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/>.
39 #include "ucioption.h"
43 volatile SignalsType Signals;
45 std::set<Move> SearchMoves;
46 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 // RootMove struct is used for moves at the root of the tree. For each root
63 // move we store a score, a node count, and a PV (really a refutation in the
64 // case of moves which fail low). Score is normally set at -VALUE_INFINITE for
70 score = prevScore = -VALUE_INFINITE;
72 pv.push_back(MOVE_NONE);
75 bool operator<(const RootMove& m) const { return score < m.score; }
76 bool operator==(const Move& m) const { return pv[0] == m; }
78 void extract_pv_from_tt(Position& pos);
79 void insert_pv_in_tt(Position& pos);
89 // Lookup table to check if a Piece is a slider and its access function
90 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
91 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
93 // Maximum depth for razoring
94 const Depth RazorDepth = 4 * ONE_PLY;
96 // Dynamic razoring margin based on depth
97 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
99 // Maximum depth for use of dynamic threat detection when null move fails low
100 const Depth ThreatDepth = 5 * ONE_PLY;
102 // Minimum depth for use of internal iterative deepening
103 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
105 // At Non-PV nodes we do an internal iterative deepening search
106 // when the static evaluation is bigger then beta - IIDMargin.
107 const Value IIDMargin = Value(0x100);
109 // Minimum depth for use of singular extension
110 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
112 // Futility margin for quiescence search
113 const Value FutilityMarginQS = Value(0x80);
115 // Futility lookup tables (initialized at startup) and their access functions
116 Value FutilityMargins[16][64]; // [depth][moveNumber]
117 int FutilityMoveCounts[32]; // [depth]
119 inline Value futility_margin(Depth d, int mn) {
121 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
122 : 2 * VALUE_INFINITE;
125 inline int futility_move_count(Depth d) {
127 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
130 // Reduction lookup tables (initialized at startup) and their access function
131 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
133 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
135 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
138 // Easy move margin. An easy move candidate must be at least this much
139 // better than the second best move.
140 const Value EasyMoveMargin = Value(0x150);
142 // This is the minimum interval in msec between two check_time() calls
143 const int TimerResolution = 5;
146 /// Namespace variables
148 std::vector<RootMove> RootMoves;
149 size_t MultiPV, UCIMultiPV, PVIdx;
153 bool SkillLevelEnabled, Chess960;
159 template <NodeType NT>
160 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
162 template <NodeType NT>
163 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
165 void id_loop(Position& pos);
166 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
167 bool connected_moves(const Position& pos, Move m1, Move m2);
168 Value value_to_tt(Value v, int ply);
169 Value value_from_tt(Value v, int ply);
170 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
171 bool connected_threat(const Position& pos, Move m, Move threat);
172 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
173 Move do_skill_level();
174 int elapsed_time(bool reset = false);
175 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
176 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
177 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
179 // MovePickerExt class template extends MovePicker and allows to choose at
180 // compile time the proper moves source according to the type of node. In the
181 // default case we simply create and use a standard MovePicker object.
182 template<bool SpNode> struct MovePickerExt : public MovePicker {
184 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
185 : MovePicker(p, ttm, d, h, ss, b) {}
188 // In case of a SpNode we use split point's shared MovePicker object as moves source
189 template<> struct MovePickerExt<true> : public MovePicker {
191 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
192 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
194 Move next_move() { return mp->next_move(); }
198 // is_dangerous() checks whether a move belongs to some classes of known
199 // 'dangerous' moves so that we avoid to prune it.
200 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
202 // Test for a pawn pushed to 7th or a passed pawn move
203 if (type_of(pos.piece_moved(m)) == PAWN)
205 Color c = pos.side_to_move();
206 if ( relative_rank(c, to_sq(m)) == RANK_7
207 || pos.pawn_is_passed(c, to_sq(m)))
211 // Test for a capture that triggers a pawn endgame
212 if ( captureOrPromotion
213 && type_of(pos.piece_on(to_sq(m))) != PAWN
214 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
215 - PieceValueMidgame[pos.piece_on(to_sq(m))] == VALUE_ZERO)
225 /// Search::init() is called during startup to initialize various lookup tables
227 void Search::init() {
229 int d; // depth (ONE_PLY == 2)
230 int hd; // half depth (ONE_PLY == 1)
233 // Init reductions array
234 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
236 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
237 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
238 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
239 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
242 // Init futility margins array
243 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
244 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
246 // Init futility move count array
247 for (d = 0; d < 32; d++)
248 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
252 /// Search::perft() is our utility to verify move generation. All the leaf nodes
253 /// up to the given depth are generated and counted and the sum returned.
