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;
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 Book 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 int e = SearchTime.elapsed();
295 Log log(Options["Search Log Filename"]);
296 log << "Nodes: " << pos.nodes_searched()
297 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
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.
369 // Search starts from ss+1 to allow referencing (ss-1). This is
370 // needed by update gains and ss copy when splitting at Root.
371 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
373 // Bring to front the best move. It is critical that sorting is
374 // done with a stable algorithm because all the values but the first
375 // and eventually the new best one are set to -VALUE_INFINITE and
376 // we want to keep the same order for all the moves but the new
377 // PV that goes to the front. Note that in case of MultiPV search
378 // the already searched PV lines are preserved.
379 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
381 // In case we have found an exact score and we are going to leave
382 // the fail high/low loop then reorder the PV moves, otherwise
383 // leave the last PV move in its position so to be searched again.
384 // Of course this is needed only in MultiPV search.
385 if (PVIdx && bestValue > alpha && bestValue < beta)
386 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
388 // Write PV back to transposition table in case the relevant
389 // entries have been overwritten during the search.
390 for (size_t i = 0; i <= PVIdx; i++)
391 RootMoves[i].insert_pv_in_tt(pos);
393 // If search has been stopped exit the aspiration window loop.
394 // Sorting and writing PV back to TT is safe becuase RootMoves
395 // is still valid, although refers to previous iteration.
399 // Send full PV info to GUI if we are going to leave the loop or
400 // if we have a fail high/low and we are deep in the search.
401 if ((bestValue > alpha && bestValue < beta) || SearchTime.elapsed() > 2000)
402 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
404 // In case of failing high/low increase aspiration window and
405 // research, otherwise exit the fail high/low loop.
406 if (bestValue >= beta)
411 else if (bestValue <= alpha)
413 Signals.failedLowAtRoot = true;
414 Signals.stopOnPonderhit = false;
422 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
424 } while (abs(bestValue) < VALUE_KNOWN_WIN);
427 // Skills: Do we need to pick now the best move ?
428 if (SkillLevelEnabled && depth == 1 + SkillLevel)
429 skillBest = do_skill_level();
431 if (!Signals.stop && Options["Use Search Log"])
433 Log log(Options["Search Log Filename"]);
434 log << pretty_pv(pos, depth, bestValue, SearchTime.elapsed(), &RootMoves[0].pv[0])
438 // Filter out startup noise when monitoring best move stability
439 if (depth > 2 && BestMoveChanges)
440 bestMoveNeverChanged = false;
442 // Do we have time for the next iteration? Can we stop searching now?
443 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.use_time_management())
445 bool stop = false; // Local variable, not the volatile Signals.stop
447 // Take in account some extra time if the best move has changed
448 if (depth > 4 && depth < 50)
449 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
451 // Stop search if most of available time is already consumed. We
452 // probably don't have enough time to search the first move at the
453 // next iteration anyway.
454 if (SearchTime.elapsed() > (TimeMgr.available_time() * 62) / 100)
457 // Stop search early if one move seems to be much better than others
460 && ( (bestMoveNeverChanged && pos.captured_piece_type())
461 || SearchTime.elapsed() > (TimeMgr.available_time() * 40) / 100))
463 Value rBeta = bestValue - EasyMoveMargin;
464 (ss+1)->excludedMove = RootMoves[0].pv[0];
465 (ss+1)->skipNullMove = true;
466 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
467 (ss+1)->skipNullMove = false;
468 (ss+1)->excludedMove = MOVE_NONE;
476 // If we are allowed to ponder do not stop the search now but
477 // keep pondering until GUI sends "ponderhit" or "stop".
479 Signals.stopOnPonderhit = true;
486 // When using skills swap best PV line with the sub-optimal one
487 if (SkillLevelEnabled)
489 if (skillBest == MOVE_NONE) // Still unassigned ?
490 skillBest = do_skill_level();
492 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), skillBest));
497 // search<>() is the main search function for both PV and non-PV nodes and for
498 // normal and SplitPoint nodes. When called just after a split point the search
499 // is simpler because we have already probed the hash table, done a null move
500 // search, and searched the first move before splitting, we don't have to repeat
501 // all this work again. We also don't need to store anything to the hash table
502 // here: This is taken care of after we return from the split point.
