.infinite))
258 Signals.stopOnPonderhit = true;
259 RootPos.this_thread()->wait_for(Signals.stop);
262 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
264 if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
265 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
267 std::cout << sync_endl;
273 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
274 // with increasing depth until the allocated thinking time has been consumed,
275 // user stops the search, or the maximum search depth is reached.
277 void id_loop(Position& pos) {
279 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
281 Value bestValue, alpha, beta, delta;
283 std::memset(ss-2, 0, 5 * sizeof(Stack));
287 bestValue = delta = alpha = -VALUE_INFINITE;
288 beta = VALUE_INFINITE;
293 Countermoves.clear();
294 Followupmoves.clear();
296 size_t multiPV = Options["MultiPV"];
297 Skill skill(Options["Skill Level"]);
299 // When playing with strength handicap enable MultiPV search that we will
300 // use behind the scenes to retrieve a set of possible moves.
302 multiPV = std::max(multiPV, (size_t)4);
304 multiPV = std::min(multiPV, RootMoves.size());
306 // Iterative deepening loop until requested to stop or target depth reached
307 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
309 // Age out PV variability metric
310 BestMoveChanges *= 0.5;
312 // Save the last iteration's scores before first PV line is searched and
313 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
314 for (RootMove& rm : RootMoves)
315 rm.previousScore = rm.score;
317 // MultiPV loop. We perform a full root search for each PV line
318 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
320 // Reset aspiration window starting size
321 if (depth >= 5 * ONE_PLY)
324 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
325 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
328 // Start with a small aspiration window and, in the case of a fail
329 // high/low, re-search with a bigger window until we're not failing
333 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
335 // Bring the best move to the front. It is critical that sorting
336 // is done with a stable algorithm because all the values but the
337 // first and eventually the new best one are set to -VALUE_INFINITE
338 // and we want to keep the same order for all the moves except the
339 // new PV that goes to the front. Note that in case of MultiPV
340 // search the already searched PV lines are preserved.
341 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
343 // Write PV back to transposition table in case the relevant
344 // entries have been overwritten during the search.
345 for (size_t i = 0; i <= PVIdx; ++i)
346 RootMoves[i].insert_pv_in_tt(pos);
348 // If search has been stopped break immediately. Sorting and
349 // writing PV back to TT is safe because RootMoves is still
350 // valid, although it refers to previous iteration.
354 // When failing high/low give some update (without cluttering
355 // the UI) before a re-search.
357 && (bestValue <= alpha || bestValue >= beta)
358 && Time::now() - SearchTime > 3000)
359 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
361 // In case of failing low/high increase aspiration window and
362 // re-search, otherwise exit the loop.
363 if (bestValue <= alpha)
365 beta = (alpha + beta) / 2;
366 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
368 Signals.failedLowAtRoot = true;
369 Signals.stopOnPonderhit = false;
371 else if (bestValue >= beta)
373 alpha = (alpha + beta) / 2;
374 beta = std::min(bestValue + delta, VALUE_INFINITE);
381 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
384 // Sort the PV lines searched so far and update the GUI
385 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
388 sync_cout << "info nodes " << RootPos.nodes_searched()
389 << " time " << Time::now() - SearchTime << sync_endl;
391 else if (PVIdx + 1 == multiPV || Time::now() - SearchTime > 3000)
392 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
395 // If skill level is enabled and time is up, pick a sub-optimal best move
396 if (skill.enabled() && skill.time_to_pick(depth))
397 skill.pick_best(multiPV);
399 // Have we found a "mate in x"?
401 && bestValue >= VALUE_MATE_IN_MAX_PLY
402 && VALUE_MATE - bestValue <= 2 * Limits.mate)
405 // Do we have time for the next iteration? Can we stop searching now?
406 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
408 // Take some extra time if the best move has changed
409 if (depth > 4 * ONE_PLY && multiPV == 1)
410 TimeMgr.pv_instability(BestMoveChanges);
412 // Stop the search if only one legal move is available or all
413 // of the available time has been used.
