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[32]; // [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][64][64]; // [pv][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
82 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
88 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(const Position& pos, Move move, Value futilityBase, Value beta);
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][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.game_ply(), 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 the threads, still sleeping: will be wake up at split time
231 for (size_t i = 0; i < Threads.size(); i++)
232 Threads[i]->maxPly = 0;
234 Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
236 // Set best timer interval to avoid lagging under time pressure. Timer is
237 // used to check for remaining available thinking time.
238 Threads.timer->msec =
239 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
240 Limits.nodes ? 2 * TimerResolution
243 Threads.timer->notify_one(); // Wake up the recurring timer
245 id_loop(RootPos); // Let's start searching !
247 Threads.timer->msec = 0; // Stop the timer
248 Threads.sleepWhileIdle = true; // Send idle threads to sleep
250 if (Options["Use Search Log"])
252 Time::point elapsed = Time::now() - SearchTime + 1;
254 Log log(Options["Search Log Filename"]);
255 log << "Nodes: " << RootPos.nodes_searched()
256 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
257 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
260 RootPos.do_move(RootMoves[0].pv[0], st);
261 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
262 RootPos.undo_move(RootMoves[0].pv[0]);
267 // When we reach max depth we arrive here even without Signals.stop is raised,
268 // but if we are pondering or in infinite search, according to UCI protocol,
269 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
270 // command. We simply wait here until GUI sends one of those commands (that
271 // raise Signals.stop).
272 if (!Signals.stop && (Limits.ponder || Limits.infinite))
274 Signals.stopOnPonderhit = true;
275 RootPos.this_thread()->wait_for(Signals.stop);
278 // Best move could be MOVE_NONE when searching on a stalemate position
279 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
280 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
287 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
288 // with increasing depth until the allocated thinking time has been consumed,
289 // user stops the search, or the maximum search depth is reached.
291 void id_loop(Position& pos) {
293 Stack ss[MAX_PLY_PLUS_2];
294 int depth, prevBestMoveChanges;
295 Value bestValue, alpha, beta, delta;
297 memset(ss, 0, 4 * sizeof(Stack));
298 depth = BestMoveChanges = 0;
299 bestValue = delta = -VALUE_INFINITE;
300 ss->currentMove = MOVE_NULL; // Hack to skip update gains
305 PVSize = Options["MultiPV"];
306 Skill skill(Options["Skill Level"]);
308 // Do we have to play with skill handicap? In this case enable MultiPV search
309 // that we will use behind the scenes to retrieve a set of possible moves.
310 if (skill.enabled() && PVSize < 4)
313 PVSize = std::min(PVSize, RootMoves.size());
315 // Iterative deepening loop until requested to stop or target depth reached
316 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
318 // Save last iteration's scores before first PV line is searched and all
319 // the move scores but the (new) PV are set to -VALUE_INFINITE.
320 for (size_t i = 0; i < RootMoves.size(); i++)
321 RootMoves[i].prevScore = RootMoves[i].score;
323 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
326 // MultiPV loop. We perform a full root search for each PV line
327 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
329 // Set aspiration window default width
330 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
333 alpha = RootMoves[PVIdx].prevScore - delta;
334 beta = RootMoves[PVIdx].prevScore + delta;
338 alpha = -VALUE_INFINITE;
339 beta = VALUE_INFINITE;
342 // Start with a small aspiration window and, in case of fail high/low,
343 // research with bigger window until not failing high/low anymore.
346 // Search starts from ss+1 to allow referencing (ss-1). This is
347 // needed by update gains and ss copy when splitting at Root.
348 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
350 // Bring to front the best move. It is critical that sorting is
351 // done with a stable algorithm because all the values but the first
352 // and eventually the new best one are set to -VALUE_INFINITE and
353 // we want to keep the same order for all the moves but the new
354 // PV that goes to the front. Note that in case of MultiPV search
355 // the already searched PV lines are preserved.
356 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
358 // Write PV back to transposition table in case the relevant
359 // entries have been overwritten during the search.
360 for (size_t i = 0; i <= PVIdx; i++)
361 RootMoves[i].insert_pv_in_tt(pos);
363 // If search has been stopped return immediately. Sorting and
364 // writing PV back to TT is safe becuase RootMoves is still
365 // valid, although refers to previous iteration.
369 // In case of failing high/low increase aspiration window and
370 // research, otherwise exit the loop.
