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 // Dynamic razoring margin based on depth
66 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
68 // Futility lookup tables (initialized at startup) and their access functions
69 Value FutilityMargins[16][64]; // [depth][moveNumber]
70 int FutilityMoveCounts[32]; // [depth]
72 inline Value futility_margin(Depth d, int mn) {
74 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
78 // Reduction lookup tables (initialized at startup) and their access function
79 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
81 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
83 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
89 Value DrawValue[COLOR_NB];
92 template <NodeType NT>
93 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
95 template <NodeType NT, bool InCheck>
96 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
98 void id_loop(Position& pos);
99 Value value_to_tt(Value v, int ply);
100 Value value_from_tt(Value v, int ply);
101 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta);
102 bool allows_move(const Position& pos, Move first, Move second);
103 bool prevents_move(const Position& pos, Move first, Move second);
104 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
107 Skill(int l) : level(l), best(MOVE_NONE) {}
109 if (enabled()) // Swap best PV line with the sub-optimal one
110 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
111 RootMoves.end(), best ? best : pick_move()));
114 bool enabled() const { return level < 20; }
115 bool time_to_pick(int depth) const { return depth == 1 + level; }
125 /// Search::init() is called during startup to initialize various lookup tables
127 void Search::init() {
129 int d; // depth (ONE_PLY == 2)
130 int hd; // half depth (ONE_PLY == 1)
133 // Init reductions array
134 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
136 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
137 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
138 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
139 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
142 // Init futility margins array
143 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
144 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
146 // Init futility move count array
147 for (d = 0; d < 32; d++)
148 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(double(d), 2.0));
152 /// Search::perft() is our utility to verify move generation. All the leaf nodes
153 /// up to the given depth are generated and counted and the sum returned.
155 size_t Search::perft(Position& pos, Depth depth) {
157 // At the last ply just return the number of legal moves (leaf nodes)
158 if (depth == ONE_PLY)
159 return MoveList<LEGAL>(pos).size();
165 for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
167 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
168 cnt += perft(pos, depth - ONE_PLY);
169 pos.undo_move(ml.move());
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 static PolyglotBook book; // Defined static to initialize the PRNG only once
184 RootColor = RootPos.side_to_move();
185 TimeMgr.init(Limits, RootPos.startpos_ply_counter(), RootColor);
187 if (RootMoves.empty())
189 RootMoves.push_back(MOVE_NONE);
190 sync_cout << "info depth 0 score "
191 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
197 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
199 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
201 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
203 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
208 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
210 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
211 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
212 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
213 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
216 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
218 if (Options["Use Search Log"])
220 Log log(Options["Search Log Filename"]);
221 log << "\nSearching: " << RootPos.fen()
222 << "\ninfinite: " << Limits.infinite
223 << " ponder: " << Limits.ponder
224 << " time: " << Limits.time[RootColor]
225 << " increment: " << Limits.inc[RootColor]
226 << " moves to go: " << Limits.movestogo
232 // Set best timer interval to avoid lagging under time pressure. Timer is
233 // used to check for remaining available thinking time.
234 if (Limits.use_time_management())
235 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16,
237 else if (Limits.nodes)
238 Threads.set_timer(2 * TimerResolution);
240 Threads.set_timer(100);
242 id_loop(RootPos); // Let's start searching !
244 Threads.set_timer(0); // Stop timer
247 if (Options["Use Search Log"])
249 Time::point elapsed = Time::now() - SearchTime + 1;
251 Log log(Options["Search Log Filename"]);
252 log << "Nodes: " << RootPos.nodes_searched()
253 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
254 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
257 RootPos.do_move(RootMoves[0].pv[0], st);
258 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
259 RootPos.undo_move(RootMoves[0].pv[0]);
264 // When we reach max depth we arrive here even without Signals.stop is raised,
265 // but if we are pondering or in infinite search, we shouldn't print the best
266 // move before we are told to do so.
267 if (!Signals.stop && (Limits.ponder || Limits.infinite))
268 RootPos.this_thread()->wait_for_stop_or_ponderhit();
270 // Best move could be MOVE_NONE when searching on a stalemate position
271 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
272 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
279 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
280 // with increasing depth until the allocated thinking time has been consumed,
281 // user stops the search, or the maximum search depth is reached.
283 void id_loop(Position& pos) {
285 Stack ss[MAX_PLY_PLUS_2];
286 int depth, prevBestMoveChanges;
287 Value bestValue, alpha, beta, delta;
288 bool bestMoveNeverChanged = true;
290 memset(ss, 0, 4 * sizeof(Stack));
291 depth = BestMoveChanges = 0;
292 bestValue = delta = -VALUE_INFINITE;
293 ss->currentMove = MOVE_NULL; // Hack to skip update gains
297 PVSize = Options["MultiPV"];
298 Skill skill(Options["Skill Level"]);
300 // Do we have to play with skill handicap? In this case enable MultiPV search
301 // that we will use behind the scenes to retrieve a set of possible moves.
