2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2012 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/>.
37 #include "ucioption.h"
41 volatile SignalsType Signals;
43 std::vector<RootMove> RootMoves;
46 Time::point SearchTime;
47 StateStackPtr SetupStates;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // This is the minimum interval in msec between two check_time() calls
60 const int TimerResolution = 5;
62 // Different node types, used as template parameter
63 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
65 // Lookup table to check if a Piece is a slider and its access function
66 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
67 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
69 // Dynamic razoring margin based on depth
70 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
72 // Futility lookup tables (initialized at startup) and their access functions
73 Value FutilityMargins[16][64]; // [depth][moveNumber]
74 int FutilityMoveCounts[32]; // [depth]
76 inline Value futility_margin(Depth d, int mn) {
78 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
82 // Reduction lookup tables (initialized at startup) and their access function
83 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
85 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
87 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
93 Value DrawValue[COLOR_NB];
96 template <NodeType NT>
97 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
99 template <NodeType NT>
100 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
102 void id_loop(Position& pos);
103 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta);
104 bool connected_moves(const Position& pos, Move m1, Move m2);
105 Value value_to_tt(Value v, int ply);
106 Value value_from_tt(Value v, int ply);
107 bool connected_threat(const Position& pos, Move m, Move threat);
108 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
111 Skill(int l) : level(l), best(MOVE_NONE) {}
113 if (enabled()) // Swap best PV line with the sub-optimal one
114 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
115 RootMoves.end(), best ? best : pick_move()));
118 bool enabled() const { return level < 20; }
119 bool time_to_pick(int depth) const { return depth == 1 + level; }
129 /// Search::init() is called during startup to initialize various lookup tables
131 void Search::init() {
133 int d; // depth (ONE_PLY == 2)
134 int hd; // half depth (ONE_PLY == 1)
137 // Init reductions array
138 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
140 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
141 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
142 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
143 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
146 // Init futility margins array
147 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
148 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
150 // Init futility move count array
151 for (d = 0; d < 32; d++)
152 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(double(d), 2.0));
156 /// Search::perft() is our utility to verify move generation. All the leaf nodes
157 /// up to the given depth are generated and counted and the sum returned.
159 size_t Search::perft(Position& pos, Depth depth) {
161 // At the last ply just return the number of legal moves (leaf nodes)
162 if (depth == ONE_PLY)
163 return MoveList<LEGAL>(pos).size();
169 for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
171 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
172 cnt += perft(pos, depth - ONE_PLY);
173 pos.undo_move(ml.move());
180 /// Search::think() is the external interface to Stockfish's search, and is
181 /// called by the main thread when the program receives the UCI 'go' command. It
182 /// searches from RootPos and at the end prints the "bestmove" to output.
184 void Search::think() {
186 static PolyglotBook book; // Defined static to initialize the PRNG only once
188 RootColor = RootPos.side_to_move();
189 TimeMgr.init(Limits, RootPos.startpos_ply_counter(), RootColor);
191 if (RootMoves.empty())
193 RootMoves.push_back(MOVE_NONE);
194 sync_cout << "info depth 0 score "
195 << score_to_uci(RootPos.in_check() ? -VALUE_MATE : VALUE_DRAW)
201 if (Options["OwnBook"] && !Limits.infinite)
203 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
205 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
207 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
212 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
214 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
215 cf = cf * MaterialTable::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
216 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
217 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
220 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
222 if (Options["Use Search Log"])
224 Log log(Options["Search Log Filename"]);
225 log << "\nSearching: " << RootPos.to_fen()
226 << "\ninfinite: " << Limits.infinite
227 << " ponder: " << Limits.ponder
228 << " time: " << Limits.time[RootColor]
229 << " increment: " << Limits.inc[RootColor]
230 << " moves to go: " << Limits.movestogo
236 // Set best timer interval to avoid lagging under time pressure. Timer is
237 // used to check for remaining available thinking time.
238 if (Limits.use_time_management())
239 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16,
241 else if (Limits.nodes)
242 Threads.set_timer(2 * TimerResolution);
244 Threads.set_timer(100);
246 id_loop(RootPos); // Let's start searching !
248 Threads.set_timer(0); // Stop timer
251 if (Options["Use Search Log"])
253 Time::point elapsed = Time::now() - SearchTime + 1;
255 Log log(Options["Search Log Filename"]);
256 log << "Nodes: " << RootPos.nodes_searched()
257 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
258 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
261 RootPos.do_move(RootMoves[0].pv[0], st);
262 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
263 RootPos.undo_move(RootMoves[0].pv[0]);
268 // When we reach max depth we arrive here even without Signals.stop is raised,
269 // but if we are pondering or in infinite search, we shouldn't print the best
270 // move before we are told to do so.
271 if (!Signals.stop && (Limits.ponder || Limits.infinite))
272 RootPos.this_thread()->wait_for_stop_or_ponderhit();
274 // Best move could be MOVE_NONE when searching on a stalemate position
275 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
276 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
283 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
284 // with increasing depth until the allocated thinking time has been consumed,
285 // user stops the search, or the maximum search depth is reached.