371 if (bestValue > alpha && bestValue < beta)
374 // Give some update (without cluttering the UI) before to research
375 if (Time::now() - SearchTime > 3000)
376 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
378 if (abs(bestValue) >= VALUE_KNOWN_WIN)
380 alpha = -VALUE_INFINITE;
381 beta = VALUE_INFINITE;
383 else if (bestValue >= beta)
390 Signals.failedLowAtRoot = true;
391 Signals.stopOnPonderhit = false;
397 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
400 // Sort the PV lines searched so far and update the GUI
401 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
403 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
404 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
407 // Do we need to pick now the sub-optimal best move ?
408 if (skill.enabled() && skill.time_to_pick(depth))
411 if (Options["Use Search Log"])
413 Log log(Options["Search Log Filename"]);
414 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
418 // Do we have found a "mate in x"?
420 && bestValue >= VALUE_MATE_IN_MAX_PLY
421 && VALUE_MATE - bestValue <= 2 * Limits.mate)
424 // Do we have time for the next iteration? Can we stop searching now?
425 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
427 bool stop = false; // Local variable, not the volatile Signals.stop
429 // Take in account some extra time if the best move has changed
430 if (depth > 4 && depth < 50 && PVSize == 1)
431 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
433 // Stop search if most of available time is already consumed. We
434 // probably don't have enough time to search the first move at the
435 // next iteration anyway.
436 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
439 // Stop search early if one move seems to be much better than others
443 && bestValue > VALUE_MATED_IN_MAX_PLY
444 && ( RootMoves.size() == 1
445 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
447 Value rBeta = bestValue - 2 * PawnValueMg;
448 (ss+1)->excludedMove = RootMoves[0].pv[0];
449 (ss+1)->skipNullMove = true;
450 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
451 (ss+1)->skipNullMove = false;
452 (ss+1)->excludedMove = MOVE_NONE;
460 // If we are allowed to ponder do not stop the search now but
461 // keep pondering until GUI sends "ponderhit" or "stop".
463 Signals.stopOnPonderhit = true;
472 // search<>() is the main search function for both PV and non-PV nodes and for
473 // normal and SplitPoint nodes. When called just after a split point the search
474 // is simpler because we have already probed the hash table, done a null move
475 // search, and searched the first move before splitting, we don't have to repeat
476 // all this work again. We also don't need to store anything to the hash table
477 // here: This is taken care of after we return from the split point.
479 template <NodeType NT>
480 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
482 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
483 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
484 const bool RootNode = (NT == Root || NT == SplitPointRoot);
486 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
487 assert(PvNode || (alpha == beta - 1));
488 assert(depth > DEPTH_ZERO);
490 Move movesSearched[64];
493 SplitPoint* splitPoint;
495 Move ttMove, move, excludedMove, bestMove, threatMove;
497 Value bestValue, value, ttValue;
498 Value eval, nullValue, futilityValue;
499 bool inCheck, givesCheck, pvMove, singularExtensionNode;
500 bool captureOrPromotion, dangerous, doFullDepthSearch;
501 int moveCount, playedMoveCount;
503 // Step 1. Initialize node
504 Thread* thisThread = pos.this_thread();
505 moveCount = playedMoveCount = 0;
506 inCheck = pos.checkers();
510 splitPoint = ss->splitPoint;
511 bestMove = splitPoint->bestMove;
512 threatMove = splitPoint->threatMove;
513 bestValue = splitPoint->bestValue;
515 ttMove = excludedMove = MOVE_NONE;
516 ttValue = VALUE_NONE;
518 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
520 goto split_point_start;
523 bestValue = -VALUE_INFINITE;
524 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
525 ss->ply = (ss-1)->ply + 1;
526 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
527 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
529 // Used to send selDepth info to GUI
530 if (PvNode && thisThread->maxPly < ss->ply)
531 thisThread->maxPly = ss->ply;
535 // Step 2. Check for aborted search and immediate draw
536 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
537 return DrawValue[pos.side_to_move()];
539 // Step 3. Mate distance pruning. Even if we mate at the next move our score
540 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
541 // a shorter mate was found upward in the tree then there is no need to search
542 // further, we will never beat current alpha. Same logic but with reversed signs
543 // applies also in the opposite condition of being mated instead of giving mate,
544 // in this case return a fail-high score.
545 alpha = std::max(mated_in(ss->ply), alpha);
546 beta = std::min(mate_in(ss->ply+1), beta);
551 // Step 4. Transposition table lookup
552 // We don't want the score of a partial search to overwrite a previous full search
553 // TT value, so we use a different position key in case of an excluded move.
554 excludedMove = ss->excludedMove;
555 posKey = excludedMove ? pos.exclusion_key() : pos.key();
556 tte = TT.probe(posKey);
557 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
558 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
560 // At PV nodes we check for exact scores, while at non-PV nodes we check for
561 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
562 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
563 // we should also update RootMoveList to avoid bogus output.
