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 // This is the minimum interval in msec between two check_time() calls
60 const int TimerResolution = 5;
62 // Different node types, used as template parameter
63 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
65 // Dynamic razoring margin based on depth
66 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
68 // Futility lookup tables (initialized at startup) and their access functions
69 Value FutilityMargins[16][64]; // [depth][moveNumber]
70 int FutilityMoveCounts[2][32]; // [improving][depth]
72 inline Value futility_margin(Depth d, int mn) {
74 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
78 // Reduction lookup tables (initialized at startup) and their access function
79 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
81 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
83 return (Depth) Reductions[PvNode][i][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
89 Value DrawValue[COLOR_NB];
92 CountermovesStats Countermoves;
94 template <NodeType NT>
95 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
97 template <NodeType NT, bool InCheck>
98 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
100 void id_loop(Position& pos);
101 Value value_to_tt(Value v, int ply);
102 Value value_from_tt(Value v, int ply);
103 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta);
104 bool allows(const Position& pos, Move first, Move second);
105 bool refutes(const Position& pos, Move first, Move second);
106 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
108 class stop : public std::exception {};
111 Skill(int l) : level(l), best(MOVE_NONE) {}
113 if (enabled()) // Swap best PV line with the sub-optimal one
114 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
115 RootMoves.end(), best ? best : pick_move()));
118 bool enabled() const { return level < 20; }
119 bool time_to_pick(int depth) const { return depth == 1 + level; }
129 /// Search::init() is called during startup to initialize various lookup tables
131 void Search::init() {
133 int d; // depth (ONE_PLY == 2)
134 int hd; // half depth (ONE_PLY == 1)
137 // Init reductions array
138 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
140 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
141 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
142 Reductions[1][1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
143 Reductions[0][1][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
145 Reductions[1][0][hd][mc] = Reductions[1][1][hd][mc];
146 Reductions[0][0][hd][mc] = Reductions[0][1][hd][mc];
148 if (Reductions[0][0][hd][mc] > 2 * ONE_PLY)
149 Reductions[0][0][hd][mc] += ONE_PLY;
152 // Init futility margins array
153 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
154 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
156 // Init futility move count array
157 for (d = 0; d < 32; d++)
159 FutilityMoveCounts[1][d] = int(3.001 + 0.3 * pow(double(d), 1.8));
160 FutilityMoveCounts[0][d] = d < 5 ? FutilityMoveCounts[1][d]
161 : 3 * FutilityMoveCounts[1][d] / 4;
166 /// Search::perft() is our utility to verify move generation. All the leaf nodes
167 /// up to the given depth are generated and counted and the sum returned.
169 static size_t perft(Position& pos, Depth depth) {
174 const bool leaf = depth == 2 * ONE_PLY;
176 for (MoveList<LEGAL> it(pos); *it; ++it)
178 pos.do_move(*it, st, ci, pos.move_gives_check(*it, ci));
179 cnt += leaf ? MoveList<LEGAL>(pos).size() : ::perft(pos, depth - ONE_PLY);
185 size_t Search::perft(Position& pos, Depth depth) {
186 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
189 /// Search::think() is the external interface to Stockfish's search, and is
190 /// called by the main thread when the program receives the UCI 'go' command. It
191 /// searches from RootPos and at the end prints the "bestmove" to output.
193 void Search::think() {
195 static PolyglotBook book; // Defined static to initialize the PRNG only once
197 RootColor = RootPos.side_to_move();
198 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
200 if (RootMoves.empty())
202 RootMoves.push_back(MOVE_NONE);
203 sync_cout << "info depth 0 score "
204 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
210 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
212 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
214 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
216 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
221 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
223 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
224 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
225 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
226 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
229 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
231 if (Options["Write Search Log"])
233 Log log(Options["Search Log Filename"]);
234 log << "\nSearching: " << RootPos.fen()
235 << "\ninfinite: " << Limits.infinite
236 << " ponder: " << Limits.ponder
237 << " time: " << Limits.time[RootColor]
238 << " increment: " << Limits.inc[RootColor]
239 << " moves to go: " << Limits.movestogo
243 // Reset the threads, still sleeping: will be wake up at split time
244 for (size_t i = 0; i < Threads.size(); i++)
245 Threads[i]->maxPly = 0;
247 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
249 // Set best timer interval to avoid lagging under time pressure. Timer is
250 // used to check for remaining available thinking time.
251 Threads.timer->msec =
252 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
253 Limits.nodes ? 2 * TimerResolution
256 Threads.timer->notify_one(); // Wake up the recurring timer
258 id_loop(RootPos); // Let's start searching !
260 Threads.timer->msec = 0; // Stop the timer
261 Threads.sleepWhileIdle = true; // Send idle threads to sleep
263 if (Options["Write Search Log"])
265 Time::point elapsed = Time::now() - SearchTime + 1;
267 Log log(Options["Search Log Filename"]);
268 log << "Nodes: " << RootPos.nodes_searched()
269 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
270 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
273 RootPos.do_move(RootMoves[0].pv[0], st);
274 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
275 RootPos.undo_move(RootMoves[0].pv[0]);
280 // When search is stopped this info is not printed
281 sync_cout << "info nodes " << RootPos.nodes_searched()
282 << " time " << Time::now() - SearchTime + 1 << sync_endl;
284 // When we reach max depth we arrive here even without Signals.stop is raised,
285 // but if we are pondering or in infinite search, according to UCI protocol,
286 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
287 // command. We simply wait here until GUI sends one of those commands (that
288 // raise Signals.stop).
