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;
44 Position RootPosition;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // Different node types, used as template parameter
59 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
61 // Lookup table to check if a Piece is a slider and its access function
62 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
63 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
65 // Maximum depth for razoring
66 const Depth RazorDepth = 4 * ONE_PLY;
68 // Dynamic razoring margin based on depth
69 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
71 // Maximum depth for use of dynamic threat detection when null move fails low
72 const Depth ThreatDepth = 5 * ONE_PLY;
74 // Minimum depth for use of internal iterative deepening
75 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
77 // At Non-PV nodes we do an internal iterative deepening search
78 // when the static evaluation is bigger then beta - IIDMargin.
79 const Value IIDMargin = Value(256);
81 // Minimum depth for use of singular extension
82 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
84 // Futility margin for quiescence search
85 const Value FutilityMarginQS = Value(128);
87 // Futility lookup tables (initialized at startup) and their access functions
88 Value FutilityMargins[16][64]; // [depth][moveNumber]
89 int FutilityMoveCounts[32]; // [depth]
91 inline Value futility_margin(Depth d, int mn) {
93 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
97 inline int futility_move_count(Depth d) {
99 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
102 // Reduction lookup tables (initialized at startup) and their access function
103 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
105 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
107 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
110 // Easy move margin. An easy move candidate must be at least this much better
111 // than the second best move.
112 const Value EasyMoveMargin = Value(0x150);
114 // This is the minimum interval in msec between two check_time() calls
115 const int TimerResolution = 5;
118 size_t MultiPV, UCIMultiPV, PVIdx;
122 bool SkillLevelEnabled, Chess960;
126 template <NodeType NT>
127 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
129 template <NodeType NT>
130 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
132 void id_loop(Position& pos);
133 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta);
134 bool connected_moves(const Position& pos, Move m1, Move m2);
135 Value value_to_tt(Value v, int ply);
136 Value value_from_tt(Value v, int ply);
137 bool can_return_tt(const TTEntry* tte, Depth depth, Value ttValue, Value beta);
138 bool connected_threat(const Position& pos, Move m, Move threat);
139 Value refine_eval(const TTEntry* tte, Value ttValue, Value defaultEval);
140 Move do_skill_level();
141 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
143 // is_dangerous() checks whether a move belongs to some classes of known
144 // 'dangerous' moves so that we avoid to prune it.
145 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
148 if (type_of(m) == CASTLE)
152 if ( type_of(pos.piece_moved(m)) == PAWN
153 && pos.pawn_is_passed(pos.side_to_move(), to_sq(m)))
156 // Entering a pawn endgame?
157 if ( captureOrPromotion
158 && type_of(pos.piece_on(to_sq(m))) != PAWN
159 && type_of(m) == NORMAL
160 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
161 - PieceValue[Mg][pos.piece_on(to_sq(m))] == VALUE_ZERO))
170 /// Search::init() is called during startup to initialize various lookup tables
172 void Search::init() {
174 int d; // depth (ONE_PLY == 2)
175 int hd; // half depth (ONE_PLY == 1)
178 // Init reductions array
179 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
181 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
182 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
183 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
184 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
187 // Init futility margins array
188 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
189 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
191 // Init futility move count array
192 for (d = 0; d < 32; d++)
193 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
197 /// Search::perft() is our utility to verify move generation. All the leaf nodes
198 /// up to the given depth are generated and counted and the sum returned.
200 size_t Search::perft(Position& pos, Depth depth) {
202 // At the last ply just return the number of legal moves (leaf nodes)
203 if (depth == ONE_PLY)
204 return MoveList<LEGAL>(pos).size();
210 for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
212 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
213 cnt += perft(pos, depth - ONE_PLY);
214 pos.undo_move(ml.move());
221 /// Search::think() is the external interface to Stockfish's search, and is
222 /// called by the main thread when the program receives the UCI 'go' command. It
223 /// searches from RootPosition and at the end prints the "bestmove" to output.
225 void Search::think() {
227 static PolyglotBook book; // Defined static to initialize the PRNG only once
229 Position& pos = RootPosition;
230 Chess960 = pos.is_chess960();
231 Eval::RootColor = pos.side_to_move();
232 TimeMgr.init(Limits, pos.startpos_ply_counter(), pos.side_to_move());
236 if (RootMoves.empty())
238 sync_cout << "info depth 0 score "
239 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << sync_endl;
241 RootMoves.push_back(MOVE_NONE);
245 if (Options["OwnBook"] && !Limits.infinite)
247 Move bookMove = book.probe(pos, Options["Book File"], Options["Best Book Move"]);
249 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
251 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
256 UCIMultiPV = Options["MultiPV"];
257 SkillLevel = Options["Skill Level"];
259 // Do we have to play with skill handicap? In this case enable MultiPV that
260 // we will use behind the scenes to retrieve a set of possible moves.
261 SkillLevelEnabled = (SkillLevel < 20);
262 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
264 if (Options["Use Search Log"])
266 Log log(Options["Search Log Filename"]);
267 log << "\nSearching: " << pos.to_fen()
268 << "\ninfinite: " << Limits.infinite
269 << " ponder: " << Limits.ponder
270 << " time: " << Limits.time[pos.side_to_move()]
271 << " increment: " << Limits.inc[pos.side_to_move()]
272 << " moves to go: " << Limits.movestogo
278 // Set best timer interval to avoid lagging under time pressure. Timer is
279 // used to check for remaining available thinking time.
