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 // Different node types, used as template parameter
59 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
61 // Dynamic razoring margin based on depth
62 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
64 // Futility lookup tables (initialized at startup) and their access functions
65 Value FutilityMargins[16][64]; // [depth][moveNumber]
66 int FutilityMoveCounts[2][32]; // [improving][depth]
68 inline Value futility_margin(Depth d, int mn) {
70 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
74 // Reduction lookup tables (initialized at startup) and their access function
75 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
77 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
79 return (Depth) Reductions[PvNode][i][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
84 double BestMoveChanges;
85 Value DrawValue[COLOR_NB];
88 CountermovesStats Countermoves;
90 template <NodeType NT>
91 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
93 template <NodeType NT, bool InCheck>
94 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
96 void id_loop(Position& pos);
97 Value value_to_tt(Value v, int ply);
98 Value value_from_tt(Value v, int ply);
99 bool allows(const Position& pos, Move first, Move second);
100 bool refutes(const Position& pos, Move first, Move second);
101 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
104 Skill(int l) : level(l), best(MOVE_NONE) {}
106 if (enabled()) // Swap best PV line with the sub-optimal one
107 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
108 RootMoves.end(), best ? best : pick_move()));
111 bool enabled() const { return level < 20; }
112 bool time_to_pick(int depth) const { return depth == 1 + level; }
122 /// Search::init() is called during startup to initialize various lookup tables
124 void Search::init() {
126 int d; // depth (ONE_PLY == 2)
127 int hd; // half depth (ONE_PLY == 1)
130 // Init reductions array
131 for (hd = 1; hd < 64; ++hd) for (mc = 1; mc < 64; ++mc)
133 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
134 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
135 Reductions[1][1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
136 Reductions[0][1][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
138 Reductions[1][0][hd][mc] = Reductions[1][1][hd][mc];
139 Reductions[0][0][hd][mc] = Reductions[0][1][hd][mc];
141 if (Reductions[0][0][hd][mc] > 2 * ONE_PLY)
142 Reductions[0][0][hd][mc] += ONE_PLY;
144 else if (Reductions[0][0][hd][mc] > 1 * ONE_PLY)
145 Reductions[0][0][hd][mc] += ONE_PLY / 2;
148 // Init futility margins array
149 for (d = 1; d < 16; ++d) for (mc = 0; mc < 64; ++mc)
150 FutilityMargins[d][mc] = Value(112 * int(2.9 * log(double(d))) - 8 * mc + 45);
152 // Init futility move count array
153 for (d = 0; d < 32; ++d)
155 FutilityMoveCounts[0][d] = int(2.4 + 0.222 * pow(d + 0.0, 1.8));
156 FutilityMoveCounts[1][d] = int(3.0 + 0.3 * pow(d + 0.98, 1.8));
161 /// Search::perft() is our utility to verify move generation. All the leaf nodes
162 /// up to the given depth are generated and counted and the sum returned.
164 static size_t perft(Position& pos, Depth depth) {
169 const bool leaf = depth == 2 * ONE_PLY;
171 for (MoveList<LEGAL> it(pos); *it; ++it)
173 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
174 cnt += leaf ? MoveList<LEGAL>(pos).size() : ::perft(pos, depth - ONE_PLY);
180 size_t Search::perft(Position& pos, Depth depth) {
181 return depth > ONE_PLY ? ::perft(pos, depth) : MoveList<LEGAL>(pos).size();
184 /// Search::think() is the external interface to Stockfish's search, and is
185 /// called by the main thread when the program receives the UCI 'go' command. It
186 /// searches from RootPos and at the end prints the "bestmove" to output.
188 void Search::think() {
190 static PolyglotBook book; // Defined static to initialize the PRNG only once
192 RootColor = RootPos.side_to_move();
193 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
195 if (RootMoves.empty())
197 RootMoves.push_back(MOVE_NONE);
198 sync_cout << "info depth 0 score "
199 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
205 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
207 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
209 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
211 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
216 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
218 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
219 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
220 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
221 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
224 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
226 if (Options["Write Search Log"])
228 Log log(Options["Search Log Filename"]);
229 log << "\nSearching: " << RootPos.fen()
230 << "\ninfinite: " << Limits.infinite
231 << " ponder: " << Limits.ponder
232 << " time: " << Limits.time[RootColor]
233 << " increment: " << Limits.inc[RootColor]
234 << " moves to go: " << Limits.movestogo
238 // Reset the threads, still sleeping: will be wake up at split time
239 for (size_t i = 0; i < Threads.size(); ++i)
240 Threads[i]->maxPly = 0;
242 Threads.sleepWhileIdle = Options["Idle Threads Sleep"];
243 Threads.timer->run = true;
244 Threads.timer->notify_one(); // Wake up the recurring timer
246 id_loop(RootPos); // Let's start searching !
