2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // This is the minimum interval in msec between two check_time() calls
59 const int TimerResolution = 5;
61 // Different node types, used as template parameter
62 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
64 // Dynamic razoring margin based on depth
65 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
67 // Futility lookup tables (initialized at startup) and their access functions
68 Value FutilityMargins[16][64]; // [depth][moveNumber]
69 int FutilityMoveCounts[32]; // [depth]
71 inline Value futility_margin(Depth d, int mn) {
73 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
77 // Reduction lookup tables (initialized at startup) and their access function
78 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
82 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
88 Value DrawValue[COLOR_NB];
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++)
149 FutilityMoveCounts[d] = int(3.001 + 0.3 * pow(double(d), 1.8));
153 /// Search::perft() is our utility to verify move generation. All the leaf nodes
154 /// up to the given depth are generated and counted and the sum returned.
156 size_t Search::perft(Position& pos, Depth depth) {
161 const bool leaf = depth == 2 * ONE_PLY;
163 for (MoveList<LEGAL> it(pos); *it; ++it)
165 pos.do_move(*it, st, ci, pos.move_gives_check(*it, ci));
166 cnt += leaf ? MoveList<LEGAL>(pos).size() : perft(pos, depth - ONE_PLY);
173 /// Search::think() is the external interface to Stockfish's search, and is
174 /// called by the main thread when the program receives the UCI 'go' command. It
175 /// searches from RootPos and at the end prints the "bestmove" to output.
177 void Search::think() {
179 static PolyglotBook book; // Defined static to initialize the PRNG only once
181 RootColor = RootPos.side_to_move();
182 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
184 if (RootMoves.empty())
186 RootMoves.push_back(MOVE_NONE);
187 sync_cout << "info depth 0 score "
188 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
194 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
196 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
198 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
200 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
205 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
207 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
208 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
209 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
210 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
213 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
215 if (Options["Use Search Log"])
217 Log log(Options["Search Log Filename"]);
218 log << "\nSearching: " << RootPos.fen()
219 << "\ninfinite: " << Limits.infinite
220 << " ponder: " << Limits.ponder
221 << " time: " << Limits.time[RootColor]
222 << " increment: " << Limits.inc[RootColor]
223 << " moves to go: " << Limits.movestogo
227 // Reset the threads, still sleeping: will be wake up at split time
228 for (size_t i = 0; i < Threads.size(); i++)
229 Threads[i]->maxPly = 0;
231 Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
233 // Set best timer interval to avoid lagging under time pressure. Timer is
234 // used to check for remaining available thinking time.
235 Threads.timer->msec =
236 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
237 Limits.nodes ? 2 * TimerResolution
240 Threads.timer->notify_one(); // Wake up the recurring timer
242 id_loop(RootPos); // Let's start searching !
244 Threads.timer->msec = 0; // Stop the timer
245 Threads.sleepWhileIdle = true; // Send idle threads to sleep
247 if (Options["Use Search Log"])
249 Time::point elapsed = Time::now() - SearchTime + 1;
251 Log log(Options["Search Log Filename"]);
252 log << "Nodes: " << RootPos.nodes_searched()
253 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
254 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
257 RootPos.do_move(RootMoves[0].pv[0], st);
258 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
259 RootPos.undo_move(RootMoves[0].pv[0]);
264 // When search is stopped this info is not printed
265 sync_cout << "info nodes " << RootPos.nodes_searched()
266 << " time " << Time::now() - SearchTime + 1 << sync_endl;
268 // When we reach max depth we arrive here even without Signals.stop is raised,
269 // but if we are pondering or in infinite search, according to UCI protocol,
270 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
271 // command. We simply wait here until GUI sends one of those commands (that
272 // raise Signals.stop).
273 if (!Signals.stop && (Limits.ponder || Limits.infinite))
275 Signals.stopOnPonderhit = true;
276 RootPos.this_thread()->wait_for(Signals.stop);
279 // Best move could be MOVE_NONE when searching on a stalemate position
280 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
281 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
288 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
289 // with increasing depth until the allocated thinking time has been consumed,
290 // user stops the search, or the maximum search depth is reached.
