2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // This is the minimum interval in msec between two check_time() calls
59 const int TimerResolution = 5;
61 // Different node types, used as template parameter
62 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
64 // Dynamic razoring margin based on depth
65 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
67 // Futility lookup tables (initialized at startup) and their access functions
68 Value FutilityMargins[16][64]; // [depth][moveNumber]
69 int FutilityMoveCounts[32]; // [depth]
71 inline Value futility_margin(Depth d, int mn) {
73 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
77 // Reduction lookup tables (initialized at startup) and their access function
78 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
82 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
88 Value DrawValue[COLOR_NB];
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(const Position& pos, Move move, Value futilityBase, Value beta);
102 bool allows(const Position& pos, Move first, Move second);
103 bool refutes(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 + 0.3 * pow(double(d), 1.8));
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.game_ply(), RootColor);
187 if (RootMoves.empty())
189 RootMoves.push_back(MOVE_NONE);
190 sync_cout << "info depth 0 score "
191 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
197 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
199 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
201 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
203 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
208 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
210 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
211 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
212 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
213 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
216 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
218 if (Options["Use Search Log"])
220 Log log(Options["Search Log Filename"]);
221 log << "\nSearching: " << RootPos.fen()
222 << "\ninfinite: " << Limits.infinite
223 << " ponder: " << Limits.ponder
224 << " time: " << Limits.time[RootColor]
225 << " increment: " << Limits.inc[RootColor]
226 << " moves to go: " << Limits.movestogo
230 // Reset the threads, still sleeping: will be wake up at split time
231 for (size_t i = 0; i < Threads.size(); i++)
232 Threads[i]->maxPly = 0;
234 Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
236 // Set best timer interval to avoid lagging under time pressure. Timer is
237 // used to check for remaining available thinking time.
238 Threads.timer->msec =
239 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
240 Limits.nodes ? 2 * TimerResolution
243 Threads.timer->notify_one(); // Wake up the recurring timer
245 id_loop(RootPos); // Let's start searching !
247 Threads.timer->msec = 0; // Stop the timer
248 Threads.sleepWhileIdle = true; // Send idle threads to sleep
250 if (Options["Use Search Log"])
252 Time::point elapsed = Time::now() - SearchTime + 1;
254 Log log(Options["Search Log Filename"]);
255 log << "Nodes: " << RootPos.nodes_searched()
256 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
257 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
260 RootPos.do_move(RootMoves[0].pv[0], st);
261 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
262 RootPos.undo_move(RootMoves[0].pv[0]);
267 // When we reach max depth we arrive here even without Signals.stop is raised,
268 // but if we are pondering or in infinite search, according to UCI protocol,
269 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
270 // command. We simply wait here until GUI sends one of those commands (that
271 // raise Signals.stop).
272 if (!Signals.stop && (Limits.ponder || Limits.infinite))
274 Signals.stopOnPonderhit = true;
275 RootPos.this_thread()->wait_for(Signals.stop);
278 // Best move could be MOVE_NONE when searching on a stalemate position
279 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
280 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
287 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
288 // with increasing depth until the allocated thinking time has been consumed,
289 // user stops the search, or the maximum search depth is reached.
291 void id_loop(Position& pos) {
293 Stack ss[MAX_PLY_PLUS_2];
294 int depth, prevBestMoveChanges;
295 Value bestValue, alpha, beta, delta;
297 memset(ss, 0, 4 * sizeof(Stack));
298 depth = BestMoveChanges = 0;
299 bestValue = delta = -VALUE_INFINITE;
300 ss->currentMove = MOVE_NULL; // Hack to skip update gains
305 PVSize = Options["MultiPV"];
306 Skill skill(Options["Skill Level"]);
308 // Do we have to play with skill handicap? In this case enable MultiPV search
309 // that we will use behind the scenes to retrieve a set of possible moves.
310 if (skill.enabled() && PVSize < 4)
313 PVSize = std::min(PVSize, RootMoves.size());
315 // Iterative deepening loop until requested to stop or target depth reached
316 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
318 // Save last iteration's scores before first PV line is searched and all
319 // the move scores but the (new) PV are set to -VALUE_INFINITE.
320 for (size_t i = 0; i < RootMoves.size(); i++)
321 RootMoves[i].prevScore = RootMoves[i].score;
323 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
326 // MultiPV loop. We perform a full root search for each PV line
327 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
329 // Set aspiration window default width
330 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
333 alpha = RootMoves[PVIdx].prevScore - delta;
334 beta = RootMoves[PVIdx].prevScore + delta;
338 alpha = -VALUE_INFINITE;
339 beta = VALUE_INFINITE;
342 // Start with a small aspiration window and, in case of fail high/low,
343 // research with bigger window until not failing high/low anymore.
