2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 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/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // This is the minimum interval in msec between two check_time() calls
59 const int TimerResolution = 5;
61 // Different node types, used as template parameter
62 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
64 // Dynamic razoring margin based on depth
65 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
67 // Futility lookup tables (initialized at startup) and their access functions
68 Value FutilityMargins[16][64]; // [depth][moveNumber]
69 int FutilityMoveCounts[2][32]; // [improving][depth]
71 inline Value futility_margin(Depth d, int mn) {
73 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
77 // Reduction lookup tables (initialized at startup) and their access function
78 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
82 return (Depth) Reductions[PvNode][i][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
87 double BestMoveChanges;
88 Value DrawValue[COLOR_NB];
91 CountermovesStats Countermoves;
93 template <NodeType NT>
94 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
96 template <NodeType NT, bool InCheck>
97 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
99 void id_loop(Position& pos);
100 Value value_to_tt(Value v, int ply);
101 Value value_from_tt(Value v, int ply);
102 bool allows(const Position& pos, Move first, Move second);
103 bool refutes(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][1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
139 Reductions[0][1][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
141 Reductions[1][0][hd][mc] = Reductions[1][1][hd][mc];
142 Reductions[0][0][hd][mc] = Reductions[0][1][hd][mc];
144 if (Reductions[0][0][hd][mc] > 2 * ONE_PLY)
145 Reductions[0][0][hd][mc] += ONE_PLY;
147 else if (Reductions[0][0][hd][mc] > 1 * ONE_PLY)
148 Reductions[0][0][hd][mc] += ONE_PLY / 2;
151 // Init futility margins array
152 for (d = 1; d < 16; ++d) for (mc = 0; mc < 64; ++mc)
153 FutilityMargins[d][mc] = Value(112 * int(2.9 * log(double(d))) - 8 * mc + 45);
155 // Init futility move count array
156 for (d = 0; d < 32; ++d)
158 FutilityMoveCounts[0][d] = int(2.4 + 0.222 * pow(d + 0.0, 1.8));
159 FutilityMoveCounts[1][d] = int(3.0 + 0.3 * pow(d + 0.98, 1.8));
164 /// Search::perft() is our utility to verify move generation. All the leaf nodes
165 /// up to the given depth are generated and counted and the sum returned.
167 static size_t perft(Position& pos, Depth depth) {
172 const bool leaf = depth == 2 * ONE_PLY;
174 for (MoveList<LEGAL> it(pos); *it; ++it)
176 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
177 cnt += leaf ? MoveList<LEGAL>(pos).size() : ::perft(pos, depth - ONE_PLY);
183 size_t Search::perft(Position& pos, Depth depth) {
184 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
187 /// Search::think() is the external interface to Stockfish's search, and is
188 /// called by the main thread when the program receives the UCI 'go' command. It
189 /// searches from RootPos and at the end prints the "bestmove" to output.
191 void Search::think() {
193 static PolyglotBook book; // Defined static to initialize the PRNG only once
195 RootColor = RootPos.side_to_move();
196 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
198 if (RootMoves.empty())
200 RootMoves.push_back(MOVE_NONE);
201 sync_cout << "info depth 0 score "
202 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
208 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
210 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
212 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
214 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
219 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
221 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
222 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
223 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
224 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
227 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
229 if (Options["Write Search Log"])
231 Log log(Options["Search Log Filename"]);
232 log << "\nSearching: " << RootPos.fen()
233 << "\ninfinite: " << Limits.infinite
234 << " ponder: " << Limits.ponder
235 << " time: " << Limits.time[RootColor]
236 << " increment: " << Limits.inc[RootColor]
237 << " moves to go: " << Limits.movestogo
241 // Reset the threads, still sleeping: will be wake up at split time
242 for (size_t i = 0; i < Threads.size(); ++i)
243 Threads[i]->maxPly = 0;
245 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
247 // Set best timer interval to avoid lagging under time pressure. Timer is
248 // used to check for remaining available thinking time.
249 Threads.timer->msec =
250 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
251 Limits.nodes ? 2 * TimerResolution
254 Threads.timer->notify_one(); // Wake up the recurring timer
256 id_loop(RootPos); // Let's start searching !
258 Threads.timer->msec = 0; // Stop the timer
259 Threads.sleepWhileIdle = true; // Send idle threads to sleep
261 if (Options["Write Search Log"])
263 Time::point elapsed = Time::now() - SearchTime + 1;
265 Log log(Options["Search Log Filename"]);
266 log << "Nodes: " << RootPos.nodes_searched()
267 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
268 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
271 RootPos.do_move(RootMoves[0].pv[0], st);
272 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
273 RootPos.undo_move(RootMoves[0].pv[0]);
278 // When search is stopped this info is not printed
279 sync_cout << "info nodes " << RootPos.nodes_searched()
280 << " time " << Time::now() - SearchTime + 1 << sync_endl;
282 // When we reach max depth we arrive here even without Signals.stop is raised,
283 // but if we are pondering or in infinite search, according to UCI protocol,
284 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
285 // command. We simply wait here until GUI sends one of those commands (that
286 // raise Signals.stop).