287 void id_loop(Position& pos) {
289 Stack ss[MAX_PLY_PLUS_2];
290 int depth, prevBestMoveChanges;
291 Value bestValue, alpha, beta, delta;
292 bool bestMoveNeverChanged = true;
294 memset(ss, 0, 4 * sizeof(Stack));
295 depth = BestMoveChanges = 0;
296 bestValue = delta = -VALUE_INFINITE;
297 ss->currentMove = MOVE_NULL; // Hack to skip update gains
301 PVSize = Options["MultiPV"];
302 Skill skill(Options["Skill Level"]);
304 // Do we have to play with skill handicap? In this case enable MultiPV search
305 // that we will use behind the scenes to retrieve a set of possible moves.
306 if (skill.enabled() && PVSize < 4)
309 PVSize = std::min(PVSize, RootMoves.size());
311 // Iterative deepening loop until requested to stop or target depth reached
312 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
314 // Save last iteration's scores before first PV line is searched and all
315 // the move scores but the (new) PV are set to -VALUE_INFINITE.
316 for (size_t i = 0; i < RootMoves.size(); i++)
317 RootMoves[i].prevScore = RootMoves[i].score;
319 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
322 // MultiPV loop. We perform a full root search for each PV line
323 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
325 // Set aspiration window default width
326 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
329 alpha = RootMoves[PVIdx].prevScore - delta;
330 beta = RootMoves[PVIdx].prevScore + delta;
334 alpha = -VALUE_INFINITE;
335 beta = VALUE_INFINITE;
338 // Start with a small aspiration window and, in case of fail high/low,
339 // research with bigger window until not failing high/low anymore.
342 // Search starts from ss+1 to allow referencing (ss-1). This is
343 // needed by update gains and ss copy when splitting at Root.
344 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
346 // Bring to front the best move. It is critical that sorting is
347 // done with a stable algorithm because all the values but the first
348 // and eventually the new best one are set to -VALUE_INFINITE and
349 // we want to keep the same order for all the moves but the new
350 // PV that goes to the front. Note that in case of MultiPV search
351 // the already searched PV lines are preserved.
352 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
354 // Write PV back to transposition table in case the relevant
355 // entries have been overwritten during the search.
356 for (size_t i = 0; i <= PVIdx; i++)
357 RootMoves[i].insert_pv_in_tt(pos);
359 // If search has been stopped return immediately. Sorting and
360 // writing PV back to TT is safe becuase RootMoves is still
361 // valid, although refers to previous iteration.
365 // In case of failing high/low increase aspiration window and
366 // research, otherwise exit the loop.
367 if (bestValue > alpha && bestValue < beta)
370 // Give some update (without cluttering the UI) before to research
371 if (Time::now() - SearchTime > 3000)
372 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
374 if (abs(bestValue) >= VALUE_KNOWN_WIN)
376 alpha = -VALUE_INFINITE;
377 beta = VALUE_INFINITE;
379 else if (bestValue >= beta)
386 Signals.failedLowAtRoot = true;
387 Signals.stopOnPonderhit = false;
393 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
396 // Sort the PV lines searched so far and update the GUI
397 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
398 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
401 // Do we need to pick now the sub-optimal best move ?
402 if (skill.enabled() && skill.time_to_pick(depth))
405 if (Options["Use Search Log"])
407 Log log(Options["Search Log Filename"]);
408 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
412 // Filter out startup noise when monitoring best move stability
413 if (depth > 2 && BestMoveChanges)
414 bestMoveNeverChanged = false;
416 // Do we have time for the next iteration? Can we stop searching now?
417 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
419 bool stop = false; // Local variable, not the volatile Signals.stop
421 // Take in account some extra time if the best move has changed
422 if (depth > 4 && depth < 50 && PVSize == 1)
423 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
425 // Stop search if most of available time is already consumed. We
426 // probably don't have enough time to search the first move at the
427 // next iteration anyway.
428 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
431 // Stop search early if one move seems to be much better than others
435 && ( (bestMoveNeverChanged && pos.captured_piece_type())
436 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
438 Value rBeta = bestValue - 2 * PawnValueMg;
439 (ss+1)->excludedMove = RootMoves[0].pv[0];
440 (ss+1)->skipNullMove = true;
441 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
442 (ss+1)->skipNullMove = false;
443 (ss+1)->excludedMove = MOVE_NONE;
451 // If we are allowed to ponder do not stop the search now but
452 // keep pondering until GUI sends "ponderhit" or "stop".
454 Signals.stopOnPonderhit = true;
463 // search<>() is the main search function for both PV and non-PV nodes and for
464 // normal and SplitPoint nodes. When called just after a split point the search
465 // is simpler because we have already probed the hash table, done a null move
466 // search, and searched the first move before splitting, we don't have to repeat
467 // all this work again. We also don't need to store anything to the hash table
468 // here: This is taken care of after we return from the split point.
