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 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
301 depth = BestMoveChanges = 0;
302 bestValue = delta = alpha = -VALUE_INFINITE;
303 beta = VALUE_INFINITE;
308 Countermoves.clear();
310 PVSize = Options["MultiPV"];
311 Skill skill(Options["Skill Level"]);
313 // Do we have to play with skill handicap? In this case enable MultiPV search
314 // that we will use behind the scenes to retrieve a set of possible moves.
315 if (skill.enabled() && PVSize < 4)
318 PVSize = std::min(PVSize, RootMoves.size());
320 // Iterative deepening loop until requested to stop or target depth reached
321 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
323 // Save last iteration's scores before first PV line is searched and all
324 // the move scores but the (new) PV are set to -VALUE_INFINITE.
325 for (size_t i = 0; i < RootMoves.size(); i++)
326 RootMoves[i].prevScore = RootMoves[i].score;
328 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
331 // MultiPV loop. We perform a full root search for each PV line
332 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
334 // Reset aspiration window starting size
338 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
339 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
342 // Start with a small aspiration window and, in case of fail high/low,
343 // research with bigger window until not failing high/low anymore.
346 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
348 // Bring to front the best move. It is critical that sorting is
349 // done with a stable algorithm because all the values but the first
350 // and eventually the new best one are set to -VALUE_INFINITE and
351 // we want to keep the same order for all the moves but the new
352 // PV that goes to the front. Note that in case of MultiPV search
353 // the already searched PV lines are preserved.
354 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
356 // Write PV back to transposition table in case the relevant
357 // entries have been overwritten during the search.
358 for (size_t i = 0; i <= PVIdx; i++)
359 RootMoves[i].insert_pv_in_tt(pos);
361 // If search has been stopped return immediately. Sorting and
362 // writing PV back to TT is safe becuase RootMoves is still
363 // valid, although refers to previous iteration.
367 // In case of failing low/high increase aspiration window and
368 // research, otherwise exit the loop.
369 if (bestValue <= alpha)
371 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
373 Signals.failedLowAtRoot = true;
374 Signals.stopOnPonderhit = false;
376 else if (bestValue >= beta)
377 beta = std::min(bestValue + delta, VALUE_INFINITE);
384 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
386 // Give some update (without cluttering the UI) before to research
387 if (Time::now() - SearchTime > 3000)
388 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
391 // Sort the PV lines searched so far and update the GUI
392 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
394 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
395 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
398 // Do we need to pick now the sub-optimal best move ?
399 if (skill.enabled() && skill.time_to_pick(depth))
402 if (Options["Use Search Log"])
404 RootMove& rm = RootMoves[0];
405 if (skill.best != MOVE_NONE)
406 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
408 Log log(Options["Search Log Filename"]);
409 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
413 // Do we have found a "mate in x"?
415 && bestValue >= VALUE_MATE_IN_MAX_PLY
416 && VALUE_MATE - bestValue <= 2 * Limits.mate)
419 // Do we have time for the next iteration? Can we stop searching now?
420 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
422 bool stop = false; // Local variable, not the volatile Signals.stop
424 // Take in account some extra time if the best move has changed
425 if (depth > 4 && depth < 50 && PVSize == 1)
426 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
428 // Stop search if most of available time is already consumed. We
429 // probably don't have enough time to search the first move at the
430 // next iteration anyway.
431 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
434 // Stop search early if one move seems to be much better than others
438 && bestValue > VALUE_MATED_IN_MAX_PLY
439 && ( RootMoves.size() == 1
440 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
442 Value rBeta = bestValue - 2 * PawnValueMg;
443 ss->excludedMove = RootMoves[0].pv[0];
444 ss->skipNullMove = true;
445 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
446 ss->skipNullMove = false;
447 ss->excludedMove = MOVE_NONE;
455 // If we are allowed to ponder do not stop the search now but
456 // keep pondering until GUI sends "ponderhit" or "stop".
458 Signals.stopOnPonderhit = true;
467 // search<>() is the main search function for both PV and non-PV nodes and for
468 // normal and SplitPoint nodes. When called just after a split point the search
469 // is simpler because we have already probed the hash table, done a null move
470 // search, and searched the first move before splitting, we don't have to repeat
471 // all this work again. We also don't need to store anything to the hash table
472 // here: This is taken care of after we return from the split point.
