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;
47 MovesVectPtr SetupMoves;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // This is the minimum interval in msec between two check_time() calls
60 const int TimerResolution = 5;
62 // Different node types, used as template parameter
63 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
65 // Dynamic razoring margin based on depth
66 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
68 // Futility lookup tables (initialized at startup) and their access functions
69 Value FutilityMargins[16][64]; // [depth][moveNumber]
70 int FutilityMoveCounts[32]; // [depth]
72 inline Value futility_margin(Depth d, int mn) {
74 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
78 // Reduction lookup tables (initialized at startup) and their access function
79 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
81 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
83 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
89 Value DrawValue[COLOR_NB];
93 template <NodeType NT>
94 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
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 + 0.25 * pow(double(d), 2.0));
153 /// Search::perft() is our utility to verify move generation. All the leaf nodes
154 /// up to the given depth are generated and counted and the sum returned.
156 size_t Search::perft(Position& pos, Depth depth) {
158 // At the last ply just return the number of legal moves (leaf nodes)
159 if (depth == ONE_PLY)
160 return MoveList<LEGAL>(pos).size();
166 for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
168 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
169 cnt += perft(pos, depth - ONE_PLY);
170 pos.undo_move(ml.move());
177 /// Search::think() is the external interface to Stockfish's search, and is
178 /// called by the main thread when the program receives the UCI 'go' command. It
179 /// searches from RootPos and at the end prints the "bestmove" to output.
181 void Search::think() {
183 static PolyglotBook book; // Defined static to initialize the PRNG only once
185 RootColor = RootPos.side_to_move();
186 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
188 if (RootMoves.empty())
190 RootMoves.push_back(MOVE_NONE);
191 sync_cout << "info depth 0 score "
192 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
198 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
200 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
202 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
204 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
209 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
211 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
212 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
213 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
214 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
217 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
219 if (Options["Use Search Log"])
221 Log log(Options["Search Log Filename"]);
222 log << "\nSearching: " << RootPos.fen()
223 << "\ninfinite: " << Limits.infinite
224 << " ponder: " << Limits.ponder
225 << " time: " << Limits.time[RootColor]
226 << " increment: " << Limits.inc[RootColor]
227 << " moves to go: " << Limits.movestogo
231 // Reset the threads, still sleeping: will be wake up at split time
232 for (size_t i = 0; i < Threads.size(); i++)
233 Threads[i]->maxPly = 0;
235 Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
237 // Set best timer interval to avoid lagging under time pressure. Timer is
238 // used to check for remaining available thinking time.
239 Threads.timer->msec =
240 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
241 Limits.nodes ? 2 * TimerResolution
244 Threads.timer->notify_one(); // Wake up the recurring timer
246 id_loop(RootPos); // Let's start searching !
248 Threads.timer->msec = 0; // Stop the timer
249 Threads.sleepWhileIdle = true; // Send idle threads to sleep
251 if (Options["Use 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 we reach max depth we arrive here even without Signals.stop is raised,
269 // but if we are pondering or in infinite search, according to UCI protocol,
270 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
271 // command. We simply wait here until GUI sends one of those commands (that
272 // raise Signals.stop).
273 if (!Signals.stop && (Limits.ponder || Limits.infinite))
275 Signals.stopOnPonderhit = true;
276 RootPos.this_thread()->wait_for(Signals.stop);
279 // Best move could be MOVE_NONE when searching on a stalemate position
280 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
281 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
288 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
289 // with increasing depth until the allocated thinking time has been consumed,
290 // user stops the search, or the maximum search depth is reached.
292 void id_loop(Position& pos) {
294 Stack ss[MAX_PLY_PLUS_2];
295 int depth, prevBestMoveChanges;
296 Value bestValue, alpha, beta, delta;
297 bool bestMoveNeverChanged = true;
299 memset(ss, 0, 4 * sizeof(Stack));
300 depth = BestMoveChanges = 0;
301 bestValue = delta = -VALUE_INFINITE;
302 ss->currentMove = MOVE_NULL; // Hack to skip update gains
307 PVSize = Options["MultiPV"];
308 Skill skill(Options["Skill Level"]);
310 // Do we have to play with skill handicap? In this case enable MultiPV search
311 // that we will use behind the scenes to retrieve a set of possible moves.
312 if (skill.enabled() && PVSize < 4)
315 PVSize = std::min(PVSize, RootMoves.size());
317 // Iterative deepening loop until requested to stop or target depth reached
318 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
320 // Save last iteration's scores before first PV line is searched and all
321 // the move scores but the (new) PV are set to -VALUE_INFINITE.
