2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // This is the minimum interval in msec between two check_time() calls
59 const int TimerResolution = 5;
61 // Different node types, used as template parameter
62 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
64 // Dynamic razoring margin based on depth
65 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
67 // Futility lookup tables (initialized at startup) and their access functions
68 Value FutilityMargins[16][64]; // [depth][moveNumber]
69 int FutilityMoveCounts[2][32]; // [improving][depth]
71 inline Value futility_margin(Depth d, int mn) {
73 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
77 // Reduction lookup tables (initialized at startup) and their access function
78 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
82 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
88 Value DrawValue[COLOR_NB];
91 CountermovesStats Countermoves;
93 template <NodeType NT>
94 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
96 template <NodeType NT, bool InCheck>
97 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
99 void id_loop(Position& pos);
100 Value value_to_tt(Value v, int ply);
101 Value value_from_tt(Value v, int ply);
102 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta);
103 bool allows(const Position& pos, Move first, Move second);
104 bool refutes(const Position& pos, Move first, Move second);
105 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
108 Skill(int l) : level(l), best(MOVE_NONE) {}
110 if (enabled()) // Swap best PV line with the sub-optimal one
111 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
112 RootMoves.end(), best ? best : pick_move()));
115 bool enabled() const { return level < 20; }
116 bool time_to_pick(int depth) const { return depth == 1 + level; }
126 /// Search::init() is called during startup to initialize various lookup tables
128 void Search::init() {
130 int d; // depth (ONE_PLY == 2)
131 int hd; // half depth (ONE_PLY == 1)
134 // Init reductions array
135 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
137 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
138 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
139 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
140 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
143 // Init futility margins array
144 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
145 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
147 // Init futility move count array
148 for (d = 0; d < 32; d++)
150 FutilityMoveCounts[1][d] = int(3.001 + 0.3 * pow(double(d), 1.8));
151 FutilityMoveCounts[0][d] = d < 5 ? FutilityMoveCounts[1][d]
152 : 3 * FutilityMoveCounts[1][d] / 4;
157 /// Search::perft() is our utility to verify move generation. All the leaf nodes
158 /// up to the given depth are generated and counted and the sum returned.
160 static size_t perft(Position& pos, Depth depth) {
165 const bool leaf = depth == 2 * ONE_PLY;
167 for (MoveList<LEGAL> it(pos); *it; ++it)
169 pos.do_move(*it, st, ci, pos.move_gives_check(*it, ci));
170 cnt += leaf ? MoveList<LEGAL>(pos).size() : ::perft(pos, depth - ONE_PLY);
176 size_t Search::perft(Position& pos, Depth depth) {
177 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
180 /// Search::think() is the external interface to Stockfish's search, and is
181 /// called by the main thread when the program receives the UCI 'go' command. It
182 /// searches from RootPos and at the end prints the "bestmove" to output.
184 void Search::think() {
186 static PolyglotBook book; // Defined static to initialize the PRNG only once
188 RootColor = RootPos.side_to_move();
189 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
191 if (RootMoves.empty())
193 RootMoves.push_back(MOVE_NONE);
194 sync_cout << "info depth 0 score "
195 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
201 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
203 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
205 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
207 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
212 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
214 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
215 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
216 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
217 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
220 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
222 if (Options["Write Search Log"])
224 Log log(Options["Search Log Filename"]);
225 log << "\nSearching: " << RootPos.fen()
226 << "\ninfinite: " << Limits.infinite
227 << " ponder: " << Limits.ponder
228 << " time: " << Limits.time[RootColor]
229 << " increment: " << Limits.inc[RootColor]
230 << " moves to go: " << Limits.movestogo
234 // Reset the threads, still sleeping: will be wake up at split time
235 for (size_t i = 0; i < Threads.size(); i++)
236 Threads[i]->maxPly = 0;
238 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
240 // Set best timer interval to avoid lagging under time pressure. Timer is
241 // used to check for remaining available thinking time.
242 Threads.timer->msec =
243 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
244 Limits.nodes ? 2 * TimerResolution
247 Threads.timer->notify_one(); // Wake up the recurring timer
249 id_loop(RootPos); // Let's start searching !
