2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2012 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/>.
37 #include "ucioption.h"
41 volatile SignalsType Signals;
43 std::vector<RootMove> RootMoves;
46 Time::point SearchTime;
47 StateStackPtr SetupStates;
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];
92 template <NodeType NT>
93 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
95 template <NodeType NT, bool InCheck>
96 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
98 void id_loop(Position& pos);
99 Value value_to_tt(Value v, int ply);
100 Value value_from_tt(Value v, int ply);
101 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta);
102 bool allows_move(const Position& pos, Move first, Move second);
103 bool prevents_move(const Position& pos, Move first, Move second);
104 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
107 Skill(int l) : level(l), best(MOVE_NONE) {}
109 if (enabled()) // Swap best PV line with the sub-optimal one
110 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
111 RootMoves.end(), best ? best : pick_move()));
114 bool enabled() const { return level < 20; }
115 bool time_to_pick(int depth) const { return depth == 1 + level; }
125 /// Search::init() is called during startup to initialize various lookup tables
127 void Search::init() {
129 int d; // depth (ONE_PLY == 2)
130 int hd; // half depth (ONE_PLY == 1)
133 // Init reductions array
134 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
136 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
137 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
138 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
139 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
142 // Init futility margins array
143 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
144 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
146 // Init futility move count array
147 for (d = 0; d < 32; d++)
148 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(double(d), 2.0));
152 /// Search::perft() is our utility to verify move generation. All the leaf nodes
153 /// up to the given depth are generated and counted and the sum returned.
155 size_t Search::perft(Position& pos, Depth depth) {
157 // At the last ply just return the number of legal moves (leaf nodes)
158 if (depth == ONE_PLY)
159 return MoveList<LEGAL>(pos).size();
165 for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
167 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
168 cnt += perft(pos, depth - ONE_PLY);
169 pos.undo_move(ml.move());
176 /// Search::think() is the external interface to Stockfish's search, and is
177 /// called by the main thread when the program receives the UCI 'go' command. It
178 /// searches from RootPos and at the end prints the "bestmove" to output.
180 void Search::think() {
182 static PolyglotBook book; // Defined static to initialize the PRNG only once
184 RootColor = RootPos.side_to_move();
185 TimeMgr.init(Limits, RootPos.startpos_ply_counter(), RootColor);
187 if (RootMoves.empty())
189 RootMoves.push_back(MOVE_NONE);
190 sync_cout << "info depth 0 score "
191 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
197 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
199 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
201 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
203 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
208 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
210 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
211 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
212 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
213 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
216 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
218 if (Options["Use Search Log"])
220 Log log(Options["Search Log Filename"]);
221 log << "\nSearching: " << RootPos.fen()
222 << "\ninfinite: " << Limits.infinite
223 << " ponder: " << Limits.ponder
224 << " time: " << Limits.time[RootColor]
225 << " increment: " << Limits.inc[RootColor]
226 << " moves to go: " << Limits.movestogo
230 // Reset and wake up the threads
231 for (size_t i = 0; i < Threads.size(); i++)
233 Threads[i].maxPly = 0;
234 Threads[i].do_sleep = false;
236 if (!Threads.use_sleeping_threads())
237 Threads[i].notify_one();
240 // Set best timer interval to avoid lagging under time pressure. Timer is
241 // used to check for remaining available thinking time.
242 if (Limits.use_time_management())
243 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16,
245 else if (Limits.nodes)
246 Threads.set_timer(2 * TimerResolution);
248 Threads.set_timer(100);
250 id_loop(RootPos); // Let's start searching !
252 Threads.set_timer(0); // Stop timer
254 // Main thread will go to sleep by itself to avoid a race with start_searching()
255 for (size_t i = 0; i < Threads.size(); i++)
256 if (&Threads[i] != Threads.main_thread())
257 Threads[i].do_sleep = true;
259 if (Options["Use Search Log"])
261 Time::point elapsed = Time::now() - SearchTime + 1;
263 Log log(Options["Search Log Filename"]);
264 log << "Nodes: " << RootPos.nodes_searched()
265 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
266 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
269 RootPos.do_move(RootMoves[0].pv[0], st);
270 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
271 RootPos.undo_move(RootMoves[0].pv[0]);
276 // When we reach max depth we arrive here even without Signals.stop is raised,
277 // but if we are pondering or in infinite search, according to UCI protocol,
278 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
279 // command. We simply wait here until GUI sends one of those commands (that
280 // raise Signals.stop).
281 if (!Signals.stop && (Limits.ponder || Limits.infinite))
283 Signals.stopOnPonderhit = true;
284 RootPos.this_thread()->wait_for(Signals.stop);
287 // Best move could be MOVE_NONE when searching on a stalemate position
288 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
289 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
296 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
297 // with increasing depth until the allocated thinking time has been consumed,
298 // user stops the search, or the maximum search depth is reached.