248 Threads.timer->run = false; // Stop the timer
249 Threads.sleepWhileIdle = true; // Send idle threads to sleep
251 if (Options["Write Search Log"])
253 Time::point elapsed = Time::now() - SearchTime + 1;
255 Log log(Options["Search Log Filename"]);
256 log << "Nodes: " << RootPos.nodes_searched()
257 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
258 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
261 RootPos.do_move(RootMoves[0].pv[0], st);
262 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
263 RootPos.undo_move(RootMoves[0].pv[0]);
268 // When search is stopped this info is not printed
269 sync_cout << "info nodes " << RootPos.nodes_searched()
270 << " time " << Time::now() - SearchTime + 1 << sync_endl;
272 // When we reach max depth we arrive here even without Signals.stop is raised,
273 // but if we are pondering or in infinite search, according to UCI protocol,
274 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
275 // command. We simply wait here until GUI sends one of those commands (that
276 // raise Signals.stop).
277 if (!Signals.stop && (Limits.ponder || Limits.infinite))
279 Signals.stopOnPonderhit = true;
280 RootPos.this_thread()->wait_for(Signals.stop);
283 // Best move could be MOVE_NONE when searching on a stalemate position
284 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
285 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
292 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
293 // with increasing depth until the allocated thinking time has been consumed,
294 // user stops the search, or the maximum search depth is reached.
296 void id_loop(Position& pos) {
298 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
300 Value bestValue, alpha, beta, delta;
302 std::memset(ss-2, 0, 5 * sizeof(Stack));
303 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
307 bestValue = delta = alpha = -VALUE_INFINITE;
308 beta = VALUE_INFINITE;
313 Countermoves.clear();
315 PVSize = Options["MultiPV"];
316 Skill skill(Options["Skill Level"]);
318 // Do we have to play with skill handicap? In this case enable MultiPV search
319 // that we will use behind the scenes to retrieve a set of possible moves.
320 if (skill.enabled() && PVSize < 4)
323 PVSize = std::min(PVSize, RootMoves.size());
325 // Iterative deepening loop until requested to stop or target depth reached
326 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
328 // Age out PV variability metric
329 BestMoveChanges *= 0.8;
331 // Save last iteration's scores before first PV line is searched and all
332 // the move scores but the (new) PV are set to -VALUE_INFINITE.
333 for (size_t i = 0; i < RootMoves.size(); ++i)
334 RootMoves[i].prevScore = RootMoves[i].score;
336 // MultiPV loop. We perform a full root search for each PV line
337 for (PVIdx = 0; PVIdx < PVSize; ++PVIdx)
339 // Reset aspiration window starting size
343 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
344 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
347 // Start with a small aspiration window and, in case of fail high/low,
348 // research with bigger window until not failing high/low anymore.
351 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
353 // Bring to front the best move. It is critical that sorting is
354 // done with a stable algorithm because all the values but the first
355 // and eventually the new best one are set to -VALUE_INFINITE and
356 // we want to keep the same order for all the moves but the new
357 // PV that goes to the front. Note that in case of MultiPV search
358 // the already searched PV lines are preserved.
359 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
361 // Write PV back to transposition table in case the relevant
362 // entries have been overwritten during the search.
363 for (size_t i = 0; i <= PVIdx; ++i)
364 RootMoves[i].insert_pv_in_tt(pos);
366 // If search has been stopped return immediately. Sorting and
367 // writing PV back to TT is safe becuase RootMoves is still
368 // valid, although refers to previous iteration.
372 // When failing high/low give some update (without cluttering
373 // the UI) before to research.
374 if ( (bestValue <= alpha || bestValue >= beta)
375 && Time::now() - SearchTime > 3000)
376 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
378 // In case of failing low/high increase aspiration window and
379 // research, otherwise exit the loop.
380 if (bestValue <= alpha)
382 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
384 Signals.failedLowAtRoot = true;
385 Signals.stopOnPonderhit = false;
387 else if (bestValue >= beta)
388 beta = std::min(bestValue + delta, VALUE_INFINITE);
395 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
398 // Sort the PV lines searched so far and update the GUI
399 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
401 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
402 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
405 // Do we need to pick now the sub-optimal best move ?
406 if (skill.enabled() && skill.time_to_pick(depth))
409 if (Options["Write Search Log"])
411 RootMove& rm = RootMoves[0];
412 if (skill.best != MOVE_NONE)
413 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
415 Log log(Options["Search Log Filename"]);
416 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
420 // Do we have found a "mate in x"?
422 && bestValue >= VALUE_MATE_IN_MAX_PLY
423 && VALUE_MATE - bestValue <= 2 * Limits.mate)
426 // Do we have time for the next iteration? Can we stop searching now?
427 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
429 bool stop = false; // Local variable, not the volatile Signals.stop
431 // Take in account some extra time if the best move has changed
432 if (depth > 4 && depth < 50 && PVSize == 1)
433 TimeMgr.pv_instability(BestMoveChanges);
435 // Stop search if most of available time is already consumed. We
436 // probably don't have enough time to search the first move at the
437 // next iteration anyway.
438 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
441 // Stop search early if one move seems to be much better than others
445 && bestValue > VALUE_MATED_IN_MAX_PLY
446 && ( RootMoves.size() == 1
447 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
449 Value rBeta = bestValue - 2 * PawnValueMg;
450 ss->excludedMove = RootMoves[0].pv[0];
451 ss->skipNullMove = true;
452 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
453 ss->skipNullMove = false;
454 ss->excludedMove = MOVE_NONE;
462 // If we are allowed to ponder do not stop the search now but
463 // keep pondering until GUI sends "ponderhit" or "stop".
