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);
112 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta);
117 /// Search::init() is called during startup to initialize various lookup tables
119 void Search::init() {
121 const double K[][2] = {{ 0.83, 2.25 }, { 0.50, 3.00 }};
123 for (int pv = 0; pv <= 1; ++pv)
124 for (int imp = 0; imp <= 1; ++imp)
125 for (int d = 1; d < 64; ++d)
126 for (int mc = 1; mc < 64; ++mc)
128 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
131 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
133 // Increase reduction when eval is not improving
134 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
135 Reductions[pv][imp][d][mc] += ONE_PLY;
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 (const ExtMove& ms : MoveList<LEGAL>(pos))
158 if (Root && depth <= ONE_PLY)
162 pos.do_move(ms.move, st, ci, pos.gives_check(ms.move, ci));
163 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
165 pos.undo_move(ms.move);
168 sync_cout << UCI::move(ms.move, 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.side_to_move(), RootPos.game_ply());
184 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
185 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
186 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
189 TB::RootInTB = false;
190 TB::UseRule50 = Options["Syzygy50MoveRule"];
191 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
192 TB::Cardinality = Options["SyzygyProbeLimit"];
194 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
195 if (TB::Cardinality > TB::MaxCardinality)
197 TB::Cardinality = TB::MaxCardinality;
198 TB::ProbeDepth = DEPTH_ZERO;
201 if (RootMoves.empty())
203 RootMoves.push_back(MOVE_NONE);
204 sync_cout << "info depth 0 score "
205 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
210 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
211 + RootPos.count<ALL_PIECES>(BLACK))
213 // If the current root position is in the tablebases then RootMoves
214 // contains only moves that preserve the draw or win.
215 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
218 TB::Cardinality = 0; // Do not probe tablebases during the search
220 else // If DTZ tables are missing, use WDL tables as a fallback
222 // Filter out moves that do not preserve a draw or win
223 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
225 // Only probe during search if winning
226 if (TB::Score <= VALUE_DRAW)
232 TB::Hits = RootMoves.size();
235 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
236 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
241 for (Thread* th : Threads)
244 Threads.timer->run = true;
245 Threads.timer->notify_one(); // Wake up the recurring timer
247 id_loop(RootPos); // Let's start searching !
249 Threads.timer->run = false;
252 // When we reach the maximum depth, we can arrive here without a raise of
253 // Signals.stop. However, if we are pondering or in an infinite search,
254 // the UCI protocol states that we shouldn't print the best move before the
255 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
256 // until the GUI sends one of those commands (which also raises Signals.stop).
257 if (!Signals.stop && (Limits.ponder || Limits.infinite))
259 Signals.stopOnPonderhit = true;
260 RootPos.this_thread()->wait_for(Signals.stop);
263 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
265 if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
266 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
268 std::cout << sync_endl;
274 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
275 // with increasing depth until the allocated thinking time has been consumed,
276 // user stops the search, or the maximum search depth is reached.
278 void id_loop(Position& pos) {
280 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
282 Value bestValue, alpha, beta, delta;
284 std::memset(ss-2, 0, 5 * sizeof(Stack));
288 bestValue = delta = alpha = -VALUE_INFINITE;
289 beta = VALUE_INFINITE;
294 Countermoves.clear();
295 Followupmoves.clear();
297 size_t multiPV = Options["MultiPV"];
298 Skill skill(Options["Skill Level"]);
300 // When playing with strength handicap enable MultiPV search that we will
301 // use behind the scenes to retrieve a set of possible moves.
303 multiPV = std::max(multiPV, (size_t)4);
305 multiPV = std::min(multiPV, RootMoves.size());
307 // Iterative deepening loop until requested to stop or target depth reached
308 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
310 // Age out PV variability metric
311 BestMoveChanges *= 0.5;
313 // Save the last iteration's scores before first PV line is searched and
314 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
315 for (RootMove& rm : RootMoves)
316 rm.previousScore = rm.score;
318 // MultiPV loop. We perform a full root search for each PV line
319 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
321 // Reset aspiration window starting size
322 if (depth >= 5 * ONE_PLY)
325 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
326 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
329 // Start with a small aspiration window and, in the case of a fail
330 // high/low, re-search with a bigger window until we're not failing
334 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
336 // Bring the best move to the front. It is critical that sorting
337 // is done with a stable algorithm because all the values but the
338 // first and eventually the new best one are set to -VALUE_INFINITE
339 // and we want to keep the same order for all the moves except the
340 // new PV that goes to the front. Note that in case of MultiPV
341 // search the already searched PV lines are preserved.