470 template <NodeType NT>
471 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
473 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
474 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
475 const bool RootNode = (NT == Root || NT == SplitPointRoot);
477 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
478 assert(PvNode || (alpha == beta - 1));
479 assert(depth > DEPTH_ZERO);
481 Move movesSearched[64];
486 Move ttMove, move, excludedMove, bestMove, threatMove;
488 Value bestValue, value, ttValue;
489 Value eval, nullValue, futilityValue;
490 bool inCheck, givesCheck, pvMove, singularExtensionNode;
491 bool captureOrPromotion, dangerous, doFullDepthSearch;
492 int moveCount, playedMoveCount;
494 // Step 1. Initialize node
495 Thread* thisThread = pos.this_thread();
496 moveCount = playedMoveCount = 0;
497 inCheck = pos.in_check();
502 bestMove = sp->bestMove;
503 threatMove = sp->threatMove;
504 bestValue = sp->bestValue;
506 ttMove = excludedMove = MOVE_NONE;
507 ttValue = VALUE_NONE;
509 assert(sp->bestValue > -VALUE_INFINITE && sp->moveCount > 0);
511 goto split_point_start;
514 bestValue = -VALUE_INFINITE;
515 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
516 ss->ply = (ss-1)->ply + 1;
517 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
518 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
520 // Used to send selDepth info to GUI
521 if (PvNode && thisThread->maxPly < ss->ply)
522 thisThread->maxPly = ss->ply;
526 // Step 2. Check for aborted search and immediate draw
527 if (Signals.stop || pos.is_draw<false,true>() || ss->ply > MAX_PLY)
528 return DrawValue[pos.side_to_move()];
530 // Step 3. Mate distance pruning. Even if we mate at the next move our score
531 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
532 // a shorter mate was found upward in the tree then there is no need to search
533 // further, we will never beat current alpha. Same logic but with reversed signs
534 // applies also in the opposite condition of being mated instead of giving mate,
535 // in this case return a fail-high score.
536 alpha = std::max(mated_in(ss->ply), alpha);
537 beta = std::min(mate_in(ss->ply+1), beta);
543 if(pos.is_draw<false,false>()) return DrawValue[pos.side_to_move()];
546 // Step 4. Transposition table lookup
547 // We don't want the score of a partial search to overwrite a previous full search
548 // TT value, so we use a different position key in case of an excluded move.
549 excludedMove = ss->excludedMove;
550 posKey = excludedMove ? pos.exclusion_key() : pos.key();
551 tte = TT.probe(posKey);
552 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
553 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
555 // At PV nodes we check for exact scores, while at non-PV nodes we check for
556 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
557 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
558 // we should also update RootMoveList to avoid bogus output.
560 && tte && tte->depth() >= depth
561 && ( PvNode ? tte->type() == BOUND_EXACT
562 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
563 : (tte->type() & BOUND_UPPER)))
565 assert(ttValue != VALUE_NONE); // Due to depth > DEPTH_NONE
568 ss->currentMove = ttMove; // Can be MOVE_NONE
572 && !pos.is_capture_or_promotion(ttMove)
573 && ttMove != ss->killers[0])
575 ss->killers[1] = ss->killers[0];
576 ss->killers[0] = ttMove;
581 // Step 5. Evaluate the position statically and update parent's gain statistics
583 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
587 assert(tte->static_value() != VALUE_NONE);
588 assert(ttValue != VALUE_NONE || tte->type() == BOUND_NONE);
590 ss->staticEval = eval = tte->static_value();
591 ss->evalMargin = tte->static_value_margin();
593 // Can ttValue be used as a better position evaluation?
594 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
595 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
600 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
601 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
602 ss->staticEval, ss->evalMargin);
605 // Update gain for the parent non-capture move given the static position
606 // evaluation before and after the move.
607 if ( (move = (ss-1)->currentMove) != MOVE_NULL
608 && (ss-1)->staticEval != VALUE_NONE
609 && ss->staticEval != VALUE_NONE
610 && !pos.captured_piece_type()
611 && type_of(move) == NORMAL)
613 Square to = to_sq(move);
614 H.update_gain(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
617 // Step 6. Razoring (is omitted in PV nodes)
619 && depth < 4 * ONE_PLY
621 && eval + razor_margin(depth) < beta
622 && ttMove == MOVE_NONE
623 && abs(beta) < VALUE_MATE_IN_MAX_PLY
624 && !pos.pawn_on_7th(pos.side_to_move()))
626 Value rbeta = beta - razor_margin(depth);
627 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
629 // Logically we should return (v + razor_margin(depth)), but
630 // surprisingly this did slightly weaker in tests.
634 // Step 7. Static null move pruning (is omitted in PV nodes)
635 // We're betting that the opponent doesn't have a move that will reduce
636 // the score by more than futility_margin(depth) if we do a null move.