302 if (skill.enabled() && PVSize < 4)
305 PVSize = std::min(PVSize, RootMoves.size());
307 // Iterative deepening loop until requested to stop or target depth reached
308 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
310 // Save last iteration's scores before first PV line is searched and all
311 // the move scores but the (new) PV are set to -VALUE_INFINITE.
312 for (size_t i = 0; i < RootMoves.size(); i++)
313 RootMoves[i].prevScore = RootMoves[i].score;
315 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
318 // MultiPV loop. We perform a full root search for each PV line
319 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
321 // Set aspiration window default width
322 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
325 alpha = RootMoves[PVIdx].prevScore - delta;
326 beta = RootMoves[PVIdx].prevScore + delta;
330 alpha = -VALUE_INFINITE;
331 beta = VALUE_INFINITE;
334 // Start with a small aspiration window and, in case of fail high/low,
335 // research with bigger window until not failing high/low anymore.
338 // Search starts from ss+1 to allow referencing (ss-1). This is
339 // needed by update gains and ss copy when splitting at Root.
340 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
342 // Bring to front the best move. It is critical that sorting is
343 // done with a stable algorithm because all the values but the first
344 // and eventually the new best one are set to -VALUE_INFINITE and
345 // we want to keep the same order for all the moves but the new
346 // PV that goes to the front. Note that in case of MultiPV search
347 // the already searched PV lines are preserved.
348 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
350 // Write PV back to transposition table in case the relevant
351 // entries have been overwritten during the search.
352 for (size_t i = 0; i <= PVIdx; i++)
353 RootMoves[i].insert_pv_in_tt(pos);
355 // If search has been stopped return immediately. Sorting and
356 // writing PV back to TT is safe becuase RootMoves is still
357 // valid, although refers to previous iteration.
361 // In case of failing high/low increase aspiration window and
362 // research, otherwise exit the loop.
363 if (bestValue > alpha && bestValue < beta)
366 // Give some update (without cluttering the UI) before to research
367 if (Time::now() - SearchTime > 3000)
368 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
370 if (abs(bestValue) >= VALUE_KNOWN_WIN)
372 alpha = -VALUE_INFINITE;
373 beta = VALUE_INFINITE;
375 else if (bestValue >= beta)
382 Signals.failedLowAtRoot = true;
383 Signals.stopOnPonderhit = false;
389 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
392 // Sort the PV lines searched so far and update the GUI
393 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
394 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
395 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
398 // Do we need to pick now the sub-optimal best move ?
399 if (skill.enabled() && skill.time_to_pick(depth))
402 if (Options["Use Search Log"])
404 Log log(Options["Search Log Filename"]);
405 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
409 // Filter out startup noise when monitoring best move stability
410 if (depth > 2 && BestMoveChanges)
411 bestMoveNeverChanged = false;
413 // Do we have found a "mate in x"?
415 && bestValue >= VALUE_MATE_IN_MAX_PLY
416 && VALUE_MATE - bestValue <= 2 * Limits.mate)
419 // Do we have time for the next iteration? Can we stop searching now?
420 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
422 bool stop = false; // Local variable, not the volatile Signals.stop
424 // Take in account some extra time if the best move has changed
425 if (depth > 4 && depth < 50 && PVSize == 1)
426 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
428 // Stop search if most of available time is already consumed. We
429 // probably don't have enough time to search the first move at the
430 // next iteration anyway.
431 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
434 // Stop search early if one move seems to be much better than others
438 && ( (bestMoveNeverChanged && pos.captured_piece_type())
439 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
441 Value rBeta = bestValue - 2 * PawnValueMg;
442 (ss+1)->excludedMove = RootMoves[0].pv[0];
443 (ss+1)->skipNullMove = true;
444 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
445 (ss+1)->skipNullMove = false;
446 (ss+1)->excludedMove = MOVE_NONE;
454 // If we are allowed to ponder do not stop the search now but
455 // keep pondering until GUI sends "ponderhit" or "stop".
457 Signals.stopOnPonderhit = true;
466 // search<>() is the main search function for both PV and non-PV nodes and for
467 // normal and SplitPoint nodes. When called just after a split point the search
468 // is simpler because we have already probed the hash table, done a null move
469 // search, and searched the first move before splitting, we don't have to repeat
470 // all this work again. We also don't need to store anything to the hash table
471 // here: This is taken care of after we return from the split point.