292 void id_loop(Position& pos) {
294 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
295 int depth, prevBestMoveChanges;
296 Value bestValue, alpha, beta, delta;
298 memset(ss-1, 0, 4 * sizeof(Stack));
299 depth = BestMoveChanges = 0;
300 bestValue = delta = -VALUE_INFINITE;
301 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
305 Countermoves.clear();
307 PVSize = Options["MultiPV"];
308 Skill skill(Options["Skill Level"]);
310 // Do we have to play with skill handicap? In this case enable MultiPV search
311 // that we will use behind the scenes to retrieve a set of possible moves.
312 if (skill.enabled() && PVSize < 4)
315 PVSize = std::min(PVSize, RootMoves.size());
317 // Iterative deepening loop until requested to stop or target depth reached
318 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
320 // Save last iteration's scores before first PV line is searched and all
321 // the move scores but the (new) PV are set to -VALUE_INFINITE.
322 for (size_t i = 0; i < RootMoves.size(); i++)
323 RootMoves[i].prevScore = RootMoves[i].score;
325 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
328 // MultiPV loop. We perform a full root search for each PV line
329 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
331 // Set aspiration window default width
332 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
335 alpha = RootMoves[PVIdx].prevScore - delta;
336 beta = RootMoves[PVIdx].prevScore + delta;
340 alpha = -VALUE_INFINITE;
341 beta = VALUE_INFINITE;
344 // Start with a small aspiration window and, in case of fail high/low,
345 // research with bigger window until not failing high/low anymore.
348 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
350 // Bring to front the best move. It is critical that sorting is
351 // done with a stable algorithm because all the values but the first
352 // and eventually the new best one are set to -VALUE_INFINITE and
353 // we want to keep the same order for all the moves but the new
354 // PV that goes to the front. Note that in case of MultiPV search
355 // the already searched PV lines are preserved.
356 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
358 // Write PV back to transposition table in case the relevant
359 // entries have been overwritten during the search.
360 for (size_t i = 0; i <= PVIdx; i++)
361 RootMoves[i].insert_pv_in_tt(pos);
363 // If search has been stopped return immediately. Sorting and
364 // writing PV back to TT is safe becuase RootMoves is still
365 // valid, although refers to previous iteration.
369 // In case of failing high/low increase aspiration window and
370 // research, otherwise exit the loop.
371 if (bestValue > alpha && bestValue < beta)
374 // Give some update (without cluttering the UI) before to research
375 if (Time::now() - SearchTime > 3000)
376 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
378 if (abs(bestValue) >= VALUE_KNOWN_WIN)
380 alpha = -VALUE_INFINITE;
381 beta = VALUE_INFINITE;
383 else if (bestValue >= beta)
390 Signals.failedLowAtRoot = true;
391 Signals.stopOnPonderhit = false;
397 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
400 // Sort the PV lines searched so far and update the GUI
401 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
403 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
404 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
407 // Do we need to pick now the sub-optimal best move ?
408 if (skill.enabled() && skill.time_to_pick(depth))
411 if (Options["Use Search Log"])
413 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;
504 bool captureOrPromotion, dangerous, doFullDepthSearch;
505 int moveCount, quietCount;
507 // Step 1. Initialize node
508 Thread* thisThread = pos.this_thread();
509 moveCount = quietCount = 0;
510 inCheck = pos.checkers();
514 splitPoint = ss->splitPoint;
515 bestMove = splitPoint->bestMove;
516 threatMove = splitPoint->threatMove;
517 bestValue = splitPoint->bestValue;
519 ttMove = excludedMove = MOVE_NONE;
520 ttValue = VALUE_NONE;
522 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
524 goto split_point_start;
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
593 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
597 // Never assume anything on values stored in TT
598 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
599 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
600 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
602 // Can ttValue be used as a better position evaluation?
603 if (ttValue != VALUE_NONE)
604 if ( ((tte->bound() & BOUND_LOWER) && ttValue > eval)
605 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
610 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
611 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
612 ss->staticEval, ss->evalMargin);
615 // Update gain for the parent non-capture move given the static position
616 // evaluation before and after the move.
