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
230 // Reset and wake up the threads
231 for (size_t i = 0; i < Threads.size(); i++)
233 Threads[i].maxPly = 0;
234 Threads[i].do_sleep = false;
236 if (!Threads.use_sleeping_threads())
237 Threads[i].notify_one();
240 // Set best timer interval to avoid lagging under time pressure. Timer is
241 // used to check for remaining available thinking time.
242 Threads.timer_thread()->maxPly = /* Hack: we use maxPly to set timer interval */
243 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
244 Limits.nodes ? 2 * TimerResolution
247 Threads.timer_thread()->notify_one(); // Wake up the recurring timer
249 id_loop(RootPos); // Let's start searching !
251 Threads.timer_thread()->maxPly = 0; // Stop the timer
253 // Main thread will go to sleep by itself to avoid a race with start_searching()
254 for (size_t i = 0; i < Threads.size(); i++)
255 if (&Threads[i] != Threads.main_thread())
256 Threads[i].do_sleep = true;
258 if (Options["Use Search Log"])
260 Time::point elapsed = Time::now() - SearchTime + 1;
262 Log log(Options["Search Log Filename"]);
263 log << "Nodes: " << RootPos.nodes_searched()
264 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
265 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
268 RootPos.do_move(RootMoves[0].pv[0], st);
269 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
270 RootPos.undo_move(RootMoves[0].pv[0]);
275 // When we reach max depth we arrive here even without Signals.stop is raised,
276 // but if we are pondering or in infinite search, according to UCI protocol,
277 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
278 // command. We simply wait here until GUI sends one of those commands (that
279 // raise Signals.stop).
280 if (!Signals.stop && (Limits.ponder || Limits.infinite))
282 Signals.stopOnPonderhit = true;
283 RootPos.this_thread()->wait_for(Signals.stop);
286 // Best move could be MOVE_NONE when searching on a stalemate position
287 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
288 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
295 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
296 // with increasing depth until the allocated thinking time has been consumed,
297 // user stops the search, or the maximum search depth is reached.
299 void id_loop(Position& pos) {
301 Stack ss[MAX_PLY_PLUS_2];
302 int depth, prevBestMoveChanges;
303 Value bestValue, alpha, beta, delta;
304 bool bestMoveNeverChanged = true;
306 memset(ss, 0, 4 * sizeof(Stack));
307 depth = BestMoveChanges = 0;
308 bestValue = delta = -VALUE_INFINITE;
309 ss->currentMove = MOVE_NULL; // Hack to skip update gains
313 PVSize = Options["MultiPV"];
314 Skill skill(Options["Skill Level"]);
316 // Do we have to play with skill handicap? In this case enable MultiPV search
317 // that we will use behind the scenes to retrieve a set of possible moves.
318 if (skill.enabled() && PVSize < 4)
321 PVSize = std::min(PVSize, RootMoves.size());
323 // Iterative deepening loop until requested to stop or target depth reached
324 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
326 // Save last iteration's scores before first PV line is searched and all
327 // the move scores but the (new) PV are set to -VALUE_INFINITE.
328 for (size_t i = 0; i < RootMoves.size(); i++)
329 RootMoves[i].prevScore = RootMoves[i].score;
331 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
334 // MultiPV loop. We perform a full root search for each PV line
335 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
337 // Set aspiration window default width
338 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
341 alpha = RootMoves[PVIdx].prevScore - delta;
342 beta = RootMoves[PVIdx].prevScore + delta;
346 alpha = -VALUE_INFINITE;
347 beta = VALUE_INFINITE;
350 // Start with a small aspiration window and, in case of fail high/low,
351 // research with bigger window until not failing high/low anymore.
354 // Search starts from ss+1 to allow referencing (ss-1). This is
355 // needed by update gains and ss copy when splitting at Root.
356 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
358 // Bring to front the best move. It is critical that sorting is
359 // done with a stable algorithm because all the values but the first
360 // and eventually the new best one are set to -VALUE_INFINITE and
361 // we want to keep the same order for all the moves but the new
362 // PV that goes to the front. Note that in case of MultiPV search
363 // the already searched PV lines are preserved.
364 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
366 // Write PV back to transposition table in case the relevant
367 // entries have been overwritten during the search.
368 for (size_t i = 0; i <= PVIdx; i++)
369 RootMoves[i].insert_pv_in_tt(pos);
371 // If search has been stopped return immediately. Sorting and
372 // writing PV back to TT is safe becuase RootMoves is still
373 // valid, although refers to previous iteration.
377 // In case of failing high/low increase aspiration window and
378 // research, otherwise exit the loop.