465 Signals.stopOnPonderhit = true;
474 // search<>() is the main search function for both PV and non-PV nodes and for
475 // normal and SplitPoint nodes. When called just after a split point the search
476 // is simpler because we have already probed the hash table, done a null move
477 // search, and searched the first move before splitting, we don't have to repeat
478 // all this work again. We also don't need to store anything to the hash table
479 // here: This is taken care of after we return from the split point.
481 template <NodeType NT>
482 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
484 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
485 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
486 const bool RootNode = (NT == Root || NT == SplitPointRoot);
488 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
489 assert(PvNode || (alpha == beta - 1));
490 assert(depth > DEPTH_ZERO);
492 Move quietsSearched[64];
495 SplitPoint* splitPoint;
497 Move ttMove, move, excludedMove, bestMove, threatMove;
499 Value bestValue, value, ttValue;
500 Value eval, nullValue, futilityValue;
501 bool inCheck, givesCheck, pvMove, singularExtensionNode, improving;
502 bool captureOrPromotion, dangerous, doFullDepthSearch;
503 int moveCount, quietCount;
505 // Step 1. Initialize node
506 Thread* thisThread = pos.this_thread();
507 inCheck = pos.checkers();
511 splitPoint = ss->splitPoint;
512 bestMove = splitPoint->bestMove;
513 threatMove = splitPoint->threatMove;
514 bestValue = splitPoint->bestValue;
516 ttMove = excludedMove = MOVE_NONE;
517 ttValue = VALUE_NONE;
519 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
524 moveCount = quietCount = 0;
525 bestValue = -VALUE_INFINITE;
526 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
527 ss->ply = (ss-1)->ply + 1;
528 ss->futilityMoveCount = 0;
529 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
530 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
532 // Used to send selDepth info to GUI
533 if (PvNode && thisThread->maxPly < ss->ply)
534 thisThread->maxPly = ss->ply;
538 // Step 2. Check for aborted search and immediate draw
539 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
540 return DrawValue[pos.side_to_move()];
542 // Step 3. Mate distance pruning. Even if we mate at the next move our score
543 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
544 // a shorter mate was found upward in the tree then there is no need to search
545 // further, we will never beat current alpha. Same logic but with reversed signs
546 // applies also in the opposite condition of being mated instead of giving mate,
547 // in this case return a fail-high score.
548 alpha = std::max(mated_in(ss->ply), alpha);
549 beta = std::min(mate_in(ss->ply+1), beta);
554 // Step 4. Transposition table lookup
555 // We don't want the score of a partial search to overwrite a previous full search
556 // TT value, so we use a different position key in case of an excluded move.
557 excludedMove = ss->excludedMove;
558 posKey = excludedMove ? pos.exclusion_key() : pos.key();
559 tte = TT.probe(posKey);
560 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
561 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
563 // At PV nodes we check for exact scores, while at non-PV nodes we check for
564 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
565 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
566 // we should also update RootMoveList to avoid bogus output.
569 && tte->depth() >= depth
570 && ttValue != VALUE_NONE // Only in case of TT access race
571 && ( PvNode ? tte->bound() == BOUND_EXACT
572 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
573 : (tte->bound() & BOUND_UPPER)))
576 ss->currentMove = ttMove; // Can be MOVE_NONE
580 && !pos.capture_or_promotion(ttMove)
581 && ttMove != ss->killers[0])
583 ss->killers[1] = ss->killers[0];
584 ss->killers[0] = ttMove;
589 // Step 5. Evaluate the position statically and update parent's gain statistics
592 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
598 // Never assume anything on values stored in TT
599 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
600 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
601 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
603 // Can ttValue be used as a better position evaluation?
604 if (ttValue != VALUE_NONE)
605 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
610 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
611 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
612 ss->staticEval, ss->evalMargin);
615 // Update gain for the parent non-capture move given the static position
616 // evaluation before and after the move.
617 if ( !pos.captured_piece_type()
618 && ss->staticEval != VALUE_NONE
619 && (ss-1)->staticEval != VALUE_NONE
620 && (move = (ss-1)->currentMove) != MOVE_NULL
621 && type_of(move) == NORMAL)
623 Square to = to_sq(move);
624 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
627 // Step 6. Razoring (skipped when in check)
629 && depth < 4 * ONE_PLY
630 && eval + razor_margin(depth) < beta
631 && ttMove == MOVE_NONE
632 && abs(beta) < VALUE_MATE_IN_MAX_PLY
633 && !pos.pawn_on_7th(pos.side_to_move()))
635 Value rbeta = beta - razor_margin(depth);
636 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
638 // Logically we should return (v + razor_margin(depth)), but
639 // surprisingly this did slightly weaker in tests.
643 // Step 7. Static null move pruning (skipped when in check)
644 // We're betting that the opponent doesn't have a move that will reduce
645 // the score by more than futility_margin(depth) if we do a null move.