300 void id_loop(Position& pos) {
302 Stack ss[MAX_PLY_PLUS_2];
303 int depth, prevBestMoveChanges;
304 Value bestValue, alpha, beta, delta;
305 bool bestMoveNeverChanged = true;
307 memset(ss, 0, 4 * sizeof(Stack));
308 depth = BestMoveChanges = 0;
309 bestValue = delta = -VALUE_INFINITE;
310 ss->currentMove = MOVE_NULL; // Hack to skip update gains
314 PVSize = Options["MultiPV"];
315 Skill skill(Options["Skill Level"]);
317 // Do we have to play with skill handicap? In this case enable MultiPV search
318 // that we will use behind the scenes to retrieve a set of possible moves.
319 if (skill.enabled() && PVSize < 4)
322 PVSize = std::min(PVSize, RootMoves.size());
324 // Iterative deepening loop until requested to stop or target depth reached
325 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
327 // Save last iteration's scores before first PV line is searched and all
328 // the move scores but the (new) PV are set to -VALUE_INFINITE.
329 for (size_t i = 0; i < RootMoves.size(); i++)
330 RootMoves[i].prevScore = RootMoves[i].score;
332 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
335 // MultiPV loop. We perform a full root search for each PV line
336 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
338 // Set aspiration window default width
339 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
342 alpha = RootMoves[PVIdx].prevScore - delta;
343 beta = RootMoves[PVIdx].prevScore + delta;
347 alpha = -VALUE_INFINITE;
348 beta = VALUE_INFINITE;
351 // Start with a small aspiration window and, in case of fail high/low,
352 // research with bigger window until not failing high/low anymore.
355 // Search starts from ss+1 to allow referencing (ss-1). This is
356 // needed by update gains and ss copy when splitting at Root.
357 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
359 // Bring to front the best move. It is critical that sorting is
360 // done with a stable algorithm because all the values but the first
361 // and eventually the new best one are set to -VALUE_INFINITE and
362 // we want to keep the same order for all the moves but the new
363 // PV that goes to the front. Note that in case of MultiPV search
364 // the already searched PV lines are preserved.
365 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
367 // Write PV back to transposition table in case the relevant
368 // entries have been overwritten during the search.
369 for (size_t i = 0; i <= PVIdx; i++)
370 RootMoves[i].insert_pv_in_tt(pos);
372 // If search has been stopped return immediately. Sorting and
373 // writing PV back to TT is safe becuase RootMoves is still
374 // valid, although refers to previous iteration.
378 // In case of failing high/low increase aspiration window and
379 // research, otherwise exit the loop.
380 if (bestValue > alpha && bestValue < beta)
383 // Give some update (without cluttering the UI) before to research
384 if (Time::now() - SearchTime > 3000)
385 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
387 if (abs(bestValue) >= VALUE_KNOWN_WIN)
389 alpha = -VALUE_INFINITE;
390 beta = VALUE_INFINITE;
392 else if (bestValue >= beta)
399 Signals.failedLowAtRoot = true;
400 Signals.stopOnPonderhit = false;
406 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
409 // Sort the PV lines searched so far and update the GUI
410 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
411 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
412 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
415 // Do we need to pick now the sub-optimal best move ?
416 if (skill.enabled() && skill.time_to_pick(depth))
419 if (Options["Use Search Log"])
421 Log log(Options["Search Log Filename"]);
422 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
426 // Filter out startup noise when monitoring best move stability
427 if (depth > 2 && BestMoveChanges)
428 bestMoveNeverChanged = false;
430 // Do we have found a "mate in x"?
432 && bestValue >= VALUE_MATE_IN_MAX_PLY
433 && VALUE_MATE - bestValue <= 2 * Limits.mate)
436 // Do we have time for the next iteration? Can we stop searching now?
437 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
439 bool stop = false; // Local variable, not the volatile Signals.stop
441 // Take in account some extra time if the best move has changed
442 if (depth > 4 && depth < 50 && PVSize == 1)
443 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
445 // Stop search if most of available time is already consumed. We
446 // probably don't have enough time to search the first move at the
447 // next iteration anyway.
448 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
451 // Stop search early if one move seems to be much better than others
455 && ( (bestMoveNeverChanged && pos.captured_piece_type())
456 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
458 Value rBeta = bestValue - 2 * PawnValueMg;
459 (ss+1)->excludedMove = RootMoves[0].pv[0];
460 (ss+1)->skipNullMove = true;
461 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
462 (ss+1)->skipNullMove = false;
463 (ss+1)->excludedMove = MOVE_NONE;
471 // If we are allowed to ponder do not stop the search now but
472 // keep pondering until GUI sends "ponderhit" or "stop".
474 Signals.stopOnPonderhit = true;
483 // search<>() is the main search function for both PV and non-PV nodes and for
484 // normal and SplitPoint nodes. When called just after a split point the search
485 // is simpler because we have already probed the hash table, done a null move
486 // search, and searched the first move before splitting, we don't have to repeat
487 // all this work again. We also don't need to store anything to the hash table
488 // here: This is taken care of after we return from the split point.
