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))
269 Signals.stopOnPonderhit = true;
270 RootPos.this_thread()->wait_for_stop();
273 // Best move could be MOVE_NONE when searching on a stalemate position
274 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
275 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
282 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
283 // with increasing depth until the allocated thinking time has been consumed,
284 // user stops the search, or the maximum search depth is reached.
286 void id_loop(Position& pos) {
288 Stack ss[MAX_PLY_PLUS_2];
289 int depth, prevBestMoveChanges;
290 Value bestValue, alpha, beta, delta;
291 bool bestMoveNeverChanged = true;
293 memset(ss, 0, 4 * sizeof(Stack));
294 depth = BestMoveChanges = 0;
295 bestValue = delta = -VALUE_INFINITE;
296 ss->currentMove = MOVE_NULL; // Hack to skip update gains
300 PVSize = Options["MultiPV"];
301 Skill skill(Options["Skill Level"]);
303 // Do we have to play with skill handicap? In this case enable MultiPV search
304 // that we will use behind the scenes to retrieve a set of possible moves.
305 if (skill.enabled() && PVSize < 4)
308 PVSize = std::min(PVSize, RootMoves.size());
310 // Iterative deepening loop until requested to stop or target depth reached
311 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
313 // Save last iteration's scores before first PV line is searched and all
314 // the move scores but the (new) PV are set to -VALUE_INFINITE.
315 for (size_t i = 0; i < RootMoves.size(); i++)
316 RootMoves[i].prevScore = RootMoves[i].score;
318 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
321 // MultiPV loop. We perform a full root search for each PV line
322 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
324 // Set aspiration window default width
325 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
328 alpha = RootMoves[PVIdx].prevScore - delta;
329 beta = RootMoves[PVIdx].prevScore + delta;
333 alpha = -VALUE_INFINITE;
334 beta = VALUE_INFINITE;
337 // Start with a small aspiration window and, in case of fail high/low,
338 // research with bigger window until not failing high/low anymore.
341 // Search starts from ss+1 to allow referencing (ss-1). This is
342 // needed by update gains and ss copy when splitting at Root.
343 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
345 // Bring to front the best move. It is critical that sorting is
346 // done with a stable algorithm because all the values but the first
347 // and eventually the new best one are set to -VALUE_INFINITE and
348 // we want to keep the same order for all the moves but the new
349 // PV that goes to the front. Note that in case of MultiPV search
350 // the already searched PV lines are preserved.
351 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
353 // Write PV back to transposition table in case the relevant
354 // entries have been overwritten during the search.
355 for (size_t i = 0; i <= PVIdx; i++)
356 RootMoves[i].insert_pv_in_tt(pos);
358 // If search has been stopped return immediately. Sorting and
359 // writing PV back to TT is safe becuase RootMoves is still
360 // valid, although refers to previous iteration.
364 // In case of failing high/low increase aspiration window and
365 // research, otherwise exit the loop.
366 if (bestValue > alpha && bestValue < beta)
369 // Give some update (without cluttering the UI) before to research
370 if (Time::now() - SearchTime > 3000)
371 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
373 if (abs(bestValue) >= VALUE_KNOWN_WIN)
375 alpha = -VALUE_INFINITE;
376 beta = VALUE_INFINITE;
378 else if (bestValue >= beta)
385 Signals.failedLowAtRoot = true;
386 Signals.stopOnPonderhit = false;
392 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
395 // Sort the PV lines searched so far and update the GUI
396 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
397 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
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 found a "mate in x"?
418 && bestValue >= VALUE_MATE_IN_MAX_PLY
419 && VALUE_MATE - bestValue <= 2 * Limits.mate)
422 // Do we have time for the next iteration? Can we stop searching now?
423 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
425 bool stop = false; // Local variable, not the volatile Signals.stop
427 // Take in account some extra time if the best move has changed
428 if (depth > 4 && depth < 50 && PVSize == 1)
429 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
431 // Stop search if most of available time is already consumed. We
432 // probably don't have enough time to search the first move at the
433 // next iteration anyway.
434 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
437 // Stop search early if one move seems to be much better than others
441 && ( (bestMoveNeverChanged && pos.captured_piece_type())
442 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
444 Value rBeta = bestValue - 2 * PawnValueMg;
445 (ss+1)->excludedMove = RootMoves[0].pv[0];
446 (ss+1)->skipNullMove = true;
447 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
448 (ss+1)->skipNullMove = false;
449 (ss+1)->excludedMove = MOVE_NONE;
457 // If we are allowed to ponder do not stop the search now but
458 // keep pondering until GUI sends "ponderhit" or "stop".