414 if ( RootMoves.size() == 1
415 || Time::now() - SearchTime > TimeMgr.available_time())
417 // If we are allowed to ponder do not stop the search now but
418 // keep pondering until the GUI sends "ponderhit" or "stop".
420 Signals.stopOnPonderhit = true;
427 // If skill level is enabled, swap best PV line with the sub-optimal one
429 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
430 RootMoves.end(), skill.best_move(multiPV)));
434 // search<>() is the main search function for both PV and non-PV nodes and for
435 // normal and SplitPoint nodes. When called just after a split point the search
436 // is simpler because we have already probed the hash table, done a null move
437 // search, and searched the first move before splitting, so we don't have to
438 // repeat all this work again. We also don't need to store anything to the hash
439 // table here: This is taken care of after we return from the split point.
441 template <NodeType NT, bool SpNode>
442 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
444 const bool RootNode = NT == Root;
445 const bool PvNode = NT == PV || NT == Root;
447 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
448 assert(PvNode || (alpha == beta - 1));
449 assert(depth > DEPTH_ZERO);
451 Move pv[MAX_PLY+1], quietsSearched[64];
454 SplitPoint* splitPoint;
456 Move ttMove, move, excludedMove, bestMove;
457 Depth extension, newDepth, predictedDepth;
458 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
459 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
460 bool captureOrPromotion, dangerous, doFullDepthSearch;
461 int moveCount, quietCount;
463 // Step 1. Initialize node
464 Thread* thisThread = pos.this_thread();
465 inCheck = pos.checkers();
469 splitPoint = ss->splitPoint;
470 bestMove = splitPoint->bestMove;
471 bestValue = splitPoint->bestValue;
474 ttMove = excludedMove = MOVE_NONE;
475 ttValue = VALUE_NONE;
477 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
482 moveCount = quietCount = 0;
483 bestValue = -VALUE_INFINITE;
484 ss->ply = (ss-1)->ply + 1;
486 // Used to send selDepth info to GUI
487 if (PvNode && thisThread->maxPly < ss->ply)
488 thisThread->maxPly = ss->ply;
492 // Step 2. Check for aborted search and immediate draw
493 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
494 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
496 // Step 3. Mate distance pruning. Even if we mate at the next move our score
497 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
498 // a shorter mate was found upward in the tree then there is no need to search
499 // because we will never beat the current alpha. Same logic but with reversed
500 // signs applies also in the opposite condition of being mated instead of giving
501 // mate. In this case return a fail-high score.
502 alpha = std::max(mated_in(ss->ply), alpha);
503 beta = std::min(mate_in(ss->ply+1), beta);
508 assert(0 <= ss->ply && ss->ply < MAX_PLY);
510 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
511 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
512 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
514 // Step 4. Transposition table lookup
515 // We don't want the score of a partial search to overwrite a previous full search
516 // TT value, so we use a different position key in case of an excluded move.
517 excludedMove = ss->excludedMove;
518 posKey = excludedMove ? pos.exclusion_key() : pos.key();
519 tte = TT.probe(posKey, ttHit);
520 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
521 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
523 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
526 && tte->depth() >= depth
527 && ttValue != VALUE_NONE // Only in case of TT access race
528 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
529 : (tte->bound() & BOUND_UPPER)))
531 ss->currentMove = ttMove; // Can be MOVE_NONE
533 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
534 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
535 update_stats(pos, ss, ttMove, depth, nullptr, 0);
540 // Step 4a. Tablebase probe
541 if (!RootNode && TB::Cardinality)
543 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
545 if ( piecesCnt <= TB::Cardinality
546 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
547 && pos.rule50_count() == 0)
549 int found, v = Tablebases::probe_wdl(pos, &found);
555 int drawScore = TB::UseRule50 ? 1 : 0;
557 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
558 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
559 : VALUE_DRAW + 2 * v * drawScore;
561 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
562 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
563 MOVE_NONE, VALUE_NONE, TT.generation());
570 // Step 5. Evaluate the position statically and update parent's gain statistics
573 ss->staticEval = eval = VALUE_NONE;
579 // Never assume anything on values stored in TT
580 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
581 eval = ss->staticEval = evaluate(pos);
583 // Can ttValue be used as a better position evaluation?