504 template <NodeType NT>
505 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
507 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
508 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
509 const bool RootNode = (NT == Root || NT == SplitPointRoot);
511 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
512 assert((alpha == beta - 1) || PvNode);
513 assert(depth > DEPTH_ZERO);
515 Move movesSearched[64];
519 Move ttMove, move, excludedMove, bestMove, threatMove;
522 Value bestValue, value, oldAlpha, ttValue;
523 Value refinedValue, nullValue, futilityBase, futilityValue;
524 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
525 bool captureOrPromotion, dangerous, doFullDepthSearch;
526 int moveCount = 0, playedMoveCount = 0;
527 Thread* thisThread = pos.this_thread();
528 SplitPoint* sp = NULL;
530 refinedValue = bestValue = value = -VALUE_INFINITE;
532 inCheck = pos.in_check();
533 ss->ply = (ss-1)->ply + 1;
535 // Used to send selDepth info to GUI
536 if (PvNode && thisThread->maxPly < ss->ply)
537 thisThread->maxPly = ss->ply;
539 // Step 1. Initialize node
543 ttMove = excludedMove = MOVE_NONE;
544 ttValue = VALUE_ZERO;
546 bestMove = sp->bestMove;
547 threatMove = sp->threatMove;
548 bestValue = sp->bestValue;
549 moveCount = sp->moveCount; // Lock must be held here
551 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
553 goto split_point_start;
557 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
558 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
559 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
563 // Step 2. Check for aborted search and immediate draw
564 // Enforce node limit here. FIXME: This only works with 1 search thread.
565 if (Limits.nodes && pos.nodes_searched() >= Limits.nodes)
569 || pos.is_draw<false>()
570 || ss->ply > MAX_PLY) && !RootNode)
573 // Step 3. Mate distance pruning. Even if we mate at the next move our score
574 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
575 // a shorter mate was found upward in the tree then there is no need to search
576 // further, we will never beat current alpha. Same logic but with reversed signs
577 // applies also in the opposite condition of being mated instead of giving mate,
578 // in this case return a fail-high score.
581 alpha = std::max(mated_in(ss->ply), alpha);
582 beta = std::min(mate_in(ss->ply+1), beta);
587 // Step 4. Transposition table lookup
588 // We don't want the score of a partial search to overwrite a previous full search
589 // TT value, so we use a different position key in case of an excluded move.
590 excludedMove = ss->excludedMove;
591 posKey = excludedMove ? pos.exclusion_key() : pos.key();
592 tte = TT.probe(posKey);
593 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
594 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_ZERO;
596 // At PV nodes we check for exact scores, while at non-PV nodes we check for
597 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
598 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
599 // we should also update RootMoveList to avoid bogus output.
600 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == BOUND_EXACT
601 : can_return_tt(tte, depth, ttValue, beta)))
604 ss->currentMove = ttMove; // Can be MOVE_NONE
608 && !pos.is_capture_or_promotion(ttMove)
609 && ttMove != ss->killers[0])
611 ss->killers[1] = ss->killers[0];
612 ss->killers[0] = ttMove;
617 // Step 5. Evaluate the position statically and update parent's gain statistics
619 ss->eval = ss->evalMargin = VALUE_NONE;
622 assert(tte->static_value() != VALUE_NONE);
624 ss->eval = tte->static_value();
625 ss->evalMargin = tte->static_value_margin();
626 refinedValue = refine_eval(tte, ttValue, ss->eval);
630 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
631 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
634 // Update gain for the parent non-capture move given the static position
635 // evaluation before and after the move.
636 if ( (move = (ss-1)->currentMove) != MOVE_NULL
637 && (ss-1)->eval != VALUE_NONE
638 && ss->eval != VALUE_NONE
639 && !pos.captured_piece_type()
640 && type_of(move) == NORMAL)
642 Square to = to_sq(move);
643 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
646 // Step 6. Razoring (is omitted in PV nodes)
648 && depth < RazorDepth
650 && refinedValue + razor_margin(depth) < beta
651 && ttMove == MOVE_NONE
652 && abs(beta) < VALUE_MATE_IN_MAX_PLY
653 && !pos.pawn_on_7th(pos.side_to_move()))
655 Value rbeta = beta - razor_margin(depth);
656 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
658 // Logically we should return (v + razor_margin(depth)), but
659 // surprisingly this did slightly weaker in tests.
663 // Step 7. Static null move pruning (is omitted in PV nodes)
664 // We're betting that the opponent doesn't have a move that will reduce
665 // the score by more than futility_margin(depth) if we do a null move.