255 int64_t Search::perft(Position& pos, Depth depth) {
260 MoveList<MV_LEGAL> ml(pos);
262 // At the last ply just return the number of moves (leaf nodes)
263 if (depth == ONE_PLY)
267 for ( ; !ml.end(); ++ml)
269 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
270 cnt += perft(pos, depth - ONE_PLY);
271 pos.undo_move(ml.move());
277 /// Search::think() is the external interface to Stockfish's search, and is
278 /// called by the main thread when the program receives the UCI 'go' command. It
279 /// searches from RootPosition and at the end prints the "bestmove" to output.
281 void Search::think() {
283 static Book book; // Defined static to initialize the PRNG only once
286 Position& pos = RootPosition;
287 Chess960 = pos.is_chess960();
289 TimeMgr.init(Limits, pos.startpos_ply_counter());
294 // Populate RootMoves with all the legal moves (default) or, if a SearchMoves
295 // is given, with the subset of legal moves to search.
296 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
297 if (SearchMoves.empty() || SearchMoves.count(ml.move()))
298 RootMoves.push_back(RootMove(ml.move()));
300 if (RootMoves.empty())
302 cout << "info depth 0 score "
303 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
305 RootMoves.push_back(MOVE_NONE);
309 if ( Options["OwnBook"]
310 && (bm = book.probe(pos, Options["Book File"], Options["Best Book Move"])) != MOVE_NONE
311 && count(RootMoves.begin(), RootMoves.end(), bm))
313 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), bm));
317 // Read UCI options: GUI could change UCI parameters during the game
318 read_evaluation_uci_options(pos.side_to_move());
319 Threads.read_uci_options();
321 TT.set_size(Options["Hash"]);
322 if (Options["Clear Hash"])
324 Options["Clear Hash"] = false;
328 UCIMultiPV = Options["MultiPV"];
329 SkillLevel = Options["Skill Level"];
331 // Do we have to play with skill handicap? In this case enable MultiPV that
332 // we will use behind the scenes to retrieve a set of possible moves.
333 SkillLevelEnabled = (SkillLevel < 20);
334 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
336 if (Options["Use Search Log"])
338 Log log(Options["Search Log Filename"]);
339 log << "\nSearching: " << pos.to_fen()
340 << "\ninfinite: " << Limits.infinite
341 << " ponder: " << Limits.ponder
342 << " time: " << Limits.time
343 << " increment: " << Limits.increment
344 << " moves to go: " << Limits.movesToGo
348 for (int i = 0; i < Threads.size(); i++)
350 Threads[i].maxPly = 0;
351 Threads[i].wake_up();
354 // Set best timer interval to avoid lagging under time pressure. Timer is
355 // used to check for remaining available thinking time.
356 if (Limits.use_time_management())
357 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)));
359 Threads.set_timer(100);
361 // We're ready to start searching. Call the iterative deepening loop function
364 // Stop timer and send all the slaves to sleep, if not already sleeping
365 Threads.set_timer(0);
368 if (Options["Use Search Log"])
370 int e = elapsed_time();
372 Log log(Options["Search Log Filename"]);
373 log << "Nodes: " << pos.nodes_searched()
374 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
375 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
378 pos.do_move(RootMoves[0].pv[0], st);
379 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
380 pos.undo_move(RootMoves[0].pv[0]);
385 // When we reach max depth we arrive here even without Signals.stop is raised,
386 // but if we are pondering or in infinite search, we shouldn't print the best
387 // move before we are told to do so.
388 if (!Signals.stop && (Limits.ponder || Limits.infinite))
389 Threads.wait_for_stop_or_ponderhit();
391 // Best move could be MOVE_NONE when searching on a stalemate position
392 cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], Chess960)
393 << " ponder " << move_to_uci(RootMoves[0].pv[1], Chess960) << endl;
399 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
400 // with increasing depth until the allocated thinking time has been consumed,
401 // user stops the search, or the maximum search depth is reached.
403 void id_loop(Position& pos) {
405 Stack ss[MAX_PLY_PLUS_2];
406 int depth, prevBestMoveChanges;
407 Value bestValue, alpha, beta, delta;
408 bool bestMoveNeverChanged = true;
409 Move skillBest = MOVE_NONE;
411 memset(ss, 0, 4 * sizeof(Stack));
412 depth = BestMoveChanges = 0;
413 bestValue = delta = -VALUE_INFINITE;
414 ss->currentMove = MOVE_NULL; // Hack to skip update gains
416 // Iterative deepening loop until requested to stop or target depth reached
417 while (!Signals.stop && ++depth <= MAX_PLY && (!Limits.maxDepth || depth <= Limits.maxDepth))
419 // Save last iteration's scores before first PV line is searched and all
420 // the move scores but the (new) PV are set to -VALUE_INFINITE.
