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 static size_t 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);
172 size_t Search::perft(Position& pos, Depth depth) {
173 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
176 /// Search::think() is the external interface to Stockfish's search, and is
177 /// called by the main thread when the program receives the UCI 'go' command. It
178 /// searches from RootPos and at the end prints the "bestmove" to output.
180 void Search::think() {
182 static PolyglotBook book; // Defined static to initialize the PRNG only once
184 RootColor = RootPos.side_to_move();
185 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
187 if (RootMoves.empty())
189 RootMoves.push_back(MOVE_NONE);
190 sync_cout << "info depth 0 score "
191 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
197 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
199 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
201 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
203 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
208 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
210 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
211 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
212 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
213 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
216 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
218 if (Options["Write Search Log"])
220 Log log(Options["Search Log Filename"]);
221 log << "\nSearching: " << RootPos.fen()
222 << "\ninfinite: " << Limits.infinite
223 << " ponder: " << Limits.ponder
224 << " time: " << Limits.time[RootColor]
225 << " increment: " << Limits.inc[RootColor]
226 << " moves to go: " << Limits.movestogo
230 // Reset the threads, still sleeping: will be wake up at split time
231 for (size_t i = 0; i < Threads.size(); i++)
232 Threads[i]->maxPly = 0;
234 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
236 // Set best timer interval to avoid lagging under time pressure. Timer is
237 // used to check for remaining available thinking time.
238 Threads.timer->msec =
239 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
240 Limits.nodes ? 2 * TimerResolution
243 Threads.timer->notify_one(); // Wake up the recurring timer
245 id_loop(RootPos); // Let's start searching !
247 Threads.timer->msec = 0; // Stop the timer
248 Threads.sleepWhileIdle = true; // Send idle threads to sleep
250 if (Options["Write Search Log"])
252 Time::point elapsed = Time::now() - SearchTime + 1;
254 Log log(Options["Search Log Filename"]);
255 log << "Nodes: " << RootPos.nodes_searched()
256 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
257 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
260 RootPos.do_move(RootMoves[0].pv[0], st);
261 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
262 RootPos.undo_move(RootMoves[0].pv[0]);
267 // When search is stopped this info is not printed
268 sync_cout << "info nodes " << RootPos.nodes_searched()
269 << " time " << Time::now() - SearchTime + 1 << sync_endl;
271 // When we reach max depth we arrive here even without Signals.stop is raised,
272 // but if we are pondering or in infinite search, according to UCI protocol,
273 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
274 // command. We simply wait here until GUI sends one of those commands (that
275 // raise Signals.stop).
276 if (!Signals.stop && (Limits.ponder || Limits.infinite))
278 Signals.stopOnPonderhit = true;
279 RootPos.this_thread()->wait_for(Signals.stop);
282 // Best move could be MOVE_NONE when searching on a stalemate position
283 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
284 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
291 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
292 // with increasing depth until the allocated thinking time has been consumed,
293 // user stops the search, or the maximum search depth is reached.
295 void id_loop(Position& pos) {
297 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
298 int depth, prevBestMoveChanges;
299 Value bestValue, alpha, beta, delta;
301 std::memset(ss-1, 0, 4 * sizeof(Stack));
302 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
304 depth = BestMoveChanges = 0;
305 bestValue = delta = alpha = -VALUE_INFINITE;
306 beta = VALUE_INFINITE;
311 Countermoves.clear();
313 PVSize = Options["MultiPV"];
314 Skill skill(Options["Skill Level"]);
316 // Do we have to play with skill handicap? In this case enable MultiPV search
317 // that we will use behind the scenes to retrieve a set of possible moves.
318 if (skill.enabled() && PVSize < 4)
321 PVSize = std::min(PVSize, RootMoves.size());
323 // Iterative deepening loop until requested to stop or target depth reached
324 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
326 // Save last iteration's scores before first PV line is searched and all
327 // the move scores but the (new) PV are set to -VALUE_INFINITE.
328 for (size_t i = 0; i < RootMoves.size(); i++)
329 RootMoves[i].prevScore = RootMoves[i].score;
331 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
334 // MultiPV loop. We perform a full root search for each PV line
335 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
337 // Reset aspiration window starting size
341 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
342 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
345 // Start with a small aspiration window and, in case of fail high/low,
346 // research with bigger window until not failing high/low anymore.
349 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
351 // Bring to front the best move. It is critical that sorting is
352 // done with a stable algorithm because all the values but the first
353 // and eventually the new best one are set to -VALUE_INFINITE and
354 // we want to keep the same order for all the moves but the new
355 // PV that goes to the front. Note that in case of MultiPV search
356 // the already searched PV lines are preserved.
357 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
359 // Write PV back to transposition table in case the relevant
360 // entries have been overwritten during the search.