379 if (bestValue > alpha && bestValue < beta)
382 // Give some update (without cluttering the UI) before to research
383 if (Time::now() - SearchTime > 3000)
384 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
386 if (abs(bestValue) >= VALUE_KNOWN_WIN)
388 alpha = -VALUE_INFINITE;
389 beta = VALUE_INFINITE;
391 else if (bestValue >= beta)
398 Signals.failedLowAtRoot = true;
399 Signals.stopOnPonderhit = false;
405 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
408 // Sort the PV lines searched so far and update the GUI
409 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
410 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
411 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
414 // Do we need to pick now the sub-optimal best move ?
415 if (skill.enabled() && skill.time_to_pick(depth))
418 if (Options["Use Search Log"])
420 Log log(Options["Search Log Filename"]);
421 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
425 // Filter out startup noise when monitoring best move stability
426 if (depth > 2 && BestMoveChanges)
427 bestMoveNeverChanged = false;
429 // Do we have found a "mate in x"?
431 && bestValue >= VALUE_MATE_IN_MAX_PLY
432 && VALUE_MATE - bestValue <= 2 * Limits.mate)
435 // Do we have time for the next iteration? Can we stop searching now?
436 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
438 bool stop = false; // Local variable, not the volatile Signals.stop
440 // Take in account some extra time if the best move has changed
441 if (depth > 4 && depth < 50 && PVSize == 1)
442 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
444 // Stop search if most of available time is already consumed. We
445 // probably don't have enough time to search the first move at the
446 // next iteration anyway.
447 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
450 // Stop search early if one move seems to be much better than others
454 && ( (bestMoveNeverChanged && pos.captured_piece_type())
455 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
457 Value rBeta = bestValue - 2 * PawnValueMg;
458 (ss+1)->excludedMove = RootMoves[0].pv[0];
459 (ss+1)->skipNullMove = true;
460 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
461 (ss+1)->skipNullMove = false;
462 (ss+1)->excludedMove = MOVE_NONE;
470 // If we are allowed to ponder do not stop the search now but
471 // keep pondering until GUI sends "ponderhit" or "stop".
473 Signals.stopOnPonderhit = true;
482 // search<>() is the main search function for both PV and non-PV nodes and for
483 // normal and SplitPoint nodes. When called just after a split point the search
484 // is simpler because we have already probed the hash table, done a null move
485 // search, and searched the first move before splitting, we don't have to repeat
486 // all this work again. We also don't need to store anything to the hash table
487 // here: This is taken care of after we return from the split point.
489 template <NodeType NT>
490 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
492 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
493 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
494 const bool RootNode = (NT == Root || NT == SplitPointRoot);
496 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
497 assert(PvNode || (alpha == beta - 1));
498 assert(depth > DEPTH_ZERO);
500 Move movesSearched[64];
505 Move ttMove, move, excludedMove, bestMove, threatMove;
507 Value bestValue, value, ttValue;
508 Value eval, nullValue, futilityValue;
509 bool inCheck, givesCheck, pvMove, singularExtensionNode;
510 bool captureOrPromotion, dangerous, doFullDepthSearch, threatExtension;
511 int moveCount, playedMoveCount;
513 // Step 1. Initialize node
514 Thread* thisThread = pos.this_thread();
515 moveCount = playedMoveCount = 0;
516 threatExtension = false;
517 inCheck = pos.checkers();
522 bestMove = sp->bestMove;
523 threatMove = sp->threatMove;
524 bestValue = sp->bestValue;
526 ttMove = excludedMove = MOVE_NONE;
527 ttValue = VALUE_NONE;
529 assert(sp->bestValue > -VALUE_INFINITE && sp->moveCount > 0);
531 goto split_point_start;
534 bestValue = -VALUE_INFINITE;
535 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
536 ss->ply = (ss-1)->ply + 1;
537 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
538 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
540 // Used to send selDepth info to GUI
541 if (PvNode && thisThread->maxPly < ss->ply)
542 thisThread->maxPly = ss->ply;
546 // Step 2. Check for aborted search and immediate draw
547 if (Signals.stop || pos.is_draw<true, PvNode>() || ss->ply > MAX_PLY)
548 return DrawValue[pos.side_to_move()];
550 // Step 3. Mate distance pruning. Even if we mate at the next move our score
551 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
552 // a shorter mate was found upward in the tree then there is no need to search
553 // further, we will never beat current alpha. Same logic but with reversed signs
554 // applies also in the opposite condition of being mated instead of giving mate,
555 // in this case return a fail-high score.