474 template <NodeType NT>
475 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
477 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
478 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
479 const bool RootNode = (NT == Root || NT == SplitPointRoot);
481 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
482 assert(PvNode || (alpha == beta - 1));
483 assert(depth > DEPTH_ZERO);
485 Move quietsSearched[64];
488 SplitPoint* splitPoint;
490 Move ttMove, move, excludedMove, bestMove, threatMove;
492 Value bestValue, value, ttValue;
493 Value eval, nullValue, futilityValue;
494 bool inCheck, givesCheck, pvMove, singularExtensionNode;
495 bool captureOrPromotion, dangerous, doFullDepthSearch;
496 int moveCount, quietCount;
498 // Step 1. Initialize node
499 Thread* thisThread = pos.this_thread();
500 moveCount = quietCount = 0;
501 inCheck = pos.checkers();
505 splitPoint = ss->splitPoint;
506 bestMove = splitPoint->bestMove;
507 threatMove = splitPoint->threatMove;
508 bestValue = splitPoint->bestValue;
510 ttMove = excludedMove = MOVE_NONE;
511 ttValue = VALUE_NONE;
513 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
518 bestValue = -VALUE_INFINITE;
519 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
520 ss->ply = (ss-1)->ply + 1;
521 ss->futilityMoveCount = 0;
522 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
523 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
525 // Used to send selDepth info to GUI
526 if (PvNode && thisThread->maxPly < ss->ply)
527 thisThread->maxPly = ss->ply;
531 // Step 2. Check for aborted search and immediate draw
532 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
533 return DrawValue[pos.side_to_move()];
535 // Step 3. Mate distance pruning. Even if we mate at the next move our score
536 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
537 // a shorter mate was found upward in the tree then there is no need to search
538 // further, we will never beat current alpha. Same logic but with reversed signs
539 // applies also in the opposite condition of being mated instead of giving mate,
540 // in this case return a fail-high score.
541 alpha = std::max(mated_in(ss->ply), alpha);
542 beta = std::min(mate_in(ss->ply+1), beta);
547 // Step 4. Transposition table lookup
548 // We don't want the score of a partial search to overwrite a previous full search
549 // TT value, so we use a different position key in case of an excluded move.
550 excludedMove = ss->excludedMove;
551 posKey = excludedMove ? pos.exclusion_key() : pos.key();
552 tte = TT.probe(posKey);
553 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
554 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
556 // At PV nodes we check for exact scores, while at non-PV nodes we check for
557 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
558 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
559 // we should also update RootMoveList to avoid bogus output.
562 && tte->depth() >= depth
563 && ttValue != VALUE_NONE // Only in case of TT access race
564 && ( PvNode ? tte->bound() == BOUND_EXACT
565 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
566 : (tte->bound() & BOUND_UPPER)))
569 ss->currentMove = ttMove; // Can be MOVE_NONE
573 && !pos.is_capture_or_promotion(ttMove)
574 && ttMove != ss->killers[0])
576 ss->killers[1] = ss->killers[0];
577 ss->killers[0] = ttMove;
582 // Step 5. Evaluate the position statically and update parent's gain statistics
585 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
591 // Never assume anything on values stored in TT
592 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
593 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
594 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
596 // Can ttValue be used as a better position evaluation?
597 if (ttValue != VALUE_NONE)
598 if ( ((tte->bound() & BOUND_LOWER) && ttValue > eval)
599 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
604 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
605 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
606 ss->staticEval, ss->evalMargin);
609 // Update gain for the parent non-capture move given the static position
610 // evaluation before and after the move.
611 if ( !pos.captured_piece_type()
612 && ss->staticEval != VALUE_NONE
613 && (ss-1)->staticEval != VALUE_NONE
614 && (move = (ss-1)->currentMove) != MOVE_NULL
615 && type_of(move) == NORMAL)
617 Square to = to_sq(move);
618 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
621 // Step 6. Razoring (is omitted in PV nodes)
623 && depth < 4 * ONE_PLY
624 && eval + razor_margin(depth) < beta
625 && ttMove == MOVE_NONE
626 && abs(beta) < VALUE_MATE_IN_MAX_PLY
627 && !pos.pawn_on_7th(pos.side_to_move()))
629 Value rbeta = beta - razor_margin(depth);
630 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
632 // Logically we should return (v + razor_margin(depth)), but
633 // surprisingly this did slightly weaker in tests.