346 // Search starts from ss+1 to allow referencing (ss-1). This is
347 // needed by update gains and ss copy when splitting at Root.
348 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
350 // Bring to front the best move. It is critical that sorting is
351 // done with a stable algorithm because all the values but the first
352 // and eventually the new best one are set to -VALUE_INFINITE and
353 // we want to keep the same order for all the moves but the new
354 // PV that goes to the front. Note that in case of MultiPV search
355 // the already searched PV lines are preserved.
356 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
358 // Write PV back to transposition table in case the relevant
359 // entries have been overwritten during the search.
360 for (size_t i = 0; i <= PVIdx; i++)
361 RootMoves[i].insert_pv_in_tt(pos);
363 // If search has been stopped return immediately. Sorting and
364 // writing PV back to TT is safe becuase RootMoves is still
365 // valid, although refers to previous iteration.
369 // In case of failing high/low increase aspiration window and
370 // research, otherwise exit the loop.
371 if (bestValue > alpha && bestValue < beta)
374 // Give some update (without cluttering the UI) before to research
375 if (Time::now() - SearchTime > 3000)
376 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
378 if (abs(bestValue) >= VALUE_KNOWN_WIN)
380 alpha = -VALUE_INFINITE;
381 beta = VALUE_INFINITE;
383 else if (bestValue >= beta)
390 Signals.failedLowAtRoot = true;
391 Signals.stopOnPonderhit = false;
397 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
400 // Sort the PV lines searched so far and update the GUI
401 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
403 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
404 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
407 // Do we need to pick now the sub-optimal best move ?
408 if (skill.enabled() && skill.time_to_pick(depth))
411 if (Options["Use Search Log"])
413 Log log(Options["Search Log Filename"]);
414 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
418 // Do we have found a "mate in x"?
420 && bestValue >= VALUE_MATE_IN_MAX_PLY
421 && VALUE_MATE - bestValue <= 2 * Limits.mate)
424 // Do we have time for the next iteration? Can we stop searching now?
425 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
427 bool stop = false; // Local variable, not the volatile Signals.stop
429 // Take in account some extra time if the best move has changed
430 if (depth > 4 && depth < 50 && PVSize == 1)
431 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
433 // Stop search if most of available time is already consumed. We
434 // probably don't have enough time to search the first move at the
435 // next iteration anyway.
436 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
439 // Stop search early if one move seems to be much better than others
443 && bestValue > VALUE_MATED_IN_MAX_PLY
444 && ( RootMoves.size() == 1
445 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
447 Value rBeta = bestValue - 2 * PawnValueMg;
448 (ss+1)->excludedMove = RootMoves[0].pv[0];
449 (ss+1)->skipNullMove = true;
450 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
451 (ss+1)->skipNullMove = false;
452 (ss+1)->excludedMove = MOVE_NONE;
460 // If we are allowed to ponder do not stop the search now but
461 // keep pondering until GUI sends "ponderhit" or "stop".
463 Signals.stopOnPonderhit = true;
472 // search<>() is the main search function for both PV and non-PV nodes and for
473 // normal and SplitPoint nodes. When called just after a split point the search
474 // is simpler because we have already probed the hash table, done a null move
475 // search, and searched the first move before splitting, we don't have to repeat
476 // all this work again. We also don't need to store anything to the hash table
477 // here: This is taken care of after we return from the split point.
479 template <NodeType NT>
480 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
482 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
483 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
484 const bool RootNode = (NT == Root || NT == SplitPointRoot);
486 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
487 assert(PvNode || (alpha == beta - 1));
488 assert(depth > DEPTH_ZERO);
490 Move movesSearched[64];
493 SplitPoint* splitPoint;
495 Move ttMove, move, excludedMove, bestMove, threatMove;
497 Value bestValue, value, ttValue;
498 Value eval, nullValue, futilityValue;
499 bool inCheck, givesCheck, pvMove, singularExtensionNode;
500 bool captureOrPromotion, dangerous, doFullDepthSearch;
501 int moveCount, playedMoveCount;
503 // Step 1. Initialize node
504 Thread* thisThread = pos.this_thread();
505 moveCount = playedMoveCount = 0;
506 inCheck = pos.checkers();
510 splitPoint = ss->splitPoint;
511 bestMove = splitPoint->bestMove;
512 threatMove = splitPoint->threatMove;
513 bestValue = splitPoint->bestValue;
515 ttMove = excludedMove = MOVE_NONE;
516 ttValue = VALUE_NONE;
518 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
520 goto split_point_start;
523 bestValue = -VALUE_INFINITE;
524 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
525 ss->ply = (ss-1)->ply + 1;
526 ss->futilityMoveCount = 0;
527 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
528 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
530 // Used to send selDepth info to GUI
531 if (PvNode && thisThread->maxPly < ss->ply)
532 thisThread->maxPly = ss->ply;
536 // Step 2. Check for aborted search and immediate draw
537 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
538 return DrawValue[pos.side_to_move()];
540 // Step 3. Mate distance pruning. Even if we mate at the next move our score
541 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
542 // a shorter mate was found upward in the tree then there is no need to search
543 // further, we will never beat current alpha. Same logic but with reversed signs
544 // applies also in the opposite condition of being mated instead of giving mate,
545 // in this case return a fail-high score.