584 if (ttValue != VALUE_NONE)
585 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
590 eval = ss->staticEval =
591 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
593 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
596 if (ss->skipEarlyPruning)
599 if ( !pos.captured_piece_type()
600 && ss->staticEval != VALUE_NONE
601 && (ss-1)->staticEval != VALUE_NONE
602 && (move = (ss-1)->currentMove) != MOVE_NULL
604 && type_of(move) == NORMAL)
606 Square to = to_sq(move);
607 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
610 // Step 6. Razoring (skipped when in check)
612 && depth < 4 * ONE_PLY
613 && eval + razor_margin(depth) <= alpha
614 && ttMove == MOVE_NONE
615 && !pos.pawn_on_7th(pos.side_to_move()))
617 if ( depth <= ONE_PLY
618 && eval + razor_margin(3 * ONE_PLY) <= alpha)
619 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
621 Value ralpha = alpha - razor_margin(depth);
622 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
627 // Step 7. Futility pruning: child node (skipped when in check)
629 && depth < 7 * ONE_PLY
630 && eval - futility_margin(depth) >= beta
631 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
632 && pos.non_pawn_material(pos.side_to_move()))
633 return eval - futility_margin(depth);
635 // Step 8. Null move search with verification search (is omitted in PV nodes)
637 && depth >= 2 * ONE_PLY
639 && pos.non_pawn_material(pos.side_to_move()))
641 ss->currentMove = MOVE_NULL;
643 assert(eval - beta >= 0);
645 // Null move dynamic reduction based on depth and value
646 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
648 pos.do_null_move(st);
649 (ss+1)->skipEarlyPruning = true;
650 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
651 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
652 (ss+1)->skipEarlyPruning = false;
653 pos.undo_null_move();
655 if (nullValue >= beta)
657 // Do not return unproven mate scores
658 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
661 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
664 // Do verification search at high depths
665 ss->skipEarlyPruning = true;
666 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
667 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
668 ss->skipEarlyPruning = false;
675 // Step 9. ProbCut (skipped when in check)
676 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
677 // and a reduced search returns a value much above beta, we can (almost) safely
678 // prune the previous move.
680 && depth >= 5 * ONE_PLY
681 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
683 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
684 Depth rdepth = depth - 4 * ONE_PLY;
686 assert(rdepth >= ONE_PLY);
687 assert((ss-1)->currentMove != MOVE_NONE);
688 assert((ss-1)->currentMove != MOVE_NULL);
690 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
693 while ((move = mp.next_move<false>()) != MOVE_NONE)
694 if (pos.legal(move, ci.pinned))
696 ss->currentMove = move;
697 pos.do_move(move, st, ci, pos.gives_check(move, ci));
698 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
705 // Step 10. Internal iterative deepening (skipped when in check)
706 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
708 && (PvNode || ss->staticEval + 256 >= beta))
710 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
711 ss->skipEarlyPruning = true;
712 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
713 ss->skipEarlyPruning = false;
715 tte = TT.probe(posKey, ttHit);
716 ttMove = ttHit ? tte->move() : MOVE_NONE;
719 moves_loop: // When in check and at SpNode search starts from here
721 Square prevMoveSq = to_sq((ss-1)->currentMove);
722 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
723 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
725 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
726 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
727 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
729 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
731 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
732 improving = ss->staticEval >= (ss-2)->staticEval
733 || ss->staticEval == VALUE_NONE
734 ||(ss-2)->staticEval == VALUE_NONE;
736 singularExtensionNode = !RootNode
738 && depth >= 8 * ONE_PLY
739 && ttMove != MOVE_NONE
740 /* && ttValue != VALUE_NONE Already implicit in the next condition */
741 && abs(ttValue) < VALUE_KNOWN_WIN
742 && !excludedMove // Recursive singular search is not allowed
743 && (tte->bound() & BOUND_LOWER)
744 && tte->depth() >= depth - 3 * ONE_PLY;
746 // Step 11. Loop through moves
747 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
748 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
752 if (move == excludedMove)
755 // At root obey the "searchmoves" option and skip moves not listed in Root
756 // Move List. As a consequence any illegal move is also skipped. In MultiPV
757 // mode we also skip PV moves which have been already searched.