421 for (size_t i = 0; i < RootMoves.size(); i++)
422 RootMoves[i].prevScore = RootMoves[i].score;
424 prevBestMoveChanges = BestMoveChanges;
427 // MultiPV loop. We perform a full root search for each PV line
428 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
430 // Set aspiration window default width
431 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
434 alpha = RootMoves[PVIdx].prevScore - delta;
435 beta = RootMoves[PVIdx].prevScore + delta;
439 alpha = -VALUE_INFINITE;
440 beta = VALUE_INFINITE;
443 // Start with a small aspiration window and, in case of fail high/low,
444 // research with bigger window until not failing high/low anymore.
446 // Search starts from ss+1 to allow referencing (ss-1). This is
447 // needed by update gains and ss copy when splitting at Root.
448 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
450 // Bring to front the best move. It is critical that sorting is
451 // done with a stable algorithm because all the values but the first
452 // and eventually the new best one are set to -VALUE_INFINITE and
453 // we want to keep the same order for all the moves but the new
454 // PV that goes to the front. Note that in case of MultiPV search
455 // the already searched PV lines are preserved.
456 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
458 // In case we have found an exact score and we are going to leave
459 // the fail high/low loop then reorder the PV moves, otherwise
460 // leave the last PV move in its position so to be searched again.
461 // Of course this is needed only in MultiPV search.
462 if (PVIdx && bestValue > alpha && bestValue < beta)
463 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
465 // Write PV back to transposition table in case the relevant
466 // entries have been overwritten during the search.
467 for (size_t i = 0; i <= PVIdx; i++)
468 RootMoves[i].insert_pv_in_tt(pos);
470 // If search has been stopped exit the aspiration window loop.
471 // Sorting and writing PV back to TT is safe becuase RootMoves
472 // is still valid, although refers to previous iteration.
476 // Send full PV info to GUI if we are going to leave the loop or
477 // if we have a fail high/low and we are deep in the search.
478 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
479 pv_info_to_uci(pos, depth, alpha, beta);
481 // In case of failing high/low increase aspiration window and
482 // research, otherwise exit the fail high/low loop.
483 if (bestValue >= beta)
488 else if (bestValue <= alpha)
490 Signals.failedLowAtRoot = true;
491 Signals.stopOnPonderhit = false;
499 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
501 } while (abs(bestValue) < VALUE_KNOWN_WIN);
504 // Skills: Do we need to pick now the best move ?
505 if (SkillLevelEnabled && depth == 1 + SkillLevel)
506 skillBest = do_skill_level();
508 if (Options["Use Search Log"])
509 pv_info_to_log(pos, depth, bestValue, elapsed_time(), &RootMoves[0].pv[0]);
511 // Filter out startup noise when monitoring best move stability
512 if (depth > 2 && BestMoveChanges)
513 bestMoveNeverChanged = false;
515 // Do we have time for the next iteration? Can we stop searching now?
516 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.use_time_management())
518 bool stop = false; // Local variable, not the volatile Signals.stop
520 // Take in account some extra time if the best move has changed
521 if (depth > 4 && depth < 50)
522 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
524 // Stop search if most of available time is already consumed. We
525 // probably don't have enough time to search the first move at the
526 // next iteration anyway.
527 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
530 // Stop search early if one move seems to be much better than others
533 && ( (bestMoveNeverChanged && pos.captured_piece_type())
534 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
536 Value rBeta = bestValue - EasyMoveMargin;
537 (ss+1)->excludedMove = RootMoves[0].pv[0];
538 (ss+1)->skipNullMove = true;
539 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
540 (ss+1)->skipNullMove = false;
541 (ss+1)->excludedMove = MOVE_NONE;
549 // If we are allowed to ponder do not stop the search now but
550 // keep pondering until GUI sends "ponderhit" or "stop".
552 Signals.stopOnPonderhit = true;
559 // When using skills swap best PV line with the sub-optimal one
560 if (SkillLevelEnabled)
562 if (skillBest == MOVE_NONE) // Still unassigned ?
563 skillBest = do_skill_level();
565 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), skillBest));
570 // search<>() is the main search function for both PV and non-PV nodes and for
571 // normal and SplitPoint nodes. When called just after a split point the search
572 // is simpler because we have already probed the hash table, done a null move
573 // search, and searched the first move before splitting, we don't have to repeat
574 // all this work again. We also don't need to store anything to the hash table
575 // here: This is taken care of after we return from the split point.