566 && tte->depth() >= depth
567 && ttValue != VALUE_NONE // Only in case of TT access race
568 && ( PvNode ? tte->type() == BOUND_EXACT
569 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
570 : (tte->type() & BOUND_UPPER)))
573 ss->currentMove = ttMove; // Can be MOVE_NONE
577 && !pos.is_capture_or_promotion(ttMove)
578 && ttMove != ss->killers[0])
580 ss->killers[1] = ss->killers[0];
581 ss->killers[0] = ttMove;
586 // Step 5. Evaluate the position statically and update parent's gain statistics
588 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
592 // Never assume anything on values stored in TT
593 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
594 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
595 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
597 // Can ttValue be used as a better position evaluation?
598 if (ttValue != VALUE_NONE)
599 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
600 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
605 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
606 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
607 ss->staticEval, ss->evalMargin);
610 // Update gain for the parent non-capture move given the static position
611 // evaluation before and after the move.
612 if ( (move = (ss-1)->currentMove) != MOVE_NULL
613 && (ss-1)->staticEval != VALUE_NONE
614 && ss->staticEval != VALUE_NONE
615 && !pos.captured_piece_type()
616 && type_of(move) == NORMAL)
618 Square to = to_sq(move);
619 Gain.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
622 // Step 6. Razoring (is omitted in PV nodes)
624 && depth < 4 * ONE_PLY
626 && eval + razor_margin(depth) < beta
627 && ttMove == MOVE_NONE
628 && abs(beta) < VALUE_MATE_IN_MAX_PLY
629 && !pos.pawn_on_7th(pos.side_to_move()))
631 Value rbeta = beta - razor_margin(depth);
632 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
634 // Logically we should return (v + razor_margin(depth)), but
635 // surprisingly this did slightly weaker in tests.
639 // Step 7. Static null move pruning (is omitted in PV nodes)
640 // We're betting that the opponent doesn't have a move that will reduce
641 // the score by more than futility_margin(depth) if we do a null move.
644 && depth < 4 * ONE_PLY
646 && eval - FutilityMargins[depth][0] >= beta
647 && abs(beta) < VALUE_MATE_IN_MAX_PLY
648 && pos.non_pawn_material(pos.side_to_move()))
649 return eval - FutilityMargins[depth][0];
651 // Step 8. Null move search with verification search (is omitted in PV nodes)
657 && abs(beta) < VALUE_MATE_IN_MAX_PLY
658 && pos.non_pawn_material(pos.side_to_move()))
660 ss->currentMove = MOVE_NULL;
662 // Null move dynamic reduction based on depth
663 Depth R = 3 * ONE_PLY + depth / 4;
665 // Null move dynamic reduction based on value
666 if (eval - PawnValueMg > beta)
669 pos.do_null_move(st);
670 (ss+1)->skipNullMove = true;
671 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
672 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
673 (ss+1)->skipNullMove = false;
674 pos.undo_null_move();
676 if (nullValue >= beta)
678 // Do not return unproven mate scores
679 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
682 if (depth < 12 * ONE_PLY)
685 // Do verification search at high depths
686 ss->skipNullMove = true;
687 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
688 ss->skipNullMove = false;
695 // The null move failed low, which means that we may be faced with
696 // some kind of threat. If the previous move was reduced, check if
697 // the move that refuted the null move was somehow connected to the
698 // move which was reduced. If a connection is found, return a fail
699 // low score (which will cause the reduced move to fail high in the
700 // parent node, which will trigger a re-search with full depth).
701 threatMove = (ss+1)->currentMove;
703 if ( depth < 5 * ONE_PLY
705 && threatMove != MOVE_NONE
706 && allows(pos, (ss-1)->currentMove, threatMove))
711 // Step 9. ProbCut (is omitted in PV nodes)
712 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
713 // and a reduced search returns a value much above beta, we can (almost) safely
714 // prune the previous move.