758 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
763 // Shared counter cannot be decremented later if the move turns out to be illegal
764 if (!pos.legal(move, ci.pinned))
767 moveCount = ++splitPoint->moveCount;
768 splitPoint->mutex.unlock();
775 Signals.firstRootMove = (moveCount == 1);
777 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
778 sync_cout << "info depth " << depth / ONE_PLY
779 << " currmove " << UCI::move(move, pos.is_chess960())
780 << " currmovenumber " << moveCount + PVIdx << sync_endl;
784 (ss+1)->pv = nullptr;
786 extension = DEPTH_ZERO;
787 captureOrPromotion = pos.capture_or_promotion(move);
789 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
790 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
791 : pos.gives_check(move, ci);
793 dangerous = givesCheck
794 || type_of(move) != NORMAL
795 || pos.advanced_pawn_push(move);
797 // Step 12. Extend checks
798 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
801 // Singular extension search. If all moves but one fail low on a search of
802 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
803 // is singular and should be extended. To verify this we do a reduced search
804 // on all the other moves but the ttMove and if the result is lower than
805 // ttValue minus a margin then we extend the ttMove.
806 if ( singularExtensionNode
809 && pos.legal(move, ci.pinned))
811 Value rBeta = ttValue - 2 * depth / ONE_PLY;
812 ss->excludedMove = move;
813 ss->skipEarlyPruning = true;
814 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
815 ss->skipEarlyPruning = false;
816 ss->excludedMove = MOVE_NONE;
822 // Update the current move (this must be done after singular extension search)
823 newDepth = depth - ONE_PLY + extension;
825 // Step 13. Pruning at shallow depth
827 && !captureOrPromotion
830 && bestValue > VALUE_MATED_IN_MAX_PLY)
832 // Move count based pruning
833 if ( depth < 16 * ONE_PLY
834 && moveCount >= FutilityMoveCounts[improving][depth])
837 splitPoint->mutex.lock();
842 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
844 // Futility pruning: parent node
845 if (predictedDepth < 7 * ONE_PLY)
847 futilityValue = ss->staticEval + futility_margin(predictedDepth)
848 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
850 if (futilityValue <= alpha)
852 bestValue = std::max(bestValue, futilityValue);
856 splitPoint->mutex.lock();
857 if (bestValue > splitPoint->bestValue)
858 splitPoint->bestValue = bestValue;
864 // Prune moves with negative SEE at low depths
865 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
868 splitPoint->mutex.lock();
874 // Speculative prefetch as early as possible
875 prefetch((char*)TT.first_entry(pos.key_after(move)));
877 // Check for legality just before making the move
878 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
884 ss->currentMove = move;
885 if (!SpNode && !captureOrPromotion && quietCount < 64)
886 quietsSearched[quietCount++] = move;
888 // Step 14. Make the move
889 pos.do_move(move, st, ci, givesCheck);
891 // Step 15. Reduced depth search (LMR). If the move fails high it will be
892 // re-searched at full depth.
893 if ( depth >= 3 * ONE_PLY
895 && !captureOrPromotion
896 && move != ss->killers[0]
897 && move != ss->killers[1])
899 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
901 if ( (!PvNode && cutNode)
902 || History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO)
903 ss->reduction += ONE_PLY;
905 if (move == countermoves[0] || move == countermoves[1])
906 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
908 // Decrease reduction for moves that escape a capture
910 && type_of(move) == NORMAL
911 && type_of(pos.piece_on(to_sq(move))) != PAWN
912 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
913 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
915 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
917 alpha = splitPoint->alpha;
919 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
921 // Re-search at intermediate depth if reduction is very high
922 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
924 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
925 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
928 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
929 ss->reduction = DEPTH_ZERO;
932 doFullDepthSearch = !PvNode || moveCount > 1;
934 // Step 16. Full depth search, when LMR is skipped or fails high
935 if (doFullDepthSearch)
938 alpha = splitPoint->alpha;
940 value = newDepth < ONE_PLY ?