473 template <NodeType NT>
474 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
476 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
477 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
478 const bool RootNode = (NT == Root || NT == SplitPointRoot);
480 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
481 assert(PvNode || (alpha == beta - 1));
482 assert(depth > DEPTH_ZERO);
484 Move movesSearched[64];
489 Move ttMove, move, excludedMove, bestMove, threatMove;
491 Value bestValue, value, ttValue;
492 Value eval, nullValue, futilityValue;
493 bool inCheck, givesCheck, pvMove, singularExtensionNode;
494 bool captureOrPromotion, dangerous, doFullDepthSearch, threatExtension;
495 int moveCount, playedMoveCount;
497 // Step 1. Initialize node
498 Thread* thisThread = pos.this_thread();
499 moveCount = playedMoveCount = 0;
500 threatExtension = false;
501 inCheck = pos.checkers();
506 bestMove = sp->bestMove;
507 threatMove = sp->threatMove;
508 bestValue = sp->bestValue;
510 ttMove = excludedMove = MOVE_NONE;
511 ttValue = VALUE_NONE;
513 assert(sp->bestValue > -VALUE_INFINITE && sp->moveCount > 0);
515 goto split_point_start;
518 bestValue = -VALUE_INFINITE;
519 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
520 ss->ply = (ss-1)->ply + 1;
521 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
522 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
524 // Used to send selDepth info to GUI
525 if (PvNode && thisThread->maxPly < ss->ply)
526 thisThread->maxPly = ss->ply;
530 // Step 2. Check for aborted search and immediate draw
531 if (Signals.stop || pos.is_draw<true, PvNode>() || ss->ply > MAX_PLY)
532 return DrawValue[pos.side_to_move()];
534 // Step 3. Mate distance pruning. Even if we mate at the next move our score
535 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
536 // a shorter mate was found upward in the tree then there is no need to search
537 // further, we will never beat current alpha. Same logic but with reversed signs
538 // applies also in the opposite condition of being mated instead of giving mate,
539 // in this case return a fail-high score.
540 alpha = std::max(mated_in(ss->ply), alpha);
541 beta = std::min(mate_in(ss->ply+1), beta);
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.
561 && tte->depth() >= depth
562 && ttValue != VALUE_NONE // Only in case of TT access race
563 && ( PvNode ? tte->type() == BOUND_EXACT
564 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
565 : (tte->type() & BOUND_UPPER)))
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 // Never assume anything on values stored in TT
588 if ( (ss->staticEval = eval = tte->static_value()) == VALUE_NONE
589 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
590 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
592 // Can ttValue be used as a better position evaluation?
593 if (ttValue != VALUE_NONE)
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 // Handling of UCI command 'mate in x moves'. We simply return if after
606 // 'x' moves we still have not checkmated the opponent.
607 if (PvNode && !RootNode && !inCheck && Limits.mate && ss->ply > 2 * Limits.mate)
610 // Update gain for the parent non-capture move given the static position
611 // evaluation before and after the move.
612 if ( (move = (ss-1)->currentMove) != MOVE_NULL
613 && (ss-1)->staticEval != VALUE_NONE
614 && ss->staticEval != VALUE_NONE
615 && !pos.captured_piece_type()
616 && type_of(move) == NORMAL)
618 Square to = to_sq(move);
619 H.update_gain(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
622 // Step 6. Razoring (is omitted in PV nodes)
624 && depth < 4 * ONE_PLY
626 && eval + razor_margin(depth) < beta
627 && ttMove == MOVE_NONE
628 && abs(beta) < VALUE_MATE_IN_MAX_PLY
629 && !pos.pawn_on_7th(pos.side_to_move()))
631 Value rbeta = beta - razor_margin(depth);
632 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
634 // Logically we should return (v + razor_margin(depth)), but
635 // surprisingly this did slightly weaker in tests.
639 // Step 7. Static null move pruning (is omitted in PV nodes)
640 // We're betting that the opponent doesn't have a move that will reduce
641 // the score by more than futility_margin(depth) if we do a null move.
644 && depth < 4 * ONE_PLY
646 && eval - FutilityMargins[depth][0] >= beta
647 && abs(beta) < VALUE_MATE_IN_MAX_PLY
648 && pos.non_pawn_material(pos.side_to_move()))
649 return eval - FutilityMargins[depth][0];
651 // Step 8. Null move search with verification search (is omitted in PV nodes)
657 && abs(beta) < VALUE_MATE_IN_MAX_PLY
658 && pos.non_pawn_material(pos.side_to_move()))
660 ss->currentMove = MOVE_NULL;
662 // Null move dynamic reduction based on depth
663 Depth R = 3 * ONE_PLY + depth / 4;
665 // Null move dynamic reduction based on value
666 if (eval - PawnValueMg > beta)
669 pos.do_null_move<true>(st);
670 (ss+1)->skipNullMove = true;
671 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
672 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
673 (ss+1)->skipNullMove = false;
674 pos.do_null_move<false>(st);
676 if (nullValue >= beta)
678 // Do not return unproven mate scores
679 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
682 if (depth < 6 * ONE_PLY)
685 // Do verification search at high depths
686 ss->skipNullMove = true;
687 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
688 ss->skipNullMove = false;
695 // The null move failed low, which means that we may be faced with
696 // some kind of threat. If the previous move was reduced, check if
697 // the move that refuted the null move was somehow connected to the
698 // move which was reduced. If a connection is found extend moves that
699 // defend against threat.