490 template <NodeType NT>
491 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
493 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
494 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
495 const bool RootNode = (NT == Root || NT == SplitPointRoot);
497 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
498 assert(PvNode || (alpha == beta - 1));
499 assert(depth > DEPTH_ZERO);
501 Move movesSearched[64];
506 Move ttMove, move, excludedMove, bestMove, threatMove;
508 Value bestValue, value, ttValue;
509 Value eval, nullValue, futilityValue;
510 bool inCheck, givesCheck, pvMove, singularExtensionNode;
511 bool captureOrPromotion, dangerous, doFullDepthSearch, threatExtension;
512 int moveCount, playedMoveCount;
514 // Step 1. Initialize node
515 Thread* thisThread = pos.this_thread();
516 moveCount = playedMoveCount = 0;
517 threatExtension = false;
518 inCheck = pos.checkers();
523 bestMove = sp->bestMove;
524 threatMove = sp->threatMove;
525 bestValue = sp->bestValue;
527 ttMove = excludedMove = MOVE_NONE;
528 ttValue = VALUE_NONE;
530 assert(sp->bestValue > -VALUE_INFINITE && sp->moveCount > 0);
532 goto split_point_start;
535 bestValue = -VALUE_INFINITE;
536 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
537 ss->ply = (ss-1)->ply + 1;
538 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
539 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
541 // Used to send selDepth info to GUI
542 if (PvNode && thisThread->maxPly < ss->ply)
543 thisThread->maxPly = ss->ply;
547 // Step 2. Check for aborted search and immediate draw
548 if (Signals.stop || pos.is_draw<true, PvNode>() || ss->ply > MAX_PLY)
549 return DrawValue[pos.side_to_move()];
551 // Step 3. Mate distance pruning. Even if we mate at the next move our score
552 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
553 // a shorter mate was found upward in the tree then there is no need to search
554 // further, we will never beat current alpha. Same logic but with reversed signs
555 // applies also in the opposite condition of being mated instead of giving mate,
556 // in this case return a fail-high score.
557 alpha = std::max(mated_in(ss->ply), alpha);
558 beta = std::min(mate_in(ss->ply+1), beta);
563 // Step 4. Transposition table lookup
564 // We don't want the score of a partial search to overwrite a previous full search
565 // TT value, so we use a different position key in case of an excluded move.
566 excludedMove = ss->excludedMove;
567 posKey = excludedMove ? pos.exclusion_key() : pos.key();
568 tte = TT.probe(posKey);
569 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
570 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
572 // At PV nodes we check for exact scores, while at non-PV nodes we check for
573 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
574 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
575 // we should also update RootMoveList to avoid bogus output.
578 && tte->depth() >= depth
579 && ttValue != VALUE_NONE // Only in case of TT access race
580 && ( PvNode ? tte->type() == BOUND_EXACT
581 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
582 : (tte->type() & BOUND_UPPER)))
585 ss->currentMove = ttMove; // Can be MOVE_NONE
589 && !pos.is_capture_or_promotion(ttMove)
590 && ttMove != ss->killers[0])
592 ss->killers[1] = ss->killers[0];
593 ss->killers[0] = ttMove;
598 // Step 5. Evaluate the position statically and update parent's gain statistics
600 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
604 // Never assume anything on values stored in TT
605 if ( (ss->staticEval = eval = tte->static_value()) == VALUE_NONE
606 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
607 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
609 // Can ttValue be used as a better position evaluation?
610 if (ttValue != VALUE_NONE)
611 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
612 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
617 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
618 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
619 ss->staticEval, ss->evalMargin);
622 // Update gain for the parent non-capture move given the static position
623 // evaluation before and after the move.
624 if ( (move = (ss-1)->currentMove) != MOVE_NULL
625 && (ss-1)->staticEval != VALUE_NONE
626 && ss->staticEval != VALUE_NONE
627 && !pos.captured_piece_type()
628 && type_of(move) == NORMAL)
630 Square to = to_sq(move);
631 H.update_gain(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
634 // Step 6. Razoring (is omitted in PV nodes)
636 && depth < 4 * ONE_PLY
638 && eval + razor_margin(depth) < beta
639 && ttMove == MOVE_NONE
640 && abs(beta) < VALUE_MATE_IN_MAX_PLY
641 && !pos.pawn_on_7th(pos.side_to_move()))
643 Value rbeta = beta - razor_margin(depth);
644 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
646 // Logically we should return (v + razor_margin(depth)), but
647 // surprisingly this did slightly weaker in tests.
651 // Step 7. Static null move pruning (is omitted in PV nodes)
652 // We're betting that the opponent doesn't have a move that will reduce
653 // the score by more than futility_margin(depth) if we do a null move.