280 if (Limits.use_time_management())
281 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)));
283 Threads.set_timer(100);
285 // We're ready to start searching. Call the iterative deepening loop function
288 Threads.set_timer(0); // Stop timer
291 if (Options["Use Search Log"])
293 Time::point elapsed = Time::now() - SearchTime + 1;
295 Log log(Options["Search Log Filename"]);
296 log << "Nodes: " << pos.nodes_searched()
297 << "\nNodes/second: " << pos.nodes_searched() * 1000 / elapsed
298 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
301 pos.do_move(RootMoves[0].pv[0], st);
302 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << std::endl;
303 pos.undo_move(RootMoves[0].pv[0]);
308 // When we reach max depth we arrive here even without Signals.stop is raised,
309 // but if we are pondering or in infinite search, we shouldn't print the best
310 // move before we are told to do so.
311 if (!Signals.stop && (Limits.ponder || Limits.infinite))
312 pos.this_thread()->wait_for_stop_or_ponderhit();
314 // Best move could be MOVE_NONE when searching on a stalemate position
315 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], Chess960)
316 << " ponder " << move_to_uci(RootMoves[0].pv[1], Chess960) << sync_endl;
322 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
323 // with increasing depth until the allocated thinking time has been consumed,
324 // user stops the search, or the maximum search depth is reached.
326 void id_loop(Position& pos) {
328 Stack ss[MAX_PLY_PLUS_2];
329 int depth, prevBestMoveChanges;
330 Value bestValue, alpha, beta, delta;
331 bool bestMoveNeverChanged = true;
332 Move skillBest = MOVE_NONE;
334 memset(ss, 0, 4 * sizeof(Stack));
335 depth = BestMoveChanges = 0;
336 bestValue = delta = -VALUE_INFINITE;
337 ss->currentMove = MOVE_NULL; // Hack to skip update gains
339 // Iterative deepening loop until requested to stop or target depth reached
340 while (!Signals.stop && ++depth <= MAX_PLY && (!Limits.depth || depth <= Limits.depth))
342 // Save last iteration's scores before first PV line is searched and all
343 // the move scores but the (new) PV are set to -VALUE_INFINITE.
344 for (size_t i = 0; i < RootMoves.size(); i++)
345 RootMoves[i].prevScore = RootMoves[i].score;
347 prevBestMoveChanges = BestMoveChanges;
350 // MultiPV loop. We perform a full root search for each PV line
351 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
353 // Set aspiration window default width
354 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
357 alpha = RootMoves[PVIdx].prevScore - delta;
358 beta = RootMoves[PVIdx].prevScore + delta;
362 alpha = -VALUE_INFINITE;
363 beta = VALUE_INFINITE;
366 // Start with a small aspiration window and, in case of fail high/low,
367 // research with bigger window until not failing high/low anymore.
370 // Search starts from ss+1 to allow referencing (ss-1). This is
371 // needed by update gains and ss copy when splitting at Root.
372 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
374 // Bring to front the best move. It is critical that sorting is
375 // done with a stable algorithm because all the values but the first
376 // and eventually the new best one are set to -VALUE_INFINITE and
377 // we want to keep the same order for all the moves but the new
378 // PV that goes to the front. Note that in case of MultiPV search
379 // the already searched PV lines are preserved.
380 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
382 // In case we have found an exact score and we are going to leave
383 // the fail high/low loop then reorder the PV moves, otherwise
384 // leave the last PV move in its position so to be searched again.
385 // Of course this is needed only in MultiPV search.
386 if (PVIdx && bestValue > alpha && bestValue < beta)
387 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
389 // Write PV back to transposition table in case the relevant
390 // entries have been overwritten during the search.
391 for (size_t i = 0; i <= PVIdx; i++)
392 RootMoves[i].insert_pv_in_tt(pos);
394 // If search has been stopped exit the aspiration window loop.
395 // Sorting and writing PV back to TT is safe becuase RootMoves
396 // is still valid, although refers to previous iteration.
400 // Send full PV info to GUI if we are going to leave the loop or
401 // if we have a fail high/low and we are deep in the search.
402 if ((bestValue > alpha && bestValue < beta) || Time::now() - SearchTime > 2000)
403 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
405 // In case of failing high/low increase aspiration window and
406 // research, otherwise exit the fail high/low loop.
407 if (bestValue >= beta)
412 else if (bestValue <= alpha)
414 Signals.failedLowAtRoot = true;
415 Signals.stopOnPonderhit = false;
423 // Search with full window in case we have a win/mate score
424 if (abs(bestValue) >= VALUE_KNOWN_WIN)
426 alpha = -VALUE_INFINITE;
427 beta = VALUE_INFINITE;
430 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
434 // Skills: Do we need to pick now the best move ?
435 if (SkillLevelEnabled && depth == 1 + SkillLevel)
436 skillBest = do_skill_level();
438 if (!Signals.stop && Options["Use Search Log"])
440 Log log(Options["Search Log Filename"]);
441 log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
445 // Filter out startup noise when monitoring best move stability
446 if (depth > 2 && BestMoveChanges)
447 bestMoveNeverChanged = false;
449 // Do we have time for the next iteration? Can we stop searching now?