617 if ( !pos.captured_piece_type()
618 && ss->staticEval != VALUE_NONE
619 && (ss-1)->staticEval != VALUE_NONE
620 && (move = (ss-1)->currentMove) != MOVE_NULL
621 && type_of(move) == NORMAL)
623 Square to = to_sq(move);
624 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
627 // Step 6. Razoring (is omitted in PV nodes)
629 && depth < 4 * ONE_PLY
631 && eval + razor_margin(depth) < beta
632 && ttMove == MOVE_NONE
633 && abs(beta) < VALUE_MATE_IN_MAX_PLY
634 && !pos.pawn_on_7th(pos.side_to_move()))
636 Value rbeta = beta - razor_margin(depth);
637 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
639 // Logically we should return (v + razor_margin(depth)), but
640 // surprisingly this did slightly weaker in tests.
644 // Step 7. Static null move pruning (is omitted in PV nodes)
645 // We're betting that the opponent doesn't have a move that will reduce
646 // the score by more than futility_margin(depth) if we do a null move.
649 && depth < 4 * ONE_PLY
651 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
652 && abs(beta) < VALUE_MATE_IN_MAX_PLY
653 && abs(eval) < VALUE_KNOWN_WIN
654 && pos.non_pawn_material(pos.side_to_move()))
655 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
657 // Step 8. Null move search with verification search (is omitted in PV nodes)
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 (is omitted in PV nodes)
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
725 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
727 Value rbeta = beta + 200;
728 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
730 assert(rdepth >= ONE_PLY);
731 assert((ss-1)->currentMove != MOVE_NONE);
732 assert((ss-1)->currentMove != MOVE_NULL);
734 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
737 while ((move = mp.next_move<false>()) != MOVE_NONE)
738 if (pos.pl_move_is_legal(move, ci.pinned))
740 ss->currentMove = move;
741 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
742 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
749 // Step 10. Internal iterative deepening
750 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
751 && ttMove == MOVE_NONE
752 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
754 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
756 ss->skipNullMove = true;
757 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
758 ss->skipNullMove = false;
760 tte = TT.probe(posKey);
761 ttMove = tte ? tte->move() : MOVE_NONE;
764 split_point_start: // At split points actual search starts from here
766 Square prevMoveSq = to_sq((ss-1)->currentMove);
767 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
768 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
770 MovePicker mp(pos, ttMove, depth, History, countermoves, ss, PvNode ? -VALUE_INFINITE : beta);
772 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
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
824 || ( captureOrPromotion // Entering a pawn endgame?
825 && type_of(pos.piece_on(to_sq(move))) != PAWN
826 && type_of(move) == NORMAL
827 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
828 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
830 // Step 12. Extend checks and, in PV nodes, also dangerous moves
831 if (PvNode && dangerous)
834 else if (givesCheck && pos.see_sign(move) >= 0)
837 // Singular extension search. If all moves but one fail low on a search of
838 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
839 // is singular and should be extended. To verify this we do a reduced search
840 // on all the other moves but the ttMove, if result is lower than ttValue minus
841 // a margin then we extend ttMove.
842 if ( singularExtensionNode
845 && pos.pl_move_is_legal(move, ci.pinned)
846 && abs(ttValue) < VALUE_KNOWN_WIN)
848 assert(ttValue != VALUE_NONE);
850 Value rBeta = ttValue - int(depth);
851 ss->excludedMove = move;
852 ss->skipNullMove = true;
853 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
854 ss->skipNullMove = false;
855 ss->excludedMove = MOVE_NONE;
861 // Update current move (this must be done after singular extension search)
862 newDepth = depth - ONE_PLY + ext;
864 // Step 13. Futility pruning (is omitted in PV nodes)
866 && !captureOrPromotion
869 /* && move != ttMove Already implicit in the next condition */
870 && bestValue > VALUE_MATED_IN_MAX_PLY)
872 // Move count based pruning
873 if ( depth < 16 * ONE_PLY
874 && moveCount >= FutilityMoveCounts[depth]
875 && (!threatMove || !refutes(pos, move, threatMove)))
878 splitPoint->mutex.lock();
883 // Value based pruning
884 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
885 // but fixing this made program slightly weaker.