361 for (size_t i = 0; i <= PVIdx; i++)
362 RootMoves[i].insert_pv_in_tt(pos);
364 // If search has been stopped return immediately. Sorting and
365 // writing PV back to TT is safe becuase RootMoves is still
366 // valid, although refers to previous iteration.
370 // When failing high/low give some update (without cluttering
371 // the UI) before to research.
372 if ( (bestValue <= alpha || bestValue >= beta)
373 && Time::now() - SearchTime > 3000)
374 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
376 // In case of failing low/high increase aspiration window and
377 // research, otherwise exit the loop.
378 if (bestValue <= alpha)
380 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
382 Signals.failedLowAtRoot = true;
383 Signals.stopOnPonderhit = false;
385 else if (bestValue >= beta)
386 beta = std::min(bestValue + delta, VALUE_INFINITE);
393 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
396 // Sort the PV lines searched so far and update the GUI
397 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
399 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
400 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
403 // Do we need to pick now the sub-optimal best move ?
404 if (skill.enabled() && skill.time_to_pick(depth))
407 if (Options["Write Search Log"])
409 RootMove& rm = RootMoves[0];
410 if (skill.best != MOVE_NONE)
411 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
413 Log log(Options["Search Log Filename"]);
414 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
418 // Do we have found a "mate in x"?
420 && bestValue >= VALUE_MATE_IN_MAX_PLY
421 && VALUE_MATE - bestValue <= 2 * Limits.mate)
424 // Do we have time for the next iteration? Can we stop searching now?
425 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
427 bool stop = false; // Local variable, not the volatile Signals.stop
429 // Take in account some extra time if the best move has changed
430 if (depth > 4 && depth < 50 && PVSize == 1)
431 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
433 // Stop search if most of available time is already consumed. We
434 // probably don't have enough time to search the first move at the
435 // next iteration anyway.
436 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
439 // Stop search early if one move seems to be much better than others
443 && bestValue > VALUE_MATED_IN_MAX_PLY
444 && ( RootMoves.size() == 1
445 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
447 Value rBeta = bestValue - 2 * PawnValueMg;
448 ss->excludedMove = RootMoves[0].pv[0];
449 ss->skipNullMove = true;
450 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
451 ss->skipNullMove = false;
452 ss->excludedMove = MOVE_NONE;
460 // If we are allowed to ponder do not stop the search now but
461 // keep pondering until GUI sends "ponderhit" or "stop".
463 Signals.stopOnPonderhit = true;
472 // search<>() is the main search function for both PV and non-PV nodes and for
473 // normal and SplitPoint nodes. When called just after a split point the search
474 // is simpler because we have already probed the hash table, done a null move
475 // search, and searched the first move before splitting, we don't have to repeat
476 // all this work again. We also don't need to store anything to the hash table
477 // here: This is taken care of after we return from the split point.
479 template <NodeType NT>
480 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
482 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
483 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
484 const bool RootNode = (NT == Root || NT == SplitPointRoot);
486 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
487 assert(PvNode || (alpha == beta - 1));
488 assert(depth > DEPTH_ZERO);
490 Move quietsSearched[64];
493 SplitPoint* splitPoint;
495 Move ttMove, move, excludedMove, bestMove, threatMove;
497 Value bestValue, value, ttValue;
498 Value eval, nullValue, futilityValue;
499 bool inCheck, givesCheck, pvMove, singularExtensionNode;
500 bool captureOrPromotion, dangerous, doFullDepthSearch;
501 int moveCount, quietCount;
503 // Step 1. Initialize node
504 Thread* thisThread = pos.this_thread();
505 inCheck = pos.checkers();
509 splitPoint = ss->splitPoint;
510 bestMove = splitPoint->bestMove;
511 threatMove = splitPoint->threatMove;
512 bestValue = splitPoint->bestValue;
514 ttMove = excludedMove = MOVE_NONE;
515 ttValue = VALUE_NONE;
517 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
522 moveCount = quietCount = 0;
523 bestValue = -VALUE_INFINITE;
524 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
525 ss->ply = (ss-1)->ply + 1;
526 ss->futilityMoveCount = 0;
527 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
528 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
530 // Used to send selDepth info to GUI
531 if (PvNode && thisThread->maxPly < ss->ply)
532 thisThread->maxPly = ss->ply;
536 // Step 2. Check for aborted search and immediate draw
537 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
538 return DrawValue[pos.side_to_move()];
540 // Step 3. Mate distance pruning. Even if we mate at the next move our score
541 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
542 // a shorter mate was found upward in the tree then there is no need to search
543 // further, we will never beat current alpha. Same logic but with reversed signs
544 // applies also in the opposite condition of being mated instead of giving mate,
545 // in this case return a fail-high score.
