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;
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];
98 CounterMovesHistoryStats CounterMovesHistory;
100 MovesStats Countermoves, Followupmoves;
102 template <NodeType NT, bool SpNode>
103 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
105 template <NodeType NT, bool InCheck>
106 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
108 void id_loop(Position& pos);
109 Value value_to_tt(Value v, int ply);
110 Value value_from_tt(Value v, int ply);
111 void update_pv(Move* pv, Move move, Move* childPv);
112 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
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 auto& m : MoveList<LEGAL>(pos))
158 if (Root && depth <= ONE_PLY)
162 pos.do_move(m, st, pos.gives_check(m, ci));
163 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
168 sync_cout << UCI::move(m, 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(RootMove(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;
293 CounterMovesHistory.clear();
295 Countermoves.clear();
296 Followupmoves.clear();
298 size_t multiPV = Options["MultiPV"];
299 Skill skill(Options["Skill Level"]);
301 // When playing with strength handicap enable MultiPV search that we will
302 // use behind the scenes to retrieve a set of possible moves.
304 multiPV = std::max(multiPV, (size_t)4);
306 multiPV = std::min(multiPV, RootMoves.size());
308 // Iterative deepening loop until requested to stop or target depth reached
309 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
311 // Age out PV variability metric
312 BestMoveChanges *= 0.5;
314 // Save the last iteration's scores before first PV line is searched and
315 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
316 for (RootMove& rm : RootMoves)
317 rm.previousScore = rm.score;
319 // MultiPV loop. We perform a full root search for each PV line
320 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
322 // Reset aspiration window starting size
323 if (depth >= 5 * ONE_PLY)
326 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
327 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
330 // Start with a small aspiration window and, in the case of a fail
331 // high/low, re-search with a bigger window until we're not failing
335 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
337 // Bring the best move to the front. It is critical that sorting
338 // is done with a stable algorithm because all the values but the
339 // first and eventually the new best one are set to -VALUE_INFINITE
340 // and we want to keep the same order for all the moves except the
341 // new PV that goes to the front. Note that in case of MultiPV
342 // search the already searched PV lines are preserved.
343 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
345 // Write PV back to transposition table in case the relevant
346 // entries have been overwritten during the search.
347 for (size_t i = 0; i <= PVIdx; ++i)
348 RootMoves[i].insert_pv_in_tt(pos);
350 // If search has been stopped break immediately. Sorting and
351 // writing PV back to TT is safe because RootMoves is still
352 // valid, although it refers to previous iteration.
356 // When failing high/low give some update (without cluttering
357 // the UI) before a re-search.
359 && (bestValue <= alpha || bestValue >= beta)
360 && now() - SearchTime > 3000)
361 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
363 // In case of failing low/high increase aspiration window and
364 // re-search, otherwise exit the loop.
365 if (bestValue <= alpha)
367 beta = (alpha + beta) / 2;
368 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
370 Signals.failedLowAtRoot = true;
371 Signals.stopOnPonderhit = false;
373 else if (bestValue >= beta)
375 alpha = (alpha + beta) / 2;
376 beta = std::min(bestValue + delta, VALUE_INFINITE);
383 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
386 // Sort the PV lines searched so far and update the GUI
387 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
390 sync_cout << "info nodes " << RootPos.nodes_searched()
391 << " time " << now() - SearchTime << sync_endl;
393 else if (PVIdx + 1 == multiPV || now() - SearchTime > 3000)
394 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
397 // If skill level is enabled and time is up, pick a sub-optimal best move
398 if (skill.enabled() && skill.time_to_pick(depth))
399 skill.pick_best(multiPV);
401 // Have we found a "mate in x"?
403 && bestValue >= VALUE_MATE_IN_MAX_PLY
404 && VALUE_MATE - bestValue <= 2 * Limits.mate)
407 // Do we have time for the next iteration? Can we stop searching now?
408 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
410 // Take some extra time if the best move has changed
411 if (depth > 4 * ONE_PLY && multiPV == 1)
412 TimeMgr.pv_instability(BestMoveChanges);
414 // Stop the search if only one legal move is available or all
415 // of the available time has been used.