886 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
887 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
888 + Gains[pos.piece_moved(move)][to_sq(move)];
890 if (futilityValue < beta)
892 bestValue = std::max(bestValue, futilityValue);
896 splitPoint->mutex.lock();
897 if (bestValue > splitPoint->bestValue)
898 splitPoint->bestValue = bestValue;
903 // Prune moves with negative SEE at low depths
904 if ( predictedDepth < 4 * ONE_PLY
905 && pos.see_sign(move) < 0)
908 splitPoint->mutex.lock();
913 // We have not pruned the move that will be searched, but remember how
914 // far in the move list we are to be more aggressive in the child node.
915 ss->futilityMoveCount = moveCount;
918 ss->futilityMoveCount = 0;
920 // Check for legality only before to do the move
921 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
927 pvMove = PvNode && moveCount == 1;
928 ss->currentMove = move;
929 if (!SpNode && !captureOrPromotion && quietCount < 64)
930 quietsSearched[quietCount++] = move;
932 // Step 14. Make the move
933 pos.do_move(move, st, ci, givesCheck);
935 // Step 15. Reduced depth search (LMR). If the move fails high will be
936 // re-searched at full depth.
937 if ( depth > 3 * ONE_PLY
939 && !captureOrPromotion
942 && move != ss->killers[0]
943 && move != ss->killers[1])
945 ss->reduction = reduction<PvNode>(depth, moveCount);
947 if (!PvNode && cutNode)
948 ss->reduction += ONE_PLY;
950 if (move == countermoves[0] || move == countermoves[1])
951 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
953 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
955 alpha = splitPoint->alpha;
957 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
959 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
960 ss->reduction = DEPTH_ZERO;
963 doFullDepthSearch = !pvMove;
965 // Step 16. Full depth search, when LMR is skipped or fails high
966 if (doFullDepthSearch)
969 alpha = splitPoint->alpha;
971 value = newDepth < ONE_PLY ?
972 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
973 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
974 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
977 // Only for PV nodes do a full PV search on the first move or after a fail
978 // high, in the latter case search only if value < beta, otherwise let the
979 // parent node to fail low with value <= alpha and to try another move.
980 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
981 value = newDepth < ONE_PLY ?
982 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
983 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
984 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
985 // Step 17. Undo move
988 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
990 // Step 18. Check for new best move
993 splitPoint->mutex.lock();
994 bestValue = splitPoint->bestValue;
995 alpha = splitPoint->alpha;
998 // Finished searching the move. If Signals.stop is true, the search
999 // was aborted because the user interrupted the search or because we
1000 // ran out of time. In this case, the return value of the search cannot
1001 // be trusted, and we don't update the best move and/or PV.
1002 if (Signals.stop || thisThread->cutoff_occurred())
1003 return value; // To avoid returning VALUE_INFINITE
1007 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1009 // PV move or new best move ?
1010 if (pvMove || value > alpha)
1013 rm.extract_pv_from_tt(pos);
1015 // We record how often the best move has been changed in each
1016 // iteration. This information is used for time management: When
1017 // the best move changes frequently, we allocate some more time.
1022 // All other moves but the PV are set to the lowest value, this
1023 // is not a problem when sorting becuase sort is stable and move
1024 // position in the list is preserved, just the PV is pushed up.
1025 rm.score = -VALUE_INFINITE;
1028 if (value > bestValue)
1030 bestValue = SpNode ? splitPoint->bestValue = value : value;
1034 bestMove = SpNode ? splitPoint->bestMove = move : move;
1036 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1037 alpha = SpNode ? splitPoint->alpha = value : value;
1040 assert(value >= beta); // Fail high
1043 splitPoint->cutoff = true;
1050 // Step 19. Check for splitting the search
1052 && depth >= Threads.minimumSplitDepth
1053 && Threads.available_slave(thisThread)
1054 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1056 assert(bestValue < beta);
1058 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1059 depth, threatMove, moveCount, &mp, NT, cutNode);
1060 if (bestValue >= beta)
1068 // Step 20. Check for mate and stalemate
1069 // All legal moves have been searched and if there are no legal moves, it
1070 // must be mate or stalemate. Note that we can have a false positive in
1071 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1072 // harmless because return value is discarded anyhow in the parent nodes.