637 // Step 7. Static null move pruning (is omitted in PV nodes)
638 // We're betting that the opponent doesn't have a move that will reduce
639 // the score by more than futility_margin(depth) if we do a null move.
642 && depth < 4 * ONE_PLY
643 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
644 && abs(beta) < VALUE_MATE_IN_MAX_PLY
645 && abs(eval) < VALUE_KNOWN_WIN
646 && pos.non_pawn_material(pos.side_to_move()))
647 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
649 // Step 8. Null move search with verification search (is omitted in PV nodes)
654 && abs(beta) < VALUE_MATE_IN_MAX_PLY
655 && pos.non_pawn_material(pos.side_to_move()))
657 ss->currentMove = MOVE_NULL;
659 // Null move dynamic reduction based on depth
660 Depth R = 3 * ONE_PLY + depth / 4;
662 // Null move dynamic reduction based on value
663 if (eval - PawnValueMg > beta)
666 pos.do_null_move(st);
667 (ss+1)->skipNullMove = true;
668 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
669 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
670 (ss+1)->skipNullMove = false;
671 pos.undo_null_move();
673 if (nullValue >= beta)
675 // Do not return unproven mate scores
676 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
679 if (depth < 12 * ONE_PLY)
682 // Do verification search at high depths
683 ss->skipNullMove = true;
684 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
685 ss->skipNullMove = false;
692 // The null move failed low, which means that we may be faced with
693 // some kind of threat. If the previous move was reduced, check if
694 // the move that refuted the null move was somehow connected to the
695 // move which was reduced. If a connection is found, return a fail
696 // low score (which will cause the reduced move to fail high in the
697 // parent node, which will trigger a re-search with full depth).
698 threatMove = (ss+1)->currentMove;
700 if ( depth < 5 * ONE_PLY
702 && threatMove != MOVE_NONE
703 && allows(pos, (ss-1)->currentMove, threatMove))
708 // Step 9. ProbCut (is omitted in PV nodes)
709 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
710 // and a reduced search returns a value much above beta, we can (almost) safely
711 // prune the previous move.
713 && depth >= 5 * ONE_PLY
715 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
717 Value rbeta = beta + 200;
718 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
720 assert(rdepth >= ONE_PLY);
721 assert((ss-1)->currentMove != MOVE_NONE);
722 assert((ss-1)->currentMove != MOVE_NULL);
724 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
727 while ((move = mp.next_move<false>()) != MOVE_NONE)
728 if (pos.pl_move_is_legal(move, ci.pinned))
730 ss->currentMove = move;
731 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
732 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
739 // Step 10. Internal iterative deepening
740 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
741 && ttMove == MOVE_NONE
742 && (PvNode || ss->staticEval + Value(256) >= beta))
744 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
746 ss->skipNullMove = true;
747 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
748 ss->skipNullMove = false;
750 tte = TT.probe(posKey);
751 ttMove = tte ? tte->move() : MOVE_NONE;
754 moves_loop: // When in check and at SpNode search starts from here
756 Square prevMoveSq = to_sq((ss-1)->currentMove);
757 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
758 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
760 MovePicker mp(pos, ttMove, depth, History, countermoves, ss, PvNode ? -VALUE_INFINITE : beta);
762 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
763 singularExtensionNode = !RootNode
765 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
766 && ttMove != MOVE_NONE
767 && !excludedMove // Recursive singular search is not allowed
768 && (tte->bound() & BOUND_LOWER)
769 && tte->depth() >= depth - 3 * ONE_PLY;
771 // Step 11. Loop through moves
772 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
773 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
777 if (move == excludedMove)
780 // At root obey the "searchmoves" option and skip moves not listed in Root
781 // Move List, as a consequence any illegal move is also skipped. In MultiPV
782 // mode we also skip PV moves which have been already searched.