416 if ( RootMoves.size() == 1
417 || now() - SearchTime > TimeMgr.available_time())
419 // If we are allowed to ponder do not stop the search now but
420 // keep pondering until the GUI sends "ponderhit" or "stop".
422 Signals.stopOnPonderhit = true;
429 // If skill level is enabled, swap best PV line with the sub-optimal one
431 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
432 RootMoves.end(), skill.best_move(multiPV)));
436 // search<>() is the main search function for both PV and non-PV nodes and for
437 // normal and SplitPoint nodes. When called just after a split point the search
438 // is simpler because we have already probed the hash table, done a null move
439 // search, and searched the first move before splitting, so we don't have to
440 // repeat all this work again. We also don't need to store anything to the hash
441 // table here: This is taken care of after we return from the split point.
443 template <NodeType NT, bool SpNode>
444 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
446 const bool RootNode = NT == Root;
447 const bool PvNode = NT == PV || NT == Root;
449 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
450 assert(PvNode || (alpha == beta - 1));
451 assert(depth > DEPTH_ZERO);
453 Move pv[MAX_PLY+1], quietsSearched[64];
456 SplitPoint* splitPoint;
458 Move ttMove, move, excludedMove, bestMove;
459 Depth extension, newDepth, predictedDepth;
460 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
461 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
462 bool captureOrPromotion, dangerous, doFullDepthSearch;
463 int moveCount, quietCount;
465 // Step 1. Initialize node
466 Thread* thisThread = pos.this_thread();
467 inCheck = pos.checkers();
471 splitPoint = ss->splitPoint;
472 bestMove = splitPoint->bestMove;
473 bestValue = splitPoint->bestValue;
476 ttMove = excludedMove = MOVE_NONE;
477 ttValue = VALUE_NONE;
479 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
484 moveCount = quietCount = 0;
485 bestValue = -VALUE_INFINITE;
486 ss->ply = (ss-1)->ply + 1;
488 // Used to send selDepth info to GUI
489 if (PvNode && thisThread->maxPly < ss->ply)
490 thisThread->maxPly = ss->ply;
494 // Step 2. Check for aborted search and immediate draw
495 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
496 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
498 // Step 3. Mate distance pruning. Even if we mate at the next move our score
499 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
500 // a shorter mate was found upward in the tree then there is no need to search
501 // because we will never beat the current alpha. Same logic but with reversed
502 // signs applies also in the opposite condition of being mated instead of giving
503 // mate. In this case return a fail-high score.
504 alpha = std::max(mated_in(ss->ply), alpha);
505 beta = std::min(mate_in(ss->ply+1), beta);
510 assert(0 <= ss->ply && ss->ply < MAX_PLY);
512 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
513 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
514 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
516 // Step 4. Transposition table lookup
517 // We don't want the score of a partial search to overwrite a previous full search
518 // TT value, so we use a different position key in case of an excluded move.
519 excludedMove = ss->excludedMove;
520 posKey = excludedMove ? pos.exclusion_key() : pos.key();
521 tte = TT.probe(posKey, ttHit);
522 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
523 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
525 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
528 && tte->depth() >= depth
529 && ttValue != VALUE_NONE // Only in case of TT access race
530 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
531 : (tte->bound() & BOUND_UPPER)))
533 ss->currentMove = ttMove; // Can be MOVE_NONE
535 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
536 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
537 update_stats(pos, ss, ttMove, depth, nullptr, 0);
542 // Step 4a. Tablebase probe
543 if (!RootNode && TB::Cardinality)
545 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
547 if ( piecesCnt <= TB::Cardinality
548 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
549 && pos.rule50_count() == 0)
551 int found, v = Tablebases::probe_wdl(pos, &found);
557 int drawScore = TB::UseRule50 ? 1 : 0;
559 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
560 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
561 : VALUE_DRAW + 2 * v * drawScore;
563 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
564 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
565 MOVE_NONE, VALUE_NONE, TT.generation());
572 // Step 5. Evaluate the position statically and update parent's gain statistics
575 ss->staticEval = eval = VALUE_NONE;
581 // Never assume anything on values stored in TT
582 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
583 eval = ss->staticEval = evaluate(pos);
585 // Can ttValue be used as a better position evaluation?