289 if (!Signals.stop && (Limits.ponder || Limits.infinite))
291 Signals.stopOnPonderhit = true;
292 RootPos.this_thread()->wait_for(Signals.stop);
295 // Best move could be MOVE_NONE when searching on a stalemate position
296 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
297 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
304 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
305 // with increasing depth until the allocated thinking time has been consumed,
306 // user stops the search, or the maximum search depth is reached.
308 void id_loop(Position& pos) {
310 Stack stack[MAX_PLY_PLUS_3], *ss = stack+2; // To allow referencing (ss-2)
311 int depth, prevBestMoveChanges;
312 Value bestValue, alpha, beta, delta;
314 std::memset(ss-2, 0, 5 * sizeof(Stack));
315 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
317 depth = BestMoveChanges = 0;
318 bestValue = delta = alpha = -VALUE_INFINITE;
319 beta = VALUE_INFINITE;
324 Countermoves.clear();
326 PVSize = Options["MultiPV"];
327 Skill skill(Options["Skill Level"]);
329 // Do we have to play with skill handicap? In this case enable MultiPV search
330 // that we will use behind the scenes to retrieve a set of possible moves.
331 if (skill.enabled() && PVSize < 4)
334 PVSize = std::min(PVSize, RootMoves.size());
336 // Iterative deepening loop until requested to stop or target depth reached
337 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
339 // Save last iteration's scores before first PV line is searched and all
340 // the move scores but the (new) PV are set to -VALUE_INFINITE.
341 for (size_t i = 0; i < RootMoves.size(); i++)
342 RootMoves[i].prevScore = RootMoves[i].score;
344 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
347 // MultiPV loop. We perform a full root search for each PV line
348 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
350 // Reset aspiration window starting size
354 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
355 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
358 // Start with a small aspiration window and, in case of fail high/low,
359 // research with bigger window until not failing high/low anymore.
363 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
366 // Bring to front the best move. It is critical that sorting is
367 // done with a stable algorithm because all the values but the first
368 // and eventually the new best one are set to -VALUE_INFINITE and
369 // we want to keep the same order for all the moves but the new
370 // PV that goes to the front. Note that in case of MultiPV search
371 // the already searched PV lines are preserved.
372 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
374 // Write PV back to transposition table in case the relevant
375 // entries have been overwritten during the search.
376 for (size_t i = 0; i <= PVIdx; i++)
377 RootMoves[i].insert_pv_in_tt(pos);
379 // If search has been stopped return immediately. Sorting and
380 // writing PV back to TT is safe becuase RootMoves is still
381 // valid, although refers to previous iteration.
385 // When failing high/low give some update (without cluttering
386 // the UI) before to research.
387 if ( (bestValue <= alpha || bestValue >= beta)
388 && Time::now() - SearchTime > 3000)
389 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
391 // In case of failing low/high increase aspiration window and
392 // research, otherwise exit the loop.
393 if (bestValue <= alpha)
395 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
397 Signals.failedLowAtRoot = true;
398 Signals.stopOnPonderhit = false;
400 else if (bestValue >= beta)
401 beta = std::min(bestValue + delta, VALUE_INFINITE);
408 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
411 // Sort the PV lines searched so far and update the GUI
412 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
414 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
415 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
418 // Do we need to pick now the sub-optimal best move ?
419 if (skill.enabled() && skill.time_to_pick(depth))
422 if (Options["Write Search Log"])
424 RootMove& rm = RootMoves[0];
425 if (skill.best != MOVE_NONE)
426 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
428 Log log(Options["Search Log Filename"]);
429 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
433 // Do we have found a "mate in x"?
435 && bestValue >= VALUE_MATE_IN_MAX_PLY
436 && VALUE_MATE - bestValue <= 2 * Limits.mate)
439 // Do we have time for the next iteration? Can we stop searching now?
440 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
442 bool stop = false; // Local variable, not the volatile Signals.stop
444 // Take in account some extra time if the best move has changed
445 if (depth > 4 && depth < 50 && PVSize == 1)
446 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
448 // Stop search if most of available time is already consumed. We
449 // probably don't have enough time to search the first move at the
450 // next iteration anyway.
451 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
454 // Stop search early if one move seems to be much better than others
458 && bestValue > VALUE_MATED_IN_MAX_PLY
459 && ( RootMoves.size() == 1
460 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
462 Value rBeta = bestValue - 2 * PawnValueMg;
463 ss->excludedMove = RootMoves[0].pv[0];
464 ss->skipNullMove = true;
465 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
466 ss->skipNullMove = false;
467 ss->excludedMove = MOVE_NONE;
475 // If we are allowed to ponder do not stop the search now but
476 // keep pondering until GUI sends "ponderhit" or "stop".
