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 // Dynamic razoring margin based on depth
70 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
72 // Futility lookup tables (initialized at startup) and their access functions
73 int FutilityMoveCounts[2][16]; // [improving][depth]
75 inline Value futility_margin(Depth d) {
76 return Value(200 * d);
79 // Reduction lookup tables (initialized at startup) and their access function
80 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
82 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
83 return (Depth) Reductions[PvNode][i][std::min(int(d), 63)][std::min(mn, 63)];
88 double BestMoveChanges;
89 Value DrawValue[COLOR_NB];
92 MovesStats Countermoves, Followupmoves;
94 template <NodeType NT, bool SpNode>
95 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
97 template <NodeType NT, bool InCheck>
98 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
100 void id_loop(Position& pos);
101 Value value_to_tt(Value v, int ply);
102 Value value_from_tt(Value v, int ply);
103 void update_pv(Move* pv, Move move, Move* childPv);
104 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
105 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta);
108 Skill(int l, size_t rootSize) : level(l),
109 candidates(l < 20 ? std::min(4, (int)rootSize) : 0),
112 if (candidates) // Swap best PV line with the sub-optimal one
113 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
114 RootMoves.end(), best ? best : pick_move()));
117 size_t candidates_size() const { return candidates; }
118 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
129 /// Search::init() is called during startup to initialize various lookup tables
131 void Search::init() {
133 // Init reductions array
134 for (int d = 1; d < 64; ++d)
135 for (int mc = 1; mc < 64; ++mc)
137 double pvRed = 0.00 + log(double(d)) * log(double(mc)) / 3.00;
138 double nonPVRed = 0.33 + log(double(d)) * log(double(mc)) / 2.25;
140 Reductions[1][1][d][mc] = int8_t( pvRed >= 1.0 ? pvRed + 0.5: 0);
141 Reductions[0][1][d][mc] = int8_t(nonPVRed >= 1.0 ? nonPVRed + 0.5: 0);
143 Reductions[1][0][d][mc] = Reductions[1][1][d][mc];
144 Reductions[0][0][d][mc] = Reductions[0][1][d][mc];
146 // Increase reduction when eval is not improving
147 if (Reductions[0][0][d][mc] >= 2)
148 Reductions[0][0][d][mc] += 1;
151 // Init futility move count array
152 for (int d = 0; d < 16; ++d)
154 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
155 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
160 /// Search::perft() is our utility to verify move generation. All the leaf nodes
161 /// up to the given depth are generated and counted and the sum returned.
163 uint64_t Search::perft(Position& pos, Depth depth) {
166 uint64_t cnt, nodes = 0;
168 const bool leaf = (depth == 2 * ONE_PLY);
170 for (MoveList<LEGAL> it(pos); *it; ++it)
172 if (Root && depth <= ONE_PLY)
176 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
177 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
182 sync_cout << UCI::move(*it, pos.is_chess960()) << ": " << cnt << sync_endl;
187 template uint64_t Search::perft<true>(Position& pos, Depth depth);
190 /// Search::think() is the external interface to Stockfish's search, and is
191 /// called by the main thread when the program receives the UCI 'go' command. It
192 /// searches from RootPos and at the end prints the "bestmove" to output.
194 void Search::think() {
196 TimeMgr.init(Limits, RootPos.side_to_move(), RootPos.game_ply());
198 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
199 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
200 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
203 TB::RootInTB = false;
204 TB::UseRule50 = Options["Syzygy50MoveRule"];
205 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
206 TB::Cardinality = Options["SyzygyProbeLimit"];
208 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
209 if (TB::Cardinality > TB::MaxCardinality)
211 TB::Cardinality = TB::MaxCardinality;
212 TB::ProbeDepth = DEPTH_ZERO;
215 if (RootMoves.empty())
217 RootMoves.push_back(MOVE_NONE);
218 sync_cout << "info depth 0 score "
219 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
224 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
225 + RootPos.count<ALL_PIECES>(BLACK))
227 // If the current root position is in the tablebases then RootMoves
228 // contains only moves that preserve the draw or win.
229 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
232 TB::Cardinality = 0; // Do not probe tablebases during the search
234 else // If DTZ tables are missing, use WDL tables as a fallback
236 // Filter out moves that do not preserve a draw or win
237 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
239 // Only probe during search if winning
240 if (TB::Score <= VALUE_DRAW)
246 TB::Hits = RootMoves.size();
249 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
250 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
255 for (size_t i = 0; i < Threads.size(); ++i)
256 Threads[i]->maxPly = 0;
258 Threads.timer->run = true;
259 Threads.timer->notify_one(); // Wake up the recurring timer
261 id_loop(RootPos); // Let's start searching !