668 && depth < RazorDepth
670 && refinedValue - futility_margin(depth, 0) >= beta
671 && abs(beta) < VALUE_MATE_IN_MAX_PLY
672 && pos.non_pawn_material(pos.side_to_move()))
673 return refinedValue - futility_margin(depth, 0);
675 // Step 8. Null move search with verification search (is omitted in PV nodes)
680 && refinedValue >= beta
681 && abs(beta) < VALUE_MATE_IN_MAX_PLY
682 && pos.non_pawn_material(pos.side_to_move()))
684 ss->currentMove = MOVE_NULL;
686 // Null move dynamic reduction based on depth
687 Depth R = 3 * ONE_PLY + depth / 4;
689 // Null move dynamic reduction based on value
690 if (refinedValue - PawnValueMg > beta)
693 pos.do_null_move<true>(st);
694 (ss+1)->skipNullMove = true;
695 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
696 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
697 (ss+1)->skipNullMove = false;
698 pos.do_null_move<false>(st);
700 if (nullValue >= beta)
702 // Do not return unproven mate scores
703 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
706 if (depth < 6 * ONE_PLY)
709 // Do verification search at high depths
710 ss->skipNullMove = true;
711 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
712 ss->skipNullMove = false;
719 // The null move failed low, which means that we may be faced with
720 // some kind of threat. If the previous move was reduced, check if
721 // the move that refuted the null move was somehow connected to the
722 // move which was reduced. If a connection is found, return a fail
723 // low score (which will cause the reduced move to fail high in the
724 // parent node, which will trigger a re-search with full depth).
725 threatMove = (ss+1)->currentMove;
727 if ( depth < ThreatDepth
729 && threatMove != MOVE_NONE
730 && connected_moves(pos, (ss-1)->currentMove, threatMove))
735 // Step 9. ProbCut (is omitted in PV nodes)
736 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
737 // and a reduced search returns a value much above beta, we can (almost) safely
738 // prune the previous move.
740 && depth >= RazorDepth + ONE_PLY
743 && excludedMove == MOVE_NONE
744 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
746 Value rbeta = beta + 200;
747 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
749 assert(rdepth >= ONE_PLY);
750 assert((ss-1)->currentMove != MOVE_NONE);
751 assert((ss-1)->currentMove != MOVE_NULL);
753 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
756 while ((move = mp.next_move<false>()) != MOVE_NONE)
757 if (pos.pl_move_is_legal(move, ci.pinned))
759 ss->currentMove = move;
760 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
761 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
768 // Step 10. Internal iterative deepening
769 if ( depth >= IIDDepth[PvNode]
770 && ttMove == MOVE_NONE
771 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
773 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
775 ss->skipNullMove = true;
776 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
777 ss->skipNullMove = false;
779 tte = TT.probe(posKey);
780 ttMove = tte ? tte->move() : MOVE_NONE;
783 split_point_start: // At split points actual search starts from here
785 MovePicker mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
787 futilityBase = ss->eval + ss->evalMargin;
788 singularExtensionNode = !RootNode
790 && depth >= SingularExtensionDepth[PvNode]
791 && ttMove != MOVE_NONE
792 && !excludedMove // Recursive singular search is not allowed
793 && (tte->type() & BOUND_LOWER)
794 && tte->depth() >= depth - 3 * ONE_PLY;
796 // Step 11. Loop through moves
797 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
798 while ( bestValue < beta
799 && (move = mp.next_move<SpNode>()) != MOVE_NONE
800 && !thisThread->cutoff_occurred()
805 if (move == excludedMove)
808 // At root obey the "searchmoves" option and skip moves not listed in Root
809 // Move List, as a consequence any illegal move is also skipped. In MultiPV
810 // mode we also skip PV moves which have been already searched.
