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/>.
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 // Different node types, used as template parameter
60 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
62 // Dynamic razoring margin based on depth
63 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
65 // Futility lookup tables (initialized at startup) and their access functions
66 Value FutilityMargins[16][64]; // [depth][moveNumber]
67 int FutilityMoveCounts[2][32]; // [improving][depth]
69 inline Value futility_margin(Depth d, int mn) {
71 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
75 // Reduction lookup tables (initialized at startup) and their access function
76 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
80 return (Depth) Reductions[PvNode][i][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
85 double BestMoveChanges;
86 Value DrawValue[COLOR_NB];
89 CountermovesStats Countermoves;
91 template <NodeType NT>
92 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
94 template <NodeType NT, bool InCheck>
95 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
97 void id_loop(Position& pos);
98 Value value_to_tt(Value v, int ply);
99 Value value_from_tt(Value v, int ply);
100 bool allows(const Position& pos, Move first, Move second);
101 bool refutes(const Position& pos, Move first, Move second);
102 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
105 Skill(int l) : level(l), best(MOVE_NONE) {}
107 if (enabled()) // Swap best PV line with the sub-optimal one
108 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
109 RootMoves.end(), best ? best : pick_move()));
112 bool enabled() const { return level < 20; }
113 bool time_to_pick(int depth) const { return depth == 1 + level; }
123 /// Search::init() is called during startup to initialize various lookup tables
125 void Search::init() {
127 int d; // depth (ONE_PLY == 2)
128 int hd; // half depth (ONE_PLY == 1)
131 // Init reductions array
132 for (hd = 1; hd < 64; ++hd) for (mc = 1; mc < 64; ++mc)
134 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
135 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
136 Reductions[1][1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
137 Reductions[0][1][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
139 Reductions[1][0][hd][mc] = Reductions[1][1][hd][mc];
140 Reductions[0][0][hd][mc] = Reductions[0][1][hd][mc];
142 if (Reductions[0][0][hd][mc] > 2 * ONE_PLY)
143 Reductions[0][0][hd][mc] += ONE_PLY;
145 else if (Reductions[0][0][hd][mc] > 1 * ONE_PLY)
146 Reductions[0][0][hd][mc] += ONE_PLY / 2;
149 // Init futility margins array
150 for (d = 1; d < 16; ++d) for (mc = 0; mc < 64; ++mc)
151 FutilityMargins[d][mc] = Value(112 * int(2.9 * log(double(d))) - 8 * mc + 45);
153 // Init futility move count array
154 for (d = 0; d < 32; ++d)
156 FutilityMoveCounts[0][d] = int(2.4 + 0.222 * pow(d + 0.0, 1.8));
157 FutilityMoveCounts[1][d] = int(3.0 + 0.3 * pow(d + 0.98, 1.8));
162 /// Search::perft() is our utility to verify move generation. All the leaf nodes
163 /// up to the given depth are generated and counted and the sum returned.
165 static size_t perft(Position& pos, Depth depth) {
170 const bool leaf = depth == 2 * ONE_PLY;
172 for (MoveList<LEGAL> it(pos); *it; ++it)
174 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
175 cnt += leaf ? MoveList<LEGAL>(pos).size() : ::perft(pos, depth - ONE_PLY);
181 size_t Search::perft(Position& pos, Depth depth) {
182 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
185 /// Search::think() is the external interface to Stockfish's search, and is
186 /// called by the main thread when the program receives the UCI 'go' command. It
187 /// searches from RootPos and at the end prints the "bestmove" to output.
189 void Search::think() {
191 static PolyglotBook book; // Defined static to initialize the PRNG only once
193 RootColor = RootPos.side_to_move();
194 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
196 if (RootMoves.empty())
198 RootMoves.push_back(MOVE_NONE);
199 sync_cout << "info depth 0 score "
200 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
206 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
208 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
210 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
212 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
217 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
219 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
220 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
221 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
222 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
225 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
227 if (Options["Write Search Log"])
229 Log log(Options["Search Log Filename"]);
230 log << "\nSearching: " << RootPos.fen()
231 << "\ninfinite: " << Limits.infinite
232 << " ponder: " << Limits.ponder
233 << " time: " << Limits.time[RootColor]
234 << " increment: " << Limits.inc[RootColor]
235 << " moves to go: " << Limits.movestogo
239 // Reset the threads, still sleeping: will be wake up at split time
240 for (size_t i = 0; i < Threads.size(); ++i)
241 Threads[i]->maxPly = 0;
243 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
244 Threads.timer->run = true;
245 Threads.timer->notify_one(); // Wake up the recurring timer
247 id_loop(RootPos); // Let's start searching !
