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/>.
38 #include "ucioption.h"
42 volatile SignalsType Signals;
44 std::vector<RootMove> RootMoves;
45 Position RootPosition;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // Different node types, used as template parameter
60 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
62 // Lookup table to check if a Piece is a slider and its access function
63 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
64 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
66 // Maximum depth for razoring
67 const Depth RazorDepth = 4 * ONE_PLY;
69 // Dynamic razoring margin based on depth
70 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
72 // Maximum depth for use of dynamic threat detection when null move fails low
73 const Depth ThreatDepth = 5 * ONE_PLY;
75 // Minimum depth for use of internal iterative deepening
76 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
78 // At Non-PV nodes we do an internal iterative deepening search
79 // when the static evaluation is bigger then beta - IIDMargin.
80 const Value IIDMargin = Value(0x100);
82 // Minimum depth for use of singular extension
83 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
85 // Futility margin for quiescence search
86 const Value FutilityMarginQS = Value(0x80);
88 // Futility lookup tables (initialized at startup) and their access functions
89 Value FutilityMargins[16][64]; // [depth][moveNumber]
90 int FutilityMoveCounts[32]; // [depth]
92 inline Value futility_margin(Depth d, int mn) {
94 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
98 inline int futility_move_count(Depth d) {
100 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
103 // Reduction lookup tables (initialized at startup) and their access function
104 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
106 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
108 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
111 // Easy move margin. An easy move candidate must be at least this much better
112 // than the second best move.
113 const Value EasyMoveMargin = Value(0x150);
115 // This is the minimum interval in msec between two check_time() calls
116 const int TimerResolution = 5;
119 size_t MultiPV, UCIMultiPV, PVIdx;
124 bool SkillLevelEnabled, Chess960;
128 template <NodeType NT>
129 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
131 template <NodeType NT>
132 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
134 void id_loop(Position& pos);
135 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta);
136 bool connected_moves(const Position& pos, Move m1, Move m2);
137 Value value_to_tt(Value v, int ply);
138 Value value_from_tt(Value v, int ply);
139 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
140 bool connected_threat(const Position& pos, Move m, Move threat);
141 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
142 Move do_skill_level();
143 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
144 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
145 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
147 // MovePickerExt class template extends MovePicker and allows to choose at
148 // compile time the proper moves source according to the type of node. In the
149 // default case we simply create and use a standard MovePicker object.
150 template<bool SpNode> struct MovePickerExt : public MovePicker {
152 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
153 : MovePicker(p, ttm, d, h, ss, b) {}
156 // In case of a SpNode we use split point's shared MovePicker object as moves source
157 template<> struct MovePickerExt<true> : public MovePicker {
159 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
160 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
162 Move next_move() { return mp->next_move(); }
166 // is_dangerous() checks whether a move belongs to some classes of known
167 // 'dangerous' moves so that we avoid to prune it.
168 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
170 // Test for a pawn pushed to 7th or a passed pawn move
171 if (type_of(pos.piece_moved(m)) == PAWN)
173 Color c = pos.side_to_move();
174 if ( relative_rank(c, to_sq(m)) == RANK_7
175 || pos.pawn_is_passed(c, to_sq(m)))
179 // Test for a capture that triggers a pawn endgame
180 if ( captureOrPromotion
181 && type_of(pos.piece_on(to_sq(m))) != PAWN
182 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
183 - PieceValueMidgame[pos.piece_on(to_sq(m))] == VALUE_ZERO)
193 /// Search::init() is called during startup to initialize various lookup tables
195 void Search::init() {
197 int d; // depth (ONE_PLY == 2)
198 int hd; // half depth (ONE_PLY == 1)
201 // Init reductions array
202 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
204 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
205 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
206 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
207 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
210 // Init futility margins array
211 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
212 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
214 // Init futility move count array
215 for (d = 0; d < 32; d++)
216 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
220 /// Search::perft() is our utility to verify move generation. All the leaf nodes
221 /// up to the given depth are generated and counted and the sum returned.
223 int64_t Search::perft(Position& pos, Depth depth) {
228 MoveList<MV_LEGAL> ml(pos);
230 // At the last ply just return the number of moves (leaf nodes)
231 if (depth == ONE_PLY)
235 for ( ; !ml.end(); ++ml)
237 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
238 cnt += perft(pos, depth - ONE_PLY);
239 pos.undo_move(ml.move());
245 /// Search::think() is the external interface to Stockfish's search, and is
246 /// called by the main thread when the program receives the UCI 'go' command. It
247 /// searches from RootPosition and at the end prints the "bestmove" to output.
249 void Search::think() {
251 static Book book; // Defined static to initialize the PRNG only once
253 Position& pos = RootPosition;
254 Chess960 = pos.is_chess960();
255 Eval::RootColor = pos.side_to_move();
256 SearchTime.restart();
257 TimeMgr.init(Limits, pos.startpos_ply_counter());
261 if (RootMoves.empty())
263 cout << "info depth 0 score "
264 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
266 RootMoves.push_back(MOVE_NONE);
270 if (Options["OwnBook"])
272 Move bookMove = book.probe(pos, Options["Book File"], Options["Best Book Move"]);
274 if (bookMove && count(RootMoves.begin(), RootMoves.end(), bookMove))
276 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), bookMove));
281 UCIMultiPV = Options["MultiPV"];
282 SkillLevel = Options["Skill Level"];
284 // Do we have to play with skill handicap? In this case enable MultiPV that
285 // we will use behind the scenes to retrieve a set of possible moves.