1073 // If we are in a singular extension search then return a fail low score.
1074 // A split node has at least one move, the one tried before to be splitted.
1076 return excludedMove ? alpha
1077 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1079 // If we have pruned all the moves without searching return a fail-low score
1080 if (bestValue == -VALUE_INFINITE)
1083 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1084 bestValue >= beta ? BOUND_LOWER :
1085 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1086 depth, bestMove, ss->staticEval, ss->evalMargin);
1088 // Quiet best move: update killers, history and countermoves
1089 if ( bestValue >= beta
1090 && !pos.is_capture_or_promotion(bestMove)
1093 if (ss->killers[0] != bestMove)
1095 ss->killers[1] = ss->killers[0];
1096 ss->killers[0] = bestMove;
1099 // Increase history value of the cut-off move and decrease all the other
1100 // played non-capture moves.
1101 Value bonus = Value(int(depth) * int(depth));
1102 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1103 for (int i = 0; i < quietCount - 1; i++)
1105 Move m = quietsSearched[i];
1106 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1109 if (is_ok((ss-1)->currentMove))
1110 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1113 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1119 // qsearch() is the quiescence search function, which is called by the main
1120 // search function when the remaining depth is zero (or, to be more precise,
1121 // less than ONE_PLY).
1123 template <NodeType NT, bool InCheck>
1124 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1126 const bool PvNode = (NT == PV);
1128 assert(NT == PV || NT == NonPV);
1129 assert(InCheck == !!pos.checkers());
1130 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1131 assert(PvNode || (alpha == beta - 1));
1132 assert(depth <= DEPTH_ZERO);
1137 Move ttMove, move, bestMove;
1138 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1139 bool givesCheck, enoughMaterial, evasionPrunable;
1142 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1146 ss->currentMove = bestMove = MOVE_NONE;
1147 ss->ply = (ss-1)->ply + 1;
1149 // Check for an instant draw or maximum ply reached
1150 if (pos.is_draw() || ss->ply > MAX_PLY)
1151 return DrawValue[pos.side_to_move()];
1153 // Decide whether or not to include checks, this fixes also the type of
1154 // TT entry depth that we are going to use. Note that in qsearch we use
1155 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1156 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1157 : DEPTH_QS_NO_CHECKS;
1159 // Transposition table lookup
1161 tte = TT.probe(posKey);
1162 ttMove = tte ? tte->move() : MOVE_NONE;
1163 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1166 && tte->depth() >= ttDepth
1167 && ttValue != VALUE_NONE // Only in case of TT access race
1168 && ( PvNode ? tte->bound() == BOUND_EXACT
1169 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1170 : (tte->bound() & BOUND_UPPER)))
1172 ss->currentMove = ttMove; // Can be MOVE_NONE
1176 // Evaluate the position statically
1179 ss->staticEval = ss->evalMargin = VALUE_NONE;
1180 bestValue = futilityBase = -VALUE_INFINITE;
1181 enoughMaterial = false;
1187 // Never assume anything on values stored in TT
1188 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1189 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1190 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1193 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1195 // Stand pat. Return immediately if static value is at least beta
1196 if (bestValue >= beta)
1199 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1200 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1205 if (PvNode && bestValue > alpha)
1208 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1209 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1212 // Initialize a MovePicker object for the current position, and prepare
1213 // to search the moves. Because the depth is <= 0 here, only captures,
1214 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1216 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1219 // Loop through the moves until no moves remain or a beta cutoff occurs
1220 while ((move = mp.next_move<false>()) != MOVE_NONE)
1222 assert(is_ok(move));
1224 givesCheck = pos.move_gives_check(move, ci);
1232 && type_of(move) != PROMOTION
1233 && !pos.is_passed_pawn_push(move))
1235 futilityValue = futilityBase
1236 + PieceValue[EG][pos.piece_on(to_sq(move))]
1237 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1239 if (futilityValue < beta)
1241 bestValue = std::max(bestValue, futilityValue);
1245 // Prune moves with negative or equal SEE and also moves with positive
1246 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1247 if ( futilityBase < beta
1248 && pos.see(move, beta - futilityBase) <= 0)
1250 bestValue = std::max(bestValue, futilityBase);
1255 // Detect non-capture evasions that are candidate to be pruned