546 alpha = std::max(mated_in(ss->ply), alpha);
547 beta = std::min(mate_in(ss->ply+1), beta);
552 // Step 4. Transposition table lookup
553 // We don't want the score of a partial search to overwrite a previous full search
554 // TT value, so we use a different position key in case of an excluded move.
555 excludedMove = ss->excludedMove;
556 posKey = excludedMove ? pos.exclusion_key() : pos.key();
557 tte = TT.probe(posKey);
558 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
559 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
561 // At PV nodes we check for exact scores, while at non-PV nodes we check for
562 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
563 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
564 // we should also update RootMoveList to avoid bogus output.
567 && tte->depth() >= depth
568 && ttValue != VALUE_NONE // Only in case of TT access race
569 && ( PvNode ? tte->bound() == BOUND_EXACT
570 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
571 : (tte->bound() & BOUND_UPPER)))
574 ss->currentMove = ttMove; // Can be MOVE_NONE
578 && !pos.is_capture_or_promotion(ttMove)
579 && ttMove != ss->killers[0])
581 ss->killers[1] = ss->killers[0];
582 ss->killers[0] = ttMove;
587 // Step 5. Evaluate the position statically and update parent's gain statistics
590 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
596 // Never assume anything on values stored in TT
597 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
598 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
599 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
601 // Can ttValue be used as a better position evaluation?
602 if (ttValue != VALUE_NONE)
603 if ( ((tte->bound() & BOUND_LOWER) && ttValue > eval)
604 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
609 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
610 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
611 ss->staticEval, ss->evalMargin);
614 // Update gain for the parent non-capture move given the static position
615 // evaluation before and after the move.
616 if ( !pos.captured_piece_type()
617 && ss->staticEval != VALUE_NONE
618 && (ss-1)->staticEval != VALUE_NONE
619 && (move = (ss-1)->currentMove) != MOVE_NULL
620 && type_of(move) == NORMAL)
622 Square to = to_sq(move);
623 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
626 // Step 6. Razoring (skipped when in check)
628 && depth < 4 * ONE_PLY
629 && eval + razor_margin(depth) < beta
630 && ttMove == MOVE_NONE
631 && abs(beta) < VALUE_MATE_IN_MAX_PLY
632 && !pos.pawn_on_7th(pos.side_to_move()))
634 Value rbeta = beta - razor_margin(depth);
635 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
637 // Logically we should return (v + razor_margin(depth)), but
638 // surprisingly this did slightly weaker in tests.
642 // Step 7. Static null move pruning (skipped when in check)
643 // We're betting that the opponent doesn't have a move that will reduce
644 // the score by more than futility_margin(depth) if we do a null move.
647 && depth < 4 * ONE_PLY
648 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
649 && abs(beta) < VALUE_MATE_IN_MAX_PLY
650 && abs(eval) < VALUE_KNOWN_WIN
651 && pos.non_pawn_material(pos.side_to_move()))
652 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
654 // Step 8. Null move search with verification search (is omitted in PV nodes)
659 && abs(beta) < VALUE_MATE_IN_MAX_PLY
660 && pos.non_pawn_material(pos.side_to_move()))
662 ss->currentMove = MOVE_NULL;
664 // Null move dynamic reduction based on depth
665 Depth R = 3 * ONE_PLY + depth / 4;
667 // Null move dynamic reduction based on value
668 if (eval - PawnValueMg > beta)
671 pos.do_null_move(st);
672 (ss+1)->skipNullMove = true;
673 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
674 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
675 (ss+1)->skipNullMove = false;
676 pos.undo_null_move();
678 if (nullValue >= beta)
680 // Do not return unproven mate scores
681 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
684 if (depth < 12 * ONE_PLY)
687 // Do verification search at high depths
688 ss->skipNullMove = true;
689 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
690 ss->skipNullMove = false;
697 // The null move failed low, which means that we may be faced with
698 // some kind of threat. If the previous move was reduced, check if
699 // the move that refuted the null move was somehow connected to the
700 // move which was reduced. If a connection is found, return a fail
701 // low score (which will cause the reduced move to fail high in the
702 // parent node, which will trigger a re-search with full depth).
703 threatMove = (ss+1)->currentMove;
705 if ( depth < 5 * ONE_PLY
707 && threatMove != MOVE_NONE
708 && allows(pos, (ss-1)->currentMove, threatMove))
713 // Step 9. ProbCut (skipped when in check)
714 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
715 // and a reduced search returns a value much above beta, we can (almost) safely
716 // prune the previous move.