322 for (size_t i = 0; i < RootMoves.size(); i++)
323 RootMoves[i].prevScore = RootMoves[i].score;
325 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
328 // MultiPV loop. We perform a full root search for each PV line
329 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
331 // Set aspiration window default width
332 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
335 alpha = RootMoves[PVIdx].prevScore - delta;
336 beta = RootMoves[PVIdx].prevScore + delta;
340 alpha = -VALUE_INFINITE;
341 beta = VALUE_INFINITE;
344 // Start with a small aspiration window and, in case of fail high/low,
345 // research with bigger window until not failing high/low anymore.
348 // Search starts from ss+1 to allow referencing (ss-1). This is
349 // needed by update gains and ss copy when splitting at Root.
350 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
352 // Bring to front the best move. It is critical that sorting is
353 // done with a stable algorithm because all the values but the first
354 // and eventually the new best one are set to -VALUE_INFINITE and
355 // we want to keep the same order for all the moves but the new
356 // PV that goes to the front. Note that in case of MultiPV search
357 // the already searched PV lines are preserved.
358 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
360 // Write PV back to transposition table in case the relevant
361 // entries have been overwritten during the search.
362 for (size_t i = 0; i <= PVIdx; i++)
363 RootMoves[i].insert_pv_in_tt(pos);
365 // If search has been stopped return immediately. Sorting and
366 // writing PV back to TT is safe becuase RootMoves is still
367 // valid, although refers to previous iteration.
371 // In case of failing high/low increase aspiration window and
372 // research, otherwise exit the loop.
373 if (bestValue > alpha && bestValue < beta)
376 // Give some update (without cluttering the UI) before to research
377 if (Time::now() - SearchTime > 3000)
378 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
380 if (abs(bestValue) >= VALUE_KNOWN_WIN)
382 alpha = -VALUE_INFINITE;
383 beta = VALUE_INFINITE;
385 else if (bestValue >= beta)
392 Signals.failedLowAtRoot = true;
393 Signals.stopOnPonderhit = false;
399 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
402 // Sort the PV lines searched so far and update the GUI
403 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
405 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
406 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
409 // Do we need to pick now the sub-optimal best move ?
410 if (skill.enabled() && skill.time_to_pick(depth))
413 if (Options["Use Search Log"])
415 Log log(Options["Search Log Filename"]);
416 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
420 // Filter out startup noise when monitoring best move stability
421 if (depth > 2 && BestMoveChanges)
422 bestMoveNeverChanged = false;
424 // Do we have found a "mate in x"?
426 && bestValue >= VALUE_MATE_IN_MAX_PLY
427 && VALUE_MATE - bestValue <= 2 * Limits.mate)
430 // Do we have time for the next iteration? Can we stop searching now?
431 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
433 bool stop = false; // Local variable, not the volatile Signals.stop
435 // Take in account some extra time if the best move has changed
436 if (depth > 4 && depth < 50 && PVSize == 1)
437 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
439 // Stop search if most of available time is already consumed. We
440 // probably don't have enough time to search the first move at the
441 // next iteration anyway.
442 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
445 // Stop search early if one move seems to be much better than others
449 && ( (bestMoveNeverChanged && pos.captured_piece_type())
450 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
452 Value rBeta = bestValue - 2 * PawnValueMg;
453 (ss+1)->excludedMove = RootMoves[0].pv[0];
454 (ss+1)->skipNullMove = true;
455 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
456 (ss+1)->skipNullMove = false;
457 (ss+1)->excludedMove = MOVE_NONE;
465 // If we are allowed to ponder do not stop the search now but
466 // keep pondering until GUI sends "ponderhit" or "stop".
468 Signals.stopOnPonderhit = true;
477 // search<>() is the main search function for both PV and non-PV nodes and for
478 // normal and SplitPoint nodes. When called just after a split point the search
479 // is simpler because we have already probed the hash table, done a null move
480 // search, and searched the first move before splitting, we don't have to repeat
481 // all this work again. We also don't need to store anything to the hash table
482 // here: This is taken care of after we return from the split point.