251 Threads.timer->msec = 0; // Stop the timer
252 Threads.sleepWhileIdle = true; // Send idle threads to sleep
254 if (Options["Write Search Log"])
256 Time::point elapsed = Time::now() - SearchTime + 1;
258 Log log(Options["Search Log Filename"]);
259 log << "Nodes: " << RootPos.nodes_searched()
260 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
261 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
264 RootPos.do_move(RootMoves[0].pv[0], st);
265 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
266 RootPos.undo_move(RootMoves[0].pv[0]);
271 // When search is stopped this info is not printed
272 sync_cout << "info nodes " << RootPos.nodes_searched()
273 << " time " << Time::now() - SearchTime + 1 << sync_endl;
275 // When we reach max depth we arrive here even without Signals.stop is raised,
276 // but if we are pondering or in infinite search, according to UCI protocol,
277 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
278 // command. We simply wait here until GUI sends one of those commands (that
279 // raise Signals.stop).
280 if (!Signals.stop && (Limits.ponder || Limits.infinite))
282 Signals.stopOnPonderhit = true;
283 RootPos.this_thread()->wait_for(Signals.stop);
286 // Best move could be MOVE_NONE when searching on a stalemate position
287 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
288 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
295 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
296 // with increasing depth until the allocated thinking time has been consumed,
297 // user stops the search, or the maximum search depth is reached.
299 void id_loop(Position& pos) {
301 Stack stack[MAX_PLY_PLUS_3], *ss = stack+2; // To allow referencing (ss-2)
302 int depth, prevBestMoveChanges;
303 Value bestValue, alpha, beta, delta;
305 std::memset(ss-2, 0, 5 * sizeof(Stack));
306 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
308 depth = BestMoveChanges = 0;
309 bestValue = delta = alpha = -VALUE_INFINITE;
310 beta = VALUE_INFINITE;
315 Countermoves.clear();
317 PVSize = Options["MultiPV"];
318 Skill skill(Options["Skill Level"]);
320 // Do we have to play with skill handicap? In this case enable MultiPV search
321 // that we will use behind the scenes to retrieve a set of possible moves.
322 if (skill.enabled() && PVSize < 4)
325 PVSize = std::min(PVSize, RootMoves.size());
327 // Iterative deepening loop until requested to stop or target depth reached
328 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
330 // Save last iteration's scores before first PV line is searched and all
331 // the move scores but the (new) PV are set to -VALUE_INFINITE.
332 for (size_t i = 0; i < RootMoves.size(); i++)
333 RootMoves[i].prevScore = RootMoves[i].score;
335 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
338 // MultiPV loop. We perform a full root search for each PV line
339 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
341 // Reset aspiration window starting size
345 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
346 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
349 // Start with a small aspiration window and, in case of fail high/low,
350 // research with bigger window until not failing high/low anymore.
353 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
355 // Bring to front the best move. It is critical that sorting is
356 // done with a stable algorithm because all the values but the first
357 // and eventually the new best one are set to -VALUE_INFINITE and
358 // we want to keep the same order for all the moves but the new
359 // PV that goes to the front. Note that in case of MultiPV search
360 // the already searched PV lines are preserved.
361 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
363 // Write PV back to transposition table in case the relevant
364 // entries have been overwritten during the search.
365 for (size_t i = 0; i <= PVIdx; i++)
366 RootMoves[i].insert_pv_in_tt(pos);
368 // If search has been stopped return immediately. Sorting and
369 // writing PV back to TT is safe becuase RootMoves is still
370 // valid, although refers to previous iteration.
374 // When failing high/low give some update (without cluttering
375 // the UI) before to research.
376 if ( (bestValue <= alpha || bestValue >= beta)
377 && Time::now() - SearchTime > 3000)
378 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
380 // In case of failing low/high increase aspiration window and
381 // research, otherwise exit the loop.
382 if (bestValue <= alpha)
384 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
386 Signals.failedLowAtRoot = true;
387 Signals.stopOnPonderhit = false;
389 else if (bestValue >= beta)
390 beta = std::min(bestValue + delta, VALUE_INFINITE);
397 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
400 // Sort the PV lines searched so far and update the GUI
401 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
403 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
404 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
407 // Do we need to pick now the sub-optimal best move ?
408 if (skill.enabled() && skill.time_to_pick(depth))
411 if (Options["Write Search Log"])
413 RootMove& rm = RootMoves[0];
414 if (skill.best != MOVE_NONE)
415 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
417 Log log(Options["Search Log Filename"]);
418 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
422 // Do we have found a "mate in x"?
424 && bestValue >= VALUE_MATE_IN_MAX_PLY
425 && VALUE_MATE - bestValue <= 2 * Limits.mate)
428 // Do we have time for the next iteration? Can we stop searching now?
429 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
431 bool stop = false; // Local variable, not the volatile Signals.stop
433 // Take in account some extra time if the best move has changed
434 if (depth > 4 && depth < 50 && PVSize == 1)
435 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
437 // Stop search if most of available time is already consumed. We
438 // probably don't have enough time to search the first move at the
439 // next iteration anyway.