263 Threads.timer->run = false;
266 // When we reach the maximum depth, we can arrive here without a raise of
267 // Signals.stop. However, if we are pondering or in an infinite search,
268 // the UCI protocol states that we shouldn't print the best move before the
269 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
270 // until the GUI sends one of those commands (which also raises Signals.stop).
271 if (!Signals.stop && (Limits.ponder || Limits.infinite))
273 Signals.stopOnPonderhit = true;
274 RootPos.this_thread()->wait_for(Signals.stop);
277 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
279 if (RootMoves[0].pv.size() > 1)
280 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
282 std::cout << sync_endl;
288 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
289 // with increasing depth until the allocated thinking time has been consumed,
290 // user stops the search, or the maximum search depth is reached.
292 void id_loop(Position& pos) {
294 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
296 Value bestValue, alpha, beta, delta;
298 std::memset(ss-2, 0, 5 * sizeof(Stack));
302 bestValue = delta = alpha = -VALUE_INFINITE;
303 beta = VALUE_INFINITE;
308 Countermoves.clear();
309 Followupmoves.clear();
311 size_t multiPV = Options["MultiPV"];
312 Skill skill(Options["Skill Level"], RootMoves.size());
314 // Do we have to play with skill handicap? In this case enable MultiPV search
315 // that we will use behind the scenes to retrieve a set of possible moves.
316 multiPV = std::max(multiPV, skill.candidates_size());
318 // Iterative deepening loop until requested to stop or target depth reached
319 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
321 // Age out PV variability metric
322 BestMoveChanges *= 0.5;
324 // Save the last iteration's scores before first PV line is searched and
325 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
326 for (size_t i = 0; i < RootMoves.size(); ++i)
327 RootMoves[i].previousScore = RootMoves[i].score;
329 // MultiPV loop. We perform a full root search for each PV line
330 for (PVIdx = 0; PVIdx < std::min(multiPV, RootMoves.size()) && !Signals.stop; ++PVIdx)
332 // Reset aspiration window starting size
333 if (depth >= 5 * ONE_PLY)
336 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
337 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
340 // Start with a small aspiration window and, in the case of a fail
341 // high/low, re-search with a bigger window until we're not failing
345 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
347 // Bring the best move to the front. It is critical that sorting
348 // is done with a stable algorithm because all the values but the
349 // first and eventually the new best one are set to -VALUE_INFINITE
350 // and we want to keep the same order for all the moves except the
351 // new PV that goes to the front. Note that in case of MultiPV
352 // search the already searched PV lines are preserved.
353 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
355 // Write PV back to transposition table in case the relevant
356 // entries have been overwritten during the search.
357 for (size_t i = 0; i <= PVIdx; ++i)
358 RootMoves[i].insert_pv_in_tt(pos);
360 // If search has been stopped break immediately. Sorting and
361 // writing PV back to TT is safe because RootMoves is still
362 // valid, although it refers to previous iteration.
366 // When failing high/low give some update (without cluttering
367 // the UI) before a re-search.
368 if ( (bestValue <= alpha || bestValue >= beta)
369 && Time::now() - SearchTime > 3000)
370 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
372 // In case of failing low/high increase aspiration window and
373 // re-search, otherwise exit the loop.
374 if (bestValue <= alpha)
376 beta = (alpha + beta) / 2;
377 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
379 Signals.failedLowAtRoot = true;
380 Signals.stopOnPonderhit = false;
382 else if (bestValue >= beta)
384 alpha = (alpha + beta) / 2;
385 beta = std::min(bestValue + delta, VALUE_INFINITE);
392 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
395 // Sort the PV lines searched so far and update the GUI
396 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
399 sync_cout << "info nodes " << RootPos.nodes_searched()
400 << " time " << Time::now() - SearchTime << sync_endl;
402 else if ( PVIdx + 1 == std::min(multiPV, RootMoves.size())
403 || Time::now() - SearchTime > 3000)
404 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
407 // If skill levels are enabled and time is up, pick a sub-optimal best move
408 if (skill.candidates_size() && skill.time_to_pick(depth))
411 // Have we found a "mate in x"?
413 && bestValue >= VALUE_MATE_IN_MAX_PLY
414 && VALUE_MATE - bestValue <= 2 * Limits.mate)
417 // Do we have time for the next iteration? Can we stop searching now?
418 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
420 // Take some extra time if the best move has changed
421 if (depth > 4 * ONE_PLY && multiPV == 1)
422 TimeMgr.pv_instability(BestMoveChanges);
424 // Stop the search if only one legal move is available or all
425 // of the available time has been used.
426 if ( RootMoves.size() == 1
427 || Time::now() - SearchTime > TimeMgr.available_time())
429 // If we are allowed to ponder do not stop the search now but
430 // keep pondering until the GUI sends "ponderhit" or "stop".