450 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.use_time_management())
452 bool stop = false; // Local variable, not the volatile Signals.stop
454 // Take in account some extra time if the best move has changed
455 if (depth > 4 && depth < 50)
456 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
458 // Stop search if most of available time is already consumed. We
459 // probably don't have enough time to search the first move at the
460 // next iteration anyway.
461 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
464 // Stop search early if one move seems to be much better than others
467 && ( (bestMoveNeverChanged && pos.captured_piece_type())
468 || Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 100))
470 Value rBeta = bestValue - EasyMoveMargin;
471 (ss+1)->excludedMove = RootMoves[0].pv[0];
472 (ss+1)->skipNullMove = true;
473 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
474 (ss+1)->skipNullMove = false;
475 (ss+1)->excludedMove = MOVE_NONE;
483 // If we are allowed to ponder do not stop the search now but
484 // keep pondering until GUI sends "ponderhit" or "stop".
486 Signals.stopOnPonderhit = true;
493 // When using skills swap best PV line with the sub-optimal one
494 if (SkillLevelEnabled)
496 if (skillBest == MOVE_NONE) // Still unassigned ?
497 skillBest = do_skill_level();
499 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), skillBest));
504 // search<>() is the main search function for both PV and non-PV nodes and for
505 // normal and SplitPoint nodes. When called just after a split point the search
506 // is simpler because we have already probed the hash table, done a null move
507 // search, and searched the first move before splitting, we don't have to repeat
508 // all this work again. We also don't need to store anything to the hash table
509 // here: This is taken care of after we return from the split point.
511 template <NodeType NT>
512 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
514 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
515 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
516 const bool RootNode = (NT == Root || NT == SplitPointRoot);
518 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
519 assert(PvNode || (alpha == beta - 1));
520 assert(depth > DEPTH_ZERO);
522 Move movesSearched[64];
527 Move ttMove, move, excludedMove, bestMove, threatMove;
530 Value bestValue, value, oldAlpha, ttValue;
531 Value refinedValue, nullValue, futilityValue;
532 bool pvMove, inCheck, singularExtensionNode, givesCheck;
533 bool captureOrPromotion, dangerous, doFullDepthSearch;
534 int moveCount, playedMoveCount;
536 Thread* thisThread = pos.this_thread();
537 moveCount = playedMoveCount = 0;
539 inCheck = pos.in_check();
540 ss->ply = (ss-1)->ply + 1;
542 // Used to send selDepth info to GUI
543 if (PvNode && thisThread->maxPly < ss->ply)
544 thisThread->maxPly = ss->ply;
546 // Step 1. Initialize node
550 ttMove = excludedMove = MOVE_NONE;
551 ttValue = VALUE_NONE;
553 bestMove = sp->bestMove;
554 threatMove = sp->threatMove;
555 bestValue = sp->bestValue;
557 assert(bestValue > -VALUE_INFINITE && sp->moveCount > 0);
559 goto split_point_start;
563 bestValue = -VALUE_INFINITE;
564 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
565 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
566 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
569 // Enforce node limit here. FIXME: This only works with 1 search thread.
570 if (Limits.nodes && pos.nodes_searched() >= Limits.nodes)
575 // Step 2. Check for aborted search and immediate draw
576 if (Signals.stop || pos.is_draw<false>() || ss->ply > MAX_PLY)
579 // Step 3. Mate distance pruning. Even if we mate at the next move our score
580 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
581 // a shorter mate was found upward in the tree then there is no need to search
582 // further, we will never beat current alpha. Same logic but with reversed signs
583 // applies also in the opposite condition of being mated instead of giving mate,
584 // in this case return a fail-high score.
585 alpha = std::max(mated_in(ss->ply), alpha);
586 beta = std::min(mate_in(ss->ply+1), beta);
591 // Step 4. Transposition table lookup
592 // We don't want the score of a partial search to overwrite a previous full search
593 // TT value, so we use a different position key in case of an excluded move.
594 excludedMove = ss->excludedMove;
595 posKey = excludedMove ? pos.exclusion_key() : pos.key();
596 tte = TT.probe(posKey);
597 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
598 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
600 // At PV nodes we check for exact scores, while at non-PV nodes we check for
601 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
602 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
603 // we should also update RootMoveList to avoid bogus output.
604 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == BOUND_EXACT
605 : can_return_tt(tte, depth, ttValue, beta)))
608 ss->currentMove = ttMove; // Can be MOVE_NONE
612 && !pos.is_capture_or_promotion(ttMove)
613 && ttMove != ss->killers[0])
615 ss->killers[1] = ss->killers[0];
616 ss->killers[0] = ttMove;
621 // Step 5. Evaluate the position statically and update parent's gain statistics
623 ss->eval = ss->evalMargin = refinedValue = VALUE_NONE;
626 assert(tte->static_value() != VALUE_NONE);
628 ss->eval = tte->static_value();
629 ss->evalMargin = tte->static_value_margin();
630 refinedValue = refine_eval(tte, ttValue, ss->eval);
634 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
635 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
638 // Update gain for the parent non-capture move given the static position
639 // evaluation before and after the move.