700 threatMove = (ss+1)->currentMove;
702 if ( depth < 5 * ONE_PLY
704 && threatMove != MOVE_NONE
705 && allows_move(pos, (ss-1)->currentMove, threatMove))
706 threatExtension = true;
710 // Step 9. ProbCut (is omitted in PV nodes)
711 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
712 // and a reduced search returns a value much above beta, we can (almost) safely
713 // prune the previous move.
715 && depth >= 5 * ONE_PLY
718 && excludedMove == MOVE_NONE
719 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
721 Value rbeta = beta + 200;
722 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
724 assert(rdepth >= ONE_PLY);
725 assert((ss-1)->currentMove != MOVE_NONE);
726 assert((ss-1)->currentMove != MOVE_NULL);
728 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
731 while ((move = mp.next_move<false>()) != MOVE_NONE)
732 if (pos.pl_move_is_legal(move, ci.pinned))
734 ss->currentMove = move;
735 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
736 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
743 // Step 10. Internal iterative deepening
744 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
745 && ttMove == MOVE_NONE
746 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
748 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
750 ss->skipNullMove = true;
751 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
752 ss->skipNullMove = false;
754 tte = TT.probe(posKey);
755 ttMove = tte ? tte->move() : MOVE_NONE;
758 split_point_start: // At split points actual search starts from here
760 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
762 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
763 singularExtensionNode = !RootNode
765 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
766 && ttMove != MOVE_NONE
767 && !excludedMove // Recursive singular search is not allowed
768 && (tte->type() & BOUND_LOWER)
769 && tte->depth() >= depth - 3 * ONE_PLY;
771 // Step 11. Loop through moves
772 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
773 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
777 if (move == excludedMove)
780 // At root obey the "searchmoves" option and skip moves not listed in Root
781 // Move List, as a consequence any illegal move is also skipped. In MultiPV
782 // mode we also skip PV moves which have been already searched.
783 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
788 // Shared counter cannot be decremented later if move turns out to be illegal
789 if (!pos.pl_move_is_legal(move, ci.pinned))
792 moveCount = ++sp->moveCount;
800 Signals.firstRootMove = (moveCount == 1);
802 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
803 sync_cout << "info depth " << depth / ONE_PLY
804 << " currmove " << move_to_uci(move, pos.is_chess960())
805 << " currmovenumber " << moveCount + PVIdx << sync_endl;
809 captureOrPromotion = pos.is_capture_or_promotion(move);
810 givesCheck = pos.move_gives_check(move, ci);
811 dangerous = givesCheck
812 || pos.is_passed_pawn_push(move)
813 || type_of(move) == CASTLE
814 || ( captureOrPromotion // Entering a pawn endgame?
815 && type_of(pos.piece_on(to_sq(move))) != PAWN
816 && type_of(move) == NORMAL
817 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
818 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
820 // Step 12. Extend checks and, in PV nodes, also dangerous moves
821 if (PvNode && dangerous)
824 else if (threatExtension && prevents_move(pos, move, threatMove))
827 else if (givesCheck && pos.see_sign(move) >= 0)
830 // Singular extension search. If all moves but one fail low on a search of
831 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
832 // is singular and should be extended. To verify this we do a reduced search
833 // on all the other moves but the ttMove, if result is lower than ttValue minus
834 // a margin then we extend ttMove.
835 if ( singularExtensionNode
838 && pos.pl_move_is_legal(move, ci.pinned)
839 && abs(ttValue) < VALUE_KNOWN_WIN)
841 assert(ttValue != VALUE_NONE);
843 Value rBeta = ttValue - int(depth);
844 ss->excludedMove = move;
845 ss->skipNullMove = true;
846 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
847 ss->skipNullMove = false;
848 ss->excludedMove = MOVE_NONE;
851 ext = rBeta >= beta ? ONE_PLY + ONE_PLY / 2 : ONE_PLY;
854 // Update current move (this must be done after singular extension search)
855 newDepth = depth - ONE_PLY + ext;
857 // Step 13. Futility pruning (is omitted in PV nodes)
859 && !captureOrPromotion
863 && (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
864 && alpha > VALUE_MATED_IN_MAX_PLY)))
866 // Move count based pruning
867 if ( depth < 16 * ONE_PLY
868 && moveCount >= FutilityMoveCounts[depth]
869 && (!threatMove || !prevents_move(pos, move, threatMove)))
877 // Value based pruning
878 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
879 // but fixing this made program slightly weaker.