342 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
344 // Write PV back to transposition table in case the relevant
345 // entries have been overwritten during the search.
346 for (size_t i = 0; i <= PVIdx; ++i)
347 RootMoves[i].insert_pv_in_tt(pos);
349 // If search has been stopped break immediately. Sorting and
350 // writing PV back to TT is safe because RootMoves is still
351 // valid, although it refers to previous iteration.
355 // When failing high/low give some update (without cluttering
356 // the UI) before a re-search.
358 && (bestValue <= alpha || bestValue >= beta)
359 && Time::now() - SearchTime > 3000)
360 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
362 // In case of failing low/high increase aspiration window and
363 // re-search, otherwise exit the loop.
364 if (bestValue <= alpha)
366 beta = (alpha + beta) / 2;
367 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
369 Signals.failedLowAtRoot = true;
370 Signals.stopOnPonderhit = false;
372 else if (bestValue >= beta)
374 alpha = (alpha + beta) / 2;
375 beta = std::min(bestValue + delta, VALUE_INFINITE);
382 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
385 // Sort the PV lines searched so far and update the GUI
386 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
389 sync_cout << "info nodes " << RootPos.nodes_searched()
390 << " time " << Time::now() - SearchTime << sync_endl;
392 else if (PVIdx + 1 == multiPV || Time::now() - SearchTime > 3000)
393 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
396 // If skill level is enabled and time is up, pick a sub-optimal best move
397 if (skill.enabled() && skill.time_to_pick(depth))
398 skill.pick_best(multiPV);
400 // Have we found a "mate in x"?
402 && bestValue >= VALUE_MATE_IN_MAX_PLY
403 && VALUE_MATE - bestValue <= 2 * Limits.mate)
406 // Do we have time for the next iteration? Can we stop searching now?
407 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
409 // Take some extra time if the best move has changed
410 if (depth > 4 * ONE_PLY && multiPV == 1)
411 TimeMgr.pv_instability(BestMoveChanges);
413 // Stop the search if only one legal move is available or all
414 // of the available time has been used.
415 if ( RootMoves.size() == 1
416 || Time::now() - SearchTime > TimeMgr.available_time())
418 // If we are allowed to ponder do not stop the search now but
419 // keep pondering until the GUI sends "ponderhit" or "stop".
421 Signals.stopOnPonderhit = true;
428 // If skill level is enabled, swap best PV line with the sub-optimal one
430 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
431 RootMoves.end(), skill.best_move(multiPV)));
435 // search<>() is the main search function for both PV and non-PV nodes and for
436 // normal and SplitPoint nodes. When called just after a split point the search
437 // is simpler because we have already probed the hash table, done a null move
438 // search, and searched the first move before splitting, so we don't have to
439 // repeat all this work again. We also don't need to store anything to the hash
440 // table here: This is taken care of after we return from the split point.
442 template <NodeType NT, bool SpNode>
443 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
445 const bool RootNode = NT == Root;
446 const bool PvNode = NT == PV || NT == Root;
448 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
449 assert(PvNode || (alpha == beta - 1));
450 assert(depth > DEPTH_ZERO);
452 Move pv[MAX_PLY+1], quietsSearched[64];
455 SplitPoint* splitPoint;
457 Move ttMove, move, excludedMove, bestMove;
458 Depth extension, newDepth, predictedDepth;
459 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
460 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
461 bool captureOrPromotion, dangerous, doFullDepthSearch;
462 int moveCount, quietCount;
464 // Step 1. Initialize node
465 Thread* thisThread = pos.this_thread();
466 inCheck = pos.checkers();
470 splitPoint = ss->splitPoint;
471 bestMove = splitPoint->bestMove;
472 bestValue = splitPoint->bestValue;
475 ttMove = excludedMove = MOVE_NONE;
476 ttValue = VALUE_NONE;
478 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
483 moveCount = quietCount = 0;
484 bestValue = -VALUE_INFINITE;
485 ss->ply = (ss-1)->ply + 1;
487 // Used to send selDepth info to GUI
488 if (PvNode && thisThread->maxPly < ss->ply)
489 thisThread->maxPly = ss->ply;
493 // Step 2. Check for aborted search and immediate draw
494 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
495 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
497 // Step 3. Mate distance pruning. Even if we mate at the next move our score
498 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
499 // a shorter mate was found upward in the tree then there is no need to search
500 // because we will never beat the current alpha. Same logic but with reversed
501 // signs applies also in the opposite condition of being mated instead of giving
502 // mate. In this case return a fail-high score.