249 Threads.timer->run = false; // Stop the timer
250 Threads.sleepWhileIdle = true; // Send idle threads to sleep
252 if (Options["Write Search Log"])
254 Time::point elapsed = Time::now() - SearchTime + 1;
256 Log log(Options["Search Log Filename"]);
257 log << "Nodes: " << RootPos.nodes_searched()
258 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
259 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
262 RootPos.do_move(RootMoves[0].pv[0], st);
263 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
264 RootPos.undo_move(RootMoves[0].pv[0]);
269 // When search is stopped this info is not printed
270 sync_cout << "info nodes " << RootPos.nodes_searched()
271 << " time " << Time::now() - SearchTime + 1 << sync_endl;
273 // When we reach max depth we arrive here even without Signals.stop is raised,
274 // but if we are pondering or in infinite search, according to UCI protocol,
275 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
276 // command. We simply wait here until GUI sends one of those commands (that
277 // raise Signals.stop).
278 if (!Signals.stop && (Limits.ponder || Limits.infinite))
280 Signals.stopOnPonderhit = true;
281 RootPos.this_thread()->wait_for(Signals.stop);
284 // Best move could be MOVE_NONE when searching on a stalemate position
285 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
286 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
293 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
294 // with increasing depth until the allocated thinking time has been consumed,
295 // user stops the search, or the maximum search depth is reached.
297 void id_loop(Position& pos) {
299 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
301 Value bestValue, alpha, beta, delta;
303 std::memset(ss-2, 0, 5 * sizeof(Stack));
304 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
308 bestValue = delta = alpha = -VALUE_INFINITE;
309 beta = VALUE_INFINITE;
314 Countermoves.clear();
316 PVSize = Options["MultiPV"];
317 Skill skill(Options["Skill Level"]);
319 // Do we have to play with skill handicap? In this case enable MultiPV search
320 // that we will use behind the scenes to retrieve a set of possible moves.
321 if (skill.enabled() && PVSize < 4)
324 PVSize = std::min(PVSize, RootMoves.size());
326 // Iterative deepening loop until requested to stop or target depth reached
327 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
329 // Age out PV variability metric
330 BestMoveChanges *= 0.8;
332 // Save last iteration's scores before first PV line is searched and all
333 // the move scores but the (new) PV are set to -VALUE_INFINITE.
334 for (size_t i = 0; i < RootMoves.size(); ++i)
335 RootMoves[i].prevScore = RootMoves[i].score;
337 // MultiPV loop. We perform a full root search for each PV line
338 for (PVIdx = 0; PVIdx < PVSize; ++PVIdx)
340 // Reset aspiration window starting size
344 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
345 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
348 // Start with a small aspiration window and, in case of fail high/low,
349 // research with bigger window until not failing high/low anymore.
352 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
354 // Bring to front the best move. It is critical that sorting is
355 // done with a stable algorithm because all the values but the first
356 // and eventually the new best one are set to -VALUE_INFINITE and
357 // we want to keep the same order for all the moves but the new
358 // PV that goes to the front. Note that in case of MultiPV search
359 // the already searched PV lines are preserved.
360 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
362 // Write PV back to transposition table in case the relevant
363 // entries have been overwritten during the search.
364 for (size_t i = 0; i <= PVIdx; ++i)
365 RootMoves[i].insert_pv_in_tt(pos);
367 // If search has been stopped return immediately. Sorting and
368 // writing PV back to TT is safe becuase RootMoves is still
369 // valid, although refers to previous iteration.
373 // When failing high/low give some update (without cluttering
374 // the UI) before to research.
375 if ( (bestValue <= alpha || bestValue >= beta)
376 && Time::now() - SearchTime > 3000)
377 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
379 // In case of failing low/high increase aspiration window and
380 // research, otherwise exit the loop.
381 if (bestValue <= alpha)
383 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
385 Signals.failedLowAtRoot = true;
386 Signals.stopOnPonderhit = false;
388 else if (bestValue >= beta)
389 beta = std::min(bestValue + delta, VALUE_INFINITE);
396 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
399 // Sort the PV lines searched so far and update the GUI
400 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
402 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
403 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
406 // Do we need to pick now the sub-optimal best move ?
407 if (skill.enabled() && skill.time_to_pick(depth))
410 if (Options["Write Search Log"])
412 RootMove& rm = RootMoves[0];
413 if (skill.best != MOVE_NONE)
414 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
416 Log log(Options["Search Log Filename"]);
417 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
421 // Do we have found a "mate in x"?
423 && bestValue >= VALUE_MATE_IN_MAX_PLY
424 && VALUE_MATE - bestValue <= 2 * Limits.mate)
427 // Do we have time for the next iteration? Can we stop searching now?
428 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
430 bool stop = false; // Local variable, not the volatile Signals.stop
432 // Take in account some extra time if the best move has changed
433 if (depth > 4 && depth < 50 && PVSize == 1)
434 TimeMgr.pv_instability(BestMoveChanges);
436 // Stop search if most of available time is already consumed. We
437 // probably don't have enough time to search the first move at the
438 // next iteration anyway.
439 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
442 // Stop search early if one move seems to be much better than others
444 && BestMoveChanges <= DBL_EPSILON
447 && bestValue > VALUE_MATED_IN_MAX_PLY
448 && ( RootMoves.size() == 1
449 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
451 Value rBeta = bestValue - 2 * PawnValueMg;
452 ss->excludedMove = RootMoves[0].pv[0];
453 ss->skipNullMove = true;
454 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
455 ss->skipNullMove = false;
456 ss->excludedMove = MOVE_NONE;
464 // If we are allowed to ponder do not stop the search now but
465 // keep pondering until GUI sends "ponderhit" or "stop".