577 template <NodeType NT>
578 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
580 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
581 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
582 const bool RootNode = (NT == Root || NT == SplitPointRoot);
584 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
585 assert(PvNode == (alpha != beta - 1));
586 assert(depth > DEPTH_ZERO);
587 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
589 Move movesSearched[MAX_MOVES];
593 Move ttMove, move, excludedMove, threatMove;
596 Value bestValue, value, oldAlpha;
597 Value refinedValue, nullValue, futilityBase, futilityValue;
598 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
599 bool captureOrPromotion, dangerous, doFullDepthSearch;
600 int moveCount = 0, playedMoveCount = 0;
601 Thread& thread = Threads[pos.thread()];
602 SplitPoint* sp = NULL;
604 refinedValue = bestValue = value = -VALUE_INFINITE;
606 inCheck = pos.in_check();
607 ss->ply = (ss-1)->ply + 1;
609 // Used to send selDepth info to GUI
610 if (PvNode && thread.maxPly < ss->ply)
611 thread.maxPly = ss->ply;
613 // Step 1. Initialize node
616 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
617 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
618 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
624 ttMove = excludedMove = MOVE_NONE;
625 threatMove = sp->threatMove;
626 goto split_point_start;
629 // Step 2. Check for aborted search and immediate draw
631 || pos.is_draw<false>()
632 || ss->ply > MAX_PLY) && !RootNode)
635 // Step 3. Mate distance pruning. Even if we mate at the next move our score
636 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
637 // a shorter mate was found upward in the tree then there is no need to search
638 // further, we will never beat current alpha. Same logic but with reversed signs
639 // applies also in the opposite condition of being mated instead of giving mate,
640 // in this case return a fail-high score.
643 alpha = std::max(mated_in(ss->ply), alpha);
644 beta = std::min(mate_in(ss->ply+1), beta);
649 // Step 4. Transposition table lookup
650 // We don't want the score of a partial search to overwrite a previous full search
651 // TT value, so we use a different position key in case of an excluded move.
652 excludedMove = ss->excludedMove;
653 posKey = excludedMove ? pos.exclusion_key() : pos.key();
654 tte = TT.probe(posKey);
655 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
657 // At PV nodes we check for exact scores, while at non-PV nodes we check for
658 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
659 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
660 // we should also update RootMoveList to avoid bogus output.
661 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
662 : can_return_tt(tte, depth, beta, ss->ply)))
665 ss->bestMove = move = ttMove; // Can be MOVE_NONE
666 value = value_from_tt(tte->value(), ss->ply);
670 && !pos.is_capture_or_promotion(move)
671 && move != ss->killers[0])
673 ss->killers[1] = ss->killers[0];
674 ss->killers[0] = move;
679 // Step 5. Evaluate the position statically and update parent's gain statistics
681 ss->eval = ss->evalMargin = VALUE_NONE;
684 assert(tte->static_value() != VALUE_NONE);
686 ss->eval = tte->static_value();
687 ss->evalMargin = tte->static_value_margin();
688 refinedValue = refine_eval(tte, ss->eval, ss->ply);
692 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
693 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
696 // Update gain for the parent non-capture move given the static position
697 // evaluation before and after the move.
698 if ( (move = (ss-1)->currentMove) != MOVE_NULL
699 && (ss-1)->eval != VALUE_NONE
700 && ss->eval != VALUE_NONE
701 && !pos.captured_piece_type()
702 && !is_special(move))
704 Square to = to_sq(move);
705 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
708 // Step 6. Razoring (is omitted in PV nodes)
710 && depth < RazorDepth
712 && refinedValue + razor_margin(depth) < beta
713 && ttMove == MOVE_NONE
714 && abs(beta) < VALUE_MATE_IN_MAX_PLY
715 && !pos.has_pawn_on_7th(pos.side_to_move()))
717 Value rbeta = beta - razor_margin(depth);
718 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
720 // Logically we should return (v + razor_margin(depth)), but
721 // surprisingly this did slightly weaker in tests.
725 // Step 7. Static null move pruning (is omitted in PV nodes)
726 // We're betting that the opponent doesn't have a move that will reduce
727 // the score by more than futility_margin(depth) if we do a null move.