656 && depth < 4 * ONE_PLY
658 && eval - FutilityMargins[depth][0] >= beta
659 && abs(beta) < VALUE_MATE_IN_MAX_PLY
660 && pos.non_pawn_material(pos.side_to_move()))
661 return eval - FutilityMargins[depth][0];
663 // Step 8. Null move search with verification search (is omitted in PV nodes)
669 && abs(beta) < VALUE_MATE_IN_MAX_PLY
670 && pos.non_pawn_material(pos.side_to_move()))
672 ss->currentMove = MOVE_NULL;
674 // Null move dynamic reduction based on depth
675 Depth R = 3 * ONE_PLY + depth / 4;
677 // Null move dynamic reduction based on value
678 if (eval - PawnValueMg > beta)
681 pos.do_null_move<true>(st);
682 (ss+1)->skipNullMove = true;
683 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
684 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
685 (ss+1)->skipNullMove = false;
686 pos.do_null_move<false>(st);
688 if (nullValue >= beta)
690 // Do not return unproven mate scores
691 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
694 if (depth < 6 * ONE_PLY)
697 // Do verification search at high depths
698 ss->skipNullMove = true;
699 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
700 ss->skipNullMove = false;
707 // The null move failed low, which means that we may be faced with
708 // some kind of threat. If the previous move was reduced, check if
709 // the move that refuted the null move was somehow connected to the
710 // move which was reduced. If a connection is found extend moves that
711 // defend against threat.
712 threatMove = (ss+1)->currentMove;
714 if ( depth < 5 * ONE_PLY
716 && threatMove != MOVE_NONE
717 && allows_move(pos, (ss-1)->currentMove, threatMove))
718 threatExtension = true;
722 // Step 9. ProbCut (is omitted in PV nodes)
723 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
724 // and a reduced search returns a value much above beta, we can (almost) safely
725 // prune the previous move.
727 && depth >= 5 * ONE_PLY
730 && excludedMove == MOVE_NONE
731 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
733 Value rbeta = beta + 200;
734 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
736 assert(rdepth >= ONE_PLY);
737 assert((ss-1)->currentMove != MOVE_NONE);
738 assert((ss-1)->currentMove != MOVE_NULL);
740 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
743 while ((move = mp.next_move<false>()) != MOVE_NONE)
744 if (pos.pl_move_is_legal(move, ci.pinned))
746 ss->currentMove = move;
747 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
748 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
755 // Step 10. Internal iterative deepening
756 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
757 && ttMove == MOVE_NONE
758 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
760 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
762 ss->skipNullMove = true;
763 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
764 ss->skipNullMove = false;
766 tte = TT.probe(posKey);
767 ttMove = tte ? tte->move() : MOVE_NONE;
770 split_point_start: // At split points actual search starts from here
772 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
774 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
775 singularExtensionNode = !RootNode
777 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
778 && ttMove != MOVE_NONE
779 && !excludedMove // Recursive singular search is not allowed
780 && (tte->type() & BOUND_LOWER)
781 && tte->depth() >= depth - 3 * ONE_PLY;
783 // Step 11. Loop through moves
784 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
785 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
789 if (move == excludedMove)
792 // At root obey the "searchmoves" option and skip moves not listed in Root
793 // Move List, as a consequence any illegal move is also skipped. In MultiPV
794 // mode we also skip PV moves which have been already searched.
795 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
800 // Shared counter cannot be decremented later if move turns out to be illegal
801 if (!pos.pl_move_is_legal(move, ci.pinned))
804 moveCount = ++sp->moveCount;
812 Signals.firstRootMove = (moveCount == 1);
814 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
815 sync_cout << "info depth " << depth / ONE_PLY
816 << " currmove " << move_to_uci(move, pos.is_chess960())
817 << " currmovenumber " << moveCount + PVIdx << sync_endl;
821 captureOrPromotion = pos.is_capture_or_promotion(move);
822 givesCheck = pos.move_gives_check(move, ci);
823 dangerous = givesCheck
824 || pos.is_passed_pawn_push(move)
825 || type_of(move) == CASTLE
826 || ( captureOrPromotion // Entering a pawn endgame?
827 && type_of(pos.piece_on(to_sq(move))) != PAWN
828 && type_of(move) == NORMAL
829 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
830 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
832 // Step 12. Extend checks and, in PV nodes, also dangerous moves
833 if (PvNode && dangerous)
836 else if (threatExtension && prevents_move(pos, move, threatMove))
839 else if (givesCheck && pos.see_sign(move) >= 0)
842 // Singular extension search. If all moves but one fail low on a search of
843 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
844 // is singular and should be extended. To verify this we do a reduced search
845 // on all the other moves but the ttMove, if result is lower than ttValue minus
846 // a margin then we extend ttMove.