440 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
443 // Stop search early if one move seems to be much better than others
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.is_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() & BOUND_LOWER) && ttValue > eval)
608 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
613 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
614 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
615 ss->staticEval, ss->evalMargin);
618 // Update gain for the parent non-capture move given the static position
619 // evaluation before and after the move.
620 if ( !pos.captured_piece_type()
621 && ss->staticEval != VALUE_NONE
622 && (ss-1)->staticEval != VALUE_NONE
623 && (move = (ss-1)->currentMove) != MOVE_NULL
624 && type_of(move) == NORMAL)
626 Square to = to_sq(move);
627 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
630 // Step 6. Razoring (skipped when in check)
632 && depth < 4 * ONE_PLY
633 && eval + razor_margin(depth) < beta
634 && ttMove == MOVE_NONE
635 && abs(beta) < VALUE_MATE_IN_MAX_PLY
636 && !pos.pawn_on_7th(pos.side_to_move()))
638 Value rbeta = beta - razor_margin(depth);
639 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
641 // Logically we should return (v + razor_margin(depth)), but
642 // surprisingly this did slightly weaker in tests.
646 // Step 7. Static null move pruning (skipped when in check)
647 // We're betting that the opponent doesn't have a move that will reduce
648 // the score by more than futility_margin(depth) if we do a null move.
651 && depth < 4 * ONE_PLY
652 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
653 && abs(beta) < VALUE_MATE_IN_MAX_PLY
654 && abs(eval) < VALUE_KNOWN_WIN
655 && pos.non_pawn_material(pos.side_to_move()))
656 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
658 // Step 8. Null move search with verification search (is omitted in PV nodes)
663 && abs(beta) < VALUE_MATE_IN_MAX_PLY
664 && pos.non_pawn_material(pos.side_to_move()))
666 ss->currentMove = MOVE_NULL;
668 // Null move dynamic reduction based on depth
669 Depth R = 3 * ONE_PLY + depth / 4;
671 // Null move dynamic reduction based on value
672 if (eval - PawnValueMg > beta)
675 pos.do_null_move(st);
676 (ss+1)->skipNullMove = true;
677 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
678 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
679 (ss+1)->skipNullMove = false;
680 pos.undo_null_move();
682 if (nullValue >= beta)
684 // Do not return unproven mate scores
685 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
688 if (depth < 12 * ONE_PLY)
691 // Do verification search at high depths
692 ss->skipNullMove = true;
693 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
694 ss->skipNullMove = false;
701 // The null move failed low, which means that we may be faced with
702 // some kind of threat. If the previous move was reduced, check if
703 // the move that refuted the null move was somehow connected to the
704 // move which was reduced. If a connection is found, return a fail
705 // low score (which will cause the reduced move to fail high in the
706 // parent node, which will trigger a re-search with full depth).
707 threatMove = (ss+1)->currentMove;
709 if ( depth < 5 * ONE_PLY
711 && threatMove != MOVE_NONE
712 && allows(pos, (ss-1)->currentMove, threatMove))
717 // Step 9. ProbCut (skipped when in check)
718 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
719 // and a reduced search returns a value much above beta, we can (almost) safely
720 // prune the previous move.
722 && depth >= 5 * ONE_PLY
724 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
726 Value rbeta = beta + 200;
727 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
729 assert(rdepth >= ONE_PLY);
730 assert((ss-1)->currentMove != MOVE_NONE);
731 assert((ss-1)->currentMove != MOVE_NULL);
733 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
736 while ((move = mp.next_move<false>()) != MOVE_NONE)
737 if (pos.pl_move_is_legal(move, ci.pinned))
739 ss->currentMove = move;
740 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
741 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
748 // Step 10. Internal iterative deepening (skipped when in check)
749 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
750 && ttMove == MOVE_NONE
751 && (PvNode || ss->staticEval + Value(256) >= beta))
753 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
755 ss->skipNullMove = true;
756 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
757 ss->skipNullMove = false;
759 tte = TT.probe(posKey);
760 ttMove = tte ? tte->move() : MOVE_NONE;
763 moves_loop: // When in check and at SpNode search starts from here
765 Square prevMoveSq = to_sq((ss-1)->currentMove);
766 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
767 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
769 MovePicker mp(pos, ttMove, depth, History, countermoves, ss);
771 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
772 improving = ss->staticEval >= (ss-2)->staticEval;
773 singularExtensionNode = !RootNode
775 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
776 && ttMove != MOVE_NONE
777 && !excludedMove // Recursive singular search is not allowed
778 && (tte->bound() & BOUND_LOWER)
779 && tte->depth() >= depth - 3 * ONE_PLY;
781 // Step 11. Loop through moves
782 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
783 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
787 if (move == excludedMove)
790 // At root obey the "searchmoves" option and skip moves not listed in Root
791 // Move List, as a consequence any illegal move is also skipped. In MultiPV
792 // mode we also skip PV moves which have been already searched.