880 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
881 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
882 + H.gain(pos.piece_moved(move), to_sq(move));
884 if (futilityValue < beta)
892 // Prune moves with negative SEE at low depths
893 if ( predictedDepth < 2 * ONE_PLY
894 && pos.see_sign(move) < 0)
903 // Check for legality only before to do the move
904 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
910 pvMove = PvNode ? moveCount == 1 : false;
911 ss->currentMove = move;
912 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
913 movesSearched[playedMoveCount++] = move;
915 // Step 14. Make the move
916 pos.do_move(move, st, ci, givesCheck);
918 // Step 15. Reduced depth search (LMR). If the move fails high will be
919 // re-searched at full depth.
920 if ( depth > 3 * ONE_PLY
922 && !captureOrPromotion
924 && ss->killers[0] != move
925 && ss->killers[1] != move)
927 ss->reduction = reduction<PvNode>(depth, moveCount);
928 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
929 alpha = SpNode ? sp->alpha : alpha;
931 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
933 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
934 ss->reduction = DEPTH_ZERO;
937 doFullDepthSearch = !pvMove;
939 // Step 16. Full depth search, when LMR is skipped or fails high
940 if (doFullDepthSearch)
942 alpha = SpNode ? sp->alpha : alpha;
943 value = newDepth < ONE_PLY ?
944 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
945 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
946 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
949 // Only for PV nodes do a full PV search on the first move or after a fail
950 // high, in the latter case search only if value < beta, otherwise let the
951 // parent node to fail low with value <= alpha and to try another move.
952 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
953 value = newDepth < ONE_PLY ?
954 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
955 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
956 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
957 // Step 17. Undo move
960 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
962 // Step 18. Check for new best move
966 bestValue = sp->bestValue;
970 // Finished searching the move. If Signals.stop is true, the search
971 // was aborted because the user interrupted the search or because we
972 // ran out of time. In this case, the return value of the search cannot
973 // be trusted, and we don't update the best move and/or PV.
974 if (Signals.stop || thisThread->cutoff_occurred())
975 return value; // To avoid returning VALUE_INFINITE
979 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
981 // PV move or new best move ?
982 if (pvMove || value > alpha)
985 rm.extract_pv_from_tt(pos);
987 // We record how often the best move has been changed in each
988 // iteration. This information is used for time management: When
989 // the best move changes frequently, we allocate some more time.
994 // All other moves but the PV are set to the lowest value, this
995 // is not a problem when sorting becuase sort is stable and move
996 // position in the list is preserved, just the PV is pushed up.
997 rm.score = -VALUE_INFINITE;
1000 if (value > bestValue)
1003 if (SpNode) sp->bestValue = value;
1008 if (SpNode) sp->bestMove = move;
1010 if (PvNode && value < beta)
1012 alpha = value; // Update alpha here! Always alpha < beta
1013 if (SpNode) sp->alpha = value;
1017 assert(value >= beta); // Fail high
1019 if (SpNode) sp->cutoff = true;
1025 // Step 19. Check for splitting the search
1027 && depth >= Threads.min_split_depth()
1028 && Threads.available_slave_exists(thisThread))
1030 assert(bestValue < beta);
1032 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1033 depth, threatMove, moveCount, mp, NT);
1034 if (bestValue >= beta)
1042 // Step 20. Check for mate and stalemate
1043 // All legal moves have been searched and if there are no legal moves, it
1044 // must be mate or stalemate. Note that we can have a false positive in
1045 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1046 // harmless because return value is discarded anyhow in the parent nodes.
1047 // If we are in a singular extension search then return a fail low score.
1048 // A split node has at least one move, the one tried before to be splitted.
1050 return excludedMove ? alpha
1051 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1053 // If we have pruned all the moves without searching return a fail-low score
1054 if (bestValue == -VALUE_INFINITE)
1056 assert(!playedMoveCount);
1061 if (bestValue >= beta) // Failed high
1063 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1064 bestMove, ss->staticEval, ss->evalMargin);
1066 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1068 if (bestMove != ss->killers[0])
1070 ss->killers[1] = ss->killers[0];
1071 ss->killers[0] = bestMove;
1074 // Increase history value of the cut-off move
1075 Value bonus = Value(int(depth) * int(depth));
1076 H.add(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1078 // Decrease history of all the other played non-capture moves
1079 for (int i = 0; i < playedMoveCount - 1; i++)
1081 Move m = movesSearched[i];
1082 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1086 else // Failed low or PV search
1087 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1088 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1089 depth, bestMove, ss->staticEval, ss->evalMargin);
1091 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1097 // qsearch() is the quiescence search function, which is called by the main
1098 // search function when the remaining depth is zero (or, to be more precise,
1099 // less than ONE_PLY).