287 if (!Signals.stop && (Limits.ponder || Limits.infinite))
289 Signals.stopOnPonderhit = true;
290 RootPos.this_thread()->wait_for(Signals.stop);
293 // Best move could be MOVE_NONE when searching on a stalemate position
294 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
295 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
302 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
303 // with increasing depth until the allocated thinking time has been consumed,
304 // user stops the search, or the maximum search depth is reached.
306 void id_loop(Position& pos) {
308 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
310 Value bestValue, alpha, beta, delta;
312 std::memset(ss-2, 0, 5 * sizeof(Stack));
313 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
317 bestValue = delta = alpha = -VALUE_INFINITE;
318 beta = VALUE_INFINITE;
323 Countermoves.clear();
325 PVSize = Options["MultiPV"];
326 Skill skill(Options["Skill Level"]);
328 // Do we have to play with skill handicap? In this case enable MultiPV search
329 // that we will use behind the scenes to retrieve a set of possible moves.
330 if (skill.enabled() && PVSize < 4)
333 PVSize = std::min(PVSize, RootMoves.size());
335 // Iterative deepening loop until requested to stop or target depth reached
336 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
338 // Age out PV variability metric
339 BestMoveChanges *= 0.8;
341 // Save last iteration's scores before first PV line is searched and all
342 // the move scores but the (new) PV are set to -VALUE_INFINITE.
343 for (size_t i = 0; i < RootMoves.size(); ++i)
344 RootMoves[i].prevScore = RootMoves[i].score;
346 // MultiPV loop. We perform a full root search for each PV line
347 for (PVIdx = 0; PVIdx < PVSize; ++PVIdx)
349 // Reset aspiration window starting size
353 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
354 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
357 // Start with a small aspiration window and, in case of fail high/low,
358 // research with bigger window until not failing high/low anymore.
361 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
363 // Bring to front the best move. It is critical that sorting is
364 // done with a stable algorithm because all the values but the first
365 // and eventually the new best one are set to -VALUE_INFINITE and
366 // we want to keep the same order for all the moves but the new
367 // PV that goes to the front. Note that in case of MultiPV search
368 // the already searched PV lines are preserved.
369 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
371 // Write PV back to transposition table in case the relevant
372 // entries have been overwritten during the search.
373 for (size_t i = 0; i <= PVIdx; ++i)
374 RootMoves[i].insert_pv_in_tt(pos);
376 // If search has been stopped return immediately. Sorting and
377 // writing PV back to TT is safe becuase RootMoves is still
378 // valid, although refers to previous iteration.
382 // When failing high/low give some update (without cluttering
383 // the UI) before to research.
384 if ( (bestValue <= alpha || bestValue >= beta)
385 && Time::now() - SearchTime > 3000)
386 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
388 // In case of failing low/high increase aspiration window and
389 // research, otherwise exit the loop.
390 if (bestValue <= alpha)
392 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
394 Signals.failedLowAtRoot = true;
395 Signals.stopOnPonderhit = false;
397 else if (bestValue >= beta)
398 beta = std::min(bestValue + delta, VALUE_INFINITE);
405 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
408 // Sort the PV lines searched so far and update the GUI
409 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
411 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
412 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
415 // Do we need to pick now the sub-optimal best move ?
416 if (skill.enabled() && skill.time_to_pick(depth))
419 if (Options["Write Search Log"])
421 RootMove& rm = RootMoves[0];
422 if (skill.best != MOVE_NONE)
423 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
425 Log log(Options["Search Log Filename"]);
426 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
430 // Do we have found a "mate in x"?
432 && bestValue >= VALUE_MATE_IN_MAX_PLY
433 && VALUE_MATE - bestValue <= 2 * Limits.mate)
436 // Do we have time for the next iteration? Can we stop searching now?
437 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
439 bool stop = false; // Local variable, not the volatile Signals.stop
441 // Take in account some extra time if the best move has changed
442 if (depth > 4 && depth < 50 && PVSize == 1)
443 TimeMgr.pv_instability(BestMoveChanges);
445 // Stop search if most of available time is already consumed. We
446 // probably don't have enough time to search the first move at the
447 // next iteration anyway.
448 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
451 // Stop search early if one move seems to be much better than others
455 && bestValue > VALUE_MATED_IN_MAX_PLY
456 && ( RootMoves.size() == 1
457 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
459 Value rBeta = bestValue - 2 * PawnValueMg;
460 ss->excludedMove = RootMoves[0].pv[0];
461 ss->skipNullMove = true;
462 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
463 ss->skipNullMove = false;
464 ss->excludedMove = MOVE_NONE;
472 // If we are allowed to ponder do not stop the search now but
473 // keep pondering until GUI sends "ponderhit" or "stop".
475 Signals.stopOnPonderhit = true;
484 // search<>() is the main search function for both PV and non-PV nodes and for
485 // normal and SplitPoint nodes. When called just after a split point the search
486 // is simpler because we have already probed the hash table, done a null move
487 // search, and searched the first move before splitting, we don't have to repeat
488 // all this work again. We also don't need to store anything to the hash table
489 // here: This is taken care of after we return from the split point.