1256 evasionPrunable = !PvNode
1258 && bestValue > VALUE_MATED_IN_MAX_PLY
1259 && !pos.is_capture(move)
1260 && !pos.can_castle(pos.side_to_move());
1262 // Don't search moves with negative SEE values
1264 && (!InCheck || evasionPrunable)
1266 && type_of(move) != PROMOTION
1267 && pos.see_sign(move) < 0)
1270 // Don't search useless checks
1275 && !pos.is_capture_or_promotion(move)
1276 && ss->staticEval + PawnValueMg / 4 < beta
1277 && !check_is_dangerous(pos, move, futilityBase, beta))
1280 // Check for legality only before to do the move
1281 if (!pos.pl_move_is_legal(move, ci.pinned))
1284 ss->currentMove = move;
1286 // Make and search the move
1287 pos.do_move(move, st, ci, givesCheck);
1288 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1289 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1290 pos.undo_move(move);
1292 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1294 // Check for new best move
1295 if (value > bestValue)
1301 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1308 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1309 ttDepth, move, ss->staticEval, ss->evalMargin);
1317 // All legal moves have been searched. A special case: If we're in check
1318 // and no legal moves were found, it is checkmate.
1319 if (InCheck && bestValue == -VALUE_INFINITE)
1320 return mated_in(ss->ply); // Plies to mate from the root
1322 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1323 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1324 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1326 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1332 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1333 // "plies to mate from the current position". Non-mate scores are unchanged.
1334 // The function is called before storing a value to the transposition table.
1336 Value value_to_tt(Value v, int ply) {
1338 assert(v != VALUE_NONE);
1340 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1341 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1345 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1346 // from the transposition table (where refers to the plies to mate/be mated
1347 // from current position) to "plies to mate/be mated from the root".
1349 Value value_from_tt(Value v, int ply) {
1351 return v == VALUE_NONE ? VALUE_NONE
1352 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1353 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1357 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1359 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1361 Piece pc = pos.piece_moved(move);
1362 Square from = from_sq(move);
1363 Square to = to_sq(move);
1364 Color them = ~pos.side_to_move();
1365 Square ksq = pos.king_square(them);
1366 Bitboard enemies = pos.pieces(them);
1367 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1368 Bitboard occ = pos.pieces() ^ from ^ ksq;
1369 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1370 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1372 // Checks which give opponent's king at most one escape square are dangerous
1373 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1376 // Queen contact check is very dangerous
1377 if (type_of(pc) == QUEEN && (kingAtt & to))
1380 // Creating new double threats with checks is dangerous
1381 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1384 // Note that here we generate illegal "double move"!
1385 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1393 // allows() tests whether the 'first' move at previous ply somehow makes the
1394 // 'second' move possible, for instance if the moving piece is the same in
1395 // both moves. Normally the second move is the threat (the best move returned
1396 // from a null search that fails low).
1398 bool allows(const Position& pos, Move first, Move second) {
1400 assert(is_ok(first));
1401 assert(is_ok(second));
1402 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1403 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1405 Square m1from = from_sq(first);
1406 Square m2from = from_sq(second);
1407 Square m1to = to_sq(first);
1408 Square m2to = to_sq(second);
1410 // The piece is the same or second's destination was vacated by the first move
1411 if (m1to == m2from || m2to == m1from)
1414 // Second one moves through the square vacated by first one
1415 if (between_bb(m2from, m2to) & m1from)
1418 // Second's destination is defended by the first move's piece
1419 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1423 // Second move gives a discovered check through the first's checking piece
1424 if (m1att & pos.king_square(pos.side_to_move()))
1426 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1434 // refutes() tests whether a 'first' move is able to defend against a 'second'
1435 // opponent's move. In this case will not be pruned. Normally the second move
1436 // is the threat (the best move returned from a null search that fails low).