640 if ( (move = (ss-1)->currentMove) != MOVE_NULL
641 && (ss-1)->eval != VALUE_NONE
642 && ss->eval != VALUE_NONE
643 && !pos.captured_piece_type()
644 && type_of(move) == NORMAL)
646 Square to = to_sq(move);
647 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
650 // Step 6. Razoring (is omitted in PV nodes)
652 && depth < RazorDepth
654 && refinedValue + razor_margin(depth) < beta
655 && ttMove == MOVE_NONE
656 && abs(beta) < VALUE_MATE_IN_MAX_PLY
657 && !pos.pawn_on_7th(pos.side_to_move()))
659 Value rbeta = beta - razor_margin(depth);
660 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
662 // Logically we should return (v + razor_margin(depth)), but
663 // surprisingly this did slightly weaker in tests.
667 // Step 7. Static null move pruning (is omitted in PV nodes)
668 // We're betting that the opponent doesn't have a move that will reduce
669 // the score by more than futility_margin(depth) if we do a null move.
672 && depth < RazorDepth
674 && refinedValue - futility_margin(depth, 0) >= beta
675 && abs(beta) < VALUE_MATE_IN_MAX_PLY
676 && pos.non_pawn_material(pos.side_to_move()))
677 return refinedValue - futility_margin(depth, 0);
679 // Step 8. Null move search with verification search (is omitted in PV nodes)
684 && refinedValue >= beta
685 && abs(beta) < VALUE_MATE_IN_MAX_PLY
686 && pos.non_pawn_material(pos.side_to_move()))
688 ss->currentMove = MOVE_NULL;
690 // Null move dynamic reduction based on depth
691 Depth R = 3 * ONE_PLY + depth / 4;
693 // Null move dynamic reduction based on value
694 if (refinedValue - PawnValueMg > beta)
697 pos.do_null_move<true>(st);
698 (ss+1)->skipNullMove = true;
699 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
700 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
701 (ss+1)->skipNullMove = false;
702 pos.do_null_move<false>(st);
704 if (nullValue >= beta)
706 // Do not return unproven mate scores
707 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
710 if (depth < 6 * ONE_PLY)
713 // Do verification search at high depths
714 ss->skipNullMove = true;
715 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
716 ss->skipNullMove = false;
723 // The null move failed low, which means that we may be faced with
724 // some kind of threat. If the previous move was reduced, check if
725 // the move that refuted the null move was somehow connected to the
726 // move which was reduced. If a connection is found, return a fail
727 // low score (which will cause the reduced move to fail high in the
728 // parent node, which will trigger a re-search with full depth).
729 threatMove = (ss+1)->currentMove;
731 if ( depth < ThreatDepth
733 && threatMove != MOVE_NONE
734 && connected_moves(pos, (ss-1)->currentMove, threatMove))
739 // Step 9. ProbCut (is omitted in PV nodes)
740 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
741 // and a reduced search returns a value much above beta, we can (almost) safely
742 // prune the previous move.
744 && depth >= RazorDepth + ONE_PLY
747 && excludedMove == MOVE_NONE
748 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
750 Value rbeta = beta + 200;
751 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
753 assert(rdepth >= ONE_PLY);
754 assert((ss-1)->currentMove != MOVE_NONE);
755 assert((ss-1)->currentMove != MOVE_NULL);
757 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
760 while ((move = mp.next_move<false>()) != MOVE_NONE)
761 if (pos.pl_move_is_legal(move, ci.pinned))
763 ss->currentMove = move;
764 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
765 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
772 // Step 10. Internal iterative deepening
773 if ( depth >= IIDDepth[PvNode]
774 && ttMove == MOVE_NONE
775 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
777 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
779 ss->skipNullMove = true;
780 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
781 ss->skipNullMove = false;
783 tte = TT.probe(posKey);
784 ttMove = tte ? tte->move() : MOVE_NONE;
787 split_point_start: // At split points actual search starts from here
789 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
791 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
792 singularExtensionNode = !RootNode
794 && depth >= SingularExtensionDepth[PvNode]
795 && ttMove != MOVE_NONE
796 && !excludedMove // Recursive singular search is not allowed
797 && (tte->type() & BOUND_LOWER)
798 && tte->depth() >= depth - 3 * ONE_PLY;
800 // Step 11. Loop through moves
801 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
802 while ( bestValue < beta
803 && (move = mp.next_move<SpNode>()) != MOVE_NONE
804 && !thisThread->cutoff_occurred()
809 if (move == excludedMove)
812 // At root obey the "searchmoves" option and skip moves not listed in Root
813 // Move List, as a consequence any illegal move is also skipped. In MultiPV
814 // mode we also skip PV moves which have been already searched.
815 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
820 // Shared counter cannot be decremented later if move turns out to be illegal
821 if (!pos.pl_move_is_legal(move, ci.pinned))
824 moveCount = ++sp->moveCount;
832 Signals.firstRootMove = (moveCount == 1);
834 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 2000)
835 sync_cout << "info depth " << depth / ONE_PLY
836 << " currmove " << move_to_uci(move, Chess960)
837 << " currmovenumber " << moveCount + PVIdx << sync_endl;
840 captureOrPromotion = pos.is_capture_or_promotion(move);
841 givesCheck = pos.move_gives_check(move, ci);
842 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
845 // Step 12. Extend checks and, in PV nodes, also dangerous moves
846 if (PvNode && dangerous)
849 else if (givesCheck && pos.see_sign(move) >= 0)
852 // Singular extension search. If all moves but one fail low on a search of
853 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
854 // is singular and should be extended. To verify this we do a reduced search
855 // on all the other moves but the ttMove, if result is lower than ttValue minus
856 // a margin then we extend ttMove.