730 && depth < RazorDepth
732 && refinedValue - futility_margin(depth, 0) >= beta
733 && abs(beta) < VALUE_MATE_IN_MAX_PLY
734 && pos.non_pawn_material(pos.side_to_move()))
735 return refinedValue - futility_margin(depth, 0);
737 // Step 8. Null move search with verification search (is omitted in PV nodes)
742 && refinedValue >= beta
743 && abs(beta) < VALUE_MATE_IN_MAX_PLY
744 && pos.non_pawn_material(pos.side_to_move()))
746 ss->currentMove = MOVE_NULL;
748 // Null move dynamic reduction based on depth
749 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
751 // Null move dynamic reduction based on value
752 if (refinedValue - PawnValueMidgame > beta)
755 pos.do_null_move<true>(st);
756 (ss+1)->skipNullMove = true;
757 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
758 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
759 (ss+1)->skipNullMove = false;
760 pos.do_null_move<false>(st);
762 if (nullValue >= beta)
764 // Do not return unproven mate scores
765 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
768 if (depth < 6 * ONE_PLY)
771 // Do verification search at high depths
772 ss->skipNullMove = true;
773 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
774 ss->skipNullMove = false;
781 // The null move failed low, which means that we may be faced with
782 // some kind of threat. If the previous move was reduced, check if
783 // the move that refuted the null move was somehow connected to the
784 // move which was reduced. If a connection is found, return a fail
785 // low score (which will cause the reduced move to fail high in the
786 // parent node, which will trigger a re-search with full depth).
787 threatMove = (ss+1)->bestMove;
789 if ( depth < ThreatDepth
791 && threatMove != MOVE_NONE
792 && connected_moves(pos, (ss-1)->currentMove, threatMove))
797 // Step 9. ProbCut (is omitted in PV nodes)
798 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
799 // and a reduced search returns a value much above beta, we can (almost) safely
800 // prune the previous move.
802 && depth >= RazorDepth + ONE_PLY
805 && excludedMove == MOVE_NONE
806 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
808 Value rbeta = beta + 200;
809 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
811 assert(rdepth >= ONE_PLY);
813 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
816 while ((move = mp.next_move()) != MOVE_NONE)
817 if (pos.pl_move_is_legal(move, ci.pinned))
819 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
820 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
827 // Step 10. Internal iterative deepening
828 if ( depth >= IIDDepth[PvNode]
829 && ttMove == MOVE_NONE
830 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
832 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
834 ss->skipNullMove = true;
835 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
836 ss->skipNullMove = false;
838 tte = TT.probe(posKey);
839 ttMove = tte ? tte->move() : MOVE_NONE;
842 split_point_start: // At split points actual search starts from here
844 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
846 ss->bestMove = MOVE_NONE;
847 futilityBase = ss->eval + ss->evalMargin;
848 singularExtensionNode = !RootNode
850 && depth >= SingularExtensionDepth[PvNode]
851 && ttMove != MOVE_NONE
852 && !excludedMove // Recursive singular search is not allowed
853 && (tte->type() & VALUE_TYPE_LOWER)
854 && tte->depth() >= depth - 3 * ONE_PLY;
857 lock_grab(&(sp->lock));
858 bestValue = sp->bestValue;
859 moveCount = sp->moveCount;
861 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
864 // Step 11. Loop through moves
865 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
866 while ( bestValue < beta
867 && (move = mp.next_move()) != MOVE_NONE
868 && !thread.cutoff_occurred()
873 if (move == excludedMove)
876 // At root obey the "searchmoves" option and skip moves not listed in Root
877 // Move List, as a consequence any illegal move is also skipped. In MultiPV
878 // mode we also skip PV moves which have been already searched.
879 if (RootNode && !count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
882 // At PV and SpNode nodes we want all moves to be legal since the beginning
883 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
888 moveCount = ++sp->moveCount;
889 lock_release(&(sp->lock));
896 Signals.firstRootMove = (moveCount == 1);
898 if (pos.thread() == 0 && elapsed_time() > 2000)
899 cout << "info depth " << depth / ONE_PLY
900 << " currmove " << move_to_uci(move, Chess960)
901 << " currmovenumber " << moveCount + PVIdx << endl;
904 isPvMove = (PvNode && moveCount <= 1);
905 captureOrPromotion = pos.is_capture_or_promotion(move);
906 givesCheck = pos.move_gives_check(move, ci);
907 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
910 // Step 12. Extend checks and, in PV nodes, also dangerous moves
911 if (PvNode && dangerous)
914 else if (givesCheck && pos.see_sign(move) >= 0)
915 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
917 // Singular extension search. If all moves but one fail low on a search of
918 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
919 // is singular and should be extended. To verify this we do a reduced search
920 // on all the other moves but the ttMove, if result is lower than ttValue minus
921 // a margin then we extend ttMove.