716 && depth >= 5 * ONE_PLY
719 && excludedMove == MOVE_NONE
720 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
722 Value rbeta = beta + 200;
723 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
725 assert(rdepth >= ONE_PLY);
726 assert((ss-1)->currentMove != MOVE_NONE);
727 assert((ss-1)->currentMove != MOVE_NULL);
729 MovePicker mp(pos, ttMove, Hist, pos.captured_piece_type());
732 while ((move = mp.next_move<false>()) != MOVE_NONE)
733 if (pos.pl_move_is_legal(move, ci.pinned))
735 ss->currentMove = move;
736 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
737 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
744 // Step 10. Internal iterative deepening
745 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
746 && ttMove == MOVE_NONE
747 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
749 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
751 ss->skipNullMove = true;
752 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
753 ss->skipNullMove = false;
755 tte = TT.probe(posKey);
756 ttMove = tte ? tte->move() : MOVE_NONE;
759 split_point_start: // At split points actual search starts from here
761 MovePicker mp(pos, ttMove, depth, Hist, ss, PvNode ? -VALUE_INFINITE : beta);
763 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
764 singularExtensionNode = !RootNode
766 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
767 && ttMove != MOVE_NONE
768 && !excludedMove // Recursive singular search is not allowed
769 && (tte->type() & BOUND_LOWER)
770 && tte->depth() >= depth - 3 * ONE_PLY;
772 // Step 11. Loop through moves
773 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
774 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
778 if (move == excludedMove)
781 // At root obey the "searchmoves" option and skip moves not listed in Root
782 // Move List, as a consequence any illegal move is also skipped. In MultiPV
783 // mode we also skip PV moves which have been already searched.
784 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
789 // Shared counter cannot be decremented later if move turns out to be illegal
790 if (!pos.pl_move_is_legal(move, ci.pinned))
793 moveCount = ++splitPoint->moveCount;
794 splitPoint->mutex.unlock();
801 Signals.firstRootMove = (moveCount == 1);
803 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
804 sync_cout << "info depth " << depth / ONE_PLY
805 << " currmove " << move_to_uci(move, pos.is_chess960())
806 << " currmovenumber " << moveCount + PVIdx << sync_endl;
810 captureOrPromotion = pos.is_capture_or_promotion(move);
811 givesCheck = pos.move_gives_check(move, ci);
812 dangerous = givesCheck
813 || pos.is_passed_pawn_push(move)
814 || type_of(move) == CASTLE
815 || ( captureOrPromotion // Entering a pawn endgame?
816 && type_of(pos.piece_on(to_sq(move))) != PAWN
817 && type_of(move) == NORMAL
818 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
819 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
821 // Step 12. Extend checks and, in PV nodes, also dangerous moves
822 if (PvNode && dangerous)
825 else if (givesCheck && pos.see_sign(move) >= 0)
828 // Singular extension search. If all moves but one fail low on a search of
829 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
830 // is singular and should be extended. To verify this we do a reduced search
831 // on all the other moves but the ttMove, if result is lower than ttValue minus
832 // a margin then we extend ttMove.
833 if ( singularExtensionNode
836 && pos.pl_move_is_legal(move, ci.pinned)
837 && abs(ttValue) < VALUE_KNOWN_WIN)
839 assert(ttValue != VALUE_NONE);
841 Value rBeta = ttValue - int(depth);
842 ss->excludedMove = move;
843 ss->skipNullMove = true;
844 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
845 ss->skipNullMove = false;
846 ss->excludedMove = MOVE_NONE;
852 // Update current move (this must be done after singular extension search)
853 newDepth = depth - ONE_PLY + ext;
855 // Step 13. Futility pruning (is omitted in PV nodes)
857 && !captureOrPromotion
860 /* && move != ttMove Already implicit in the next condition */
861 && bestValue > VALUE_MATED_IN_MAX_PLY)
863 // Move count based pruning
864 if ( depth < 16 * ONE_PLY
865 && moveCount >= FutilityMoveCounts[depth]
866 && (!threatMove || !refutes(pos, move, threatMove)))
869 splitPoint->mutex.lock();
874 // Value based pruning
875 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
876 // but fixing this made program slightly weaker.
877 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
878 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
879 + Gain[pos.piece_moved(move)][to_sq(move)];
881 if (futilityValue < beta)
883 bestValue = std::max(bestValue, futilityValue);
887 splitPoint->mutex.lock();
888 if (bestValue > splitPoint->bestValue)
889 splitPoint->bestValue = bestValue;
894 // Prune moves with negative SEE at low depths
895 if ( predictedDepth < 4 * ONE_PLY
896 && pos.see_sign(move) < 0)
899 splitPoint->mutex.lock();
905 // Check for legality only before to do the move
906 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
912 pvMove = PvNode && moveCount == 1;
913 ss->currentMove = move;
914 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
915 movesSearched[playedMoveCount++] = move;
917 // Step 14. Make the move
918 pos.do_move(move, st, ci, givesCheck);
920 // Step 15. Reduced depth search (LMR). If the move fails high will be
921 // re-searched at full depth.
922 if ( depth > 3 * ONE_PLY
924 && !captureOrPromotion
927 && move != ss->killers[0]
928 && move != ss->killers[1])
930 ss->reduction = reduction<PvNode>(depth, moveCount);
931 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
933 alpha = splitPoint->alpha;
935 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
937 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
938 ss->reduction = DEPTH_ZERO;
941 doFullDepthSearch = !pvMove;
943 // Step 16. Full depth search, when LMR is skipped or fails high
944 if (doFullDepthSearch)
947 alpha = splitPoint->alpha;
949 value = newDepth < ONE_PLY ?