286 SkillLevelEnabled = (SkillLevel < 20);
287 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
289 if (Options["Use Search Log"])
291 Log log(Options["Search Log Filename"]);
292 log << "\nSearching: " << pos.to_fen()
293 << "\ninfinite: " << Limits.infinite
294 << " ponder: " << Limits.ponder
295 << " time: " << Limits.time
296 << " increment: " << Limits.increment
297 << " moves to go: " << Limits.movesToGo
301 Threads.set_size(Options["Threads"]);
303 // Set best timer interval to avoid lagging under time pressure. Timer is
304 // used to check for remaining available thinking time.
305 if (Limits.use_time_management())
306 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)));
308 Threads.set_timer(100);
310 // We're ready to start searching. Call the iterative deepening loop function
313 // Stop timer and send all the slaves to sleep, if not already sleeping
314 Threads.set_timer(0);
317 if (Options["Use Search Log"])
319 int e = SearchTime.elapsed();
321 Log log(Options["Search Log Filename"]);
322 log << "Nodes: " << pos.nodes_searched()
323 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
324 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
327 pos.do_move(RootMoves[0].pv[0], st);
328 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
329 pos.undo_move(RootMoves[0].pv[0]);
334 // When we reach max depth we arrive here even without Signals.stop is raised,
335 // but if we are pondering or in infinite search, we shouldn't print the best
336 // move before we are told to do so.
337 if (!Signals.stop && (Limits.ponder || Limits.infinite))
338 Threads[pos.thread()].wait_for_stop_or_ponderhit();
340 // Best move could be MOVE_NONE when searching on a stalemate position
341 cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], Chess960)
342 << " ponder " << move_to_uci(RootMoves[0].pv[1], Chess960) << endl;
348 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
349 // with increasing depth until the allocated thinking time has been consumed,
350 // user stops the search, or the maximum search depth is reached.
352 void id_loop(Position& pos) {
354 Stack ss[MAX_PLY_PLUS_2];
355 int depth, prevBestMoveChanges;
356 Value bestValue, alpha, beta, delta;
357 bool bestMoveNeverChanged = true;
358 Move skillBest = MOVE_NONE;
360 memset(ss, 0, 4 * sizeof(Stack));
361 depth = BestMoveChanges = 0;
362 bestValue = delta = -VALUE_INFINITE;
363 ss->currentMove = MOVE_NULL; // Hack to skip update gains
365 // Iterative deepening loop until requested to stop or target depth reached
366 while (!Signals.stop && ++depth <= MAX_PLY && (!Limits.maxDepth || depth <= Limits.maxDepth))
368 // Save last iteration's scores before first PV line is searched and all
369 // the move scores but the (new) PV are set to -VALUE_INFINITE.
370 for (size_t i = 0; i < RootMoves.size(); i++)
371 RootMoves[i].prevScore = RootMoves[i].score;
373 prevBestMoveChanges = BestMoveChanges;
376 // MultiPV loop. We perform a full root search for each PV line
377 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
379 // Set aspiration window default width
380 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
383 alpha = RootMoves[PVIdx].prevScore - delta;
384 beta = RootMoves[PVIdx].prevScore + delta;
388 alpha = -VALUE_INFINITE;
389 beta = VALUE_INFINITE;
392 // Start with a small aspiration window and, in case of fail high/low,
393 // research with bigger window until not failing high/low anymore.
395 // Search starts from ss+1 to allow referencing (ss-1). This is
396 // needed by update gains and ss copy when splitting at Root.
397 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
399 // Bring to front the best move. It is critical that sorting is
400 // done with a stable algorithm because all the values but the first
401 // and eventually the new best one are set to -VALUE_INFINITE and
402 // we want to keep the same order for all the moves but the new
403 // PV that goes to the front. Note that in case of MultiPV search
404 // the already searched PV lines are preserved.
405 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
407 // In case we have found an exact score and we are going to leave
408 // the fail high/low loop then reorder the PV moves, otherwise
409 // leave the last PV move in its position so to be searched again.
410 // Of course this is needed only in MultiPV search.
411 if (PVIdx && bestValue > alpha && bestValue < beta)
412 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
414 // Write PV back to transposition table in case the relevant
415 // entries have been overwritten during the search.
416 for (size_t i = 0; i <= PVIdx; i++)
417 RootMoves[i].insert_pv_in_tt(pos);
419 // If search has been stopped exit the aspiration window loop.
420 // Sorting and writing PV back to TT is safe becuase RootMoves
421 // is still valid, although refers to previous iteration.
425 // Send full PV info to GUI if we are going to leave the loop or
426 // if we have a fail high/low and we are deep in the search.
427 if ((bestValue > alpha && bestValue < beta) || SearchTime.elapsed() > 2000)
428 pv_info_to_uci(pos, depth, alpha, beta);
430 // In case of failing high/low increase aspiration window and
431 // research, otherwise exit the fail high/low loop.
432 if (bestValue >= beta)
437 else if (bestValue <= alpha)
439 Signals.failedLowAtRoot = true;
440 Signals.stopOnPonderhit = false;
448 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
450 } while (abs(bestValue) < VALUE_KNOWN_WIN);
453 // Skills: Do we need to pick now the best move ?
454 if (SkillLevelEnabled && depth == 1 + SkillLevel)
455 skillBest = do_skill_level();
457 if (!Signals.stop && Options["Use Search Log"])
458 pv_info_to_log(pos, depth, bestValue, SearchTime.elapsed(), &RootMoves[0].pv[0]);
460 // Filter out startup noise when monitoring best move stability
461 if (depth > 2 && BestMoveChanges)
462 bestMoveNeverChanged = false;
464 // Do we have time for the next iteration? Can we stop searching now?
465 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.use_time_management())
467 bool stop = false; // Local variable, not the volatile Signals.stop
469 // Take in account some extra time if the best move has changed
470 if (depth > 4 && depth < 50)
471 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
473 // Stop search if most of available time is already consumed. We
474 // probably don't have enough time to search the first move at the
475 // next iteration anyway.