718 && depth >= 5 * ONE_PLY
720 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
722 Value rbeta = beta + 200;
723 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
725 assert(rdepth >= ONE_PLY);
726 assert((ss-1)->currentMove != MOVE_NONE);
727 assert((ss-1)->currentMove != MOVE_NULL);
729 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
732 while ((move = mp.next_move<false>()) != MOVE_NONE)
733 if (pos.pl_move_is_legal(move, ci.pinned))
735 ss->currentMove = move;
736 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
737 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
744 // Step 10. Internal iterative deepening (skipped when in check)
745 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
746 && ttMove == MOVE_NONE
747 && (PvNode || ss->staticEval + Value(256) >= beta))
749 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
751 ss->skipNullMove = true;
752 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
753 ss->skipNullMove = false;
755 tte = TT.probe(posKey);
756 ttMove = tte ? tte->move() : MOVE_NONE;
759 moves_loop: // When in check and at SpNode search starts from here
761 Square prevMoveSq = to_sq((ss-1)->currentMove);
762 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
763 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
765 MovePicker mp(pos, ttMove, depth, History, countermoves, ss, PvNode ? -VALUE_INFINITE : beta);
767 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
768 singularExtensionNode = !RootNode
770 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
771 && ttMove != MOVE_NONE
772 && !excludedMove // Recursive singular search is not allowed
773 && (tte->bound() & BOUND_LOWER)
774 && tte->depth() >= depth - 3 * ONE_PLY;
776 // Step 11. Loop through moves
777 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
778 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
782 if (move == excludedMove)
785 // At root obey the "searchmoves" option and skip moves not listed in Root
786 // Move List, as a consequence any illegal move is also skipped. In MultiPV
787 // mode we also skip PV moves which have been already searched.
788 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
793 // Shared counter cannot be decremented later if move turns out to be illegal
794 if (!pos.pl_move_is_legal(move, ci.pinned))
797 moveCount = ++splitPoint->moveCount;
798 splitPoint->mutex.unlock();
805 Signals.firstRootMove = (moveCount == 1);
807 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
808 sync_cout << "info depth " << depth / ONE_PLY
809 << " currmove " << move_to_uci(move, pos.is_chess960())
810 << " currmovenumber " << moveCount + PVIdx << sync_endl;
814 captureOrPromotion = pos.is_capture_or_promotion(move);
815 givesCheck = pos.move_gives_check(move, ci);
816 dangerous = givesCheck
817 || pos.is_passed_pawn_push(move)
818 || type_of(move) == CASTLE;
820 // Step 12. Extend checks and, in PV nodes, also dangerous moves
821 if (PvNode && dangerous)
824 else if (givesCheck && pos.see_sign(move) >= 0)
827 // Singular extension search. If all moves but one fail low on a search of
828 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
829 // is singular and should be extended. To verify this we do a reduced search
830 // on all the other moves but the ttMove, if result is lower than ttValue minus
831 // a margin then we extend ttMove.
832 if ( singularExtensionNode
835 && pos.pl_move_is_legal(move, ci.pinned)
836 && abs(ttValue) < VALUE_KNOWN_WIN)
838 assert(ttValue != VALUE_NONE);
840 Value rBeta = ttValue - int(depth);
841 ss->excludedMove = move;
842 ss->skipNullMove = true;
843 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
844 ss->skipNullMove = false;
845 ss->excludedMove = MOVE_NONE;
851 // Update current move (this must be done after singular extension search)
852 newDepth = depth - ONE_PLY + ext;
854 // Step 13. Futility pruning (is omitted in PV nodes)
856 && !captureOrPromotion
859 /* && move != ttMove Already implicit in the next condition */
860 && bestValue > VALUE_MATED_IN_MAX_PLY)
862 // Move count based pruning
863 if ( depth < 16 * ONE_PLY
864 && moveCount >= FutilityMoveCounts[depth]
865 && (!threatMove || !refutes(pos, move, threatMove)))
868 splitPoint->mutex.lock();
873 // Value based pruning
874 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
875 // but fixing this made program slightly weaker.
876 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
877 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
878 + Gains[pos.piece_moved(move)][to_sq(move)];
880 if (futilityValue < beta)
882 bestValue = std::max(bestValue, futilityValue);
886 splitPoint->mutex.lock();
887 if (bestValue > splitPoint->bestValue)
888 splitPoint->bestValue = bestValue;
893 // Prune moves with negative SEE at low depths
894 if ( predictedDepth < 4 * ONE_PLY
895 && pos.see_sign(move) < 0)
898 splitPoint->mutex.lock();
903 // We have not pruned the move that will be searched, but remember how
904 // far in the move list we are to be more aggressive in the child node.
905 ss->futilityMoveCount = moveCount;
908 ss->futilityMoveCount = 0;
910 // Check for legality only before to do the move
911 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
917 pvMove = PvNode && moveCount == 1;
918 ss->currentMove = move;
919 if (!SpNode && !captureOrPromotion && quietCount < 64)
920 quietsSearched[quietCount++] = move;
922 // Step 14. Make the move
923 pos.do_move(move, st, ci, givesCheck);
925 // Step 15. Reduced depth search (LMR). If the move fails high will be
926 // re-searched at full depth.