648 && depth < 4 * ONE_PLY
649 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
650 && abs(beta) < VALUE_MATE_IN_MAX_PLY
651 && abs(eval) < VALUE_KNOWN_WIN
652 && pos.non_pawn_material(pos.side_to_move()))
653 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
655 // Step 8. Null move search with verification search (is omitted in PV nodes)
658 && depth >= 2 * ONE_PLY
660 && abs(beta) < VALUE_MATE_IN_MAX_PLY
661 && pos.non_pawn_material(pos.side_to_move()))
663 ss->currentMove = MOVE_NULL;
665 // Null move dynamic reduction based on depth
666 Depth R = 3 * ONE_PLY + depth / 4;
668 // Null move dynamic reduction based on value
669 if (eval - PawnValueMg > beta)
672 pos.do_null_move(st);
673 (ss+1)->skipNullMove = true;
674 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
675 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
676 (ss+1)->skipNullMove = false;
677 pos.undo_null_move();
679 if (nullValue >= beta)
681 // Do not return unproven mate scores
682 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
685 if (depth < 12 * ONE_PLY)
688 // Do verification search at high depths
689 ss->skipNullMove = true;
690 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
691 ss->skipNullMove = false;
698 // The null move failed low, which means that we may be faced with
699 // some kind of threat. If the previous move was reduced, check if
700 // the move that refuted the null move was somehow connected to the
701 // move which was reduced. If a connection is found, return a fail
702 // low score (which will cause the reduced move to fail high in the
703 // parent node, which will trigger a re-search with full depth).
704 threatMove = (ss+1)->currentMove;
706 if ( depth < 5 * ONE_PLY
708 && threatMove != MOVE_NONE
709 && allows(pos, (ss-1)->currentMove, threatMove))
714 // Step 9. ProbCut (skipped when in check)
715 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
716 // and a reduced search returns a value much above beta, we can (almost) safely
717 // prune the previous move.
719 && depth >= 5 * ONE_PLY
721 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
723 Value rbeta = beta + 200;
724 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
726 assert(rdepth >= ONE_PLY);
727 assert((ss-1)->currentMove != MOVE_NONE);
728 assert((ss-1)->currentMove != MOVE_NULL);
730 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
733 while ((move = mp.next_move<false>()) != MOVE_NONE)
734 if (pos.legal(move, ci.pinned))
736 ss->currentMove = move;
737 pos.do_move(move, st, ci, pos.gives_check(move, ci));
738 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
745 // Step 10. Internal iterative deepening (skipped when in check)
746 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
747 && ttMove == MOVE_NONE
748 && (PvNode || ss->staticEval + Value(256) >= beta))
750 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
752 ss->skipNullMove = true;
753 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
754 ss->skipNullMove = false;
756 tte = TT.probe(posKey);
757 ttMove = tte ? tte->move() : MOVE_NONE;
760 moves_loop: // When in check and at SpNode search starts from here
762 Square prevMoveSq = to_sq((ss-1)->currentMove);
763 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
764 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
766 MovePicker mp(pos, ttMove, depth, History, countermoves, ss);
768 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
769 improving = ss->staticEval >= (ss-2)->staticEval
770 || ss->staticEval == VALUE_NONE
771 ||(ss-2)->staticEval == VALUE_NONE;
773 singularExtensionNode = !RootNode
775 && depth >= 8 * ONE_PLY
776 && ttMove != MOVE_NONE
777 && !excludedMove // Recursive singular search is not allowed
778 && (tte->bound() & BOUND_LOWER)
779 && tte->depth() >= depth - 3 * ONE_PLY;
781 // Step 11. Loop through moves
782 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
783 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
787 if (move == excludedMove)
790 // At root obey the "searchmoves" option and skip moves not listed in Root
791 // Move List, as a consequence any illegal move is also skipped. In MultiPV
792 // mode we also skip PV moves which have been already searched.
793 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
798 // Shared counter cannot be decremented later if move turns out to be illegal
799 if (!pos.legal(move, ci.pinned))
802 moveCount = ++splitPoint->moveCount;
803 splitPoint->mutex.unlock();
810 Signals.firstRootMove = (moveCount == 1);
812 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
813 sync_cout << "info depth " << depth / ONE_PLY
814 << " currmove " << move_to_uci(move, pos.is_chess960())
815 << " currmovenumber " << moveCount + PVIdx << sync_endl;
819 captureOrPromotion = pos.capture_or_promotion(move);
820 givesCheck = pos.gives_check(move, ci);
821 dangerous = givesCheck
822 || pos.passed_pawn_push(move)
823 || type_of(move) == CASTLE;
825 // Step 12. Extend checks
826 if (givesCheck && pos.see_sign(move) >= 0)
829 // Singular extension search. If all moves but one fail low on a search of
830 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
831 // is singular and should be extended. To verify this we do a reduced search
832 // on all the other moves but the ttMove, if result is lower than ttValue minus
833 // a margin then we extend ttMove.