586 if (ttValue != VALUE_NONE)
587 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
592 eval = ss->staticEval =
593 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
595 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
598 if (ss->skipEarlyPruning)
601 if ( !pos.captured_piece_type()
602 && ss->staticEval != VALUE_NONE
603 && (ss-1)->staticEval != VALUE_NONE
604 && (move = (ss-1)->currentMove) != MOVE_NULL
606 && type_of(move) == NORMAL)
608 Square to = to_sq(move);
609 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
612 // Step 6. Razoring (skipped when in check)
614 && depth < 4 * ONE_PLY
615 && eval + razor_margin(depth) <= alpha
616 && ttMove == MOVE_NONE
617 && !pos.pawn_on_7th(pos.side_to_move()))
619 if ( depth <= ONE_PLY
620 && eval + razor_margin(3 * ONE_PLY) <= alpha)
621 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
623 Value ralpha = alpha - razor_margin(depth);
624 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
629 // Step 7. Futility pruning: child node (skipped when in check)
631 && depth < 7 * ONE_PLY
632 && eval - futility_margin(depth) >= beta
633 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
634 && pos.non_pawn_material(pos.side_to_move()))
635 return eval - futility_margin(depth);
637 // Step 8. Null move search with verification search (is omitted in PV nodes)
639 && depth >= 2 * ONE_PLY
641 && pos.non_pawn_material(pos.side_to_move()))
643 ss->currentMove = MOVE_NULL;
645 assert(eval - beta >= 0);
647 // Null move dynamic reduction based on depth and value
648 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
650 pos.do_null_move(st);
651 (ss+1)->skipEarlyPruning = true;
652 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
653 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
654 (ss+1)->skipEarlyPruning = false;
655 pos.undo_null_move();
657 if (nullValue >= beta)
659 // Do not return unproven mate scores
660 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
663 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
666 // Do verification search at high depths
667 ss->skipEarlyPruning = true;
668 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
669 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
670 ss->skipEarlyPruning = false;
677 // Step 9. ProbCut (skipped when in check)
678 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
679 // and a reduced search returns a value much above beta, we can (almost) safely
680 // prune the previous move.
682 && depth >= 5 * ONE_PLY
683 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
685 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
686 Depth rdepth = depth - 4 * ONE_PLY;
688 assert(rdepth >= ONE_PLY);
689 assert((ss-1)->currentMove != MOVE_NONE);
690 assert((ss-1)->currentMove != MOVE_NULL);
692 MovePicker mp(pos, ttMove, History, CounterMovesHistory, pos.captured_piece_type());
695 while ((move = mp.next_move<false>()) != MOVE_NONE)
696 if (pos.legal(move, ci.pinned))
698 ss->currentMove = move;
699 pos.do_move(move, st, pos.gives_check(move, ci));
700 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
707 // Step 10. Internal iterative deepening (skipped when in check)
708 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
710 && (PvNode || ss->staticEval + 256 >= beta))
712 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
713 ss->skipEarlyPruning = true;
714 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
715 ss->skipEarlyPruning = false;
717 tte = TT.probe(posKey, ttHit);
718 ttMove = ttHit ? tte->move() : MOVE_NONE;
721 moves_loop: // When in check and at SpNode search starts from here
723 Square prevMoveSq = to_sq((ss-1)->currentMove);
724 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
725 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
727 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
728 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
729 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
731 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, countermoves, followupmoves, ss);
733 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
734 improving = ss->staticEval >= (ss-2)->staticEval
735 || ss->staticEval == VALUE_NONE
736 ||(ss-2)->staticEval == VALUE_NONE;
738 singularExtensionNode = !RootNode
740 && depth >= 8 * ONE_PLY
741 && ttMove != MOVE_NONE
742 /* && ttValue != VALUE_NONE Already implicit in the next condition */
743 && abs(ttValue) < VALUE_KNOWN_WIN
744 && !excludedMove // Recursive singular search is not allowed
745 && (tte->bound() & BOUND_LOWER)
746 && tte->depth() >= depth - 3 * ONE_PLY;
748 // Step 11. Loop through moves
749 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
750 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
754 if (move == excludedMove)
757 // At root obey the "searchmoves" option and skip moves not listed in Root
758 // Move List. As a consequence any illegal move is also skipped. In MultiPV
759 // mode we also skip PV moves which have been already searched.