467 Signals.stopOnPonderhit = true;
476 // search<>() is the main search function for both PV and non-PV nodes and for
477 // normal and SplitPoint nodes. When called just after a split point the search
478 // is simpler because we have already probed the hash table, done a null move
479 // search, and searched the first move before splitting, we don't have to repeat
480 // all this work again. We also don't need to store anything to the hash table
481 // here: This is taken care of after we return from the split point.
483 template <NodeType NT>
484 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
486 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
487 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
488 const bool RootNode = (NT == Root || NT == SplitPointRoot);
490 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
491 assert(PvNode || (alpha == beta - 1));
492 assert(depth > DEPTH_ZERO);
494 Move quietsSearched[64];
497 SplitPoint* splitPoint;
499 Move ttMove, move, excludedMove, bestMove, threatMove;
501 Value bestValue, value, ttValue;
502 Value eval, nullValue, futilityValue;
503 bool inCheck, givesCheck, pvMove, singularExtensionNode, improving;
504 bool captureOrPromotion, dangerous, doFullDepthSearch;
505 int moveCount, quietCount;
507 // Step 1. Initialize node
508 Thread* thisThread = pos.this_thread();
509 inCheck = pos.checkers();
513 splitPoint = ss->splitPoint;
514 bestMove = splitPoint->bestMove;
515 threatMove = splitPoint->threatMove;
516 bestValue = splitPoint->bestValue;
518 ttMove = excludedMove = MOVE_NONE;
519 ttValue = VALUE_NONE;
521 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
526 moveCount = quietCount = 0;
527 bestValue = -VALUE_INFINITE;
528 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
529 ss->ply = (ss-1)->ply + 1;
530 ss->futilityMoveCount = 0;
531 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
532 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
534 // Used to send selDepth info to GUI
535 if (PvNode && thisThread->maxPly < ss->ply)
536 thisThread->maxPly = ss->ply;
540 // Step 2. Check for aborted search and immediate draw
541 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
542 return DrawValue[pos.side_to_move()];
544 // Step 3. Mate distance pruning. Even if we mate at the next move our score
545 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
546 // a shorter mate was found upward in the tree then there is no need to search
547 // further, we will never beat current alpha. Same logic but with reversed signs
548 // applies also in the opposite condition of being mated instead of giving mate,
549 // in this case return a fail-high score.
550 alpha = std::max(mated_in(ss->ply), alpha);
551 beta = std::min(mate_in(ss->ply+1), beta);
556 // Step 4. Transposition table lookup
557 // We don't want the score of a partial search to overwrite a previous full search
558 // TT value, so we use a different position key in case of an excluded move.
559 excludedMove = ss->excludedMove;
560 posKey = excludedMove ? pos.exclusion_key() : pos.key();
561 tte = TT.probe(posKey);
562 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
563 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
565 // At PV nodes we check for exact scores, while at non-PV nodes we check for
566 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
567 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
568 // we should also update RootMoveList to avoid bogus output.
571 && tte->depth() >= depth
572 && ttValue != VALUE_NONE // Only in case of TT access race
573 && ( PvNode ? tte->bound() == BOUND_EXACT
574 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
575 : (tte->bound() & BOUND_UPPER)))
578 ss->currentMove = ttMove; // Can be MOVE_NONE
582 && !pos.capture_or_promotion(ttMove)
583 && ttMove != ss->killers[0])
585 ss->killers[1] = ss->killers[0];
586 ss->killers[0] = ttMove;
591 // Step 5. Evaluate the position statically and update parent's gain statistics
594 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
600 // Never assume anything on values stored in TT
601 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
602 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
603 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
605 // Can ttValue be used as a better position evaluation?
606 if (ttValue != VALUE_NONE)
607 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
612 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
613 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
614 ss->staticEval, ss->evalMargin);
617 // Update gain for the parent non-capture move given the static position
618 // evaluation before and after the move.
619 if ( !pos.captured_piece_type()
620 && ss->staticEval != VALUE_NONE
621 && (ss-1)->staticEval != VALUE_NONE
622 && (move = (ss-1)->currentMove) != MOVE_NULL
623 && type_of(move) == NORMAL)
625 Square to = to_sq(move);
626 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
629 // Step 6. Razoring (skipped when in check)
631 && depth < 4 * ONE_PLY
632 && eval + razor_margin(depth) < beta
633 && ttMove == MOVE_NONE
634 && abs(beta) < VALUE_MATE_IN_MAX_PLY
635 && !pos.pawn_on_7th(pos.side_to_move()))
637 Value rbeta = beta - razor_margin(depth);
638 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
640 // Logically we should return (v + razor_margin(depth)), but
641 // surprisingly this did slightly weaker in tests.
645 // Step 7. Static null move pruning (skipped when in check)
646 // We're betting that the opponent doesn't have a move that will reduce
647 // the score by more than futility_margin(depth) if we do a null move.