834 if ( singularExtensionNode
837 && pos.legal(move, ci.pinned)
838 && abs(ttValue) < VALUE_KNOWN_WIN)
840 assert(ttValue != VALUE_NONE);
842 Value rBeta = ttValue - int(depth);
843 ss->excludedMove = move;
844 ss->skipNullMove = true;
845 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
846 ss->skipNullMove = false;
847 ss->excludedMove = MOVE_NONE;
853 // Update current move (this must be done after singular extension search)
854 newDepth = depth - ONE_PLY + ext;
856 // Step 13. Futility pruning (is omitted in PV nodes)
858 && !captureOrPromotion
861 /* && move != ttMove Already implicit in the next condition */
862 && bestValue > VALUE_MATED_IN_MAX_PLY)
864 // Move count based pruning
865 if ( depth < 16 * ONE_PLY
866 && moveCount >= FutilityMoveCounts[improving][depth]
867 && (!threatMove || !refutes(pos, move, threatMove)))
870 splitPoint->mutex.lock();
875 // Value based pruning
876 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
877 // but fixing this made program slightly weaker.
878 Depth predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
879 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
880 + Gains[pos.moved_piece(move)][to_sq(move)];
882 if (futilityValue < beta)
884 bestValue = std::max(bestValue, futilityValue);
888 splitPoint->mutex.lock();
889 if (bestValue > splitPoint->bestValue)
890 splitPoint->bestValue = bestValue;
895 // Prune moves with negative SEE at low depths
896 if ( predictedDepth < 4 * ONE_PLY
897 && pos.see_sign(move) < 0)
900 splitPoint->mutex.lock();
905 // We have not pruned the move that will be searched, but remember how
906 // far in the move list we are to be more aggressive in the child node.
907 ss->futilityMoveCount = moveCount;
910 ss->futilityMoveCount = 0;
912 // Check for legality only before to do the move
913 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
919 pvMove = PvNode && moveCount == 1;
920 ss->currentMove = move;
921 if (!SpNode && !captureOrPromotion && quietCount < 64)
922 quietsSearched[quietCount++] = move;
924 // Step 14. Make the move
925 pos.do_move(move, st, ci, givesCheck);
927 // Step 15. Reduced depth search (LMR). If the move fails high will be
928 // re-searched at full depth.
929 if ( depth >= 3 * ONE_PLY
931 && !captureOrPromotion
933 && move != ss->killers[0]
934 && move != ss->killers[1])
936 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
938 if (!PvNode && cutNode)
939 ss->reduction += ONE_PLY;
941 else if (History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
942 ss->reduction += ONE_PLY / 2;
944 if (move == countermoves[0] || move == countermoves[1])
945 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
947 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
949 alpha = splitPoint->alpha;
951 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
953 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
954 ss->reduction = DEPTH_ZERO;
957 doFullDepthSearch = !pvMove;
959 // Step 16. Full depth search, when LMR is skipped or fails high
960 if (doFullDepthSearch)
963 alpha = splitPoint->alpha;
965 value = newDepth < ONE_PLY ?
966 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
967 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
968 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
971 // Only for PV nodes do a full PV search on the first move or after a fail
972 // high, in the latter case search only if value < beta, otherwise let the
973 // parent node to fail low with value <= alpha and to try another move.
974 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
975 value = newDepth < ONE_PLY ?
976 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
977 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
978 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
979 // Step 17. Undo move
982 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
984 // Step 18. Check for new best move
987 splitPoint->mutex.lock();
988 bestValue = splitPoint->bestValue;
989 alpha = splitPoint->alpha;
992 // Finished searching the move. If Signals.stop is true, the search
993 // was aborted because the user interrupted the search or because we
994 // ran out of time. In this case, the return value of the search cannot
995 // be trusted, and we don't update the best move and/or PV.
996 if (Signals.stop || thisThread->cutoff_occurred())
997 return value; // To avoid returning VALUE_INFINITE
1001 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1003 // PV move or new best move ?
1004 if (pvMove || value > alpha)
1007 rm.extract_pv_from_tt(pos);
1009 // We record how often the best move has been changed in each
1010 // iteration. This information is used for time management: When
1011 // the best move changes frequently, we allocate some more time.
1016 // All other moves but the PV are set to the lowest value, this
1017 // is not a problem when sorting becuase sort is stable and move
1018 // position in the list is preserved, just the PV is pushed up.
1019 rm.score = -VALUE_INFINITE;
1022 if (value > bestValue)
1024 bestValue = SpNode ? splitPoint->bestValue = value : value;
1028 bestMove = SpNode ? splitPoint->bestMove = move : move;
1030 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1031 alpha = SpNode ? splitPoint->alpha = value : value;
1034 assert(value >= beta); // Fail high
1037 splitPoint->cutoff = true;
1044 // Step 19. Check for splitting the search
1046 && depth >= Threads.minimumSplitDepth
1047 && Threads.available_slave(thisThread)
1048 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1050 assert(bestValue < beta);
1052 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1053 depth, threatMove, moveCount, &mp, NT, cutNode);
1054 if (bestValue >= beta)
1062 // Step 20. Check for mate and stalemate
1063 // All legal moves have been searched and if there are no legal moves, it
1064 // must be mate or stalemate. Note that we can have a false positive in
1065 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1066 // harmless because return value is discarded anyhow in the parent nodes.
1067 // If we are in a singular extension search then return a fail low score.
1068 // A split node has at least one move, the one tried before to be splitted.