1438 bool refutes(const Position& pos, Move first, Move second) {
1440 assert(is_ok(first));
1441 assert(is_ok(second));
1443 Square m1from = from_sq(first);
1444 Square m2from = from_sq(second);
1445 Square m1to = to_sq(first);
1446 Square m2to = to_sq(second);
1448 // Don't prune moves of the threatened piece
1452 // If the threatened piece has value less than or equal to the value of the
1453 // threat piece, don't prune moves which defend it.
1454 if ( pos.is_capture(second)
1455 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1456 || type_of(pos.piece_on(m2from)) == KING))
1458 // Update occupancy as if the piece and the threat are moving
1459 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1460 Piece pc = pos.piece_on(m1from);
1462 // The moved piece attacks the square 'tto' ?
1463 if (pos.attacks_from(pc, m1to, occ) & m2to)
1466 // Scan for possible X-ray attackers behind the moved piece
1467 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1468 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1470 // Verify attackers are triggered by our move and not already existing
1471 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1475 // Don't prune safe moves which block the threat path
1476 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1483 // When playing with strength handicap choose best move among the MultiPV set
1484 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1486 Move Skill::pick_move() {
1490 // PRNG sequence should be not deterministic
1491 for (int i = Time::now() % 50; i > 0; i--)
1492 rk.rand<unsigned>();
1494 // RootMoves are already sorted by score in descending order
1495 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1496 int weakness = 120 - 2 * level;
1497 int max_s = -VALUE_INFINITE;
1500 // Choose best move. For each move score we add two terms both dependent on
1501 // weakness, one deterministic and bigger for weaker moves, and one random,
1502 // then we choose the move with the resulting highest score.
1503 for (size_t i = 0; i < PVSize; i++)
1505 int s = RootMoves[i].score;
1507 // Don't allow crazy blunders even at very low skills
1508 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1511 // This is our magic formula
1512 s += ( weakness * int(RootMoves[0].score - s)
1513 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1518 best = RootMoves[i].pv[0];
1525 // uci_pv() formats PV information according to UCI protocol. UCI requires
1526 // to send all the PV lines also if are still to be searched and so refer to
1527 // the previous search score.
1529 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1531 std::stringstream s;
1532 Time::point elapsed = Time::now() - SearchTime + 1;
1533 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1536 for (size_t i = 0; i < Threads.size(); i++)
1537 if (Threads[i]->maxPly > selDepth)
1538 selDepth = Threads[i]->maxPly;
1540 for (size_t i = 0; i < uciPVSize; i++)
1542 bool updated = (i <= PVIdx);
1544 if (depth == 1 && !updated)
1547 int d = updated ? depth : depth - 1;
1548 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1550 if (s.rdbuf()->in_avail()) // Not at first line
1553 s << "info depth " << d
1554 << " seldepth " << selDepth
1555 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1556 << " nodes " << pos.nodes_searched()
1557 << " nps " << pos.nodes_searched() * 1000 / elapsed
1558 << " time " << elapsed
1559 << " multipv " << i + 1
1562 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1563 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1572 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1573 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1574 /// allow to always have a ponder move even when we fail high at root, and a
1575 /// long PV to print that is important for position analysis.
1577 void RootMove::extract_pv_from_tt(Position& pos) {
1579 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1589 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1591 pos.do_move(pv[ply++], *st++);
1592 tte = TT.probe(pos.key());
1595 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1596 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1598 && (!pos.is_draw() || ply < 2));
1600 pv.push_back(MOVE_NONE); // Must be zero-terminating
1602 while (ply) pos.undo_move(pv[--ply]);
1606 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1607 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1608 /// first, even if the old TT entries have been overwritten.