460 Signals.stopOnPonderhit = true;
469 // search<>() is the main search function for both PV and non-PV nodes and for
470 // normal and SplitPoint nodes. When called just after a split point the search
471 // is simpler because we have already probed the hash table, done a null move
472 // search, and searched the first move before splitting, we don't have to repeat
473 // all this work again. We also don't need to store anything to the hash table
474 // here: This is taken care of after we return from the split point.
476 template <NodeType NT>
477 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
479 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
480 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
481 const bool RootNode = (NT == Root || NT == SplitPointRoot);
483 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
484 assert(PvNode || (alpha == beta - 1));
485 assert(depth > DEPTH_ZERO);
487 Move movesSearched[64];
492 Move ttMove, move, excludedMove, bestMove, threatMove;
494 Value bestValue, value, ttValue;
495 Value eval, nullValue, futilityValue;
496 bool inCheck, givesCheck, pvMove, singularExtensionNode;
497 bool captureOrPromotion, dangerous, doFullDepthSearch, threatExtension;
498 int moveCount, playedMoveCount;
500 // Step 1. Initialize node
501 Thread* thisThread = pos.this_thread();
502 moveCount = playedMoveCount = 0;
503 threatExtension = false;
504 inCheck = pos.checkers();
509 bestMove = sp->bestMove;
510 threatMove = sp->threatMove;
511 bestValue = sp->bestValue;
513 ttMove = excludedMove = MOVE_NONE;
514 ttValue = VALUE_NONE;
516 assert(sp->bestValue > -VALUE_INFINITE && sp->moveCount > 0);
518 goto split_point_start;
521 bestValue = -VALUE_INFINITE;
522 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
523 ss->ply = (ss-1)->ply + 1;
524 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
525 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
527 // Used to send selDepth info to GUI
528 if (PvNode && thisThread->maxPly < ss->ply)
529 thisThread->maxPly = ss->ply;
533 // Step 2. Check for aborted search and immediate draw
534 if (Signals.stop || pos.is_draw<true, PvNode>() || ss->ply > MAX_PLY)
535 return DrawValue[pos.side_to_move()];
537 // Step 3. Mate distance pruning. Even if we mate at the next move our score
538 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
539 // a shorter mate was found upward in the tree then there is no need to search
540 // further, we will never beat current alpha. Same logic but with reversed signs
541 // applies also in the opposite condition of being mated instead of giving mate,
542 // in this case return a fail-high score.
543 alpha = std::max(mated_in(ss->ply), alpha);
544 beta = std::min(mate_in(ss->ply+1), beta);
549 // Step 4. Transposition table lookup
550 // We don't want the score of a partial search to overwrite a previous full search
551 // TT value, so we use a different position key in case of an excluded move.
552 excludedMove = ss->excludedMove;
553 posKey = excludedMove ? pos.exclusion_key() : pos.key();
554 tte = TT.probe(posKey);
555 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
556 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
558 // At PV nodes we check for exact scores, while at non-PV nodes we check for
559 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
560 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
561 // we should also update RootMoveList to avoid bogus output.
564 && tte->depth() >= depth
565 && ttValue != VALUE_NONE // Only in case of TT access race
566 && ( PvNode ? tte->type() == BOUND_EXACT
567 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
568 : (tte->type() & BOUND_UPPER)))
571 ss->currentMove = ttMove; // Can be MOVE_NONE
575 && !pos.is_capture_or_promotion(ttMove)
576 && ttMove != ss->killers[0])
578 ss->killers[1] = ss->killers[0];
579 ss->killers[0] = ttMove;
584 // Step 5. Evaluate the position statically and update parent's gain statistics
586 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
590 // Never assume anything on values stored in TT
591 if ( (ss->staticEval = eval = tte->static_value()) == VALUE_NONE
592 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
593 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
595 // Can ttValue be used as a better position evaluation?
596 if (ttValue != VALUE_NONE)
597 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
598 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
603 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
604 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
605 ss->staticEval, ss->evalMargin);
608 // Update gain for the parent non-capture move given the static position
609 // evaluation before and after the move.