1070 return excludedMove ? alpha
1071 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1073 // If we have pruned all the moves without searching return a fail-low score
1074 if (bestValue == -VALUE_INFINITE)
1077 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1078 bestValue >= beta ? BOUND_LOWER :
1079 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1080 depth, bestMove, ss->staticEval, ss->evalMargin);
1082 // Quiet best move: update killers, history and countermoves
1083 if ( bestValue >= beta
1084 && !pos.capture_or_promotion(bestMove)
1087 if (ss->killers[0] != bestMove)
1089 ss->killers[1] = ss->killers[0];
1090 ss->killers[0] = bestMove;
1093 // Increase history value of the cut-off move and decrease all the other
1094 // played non-capture moves.
1095 Value bonus = Value(int(depth) * int(depth));
1096 History.update(pos.moved_piece(bestMove), to_sq(bestMove), bonus);
1097 for (int i = 0; i < quietCount - 1; ++i)
1099 Move m = quietsSearched[i];
1100 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1103 if (is_ok((ss-1)->currentMove))
1104 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1107 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1113 // qsearch() is the quiescence search function, which is called by the main
1114 // search function when the remaining depth is zero (or, to be more precise,
1115 // less than ONE_PLY).
1117 template <NodeType NT, bool InCheck>
1118 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1120 const bool PvNode = (NT == PV);
1122 assert(NT == PV || NT == NonPV);
1123 assert(InCheck == !!pos.checkers());
1124 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1125 assert(PvNode || (alpha == beta - 1));
1126 assert(depth <= DEPTH_ZERO);
1131 Move ttMove, move, bestMove;
1132 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1133 bool givesCheck, evasionPrunable;
1136 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1140 ss->currentMove = bestMove = MOVE_NONE;
1141 ss->ply = (ss-1)->ply + 1;
1143 // Check for an instant draw or maximum ply reached
1144 if (pos.is_draw() || ss->ply > MAX_PLY)
1145 return DrawValue[pos.side_to_move()];
1147 // Decide whether or not to include checks, this fixes also the type of
1148 // TT entry depth that we are going to use. Note that in qsearch we use
1149 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1150 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1151 : DEPTH_QS_NO_CHECKS;
1153 // Transposition table lookup
1155 tte = TT.probe(posKey);
1156 ttMove = tte ? tte->move() : MOVE_NONE;
1157 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1160 && tte->depth() >= ttDepth
1161 && ttValue != VALUE_NONE // Only in case of TT access race
1162 && ( PvNode ? tte->bound() == BOUND_EXACT
1163 : ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1164 : (tte->bound() & BOUND_UPPER)))
1166 ss->currentMove = ttMove; // Can be MOVE_NONE
1170 // Evaluate the position statically
1173 ss->staticEval = ss->evalMargin = VALUE_NONE;
1174 bestValue = futilityBase = -VALUE_INFINITE;
1180 // Never assume anything on values stored in TT
1181 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1182 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1183 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1185 // Can ttValue be used as a better position evaluation?
1186 if (ttValue != VALUE_NONE)
1187 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1188 bestValue = ttValue;
1191 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1193 // Stand pat. Return immediately if static value is at least beta
1194 if (bestValue >= beta)
1197 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1198 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1203 if (PvNode && bestValue > alpha)
1206 futilityBase = bestValue + ss->evalMargin + Value(128);
1209 // Initialize a MovePicker object for the current position, and prepare
1210 // to search the moves. Because the depth is <= 0 here, only captures,
1211 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1213 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1216 // Loop through the moves until no moves remain or a beta cutoff occurs
1217 while ((move = mp.next_move<false>()) != MOVE_NONE)
1219 assert(is_ok(move));
1221 givesCheck = pos.gives_check(move, ci);
1228 && type_of(move) != PROMOTION
1229 && futilityBase > -VALUE_KNOWN_WIN
1230 && !pos.passed_pawn_push(move))
1232 futilityValue = futilityBase
1233 + PieceValue[EG][pos.piece_on(to_sq(move))]
1234 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1236 if (futilityValue < beta)
1238 bestValue = std::max(bestValue, futilityValue);
1242 // Prune moves with negative or equal SEE and also moves with positive
1243 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1244 if ( futilityBase < beta
1245 && pos.see(move, beta - futilityBase) <= 0)
1247 bestValue = std::max(bestValue, futilityBase);
1252 // Detect non-capture evasions that are candidate to be pruned
1253 evasionPrunable = InCheck
1254 && bestValue > VALUE_MATED_IN_MAX_PLY
1255 && !pos.capture(move)
1256 && !pos.can_castle(pos.side_to_move());
1258 // Don't search moves with negative SEE values
1260 && (!InCheck || evasionPrunable)
1262 && type_of(move) != PROMOTION
1263 && pos.see_sign(move) < 0)
1266 // Check for legality only before to do the move
1267 if (!pos.legal(move, ci.pinned))
1270 ss->currentMove = move;
1272 // Make and search the move
1273 pos.do_move(move, st, ci, givesCheck);
1274 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1275 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1276 pos.undo_move(move);
1278 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1280 // Check for new best move
1281 if (value > bestValue)
1287 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1294 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1295 ttDepth, move, ss->staticEval, ss->evalMargin);
1303 // All legal moves have been searched. A special case: If we're in check
1304 // and no legal moves were found, it is checkmate.