811 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
814 // At PV and SpNode nodes we want all moves to be legal since the beginning
815 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
820 moveCount = ++sp->moveCount;
828 Signals.firstRootMove = (moveCount == 1);
830 if (thisThread == Threads.main_thread() && SearchTime.elapsed() > 2000)
831 sync_cout << "info depth " << depth / ONE_PLY
832 << " currmove " << move_to_uci(move, Chess960)
833 << " currmovenumber " << moveCount + PVIdx << sync_endl;
836 isPvMove = (PvNode && moveCount <= 1);
837 captureOrPromotion = pos.is_capture_or_promotion(move);
838 givesCheck = pos.move_gives_check(move, ci);
839 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
842 // Step 12. Extend checks and, in PV nodes, also dangerous moves
843 if (PvNode && dangerous)
846 else if (givesCheck && pos.see_sign(move) >= 0)
849 // Singular extension search. If all moves but one fail low on a search of
850 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
851 // is singular and should be extended. To verify this we do a reduced search
852 // on all the other moves but the ttMove, if result is lower than ttValue minus
853 // a margin then we extend ttMove.
854 if ( singularExtensionNode
857 && pos.pl_move_is_legal(move, ci.pinned)
858 && abs(ttValue) < VALUE_KNOWN_WIN)
860 Value rBeta = ttValue - int(depth);
861 ss->excludedMove = move;
862 ss->skipNullMove = true;
863 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
864 ss->skipNullMove = false;
865 ss->excludedMove = MOVE_NONE;
871 // Update current move (this must be done after singular extension search)
872 newDepth = depth - ONE_PLY + ext;
874 // Step 13. Futility pruning (is omitted in PV nodes)
876 && !captureOrPromotion
880 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
882 // Move count based pruning
883 if ( moveCount >= futility_move_count(depth)
884 && (!threatMove || !connected_threat(pos, move, threatMove)))
892 // Value based pruning
893 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
894 // but fixing this made program slightly weaker.
895 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
896 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
897 + H.gain(pos.piece_moved(move), to_sq(move));
899 if (futilityValue < beta)
907 // Prune moves with negative SEE at low depths
908 if ( predictedDepth < 2 * ONE_PLY
909 && pos.see_sign(move) < 0)
918 // Check for legality only before to do the move
919 if (!pos.pl_move_is_legal(move, ci.pinned))
925 ss->currentMove = move;
926 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
927 movesSearched[playedMoveCount++] = move;
929 // Step 14. Make the move
930 pos.do_move(move, st, ci, givesCheck);
932 // Step 15. Reduced depth search (LMR). If the move fails high will be
933 // re-searched at full depth.
934 if ( depth > 3 * ONE_PLY
936 && !captureOrPromotion
938 && ss->killers[0] != move
939 && ss->killers[1] != move)
941 ss->reduction = reduction<PvNode>(depth, moveCount);
942 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
943 alpha = SpNode ? sp->alpha : alpha;
945 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
947 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
948 ss->reduction = DEPTH_ZERO;
951 doFullDepthSearch = !isPvMove;
953 // Step 16. Full depth search, when LMR is skipped or fails high
954 if (doFullDepthSearch)
956 alpha = SpNode ? sp->alpha : alpha;
957 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
958 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
961 // Only for PV nodes do a full PV search on the first move or after a fail
962 // high, in the latter case search only if value < beta, otherwise let the
963 // parent node to fail low with value <= alpha and to try another move.
964 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
965 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
966 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
968 // Step 17. Undo move
971 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
973 // Step 18. Check for new best move
977 bestValue = sp->bestValue;
981 // Finished searching the move. If Signals.stop is true, the search
982 // was aborted because the user interrupted the search or because we
983 // ran out of time. In this case, the return value of the search cannot
984 // be trusted, and we don't update the best move and/or PV.
985 if (RootNode && !Signals.stop)
987 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
989 // PV move or new best move ?
990 if (isPvMove || value > alpha)
993 rm.extract_pv_from_tt(pos);
995 // We record how often the best move has been changed in each
996 // iteration. This information is used for time management: When
997 // the best move changes frequently, we allocate some more time.
998 if (!isPvMove && MultiPV == 1)
1002 // All other moves but the PV are set to the lowest value, this
1003 // is not a problem when sorting becuase sort is stable and move
1004 // position in the list is preserved, just the PV is pushed up.
1005 rm.score = -VALUE_INFINITE;
1009 if (value > bestValue)
1016 && value < beta) // We want always alpha < beta
1019 if (SpNode && !thisThread->cutoff_occurred())
1021 sp->bestValue = value;
1022 sp->bestMove = move;
1030 // Step 19. Check for split
1032 && depth >= Threads.min_split_depth()
1034 && Threads.available_slave_exists(thisThread)
1036 && !thisThread->cutoff_occurred())
1037 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1038 depth, threatMove, moveCount, &mp, NT);
1041 // Step 20. Check for mate and stalemate
1042 // All legal moves have been searched and if there are no legal moves, it
1043 // must be mate or stalemate. Note that we can have a false positive in
1044 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1045 // harmless because return value is discarded anyhow in the parent nodes.