783 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
788 // Shared counter cannot be decremented later if move turns out to be illegal
789 if (!pos.pl_move_is_legal(move, ci.pinned))
792 moveCount = ++splitPoint->moveCount;
793 splitPoint->mutex.unlock();
800 Signals.firstRootMove = (moveCount == 1);
802 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
803 sync_cout << "info depth " << depth / ONE_PLY
804 << " currmove " << move_to_uci(move, pos.is_chess960())
805 << " currmovenumber " << moveCount + PVIdx << sync_endl;
809 captureOrPromotion = pos.is_capture_or_promotion(move);
810 givesCheck = pos.move_gives_check(move, ci);
811 dangerous = givesCheck
812 || pos.is_passed_pawn_push(move)
813 || type_of(move) == CASTLE;
815 // Step 12. Extend checks and, in PV nodes, also dangerous moves
816 if (PvNode && dangerous)
819 else if (givesCheck && pos.see_sign(move) >= 0)
822 // Singular extension search. If all moves but one fail low on a search of
823 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
824 // is singular and should be extended. To verify this we do a reduced search
825 // on all the other moves but the ttMove, if result is lower than ttValue minus
826 // a margin then we extend ttMove.
827 if ( singularExtensionNode
830 && pos.pl_move_is_legal(move, ci.pinned)
831 && abs(ttValue) < VALUE_KNOWN_WIN)
833 assert(ttValue != VALUE_NONE);
835 Value rBeta = ttValue - int(depth);
836 ss->excludedMove = move;
837 ss->skipNullMove = true;
838 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
839 ss->skipNullMove = false;
840 ss->excludedMove = MOVE_NONE;
846 // Update current move (this must be done after singular extension search)
847 newDepth = depth - ONE_PLY + ext;
849 // Step 13. Futility pruning (is omitted in PV nodes)
851 && !captureOrPromotion
854 /* && move != ttMove Already implicit in the next condition */
855 && bestValue > VALUE_MATED_IN_MAX_PLY)
857 // Move count based pruning
858 if ( depth < 16 * ONE_PLY
859 && moveCount >= FutilityMoveCounts[depth]
860 && (!threatMove || !refutes(pos, move, threatMove)))
863 splitPoint->mutex.lock();
868 // Value based pruning
869 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
870 // but fixing this made program slightly weaker.
871 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
872 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
873 + Gains[pos.piece_moved(move)][to_sq(move)];
875 if (futilityValue < beta)
877 bestValue = std::max(bestValue, futilityValue);
881 splitPoint->mutex.lock();
882 if (bestValue > splitPoint->bestValue)
883 splitPoint->bestValue = bestValue;
888 // Prune moves with negative SEE at low depths
889 if ( predictedDepth < 4 * ONE_PLY
890 && pos.see_sign(move) < 0)
893 splitPoint->mutex.lock();
898 // We have not pruned the move that will be searched, but remember how
899 // far in the move list we are to be more aggressive in the child node.
900 ss->futilityMoveCount = moveCount;
903 ss->futilityMoveCount = 0;
905 // Check for legality only before to do the move
906 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
912 pvMove = PvNode && moveCount == 1;
913 ss->currentMove = move;
914 if (!SpNode && !captureOrPromotion && quietCount < 64)
915 quietsSearched[quietCount++] = move;
917 // Step 14. Make the move
918 pos.do_move(move, st, ci, givesCheck);
920 // Step 15. Reduced depth search (LMR). If the move fails high will be
921 // re-searched at full depth.
922 if ( depth > 3 * ONE_PLY
924 && !captureOrPromotion
927 && move != ss->killers[0]
928 && move != ss->killers[1])
930 ss->reduction = reduction<PvNode>(depth, moveCount);
932 if (!PvNode && cutNode)
933 ss->reduction += ONE_PLY;
935 if (move == countermoves[0] || move == countermoves[1])
936 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
938 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
940 alpha = splitPoint->alpha;
942 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
944 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
945 ss->reduction = DEPTH_ZERO;
948 doFullDepthSearch = !pvMove;
950 // Step 16. Full depth search, when LMR is skipped or fails high
951 if (doFullDepthSearch)
954 alpha = splitPoint->alpha;
956 value = newDepth < ONE_PLY ?
957 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
958 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
959 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
962 // Only for PV nodes do a full PV search on the first move or after a fail
963 // high, in the latter case search only if value < beta, otherwise let the
964 // parent node to fail low with value <= alpha and to try another move.
965 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
966 value = newDepth < ONE_PLY ?
967 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
968 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
969 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
970 // Step 17. Undo move
973 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
975 // Step 18. Check for new best move
978 splitPoint->mutex.lock();
979 bestValue = splitPoint->bestValue;
980 alpha = splitPoint->alpha;
983 // Finished searching the move. If Signals.stop is true, the search
984 // was aborted because the user interrupted the search or because we
985 // ran out of time. In this case, the return value of the search cannot
986 // be trusted, and we don't update the best move and/or PV.