503 alpha = std::max(mated_in(ss->ply), alpha);
504 beta = std::min(mate_in(ss->ply+1), beta);
509 assert(0 <= ss->ply && ss->ply < MAX_PLY);
511 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
512 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
513 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
515 // Step 4. Transposition table lookup
516 // We don't want the score of a partial search to overwrite a previous full search
517 // TT value, so we use a different position key in case of an excluded move.
518 excludedMove = ss->excludedMove;
519 posKey = excludedMove ? pos.exclusion_key() : pos.key();
520 tte = TT.probe(posKey, ttHit);
521 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
522 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
524 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
527 && tte->depth() >= depth
528 && ttValue != VALUE_NONE // Only in case of TT access race
529 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
530 : (tte->bound() & BOUND_UPPER)))
532 ss->currentMove = ttMove; // Can be MOVE_NONE
534 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
535 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
536 update_stats(pos, ss, ttMove, depth, nullptr, 0);
541 // Step 4a. Tablebase probe
542 if (!RootNode && TB::Cardinality)
544 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
546 if ( piecesCnt <= TB::Cardinality
547 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
548 && pos.rule50_count() == 0)
550 int found, v = Tablebases::probe_wdl(pos, &found);
556 int drawScore = TB::UseRule50 ? 1 : 0;
558 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
559 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
560 : VALUE_DRAW + 2 * v * drawScore;
562 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
563 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
564 MOVE_NONE, VALUE_NONE, TT.generation());
571 // Step 5. Evaluate the position statically and update parent's gain statistics
574 ss->staticEval = eval = VALUE_NONE;
580 // Never assume anything on values stored in TT
581 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
582 eval = ss->staticEval = evaluate(pos);
584 // Can ttValue be used as a better position evaluation?
585 if (ttValue != VALUE_NONE)
586 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
591 eval = ss->staticEval =
592 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
594 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
597 if (ss->skipEarlyPruning)
600 if ( !pos.captured_piece_type()
601 && ss->staticEval != VALUE_NONE
602 && (ss-1)->staticEval != VALUE_NONE
603 && (move = (ss-1)->currentMove) != MOVE_NULL
605 && type_of(move) == NORMAL)
607 Square to = to_sq(move);
608 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
611 // Step 6. Razoring (skipped when in check)
613 && depth < 4 * ONE_PLY
614 && eval + razor_margin(depth) <= alpha
615 && ttMove == MOVE_NONE
616 && !pos.pawn_on_7th(pos.side_to_move()))
618 if ( depth <= ONE_PLY
619 && eval + razor_margin(3 * ONE_PLY) <= alpha)
620 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
622 Value ralpha = alpha - razor_margin(depth);
623 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
628 // Step 7. Futility pruning: child node (skipped when in check)
630 && depth < 7 * ONE_PLY
631 && eval - futility_margin(depth) >= beta
632 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
633 && pos.non_pawn_material(pos.side_to_move()))
634 return eval - futility_margin(depth);
636 // Step 8. Null move search with verification search (is omitted in PV nodes)
638 && depth >= 2 * ONE_PLY
640 && pos.non_pawn_material(pos.side_to_move()))
642 ss->currentMove = MOVE_NULL;
644 assert(eval - beta >= 0);
646 // Null move dynamic reduction based on depth and value
647 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
649 pos.do_null_move(st);
650 (ss+1)->skipEarlyPruning = true;
651 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
652 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
653 (ss+1)->skipEarlyPruning = false;
654 pos.undo_null_move();
656 if (nullValue >= beta)
658 // Do not return unproven mate scores
659 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
662 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
665 // Do verification search at high depths
666 ss->skipEarlyPruning = true;
667 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
668 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
669 ss->skipEarlyPruning = false;
676 // Step 9. ProbCut (skipped when in check)
677 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
678 // and a reduced search returns a value much above beta, we can (almost) safely
679 // prune the previous move.