491 template <NodeType NT>
492 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
494 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
495 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
496 const bool RootNode = (NT == Root || NT == SplitPointRoot);
498 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
499 assert(PvNode || (alpha == beta - 1));
500 assert(depth > DEPTH_ZERO);
502 Move quietsSearched[64];
505 SplitPoint* splitPoint;
507 Move ttMove, move, excludedMove, bestMove, threatMove;
509 Value bestValue, value, ttValue;
510 Value eval, nullValue, futilityValue;
511 bool inCheck, givesCheck, pvMove, singularExtensionNode, improving;
512 bool captureOrPromotion, dangerous, doFullDepthSearch;
513 int moveCount, quietCount;
515 // Step 1. Initialize node
516 Thread* thisThread = pos.this_thread();
517 inCheck = pos.checkers();
521 splitPoint = ss->splitPoint;
522 bestMove = splitPoint->bestMove;
523 threatMove = splitPoint->threatMove;
524 bestValue = splitPoint->bestValue;
526 ttMove = excludedMove = MOVE_NONE;
527 ttValue = VALUE_NONE;
529 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
534 moveCount = quietCount = 0;
535 bestValue = -VALUE_INFINITE;
536 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
537 ss->ply = (ss-1)->ply + 1;
538 ss->futilityMoveCount = 0;
539 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
540 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
542 // Used to send selDepth info to GUI
543 if (PvNode && thisThread->maxPly < ss->ply)
544 thisThread->maxPly = ss->ply;
548 // Step 2. Check for aborted search and immediate draw
549 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
550 return DrawValue[pos.side_to_move()];
552 // Step 3. Mate distance pruning. Even if we mate at the next move our score
553 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
554 // a shorter mate was found upward in the tree then there is no need to search
555 // further, we will never beat current alpha. Same logic but with reversed signs
556 // applies also in the opposite condition of being mated instead of giving mate,
557 // in this case return a fail-high score.
558 alpha = std::max(mated_in(ss->ply), alpha);
559 beta = std::min(mate_in(ss->ply+1), beta);
564 // Step 4. Transposition table lookup
565 // We don't want the score of a partial search to overwrite a previous full search
566 // TT value, so we use a different position key in case of an excluded move.
567 excludedMove = ss->excludedMove;
568 posKey = excludedMove ? pos.exclusion_key() : pos.key();
569 tte = TT.probe(posKey);
570 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
571 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
573 // At PV nodes we check for exact scores, while at non-PV nodes we check for
574 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
575 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
576 // we should also update RootMoveList to avoid bogus output.
579 && tte->depth() >= depth
580 && ttValue != VALUE_NONE // Only in case of TT access race
581 && ( PvNode ? tte->bound() == BOUND_EXACT
582 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
583 : (tte->bound() & BOUND_UPPER)))
586 ss->currentMove = ttMove; // Can be MOVE_NONE
590 && !pos.capture_or_promotion(ttMove)
591 && ttMove != ss->killers[0])
593 ss->killers[1] = ss->killers[0];
594 ss->killers[0] = ttMove;
599 // Step 5. Evaluate the position statically and update parent's gain statistics
602 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
608 // Never assume anything on values stored in TT
609 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
610 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
611 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
613 // Can ttValue be used as a better position evaluation?
614 if (ttValue != VALUE_NONE)
615 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
620 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
621 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
622 ss->staticEval, ss->evalMargin);
625 // Update gain for the parent non-capture move given the static position
626 // evaluation before and after the move.
627 if ( !pos.captured_piece_type()
628 && ss->staticEval != VALUE_NONE
629 && (ss-1)->staticEval != VALUE_NONE
630 && (move = (ss-1)->currentMove) != MOVE_NULL
631 && type_of(move) == NORMAL)
633 Square to = to_sq(move);
634 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
637 // Step 6. Razoring (skipped when in check)
639 && depth < 4 * ONE_PLY
640 && eval + razor_margin(depth) < beta
641 && ttMove == MOVE_NONE
642 && abs(beta) < VALUE_MATE_IN_MAX_PLY
643 && !pos.pawn_on_7th(pos.side_to_move()))
645 Value rbeta = beta - razor_margin(depth);
646 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
648 // Logically we should return (v + razor_margin(depth)), but
649 // surprisingly this did slightly weaker in tests.
653 // Step 7. Static null move pruning (skipped when in check)
654 // We're betting that the opponent doesn't have a move that will reduce
655 // the score by more than futility_margin(depth) if we do a null move.