987 if (Signals.stop || thisThread->cutoff_occurred())
988 return value; // To avoid returning VALUE_INFINITE
992 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
994 // PV move or new best move ?
995 if (pvMove || value > alpha)
998 rm.extract_pv_from_tt(pos);
1000 // We record how often the best move has been changed in each
1001 // iteration. This information is used for time management: When
1002 // the best move changes frequently, we allocate some more time.
1007 // All other moves but the PV are set to the lowest value, this
1008 // is not a problem when sorting becuase sort is stable and move
1009 // position in the list is preserved, just the PV is pushed up.
1010 rm.score = -VALUE_INFINITE;
1013 if (value > bestValue)
1015 bestValue = SpNode ? splitPoint->bestValue = value : value;
1019 bestMove = SpNode ? splitPoint->bestMove = move : move;
1021 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1022 alpha = SpNode ? splitPoint->alpha = value : value;
1025 assert(value >= beta); // Fail high
1028 splitPoint->cutoff = true;
1035 // Step 19. Check for splitting the search
1037 && depth >= Threads.minimumSplitDepth
1038 && Threads.available_slave(thisThread)
1039 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1041 assert(bestValue < beta);
1043 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1044 depth, threatMove, moveCount, &mp, NT, cutNode);
1045 if (bestValue >= beta)
1053 // Step 20. Check for mate and stalemate
1054 // All legal moves have been searched and if there are no legal moves, it
1055 // must be mate or stalemate. Note that we can have a false positive in
1056 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1057 // harmless because return value is discarded anyhow in the parent nodes.
1058 // If we are in a singular extension search then return a fail low score.
1059 // A split node has at least one move, the one tried before to be splitted.
1061 return excludedMove ? alpha
1062 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1064 // If we have pruned all the moves without searching return a fail-low score
1065 if (bestValue == -VALUE_INFINITE)
1068 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1069 bestValue >= beta ? BOUND_LOWER :
1070 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1071 depth, bestMove, ss->staticEval, ss->evalMargin);
1073 // Quiet best move: update killers, history and countermoves
1074 if ( bestValue >= beta
1075 && !pos.is_capture_or_promotion(bestMove)
1078 if (ss->killers[0] != bestMove)
1080 ss->killers[1] = ss->killers[0];
1081 ss->killers[0] = bestMove;
1084 // Increase history value of the cut-off move and decrease all the other
1085 // played non-capture moves.
1086 Value bonus = Value(int(depth) * int(depth));
1087 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1088 for (int i = 0; i < quietCount - 1; i++)
1090 Move m = quietsSearched[i];
1091 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1094 if (is_ok((ss-1)->currentMove))
1095 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1098 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1104 // qsearch() is the quiescence search function, which is called by the main
1105 // search function when the remaining depth is zero (or, to be more precise,
1106 // less than ONE_PLY).
1108 template <NodeType NT, bool InCheck>
1109 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1111 const bool PvNode = (NT == PV);
1113 assert(NT == PV || NT == NonPV);
1114 assert(InCheck == !!pos.checkers());
1115 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1116 assert(PvNode || (alpha == beta - 1));
1117 assert(depth <= DEPTH_ZERO);
1122 Move ttMove, move, bestMove;
1123 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1124 bool givesCheck, enoughMaterial, evasionPrunable;
1127 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1131 ss->currentMove = bestMove = MOVE_NONE;
1132 ss->ply = (ss-1)->ply + 1;
1134 // Check for an instant draw or maximum ply reached
1135 if (pos.is_draw() || ss->ply > MAX_PLY)
1136 return DrawValue[pos.side_to_move()];
1138 // Decide whether or not to include checks, this fixes also the type of
1139 // TT entry depth that we are going to use. Note that in qsearch we use
1140 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1141 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1142 : DEPTH_QS_NO_CHECKS;
1144 // Transposition table lookup
1146 tte = TT.probe(posKey);
1147 ttMove = tte ? tte->move() : MOVE_NONE;
1148 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1151 && tte->depth() >= ttDepth
1152 && ttValue != VALUE_NONE // Only in case of TT access race
1153 && ( PvNode ? tte->bound() == BOUND_EXACT
1154 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1155 : (tte->bound() & BOUND_UPPER)))
1157 ss->currentMove = ttMove; // Can be MOVE_NONE
1161 // Evaluate the position statically
1164 ss->staticEval = ss->evalMargin = VALUE_NONE;
1165 bestValue = futilityBase = -VALUE_INFINITE;
1166 enoughMaterial = false;
1172 // Never assume anything on values stored in TT
1173 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1174 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1175 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1178 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1180 // Stand pat. Return immediately if static value is at least beta