922 if ( singularExtensionNode
925 && pos.pl_move_is_legal(move, ci.pinned))
927 Value ttValue = value_from_tt(tte->value(), ss->ply);
929 if (abs(ttValue) < VALUE_KNOWN_WIN)
931 Value rBeta = ttValue - int(depth);
932 ss->excludedMove = move;
933 ss->skipNullMove = true;
934 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
935 ss->skipNullMove = false;
936 ss->excludedMove = MOVE_NONE;
937 ss->bestMove = MOVE_NONE;
943 // Update current move (this must be done after singular extension search)
944 newDepth = depth - ONE_PLY + ext;
946 // Step 13. Futility pruning (is omitted in PV nodes)
948 && !captureOrPromotion
953 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
955 // Move count based pruning
956 if ( moveCount >= futility_move_count(depth)
957 && (!threatMove || !connected_threat(pos, move, threatMove)))
960 lock_grab(&(sp->lock));
965 // Value based pruning
966 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
967 // but fixing this made program slightly weaker.
968 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
969 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
970 + H.gain(pos.piece_moved(move), to_sq(move));
972 if (futilityValue < beta)
975 lock_grab(&(sp->lock));
980 // Prune moves with negative SEE at low depths
981 if ( predictedDepth < 2 * ONE_PLY
982 && pos.see_sign(move) < 0)
985 lock_grab(&(sp->lock));
991 // Check for legality only before to do the move
992 if (!pos.pl_move_is_legal(move, ci.pinned))
998 ss->currentMove = move;
999 if (!SpNode && !captureOrPromotion)
1000 movesSearched[playedMoveCount++] = move;
1002 // Step 14. Make the move
1003 pos.do_move(move, st, ci, givesCheck);
1005 // Step 15. Reduced depth search (LMR). If the move fails high will be
1006 // re-searched at full depth.
1007 if ( depth > 3 * ONE_PLY
1009 && !captureOrPromotion
1012 && ss->killers[0] != move
1013 && ss->killers[1] != move)
1015 ss->reduction = reduction<PvNode>(depth, moveCount);
1016 Depth d = newDepth - ss->reduction;
1017 alpha = SpNode ? sp->alpha : alpha;
1019 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1020 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1022 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1023 ss->reduction = DEPTH_ZERO;
1026 doFullDepthSearch = !isPvMove;
1028 // Step 16. Full depth search, when LMR is skipped or fails high
1029 if (doFullDepthSearch)
1031 alpha = SpNode ? sp->alpha : alpha;
1032 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1033 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1036 // Only for PV nodes do a full PV search on the first move or after a fail
1037 // high, in the latter case search only if value < beta, otherwise let the
1038 // parent node to fail low with value <= alpha and to try another move.
1039 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1040 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1041 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1043 // Step 17. Undo move
1044 pos.undo_move(move);
1046 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1048 // Step 18. Check for new best move
1051 lock_grab(&(sp->lock));
1052 bestValue = sp->bestValue;
1056 // Finished searching the move. If StopRequest is true, the search
1057 // was aborted because the user interrupted the search or because we
1058 // ran out of time. In this case, the return value of the search cannot
1059 // be trusted, and we don't update the best move and/or PV.
1060 if (RootNode && !Signals.stop)
1062 RootMove& rm = *find(RootMoves.begin(), RootMoves.end(), move);
1064 // PV move or new best move ?
1065 if (isPvMove || value > alpha)
1068 rm.extract_pv_from_tt(pos);
1070 // We record how often the best move has been changed in each
1071 // iteration. This information is used for time management: When
1072 // the best move changes frequently, we allocate some more time.
1073 if (!isPvMove && MultiPV == 1)
1077 // All other moves but the PV are set to the lowest value, this
1078 // is not a problem when sorting becuase sort is stable and move
1079 // position in the list is preserved, just the PV is pushed up.
1080 rm.score = -VALUE_INFINITE;
1084 if (value > bestValue)
1087 ss->bestMove = move;
1091 && value < beta) // We want always alpha < beta
1094 if (SpNode && !thread.cutoff_occurred())
1096 sp->bestValue = value;
1097 sp->ss->bestMove = move;
1099 sp->is_betaCutoff = (value >= beta);
1103 // Step 19. Check for split
1105 && depth >= Threads.min_split_depth()
1107 && Threads.available_slave_exists(pos.thread())
1109 && !thread.cutoff_occurred())
1110 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1111 threatMove, moveCount, &mp, NT);
1114 // Step 20. Check for mate and stalemate
1115 // All legal moves have been searched and if there are no legal moves, it
1116 // must be mate or stalemate. Note that we can have a false positive in
1117 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1118 // harmless because return value is discarded anyhow in the parent nodes.
1119 // If we are in a singular extension search then return a fail low score.