1046 // If we are in a singular extension search then return a fail low score.
1048 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1050 // If we have pruned all the moves without searching return a fail-low score
1051 if (bestValue == -VALUE_INFINITE)
1053 assert(!playedMoveCount);
1055 bestValue = oldAlpha;
1058 // Step 21. Update tables
1059 // Update transposition table entry, killers and history
1060 if (!SpNode && !Signals.stop && !thisThread->cutoff_occurred())
1062 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1063 bt = bestValue <= oldAlpha ? BOUND_UPPER
1064 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1066 TT.store(posKey, value_to_tt(bestValue, ss->ply), bt, depth, move, ss->eval, ss->evalMargin);
1068 // Update killers and history for non capture cut-off moves
1069 if ( bestValue >= beta
1070 && !pos.is_capture_or_promotion(move)
1073 if (move != ss->killers[0])
1075 ss->killers[1] = ss->killers[0];
1076 ss->killers[0] = move;
1079 // Increase history value of the cut-off move
1080 Value bonus = Value(int(depth) * int(depth));
1081 H.add(pos.piece_moved(move), to_sq(move), bonus);
1083 // Decrease history of all the other played non-capture moves
1084 for (int i = 0; i < playedMoveCount - 1; i++)
1086 Move m = movesSearched[i];
1087 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1092 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1098 // qsearch() is the quiescence search function, which is called by the main
1099 // search function when the remaining depth is zero (or, to be more precise,
1100 // less than ONE_PLY).
1102 template <NodeType NT>
1103 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1105 const bool PvNode = (NT == PV);
1107 assert(NT == PV || NT == NonPV);
1108 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1109 assert((alpha == beta - 1) || PvNode);
1110 assert(depth <= DEPTH_ZERO);
1113 Move ttMove, move, bestMove;
1114 Value ttValue, bestValue, value, evalMargin, futilityValue, futilityBase;
1115 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1119 Value oldAlpha = alpha;
1121 ss->currentMove = bestMove = MOVE_NONE;
1122 ss->ply = (ss-1)->ply + 1;
1124 // Check for an instant draw or maximum ply reached
1125 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1128 // Decide whether or not to include checks, this fixes also the type of
1129 // TT entry depth that we are going to use. Note that in qsearch we use
1130 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1131 inCheck = pos.in_check();
1132 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1134 // Transposition table lookup. At PV nodes, we don't use the TT for
1135 // pruning, but only for move ordering.
1136 tte = TT.probe(pos.key());
1137 ttMove = (tte ? tte->move() : MOVE_NONE);
1138 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_ZERO;
1140 if (!PvNode && tte && can_return_tt(tte, ttDepth, ttValue, beta))
1142 ss->currentMove = ttMove; // Can be MOVE_NONE
1146 // Evaluate the position statically
1149 bestValue = futilityBase = -VALUE_INFINITE;
1150 ss->eval = evalMargin = VALUE_NONE;
1151 enoughMaterial = false;
1157 assert(tte->static_value() != VALUE_NONE);
1159 evalMargin = tte->static_value_margin();
1160 ss->eval = bestValue = tte->static_value();
1163 ss->eval = bestValue = evaluate(pos, evalMargin);
1165 // Stand pat. Return immediately if static value is at least beta
1166 if (bestValue >= beta)
1169 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1174 if (PvNode && bestValue > alpha)
1177 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1178 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1181 // Initialize a MovePicker object for the current position, and prepare
1182 // to search the moves. Because the depth is <= 0 here, only captures,
1183 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1185 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1188 // Loop through the moves until no moves remain or a beta cutoff occurs
1189 while ( bestValue < beta
1190 && (move = mp.next_move<false>()) != MOVE_NONE)
1192 assert(is_ok(move));
1194 givesCheck = pos.move_gives_check(move, ci);
1202 && type_of(move) != PROMOTION
1203 && !pos.is_passed_pawn_push(move))
1205 futilityValue = futilityBase
1206 + PieceValue[Eg][pos.piece_on(to_sq(move))]
1207 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1209 if (futilityValue < beta)
1211 if (futilityValue > bestValue)
1212 bestValue = futilityValue;
1217 // Prune moves with negative or equal SEE
