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["Use 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["Use Sleeping Threads"];
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["Use 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 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 // In case of failing low/high increase aspiration window and
371 // research, otherwise exit the loop.
372 if (bestValue <= alpha)
374 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
376 Signals.failedLowAtRoot = true;
377 Signals.stopOnPonderhit = false;
379 else if (bestValue >= beta)
380 beta = std::min(bestValue + delta, VALUE_INFINITE);
387 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
389 // Give some update (without cluttering the UI) before to research
390 if (Time::now() - SearchTime > 3000)
391 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
394 // Sort the PV lines searched so far and update the GUI
395 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
397 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
398 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
401 // Do we need to pick now the sub-optimal best move ?
402 if (skill.enabled() && skill.time_to_pick(depth))
405 if (Options["Use Search Log"])
407 RootMove& rm = RootMoves[0];
408 if (skill.best != MOVE_NONE)
409 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
411 Log log(Options["Search Log Filename"]);
412 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
416 // Do we have found a "mate in x"?
418 && bestValue >= VALUE_MATE_IN_MAX_PLY
419 && VALUE_MATE - bestValue <= 2 * Limits.mate)
422 // Do we have time for the next iteration? Can we stop searching now?
423 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
425 bool stop = false; // Local variable, not the volatile Signals.stop
427 // Take in account some extra time if the best move has changed
428 if (depth > 4 && depth < 50 && PVSize == 1)
429 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
431 // Stop search if most of available time is already consumed. We
432 // probably don't have enough time to search the first move at the
433 // next iteration anyway.
434 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
437 // Stop search early if one move seems to be much better than others
441 && bestValue > VALUE_MATED_IN_MAX_PLY
442 && ( RootMoves.size() == 1
443 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
445 Value rBeta = bestValue - 2 * PawnValueMg;
446 ss->excludedMove = RootMoves[0].pv[0];
447 ss->skipNullMove = true;
448 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
449 ss->skipNullMove = false;
450 ss->excludedMove = MOVE_NONE;
458 // If we are allowed to ponder do not stop the search now but
459 // keep pondering until GUI sends "ponderhit" or "stop".
461 Signals.stopOnPonderhit = true;
470 // search<>() is the main search function for both PV and non-PV nodes and for
471 // normal and SplitPoint nodes. When called just after a split point the search
472 // is simpler because we have already probed the hash table, done a null move
473 // search, and searched the first move before splitting, we don't have to repeat
474 // all this work again. We also don't need to store anything to the hash table
475 // here: This is taken care of after we return from the split point.
477 template <NodeType NT>
478 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
480 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
481 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
482 const bool RootNode = (NT == Root || NT == SplitPointRoot);
484 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
485 assert(PvNode || (alpha == beta - 1));
486 assert(depth > DEPTH_ZERO);
488 Move quietsSearched[64];
491 SplitPoint* splitPoint;
493 Move ttMove, move, excludedMove, bestMove, threatMove;
495 Value bestValue, value, ttValue;
496 Value eval, nullValue, futilityValue;
497 bool inCheck, givesCheck, pvMove, singularExtensionNode;
498 bool captureOrPromotion, dangerous, doFullDepthSearch;
499 int moveCount, quietCount;
501 // Step 1. Initialize node
502 Thread* thisThread = pos.this_thread();
503 moveCount = quietCount = 0;
504 inCheck = pos.checkers();
508 splitPoint = ss->splitPoint;
509 bestMove = splitPoint->bestMove;
510 threatMove = splitPoint->threatMove;
511 bestValue = splitPoint->bestValue;
513 ttMove = excludedMove = MOVE_NONE;
514 ttValue = VALUE_NONE;
516 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
521 bestValue = -VALUE_INFINITE;
522 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
523 ss->ply = (ss-1)->ply + 1;
524 ss->futilityMoveCount = 0;
525 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
526 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
528 // Used to send selDepth info to GUI
529 if (PvNode && thisThread->maxPly < ss->ply)
530 thisThread->maxPly = ss->ply;
534 // Step 2. Check for aborted search and immediate draw
535 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
536 return DrawValue[pos.side_to_move()];
538 // Step 3. Mate distance pruning. Even if we mate at the next move our score
539 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
540 // a shorter mate was found upward in the tree then there is no need to search
541 // further, we will never beat current alpha. Same logic but with reversed signs
542 // applies also in the opposite condition of being mated instead of giving mate,
543 // in this case return a fail-high score.