1101 template <NodeType NT, bool InCheck>
1102 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1104 const bool PvNode = (NT == PV);
1106 assert(NT == PV || NT == NonPV);
1107 assert(InCheck == !!pos.checkers());
1108 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1109 assert(PvNode || (alpha == beta - 1));
1110 assert(depth <= DEPTH_ZERO);
1115 Move ttMove, move, bestMove;
1116 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1117 bool givesCheck, enoughMaterial, evasionPrunable, fromNull;
1120 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1124 ss->currentMove = bestMove = MOVE_NONE;
1125 ss->ply = (ss-1)->ply + 1;
1126 fromNull = (ss-1)->currentMove == MOVE_NULL;
1128 // Check for an instant draw or maximum ply reached
1129 if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)
1130 return DrawValue[pos.side_to_move()];
1132 // Transposition table lookup. At PV nodes, we don't use the TT for
1133 // pruning, but only for move ordering.
1135 tte = TT.probe(posKey);
1136 ttMove = tte ? tte->move() : MOVE_NONE;
1137 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1139 // Decide whether or not to include checks, this fixes also the type of
1140 // TT entry depth that we are going to use. Note that in qsearch we use
1141 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1142 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1143 : DEPTH_QS_NO_CHECKS;
1145 && tte->depth() >= ttDepth
1146 && ttValue != VALUE_NONE // Only in case of TT access race
1147 && ( PvNode ? tte->type() == BOUND_EXACT
1148 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1149 : (tte->type() & BOUND_UPPER)))
1151 ss->currentMove = ttMove; // Can be MOVE_NONE
1155 // Evaluate the position statically
1158 ss->staticEval = ss->evalMargin = VALUE_NONE;
1159 bestValue = futilityBase = -VALUE_INFINITE;
1160 enoughMaterial = false;
1166 // Approximated score. Real one is slightly higher due to tempo
1167 ss->staticEval = bestValue = -(ss-1)->staticEval;
1168 ss->evalMargin = VALUE_ZERO;
1172 // Never assume anything on values stored in TT
1173 if ( (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE
1174 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
1175 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1178 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1180 // Stand pat. Return immediately if static value is at least beta
1181 if (bestValue >= beta)
1184 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1185 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1190 if (PvNode && bestValue > alpha)
1193 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1194 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1197 // Initialize a MovePicker object for the current position, and prepare
1198 // to search the moves. Because the depth is <= 0 here, only captures,
1199 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1201 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1204 // Loop through the moves until no moves remain or a beta cutoff occurs
1205 while ((move = mp.next_move<false>()) != MOVE_NONE)
1207 assert(is_ok(move));
1209 givesCheck = pos.move_gives_check(move, ci);
1218 && type_of(move) != PROMOTION
1219 && !pos.is_passed_pawn_push(move))
1221 futilityValue = futilityBase
1222 + PieceValue[EG][pos.piece_on(to_sq(move))]
1223 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1225 if (futilityValue < beta)
1227 bestValue = std::max(bestValue, futilityValue);
1231 // Prune moves with negative or equal SEE
1232 if ( futilityBase < beta
1233 && depth < DEPTH_ZERO
1234 && pos.see(move) <= 0)
1236 bestValue = std::max(bestValue, futilityBase);
1241 // Detect non-capture evasions that are candidate to be pruned
1242 evasionPrunable = !PvNode
1244 && bestValue > VALUE_MATED_IN_MAX_PLY
1245 && !pos.is_capture(move)
1246 && !pos.can_castle(pos.side_to_move());
1248 // Don't search moves with negative SEE values
1250 && (!InCheck || evasionPrunable)
1252 && type_of(move) != PROMOTION
1253 && pos.see_sign(move) < 0)
1256 // Don't search useless checks
1261 && !pos.is_capture_or_promotion(move)
1262 && ss->staticEval + PawnValueMg / 4 < beta
1263 && !check_is_dangerous(pos, move, futilityBase, beta))
1266 // Check for legality only before to do the move
1267 if (!pos.pl_move_is_legal(move, ci.pinned))
1270 ss->currentMove = move;
1272 // Make and search the move
1273 pos.do_move(move, st, ci, givesCheck);
1274 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1275 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1276 pos.undo_move(move);
1278 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1280 // Check for new best move
1281 if (value > bestValue)
1287 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1294 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1295 ttDepth, move, ss->staticEval, ss->evalMargin);
1303 // All legal moves have been searched. A special case: If we're in check
1304 // and no legal moves were found, it is checkmate.
1305 if (InCheck && bestValue == -VALUE_INFINITE)
1306 return mated_in(ss->ply); // Plies to mate from the root
1308 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1309 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1310 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1312 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1318 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1319 // "plies to mate from the current position". Non-mate scores are unchanged.
1320 // The function is called before storing a value to the transposition table.
1322 Value value_to_tt(Value v, int ply) {
1324 assert(v != VALUE_NONE);
1326 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1327 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1331 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1332 // from the transposition table (where refers to the plies to mate/be mated
1333 // from current position) to "plies to mate/be mated from the root".