658 && depth < 4 * ONE_PLY
659 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
660 && abs(beta) < VALUE_MATE_IN_MAX_PLY
661 && abs(eval) < VALUE_KNOWN_WIN
662 && pos.non_pawn_material(pos.side_to_move()))
663 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
665 // Step 8. Null move search with verification search (is omitted in PV nodes)
668 && depth >= 2 * ONE_PLY
670 && abs(beta) < VALUE_MATE_IN_MAX_PLY
671 && pos.non_pawn_material(pos.side_to_move()))
673 ss->currentMove = MOVE_NULL;
675 // Null move dynamic reduction based on depth
676 Depth R = 3 * ONE_PLY + depth / 4;
678 // Null move dynamic reduction based on value
679 if (eval - PawnValueMg > beta)
682 pos.do_null_move(st);
683 (ss+1)->skipNullMove = true;
684 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
685 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
686 (ss+1)->skipNullMove = false;
687 pos.undo_null_move();
689 if (nullValue >= beta)
691 // Do not return unproven mate scores
692 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
695 if (depth < 12 * ONE_PLY)
698 // Do verification search at high depths
699 ss->skipNullMove = true;
700 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
701 ss->skipNullMove = false;
708 // The null move failed low, which means that we may be faced with
709 // some kind of threat. If the previous move was reduced, check if
710 // the move that refuted the null move was somehow connected to the
711 // move which was reduced. If a connection is found, return a fail
712 // low score (which will cause the reduced move to fail high in the
713 // parent node, which will trigger a re-search with full depth).
714 threatMove = (ss+1)->currentMove;
716 if ( depth < 5 * ONE_PLY
718 && threatMove != MOVE_NONE
719 && allows(pos, (ss-1)->currentMove, threatMove))
724 // Step 9. ProbCut (skipped when in check)
725 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
726 // and a reduced search returns a value much above beta, we can (almost) safely
727 // prune the previous move.
729 && depth >= 5 * ONE_PLY
731 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
733 Value rbeta = beta + 200;
734 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
736 assert(rdepth >= ONE_PLY);
737 assert((ss-1)->currentMove != MOVE_NONE);
738 assert((ss-1)->currentMove != MOVE_NULL);
740 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
743 while ((move = mp.next_move<false>()) != MOVE_NONE)
744 if (pos.legal(move, ci.pinned))
746 ss->currentMove = move;
747 pos.do_move(move, st, ci, pos.gives_check(move, ci));
748 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
755 // Step 10. Internal iterative deepening (skipped when in check)
756 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
757 && ttMove == MOVE_NONE
758 && (PvNode || ss->staticEval + Value(256) >= beta))
760 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
762 ss->skipNullMove = true;
763 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
764 ss->skipNullMove = false;
766 tte = TT.probe(posKey);
767 ttMove = tte ? tte->move() : MOVE_NONE;
770 moves_loop: // When in check and at SpNode search starts from here
772 Square prevMoveSq = to_sq((ss-1)->currentMove);
773 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
774 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
776 MovePicker mp(pos, ttMove, depth, History, countermoves, ss);
778 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
779 improving = ss->staticEval >= (ss-2)->staticEval
780 || ss->staticEval == VALUE_NONE
781 ||(ss-2)->staticEval == VALUE_NONE;
783 singularExtensionNode = !RootNode
785 && depth >= 8 * ONE_PLY
786 && ttMove != MOVE_NONE
787 && !excludedMove // Recursive singular search is not allowed
788 && (tte->bound() & BOUND_LOWER)
789 && tte->depth() >= depth - 3 * ONE_PLY;
791 // Step 11. Loop through moves
792 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
793 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
797 if (move == excludedMove)
800 // At root obey the "searchmoves" option and skip moves not listed in Root
801 // Move List, as a consequence any illegal move is also skipped. In MultiPV
802 // mode we also skip PV moves which have been already searched.
803 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
808 // Shared counter cannot be decremented later if move turns out to be illegal
809 if (!pos.legal(move, ci.pinned))
812 moveCount = ++splitPoint->moveCount;
813 splitPoint->mutex.unlock();
820 Signals.firstRootMove = (moveCount == 1);
822 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
823 sync_cout << "info depth " << depth / ONE_PLY
824 << " currmove " << move_to_uci(move, pos.is_chess960())
825 << " currmovenumber " << moveCount + PVIdx << sync_endl;
829 captureOrPromotion = pos.capture_or_promotion(move);
830 givesCheck = pos.gives_check(move, ci);
831 dangerous = givesCheck
832 || pos.passed_pawn_push(move)
833 || type_of(move) == CASTLE;
835 // Step 12. Extend checks
836 if (givesCheck && pos.see_sign(move) >= 0)
839 // Singular extension search. If all moves but one fail low on a search of
840 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
841 // is singular and should be extended. To verify this we do a reduced search
842 // on all the other moves but the ttMove, if result is lower than ttValue minus
843 // a margin then we extend ttMove.
844 if ( singularExtensionNode
847 && pos.legal(move, ci.pinned)
848 && abs(ttValue) < VALUE_KNOWN_WIN)
850 assert(ttValue != VALUE_NONE);
852 Value rBeta = ttValue - int(depth);
853 ss->excludedMove = move;
854 ss->skipNullMove = true;
855 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
856 ss->skipNullMove = false;
857 ss->excludedMove = MOVE_NONE;
863 // Update current move (this must be done after singular extension search)
864 newDepth = depth - ONE_PLY + ext;
866 // Step 13. Futility pruning (is omitted in PV nodes)
868 && !captureOrPromotion
871 /* && move != ttMove Already implicit in the next condition */
872 && bestValue > VALUE_MATED_IN_MAX_PLY)
874 // Move count based pruning
875 if ( depth < 16 * ONE_PLY
876 && moveCount >= FutilityMoveCounts[improving][depth]
877 && (!threatMove || !refutes(pos, move, threatMove)))
880 splitPoint->mutex.lock();
885 // Value based pruning
886 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
887 // but fixing this made program slightly weaker.