476 if (SearchTime.elapsed() > (TimeMgr.available_time() * 62) / 100)
479 // Stop search early if one move seems to be much better than others
482 && ( (bestMoveNeverChanged && pos.captured_piece_type())
483 || SearchTime.elapsed() > (TimeMgr.available_time() * 40) / 100))
485 Value rBeta = bestValue - EasyMoveMargin;
486 (ss+1)->excludedMove = RootMoves[0].pv[0];
487 (ss+1)->skipNullMove = true;
488 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
489 (ss+1)->skipNullMove = false;
490 (ss+1)->excludedMove = MOVE_NONE;
498 // If we are allowed to ponder do not stop the search now but
499 // keep pondering until GUI sends "ponderhit" or "stop".
501 Signals.stopOnPonderhit = true;
508 // When using skills swap best PV line with the sub-optimal one
509 if (SkillLevelEnabled)
511 if (skillBest == MOVE_NONE) // Still unassigned ?
512 skillBest = do_skill_level();
514 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), skillBest));
519 // search<>() is the main search function for both PV and non-PV nodes and for
520 // normal and SplitPoint nodes. When called just after a split point the search
521 // is simpler because we have already probed the hash table, done a null move
522 // search, and searched the first move before splitting, we don't have to repeat
523 // all this work again. We also don't need to store anything to the hash table
524 // here: This is taken care of after we return from the split point.
526 template <NodeType NT>
527 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
529 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
530 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
531 const bool RootNode = (NT == Root || NT == SplitPointRoot);
533 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
534 assert((alpha == beta - 1) || PvNode);
535 assert(depth > DEPTH_ZERO);
536 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
538 Move movesSearched[MAX_MOVES];
542 Move ttMove, move, excludedMove, bestMove, threatMove;
545 Value bestValue, value, oldAlpha;
546 Value refinedValue, nullValue, futilityBase, futilityValue;
547 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
548 bool captureOrPromotion, dangerous, doFullDepthSearch;
549 int moveCount = 0, playedMoveCount = 0;
550 Thread& thread = Threads[pos.thread()];
551 SplitPoint* sp = NULL;
553 refinedValue = bestValue = value = -VALUE_INFINITE;
555 inCheck = pos.in_check();
556 ss->ply = (ss-1)->ply + 1;
558 // Used to send selDepth info to GUI
559 if (PvNode && thread.maxPly < ss->ply)
560 thread.maxPly = ss->ply;
562 // Step 1. Initialize node
566 ttMove = excludedMove = MOVE_NONE;
568 bestMove = sp->bestMove;
569 threatMove = sp->threatMove;
570 bestValue = sp->bestValue;
571 moveCount = sp->moveCount; // Lock must be held here
573 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
575 goto split_point_start;
579 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
580 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
581 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
585 // Step 2. Check for aborted search and immediate draw
586 // Enforce node limit here. FIXME: This only works with 1 search thread.
587 if (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)
591 || pos.is_draw<false>()
592 || ss->ply > MAX_PLY) && !RootNode)
595 // Step 3. Mate distance pruning. Even if we mate at the next move our score
596 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
597 // a shorter mate was found upward in the tree then there is no need to search
598 // further, we will never beat current alpha. Same logic but with reversed signs
599 // applies also in the opposite condition of being mated instead of giving mate,
600 // in this case return a fail-high score.
603 alpha = std::max(mated_in(ss->ply), alpha);
604 beta = std::min(mate_in(ss->ply+1), beta);
609 // Step 4. Transposition table lookup
610 // We don't want the score of a partial search to overwrite a previous full search
611 // TT value, so we use a different position key in case of an excluded move.
612 excludedMove = ss->excludedMove;
613 posKey = excludedMove ? pos.exclusion_key() : pos.key();
614 tte = TT.probe(posKey);
615 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
617 // At PV nodes we check for exact scores, while at non-PV nodes we check for
618 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
619 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
620 // we should also update RootMoveList to avoid bogus output.
621 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == BOUND_EXACT
622 : can_return_tt(tte, depth, beta, ss->ply)))
625 ss->currentMove = ttMove; // Can be MOVE_NONE
626 value = value_from_tt(tte->value(), ss->ply);
630 && !pos.is_capture_or_promotion(ttMove)
631 && ttMove != ss->killers[0])
633 ss->killers[1] = ss->killers[0];
634 ss->killers[0] = ttMove;
639 // Step 5. Evaluate the position statically and update parent's gain statistics
641 ss->eval = ss->evalMargin = VALUE_NONE;
644 assert(tte->static_value() != VALUE_NONE);
646 ss->eval = tte->static_value();
647 ss->evalMargin = tte->static_value_margin();
648 refinedValue = refine_eval(tte, ss->eval, ss->ply);
652 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
653 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
656 // Update gain for the parent non-capture move given the static position
657 // evaluation before and after the move.
658 if ( (move = (ss-1)->currentMove) != MOVE_NULL
659 && (ss-1)->eval != VALUE_NONE
660 && ss->eval != VALUE_NONE
661 && !pos.captured_piece_type()
662 && !is_special(move))
664 Square to = to_sq(move);
665 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
668 // Step 6. Razoring (is omitted in PV nodes)
670 && depth < RazorDepth
672 && refinedValue + razor_margin(depth) < beta
673 && ttMove == MOVE_NONE
674 && abs(beta) < VALUE_MATE_IN_MAX_PLY
675 && !pos.has_pawn_on_7th(pos.side_to_move()))
677 Value rbeta = beta - razor_margin(depth);
678 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
680 // Logically we should return (v + razor_margin(depth)), but
681 // surprisingly this did slightly weaker in tests.