650 && depth < 4 * ONE_PLY
651 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
652 && abs(beta) < VALUE_MATE_IN_MAX_PLY
653 && abs(eval) < VALUE_KNOWN_WIN
654 && pos.non_pawn_material(pos.side_to_move()))
655 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
657 // Step 8. Null move search with verification search (is omitted in PV nodes)
660 && depth >= 2 * ONE_PLY
662 && abs(beta) < VALUE_MATE_IN_MAX_PLY
663 && pos.non_pawn_material(pos.side_to_move()))
665 ss->currentMove = MOVE_NULL;
667 // Null move dynamic reduction based on depth
668 Depth R = 3 * ONE_PLY + depth / 4;
670 // Null move dynamic reduction based on value
671 if (eval - PawnValueMg > beta)
674 pos.do_null_move(st);
675 (ss+1)->skipNullMove = true;
676 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
677 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
678 (ss+1)->skipNullMove = false;
679 pos.undo_null_move();
681 if (nullValue >= beta)
683 // Do not return unproven mate scores
684 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
687 if (depth < 12 * ONE_PLY)
690 // Do verification search at high depths
691 ss->skipNullMove = true;
692 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
693 ss->skipNullMove = false;
700 // The null move failed low, which means that we may be faced with
701 // some kind of threat. If the previous move was reduced, check if
702 // the move that refuted the null move was somehow connected to the
703 // move which was reduced. If a connection is found, return a fail
704 // low score (which will cause the reduced move to fail high in the
705 // parent node, which will trigger a re-search with full depth).
706 threatMove = (ss+1)->currentMove;
708 if ( depth < 5 * ONE_PLY
710 && threatMove != MOVE_NONE
711 && allows(pos, (ss-1)->currentMove, threatMove))
716 // Step 9. ProbCut (skipped when in check)
717 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
718 // and a reduced search returns a value much above beta, we can (almost) safely
719 // prune the previous move.
721 && depth >= 5 * ONE_PLY
723 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
725 Value rbeta = beta + 200;
726 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
728 assert(rdepth >= ONE_PLY);
729 assert((ss-1)->currentMove != MOVE_NONE);
730 assert((ss-1)->currentMove != MOVE_NULL);
732 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
735 while ((move = mp.next_move<false>()) != MOVE_NONE)
736 if (pos.legal(move, ci.pinned))
738 ss->currentMove = move;
739 pos.do_move(move, st, ci, pos.gives_check(move, ci));
740 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
747 // Step 10. Internal iterative deepening (skipped when in check)
748 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
749 && ttMove == MOVE_NONE
750 && (PvNode || ss->staticEval + Value(256) >= beta))
752 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
754 ss->skipNullMove = true;
755 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
756 ss->skipNullMove = false;
758 tte = TT.probe(posKey);
759 ttMove = tte ? tte->move() : MOVE_NONE;
762 moves_loop: // When in check and at SpNode search starts from here
764 Square prevMoveSq = to_sq((ss-1)->currentMove);
765 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
766 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
768 MovePicker mp(pos, ttMove, depth, History, countermoves, ss);
770 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
771 improving = ss->staticEval >= (ss-2)->staticEval
772 || ss->staticEval == VALUE_NONE
773 ||(ss-2)->staticEval == VALUE_NONE;
775 singularExtensionNode = !RootNode
777 && depth >= 8 * ONE_PLY
778 && ttMove != MOVE_NONE
779 && !excludedMove // Recursive singular search is not allowed
780 && (tte->bound() & BOUND_LOWER)
781 && tte->depth() >= depth - 3 * ONE_PLY;
783 // Step 11. Loop through moves
784 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
785 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
789 if (move == excludedMove)
792 // At root obey the "searchmoves" option and skip moves not listed in Root
793 // Move List, as a consequence any illegal move is also skipped. In MultiPV
794 // mode we also skip PV moves which have been already searched.
795 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
800 // Shared counter cannot be decremented later if move turns out to be illegal
801 if (!pos.legal(move, ci.pinned))
804 moveCount = ++splitPoint->moveCount;
805 splitPoint->mutex.unlock();
812 Signals.firstRootMove = (moveCount == 1);
814 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
815 sync_cout << "info depth " << depth / ONE_PLY
816 << " currmove " << move_to_uci(move, pos.is_chess960())
817 << " currmovenumber " << moveCount + PVIdx << sync_endl;
821 captureOrPromotion = pos.capture_or_promotion(move);
822 givesCheck = pos.gives_check(move, ci);
823 dangerous = givesCheck
824 || pos.passed_pawn_push(move)
825 || type_of(move) == CASTLE;
827 // Step 12. Extend checks
828 if (givesCheck && pos.see_sign(move) >= 0)
831 // Singular extension search. If all moves but one fail low on a search of
832 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
833 // is singular and should be extended. To verify this we do a reduced search
834 // on all the other moves but the ttMove, if result is lower than ttValue minus
835 // a margin then we extend ttMove.