432 Signals.stopOnPonderhit = true;
441 // search<>() is the main search function for both PV and non-PV nodes and for
442 // normal and SplitPoint nodes. When called just after a split point the search
443 // is simpler because we have already probed the hash table, done a null move
444 // search, and searched the first move before splitting, so we don't have to
445 // repeat all this work again. We also don't need to store anything to the hash
446 // table here: This is taken care of after we return from the split point.
448 template <NodeType NT, bool SpNode>
449 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
451 const bool RootNode = NT == Root;
452 const bool PvNode = NT == PV || NT == Root;
454 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
455 assert(PvNode || (alpha == beta - 1));
456 assert(depth > DEPTH_ZERO);
458 Move pv[MAX_PLY+1], quietsSearched[64];
461 SplitPoint* splitPoint;
463 Move ttMove, move, excludedMove, bestMove;
464 Depth extension, newDepth, predictedDepth;
465 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
466 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
467 bool captureOrPromotion, dangerous, doFullDepthSearch;
468 int moveCount, quietCount;
470 // Step 1. Initialize node
471 Thread* thisThread = pos.this_thread();
472 inCheck = pos.checkers();
476 splitPoint = ss->splitPoint;
477 bestMove = splitPoint->bestMove;
478 bestValue = splitPoint->bestValue;
481 ttMove = excludedMove = MOVE_NONE;
482 ttValue = VALUE_NONE;
484 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
489 moveCount = quietCount = 0;
490 bestValue = -VALUE_INFINITE;
491 ss->ply = (ss-1)->ply + 1;
493 // Used to send selDepth info to GUI
494 if (PvNode && thisThread->maxPly < ss->ply)
495 thisThread->maxPly = ss->ply;
499 // Step 2. Check for aborted search and immediate draw
500 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
501 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
503 // Step 3. Mate distance pruning. Even if we mate at the next move our score
504 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
505 // a shorter mate was found upward in the tree then there is no need to search
506 // because we will never beat the current alpha. Same logic but with reversed
507 // signs applies also in the opposite condition of being mated instead of giving
508 // mate. In this case return a fail-high score.
509 alpha = std::max(mated_in(ss->ply), alpha);
510 beta = std::min(mate_in(ss->ply+1), beta);
515 assert(0 <= ss->ply && ss->ply < MAX_PLY);
517 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
518 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
519 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
521 // Step 4. Transposition table lookup
522 // We don't want the score of a partial search to overwrite a previous full search
523 // TT value, so we use a different position key in case of an excluded move.
524 excludedMove = ss->excludedMove;
525 posKey = excludedMove ? pos.exclusion_key() : pos.key();
526 tte = TT.probe(posKey, ttHit);
527 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
528 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
530 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
533 && tte->depth() >= depth
534 && ttValue != VALUE_NONE // Only in case of TT access race
535 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
536 : (tte->bound() & BOUND_UPPER)))
538 ss->currentMove = ttMove; // Can be MOVE_NONE
540 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
541 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
542 update_stats(pos, ss, ttMove, depth, NULL, 0);
547 // Step 4a. Tablebase probe
548 if (!RootNode && TB::Cardinality)
550 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
552 if ( piecesCnt <= TB::Cardinality
553 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
554 && pos.rule50_count() == 0)
556 int found, v = Tablebases::probe_wdl(pos, &found);
562 int drawScore = TB::UseRule50 ? 1 : 0;
564 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
565 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
566 : VALUE_DRAW + 2 * v * drawScore;
568 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
569 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
570 MOVE_NONE, VALUE_NONE, TT.generation());
577 // Step 5. Evaluate the position statically and update parent's gain statistics
580 ss->staticEval = eval = VALUE_NONE;
586 // Never assume anything on values stored in TT
587 if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
588 eval = ss->staticEval = evaluate(pos);
590 // Can ttValue be used as a better position evaluation?