484 template <NodeType NT>
485 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
487 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
488 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
489 const bool RootNode = (NT == Root || NT == SplitPointRoot);
491 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
492 assert(PvNode || (alpha == beta - 1));
493 assert(depth > DEPTH_ZERO);
495 Move movesSearched[64];
498 SplitPoint* splitPoint;
500 Move ttMove, move, excludedMove, bestMove, threatMove;
502 Value bestValue, value, ttValue;
503 Value eval, nullValue, futilityValue;
504 bool inCheck, givesCheck, pvMove, singularExtensionNode;
505 bool captureOrPromotion, dangerous, doFullDepthSearch;
506 int moveCount, playedMoveCount;
508 // Step 1. Initialize node
509 Thread* thisThread = pos.this_thread();
510 moveCount = playedMoveCount = 0;
511 inCheck = pos.checkers();
515 splitPoint = ss->splitPoint;
516 bestMove = splitPoint->bestMove;
517 threatMove = splitPoint->threatMove;
518 bestValue = splitPoint->bestValue;
520 ttMove = excludedMove = MOVE_NONE;
521 ttValue = VALUE_NONE;
523 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
525 goto split_point_start;
528 bestValue = -VALUE_INFINITE;
529 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
530 ss->ply = (ss-1)->ply + 1;
531 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
532 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
534 // Used to send selDepth info to GUI
535 if (PvNode && thisThread->maxPly < ss->ply)
536 thisThread->maxPly = ss->ply;
540 // Step 2. Check for aborted search and immediate draw
541 if (Signals.stop || pos.is_draw<false>() || ss->ply > MAX_PLY)
542 return DrawValue[pos.side_to_move()];
544 // Step 3. Mate distance pruning. Even if we mate at the next move our score
545 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
546 // a shorter mate was found upward in the tree then there is no need to search
547 // further, we will never beat current alpha. Same logic but with reversed signs
548 // applies also in the opposite condition of being mated instead of giving mate,
549 // in this case return a fail-high score.
550 alpha = std::max(mated_in(ss->ply), alpha);
551 beta = std::min(mate_in(ss->ply+1), beta);
556 // Step 4. Transposition table lookup
557 // We don't want the score of a partial search to overwrite a previous full search
558 // TT value, so we use a different position key in case of an excluded move.
559 excludedMove = ss->excludedMove;
560 posKey = excludedMove ? pos.exclusion_key() : pos.key();
561 tte = TT.probe(posKey);
562 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
563 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
565 // At PV nodes we check for exact scores, while at non-PV nodes we check for
566 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
567 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
568 // we should also update RootMoveList to avoid bogus output.
571 && tte->depth() >= depth
572 && ttValue != VALUE_NONE // Only in case of TT access race
573 && ( PvNode ? tte->type() == BOUND_EXACT
574 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
575 : (tte->type() & BOUND_UPPER)))
578 ss->currentMove = ttMove; // Can be MOVE_NONE
582 && !pos.is_capture_or_promotion(ttMove)
583 && ttMove != ss->killers[0])
585 ss->killers[1] = ss->killers[0];
586 ss->killers[0] = ttMove;
591 // Step 5. Evaluate the position statically and update parent's gain statistics
593 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
597 // Never assume anything on values stored in TT
598 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
599 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
600 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
602 // Can ttValue be used as a better position evaluation?
603 if (ttValue != VALUE_NONE)
604 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
605 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
610 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
611 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
612 ss->staticEval, ss->evalMargin);
615 // Update gain for the parent non-capture move given the static position
616 // evaluation before and after the move.
617 if ( (move = (ss-1)->currentMove) != MOVE_NULL
618 && (ss-1)->staticEval != VALUE_NONE
619 && ss->staticEval != VALUE_NONE
620 && !pos.captured_piece_type()
621 && type_of(move) == NORMAL)
623 Square to = to_sq(move);
624 Gain.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
627 // Step 6. Razoring (is omitted in PV nodes)
629 && depth < 4 * ONE_PLY
631 && eval + razor_margin(depth) < beta
632 && ttMove == MOVE_NONE
633 && abs(beta) < VALUE_MATE_IN_MAX_PLY
634 && !pos.pawn_on_7th(pos.side_to_move()))
636 Value rbeta = beta - razor_margin(depth);
637 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
639 // Logically we should return (v + razor_margin(depth)), but
640 // surprisingly this did slightly weaker in tests.
644 // Step 7. Static null move pruning (is omitted in PV nodes)
645 // We're betting that the opponent doesn't have a move that will reduce
646 // the score by more than futility_margin(depth) if we do a null move.
649 && depth < 4 * ONE_PLY
651 && eval - FutilityMargins[depth][0] >= beta
652 && abs(beta) < VALUE_MATE_IN_MAX_PLY
653 && pos.non_pawn_material(pos.side_to_move()))
654 return eval - FutilityMargins[depth][0];
656 // Step 8. Null move search with verification search (is omitted in PV nodes)
662 && abs(beta) < VALUE_MATE_IN_MAX_PLY
663 && pos.non_pawn_material(pos.side_to_move()))
665 ss->currentMove = MOVE_NULL;
667 // Null move dynamic reduction based on depth
668 Depth R = 3 * ONE_PLY + depth / 4;
670 // Null move dynamic reduction based on value
671 if (eval - PawnValueMg > beta)
674 pos.do_null_move(st);
675 (ss+1)->skipNullMove = true;
676 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
677 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
678 (ss+1)->skipNullMove = false;
679 pos.undo_null_move();
681 if (nullValue >= beta)
683 // Do not return unproven mate scores
684 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
687 if (depth < 6 * ONE_PLY)
690 // Do verification search at high depths
691 ss->skipNullMove = true;
692 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
693 ss->skipNullMove = false;
700 // The null move failed low, which means that we may be faced with
701 // some kind of threat. If the previous move was reduced, check if
702 // the move that refuted the null move was somehow connected to the
703 // move which was reduced. If a connection is found, return a fail
704 // low score (which will cause the reduced move to fail high in the
705 // parent node, which will trigger a re-search with full depth).