610 if ( (move = (ss-1)->currentMove) != MOVE_NULL
611 && (ss-1)->staticEval != VALUE_NONE
612 && ss->staticEval != VALUE_NONE
613 && !pos.captured_piece_type()
614 && type_of(move) == NORMAL)
616 Square to = to_sq(move);
617 H.update_gain(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
620 // Step 6. Razoring (is omitted in PV nodes)
622 && depth < 4 * ONE_PLY
624 && eval + razor_margin(depth) < beta
625 && ttMove == MOVE_NONE
626 && abs(beta) < VALUE_MATE_IN_MAX_PLY
627 && !pos.pawn_on_7th(pos.side_to_move()))
629 Value rbeta = beta - razor_margin(depth);
630 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
632 // Logically we should return (v + razor_margin(depth)), but
633 // surprisingly this did slightly weaker in tests.
637 // Step 7. Static null move pruning (is omitted in PV nodes)
638 // We're betting that the opponent doesn't have a move that will reduce
639 // the score by more than futility_margin(depth) if we do a null move.
642 && depth < 4 * ONE_PLY
644 && eval - FutilityMargins[depth][0] >= beta
645 && abs(beta) < VALUE_MATE_IN_MAX_PLY
646 && pos.non_pawn_material(pos.side_to_move()))
647 return eval - FutilityMargins[depth][0];
649 // Step 8. Null move search with verification search (is omitted in PV nodes)
655 && abs(beta) < VALUE_MATE_IN_MAX_PLY
656 && pos.non_pawn_material(pos.side_to_move()))
658 ss->currentMove = MOVE_NULL;
660 // Null move dynamic reduction based on depth
661 Depth R = 3 * ONE_PLY + depth / 4;
663 // Null move dynamic reduction based on value
664 if (eval - PawnValueMg > beta)
667 pos.do_null_move<true>(st);
668 (ss+1)->skipNullMove = true;
669 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
670 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
671 (ss+1)->skipNullMove = false;
672 pos.do_null_move<false>(st);
674 if (nullValue >= beta)
676 // Do not return unproven mate scores
677 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
680 if (depth < 6 * ONE_PLY)
683 // Do verification search at high depths
684 ss->skipNullMove = true;
685 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
686 ss->skipNullMove = false;
693 // The null move failed low, which means that we may be faced with
694 // some kind of threat. If the previous move was reduced, check if
695 // the move that refuted the null move was somehow connected to the
696 // move which was reduced. If a connection is found extend moves that
697 // defend against threat.
698 threatMove = (ss+1)->currentMove;
700 if ( depth < 5 * ONE_PLY
702 && threatMove != MOVE_NONE
703 && allows_move(pos, (ss-1)->currentMove, threatMove))
704 threatExtension = true;
708 // Step 9. ProbCut (is omitted in PV nodes)
709 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
710 // and a reduced search returns a value much above beta, we can (almost) safely
711 // prune the previous move.
713 && depth >= 5 * ONE_PLY
716 && excludedMove == MOVE_NONE
717 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
719 Value rbeta = beta + 200;
720 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
722 assert(rdepth >= ONE_PLY);
723 assert((ss-1)->currentMove != MOVE_NONE);
724 assert((ss-1)->currentMove != MOVE_NULL);
726 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
729 while ((move = mp.next_move<false>()) != MOVE_NONE)
730 if (pos.pl_move_is_legal(move, ci.pinned))
732 ss->currentMove = move;
733 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
734 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
741 // Step 10. Internal iterative deepening
742 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
743 && ttMove == MOVE_NONE
744 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
746 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
748 ss->skipNullMove = true;
749 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
750 ss->skipNullMove = false;
752 tte = TT.probe(posKey);
753 ttMove = tte ? tte->move() : MOVE_NONE;
756 split_point_start: // At split points actual search starts from here
758 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
760 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
761 singularExtensionNode = !RootNode
763 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
764 && ttMove != MOVE_NONE
765 && !excludedMove // Recursive singular search is not allowed
766 && (tte->type() & BOUND_LOWER)
767 && tte->depth() >= depth - 3 * ONE_PLY;
769 // Step 11. Loop through moves
770 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
771 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
775 if (move == excludedMove)
778 // At root obey the "searchmoves" option and skip moves not listed in Root
779 // Move List, as a consequence any illegal move is also skipped. In MultiPV
780 // mode we also skip PV moves which have been already searched.