1305 if (InCheck && bestValue == -VALUE_INFINITE)
1306 return mated_in(ss->ply); // Plies to mate from the root
1308 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1309 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1310 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1312 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1318 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1319 // "plies to mate from the current position". Non-mate scores are unchanged.
1320 // The function is called before storing a value to the transposition table.
1322 Value value_to_tt(Value v, int ply) {
1324 assert(v != VALUE_NONE);
1326 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1327 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1331 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1332 // from the transposition table (where refers to the plies to mate/be mated
1333 // from current position) to "plies to mate/be mated from the root".
1335 Value value_from_tt(Value v, int ply) {
1337 return v == VALUE_NONE ? VALUE_NONE
1338 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1339 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1343 // allows() tests whether the 'first' move at previous ply somehow makes the
1344 // 'second' move possible, for instance if the moving piece is the same in
1345 // both moves. Normally the second move is the threat (the best move returned
1346 // from a null search that fails low).
1348 bool allows(const Position& pos, Move first, Move second) {
1350 assert(is_ok(first));
1351 assert(is_ok(second));
1352 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1353 assert(type_of(first) == CASTLE || color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1355 Square m1from = from_sq(first);
1356 Square m2from = from_sq(second);
1357 Square m1to = to_sq(first);
1358 Square m2to = to_sq(second);
1360 // The piece is the same or second's destination was vacated by the first move
1361 // We exclude the trivial case where a sliding piece does in two moves what
1362 // it could do in one move: eg. Ra1a2, Ra2a3.
1364 || (m1to == m2from && !squares_aligned(m1from, m2from, m2to)))
1367 // Second one moves through the square vacated by first one
1368 if (between_bb(m2from, m2to) & m1from)
1371 // Second's destination is defended by the first move's piece
1372 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1376 // Second move gives a discovered check through the first's checking piece
1377 if (m1att & pos.king_square(pos.side_to_move()))
1379 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1387 // refutes() tests whether a 'first' move is able to defend against a 'second'
1388 // opponent's move. In this case will not be pruned. Normally the second move
1389 // is the threat (the best move returned from a null search that fails low).
1391 bool refutes(const Position& pos, Move first, Move second) {
1393 assert(is_ok(first));
1394 assert(is_ok(second));
1396 Square m1from = from_sq(first);
1397 Square m2from = from_sq(second);
1398 Square m1to = to_sq(first);
1399 Square m2to = to_sq(second);
1401 // Don't prune moves of the threatened piece
1405 // If the threatened piece has value less than or equal to the value of the
1406 // threat piece, don't prune moves which defend it.
1407 if ( pos.capture(second)
1408 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1409 || type_of(pos.piece_on(m2from)) == KING))
1411 // Update occupancy as if the piece and the threat are moving
1412 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1413 Piece pc = pos.piece_on(m1from);
1415 // The moved piece attacks the square 'tto' ?
1416 if (pos.attacks_from(pc, m1to, occ) & m2to)
1419 // Scan for possible X-ray attackers behind the moved piece
1420 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1421 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1423 // Verify attackers are triggered by our move and not already existing
1424 if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
1428 // Don't prune safe moves which block the threat path
1429 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1436 // When playing with strength handicap choose best move among the MultiPV set
1437 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1439 Move Skill::pick_move() {
1443 // PRNG sequence should be not deterministic
1444 for (int i = Time::now() % 50; i > 0; --i)
1445 rk.rand<unsigned>();
1447 // RootMoves are already sorted by score in descending order
1448 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1449 int weakness = 120 - 2 * level;
1450 int max_s = -VALUE_INFINITE;
1453 // Choose best move. For each move score we add two terms both dependent on
1454 // weakness, one deterministic and bigger for weaker moves, and one random,
1455 // then we choose the move with the resulting highest score.
1456 for (size_t i = 0; i < PVSize; ++i)
1458 int s = RootMoves[i].score;
1460 // Don't allow crazy blunders even at very low skills
1461 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1464 // This is our magic formula
1465 s += ( weakness * int(RootMoves[0].score - s)
1466 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1471 best = RootMoves[i].pv[0];
1478 // uci_pv() formats PV information according to UCI protocol. UCI requires
1479 // to send all the PV lines also if are still to be searched and so refer to
1480 // the previous search score.
1482 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1484 std::stringstream s;
1485 Time::point elapsed = Time::now() - SearchTime + 1;
1486 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1489 for (size_t i = 0; i < Threads.size(); ++i)
1490 if (Threads[i]->maxPly > selDepth)
1491 selDepth = Threads[i]->maxPly;
1493 for (size_t i = 0; i < uciPVSize; ++i)
1495 bool updated = (i <= PVIdx);
1497 if (depth == 1 && !updated)
1500 int d = updated ? depth : depth - 1;
1501 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1503 if (s.rdbuf()->in_avail()) // Not at first line
1506 s << "info depth " << d
1507 << " seldepth " << selDepth
1508 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1509 << " nodes " << pos.nodes_searched()
1510 << " nps " << pos.nodes_searched() * 1000 / elapsed
1511 << " time " << elapsed
1512 << " multipv " << i + 1
1515 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; ++j)
1516 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1525 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1526 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1527 /// allow to always have a ponder move even when we fail high at root, and a
1528 /// long PV to print that is important for position analysis.