546 alpha = std::max(mated_in(ss->ply), alpha);
547 beta = std::min(mate_in(ss->ply+1), beta);
552 // Step 4. Transposition table lookup
553 // We don't want the score of a partial search to overwrite a previous full search
554 // TT value, so we use a different position key in case of an excluded move.
555 excludedMove = ss->excludedMove;
556 posKey = excludedMove ? pos.exclusion_key() : pos.key();
557 tte = TT.probe(posKey);
558 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
559 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
561 // At PV nodes we check for exact scores, while at non-PV nodes we check for
562 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
563 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
564 // we should also update RootMoveList to avoid bogus output.
567 && tte->depth() >= depth
568 && ttValue != VALUE_NONE // Only in case of TT access race
569 && ( PvNode ? tte->type() == BOUND_EXACT
570 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
571 : (tte->type() & BOUND_UPPER)))
574 ss->currentMove = ttMove; // Can be MOVE_NONE
578 && !pos.is_capture_or_promotion(ttMove)
579 && ttMove != ss->killers[0])
581 ss->killers[1] = ss->killers[0];
582 ss->killers[0] = ttMove;
587 // Step 5. Evaluate the position statically and update parent's gain statistics
589 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
593 // Never assume anything on values stored in TT
594 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
595 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
596 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
598 // Can ttValue be used as a better position evaluation?
599 if (ttValue != VALUE_NONE)
600 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
601 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
606 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
607 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
608 ss->staticEval, ss->evalMargin);
611 // Update gain for the parent non-capture move given the static position
612 // evaluation before and after the move.
613 if ( (move = (ss-1)->currentMove) != MOVE_NULL
614 && (ss-1)->staticEval != VALUE_NONE
615 && ss->staticEval != VALUE_NONE
616 && !pos.captured_piece_type()
617 && type_of(move) == NORMAL)
619 Square to = to_sq(move);
620 Gain.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
623 // Step 6. Razoring (is omitted in PV nodes)
625 && depth < 4 * ONE_PLY
627 && eval + razor_margin(depth) < beta
628 && ttMove == MOVE_NONE
629 && abs(beta) < VALUE_MATE_IN_MAX_PLY
630 && !pos.pawn_on_7th(pos.side_to_move()))
632 Value rbeta = beta - razor_margin(depth);
633 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
635 // Logically we should return (v + razor_margin(depth)), but
636 // surprisingly this did slightly weaker in tests.
640 // Step 7. Static null move pruning (is omitted in PV nodes)
641 // We're betting that the opponent doesn't have a move that will reduce
642 // the score by more than futility_margin(depth) if we do a null move.
645 && depth < 4 * ONE_PLY
647 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
648 && abs(beta) < VALUE_MATE_IN_MAX_PLY
649 && abs(eval) < VALUE_KNOWN_WIN
650 && pos.non_pawn_material(pos.side_to_move()))
651 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
653 // Step 8. Null move search with verification search (is omitted in PV nodes)
659 && abs(beta) < VALUE_MATE_IN_MAX_PLY
660 && pos.non_pawn_material(pos.side_to_move()))
662 ss->currentMove = MOVE_NULL;
664 // Null move dynamic reduction based on depth
665 Depth R = 3 * ONE_PLY + depth / 4;
667 // Null move dynamic reduction based on value
668 if (eval - PawnValueMg > beta)
671 pos.do_null_move(st);
672 (ss+1)->skipNullMove = true;
673 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
674 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
675 (ss+1)->skipNullMove = false;
676 pos.undo_null_move();
678 if (nullValue >= beta)
680 // Do not return unproven mate scores
681 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
684 if (depth < 12 * ONE_PLY)
687 // Do verification search at high depths
688 ss->skipNullMove = true;
689 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
690 ss->skipNullMove = false;
697 // The null move failed low, which means that we may be faced with
698 // some kind of threat. If the previous move was reduced, check if
699 // the move that refuted the null move was somehow connected to the
700 // move which was reduced. If a connection is found, return a fail
701 // low score (which will cause the reduced move to fail high in the
702 // parent node, which will trigger a re-search with full depth).