1218 if ( futilityBase < beta
1219 && depth < DEPTH_ZERO
1220 && pos.see(move) <= 0)
1224 // Detect non-capture evasions that are candidate to be pruned
1225 evasionPrunable = !PvNode
1227 && bestValue > VALUE_MATED_IN_MAX_PLY
1228 && !pos.is_capture(move)
1229 && !pos.can_castle(pos.side_to_move());
1231 // Don't search moves with negative SEE values
1233 && (!inCheck || evasionPrunable)
1235 && type_of(move) != PROMOTION
1236 && pos.see_sign(move) < 0)
1239 // Don't search useless checks
1244 && !pos.is_capture_or_promotion(move)
1245 && ss->eval + PawnValueMg / 4 < beta
1246 && !check_is_dangerous(pos, move, futilityBase, beta))
1249 // Check for legality only before to do the move
1250 if (!pos.pl_move_is_legal(move, ci.pinned))
1253 ss->currentMove = move;
1255 // Make and search the move
1256 pos.do_move(move, st, ci, givesCheck);
1257 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1258 pos.undo_move(move);
1260 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1263 if (value > bestValue)
1270 && value < beta) // We want always alpha < beta
1275 // All legal moves have been searched. A special case: If we're in check
1276 // and no legal moves were found, it is checkmate.
1277 if (inCheck && bestValue == -VALUE_INFINITE)
1278 return mated_in(ss->ply); // Plies to mate from the root
1280 // Update transposition table
1281 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1282 bt = bestValue <= oldAlpha ? BOUND_UPPER
1283 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1285 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), bt, ttDepth, move, ss->eval, evalMargin);
1287 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1293 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1294 // bestValue is updated only when returning false because in that case move
1297 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta)
1299 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1300 Square from, to, ksq;
1304 from = from_sq(move);
1306 them = ~pos.side_to_move();
1307 ksq = pos.king_square(them);
1308 kingAtt = pos.attacks_from<KING>(ksq);
1309 pc = pos.piece_moved(move);
1311 occ = pos.pieces() ^ from ^ ksq;
1312 oldAtt = pos.attacks_from(pc, from, occ);
1313 newAtt = pos.attacks_from(pc, to, occ);
1315 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1316 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1318 if (!more_than_one(b))
1321 // Rule 2. Queen contact check is very dangerous
1322 if (type_of(pc) == QUEEN && (kingAtt & to))
1325 // Rule 3. Creating new double threats with checks
1326 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1329 // Note that here we generate illegal "double move"!
1330 if (futilityBase + PieceValue[Eg][pos.piece_on(pop_lsb(&b))] >= beta)
1338 // connected_moves() tests whether two moves are 'connected' in the sense
1339 // that the first move somehow made the second move possible (for instance
1340 // if the moving piece is the same in both moves). The first move is assumed
1341 // to be the move that was made to reach the current position, while the
1342 // second move is assumed to be a move from the current position.
1344 bool connected_moves(const Position& pos, Move m1, Move m2) {
1346 Square f1, t1, f2, t2;
1353 // Case 1: The moving piece is the same in both moves
1359 // Case 2: The destination square for m2 was vacated by m1
1365 // Case 3: Moving through the vacated square
1366 p2 = pos.piece_on(f2);
1367 if (piece_is_slider(p2) && (between_bb(f2, t2) & f1))
1370 // Case 4: The destination square for m2 is defended by the moving piece in m1
1371 p1 = pos.piece_on(t1);
1372 if (pos.attacks_from(p1, t1) & t2)
1375 // Case 5: Discovered check, checking piece is the piece moved in m1
1376 ksq = pos.king_square(pos.side_to_move());
1377 if ( piece_is_slider(p1)
1378 && (between_bb(t1, ksq) & f2)
1379 && (pos.attacks_from(p1, t1, pos.pieces() ^ f2) & ksq))
1386 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1387 // "plies to mate from the current position". Non-mate scores are unchanged.
1388 // The function is called before storing a value to the transposition table.
1390 Value value_to_tt(Value v, int ply) {
1392 if (v >= VALUE_MATE_IN_MAX_PLY)
1395 if (v <= VALUE_MATED_IN_MAX_PLY)
1402 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1403 // from the transposition table (where refers to the plies to mate/be mated
1404 // from current position) to "plies to mate/be mated from the root".