1335 Value value_from_tt(Value v, int ply) {
1337 return v == VALUE_NONE ? VALUE_NONE
1338 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1339 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1343 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1345 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta)
1347 Piece pc = pos.piece_moved(move);
1348 Square from = from_sq(move);
1349 Square to = to_sq(move);
1350 Color them = ~pos.side_to_move();
1351 Square ksq = pos.king_square(them);
1352 Bitboard enemies = pos.pieces(them);
1353 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1354 Bitboard occ = pos.pieces() ^ from ^ ksq;
1355 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1356 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1358 // Checks which give opponent's king at most one escape square are dangerous
1359 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1362 // Queen contact check is very dangerous
1363 if (type_of(pc) == QUEEN && (kingAtt & to))
1366 // Creating new double threats with checks is dangerous
1367 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1370 // Note that here we generate illegal "double move"!
1371 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1379 // allows_move() tests whether the move at previous ply (first) somehow makes a
1380 // second move possible, for instance if the moving piece is the same in both
1381 // moves. Normally the second move is the threat move (the best move returned
1382 // from a null search that fails low).
1384 bool allows_move(const Position& pos, Move first, Move second) {
1386 assert(is_ok(first));
1387 assert(is_ok(second));
1388 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1389 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1391 Square m1from = from_sq(first);
1392 Square m2from = from_sq(second);
1393 Square m1to = to_sq(first);
1394 Square m2to = to_sq(second);
1396 // The piece is the same or second's destination was vacated by the first move
1397 if (m1to == m2from || m2to == m1from)
1400 // Second one moves through the square vacated by first one
1401 if (between_bb(m2from, m2to) & m1from)
1404 // Second's destination is defended by the first move's piece
1405 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1409 // Second move gives a discovered check through the first's checking piece
1410 if (m1att & pos.king_square(pos.side_to_move()))
1412 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1420 // prevents_move() tests whether a move (first) is able to defend against an
1421 // opponent's move (second). In this case will not be pruned. Normally the
1422 // second move is the threat move (the best move returned from a null search
1425 bool prevents_move(const Position& pos, Move first, Move second) {
1427 assert(is_ok(first));
1428 assert(is_ok(second));
1430 Square m1from = from_sq(first);
1431 Square m2from = from_sq(second);
1432 Square m1to = to_sq(first);
1433 Square m2to = to_sq(second);
1435 // Don't prune moves of the threatened piece
1439 // If the threatened piece has value less than or equal to the value of the
1440 // threat piece, don't prune moves which defend it.
1441 if ( pos.is_capture(second)
1442 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1443 || type_of(pos.piece_on(m2from)) == KING))
1445 // Update occupancy as if the piece and the threat are moving
1446 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1447 Piece piece = pos.piece_on(m1from);
1449 // The moved piece attacks the square 'tto' ?
1450 if (pos.attacks_from(piece, m1to, occ) & m2to)
1453 // Scan for possible X-ray attackers behind the moved piece
1454 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1455 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1457 // Verify attackers are triggered by our move and not already existing
1458 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1462 // Don't prune safe moves which block the threat path
1463 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1470 // When playing with strength handicap choose best move among the MultiPV set
1471 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1473 Move Skill::pick_move() {
1477 // PRNG sequence should be not deterministic
1478 for (int i = Time::now() % 50; i > 0; i--)
1479 rk.rand<unsigned>();
1481 // RootMoves are already sorted by score in descending order
1482 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1483 int weakness = 120 - 2 * level;
1484 int max_s = -VALUE_INFINITE;
1487 // Choose best move. For each move score we add two terms both dependent on
1488 // weakness, one deterministic and bigger for weaker moves, and one random,
1489 // then we choose the move with the resulting highest score.
1490 for (size_t i = 0; i < PVSize; i++)
1492 int s = RootMoves[i].score;
1494 // Don't allow crazy blunders even at very low skills
1495 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1498 // This is our magic formula
1499 s += ( weakness * int(RootMoves[0].score - s)
1500 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1505 best = RootMoves[i].pv[0];
1512 // uci_pv() formats PV information according to UCI protocol. UCI requires
1513 // to send all the PV lines also if are still to be searched and so refer to
1514 // the previous search score.
1516 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1518 std::stringstream s;
1519 Time::point elaspsed = Time::now() - SearchTime + 1;
1520 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1523 for (size_t i = 0; i < Threads.size(); i++)
1524 if (Threads[i].maxPly > selDepth)
1525 selDepth = Threads[i].maxPly;
1527 for (size_t i = 0; i < uciPVSize; i++)
1529 bool updated = (i <= PVIdx);
1531 if (depth == 1 && !updated)
1534 int d = updated ? depth : depth - 1;
1535 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1537 if (s.rdbuf()->in_avail()) // Not at first line
1540 s << "info depth " << d
1541 << " seldepth " << selDepth
1542 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1543 << " nodes " << pos.nodes_searched()
1544 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1545 << " time " << elaspsed
1546 << " multipv " << i + 1
1549 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1550 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1559 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1560 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1561 /// allow to always have a ponder move even when we fail high at root, and a
1562 /// long PV to print that is important for position analysis.