950 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
951 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
952 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
955 // Only for PV nodes do a full PV search on the first move or after a fail
956 // high, in the latter case search only if value < beta, otherwise let the
957 // parent node to fail low with value <= alpha and to try another move.
958 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
959 value = newDepth < ONE_PLY ?
960 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
961 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
962 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
963 // Step 17. Undo move
966 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
968 // Step 18. Check for new best move
971 splitPoint->mutex.lock();
972 bestValue = splitPoint->bestValue;
973 alpha = splitPoint->alpha;
976 // Finished searching the move. If Signals.stop is true, the search
977 // was aborted because the user interrupted the search or because we
978 // ran out of time. In this case, the return value of the search cannot
979 // be trusted, and we don't update the best move and/or PV.
980 if (Signals.stop || thisThread->cutoff_occurred())
981 return value; // To avoid returning VALUE_INFINITE
985 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
987 // PV move or new best move ?
988 if (pvMove || value > alpha)
991 rm.extract_pv_from_tt(pos);
993 // We record how often the best move has been changed in each
994 // iteration. This information is used for time management: When
995 // the best move changes frequently, we allocate some more time.
1000 // All other moves but the PV are set to the lowest value, this
1001 // is not a problem when sorting becuase sort is stable and move
1002 // position in the list is preserved, just the PV is pushed up.
1003 rm.score = -VALUE_INFINITE;
1006 if (value > bestValue)
1008 bestValue = SpNode ? splitPoint->bestValue = value : value;
1012 bestMove = SpNode ? splitPoint->bestMove = move : move;
1014 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1015 alpha = SpNode ? splitPoint->alpha = value : value;
1018 assert(value >= beta); // Fail high
1021 splitPoint->cutoff = true;
1028 // Step 19. Check for splitting the search
1030 && depth >= Threads.minimumSplitDepth
1031 && Threads.available_slave(thisThread)
1032 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1034 assert(bestValue < beta);
1036 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1037 depth, threatMove, moveCount, &mp, NT);
1038 if (bestValue >= beta)
1046 // Step 20. Check for mate and stalemate
1047 // All legal moves have been searched and if there are no legal moves, it
1048 // must be mate or stalemate. Note that we can have a false positive in
1049 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1050 // harmless because return value is discarded anyhow in the parent nodes.
1051 // If we are in a singular extension search then return a fail low score.
1052 // A split node has at least one move, the one tried before to be splitted.
1054 return excludedMove ? alpha
1055 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1057 // If we have pruned all the moves without searching return a fail-low score
1058 if (bestValue == -VALUE_INFINITE)
1060 assert(!playedMoveCount);
1065 if (bestValue >= beta) // Failed high
1067 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1068 bestMove, ss->staticEval, ss->evalMargin);
1070 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1072 if (bestMove != ss->killers[0])
1074 ss->killers[1] = ss->killers[0];
1075 ss->killers[0] = bestMove;
1078 // Increase history value of the cut-off move
1079 Value bonus = Value(int(depth) * int(depth));
1080 Hist.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1082 // Decrease history of all the other played non-capture moves
1083 for (int i = 0; i < playedMoveCount - 1; i++)
1085 Move m = movesSearched[i];
1086 Hist.update(pos.piece_moved(m), to_sq(m), -bonus);
1090 else // Failed low or PV search
1091 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1092 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1093 depth, bestMove, ss->staticEval, ss->evalMargin);
1095 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1101 // qsearch() is the quiescence search function, which is called by the main
1102 // search function when the remaining depth is zero (or, to be more precise,
1103 // less than ONE_PLY).
1105 template <NodeType NT, bool InCheck>
1106 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1108 const bool PvNode = (NT == PV);
1110 assert(NT == PV || NT == NonPV);
1111 assert(InCheck == !!pos.checkers());
1112 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1113 assert(PvNode || (alpha == beta - 1));
1114 assert(depth <= DEPTH_ZERO);
1119 Move ttMove, move, bestMove;
1120 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1121 bool givesCheck, enoughMaterial, evasionPrunable;
1124 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1128 ss->currentMove = bestMove = MOVE_NONE;
1129 ss->ply = (ss-1)->ply + 1;
1131 // Check for an instant draw or maximum ply reached
1132 if (pos.is_draw() || ss->ply > MAX_PLY)
1133 return DrawValue[pos.side_to_move()];
1135 // Decide whether or not to include checks, this fixes also the type of
1136 // TT entry depth that we are going to use. Note that in qsearch we use
1137 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1138 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1139 : DEPTH_QS_NO_CHECKS;
1141 // Transposition table lookup. At PV nodes, we don't use the TT for
1142 // pruning, but only for move ordering.