857 if ( singularExtensionNode
860 && pos.pl_move_is_legal(move, ci.pinned)
861 && abs(ttValue) < VALUE_KNOWN_WIN)
863 Value rBeta = ttValue - int(depth);
864 ss->excludedMove = move;
865 ss->skipNullMove = true;
866 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
867 ss->skipNullMove = false;
868 ss->excludedMove = MOVE_NONE;
874 // Update current move (this must be done after singular extension search)
875 newDepth = depth - ONE_PLY + ext;
877 // Step 13. Futility pruning (is omitted in PV nodes)
879 && !captureOrPromotion
883 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
885 // Move count based pruning
886 if ( moveCount >= futility_move_count(depth)
887 && (!threatMove || !connected_threat(pos, move, threatMove)))
895 // Value based pruning
896 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
897 // but fixing this made program slightly weaker.
898 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
899 futilityValue = ss->eval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
900 + H.gain(pos.piece_moved(move), to_sq(move));
902 if (futilityValue < beta)
910 // Prune moves with negative SEE at low depths
911 if ( predictedDepth < 2 * ONE_PLY
912 && pos.see_sign(move) < 0)
921 // Check for legality only before to do the move
922 if (!pos.pl_move_is_legal(move, ci.pinned))
928 pvMove = PvNode ? moveCount == 1 : false;
929 ss->currentMove = move;
930 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
931 movesSearched[playedMoveCount++] = move;
933 // Step 14. Make the move
934 pos.do_move(move, st, ci, givesCheck);
936 // Step 15. Reduced depth search (LMR). If the move fails high will be
937 // re-searched at full depth.
938 if ( depth > 3 * ONE_PLY
940 && !captureOrPromotion
942 && ss->killers[0] != move
943 && ss->killers[1] != move)
945 ss->reduction = reduction<PvNode>(depth, moveCount);
946 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
947 alpha = SpNode ? sp->alpha : alpha;
949 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
951 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
952 ss->reduction = DEPTH_ZERO;
955 doFullDepthSearch = !pvMove;
957 // Step 16. Full depth search, when LMR is skipped or fails high
958 if (doFullDepthSearch)
960 alpha = SpNode ? sp->alpha : alpha;
961 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
962 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
965 // Only for PV nodes do a full PV search on the first move or after a fail
966 // high, in the latter case search only if value < beta, otherwise let the
967 // parent node to fail low with value <= alpha and to try another move.
968 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
969 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
970 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
972 // Step 17. Undo move
975 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
977 // Step 18. Check for new best move
981 bestValue = sp->bestValue;
985 // Finished searching the move. If Signals.stop is true, the search
986 // was aborted because the user interrupted the search or because we
987 // ran out of time. In this case, the return value of the search cannot
988 // be trusted, and we don't update the best move and/or PV.
989 if (RootNode && !Signals.stop)
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.
1002 if (!pvMove && MultiPV == 1)
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;
1013 if (value > bestValue)
1020 && value < beta) // We want always alpha < beta
1023 if (SpNode && !thisThread->cutoff_occurred())
1025 sp->bestValue = value;
1026 sp->bestMove = move;
1034 // Step 19. Check for split
1036 && depth >= Threads.min_split_depth()
1038 && Threads.available_slave_exists(thisThread)
1040 && !thisThread->cutoff_occurred())
1041 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1042 depth, threatMove, moveCount, &mp, NT);
1045 // Step 20. Check for mate and stalemate
1046 // All legal moves have been searched and if there are no legal moves, it
1047 // must be mate or stalemate. Note that we can have a false positive in
1048 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1049 // harmless because return value is discarded anyhow in the parent nodes.
1050 // If we are in a singular extension search then return a fail low score.
1052 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1054 // If we have pruned all the moves without searching return a fail-low score
1055 if (bestValue == -VALUE_INFINITE)
1057 assert(!playedMoveCount);
1059 bestValue = oldAlpha;
1062 // Step 21. Update tables
1063 // Update transposition table entry, killers and history
1064 if (!SpNode && !Signals.stop && !thisThread->cutoff_occurred())
1066 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1067 bt = bestValue <= oldAlpha ? BOUND_UPPER
1068 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1070 TT.store(posKey, value_to_tt(bestValue, ss->ply), bt, depth, move, ss->eval, ss->evalMargin);
1072 // Update killers and history for non capture cut-off moves
1073 if ( bestValue >= beta
1074 && !pos.is_capture_or_promotion(move)
1077 if (move != ss->killers[0])
1079 ss->killers[1] = ss->killers[0];
1080 ss->killers[0] = move;
1083 // Increase history value of the cut-off move
1084 Value bonus = Value(int(depth) * int(depth));
1085 H.add(pos.piece_moved(move), to_sq(move), 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);
1096 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1102 // qsearch() is the quiescence search function, which is called by the main
1103 // search function when the remaining depth is zero (or, to be more precise,
1104 // less than ONE_PLY).