888 Depth predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
889 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
890 + Gains[pos.moved_piece(move)][to_sq(move)];
892 if (futilityValue < beta)
894 bestValue = std::max(bestValue, futilityValue);
898 splitPoint->mutex.lock();
899 if (bestValue > splitPoint->bestValue)
900 splitPoint->bestValue = bestValue;
905 // Prune moves with negative SEE at low depths
906 if ( predictedDepth < 4 * ONE_PLY
907 && pos.see_sign(move) < 0)
910 splitPoint->mutex.lock();
915 // We have not pruned the move that will be searched, but remember how
916 // far in the move list we are to be more aggressive in the child node.
917 ss->futilityMoveCount = moveCount;
920 ss->futilityMoveCount = 0;
922 // Check for legality only before to do the move
923 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
929 pvMove = PvNode && moveCount == 1;
930 ss->currentMove = move;
931 if (!SpNode && !captureOrPromotion && quietCount < 64)
932 quietsSearched[quietCount++] = move;
934 // Step 14. Make the move
935 pos.do_move(move, st, ci, givesCheck);
937 // Step 15. Reduced depth search (LMR). If the move fails high will be
938 // re-searched at full depth.
939 if ( depth >= 3 * ONE_PLY
941 && !captureOrPromotion
943 && move != ss->killers[0]
944 && move != ss->killers[1])
946 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
948 if (!PvNode && cutNode)
949 ss->reduction += ONE_PLY;
951 else if (History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
952 ss->reduction += ONE_PLY / 2;
954 if (move == countermoves[0] || move == countermoves[1])
955 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
957 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
959 alpha = splitPoint->alpha;
961 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
963 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
964 ss->reduction = DEPTH_ZERO;
967 doFullDepthSearch = !pvMove;
969 // Step 16. Full depth search, when LMR is skipped or fails high
970 if (doFullDepthSearch)
973 alpha = splitPoint->alpha;
975 value = newDepth < ONE_PLY ?
976 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
977 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
978 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
981 // Only for PV nodes do a full PV search on the first move or after a fail
982 // high, in the latter case search only if value < beta, otherwise let the
983 // parent node to fail low with value <= alpha and to try another move.
984 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
985 value = newDepth < ONE_PLY ?
986 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
987 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
988 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
989 // Step 17. Undo move
992 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
994 // Step 18. Check for new best move
997 splitPoint->mutex.lock();
998 bestValue = splitPoint->bestValue;
999 alpha = splitPoint->alpha;
1002 // Finished searching the move. If Signals.stop is true, the search
1003 // was aborted because the user interrupted the search or because we
1004 // ran out of time. In this case, the return value of the search cannot
1005 // be trusted, and we don't update the best move and/or PV.
1006 if (Signals.stop || thisThread->cutoff_occurred())
1007 return value; // To avoid returning VALUE_INFINITE
1011 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1013 // PV move or new best move ?
1014 if (pvMove || value > alpha)
1017 rm.extract_pv_from_tt(pos);
1019 // We record how often the best move has been changed in each
1020 // iteration. This information is used for time management: When
1021 // the best move changes frequently, we allocate some more time.
1026 // All other moves but the PV are set to the lowest value, this
1027 // is not a problem when sorting becuase sort is stable and move
1028 // position in the list is preserved, just the PV is pushed up.
1029 rm.score = -VALUE_INFINITE;
1032 if (value > bestValue)
1034 bestValue = SpNode ? splitPoint->bestValue = value : value;
1038 bestMove = SpNode ? splitPoint->bestMove = move : move;
1040 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1041 alpha = SpNode ? splitPoint->alpha = value : value;
1044 assert(value >= beta); // Fail high
1047 splitPoint->cutoff = true;
1054 // Step 19. Check for splitting the search
1056 && depth >= Threads.minimumSplitDepth
1057 && Threads.available_slave(thisThread)
1058 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1060 assert(bestValue < beta);
1062 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1063 depth, threatMove, moveCount, &mp, NT, cutNode);
1064 if (bestValue >= beta)
1072 // Step 20. Check for mate and stalemate
1073 // All legal moves have been searched and if there are no legal moves, it
1074 // must be mate or stalemate. Note that we can have a false positive in
1075 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1076 // harmless because return value is discarded anyhow in the parent nodes.
1077 // If we are in a singular extension search then return a fail low score.
1078 // A split node has at least one move, the one tried before to be splitted.