793 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
798 // Shared counter cannot be decremented later if move turns out to be illegal
799 if (!pos.pl_move_is_legal(move, ci.pinned))
802 moveCount = ++splitPoint->moveCount;
803 splitPoint->mutex.unlock();
810 Signals.firstRootMove = (moveCount == 1);
812 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
813 sync_cout << "info depth " << depth / ONE_PLY
814 << " currmove " << move_to_uci(move, pos.is_chess960())
815 << " currmovenumber " << moveCount + PVIdx << sync_endl;
819 captureOrPromotion = pos.is_capture_or_promotion(move);
820 givesCheck = pos.move_gives_check(move, ci);
821 dangerous = givesCheck
822 || pos.is_passed_pawn_push(move)
823 || type_of(move) == CASTLE;
825 // Step 12. Extend checks and, in PV nodes, also dangerous moves
826 if (PvNode && dangerous)
829 else if (givesCheck && pos.see_sign(move) >= 0)
832 // Singular extension search. If all moves but one fail low on a search of
833 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
834 // is singular and should be extended. To verify this we do a reduced search
835 // on all the other moves but the ttMove, if result is lower than ttValue minus
836 // a margin then we extend ttMove.
837 if ( singularExtensionNode
840 && pos.pl_move_is_legal(move, ci.pinned)
841 && abs(ttValue) < VALUE_KNOWN_WIN)
843 assert(ttValue != VALUE_NONE);
845 Value rBeta = ttValue - int(depth);
846 ss->excludedMove = move;
847 ss->skipNullMove = true;
848 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
849 ss->skipNullMove = false;
850 ss->excludedMove = MOVE_NONE;
856 // Update current move (this must be done after singular extension search)
857 newDepth = depth - ONE_PLY + ext;
859 // Step 13. Futility pruning (is omitted in PV nodes)
861 && !captureOrPromotion
864 /* && move != ttMove Already implicit in the next condition */
865 && bestValue > VALUE_MATED_IN_MAX_PLY)
867 // Move count based pruning
868 if ( depth < 16 * ONE_PLY
869 && moveCount >= FutilityMoveCounts[improving][depth]
870 && (!threatMove || !refutes(pos, move, threatMove)))
873 splitPoint->mutex.lock();
878 // Value based pruning
879 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
880 // but fixing this made program slightly weaker.
881 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
882 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
883 + Gains[pos.piece_moved(move)][to_sq(move)];
885 if (futilityValue < beta)
887 bestValue = std::max(bestValue, futilityValue);
891 splitPoint->mutex.lock();
892 if (bestValue > splitPoint->bestValue)
893 splitPoint->bestValue = bestValue;
898 // Prune moves with negative SEE at low depths
899 if ( predictedDepth < 4 * ONE_PLY
900 && pos.see_sign(move) < 0)
903 splitPoint->mutex.lock();
908 // We have not pruned the move that will be searched, but remember how
909 // far in the move list we are to be more aggressive in the child node.
910 ss->futilityMoveCount = moveCount;
913 ss->futilityMoveCount = 0;
915 // Check for legality only before to do the move
916 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
922 pvMove = PvNode && moveCount == 1;
923 ss->currentMove = move;
924 if (!SpNode && !captureOrPromotion && quietCount < 64)
925 quietsSearched[quietCount++] = move;
927 // Step 14. Make the move
928 pos.do_move(move, st, ci, givesCheck);
930 // Step 15. Reduced depth search (LMR). If the move fails high will be
931 // re-searched at full depth.
932 if ( depth > 3 * ONE_PLY
934 && !captureOrPromotion
937 && move != ss->killers[0]
938 && move != ss->killers[1])
940 ss->reduction = reduction<PvNode>(depth, moveCount);
942 if (!PvNode && cutNode)
943 ss->reduction += ONE_PLY;
945 if (move == countermoves[0] || move == countermoves[1])
946 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
948 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
950 alpha = splitPoint->alpha;
952 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
954 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
955 ss->reduction = DEPTH_ZERO;
958 doFullDepthSearch = !pvMove;
960 // Step 16. Full depth search, when LMR is skipped or fails high
961 if (doFullDepthSearch)
964 alpha = splitPoint->alpha;
966 value = newDepth < ONE_PLY ?