847 if ( singularExtensionNode
850 && pos.pl_move_is_legal(move, ci.pinned)
851 && abs(ttValue) < VALUE_KNOWN_WIN)
853 assert(ttValue != VALUE_NONE);
855 Value rBeta = ttValue - int(depth);
856 ss->excludedMove = move;
857 ss->skipNullMove = true;
858 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
859 ss->skipNullMove = false;
860 ss->excludedMove = MOVE_NONE;
863 ext = rBeta >= beta ? ONE_PLY + ONE_PLY / 2 : ONE_PLY;
866 // Update current move (this must be done after singular extension search)
867 newDepth = depth - ONE_PLY + ext;
869 // Step 13. Futility pruning (is omitted in PV nodes)
871 && !captureOrPromotion
875 && (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
876 && alpha > VALUE_MATED_IN_MAX_PLY)))
878 // Move count based pruning
879 if ( depth < 16 * ONE_PLY
880 && moveCount >= FutilityMoveCounts[depth]
881 && (!threatMove || !prevents_move(pos, move, threatMove)))
889 // Value based pruning
890 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
891 // but fixing this made program slightly weaker.
892 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
893 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
894 + H.gain(pos.piece_moved(move), to_sq(move));
896 if (futilityValue < beta)
904 // Prune moves with negative SEE at low depths
905 if ( predictedDepth < 2 * ONE_PLY
906 && pos.see_sign(move) < 0)
915 // Check for legality only before to do the move
916 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
922 pvMove = PvNode && moveCount == 1;
923 ss->currentMove = move;
924 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
925 movesSearched[playedMoveCount++] = move;
927 // Step 14. Make the move
928 pos.do_move(move, st, ci, givesCheck);
930 // Step 15. Reduced depth search (LMR). If the move fails high will be
931 // re-searched at full depth.
932 if ( depth > 3 * ONE_PLY
934 && !captureOrPromotion
936 && ss->killers[0] != move
937 && ss->killers[1] != move)
939 ss->reduction = reduction<PvNode>(depth, moveCount);
940 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
941 alpha = SpNode ? sp->alpha : alpha;
943 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
945 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
946 ss->reduction = DEPTH_ZERO;
949 doFullDepthSearch = !pvMove;
951 // Step 16. Full depth search, when LMR is skipped or fails high
952 if (doFullDepthSearch)
954 alpha = SpNode ? sp->alpha : alpha;
955 value = newDepth < ONE_PLY ?
956 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
957 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
958 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
961 // Only for PV nodes do a full PV search on the first move or after a fail
962 // high, in the latter case search only if value < beta, otherwise let the
963 // parent node to fail low with value <= alpha and to try another move.
964 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
965 value = newDepth < ONE_PLY ?
966 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
967 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
968 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
969 // Step 17. Undo move
972 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
974 // Step 18. Check for new best move
978 bestValue = sp->bestValue;
982 // Finished searching the move. If Signals.stop is true, the search
983 // was aborted because the user interrupted the search or because we
984 // ran out of time. In this case, the return value of the search cannot
985 // be trusted, and we don't update the best move and/or PV.
986 if (Signals.stop || thisThread->cutoff_occurred())
987 return value; // To avoid returning VALUE_INFINITE
991 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
993 // PV move or new best move ?
994 if (pvMove || value > alpha)
997 rm.extract_pv_from_tt(pos);
999 // We record how often the best move has been changed in each
1000 // iteration. This information is used for time management: When
1001 // the best move changes frequently, we allocate some more time.
1006 // All other moves but the PV are set to the lowest value, this
1007 // is not a problem when sorting becuase sort is stable and move
1008 // position in the list is preserved, just the PV is pushed up.
1009 rm.score = -VALUE_INFINITE;
1012 if (value > bestValue)
1014 bestValue = SpNode ? sp->bestValue = value : value;
1018 bestMove = SpNode ? sp->bestMove = move : move;
1020 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1021 alpha = SpNode ? sp->alpha = value : value;
1024 assert(value >= beta); // Fail high
1034 // Step 19. Check for splitting the search
1036 && depth >= Threads.min_split_depth()
1037 && Threads.available_slave_exists(thisThread))
1039 assert(bestValue < beta);
1041 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1042 depth, threatMove, moveCount, mp, NT);
1043 if (bestValue >= beta)
1051 // Step 20. Check for mate and stalemate
1052 // All legal moves have been searched and if there are no legal moves, it
1053 // must be mate or stalemate. Note that we can have a false positive in
1054 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1055 // harmless because return value is discarded anyhow in the parent nodes.
1056 // If we are in a singular extension search then return a fail low score.
1057 // A split node has at least one move, the one tried before to be splitted.
1059 return excludedMove ? alpha
1060 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1062 // If we have pruned all the moves without searching return a fail-low score
1063 if (bestValue == -VALUE_INFINITE)
1065 assert(!playedMoveCount);
1070 if (bestValue >= beta) // Failed high
1072 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1073 bestMove, ss->staticEval, ss->evalMargin);
1075 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1077 if (bestMove != ss->killers[0])
1079 ss->killers[1] = ss->killers[0];
1080 ss->killers[0] = bestMove;
1083 // Increase history value of the cut-off move
1084 Value bonus = Value(int(depth) * int(depth));
1085 H.add(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1087 // Decrease history of all the other played non-capture moves
1088 for (int i = 0; i < playedMoveCount - 1; i++)
1090 Move m = movesSearched[i];
1091 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1095 else // Failed low or PV search
1096 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1097 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1098 depth, bestMove, ss->staticEval, ss->evalMargin);
1100 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1106 // qsearch() is the quiescence search function, which is called by the main
1107 // search function when the remaining depth is zero (or, to be more precise,
1108 // less than ONE_PLY).