760 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
765 // Shared counter cannot be decremented later if the move turns out to be illegal
766 if (!pos.legal(move, ci.pinned))
769 moveCount = ++splitPoint->moveCount;
770 splitPoint->mutex.unlock();
777 Signals.firstRootMove = (moveCount == 1);
779 if (thisThread == Threads.main() && now() - SearchTime > 3000)
780 sync_cout << "info depth " << depth / ONE_PLY
781 << " currmove " << UCI::move(move, pos.is_chess960())
782 << " currmovenumber " << moveCount + PVIdx << sync_endl;
786 (ss+1)->pv = nullptr;
788 extension = DEPTH_ZERO;
789 captureOrPromotion = pos.capture_or_promotion(move);
791 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
792 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
793 : pos.gives_check(move, ci);
795 dangerous = givesCheck
796 || type_of(move) != NORMAL
797 || pos.advanced_pawn_push(move);
799 // Step 12. Extend checks
800 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
803 // Singular extension search. If all moves but one fail low on a search of
804 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
805 // is singular and should be extended. To verify this we do a reduced search
806 // on all the other moves but the ttMove and if the result is lower than
807 // ttValue minus a margin then we extend the ttMove.
808 if ( singularExtensionNode
811 && pos.legal(move, ci.pinned))
813 Value rBeta = ttValue - 2 * depth / ONE_PLY;
814 ss->excludedMove = move;
815 ss->skipEarlyPruning = true;
816 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
817 ss->skipEarlyPruning = false;
818 ss->excludedMove = MOVE_NONE;
824 // Update the current move (this must be done after singular extension search)
825 newDepth = depth - ONE_PLY + extension;
827 // Step 13. Pruning at shallow depth
829 && !captureOrPromotion
832 && bestValue > VALUE_MATED_IN_MAX_PLY)
834 // Move count based pruning
835 if ( depth < 16 * ONE_PLY
836 && moveCount >= FutilityMoveCounts[improving][depth])
839 splitPoint->mutex.lock();
844 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
846 // Futility pruning: parent node
847 if (predictedDepth < 7 * ONE_PLY)
849 futilityValue = ss->staticEval + futility_margin(predictedDepth)
850 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
852 if (futilityValue <= alpha)
854 bestValue = std::max(bestValue, futilityValue);
858 splitPoint->mutex.lock();
859 if (bestValue > splitPoint->bestValue)
860 splitPoint->bestValue = bestValue;
866 // Prune moves with negative SEE at low depths
867 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
870 splitPoint->mutex.lock();
876 // Speculative prefetch as early as possible
877 prefetch(TT.first_entry(pos.key_after(move)));
879 // Check for legality just before making the move
880 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
886 ss->currentMove = move;
887 if (!SpNode && !captureOrPromotion && quietCount < 64)
888 quietsSearched[quietCount++] = move;
890 // Step 14. Make the move
891 pos.do_move(move, st, givesCheck);
893 // Step 15. Reduced depth search (LMR). If the move fails high it will be
894 // re-searched at full depth.
895 if ( depth >= 3 * ONE_PLY
897 && !captureOrPromotion
898 && move != ss->killers[0]
899 && move != ss->killers[1])
901 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
903 if ( (!PvNode && cutNode)
904 || History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO)
905 ss->reduction += ONE_PLY;
907 if (move == countermoves[0] || move == countermoves[1])
908 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
910 // Decrease reduction for moves that escape a capture
912 && type_of(move) == NORMAL
913 && type_of(pos.piece_on(to_sq(move))) != PAWN
914 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
915 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
917 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
919 alpha = splitPoint->alpha;
921 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
923 // Re-search at intermediate depth if reduction is very high
924 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
926 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
927 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
930 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
931 ss->reduction = DEPTH_ZERO;
934 doFullDepthSearch = !PvNode || moveCount > 1;
936 // Step 16. Full depth search, when LMR is skipped or fails high
937 if (doFullDepthSearch)
940 alpha = splitPoint->alpha;
942 value = newDepth < ONE_PLY ?