685 // Step 7. Static null move pruning (is omitted in PV nodes)
686 // We're betting that the opponent doesn't have a move that will reduce
687 // the score by more than futility_margin(depth) if we do a null move.
690 && depth < RazorDepth
692 && refinedValue - futility_margin(depth, 0) >= beta
693 && abs(beta) < VALUE_MATE_IN_MAX_PLY
694 && pos.non_pawn_material(pos.side_to_move()))
695 return refinedValue - futility_margin(depth, 0);
697 // Step 8. Null move search with verification search (is omitted in PV nodes)
702 && refinedValue >= beta
703 && abs(beta) < VALUE_MATE_IN_MAX_PLY
704 && pos.non_pawn_material(pos.side_to_move()))
706 ss->currentMove = MOVE_NULL;
708 // Null move dynamic reduction based on depth
709 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
711 // Null move dynamic reduction based on value
712 if (refinedValue - PawnValueMidgame > beta)
715 pos.do_null_move<true>(st);
716 (ss+1)->skipNullMove = true;
717 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
718 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
719 (ss+1)->skipNullMove = false;
720 pos.do_null_move<false>(st);
722 if (nullValue >= beta)
724 // Do not return unproven mate scores
725 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
728 if (depth < 6 * ONE_PLY)
731 // Do verification search at high depths
732 ss->skipNullMove = true;
733 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
734 ss->skipNullMove = false;
741 // The null move failed low, which means that we may be faced with
742 // some kind of threat. If the previous move was reduced, check if
743 // the move that refuted the null move was somehow connected to the
744 // move which was reduced. If a connection is found, return a fail
745 // low score (which will cause the reduced move to fail high in the
746 // parent node, which will trigger a re-search with full depth).
747 threatMove = (ss+1)->currentMove;
749 if ( depth < ThreatDepth
751 && threatMove != MOVE_NONE
752 && connected_moves(pos, (ss-1)->currentMove, threatMove))
757 // Step 9. ProbCut (is omitted in PV nodes)
758 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
759 // and a reduced search returns a value much above beta, we can (almost) safely
760 // prune the previous move.
762 && depth >= RazorDepth + ONE_PLY
765 && excludedMove == MOVE_NONE
766 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
768 Value rbeta = beta + 200;
769 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
771 assert(rdepth >= ONE_PLY);
772 assert((ss-1)->currentMove != MOVE_NONE);
773 assert((ss-1)->currentMove != MOVE_NULL);
775 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
778 while ((move = mp.next_move()) != MOVE_NONE)
779 if (pos.pl_move_is_legal(move, ci.pinned))
781 ss->currentMove = move;
782 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
783 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
790 // Step 10. Internal iterative deepening
791 if ( depth >= IIDDepth[PvNode]
792 && ttMove == MOVE_NONE
793 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
795 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
797 ss->skipNullMove = true;
798 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
799 ss->skipNullMove = false;
801 tte = TT.probe(posKey);
802 ttMove = tte ? tte->move() : MOVE_NONE;
805 split_point_start: // At split points actual search starts from here
807 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
809 futilityBase = ss->eval + ss->evalMargin;
810 singularExtensionNode = !RootNode
812 && depth >= SingularExtensionDepth[PvNode]
813 && ttMove != MOVE_NONE
814 && !excludedMove // Recursive singular search is not allowed
815 && (tte->type() & BOUND_LOWER)
816 && tte->depth() >= depth - 3 * ONE_PLY;
818 // Step 11. Loop through moves
819 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
820 while ( bestValue < beta
821 && (move = mp.next_move()) != MOVE_NONE
822 && !thread.cutoff_occurred()
827 if (move == excludedMove)
830 // At root obey the "searchmoves" option and skip moves not listed in Root
831 // Move List, as a consequence any illegal move is also skipped. In MultiPV
832 // mode we also skip PV moves which have been already searched.
833 if (RootNode && !count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
836 // At PV and SpNode nodes we want all moves to be legal since the beginning
837 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
842 moveCount = ++sp->moveCount;
843 lock_release(sp->lock);
850 Signals.firstRootMove = (moveCount == 1);
852 if (pos.thread() == 0 && SearchTime.elapsed() > 2000)
853 cout << "info depth " << depth / ONE_PLY
854 << " currmove " << move_to_uci(move, Chess960)
855 << " currmovenumber " << moveCount + PVIdx << endl;
858 isPvMove = (PvNode && moveCount <= 1);
859 captureOrPromotion = pos.is_capture_or_promotion(move);
860 givesCheck = pos.move_gives_check(move, ci);
861 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
864 // Step 12. Extend checks and, in PV nodes, also dangerous moves
865 if (PvNode && dangerous)
868 else if (givesCheck && pos.see_sign(move) >= 0)
869 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
871 // Singular extension search. If all moves but one fail low on a search of
872 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
873 // is singular and should be extended. To verify this we do a reduced search
874 // on all the other moves but the ttMove, if result is lower than ttValue minus
875 // a margin then we extend ttMove.
876 if ( singularExtensionNode
879 && pos.pl_move_is_legal(move, ci.pinned))
881 Value ttValue = value_from_tt(tte->value(), ss->ply);
883 if (abs(ttValue) < VALUE_KNOWN_WIN)
885 Value rBeta = ttValue - int(depth);
886 ss->excludedMove = move;
887 ss->skipNullMove = true;
888 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
889 ss->skipNullMove = false;
890 ss->excludedMove = MOVE_NONE;
896 // Update current move (this must be done after singular extension search)
897 newDepth = depth - ONE_PLY + ext;
899 // Step 13. Futility pruning (is omitted in PV nodes)
901 && !captureOrPromotion
906 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
908 // Move count based pruning
909 if ( moveCount >= futility_move_count(depth)
910 && (!threatMove || !connected_threat(pos, move, threatMove)))
918 // Value based pruning
919 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
920 // but fixing this made program slightly weaker.