836 if ( singularExtensionNode
839 && pos.legal(move, ci.pinned)
840 && abs(ttValue) < VALUE_KNOWN_WIN)
842 assert(ttValue != VALUE_NONE);
844 Value rBeta = ttValue - int(depth);
845 ss->excludedMove = move;
846 ss->skipNullMove = true;
847 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
848 ss->skipNullMove = false;
849 ss->excludedMove = MOVE_NONE;
855 // Update current move (this must be done after singular extension search)
856 newDepth = depth - ONE_PLY + ext;
858 // Step 13. Futility pruning (is omitted in PV nodes)
860 && !captureOrPromotion
863 /* && move != ttMove Already implicit in the next condition */
864 && bestValue > VALUE_MATED_IN_MAX_PLY)
866 // Move count based pruning
867 if ( depth < 16 * ONE_PLY
868 && moveCount >= FutilityMoveCounts[improving][depth]
869 && (!threatMove || !refutes(pos, move, threatMove)))
872 splitPoint->mutex.lock();
877 // Value based pruning
878 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
879 // but fixing this made program slightly weaker.
880 Depth predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
881 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
882 + Gains[pos.moved_piece(move)][to_sq(move)];
884 if (futilityValue < beta)
886 bestValue = std::max(bestValue, futilityValue);
890 splitPoint->mutex.lock();
891 if (bestValue > splitPoint->bestValue)
892 splitPoint->bestValue = bestValue;
897 // Prune moves with negative SEE at low depths
898 if ( predictedDepth < 4 * ONE_PLY
899 && pos.see_sign(move) < 0)
902 splitPoint->mutex.lock();
907 // We have not pruned the move that will be searched, but remember how
908 // far in the move list we are to be more aggressive in the child node.
909 ss->futilityMoveCount = moveCount;
912 ss->futilityMoveCount = 0;
914 // Check for legality only before to do the move
915 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
921 pvMove = PvNode && moveCount == 1;
922 ss->currentMove = move;
923 if (!SpNode && !captureOrPromotion && quietCount < 64)
924 quietsSearched[quietCount++] = 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 && move != ss->killers[0]
936 && move != ss->killers[1])
938 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
940 if (!PvNode && cutNode)
941 ss->reduction += ONE_PLY;
943 else if (History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
944 ss->reduction += ONE_PLY / 2;
946 if (move == countermoves[0] || move == countermoves[1])
947 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
949 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
951 alpha = splitPoint->alpha;
953 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
955 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
956 ss->reduction = DEPTH_ZERO;
959 doFullDepthSearch = !pvMove;
961 // Step 16. Full depth search, when LMR is skipped or fails high
962 if (doFullDepthSearch)
965 alpha = splitPoint->alpha;
967 value = newDepth < ONE_PLY ?
968 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
969 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
970 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
973 // Only for PV nodes do a full PV search on the first move or after a fail
974 // high, in the latter case search only if value < beta, otherwise let the
975 // parent node to fail low with value <= alpha and to try another move.
976 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
977 value = newDepth < ONE_PLY ?
978 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
979 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
980 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
981 // Step 17. Undo move
984 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
986 // Step 18. Check for new best move
989 splitPoint->mutex.lock();
990 bestValue = splitPoint->bestValue;
991 alpha = splitPoint->alpha;
994 // Finished searching the move. If Signals.stop is true, the search
995 // was aborted because the user interrupted the search or because we
996 // ran out of time. In this case, the return value of the search cannot
997 // be trusted, and we don't update the best move and/or PV.
998 if (Signals.stop || thisThread->cutoff_occurred())
999 return value; // To avoid returning VALUE_INFINITE
1003 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1005 // PV move or new best move ?
1006 if (pvMove || value > alpha)
1009 rm.extract_pv_from_tt(pos);
1011 // We record how often the best move has been changed in each
1012 // iteration. This information is used for time management: When
1013 // the best move changes frequently, we allocate some more time.
1018 // All other moves but the PV are set to the lowest value, this
1019 // is not a problem when sorting becuase sort is stable and move
1020 // position in the list is preserved, just the PV is pushed up.
1021 rm.score = -VALUE_INFINITE;
1024 if (value > bestValue)
1026 bestValue = SpNode ? splitPoint->bestValue = value : value;
1030 bestMove = SpNode ? splitPoint->bestMove = move : move;
1032 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1033 alpha = SpNode ? splitPoint->alpha = value : value;
1036 assert(value >= beta); // Fail high
1039 splitPoint->cutoff = true;
1046 // Step 19. Check for splitting the search
1048 && depth >= Threads.minimumSplitDepth
1049 && Threads.available_slave(thisThread)
1050 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1052 assert(bestValue < beta);
1054 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1055 depth, threatMove, moveCount, &mp, NT, cutNode);
1056 if (bestValue >= beta)
1064 // Step 20. Check for mate and stalemate
1065 // All legal moves have been searched and if there are no legal moves, it
1066 // must be mate or stalemate. Note that we can have a false positive in
1067 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1068 // harmless because return value is discarded anyhow in the parent nodes.
1069 // If we are in a singular extension search then return a fail low score.
1070 // A split node has at least one move, the one tried before to be splitted.