927 if ( depth > 3 * ONE_PLY
929 && !captureOrPromotion
932 && move != ss->killers[0]
933 && move != ss->killers[1])
935 ss->reduction = reduction<PvNode>(depth, moveCount);
937 if (!PvNode && cutNode)
938 ss->reduction += ONE_PLY;
940 if (move == countermoves[0] || move == countermoves[1])
941 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
943 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
945 alpha = splitPoint->alpha;
947 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
949 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
950 ss->reduction = DEPTH_ZERO;
953 doFullDepthSearch = !pvMove;
955 // Step 16. Full depth search, when LMR is skipped or fails high
956 if (doFullDepthSearch)
959 alpha = splitPoint->alpha;
961 value = newDepth < ONE_PLY ?
962 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
963 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
964 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
967 // Only for PV nodes do a full PV search on the first move or after a fail
968 // high, in the latter case search only if value < beta, otherwise let the
969 // parent node to fail low with value <= alpha and to try another move.
970 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
971 value = newDepth < ONE_PLY ?
972 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
973 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
974 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
975 // Step 17. Undo move
978 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
980 // Step 18. Check for new best move
983 splitPoint->mutex.lock();
984 bestValue = splitPoint->bestValue;
985 alpha = splitPoint->alpha;
988 // Finished searching the move. If Signals.stop is true, the search
989 // was aborted because the user interrupted the search or because we
990 // ran out of time. In this case, the return value of the search cannot
991 // be trusted, and we don't update the best move and/or PV.
992 if (Signals.stop || thisThread->cutoff_occurred())
993 return value; // To avoid returning VALUE_INFINITE
997 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
999 // PV move or new best move ?
1000 if (pvMove || value > alpha)
1003 rm.extract_pv_from_tt(pos);
1005 // We record how often the best move has been changed in each
1006 // iteration. This information is used for time management: When
1007 // the best move changes frequently, we allocate some more time.
1012 // All other moves but the PV are set to the lowest value, this
1013 // is not a problem when sorting becuase sort is stable and move
1014 // position in the list is preserved, just the PV is pushed up.
1015 rm.score = -VALUE_INFINITE;
1018 if (value > bestValue)
1020 bestValue = SpNode ? splitPoint->bestValue = value : value;
1024 bestMove = SpNode ? splitPoint->bestMove = move : move;
1026 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1027 alpha = SpNode ? splitPoint->alpha = value : value;
1030 assert(value >= beta); // Fail high
1033 splitPoint->cutoff = true;
1040 // Step 19. Check for splitting the search
1042 && depth >= Threads.minimumSplitDepth
1043 && Threads.available_slave(thisThread)
1044 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1046 assert(bestValue < beta);
1048 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1049 depth, threatMove, moveCount, &mp, NT, cutNode);
1050 if (bestValue >= beta)
1058 // Step 20. Check for mate and stalemate
1059 // All legal moves have been searched and if there are no legal moves, it
1060 // must be mate or stalemate. Note that we can have a false positive in
1061 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1062 // harmless because return value is discarded anyhow in the parent nodes.
1063 // If we are in a singular extension search then return a fail low score.
1064 // A split node has at least one move, the one tried before to be splitted.
1066 return excludedMove ? alpha
1067 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1069 // If we have pruned all the moves without searching return a fail-low score
1070 if (bestValue == -VALUE_INFINITE)
1073 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1074 bestValue >= beta ? BOUND_LOWER :
1075 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1076 depth, bestMove, ss->staticEval, ss->evalMargin);
1078 // Quiet best move: update killers, history and countermoves
1079 if ( bestValue >= beta
1080 && !pos.is_capture_or_promotion(bestMove)
1083 if (ss->killers[0] != bestMove)
1085 ss->killers[1] = ss->killers[0];
1086 ss->killers[0] = bestMove;
1089 // Increase history value of the cut-off move and decrease all the other
1090 // played non-capture moves.
1091 Value bonus = Value(int(depth) * int(depth));
1092 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1093 for (int i = 0; i < quietCount - 1; i++)
1095 Move m = quietsSearched[i];
1096 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1099 if (is_ok((ss-1)->currentMove))
1100 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1103 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1109 // qsearch() is the quiescence search function, which is called by the main
1110 // search function when the remaining depth is zero (or, to be more precise,
1111 // less than ONE_PLY).