943 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
944 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
945 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
948 // For PV nodes only, do a full PV search on the first move or after a fail
949 // high (in the latter case search only if value < beta), otherwise let the
950 // parent node fail low with value <= alpha and to try another move.
951 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
954 (ss+1)->pv[0] = MOVE_NONE;
956 value = newDepth < ONE_PLY ?
957 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
958 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
959 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
962 // Step 17. Undo move
965 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
967 // Step 18. Check for new best move
970 splitPoint->mutex.lock();
971 bestValue = splitPoint->bestValue;
972 alpha = splitPoint->alpha;
975 // Finished searching the move. If a stop or a cutoff occurred, the return
976 // value of the search cannot be trusted, and we return immediately without
977 // updating best move, PV and TT.
978 if (Signals.stop || thisThread->cutoff_occurred())
983 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
985 // PV move or new best move ?
986 if (moveCount == 1 || value > alpha)
993 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
996 // We record how often the best move has been changed in each
997 // iteration. This information is used for time management: When
998 // the best move changes frequently, we allocate some more time.
1003 // All other moves but the PV are set to the lowest value: this is
1004 // not a problem when sorting because the sort is stable and the
1005 // move position in the list is preserved - just the PV is pushed up.
1006 rm.score = -VALUE_INFINITE;
1009 if (value > bestValue)
1011 bestValue = SpNode ? splitPoint->bestValue = value : value;
1015 bestMove = SpNode ? splitPoint->bestMove = move : move;
1017 if (PvNode && !RootNode) // Update pv even in fail-high case
1018 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1020 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1021 alpha = SpNode ? splitPoint->alpha = value : value;
1024 assert(value >= beta); // Fail high
1027 splitPoint->cutoff = true;
1034 // Step 19. Check for splitting the search
1036 && Threads.size() >= 2
1037 && depth >= Threads.minimumSplitDepth
1038 && ( !thisThread->activeSplitPoint
1039 || !thisThread->activeSplitPoint->allSlavesSearching
1040 || ( Threads.size() > MAX_SLAVES_PER_SPLITPOINT
1041 && thisThread->activeSplitPoint->slavesMask.count() == MAX_SLAVES_PER_SPLITPOINT))
1042 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1044 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1046 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1047 depth, moveCount, &mp, NT, cutNode);
1049 if (Signals.stop || thisThread->cutoff_occurred())
1052 if (bestValue >= beta)
1060 // Following condition would detect a stop or a cutoff set only after move
1061 // loop has been completed. But in this case bestValue is valid because we
1062 // have fully searched our subtree, and we can anyhow save the result in TT.
1064 if (Signals.stop || thisThread->cutoff_occurred())
1068 // Step 20. Check for mate and stalemate
1069 // All legal moves have been searched and if there are no legal moves, it
1070 // must be mate or stalemate. If we are in a singular extension search then
1071 // return a fail low score.
1073 bestValue = excludedMove ? alpha
1074 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1076 // Quiet best move: update killers, history, countermoves and followupmoves
1077 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1078 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1080 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1081 bestValue >= beta ? BOUND_LOWER :
1082 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1083 depth, bestMove, ss->staticEval, TT.generation());
1085 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1091 // qsearch() is the quiescence search function, which is called by the main
1092 // search function when the remaining depth is zero (or, to be more precise,
1093 // less than ONE_PLY).