544 alpha = std::max(mated_in(ss->ply), alpha);
545 beta = std::min(mate_in(ss->ply+1), beta);
550 // Step 4. Transposition table lookup
551 // We don't want the score of a partial search to overwrite a previous full search
552 // TT value, so we use a different position key in case of an excluded move.
553 excludedMove = ss->excludedMove;
554 posKey = excludedMove ? pos.exclusion_key() : pos.key();
555 tte = TT.probe(posKey);
556 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
557 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
559 // At PV nodes we check for exact scores, while at non-PV nodes we check for
560 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
561 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
562 // we should also update RootMoveList to avoid bogus output.
565 && tte->depth() >= depth
566 && ttValue != VALUE_NONE // Only in case of TT access race
567 && ( PvNode ? tte->bound() == BOUND_EXACT
568 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
569 : (tte->bound() & BOUND_UPPER)))
572 ss->currentMove = ttMove; // Can be MOVE_NONE
576 && !pos.is_capture_or_promotion(ttMove)
577 && ttMove != ss->killers[0])
579 ss->killers[1] = ss->killers[0];
580 ss->killers[0] = ttMove;
585 // Step 5. Evaluate the position statically and update parent's gain statistics
588 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
594 // Never assume anything on values stored in TT
595 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
596 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
597 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
599 // Can ttValue be used as a better position evaluation?
600 if (ttValue != VALUE_NONE)
601 if ( ((tte->bound() & BOUND_LOWER) && ttValue > eval)
602 || ((tte->bound() & BOUND_UPPER) && ttValue < eval))
607 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
608 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
609 ss->staticEval, ss->evalMargin);
612 // Update gain for the parent non-capture move given the static position
613 // evaluation before and after the move.
614 if ( !pos.captured_piece_type()
615 && ss->staticEval != VALUE_NONE
616 && (ss-1)->staticEval != VALUE_NONE
617 && (move = (ss-1)->currentMove) != MOVE_NULL
618 && type_of(move) == NORMAL)
620 Square to = to_sq(move);
621 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
624 // Step 6. Razoring (is omitted in PV nodes)
626 && depth < 4 * ONE_PLY
627 && eval + razor_margin(depth) < beta
628 && ttMove == MOVE_NONE
629 && abs(beta) < VALUE_MATE_IN_MAX_PLY
630 && !pos.pawn_on_7th(pos.side_to_move()))
632 Value rbeta = beta - razor_margin(depth);
633 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
635 // Logically we should return (v + razor_margin(depth)), but
636 // surprisingly this did slightly weaker in tests.
640 // Step 7. Static null move pruning (is omitted in PV nodes)
641 // We're betting that the opponent doesn't have a move that will reduce
642 // the score by more than futility_margin(depth) if we do a null move.
645 && depth < 4 * ONE_PLY
646 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
647 && abs(beta) < VALUE_MATE_IN_MAX_PLY
648 && abs(eval) < VALUE_KNOWN_WIN
649 && pos.non_pawn_material(pos.side_to_move()))
650 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
652 // Step 8. Null move search with verification search (is omitted in PV nodes)
657 && abs(beta) < VALUE_MATE_IN_MAX_PLY
658 && pos.non_pawn_material(pos.side_to_move()))
660 ss->currentMove = MOVE_NULL;
662 // Null move dynamic reduction based on depth
663 Depth R = 3 * ONE_PLY + depth / 4;
665 // Null move dynamic reduction based on value
666 if (eval - PawnValueMg > beta)
669 pos.do_null_move(st);
670 (ss+1)->skipNullMove = true;
671 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
672 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
673 (ss+1)->skipNullMove = false;
674 pos.undo_null_move();
676 if (nullValue >= beta)
678 // Do not return unproven mate scores
679 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
682 if (depth < 12 * ONE_PLY)
685 // Do verification search at high depths
686 ss->skipNullMove = true;
687 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
688 ss->skipNullMove = false;
695 // The null move failed low, which means that we may be faced with
696 // some kind of threat. If the previous move was reduced, check if
697 // the move that refuted the null move was somehow connected to the
698 // move which was reduced. If a connection is found, return a fail
699 // low score (which will cause the reduced move to fail high in the
700 // parent node, which will trigger a re-search with full depth).