703 threatMove = (ss+1)->currentMove;
705 if ( depth < 5 * ONE_PLY
707 && threatMove != MOVE_NONE
708 && allows(pos, (ss-1)->currentMove, threatMove))
713 // Step 9. ProbCut (is omitted in PV nodes)
714 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
715 // and a reduced search returns a value much above beta, we can (almost) safely
716 // prune the previous move.
718 && depth >= 5 * ONE_PLY
721 && excludedMove == MOVE_NONE
722 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
724 Value rbeta = beta + 200;
725 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
727 assert(rdepth >= ONE_PLY);
728 assert((ss-1)->currentMove != MOVE_NONE);
729 assert((ss-1)->currentMove != MOVE_NULL);
731 MovePicker mp(pos, ttMove, Hist, pos.captured_piece_type());
734 while ((move = mp.next_move<false>()) != MOVE_NONE)
735 if (pos.pl_move_is_legal(move, ci.pinned))
737 ss->currentMove = move;
738 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
739 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
746 // Step 10. Internal iterative deepening
747 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
748 && ttMove == MOVE_NONE
749 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
751 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
753 ss->skipNullMove = true;
754 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
755 ss->skipNullMove = false;
757 tte = TT.probe(posKey);
758 ttMove = tte ? tte->move() : MOVE_NONE;
761 split_point_start: // At split points actual search starts from here
763 MovePicker mp(pos, ttMove, depth, Hist, ss, PvNode ? -VALUE_INFINITE : beta);
765 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
766 singularExtensionNode = !RootNode
768 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
769 && ttMove != MOVE_NONE
770 && !excludedMove // Recursive singular search is not allowed
771 && (tte->type() & BOUND_LOWER)
772 && tte->depth() >= depth - 3 * ONE_PLY;
774 // Step 11. Loop through moves
775 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
776 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
780 if (move == excludedMove)
783 // At root obey the "searchmoves" option and skip moves not listed in Root
784 // Move List, as a consequence any illegal move is also skipped. In MultiPV
785 // mode we also skip PV moves which have been already searched.
786 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
791 // Shared counter cannot be decremented later if move turns out to be illegal
792 if (!pos.pl_move_is_legal(move, ci.pinned))
795 moveCount = ++splitPoint->moveCount;
796 splitPoint->mutex.unlock();
803 Signals.firstRootMove = (moveCount == 1);
805 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
806 sync_cout << "info depth " << depth / ONE_PLY
807 << " currmove " << move_to_uci(move, pos.is_chess960())
808 << " currmovenumber " << moveCount + PVIdx << sync_endl;
812 captureOrPromotion = pos.is_capture_or_promotion(move);
813 givesCheck = pos.move_gives_check(move, ci);
814 dangerous = givesCheck
815 || pos.is_passed_pawn_push(move)
816 || type_of(move) == CASTLE
817 || ( captureOrPromotion // Entering a pawn endgame?
818 && type_of(pos.piece_on(to_sq(move))) != PAWN
819 && type_of(move) == NORMAL
820 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
821 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
823 // Step 12. Extend checks and, in PV nodes, also dangerous moves
824 if (PvNode && dangerous)
827 else if (givesCheck && pos.see_sign(move) >= 0)
830 // Singular extension search. If all moves but one fail low on a search of
831 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
832 // is singular and should be extended. To verify this we do a reduced search
833 // on all the other moves but the ttMove, if result is lower than ttValue minus
834 // a margin then we extend ttMove.
835 if ( singularExtensionNode
838 && pos.pl_move_is_legal(move, ci.pinned)
839 && abs(ttValue) < VALUE_KNOWN_WIN)
841 assert(ttValue != VALUE_NONE);
843 Value rBeta = ttValue - int(depth);
844 ss->excludedMove = move;
845 ss->skipNullMove = true;
846 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
847 ss->skipNullMove = false;
848 ss->excludedMove = MOVE_NONE;
854 // Update current move (this must be done after singular extension search)
855 newDepth = depth - ONE_PLY + ext;
857 // Step 13. Futility pruning (is omitted in PV nodes)
859 && !captureOrPromotion
862 /* && move != ttMove Already implicit in the next condition */
863 && bestValue > VALUE_MATED_IN_MAX_PLY)
865 // Move count based pruning
866 if ( depth < 16 * ONE_PLY
867 && moveCount >= FutilityMoveCounts[depth]
868 && (!threatMove || !refutes(pos, move, threatMove)))
871 splitPoint->mutex.lock();
876 // Value based pruning
877 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
878 // but fixing this made program slightly weaker.
879 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
880 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
881 + Gain[pos.piece_moved(move)][to_sq(move)];
883 if (futilityValue < beta)
885 bestValue = std::max(bestValue, futilityValue);
889 splitPoint->mutex.lock();
890 if (bestValue > splitPoint->bestValue)
891 splitPoint->bestValue = bestValue;
896 // Prune moves with negative SEE at low depths
897 if ( predictedDepth < 4 * ONE_PLY
898 && pos.see_sign(move) < 0)
901 splitPoint->mutex.lock();
906 // We have not pruned the move that will be searched, but remember how
907 // far in the move list we are to be more aggressive in the child node.