478 Signals.stopOnPonderhit = true;
487 // search<>() is the main search function for both PV and non-PV nodes and for
488 // normal and SplitPoint nodes. When called just after a split point the search
489 // is simpler because we have already probed the hash table, done a null move
490 // search, and searched the first move before splitting, we don't have to repeat
491 // all this work again. We also don't need to store anything to the hash table
492 // here: This is taken care of after we return from the split point.
494 template <NodeType NT>
495 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
497 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
498 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
499 const bool RootNode = (NT == Root || NT == SplitPointRoot);
501 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
502 assert(PvNode || (alpha == beta - 1));
503 assert(depth > DEPTH_ZERO);
505 Move quietsSearched[64];
508 SplitPoint* splitPoint;
510 Move ttMove, move, excludedMove, bestMove, threatMove;
512 Value bestValue, value, ttValue;
513 Value eval, nullValue, futilityValue;
514 bool inCheck, givesCheck, pvMove, singularExtensionNode, improving;
515 bool captureOrPromotion, dangerous, doFullDepthSearch;
516 int moveCount, quietCount;
518 // Step 1. Initialize node
519 Thread* thisThread = pos.this_thread();
520 inCheck = pos.checkers();
524 splitPoint = ss->splitPoint;
525 bestMove = splitPoint->bestMove;
526 threatMove = splitPoint->threatMove;
527 bestValue = splitPoint->bestValue;
529 ttMove = excludedMove = MOVE_NONE;
530 ttValue = VALUE_NONE;
532 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
537 moveCount = quietCount = 0;
538 bestValue = -VALUE_INFINITE;
539 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
540 ss->ply = (ss-1)->ply + 1;
541 ss->futilityMoveCount = 0;
542 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
543 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
545 // Used to send selDepth info to GUI
546 if (PvNode && thisThread->maxPly < ss->ply)
547 thisThread->maxPly = ss->ply;
549 if (Signals.stop || thisThread->cutoff_occurred())
554 // Step 2. Check for aborted search and immediate draw
555 if (pos.is_draw() || ss->ply > MAX_PLY)
556 return DrawValue[pos.side_to_move()];
558 // Step 3. Mate distance pruning. Even if we mate at the next move our score
559 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
560 // a shorter mate was found upward in the tree then there is no need to search
561 // further, we will never beat current alpha. Same logic but with reversed signs
562 // applies also in the opposite condition of being mated instead of giving mate,
563 // in this case return a fail-high score.
564 alpha = std::max(mated_in(ss->ply), alpha);
565 beta = std::min(mate_in(ss->ply+1), beta);
570 // Step 4. Transposition table lookup
571 // We don't want the score of a partial search to overwrite a previous full search
572 // TT value, so we use a different position key in case of an excluded move.
573 excludedMove = ss->excludedMove;
574 posKey = excludedMove ? pos.exclusion_key() : pos.key();
575 tte = TT.probe(posKey);
576 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
577 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
579 // At PV nodes we check for exact scores, while at non-PV nodes we check for
580 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
581 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
582 // we should also update RootMoveList to avoid bogus output.
585 && tte->depth() >= depth
586 && ttValue != VALUE_NONE // Only in case of TT access race
587 && ( PvNode ? tte->bound() == BOUND_EXACT
588 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
589 : (tte->bound() & BOUND_UPPER)))
592 ss->currentMove = ttMove; // Can be MOVE_NONE
596 && !pos.is_capture_or_promotion(ttMove)
597 && ttMove != ss->killers[0])
599 ss->killers[1] = ss->killers[0];
600 ss->killers[0] = ttMove;
605 // Step 5. Evaluate the position statically and update parent's gain statistics
608 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
614 // Never assume anything on values stored in TT
615 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
616 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
617 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
619 // Can ttValue be used as a better position evaluation?
620 if (ttValue != VALUE_NONE)
621 if ( ((tte->bound() & BOUND_LOWER) && ttValue > eval)
622 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
627 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
628 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
629 ss->staticEval, ss->evalMargin);
632 // Update gain for the parent non-capture move given the static position
633 // evaluation before and after the move.
634 if ( !pos.captured_piece_type()
635 && ss->staticEval != VALUE_NONE
636 && (ss-1)->staticEval != VALUE_NONE
637 && (move = (ss-1)->currentMove) != MOVE_NULL
638 && type_of(move) == NORMAL)
640 Square to = to_sq(move);
641 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
644 // Step 6. Razoring (skipped when in check)
646 && depth < 4 * ONE_PLY
647 && eval + razor_margin(depth) < beta
648 && ttMove == MOVE_NONE
649 && abs(beta) < VALUE_MATE_IN_MAX_PLY
650 && !pos.pawn_on_7th(pos.side_to_move()))
652 Value rbeta = beta - razor_margin(depth);
653 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
655 // Logically we should return (v + razor_margin(depth)), but
656 // surprisingly this did slightly weaker in tests.
660 // Step 7. Static null move pruning (skipped when in check)
661 // We're betting that the opponent doesn't have a move that will reduce
662 // the score by more than futility_margin(depth) if we do a null move.