1095 template <NodeType NT, bool InCheck>
1096 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1098 const bool PvNode = NT == PV;
1100 assert(NT == PV || NT == NonPV);
1101 assert(InCheck == !!pos.checkers());
1102 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1103 assert(PvNode || (alpha == beta - 1));
1104 assert(depth <= DEPTH_ZERO);
1110 Move ttMove, move, bestMove;
1111 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1112 bool ttHit, givesCheck, evasionPrunable;
1117 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1119 ss->pv[0] = MOVE_NONE;
1122 ss->currentMove = bestMove = MOVE_NONE;
1123 ss->ply = (ss-1)->ply + 1;
1125 // Check for an instant draw or if the maximum ply has been reached
1126 if (pos.is_draw() || ss->ply >= MAX_PLY)
1127 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1129 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1131 // Decide whether or not to include checks: this fixes also the type of
1132 // TT entry depth that we are going to use. Note that in qsearch we use
1133 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1134 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1135 : DEPTH_QS_NO_CHECKS;
1137 // Transposition table lookup
1139 tte = TT.probe(posKey, ttHit);
1140 ttMove = ttHit ? tte->move() : MOVE_NONE;
1141 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1145 && tte->depth() >= ttDepth
1146 && ttValue != VALUE_NONE // Only in case of TT access race
1147 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1148 : (tte->bound() & BOUND_UPPER)))
1150 ss->currentMove = ttMove; // Can be MOVE_NONE
1154 // Evaluate the position statically
1157 ss->staticEval = VALUE_NONE;
1158 bestValue = futilityBase = -VALUE_INFINITE;
1164 // Never assume anything on values stored in TT
1165 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1166 ss->staticEval = bestValue = evaluate(pos);
1168 // Can ttValue be used as a better position evaluation?
1169 if (ttValue != VALUE_NONE)
1170 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1171 bestValue = ttValue;
1174 ss->staticEval = bestValue =
1175 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1177 // Stand pat. Return immediately if static value is at least beta
1178 if (bestValue >= beta)
1181 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1182 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1187 if (PvNode && bestValue > alpha)
1190 futilityBase = bestValue + 128;
1193 // Initialize a MovePicker object for the current position, and prepare
1194 // to search the moves. Because the depth is <= 0 here, only captures,
1195 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1197 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, to_sq((ss-1)->currentMove));
1200 // Loop through the moves until no moves remain or a beta cutoff occurs
1201 while ((move = mp.next_move<false>()) != MOVE_NONE)
1203 assert(is_ok(move));
1205 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1206 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1207 : pos.gives_check(move, ci);
1212 && futilityBase > -VALUE_KNOWN_WIN
1213 && !pos.advanced_pawn_push(move))
1215 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1217 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1219 if (futilityValue <= alpha)
1221 bestValue = std::max(bestValue, futilityValue);
1225 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1227 bestValue = std::max(bestValue, futilityBase);
1232 // Detect non-capture evasions that are candidates to be pruned
1233 evasionPrunable = InCheck
1234 && bestValue > VALUE_MATED_IN_MAX_PLY
1235 && !pos.capture(move)
1236 && !pos.can_castle(pos.side_to_move());
1238 // Don't search moves with negative SEE values
1239 if ( (!InCheck || evasionPrunable)
1240 && type_of(move) != PROMOTION
1241 && pos.see_sign(move) < VALUE_ZERO)
1244 // Speculative prefetch as early as possible
1245 prefetch(TT.first_entry(pos.key_after(move)));
1247 // Check for legality just before making the move
1248 if (!pos.legal(move, ci.pinned))
1251 ss->currentMove = move;
1253 // Make and search the move
1254 pos.do_move(move, st, givesCheck);
1255 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1256 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1257 pos.undo_move(move);
1259 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1261 // Check for new best move
1262 if (value > bestValue)
1268 if (PvNode) // Update pv even in fail-high case
1269 update_pv(ss->pv, move, (ss+1)->pv);
1271 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1278 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1279 ttDepth, move, ss->staticEval, TT.generation());
1287 // All legal moves have been searched. A special case: If we're in check
1288 // and no legal moves were found, it is checkmate.
1289 if (InCheck && bestValue == -VALUE_INFINITE)
1290 return mated_in(ss->ply); // Plies to mate from the root
1292 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1293 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1294 ttDepth, bestMove, ss->staticEval, TT.generation());
1296 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1302 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1303 // "plies to mate from the current position". Non-mate scores are unchanged.
1304 // The function is called before storing a value in the transposition table.
1306 Value value_to_tt(Value v, int ply) {
1308 assert(v != VALUE_NONE);
1310 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1311 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1315 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1316 // from the transposition table (which refers to the plies to mate/be mated
1317 // from current position) to "plies to mate/be mated from the root".