967 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
968 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
969 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
972 // Only for PV nodes do a full PV search on the first move or after a fail
973 // high, in the latter case search only if value < beta, otherwise let the
974 // parent node to fail low with value <= alpha and to try another move.
975 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
976 value = newDepth < ONE_PLY ?
977 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
978 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
979 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
980 // Step 17. Undo move
983 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
985 // Step 18. Check for new best move
988 splitPoint->mutex.lock();
989 bestValue = splitPoint->bestValue;
990 alpha = splitPoint->alpha;
993 // Finished searching the move. If Signals.stop is true, the search
994 // was aborted because the user interrupted the search or because we
995 // ran out of time. In this case, the return value of the search cannot
996 // be trusted, and we don't update the best move and/or PV.
997 if (Signals.stop || thisThread->cutoff_occurred())
998 return value; // To avoid returning VALUE_INFINITE
1002 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1004 // PV move or new best move ?
1005 if (pvMove || value > alpha)
1008 rm.extract_pv_from_tt(pos);
1010 // We record how often the best move has been changed in each
1011 // iteration. This information is used for time management: When
1012 // the best move changes frequently, we allocate some more time.
1017 // All other moves but the PV are set to the lowest value, this
1018 // is not a problem when sorting becuase sort is stable and move
1019 // position in the list is preserved, just the PV is pushed up.
1020 rm.score = -VALUE_INFINITE;
1023 if (value > bestValue)
1025 bestValue = SpNode ? splitPoint->bestValue = value : value;
1029 bestMove = SpNode ? splitPoint->bestMove = move : move;
1031 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1032 alpha = SpNode ? splitPoint->alpha = value : value;
1035 assert(value >= beta); // Fail high
1038 splitPoint->cutoff = true;
1045 // Step 19. Check for splitting the search
1047 && depth >= Threads.minimumSplitDepth
1048 && Threads.available_slave(thisThread)
1049 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1051 assert(bestValue < beta);
1053 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1054 depth, threatMove, moveCount, &mp, NT, cutNode);
1055 if (bestValue >= beta)
1063 // Step 20. Check for mate and stalemate
1064 // All legal moves have been searched and if there are no legal moves, it
1065 // must be mate or stalemate. Note that we can have a false positive in
1066 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1067 // harmless because return value is discarded anyhow in the parent nodes.
1068 // If we are in a singular extension search then return a fail low score.
1069 // A split node has at least one move, the one tried before to be splitted.
1071 return excludedMove ? alpha
1072 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1074 // If we have pruned all the moves without searching return a fail-low score
1075 if (bestValue == -VALUE_INFINITE)
1078 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1079 bestValue >= beta ? BOUND_LOWER :
1080 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1081 depth, bestMove, ss->staticEval, ss->evalMargin);
1083 // Quiet best move: update killers, history and countermoves
1084 if ( bestValue >= beta
1085 && !pos.is_capture_or_promotion(bestMove)
1088 if (ss->killers[0] != bestMove)
1090 ss->killers[1] = ss->killers[0];
1091 ss->killers[0] = bestMove;
1094 // Increase history value of the cut-off move and decrease all the other
1095 // played non-capture moves.
1096 Value bonus = Value(int(depth) * int(depth));
1097 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1098 for (int i = 0; i < quietCount - 1; i++)
1100 Move m = quietsSearched[i];
1101 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1104 if (is_ok((ss-1)->currentMove))
1105 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1108 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1114 // qsearch() is the quiescence search function, which is called by the main
1115 // search function when the remaining depth is zero (or, to be more precise,
1116 // less than ONE_PLY).