701 threatMove = (ss+1)->currentMove;
703 if ( depth < 5 * ONE_PLY
705 && threatMove != MOVE_NONE
706 && allows(pos, (ss-1)->currentMove, threatMove))
711 // Step 9. ProbCut (is omitted in PV nodes)
712 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
713 // and a reduced search returns a value much above beta, we can (almost) safely
714 // prune the previous move.
716 && depth >= 5 * ONE_PLY
718 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
720 Value rbeta = beta + 200;
721 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
723 assert(rdepth >= ONE_PLY);
724 assert((ss-1)->currentMove != MOVE_NONE);
725 assert((ss-1)->currentMove != MOVE_NULL);
727 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
730 while ((move = mp.next_move<false>()) != MOVE_NONE)
731 if (pos.pl_move_is_legal(move, ci.pinned))
733 ss->currentMove = move;
734 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
735 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
742 // Step 10. Internal iterative deepening
743 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
744 && ttMove == MOVE_NONE
745 && (PvNode || ss->staticEval + Value(256) >= beta))
747 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
749 ss->skipNullMove = true;
750 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
751 ss->skipNullMove = false;
753 tte = TT.probe(posKey);
754 ttMove = tte ? tte->move() : MOVE_NONE;
757 moves_loop: // When in check and at SpNode search starts from here
759 Square prevMoveSq = to_sq((ss-1)->currentMove);
760 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
761 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
763 MovePicker mp(pos, ttMove, depth, History, countermoves, ss, PvNode ? -VALUE_INFINITE : beta);
765 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
766 singularExtensionNode = !RootNode
768 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
769 && ttMove != MOVE_NONE
770 && !excludedMove // Recursive singular search is not allowed
771 && (tte->bound() & BOUND_LOWER)
772 && tte->depth() >= depth - 3 * ONE_PLY;
774 // Step 11. Loop through moves
775 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
776 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
780 if (move == excludedMove)
783 // At root obey the "searchmoves" option and skip moves not listed in Root
784 // Move List, as a consequence any illegal move is also skipped. In MultiPV
785 // mode we also skip PV moves which have been already searched.
786 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
791 // Shared counter cannot be decremented later if move turns out to be illegal
792 if (!pos.pl_move_is_legal(move, ci.pinned))
795 moveCount = ++splitPoint->moveCount;
796 splitPoint->mutex.unlock();
803 Signals.firstRootMove = (moveCount == 1);
805 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
806 sync_cout << "info depth " << depth / ONE_PLY
807 << " currmove " << move_to_uci(move, pos.is_chess960())
808 << " currmovenumber " << moveCount + PVIdx << sync_endl;
812 captureOrPromotion = pos.is_capture_or_promotion(move);
813 givesCheck = pos.move_gives_check(move, ci);
814 dangerous = givesCheck
815 || pos.is_passed_pawn_push(move)
816 || type_of(move) == CASTLE;
818 // Step 12. Extend checks and, in PV nodes, also dangerous moves
819 if (PvNode && dangerous)
822 else if (givesCheck && pos.see_sign(move) >= 0)
825 // Singular extension search. If all moves but one fail low on a search of
826 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
827 // is singular and should be extended. To verify this we do a reduced search
828 // on all the other moves but the ttMove, if result is lower than ttValue minus
829 // a margin then we extend ttMove.