781 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
786 // Shared counter cannot be decremented later if move turns out to be illegal
787 if (!pos.pl_move_is_legal(move, ci.pinned))
790 moveCount = ++sp->moveCount;
798 Signals.firstRootMove = (moveCount == 1);
800 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
801 sync_cout << "info depth " << depth / ONE_PLY
802 << " currmove " << move_to_uci(move, pos.is_chess960())
803 << " currmovenumber " << moveCount + PVIdx << sync_endl;
807 captureOrPromotion = pos.is_capture_or_promotion(move);
808 givesCheck = pos.move_gives_check(move, ci);
809 dangerous = givesCheck
810 || pos.is_passed_pawn_push(move)
811 || type_of(move) == CASTLE
812 || ( captureOrPromotion // Entering a pawn endgame?
813 && type_of(pos.piece_on(to_sq(move))) != PAWN
814 && type_of(move) == NORMAL
815 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
816 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
818 // Step 12. Extend checks and, in PV nodes, also dangerous moves
819 if (PvNode && dangerous)
822 else if (threatExtension && prevents_move(pos, move, threatMove))
825 else if (givesCheck && pos.see_sign(move) >= 0)
828 // Singular extension search. If all moves but one fail low on a search of
829 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
830 // is singular and should be extended. To verify this we do a reduced search
831 // on all the other moves but the ttMove, if result is lower than ttValue minus
832 // a margin then we extend ttMove.
833 if ( singularExtensionNode
836 && pos.pl_move_is_legal(move, ci.pinned)
837 && abs(ttValue) < VALUE_KNOWN_WIN)
839 assert(ttValue != VALUE_NONE);
841 Value rBeta = ttValue - int(depth);
842 ss->excludedMove = move;
843 ss->skipNullMove = true;
844 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
845 ss->skipNullMove = false;
846 ss->excludedMove = MOVE_NONE;
849 ext = rBeta >= beta ? ONE_PLY + ONE_PLY / 2 : ONE_PLY;
852 // Update current move (this must be done after singular extension search)
853 newDepth = depth - ONE_PLY + ext;
855 // Step 13. Futility pruning (is omitted in PV nodes)
857 && !captureOrPromotion
861 && (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
862 && alpha > VALUE_MATED_IN_MAX_PLY)))
864 // Move count based pruning
865 if ( depth < 16 * ONE_PLY
866 && moveCount >= FutilityMoveCounts[depth]
867 && (!threatMove || !prevents_move(pos, move, threatMove)))
875 // Value based pruning
876 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
877 // but fixing this made program slightly weaker.
878 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
879 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
880 + H.gain(pos.piece_moved(move), to_sq(move));
882 if (futilityValue < beta)
890 // Prune moves with negative SEE at low depths
891 if ( predictedDepth < 2 * ONE_PLY
892 && pos.see_sign(move) < 0)
901 // Check for legality only before to do the move
902 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
908 pvMove = PvNode && moveCount == 1;
909 ss->currentMove = move;
910 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
911 movesSearched[playedMoveCount++] = move;
913 // Step 14. Make the move
914 pos.do_move(move, st, ci, givesCheck);
916 // Step 15. Reduced depth search (LMR). If the move fails high will be
917 // re-searched at full depth.
918 if ( depth > 3 * ONE_PLY
920 && !captureOrPromotion
922 && ss->killers[0] != move
923 && ss->killers[1] != move)
925 ss->reduction = reduction<PvNode>(depth, moveCount);
926 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
927 alpha = SpNode ? sp->alpha : alpha;
929 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
931 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
932 ss->reduction = DEPTH_ZERO;
935 doFullDepthSearch = !pvMove;
937 // Step 16. Full depth search, when LMR is skipped or fails high
938 if (doFullDepthSearch)
940 alpha = SpNode ? sp->alpha : alpha;
941 value = newDepth < ONE_PLY ?
942 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
943 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
944 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
947 // Only for PV nodes do a full PV search on the first move or after a fail
948 // high, in the latter case search only if value < beta, otherwise let the
949 // parent node to fail low with value <= alpha and to try another move.
950 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
951 value = newDepth < ONE_PLY ?
952 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
953 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
954 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
955 // Step 17. Undo move
958 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
960 // Step 18. Check for new best move
964 bestValue = sp->bestValue;
968 // Finished searching the move. If Signals.stop is true, the search
969 // was aborted because the user interrupted the search or because we
970 // ran out of time. In this case, the return value of the search cannot
971 // be trusted, and we don't update the best move and/or PV.