681 && depth >= 5 * ONE_PLY
682 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
684 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
685 Depth rdepth = depth - 4 * ONE_PLY;
687 assert(rdepth >= ONE_PLY);
688 assert((ss-1)->currentMove != MOVE_NONE);
689 assert((ss-1)->currentMove != MOVE_NULL);
691 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
694 while ((move = mp.next_move<false>()) != MOVE_NONE)
695 if (pos.legal(move, ci.pinned))
697 ss->currentMove = move;
698 pos.do_move(move, st, ci, pos.gives_check(move, ci));
699 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
706 // Step 10. Internal iterative deepening (skipped when in check)
707 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
709 && (PvNode || ss->staticEval + 256 >= beta))
711 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
712 ss->skipEarlyPruning = true;
713 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
714 ss->skipEarlyPruning = false;
716 tte = TT.probe(posKey, ttHit);
717 ttMove = ttHit ? tte->move() : MOVE_NONE;
720 moves_loop: // When in check and at SpNode search starts from here
722 Square prevMoveSq = to_sq((ss-1)->currentMove);
723 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
724 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
726 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
727 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
728 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
730 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
732 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
733 improving = ss->staticEval >= (ss-2)->staticEval
734 || ss->staticEval == VALUE_NONE
735 ||(ss-2)->staticEval == VALUE_NONE;
737 singularExtensionNode = !RootNode
739 && depth >= 8 * ONE_PLY
740 && ttMove != MOVE_NONE
741 /* && ttValue != VALUE_NONE Already implicit in the next condition */
742 && abs(ttValue) < VALUE_KNOWN_WIN
743 && !excludedMove // Recursive singular search is not allowed
744 && (tte->bound() & BOUND_LOWER)
745 && tte->depth() >= depth - 3 * ONE_PLY;
747 // Step 11. Loop through moves
748 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
749 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
753 if (move == excludedMove)
756 // At root obey the "searchmoves" option and skip moves not listed in Root
757 // Move List. As a consequence any illegal move is also skipped. In MultiPV
758 // mode we also skip PV moves which have been already searched.
759 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
764 // Shared counter cannot be decremented later if the move turns out to be illegal
765 if (!pos.legal(move, ci.pinned))
768 moveCount = ++splitPoint->moveCount;
769 splitPoint->mutex.unlock();
776 Signals.firstRootMove = (moveCount == 1);
778 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
779 sync_cout << "info depth " << depth / ONE_PLY
780 << " currmove " << UCI::move(move, pos.is_chess960())
781 << " currmovenumber " << moveCount + PVIdx << sync_endl;
785 (ss+1)->pv = nullptr;
787 extension = DEPTH_ZERO;
788 captureOrPromotion = pos.capture_or_promotion(move);
790 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
791 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
792 : pos.gives_check(move, ci);
794 dangerous = givesCheck
795 || type_of(move) != NORMAL
796 || pos.advanced_pawn_push(move);
798 // Step 12. Extend checks
799 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
802 // Singular extension search. If all moves but one fail low on a search of
803 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
804 // is singular and should be extended. To verify this we do a reduced search
805 // on all the other moves but the ttMove and if the result is lower than
806 // ttValue minus a margin then we extend the ttMove.
807 if ( singularExtensionNode
810 && pos.legal(move, ci.pinned))
812 Value rBeta = ttValue - 2 * depth / ONE_PLY;
813 ss->excludedMove = move;
814 ss->skipEarlyPruning = true;
815 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
816 ss->skipEarlyPruning = false;
817 ss->excludedMove = MOVE_NONE;
823 // Update the current move (this must be done after singular extension search)
824 newDepth = depth - ONE_PLY + extension;
826 // Step 13. Pruning at shallow depth
828 && !captureOrPromotion
831 && bestValue > VALUE_MATED_IN_MAX_PLY)
833 // Move count based pruning
834 if ( depth < 16 * ONE_PLY
835 && moveCount >= FutilityMoveCounts[improving][depth])
838 splitPoint->mutex.lock();
843 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
845 // Futility pruning: parent node
846 if (predictedDepth < 7 * ONE_PLY)
848 futilityValue = ss->staticEval + futility_margin(predictedDepth)
849 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
851 if (futilityValue <= alpha)
853 bestValue = std::max(bestValue, futilityValue);
857 splitPoint->mutex.lock();
858 if (bestValue > splitPoint->bestValue)
859 splitPoint->bestValue = bestValue;
865 // Prune moves with negative SEE at low depths
866 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
869 splitPoint->mutex.lock();
875 // Speculative prefetch as early as possible
876 prefetch((char*)TT.first_entry(pos.key_after(move)));
878 // Check for legality just before making the move
879 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
885 ss->currentMove = move;
886 if (!SpNode && !captureOrPromotion && quietCount < 64)
887 quietsSearched[quietCount++] = move;
889 // Step 14. Make the move
890 pos.do_move(move, st, ci, givesCheck);
892 // Step 15. Reduced depth search (LMR). If the move fails high it will be
893 // re-searched at full depth.