908 ss->futilityMoveCount = moveCount;
911 ss->futilityMoveCount = 0;
913 // Check for legality only before to do the move
914 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
920 pvMove = PvNode && moveCount == 1;
921 ss->currentMove = move;
922 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
923 movesSearched[playedMoveCount++] = move;
925 // Step 14. Make the move
926 pos.do_move(move, st, ci, givesCheck);
928 // Step 15. Reduced depth search (LMR). If the move fails high will be
929 // re-searched at full depth.
930 if ( depth > 3 * ONE_PLY
932 && !captureOrPromotion
935 && move != ss->killers[0]
936 && move != ss->killers[1])
938 ss->reduction = reduction<PvNode>(depth, moveCount);
939 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
941 alpha = splitPoint->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)
955 alpha = splitPoint->alpha;
957 value = newDepth < ONE_PLY ?
958 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
959 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
960 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
963 // Only for PV nodes do a full PV search on the first move or after a fail
964 // high, in the latter case search only if value < beta, otherwise let the
965 // parent node to fail low with value <= alpha and to try another move.
966 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
967 value = newDepth < ONE_PLY ?
968 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
969 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
970 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
971 // Step 17. Undo move
974 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
976 // Step 18. Check for new best move
979 splitPoint->mutex.lock();
980 bestValue = splitPoint->bestValue;
981 alpha = splitPoint->alpha;
984 // Finished searching the move. If Signals.stop is true, the search
985 // was aborted because the user interrupted the search or because we
986 // ran out of time. In this case, the return value of the search cannot
987 // be trusted, and we don't update the best move and/or PV.
988 if (Signals.stop || thisThread->cutoff_occurred())
989 return value; // To avoid returning VALUE_INFINITE
993 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
995 // PV move or new best move ?
996 if (pvMove || value > alpha)
999 rm.extract_pv_from_tt(pos);
1001 // We record how often the best move has been changed in each
1002 // iteration. This information is used for time management: When
1003 // the best move changes frequently, we allocate some more time.
1008 // All other moves but the PV are set to the lowest value, this
1009 // is not a problem when sorting becuase sort is stable and move
1010 // position in the list is preserved, just the PV is pushed up.
1011 rm.score = -VALUE_INFINITE;
1014 if (value > bestValue)
1016 bestValue = SpNode ? splitPoint->bestValue = value : value;
1020 bestMove = SpNode ? splitPoint->bestMove = move : move;
1022 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1023 alpha = SpNode ? splitPoint->alpha = value : value;
1026 assert(value >= beta); // Fail high
1029 splitPoint->cutoff = true;
1036 // Step 19. Check for splitting the search
1038 && depth >= Threads.minimumSplitDepth
1039 && Threads.available_slave(thisThread)
1040 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1042 assert(bestValue < beta);
1044 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1045 depth, threatMove, moveCount, &mp, NT);
1046 if (bestValue >= beta)
1054 // Step 20. Check for mate and stalemate
1055 // All legal moves have been searched and if there are no legal moves, it
1056 // must be mate or stalemate. Note that we can have a false positive in
1057 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1058 // harmless because return value is discarded anyhow in the parent nodes.
1059 // If we are in a singular extension search then return a fail low score.
1060 // A split node has at least one move, the one tried before to be splitted.
1062 return excludedMove ? alpha
1063 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1065 // If we have pruned all the moves without searching return a fail-low score
1066 if (bestValue == -VALUE_INFINITE)
1068 assert(!playedMoveCount);
1073 if (bestValue >= beta) // Failed high
1075 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1076 bestMove, ss->staticEval, ss->evalMargin);
1078 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1080 if (bestMove != ss->killers[0])
1082 ss->killers[1] = ss->killers[0];
1083 ss->killers[0] = bestMove;
1086 // Increase history value of the cut-off move
1087 Value bonus = Value(int(depth) * int(depth));
1088 Hist.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1090 // Decrease history of all the other played non-capture moves
1091 for (int i = 0; i < playedMoveCount - 1; i++)
1093 Move m = movesSearched[i];
1094 Hist.update(pos.piece_moved(m), to_sq(m), -bonus);
1098 else // Failed low or PV search
1099 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1100 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1101 depth, bestMove, ss->staticEval, ss->evalMargin);
1103 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1109 // qsearch() is the quiescence search function, which is called by the main
1110 // search function when the remaining depth is zero (or, to be more precise,
1111 // less than ONE_PLY).