972 if (Signals.stop || thisThread->cutoff_occurred())
973 return value; // To avoid returning VALUE_INFINITE
977 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
979 // PV move or new best move ?
980 if (pvMove || value > alpha)
983 rm.extract_pv_from_tt(pos);
985 // We record how often the best move has been changed in each
986 // iteration. This information is used for time management: When
987 // the best move changes frequently, we allocate some more time.
992 // All other moves but the PV are set to the lowest value, this
993 // is not a problem when sorting becuase sort is stable and move
994 // position in the list is preserved, just the PV is pushed up.
995 rm.score = -VALUE_INFINITE;
998 if (value > bestValue)
1000 bestValue = SpNode ? sp->bestValue = value : value;
1004 bestMove = SpNode ? sp->bestMove = move : move;
1006 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1007 alpha = SpNode ? sp->alpha = value : value;
1010 assert(value >= beta); // Fail high
1020 // Step 19. Check for splitting the search
1022 && depth >= Threads.min_split_depth()
1023 && Threads.available_slave_exists(thisThread))
1025 assert(bestValue < beta);
1027 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1028 depth, threatMove, moveCount, mp, NT);
1029 if (bestValue >= beta)
1037 // Step 20. Check for mate and stalemate
1038 // All legal moves have been searched and if there are no legal moves, it
1039 // must be mate or stalemate. Note that we can have a false positive in
1040 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1041 // harmless because return value is discarded anyhow in the parent nodes.
1042 // If we are in a singular extension search then return a fail low score.
1043 // A split node has at least one move, the one tried before to be splitted.
1045 return excludedMove ? alpha
1046 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1048 // If we have pruned all the moves without searching return a fail-low score
1049 if (bestValue == -VALUE_INFINITE)
1051 assert(!playedMoveCount);
1056 if (bestValue >= beta) // Failed high
1058 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1059 bestMove, ss->staticEval, ss->evalMargin);
1061 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1063 if (bestMove != ss->killers[0])
1065 ss->killers[1] = ss->killers[0];
1066 ss->killers[0] = bestMove;
1069 // Increase history value of the cut-off move
1070 Value bonus = Value(int(depth) * int(depth));
1071 H.add(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1073 // Decrease history of all the other played non-capture moves
1074 for (int i = 0; i < playedMoveCount - 1; i++)
1076 Move m = movesSearched[i];
1077 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1081 else // Failed low or PV search
1082 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1083 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1084 depth, bestMove, ss->staticEval, ss->evalMargin);
1086 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1092 // qsearch() is the quiescence search function, which is called by the main
1093 // search function when the remaining depth is zero (or, to be more precise,
1094 // less than ONE_PLY).
1096 template <NodeType NT, bool InCheck>
1097 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1099 const bool PvNode = (NT == PV);
1101 assert(NT == PV || NT == NonPV);
1102 assert(InCheck == !!pos.checkers());
1103 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1104 assert(PvNode || (alpha == beta - 1));
1105 assert(depth <= DEPTH_ZERO);
1110 Move ttMove, move, bestMove;
1111 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1112 bool givesCheck, enoughMaterial, evasionPrunable;
1115 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1119 ss->currentMove = bestMove = MOVE_NONE;
1120 ss->ply = (ss-1)->ply + 1;
1122 // Check for an instant draw or maximum ply reached
1123 if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)
1124 return DrawValue[pos.side_to_move()];
1126 // Transposition table lookup. At PV nodes, we don't use the TT for
1127 // pruning, but only for move ordering.