941 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
942 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
943 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
946 // For PV nodes only, do a full PV search on the first move or after a fail
947 // high (in the latter case search only if value < beta), otherwise let the
948 // parent node fail low with value <= alpha and to try another move.
949 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
952 (ss+1)->pv[0] = MOVE_NONE;
954 value = newDepth < ONE_PLY ?
955 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
956 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
957 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
960 // Step 17. Undo move
963 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
965 // Step 18. Check for new best move
968 splitPoint->mutex.lock();
969 bestValue = splitPoint->bestValue;
970 alpha = splitPoint->alpha;
973 // Finished searching the move. If a stop or a cutoff occurred, the return
974 // value of the search cannot be trusted, and we return immediately without
975 // updating best move, PV and TT.
976 if (Signals.stop || thisThread->cutoff_occurred())
981 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
983 // PV move or new best move ?
984 if (moveCount == 1 || value > alpha)
991 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
994 // We record how often the best move has been changed in each
995 // iteration. This information is used for time management: When
996 // the best move changes frequently, we allocate some more time.
1001 // All other moves but the PV are set to the lowest value: this is
1002 // not a problem when sorting because the sort is stable and the
1003 // move position in the list is preserved - just the PV is pushed up.
1004 rm.score = -VALUE_INFINITE;
1007 if (value > bestValue)
1009 bestValue = SpNode ? splitPoint->bestValue = value : value;
1013 bestMove = SpNode ? splitPoint->bestMove = move : move;
1015 if (PvNode && !RootNode) // Update pv even in fail-high case
1016 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1018 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1019 alpha = SpNode ? splitPoint->alpha = value : value;
1022 assert(value >= beta); // Fail high
1025 splitPoint->cutoff = true;
1032 // Step 19. Check for splitting the search
1034 && Threads.size() >= 2
1035 && depth >= Threads.minimumSplitDepth
1036 && ( !thisThread->activeSplitPoint
1037 || !thisThread->activeSplitPoint->allSlavesSearching)
1038 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1040 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1042 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1043 depth, moveCount, &mp, NT, cutNode);
1045 if (Signals.stop || thisThread->cutoff_occurred())
1048 if (bestValue >= beta)
1056 // Following condition would detect a stop or a cutoff set only after move
1057 // loop has been completed. But in this case bestValue is valid because we
1058 // have fully searched our subtree, and we can anyhow save the result in TT.
1060 if (Signals.stop || thisThread->cutoff_occurred())
1064 // Step 20. Check for mate and stalemate
1065 // All legal moves have been searched and if there are no legal moves, it
1066 // must be mate or stalemate. If we are in a singular extension search then
1067 // return a fail low score.
1069 bestValue = excludedMove ? alpha
1070 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1072 // Quiet best move: update killers, history, countermoves and followupmoves
1073 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1074 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1076 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1077 bestValue >= beta ? BOUND_LOWER :
1078 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1079 depth, bestMove, ss->staticEval, TT.generation());
1081 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1087 // qsearch() is the quiescence search function, which is called by the main
1088 // search function when the remaining depth is zero (or, to be more precise,
1089 // less than ONE_PLY).