665 && depth < 4 * ONE_PLY
666 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
667 && abs(beta) < VALUE_MATE_IN_MAX_PLY
668 && abs(eval) < VALUE_KNOWN_WIN
669 && pos.non_pawn_material(pos.side_to_move()))
670 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
672 // Step 8. Null move search with verification search (is omitted in PV nodes)
677 && abs(beta) < VALUE_MATE_IN_MAX_PLY
678 && pos.non_pawn_material(pos.side_to_move()))
680 ss->currentMove = MOVE_NULL;
682 // Null move dynamic reduction based on depth
683 Depth R = 3 * ONE_PLY + depth / 4;
685 // Null move dynamic reduction based on value
686 if (eval - PawnValueMg > beta)
689 pos.do_null_move(st);
690 (ss+1)->skipNullMove = true;
691 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
692 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
693 (ss+1)->skipNullMove = false;
694 pos.undo_null_move();
696 if (nullValue >= beta)
698 // Do not return unproven mate scores
699 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
702 if (depth < 12 * ONE_PLY)
705 // Do verification search at high depths
706 ss->skipNullMove = true;
707 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
708 ss->skipNullMove = false;
715 // The null move failed low, which means that we may be faced with
716 // some kind of threat. If the previous move was reduced, check if
717 // the move that refuted the null move was somehow connected to the
718 // move which was reduced. If a connection is found, return a fail
719 // low score (which will cause the reduced move to fail high in the
720 // parent node, which will trigger a re-search with full depth).
721 threatMove = (ss+1)->currentMove;
723 if ( depth < 5 * ONE_PLY
725 && threatMove != MOVE_NONE
726 && allows(pos, (ss-1)->currentMove, threatMove))
731 // Step 9. ProbCut (skipped when in check)
732 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
733 // and a reduced search returns a value much above beta, we can (almost) safely
734 // prune the previous move.
736 && depth >= 5 * ONE_PLY
738 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
740 Value rbeta = beta + 200;
741 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
743 assert(rdepth >= ONE_PLY);
744 assert((ss-1)->currentMove != MOVE_NONE);
745 assert((ss-1)->currentMove != MOVE_NULL);
747 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
750 while ((move = mp.next_move<false>()) != MOVE_NONE)
751 if (pos.pl_move_is_legal(move, ci.pinned))
753 ss->currentMove = move;
754 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
755 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
762 // Step 10. Internal iterative deepening (skipped when in check)
763 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
764 && ttMove == MOVE_NONE
765 && (PvNode || ss->staticEval + Value(256) >= beta))
767 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
769 ss->skipNullMove = true;
770 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
771 ss->skipNullMove = false;
773 tte = TT.probe(posKey);
774 ttMove = tte ? tte->move() : MOVE_NONE;
777 moves_loop: // When in check and at SpNode search starts from here
779 Square prevMoveSq = to_sq((ss-1)->currentMove);
780 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
781 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
783 MovePicker mp(pos, ttMove, depth, History, countermoves, ss);
785 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
786 improving = ss->staticEval >= (ss-2)->staticEval
787 || ss->staticEval == VALUE_NONE
788 ||(ss-2)->staticEval == VALUE_NONE;
790 singularExtensionNode = !RootNode
792 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
793 && ttMove != MOVE_NONE
794 && !excludedMove // Recursive singular search is not allowed
795 && (tte->bound() & BOUND_LOWER)
796 && tte->depth() >= depth - 3 * ONE_PLY;
798 // Step 11. Loop through moves
799 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
800 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
804 if (move == excludedMove)
807 // At root obey the "searchmoves" option and skip moves not listed in Root
808 // Move List, as a consequence any illegal move is also skipped. In MultiPV
809 // mode we also skip PV moves which have been already searched.
810 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
815 // Shared counter cannot be decremented later if move turns out to be illegal
816 if (!pos.pl_move_is_legal(move, ci.pinned))
819 moveCount = ++splitPoint->moveCount;
820 splitPoint->mutex.unlock();
827 Signals.firstRootMove = (moveCount == 1);
829 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
830 sync_cout << "info depth " << depth / ONE_PLY
831 << " currmove " << move_to_uci(move, pos.is_chess960())
832 << " currmovenumber " << moveCount + PVIdx << sync_endl;
836 captureOrPromotion = pos.is_capture_or_promotion(move);
837 givesCheck = pos.move_gives_check(move, ci);
838 dangerous = givesCheck
839 || pos.is_passed_pawn_push(move)
840 || type_of(move) == CASTLE;
842 // Step 12. Extend checks and, in PV nodes, also dangerous moves
843 if (PvNode && dangerous)
846 else if (givesCheck && pos.see_sign(move) >= 0)
849 // Singular extension search. If all moves but one fail low on a search of
850 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
851 // is singular and should be extended. To verify this we do a reduced search
852 // on all the other moves but the ttMove, if result is lower than ttValue minus
853 // a margin then we extend ttMove.