894 if ( depth >= 3 * ONE_PLY
896 && !captureOrPromotion
897 && move != ss->killers[0]
898 && move != ss->killers[1])
900 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
902 if ( (!PvNode && cutNode)
903 || History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO)
904 ss->reduction += ONE_PLY;
906 if (move == countermoves[0] || move == countermoves[1])
907 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
909 // Decrease reduction for moves that escape a capture
911 && type_of(move) == NORMAL
912 && type_of(pos.piece_on(to_sq(move))) != PAWN
913 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
914 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
916 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
918 alpha = splitPoint->alpha;
920 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
922 // Re-search at intermediate depth if reduction is very high
923 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
925 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
926 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
929 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
930 ss->reduction = DEPTH_ZERO;
933 doFullDepthSearch = !PvNode || moveCount > 1;
935 // Step 16. Full depth search, when LMR is skipped or fails high
936 if (doFullDepthSearch)
939 alpha = splitPoint->alpha;
941 value = newDepth < ONE_PLY ?
942 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
943 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
944 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
947 // For PV nodes only, do a full PV search on the first move or after a fail
948 // high (in the latter case search only if value < beta), otherwise let the
949 // parent node fail low with value <= alpha and to try another move.
950 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
953 (ss+1)->pv[0] = MOVE_NONE;
955 value = newDepth < ONE_PLY ?
956 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
957 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
958 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
961 // Step 17. Undo move
964 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
966 // Step 18. Check for new best move
969 splitPoint->mutex.lock();
970 bestValue = splitPoint->bestValue;
971 alpha = splitPoint->alpha;
974 // Finished searching the move. If a stop or a cutoff occurred, the return
975 // value of the search cannot be trusted, and we return immediately without
976 // updating best move, PV and TT.
977 if (Signals.stop || thisThread->cutoff_occurred())
982 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
984 // PV move or new best move ?
985 if (moveCount == 1 || value > alpha)
992 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
995 // We record how often the best move has been changed in each
996 // iteration. This information is used for time management: When
997 // the best move changes frequently, we allocate some more time.
1002 // All other moves but the PV are set to the lowest value: this is
1003 // not a problem when sorting because the sort is stable and the
1004 // move position in the list is preserved - just the PV is pushed up.
1005 rm.score = -VALUE_INFINITE;
1008 if (value > bestValue)
1010 bestValue = SpNode ? splitPoint->bestValue = value : value;
1014 bestMove = SpNode ? splitPoint->bestMove = move : move;
1016 if (PvNode && !RootNode) // Update pv even in fail-high case
1017 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1019 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1020 alpha = SpNode ? splitPoint->alpha = value : value;
1023 assert(value >= beta); // Fail high
1026 splitPoint->cutoff = true;
1033 // Step 19. Check for splitting the search
1035 && Threads.size() >= 2
1036 && depth >= Threads.minimumSplitDepth
1037 && ( !thisThread->activeSplitPoint
1038 || !thisThread->activeSplitPoint->allSlavesSearching)
1039 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1041 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1043 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1044 depth, moveCount, &mp, NT, cutNode);
1046 if (Signals.stop || thisThread->cutoff_occurred())
1049 if (bestValue >= beta)
1057 // Following condition would detect a stop or a cutoff set only after move
1058 // loop has been completed. But in this case bestValue is valid because we
1059 // have fully searched our subtree, and we can anyhow save the result in TT.
1061 if (Signals.stop || thisThread->cutoff_occurred())
1065 // Step 20. Check for mate and stalemate
1066 // All legal moves have been searched and if there are no legal moves, it
1067 // must be mate or stalemate. If we are in a singular extension search then
1068 // return a fail low score.