1118 template <NodeType NT, bool InCheck>
1119 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1121 const bool PvNode = (NT == PV);
1123 assert(NT == PV || NT == NonPV);
1124 assert(InCheck == !!pos.checkers());
1125 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1126 assert(PvNode || (alpha == beta - 1));
1127 assert(depth <= DEPTH_ZERO);
1132 Move ttMove, move, bestMove;
1133 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1134 bool givesCheck, evasionPrunable;
1137 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1141 ss->currentMove = bestMove = MOVE_NONE;
1142 ss->ply = (ss-1)->ply + 1;
1144 // Check for an instant draw or maximum ply reached
1145 if (pos.is_draw() || ss->ply > MAX_PLY)
1146 return DrawValue[pos.side_to_move()];
1148 // Decide whether or not to include checks, this fixes also the type of
1149 // TT entry depth that we are going to use. Note that in qsearch we use
1150 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1151 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1152 : DEPTH_QS_NO_CHECKS;
1154 // Transposition table lookup
1156 tte = TT.probe(posKey);
1157 ttMove = tte ? tte->move() : MOVE_NONE;
1158 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1161 && tte->depth() >= ttDepth
1162 && ttValue != VALUE_NONE // Only in case of TT access race
1163 && ( PvNode ? tte->bound() == BOUND_EXACT
1164 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1165 : (tte->bound() & BOUND_UPPER)))
1167 ss->currentMove = ttMove; // Can be MOVE_NONE
1171 // Evaluate the position statically
1174 ss->staticEval = ss->evalMargin = VALUE_NONE;
1175 bestValue = futilityBase = -VALUE_INFINITE;
1181 // Never assume anything on values stored in TT
1182 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1183 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1184 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1187 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1189 // Stand pat. Return immediately if static value is at least beta
1190 if (bestValue >= beta)
1193 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1194 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1199 if (PvNode && bestValue > alpha)
1202 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1205 // Initialize a MovePicker object for the current position, and prepare
1206 // to search the moves. Because the depth is <= 0 here, only captures,
1207 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1209 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1212 // Loop through the moves until no moves remain or a beta cutoff occurs
1213 while ((move = mp.next_move<false>()) != MOVE_NONE)
1215 assert(is_ok(move));
1217 givesCheck = pos.move_gives_check(move, ci);
1224 && type_of(move) != PROMOTION
1225 && !pos.is_passed_pawn_push(move))
1227 futilityValue = futilityBase
1228 + PieceValue[EG][pos.piece_on(to_sq(move))]
1229 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1231 if (futilityValue < beta)
1233 bestValue = std::max(bestValue, futilityValue);
1237 // Prune moves with negative or equal SEE and also moves with positive
1238 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1239 if ( futilityBase < beta
1240 && pos.see(move, beta - futilityBase) <= 0)
1242 bestValue = std::max(bestValue, futilityBase);
1247 // Detect non-capture evasions that are candidate to be pruned
1248 evasionPrunable = InCheck
1249 && bestValue > VALUE_MATED_IN_MAX_PLY
1250 && !pos.is_capture(move)
1251 && !pos.can_castle(pos.side_to_move());
1253 // Don't search moves with negative SEE values
1255 && (!InCheck || evasionPrunable)
1257 && type_of(move) != PROMOTION
1258 && pos.see_sign(move) < 0)
1261 // Don't search useless checks
1266 && !pos.is_capture_or_promotion(move)
1267 && ss->staticEval + PawnValueMg / 4 < beta
1268 && !check_is_dangerous(pos, move, futilityBase, beta))
1271 // Check for legality only before to do the move
1272 if (!pos.pl_move_is_legal(move, ci.pinned))
1275 ss->currentMove = move;
1277 // Make and search the move
1278 pos.do_move(move, st, ci, givesCheck);
1279 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1280 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1281 pos.undo_move(move);
1283 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1285 // Check for new best move
1286 if (value > bestValue)
1292 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1299 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1300 ttDepth, move, ss->staticEval, ss->evalMargin);
1308 // All legal moves have been searched. A special case: If we're in check
1309 // and no legal moves were found, it is checkmate.
1310 if (InCheck && bestValue == -VALUE_INFINITE)
1311 return mated_in(ss->ply); // Plies to mate from the root
1313 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1314 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1315 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1317 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1323 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1324 // "plies to mate from the current position". Non-mate scores are unchanged.
1325 // The function is called before storing a value to the transposition table.
1327 Value value_to_tt(Value v, int ply) {
1329 assert(v != VALUE_NONE);
1331 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1332 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1336 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1337 // from the transposition table (where refers to the plies to mate/be mated
1338 // from current position) to "plies to mate/be mated from the root".
1340 Value value_from_tt(Value v, int ply) {
1342 return v == VALUE_NONE ? VALUE_NONE
1343 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1344 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1348 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1350 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1352 Piece pc = pos.piece_moved(move);
1353 Square from = from_sq(move);
1354 Square to = to_sq(move);
1355 Color them = ~pos.side_to_move();
1356 Square ksq = pos.king_square(them);
1357 Bitboard enemies = pos.pieces(them);
1358 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1359 Bitboard occ = pos.pieces() ^ from ^ ksq;
1360 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1361 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1363 // Checks which give opponent's king at most one escape square are dangerous
1364 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1367 // Queen contact check is very dangerous
1368 if (type_of(pc) == QUEEN && (kingAtt & to))
1371 // Creating new double threats with checks is dangerous
1372 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1375 // Note that here we generate illegal "double move"!