854 if ( singularExtensionNode
857 && pos.pl_move_is_legal(move, ci.pinned)
858 && abs(ttValue) < VALUE_KNOWN_WIN)
860 assert(ttValue != VALUE_NONE);
862 Value rBeta = ttValue - int(depth);
863 ss->excludedMove = move;
864 ss->skipNullMove = true;
865 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
866 ss->skipNullMove = false;
867 ss->excludedMove = MOVE_NONE;
873 // Update current move (this must be done after singular extension search)
874 newDepth = depth - ONE_PLY + ext;
876 // Step 13. Futility pruning (is omitted in PV nodes)
878 && !captureOrPromotion
881 /* && move != ttMove Already implicit in the next condition */
882 && bestValue > VALUE_MATED_IN_MAX_PLY)
884 // Move count based pruning
885 if ( depth < 16 * ONE_PLY
886 && moveCount >= FutilityMoveCounts[improving][depth]
887 && (!threatMove || !refutes(pos, move, threatMove)))
890 splitPoint->mutex.lock();
895 // Value based pruning
896 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
897 // but fixing this made program slightly weaker.
898 Depth predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
899 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
900 + Gains[pos.piece_moved(move)][to_sq(move)];
902 if (futilityValue < beta)
904 bestValue = std::max(bestValue, futilityValue);
908 splitPoint->mutex.lock();
909 if (bestValue > splitPoint->bestValue)
910 splitPoint->bestValue = bestValue;
915 // Prune moves with negative SEE at low depths
916 if ( predictedDepth < 4 * ONE_PLY
917 && pos.see_sign(move) < 0)
920 splitPoint->mutex.lock();
925 // We have not pruned the move that will be searched, but remember how
926 // far in the move list we are to be more aggressive in the child node.
927 ss->futilityMoveCount = moveCount;
930 ss->futilityMoveCount = 0;
932 // Check for legality only before to do the move
933 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
939 pvMove = PvNode && moveCount == 1;
940 ss->currentMove = move;
941 if (!SpNode && !captureOrPromotion && quietCount < 64)
942 quietsSearched[quietCount++] = move;
944 // Step 14. Make the move
945 pos.do_move(move, st, ci, givesCheck);
947 // Step 15. Reduced depth search (LMR). If the move fails high will be
948 // re-searched at full depth.
949 if ( depth > 3 * ONE_PLY
951 && !captureOrPromotion
954 && move != ss->killers[0]
955 && move != ss->killers[1])
957 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
959 if (!PvNode && cutNode)
960 ss->reduction += ONE_PLY;
962 if (move == countermoves[0] || move == countermoves[1])
963 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
965 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
967 alpha = splitPoint->alpha;
969 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
971 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
972 ss->reduction = DEPTH_ZERO;
975 doFullDepthSearch = !pvMove;
977 // Step 16. Full depth search, when LMR is skipped or fails high
978 if (doFullDepthSearch)
981 alpha = splitPoint->alpha;
983 value = newDepth < ONE_PLY ?
984 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
985 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
986 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
989 // Only for PV nodes do a full PV search on the first move or after a fail
990 // high, in the latter case search only if value < beta, otherwise let the
991 // parent node to fail low with value <= alpha and to try another move.
992 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
993 value = newDepth < ONE_PLY ?
994 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
995 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
996 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
997 // Step 17. Undo move
1000 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1002 // Step 18. Check for new best move
1005 splitPoint->mutex.lock();
1006 bestValue = splitPoint->bestValue;
1007 alpha = splitPoint->alpha;
1012 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1014 // PV move or new best move ?
1015 if (pvMove || value > alpha)
1018 rm.extract_pv_from_tt(pos);
1020 // We record how often the best move has been changed in each
1021 // iteration. This information is used for time management: When
1022 // the best move changes frequently, we allocate some more time.
1027 // All other moves but the PV are set to the lowest value, this
1028 // is not a problem when sorting becuase sort is stable and move
1029 // position in the list is preserved, just the PV is pushed up.
1030 rm.score = -VALUE_INFINITE;
1033 if (value > bestValue)
1035 bestValue = SpNode ? splitPoint->bestValue = value : value;
1039 bestMove = SpNode ? splitPoint->bestMove = move : move;
1041 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1042 alpha = SpNode ? splitPoint->alpha = value : value;
1045 assert(value >= beta); // Fail high
1048 splitPoint->cutoff = true;
1055 // Step 19. Check for splitting the search
1057 && depth >= Threads.minimumSplitDepth
1058 && Threads.available_slave(thisThread)
1059 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1061 assert(bestValue < beta);
1063 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1064 depth, threatMove, moveCount, &mp, NT, cutNode);
1065 if (bestValue >= beta)
1073 // Step 20. Check for mate and stalemate
1074 // All legal moves have been searched and if there are no legal moves, it
1075 // must be mate or stalemate. Note that we can have a false positive in
1076 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1077 // harmless because return value is discarded anyhow in the parent nodes.
1078 // If we are in a singular extension search then return a fail low score.
1079 // A split node has at least one move, the one tried before to be splitted.
1081 return excludedMove ? alpha
1082 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1084 // If we have pruned all the moves without searching return a fail-low score
1085 if (bestValue == -VALUE_INFINITE)
1088 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1089 bestValue >= beta ? BOUND_LOWER :
1090 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1091 depth, bestMove, ss->staticEval, ss->evalMargin);
1093 // Quiet best move: update killers, history and countermoves
1094 if ( bestValue >= beta
1095 && !pos.is_capture_or_promotion(bestMove)
1098 if (ss->killers[0] != bestMove)
1100 ss->killers[1] = ss->killers[0];
1101 ss->killers[0] = bestMove;
1104 // Increase history value of the cut-off move and decrease all the other
1105 // played non-capture moves.