921 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
922 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
923 + H.gain(pos.piece_moved(move), to_sq(move));
925 if (futilityValue < beta)
933 // Prune moves with negative SEE at low depths
934 if ( predictedDepth < 2 * ONE_PLY
935 && pos.see_sign(move) < 0)
944 // Check for legality only before to do the move
945 if (!pos.pl_move_is_legal(move, ci.pinned))
951 ss->currentMove = move;
952 if (!SpNode && !captureOrPromotion)
953 movesSearched[playedMoveCount++] = move;
955 // Step 14. Make the move
956 pos.do_move(move, st, ci, givesCheck);
958 // Step 15. Reduced depth search (LMR). If the move fails high will be
959 // re-searched at full depth.
960 if ( depth > 3 * ONE_PLY
962 && !captureOrPromotion
965 && ss->killers[0] != move
966 && ss->killers[1] != move)
968 ss->reduction = reduction<PvNode>(depth, moveCount);
969 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
970 alpha = SpNode ? sp->alpha : alpha;
972 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
974 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
975 ss->reduction = DEPTH_ZERO;
978 doFullDepthSearch = !isPvMove;
980 // Step 16. Full depth search, when LMR is skipped or fails high
981 if (doFullDepthSearch)
983 alpha = SpNode ? sp->alpha : alpha;
984 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
985 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
988 // Only for PV nodes do a full PV search on the first move or after a fail
989 // high, in the latter case search only if value < beta, otherwise let the
990 // parent node to fail low with value <= alpha and to try another move.
991 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
992 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
993 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
995 // Step 17. Undo move
998 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1000 // Step 18. Check for new best move
1003 lock_grab(sp->lock);
1004 bestValue = sp->bestValue;
1008 // Finished searching the move. If Signals.stop is true, the search
1009 // was aborted because the user interrupted the search or because we
1010 // ran out of time. In this case, the return value of the search cannot
1011 // be trusted, and we don't update the best move and/or PV.
1012 if (RootNode && !Signals.stop)
1014 RootMove& rm = *find(RootMoves.begin(), RootMoves.end(), move);
1016 // PV move or new best move ?
1017 if (isPvMove || value > alpha)
1020 rm.extract_pv_from_tt(pos);
1022 // We record how often the best move has been changed in each
1023 // iteration. This information is used for time management: When
1024 // the best move changes frequently, we allocate some more time.
1025 if (!isPvMove && MultiPV == 1)
1029 // All other moves but the PV are set to the lowest value, this
1030 // is not a problem when sorting becuase sort is stable and move
1031 // position in the list is preserved, just the PV is pushed up.
1032 rm.score = -VALUE_INFINITE;
1036 if (value > bestValue)
1043 && value < beta) // We want always alpha < beta
1046 if (SpNode && !thread.cutoff_occurred())
1048 sp->bestValue = value;
1049 sp->bestMove = move;
1057 // Step 19. Check for split
1059 && depth >= Threads.min_split_depth()
1061 && Threads.available_slave_exists(pos.thread())
1063 && !thread.cutoff_occurred())
1064 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1065 depth, threatMove, moveCount, &mp, NT);
1068 // Step 20. Check for mate and stalemate
1069 // All legal moves have been searched and if there are no legal moves, it
1070 // must be mate or stalemate. Note that we can have a false positive in
1071 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1072 // harmless because return value is discarded anyhow in the parent nodes.
1073 // If we are in a singular extension search then return a fail low score.
1075 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1077 // If we have pruned all the moves without searching return a fail-low score
1078 if (bestValue == -VALUE_INFINITE)
1080 assert(!playedMoveCount);
1082 bestValue = oldAlpha;
1085 // Step 21. Update tables
1086 // Update transposition table entry, killers and history
1087 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1089 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1090 bt = bestValue <= oldAlpha ? BOUND_UPPER
1091 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1093 TT.store(posKey, value_to_tt(bestValue, ss->ply), bt, depth, move, ss->eval, ss->evalMargin);
1095 // Update killers and history for non capture cut-off moves
1096 if ( bestValue >= beta
1097 && !pos.is_capture_or_promotion(move)
1100 if (move != ss->killers[0])
1102 ss->killers[1] = ss->killers[0];
1103 ss->killers[0] = move;
1106 // Increase history value of the cut-off move
1107 Value bonus = Value(int(depth) * int(depth));
1108 H.add(pos.piece_moved(move), to_sq(move), bonus);
1110 // Decrease history of all the other played non-capture moves
1111 for (int i = 0; i < playedMoveCount - 1; i++)
1113 Move m = movesSearched[i];
1114 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1119 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1125 // qsearch() is the quiescence search function, which is called by the main
1126 // search function when the remaining depth is zero (or, to be more precise,
1127 // less than ONE_PLY).
1129 template <NodeType NT>
1130 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1132 const bool PvNode = (NT == PV);
1134 assert(NT == PV || NT == NonPV);
1135 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1136 assert((alpha == beta - 1) || PvNode);
1137 assert(depth <= DEPTH_ZERO);
1138 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1141 Move ttMove, move, bestMove;
1142 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1143 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1147 Value oldAlpha = alpha;
1149 ss->currentMove = bestMove = MOVE_NONE;
1150 ss->ply = (ss-1)->ply + 1;
1152 // Check for an instant draw or maximum ply reached
1153 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1156 // Decide whether or not to include checks, this fixes also the type of
1157 // TT entry depth that we are going to use. Note that in qsearch we use
1158 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1159 inCheck = pos.in_check();
1160 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1162 // Transposition table lookup. At PV nodes, we don't use the TT for
1163 // pruning, but only for move ordering.