1121 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1123 // If we have pruned all the moves without searching return a fail-low score
1124 if (bestValue == -VALUE_INFINITE)
1126 assert(!playedMoveCount);
1131 // Step 21. Update tables
1132 // Update transposition table entry, killers and history
1133 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1135 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1136 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1137 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1139 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1141 // Update killers and history for non capture cut-off moves
1142 if ( bestValue >= beta
1143 && !pos.is_capture_or_promotion(move)
1146 if (move != ss->killers[0])
1148 ss->killers[1] = ss->killers[0];
1149 ss->killers[0] = move;
1152 // Increase history value of the cut-off move
1153 Value bonus = Value(int(depth) * int(depth));
1154 H.add(pos.piece_moved(move), to_sq(move), bonus);
1156 // Decrease history of all the other played non-capture moves
1157 for (int i = 0; i < playedMoveCount - 1; i++)
1159 Move m = movesSearched[i];
1160 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1167 // Here we have the lock still grabbed
1168 sp->is_slave[pos.thread()] = false;
1169 sp->nodes += pos.nodes_searched();
1170 lock_release(&(sp->lock));
1173 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1179 // qsearch() is the quiescence search function, which is called by the main
1180 // search function when the remaining depth is zero (or, to be more precise,
1181 // less than ONE_PLY).
1183 template <NodeType NT>
1184 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1186 const bool PvNode = (NT == PV);
1188 assert(NT == PV || NT == NonPV);
1189 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1190 assert(PvNode == (alpha != beta - 1));
1191 assert(depth <= DEPTH_ZERO);
1192 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1196 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1197 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1201 Value oldAlpha = alpha;
1203 ss->bestMove = ss->currentMove = MOVE_NONE;
1204 ss->ply = (ss-1)->ply + 1;
1206 // Check for an instant draw or maximum ply reached
1207 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1210 // Decide whether or not to include checks, this fixes also the type of
1211 // TT entry depth that we are going to use. Note that in qsearch we use
1212 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1213 inCheck = pos.in_check();
1214 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1216 // Transposition table lookup. At PV nodes, we don't use the TT for
1217 // pruning, but only for move ordering.
1218 tte = TT.probe(pos.key());
1219 ttMove = (tte ? tte->move() : MOVE_NONE);
1221 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1223 ss->bestMove = ttMove; // Can be MOVE_NONE
1224 return value_from_tt(tte->value(), ss->ply);
1227 // Evaluate the position statically
1230 bestValue = futilityBase = -VALUE_INFINITE;
1231 ss->eval = evalMargin = VALUE_NONE;
1232 enoughMaterial = false;
1238 assert(tte->static_value() != VALUE_NONE);
1240 evalMargin = tte->static_value_margin();
1241 ss->eval = bestValue = tte->static_value();
1244 ss->eval = bestValue = evaluate(pos, evalMargin);
1246 // Stand pat. Return immediately if static value is at least beta
1247 if (bestValue >= beta)
1250 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1255 if (PvNode && bestValue > alpha)
1258 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1259 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1262 // Initialize a MovePicker object for the current position, and prepare
1263 // to search the moves. Because the depth is <= 0 here, only captures,
1264 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1266 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1269 // Loop through the moves until no moves remain or a beta cutoff occurs
1270 while ( bestValue < beta
1271 && (move = mp.next_move()) != MOVE_NONE)
1273 assert(is_ok(move));
1275 givesCheck = pos.move_gives_check(move, ci);
1283 && !is_promotion(move)
1284 && !pos.is_passed_pawn_push(move))
1286 futilityValue = futilityBase
1287 + PieceValueEndgame[pos.piece_on(to_sq(move))]
1288 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1290 if (futilityValue < beta)
1292 if (futilityValue > bestValue)
1293 bestValue = futilityValue;
1298 // Prune moves with negative or equal SEE
1299 if ( futilityBase < beta
1300 && depth < DEPTH_ZERO
1301 && pos.see(move) <= 0)
1305 // Detect non-capture evasions that are candidate to be pruned
1306 evasionPrunable = !PvNode
1308 && bestValue > VALUE_MATED_IN_MAX_PLY
1309 && !pos.is_capture(move)
1310 && !pos.can_castle(pos.side_to_move());
1312 // Don't search moves with negative SEE values
1314 && (!inCheck || evasionPrunable)
1316 && !is_promotion(move)
1317 && pos.see_sign(move) < 0)
1320 // Don't search useless checks
1325 && !pos.is_capture_or_promotion(move)
1326 && ss->eval + PawnValueMidgame / 4 < beta
1327 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1329 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1330 bestValue = ss->eval + PawnValueMidgame / 4;
1335 // Check for legality only before to do the move
1336 if (!pos.pl_move_is_legal(move, ci.pinned))
1339 ss->currentMove = move;
1341 // Make and search the move
1342 pos.do_move(move, st, ci, givesCheck);
1343 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1344 pos.undo_move(move);
1346 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1349 if (value > bestValue)
1352 ss->bestMove = move;
1356 && value < beta) // We want always alpha < beta
1361 // All legal moves have been searched. A special case: If we're in check
1362 // and no legal moves were found, it is checkmate.