556 alpha = std::max(mated_in(ss->ply), alpha);
557 beta = std::min(mate_in(ss->ply+1), beta);
562 // Step 4. Transposition table lookup
563 // We don't want the score of a partial search to overwrite a previous full search
564 // TT value, so we use a different position key in case of an excluded move.
565 excludedMove = ss->excludedMove;
566 posKey = excludedMove ? pos.exclusion_key() : pos.key();
567 tte = TT.probe(posKey);
568 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
569 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
571 // At PV nodes we check for exact scores, while at non-PV nodes we check for
572 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
573 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
574 // we should also update RootMoveList to avoid bogus output.
577 && tte->depth() >= depth
578 && ttValue != VALUE_NONE // Only in case of TT access race
579 && ( PvNode ? tte->type() == BOUND_EXACT
580 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
581 : (tte->type() & BOUND_UPPER)))
584 ss->currentMove = ttMove; // Can be MOVE_NONE
588 && !pos.is_capture_or_promotion(ttMove)
589 && ttMove != ss->killers[0])
591 ss->killers[1] = ss->killers[0];
592 ss->killers[0] = ttMove;
597 // Step 5. Evaluate the position statically and update parent's gain statistics
599 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
603 // Never assume anything on values stored in TT
604 if ( (ss->staticEval = eval = tte->static_value()) == VALUE_NONE
605 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
606 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
608 // Can ttValue be used as a better position evaluation?
609 if (ttValue != VALUE_NONE)
610 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
611 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
616 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
617 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
618 ss->staticEval, ss->evalMargin);
621 // Update gain for the parent non-capture move given the static position
622 // evaluation before and after the move.
623 if ( (move = (ss-1)->currentMove) != MOVE_NULL
624 && (ss-1)->staticEval != VALUE_NONE
625 && ss->staticEval != VALUE_NONE
626 && !pos.captured_piece_type()
627 && type_of(move) == NORMAL)
629 Square to = to_sq(move);
630 H.update_gain(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
633 // Step 6. Razoring (is omitted in PV nodes)
635 && depth < 4 * ONE_PLY
637 && eval + razor_margin(depth) < beta
638 && ttMove == MOVE_NONE
639 && abs(beta) < VALUE_MATE_IN_MAX_PLY
640 && !pos.pawn_on_7th(pos.side_to_move()))
642 Value rbeta = beta - razor_margin(depth);
643 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
645 // Logically we should return (v + razor_margin(depth)), but
646 // surprisingly this did slightly weaker in tests.
650 // Step 7. Static null move pruning (is omitted in PV nodes)
651 // We're betting that the opponent doesn't have a move that will reduce
652 // the score by more than futility_margin(depth) if we do a null move.
655 && depth < 4 * ONE_PLY
657 && eval - FutilityMargins[depth][0] >= beta
658 && abs(beta) < VALUE_MATE_IN_MAX_PLY
659 && pos.non_pawn_material(pos.side_to_move()))
660 return eval - FutilityMargins[depth][0];
662 // Step 8. Null move search with verification search (is omitted in PV nodes)
668 && abs(beta) < VALUE_MATE_IN_MAX_PLY
669 && pos.non_pawn_material(pos.side_to_move()))
671 ss->currentMove = MOVE_NULL;
673 // Null move dynamic reduction based on depth
674 Depth R = 3 * ONE_PLY + depth / 4;
676 // Null move dynamic reduction based on value
677 if (eval - PawnValueMg > beta)
680 pos.do_null_move<true>(st);
681 (ss+1)->skipNullMove = true;
682 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
683 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
684 (ss+1)->skipNullMove = false;
685 pos.do_null_move<false>(st);
687 if (nullValue >= beta)
689 // Do not return unproven mate scores
690 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
693 if (depth < 6 * ONE_PLY)
696 // Do verification search at high depths
697 ss->skipNullMove = true;
698 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
699 ss->skipNullMove = false;
706 // The null move failed low, which means that we may be faced with
707 // some kind of threat. If the previous move was reduced, check if
708 // the move that refuted the null move was somehow connected to the
709 // move which was reduced. If a connection is found extend moves that
710 // defend against threat.
711 threatMove = (ss+1)->currentMove;
713 if ( depth < 5 * ONE_PLY
715 && threatMove != MOVE_NONE
716 && allows_move(pos, (ss-1)->currentMove, threatMove))
717 threatExtension = true;
721 // Step 9. ProbCut (is omitted in PV nodes)
722 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
723 // and a reduced search returns a value much above beta, we can (almost) safely
724 // prune the previous move.