1091 template <NodeType NT, bool InCheck>
1092 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1094 const bool PvNode = NT == PV;
1096 assert(NT == PV || NT == NonPV);
1097 assert(InCheck == !!pos.checkers());
1098 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1099 assert(PvNode || (alpha == beta - 1));
1100 assert(depth <= DEPTH_ZERO);
1106 Move ttMove, move, bestMove;
1107 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1108 bool ttHit, givesCheck, evasionPrunable;
1113 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1115 ss->pv[0] = MOVE_NONE;
1118 ss->currentMove = bestMove = MOVE_NONE;
1119 ss->ply = (ss-1)->ply + 1;
1121 // Check for an instant draw or if the maximum ply has been reached
1122 if (pos.is_draw() || ss->ply >= MAX_PLY)
1123 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1125 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1127 // Decide whether or not to include checks: this fixes also the type of
1128 // TT entry depth that we are going to use. Note that in qsearch we use
1129 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1130 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1131 : DEPTH_QS_NO_CHECKS;
1133 // Transposition table lookup
1135 tte = TT.probe(posKey, ttHit);
1136 ttMove = ttHit ? tte->move() : MOVE_NONE;
1137 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1141 && tte->depth() >= ttDepth
1142 && ttValue != VALUE_NONE // Only in case of TT access race
1143 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1144 : (tte->bound() & BOUND_UPPER)))
1146 ss->currentMove = ttMove; // Can be MOVE_NONE
1150 // Evaluate the position statically
1153 ss->staticEval = VALUE_NONE;
1154 bestValue = futilityBase = -VALUE_INFINITE;
1160 // Never assume anything on values stored in TT
1161 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1162 ss->staticEval = bestValue = evaluate(pos);
1164 // Can ttValue be used as a better position evaluation?
1165 if (ttValue != VALUE_NONE)
1166 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1167 bestValue = ttValue;
1170 ss->staticEval = bestValue =
1171 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1173 // Stand pat. Return immediately if static value is at least beta
1174 if (bestValue >= beta)
1177 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1178 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1183 if (PvNode && bestValue > alpha)
1186 futilityBase = bestValue + 128;
1189 // Initialize a MovePicker object for the current position, and prepare
1190 // to search the moves. Because the depth is <= 0 here, only captures,
1191 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1193 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1196 // Loop through the moves until no moves remain or a beta cutoff occurs
1197 while ((move = mp.next_move<false>()) != MOVE_NONE)
1199 assert(is_ok(move));
1201 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1202 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1203 : pos.gives_check(move, ci);
1208 && futilityBase > -VALUE_KNOWN_WIN
1209 && !pos.advanced_pawn_push(move))
1211 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1213 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1215 if (futilityValue <= alpha)
1217 bestValue = std::max(bestValue, futilityValue);
1221 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1223 bestValue = std::max(bestValue, futilityBase);
1228 // Detect non-capture evasions that are candidates to be pruned
1229 evasionPrunable = InCheck
1230 && bestValue > VALUE_MATED_IN_MAX_PLY
1231 && !pos.capture(move)
1232 && !pos.can_castle(pos.side_to_move());
1234 // Don't search moves with negative SEE values
1235 if ( (!InCheck || evasionPrunable)
1236 && type_of(move) != PROMOTION
1237 && pos.see_sign(move) < VALUE_ZERO)
1240 // Speculative prefetch as early as possible
1241 prefetch((char*)TT.first_entry(pos.key_after(move)));
1243 // Check for legality just before making the move
1244 if (!pos.legal(move, ci.pinned))
1247 ss->currentMove = move;
1249 // Make and search the move
1250 pos.do_move(move, st, ci, givesCheck);
1251 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1252 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1253 pos.undo_move(move);
1255 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1257 // Check for new best move
1258 if (value > bestValue)
1264 if (PvNode) // Update pv even in fail-high case
1265 update_pv(ss->pv, move, (ss+1)->pv);
1267 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1274 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1275 ttDepth, move, ss->staticEval, TT.generation());
1283 // All legal moves have been searched. A special case: If we're in check
1284 // and no legal moves were found, it is checkmate.
1285 if (InCheck && bestValue == -VALUE_INFINITE)
1286 return mated_in(ss->ply); // Plies to mate from the root
1288 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1289 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1290 ttDepth, bestMove, ss->staticEval, TT.generation());
1292 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1298 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1299 // "plies to mate from the current position". Non-mate scores are unchanged.
1300 // The function is called before storing a value in the transposition table.
1302 Value value_to_tt(Value v, int ply) {
1304 assert(v != VALUE_NONE);
1306 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1307 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1311 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1312 // from the transposition table (which refers to the plies to mate/be mated
1313 // from current position) to "plies to mate/be mated from the root".