1106 template <NodeType NT>
1107 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1109 const bool PvNode = (NT == PV);
1111 assert(NT == PV || NT == NonPV);
1112 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1113 assert((alpha == beta - 1) || PvNode);
1114 assert(depth <= DEPTH_ZERO);
1117 Move ttMove, move, bestMove;
1118 Value ttValue, bestValue, value, evalMargin, futilityValue, futilityBase;
1119 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1123 Value oldAlpha = alpha;
1125 ss->currentMove = bestMove = MOVE_NONE;
1126 ss->ply = (ss-1)->ply + 1;
1128 // Check for an instant draw or maximum ply reached
1129 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1132 // Decide whether or not to include checks, this fixes also the type of
1133 // TT entry depth that we are going to use. Note that in qsearch we use
1134 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1135 inCheck = pos.in_check();
1136 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1138 // Transposition table lookup. At PV nodes, we don't use the TT for
1139 // pruning, but only for move ordering.
1140 tte = TT.probe(pos.key());
1141 ttMove = (tte ? tte->move() : MOVE_NONE);
1142 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_ZERO;
1144 if (!PvNode && tte && can_return_tt(tte, ttDepth, ttValue, beta))
1146 ss->currentMove = ttMove; // Can be MOVE_NONE
1150 // Evaluate the position statically
1153 bestValue = futilityBase = -VALUE_INFINITE;
1154 ss->eval = evalMargin = VALUE_NONE;
1155 enoughMaterial = false;
1161 assert(tte->static_value() != VALUE_NONE);
1163 evalMargin = tte->static_value_margin();
1164 ss->eval = bestValue = tte->static_value();
1167 ss->eval = bestValue = evaluate(pos, evalMargin);
1169 // Stand pat. Return immediately if static value is at least beta
1170 if (bestValue >= beta)
1173 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1178 if (PvNode && bestValue > alpha)
1181 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1182 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1185 // Initialize a MovePicker object for the current position, and prepare
1186 // to search the moves. Because the depth is <= 0 here, only captures,
1187 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1189 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1192 // Loop through the moves until no moves remain or a beta cutoff occurs
1193 while ( bestValue < beta
1194 && (move = mp.next_move<false>()) != MOVE_NONE)
1196 assert(is_ok(move));
1198 givesCheck = pos.move_gives_check(move, ci);
1206 && type_of(move) != PROMOTION
1207 && !pos.is_passed_pawn_push(move))
1209 futilityValue = futilityBase
1210 + PieceValue[Eg][pos.piece_on(to_sq(move))]
1211 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1213 if (futilityValue < beta)
1215 if (futilityValue > bestValue)
1216 bestValue = futilityValue;
1221 // Prune moves with negative or equal SEE
1222 if ( futilityBase < beta
1223 && depth < DEPTH_ZERO
1224 && pos.see(move) <= 0)
1228 // Detect non-capture evasions that are candidate to be pruned
1229 evasionPrunable = !PvNode
1231 && bestValue > VALUE_MATED_IN_MAX_PLY
1232 && !pos.is_capture(move)
1233 && !pos.can_castle(pos.side_to_move());
1235 // Don't search moves with negative SEE values
1237 && (!inCheck || evasionPrunable)
1239 && type_of(move) != PROMOTION
1240 && pos.see_sign(move) < 0)
1243 // Don't search useless checks
1248 && !pos.is_capture_or_promotion(move)
1249 && ss->eval + PawnValueMg / 4 < beta
1250 && !check_is_dangerous(pos, move, futilityBase, beta))
1253 // Check for legality only before to do the move
1254 if (!pos.pl_move_is_legal(move, ci.pinned))
1257 ss->currentMove = move;
1259 // Make and search the move
1260 pos.do_move(move, st, ci, givesCheck);
1261 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1262 pos.undo_move(move);
1264 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1267 if (value > bestValue)
1274 && value < beta) // We want always alpha < beta
1279 // All legal moves have been searched. A special case: If we're in check
1280 // and no legal moves were found, it is checkmate.
1281 if (inCheck && bestValue == -VALUE_INFINITE)
1282 return mated_in(ss->ply); // Plies to mate from the root
1284 // Update transposition table
1285 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1286 bt = bestValue <= oldAlpha ? BOUND_UPPER
1287 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1289 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), bt, ttDepth, move, ss->eval, evalMargin);
1291 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1297 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1298 // bestValue is updated only when returning false because in that case move
1301 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta)
1303 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1304 Square from, to, ksq;
1308 from = from_sq(move);
1310 them = ~pos.side_to_move();
1311 ksq = pos.king_square(them);
1312 kingAtt = pos.attacks_from<KING>(ksq);
1313 pc = pos.piece_moved(move);
1315 occ = pos.pieces() ^ from ^ ksq;
1316 oldAtt = pos.attacks_from(pc, from, occ);
1317 newAtt = pos.attacks_from(pc, to, occ);
1319 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1320 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1322 if (!more_than_one(b))
1325 // Rule 2. Queen contact check is very dangerous
1326 if (type_of(pc) == QUEEN && (kingAtt & to))
1329 // Rule 3. Creating new double threats with checks
1330 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1333 // Note that here we generate illegal "double move"!