591 if (ttValue != VALUE_NONE)
592 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
597 eval = ss->staticEval =
598 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
600 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
603 if (ss->skipEarlyPruning)
606 if ( !pos.captured_piece_type()
607 && ss->staticEval != VALUE_NONE
608 && (ss-1)->staticEval != VALUE_NONE
609 && (move = (ss-1)->currentMove) != MOVE_NULL
611 && type_of(move) == NORMAL)
613 Square to = to_sq(move);
614 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
617 // Step 6. Razoring (skipped when in check)
619 && depth < 4 * ONE_PLY
620 && eval + razor_margin(depth) <= alpha
621 && ttMove == MOVE_NONE
622 && !pos.pawn_on_7th(pos.side_to_move()))
624 if ( depth <= ONE_PLY
625 && eval + razor_margin(3 * ONE_PLY) <= alpha)
626 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
628 Value ralpha = alpha - razor_margin(depth);
629 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
634 // Step 7. Futility pruning: child node (skipped when in check)
636 && depth < 7 * ONE_PLY
637 && eval - futility_margin(depth) >= beta
638 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
639 && pos.non_pawn_material(pos.side_to_move()))
640 return eval - futility_margin(depth);
642 // Step 8. Null move search with verification search (is omitted in PV nodes)
644 && depth >= 2 * ONE_PLY
646 && pos.non_pawn_material(pos.side_to_move()))
648 ss->currentMove = MOVE_NULL;
650 assert(eval - beta >= 0);
652 // Null move dynamic reduction based on depth and value
653 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
655 pos.do_null_move(st);
656 (ss+1)->skipEarlyPruning = true;
657 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
658 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
659 (ss+1)->skipEarlyPruning = false;
660 pos.undo_null_move();
662 if (nullValue >= beta)
664 // Do not return unproven mate scores
665 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
668 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
671 // Do verification search at high depths
672 ss->skipEarlyPruning = true;
673 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
674 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
675 ss->skipEarlyPruning = false;
682 // Step 9. ProbCut (skipped when in check)
683 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
684 // and a reduced search returns a value much above beta, we can (almost) safely
685 // prune the previous move.
687 && depth >= 5 * ONE_PLY
688 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
690 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
691 Depth rdepth = depth - 4 * ONE_PLY;
693 assert(rdepth >= ONE_PLY);
694 assert((ss-1)->currentMove != MOVE_NONE);
695 assert((ss-1)->currentMove != MOVE_NULL);
697 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
700 while ((move = mp.next_move<false>()) != MOVE_NONE)
701 if (pos.legal(move, ci.pinned))
703 ss->currentMove = move;
704 pos.do_move(move, st, ci, pos.gives_check(move, ci));
705 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
712 // Step 10. Internal iterative deepening (skipped when in check)
713 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
715 && (PvNode || ss->staticEval + 256 >= beta))
717 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
718 ss->skipEarlyPruning = true;
719 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
720 ss->skipEarlyPruning = false;
722 tte = TT.probe(posKey, ttHit);
723 ttMove = ttHit ? tte->move() : MOVE_NONE;
726 moves_loop: // When in check and at SpNode search starts from here
728 Square prevMoveSq = to_sq((ss-1)->currentMove);
729 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
730 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
732 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
733 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
734 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
736 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
738 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
739 improving = ss->staticEval >= (ss-2)->staticEval
740 || ss->staticEval == VALUE_NONE
741 ||(ss-2)->staticEval == VALUE_NONE;
743 singularExtensionNode = !RootNode
745 && depth >= 8 * ONE_PLY
746 && ttMove != MOVE_NONE
747 /* && ttValue != VALUE_NONE Already implicit in the next condition */
748 && abs(ttValue) < VALUE_KNOWN_WIN
749 && !excludedMove // Recursive singular search is not allowed
750 && (tte->bound() & BOUND_LOWER)
751 && tte->depth() >= depth - 3 * ONE_PLY;
753 // Step 11. Loop through moves
754 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
755 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
759 if (move == excludedMove)
762 // At root obey the "searchmoves" option and skip moves not listed in Root
763 // Move List. As a consequence any illegal move is also skipped. In MultiPV
764 // mode we also skip PV moves which have been already searched.
765 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
770 // Shared counter cannot be decremented later if the move turns out to be illegal
771 if (!pos.legal(move, ci.pinned))
774 moveCount = ++splitPoint->moveCount;
775 splitPoint->mutex.unlock();
782 Signals.firstRootMove = (moveCount == 1);
784 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
785 sync_cout << "info depth " << depth / ONE_PLY
786 << " currmove " << UCI::move(move, pos.is_chess960())
787 << " currmovenumber " << moveCount + PVIdx << sync_endl;
793 extension = DEPTH_ZERO;
794 captureOrPromotion = pos.capture_or_promotion(move);
796 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
797 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
798 : pos.gives_check(move, ci);
800 dangerous = givesCheck
801 || type_of(move) != NORMAL
802 || pos.advanced_pawn_push(move);
804 // Step 12. Extend checks
805 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
808 // Singular extension search. If all moves but one fail low on a search of
809 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
810 // is singular and should be extended. To verify this we do a reduced search
811 // on all the other moves but the ttMove and if the result is lower than
812 // ttValue minus a margin then we extend the ttMove.