1070 bestValue = excludedMove ? alpha
1071 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1073 // Quiet best move: update killers, history, countermoves and followupmoves
1074 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1075 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1077 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1078 bestValue >= beta ? BOUND_LOWER :
1079 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1080 depth, bestMove, ss->staticEval, TT.generation());
1082 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1088 // qsearch() is the quiescence search function, which is called by the main
1089 // search function when the remaining depth is zero (or, to be more precise,
1090 // less than ONE_PLY).
1092 template <NodeType NT, bool InCheck>
1093 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1095 const bool PvNode = NT == PV;
1097 assert(NT == PV || NT == NonPV);
1098 assert(InCheck == !!pos.checkers());
1099 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1100 assert(PvNode || (alpha == beta - 1));
1101 assert(depth <= DEPTH_ZERO);
1107 Move ttMove, move, bestMove;
1108 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1109 bool ttHit, givesCheck, evasionPrunable;
1114 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1116 ss->pv[0] = MOVE_NONE;
1119 ss->currentMove = bestMove = MOVE_NONE;
1120 ss->ply = (ss-1)->ply + 1;
1122 // Check for an instant draw or if the maximum ply has been reached
1123 if (pos.is_draw() || ss->ply >= MAX_PLY)
1124 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1126 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1128 // Decide whether or not to include checks: this fixes also the type of
1129 // TT entry depth that we are going to use. Note that in qsearch we use
1130 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1131 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1132 : DEPTH_QS_NO_CHECKS;
1134 // Transposition table lookup
1136 tte = TT.probe(posKey, ttHit);
1137 ttMove = ttHit ? tte->move() : MOVE_NONE;
1138 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1142 && tte->depth() >= ttDepth
1143 && ttValue != VALUE_NONE // Only in case of TT access race
1144 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1145 : (tte->bound() & BOUND_UPPER)))
1147 ss->currentMove = ttMove; // Can be MOVE_NONE
1151 // Evaluate the position statically
1154 ss->staticEval = VALUE_NONE;
1155 bestValue = futilityBase = -VALUE_INFINITE;
1161 // Never assume anything on values stored in TT
1162 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1163 ss->staticEval = bestValue = evaluate(pos);
1165 // Can ttValue be used as a better position evaluation?
1166 if (ttValue != VALUE_NONE)
1167 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1168 bestValue = ttValue;
1171 ss->staticEval = bestValue =
1172 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1174 // Stand pat. Return immediately if static value is at least beta
1175 if (bestValue >= beta)
1178 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1179 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1184 if (PvNode && bestValue > alpha)
1187 futilityBase = bestValue + 128;
1190 // Initialize a MovePicker object for the current position, and prepare
1191 // to search the moves. Because the depth is <= 0 here, only captures,
1192 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1194 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1197 // Loop through the moves until no moves remain or a beta cutoff occurs
1198 while ((move = mp.next_move<false>()) != MOVE_NONE)
1200 assert(is_ok(move));
1202 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1203 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1204 : pos.gives_check(move, ci);
1209 && futilityBase > -VALUE_KNOWN_WIN
1210 && !pos.advanced_pawn_push(move))
1212 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1214 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1216 if (futilityValue <= alpha)
1218 bestValue = std::max(bestValue, futilityValue);
1222 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1224 bestValue = std::max(bestValue, futilityBase);
1229 // Detect non-capture evasions that are candidates to be pruned
1230 evasionPrunable = InCheck
1231 && bestValue > VALUE_MATED_IN_MAX_PLY
1232 && !pos.capture(move)
1233 && !pos.can_castle(pos.side_to_move());
1235 // Don't search moves with negative SEE values
1236 if ( (!InCheck || evasionPrunable)
1237 && type_of(move) != PROMOTION
1238 && pos.see_sign(move) < VALUE_ZERO)
1241 // Speculative prefetch as early as possible
1242 prefetch((char*)TT.first_entry(pos.key_after(move)));
1244 // Check for legality just before making the move
1245 if (!pos.legal(move, ci.pinned))
1248 ss->currentMove = move;
1250 // Make and search the move
1251 pos.do_move(move, st, ci, givesCheck);
1252 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1253 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1254 pos.undo_move(move);
1256 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1258 // Check for new best move
1259 if (value > bestValue)
1265 if (PvNode) // Update pv even in fail-high case
1266 update_pv(ss->pv, move, (ss+1)->pv);
1268 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1275 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1276 ttDepth, move, ss->staticEval, TT.generation());
1284 // All legal moves have been searched. A special case: If we're in check
1285 // and no legal moves were found, it is checkmate.