639 && depth < 4 * ONE_PLY
641 && eval - FutilityMargins[depth][0] >= beta
642 && abs(beta) < VALUE_MATE_IN_MAX_PLY
643 && pos.non_pawn_material(pos.side_to_move()))
644 return eval - FutilityMargins[depth][0];
646 // Step 8. Null move search with verification search (is omitted in PV nodes)
652 && abs(beta) < VALUE_MATE_IN_MAX_PLY
653 && pos.non_pawn_material(pos.side_to_move()))
655 ss->currentMove = MOVE_NULL;
657 // Null move dynamic reduction based on depth
658 Depth R = 3 * ONE_PLY + depth / 4;
660 // Null move dynamic reduction based on value
661 if (eval - PawnValueMg > beta)
664 pos.do_null_move<true>(st);
665 (ss+1)->skipNullMove = true;
666 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
667 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
668 (ss+1)->skipNullMove = false;
669 pos.do_null_move<false>(st);
671 if (nullValue >= beta)
673 // Do not return unproven mate scores
674 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
677 if (depth < 6 * ONE_PLY)
680 // Do verification search at high depths
681 ss->skipNullMove = true;
682 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
683 ss->skipNullMove = false;
690 // The null move failed low, which means that we may be faced with
691 // some kind of threat. If the previous move was reduced, check if
692 // the move that refuted the null move was somehow connected to the
693 // move which was reduced. If a connection is found, return a fail
694 // low score (which will cause the reduced move to fail high in the
695 // parent node, which will trigger a re-search with full depth).
696 threatMove = (ss+1)->currentMove;
698 if ( depth < 5 * ONE_PLY
700 && threatMove != MOVE_NONE
701 && connected_moves(pos, (ss-1)->currentMove, threatMove))
706 // Step 9. ProbCut (is omitted in PV nodes)
707 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
708 // and a reduced search returns a value much above beta, we can (almost) safely
709 // prune the previous move.
711 && depth >= 5 * ONE_PLY
714 && excludedMove == MOVE_NONE
715 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
717 Value rbeta = beta + 200;
718 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
720 assert(rdepth >= ONE_PLY);
721 assert((ss-1)->currentMove != MOVE_NONE);
722 assert((ss-1)->currentMove != MOVE_NULL);
724 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
727 while ((move = mp.next_move<false>()) != MOVE_NONE)
728 if (pos.pl_move_is_legal(move, ci.pinned))
730 ss->currentMove = move;
731 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
732 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
739 // Step 10. Internal iterative deepening
740 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
741 && ttMove == MOVE_NONE
742 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
744 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
746 ss->skipNullMove = true;
747 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
748 ss->skipNullMove = false;
750 tte = TT.probe(posKey);
751 ttMove = tte ? tte->move() : MOVE_NONE;
754 split_point_start: // At split points actual search starts from here
756 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
758 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
759 singularExtensionNode = !RootNode
761 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
762 && ttMove != MOVE_NONE
763 && !excludedMove // Recursive singular search is not allowed
764 && (tte->type() & BOUND_LOWER)
765 && tte->depth() >= depth - 3 * ONE_PLY;
767 // Step 11. Loop through moves
768 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
769 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
773 if (move == excludedMove)
776 // At root obey the "searchmoves" option and skip moves not listed in Root
777 // Move List, as a consequence any illegal move is also skipped. In MultiPV
778 // mode we also skip PV moves which have been already searched.
779 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
784 // Shared counter cannot be decremented later if move turns out to be illegal
785 if (!pos.pl_move_is_legal(move, ci.pinned))
788 moveCount = ++sp->moveCount;
796 Signals.firstRootMove = (moveCount == 1);
798 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 2000)
799 sync_cout << "info depth " << depth / ONE_PLY
800 << " currmove " << move_to_uci(move, pos.is_chess960())
801 << " currmovenumber " << moveCount + PVIdx << sync_endl;
805 captureOrPromotion = pos.is_capture_or_promotion(move);
806 givesCheck = pos.move_gives_check(move, ci);
807 dangerous = givesCheck
808 || pos.is_passed_pawn_push(move)
809 || type_of(move) == CASTLE
810 || ( captureOrPromotion // Entering a pawn endgame?
811 && type_of(pos.piece_on(to_sq(move))) != PAWN
812 && type_of(move) == NORMAL
813 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
814 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
816 // Step 12. Extend checks and, in PV nodes, also dangerous moves
817 if (PvNode && dangerous)
820 else if (givesCheck && pos.see_sign(move) >= 0)
823 // Singular extension search. If all moves but one fail low on a search of
824 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
825 // is singular and should be extended. To verify this we do a reduced search
826 // on all the other moves but the ttMove, if result is lower than ttValue minus
827 // a margin then we extend ttMove.
828 if ( singularExtensionNode
831 && pos.pl_move_is_legal(move, ci.pinned)
832 && abs(ttValue) < VALUE_KNOWN_WIN)
834 assert(ttValue != VALUE_NONE);
836 Value rBeta = ttValue - int(depth);
837 ss->excludedMove = move;
838 ss->skipNullMove = true;
839 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
840 ss->skipNullMove = false;
841 ss->excludedMove = MOVE_NONE;
844 ext = rBeta >= beta ? ONE_PLY + ONE_PLY / 2 : ONE_PLY;
847 // Update current move (this must be done after singular extension search)
848 newDepth = depth - ONE_PLY + ext;
850 // Step 13. Futility pruning (is omitted in PV nodes)
852 && !captureOrPromotion
856 && (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
857 && alpha > VALUE_MATED_IN_MAX_PLY)))
859 // Move count based pruning
860 if ( depth < 16 * ONE_PLY
861 && moveCount >= FutilityMoveCounts[depth]
862 && (!threatMove || !connected_threat(pos, move, threatMove)))
870 // Value based pruning
871 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
872 // but fixing this made program slightly weaker.