1181 if (bestValue >= beta)
1184 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1185 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1190 if (PvNode && bestValue > alpha)
1193 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1194 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1197 // Initialize a MovePicker object for the current position, and prepare
1198 // to search the moves. Because the depth is <= 0 here, only captures,
1199 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1201 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1204 // Loop through the moves until no moves remain or a beta cutoff occurs
1205 while ((move = mp.next_move<false>()) != MOVE_NONE)
1207 assert(is_ok(move));
1209 givesCheck = pos.move_gives_check(move, ci);
1217 && type_of(move) != PROMOTION
1218 && !pos.is_passed_pawn_push(move))
1220 futilityValue = futilityBase
1221 + PieceValue[EG][pos.piece_on(to_sq(move))]
1222 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1224 if (futilityValue < beta)
1226 bestValue = std::max(bestValue, futilityValue);
1230 // Prune moves with negative or equal SEE and also moves with positive
1231 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1232 if ( futilityBase < beta
1233 && pos.see(move, beta - futilityBase) <= 0)
1235 bestValue = std::max(bestValue, futilityBase);
1240 // Detect non-capture evasions that are candidate to be pruned
1241 evasionPrunable = !PvNode
1243 && bestValue > VALUE_MATED_IN_MAX_PLY
1244 && !pos.is_capture(move)
1245 && !pos.can_castle(pos.side_to_move());
1247 // Don't search moves with negative SEE values
1249 && (!InCheck || evasionPrunable)
1251 && type_of(move) != PROMOTION
1252 && pos.see_sign(move) < 0)
1255 // Don't search useless checks
1260 && !pos.is_capture_or_promotion(move)
1261 && ss->staticEval + PawnValueMg / 4 < beta
1262 && !check_is_dangerous(pos, move, futilityBase, beta))
1265 // Check for legality only before to do the move
1266 if (!pos.pl_move_is_legal(move, ci.pinned))
1269 ss->currentMove = move;
1271 // Make and search the move
1272 pos.do_move(move, st, ci, givesCheck);
1273 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1274 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1275 pos.undo_move(move);
1277 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1279 // Check for new best move
1280 if (value > bestValue)
1286 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1293 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1294 ttDepth, move, ss->staticEval, ss->evalMargin);
1302 // All legal moves have been searched. A special case: If we're in check
1303 // and no legal moves were found, it is checkmate.
1304 if (InCheck && bestValue == -VALUE_INFINITE)
1305 return mated_in(ss->ply); // Plies to mate from the root
1307 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1308 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1309 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1311 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1317 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1318 // "plies to mate from the current position". Non-mate scores are unchanged.
1319 // The function is called before storing a value to the transposition table.
1321 Value value_to_tt(Value v, int ply) {
1323 assert(v != VALUE_NONE);
1325 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1326 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1330 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1331 // from the transposition table (where refers to the plies to mate/be mated
1332 // from current position) to "plies to mate/be mated from the root".
1334 Value value_from_tt(Value v, int ply) {
1336 return v == VALUE_NONE ? VALUE_NONE
1337 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1338 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1342 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1344 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1346 Piece pc = pos.piece_moved(move);
1347 Square from = from_sq(move);
1348 Square to = to_sq(move);
1349 Color them = ~pos.side_to_move();
1350 Square ksq = pos.king_square(them);
1351 Bitboard enemies = pos.pieces(them);
1352 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1353 Bitboard occ = pos.pieces() ^ from ^ ksq;
1354 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1355 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1357 // Checks which give opponent's king at most one escape square are dangerous
1358 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1361 // Queen contact check is very dangerous
1362 if (type_of(pc) == QUEEN && (kingAtt & to))
1365 // Creating new double threats with checks is dangerous
1366 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1369 // Note that here we generate illegal "double move"!
1370 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1378 // allows() tests whether the 'first' move at previous ply somehow makes the
1379 // 'second' move possible, for instance if the moving piece is the same in
1380 // both moves. Normally the second move is the threat (the best move returned
1381 // from a null search that fails low).