1286 if (InCheck && bestValue == -VALUE_INFINITE)
1287 return mated_in(ss->ply); // Plies to mate from the root
1289 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1290 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1291 ttDepth, bestMove, ss->staticEval, TT.generation());
1293 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1299 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1300 // "plies to mate from the current position". Non-mate scores are unchanged.
1301 // The function is called before storing a value in the transposition table.
1303 Value value_to_tt(Value v, int ply) {
1305 assert(v != VALUE_NONE);
1307 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1308 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1312 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1313 // from the transposition table (which refers to the plies to mate/be mated
1314 // from current position) to "plies to mate/be mated from the root".
1316 Value value_from_tt(Value v, int ply) {
1318 return v == VALUE_NONE ? VALUE_NONE
1319 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1320 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1324 // update_pv() adds current move and appends child pv[]
1326 void update_pv(Move* pv, Move move, Move* childPv) {
1328 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1333 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1336 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1338 if (ss->killers[0] != move)
1340 ss->killers[1] = ss->killers[0];
1341 ss->killers[0] = move;
1344 // Increase history value of the cut-off move and decrease all the other
1345 // played quiet moves.
1346 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1347 History.update(pos.moved_piece(move), to_sq(move), bonus);
1349 for (int i = 0; i < quietsCnt; ++i)
1352 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1355 if (is_ok((ss-1)->currentMove))
1357 Square prevMoveSq = to_sq((ss-1)->currentMove);
1358 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1361 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1363 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1364 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1369 // When playing with strength handicap, choose best move among a set of RootMoves
1370 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1372 Move Skill::pick_best(size_t multiPV) {
1374 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1375 static PRNG rng(Time::now());
1377 // RootMoves are already sorted by score in descending order
1378 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1379 int weakness = 120 - 2 * level;
1380 int maxScore = -VALUE_INFINITE;
1382 // Choose best move. For each move score we add two terms both dependent on
1383 // weakness. One deterministic and bigger for weaker levels, and one random,
1384 // then we choose the move with the resulting highest score.
1385 for (size_t i = 0; i < multiPV; ++i)
1387 int score = RootMoves[i].score;
1389 // This is our magic formula
1390 score += ( weakness * int(RootMoves[0].score - score)
1391 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1393 if (score > maxScore)
1396 best = RootMoves[i].pv[0];
1403 // uci_pv() formats PV information according to the UCI protocol. UCI
1404 // requires that all (if any) unsearched PV lines are sent using a previous
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 uciPVSize = 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 < uciPVSize; ++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
1434 ss << "info depth " << d / ONE_PLY
1435 << " seldepth " << selDepth
1436 << " multipv " << i + 1
1437 << " score " << UCI::value(v);
1439 if (!tb && i == PVIdx)
1440 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1442 ss << " nodes " << pos.nodes_searched()
1443 << " nps " << pos.nodes_searched() * 1000 / elapsed
1444 << " tbhits " << TB::Hits
1445 << " time " << elapsed
1448 for (size_t j = 0; j < RootMoves[i].pv.size(); ++j)
1449 ss << " " << UCI::move(RootMoves[i].pv[j], pos.is_chess960());
1458 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1459 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1460 /// first, even if the old TT entries have been overwritten.
1462 void RootMove::insert_pv_in_tt(Position& pos) {
1464 StateInfo state[MAX_PLY], *st = state;
1467 for ( ; idx < pv.size(); ++idx)
1470 TTEntry* tte = TT.probe(pos.key(), ttHit);
1472 if (!ttHit || tte->move() != pv[idx]) // Don't overwrite correct entries
1473 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE, TT.generation());
1475 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1477 pos.do_move(pv[idx], *st++);
1480 while (idx) pos.undo_move(pv[--idx]);
1484 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1485 /// exiting the search, for instance in case we stop the search during a fail high at
1486 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1487 /// 'ponder on' we have nothing to think on.