706 threatMove = (ss+1)->currentMove;
708 if ( depth < 5 * ONE_PLY
710 && threatMove != MOVE_NONE
711 && allows(pos, (ss-1)->currentMove, threatMove))
716 // Step 9. ProbCut (is omitted in PV nodes)
717 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
718 // and a reduced search returns a value much above beta, we can (almost) safely
719 // prune the previous move.
721 && depth >= 5 * ONE_PLY
724 && excludedMove == MOVE_NONE
725 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
727 Value rbeta = beta + 200;
728 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
730 assert(rdepth >= ONE_PLY);
731 assert((ss-1)->currentMove != MOVE_NONE);
732 assert((ss-1)->currentMove != MOVE_NULL);
734 MovePicker mp(pos, ttMove, Hist, pos.captured_piece_type());
737 while ((move = mp.next_move<false>()) != MOVE_NONE)
738 if (pos.pl_move_is_legal(move, ci.pinned))
740 ss->currentMove = move;
741 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
742 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
749 // Step 10. Internal iterative deepening
750 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
751 && ttMove == MOVE_NONE
752 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
754 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
756 ss->skipNullMove = true;
757 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
758 ss->skipNullMove = false;
760 tte = TT.probe(posKey);
761 ttMove = tte ? tte->move() : MOVE_NONE;
764 split_point_start: // At split points actual search starts from here
766 MovePicker mp(pos, ttMove, depth, Hist, ss, PvNode ? -VALUE_INFINITE : beta);
768 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
769 singularExtensionNode = !RootNode
771 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
772 && ttMove != MOVE_NONE
773 && !excludedMove // Recursive singular search is not allowed
774 && (tte->type() & BOUND_LOWER)
775 && tte->depth() >= depth - 3 * ONE_PLY;
777 // Step 11. Loop through moves
778 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
779 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
783 if (move == excludedMove)
786 // At root obey the "searchmoves" option and skip moves not listed in Root
787 // Move List, as a consequence any illegal move is also skipped. In MultiPV
788 // mode we also skip PV moves which have been already searched.
789 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
794 // Shared counter cannot be decremented later if move turns out to be illegal
795 if (!pos.pl_move_is_legal(move, ci.pinned))
798 moveCount = ++splitPoint->moveCount;
799 splitPoint->mutex.unlock();
806 Signals.firstRootMove = (moveCount == 1);
808 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
809 sync_cout << "info depth " << depth / ONE_PLY
810 << " currmove " << move_to_uci(move, pos.is_chess960())
811 << " currmovenumber " << moveCount + PVIdx << sync_endl;
815 captureOrPromotion = pos.is_capture_or_promotion(move);
816 givesCheck = pos.move_gives_check(move, ci);
817 dangerous = givesCheck
818 || pos.is_passed_pawn_push(move)
819 || type_of(move) == CASTLE
820 || ( captureOrPromotion // Entering a pawn endgame?
821 && type_of(pos.piece_on(to_sq(move))) != PAWN
822 && type_of(move) == NORMAL
823 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
824 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
826 // Step 12. Extend checks and, in PV nodes, also dangerous moves
827 if (PvNode && dangerous)
830 else if (givesCheck && pos.see_sign(move) >= 0)
833 // Singular extension search. If all moves but one fail low on a search of
834 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
835 // is singular and should be extended. To verify this we do a reduced search
836 // on all the other moves but the ttMove, if result is lower than ttValue minus
837 // a margin then we extend ttMove.
838 if ( singularExtensionNode
841 && pos.pl_move_is_legal(move, ci.pinned)
842 && abs(ttValue) < VALUE_KNOWN_WIN)
844 assert(ttValue != VALUE_NONE);
846 Value rBeta = ttValue - int(depth);
847 ss->excludedMove = move;
848 ss->skipNullMove = true;
849 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
850 ss->skipNullMove = false;
851 ss->excludedMove = MOVE_NONE;
857 // Update current move (this must be done after singular extension search)
858 newDepth = depth - ONE_PLY + ext;
860 // Step 13. Futility pruning (is omitted in PV nodes)
861 if ( !captureOrPromotion
866 // Move count based pruning
868 && depth < 16 * ONE_PLY
869 && moveCount >= FutilityMoveCounts[depth]
870 && (!threatMove || !refutes(pos, move, threatMove)))
873 splitPoint->mutex.lock();
878 // Value based pruning
879 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
880 // but fixing this made program slightly weaker.