726 && depth >= 5 * ONE_PLY
729 && excludedMove == MOVE_NONE
730 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
732 Value rbeta = beta + 200;
733 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
735 assert(rdepth >= ONE_PLY);
736 assert((ss-1)->currentMove != MOVE_NONE);
737 assert((ss-1)->currentMove != MOVE_NULL);
739 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
742 while ((move = mp.next_move<false>()) != MOVE_NONE)
743 if (pos.pl_move_is_legal(move, ci.pinned))
745 ss->currentMove = move;
746 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
747 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
754 // Step 10. Internal iterative deepening
755 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
756 && ttMove == MOVE_NONE
757 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
759 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
761 ss->skipNullMove = true;
762 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
763 ss->skipNullMove = false;
765 tte = TT.probe(posKey);
766 ttMove = tte ? tte->move() : MOVE_NONE;
769 split_point_start: // At split points actual search starts from here
771 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
773 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
774 singularExtensionNode = !RootNode
776 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
777 && ttMove != MOVE_NONE
778 && !excludedMove // Recursive singular search is not allowed
779 && (tte->type() & BOUND_LOWER)
780 && tte->depth() >= depth - 3 * ONE_PLY;
782 // Step 11. Loop through moves
783 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
784 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
788 if (move == excludedMove)
791 // At root obey the "searchmoves" option and skip moves not listed in Root
792 // Move List, as a consequence any illegal move is also skipped. In MultiPV
793 // mode we also skip PV moves which have been already searched.
794 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
799 // Shared counter cannot be decremented later if move turns out to be illegal
800 if (!pos.pl_move_is_legal(move, ci.pinned))
803 moveCount = ++sp->moveCount;
811 Signals.firstRootMove = (moveCount == 1);
813 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
814 sync_cout << "info depth " << depth / ONE_PLY
815 << " currmove " << move_to_uci(move, pos.is_chess960())
816 << " currmovenumber " << moveCount + PVIdx << sync_endl;
820 captureOrPromotion = pos.is_capture_or_promotion(move);
821 givesCheck = pos.move_gives_check(move, ci);
822 dangerous = givesCheck
823 || pos.is_passed_pawn_push(move)
824 || type_of(move) == CASTLE
825 || ( captureOrPromotion // Entering a pawn endgame?
826 && type_of(pos.piece_on(to_sq(move))) != PAWN
827 && type_of(move) == NORMAL
828 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
829 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
831 // Step 12. Extend checks and, in PV nodes, also dangerous moves
832 if (PvNode && dangerous)
835 else if (threatExtension && prevents_move(pos, move, threatMove))
838 else if (givesCheck && pos.see_sign(move) >= 0)
841 // Singular extension search. If all moves but one fail low on a search of
842 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
843 // is singular and should be extended. To verify this we do a reduced search
844 // on all the other moves but the ttMove, if result is lower than ttValue minus
845 // a margin then we extend ttMove.
846 if ( singularExtensionNode
849 && pos.pl_move_is_legal(move, ci.pinned)
850 && abs(ttValue) < VALUE_KNOWN_WIN)
852 assert(ttValue != VALUE_NONE);
854 Value rBeta = ttValue - int(depth);
855 ss->excludedMove = move;
856 ss->skipNullMove = true;
857 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
858 ss->skipNullMove = false;
859 ss->excludedMove = MOVE_NONE;
862 ext = rBeta >= beta ? ONE_PLY + ONE_PLY / 2 : ONE_PLY;
865 // Update current move (this must be done after singular extension search)
866 newDepth = depth - ONE_PLY + ext;
868 // Step 13. Futility pruning (is omitted in PV nodes)
870 && !captureOrPromotion
874 && (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
875 && alpha > VALUE_MATED_IN_MAX_PLY)))
877 // Move count based pruning
878 if ( depth < 16 * ONE_PLY
879 && moveCount >= FutilityMoveCounts[depth]
880 && (!threatMove || !prevents_move(pos, move, threatMove)))
888 // Value based pruning
889 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
890 // but fixing this made program slightly weaker.
891 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
892 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
893 + H.gain(pos.piece_moved(move), to_sq(move));
895 if (futilityValue < beta)
903 // Prune moves with negative SEE at low depths
904 if ( predictedDepth < 2 * ONE_PLY
905 && pos.see_sign(move) < 0)
914 // Check for legality only before to do the move
915 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
921 pvMove = PvNode && moveCount == 1;
922 ss->currentMove = move;
923 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
924 movesSearched[playedMoveCount++] = move;
926 // Step 14. Make the move
927 pos.do_move(move, st, ci, givesCheck);
929 // Step 15. Reduced depth search (LMR). If the move fails high will be
930 // re-searched at full depth.