830 if ( singularExtensionNode
833 && pos.pl_move_is_legal(move, ci.pinned)
834 && abs(ttValue) < VALUE_KNOWN_WIN)
836 assert(ttValue != VALUE_NONE);
838 Value rBeta = ttValue - int(depth);
839 ss->excludedMove = move;
840 ss->skipNullMove = true;
841 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
842 ss->skipNullMove = false;
843 ss->excludedMove = MOVE_NONE;
849 // Update current move (this must be done after singular extension search)
850 newDepth = depth - ONE_PLY + ext;
852 // Step 13. Futility pruning (is omitted in PV nodes)
854 && !captureOrPromotion
857 /* && move != ttMove Already implicit in the next condition */
858 && bestValue > VALUE_MATED_IN_MAX_PLY)
860 // Move count based pruning
861 if ( depth < 16 * ONE_PLY
862 && moveCount >= FutilityMoveCounts[depth]
863 && (!threatMove || !refutes(pos, move, threatMove)))
866 splitPoint->mutex.lock();
871 // Value based pruning
872 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
873 // but fixing this made program slightly weaker.
874 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
875 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
876 + Gains[pos.piece_moved(move)][to_sq(move)];
878 if (futilityValue < beta)
880 bestValue = std::max(bestValue, futilityValue);
884 splitPoint->mutex.lock();
885 if (bestValue > splitPoint->bestValue)
886 splitPoint->bestValue = bestValue;
891 // Prune moves with negative SEE at low depths
892 if ( predictedDepth < 4 * ONE_PLY
893 && pos.see_sign(move) < 0)
896 splitPoint->mutex.lock();
901 // We have not pruned the move that will be searched, but remember how
902 // far in the move list we are to be more aggressive in the child node.
903 ss->futilityMoveCount = moveCount;
906 ss->futilityMoveCount = 0;
908 // Check for legality only before to do the move
909 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
915 pvMove = PvNode && moveCount == 1;
916 ss->currentMove = move;
917 if (!SpNode && !captureOrPromotion && quietCount < 64)
918 quietsSearched[quietCount++] = move;
920 // Step 14. Make the move
921 pos.do_move(move, st, ci, givesCheck);
923 // Step 15. Reduced depth search (LMR). If the move fails high will be
924 // re-searched at full depth.
925 if ( depth > 3 * ONE_PLY
927 && !captureOrPromotion
930 && move != ss->killers[0]
931 && move != ss->killers[1])
933 ss->reduction = reduction<PvNode>(depth, moveCount);
935 if (!PvNode && cutNode)
936 ss->reduction += ONE_PLY;
938 if (move == countermoves[0] || move == countermoves[1])
939 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
941 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
943 alpha = splitPoint->alpha;
945 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
947 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
948 ss->reduction = DEPTH_ZERO;
951 doFullDepthSearch = !pvMove;
953 // Step 16. Full depth search, when LMR is skipped or fails high
954 if (doFullDepthSearch)
957 alpha = splitPoint->alpha;
959 value = newDepth < ONE_PLY ?
960 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
961 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
962 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
965 // Only for PV nodes do a full PV search on the first move or after a fail
966 // high, in the latter case search only if value < beta, otherwise let the
967 // parent node to fail low with value <= alpha and to try another move.
968 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
969 value = newDepth < ONE_PLY ?
970 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
971 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
972 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
973 // Step 17. Undo move
976 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
978 // Step 18. Check for new best move
981 splitPoint->mutex.lock();
982 bestValue = splitPoint->bestValue;
983 alpha = splitPoint->alpha;
986 // Finished searching the move. If Signals.stop is true, the search
987 // was aborted because the user interrupted the search or because we
988 // ran out of time. In this case, the return value of the search cannot
989 // be trusted, and we don't update the best move and/or PV.
990 if (Signals.stop || thisThread->cutoff_occurred())
991 return value; // To avoid returning VALUE_INFINITE
995 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
997 // PV move or new best move ?
998 if (pvMove || value > alpha)
1001 rm.extract_pv_from_tt(pos);
1003 // We record how often the best move has been changed in each
1004 // iteration. This information is used for time management: When
1005 // the best move changes frequently, we allocate some more time.
1010 // All other moves but the PV are set to the lowest value, this
1011 // is not a problem when sorting becuase sort is stable and move
1012 // position in the list is preserved, just the PV is pushed up.