813 if ( singularExtensionNode
816 && pos.legal(move, ci.pinned))
818 Value rBeta = ttValue - 2 * depth / ONE_PLY;
819 ss->excludedMove = move;
820 ss->skipEarlyPruning = true;
821 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
822 ss->skipEarlyPruning = false;
823 ss->excludedMove = MOVE_NONE;
829 // Update the current move (this must be done after singular extension search)
830 newDepth = depth - ONE_PLY + extension;
832 // Step 13. Pruning at shallow depth (exclude PV nodes)
834 && !captureOrPromotion
837 && bestValue > VALUE_MATED_IN_MAX_PLY)
839 // Move count based pruning
840 if ( depth < 16 * ONE_PLY
841 && moveCount >= FutilityMoveCounts[improving][depth])
844 splitPoint->mutex.lock();
849 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
851 // Futility pruning: parent node
852 if (predictedDepth < 7 * ONE_PLY)
854 futilityValue = ss->staticEval + futility_margin(predictedDepth)
855 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
857 if (futilityValue <= alpha)
859 bestValue = std::max(bestValue, futilityValue);
863 splitPoint->mutex.lock();
864 if (bestValue > splitPoint->bestValue)
865 splitPoint->bestValue = bestValue;
871 // Prune moves with negative SEE at low depths
872 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
875 splitPoint->mutex.lock();
881 // Speculative prefetch as early as possible
882 prefetch((char*)TT.first_entry(pos.key_after(move)));
884 // Check for legality just before making the move
885 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
891 ss->currentMove = move;
892 if (!SpNode && !captureOrPromotion && quietCount < 64)
893 quietsSearched[quietCount++] = move;
895 // Step 14. Make the move
896 pos.do_move(move, st, ci, givesCheck);
898 // Step 15. Reduced depth search (LMR). If the move fails high it will be
899 // re-searched at full depth.
900 if ( depth >= 3 * ONE_PLY
902 && !captureOrPromotion
903 && move != ss->killers[0]
904 && move != ss->killers[1])
906 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
908 if ( (!PvNode && cutNode)
909 || History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
910 ss->reduction += ONE_PLY;
912 if (move == countermoves[0] || move == countermoves[1])
913 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
915 // Decrease reduction for moves that escape a capture
917 && type_of(move) == NORMAL
918 && type_of(pos.piece_on(to_sq(move))) != PAWN
919 && pos.see(make_move(to_sq(move), from_sq(move))) < 0)
920 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
922 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
924 alpha = splitPoint->alpha;
926 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
928 // Re-search at intermediate depth if reduction is very high
929 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
931 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
932 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
935 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
936 ss->reduction = DEPTH_ZERO;
939 doFullDepthSearch = !PvNode || moveCount > 1;
941 // Step 16. Full depth search, when LMR is skipped or fails high
942 if (doFullDepthSearch)
945 alpha = splitPoint->alpha;
947 value = newDepth < ONE_PLY ?
948 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
949 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
950 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
953 // For PV nodes only, do a full PV search on the first move or after a fail
954 // high (in the latter case search only if value < beta), otherwise let the
955 // parent node fail low with value <= alpha and to try another move.
956 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
959 (ss+1)->pv[0] = MOVE_NONE;
961 value = newDepth < ONE_PLY ?
962 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
963 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
964 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
967 // Step 17. Undo move
970 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
972 // Step 18. Check for new best move
975 splitPoint->mutex.lock();
976 bestValue = splitPoint->bestValue;
977 alpha = splitPoint->alpha;
980 // Finished searching the move. If a stop or a cutoff occurred, the return
981 // value of the search cannot be trusted, and we return immediately without
982 // updating best move, PV and TT.
983 if (Signals.stop || thisThread->cutoff_occurred())
988 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
990 // PV move or new best move ?
991 if (moveCount == 1 || value > alpha)
998 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1001 // We record how often the best move has been changed in each
1002 // iteration. This information is used for time management: When
1003 // the best move changes frequently, we allocate some more time.
1008 // All other moves but the PV are set to the lowest value: this is
1009 // not a problem when sorting because the sort is stable and the
1010 // move position in the list is preserved - just the PV is pushed up.
1011 rm.score = -VALUE_INFINITE;
1014 if (value > bestValue)
1016 bestValue = SpNode ? splitPoint->bestValue = value : value;
1020 bestMove = SpNode ? splitPoint->bestMove = move : move;
1022 if (PvNode && !RootNode) // Update pv even in fail-high case
1023 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1025 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1026 alpha = SpNode ? splitPoint->alpha = value : value;
1029 assert(value >= beta); // Fail high
1032 splitPoint->cutoff = true;
1039 // Step 19. Check for splitting the search
1041 && Threads.size() >= 2
1042 && depth >= Threads.minimumSplitDepth
1043 && ( !thisThread->activeSplitPoint
1044 || !thisThread->activeSplitPoint->allSlavesSearching)
1045 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1047 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1049 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1050 depth, moveCount, &mp, NT, cutNode);
1052 if (Signals.stop || thisThread->cutoff_occurred())
1055 if (bestValue >= beta)
1063 // Following condition would detect a stop or a cutoff set only after move
1064 // loop has been completed. But in this case bestValue is valid because we
1065 // have fully searched our subtree, and we can anyhow save the result in TT.