1072 return excludedMove ? alpha
1073 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1075 // If we have pruned all the moves without searching return a fail-low score
1076 if (bestValue == -VALUE_INFINITE)
1079 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1080 bestValue >= beta ? BOUND_LOWER :
1081 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1082 depth, bestMove, ss->staticEval, ss->evalMargin);
1084 // Quiet best move: update killers, history and countermoves
1085 if ( bestValue >= beta
1086 && !pos.capture_or_promotion(bestMove)
1089 if (ss->killers[0] != bestMove)
1091 ss->killers[1] = ss->killers[0];
1092 ss->killers[0] = bestMove;
1095 // Increase history value of the cut-off move and decrease all the other
1096 // played non-capture moves.
1097 Value bonus = Value(int(depth) * int(depth));
1098 History.update(pos.moved_piece(bestMove), to_sq(bestMove), bonus);
1099 for (int i = 0; i < quietCount - 1; ++i)
1101 Move m = quietsSearched[i];
1102 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1105 if (is_ok((ss-1)->currentMove))
1106 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1109 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1115 // qsearch() is the quiescence search function, which is called by the main
1116 // search function when the remaining depth is zero (or, to be more precise,
1117 // less than ONE_PLY).
1119 template <NodeType NT, bool InCheck>
1120 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1122 const bool PvNode = (NT == PV);
1124 assert(NT == PV || NT == NonPV);
1125 assert(InCheck == !!pos.checkers());
1126 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1127 assert(PvNode || (alpha == beta - 1));
1128 assert(depth <= DEPTH_ZERO);
1133 Move ttMove, move, bestMove;
1134 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1135 bool givesCheck, evasionPrunable;
1138 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1142 ss->currentMove = bestMove = MOVE_NONE;
1143 ss->ply = (ss-1)->ply + 1;
1145 // Check for an instant draw or maximum ply reached
1146 if (pos.is_draw() || ss->ply > MAX_PLY)
1147 return DrawValue[pos.side_to_move()];
1149 // Decide whether or not to include checks, this fixes also the type of
1150 // TT entry depth that we are going to use. Note that in qsearch we use
1151 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1152 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1153 : DEPTH_QS_NO_CHECKS;
1155 // Transposition table lookup
1157 tte = TT.probe(posKey);
1158 ttMove = tte ? tte->move() : MOVE_NONE;
1159 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1162 && tte->depth() >= ttDepth
1163 && ttValue != VALUE_NONE // Only in case of TT access race
1164 && ( PvNode ? tte->bound() == BOUND_EXACT
1165 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1166 : (tte->bound() & BOUND_UPPER)))
1168 ss->currentMove = ttMove; // Can be MOVE_NONE
1172 // Evaluate the position statically
1175 ss->staticEval = ss->evalMargin = VALUE_NONE;
1176 bestValue = futilityBase = -VALUE_INFINITE;
1182 // Never assume anything on values stored in TT
1183 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1184 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1185 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1187 // Can ttValue be used as a better position evaluation?
1188 if (ttValue != VALUE_NONE)
1189 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1190 bestValue = ttValue;
1193 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1195 // Stand pat. Return immediately if static value is at least beta
1196 if (bestValue >= beta)
1199 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1200 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1205 if (PvNode && bestValue > alpha)
1208 futilityBase = bestValue + ss->evalMargin + Value(128);
1211 // Initialize a MovePicker object for the current position, and prepare
1212 // to search the moves. Because the depth is <= 0 here, only captures,
1213 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1215 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1218 // Loop through the moves until no moves remain or a beta cutoff occurs
1219 while ((move = mp.next_move<false>()) != MOVE_NONE)
1221 assert(is_ok(move));
1223 givesCheck = pos.gives_check(move, ci);
1230 && type_of(move) != PROMOTION
1231 && futilityBase > -VALUE_KNOWN_WIN
1232 && !pos.passed_pawn_push(move))
1234 futilityValue = futilityBase
1235 + PieceValue[EG][pos.piece_on(to_sq(move))]
1236 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1238 if (futilityValue < beta)
1240 bestValue = std::max(bestValue, futilityValue);
1244 // Prune moves with negative or equal SEE and also moves with positive
1245 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1246 if ( futilityBase < beta
1247 && pos.see(move, beta - futilityBase) <= 0)
1249 bestValue = std::max(bestValue, futilityBase);
1254 // Detect non-capture evasions that are candidate to be pruned
1255 evasionPrunable = InCheck
1256 && bestValue > VALUE_MATED_IN_MAX_PLY
1257 && !pos.capture(move)
1258 && !pos.can_castle(pos.side_to_move());
1260 // Don't search moves with negative SEE values
1262 && (!InCheck || evasionPrunable)
1264 && type_of(move) != PROMOTION
1265 && pos.see_sign(move) < 0)
1268 // Check for legality only before to do the move
1269 if (!pos.legal(move, ci.pinned))
1272 ss->currentMove = move;
1274 // Make and search the move
1275 pos.do_move(move, st, ci, givesCheck);
1276 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1277 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1278 pos.undo_move(move);
1280 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1282 // Check for new best move
1283 if (value > bestValue)
1289 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1296 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1297 ttDepth, move, ss->staticEval, ss->evalMargin);
1305 // All legal moves have been searched. A special case: If we're in check
1306 // and no legal moves were found, it is checkmate.