873 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
874 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
875 + H.gain(pos.piece_moved(move), to_sq(move));
877 if (futilityValue < beta)
885 // Prune moves with negative SEE at low depths
886 if ( predictedDepth < 2 * ONE_PLY
887 && pos.see_sign(move) < 0)
896 // Check for legality only before to do the move
897 if (!pos.pl_move_is_legal(move, ci.pinned))
903 pvMove = PvNode ? moveCount == 1 : false;
904 ss->currentMove = move;
905 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
906 movesSearched[playedMoveCount++] = move;
908 // Step 14. Make the move
909 pos.do_move(move, st, ci, givesCheck);
911 // Step 15. Reduced depth search (LMR). If the move fails high will be
912 // re-searched at full depth.
913 if ( depth > 3 * ONE_PLY
915 && !captureOrPromotion
917 && ss->killers[0] != move
918 && ss->killers[1] != move)
920 ss->reduction = reduction<PvNode>(depth, moveCount);
921 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
922 alpha = SpNode ? sp->alpha : alpha;
924 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
926 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
927 ss->reduction = DEPTH_ZERO;
930 doFullDepthSearch = !pvMove;
932 // Step 16. Full depth search, when LMR is skipped or fails high
933 if (doFullDepthSearch)
935 alpha = SpNode ? sp->alpha : alpha;
936 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
937 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
940 // Only for PV nodes do a full PV search on the first move or after a fail
941 // high, in the latter case search only if value < beta, otherwise let the
942 // parent node to fail low with value <= alpha and to try another move.
943 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
944 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
945 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
947 // Step 17. Undo move
950 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
952 // Step 18. Check for new best move
956 bestValue = sp->bestValue;
960 // Finished searching the move. If Signals.stop is true, the search
961 // was aborted because the user interrupted the search or because we
962 // ran out of time. In this case, the return value of the search cannot
963 // be trusted, and we don't update the best move and/or PV.
964 if (Signals.stop || thisThread->cutoff_occurred())
965 return value; // To avoid returning VALUE_INFINITE
969 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
971 // PV move or new best move ?
972 if (pvMove || value > alpha)
975 rm.extract_pv_from_tt(pos);
977 // We record how often the best move has been changed in each
978 // iteration. This information is used for time management: When
979 // the best move changes frequently, we allocate some more time.
984 // All other moves but the PV are set to the lowest value, this
985 // is not a problem when sorting becuase sort is stable and move
986 // position in the list is preserved, just the PV is pushed up.
987 rm.score = -VALUE_INFINITE;
990 if (value > bestValue)
993 if (SpNode) sp->bestValue = value;
998 if (SpNode) sp->bestMove = move;
1000 if (PvNode && value < beta)
1002 alpha = value; // Update alpha here! Always alpha < beta
1003 if (SpNode) sp->alpha = value;
1007 if (SpNode) sp->cutoff = true;
1013 // Step 19. Check for splitting the search
1015 && depth >= Threads.min_split_depth()
1017 && Threads.available_slave_exists(thisThread))
1019 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1020 depth, threatMove, moveCount, mp, NT);
1028 // Step 20. Check for mate and stalemate
1029 // All legal moves have been searched and if there are no legal moves, it
1030 // must be mate or stalemate. Note that we can have a false positive in
1031 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1032 // harmless because return value is discarded anyhow in the parent nodes.
1033 // If we are in a singular extension search then return a fail low score.
1034 // A split node has at least one move, the one tried before to be splitted.
1036 return excludedMove ? alpha
1037 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1039 // If we have pruned all the moves without searching return a fail-low score
1040 if (bestValue == -VALUE_INFINITE)
1042 assert(!playedMoveCount);
1047 if (bestValue >= beta) // Failed high
1049 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1050 bestMove, ss->staticEval, ss->evalMargin);
1052 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1054 if (bestMove != ss->killers[0])
1056 ss->killers[1] = ss->killers[0];
1057 ss->killers[0] = bestMove;
1060 // Increase history value of the cut-off move
1061 Value bonus = Value(int(depth) * int(depth));
1062 H.add(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1064 // Decrease history of all the other played non-capture moves
1065 for (int i = 0; i < playedMoveCount - 1; i++)
1067 Move m = movesSearched[i];
1068 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1072 else // Failed low or PV search
1073 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1074 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1075 depth, bestMove, ss->staticEval, ss->evalMargin);
1077 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1083 // qsearch() is the quiescence search function, which is called by the main
1084 // search function when the remaining depth is zero (or, to be more precise,
1085 // less than ONE_PLY).