1315 Value value_from_tt(Value v, int ply) {
1317 return v == VALUE_NONE ? VALUE_NONE
1318 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1319 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1323 // update_pv() adds current move and appends child pv[]
1325 void update_pv(Move* pv, Move move, Move* childPv) {
1327 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1332 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1335 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1337 if (ss->killers[0] != move)
1339 ss->killers[1] = ss->killers[0];
1340 ss->killers[0] = move;
1343 // Increase history value of the cut-off move and decrease all the other
1344 // played quiet moves.
1345 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1346 History.update(pos.moved_piece(move), to_sq(move), bonus);
1348 for (int i = 0; i < quietsCnt; ++i)
1351 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1354 if (is_ok((ss-1)->currentMove))
1356 Square prevMoveSq = to_sq((ss-1)->currentMove);
1357 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1360 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1362 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1363 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1368 // When playing with strength handicap, choose best move among a set of RootMoves
1369 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1371 Move Skill::pick_best(size_t multiPV) {
1373 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1374 static PRNG rng(Time::now());
1376 // RootMoves are already sorted by score in descending order
1377 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1378 int weakness = 120 - 2 * level;
1379 int maxScore = -VALUE_INFINITE;
1381 // Choose best move. For each move score we add two terms both dependent on
1382 // weakness. One deterministic and bigger for weaker levels, and one random,
1383 // then we choose the move with the resulting highest score.
1384 for (size_t i = 0; i < multiPV; ++i)
1386 int score = RootMoves[i].score;
1388 // This is our magic formula
1389 score += ( weakness * int(RootMoves[0].score - score)
1390 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1392 if (score > maxScore)
1395 best = RootMoves[i].pv[0];
1404 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1405 /// that all (if any) unsearched PV lines are sent using a previous search score.
1407 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1409 std::stringstream ss;
1410 Time::point elapsed = Time::now() - SearchTime + 1;
1411 size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
1414 for (Thread* th : Threads)
1415 if (th->maxPly > selDepth)
1416 selDepth = th->maxPly;
1418 for (size_t i = 0; i < multiPV; ++i)
1420 bool updated = (i <= PVIdx);
1422 if (depth == ONE_PLY && !updated)
1425 Depth d = updated ? depth : depth - ONE_PLY;
1426 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1428 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1429 v = tb ? TB::Score : v;
1431 if (ss.rdbuf()->in_avail()) // Not at first line
1435 << " depth " << d / ONE_PLY
1436 << " seldepth " << selDepth
1437 << " multipv " << i + 1
1438 << " score " << UCI::value(v);
1440 if (!tb && i == PVIdx)
1441 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1443 ss << " nodes " << pos.nodes_searched()
1444 << " nps " << pos.nodes_searched() * 1000 / elapsed
1445 << " tbhits " << TB::Hits
1446 << " time " << elapsed
1449 for (Move m : RootMoves[i].pv)
1450 ss << " " << UCI::move(m, pos.is_chess960());
1457 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1458 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1459 /// first, even if the old TT entries have been overwritten.
1461 void RootMove::insert_pv_in_tt(Position& pos) {
1463 StateInfo state[MAX_PLY], *st = state;
1466 for ( ; idx < pv.size(); ++idx)
1469 TTEntry* tte = TT.probe(pos.key(), ttHit);
1471 if (!ttHit || tte->move() != pv[idx]) // Don't overwrite correct entries
1472 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE, TT.generation());
1474 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1476 pos.do_move(pv[idx], *st++);
1479 while (idx) pos.undo_move(pv[--idx]);
1483 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1484 /// exiting the search, for instance in case we stop the search during a fail high at
1485 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1486 /// 'ponder on' we have nothing to think on.
1488 Move RootMove::extract_ponder_from_tt(Position& pos)
1493 assert(pv.size() == 1);
1495 pos.do_move(pv[0], st);
1496 TTEntry* tte = TT.probe(pos.key(), found);
1497 Move m = found ? tte->move() : MOVE_NONE;
1498 if (!MoveList<LEGAL>(pos).contains(m))
1501 pos.undo_move(pv[0]);
1507 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1509 void Thread::idle_loop() {
1511 // Pointer 'this_sp' is not null only if we are called from split(), and not
1512 // at the thread creation. This means we are the split point's master.