1164 tte = TT.probe(pos.key());
1165 ttMove = (tte ? tte->move() : MOVE_NONE);
1167 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1169 ss->currentMove = ttMove; // Can be MOVE_NONE
1170 return value_from_tt(tte->value(), ss->ply);
1173 // Evaluate the position statically
1176 bestValue = futilityBase = -VALUE_INFINITE;
1177 ss->eval = evalMargin = VALUE_NONE;
1178 enoughMaterial = false;
1184 assert(tte->static_value() != VALUE_NONE);
1186 evalMargin = tte->static_value_margin();
1187 ss->eval = bestValue = tte->static_value();
1190 ss->eval = bestValue = evaluate(pos, evalMargin);
1192 // Stand pat. Return immediately if static value is at least beta
1193 if (bestValue >= beta)
1196 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1201 if (PvNode && bestValue > alpha)
1204 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1205 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1208 // Initialize a MovePicker object for the current position, and prepare
1209 // to search the moves. Because the depth is <= 0 here, only captures,
1210 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1212 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1215 // Loop through the moves until no moves remain or a beta cutoff occurs
1216 while ( bestValue < beta
1217 && (move = mp.next_move()) != MOVE_NONE)
1219 assert(is_ok(move));
1221 givesCheck = pos.move_gives_check(move, ci);
1229 && !is_promotion(move)
1230 && !pos.is_passed_pawn_push(move))
1232 futilityValue = futilityBase
1233 + PieceValueEndgame[pos.piece_on(to_sq(move))]
1234 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1236 if (futilityValue < beta)
1238 if (futilityValue > bestValue)
1239 bestValue = futilityValue;
1244 // Prune moves with negative or equal SEE
1245 if ( futilityBase < beta
1246 && depth < DEPTH_ZERO
1247 && pos.see(move) <= 0)
1251 // Detect non-capture evasions that are candidate to be pruned
1252 evasionPrunable = !PvNode
1254 && bestValue > VALUE_MATED_IN_MAX_PLY
1255 && !pos.is_capture(move)
1256 && !pos.can_castle(pos.side_to_move());
1258 // Don't search moves with negative SEE values
1260 && (!inCheck || evasionPrunable)
1262 && !is_promotion(move)
1263 && pos.see_sign(move) < 0)
1266 // Don't search useless checks
1271 && !pos.is_capture_or_promotion(move)
1272 && ss->eval + PawnValueMidgame / 4 < beta
1273 && !check_is_dangerous(pos, move, futilityBase, beta))
1276 // Check for legality only before to do the move
1277 if (!pos.pl_move_is_legal(move, ci.pinned))
1280 ss->currentMove = move;
1282 // Make and search the move
1283 pos.do_move(move, st, ci, givesCheck);
1284 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1285 pos.undo_move(move);
1287 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1290 if (value > bestValue)
1297 && value < beta) // We want always alpha < beta
1302 // All legal moves have been searched. A special case: If we're in check
1303 // and no legal moves were found, it is checkmate.
1304 if (inCheck && bestValue == -VALUE_INFINITE)
1305 return mated_in(ss->ply); // Plies to mate from the root
1307 // Update transposition table
1308 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1309 bt = bestValue <= oldAlpha ? BOUND_UPPER
1310 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1312 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), bt, ttDepth, move, ss->eval, evalMargin);
1314 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1320 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1321 // bestValue is updated only when returning false because in that case move
1324 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta)
1326 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1327 Square from, to, ksq;
1331 from = from_sq(move);
1333 them = ~pos.side_to_move();
1334 ksq = pos.king_square(them);
1335 kingAtt = pos.attacks_from<KING>(ksq);
1336 pc = pos.piece_moved(move);
1338 occ = pos.pieces() ^ from ^ ksq;
1339 oldAtt = pos.attacks_from(pc, from, occ);
1340 newAtt = pos.attacks_from(pc, to, occ);
1342 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1343 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1345 if (single_bit(b)) // Catches also !b
1348 // Rule 2. Queen contact check is very dangerous
1349 if (type_of(pc) == QUEEN && (kingAtt & to))
1352 // Rule 3. Creating new double threats with checks
1353 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1356 // Note that here we generate illegal "double move"!
1357 if (futilityBase + PieceValueEndgame[pos.piece_on(pop_1st_bit(&b))] >= beta)
1365 // connected_moves() tests whether two moves are 'connected' in the sense
1366 // that the first move somehow made the second move possible (for instance
1367 // if the moving piece is the same in both moves). The first move is assumed
1368 // to be the move that was made to reach the current position, while the
1369 // second move is assumed to be a move from the current position.
1371 bool connected_moves(const Position& pos, Move m1, Move m2) {
1373 Square f1, t1, f2, t2;
1380 // Case 1: The moving piece is the same in both moves
1386 // Case 2: The destination square for m2 was vacated by m1
1392 // Case 3: Moving through the vacated square
1393 p2 = pos.piece_on(f2);
1394 if (piece_is_slider(p2) && (squares_between(f2, t2) & f1))
1397 // Case 4: The destination square for m2 is defended by the moving piece in m1
1398 p1 = pos.piece_on(t1);
1399 if (pos.attacks_from(p1, t1) & t2)
1402 // Case 5: Discovered check, checking piece is the piece moved in m1
1403 ksq = pos.king_square(pos.side_to_move());
1404 if ( piece_is_slider(p1)
1405 && (squares_between(t1, ksq) & f2)
1406 && (pos.attacks_from(p1, t1, pos.pieces() ^ f2) & ksq))
1413 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1414 // "plies to mate from the current position". Non-mate scores are unchanged.