1129 tte = TT.probe(posKey);
1130 ttMove = tte ? tte->move() : MOVE_NONE;
1131 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1133 // Decide whether or not to include checks, this fixes also the type of
1134 // TT entry depth that we are going to use. Note that in qsearch we use
1135 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1136 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1137 : DEPTH_QS_NO_CHECKS;
1139 && tte->depth() >= ttDepth
1140 && ttValue != VALUE_NONE // Only in case of TT access race
1141 && ( PvNode ? tte->type() == BOUND_EXACT
1142 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1143 : (tte->type() & BOUND_UPPER)))
1145 ss->currentMove = ttMove; // Can be MOVE_NONE
1149 // Evaluate the position statically
1152 ss->staticEval = ss->evalMargin = VALUE_NONE;
1153 bestValue = futilityBase = -VALUE_INFINITE;
1154 enoughMaterial = false;
1160 // Never assume anything on values stored in TT
1161 if ( (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE
1162 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
1163 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1166 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1168 // Stand pat. Return immediately if static value is at least beta
1169 if (bestValue >= beta)
1172 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1173 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1178 if (PvNode && bestValue > alpha)
1181 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1182 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1185 // Initialize a MovePicker object for the current position, and prepare
1186 // to search the moves. Because the depth is <= 0 here, only captures,
1187 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1189 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1192 // Loop through the moves until no moves remain or a beta cutoff occurs
1193 while ((move = mp.next_move<false>()) != MOVE_NONE)
1195 assert(is_ok(move));
1197 givesCheck = pos.move_gives_check(move, ci);
1205 && type_of(move) != PROMOTION
1206 && !pos.is_passed_pawn_push(move))
1208 futilityValue = futilityBase
1209 + PieceValue[EG][pos.piece_on(to_sq(move))]
1210 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1212 if (futilityValue < beta)
1214 bestValue = std::max(bestValue, futilityValue);
1218 // Prune moves with negative or equal SEE
1219 if ( futilityBase < beta
1220 && depth < DEPTH_ZERO
1221 && pos.see(move) <= 0)
1223 bestValue = std::max(bestValue, futilityBase);
1228 // Detect non-capture evasions that are candidate to be pruned
1229 evasionPrunable = !PvNode
1231 && bestValue > VALUE_MATED_IN_MAX_PLY
1232 && !pos.is_capture(move)
1233 && !pos.can_castle(pos.side_to_move());
1235 // Don't search moves with negative SEE values
1237 && (!InCheck || evasionPrunable)
1239 && type_of(move) != PROMOTION
1240 && pos.see_sign(move) < 0)
1243 // Don't search useless checks
1248 && !pos.is_capture_or_promotion(move)
1249 && ss->staticEval + PawnValueMg / 4 < beta
1250 && !check_is_dangerous(pos, move, futilityBase, beta))
1253 // Check for legality only before to do the move
1254 if (!pos.pl_move_is_legal(move, ci.pinned))
1257 ss->currentMove = move;
1259 // Make and search the move
1260 pos.do_move(move, st, ci, givesCheck);
1261 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1262 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1263 pos.undo_move(move);
1265 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1267 // Check for new best move
1268 if (value > bestValue)
1274 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1281 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1282 ttDepth, move, ss->staticEval, ss->evalMargin);
1290 // All legal moves have been searched. A special case: If we're in check
1291 // and no legal moves were found, it is checkmate.
1292 if (InCheck && bestValue == -VALUE_INFINITE)
1293 return mated_in(ss->ply); // Plies to mate from the root
1295 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1296 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1297 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1299 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1305 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1306 // "plies to mate from the current position". Non-mate scores are unchanged.
1307 // The function is called before storing a value to the transposition table.
1309 Value value_to_tt(Value v, int ply) {
1311 assert(v != VALUE_NONE);
1313 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1314 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1318 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1319 // from the transposition table (where refers to the plies to mate/be mated
1320 // from current position) to "plies to mate/be mated from the root".
1322 Value value_from_tt(Value v, int ply) {
1324 return v == VALUE_NONE ? VALUE_NONE
1325 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1326 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1330 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1332 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta)
1334 Piece pc = pos.piece_moved(move);
1335 Square from = from_sq(move);
1336 Square to = to_sq(move);
1337 Color them = ~pos.side_to_move();
1338 Square ksq = pos.king_square(them);
1339 Bitboard enemies = pos.pieces(them);
1340 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1341 Bitboard occ = pos.pieces() ^ from ^ ksq;
1342 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1343 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1345 // Checks which give opponent's king at most one escape square are dangerous
1346 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1349 // Queen contact check is very dangerous
1350 if (type_of(pc) == QUEEN && (kingAtt & to))
1353 // Creating new double threats with checks is dangerous
1354 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1357 // Note that here we generate illegal "double move"!
1358 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1366 // allows_move() tests whether the move at previous ply (first) somehow makes a
1367 // second move possible, for instance if the moving piece is the same in both
1368 // moves. Normally the second move is the threat move (the best move returned
1369 // from a null search that fails low).