1113 template <NodeType NT, bool InCheck>
1114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1116 const bool PvNode = (NT == PV);
1118 assert(NT == PV || NT == NonPV);
1119 assert(InCheck == !!pos.checkers());
1120 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1121 assert(PvNode || (alpha == beta - 1));
1122 assert(depth <= DEPTH_ZERO);
1127 Move ttMove, move, bestMove;
1128 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1129 bool givesCheck, enoughMaterial, evasionPrunable;
1132 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1136 ss->currentMove = bestMove = MOVE_NONE;
1137 ss->ply = (ss-1)->ply + 1;
1139 // Check for an instant draw or maximum ply reached
1140 if (pos.is_draw() || ss->ply > MAX_PLY)
1141 return DrawValue[pos.side_to_move()];
1143 // Decide whether or not to include checks, this fixes also the type of
1144 // TT entry depth that we are going to use. Note that in qsearch we use
1145 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1146 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1147 : DEPTH_QS_NO_CHECKS;
1149 // Transposition table lookup. At PV nodes, we don't use the TT for
1150 // pruning, but only for move ordering.
1152 tte = TT.probe(posKey);
1153 ttMove = tte ? tte->move() : MOVE_NONE;
1154 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1157 && tte->depth() >= ttDepth
1158 && ttValue != VALUE_NONE // Only in case of TT access race
1159 && ( PvNode ? tte->type() == BOUND_EXACT
1160 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1161 : (tte->type() & BOUND_UPPER)))
1163 ss->currentMove = ttMove; // Can be MOVE_NONE
1167 // Evaluate the position statically
1170 ss->staticEval = ss->evalMargin = VALUE_NONE;
1171 bestValue = futilityBase = -VALUE_INFINITE;
1172 enoughMaterial = false;
1178 // Never assume anything on values stored in TT
1179 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1180 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1181 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1184 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1186 // Stand pat. Return immediately if static value is at least beta
1187 if (bestValue >= beta)
1190 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1191 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1196 if (PvNode && bestValue > alpha)
1199 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1200 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1203 // Initialize a MovePicker object for the current position, and prepare
1204 // to search the moves. Because the depth is <= 0 here, only captures,
1205 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1207 MovePicker mp(pos, ttMove, depth, Hist, to_sq((ss-1)->currentMove));
1210 // Loop through the moves until no moves remain or a beta cutoff occurs
1211 while ((move = mp.next_move<false>()) != MOVE_NONE)
1213 assert(is_ok(move));
1215 givesCheck = pos.move_gives_check(move, ci);
1223 && type_of(move) != PROMOTION
1224 && !pos.is_passed_pawn_push(move))
1226 futilityValue = futilityBase
1227 + PieceValue[EG][pos.piece_on(to_sq(move))]
1228 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1230 if (futilityValue < beta)
1232 bestValue = std::max(bestValue, futilityValue);
1236 // Prune moves with negative or equal SEE and also moves with positive
1237 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1238 if ( futilityBase < beta
1239 && depth < DEPTH_ZERO
1240 && pos.see(move, beta - futilityBase) <= 0)
1242 bestValue = std::max(bestValue, futilityBase);
1247 // Detect non-capture evasions that are candidate to be pruned
1248 evasionPrunable = !PvNode
1250 && bestValue > VALUE_MATED_IN_MAX_PLY
1251 && !pos.is_capture(move)
1252 && !pos.can_castle(pos.side_to_move());
1254 // Don't search moves with negative SEE values
1256 && (!InCheck || evasionPrunable)
1258 && type_of(move) != PROMOTION
1259 && pos.see_sign(move) < 0)
1262 // Don't search useless checks
1267 && !pos.is_capture_or_promotion(move)
1268 && ss->staticEval + PawnValueMg / 4 < beta
1269 && !check_is_dangerous(pos, move, futilityBase, beta))
1272 // Check for legality only before to do the move
1273 if (!pos.pl_move_is_legal(move, ci.pinned))
1276 ss->currentMove = move;
1278 // Make and search the move
1279 pos.do_move(move, st, ci, givesCheck);
1280 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1281 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1282 pos.undo_move(move);
1284 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1286 // Check for new best move
1287 if (value > bestValue)
1293 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1300 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1301 ttDepth, move, ss->staticEval, ss->evalMargin);
1309 // All legal moves have been searched. A special case: If we're in check
1310 // and no legal moves were found, it is checkmate.
1311 if (InCheck && bestValue == -VALUE_INFINITE)
1312 return mated_in(ss->ply); // Plies to mate from the root
1314 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1315 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1316 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1318 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1324 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1325 // "plies to mate from the current position". Non-mate scores are unchanged.
1326 // The function is called before storing a value to the transposition table.
1328 Value value_to_tt(Value v, int ply) {
1330 assert(v != VALUE_NONE);
1332 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1333 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1337 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1338 // from the transposition table (where refers to the plies to mate/be mated
1339 // from current position) to "plies to mate/be mated from the root".