881 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
882 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
883 + Gain[pos.piece_moved(move)][to_sq(move)];
885 if (!PvNode && futilityValue < beta)
888 splitPoint->mutex.lock();
893 // Prune moves with negative SEE at low depths
894 if ( predictedDepth < 2 * ONE_PLY
895 && pos.see_sign(move) < 0)
898 splitPoint->mutex.lock();
904 // Check for legality only before to do the move
905 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
911 pvMove = PvNode && moveCount == 1;
912 ss->currentMove = move;
913 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
914 movesSearched[playedMoveCount++] = move;
916 // Step 14. Make the move
917 pos.do_move(move, st, ci, givesCheck);
919 // Step 15. Reduced depth search (LMR). If the move fails high will be
920 // re-searched at full depth.
921 if ( depth > 3 * ONE_PLY
923 && !captureOrPromotion
926 && move != ss->killers[0]
927 && move != ss->killers[1])
929 ss->reduction = reduction<PvNode>(depth, moveCount);
930 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
932 alpha = splitPoint->alpha;
934 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
936 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
937 ss->reduction = DEPTH_ZERO;
940 doFullDepthSearch = !pvMove;
942 // Step 16. Full depth search, when LMR is skipped or fails high
943 if (doFullDepthSearch)
946 alpha = splitPoint->alpha;
948 value = newDepth < ONE_PLY ?
949 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
950 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
951 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
954 // Only for PV nodes do a full PV search on the first move or after a fail
955 // high, in the latter case search only if value < beta, otherwise let the
956 // parent node to fail low with value <= alpha and to try another move.
957 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
958 value = newDepth < ONE_PLY ?
959 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
960 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
961 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
962 // Step 17. Undo move
965 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
967 // Step 18. Check for new best move
970 splitPoint->mutex.lock();
971 bestValue = splitPoint->bestValue;
972 alpha = splitPoint->alpha;
975 // Finished searching the move. If Signals.stop is true, the search
976 // was aborted because the user interrupted the search or because we
977 // ran out of time. In this case, the return value of the search cannot
978 // be trusted, and we don't update the best move and/or PV.
979 if (Signals.stop || thisThread->cutoff_occurred())
980 return value; // To avoid returning VALUE_INFINITE
984 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
986 // PV move or new best move ?
987 if (pvMove || value > alpha)
990 rm.extract_pv_from_tt(pos);
992 // We record how often the best move has been changed in each
993 // iteration. This information is used for time management: When
994 // the best move changes frequently, we allocate some more time.
999 // All other moves but the PV are set to the lowest value, this
1000 // is not a problem when sorting becuase sort is stable and move
1001 // position in the list is preserved, just the PV is pushed up.
1002 rm.score = -VALUE_INFINITE;
1005 if (value > bestValue)
1007 bestValue = SpNode ? splitPoint->bestValue = value : value;
1011 bestMove = SpNode ? splitPoint->bestMove = move : move;
1013 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1014 alpha = SpNode ? splitPoint->alpha = value : value;
1017 assert(value >= beta); // Fail high
1020 splitPoint->cutoff = true;
1027 // Step 19. Check for splitting the search
1029 && depth >= Threads.minimumSplitDepth
1030 && Threads.available_slave(thisThread)
1031 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1033 assert(bestValue < beta);
1035 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1036 depth, threatMove, moveCount, &mp, NT);
1037 if (bestValue >= beta)
1045 // Step 20. Check for mate and stalemate
1046 // All legal moves have been searched and if there are no legal moves, it
1047 // must be mate or stalemate. Note that we can have a false positive in
1048 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1049 // harmless because return value is discarded anyhow in the parent nodes.
1050 // If we are in a singular extension search then return a fail low score.
1051 // A split node has at least one move, the one tried before to be splitted.
1053 return excludedMove ? alpha
1054 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1056 // If we have pruned all the moves without searching return a fail-low score
1057 if (bestValue == -VALUE_INFINITE)
1059 assert(!playedMoveCount);
1064 if (bestValue >= beta) // Failed high
1066 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1067 bestMove, ss->staticEval, ss->evalMargin);
1069 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1071 if (bestMove != ss->killers[0])
1073 ss->killers[1] = ss->killers[0];
1074 ss->killers[0] = bestMove;
1077 // Increase history value of the cut-off move
1078 Value bonus = Value(int(depth) * int(depth));
1079 Hist.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1081 // Decrease history of all the other played non-capture moves
1082 for (int i = 0; i < playedMoveCount - 1; i++)
1084 Move m = movesSearched[i];
1085 Hist.update(pos.piece_moved(m), to_sq(m), -bonus);
1089 else // Failed low or PV search
1090 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1091 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1092 depth, bestMove, ss->staticEval, ss->evalMargin);
1094 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1100 // qsearch() is the quiescence search function, which is called by the main
1101 // search function when the remaining depth is zero (or, to be more precise,
1102 // less than ONE_PLY).