1307 if (InCheck && bestValue == -VALUE_INFINITE)
1308 return mated_in(ss->ply); // Plies to mate from the root
1310 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1311 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1312 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1314 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1320 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1321 // "plies to mate from the current position". Non-mate scores are unchanged.
1322 // The function is called before storing a value to the transposition table.
1324 Value value_to_tt(Value v, int ply) {
1326 assert(v != VALUE_NONE);
1328 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1329 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1333 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1334 // from the transposition table (where refers to the plies to mate/be mated
1335 // from current position) to "plies to mate/be mated from the root".
1337 Value value_from_tt(Value v, int ply) {
1339 return v == VALUE_NONE ? VALUE_NONE
1340 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1341 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1345 // allows() tests whether the 'first' move at previous ply somehow makes the
1346 // 'second' move possible, for instance if the moving piece is the same in
1347 // both moves. Normally the second move is the threat (the best move returned
1348 // from a null search that fails low).
1350 bool allows(const Position& pos, Move first, Move second) {
1352 assert(is_ok(first));
1353 assert(is_ok(second));
1354 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1355 assert(type_of(first) == CASTLE || color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1357 Square m1from = from_sq(first);
1358 Square m2from = from_sq(second);
1359 Square m1to = to_sq(first);
1360 Square m2to = to_sq(second);
1362 // The piece is the same or second's destination was vacated by the first move
1363 // We exclude the trivial case where a sliding piece does in two moves what
1364 // it could do in one move: eg. Ra1a2, Ra2a3.
1366 || (m1to == m2from && !squares_aligned(m1from, m2from, m2to)))
1369 // Second one moves through the square vacated by first one
1370 if (between_bb(m2from, m2to) & m1from)
1373 // Second's destination is defended by the first move's piece
1374 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1378 // Second move gives a discovered check through the first's checking piece
1379 if (m1att & pos.king_square(pos.side_to_move()))
1381 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1389 // refutes() tests whether a 'first' move is able to defend against a 'second'
1390 // opponent's move. In this case will not be pruned. Normally the second move
1391 // is the threat (the best move returned from a null search that fails low).
1393 bool refutes(const Position& pos, Move first, Move second) {
1395 assert(is_ok(first));
1396 assert(is_ok(second));
1398 Square m1from = from_sq(first);
1399 Square m2from = from_sq(second);
1400 Square m1to = to_sq(first);
1401 Square m2to = to_sq(second);
1403 // Don't prune moves of the threatened piece
1407 // If the threatened piece has value less than or equal to the value of the
1408 // threat piece, don't prune moves which defend it.
1409 if ( pos.capture(second)
1410 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1411 || type_of(pos.piece_on(m2from)) == KING))
1413 // Update occupancy as if the piece and the threat are moving
1414 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1415 Piece pc = pos.piece_on(m1from);
1417 // The moved piece attacks the square 'tto' ?
1418 if (pos.attacks_from(pc, m1to, occ) & m2to)
1421 // Scan for possible X-ray attackers behind the moved piece
1422 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1423 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1425 // Verify attackers are triggered by our move and not already existing
1426 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1430 // Don't prune safe moves which block the threat path
1431 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1438 // When playing with strength handicap choose best move among the MultiPV set
1439 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1441 Move Skill::pick_move() {
1445 // PRNG sequence should be not deterministic
1446 for (int i = Time::now() % 50; i > 0; --i)
1447 rk.rand<unsigned>();
1449 // RootMoves are already sorted by score in descending order
1450 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1451 int weakness = 120 - 2 * level;
1452 int max_s = -VALUE_INFINITE;
1455 // Choose best move. For each move score we add two terms both dependent on
1456 // weakness, one deterministic and bigger for weaker moves, and one random,
1457 // then we choose the move with the resulting highest score.
1458 for (size_t i = 0; i < PVSize; ++i)
1460 int s = RootMoves[i].score;
1462 // Don't allow crazy blunders even at very low skills
1463 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1466 // This is our magic formula
1467 s += ( weakness * int(RootMoves[0].score - s)
1468 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1473 best = RootMoves[i].pv[0];
1480 // uci_pv() formats PV information according to UCI protocol. UCI requires
1481 // to send all the PV lines also if are still to be searched and so refer to
1482 // the previous search score.
1484 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1486 std::stringstream s;
1487 Time::point elapsed = Time::now() - SearchTime + 1;
1488 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1491 for (size_t i = 0; i < Threads.size(); ++i)
1492 if (Threads[i]->maxPly > selDepth)
1493 selDepth = Threads[i]->maxPly;
1495 for (size_t i = 0; i < uciPVSize; ++i)
1497 bool updated = (i <= PVIdx);
1499 if (depth == 1 && !updated)
1502 int d = updated ? depth : depth - 1;
1503 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1505 if (s.rdbuf()->in_avail()) // Not at first line
1508 s << "info depth " << d
1509 << " seldepth " << selDepth
1510 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1511 << " nodes " << pos.nodes_searched()
1512 << " nps " << pos.nodes_searched() * 1000 / elapsed
1513 << " time " << elapsed
1514 << " multipv " << i + 1
1517 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; ++j)
1518 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1527 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1528 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1529 /// allow to always have a ponder move even when we fail high at root, and a
1530 /// long PV to print that is important for position analysis.