1341 Value value_from_tt(Value v, int ply) {
1343 return v == VALUE_NONE ? VALUE_NONE
1344 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1345 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1349 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1351 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1353 Piece pc = pos.piece_moved(move);
1354 Square from = from_sq(move);
1355 Square to = to_sq(move);
1356 Color them = ~pos.side_to_move();
1357 Square ksq = pos.king_square(them);
1358 Bitboard enemies = pos.pieces(them);
1359 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1360 Bitboard occ = pos.pieces() ^ from ^ ksq;
1361 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1362 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1364 // Checks which give opponent's king at most one escape square are dangerous
1365 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1368 // Queen contact check is very dangerous
1369 if (type_of(pc) == QUEEN && (kingAtt & to))
1372 // Creating new double threats with checks is dangerous
1373 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1376 // Note that here we generate illegal "double move"!
1377 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1385 // allows() tests whether the 'first' move at previous ply somehow makes the
1386 // 'second' move possible, for instance if the moving piece is the same in
1387 // both moves. Normally the second move is the threat (the best move returned
1388 // from a null search that fails low).
1390 bool allows(const Position& pos, Move first, Move second) {
1392 assert(is_ok(first));
1393 assert(is_ok(second));
1394 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1395 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1397 Square m1from = from_sq(first);
1398 Square m2from = from_sq(second);
1399 Square m1to = to_sq(first);
1400 Square m2to = to_sq(second);
1402 // The piece is the same or second's destination was vacated by the first move
1403 if (m1to == m2from || m2to == m1from)
1406 // Second one moves through the square vacated by first one
1407 if (between_bb(m2from, m2to) & m1from)
1410 // Second's destination is defended by the first move's piece
1411 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1415 // Second move gives a discovered check through the first's checking piece
1416 if (m1att & pos.king_square(pos.side_to_move()))
1418 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1426 // refutes() tests whether a 'first' move is able to defend against a 'second'
1427 // opponent's move. In this case will not be pruned. Normally the second move
1428 // is the threat (the best move returned from a null search that fails low).
1430 bool refutes(const Position& pos, Move first, Move second) {
1432 assert(is_ok(first));
1433 assert(is_ok(second));
1435 Square m1from = from_sq(first);
1436 Square m2from = from_sq(second);
1437 Square m1to = to_sq(first);
1438 Square m2to = to_sq(second);
1440 // Don't prune moves of the threatened piece
1444 // If the threatened piece has value less than or equal to the value of the
1445 // threat piece, don't prune moves which defend it.
1446 if ( pos.is_capture(second)
1447 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1448 || type_of(pos.piece_on(m2from)) == KING))
1450 // Update occupancy as if the piece and the threat are moving
1451 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1452 Piece piece = pos.piece_on(m1from);
1454 // The moved piece attacks the square 'tto' ?
1455 if (pos.attacks_from(piece, m1to, occ) & m2to)
1458 // Scan for possible X-ray attackers behind the moved piece
1459 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
1460 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
1462 // Verify attackers are triggered by our move and not already existing
1463 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1467 // Don't prune safe moves which block the threat path
1468 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1475 // When playing with strength handicap choose best move among the MultiPV set
1476 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1478 Move Skill::pick_move() {
1482 // PRNG sequence should be not deterministic
1483 for (int i = Time::now() % 50; i > 0; i--)
1484 rk.rand<unsigned>();
1486 // RootMoves are already sorted by score in descending order
1487 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1488 int weakness = 120 - 2 * level;
1489 int max_s = -VALUE_INFINITE;
1492 // Choose best move. For each move score we add two terms both dependent on
1493 // weakness, one deterministic and bigger for weaker moves, and one random,
1494 // then we choose the move with the resulting highest score.
1495 for (size_t i = 0; i < PVSize; i++)
1497 int s = RootMoves[i].score;
1499 // Don't allow crazy blunders even at very low skills
1500 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1503 // This is our magic formula
1504 s += ( weakness * int(RootMoves[0].score - s)
1505 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1510 best = RootMoves[i].pv[0];
1517 // uci_pv() formats PV information according to UCI protocol. UCI requires
1518 // to send all the PV lines also if are still to be searched and so refer to
1519 // the previous search score.