1104 template <NodeType NT, bool InCheck>
1105 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1107 const bool PvNode = (NT == PV);
1109 assert(NT == PV || NT == NonPV);
1110 assert(InCheck == !!pos.checkers());
1111 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1112 assert(PvNode || (alpha == beta - 1));
1113 assert(depth <= DEPTH_ZERO);
1118 Move ttMove, move, bestMove;
1119 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1120 bool givesCheck, enoughMaterial, evasionPrunable;
1123 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1127 ss->currentMove = bestMove = MOVE_NONE;
1128 ss->ply = (ss-1)->ply + 1;
1130 // Check for an instant draw or maximum ply reached
1131 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1132 return DrawValue[pos.side_to_move()];
1134 // Transposition table lookup. At PV nodes, we don't use the TT for
1135 // pruning, but only for move ordering.
1137 tte = TT.probe(posKey);
1138 ttMove = tte ? tte->move() : MOVE_NONE;
1139 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
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 && tte->depth() >= ttDepth
1148 && ttValue != VALUE_NONE // Only in case of TT access race
1149 && ( PvNode ? tte->type() == BOUND_EXACT
1150 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1151 : (tte->type() & BOUND_UPPER)))
1153 ss->currentMove = ttMove; // Can be MOVE_NONE
1157 // Evaluate the position statically
1160 ss->staticEval = ss->evalMargin = VALUE_NONE;
1161 bestValue = futilityBase = -VALUE_INFINITE;
1162 enoughMaterial = false;
1168 // Never assume anything on values stored in TT
1169 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1170 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1171 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1174 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1176 // Stand pat. Return immediately if static value is at least beta
1177 if (bestValue >= beta)
1180 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1181 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1186 if (PvNode && bestValue > alpha)
1189 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1190 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1193 // Initialize a MovePicker object for the current position, and prepare
1194 // to search the moves. Because the depth is <= 0 here, only captures,
1195 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1197 MovePicker mp(pos, ttMove, depth, Hist, to_sq((ss-1)->currentMove));
1200 // Loop through the moves until no moves remain or a beta cutoff occurs
1201 while ((move = mp.next_move<false>()) != MOVE_NONE)
1203 assert(is_ok(move));
1205 givesCheck = pos.move_gives_check(move, ci);
1213 && type_of(move) != PROMOTION
1214 && !pos.is_passed_pawn_push(move))
1216 futilityValue = futilityBase
1217 + PieceValue[EG][pos.piece_on(to_sq(move))]
1218 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1220 if (futilityValue < beta)
1222 bestValue = std::max(bestValue, futilityValue);
1226 // Prune moves with negative or equal SEE
1227 if ( futilityBase < beta
1228 && depth < DEPTH_ZERO
1229 && pos.see(move) <= 0)
1231 bestValue = std::max(bestValue, futilityBase);
1236 // Detect non-capture evasions that are candidate to be pruned
1237 evasionPrunable = !PvNode
1239 && bestValue > VALUE_MATED_IN_MAX_PLY
1240 && !pos.is_capture(move)
1241 && !pos.can_castle(pos.side_to_move());
1243 // Don't search moves with negative SEE values
1245 && (!InCheck || evasionPrunable)
1247 && type_of(move) != PROMOTION
1248 && pos.see_sign(move) < 0)
1251 // Don't search useless checks
1256 && !pos.is_capture_or_promotion(move)
1257 && ss->staticEval + PawnValueMg / 4 < beta
1258 && !check_is_dangerous(pos, move, futilityBase, beta))
1261 // Check for legality only before to do the move
1262 if (!pos.pl_move_is_legal(move, ci.pinned))
1265 ss->currentMove = move;
1267 // Make and search the move
1268 pos.do_move(move, st, ci, givesCheck);
1269 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1270 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1271 pos.undo_move(move);
1273 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1275 // Check for new best move
1276 if (value > bestValue)
1282 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1289 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1290 ttDepth, move, ss->staticEval, ss->evalMargin);
1298 // All legal moves have been searched. A special case: If we're in check
1299 // and no legal moves were found, it is checkmate.
1300 if (InCheck && bestValue == -VALUE_INFINITE)
1301 return mated_in(ss->ply); // Plies to mate from the root
1303 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1304 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1305 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1307 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1313 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1314 // "plies to mate from the current position". Non-mate scores are unchanged.
1315 // The function is called before storing a value to the transposition table.
1317 Value value_to_tt(Value v, int ply) {
1319 assert(v != VALUE_NONE);
1321 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1322 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1326 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1327 // from the transposition table (where refers to the plies to mate/be mated
1328 // from current position) to "plies to mate/be mated from the root".