1106 Value bonus = Value(int(depth) * int(depth));
1107 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1108 for (int i = 0; i < quietCount - 1; i++)
1110 Move m = quietsSearched[i];
1111 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1114 if (is_ok((ss-1)->currentMove))
1115 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1118 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1124 // qsearch() is the quiescence search function, which is called by the main
1125 // search function when the remaining depth is zero (or, to be more precise,
1126 // less than ONE_PLY).
1128 template <NodeType NT, bool InCheck>
1129 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1131 const bool PvNode = (NT == PV);
1133 assert(NT == PV || NT == NonPV);
1134 assert(InCheck == !!pos.checkers());
1135 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1136 assert(PvNode || (alpha == beta - 1));
1137 assert(depth <= DEPTH_ZERO);
1142 Move ttMove, move, bestMove;
1143 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1144 bool givesCheck, evasionPrunable;
1147 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1151 ss->currentMove = bestMove = MOVE_NONE;
1152 ss->ply = (ss-1)->ply + 1;
1154 // Check for an instant draw or maximum ply reached
1155 if (pos.is_draw() || ss->ply > MAX_PLY)
1156 return DrawValue[pos.side_to_move()];
1158 // Decide whether or not to include checks, this fixes also the type of
1159 // TT entry depth that we are going to use. Note that in qsearch we use
1160 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1161 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1162 : DEPTH_QS_NO_CHECKS;
1164 // Transposition table lookup
1166 tte = TT.probe(posKey);
1167 ttMove = tte ? tte->move() : MOVE_NONE;
1168 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1171 && tte->depth() >= ttDepth
1172 && ttValue != VALUE_NONE // Only in case of TT access race
1173 && ( PvNode ? tte->bound() == BOUND_EXACT
1174 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1175 : (tte->bound() & BOUND_UPPER)))
1177 ss->currentMove = ttMove; // Can be MOVE_NONE
1181 // Evaluate the position statically
1184 ss->staticEval = ss->evalMargin = VALUE_NONE;
1185 bestValue = futilityBase = -VALUE_INFINITE;
1191 // Never assume anything on values stored in TT
1192 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1193 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1194 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1197 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1199 // Stand pat. Return immediately if static value is at least beta
1200 if (bestValue >= beta)
1203 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1204 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1209 if (PvNode && bestValue > alpha)
1212 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1215 // Initialize a MovePicker object for the current position, and prepare
1216 // to search the moves. Because the depth is <= 0 here, only captures,
1217 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1219 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1222 // Loop through the moves until no moves remain or a beta cutoff occurs
1223 while ((move = mp.next_move<false>()) != MOVE_NONE)
1225 assert(is_ok(move));
1227 givesCheck = pos.move_gives_check(move, ci);
1234 && type_of(move) != PROMOTION
1235 && !pos.is_passed_pawn_push(move))
1237 futilityValue = futilityBase
1238 + PieceValue[EG][pos.piece_on(to_sq(move))]
1239 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1241 if (futilityValue < beta)
1243 bestValue = std::max(bestValue, futilityValue);
1247 // Prune moves with negative or equal SEE and also moves with positive
1248 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1249 if ( futilityBase < beta
1250 && pos.see(move, beta - futilityBase) <= 0)
1252 bestValue = std::max(bestValue, futilityBase);
1257 // Detect non-capture evasions that are candidate to be pruned
1258 evasionPrunable = InCheck
1259 && bestValue > VALUE_MATED_IN_MAX_PLY
1260 && !pos.is_capture(move)
1261 && !pos.can_castle(pos.side_to_move());
1263 // Don't search moves with negative SEE values
1265 && (!InCheck || evasionPrunable)
1267 && type_of(move) != PROMOTION
1268 && pos.see_sign(move) < 0)
1271 // Don't search useless checks
1276 && !pos.is_capture_or_promotion(move)
1277 && ss->staticEval + PawnValueMg / 4 < beta
1278 && !check_is_dangerous(pos, move, futilityBase, beta))
1281 // Check for legality only before to do the move
1282 if (!pos.pl_move_is_legal(move, ci.pinned))
1285 ss->currentMove = move;
1287 // Make and search the move
1288 pos.do_move(move, st, ci, givesCheck);
1289 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1290 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1291 pos.undo_move(move);
1293 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1295 // Check for new best move
1296 if (value > bestValue)
1302 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1309 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1310 ttDepth, move, ss->staticEval, ss->evalMargin);
1318 // All legal moves have been searched. A special case: If we're in check
1319 // and no legal moves were found, it is checkmate.
1320 if (InCheck && bestValue == -VALUE_INFINITE)
1321 return mated_in(ss->ply); // Plies to mate from the root
1323 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1324 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1325 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1327 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1333 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1334 // "plies to mate from the current position". Non-mate scores are unchanged.
1335 // The function is called before storing a value to the transposition table.