1013 rm.score = -VALUE_INFINITE;
1016 if (value > bestValue)
1018 bestValue = SpNode ? splitPoint->bestValue = value : value;
1022 bestMove = SpNode ? splitPoint->bestMove = move : move;
1024 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1025 alpha = SpNode ? splitPoint->alpha = value : value;
1028 assert(value >= beta); // Fail high
1031 splitPoint->cutoff = true;
1038 // Step 19. Check for splitting the search
1040 && depth >= Threads.minimumSplitDepth
1041 && Threads.available_slave(thisThread)
1042 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1044 assert(bestValue < beta);
1046 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1047 depth, threatMove, moveCount, &mp, NT, cutNode);
1048 if (bestValue >= beta)
1056 // Step 20. Check for mate and stalemate
1057 // All legal moves have been searched and if there are no legal moves, it
1058 // must be mate or stalemate. Note that we can have a false positive in
1059 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1060 // harmless because return value is discarded anyhow in the parent nodes.
1061 // If we are in a singular extension search then return a fail low score.
1062 // A split node has at least one move, the one tried before to be splitted.
1064 return excludedMove ? alpha
1065 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1067 // If we have pruned all the moves without searching return a fail-low score
1068 if (bestValue == -VALUE_INFINITE)
1071 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1072 bestValue >= beta ? BOUND_LOWER :
1073 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1074 depth, bestMove, ss->staticEval, ss->evalMargin);
1076 // Quiet best move: update killers, history and countermoves
1077 if ( bestValue >= beta
1078 && !pos.is_capture_or_promotion(bestMove)
1081 if (ss->killers[0] != bestMove)
1083 ss->killers[1] = ss->killers[0];
1084 ss->killers[0] = bestMove;
1087 // Increase history value of the cut-off move and decrease all the other
1088 // played non-capture moves.
1089 Value bonus = Value(int(depth) * int(depth));
1090 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1091 for (int i = 0; i < quietCount - 1; i++)
1093 Move m = quietsSearched[i];
1094 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1097 if (is_ok((ss-1)->currentMove))
1098 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1101 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1107 // qsearch() is the quiescence search function, which is called by the main
1108 // search function when the remaining depth is zero (or, to be more precise,
1109 // less than ONE_PLY).
1111 template <NodeType NT, bool InCheck>
1112 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1114 const bool PvNode = (NT == PV);
1116 assert(NT == PV || NT == NonPV);
1117 assert(InCheck == !!pos.checkers());
1118 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1119 assert(PvNode || (alpha == beta - 1));
1120 assert(depth <= DEPTH_ZERO);
1125 Move ttMove, move, bestMove;
1126 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1127 bool givesCheck, enoughMaterial, evasionPrunable;
1130 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1134 ss->currentMove = bestMove = MOVE_NONE;
1135 ss->ply = (ss-1)->ply + 1;
1137 // Check for an instant draw or maximum ply reached
1138 if (pos.is_draw() || ss->ply > MAX_PLY)
1139 return DrawValue[pos.side_to_move()];
1141 // Decide whether or not to include checks, this fixes also the type of
1142 // TT entry depth that we are going to use. Note that in qsearch we use
1143 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1144 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1145 : DEPTH_QS_NO_CHECKS;
1147 // Transposition table lookup
1149 tte = TT.probe(posKey);
1150 ttMove = tte ? tte->move() : MOVE_NONE;
1151 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1154 && tte->depth() >= ttDepth
1155 && ttValue != VALUE_NONE // Only in case of TT access race
1156 && ( PvNode ? tte->bound() == BOUND_EXACT
1157 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1158 : (tte->bound() & BOUND_UPPER)))
1160 ss->currentMove = ttMove; // Can be MOVE_NONE
1164 // Evaluate the position statically
1167 ss->staticEval = ss->evalMargin = VALUE_NONE;
1168 bestValue = futilityBase = -VALUE_INFINITE;
1169 enoughMaterial = false;
1175 // Never assume anything on values stored in TT
1176 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1177 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1178 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1181 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1183 // Stand pat. Return immediately if static value is at least beta
1184 if (bestValue >= beta)
1187 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1188 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1193 if (PvNode && bestValue > alpha)
1196 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1197 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
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);
1220 && type_of(move) != PROMOTION
1221 && !pos.is_passed_pawn_push(move))
1223 futilityValue = futilityBase
1224 + PieceValue[EG][pos.piece_on(to_sq(move))]
1225 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1227 if (futilityValue < beta)