931 if ( depth > 3 * ONE_PLY
933 && !captureOrPromotion
935 && ss->killers[0] != move
936 && ss->killers[1] != move)
938 ss->reduction = reduction<PvNode>(depth, moveCount);
939 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
940 alpha = SpNode ? sp->alpha : alpha;
942 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
944 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
945 ss->reduction = DEPTH_ZERO;
948 doFullDepthSearch = !pvMove;
950 // Step 16. Full depth search, when LMR is skipped or fails high
951 if (doFullDepthSearch)
953 alpha = SpNode ? sp->alpha : alpha;
954 value = newDepth < ONE_PLY ?
955 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
956 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
957 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
960 // Only for PV nodes do a full PV search on the first move or after a fail
961 // high, in the latter case search only if value < beta, otherwise let the
962 // parent node to fail low with value <= alpha and to try another move.
963 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
964 value = newDepth < ONE_PLY ?
965 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
966 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
967 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
968 // Step 17. Undo move
971 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
973 // Step 18. Check for new best move
977 bestValue = sp->bestValue;
981 // Finished searching the move. If Signals.stop is true, the search
982 // was aborted because the user interrupted the search or because we
983 // ran out of time. In this case, the return value of the search cannot
984 // be trusted, and we don't update the best move and/or PV.
985 if (Signals.stop || thisThread->cutoff_occurred())
986 return value; // To avoid returning VALUE_INFINITE
990 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
992 // PV move or new best move ?
993 if (pvMove || value > alpha)
996 rm.extract_pv_from_tt(pos);
998 // We record how often the best move has been changed in each
999 // iteration. This information is used for time management: When
1000 // the best move changes frequently, we allocate some more time.
1005 // All other moves but the PV are set to the lowest value, this
1006 // is not a problem when sorting becuase sort is stable and move
1007 // position in the list is preserved, just the PV is pushed up.
1008 rm.score = -VALUE_INFINITE;
1011 if (value > bestValue)
1013 bestValue = SpNode ? sp->bestValue = value : value;
1017 bestMove = SpNode ? sp->bestMove = move : move;
1019 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1020 alpha = SpNode ? sp->alpha = value : value;
1023 assert(value >= beta); // Fail high
1033 // Step 19. Check for splitting the search
1035 && depth >= Threads.min_split_depth()
1036 && Threads.available_slave_exists(thisThread))
1038 assert(bestValue < beta);
1040 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1041 depth, threatMove, moveCount, mp, NT);
1042 if (bestValue >= beta)
1050 // Step 20. Check for mate and stalemate
1051 // All legal moves have been searched and if there are no legal moves, it
1052 // must be mate or stalemate. Note that we can have a false positive in
1053 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1054 // harmless because return value is discarded anyhow in the parent nodes.
1055 // If we are in a singular extension search then return a fail low score.
1056 // A split node has at least one move, the one tried before to be splitted.
1058 return excludedMove ? alpha
1059 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1061 // If we have pruned all the moves without searching return a fail-low score
1062 if (bestValue == -VALUE_INFINITE)
1064 assert(!playedMoveCount);
1069 if (bestValue >= beta) // Failed high
1071 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1072 bestMove, ss->staticEval, ss->evalMargin);
1074 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1076 if (bestMove != ss->killers[0])
1078 ss->killers[1] = ss->killers[0];
1079 ss->killers[0] = bestMove;
1082 // Increase history value of the cut-off move
1083 Value bonus = Value(int(depth) * int(depth));
1084 H.add(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1086 // Decrease history of all the other played non-capture moves
1087 for (int i = 0; i < playedMoveCount - 1; i++)
1089 Move m = movesSearched[i];
1090 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1094 else // Failed low or PV search
1095 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1096 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1097 depth, bestMove, ss->staticEval, ss->evalMargin);
1099 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1105 // qsearch() is the quiescence search function, which is called by the main
1106 // search function when the remaining depth is zero (or, to be more precise,
1107 // less than ONE_PLY).
1109 template <NodeType NT, bool InCheck>
1110 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1112 const bool PvNode = (NT == PV);
1114 assert(NT == PV || NT == NonPV);
1115 assert(InCheck == !!pos.checkers());
1116 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1117 assert(PvNode || (alpha == beta - 1));
1118 assert(depth <= DEPTH_ZERO);
1123 Move ttMove, move, bestMove;
1124 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1125 bool givesCheck, enoughMaterial, evasionPrunable;
1128 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1132 ss->currentMove = bestMove = MOVE_NONE;
1133 ss->ply = (ss-1)->ply + 1;
1135 // Check for an instant draw or maximum ply reached
1136 if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)
1137 return DrawValue[pos.side_to_move()];
1139 // Transposition table lookup. At PV nodes, we don't use the TT for
1140 // pruning, but only for move ordering.