1530 void RootMove::extract_pv_from_tt(Position& pos) {
1532 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1542 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1544 pos.do_move(pv[ply++], *st++);
1545 tte = TT.probe(pos.key());
1548 && pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
1549 && pos.legal(m, pos.pinned_pieces())
1551 && (!pos.is_draw() || ply < 2));
1553 pv.push_back(MOVE_NONE); // Must be zero-terminating
1555 while (ply) pos.undo_move(pv[--ply]);
1559 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1560 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1561 /// first, even if the old TT entries have been overwritten.
1563 void RootMove::insert_pv_in_tt(Position& pos) {
1565 StateInfo state[MAX_PLY_PLUS_6], *st = state;
1570 tte = TT.probe(pos.key());
1572 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1573 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1575 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1577 pos.do_move(pv[ply++], *st++);
1579 } while (pv[ply] != MOVE_NONE);
1581 while (ply) pos.undo_move(pv[--ply]);
1585 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1587 void Thread::idle_loop() {
1589 // Pointer 'this_sp' is not null only if we are called from split(), and not
1590 // at the thread creation. So it means we are the split point's master.
1591 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1593 assert(!this_sp || (this_sp->masterThread == this && searching));
1597 // If we are not searching, wait for a condition to be signaled instead of
1598 // wasting CPU time polling for work.
1599 while ((!searching && Threads.sleepWhileIdle) || exit)
1607 // Grab the lock to avoid races with Thread::notify_one()
1610 // If we are master and all slaves have finished then exit idle_loop
1611 if (this_sp && !this_sp->slavesMask)
1617 // Do sleep after retesting sleep conditions under lock protection, in
1618 // particular we need to avoid a deadlock in case a master thread has,
1619 // in the meanwhile, allocated us and sent the notify_one() call before
1620 // we had the chance to grab the lock.
1621 if (!searching && !exit)
1622 sleepCondition.wait(mutex);
1627 // If this thread has been assigned work, launch a search
1632 Threads.mutex.lock();
1635 assert(activeSplitPoint);
1636 SplitPoint* sp = activeSplitPoint;
1638 Threads.mutex.unlock();
1640 Stack stack[MAX_PLY_PLUS_6], *ss = stack+2; // To allow referencing (ss-2)
1641 Position pos(*sp->pos, this);
1643 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1644 ss->splitPoint = sp;
1648 assert(activePosition == NULL);
1650 activePosition = &pos;
1652 switch (sp->nodeType) {
1654 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1657 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1660 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1669 activePosition = NULL;
1670 sp->slavesMask &= ~(1ULL << idx);
1671 sp->nodes += pos.nodes_searched();
1673 // Wake up master thread so to allow it to return from the idle loop
1674 // in case we are the last slave of the split point.
1675 if ( Threads.sleepWhileIdle
1676 && this != sp->masterThread
1679 assert(!sp->masterThread->searching);
1680 sp->masterThread->notify_one();
1683 // After releasing the lock we cannot access anymore any SplitPoint
1684 // related data in a safe way becuase it could have been released under
1685 // our feet by the sp master. Also accessing other Thread objects is
1686 // unsafe because if we are exiting there is a chance are already freed.
1690 // If this thread is the master of a split point and all slaves have finished
1691 // their work at this split point, return from the idle loop.
1692 if (this_sp && !this_sp->slavesMask)
1694 this_sp->mutex.lock();
1695 bool finished = !this_sp->slavesMask; // Retest under lock protection
1696 this_sp->mutex.unlock();
1704 /// check_time() is called by the timer thread when the timer triggers. It is
1705 /// used to print debug info and, more important, to detect when we are out of
1706 /// available time and so stop the search.
1710 static Time::point lastInfoTime = Time::now();
1711 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1713 if (Time::now() - lastInfoTime >= 1000)
1715 lastInfoTime = Time::now();
1724 Threads.mutex.lock();
1726 nodes = RootPos.nodes_searched();
1728 // Loop across all split points and sum accumulated SplitPoint nodes plus
1729 // all the currently active positions nodes.
1730 for (size_t i = 0; i < Threads.size(); ++i)
1731 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1733 SplitPoint& sp = Threads[i]->splitPoints[j];
1738 Bitboard sm = sp.slavesMask;
1741 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1743 nodes += pos->nodes_searched();
1749 Threads.mutex.unlock();
1752 Time::point elapsed = Time::now() - SearchTime;
1753 bool stillAtFirstMove = Signals.firstRootMove
1754 && !Signals.failedLowAtRoot
1755 && elapsed > TimeMgr.available_time();
1757 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution
1758 || stillAtFirstMove;
1760 if ( (Limits.use_time_management() && noMoreTime)
1761 || (Limits.movetime && elapsed >= Limits.movetime)
1762 || (Limits.nodes && nodes >= Limits.nodes))
1763 Signals.stop = true;