1113 template <NodeType NT, bool InCheck>
1114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1116 const bool PvNode = (NT == PV);
1118 assert(NT == PV || NT == NonPV);
1119 assert(InCheck == !!pos.checkers());
1120 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1121 assert(PvNode || (alpha == beta - 1));
1122 assert(depth <= DEPTH_ZERO);
1127 Move ttMove, move, bestMove;
1128 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1129 bool givesCheck, evasionPrunable;
1132 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1136 ss->currentMove = bestMove = MOVE_NONE;
1137 ss->ply = (ss-1)->ply + 1;
1139 // Check for an instant draw or maximum ply reached
1140 if (pos.is_draw() || ss->ply > MAX_PLY)
1141 return DrawValue[pos.side_to_move()];
1143 // Decide whether or not to include checks, this fixes also the type of
1144 // TT entry depth that we are going to use. Note that in qsearch we use
1145 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1146 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1147 : DEPTH_QS_NO_CHECKS;
1149 // Transposition table lookup
1151 tte = TT.probe(posKey);
1152 ttMove = tte ? tte->move() : MOVE_NONE;
1153 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1156 && tte->depth() >= ttDepth
1157 && ttValue != VALUE_NONE // Only in case of TT access race
1158 && ( PvNode ? tte->bound() == BOUND_EXACT
1159 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1160 : (tte->bound() & BOUND_UPPER)))
1162 ss->currentMove = ttMove; // Can be MOVE_NONE
1166 // Evaluate the position statically
1169 ss->staticEval = ss->evalMargin = VALUE_NONE;
1170 bestValue = futilityBase = -VALUE_INFINITE;
1176 // Never assume anything on values stored in TT
1177 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1178 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1179 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1182 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1184 // Stand pat. Return immediately if static value is at least beta
1185 if (bestValue >= beta)
1188 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1189 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1194 if (PvNode && bestValue > alpha)
1197 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1200 // Initialize a MovePicker object for the current position, and prepare
1201 // to search the moves. Because the depth is <= 0 here, only captures,
1202 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1204 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1207 // Loop through the moves until no moves remain or a beta cutoff occurs
1208 while ((move = mp.next_move<false>()) != MOVE_NONE)
1210 assert(is_ok(move));
1212 givesCheck = pos.move_gives_check(move, ci);
1219 && type_of(move) != PROMOTION
1220 && !pos.is_passed_pawn_push(move))
1222 futilityValue = futilityBase
1223 + PieceValue[EG][pos.piece_on(to_sq(move))]
1224 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1226 if (futilityValue < beta)
1228 bestValue = std::max(bestValue, futilityValue);
1232 // Prune moves with negative or equal SEE and also moves with positive
1233 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1234 if ( futilityBase < beta
1235 && pos.see(move, beta - futilityBase) <= 0)
1237 bestValue = std::max(bestValue, futilityBase);
1242 // Detect non-capture evasions that are candidate to be pruned
1243 evasionPrunable = InCheck
1244 && bestValue > VALUE_MATED_IN_MAX_PLY
1245 && !pos.is_capture(move)
1246 && !pos.can_castle(pos.side_to_move());
1248 // Don't search moves with negative SEE values
1250 && (!InCheck || evasionPrunable)
1252 && type_of(move) != PROMOTION
1253 && pos.see_sign(move) < 0)
1256 // Don't search useless checks
1261 && !pos.is_capture_or_promotion(move)
1262 && ss->staticEval + PawnValueMg / 4 < beta
1263 && !check_is_dangerous(pos, move, futilityBase, beta))
1266 // Check for legality only before to do the move
1267 if (!pos.pl_move_is_legal(move, ci.pinned))
1270 ss->currentMove = move;
1272 // Make and search the move
1273 pos.do_move(move, st, ci, givesCheck);
1274 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1275 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1276 pos.undo_move(move);
1278 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1280 // Check for new best move
1281 if (value > bestValue)
1287 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1294 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1295 ttDepth, move, ss->staticEval, ss->evalMargin);
1303 // All legal moves have been searched. A special case: If we're in check
1304 // and no legal moves were found, it is checkmate.
1305 if (InCheck && bestValue == -VALUE_INFINITE)
1306 return mated_in(ss->ply); // Plies to mate from the root
1308 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1309 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1310 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1312 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1318 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1319 // "plies to mate from the current position". Non-mate scores are unchanged.
1320 // The function is called before storing a value to the transposition table.
1322 Value value_to_tt(Value v, int ply) {
1324 assert(v != VALUE_NONE);
1326 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1327 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1331 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1332 // from the transposition table (where refers to the plies to mate/be mated
1333 // from current position) to "plies to mate/be mated from the root".