1330 Value value_from_tt(Value v, int ply) {
1332 return v == VALUE_NONE ? VALUE_NONE
1333 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1334 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1338 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1340 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1342 Piece pc = pos.piece_moved(move);
1343 Square from = from_sq(move);
1344 Square to = to_sq(move);
1345 Color them = ~pos.side_to_move();
1346 Square ksq = pos.king_square(them);
1347 Bitboard enemies = pos.pieces(them);
1348 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1349 Bitboard occ = pos.pieces() ^ from ^ ksq;
1350 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1351 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1353 // Checks which give opponent's king at most one escape square are dangerous
1354 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1357 // Queen contact check is very dangerous
1358 if (type_of(pc) == QUEEN && (kingAtt & to))
1361 // Creating new double threats with checks is dangerous
1362 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1365 // Note that here we generate illegal "double move"!
1366 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1374 // allows() tests whether the 'first' move at previous ply somehow makes the
1375 // 'second' move possible, for instance if the moving piece is the same in
1376 // both moves. Normally the second move is the threat (the best move returned
1377 // from a null search that fails low).
1379 bool allows(const Position& pos, Move first, Move second) {
1381 assert(is_ok(first));
1382 assert(is_ok(second));
1383 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1384 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1386 Square m1from = from_sq(first);
1387 Square m2from = from_sq(second);
1388 Square m1to = to_sq(first);
1389 Square m2to = to_sq(second);
1391 // The piece is the same or second's destination was vacated by the first move
1392 if (m1to == m2from || m2to == m1from)
1395 // Second one moves through the square vacated by first one
1396 if (between_bb(m2from, m2to) & m1from)
1399 // Second's destination is defended by the first move's piece
1400 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1404 // Second move gives a discovered check through the first's checking piece
1405 if (m1att & pos.king_square(pos.side_to_move()))
1407 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1415 // refutes() tests whether a 'first' move is able to defend against a 'second'
1416 // opponent's move. In this case will not be pruned. Normally the second move
1417 // is the threat (the best move returned from a null search that fails low).
1419 bool refutes(const Position& pos, Move first, Move second) {
1421 assert(is_ok(first));
1422 assert(is_ok(second));
1424 Square m1from = from_sq(first);
1425 Square m2from = from_sq(second);
1426 Square m1to = to_sq(first);
1427 Square m2to = to_sq(second);
1429 // Don't prune moves of the threatened piece
1433 // If the threatened piece has value less than or equal to the value of the
1434 // threat piece, don't prune moves which defend it.
1435 if ( pos.is_capture(second)
1436 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1437 || type_of(pos.piece_on(m2from)) == KING))
1439 // Update occupancy as if the piece and the threat are moving
1440 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1441 Piece piece = pos.piece_on(m1from);
1443 // The moved piece attacks the square 'tto' ?
1444 if (pos.attacks_from(piece, m1to, occ) & m2to)
1447 // Scan for possible X-ray attackers behind the moved piece
1448 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1449 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1451 // Verify attackers are triggered by our move and not already existing
1452 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1456 // Don't prune safe moves which block the threat path
1457 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1464 // When playing with strength handicap choose best move among the MultiPV set
1465 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1467 Move Skill::pick_move() {
1471 // PRNG sequence should be not deterministic
1472 for (int i = Time::now() % 50; i > 0; i--)
1473 rk.rand<unsigned>();
1475 // RootMoves are already sorted by score in descending order
1476 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1477 int weakness = 120 - 2 * level;
1478 int max_s = -VALUE_INFINITE;
1481 // Choose best move. For each move score we add two terms both dependent on
1482 // weakness, one deterministic and bigger for weaker moves, and one random,
1483 // then we choose the move with the resulting highest score.
1484 for (size_t i = 0; i < PVSize; i++)
1486 int s = RootMoves[i].score;
1488 // Don't allow crazy blunders even at very low skills
1489 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1492 // This is our magic formula
1493 s += ( weakness * int(RootMoves[0].score - s)
1494 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1499 best = RootMoves[i].pv[0];
1506 // uci_pv() formats PV information according to UCI protocol. UCI requires
1507 // to send all the PV lines also if are still to be searched and so refer to
1508 // the previous search score.
1510 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1512 std::stringstream s;
1513 Time::point elaspsed = Time::now() - SearchTime + 1;
1514 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1517 for (size_t i = 0; i < Threads.size(); i++)
1518 if (Threads[i]->maxPly > selDepth)
1519 selDepth = Threads[i]->maxPly;
1521 for (size_t i = 0; i < uciPVSize; i++)
1523 bool updated = (i <= PVIdx);
1525 if (depth == 1 && !updated)
1528 int d = updated ? depth : depth - 1;
1529 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1531 if (s.rdbuf()->in_avail()) // Not at first line
1534 s << "info depth " << d
1535 << " seldepth " << selDepth
1536 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1537 << " nodes " << pos.nodes_searched()
1538 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1539 << " time " << elaspsed
1540 << " multipv " << i + 1
1543 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1544 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1553 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1554 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1555 /// allow to always have a ponder move even when we fail high at root, and a
1556 /// long PV to print that is important for position analysis.