1229 bestValue = std::max(bestValue, futilityValue);
1233 // Prune moves with negative or equal SEE and also moves with positive
1234 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1235 if ( futilityBase < beta
1236 && pos.see(move, beta - futilityBase) <= 0)
1238 bestValue = std::max(bestValue, futilityBase);
1243 // Detect non-capture evasions that are candidate to be pruned
1244 evasionPrunable = !PvNode
1246 && bestValue > VALUE_MATED_IN_MAX_PLY
1247 && !pos.is_capture(move)
1248 && !pos.can_castle(pos.side_to_move());
1250 // Don't search moves with negative SEE values
1252 && (!InCheck || evasionPrunable)
1254 && type_of(move) != PROMOTION
1255 && pos.see_sign(move) < 0)
1258 // Don't search useless checks
1263 && !pos.is_capture_or_promotion(move)
1264 && ss->staticEval + PawnValueMg / 4 < beta
1265 && !check_is_dangerous(pos, move, futilityBase, beta))
1268 // Check for legality only before to do the move
1269 if (!pos.pl_move_is_legal(move, ci.pinned))
1272 ss->currentMove = move;
1274 // Make and search the move
1275 pos.do_move(move, st, ci, givesCheck);
1276 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1277 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1278 pos.undo_move(move);
1280 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1282 // Check for new best move
1283 if (value > bestValue)
1289 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1296 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1297 ttDepth, move, ss->staticEval, ss->evalMargin);
1305 // All legal moves have been searched. A special case: If we're in check
1306 // and no legal moves were found, it is checkmate.
1307 if (InCheck && bestValue == -VALUE_INFINITE)
1308 return mated_in(ss->ply); // Plies to mate from the root
1310 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1311 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1312 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1314 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1320 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1321 // "plies to mate from the current position". Non-mate scores are unchanged.
1322 // The function is called before storing a value to the transposition table.
1324 Value value_to_tt(Value v, int ply) {
1326 assert(v != VALUE_NONE);
1328 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1329 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1333 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1334 // from the transposition table (where refers to the plies to mate/be mated
1335 // from current position) to "plies to mate/be mated from the root".
1337 Value value_from_tt(Value v, int ply) {
1339 return v == VALUE_NONE ? VALUE_NONE
1340 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1341 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1345 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1347 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1349 Piece pc = pos.piece_moved(move);
1350 Square from = from_sq(move);
1351 Square to = to_sq(move);
1352 Color them = ~pos.side_to_move();
1353 Square ksq = pos.king_square(them);
1354 Bitboard enemies = pos.pieces(them);
1355 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1356 Bitboard occ = pos.pieces() ^ from ^ ksq;
1357 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1358 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1360 // Checks which give opponent's king at most one escape square are dangerous
1361 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1364 // Queen contact check is very dangerous
1365 if (type_of(pc) == QUEEN && (kingAtt & to))
1368 // Creating new double threats with checks is dangerous
1369 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1372 // Note that here we generate illegal "double move"!
1373 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1381 // allows() tests whether the 'first' move at previous ply somehow makes the
1382 // 'second' move possible, for instance if the moving piece is the same in
1383 // both moves. Normally the second move is the threat (the best move returned
1384 // from a null search that fails low).
1386 bool allows(const Position& pos, Move first, Move second) {
1388 assert(is_ok(first));
1389 assert(is_ok(second));
1390 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1391 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1393 Square m1from = from_sq(first);
1394 Square m2from = from_sq(second);
1395 Square m1to = to_sq(first);
1396 Square m2to = to_sq(second);
1398 // The piece is the same or second's destination was vacated by the first move
1399 if (m1to == m2from || m2to == m1from)
1402 // Second one moves through the square vacated by first one
1403 if (between_bb(m2from, m2to) & m1from)
1406 // Second's destination is defended by the first move's piece
1407 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1411 // Second move gives a discovered check through the first's checking piece
1412 if (m1att & pos.king_square(pos.side_to_move()))
1414 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1422 // refutes() tests whether a 'first' move is able to defend against a 'second'
1423 // opponent's move. In this case will not be pruned. Normally the second move
1424 // is the threat (the best move returned from a null search that fails low).