1489 Move RootMove::extract_ponder_from_tt(Position& pos)
1494 assert(pv.size() == 1);
1496 pos.do_move(pv[0], st);
1497 TTEntry* tte = TT.probe(pos.key(), found);
1498 Move m = found ? tte->move() : MOVE_NONE;
1499 if (!MoveList<LEGAL>(pos).contains(m))
1502 pos.undo_move(pv[0]);
1508 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1510 void Thread::idle_loop() {
1512 // Pointer 'this_sp' is not null only if we are called from split(), and not
1513 // at the thread creation. This means we are the split point's master.
1514 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : nullptr;
1516 assert(!this_sp || (this_sp->masterThread == this && searching));
1520 // If this thread has been assigned work, launch a search
1523 Threads.mutex.lock();
1525 assert(activeSplitPoint);
1526 SplitPoint* sp = activeSplitPoint;
1528 Threads.mutex.unlock();
1530 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1531 Position pos(*sp->pos, this);
1533 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1534 ss->splitPoint = sp;
1538 assert(activePosition == nullptr);
1540 activePosition = &pos;
1542 if (sp->nodeType == NonPV)
1543 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1545 else if (sp->nodeType == PV)
1546 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1548 else if (sp->nodeType == Root)
1549 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1557 activePosition = nullptr;
1558 sp->slavesMask.reset(idx);
1559 sp->allSlavesSearching = false;
1560 sp->nodes += pos.nodes_searched();
1562 // Wake up the master thread so to allow it to return from the idle
1563 // loop in case we are the last slave of the split point.
1564 if ( this != sp->masterThread
1565 && sp->slavesMask.none())
1567 assert(!sp->masterThread->searching);
1568 sp->masterThread->notify_one();
1571 // After releasing the lock we can't access any SplitPoint related data
1572 // in a safe way because it could have been released under our feet by
1576 // Try to late join to another split point if none of its slaves has
1577 // already finished.
1578 if (Threads.size() > 2)
1579 for (size_t i = 0; i < Threads.size(); ++i)
1581 const int size = Threads[i]->splitPointsSize; // Local copy
1582 sp = size ? &Threads[i]->splitPoints[size - 1] : nullptr;
1585 && sp->allSlavesSearching
1586 && available_to(Threads[i]))
1588 // Recheck the conditions under lock protection
1589 Threads.mutex.lock();
1592 if ( sp->allSlavesSearching
1593 && available_to(Threads[i]))
1595 sp->slavesMask.set(idx);
1596 activeSplitPoint = sp;
1601 Threads.mutex.unlock();
1603 break; // Just a single attempt
1608 // Grab the lock to avoid races with Thread::notify_one()
1609 std::unique_lock<std::mutex> lk(mutex);
1611 // If we are master and all slaves have finished then exit idle_loop
1612 if (this_sp && this_sp->slavesMask.none())
1618 // If we are not searching, wait for a condition to be signaled instead of
1619 // wasting CPU time polling for work.
1620 if (!searching && !exit)
1621 sleepCondition.wait(lk);
1626 /// check_time() is called by the timer thread when the timer triggers. It is
1627 /// used to print debug info and, more importantly, to detect when we are out of
1628 /// available time and thus stop the search.
1632 static Time::point lastInfoTime = Time::now();
1633 Time::point elapsed = Time::now() - SearchTime;
1635 if (Time::now() - lastInfoTime >= 1000)
1637 lastInfoTime = Time::now();
1641 // An engine may not stop pondering until told so by the GUI
1645 if (Limits.use_time_management())
1647 bool stillAtFirstMove = Signals.firstRootMove
1648 && !Signals.failedLowAtRoot
1649 && elapsed > TimeMgr.available_time() * 75 / 100;
1651 if ( stillAtFirstMove
1652 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1653 Signals.stop = true;
1655 else if (Limits.movetime && elapsed >= Limits.movetime)
1656 Signals.stop = true;
1658 else if (Limits.nodes)
1660 Threads.mutex.lock();
1662 int64_t nodes = RootPos.nodes_searched();
1664 // Loop across all split points and sum accumulated SplitPoint nodes plus
1665 // all the currently active positions nodes.
1666 for (Thread* th : Threads)
1667 for (int i = 0; i < th->splitPointsSize; ++i)
1669 SplitPoint& sp = th->splitPoints[i];
1675 for (size_t idx = 0; idx < Threads.size(); ++idx)
1676 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1677 nodes += Threads[idx]->activePosition->nodes_searched();
1682 Threads.mutex.unlock();
1684 if (nodes >= Limits.nodes)
1685 Signals.stop = true;