1371 bool allows_move(const Position& pos, Move first, Move second) {
1373 assert(is_ok(first));
1374 assert(is_ok(second));
1375 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1376 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1378 Square m1from = from_sq(first);
1379 Square m2from = from_sq(second);
1380 Square m1to = to_sq(first);
1381 Square m2to = to_sq(second);
1383 // The piece is the same or second's destination was vacated by the first move
1384 if (m1to == m2from || m2to == m1from)
1387 // Second one moves through the square vacated by first one
1388 if (between_bb(m2from, m2to) & m1from)
1391 // Second's destination is defended by the first move's piece
1392 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1396 // Second move gives a discovered check through the first's checking piece
1397 if (m1att & pos.king_square(pos.side_to_move()))
1399 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1407 // prevents_move() tests whether a move (first) is able to defend against an
1408 // opponent's move (second). In this case will not be pruned. Normally the
1409 // second move is the threat move (the best move returned from a null search
1412 bool prevents_move(const Position& pos, Move first, Move second) {
1414 assert(is_ok(first));
1415 assert(is_ok(second));
1417 Square m1from = from_sq(first);
1418 Square m2from = from_sq(second);
1419 Square m1to = to_sq(first);
1420 Square m2to = to_sq(second);
1422 // Don't prune moves of the threatened piece
1426 // If the threatened piece has value less than or equal to the value of the
1427 // threat piece, don't prune moves which defend it.
1428 if ( pos.is_capture(second)
1429 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1430 || type_of(pos.piece_on(m2from)) == KING))
1432 // Update occupancy as if the piece and the threat are moving
1433 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1434 Piece piece = pos.piece_on(m1from);
1436 // The moved piece attacks the square 'tto' ?
1437 if (pos.attacks_from(piece, m1to, occ) & m2to)
1440 // Scan for possible X-ray attackers behind the moved piece
1441 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1442 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1444 // Verify attackers are triggered by our move and not already existing
1445 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1449 // Don't prune safe moves which block the threat path
1450 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1457 // When playing with strength handicap choose best move among the MultiPV set
1458 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1460 Move Skill::pick_move() {
1464 // PRNG sequence should be not deterministic
1465 for (int i = Time::now() % 50; i > 0; i--)
1466 rk.rand<unsigned>();
1468 // RootMoves are already sorted by score in descending order
1469 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1470 int weakness = 120 - 2 * level;
1471 int max_s = -VALUE_INFINITE;
1474 // Choose best move. For each move score we add two terms both dependent on
1475 // weakness, one deterministic and bigger for weaker moves, and one random,
1476 // then we choose the move with the resulting highest score.
1477 for (size_t i = 0; i < PVSize; i++)
1479 int s = RootMoves[i].score;
1481 // Don't allow crazy blunders even at very low skills
1482 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1485 // This is our magic formula
1486 s += ( weakness * int(RootMoves[0].score - s)
1487 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1492 best = RootMoves[i].pv[0];
1499 // uci_pv() formats PV information according to UCI protocol. UCI requires
1500 // to send all the PV lines also if are still to be searched and so refer to
1501 // the previous search score.
1503 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1505 std::stringstream s;
1506 Time::point elaspsed = Time::now() - SearchTime + 1;
1507 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1510 for (size_t i = 0; i < Threads.size(); i++)
1511 if (Threads[i].maxPly > selDepth)
1512 selDepth = Threads[i].maxPly;
1514 for (size_t i = 0; i < uciPVSize; i++)
1516 bool updated = (i <= PVIdx);
1518 if (depth == 1 && !updated)
1521 int d = updated ? depth : depth - 1;
1522 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1524 if (s.rdbuf()->in_avail()) // Not at first line
1527 s << "info depth " << d
1528 << " seldepth " << selDepth
1529 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1530 << " nodes " << pos.nodes_searched()
1531 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1532 << " time " << elaspsed
1533 << " multipv " << i + 1
1536 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1537 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1546 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1547 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1548 /// allow to always have a ponder move even when we fail high at root, and a
1549 /// long PV to print that is important for position analysis.
1551 void RootMove::extract_pv_from_tt(Position& pos) {
1553 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1563 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1565 pos.do_move(pv[ply++], *st++);
1566 tte = TT.probe(pos.key());
1569 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1570 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1572 && (!pos.is_draw<true, true>() || ply < 2));
1574 pv.push_back(MOVE_NONE); // Must be zero-terminating
1576 while (ply) pos.undo_move(pv[--ply]);
1580 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1581 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1582 /// first, even if the old TT entries have been overwritten.