1337 Value value_to_tt(Value v, int ply) {
1339 assert(v != VALUE_NONE);
1341 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1342 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1346 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1347 // from the transposition table (where refers to the plies to mate/be mated
1348 // from current position) to "plies to mate/be mated from the root".
1350 Value value_from_tt(Value v, int ply) {
1352 return v == VALUE_NONE ? VALUE_NONE
1353 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1354 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1358 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1360 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1362 Piece pc = pos.piece_moved(move);
1363 Square from = from_sq(move);
1364 Square to = to_sq(move);
1365 Color them = ~pos.side_to_move();
1366 Square ksq = pos.king_square(them);
1367 Bitboard enemies = pos.pieces(them);
1368 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1369 Bitboard occ = pos.pieces() ^ from ^ ksq;
1370 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1371 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1373 // Checks which give opponent's king at most one escape square are dangerous
1374 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1377 // Queen contact check is very dangerous
1378 if (type_of(pc) == QUEEN && (kingAtt & to))
1381 // Creating new double threats with checks is dangerous
1382 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1385 // Note that here we generate illegal "double move"!
1386 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1394 // allows() tests whether the 'first' move at previous ply somehow makes the
1395 // 'second' move possible, for instance if the moving piece is the same in
1396 // both moves. Normally the second move is the threat (the best move returned
1397 // from a null search that fails low).
1399 bool allows(const Position& pos, Move first, Move second) {
1401 assert(is_ok(first));
1402 assert(is_ok(second));
1403 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1404 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1406 Square m1from = from_sq(first);
1407 Square m2from = from_sq(second);
1408 Square m1to = to_sq(first);
1409 Square m2to = to_sq(second);
1411 // The piece is the same or second's destination was vacated by the first move
1412 if (m1to == m2from || m2to == m1from)
1415 // Second one moves through the square vacated by first one
1416 if (between_bb(m2from, m2to) & m1from)
1419 // Second's destination is defended by the first move's piece
1420 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1424 // Second move gives a discovered check through the first's checking piece
1425 if (m1att & pos.king_square(pos.side_to_move()))
1427 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1435 // refutes() tests whether a 'first' move is able to defend against a 'second'
1436 // opponent's move. In this case will not be pruned. Normally the second move
1437 // is the threat (the best move returned from a null search that fails low).
1439 bool refutes(const Position& pos, Move first, Move second) {
1441 assert(is_ok(first));
1442 assert(is_ok(second));
1444 Square m1from = from_sq(first);
1445 Square m2from = from_sq(second);
1446 Square m1to = to_sq(first);
1447 Square m2to = to_sq(second);
1449 // Don't prune moves of the threatened piece
1453 // If the threatened piece has value less than or equal to the value of the
1454 // threat piece, don't prune moves which defend it.
1455 if ( pos.is_capture(second)
1456 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1457 || type_of(pos.piece_on(m2from)) == KING))
1459 // Update occupancy as if the piece and the threat are moving
1460 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1461 Piece pc = pos.piece_on(m1from);
1463 // The moved piece attacks the square 'tto' ?
1464 if (pos.attacks_from(pc, m1to, occ) & m2to)
1467 // Scan for possible X-ray attackers behind the moved piece
1468 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1469 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1471 // Verify attackers are triggered by our move and not already existing
1472 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1476 // Don't prune safe moves which block the threat path
1477 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1484 // When playing with strength handicap choose best move among the MultiPV set
1485 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1487 Move Skill::pick_move() {
1491 // PRNG sequence should be not deterministic
1492 for (int i = Time::now() % 50; i > 0; i--)
1493 rk.rand<unsigned>();
1495 // RootMoves are already sorted by score in descending order
1496 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1497 int weakness = 120 - 2 * level;
1498 int max_s = -VALUE_INFINITE;
1501 // Choose best move. For each move score we add two terms both dependent on
1502 // weakness, one deterministic and bigger for weaker moves, and one random,
1503 // then we choose the move with the resulting highest score.
1504 for (size_t i = 0; i < PVSize; i++)
1506 int s = RootMoves[i].score;
1508 // Don't allow crazy blunders even at very low skills
1509 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1512 // This is our magic formula
1513 s += ( weakness * int(RootMoves[0].score - s)
1514 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1519 best = RootMoves[i].pv[0];
1526 // uci_pv() formats PV information according to UCI protocol. UCI requires
1527 // to send all the PV lines also if are still to be searched and so refer to
1528 // the previous search score.
1530 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1532 std::stringstream s;
1533 Time::point elapsed = Time::now() - SearchTime + 1;
1534 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1537 for (size_t i = 0; i < Threads.size(); i++)
1538 if (Threads[i]->maxPly > selDepth)
1539 selDepth = Threads[i]->maxPly;
1541 for (size_t i = 0; i < uciPVSize; i++)
1543 bool updated = (i <= PVIdx);
1545 if (depth == 1 && !updated)
1548 int d = updated ? depth : depth - 1;
1549 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1551 if (s.rdbuf()->in_avail()) // Not at first line
1554 s << "info depth " << d
1555 << " seldepth " << selDepth
1556 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1557 << " nodes " << pos.nodes_searched()
1558 << " nps " << pos.nodes_searched() * 1000 / elapsed
1559 << " time " << elapsed
1560 << " multipv " << i + 1
1563 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1564 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1573 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1574 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1575 /// allow to always have a ponder move even when we fail high at root, and a
1576 /// long PV to print that is important for position analysis.