1087 template <NodeType NT>
1088 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1090 const bool PvNode = (NT == PV);
1092 assert(NT == PV || NT == NonPV);
1093 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1094 assert(PvNode || (alpha == beta - 1));
1095 assert(depth <= DEPTH_ZERO);
1100 Move ttMove, move, bestMove;
1101 Value bestValue, value, ttValue, futilityValue, futilityBase;
1102 bool inCheck, givesCheck, enoughMaterial, evasionPrunable;
1105 inCheck = pos.in_check();
1106 ss->currentMove = bestMove = MOVE_NONE;
1107 ss->ply = (ss-1)->ply + 1;
1109 // Check for an instant draw or maximum ply reached
1110 if (pos.is_draw<true,true>() || ss->ply > MAX_PLY)
1111 return DrawValue[pos.side_to_move()];
1113 // Transposition table lookup. At PV nodes, we don't use the TT for
1114 // pruning, but only for move ordering.
1116 tte = TT.probe(posKey);
1117 ttMove = tte ? tte->move() : MOVE_NONE;
1118 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1120 // Decide whether or not to include checks, this fixes also the type of
1121 // TT entry depth that we are going to use. Note that in qsearch we use
1122 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1123 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1124 : DEPTH_QS_NO_CHECKS;
1125 if ( tte && tte->depth() >= ttDepth
1126 && ( PvNode ? tte->type() == BOUND_EXACT
1127 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1128 : (tte->type() & BOUND_UPPER)))
1130 assert(ttValue != VALUE_NONE); // Due to ttDepth > DEPTH_NONE
1132 ss->currentMove = ttMove; // Can be MOVE_NONE
1136 // Evaluate the position statically
1139 ss->staticEval = ss->evalMargin = VALUE_NONE;
1140 bestValue = futilityBase = -VALUE_INFINITE;
1141 enoughMaterial = false;
1147 assert(tte->static_value() != VALUE_NONE);
1149 ss->staticEval = bestValue = tte->static_value();
1150 ss->evalMargin = tte->static_value_margin();
1153 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1155 // Stand pat. Return immediately if static value is at least beta
1156 if (bestValue >= beta)
1159 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1160 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1165 if (PvNode && bestValue > alpha)
1168 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1169 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1172 // Initialize a MovePicker object for the current position, and prepare
1173 // to search the moves. Because the depth is <= 0 here, only captures,
1174 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1176 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1179 // Loop through the moves until no moves remain or a beta cutoff occurs
1180 while ((move = mp.next_move<false>()) != MOVE_NONE)
1182 assert(is_ok(move));
1184 givesCheck = pos.move_gives_check(move, ci);
1192 && type_of(move) != PROMOTION
1193 && !pos.is_passed_pawn_push(move))
1195 futilityValue = futilityBase
1196 + PieceValue[EG][pos.piece_on(to_sq(move))]
1197 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1199 if (futilityValue < beta)
1201 if (futilityValue > bestValue)
1202 bestValue = futilityValue;
1207 // Prune moves with negative or equal SEE
1208 if ( futilityBase < beta
1209 && depth < DEPTH_ZERO
1210 && pos.see(move) <= 0)
1214 // Detect non-capture evasions that are candidate to be pruned
1215 evasionPrunable = !PvNode
1217 && bestValue > VALUE_MATED_IN_MAX_PLY
1218 && !pos.is_capture(move)
1219 && !pos.can_castle(pos.side_to_move());
1221 // Don't search moves with negative SEE values
1223 && (!inCheck || evasionPrunable)
1225 && type_of(move) != PROMOTION
1226 && pos.see_sign(move) < 0)
1229 // Don't search useless checks
1234 && !pos.is_capture_or_promotion(move)
1235 && ss->staticEval + PawnValueMg / 4 < beta
1236 && !check_is_dangerous(pos, move, futilityBase, beta))
1239 // Check for legality only before to do the move
1240 if (!pos.pl_move_is_legal(move, ci.pinned))
1243 ss->currentMove = move;
1245 // Make and search the move
1246 pos.do_move(move, st, ci, givesCheck);
1247 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1248 pos.undo_move(move);
1250 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1252 // Check for new best move
1253 if (value > bestValue)
1259 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1266 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1267 ttDepth, move, ss->staticEval, ss->evalMargin);
1275 // All legal moves have been searched. A special case: If we're in check
1276 // and no legal moves were found, it is checkmate.
1277 if (inCheck && bestValue == -VALUE_INFINITE)
1278 return mated_in(ss->ply); // Plies to mate from the root
1280 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1281 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1282 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1284 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1290 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1291 // bestValue is updated only when returning false because in that case move
1294 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta)
1296 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1297 Square from, to, ksq;
1301 from = from_sq(move);
1303 them = ~pos.side_to_move();
1304 ksq = pos.king_square(them);
1305 kingAtt = pos.attacks_from<KING>(ksq);
1306 pc = pos.piece_moved(move);
1308 occ = pos.pieces() ^ from ^ ksq;
1309 oldAtt = pos.attacks_from(pc, from, occ);
1310 newAtt = pos.attacks_from(pc, to, occ);
1312 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1313 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1315 if (!more_than_one(b))
1318 // Rule 2. Queen contact check is very dangerous
1319 if (type_of(pc) == QUEEN && (kingAtt & to))
1322 // Rule 3. Creating new double threats with checks
1323 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1326 // Note that here we generate illegal "double move"!