1383 bool allows(const Position& pos, Move first, Move second) {
1385 assert(is_ok(first));
1386 assert(is_ok(second));
1387 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1388 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1390 Square m1from = from_sq(first);
1391 Square m2from = from_sq(second);
1392 Square m1to = to_sq(first);
1393 Square m2to = to_sq(second);
1395 // The piece is the same or second's destination was vacated by the first move
1396 if (m1to == m2from || m2to == m1from)
1399 // Second one moves through the square vacated by first one
1400 if (between_bb(m2from, m2to) & m1from)
1403 // Second's destination is defended by the first move's piece
1404 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1408 // Second move gives a discovered check through the first's checking piece
1409 if (m1att & pos.king_square(pos.side_to_move()))
1411 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1419 // refutes() tests whether a 'first' move is able to defend against a 'second'
1420 // opponent's move. In this case will not be pruned. Normally the second move
1421 // is the threat (the best move returned from a null search that fails low).
1423 bool refutes(const Position& pos, Move first, Move second) {
1425 assert(is_ok(first));
1426 assert(is_ok(second));
1428 Square m1from = from_sq(first);
1429 Square m2from = from_sq(second);
1430 Square m1to = to_sq(first);
1431 Square m2to = to_sq(second);
1433 // Don't prune moves of the threatened piece
1437 // If the threatened piece has value less than or equal to the value of the
1438 // threat piece, don't prune moves which defend it.
1439 if ( pos.is_capture(second)
1440 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1441 || type_of(pos.piece_on(m2from)) == KING))
1443 // Update occupancy as if the piece and the threat are moving
1444 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1445 Piece pc = pos.piece_on(m1from);
1447 // The moved piece attacks the square 'tto' ?
1448 if (pos.attacks_from(pc, m1to, occ) & m2to)
1451 // Scan for possible X-ray attackers behind the moved piece
1452 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1453 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1455 // Verify attackers are triggered by our move and not already existing
1456 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1460 // Don't prune safe moves which block the threat path
1461 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1468 // When playing with strength handicap choose best move among the MultiPV set
1469 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1471 Move Skill::pick_move() {
1475 // PRNG sequence should be not deterministic
1476 for (int i = Time::now() % 50; i > 0; i--)
1477 rk.rand<unsigned>();
1479 // RootMoves are already sorted by score in descending order
1480 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1481 int weakness = 120 - 2 * level;
1482 int max_s = -VALUE_INFINITE;
1485 // Choose best move. For each move score we add two terms both dependent on
1486 // weakness, one deterministic and bigger for weaker moves, and one random,
1487 // then we choose the move with the resulting highest score.
1488 for (size_t i = 0; i < PVSize; i++)
1490 int s = RootMoves[i].score;
1492 // Don't allow crazy blunders even at very low skills
1493 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1496 // This is our magic formula
1497 s += ( weakness * int(RootMoves[0].score - s)
1498 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1503 best = RootMoves[i].pv[0];
1510 // uci_pv() formats PV information according to UCI protocol. UCI requires
1511 // to send all the PV lines also if are still to be searched and so refer to
1512 // the previous search score.
1514 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1516 std::stringstream s;
1517 Time::point elapsed = Time::now() - SearchTime + 1;
1518 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1521 for (size_t i = 0; i < Threads.size(); i++)
1522 if (Threads[i]->maxPly > selDepth)
1523 selDepth = Threads[i]->maxPly;
1525 for (size_t i = 0; i < uciPVSize; i++)
1527 bool updated = (i <= PVIdx);
1529 if (depth == 1 && !updated)
1532 int d = updated ? depth : depth - 1;
1533 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1535 if (s.rdbuf()->in_avail()) // Not at first line
1538 s << "info depth " << d
1539 << " seldepth " << selDepth
1540 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1541 << " nodes " << pos.nodes_searched()
1542 << " nps " << pos.nodes_searched() * 1000 / elapsed
1543 << " time " << elapsed
1544 << " multipv " << i + 1
1547 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1548 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1557 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1558 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1559 /// allow to always have a ponder move even when we fail high at root, and a
1560 /// long PV to print that is important for position analysis.
1562 void RootMove::extract_pv_from_tt(Position& pos) {
1564 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1574 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1576 pos.do_move(pv[ply++], *st++);
1577 tte = TT.probe(pos.key());
1580 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1581 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1583 && (!pos.is_draw() || ply < 2));
1585 pv.push_back(MOVE_NONE); // Must be zero-terminating
1587 while (ply) pos.undo_move(pv[--ply]);
1591 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1592 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1593 /// first, even if the old TT entries have been overwritten.