1564 void RootMove::extract_pv_from_tt(Position& pos) {
1566 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1576 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1578 pos.do_move(pv[ply++], *st++);
1579 tte = TT.probe(pos.key());
1582 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1583 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1585 && (!pos.is_draw<true, true>() || ply < 2));
1587 pv.push_back(MOVE_NONE); // Must be zero-terminating
1589 while (ply) pos.undo_move(pv[--ply]);
1593 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1594 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1595 /// first, even if the old TT entries have been overwritten.
1597 void RootMove::insert_pv_in_tt(Position& pos) {
1599 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1604 tte = TT.probe(pos.key());
1606 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1607 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1609 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1611 pos.do_move(pv[ply++], *st++);
1613 } while (pv[ply] != MOVE_NONE);
1615 while (ply) pos.undo_move(pv[--ply]);
1619 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1621 void Thread::idle_loop() {
1623 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1624 // object for which the thread is the master.
1625 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1627 assert(!sp_master || (sp_master->master == this && is_searching));
1629 // If this thread is the master of a split point and all slaves have
1630 // finished their work at this split point, return from the idle loop.
1631 while (!sp_master || sp_master->slavesMask)
1633 // If we are not searching, wait for a condition to be signaled
1634 // instead of wasting CPU time polling for work.
1637 || (!is_searching && Threads.use_sleeping_threads()))
1645 // Grab the lock to avoid races with Thread::wake_up()
1648 // If we are master and all slaves have finished don't go to sleep
1649 if (sp_master && !sp_master->slavesMask)
1655 // Do sleep after retesting sleep conditions under lock protection, in
1656 // particular we need to avoid a deadlock in case a master thread has,
1657 // in the meanwhile, allocated us and sent the wake_up() call before we
1658 // had the chance to grab the lock.
1659 if (do_sleep || !is_searching)
1660 sleepCondition.wait(mutex);
1665 // If this thread has been assigned work, launch a search
1668 assert(!do_sleep && !do_exit);
1670 Threads.mutex.lock();
1672 assert(is_searching);
1673 SplitPoint* sp = curSplitPoint;
1675 Threads.mutex.unlock();
1677 Stack ss[MAX_PLY_PLUS_2];
1678 Position pos(*sp->pos, this);
1680 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1685 assert(sp->activePositions[idx] == NULL);
1687 sp->activePositions[idx] = &pos;
1689 if (sp->nodeType == Root)
1690 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1691 else if (sp->nodeType == PV)
1692 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1693 else if (sp->nodeType == NonPV)
1694 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1698 assert(is_searching);
1700 is_searching = false;
1701 sp->activePositions[idx] = NULL;
1702 sp->slavesMask &= ~(1ULL << idx);
1703 sp->nodes += pos.nodes_searched();
1705 // Wake up master thread so to allow it to return from the idle loop in
1706 // case we are the last slave of the split point.
1707 if ( Threads.use_sleeping_threads()
1708 && this != sp->master
1711 assert(!sp->master->is_searching);
1712 sp->master->wake_up();
1715 // After releasing the lock we cannot access anymore any SplitPoint
1716 // related data in a safe way becuase it could have been released under
1717 // our feet by the sp master. Also accessing other Thread objects is
1718 // unsafe because if we are exiting there is a chance are already freed.
1725 /// check_time() is called by the timer thread when the timer triggers. It is
1726 /// used to print debug info and, more important, to detect when we are out of
1727 /// available time and so stop the search.
1731 static Time::point lastInfoTime = Time::now();
1732 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1734 if (Time::now() - lastInfoTime >= 1000)
1736 lastInfoTime = Time::now();
1745 Threads.mutex.lock();
1747 nodes = RootPos.nodes_searched();
1749 // Loop across all split points and sum accumulated SplitPoint nodes plus
1750 // all the currently active slaves positions.
1751 for (size_t i = 0; i < Threads.size(); i++)
1752 for (int j = 0; j < Threads[i].splitPointsCnt; j++)
1754 SplitPoint& sp = Threads[i].splitPoints[j];
1759 Bitboard sm = sp.slavesMask;
1762 Position* pos = sp.activePositions[pop_lsb(&sm)];
1763 nodes += pos ? pos->nodes_searched() : 0;
1769 Threads.mutex.unlock();
1772 Time::point elapsed = Time::now() - SearchTime;
1773 bool stillAtFirstMove = Signals.firstRootMove
1774 && !Signals.failedLowAtRoot
1775 && elapsed > TimeMgr.available_time();
1777 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1778 || stillAtFirstMove;
1780 if ( (Limits.use_time_management() && noMoreTime)
1781 || (Limits.movetime && elapsed >= Limits.movetime)
1782 || (Limits.nodes && nodes >= Limits.nodes))
1783 Signals.stop = true;