1142 tte = TT.probe(posKey);
1143 ttMove = tte ? tte->move() : MOVE_NONE;
1144 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1146 // Decide whether or not to include checks, this fixes also the type of
1147 // TT entry depth that we are going to use. Note that in qsearch we use
1148 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1149 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1150 : DEPTH_QS_NO_CHECKS;
1152 && tte->depth() >= ttDepth
1153 && ttValue != VALUE_NONE // Only in case of TT access race
1154 && ( PvNode ? tte->type() == BOUND_EXACT
1155 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1156 : (tte->type() & BOUND_UPPER)))
1158 ss->currentMove = ttMove; // Can be MOVE_NONE
1162 // Evaluate the position statically
1165 ss->staticEval = ss->evalMargin = VALUE_NONE;
1166 bestValue = futilityBase = -VALUE_INFINITE;
1167 enoughMaterial = false;
1173 // Never assume anything on values stored in TT
1174 if ( (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE
1175 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
1176 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1179 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1181 // Stand pat. Return immediately if static value is at least beta
1182 if (bestValue >= beta)
1185 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1186 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1191 if (PvNode && bestValue > alpha)
1194 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1195 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1198 // Initialize a MovePicker object for the current position, and prepare
1199 // to search the moves. Because the depth is <= 0 here, only captures,
1200 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1202 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1205 // Loop through the moves until no moves remain or a beta cutoff occurs
1206 while ((move = mp.next_move<false>()) != MOVE_NONE)
1208 assert(is_ok(move));
1210 givesCheck = pos.move_gives_check(move, ci);
1218 && type_of(move) != PROMOTION
1219 && !pos.is_passed_pawn_push(move))
1221 futilityValue = futilityBase
1222 + PieceValue[EG][pos.piece_on(to_sq(move))]
1223 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1225 if (futilityValue < beta)
1227 bestValue = std::max(bestValue, futilityValue);
1231 // Prune moves with negative or equal SEE
1232 if ( futilityBase < beta
1233 && depth < DEPTH_ZERO
1234 && pos.see(move) <= 0)
1236 bestValue = std::max(bestValue, futilityBase);
1241 // Detect non-capture evasions that are candidate to be pruned
1242 evasionPrunable = !PvNode
1244 && bestValue > VALUE_MATED_IN_MAX_PLY
1245 && !pos.is_capture(move)
1246 && !pos.can_castle(pos.side_to_move());
1248 // Don't search moves with negative SEE values
1250 && (!InCheck || evasionPrunable)
1252 && type_of(move) != PROMOTION
1253 && pos.see_sign(move) < 0)
1256 // Don't search useless checks
1261 && !pos.is_capture_or_promotion(move)
1262 && ss->staticEval + PawnValueMg / 4 < beta
1263 && !check_is_dangerous(pos, move, futilityBase, beta))
1266 // Check for legality only before to do the move
1267 if (!pos.pl_move_is_legal(move, ci.pinned))
1270 ss->currentMove = move;
1272 // Make and search the move
1273 pos.do_move(move, st, ci, givesCheck);
1274 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1275 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1276 pos.undo_move(move);
1278 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1280 // Check for new best move
1281 if (value > bestValue)
1287 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1294 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1295 ttDepth, move, ss->staticEval, ss->evalMargin);
1303 // All legal moves have been searched. A special case: If we're in check
1304 // and no legal moves were found, it is checkmate.
1305 if (InCheck && bestValue == -VALUE_INFINITE)
1306 return mated_in(ss->ply); // Plies to mate from the root
1308 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1309 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1310 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1312 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1318 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1319 // "plies to mate from the current position". Non-mate scores are unchanged.
1320 // The function is called before storing a value to the transposition table.
1322 Value value_to_tt(Value v, int ply) {
1324 assert(v != VALUE_NONE);
1326 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1327 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1331 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1332 // from the transposition table (where refers to the plies to mate/be mated
1333 // from current position) to "plies to mate/be mated from the root".
1335 Value value_from_tt(Value v, int ply) {
1337 return v == VALUE_NONE ? VALUE_NONE
1338 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1339 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1343 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1345 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta)
1347 Piece pc = pos.piece_moved(move);
1348 Square from = from_sq(move);
1349 Square to = to_sq(move);
1350 Color them = ~pos.side_to_move();
1351 Square ksq = pos.king_square(them);
1352 Bitboard enemies = pos.pieces(them);
1353 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1354 Bitboard occ = pos.pieces() ^ from ^ ksq;
1355 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1356 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1358 // Checks which give opponent's king at most one escape square are dangerous
1359 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1362 // Queen contact check is very dangerous
1363 if (type_of(pc) == QUEEN && (kingAtt & to))
1366 // Creating new double threats with checks is dangerous
1367 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1370 // Note that here we generate illegal "double move"!