1080 return excludedMove ? alpha
1081 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1083 // If we have pruned all the moves without searching return a fail-low score
1084 if (bestValue == -VALUE_INFINITE)
1087 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1088 bestValue >= beta ? BOUND_LOWER :
1089 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1090 depth, bestMove, ss->staticEval, ss->evalMargin);
1092 // Quiet best move: update killers, history and countermoves
1093 if ( bestValue >= beta
1094 && !pos.capture_or_promotion(bestMove)
1097 if (ss->killers[0] != bestMove)
1099 ss->killers[1] = ss->killers[0];
1100 ss->killers[0] = bestMove;
1103 // Increase history value of the cut-off move and decrease all the other
1104 // played non-capture moves.
1105 Value bonus = Value(int(depth) * int(depth));
1106 History.update(pos.moved_piece(bestMove), to_sq(bestMove), bonus);
1107 for (int i = 0; i < quietCount - 1; ++i)
1109 Move m = quietsSearched[i];
1110 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1113 if (is_ok((ss-1)->currentMove))
1114 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1117 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1123 // qsearch() is the quiescence search function, which is called by the main
1124 // search function when the remaining depth is zero (or, to be more precise,
1125 // less than ONE_PLY).
1127 template <NodeType NT, bool InCheck>
1128 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1130 const bool PvNode = (NT == PV);
1132 assert(NT == PV || NT == NonPV);
1133 assert(InCheck == !!pos.checkers());
1134 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1135 assert(PvNode || (alpha == beta - 1));
1136 assert(depth <= DEPTH_ZERO);
1141 Move ttMove, move, bestMove;
1142 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1143 bool givesCheck, evasionPrunable;
1146 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1150 ss->currentMove = bestMove = MOVE_NONE;
1151 ss->ply = (ss-1)->ply + 1;
1153 // Check for an instant draw or maximum ply reached
1154 if (pos.is_draw() || ss->ply > MAX_PLY)
1155 return DrawValue[pos.side_to_move()];
1157 // Decide whether or not to include checks, this fixes also the type of
1158 // TT entry depth that we are going to use. Note that in qsearch we use
1159 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1160 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1161 : DEPTH_QS_NO_CHECKS;
1163 // Transposition table lookup
1165 tte = TT.probe(posKey);
1166 ttMove = tte ? tte->move() : MOVE_NONE;
1167 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1170 && tte->depth() >= ttDepth
1171 && ttValue != VALUE_NONE // Only in case of TT access race
1172 && ( PvNode ? tte->bound() == BOUND_EXACT
1173 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1174 : (tte->bound() & BOUND_UPPER)))
1176 ss->currentMove = ttMove; // Can be MOVE_NONE
1180 // Evaluate the position statically
1183 ss->staticEval = ss->evalMargin = VALUE_NONE;
1184 bestValue = futilityBase = -VALUE_INFINITE;
1190 // Never assume anything on values stored in TT
1191 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1192 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1193 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1195 // Can ttValue be used as a better position evaluation?
1196 if (ttValue != VALUE_NONE)
1197 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1198 bestValue = ttValue;
1201 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1203 // Stand pat. Return immediately if static value is at least beta
1204 if (bestValue >= beta)
1207 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1208 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1213 if (PvNode && bestValue > alpha)
1216 futilityBase = bestValue + ss->evalMargin + Value(128);
1219 // Initialize a MovePicker object for the current position, and prepare
1220 // to search the moves. Because the depth is <= 0 here, only captures,
1221 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1223 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1226 // Loop through the moves until no moves remain or a beta cutoff occurs
1227 while ((move = mp.next_move<false>()) != MOVE_NONE)
1229 assert(is_ok(move));
1231 givesCheck = pos.gives_check(move, ci);
1238 && type_of(move) != PROMOTION
1239 && futilityBase > -VALUE_KNOWN_WIN
1240 && !pos.passed_pawn_push(move))
1242 futilityValue = futilityBase
1243 + PieceValue[EG][pos.piece_on(to_sq(move))]
1244 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1246 if (futilityValue < beta)
1248 bestValue = std::max(bestValue, futilityValue);
1252 // Prune moves with negative or equal SEE and also moves with positive
1253 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1254 if ( futilityBase < beta
1255 && pos.see(move, beta - futilityBase) <= 0)
1257 bestValue = std::max(bestValue, futilityBase);
1262 // Detect non-capture evasions that are candidate to be pruned
1263 evasionPrunable = InCheck
1264 && bestValue > VALUE_MATED_IN_MAX_PLY
1265 && !pos.capture(move)
1266 && !pos.can_castle(pos.side_to_move());
1268 // Don't search moves with negative SEE values
1270 && (!InCheck || evasionPrunable)
1272 && type_of(move) != PROMOTION
1273 && pos.see_sign(move) < 0)
1276 // Check for legality only before to do the move
1277 if (!pos.legal(move, ci.pinned))
1280 ss->currentMove = move;
1282 // Make and search the move
1283 pos.do_move(move, st, ci, givesCheck);
1284 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1285 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1286 pos.undo_move(move);
1288 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1290 // Check for new best move
1291 if (value > bestValue)
1297 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1304 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1305 ttDepth, move, ss->staticEval, ss->evalMargin);
1313 // All legal moves have been searched. A special case: If we're in check
1314 // and no legal moves were found, it is checkmate.