1376 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1384 // allows() tests whether the 'first' move at previous ply somehow makes the
1385 // 'second' move possible, for instance if the moving piece is the same in
1386 // both moves. Normally the second move is the threat (the best move returned
1387 // from a null search that fails low).
1389 bool allows(const Position& pos, Move first, Move second) {
1391 assert(is_ok(first));
1392 assert(is_ok(second));
1393 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1394 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1396 Square m1from = from_sq(first);
1397 Square m2from = from_sq(second);
1398 Square m1to = to_sq(first);
1399 Square m2to = to_sq(second);
1401 // The piece is the same or second's destination was vacated by the first move
1402 if (m1to == m2from || m2to == m1from)
1405 // Second one moves through the square vacated by first one
1406 if (between_bb(m2from, m2to) & m1from)
1409 // Second's destination is defended by the first move's piece
1410 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1414 // Second move gives a discovered check through the first's checking piece
1415 if (m1att & pos.king_square(pos.side_to_move()))
1417 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1425 // refutes() tests whether a 'first' move is able to defend against a 'second'
1426 // opponent's move. In this case will not be pruned. Normally the second move
1427 // is the threat (the best move returned from a null search that fails low).
1429 bool refutes(const Position& pos, Move first, Move second) {
1431 assert(is_ok(first));
1432 assert(is_ok(second));
1434 Square m1from = from_sq(first);
1435 Square m2from = from_sq(second);
1436 Square m1to = to_sq(first);
1437 Square m2to = to_sq(second);
1439 // Don't prune moves of the threatened piece
1443 // If the threatened piece has value less than or equal to the value of the
1444 // threat piece, don't prune moves which defend it.
1445 if ( pos.is_capture(second)
1446 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1447 || type_of(pos.piece_on(m2from)) == KING))
1449 // Update occupancy as if the piece and the threat are moving
1450 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1451 Piece pc = pos.piece_on(m1from);
1453 // The moved piece attacks the square 'tto' ?
1454 if (pos.attacks_from(pc, m1to, occ) & m2to)
1457 // Scan for possible X-ray attackers behind the moved piece
1458 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1459 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1461 // Verify attackers are triggered by our move and not already existing
1462 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1466 // Don't prune safe moves which block the threat path
1467 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1474 // When playing with strength handicap choose best move among the MultiPV set
1475 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1477 Move Skill::pick_move() {
1481 // PRNG sequence should be not deterministic
1482 for (int i = Time::now() % 50; i > 0; i--)
1483 rk.rand<unsigned>();
1485 // RootMoves are already sorted by score in descending order
1486 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1487 int weakness = 120 - 2 * level;
1488 int max_s = -VALUE_INFINITE;
1491 // Choose best move. For each move score we add two terms both dependent on
1492 // weakness, one deterministic and bigger for weaker moves, and one random,
1493 // then we choose the move with the resulting highest score.
1494 for (size_t i = 0; i < PVSize; i++)
1496 int s = RootMoves[i].score;
1498 // Don't allow crazy blunders even at very low skills
1499 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1502 // This is our magic formula
1503 s += ( weakness * int(RootMoves[0].score - s)
1504 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1509 best = RootMoves[i].pv[0];
1516 // uci_pv() formats PV information according to UCI protocol. UCI requires
1517 // to send all the PV lines also if are still to be searched and so refer to
1518 // the previous search score.
1520 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1522 std::stringstream s;
1523 Time::point elapsed = Time::now() - SearchTime + 1;
1524 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1527 for (size_t i = 0; i < Threads.size(); i++)
1528 if (Threads[i]->maxPly > selDepth)
1529 selDepth = Threads[i]->maxPly;
1531 for (size_t i = 0; i < uciPVSize; i++)
1533 bool updated = (i <= PVIdx);
1535 if (depth == 1 && !updated)
1538 int d = updated ? depth : depth - 1;
1539 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1541 if (s.rdbuf()->in_avail()) // Not at first line
1544 s << "info depth " << d
1545 << " seldepth " << selDepth
1546 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1547 << " nodes " << pos.nodes_searched()
1548 << " nps " << pos.nodes_searched() * 1000 / elapsed
1549 << " time " << elapsed
1550 << " multipv " << i + 1
1553 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1554 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1563 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1564 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1565 /// allow to always have a ponder move even when we fail high at root, and a
1566 /// long PV to print that is important for position analysis.