1067 if (Signals.stop || thisThread->cutoff_occurred())
1071 // Step 20. Check for mate and stalemate
1072 // All legal moves have been searched and if there are no legal moves, it
1073 // must be mate or stalemate. If we are in a singular extension search then
1074 // return a fail low score.
1076 bestValue = excludedMove ? alpha
1077 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1079 // Quiet best move: update killers, history, countermoves and followupmoves
1080 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1081 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1083 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1084 bestValue >= beta ? BOUND_LOWER :
1085 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1086 depth, bestMove, ss->staticEval, TT.generation());
1088 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1094 // qsearch() is the quiescence search function, which is called by the main
1095 // search function when the remaining depth is zero (or, to be more precise,
1096 // less than ONE_PLY).
1098 template <NodeType NT, bool InCheck>
1099 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1101 const bool PvNode = NT == PV;
1103 assert(NT == PV || NT == NonPV);
1104 assert(InCheck == !!pos.checkers());
1105 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1106 assert(PvNode || (alpha == beta - 1));
1107 assert(depth <= DEPTH_ZERO);
1113 Move ttMove, move, bestMove;
1114 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1115 bool ttHit, givesCheck, evasionPrunable;
1120 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1122 ss->pv[0] = MOVE_NONE;
1125 ss->currentMove = bestMove = MOVE_NONE;
1126 ss->ply = (ss-1)->ply + 1;
1128 // Check for an instant draw or if the maximum ply has been reached
1129 if (pos.is_draw() || ss->ply >= MAX_PLY)
1130 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1132 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1134 // Decide whether or not to include checks: this fixes also the type of
1135 // TT entry depth that we are going to use. Note that in qsearch we use
1136 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1137 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1138 : DEPTH_QS_NO_CHECKS;
1140 // Transposition table lookup
1142 tte = TT.probe(posKey, ttHit);
1143 ttMove = ttHit ? tte->move() : MOVE_NONE;
1144 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1148 && tte->depth() >= ttDepth
1149 && ttValue != VALUE_NONE // Only in case of TT access race
1150 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1151 : (tte->bound() & BOUND_UPPER)))
1153 ss->currentMove = ttMove; // Can be MOVE_NONE
1157 // Evaluate the position statically
1160 ss->staticEval = VALUE_NONE;
1161 bestValue = futilityBase = -VALUE_INFINITE;
1167 // Never assume anything on values stored in TT
1168 if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
1169 ss->staticEval = bestValue = evaluate(pos);
1171 // Can ttValue be used as a better position evaluation?
1172 if (ttValue != VALUE_NONE)
1173 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1174 bestValue = ttValue;
1177 ss->staticEval = bestValue =
1178 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1180 // Stand pat. Return immediately if static value is at least beta
1181 if (bestValue >= beta)
1184 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1185 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1190 if (PvNode && bestValue > alpha)
1193 futilityBase = bestValue + 128;
1196 // Initialize a MovePicker object for the current position, and prepare
1197 // to search the moves. Because the depth is <= 0 here, only captures,
1198 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1200 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1203 // Loop through the moves until no moves remain or a beta cutoff occurs
1204 while ((move = mp.next_move<false>()) != MOVE_NONE)
1206 assert(is_ok(move));
1208 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1209 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1210 : pos.gives_check(move, ci);
1216 && futilityBase > -VALUE_KNOWN_WIN
1217 && !pos.advanced_pawn_push(move))
1219 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1221 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1223 if (futilityValue < beta)
1225 bestValue = std::max(bestValue, futilityValue);
1229 if (futilityBase < beta && pos.see(move) <= VALUE_ZERO)
1231 bestValue = std::max(bestValue, futilityBase);
1236 // Detect non-capture evasions that are candidates to be pruned
1237 evasionPrunable = InCheck
1238 && bestValue > VALUE_MATED_IN_MAX_PLY
1239 && !pos.capture(move)
1240 && !pos.can_castle(pos.side_to_move());
1242 // Don't search moves with negative SEE values
1244 && (!InCheck || evasionPrunable)
1245 && type_of(move) != PROMOTION
1246 && pos.see_sign(move) < VALUE_ZERO)
1249 // Speculative prefetch as early as possible
1250 prefetch((char*)TT.first_entry(pos.key_after(move)));
1252 // Check for legality just before making the move
1253 if (!pos.legal(move, ci.pinned))
1256 ss->currentMove = move;
1258 // Make and search the move
1259 pos.do_move(move, st, ci, givesCheck);
1260 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1261 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1262 pos.undo_move(move);
1264 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1266 // Check for new best move
1267 if (value > bestValue)
1273 if (PvNode) // Update pv even in fail-high case
1274 update_pv(ss->pv, move, (ss+1)->pv);
1276 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1283 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1284 ttDepth, move, ss->staticEval, TT.generation());
1292 // All legal moves have been searched. A special case: If we're in check
1293 // and no legal moves were found, it is checkmate.