1315 if (InCheck && bestValue == -VALUE_INFINITE)
1316 return mated_in(ss->ply); // Plies to mate from the root
1318 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1319 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1320 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1322 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1328 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1329 // "plies to mate from the current position". Non-mate scores are unchanged.
1330 // The function is called before storing a value to the transposition table.
1332 Value value_to_tt(Value v, int ply) {
1334 assert(v != VALUE_NONE);
1336 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1337 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1341 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1342 // from the transposition table (where refers to the plies to mate/be mated
1343 // from current position) to "plies to mate/be mated from the root".
1345 Value value_from_tt(Value v, int ply) {
1347 return v == VALUE_NONE ? VALUE_NONE
1348 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1349 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1353 // allows() tests whether the 'first' move at previous ply somehow makes the
1354 // 'second' move possible, for instance if the moving piece is the same in
1355 // both moves. Normally the second move is the threat (the best move returned
1356 // from a null search that fails low).
1358 bool allows(const Position& pos, Move first, Move second) {
1360 assert(is_ok(first));
1361 assert(is_ok(second));
1362 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1363 assert(type_of(first) == CASTLE || color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1365 Square m1from = from_sq(first);
1366 Square m2from = from_sq(second);
1367 Square m1to = to_sq(first);
1368 Square m2to = to_sq(second);
1370 // The piece is the same or second's destination was vacated by the first move
1371 // We exclude the trivial case where a sliding piece does in two moves what
1372 // it could do in one move: eg. Ra1a2, Ra2a3.
1374 || (m1to == m2from && !squares_aligned(m1from, m2from, m2to)))
1377 // Second one moves through the square vacated by first one
1378 if (between_bb(m2from, m2to) & m1from)
1381 // Second's destination is defended by the first move's piece
1382 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1386 // Second move gives a discovered check through the first's checking piece
1387 if (m1att & pos.king_square(pos.side_to_move()))
1389 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1397 // refutes() tests whether a 'first' move is able to defend against a 'second'
1398 // opponent's move. In this case will not be pruned. Normally the second move
1399 // is the threat (the best move returned from a null search that fails low).
1401 bool refutes(const Position& pos, Move first, Move second) {
1403 assert(is_ok(first));
1404 assert(is_ok(second));
1406 Square m1from = from_sq(first);
1407 Square m2from = from_sq(second);
1408 Square m1to = to_sq(first);
1409 Square m2to = to_sq(second);
1411 // Don't prune moves of the threatened piece
1415 // If the threatened piece has value less than or equal to the value of the
1416 // threat piece, don't prune moves which defend it.
1417 if ( pos.capture(second)
1418 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1419 || type_of(pos.piece_on(m2from)) == KING))
1421 // Update occupancy as if the piece and the threat are moving
1422 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1423 Piece pc = pos.piece_on(m1from);
1425 // The moved piece attacks the square 'tto' ?
1426 if (pos.attacks_from(pc, m1to, occ) & m2to)
1429 // Scan for possible X-ray attackers behind the moved piece
1430 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1431 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1433 // Verify attackers are triggered by our move and not already existing
1434 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1438 // Don't prune safe moves which block the threat path
1439 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1446 // When playing with strength handicap choose best move among the MultiPV set
1447 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1449 Move Skill::pick_move() {
1453 // PRNG sequence should be not deterministic
1454 for (int i = Time::now() % 50; i > 0; --i)
1455 rk.rand<unsigned>();
1457 // RootMoves are already sorted by score in descending order
1458 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1459 int weakness = 120 - 2 * level;
1460 int max_s = -VALUE_INFINITE;
1463 // Choose best move. For each move score we add two terms both dependent on
1464 // weakness, one deterministic and bigger for weaker moves, and one random,
1465 // then we choose the move with the resulting highest score.
1466 for (size_t i = 0; i < PVSize; ++i)
1468 int s = RootMoves[i].score;
1470 // Don't allow crazy blunders even at very low skills
1471 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1474 // This is our magic formula
1475 s += ( weakness * int(RootMoves[0].score - s)
1476 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1481 best = RootMoves[i].pv[0];
1488 // uci_pv() formats PV information according to UCI protocol. UCI requires
1489 // to send all the PV lines also if are still to be searched and so refer to
1490 // the previous search score.
1492 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1494 std::stringstream s;
1495 Time::point elapsed = Time::now() - SearchTime + 1;
1496 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1499 for (size_t i = 0; i < Threads.size(); ++i)
1500 if (Threads[i]->maxPly > selDepth)
1501 selDepth = Threads[i]->maxPly;
1503 for (size_t i = 0; i < uciPVSize; ++i)
1505 bool updated = (i <= PVIdx);
1507 if (depth == 1 && !updated)
1510 int d = updated ? depth : depth - 1;
1511 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1513 if (s.rdbuf()->in_avail()) // Not at first line
1516 s << "info depth " << d
1517 << " seldepth " << selDepth
1518 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1519 << " nodes " << pos.nodes_searched()
1520 << " nps " << pos.nodes_searched() * 1000 / elapsed
1521 << " time " << elapsed
1522 << " multipv " << i + 1
1525 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; ++j)
1526 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1535 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1536 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1537 /// allow to always have a ponder move even when we fail high at root, and a
1538 /// long PV to print that is important for position analysis.