1415 // The function is called before storing a value to the transposition table.
1417 Value value_to_tt(Value v, int ply) {
1419 if (v >= VALUE_MATE_IN_MAX_PLY)
1422 if (v <= VALUE_MATED_IN_MAX_PLY)
1429 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1430 // from the transposition table (where refers to the plies to mate/be mated
1431 // from current position) to "plies to mate/be mated from the root".
1433 Value value_from_tt(Value v, int ply) {
1435 if (v >= VALUE_MATE_IN_MAX_PLY)
1438 if (v <= VALUE_MATED_IN_MAX_PLY)
1445 // connected_threat() tests whether it is safe to forward prune a move or if
1446 // is somehow connected to the threat move returned by null search.
1448 bool connected_threat(const Position& pos, Move m, Move threat) {
1451 assert(is_ok(threat));
1452 assert(!pos.is_capture_or_promotion(m));
1453 assert(!pos.is_passed_pawn_push(m));
1455 Square mfrom, mto, tfrom, tto;
1459 tfrom = from_sq(threat);
1460 tto = to_sq(threat);
1462 // Case 1: Don't prune moves which move the threatened piece
1466 // Case 2: If the threatened piece has value less than or equal to the
1467 // value of the threatening piece, don't prune moves which defend it.
1468 if ( pos.is_capture(threat)
1469 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1470 || type_of(pos.piece_on(tfrom)) == KING)
1471 && pos.move_attacks_square(m, tto))
1474 // Case 3: If the moving piece in the threatened move is a slider, don't
1475 // prune safe moves which block its ray.
1476 if ( piece_is_slider(pos.piece_on(tfrom))
1477 && (squares_between(tfrom, tto) & mto)
1478 && pos.see_sign(m) >= 0)
1485 // can_return_tt() returns true if a transposition table score can be used to
1486 // cut-off at a given point in search.
1488 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1490 Value v = value_from_tt(tte->value(), ply);
1492 return ( tte->depth() >= depth
1493 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1494 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1496 && ( ((tte->type() & BOUND_LOWER) && v >= beta)
1497 || ((tte->type() & BOUND_UPPER) && v < beta));
1501 // refine_eval() returns the transposition table score if possible, otherwise
1502 // falls back on static position evaluation.
1504 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1508 Value v = value_from_tt(tte->value(), ply);
1510 if ( ((tte->type() & BOUND_LOWER) && v >= defaultEval)
1511 || ((tte->type() & BOUND_UPPER) && v < defaultEval))
1518 // score_to_uci() converts a value to a string suitable for use with the UCI
1519 // protocol specifications:
1521 // cp <x> The score from the engine's point of view in centipawns.
1522 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1523 // use negative values for y.
1525 string score_to_uci(Value v, Value alpha, Value beta) {
1527 std::stringstream s;
1529 if (abs(v) < VALUE_MATE_IN_MAX_PLY)
1530 s << "cp " << v * 100 / int(PawnValueMidgame);
1532 s << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1534 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1540 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1541 // the PV lines also if are still to be searched and so refer to the previous
1544 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1546 int t = SearchTime.elapsed();
1549 for (int i = 0; i < Threads.size(); i++)
1550 if (Threads[i].maxPly > selDepth)
1551 selDepth = Threads[i].maxPly;
1553 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1555 bool updated = (i <= PVIdx);
1557 if (depth == 1 && !updated)
1560 int d = (updated ? depth : depth - 1);
1561 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1562 std::stringstream s;
1564 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1565 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1567 cout << "info depth " << d
1568 << " seldepth " << selDepth
1569 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1570 << " nodes " << pos.nodes_searched()
1571 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1573 << " multipv " << i + 1
1574 << " pv" << s.str() << endl;
1579 // pv_info_to_log() writes human-readable search information to the log file
1580 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1581 // uses the two below helpers to pretty format time and score respectively.
1583 string time_to_string(int millisecs) {
1585 const int MSecMinute = 1000 * 60;
1586 const int MSecHour = 1000 * 60 * 60;
1588 int hours = millisecs / MSecHour;
1589 int minutes = (millisecs % MSecHour) / MSecMinute;
1590 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1592 std::stringstream s;
1597 s << std::setfill('0') << std::setw(2) << minutes << ':'
1598 << std::setw(2) << seconds;
1602 string score_to_string(Value v) {
1604 std::stringstream s;
1606 if (v >= VALUE_MATE_IN_MAX_PLY)
1607 s << "#" << (VALUE_MATE - v + 1) / 2;
1608 else if (v <= VALUE_MATED_IN_MAX_PLY)
1609 s << "-#" << (VALUE_MATE + v) / 2;
1611 s << std::setprecision(2) << std::fixed << std::showpos
1612 << float(v) / PawnValueMidgame;
1617 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1619 const int64_t K = 1000;
1620 const int64_t M = 1000000;
1622 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1624 string san, padding;
1626 std::stringstream s;
1628 s << std::setw(2) << depth
1629 << std::setw(8) << score_to_string(value)
1630 << std::setw(8) << time_to_string(time);
1632 if (pos.nodes_searched() < M)
1633 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1635 else if (pos.nodes_searched() < K * M)
1636 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1639 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1641 padding = string(s.str().length(), ' ');
1642 length = padding.length();
1644 while (*m != MOVE_NONE)
1646 san = move_to_san(pos, *m);
1648 if (length + san.length() > 80)
1650 s << "\n" + padding;
1651 length = padding.length();
1655 length += san.length() + 1;
1657 pos.do_move(*m++, *st++);
1661 pos.undo_move(*--m);
1663 Log l(Options["Search Log Filename"]);
1664 l << s.str() << endl;
1668 // When playing with strength handicap choose best move among the MultiPV set
1669 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1671 Move do_skill_level() {
1673 assert(MultiPV > 1);
1677 // PRNG sequence should be not deterministic
1678 for (int i = Time::current_time().msec() % 50; i > 0; i--)
1679 rk.rand<unsigned>();
1681 // RootMoves are already sorted by score in descending order
1682 size_t size = std::min(MultiPV, RootMoves.size());
1683 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1684 int weakness = 120 - 2 * SkillLevel;
1685 int max_s = -VALUE_INFINITE;
1686 Move best = MOVE_NONE;
1688 // Choose best move. For each move score we add two terms both dependent on
1689 // weakness, one deterministic and bigger for weaker moves, and one random,
1690 // then we choose the move with the resulting highest score.