1110 template <NodeType NT, bool InCheck>
1111 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1113 const bool PvNode = (NT == PV);
1115 assert(NT == PV || NT == NonPV);
1116 assert(InCheck == !!pos.checkers());
1117 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1118 assert(PvNode || (alpha == beta - 1));
1119 assert(depth <= DEPTH_ZERO);
1124 Move ttMove, move, bestMove;
1125 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1126 bool givesCheck, enoughMaterial, evasionPrunable;
1129 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1133 ss->currentMove = bestMove = MOVE_NONE;
1134 ss->ply = (ss-1)->ply + 1;
1136 // Check for an instant draw or maximum ply reached
1137 if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)
1138 return DrawValue[pos.side_to_move()];
1140 // Transposition table lookup. At PV nodes, we don't use the TT for
1141 // pruning, but only for move ordering.
1143 tte = TT.probe(posKey);
1144 ttMove = tte ? tte->move() : MOVE_NONE;
1145 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
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 && tte->depth() >= ttDepth
1154 && ttValue != VALUE_NONE // Only in case of TT access race
1155 && ( PvNode ? tte->type() == BOUND_EXACT
1156 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1157 : (tte->type() & BOUND_UPPER)))
1159 ss->currentMove = ttMove; // Can be MOVE_NONE
1163 // Evaluate the position statically
1166 ss->staticEval = ss->evalMargin = VALUE_NONE;
1167 bestValue = futilityBase = -VALUE_INFINITE;
1168 enoughMaterial = false;
1174 // Never assume anything on values stored in TT
1175 if ( (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE
1176 ||(ss->evalMargin = tte->static_value_margin()) == VALUE_NONE)
1177 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1180 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1182 // Stand pat. Return immediately if static value is at least beta
1183 if (bestValue >= beta)
1186 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1187 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1192 if (PvNode && bestValue > alpha)
1195 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1196 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1199 // Initialize a MovePicker object for the current position, and prepare
1200 // to search the moves. Because the depth is <= 0 here, only captures,
1201 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1203 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1206 // Loop through the moves until no moves remain or a beta cutoff occurs
1207 while ((move = mp.next_move<false>()) != MOVE_NONE)
1209 assert(is_ok(move));
1211 givesCheck = pos.move_gives_check(move, ci);
1219 && type_of(move) != PROMOTION
1220 && !pos.is_passed_pawn_push(move))
1222 futilityValue = futilityBase
1223 + PieceValue[EG][pos.piece_on(to_sq(move))]
1224 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1226 if (futilityValue < beta)
1228 bestValue = std::max(bestValue, futilityValue);
1232 // Prune moves with negative or equal SEE
1233 if ( futilityBase < beta
1234 && depth < DEPTH_ZERO
1235 && pos.see(move) <= 0)
1237 bestValue = std::max(bestValue, futilityBase);
1242 // Detect non-capture evasions that are candidate to be pruned
1243 evasionPrunable = !PvNode
1245 && bestValue > VALUE_MATED_IN_MAX_PLY
1246 && !pos.is_capture(move)
1247 && !pos.can_castle(pos.side_to_move());
1249 // Don't search moves with negative SEE values
1251 && (!InCheck || evasionPrunable)
1253 && type_of(move) != PROMOTION
1254 && pos.see_sign(move) < 0)
1257 // Don't search useless checks
1262 && !pos.is_capture_or_promotion(move)
1263 && ss->staticEval + PawnValueMg / 4 < beta
1264 && !check_is_dangerous(pos, move, futilityBase, beta))
1267 // Check for legality only before to do the move
1268 if (!pos.pl_move_is_legal(move, ci.pinned))
1271 ss->currentMove = move;
1273 // Make and search the move
1274 pos.do_move(move, st, ci, givesCheck);
1275 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1276 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1277 pos.undo_move(move);
1279 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1281 // Check for new best move
1282 if (value > bestValue)
1288 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1295 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1296 ttDepth, move, ss->staticEval, ss->evalMargin);
1304 // All legal moves have been searched. A special case: If we're in check
1305 // and no legal moves were found, it is checkmate.
1306 if (InCheck && bestValue == -VALUE_INFINITE)
1307 return mated_in(ss->ply); // Plies to mate from the root
1309 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1310 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1311 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1313 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1319 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1320 // "plies to mate from the current position". Non-mate scores are unchanged.
1321 // The function is called before storing a value to the transposition table.
1323 Value value_to_tt(Value v, int ply) {
1325 assert(v != VALUE_NONE);
1327 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1328 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1332 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1333 // from the transposition table (where refers to the plies to mate/be mated
1334 // from current position) to "plies to mate/be mated from the root".