1521 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1523 std::stringstream s;
1524 Time::point elaspsed = Time::now() - SearchTime + 1;
1525 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1528 for (size_t i = 0; i < Threads.size(); i++)
1529 if (Threads[i]->maxPly > selDepth)
1530 selDepth = Threads[i]->maxPly;
1532 for (size_t i = 0; i < uciPVSize; i++)
1534 bool updated = (i <= PVIdx);
1536 if (depth == 1 && !updated)
1539 int d = updated ? depth : depth - 1;
1540 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1542 if (s.rdbuf()->in_avail()) // Not at first line
1545 s << "info depth " << d
1546 << " seldepth " << selDepth
1547 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1548 << " nodes " << pos.nodes_searched()
1549 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1550 << " time " << elaspsed
1551 << " multipv " << i + 1
1554 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1555 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1564 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1565 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1566 /// allow to always have a ponder move even when we fail high at root, and a
1567 /// long PV to print that is important for position analysis.
1569 void RootMove::extract_pv_from_tt(Position& pos) {
1571 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1581 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1583 pos.do_move(pv[ply++], *st++);
1584 tte = TT.probe(pos.key());
1587 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1588 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1590 && (!pos.is_draw() || ply < 2));
1592 pv.push_back(MOVE_NONE); // Must be zero-terminating
1594 while (ply) pos.undo_move(pv[--ply]);
1598 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1599 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1600 /// first, even if the old TT entries have been overwritten.
1602 void RootMove::insert_pv_in_tt(Position& pos) {
1604 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1609 tte = TT.probe(pos.key());
1611 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1612 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1614 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1616 pos.do_move(pv[ply++], *st++);
1618 } while (pv[ply] != MOVE_NONE);
1620 while (ply) pos.undo_move(pv[--ply]);
1624 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1626 void Thread::idle_loop() {
1628 // Pointer 'this_sp' is not null only if we are called from split(), and not
1629 // at the thread creation. So it means we are the split point's master.
1630 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1632 assert(!this_sp || (this_sp->masterThread == this && searching));
1636 // If we are not searching, wait for a condition to be signaled instead of
1637 // wasting CPU time polling for work.
1638 while ((!searching && Threads.sleepWhileIdle) || exit)
1646 // Grab the lock to avoid races with Thread::notify_one()
1649 // If we are master and all slaves have finished then exit idle_loop
1650 if (this_sp && !this_sp->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 notify_one() call before
1659 // we had the chance to grab the lock.
1660 if (!searching && !exit)
1661 sleepCondition.wait(mutex);
1666 // If this thread has been assigned work, launch a search
1671 Threads.mutex.lock();
1674 SplitPoint* sp = activeSplitPoint;
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));
1682 (ss+1)->splitPoint = sp;
1686 assert(activePosition == NULL);
1688 activePosition = &pos;
1690 switch (sp->nodeType) {
1692 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1695 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1698 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1707 activePosition = NULL;
1708 sp->slavesMask &= ~(1ULL << idx);
1709 sp->nodes += pos.nodes_searched();
1711 // Wake up master thread so to allow it to return from the idle loop
1712 // in case we are the last slave of the split point.
1713 if ( Threads.sleepWhileIdle
1714 && this != sp->masterThread
1717 assert(!sp->masterThread->searching);
1718 sp->masterThread->notify_one();
1721 // After releasing the lock we cannot access anymore any SplitPoint
1722 // related data in a safe way becuase it could have been released under
1723 // our feet by the sp master. Also accessing other Thread objects is
1724 // unsafe because if we are exiting there is a chance are already freed.
1728 // If this thread is the master of a split point and all slaves have finished
1729 // their work at this split point, return from the idle loop.
1730 if (this_sp && !this_sp->slavesMask)
1732 this_sp->mutex.lock();
1733 bool finished = !this_sp->slavesMask; // Retest under lock protection
1734 this_sp->mutex.unlock();
1742 /// check_time() is called by the timer thread when the timer triggers. It is
1743 /// used to print debug info and, more important, to detect when we are out of
1744 /// available time and so stop the search.
1748 static Time::point lastInfoTime = Time::now();
1749 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1751 if (Time::now() - lastInfoTime >= 1000)
1753 lastInfoTime = Time::now();
1762 Threads.mutex.lock();
1764 nodes = RootPos.nodes_searched();
1766 // Loop across all split points and sum accumulated SplitPoint nodes plus
1767 // all the currently active positions nodes.
1768 for (size_t i = 0; i < Threads.size(); i++)
1769 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1771 SplitPoint& sp = Threads[i]->splitPoints[j];
1776 Bitboard sm = sp.slavesMask;
1779 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1781 nodes += pos->nodes_searched();
1787 Threads.mutex.unlock();
1790 Time::point elapsed = Time::now() - SearchTime;
1791 bool stillAtFirstMove = Signals.firstRootMove
1792 && !Signals.failedLowAtRoot
1793 && elapsed > TimeMgr.available_time();
1795 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1796 || stillAtFirstMove;
1798 if ( (Limits.use_time_management() && noMoreTime)
1799 || (Limits.movetime && elapsed >= Limits.movetime)
1800 || (Limits.nodes && nodes >= Limits.nodes))
1801 Signals.stop = true;