1691 for (size_t i = 0; i < size; i++)
1693 int s = RootMoves[i].score;
1695 // Don't allow crazy blunders even at very low skills
1696 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1699 // This is our magic formula
1700 s += ( weakness * int(RootMoves[0].score - s)
1701 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1706 best = RootMoves[i].pv[0];
1715 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1716 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1717 /// allow to always have a ponder move even when we fail high at root, and a
1718 /// long PV to print that is important for position analysis.
1720 void RootMove::extract_pv_from_tt(Position& pos) {
1722 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1727 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1731 pos.do_move(m, *st++);
1733 while ( (tte = TT.probe(pos.key())) != NULL
1734 && (m = tte->move()) != MOVE_NONE // Local copy, TT entry could change
1735 && pos.is_pseudo_legal(m)
1736 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1738 && (!pos.is_draw<false>() || ply < 2))
1741 pos.do_move(m, *st++);
1744 pv.push_back(MOVE_NONE);
1746 do pos.undo_move(pv[--ply]); while (ply);
1750 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1751 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1752 /// first, even if the old TT entries have been overwritten.
1754 void RootMove::insert_pv_in_tt(Position& pos) {
1756 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1759 Value v, m = VALUE_NONE;
1762 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1768 // Don't overwrite existing correct entries
1769 if (!tte || tte->move() != pv[ply])
1771 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1772 TT.store(k, VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], v, m);
1774 pos.do_move(pv[ply], *st++);
1776 } while (pv[++ply] != MOVE_NONE);
1778 do pos.undo_move(pv[--ply]); while (ply);
1782 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1783 /// The parameter 'master_sp', if non-NULL, is a pointer to an active SplitPoint
1784 /// object for which the thread is the master.
1786 void Thread::idle_loop(SplitPoint* sp_master) {
1788 // If this thread is the master of a split point and all slaves have
1789 // finished their work at this split point, return from the idle loop.
1790 while (!sp_master || sp_master->slavesMask)
1792 // If we are not searching, wait for a condition to be signaled
1793 // instead of wasting CPU time polling for work.
1796 || (!is_searching && Threads.use_sleeping_threads()))
1804 // Grab the lock to avoid races with Thread::wake_up()
1805 lock_grab(sleepLock);
1807 // If we are master and all slaves have finished don't go to sleep
1808 if (sp_master && !sp_master->slavesMask)
1810 lock_release(sleepLock);
1814 // Do sleep after retesting sleep conditions under lock protection, in
1815 // particular we need to avoid a deadlock in case a master thread has,
1816 // in the meanwhile, allocated us and sent the wake_up() call before we
1817 // had the chance to grab the lock.
1818 if (do_sleep || !is_searching)
1819 cond_wait(sleepCond, sleepLock);
1821 lock_release(sleepLock);
1824 // If this thread has been assigned work, launch a search
1827 assert(!do_sleep && !do_exit);
1829 lock_grab(Threads.splitLock);
1831 assert(is_searching);
1832 SplitPoint* sp = curSplitPoint;
1834 lock_release(Threads.splitLock);
1836 Stack ss[MAX_PLY_PLUS_2];
1837 Position pos(*sp->pos, threadID);
1838 int master = sp->master;
1840 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1843 lock_grab(sp->lock);
1845 if (sp->nodeType == Root)
1846 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1847 else if (sp->nodeType == PV)
1848 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1849 else if (sp->nodeType == NonPV)
1850 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1854 assert(is_searching);
1856 is_searching = false;
1857 sp->slavesMask &= ~(1ULL << threadID);
1858 sp->nodes += pos.nodes_searched();
1860 // After releasing the lock we cannot access anymore any SplitPoint
1861 // related data in a reliably way becuase it could have been released
1862 // under our feet by the sp master.
1863 lock_release(sp->lock);
1865 // Wake up master thread so to allow it to return from the idle loop in
1866 // case we are the last slave of the split point.
1867 if ( Threads.use_sleeping_threads()
1868 && threadID != master
1869 && !Threads[master].is_searching)
1870 Threads[master].wake_up();
1876 /// check_time() is called by the timer thread when the timer triggers. It is
1877 /// used to print debug info and, more important, to detect when we are out of
1878 /// available time and so stop the search.
1882 static Time lastInfoTime = Time::current_time();
1884 if (lastInfoTime.elapsed() >= 1000)
1886 lastInfoTime.restart();
1893 int e = SearchTime.elapsed();
1894 bool stillAtFirstMove = Signals.firstRootMove
1895 && !Signals.failedLowAtRoot
1896 && e > TimeMgr.available_time();
1898 bool noMoreTime = e > TimeMgr.maximum_time() - 2 * TimerResolution
1899 || stillAtFirstMove;
1901 if ( (Limits.use_time_management() && noMoreTime)
1902 || (Limits.maxTime && e >= Limits.maxTime))
1903 Signals.stop = true;