1578 void RootMove::extract_pv_from_tt(Position& pos) {
1580 StateInfo state[MAX_PLY_PLUS_3], *st = state;
1590 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1592 pos.do_move(pv[ply++], *st++);
1593 tte = TT.probe(pos.key());
1596 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1597 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1599 && (!pos.is_draw() || ply < 2));
1601 pv.push_back(MOVE_NONE); // Must be zero-terminating
1603 while (ply) pos.undo_move(pv[--ply]);
1607 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1608 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1609 /// first, even if the old TT entries have been overwritten.
1611 void RootMove::insert_pv_in_tt(Position& pos) {
1613 StateInfo state[MAX_PLY_PLUS_3], *st = state;
1618 tte = TT.probe(pos.key());
1620 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1621 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1623 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1625 pos.do_move(pv[ply++], *st++);
1627 } while (pv[ply] != MOVE_NONE);
1629 while (ply) pos.undo_move(pv[--ply]);
1633 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1635 void Thread::idle_loop() {
1637 // Pointer 'this_sp' is not null only if we are called from split(), and not
1638 // at the thread creation. So it means we are the split point's master.
1639 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1641 assert(!this_sp || (this_sp->masterThread == this && searching));
1645 // If we are not searching, wait for a condition to be signaled instead of
1646 // wasting CPU time polling for work.
1647 while ((!searching && Threads.sleepWhileIdle) || exit)
1655 // Grab the lock to avoid races with Thread::notify_one()
1658 // If we are master and all slaves have finished then exit idle_loop
1659 if (this_sp && !this_sp->slavesMask)
1665 // Do sleep after retesting sleep conditions under lock protection, in
1666 // particular we need to avoid a deadlock in case a master thread has,
1667 // in the meanwhile, allocated us and sent the notify_one() call before
1668 // we had the chance to grab the lock.
1669 if (!searching && !exit)
1670 sleepCondition.wait(mutex);
1675 // If this thread has been assigned work, launch a search
1680 Threads.mutex.lock();
1683 assert(activeSplitPoint);
1684 SplitPoint* sp = activeSplitPoint;
1686 Threads.mutex.unlock();
1688 Stack stack[MAX_PLY_PLUS_3], *ss = stack+2; // To allow referencing (ss-2)
1689 Position pos(*sp->pos, this);
1691 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1692 ss->splitPoint = sp;
1696 assert(activePosition == NULL);
1698 activePosition = &pos;
1701 switch (sp->nodeType) {
1703 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1706 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1709 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1718 sp->mutex.lock(); // Exception is thrown out of lock
1722 activePosition = NULL;
1723 sp->slavesMask &= ~(1ULL << idx);
1724 sp->nodes += pos.nodes_searched();
1726 // Wake up master thread so to allow it to return from the idle loop
1727 // in case we are the last slave of the split point.
1728 if ( Threads.sleepWhileIdle
1729 && this != sp->masterThread
1732 assert(!sp->masterThread->searching);
1733 sp->masterThread->notify_one();
1736 // After releasing the lock we cannot access anymore any SplitPoint
1737 // related data in a safe way becuase it could have been released under
1738 // our feet by the sp master. Also accessing other Thread objects is
1739 // unsafe because if we are exiting there is a chance are already freed.
1743 // If this thread is the master of a split point and all slaves have finished
1744 // their work at this split point, return from the idle loop.
1745 if (this_sp && !this_sp->slavesMask)
1747 this_sp->mutex.lock();
1748 bool finished = !this_sp->slavesMask; // Retest under lock protection
1749 this_sp->mutex.unlock();
1757 /// check_time() is called by the timer thread when the timer triggers. It is
1758 /// used to print debug info and, more important, to detect when we are out of
1759 /// available time and so stop the search.
1763 static Time::point lastInfoTime = Time::now();
1764 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1766 if (Time::now() - lastInfoTime >= 1000)
1768 lastInfoTime = Time::now();
1777 Threads.mutex.lock();
1779 nodes = RootPos.nodes_searched();
1781 // Loop across all split points and sum accumulated SplitPoint nodes plus
1782 // all the currently active positions nodes.
1783 for (size_t i = 0; i < Threads.size(); i++)
1784 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1786 SplitPoint& sp = Threads[i]->splitPoints[j];
1791 Bitboard sm = sp.slavesMask;
1794 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1796 nodes += pos->nodes_searched();
1802 Threads.mutex.unlock();
1805 Time::point elapsed = Time::now() - SearchTime;
1806 bool stillAtFirstMove = Signals.firstRootMove
1807 && !Signals.failedLowAtRoot
1808 && elapsed > TimeMgr.available_time();
1810 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1811 || stillAtFirstMove;
1813 if ( (Limits.use_time_management() && noMoreTime)
1814 || (Limits.movetime && elapsed >= Limits.movetime)
1815 || (Limits.nodes && nodes >= Limits.nodes))
1816 Signals.stop = true;