1406 Value value_from_tt(Value v, int ply) {
1408 if (v >= VALUE_MATE_IN_MAX_PLY)
1411 if (v <= VALUE_MATED_IN_MAX_PLY)
1418 // connected_threat() tests whether it is safe to forward prune a move or if
1419 // is somehow connected to the threat move returned by null search.
1421 bool connected_threat(const Position& pos, Move m, Move threat) {
1424 assert(is_ok(threat));
1425 assert(!pos.is_capture_or_promotion(m));
1426 assert(!pos.is_passed_pawn_push(m));
1428 Square mfrom, mto, tfrom, tto;
1432 tfrom = from_sq(threat);
1433 tto = to_sq(threat);
1435 // Case 1: Don't prune moves which move the threatened piece
1439 // Case 2: If the threatened piece has value less than or equal to the
1440 // value of the threatening piece, don't prune moves which defend it.
1441 if ( pos.is_capture(threat)
1442 && ( PieceValue[Mg][pos.piece_on(tfrom)] >= PieceValue[Mg][pos.piece_on(tto)]
1443 || type_of(pos.piece_on(tfrom)) == KING)
1444 && pos.move_attacks_square(m, tto))
1447 // Case 3: If the moving piece in the threatened move is a slider, don't
1448 // prune safe moves which block its ray.
1449 if ( piece_is_slider(pos.piece_on(tfrom))
1450 && (between_bb(tfrom, tto) & mto)
1451 && pos.see_sign(m) >= 0)
1458 // can_return_tt() returns true if a transposition table score can be used to
1459 // cut-off at a given point in search.
1461 bool can_return_tt(const TTEntry* tte, Depth depth, Value v, Value beta) {
1463 return ( tte->depth() >= depth
1464 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1465 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1467 && ( ((tte->type() & BOUND_LOWER) && v >= beta)
1468 || ((tte->type() & BOUND_UPPER) && v < beta));
1472 // refine_eval() returns the transposition table score if possible, otherwise
1473 // falls back on static position evaluation.
1475 Value refine_eval(const TTEntry* tte, Value v, Value defaultEval) {
1479 if ( ((tte->type() & BOUND_LOWER) && v >= defaultEval)
1480 || ((tte->type() & BOUND_UPPER) && v < defaultEval))
1487 // When playing with strength handicap choose best move among the MultiPV set
1488 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1490 Move do_skill_level() {
1492 assert(MultiPV > 1);
1496 // PRNG sequence should be not deterministic
1497 for (int i = Time::current_time().msec() % 50; i > 0; i--)
1498 rk.rand<unsigned>();
1500 // RootMoves are already sorted by score in descending order
1501 size_t size = std::min(MultiPV, RootMoves.size());
1502 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMg);
1503 int weakness = 120 - 2 * SkillLevel;
1504 int max_s = -VALUE_INFINITE;
1505 Move best = MOVE_NONE;
1507 // Choose best move. For each move score we add two terms both dependent on
1508 // weakness, one deterministic and bigger for weaker moves, and one random,
1509 // then we choose the move with the resulting highest score.
1510 for (size_t i = 0; i < size; i++)
1512 int s = RootMoves[i].score;
1514 // Don't allow crazy blunders even at very low skills
1515 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1518 // This is our magic formula
1519 s += ( weakness * int(RootMoves[0].score - s)
1520 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1525 best = RootMoves[i].pv[0];
1532 // uci_pv() formats PV information according to UCI protocol. UCI requires
1533 // to send all the PV lines also if are still to be searched and so refer to
1534 // the previous search score.
1536 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1538 std::stringstream s;
1539 int t = SearchTime.elapsed();
1542 for (size_t i = 0; i < Threads.size(); i++)
1543 if (Threads[i].maxPly > selDepth)
1544 selDepth = Threads[i].maxPly;
1546 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1548 bool updated = (i <= PVIdx);
1550 if (depth == 1 && !updated)
1553 int d = (updated ? depth : depth - 1);
1554 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1556 if (s.rdbuf()->in_avail())
1559 s << "info depth " << d
1560 << " seldepth " << selDepth
1561 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1562 << " nodes " << pos.nodes_searched()
1563 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1565 << " multipv " << i + 1
1568 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1569 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1578 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1579 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1580 /// allow to always have a ponder move even when we fail high at root, and a
1581 /// long PV to print that is important for position analysis.