1426 bool refutes(const Position& pos, Move first, Move second) {
1428 assert(is_ok(first));
1429 assert(is_ok(second));
1431 Square m1from = from_sq(first);
1432 Square m2from = from_sq(second);
1433 Square m1to = to_sq(first);
1434 Square m2to = to_sq(second);
1436 // Don't prune moves of the threatened piece
1440 // If the threatened piece has value less than or equal to the value of the
1441 // threat piece, don't prune moves which defend it.
1442 if ( pos.is_capture(second)
1443 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1444 || type_of(pos.piece_on(m2from)) == KING))
1446 // Update occupancy as if the piece and the threat are moving
1447 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1448 Piece pc = pos.piece_on(m1from);
1450 // The moved piece attacks the square 'tto' ?
1451 if (pos.attacks_from(pc, m1to, occ) & m2to)
1454 // Scan for possible X-ray attackers behind the moved piece
1455 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1456 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1458 // Verify attackers are triggered by our move and not already existing
1459 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1463 // Don't prune safe moves which block the threat path
1464 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1471 // When playing with strength handicap choose best move among the MultiPV set
1472 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1474 Move Skill::pick_move() {
1478 // PRNG sequence should be not deterministic
1479 for (int i = Time::now() % 50; i > 0; i--)
1480 rk.rand<unsigned>();
1482 // RootMoves are already sorted by score in descending order
1483 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1484 int weakness = 120 - 2 * level;
1485 int max_s = -VALUE_INFINITE;
1488 // Choose best move. For each move score we add two terms both dependent on
1489 // weakness, one deterministic and bigger for weaker moves, and one random,
1490 // then we choose the move with the resulting highest score.
1491 for (size_t i = 0; i < PVSize; i++)
1493 int s = RootMoves[i].score;
1495 // Don't allow crazy blunders even at very low skills
1496 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1499 // This is our magic formula
1500 s += ( weakness * int(RootMoves[0].score - s)
1501 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1506 best = RootMoves[i].pv[0];
1513 // uci_pv() formats PV information according to UCI protocol. UCI requires
1514 // to send all the PV lines also if are still to be searched and so refer to
1515 // the previous search score.
1517 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1519 std::stringstream s;
1520 Time::point elapsed = Time::now() - SearchTime + 1;
1521 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1524 for (size_t i = 0; i < Threads.size(); i++)
1525 if (Threads[i]->maxPly > selDepth)
1526 selDepth = Threads[i]->maxPly;
1528 for (size_t i = 0; i < uciPVSize; i++)
1530 bool updated = (i <= PVIdx);
1532 if (depth == 1 && !updated)
1535 int d = updated ? depth : depth - 1;
1536 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1538 if (s.rdbuf()->in_avail()) // Not at first line
1541 s << "info depth " << d
1542 << " seldepth " << selDepth
1543 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1544 << " nodes " << pos.nodes_searched()
1545 << " nps " << pos.nodes_searched() * 1000 / elapsed
1546 << " time " << elapsed
1547 << " multipv " << i + 1
1550 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1551 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1560 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1561 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1562 /// allow to always have a ponder move even when we fail high at root, and a
1563 /// long PV to print that is important for position analysis.
1565 void RootMove::extract_pv_from_tt(Position& pos) {
1567 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1577 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1579 pos.do_move(pv[ply++], *st++);
1580 tte = TT.probe(pos.key());
1583 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1584 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1586 && (!pos.is_draw() || ply < 2));
1588 pv.push_back(MOVE_NONE); // Must be zero-terminating
1590 while (ply) pos.undo_move(pv[--ply]);
1594 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1595 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1596 /// first, even if the old TT entries have been overwritten.