1540 void RootMove::extract_pv_from_tt(Position& pos) {
1542 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1552 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1554 pos.do_move(pv[ply++], *st++);
1555 tte = TT.probe(pos.key());
1558 && pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
1559 && pos.legal(m, pos.pinned_pieces())
1561 && (!pos.is_draw() || ply < 2));
1563 pv.push_back(MOVE_NONE); // Must be zero-terminating
1565 while (ply) pos.undo_move(pv[--ply]);
1569 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1570 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1571 /// first, even if the old TT entries have been overwritten.
1573 void RootMove::insert_pv_in_tt(Position& pos) {
1575 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1580 tte = TT.probe(pos.key());
1582 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1583 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1585 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1587 pos.do_move(pv[ply++], *st++);
1589 } while (pv[ply] != MOVE_NONE);
1591 while (ply) pos.undo_move(pv[--ply]);
1595 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1597 void Thread::idle_loop() {
1599 // Pointer 'this_sp' is not null only if we are called from split(), and not
1600 // at the thread creation. So it means we are the split point's master.
1601 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1603 assert(!this_sp || (this_sp->masterThread == this && searching));
1607 // If we are not searching, wait for a condition to be signaled instead of
1608 // wasting CPU time polling for work.
1609 while ((!searching && Threads.sleepWhileIdle) || exit)
1617 // Grab the lock to avoid races with Thread::notify_one()
1620 // If we are master and all slaves have finished then exit idle_loop
1621 if (this_sp && !this_sp->slavesMask)
1627 // Do sleep after retesting sleep conditions under lock protection, in
1628 // particular we need to avoid a deadlock in case a master thread has,
1629 // in the meanwhile, allocated us and sent the notify_one() call before
1630 // we had the chance to grab the lock.
1631 if (!searching && !exit)
1632 sleepCondition.wait(mutex);
1637 // If this thread has been assigned work, launch a search
1642 Threads.mutex.lock();
1645 assert(activeSplitPoint);
1646 SplitPoint* sp = activeSplitPoint;
1648 Threads.mutex.unlock();
1650 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
1651 Position pos(*sp->pos, this);
1653 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1654 ss->splitPoint = sp;
1658 assert(activePosition == NULL);
1660 activePosition = &pos;
1662 switch (sp->nodeType) {
1664 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1667 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1670 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1679 activePosition = NULL;
1680 sp->slavesMask &= ~(1ULL << idx);
1681 sp->nodes += pos.nodes_searched();
1683 // Wake up master thread so to allow it to return from the idle loop
1684 // in case we are the last slave of the split point.
1685 if ( Threads.sleepWhileIdle
1686 && this != sp->masterThread
1689 assert(!sp->masterThread->searching);
1690 sp->masterThread->notify_one();
1693 // After releasing the lock we cannot access anymore any SplitPoint
1694 // related data in a safe way becuase it could have been released under
1695 // our feet by the sp master. Also accessing other Thread objects is
1696 // unsafe because if we are exiting there is a chance are already freed.
1700 // If this thread is the master of a split point and all slaves have finished
1701 // their work at this split point, return from the idle loop.
1702 if (this_sp && !this_sp->slavesMask)
1704 this_sp->mutex.lock();
1705 bool finished = !this_sp->slavesMask; // Retest under lock protection
1706 this_sp->mutex.unlock();
1714 /// check_time() is called by the timer thread when the timer triggers. It is
1715 /// used to print debug info and, more important, to detect when we are out of
1716 /// available time and so stop the search.
1720 static Time::point lastInfoTime = Time::now();
1721 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1723 if (Time::now() - lastInfoTime >= 1000)
1725 lastInfoTime = Time::now();
1734 Threads.mutex.lock();
1736 nodes = RootPos.nodes_searched();
1738 // Loop across all split points and sum accumulated SplitPoint nodes plus
1739 // all the currently active positions nodes.
1740 for (size_t i = 0; i < Threads.size(); ++i)
1741 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1743 SplitPoint& sp = Threads[i]->splitPoints[j];
1748 Bitboard sm = sp.slavesMask;
1751 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1753 nodes += pos->nodes_searched();
1759 Threads.mutex.unlock();
1762 Time::point elapsed = Time::now() - SearchTime;
1763 bool stillAtFirstMove = Signals.firstRootMove
1764 && !Signals.failedLowAtRoot
1765 && elapsed > TimeMgr.available_time();
1767 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1768 || stillAtFirstMove;
1770 if ( (Limits.use_time_management() && noMoreTime)
1771 || (Limits.movetime && elapsed >= Limits.movetime)
1772 || (Limits.nodes && nodes >= Limits.nodes))
1773 Signals.stop = true;