1610 void RootMove::insert_pv_in_tt(Position& pos) {
1612 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1617 tte = TT.probe(pos.key());
1619 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1620 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1622 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1624 pos.do_move(pv[ply++], *st++);
1626 } while (pv[ply] != MOVE_NONE);
1628 while (ply) pos.undo_move(pv[--ply]);
1632 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1634 void Thread::idle_loop() {
1636 // Pointer 'this_sp' is not null only if we are called from split(), and not
1637 // at the thread creation. So it means we are the split point's master.
1638 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1640 assert(!this_sp || (this_sp->masterThread == this && searching));
1644 // If we are not searching, wait for a condition to be signaled instead of
1645 // wasting CPU time polling for work.
1646 while ((!searching && Threads.sleepWhileIdle) || exit)
1654 // Grab the lock to avoid races with Thread::notify_one()
1657 // If we are master and all slaves have finished then exit idle_loop
1658 if (this_sp && !this_sp->slavesMask)
1664 // Do sleep after retesting sleep conditions under lock protection, in
1665 // particular we need to avoid a deadlock in case a master thread has,
1666 // in the meanwhile, allocated us and sent the notify_one() call before
1667 // we had the chance to grab the lock.
1668 if (!searching && !exit)
1669 sleepCondition.wait(mutex);
1674 // If this thread has been assigned work, launch a search
1679 Threads.mutex.lock();
1682 SplitPoint* sp = activeSplitPoint;
1684 Threads.mutex.unlock();
1686 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
1687 Position pos(*sp->pos, this);
1689 memcpy(ss-1, sp->ss-1, 4 * sizeof(Stack));
1690 ss->splitPoint = sp;
1694 assert(activePosition == NULL);
1696 activePosition = &pos;
1698 switch (sp->nodeType) {
1700 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1703 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1706 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1715 activePosition = NULL;
1716 sp->slavesMask &= ~(1ULL << idx);
1717 sp->nodes += pos.nodes_searched();
1719 // Wake up master thread so to allow it to return from the idle loop
1720 // in case we are the last slave of the split point.
1721 if ( Threads.sleepWhileIdle
1722 && this != sp->masterThread
1725 assert(!sp->masterThread->searching);
1726 sp->masterThread->notify_one();
1729 // After releasing the lock we cannot access anymore any SplitPoint
1730 // related data in a safe way becuase it could have been released under
1731 // our feet by the sp master. Also accessing other Thread objects is
1732 // unsafe because if we are exiting there is a chance are already freed.
1736 // If this thread is the master of a split point and all slaves have finished
1737 // their work at this split point, return from the idle loop.
1738 if (this_sp && !this_sp->slavesMask)
1740 this_sp->mutex.lock();
1741 bool finished = !this_sp->slavesMask; // Retest under lock protection
1742 this_sp->mutex.unlock();
1750 /// check_time() is called by the timer thread when the timer triggers. It is
1751 /// used to print debug info and, more important, to detect when we are out of
1752 /// available time and so stop the search.
1756 static Time::point lastInfoTime = Time::now();
1757 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1759 if (Time::now() - lastInfoTime >= 1000)
1761 lastInfoTime = Time::now();
1770 Threads.mutex.lock();
1772 nodes = RootPos.nodes_searched();
1774 // Loop across all split points and sum accumulated SplitPoint nodes plus
1775 // all the currently active positions nodes.
1776 for (size_t i = 0; i < Threads.size(); i++)
1777 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1779 SplitPoint& sp = Threads[i]->splitPoints[j];
1784 Bitboard sm = sp.slavesMask;
1787 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1789 nodes += pos->nodes_searched();
1795 Threads.mutex.unlock();
1798 Time::point elapsed = Time::now() - SearchTime;
1799 bool stillAtFirstMove = Signals.firstRootMove
1800 && !Signals.failedLowAtRoot
1801 && elapsed > TimeMgr.available_time();
1803 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1804 || stillAtFirstMove;
1806 if ( (Limits.use_time_management() && noMoreTime)
1807 || (Limits.movetime && elapsed >= Limits.movetime)
1808 || (Limits.nodes && nodes >= Limits.nodes))
1809 Signals.stop = true;