1583 void RootMove::extract_pv_from_tt(Position& pos) {
1585 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1590 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1594 pos.do_move(m, *st++);
1596 while ( (tte = TT.probe(pos.key())) != NULL
1597 && (m = tte->move()) != MOVE_NONE // Local copy, TT entry could change
1598 && pos.is_pseudo_legal(m)
1599 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1601 && (!pos.is_draw<false>() || ply < 2))
1604 pos.do_move(m, *st++);
1607 pv.push_back(MOVE_NONE);
1609 do pos.undo_move(pv[--ply]); while (ply);
1613 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1614 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1615 /// first, even if the old TT entries have been overwritten.
1617 void RootMove::insert_pv_in_tt(Position& pos) {
1619 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1622 Value v, m = VALUE_NONE;
1625 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1631 // Don't overwrite existing correct entries
1632 if (!tte || tte->move() != pv[ply])
1634 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1635 TT.store(k, VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], v, m);
1637 pos.do_move(pv[ply], *st++);
1639 } while (pv[++ply] != MOVE_NONE);
1641 do pos.undo_move(pv[--ply]); while (ply);
1645 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1647 void Thread::idle_loop() {
1649 // Pointer 'sp_master', if non-NULL, points to the active SplitPoint
1650 // object for which the thread is the master.
1651 const SplitPoint* sp_master = splitPointsCnt ? curSplitPoint : NULL;
1653 assert(!sp_master || (sp_master->master == this && is_searching));
1655 // If this thread is the master of a split point and all slaves have
1656 // finished their work at this split point, return from the idle loop.
1657 while (!sp_master || sp_master->slavesMask)
1659 // If we are not searching, wait for a condition to be signaled
1660 // instead of wasting CPU time polling for work.
1663 || (!is_searching && Threads.use_sleeping_threads()))
1671 // Grab the lock to avoid races with Thread::wake_up()
1674 // If we are master and all slaves have finished don't go to sleep
1675 if (sp_master && !sp_master->slavesMask)
1681 // Do sleep after retesting sleep conditions under lock protection, in
1682 // particular we need to avoid a deadlock in case a master thread has,
1683 // in the meanwhile, allocated us and sent the wake_up() call before we
1684 // had the chance to grab the lock.
1685 if (do_sleep || !is_searching)
1686 sleepCondition.wait(mutex);
1691 // If this thread has been assigned work, launch a search
1694 assert(!do_sleep && !do_exit);
1696 Threads.mutex.lock();
1698 assert(is_searching);
1699 SplitPoint* sp = curSplitPoint;
1701 Threads.mutex.unlock();
1703 Stack ss[MAX_PLY_PLUS_2];
1704 Position pos(*sp->pos, this);
1706 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1711 if (sp->nodeType == Root)
1712 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1713 else if (sp->nodeType == PV)
1714 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1715 else if (sp->nodeType == NonPV)
1716 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1720 assert(is_searching);
1722 is_searching = false;
1723 sp->slavesMask &= ~(1ULL << idx);
1724 sp->nodes += pos.nodes_searched();
1726 // Wake up master thread so to allow it to return from the idle loop in
1727 // case we are the last slave of the split point.
1728 if ( Threads.use_sleeping_threads()
1729 && this != sp->master
1732 assert(!sp->master->is_searching);
1733 sp->master->wake_up();
1736 // After releasing the lock we cannot access anymore any SplitPoint
1737 // related data in a safe way becuase it could have been released under
1738 // our feet by the sp master. Also accessing other Thread objects is
1739 // unsafe because if we are exiting there is a chance are already freed.
1746 /// check_time() is called by the timer thread when the timer triggers. It is
1747 /// used to print debug info and, more important, to detect when we are out of
1748 /// available time and so stop the search.
1752 static Time lastInfoTime = Time::current_time();
1754 if (lastInfoTime.elapsed() >= 1000)
1756 lastInfoTime = Time::current_time();
1763 int e = SearchTime.elapsed();
1764 bool stillAtFirstMove = Signals.firstRootMove
1765 && !Signals.failedLowAtRoot
1766 && e > TimeMgr.available_time();
1768 bool noMoreTime = e > TimeMgr.maximum_time() - 2 * TimerResolution
1769 || stillAtFirstMove;
1771 if ( (Limits.use_time_management() && noMoreTime)
1772 || (Limits.movetime && e >= Limits.movetime))
1773 Signals.stop = true;