1319 Value value_from_tt(Value v, int ply) {
1321 return v == VALUE_NONE ? VALUE_NONE
1322 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1323 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1327 // update_pv() adds current move and appends child pv[]
1329 void update_pv(Move* pv, Move move, Move* childPv) {
1331 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1336 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1339 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1341 if (ss->killers[0] != move)
1343 ss->killers[1] = ss->killers[0];
1344 ss->killers[0] = move;
1347 // Increase history value of the cut-off move and decrease all the other
1348 // played quiet moves.
1349 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1350 History.update(pos.moved_piece(move), to_sq(move), bonus);
1352 for (int i = 0; i < quietsCnt; ++i)
1355 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1358 if (is_ok((ss-1)->currentMove))
1360 Square prevMoveSq = to_sq((ss-1)->currentMove);
1361 Piece prevMovePiece = pos.piece_on(prevMoveSq);
1362 Countermoves.update(prevMovePiece, prevMoveSq, move);
1364 HistoryStats& cmh = CounterMovesHistory[prevMovePiece][prevMoveSq];
1365 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1366 for (int i = 0; i < quietsCnt; ++i)
1369 cmh.update(pos.moved_piece(m), to_sq(m), -bonus);
1373 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1375 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1376 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1381 // When playing with strength handicap, choose best move among a set of RootMoves
1382 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1384 Move Skill::pick_best(size_t multiPV) {
1386 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1387 static PRNG rng(now());
1389 // RootMoves are already sorted by score in descending order
1390 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1391 int weakness = 120 - 2 * level;
1392 int maxScore = -VALUE_INFINITE;
1394 // Choose best move. For each move score we add two terms both dependent on
1395 // weakness. One deterministic and bigger for weaker levels, and one random,
1396 // then we choose the move with the resulting highest score.
1397 for (size_t i = 0; i < multiPV; ++i)
1399 // This is our magic formula
1400 int push = ( weakness * int(RootMoves[0].score - RootMoves[i].score)
1401 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1403 if (RootMoves[i].score + push > maxScore)
1405 maxScore = RootMoves[i].score + push;
1406 best = RootMoves[i].pv[0];
1415 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1416 /// that all (if any) unsearched PV lines are sent using a previous search score.
1418 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1420 std::stringstream ss;
1421 TimePoint elapsed = now() - SearchTime + 1;
1422 size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
1425 for (Thread* th : Threads)
1426 if (th->maxPly > selDepth)
1427 selDepth = th->maxPly;
1429 for (size_t i = 0; i < multiPV; ++i)
1431 bool updated = (i <= PVIdx);
1433 if (depth == ONE_PLY && !updated)
1436 Depth d = updated ? depth : depth - ONE_PLY;
1437 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1439 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1440 v = tb ? TB::Score : v;
1442 if (ss.rdbuf()->in_avail()) // Not at first line
1446 << " 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;
1457 if (elapsed > 1000) // Earlier makes little sense
1458 ss << " hashfull " << TT.hashfull();
1460 ss << " tbhits " << TB::Hits
1461 << " time " << elapsed
1464 for (Move m : RootMoves[i].pv)
1465 ss << " " << UCI::move(m, pos.is_chess960());
1472 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1473 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1474 /// first, even if the old TT entries have been overwritten.
1476 void RootMove::insert_pv_in_tt(Position& pos) {
1478 StateInfo state[MAX_PLY], *st = state;
1483 assert(MoveList<LEGAL>(pos).contains(m));
1485 TTEntry* tte = TT.probe(pos.key(), ttHit);
1487 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1488 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
1490 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1493 for (size_t i = pv.size(); i > 0; )
1494 pos.undo_move(pv[--i]);
1498 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1499 /// exiting the search, for instance in case we stop the search during a fail high at
1500 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1501 /// 'ponder on' we have nothing to think on.
1503 bool RootMove::extract_ponder_from_tt(Position& pos)
1508 assert(pv.size() == 1);
1510 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1511 TTEntry* tte = TT.probe(pos.key(), ttHit);
1512 pos.undo_move(pv[0]);
1516 Move m = tte->move(); // Local copy to be SMP safe
1517 if (MoveList<LEGAL>(pos).contains(m))
1518 return pv.push_back(m), true;
1525 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1527 void Thread::idle_loop() {
1529 // Pointer 'this_sp' is not null only if we are called from split(), and not
1530 // at the thread creation. This means we are the split point's master.