1336 Value value_from_tt(Value v, int ply) {
1338 return v == VALUE_NONE ? VALUE_NONE
1339 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1340 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1344 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1346 bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta)
1348 Piece pc = pos.piece_moved(move);
1349 Square from = from_sq(move);
1350 Square to = to_sq(move);
1351 Color them = ~pos.side_to_move();
1352 Square ksq = pos.king_square(them);
1353 Bitboard enemies = pos.pieces(them);
1354 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1355 Bitboard occ = pos.pieces() ^ from ^ ksq;
1356 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1357 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1359 // Checks which give opponent's king at most one escape square are dangerous
1360 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1363 // Queen contact check is very dangerous
1364 if (type_of(pc) == QUEEN && (kingAtt & to))
1367 // Creating new double threats with checks is dangerous
1368 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1371 // Note that here we generate illegal "double move"!
1372 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1380 // allows_move() tests whether the move at previous ply (first) somehow makes a
1381 // second move possible, for instance if the moving piece is the same in both
1382 // moves. Normally the second move is the threat move (the best move returned
1383 // from a null search that fails low).
1385 bool allows_move(const Position& pos, Move first, Move second) {
1387 assert(is_ok(first));
1388 assert(is_ok(second));
1389 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1390 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1392 Square m1from = from_sq(first);
1393 Square m2from = from_sq(second);
1394 Square m1to = to_sq(first);
1395 Square m2to = to_sq(second);
1397 // The piece is the same or second's destination was vacated by the first move
1398 if (m1to == m2from || m2to == m1from)
1401 // Second one moves through the square vacated by first one
1402 if (between_bb(m2from, m2to) & m1from)
1405 // Second's destination is defended by the first move's piece
1406 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1410 // Second move gives a discovered check through the first's checking piece
1411 if (m1att & pos.king_square(pos.side_to_move()))
1413 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1421 // prevents_move() tests whether a move (first) is able to defend against an
1422 // opponent's move (second). In this case will not be pruned. Normally the
1423 // second move is the threat move (the best move returned from a null search
1426 bool prevents_move(const Position& pos, Move first, Move second) {
1428 assert(is_ok(first));
1429 assert(is_ok(second));
1431 Square m1from = from_sq(first);
1432 Square m2from = from_sq(second);
1433 Square m1to = to_sq(first);
1434 Square m2to = to_sq(second);
1436 // Don't prune moves of the threatened piece
1440 // If the threatened piece has value less than or equal to the value of the
1441 // threat piece, don't prune moves which defend it.
1442 if ( pos.is_capture(second)
1443 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1444 || type_of(pos.piece_on(m2from)) == KING))
1446 // Update occupancy as if the piece and the threat are moving
1447 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1448 Piece piece = pos.piece_on(m1from);
1450 // The moved piece attacks the square 'tto' ?
1451 if (pos.attacks_from(piece, m1to, occ) & m2to)
1454 // Scan for possible X-ray attackers behind the moved piece
1455 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1456 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1458 // Verify attackers are triggered by our move and not already existing
1459 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1463 // Don't prune safe moves which block the threat path
1464 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1471 // When playing with strength handicap choose best move among the MultiPV set
1472 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1474 Move Skill::pick_move() {
1478 // PRNG sequence should be not deterministic
1479 for (int i = Time::now() % 50; i > 0; i--)
1480 rk.rand<unsigned>();
1482 // RootMoves are already sorted by score in descending order
1483 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1484 int weakness = 120 - 2 * level;
1485 int max_s = -VALUE_INFINITE;
1488 // Choose best move. For each move score we add two terms both dependent on
1489 // weakness, one deterministic and bigger for weaker moves, and one random,
1490 // then we choose the move with the resulting highest score.
1491 for (size_t i = 0; i < PVSize; i++)
1493 int s = RootMoves[i].score;
1495 // Don't allow crazy blunders even at very low skills
1496 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1499 // This is our magic formula
1500 s += ( weakness * int(RootMoves[0].score - s)
1501 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1506 best = RootMoves[i].pv[0];
1513 // uci_pv() formats PV information according to UCI protocol. UCI requires
1514 // to send all the PV lines also if are still to be searched and so refer to
1515 // the previous search score.
1517 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1519 std::stringstream s;
1520 Time::point elaspsed = Time::now() - SearchTime + 1;
1521 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1524 for (size_t i = 0; i < Threads.size(); i++)
1525 if (Threads[i].maxPly > selDepth)
1526 selDepth = Threads[i].maxPly;
1528 for (size_t i = 0; i < uciPVSize; i++)
1530 bool updated = (i <= PVIdx);
1532 if (depth == 1 && !updated)
1535 int d = updated ? depth : depth - 1;
1536 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1538 if (s.rdbuf()->in_avail()) // Not at first line
1541 s << "info depth " << d
1542 << " seldepth " << selDepth
1543 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1544 << " nodes " << pos.nodes_searched()
1545 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1546 << " time " << elaspsed
1547 << " multipv " << i + 1
1550 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1551 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1560 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1561 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1562 /// allow to always have a ponder move even when we fail high at root, and a
1563 /// long PV to print that is important for position analysis.