1327 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1335 // connected_moves() tests whether two moves are 'connected' in the sense
1336 // that the first move somehow made the second move possible (for instance
1337 // if the moving piece is the same in both moves). The first move is assumed
1338 // to be the move that was made to reach the current position, while the
1339 // second move is assumed to be a move from the current position.
1341 bool connected_moves(const Position& pos, Move m1, Move m2) {
1343 Square f1, t1, f2, t2;
1350 // Case 1: The moving piece is the same in both moves
1356 // Case 2: The destination square for m2 was vacated by m1
1362 // Case 3: Moving through the vacated square
1363 p2 = pos.piece_on(f2);
1364 if (piece_is_slider(p2) && (between_bb(f2, t2) & f1))
1367 // Case 4: The destination square for m2 is defended by the moving piece in m1
1368 p1 = pos.piece_on(t1);
1369 if (pos.attacks_from(p1, t1) & t2)
1372 // Case 5: Discovered check, checking piece is the piece moved in m1
1373 ksq = pos.king_square(pos.side_to_move());
1374 if ( piece_is_slider(p1)
1375 && (between_bb(t1, ksq) & f2)
1376 && (pos.attacks_from(p1, t1, pos.pieces() ^ f2) & ksq))
1383 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1384 // "plies to mate from the current position". Non-mate scores are unchanged.
1385 // The function is called before storing a value to the transposition table.
1387 Value value_to_tt(Value v, int ply) {
1389 assert(v != VALUE_NONE);
1391 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1392 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1396 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1397 // from the transposition table (where refers to the plies to mate/be mated
1398 // from current position) to "plies to mate/be mated from the root".
1400 Value value_from_tt(Value v, int ply) {
1402 return v == VALUE_NONE ? VALUE_NONE
1403 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1404 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1408 // connected_threat() tests whether it is safe to forward prune a move or if
1409 // is somehow connected to the threat move returned by null search.
1411 bool connected_threat(const Position& pos, Move m, Move threat) {
1414 assert(is_ok(threat));
1415 assert(!pos.is_capture_or_promotion(m));
1416 assert(!pos.is_passed_pawn_push(m));
1418 Square mfrom, mto, tfrom, tto;
1422 tfrom = from_sq(threat);
1423 tto = to_sq(threat);
1425 // Case 1: Don't prune moves which move the threatened piece
1429 // Case 2: If the threatened piece has value less than or equal to the
1430 // value of the threatening piece, don't prune moves which defend it.
1431 if ( pos.is_capture(threat)
1432 && ( PieceValue[MG][pos.piece_on(tfrom)] >= PieceValue[MG][pos.piece_on(tto)]
1433 || type_of(pos.piece_on(tfrom)) == KING)
1434 && pos.move_attacks_square(m, tto))
1437 // Case 3: If the moving piece in the threatened move is a slider, don't
1438 // prune safe moves which block its ray.
1439 if ( piece_is_slider(pos.piece_on(tfrom))
1440 && (between_bb(tfrom, tto) & mto)
1441 && pos.see_sign(m) >= 0)
1448 // When playing with strength handicap choose best move among the MultiPV set
1449 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1451 Move Skill::pick_move() {
1455 // PRNG sequence should be not deterministic
1456 for (int i = Time::now() % 50; i > 0; i--)
1457 rk.rand<unsigned>();
1459 // RootMoves are already sorted by score in descending order
1460 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1461 int weakness = 120 - 2 * level;
1462 int max_s = -VALUE_INFINITE;
1465 // Choose best move. For each move score we add two terms both dependent on
1466 // weakness, one deterministic and bigger for weaker moves, and one random,
1467 // then we choose the move with the resulting highest score.
1468 for (size_t i = 0; i < PVSize; i++)
1470 int s = RootMoves[i].score;
1472 // Don't allow crazy blunders even at very low skills
1473 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1476 // This is our magic formula
1477 s += ( weakness * int(RootMoves[0].score - s)
1478 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1483 best = RootMoves[i].pv[0];
1490 // uci_pv() formats PV information according to UCI protocol. UCI requires
1491 // to send all the PV lines also if are still to be searched and so refer to
1492 // the previous search score.
1494 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1496 std::stringstream s;
1497 Time::point elaspsed = Time::now() - SearchTime + 1;
1500 for (size_t i = 0; i < Threads.size(); i++)
1501 if (Threads[i].maxPly > selDepth)
1502 selDepth = Threads[i].maxPly;
1504 for (size_t i = 0; i < std::min((size_t)Options["MultiPV"], RootMoves.size()); i++)
1506 bool updated = (i <= PVIdx);
1508 if (depth == 1 && !updated)
1511 int d = (updated ? depth : depth - 1);
1512 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1514 if (s.rdbuf()->in_avail())
1517 s << "info depth " << d
1518 << " seldepth " << selDepth
1519 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1520 << " nodes " << pos.nodes_searched()
1521 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1522 << " time " << elaspsed
1523 << " multipv " << i + 1
1526 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1527 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1536 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1537 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1538 /// allow to always have a ponder move even when we fail high at root, and a
1539 /// long PV to print that is important for position analysis.