1335 Value value_from_tt(Value v, int ply) {
1337 return v == VALUE_NONE ? VALUE_NONE
1338 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1339 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1343 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1345 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1347 Piece pc = pos.piece_moved(move);
1348 Square from = from_sq(move);
1349 Square to = to_sq(move);
1350 Color them = ~pos.side_to_move();
1351 Square ksq = pos.king_square(them);
1352 Bitboard enemies = pos.pieces(them);
1353 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1354 Bitboard occ = pos.pieces() ^ from ^ ksq;
1355 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1356 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1358 // Checks which give opponent's king at most one escape square are dangerous
1359 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1362 // Queen contact check is very dangerous
1363 if (type_of(pc) == QUEEN && (kingAtt & to))
1366 // Creating new double threats with checks is dangerous
1367 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1370 // Note that here we generate illegal "double move"!
1371 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1379 // allows() tests whether the 'first' move at previous ply somehow makes the
1380 // 'second' move possible, for instance if the moving piece is the same in
1381 // both moves. Normally the second move is the threat (the best move returned
1382 // from a null search that fails low).
1384 bool allows(const Position& pos, Move first, Move second) {
1386 assert(is_ok(first));
1387 assert(is_ok(second));
1388 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1389 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1391 Square m1from = from_sq(first);
1392 Square m2from = from_sq(second);
1393 Square m1to = to_sq(first);
1394 Square m2to = to_sq(second);
1396 // The piece is the same or second's destination was vacated by the first move
1397 if (m1to == m2from || m2to == m1from)
1400 // Second one moves through the square vacated by first one
1401 if (between_bb(m2from, m2to) & m1from)
1404 // Second's destination is defended by the first move's piece
1405 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1409 // Second move gives a discovered check through the first's checking piece
1410 if (m1att & pos.king_square(pos.side_to_move()))
1412 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1420 // refutes() tests whether a 'first' move is able to defend against a 'second'
1421 // opponent's move. In this case will not be pruned. Normally the second move
1422 // is the threat (the best move returned from a null search that fails low).
1424 bool refutes(const Position& pos, Move first, Move second) {
1426 assert(is_ok(first));
1427 assert(is_ok(second));
1429 Square m1from = from_sq(first);
1430 Square m2from = from_sq(second);
1431 Square m1to = to_sq(first);
1432 Square m2to = to_sq(second);
1434 // Don't prune moves of the threatened piece
1438 // If the threatened piece has value less than or equal to the value of the
1439 // threat piece, don't prune moves which defend it.
1440 if ( pos.is_capture(second)
1441 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1442 || type_of(pos.piece_on(m2from)) == KING))
1444 // Update occupancy as if the piece and the threat are moving
1445 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1446 Piece pc = pos.piece_on(m1from);
1448 // The moved piece attacks the square 'tto' ?
1449 if (pos.attacks_from(pc, m1to, occ) & m2to)
1452 // Scan for possible X-ray attackers behind the moved piece
1453 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1454 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1456 // Verify attackers are triggered by our move and not already existing
1457 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1461 // Don't prune safe moves which block the threat path
1462 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1469 // When playing with strength handicap choose best move among the MultiPV set
1470 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1472 Move Skill::pick_move() {
1476 // PRNG sequence should be not deterministic
1477 for (int i = Time::now() % 50; i > 0; i--)
1478 rk.rand<unsigned>();
1480 // RootMoves are already sorted by score in descending order
1481 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1482 int weakness = 120 - 2 * level;
1483 int max_s = -VALUE_INFINITE;
1486 // Choose best move. For each move score we add two terms both dependent on
1487 // weakness, one deterministic and bigger for weaker moves, and one random,
1488 // then we choose the move with the resulting highest score.
1489 for (size_t i = 0; i < PVSize; i++)
1491 int s = RootMoves[i].score;
1493 // Don't allow crazy blunders even at very low skills
1494 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1497 // This is our magic formula
1498 s += ( weakness * int(RootMoves[0].score - s)
1499 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1504 best = RootMoves[i].pv[0];
1511 // uci_pv() formats PV information according to UCI protocol. UCI requires
1512 // to send all the PV lines also if are still to be searched and so refer to
1513 // the previous search score.
1515 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1517 std::stringstream s;
1518 Time::point elapsed = Time::now() - SearchTime + 1;
1519 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1522 for (size_t i = 0; i < Threads.size(); i++)
1523 if (Threads[i]->maxPly > selDepth)
1524 selDepth = Threads[i]->maxPly;
1526 for (size_t i = 0; i < uciPVSize; i++)
1528 bool updated = (i <= PVIdx);
1530 if (depth == 1 && !updated)
1533 int d = updated ? depth : depth - 1;
1534 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1536 if (s.rdbuf()->in_avail()) // Not at first line
1539 s << "info depth " << d
1540 << " seldepth " << selDepth
1541 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1542 << " nodes " << pos.nodes_searched()
1543 << " nps " << pos.nodes_searched() * 1000 / elapsed
1544 << " time " << elapsed
1545 << " multipv " << i + 1
1548 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1549 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1558 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1559 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1560 /// allow to always have a ponder move even when we fail high at root, and a
1561 /// long PV to print that is important for position analysis.