1568 void RootMove::extract_pv_from_tt(Position& pos) {
1570 StateInfo state[MAX_PLY_PLUS_3], *st = state;
1580 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1582 pos.do_move(pv[ply++], *st++);
1583 tte = TT.probe(pos.key());
1586 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1587 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1589 && (!pos.is_draw() || ply < 2));
1591 pv.push_back(MOVE_NONE); // Must be zero-terminating
1593 while (ply) pos.undo_move(pv[--ply]);
1597 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1598 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1599 /// first, even if the old TT entries have been overwritten.
1601 void RootMove::insert_pv_in_tt(Position& pos) {
1603 StateInfo state[MAX_PLY_PLUS_3], *st = state;
1608 tte = TT.probe(pos.key());
1610 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1611 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1613 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1615 pos.do_move(pv[ply++], *st++);
1617 } while (pv[ply] != MOVE_NONE);
1619 while (ply) pos.undo_move(pv[--ply]);
1623 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1625 void Thread::idle_loop() {
1627 // Pointer 'this_sp' is not null only if we are called from split(), and not
1628 // at the thread creation. So it means we are the split point's master.
1629 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1631 assert(!this_sp || (this_sp->masterThread == this && searching));
1635 // If we are not searching, wait for a condition to be signaled instead of
1636 // wasting CPU time polling for work.
1637 while ((!searching && Threads.sleepWhileIdle) || exit)
1645 // Grab the lock to avoid races with Thread::notify_one()
1648 // If we are master and all slaves have finished then exit idle_loop
1649 if (this_sp && !this_sp->slavesMask)
1655 // Do sleep after retesting sleep conditions under lock protection, in
1656 // particular we need to avoid a deadlock in case a master thread has,
1657 // in the meanwhile, allocated us and sent the notify_one() call before
1658 // we had the chance to grab the lock.
1659 if (!searching && !exit)
1660 sleepCondition.wait(mutex);
1665 // If this thread has been assigned work, launch a search
1670 Threads.mutex.lock();
1673 assert(activeSplitPoint);
1674 SplitPoint* sp = activeSplitPoint;
1676 Threads.mutex.unlock();
1678 Stack stack[MAX_PLY_PLUS_3], *ss = stack+2; // To allow referencing (ss-2)
1679 Position pos(*sp->pos, this);
1681 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1682 ss->splitPoint = sp;
1686 assert(activePosition == NULL);
1688 activePosition = &pos;
1690 switch (sp->nodeType) {
1692 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1695 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1698 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1707 activePosition = NULL;
1708 sp->slavesMask &= ~(1ULL << idx);
1709 sp->nodes += pos.nodes_searched();
1711 // Wake up master thread so to allow it to return from the idle loop
1712 // in case we are the last slave of the split point.
1713 if ( Threads.sleepWhileIdle
1714 && this != sp->masterThread
1717 assert(!sp->masterThread->searching);
1718 sp->masterThread->notify_one();
1721 // After releasing the lock we cannot access anymore any SplitPoint
1722 // related data in a safe way becuase it could have been released under
1723 // our feet by the sp master. Also accessing other Thread objects is
1724 // unsafe because if we are exiting there is a chance are already freed.
1728 // If this thread is the master of a split point and all slaves have finished
1729 // their work at this split point, return from the idle loop.
1730 if (this_sp && !this_sp->slavesMask)
1732 this_sp->mutex.lock();
1733 bool finished = !this_sp->slavesMask; // Retest under lock protection
1734 this_sp->mutex.unlock();
1742 /// check_time() is called by the timer thread when the timer triggers. It is
1743 /// used to print debug info and, more important, to detect when we are out of
1744 /// available time and so stop the search.
1748 static Time::point lastInfoTime = Time::now();
1749 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1751 if (Time::now() - lastInfoTime >= 1000)
1753 lastInfoTime = Time::now();
1762 Threads.mutex.lock();
1764 nodes = RootPos.nodes_searched();
1766 // Loop across all split points and sum accumulated SplitPoint nodes plus
1767 // all the currently active positions nodes.
1768 for (size_t i = 0; i < Threads.size(); i++)
1769 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1771 SplitPoint& sp = Threads[i]->splitPoints[j];
1776 Bitboard sm = sp.slavesMask;
1779 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1781 nodes += pos->nodes_searched();
1787 Threads.mutex.unlock();
1790 Time::point elapsed = Time::now() - SearchTime;
1791 bool stillAtFirstMove = Signals.firstRootMove
1792 && !Signals.failedLowAtRoot
1793 && elapsed > TimeMgr.available_time();
1795 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1796 || stillAtFirstMove;
1798 if ( (Limits.use_time_management() && noMoreTime)
1799 || (Limits.movetime && elapsed >= Limits.movetime)
1800 || (Limits.nodes && nodes >= Limits.nodes))
1801 Signals.stop = true;