1541 void RootMove::extract_pv_from_tt(Position& pos) {
1543 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1548 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1552 pos.do_move(m, *st++);
1554 while ( (tte = TT.probe(pos.key())) != NULL
1555 && (m = tte->move()) != MOVE_NONE // Local copy, TT entry could change
1556 && pos.is_pseudo_legal(m)
1557 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1559 && (!pos.is_draw<false,true>() || ply < 2))
1562 pos.do_move(m, *st++);
1565 pv.push_back(MOVE_NONE);
1567 do pos.undo_move(pv[--ply]); while (ply);
1571 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1572 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1573 /// first, even if the old TT entries have been overwritten.
1575 void RootMove::insert_pv_in_tt(Position& pos) {
1577 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1580 Value v, m = VALUE_NONE;
1583 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1589 // Don't overwrite existing correct entries
1590 if (!tte || tte->move() != pv[ply])
1592 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1593 TT.store(k, VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], v, m);
1595 pos.do_move(pv[ply], *st++);
1597 } while (pv[++ply] != MOVE_NONE);
1599 do pos.undo_move(pv[--ply]); while (ply);
1603 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1605 void Thread::idle_loop() {
1607 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1608 // object for which the thread is the master.
1609 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1611 assert(!sp_master || (sp_master->master == this && is_searching));
1613 // If this thread is the master of a split point and all slaves have
1614 // finished their work at this split point, return from the idle loop.
1615 while (!sp_master || sp_master->slavesMask)
1617 // If we are not searching, wait for a condition to be signaled
1618 // instead of wasting CPU time polling for work.
1621 || (!is_searching && Threads.use_sleeping_threads()))
1629 // Grab the lock to avoid races with Thread::wake_up()
1632 // If we are master and all slaves have finished don't go to sleep
1633 if (sp_master && !sp_master->slavesMask)
1639 // Do sleep after retesting sleep conditions under lock protection, in
1640 // particular we need to avoid a deadlock in case a master thread has,
1641 // in the meanwhile, allocated us and sent the wake_up() call before we
1642 // had the chance to grab the lock.
1643 if (do_sleep || !is_searching)
1644 sleepCondition.wait(mutex);
1649 // If this thread has been assigned work, launch a search
1652 assert(!do_sleep && !do_exit);
1654 Threads.mutex.lock();
1656 assert(is_searching);
1657 SplitPoint* sp = curSplitPoint;
1659 Threads.mutex.unlock();
1661 Stack ss[MAX_PLY_PLUS_2];
1662 Position pos(*sp->pos, this);
1664 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1669 assert(sp->activePositions[idx] == NULL);
1671 sp->activePositions[idx] = &pos;
1673 if (sp->nodeType == Root)
1674 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1675 else if (sp->nodeType == PV)
1676 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1677 else if (sp->nodeType == NonPV)
1678 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1682 assert(is_searching);
1684 is_searching = false;
1685 sp->activePositions[idx] = NULL;
1686 sp->slavesMask &= ~(1ULL << idx);
1687 sp->nodes += pos.nodes_searched();
1689 // Wake up master thread so to allow it to return from the idle loop in
1690 // case we are the last slave of the split point.
1691 if ( Threads.use_sleeping_threads()
1692 && this != sp->master
1695 assert(!sp->master->is_searching);
1696 sp->master->wake_up();
1699 // After releasing the lock we cannot access anymore any SplitPoint
1700 // related data in a safe way becuase it could have been released under
1701 // our feet by the sp master. Also accessing other Thread objects is
1702 // unsafe because if we are exiting there is a chance are already freed.
1709 /// check_time() is called by the timer thread when the timer triggers. It is
1710 /// used to print debug info and, more important, to detect when we are out of
1711 /// available time and so stop the search.
1715 static Time::point lastInfoTime = Time::now();
1716 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1718 if (Time::now() - lastInfoTime >= 1000)
1720 lastInfoTime = Time::now();
1729 Threads.mutex.lock();
1731 nodes = RootPos.nodes_searched();
1733 // Loop across all split points and sum accumulated SplitPoint nodes plus
1734 // all the currently active slaves positions.
1735 for (size_t i = 0; i < Threads.size(); i++)
1736 for (int j = 0; j < Threads[i].splitPointsCnt; j++)
1738 SplitPoint& sp = Threads[i].splitPoints[j];
1743 Bitboard sm = sp.slavesMask;
1746 Position* pos = sp.activePositions[pop_lsb(&sm)];
1747 nodes += pos ? pos->nodes_searched() : 0;
1753 Threads.mutex.unlock();
1756 Time::point elapsed = Time::now() - SearchTime;
1757 bool stillAtFirstMove = Signals.firstRootMove
1758 && !Signals.failedLowAtRoot
1759 && elapsed > TimeMgr.available_time();
1761 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1762 || stillAtFirstMove;
1764 if ( (Limits.use_time_management() && noMoreTime)
1765 || (Limits.movetime && elapsed >= Limits.movetime)
1766 || (Limits.nodes && nodes >= Limits.nodes))
1767 Signals.stop = true;