1563 void RootMove::extract_pv_from_tt(Position& pos) {
1565 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1575 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1577 pos.do_move(pv[ply++], *st++);
1578 tte = TT.probe(pos.key());
1581 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1582 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1584 && (!pos.is_draw() || ply < 2));
1586 pv.push_back(MOVE_NONE); // Must be zero-terminating
1588 while (ply) pos.undo_move(pv[--ply]);
1592 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1593 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1594 /// first, even if the old TT entries have been overwritten.
1596 void RootMove::insert_pv_in_tt(Position& pos) {
1598 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1603 tte = TT.probe(pos.key());
1605 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1606 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1608 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1610 pos.do_move(pv[ply++], *st++);
1612 } while (pv[ply] != MOVE_NONE);
1614 while (ply) pos.undo_move(pv[--ply]);
1618 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1620 void Thread::idle_loop() {
1622 // Pointer 'this_sp' is not null only if we are called from split(), and not
1623 // at the thread creation. So it means we are the split point's master.
1624 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1626 assert(!this_sp || (this_sp->masterThread == this && searching));
1630 // If we are not searching, wait for a condition to be signaled instead of
1631 // wasting CPU time polling for work.
1632 while ((!searching && Threads.sleepWhileIdle) || exit)
1640 // Grab the lock to avoid races with Thread::notify_one()
1643 // If we are master and all slaves have finished then exit idle_loop
1644 if (this_sp && !this_sp->slavesMask)
1650 // Do sleep after retesting sleep conditions under lock protection, in
1651 // particular we need to avoid a deadlock in case a master thread has,
1652 // in the meanwhile, allocated us and sent the notify_one() call before
1653 // we had the chance to grab the lock.
1654 if (!searching && !exit)
1655 sleepCondition.wait(mutex);
1660 // If this thread has been assigned work, launch a search
1665 Threads.mutex.lock();
1668 assert(activeSplitPoint);
1669 SplitPoint* sp = activeSplitPoint;
1671 Threads.mutex.unlock();
1673 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
1674 Position pos(*sp->pos, this);
1676 std::memcpy(ss-1, sp->ss-1, 4 * sizeof(Stack));
1677 ss->splitPoint = sp;
1681 assert(activePosition == NULL);
1683 activePosition = &pos;
1685 switch (sp->nodeType) {
1687 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1690 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1693 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1702 activePosition = NULL;
1703 sp->slavesMask &= ~(1ULL << idx);
1704 sp->nodes += pos.nodes_searched();
1706 // Wake up master thread so to allow it to return from the idle loop
1707 // in case we are the last slave of the split point.
1708 if ( Threads.sleepWhileIdle
1709 && this != sp->masterThread
1712 assert(!sp->masterThread->searching);
1713 sp->masterThread->notify_one();
1716 // After releasing the lock we cannot access anymore any SplitPoint
1717 // related data in a safe way becuase it could have been released under
1718 // our feet by the sp master. Also accessing other Thread objects is
1719 // unsafe because if we are exiting there is a chance are already freed.
1723 // If this thread is the master of a split point and all slaves have finished
1724 // their work at this split point, return from the idle loop.
1725 if (this_sp && !this_sp->slavesMask)
1727 this_sp->mutex.lock();
1728 bool finished = !this_sp->slavesMask; // Retest under lock protection
1729 this_sp->mutex.unlock();
1737 /// check_time() is called by the timer thread when the timer triggers. It is
1738 /// used to print debug info and, more important, to detect when we are out of
1739 /// available time and so stop the search.
1743 static Time::point lastInfoTime = Time::now();
1744 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1746 if (Time::now() - lastInfoTime >= 1000)
1748 lastInfoTime = Time::now();
1757 Threads.mutex.lock();
1759 nodes = RootPos.nodes_searched();
1761 // Loop across all split points and sum accumulated SplitPoint nodes plus
1762 // all the currently active positions nodes.
1763 for (size_t i = 0; i < Threads.size(); i++)
1764 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1766 SplitPoint& sp = Threads[i]->splitPoints[j];
1771 Bitboard sm = sp.slavesMask;
1774 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1776 nodes += pos->nodes_searched();
1782 Threads.mutex.unlock();
1785 Time::point elapsed = Time::now() - SearchTime;
1786 bool stillAtFirstMove = Signals.firstRootMove
1787 && !Signals.failedLowAtRoot
1788 && elapsed > TimeMgr.available_time();
1790 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1791 || stillAtFirstMove;
1793 if ( (Limits.use_time_management() && noMoreTime)
1794 || (Limits.movetime && elapsed >= Limits.movetime)
1795 || (Limits.nodes && nodes >= Limits.nodes))
1796 Signals.stop = true;