1513 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : nullptr;
1515 assert(!this_sp || (this_sp->masterThread == this && searching));
1519 // If this thread has been assigned work, launch a search
1522 Threads.mutex.lock();
1524 assert(activeSplitPoint);
1525 SplitPoint* sp = activeSplitPoint;
1527 Threads.mutex.unlock();
1529 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1530 Position pos(*sp->pos, this);
1532 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1533 ss->splitPoint = sp;
1537 assert(activePosition == nullptr);
1539 activePosition = &pos;
1541 if (sp->nodeType == NonPV)
1542 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1544 else if (sp->nodeType == PV)
1545 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1547 else if (sp->nodeType == Root)
1548 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1556 activePosition = nullptr;
1557 sp->slavesMask.reset(idx);
1558 sp->allSlavesSearching = false;
1559 sp->nodes += pos.nodes_searched();
1561 // Wake up the master thread so to allow it to return from the idle
1562 // loop in case we are the last slave of the split point.
1563 if ( this != sp->masterThread
1564 && sp->slavesMask.none())
1566 assert(!sp->masterThread->searching);
1567 sp->masterThread->notify_one();
1570 // After releasing the lock we can't access any SplitPoint related data
1571 // in a safe way because it could have been released under our feet by
1575 // Try to late join to another split point if none of its slaves has
1576 // already finished.
1577 if (Threads.size() > 2)
1578 for (size_t i = 0; i < Threads.size(); ++i)
1580 const int size = Threads[i]->splitPointsSize; // Local copy
1581 sp = size ? &Threads[i]->splitPoints[size - 1] : nullptr;
1584 && sp->allSlavesSearching
1585 && available_to(Threads[i]))
1587 // Recheck the conditions under lock protection
1588 Threads.mutex.lock();
1591 if ( sp->allSlavesSearching
1592 && available_to(Threads[i]))
1594 sp->slavesMask.set(idx);
1595 activeSplitPoint = sp;
1600 Threads.mutex.unlock();
1602 break; // Just a single attempt
1607 // Grab the lock to avoid races with Thread::notify_one()
1608 std::unique_lock<std::mutex> lk(mutex);
1610 // If we are master and all slaves have finished then exit idle_loop
1611 if (this_sp && this_sp->slavesMask.none())
1617 // If we are not searching, wait for a condition to be signaled instead of
1618 // wasting CPU time polling for work.
1619 if (!searching && !exit)
1620 sleepCondition.wait(lk);
1625 /// check_time() is called by the timer thread when the timer triggers. It is
1626 /// used to print debug info and, more importantly, to detect when we are out of
1627 /// available time and thus stop the search.
1631 static Time::point lastInfoTime = Time::now();
1632 Time::point elapsed = Time::now() - SearchTime;
1634 if (Time::now() - lastInfoTime >= 1000)
1636 lastInfoTime = Time::now();
1640 // An engine may not stop pondering until told so by the GUI
1644 if (Limits.use_time_management())
1646 bool stillAtFirstMove = Signals.firstRootMove
1647 && !Signals.failedLowAtRoot
1648 && elapsed > TimeMgr.available_time() * 75 / 100;
1650 if ( stillAtFirstMove
1651 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1652 Signals.stop = true;
1654 else if (Limits.movetime && elapsed >= Limits.movetime)
1655 Signals.stop = true;
1657 else if (Limits.nodes)
1659 Threads.mutex.lock();
1661 int64_t nodes = RootPos.nodes_searched();
1663 // Loop across all split points and sum accumulated SplitPoint nodes plus
1664 // all the currently active positions nodes.
1665 for (Thread* th : Threads)
1666 for (int i = 0; i < th->splitPointsSize; ++i)
1668 SplitPoint& sp = th->splitPoints[i];
1674 for (size_t idx = 0; idx < Threads.size(); ++idx)
1675 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1676 nodes += Threads[idx]->activePosition->nodes_searched();
1681 Threads.mutex.unlock();
1683 if (nodes >= Limits.nodes)
1684 Signals.stop = true;