1294 if (InCheck && bestValue == -VALUE_INFINITE)
1295 return mated_in(ss->ply); // Plies to mate from the root
1297 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1298 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1299 ttDepth, bestMove, ss->staticEval, TT.generation());
1301 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1307 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1308 // "plies to mate from the current position". Non-mate scores are unchanged.
1309 // The function is called before storing a value in the transposition table.
1311 Value value_to_tt(Value v, int ply) {
1313 assert(v != VALUE_NONE);
1315 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1316 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1320 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1321 // from the transposition table (which refers to the plies to mate/be mated
1322 // from current position) to "plies to mate/be mated from the root".
1324 Value value_from_tt(Value v, int ply) {
1326 return v == VALUE_NONE ? VALUE_NONE
1327 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1328 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1332 // update_pv() adds current move and appends child pv[]
1334 void update_pv(Move* pv, Move move, Move* childPv) {
1336 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1341 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1344 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1346 if (ss->killers[0] != move)
1348 ss->killers[1] = ss->killers[0];
1349 ss->killers[0] = move;
1352 // Increase history value of the cut-off move and decrease all the other
1353 // played quiet moves.
1354 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1355 History.update(pos.moved_piece(move), to_sq(move), bonus);
1356 for (int i = 0; i < quietsCnt; ++i)
1359 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1362 if (is_ok((ss-1)->currentMove))
1364 Square prevMoveSq = to_sq((ss-1)->currentMove);
1365 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1368 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1370 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1371 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1376 // When playing with a strength handicap, choose best move among the first 'candidates'
1377 // RootMoves using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1379 Move Skill::pick_move() {
1381 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1382 static PRNG rng(Time::now());
1384 // RootMoves are already sorted by score in descending order
1385 int variance = std::min(RootMoves[0].score - RootMoves[candidates - 1].score, PawnValueMg);
1386 int weakness = 120 - 2 * level;
1387 int maxScore = -VALUE_INFINITE;
1390 // Choose best move. For each move score we add two terms both dependent on
1391 // weakness. One deterministic and bigger for weaker moves, and one random,
1392 // then we choose the move with the resulting highest score.
1393 for (size_t i = 0; i < candidates; ++i)
1395 int score = RootMoves[i].score;
1397 // Don't allow crazy blunders even at very low skills
1398 if (i > 0 && RootMoves[i - 1].score > score + 2 * PawnValueMg)
1401 // This is our magic formula
1402 score += ( weakness * int(RootMoves[0].score - score)
1403 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1405 if (score > maxScore)
1408 best = RootMoves[i].pv[0];
1415 // uci_pv() formats PV information according to the UCI protocol. UCI
1416 // requires that all (if any) unsearched PV lines are sent using a previous
1419 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1421 std::stringstream ss;
1422 Time::point elapsed = Time::now() - SearchTime + 1;
1423 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1426 for (size_t i = 0; i < Threads.size(); ++i)
1427 if (Threads[i]->maxPly > selDepth)
1428 selDepth = Threads[i]->maxPly;
1430 for (size_t i = 0; i < uciPVSize; ++i)
1432 bool updated = (i <= PVIdx);
1434 if (depth == ONE_PLY && !updated)
1437 Depth d = updated ? depth : depth - ONE_PLY;
1438 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1440 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1441 v = tb ? TB::Score : v;
1443 if (ss.rdbuf()->in_avail()) // Not at first line
1446 ss << "info depth " << d / ONE_PLY
1447 << " seldepth " << selDepth
1448 << " multipv " << i + 1
1449 << " score " << UCI::value(v);
1451 if (!tb && i == PVIdx)
1452 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1454 ss << " nodes " << pos.nodes_searched()
1455 << " nps " << pos.nodes_searched() * 1000 / elapsed
1456 << " tbhits " << TB::Hits
1457 << " time " << elapsed
1460 for (size_t j = 0; j < RootMoves[i].pv.size(); ++j)
1461 ss << " " << UCI::move(RootMoves[i].pv[j], pos.is_chess960());
1470 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1471 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1472 /// first, even if the old TT entries have been overwritten.