1144 tte = TT.probe(posKey);
1145 ttMove = tte ? tte->move() : MOVE_NONE;
1146 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1149 && tte->depth() >= ttDepth
1150 && ttValue != VALUE_NONE // Only in case of TT access race
1151 && ( PvNode ? tte->type() == BOUND_EXACT
1152 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1153 : (tte->type() & BOUND_UPPER)))
1155 ss->currentMove = ttMove; // Can be MOVE_NONE
1159 // Evaluate the position statically
1162 ss->staticEval = ss->evalMargin = VALUE_NONE;
1163 bestValue = futilityBase = -VALUE_INFINITE;
1164 enoughMaterial = false;
1170 // Never assume anything on values stored in TT
1171 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1172 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1173 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1176 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1178 // Stand pat. Return immediately if static value is at least beta
1179 if (bestValue >= beta)
1182 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1183 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1188 if (PvNode && bestValue > alpha)
1191 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1192 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1195 // Initialize a MovePicker object for the current position, and prepare
1196 // to search the moves. Because the depth is <= 0 here, only captures,
1197 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1199 MovePicker mp(pos, ttMove, depth, Hist, to_sq((ss-1)->currentMove));
1202 // Loop through the moves until no moves remain or a beta cutoff occurs
1203 while ((move = mp.next_move<false>()) != MOVE_NONE)
1205 assert(is_ok(move));
1207 givesCheck = pos.move_gives_check(move, ci);
1215 && type_of(move) != PROMOTION
1216 && !pos.is_passed_pawn_push(move))
1218 futilityValue = futilityBase
1219 + PieceValue[EG][pos.piece_on(to_sq(move))]
1220 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1222 if (futilityValue < beta)
1224 bestValue = std::max(bestValue, futilityValue);
1228 // Prune moves with negative or equal SEE and also moves with positive
1229 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1230 if ( futilityBase < beta
1231 && depth < DEPTH_ZERO
1232 && pos.see(move, beta - futilityBase) <= 0)
1234 bestValue = std::max(bestValue, futilityBase);
1239 // Detect non-capture evasions that are candidate to be pruned
1240 evasionPrunable = !PvNode
1242 && bestValue > VALUE_MATED_IN_MAX_PLY
1243 && !pos.is_capture(move)
1244 && !pos.can_castle(pos.side_to_move());
1246 // Don't search moves with negative SEE values
1248 && (!InCheck || evasionPrunable)
1250 && type_of(move) != PROMOTION
1251 && pos.see_sign(move) < 0)
1254 // Don't search useless checks
1259 && !pos.is_capture_or_promotion(move)
1260 && ss->staticEval + PawnValueMg / 4 < beta
1261 && !check_is_dangerous(pos, move, futilityBase, beta))
1264 // Check for legality only before to do the move
1265 if (!pos.pl_move_is_legal(move, ci.pinned))
1268 ss->currentMove = move;
1270 // Make and search the move
1271 pos.do_move(move, st, ci, givesCheck);
1272 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1273 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1274 pos.undo_move(move);
1276 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1278 // Check for new best move
1279 if (value > bestValue)
1285 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1292 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1293 ttDepth, move, ss->staticEval, ss->evalMargin);
1301 // All legal moves have been searched. A special case: If we're in check
1302 // and no legal moves were found, it is checkmate.
1303 if (InCheck && bestValue == -VALUE_INFINITE)
1304 return mated_in(ss->ply); // Plies to mate from the root
1306 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1307 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1308 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1310 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1316 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1317 // "plies to mate from the current position". Non-mate scores are unchanged.
1318 // The function is called before storing a value to the transposition table.
1320 Value value_to_tt(Value v, int ply) {
1322 assert(v != VALUE_NONE);
1324 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1325 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1329 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1330 // from the transposition table (where refers to the plies to mate/be mated
1331 // from current position) to "plies to mate/be mated from the root".
1333 Value value_from_tt(Value v, int ply) {
1335 return v == VALUE_NONE ? VALUE_NONE
1336 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1337 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1341 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1343 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1345 Piece pc = pos.piece_moved(move);
1346 Square from = from_sq(move);
1347 Square to = to_sq(move);
1348 Color them = ~pos.side_to_move();
1349 Square ksq = pos.king_square(them);
1350 Bitboard enemies = pos.pieces(them);
1351 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1352 Bitboard occ = pos.pieces() ^ from ^ ksq;
1353 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1354 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1356 // Checks which give opponent's king at most one escape square are dangerous
1357 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1360 // Queen contact check is very dangerous
1361 if (type_of(pc) == QUEEN && (kingAtt & to))
1364 // Creating new double threats with checks is dangerous
1365 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1368 // Note that here we generate illegal "double move"!