1531 SplitPoint* this_sp = activeSplitPoint;
1533 assert(!this_sp || (this_sp->master == this && searching));
1537 // If this thread has been assigned work, launch a search
1542 assert(activeSplitPoint);
1543 SplitPoint* sp = activeSplitPoint;
1547 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1548 Position pos(*sp->pos, this);
1550 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1551 ss->splitPoint = sp;
1555 assert(activePosition == nullptr);
1557 activePosition = &pos;
1559 if (sp->nodeType == NonPV)
1560 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1562 else if (sp->nodeType == PV)
1563 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1565 else if (sp->nodeType == Root)
1566 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1574 activePosition = nullptr;
1575 sp->slavesMask.reset(idx);
1576 sp->allSlavesSearching = false;
1577 sp->nodes += pos.nodes_searched();
1579 // Wake up the master thread so to allow it to return from the idle
1580 // loop in case we are the last slave of the split point.
1581 if (this != sp->master && sp->slavesMask.none())
1583 assert(!sp->master->searching);
1585 sp->master->notify_one();
1588 // After releasing the lock we can't access any SplitPoint related data
1589 // in a safe way because it could have been released under our feet by
1593 // Try to late join to another split point if none of its slaves has
1594 // already finished.
1595 SplitPoint* bestSp = NULL;
1596 int minLevel = INT_MAX;
1598 for (Thread* th : Threads)
1600 const size_t size = th->splitPointsSize; // Local copy
1601 sp = size ? &th->splitPoints[size - 1] : nullptr;
1604 && sp->allSlavesSearching
1605 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
1609 assert(!(this_sp && this_sp->slavesMask.none()));
1610 assert(Threads.size() > 2);
1612 // Prefer to join to SP with few parents to reduce the probability
1613 // that a cut-off occurs above us, and hence we waste our work.
1615 for (SplitPoint* p = th->activeSplitPoint; p; p = p->parentSplitPoint)
1618 if (level < minLevel)
1630 // Recheck the conditions under lock protection
1633 if ( sp->allSlavesSearching
1634 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT)
1640 sp->slavesMask.set(idx);
1641 activeSplitPoint = sp;
1652 // Avoid races with notify_one() fired from last slave of the split point
1653 std::unique_lock<Mutex> lk(mutex);
1655 // If we are master and all slaves have finished then exit idle_loop
1656 if (this_sp && this_sp->slavesMask.none())
1662 // If we are not searching, wait for a condition to be signaled instead of
1663 // wasting CPU time polling for work.
1664 if (!searching && !exit)
1665 sleepCondition.wait(lk);
1670 /// check_time() is called by the timer thread when the timer triggers. It is
1671 /// used to print debug info and, more importantly, to detect when we are out of
1672 /// available time and thus stop the search.
1676 static TimePoint lastInfoTime = now();
1677 TimePoint elapsed = now() - SearchTime;
1679 if (now() - lastInfoTime >= 1000)
1681 lastInfoTime = now();
1685 // An engine may not stop pondering until told so by the GUI
1689 if (Limits.use_time_management())
1691 bool stillAtFirstMove = Signals.firstRootMove
1692 && !Signals.failedLowAtRoot
1693 && elapsed > TimeMgr.available_time() * 75 / 100;
1695 if ( stillAtFirstMove
1696 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1697 Signals.stop = true;
1699 else if (Limits.movetime && elapsed >= Limits.movetime)
1700 Signals.stop = true;
1702 else if (Limits.nodes)
1704 int64_t nodes = RootPos.nodes_searched();
1706 // Loop across all split points and sum accumulated SplitPoint nodes plus
1707 // all the currently active positions nodes.
1709 for (Thread* th : Threads)
1710 for (size_t i = 0; i < th->splitPointsSize; ++i)
1712 SplitPoint& sp = th->splitPoints[i];
1718 for (size_t idx = 0; idx < Threads.size(); ++idx)
1719 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1720 nodes += Threads[idx]->activePosition->nodes_searched();
1725 if (nodes >= Limits.nodes)
1726 Signals.stop = true;