1532 void RootMove::extract_pv_from_tt(Position& pos) {
1534 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1544 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1546 pos.do_move(pv[ply++], *st++);
1547 tte = TT.probe(pos.key());
1550 && pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
1551 && pos.legal(m, pos.pinned_pieces())
1553 && (!pos.is_draw() || ply < 2));
1555 pv.push_back(MOVE_NONE); // Must be zero-terminating
1557 while (ply) pos.undo_move(pv[--ply]);
1561 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1562 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1563 /// first, even if the old TT entries have been overwritten.
1565 void RootMove::insert_pv_in_tt(Position& pos) {
1567 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1572 tte = TT.probe(pos.key());
1574 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1575 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1577 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1579 pos.do_move(pv[ply++], *st++);
1581 } while (pv[ply] != MOVE_NONE);
1583 while (ply) pos.undo_move(pv[--ply]);
1587 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1589 void Thread::idle_loop() {
1591 // Pointer 'this_sp' is not null only if we are called from split(), and not
1592 // at the thread creation. So it means we are the split point's master.
1593 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1595 assert(!this_sp || (this_sp->masterThread == this && searching));
1599 // If we are not searching, wait for a condition to be signaled instead of
1600 // wasting CPU time polling for work.
1601 while ((!searching && Threads.sleepWhileIdle) || exit)
1609 // Grab the lock to avoid races with Thread::notify_one()
1612 // If we are master and all slaves have finished then exit idle_loop
1613 if (this_sp && !this_sp->slavesMask)
1619 // Do sleep after retesting sleep conditions under lock protection, in
1620 // particular we need to avoid a deadlock in case a master thread has,
1621 // in the meanwhile, allocated us and sent the notify_one() call before
1622 // we had the chance to grab the lock.
1623 if (!searching && !exit)
1624 sleepCondition.wait(mutex);
1629 // If this thread has been assigned work, launch a search
1634 Threads.mutex.lock();
1637 assert(activeSplitPoint);
1638 SplitPoint* sp = activeSplitPoint;
1640 Threads.mutex.unlock();
1642 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
1643 Position pos(*sp->pos, this);
1645 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1646 ss->splitPoint = sp;
1650 assert(activePosition == NULL);
1652 activePosition = &pos;
1654 switch (sp->nodeType) {
1656 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1659 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1662 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1671 activePosition = NULL;
1672 sp->slavesMask &= ~(1ULL << idx);
1673 sp->nodes += pos.nodes_searched();
1675 // Wake up master thread so to allow it to return from the idle loop
1676 // in case we are the last slave of the split point.
1677 if ( Threads.sleepWhileIdle
1678 && this != sp->masterThread
1681 assert(!sp->masterThread->searching);
1682 sp->masterThread->notify_one();
1685 // After releasing the lock we cannot access anymore any SplitPoint
1686 // related data in a safe way becuase it could have been released under
1687 // our feet by the sp master. Also accessing other Thread objects is
1688 // unsafe because if we are exiting there is a chance are already freed.
1692 // If this thread is the master of a split point and all slaves have finished
1693 // their work at this split point, return from the idle loop.
1694 if (this_sp && !this_sp->slavesMask)
1696 this_sp->mutex.lock();
1697 bool finished = !this_sp->slavesMask; // Retest under lock protection
1698 this_sp->mutex.unlock();
1706 /// check_time() is called by the timer thread when the timer triggers. It is
1707 /// used to print debug info and, more important, to detect when we are out of
1708 /// available time and so stop the search.
1712 static Time::point lastInfoTime = Time::now();
1713 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1715 if (Time::now() - lastInfoTime >= 1000)
1717 lastInfoTime = Time::now();
1726 Threads.mutex.lock();
1728 nodes = RootPos.nodes_searched();
1730 // Loop across all split points and sum accumulated SplitPoint nodes plus
1731 // all the currently active positions nodes.
1732 for (size_t i = 0; i < Threads.size(); ++i)
1733 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1735 SplitPoint& sp = Threads[i]->splitPoints[j];
1740 Bitboard sm = sp.slavesMask;
1743 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1745 nodes += pos->nodes_searched();
1751 Threads.mutex.unlock();
1754 Time::point elapsed = Time::now() - SearchTime;
1755 bool stillAtFirstMove = Signals.firstRootMove
1756 && !Signals.failedLowAtRoot
1757 && elapsed > TimeMgr.available_time();
1759 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution
1760 || stillAtFirstMove;
1762 if ( (Limits.use_time_management() && noMoreTime)
1763 || (Limits.movetime && elapsed >= Limits.movetime)
1764 || (Limits.nodes && nodes >= Limits.nodes))
1765 Signals.stop = true;