1369 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1377 // allows() tests whether the 'first' move at previous ply somehow makes the
1378 // 'second' move possible, for instance if the moving piece is the same in
1379 // both moves. Normally the second move is the threat (the best move returned
1380 // from a null search that fails low).
1382 bool allows(const Position& pos, Move first, Move second) {
1384 assert(is_ok(first));
1385 assert(is_ok(second));
1386 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1387 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1389 Square m1from = from_sq(first);
1390 Square m2from = from_sq(second);
1391 Square m1to = to_sq(first);
1392 Square m2to = to_sq(second);
1394 // The piece is the same or second's destination was vacated by the first move
1395 if (m1to == m2from || m2to == m1from)
1398 // Second one moves through the square vacated by first one
1399 if (between_bb(m2from, m2to) & m1from)
1402 // Second's destination is defended by the first move's piece
1403 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1407 // Second move gives a discovered check through the first's checking piece
1408 if (m1att & pos.king_square(pos.side_to_move()))
1410 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1418 // refutes() tests whether a 'first' move is able to defend against a 'second'
1419 // opponent's move. In this case will not be pruned. Normally the second move
1420 // is the threat (the best move returned from a null search that fails low).
1422 bool refutes(const Position& pos, Move first, Move second) {
1424 assert(is_ok(first));
1425 assert(is_ok(second));
1427 Square m1from = from_sq(first);
1428 Square m2from = from_sq(second);
1429 Square m1to = to_sq(first);
1430 Square m2to = to_sq(second);
1432 // Don't prune moves of the threatened piece
1436 // If the threatened piece has value less than or equal to the value of the
1437 // threat piece, don't prune moves which defend it.
1438 if ( pos.is_capture(second)
1439 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1440 || type_of(pos.piece_on(m2from)) == KING))
1442 // Update occupancy as if the piece and the threat are moving
1443 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1444 Piece piece = pos.piece_on(m1from);
1446 // The moved piece attacks the square 'tto' ?
1447 if (pos.attacks_from(piece, m1to, occ) & m2to)
1450 // Scan for possible X-ray attackers behind the moved piece
1451 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1452 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1454 // Verify attackers are triggered by our move and not already existing
1455 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1459 // Don't prune safe moves which block the threat path
1460 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1467 // When playing with strength handicap choose best move among the MultiPV set
1468 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1470 Move Skill::pick_move() {
1474 // PRNG sequence should be not deterministic
1475 for (int i = Time::now() % 50; i > 0; i--)
1476 rk.rand<unsigned>();
1478 // RootMoves are already sorted by score in descending order
1479 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1480 int weakness = 120 - 2 * level;
1481 int max_s = -VALUE_INFINITE;
1484 // Choose best move. For each move score we add two terms both dependent on
1485 // weakness, one deterministic and bigger for weaker moves, and one random,
1486 // then we choose the move with the resulting highest score.
1487 for (size_t i = 0; i < PVSize; i++)
1489 int s = RootMoves[i].score;
1491 // Don't allow crazy blunders even at very low skills
1492 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1495 // This is our magic formula
1496 s += ( weakness * int(RootMoves[0].score - s)
1497 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1502 best = RootMoves[i].pv[0];
1509 // uci_pv() formats PV information according to UCI protocol. UCI requires
1510 // to send all the PV lines also if are still to be searched and so refer to
1511 // the previous search score.
1513 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1515 std::stringstream s;
1516 Time::point elaspsed = Time::now() - SearchTime + 1;
1517 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1520 for (size_t i = 0; i < Threads.size(); i++)
1521 if (Threads[i]->maxPly > selDepth)
1522 selDepth = Threads[i]->maxPly;
1524 for (size_t i = 0; i < uciPVSize; i++)
1526 bool updated = (i <= PVIdx);
1528 if (depth == 1 && !updated)
1531 int d = updated ? depth : depth - 1;
1532 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1534 if (s.rdbuf()->in_avail()) // Not at first line
1537 s << "info depth " << d
1538 << " seldepth " << selDepth
1539 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1540 << " nodes " << pos.nodes_searched()
1541 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1542 << " time " << elaspsed
1543 << " multipv " << i + 1
1546 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1547 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1556 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1557 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1558 /// allow to always have a ponder move even when we fail high at root, and a
1559 /// long PV to print that is important for position analysis.