1558 void RootMove::extract_pv_from_tt(Position& pos) {
1560 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1570 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1572 pos.do_move(pv[ply++], *st++);
1573 tte = TT.probe(pos.key());
1576 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1577 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1579 && (!pos.is_draw<false>() || ply < 2));
1581 pv.push_back(MOVE_NONE); // Must be zero-terminating
1583 while (ply) pos.undo_move(pv[--ply]);
1587 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1588 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1589 /// first, even if the old TT entries have been overwritten.
1591 void RootMove::insert_pv_in_tt(Position& pos) {
1593 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1598 tte = TT.probe(pos.key());
1600 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1601 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1603 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1605 pos.do_move(pv[ply++], *st++);
1607 } while (pv[ply] != MOVE_NONE);
1609 while (ply) pos.undo_move(pv[--ply]);
1613 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1615 void Thread::idle_loop() {
1617 // Pointer 'this_sp' is not null only if we are called from split(), and not
1618 // at the thread creation. So it means we are the split point's master.
1619 const SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1621 assert(!this_sp || (this_sp->masterThread == this && searching));
1623 // If this thread is the master of a split point and all slaves have finished
1624 // their work at this split point, return from the idle loop.
1625 while (!this_sp || this_sp->slavesMask)
1627 // If we are not searching, wait for a condition to be signaled instead of
1628 // wasting CPU time polling for work.
1629 while ((!searching && Threads.sleepWhileIdle) || exit)
1637 // Grab the lock to avoid races with Thread::notify_one()
1640 // If we are master and all slaves have finished then exit idle_loop
1641 if (this_sp && !this_sp->slavesMask)
1647 // Do sleep after retesting sleep conditions under lock protection, in
1648 // particular we need to avoid a deadlock in case a master thread has,
1649 // in the meanwhile, allocated us and sent the notify_one() call before
1650 // we had the chance to grab the lock.
1651 if (!searching && !exit)
1652 sleepCondition.wait(mutex);
1657 // If this thread has been assigned work, launch a search
1662 Threads.mutex.lock();
1665 SplitPoint* sp = activeSplitPoint;
1667 Threads.mutex.unlock();
1669 Stack ss[MAX_PLY_PLUS_2];
1670 Position pos(*sp->pos, this);
1672 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1673 (ss+1)->splitPoint = sp;
1677 assert(activePosition == NULL);
1679 activePosition = &pos;
1681 switch (sp->nodeType) {
1683 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1686 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1689 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1698 activePosition = NULL;
1699 sp->slavesMask &= ~(1ULL << idx);
1700 sp->nodes += pos.nodes_searched();
1702 // Wake up master thread so to allow it to return from the idle loop
1703 // in case we are the last slave of the split point.
1704 if ( Threads.sleepWhileIdle
1705 && this != sp->masterThread
1708 assert(!sp->masterThread->searching);
1709 sp->masterThread->notify_one();
1712 // After releasing the lock we cannot access anymore any SplitPoint
1713 // related data in a safe way becuase it could have been released under
1714 // our feet by the sp master. Also accessing other Thread objects is
1715 // unsafe because if we are exiting there is a chance are already freed.
1722 /// check_time() is called by the timer thread when the timer triggers. It is
1723 /// used to print debug info and, more important, to detect when we are out of
1724 /// available time and so stop the search.
1728 static Time::point lastInfoTime = Time::now();
1729 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1731 if (Time::now() - lastInfoTime >= 1000)
1733 lastInfoTime = Time::now();
1742 Threads.mutex.lock();
1744 nodes = RootPos.nodes_searched();
1746 // Loop across all split points and sum accumulated SplitPoint nodes plus
1747 // all the currently active positions nodes.
1748 for (size_t i = 0; i < Threads.size(); i++)
1749 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1751 SplitPoint& sp = Threads[i]->splitPoints[j];
1756 Bitboard sm = sp.slavesMask;
1759 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1761 nodes += pos->nodes_searched();
1767 Threads.mutex.unlock();
1770 Time::point elapsed = Time::now() - SearchTime;
1771 bool stillAtFirstMove = Signals.firstRootMove
1772 && !Signals.failedLowAtRoot
1773 && elapsed > TimeMgr.available_time();
1775 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1776 || stillAtFirstMove;
1778 if ( (Limits.use_time_management() && noMoreTime)
1779 || (Limits.movetime && elapsed >= Limits.movetime)
1780 || (Limits.nodes && nodes >= Limits.nodes))
1781 Signals.stop = true;