1584 void RootMove::insert_pv_in_tt(Position& pos) {
1586 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1591 tte = TT.probe(pos.key());
1593 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1594 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1596 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1598 pos.do_move(pv[ply++], *st++);
1600 } while (pv[ply] != MOVE_NONE);
1602 while (ply) pos.undo_move(pv[--ply]);
1606 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1608 void Thread::idle_loop() {
1610 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1611 // object for which the thread is the master.
1612 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1614 assert(!sp_master || (sp_master->master == this && is_searching));
1616 // If this thread is the master of a split point and all slaves have
1617 // finished their work at this split point, return from the idle loop.
1618 while (!sp_master || sp_master->slavesMask)
1620 // If we are not searching, wait for a condition to be signaled
1621 // instead of wasting CPU time polling for work.
1624 || (!is_searching && Threads.use_sleeping_threads()))
1632 // Grab the lock to avoid races with Thread::wake_up()
1635 // If we are master and all slaves have finished don't go to sleep
1636 if (sp_master && !sp_master->slavesMask)
1642 // Do sleep after retesting sleep conditions under lock protection, in
1643 // particular we need to avoid a deadlock in case a master thread has,
1644 // in the meanwhile, allocated us and sent the wake_up() call before we
1645 // had the chance to grab the lock.
1646 if (do_sleep || !is_searching)
1647 sleepCondition.wait(mutex);
1652 // If this thread has been assigned work, launch a search
1655 assert(!do_sleep && !do_exit);
1657 Threads.mutex.lock();
1659 assert(is_searching);
1660 SplitPoint* sp = curSplitPoint;
1662 Threads.mutex.unlock();
1664 Stack ss[MAX_PLY_PLUS_2];
1665 Position pos(*sp->pos, this);
1667 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1672 assert(sp->activePositions[idx] == NULL);
1674 sp->activePositions[idx] = &pos;
1676 if (sp->nodeType == Root)
1677 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1678 else if (sp->nodeType == PV)
1679 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1680 else if (sp->nodeType == NonPV)
1681 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1685 assert(is_searching);
1687 is_searching = false;
1688 sp->activePositions[idx] = NULL;
1689 sp->slavesMask &= ~(1ULL << idx);
1690 sp->nodes += pos.nodes_searched();
1692 // Wake up master thread so to allow it to return from the idle loop in
1693 // case we are the last slave of the split point.
1694 if ( Threads.use_sleeping_threads()
1695 && this != sp->master
1698 assert(!sp->master->is_searching);
1699 sp->master->wake_up();
1702 // After releasing the lock we cannot access anymore any SplitPoint
1703 // related data in a safe way becuase it could have been released under
1704 // our feet by the sp master. Also accessing other Thread objects is
1705 // unsafe because if we are exiting there is a chance are already freed.
1712 /// check_time() is called by the timer thread when the timer triggers. It is
1713 /// used to print debug info and, more important, to detect when we are out of
1714 /// available time and so stop the search.
1718 static Time::point lastInfoTime = Time::now();
1719 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1721 if (Time::now() - lastInfoTime >= 1000)
1723 lastInfoTime = Time::now();
1732 Threads.mutex.lock();
1734 nodes = RootPos.nodes_searched();
1736 // Loop across all split points and sum accumulated SplitPoint nodes plus
1737 // all the currently active slaves positions.
1738 for (size_t i = 0; i < Threads.size(); i++)
1739 for (int j = 0; j < Threads[i].splitPointsCnt; j++)
1741 SplitPoint& sp = Threads[i].splitPoints[j];
1746 Bitboard sm = sp.slavesMask;
1749 Position* pos = sp.activePositions[pop_lsb(&sm)];
1750 nodes += pos ? pos->nodes_searched() : 0;
1756 Threads.mutex.unlock();
1759 Time::point elapsed = Time::now() - SearchTime;
1760 bool stillAtFirstMove = Signals.firstRootMove
1761 && !Signals.failedLowAtRoot
1762 && elapsed > TimeMgr.available_time();
1764 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1765 || stillAtFirstMove;
1767 if ( (Limits.use_time_management() && noMoreTime)
1768 || (Limits.movetime && elapsed >= Limits.movetime)
1769 || (Limits.nodes && nodes >= Limits.nodes))
1770 Signals.stop = true;