1561 void RootMove::extract_pv_from_tt(Position& pos) {
1563 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1573 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1575 pos.do_move(pv[ply++], *st++);
1576 tte = TT.probe(pos.key());
1579 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1580 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1582 && (!pos.is_draw() || ply < 2));
1584 pv.push_back(MOVE_NONE); // Must be zero-terminating
1586 while (ply) pos.undo_move(pv[--ply]);
1590 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1591 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1592 /// first, even if the old TT entries have been overwritten.
1594 void RootMove::insert_pv_in_tt(Position& pos) {
1596 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1601 tte = TT.probe(pos.key());
1603 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1604 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1606 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1608 pos.do_move(pv[ply++], *st++);
1610 } while (pv[ply] != MOVE_NONE);
1612 while (ply) pos.undo_move(pv[--ply]);
1616 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1618 void Thread::idle_loop() {
1620 // Pointer 'this_sp' is not null only if we are called from split(), and not
1621 // at the thread creation. So it means we are the split point's master.
1622 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1624 assert(!this_sp || (this_sp->masterThread == this && searching));
1628 // If we are not searching, wait for a condition to be signaled instead of
1629 // wasting CPU time polling for work.
1630 while ((!searching && Threads.sleepWhileIdle) || exit)
1638 // Grab the lock to avoid races with Thread::notify_one()
1641 // If we are master and all slaves have finished then exit idle_loop
1642 if (this_sp && !this_sp->slavesMask)
1648 // Do sleep after retesting sleep conditions under lock protection, in
1649 // particular we need to avoid a deadlock in case a master thread has,
1650 // in the meanwhile, allocated us and sent the notify_one() call before
1651 // we had the chance to grab the lock.
1652 if (!searching && !exit)
1653 sleepCondition.wait(mutex);
1658 // If this thread has been assigned work, launch a search
1663 Threads.mutex.lock();
1666 SplitPoint* sp = activeSplitPoint;
1668 Threads.mutex.unlock();
1670 Stack ss[MAX_PLY_PLUS_2];
1671 Position pos(*sp->pos, this);
1673 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1674 (ss+1)->splitPoint = sp;
1678 assert(activePosition == NULL);
1680 activePosition = &pos;
1682 switch (sp->nodeType) {
1684 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1687 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1690 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1699 activePosition = NULL;
1700 sp->slavesMask &= ~(1ULL << idx);
1701 sp->nodes += pos.nodes_searched();
1703 // Wake up master thread so to allow it to return from the idle loop
1704 // in case we are the last slave of the split point.
1705 if ( Threads.sleepWhileIdle
1706 && this != sp->masterThread
1709 assert(!sp->masterThread->searching);
1710 sp->masterThread->notify_one();
1713 // After releasing the lock we cannot access anymore any SplitPoint
1714 // related data in a safe way becuase it could have been released under
1715 // our feet by the sp master. Also accessing other Thread objects is
1716 // unsafe because if we are exiting there is a chance are already freed.
1720 // If this thread is the master of a split point and all slaves have finished
1721 // their work at this split point, return from the idle loop.
1722 if (this_sp && !this_sp->slavesMask)
1724 this_sp->mutex.lock();
1725 bool finished = !this_sp->slavesMask; // Retest under lock protection
1726 this_sp->mutex.unlock();
1734 /// check_time() is called by the timer thread when the timer triggers. It is
1735 /// used to print debug info and, more important, to detect when we are out of
1736 /// available time and so stop the search.
1740 static Time::point lastInfoTime = Time::now();
1741 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1743 if (Time::now() - lastInfoTime >= 1000)
1745 lastInfoTime = Time::now();
1754 Threads.mutex.lock();
1756 nodes = RootPos.nodes_searched();
1758 // Loop across all split points and sum accumulated SplitPoint nodes plus
1759 // all the currently active positions nodes.
1760 for (size_t i = 0; i < Threads.size(); i++)
1761 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1763 SplitPoint& sp = Threads[i]->splitPoints[j];
1768 Bitboard sm = sp.slavesMask;
1771 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1773 nodes += pos->nodes_searched();
1779 Threads.mutex.unlock();
1782 Time::point elapsed = Time::now() - SearchTime;
1783 bool stillAtFirstMove = Signals.firstRootMove
1784 && !Signals.failedLowAtRoot
1785 && elapsed > TimeMgr.available_time();
1787 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1788 || stillAtFirstMove;
1790 if ( (Limits.use_time_management() && noMoreTime)
1791 || (Limits.movetime && elapsed >= Limits.movetime)
1792 || (Limits.nodes && nodes >= Limits.nodes))
1793 Signals.stop = true;