1474 void RootMove::insert_pv_in_tt(Position& pos) {
1476 StateInfo state[MAX_PLY], *st = state;
1479 for ( ; idx < pv.size(); ++idx)
1482 TTEntry* tte = TT.probe(pos.key(), ttHit);
1484 if (!ttHit || tte->move() != pv[idx]) // Don't overwrite correct entries
1485 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE, TT.generation());
1487 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1489 pos.do_move(pv[idx], *st++);
1492 while (idx) pos.undo_move(pv[--idx]);
1496 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1498 void Thread::idle_loop() {
1500 // Pointer 'this_sp' is not null only if we are called from split(), and not
1501 // at the thread creation. This means we are the split point's master.
1502 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1504 assert(!this_sp || (this_sp->masterThread == this && searching));
1508 // If this thread has been assigned work, launch a search
1511 Threads.mutex.lock();
1513 assert(activeSplitPoint);
1514 SplitPoint* sp = activeSplitPoint;
1516 Threads.mutex.unlock();
1518 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1519 Position pos(*sp->pos, this);
1521 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1522 ss->splitPoint = sp;
1526 assert(activePosition == NULL);
1528 activePosition = &pos;
1530 if (sp->nodeType == NonPV)
1531 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1533 else if (sp->nodeType == PV)
1534 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1536 else if (sp->nodeType == Root)
1537 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1545 activePosition = NULL;
1546 sp->slavesMask.reset(idx);
1547 sp->allSlavesSearching = false;
1548 sp->nodes += pos.nodes_searched();
1550 // Wake up the master thread so to allow it to return from the idle
1551 // loop in case we are the last slave of the split point.
1552 if ( this != sp->masterThread
1553 && sp->slavesMask.none())
1555 assert(!sp->masterThread->searching);
1556 sp->masterThread->notify_one();
1559 // After releasing the lock we can't access any SplitPoint related data
1560 // in a safe way because it could have been released under our feet by
1564 // Try to late join to another split point if none of its slaves has
1565 // already finished.
1566 if (Threads.size() > 2)
1567 for (size_t i = 0; i < Threads.size(); ++i)
1569 const int size = Threads[i]->splitPointsSize; // Local copy
1570 sp = size ? &Threads[i]->splitPoints[size - 1] : NULL;
1573 && sp->allSlavesSearching
1574 && available_to(Threads[i]))
1576 // Recheck the conditions under lock protection
1577 Threads.mutex.lock();
1580 if ( sp->allSlavesSearching
1581 && available_to(Threads[i]))
1583 sp->slavesMask.set(idx);
1584 activeSplitPoint = sp;
1589 Threads.mutex.unlock();
1591 break; // Just a single attempt
1596 // Grab the lock to avoid races with Thread::notify_one()
1599 // If we are master and all slaves have finished then exit idle_loop
1600 if (this_sp && this_sp->slavesMask.none())
1607 // If we are not searching, wait for a condition to be signaled instead of
1608 // wasting CPU time polling for work.
1609 if (!searching && !exit)
1610 sleepCondition.wait(mutex);
1617 /// check_time() is called by the timer thread when the timer triggers. It is
1618 /// used to print debug info and, more importantly, to detect when we are out of
1619 /// available time and thus stop the search.
1623 static Time::point lastInfoTime = Time::now();
1624 Time::point elapsed = Time::now() - SearchTime;
1626 if (Time::now() - lastInfoTime >= 1000)
1628 lastInfoTime = Time::now();
1632 // An engine may not stop pondering until told so by the GUI
1636 if (Limits.use_time_management())
1638 bool stillAtFirstMove = Signals.firstRootMove
1639 && !Signals.failedLowAtRoot
1640 && elapsed > TimeMgr.available_time() * 75 / 100;
1642 if ( stillAtFirstMove
1643 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1644 Signals.stop = true;
1646 else if (Limits.movetime && elapsed >= Limits.movetime)
1647 Signals.stop = true;
1649 else if (Limits.nodes)
1651 Threads.mutex.lock();
1653 int64_t nodes = RootPos.nodes_searched();
1655 // Loop across all split points and sum accumulated SplitPoint nodes plus
1656 // all the currently active positions nodes.
1657 for (size_t i = 0; i < Threads.size(); ++i)
1658 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1660 SplitPoint& sp = Threads[i]->splitPoints[j];
1666 for (size_t idx = 0; idx < Threads.size(); ++idx)
1667 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1668 nodes += Threads[idx]->activePosition->nodes_searched();
1673 Threads.mutex.unlock();
1675 if (nodes >= Limits.nodes)
1676 Signals.stop = true;