1371 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1379 // allows_move() tests whether the move at previous ply (first) somehow makes a
1380 // second move possible, for instance if the moving piece is the same in both
1381 // moves. Normally the second move is the threat move (the best move returned
1382 // from a null search that fails low).
1384 bool allows_move(const Position& pos, Move first, Move second) {
1386 assert(is_ok(first));
1387 assert(is_ok(second));
1388 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1389 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1391 Square m1from = from_sq(first);
1392 Square m2from = from_sq(second);
1393 Square m1to = to_sq(first);
1394 Square m2to = to_sq(second);
1396 // The piece is the same or second's destination was vacated by the first move
1397 if (m1to == m2from || m2to == m1from)
1400 // Second one moves through the square vacated by first one
1401 if (between_bb(m2from, m2to) & m1from)
1404 // Second's destination is defended by the first move's piece
1405 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1409 // Second move gives a discovered check through the first's checking piece
1410 if (m1att & pos.king_square(pos.side_to_move()))
1412 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1420 // prevents_move() tests whether a move (first) is able to defend against an
1421 // opponent's move (second). In this case will not be pruned. Normally the
1422 // second move is the threat move (the best move returned from a null search
1425 bool prevents_move(const Position& pos, Move first, Move second) {
1427 assert(is_ok(first));
1428 assert(is_ok(second));
1430 Square m1from = from_sq(first);
1431 Square m2from = from_sq(second);
1432 Square m1to = to_sq(first);
1433 Square m2to = to_sq(second);
1435 // Don't prune moves of the threatened piece
1439 // If the threatened piece has value less than or equal to the value of the
1440 // threat piece, don't prune moves which defend it.
1441 if ( pos.is_capture(second)
1442 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1443 || type_of(pos.piece_on(m2from)) == KING))
1445 // Update occupancy as if the piece and the threat are moving
1446 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1447 Piece piece = pos.piece_on(m1from);
1449 // The moved piece attacks the square 'tto' ?
1450 if (pos.attacks_from(piece, m1to, occ) & m2to)
1453 // Scan for possible X-ray attackers behind the moved piece
1454 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1455 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1457 // Verify attackers are triggered by our move and not already existing
1458 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1462 // Don't prune safe moves which block the threat path
1463 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1470 // When playing with strength handicap choose best move among the MultiPV set
1471 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1473 Move Skill::pick_move() {
1477 // PRNG sequence should be not deterministic
1478 for (int i = Time::now() % 50; i > 0; i--)
1479 rk.rand<unsigned>();
1481 // RootMoves are already sorted by score in descending order
1482 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1483 int weakness = 120 - 2 * level;
1484 int max_s = -VALUE_INFINITE;
1487 // Choose best move. For each move score we add two terms both dependent on
1488 // weakness, one deterministic and bigger for weaker moves, and one random,
1489 // then we choose the move with the resulting highest score.
1490 for (size_t i = 0; i < PVSize; i++)
1492 int s = RootMoves[i].score;
1494 // Don't allow crazy blunders even at very low skills
1495 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1498 // This is our magic formula
1499 s += ( weakness * int(RootMoves[0].score - s)
1500 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1505 best = RootMoves[i].pv[0];
1512 // uci_pv() formats PV information according to UCI protocol. UCI requires
1513 // to send all the PV lines also if are still to be searched and so refer to
1514 // the previous search score.
1516 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1518 std::stringstream s;
1519 Time::point elaspsed = Time::now() - SearchTime + 1;
1520 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1523 for (size_t i = 0; i < Threads.size(); i++)
1524 if (Threads[i].maxPly > selDepth)
1525 selDepth = Threads[i].maxPly;
1527 for (size_t i = 0; i < uciPVSize; i++)
1529 bool updated = (i <= PVIdx);
1531 if (depth == 1 && !updated)
1534 int d = updated ? depth : depth - 1;
1535 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1537 if (s.rdbuf()->in_avail()) // Not at first line
1540 s << "info depth " << d
1541 << " seldepth " << selDepth
1542 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1543 << " nodes " << pos.nodes_searched()
1544 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1545 << " time " << elaspsed
1546 << " multipv " << i + 1
1549 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1550 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1559 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1560 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1561 /// allow to always have a ponder move even when we fail high at root, and a
1562 /// long PV to print that is important for position analysis.