1334 if (futilityBase + PieceValue[Eg][pos.piece_on(pop_lsb(&b))] >= beta)
1342 // connected_moves() tests whether two moves are 'connected' in the sense
1343 // that the first move somehow made the second move possible (for instance
1344 // if the moving piece is the same in both moves). The first move is assumed
1345 // to be the move that was made to reach the current position, while the
1346 // second move is assumed to be a move from the current position.
1348 bool connected_moves(const Position& pos, Move m1, Move m2) {
1350 Square f1, t1, f2, t2;
1357 // Case 1: The moving piece is the same in both moves
1363 // Case 2: The destination square for m2 was vacated by m1
1369 // Case 3: Moving through the vacated square
1370 p2 = pos.piece_on(f2);
1371 if (piece_is_slider(p2) && (between_bb(f2, t2) & f1))
1374 // Case 4: The destination square for m2 is defended by the moving piece in m1
1375 p1 = pos.piece_on(t1);
1376 if (pos.attacks_from(p1, t1) & t2)
1379 // Case 5: Discovered check, checking piece is the piece moved in m1
1380 ksq = pos.king_square(pos.side_to_move());
1381 if ( piece_is_slider(p1)
1382 && (between_bb(t1, ksq) & f2)
1383 && (pos.attacks_from(p1, t1, pos.pieces() ^ f2) & ksq))
1390 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1391 // "plies to mate from the current position". Non-mate scores are unchanged.
1392 // The function is called before storing a value to the transposition table.
1394 Value value_to_tt(Value v, int ply) {
1396 if (v >= VALUE_MATE_IN_MAX_PLY)
1399 if (v <= VALUE_MATED_IN_MAX_PLY)
1406 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1407 // from the transposition table (where refers to the plies to mate/be mated
1408 // from current position) to "plies to mate/be mated from the root".
1410 Value value_from_tt(Value v, int ply) {
1412 if (v >= VALUE_MATE_IN_MAX_PLY)
1415 if (v <= VALUE_MATED_IN_MAX_PLY)
1422 // connected_threat() tests whether it is safe to forward prune a move or if
1423 // is somehow connected to the threat move returned by null search.
1425 bool connected_threat(const Position& pos, Move m, Move threat) {
1428 assert(is_ok(threat));
1429 assert(!pos.is_capture_or_promotion(m));
1430 assert(!pos.is_passed_pawn_push(m));
1432 Square mfrom, mto, tfrom, tto;
1436 tfrom = from_sq(threat);
1437 tto = to_sq(threat);
1439 // Case 1: Don't prune moves which move the threatened piece
1443 // Case 2: If the threatened piece has value less than or equal to the
1444 // value of the threatening piece, don't prune moves which defend it.
1445 if ( pos.is_capture(threat)
1446 && ( PieceValue[Mg][pos.piece_on(tfrom)] >= PieceValue[Mg][pos.piece_on(tto)]
1447 || type_of(pos.piece_on(tfrom)) == KING)
1448 && pos.move_attacks_square(m, tto))
1451 // Case 3: If the moving piece in the threatened move is a slider, don't
1452 // prune safe moves which block its ray.
1453 if ( piece_is_slider(pos.piece_on(tfrom))
1454 && (between_bb(tfrom, tto) & mto)
1455 && pos.see_sign(m) >= 0)
1462 // can_return_tt() returns true if a transposition table score can be used to
1463 // cut-off at a given point in search.
1465 bool can_return_tt(const TTEntry* tte, Depth depth, Value v, Value beta) {
1467 return ( tte->depth() >= depth
1468 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1469 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1471 && ( ((tte->type() & BOUND_LOWER) && v >= beta)
1472 || ((tte->type() & BOUND_UPPER) && v < beta));
1476 // refine_eval() returns the transposition table score if possible, otherwise
1477 // falls back on static position evaluation.
1479 Value refine_eval(const TTEntry* tte, Value v, Value defaultEval) {
1483 if ( ((tte->type() & BOUND_LOWER) && v >= defaultEval)
1484 || ((tte->type() & BOUND_UPPER) && v < defaultEval))
1491 // When playing with strength handicap choose best move among the MultiPV set
1492 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1494 Move do_skill_level() {
1496 assert(MultiPV > 1);
1500 // PRNG sequence should be not deterministic
1501 for (int i = Time::now() % 50; i > 0; i--)
1502 rk.rand<unsigned>();
1504 // RootMoves are already sorted by score in descending order
1505 size_t size = std::min(MultiPV, RootMoves.size());
1506 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMg);
1507 int weakness = 120 - 2 * SkillLevel;
1508 int max_s = -VALUE_INFINITE;
1509 Move best = MOVE_NONE;
1511 // Choose best move. For each move score we add two terms both dependent on
1512 // weakness, one deterministic and bigger for weaker moves, and one random,
1513 // then we choose the move with the resulting highest score.
1514 for (size_t i = 0; i < size; i++)
1516 int s = RootMoves[i].score;
1518 // Don't allow crazy blunders even at very low skills
1519 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1522 // This is our magic formula
1523 s += ( weakness * int(RootMoves[0].score - s)
1524 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1529 best = RootMoves[i].pv[0];
1536 // uci_pv() formats PV information according to UCI protocol. UCI requires
1537 // to send all the PV lines also if are still to be searched and so refer to
1538 // the previous search score.