1565 void RootMove::extract_pv_from_tt(Position& pos) {
1567 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1577 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1579 pos.do_move(pv[ply++], *st++);
1580 tte = TT.probe(pos.key());
1583 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1584 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1586 && (!pos.is_draw<true, true>() || ply < 2));
1588 pv.push_back(MOVE_NONE); // Must be zero-terminating
1590 while (ply) pos.undo_move(pv[--ply]);
1594 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1595 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1596 /// first, even if the old TT entries have been overwritten.
1598 void RootMove::insert_pv_in_tt(Position& pos) {
1600 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1605 tte = TT.probe(pos.key());
1607 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1608 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1610 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1612 pos.do_move(pv[ply++], *st++);
1614 } while (pv[ply] != MOVE_NONE);
1616 while (ply) pos.undo_move(pv[--ply]);
1620 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1622 void Thread::idle_loop() {
1624 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1625 // object for which the thread is the master.
1626 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1628 assert(!sp_master || (sp_master->master == this && is_searching));
1630 // If this thread is the master of a split point and all slaves have
1631 // finished their work at this split point, return from the idle loop.
1632 while (!sp_master || sp_master->slavesMask)
1634 // If we are not searching, wait for a condition to be signaled
1635 // instead of wasting CPU time polling for work.
1638 || (!is_searching && Threads.use_sleeping_threads()))
1646 // Grab the lock to avoid races with Thread::wake_up()
1649 // If we are master and all slaves have finished don't go to sleep
1650 if (sp_master && !sp_master->slavesMask)
1656 // Do sleep after retesting sleep conditions under lock protection, in
1657 // particular we need to avoid a deadlock in case a master thread has,
1658 // in the meanwhile, allocated us and sent the wake_up() call before we
1659 // had the chance to grab the lock.
1660 if (do_sleep || !is_searching)
1661 sleepCondition.wait(mutex);
1666 // If this thread has been assigned work, launch a search
1669 assert(!do_sleep && !do_exit);
1671 Threads.mutex.lock();
1673 assert(is_searching);
1674 SplitPoint* sp = curSplitPoint;
1676 Threads.mutex.unlock();
1678 Stack ss[MAX_PLY_PLUS_2];
1679 Position pos(*sp->pos, this);
1681 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1686 assert(sp->activePositions[idx] == NULL);
1688 sp->activePositions[idx] = &pos;
1690 if (sp->nodeType == Root)
1691 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1692 else if (sp->nodeType == PV)
1693 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1694 else if (sp->nodeType == NonPV)
1695 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1699 assert(is_searching);
1701 is_searching = false;
1702 sp->activePositions[idx] = NULL;
1703 sp->slavesMask &= ~(1ULL << idx);
1704 sp->nodes += pos.nodes_searched();
1706 // Wake up master thread so to allow it to return from the idle loop in
1707 // case we are the last slave of the split point.
1708 if ( Threads.use_sleeping_threads()
1709 && this != sp->master
1712 assert(!sp->master->is_searching);
1713 sp->master->notify_one();
1716 // After releasing the lock we cannot access anymore any SplitPoint
1717 // related data in a safe way becuase it could have been released under
1718 // our feet by the sp master. Also accessing other Thread objects is
1719 // unsafe because if we are exiting there is a chance are already freed.
1726 /// check_time() is called by the timer thread when the timer triggers. It is
1727 /// used to print debug info and, more important, to detect when we are out of
1728 /// available time and so stop the search.
1732 static Time::point lastInfoTime = Time::now();
1733 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1735 if (Time::now() - lastInfoTime >= 1000)
1737 lastInfoTime = Time::now();
1746 Threads.mutex.lock();
1748 nodes = RootPos.nodes_searched();
1750 // Loop across all split points and sum accumulated SplitPoint nodes plus
1751 // all the currently active slaves positions.
1752 for (size_t i = 0; i < Threads.size(); i++)
1753 for (int j = 0; j < Threads[i].splitPointsCnt; j++)
1755 SplitPoint& sp = Threads[i].splitPoints[j];
1760 Bitboard sm = sp.slavesMask;
1763 Position* pos = sp.activePositions[pop_lsb(&sm)];
1764 nodes += pos ? pos->nodes_searched() : 0;
1770 Threads.mutex.unlock();
1773 Time::point elapsed = Time::now() - SearchTime;
1774 bool stillAtFirstMove = Signals.firstRootMove
1775 && !Signals.failedLowAtRoot
1776 && elapsed > TimeMgr.available_time();
1778 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1779 || stillAtFirstMove;
1781 if ( (Limits.use_time_management() && noMoreTime)
1782 || (Limits.movetime && elapsed >= Limits.movetime)
1783 || (Limits.nodes && nodes >= Limits.nodes))
1784 Signals.stop = true;