1595 void RootMove::insert_pv_in_tt(Position& pos) {
1597 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1602 tte = TT.probe(pos.key());
1604 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1605 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1607 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1609 pos.do_move(pv[ply++], *st++);
1611 } while (pv[ply] != MOVE_NONE);
1613 while (ply) pos.undo_move(pv[--ply]);
1617 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1619 void Thread::idle_loop() {
1621 // Pointer 'this_sp' is not null only if we are called from split(), and not
1622 // at the thread creation. So it means we are the split point's master.
1623 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1625 assert(!this_sp || (this_sp->masterThread == this && searching));
1629 // If we are not searching, wait for a condition to be signaled instead of
1630 // wasting CPU time polling for work.
1631 while ((!searching && Threads.sleepWhileIdle) || exit)
1639 // Grab the lock to avoid races with Thread::notify_one()
1642 // If we are master and all slaves have finished then exit idle_loop
1643 if (this_sp && !this_sp->slavesMask)
1649 // Do sleep after retesting sleep conditions under lock protection, in
1650 // particular we need to avoid a deadlock in case a master thread has,
1651 // in the meanwhile, allocated us and sent the notify_one() call before
1652 // we had the chance to grab the lock.
1653 if (!searching && !exit)
1654 sleepCondition.wait(mutex);
1659 // If this thread has been assigned work, launch a search
1664 Threads.mutex.lock();
1667 SplitPoint* sp = activeSplitPoint;
1669 Threads.mutex.unlock();
1671 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
1672 Position pos(*sp->pos, this);
1674 memcpy(ss-1, sp->ss-1, 4 * sizeof(Stack));
1675 ss->splitPoint = sp;
1679 assert(activePosition == NULL);
1681 activePosition = &pos;
1683 switch (sp->nodeType) {
1685 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1688 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1691 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1700 activePosition = NULL;
1701 sp->slavesMask &= ~(1ULL << idx);
1702 sp->nodes += pos.nodes_searched();
1704 // Wake up master thread so to allow it to return from the idle loop
1705 // in case we are the last slave of the split point.
1706 if ( Threads.sleepWhileIdle
1707 && this != sp->masterThread
1710 assert(!sp->masterThread->searching);
1711 sp->masterThread->notify_one();
1714 // After releasing the lock we cannot access anymore any SplitPoint
1715 // related data in a safe way becuase it could have been released under
1716 // our feet by the sp master. Also accessing other Thread objects is
1717 // unsafe because if we are exiting there is a chance are already freed.
1721 // If this thread is the master of a split point and all slaves have finished
1722 // their work at this split point, return from the idle loop.
1723 if (this_sp && !this_sp->slavesMask)
1725 this_sp->mutex.lock();
1726 bool finished = !this_sp->slavesMask; // Retest under lock protection
1727 this_sp->mutex.unlock();
1735 /// check_time() is called by the timer thread when the timer triggers. It is
1736 /// used to print debug info and, more important, to detect when we are out of
1737 /// available time and so stop the search.
1741 static Time::point lastInfoTime = Time::now();
1742 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1744 if (Time::now() - lastInfoTime >= 1000)
1746 lastInfoTime = Time::now();
1755 Threads.mutex.lock();
1757 nodes = RootPos.nodes_searched();
1759 // Loop across all split points and sum accumulated SplitPoint nodes plus
1760 // all the currently active positions nodes.
1761 for (size_t i = 0; i < Threads.size(); i++)
1762 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1764 SplitPoint& sp = Threads[i]->splitPoints[j];
1769 Bitboard sm = sp.slavesMask;
1772 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1774 nodes += pos->nodes_searched();
1780 Threads.mutex.unlock();
1783 Time::point elapsed = Time::now() - SearchTime;
1784 bool stillAtFirstMove = Signals.firstRootMove
1785 && !Signals.failedLowAtRoot
1786 && elapsed > TimeMgr.available_time();
1788 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1789 || stillAtFirstMove;
1791 if ( (Limits.use_time_management() && noMoreTime)
1792 || (Limits.movetime && elapsed >= Limits.movetime)
1793 || (Limits.nodes && nodes >= Limits.nodes))
1794 Signals.stop = true;