1564 void RootMove::extract_pv_from_tt(Position& pos) {
1566 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1576 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1578 pos.do_move(pv[ply++], *st++);
1579 tte = TT.probe(pos.key());
1582 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1583 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1585 && (!pos.is_draw<true, true>() || ply < 2));
1587 pv.push_back(MOVE_NONE); // Must be zero-terminating
1589 while (ply) pos.undo_move(pv[--ply]);
1593 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1594 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1595 /// first, even if the old TT entries have been overwritten.
1597 void RootMove::insert_pv_in_tt(Position& pos) {
1599 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1604 tte = TT.probe(pos.key());
1606 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1607 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1609 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1611 pos.do_move(pv[ply++], *st++);
1613 } while (pv[ply] != MOVE_NONE);
1615 while (ply) pos.undo_move(pv[--ply]);
1619 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1621 void Thread::idle_loop() {
1623 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1624 // object for which the thread is the master.
1625 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1627 assert(!sp_master || (sp_master->master == this && is_searching));
1629 // If this thread is the master of a split point and all slaves have
1630 // finished their work at this split point, return from the idle loop.
1631 while (!sp_master || sp_master->slavesMask)
1633 // If we are not searching, wait for a condition to be signaled
1634 // instead of wasting CPU time polling for work.
1637 || (!is_searching && Threads.use_sleeping_threads()))
1645 // Grab the lock to avoid races with Thread::wake_up()
1648 // If we are master and all slaves have finished don't go to sleep
1649 if (sp_master && !sp_master->slavesMask)
1655 // Do sleep after retesting sleep conditions under lock protection, in
1656 // particular we need to avoid a deadlock in case a master thread has,
1657 // in the meanwhile, allocated us and sent the wake_up() call before we
1658 // had the chance to grab the lock.
1659 if (do_sleep || !is_searching)
1660 sleepCondition.wait(mutex);
1665 // If this thread has been assigned work, launch a search
1668 assert(!do_sleep && !do_exit);
1670 Threads.mutex.lock();
1672 assert(is_searching);
1673 SplitPoint* sp = curSplitPoint;
1675 Threads.mutex.unlock();
1677 Stack ss[MAX_PLY_PLUS_2];
1678 Position pos(*sp->pos, this);
1680 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1685 assert(sp->activePositions[idx] == NULL);
1687 sp->activePositions[idx] = &pos;
1689 if (sp->nodeType == Root)
1690 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1691 else if (sp->nodeType == PV)
1692 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1693 else if (sp->nodeType == NonPV)
1694 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1698 assert(is_searching);
1700 is_searching = false;
1701 sp->activePositions[idx] = NULL;
1702 sp->slavesMask &= ~(1ULL << idx);
1703 sp->nodes += pos.nodes_searched();
1705 // Wake up master thread so to allow it to return from the idle loop in
1706 // case we are the last slave of the split point.
1707 if ( Threads.use_sleeping_threads()
1708 && this != sp->master
1711 assert(!sp->master->is_searching);
1712 sp->master->notify_one();
1715 // After releasing the lock we cannot access anymore any SplitPoint
1716 // related data in a safe way becuase it could have been released under
1717 // our feet by the sp master. Also accessing other Thread objects is
1718 // unsafe because if we are exiting there is a chance are already freed.
1725 /// check_time() is called by the timer thread when the timer triggers. It is
1726 /// used to print debug info and, more important, to detect when we are out of
1727 /// available time and so stop the search.
1731 static Time::point lastInfoTime = Time::now();
1732 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1734 if (Time::now() - lastInfoTime >= 1000)
1736 lastInfoTime = Time::now();
1745 Threads.mutex.lock();
1747 nodes = RootPos.nodes_searched();
1749 // Loop across all split points and sum accumulated SplitPoint nodes plus
1750 // all the currently active slaves positions.
1751 for (size_t i = 0; i < Threads.size(); i++)
1752 for (int j = 0; j < Threads[i].splitPointsCnt; j++)
1754 SplitPoint& sp = Threads[i].splitPoints[j];
1759 Bitboard sm = sp.slavesMask;
1762 Position* pos = sp.activePositions[pop_lsb(&sm)];
1763 nodes += pos ? pos->nodes_searched() : 0;
1769 Threads.mutex.unlock();
1772 Time::point elapsed = Time::now() - SearchTime;
1773 bool stillAtFirstMove = Signals.firstRootMove
1774 && !Signals.failedLowAtRoot
1775 && elapsed > TimeMgr.available_time();
1777 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1778 || stillAtFirstMove;
1780 if ( (Limits.use_time_management() && noMoreTime)
1781 || (Limits.movetime && elapsed >= Limits.movetime)
1782 || (Limits.nodes && nodes >= Limits.nodes))
1783 Signals.stop = true;