1540 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1542 std::stringstream s;
1543 Time::point elaspsed = Time::now() - SearchTime + 1;
1546 for (size_t i = 0; i < Threads.size(); i++)
1547 if (Threads[i].maxPly > selDepth)
1548 selDepth = Threads[i].maxPly;
1550 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1552 bool updated = (i <= PVIdx);
1554 if (depth == 1 && !updated)
1557 int d = (updated ? depth : depth - 1);
1558 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1560 if (s.rdbuf()->in_avail())
1563 s << "info depth " << d
1564 << " seldepth " << selDepth
1565 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1566 << " nodes " << pos.nodes_searched()
1567 << " nps " << pos.nodes_searched() * 1000 / elaspsed
1568 << " time " << elaspsed
1569 << " multipv " << i + 1
1572 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1573 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1582 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1583 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1584 /// allow to always have a ponder move even when we fail high at root, and a
1585 /// long PV to print that is important for position analysis.
1587 void RootMove::extract_pv_from_tt(Position& pos) {
1589 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1594 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1598 pos.do_move(m, *st++);
1600 while ( (tte = TT.probe(pos.key())) != NULL
1601 && (m = tte->move()) != MOVE_NONE // Local copy, TT entry could change
1602 && pos.is_pseudo_legal(m)
1603 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1605 && (!pos.is_draw<false>() || ply < 2))
1608 pos.do_move(m, *st++);
1611 pv.push_back(MOVE_NONE);
1613 do pos.undo_move(pv[--ply]); while (ply);
1617 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1618 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1619 /// first, even if the old TT entries have been overwritten.
1621 void RootMove::insert_pv_in_tt(Position& pos) {
1623 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1626 Value v, m = VALUE_NONE;
1629 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1635 // Don't overwrite existing correct entries
1636 if (!tte || tte->move() != pv[ply])
1638 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1639 TT.store(k, VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], v, m);
1641 pos.do_move(pv[ply], *st++);
1643 } while (pv[++ply] != MOVE_NONE);
1645 do pos.undo_move(pv[--ply]); while (ply);
1649 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1651 void Thread::idle_loop() {
1653 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1654 // object for which the thread is the master.
1655 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1657 assert(!sp_master || (sp_master->master == this && is_searching));
1659 // If this thread is the master of a split point and all slaves have
1660 // finished their work at this split point, return from the idle loop.
1661 while (!sp_master || sp_master->slavesMask)
1663 // If we are not searching, wait for a condition to be signaled
1664 // instead of wasting CPU time polling for work.
1667 || (!is_searching && Threads.use_sleeping_threads()))
1675 // Grab the lock to avoid races with Thread::wake_up()
1678 // If we are master and all slaves have finished don't go to sleep
1679 if (sp_master && !sp_master->slavesMask)
1685 // Do sleep after retesting sleep conditions under lock protection, in
1686 // particular we need to avoid a deadlock in case a master thread has,
1687 // in the meanwhile, allocated us and sent the wake_up() call before we
1688 // had the chance to grab the lock.
1689 if (do_sleep || !is_searching)
1690 sleepCondition.wait(mutex);
1695 // If this thread has been assigned work, launch a search
1698 assert(!do_sleep && !do_exit);
1700 Threads.mutex.lock();
1702 assert(is_searching);
1703 SplitPoint* sp = curSplitPoint;
1705 Threads.mutex.unlock();
1707 Stack ss[MAX_PLY_PLUS_2];
1708 Position pos(*sp->pos, this);
1710 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1715 if (sp->nodeType == Root)
1716 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1717 else if (sp->nodeType == PV)
1718 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1719 else if (sp->nodeType == NonPV)
1720 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1724 assert(is_searching);
1726 is_searching = false;
1727 sp->slavesMask &= ~(1ULL << idx);
1728 sp->nodes += pos.nodes_searched();
1730 // Wake up master thread so to allow it to return from the idle loop in
1731 // case we are the last slave of the split point.
1732 if ( Threads.use_sleeping_threads()
1733 && this != sp->master
1736 assert(!sp->master->is_searching);
1737 sp->master->wake_up();
1740 // After releasing the lock we cannot access anymore any SplitPoint
1741 // related data in a safe way becuase it could have been released under
1742 // our feet by the sp master. Also accessing other Thread objects is
1743 // unsafe because if we are exiting there is a chance are already freed.
1750 /// check_time() is called by the timer thread when the timer triggers. It is
1751 /// used to print debug info and, more important, to detect when we are out of
1752 /// available time and so stop the search.
1756 static Time::point lastInfoTime = Time::now();
1758 if (Time::now() - lastInfoTime >= 1000)
1760 lastInfoTime = Time::now();
1767 Time::point elapsed = Time::now() - SearchTime;
1768 bool stillAtFirstMove = Signals.firstRootMove
1769 && !Signals.failedLowAtRoot
1770 && elapsed > TimeMgr.available_time();
1772 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1773 || stillAtFirstMove;
1775 if ( (Limits.use_time_management() && noMoreTime)
1776 || (Limits.movetime && elapsed >= Limits.movetime))
1777 Signals.stop = true;