1598 void RootMove::insert_pv_in_tt(Position& pos) {
1600 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1605 tte = TT.probe(pos.key());
1607 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1608 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1610 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1612 pos.do_move(pv[ply++], *st++);
1614 } while (pv[ply] != MOVE_NONE);
1616 while (ply) pos.undo_move(pv[--ply]);
1620 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1622 void Thread::idle_loop() {
1624 // Pointer 'this_sp' is not null only if we are called from split(), and not
1625 // at the thread creation. So it means we are the split point's master.
1626 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1628 assert(!this_sp || (this_sp->masterThread == this && searching));
1632 // If we are not searching, wait for a condition to be signaled instead of
1633 // wasting CPU time polling for work.
1634 while ((!searching && Threads.sleepWhileIdle) || exit)
1642 // Grab the lock to avoid races with Thread::notify_one()
1645 // If we are master and all slaves have finished then exit idle_loop
1646 if (this_sp && !this_sp->slavesMask)
1652 // Do sleep after retesting sleep conditions under lock protection, in
1653 // particular we need to avoid a deadlock in case a master thread has,
1654 // in the meanwhile, allocated us and sent the notify_one() call before
1655 // we had the chance to grab the lock.
1656 if (!searching && !exit)
1657 sleepCondition.wait(mutex);
1662 // If this thread has been assigned work, launch a search
1667 Threads.mutex.lock();
1670 SplitPoint* sp = activeSplitPoint;
1672 Threads.mutex.unlock();
1674 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
1675 Position pos(*sp->pos, this);
1677 memcpy(ss-1, sp->ss-1, 4 * sizeof(Stack));
1678 ss->splitPoint = sp;
1682 assert(activePosition == NULL);
1684 activePosition = &pos;
1686 switch (sp->nodeType) {
1688 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1691 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1694 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1703 activePosition = NULL;
1704 sp->slavesMask &= ~(1ULL << idx);
1705 sp->nodes += pos.nodes_searched();
1707 // Wake up master thread so to allow it to return from the idle loop
1708 // in case we are the last slave of the split point.
1709 if ( Threads.sleepWhileIdle
1710 && this != sp->masterThread
1713 assert(!sp->masterThread->searching);
1714 sp->masterThread->notify_one();
1717 // After releasing the lock we cannot access anymore any SplitPoint
1718 // related data in a safe way becuase it could have been released under
1719 // our feet by the sp master. Also accessing other Thread objects is
1720 // unsafe because if we are exiting there is a chance are already freed.
1724 // If this thread is the master of a split point and all slaves have finished
1725 // their work at this split point, return from the idle loop.
1726 if (this_sp && !this_sp->slavesMask)
1728 this_sp->mutex.lock();
1729 bool finished = !this_sp->slavesMask; // Retest under lock protection
1730 this_sp->mutex.unlock();
1738 /// check_time() is called by the timer thread when the timer triggers. It is
1739 /// used to print debug info and, more important, to detect when we are out of
1740 /// available time and so stop the search.
1744 static Time::point lastInfoTime = Time::now();
1745 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1747 if (Time::now() - lastInfoTime >= 1000)
1749 lastInfoTime = Time::now();
1758 Threads.mutex.lock();
1760 nodes = RootPos.nodes_searched();
1762 // Loop across all split points and sum accumulated SplitPoint nodes plus
1763 // all the currently active positions nodes.
1764 for (size_t i = 0; i < Threads.size(); i++)
1765 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1767 SplitPoint& sp = Threads[i]->splitPoints[j];
1772 Bitboard sm = sp.slavesMask;
1775 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1777 nodes += pos->nodes_searched();
1783 Threads.mutex.unlock();
1786 Time::point elapsed = Time::now() - SearchTime;
1787 bool stillAtFirstMove = Signals.firstRootMove
1788 && !Signals.failedLowAtRoot
1789 && elapsed > TimeMgr.available_time();
1791 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1792 || stillAtFirstMove;
1794 if ( (Limits.use_time_management() && noMoreTime)
1795 || (Limits.movetime && elapsed >= Limits.movetime)
1796 || (Limits.nodes && nodes >= Limits.nodes))
1797 Signals.stop = true;