1363 if (inCheck && bestValue == -VALUE_INFINITE)
1364 return mated_in(ss->ply); // Plies to mate from the root
1366 // Update transposition table
1367 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1368 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1369 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1371 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1373 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1379 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1380 // bestValue is updated only when returning false because in that case move
1383 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1385 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1386 Square from, to, ksq, victimSq;
1389 Value futilityValue, bv = *bestValue;
1391 from = from_sq(move);
1393 them = ~pos.side_to_move();
1394 ksq = pos.king_square(them);
1395 kingAtt = pos.attacks_from<KING>(ksq);
1396 pc = pos.piece_on(from);
1398 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1399 oldAtt = pos.attacks_from(pc, from, occ);
1400 newAtt = pos.attacks_from(pc, to, occ);
1402 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1403 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1405 if (!(b && (b & (b - 1))))
1408 // Rule 2. Queen contact check is very dangerous
1409 if ( type_of(pc) == QUEEN
1410 && bit_is_set(kingAtt, to))
1413 // Rule 3. Creating new double threats with checks
1414 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1418 victimSq = pop_1st_bit(&b);
1419 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1421 // Note that here we generate illegal "double move"!
1422 if ( futilityValue >= beta
1423 && pos.see_sign(make_move(from, victimSq)) >= 0)
1426 if (futilityValue > bv)
1430 // Update bestValue only if check is not dangerous (because we will prune the move)
1436 // connected_moves() tests whether two moves are 'connected' in the sense
1437 // that the first move somehow made the second move possible (for instance
1438 // if the moving piece is the same in both moves). The first move is assumed
1439 // to be the move that was made to reach the current position, while the
1440 // second move is assumed to be a move from the current position.
1442 bool connected_moves(const Position& pos, Move m1, Move m2) {
1444 Square f1, t1, f2, t2;
1451 // Case 1: The moving piece is the same in both moves
1457 // Case 2: The destination square for m2 was vacated by m1
1463 // Case 3: Moving through the vacated square
1464 p2 = pos.piece_on(f2);
1465 if ( piece_is_slider(p2)
1466 && bit_is_set(squares_between(f2, t2), f1))
1469 // Case 4: The destination square for m2 is defended by the moving piece in m1
1470 p1 = pos.piece_on(t1);
1471 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1474 // Case 5: Discovered check, checking piece is the piece moved in m1
1475 ksq = pos.king_square(pos.side_to_move());
1476 if ( piece_is_slider(p1)
1477 && bit_is_set(squares_between(t1, ksq), f2))
1479 Bitboard occ = pos.occupied_squares();
1480 clear_bit(&occ, f2);
1481 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1488 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1489 // "plies to mate from the current position". Non-mate scores are unchanged.
1490 // The function is called before storing a value to the transposition table.
1492 Value value_to_tt(Value v, int ply) {
1494 if (v >= VALUE_MATE_IN_MAX_PLY)
1497 if (v <= VALUE_MATED_IN_MAX_PLY)
1504 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1505 // from the transposition table (where refers to the plies to mate/be mated
1506 // from current position) to "plies to mate/be mated from the root".
1508 Value value_from_tt(Value v, int ply) {
1510 if (v >= VALUE_MATE_IN_MAX_PLY)
1513 if (v <= VALUE_MATED_IN_MAX_PLY)
1520 // connected_threat() tests whether it is safe to forward prune a move or if
1521 // is somehow connected to the threat move returned by null search.
1523 bool connected_threat(const Position& pos, Move m, Move threat) {
1526 assert(is_ok(threat));
1527 assert(!pos.is_capture_or_promotion(m));
1528 assert(!pos.is_passed_pawn_push(m));
1530 Square mfrom, mto, tfrom, tto;
1534 tfrom = from_sq(threat);
1535 tto = to_sq(threat);
1537 // Case 1: Don't prune moves which move the threatened piece
1541 // Case 2: If the threatened piece has value less than or equal to the
1542 // value of the threatening piece, don't prune moves which defend it.
1543 if ( pos.is_capture(threat)
1544 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1545 || type_of(pos.piece_on(tfrom)) == KING)
1546 && pos.move_attacks_square(m, tto))
1549 // Case 3: If the moving piece in the threatened move is a slider, don't
1550 // prune safe moves which block its ray.
1551 if ( piece_is_slider(pos.piece_on(tfrom))
1552 && bit_is_set(squares_between(tfrom, tto), mto)
1553 && pos.see_sign(m) >= 0)
1560 // can_return_tt() returns true if a transposition table score can be used to
1561 // cut-off at a given point in search.
1563 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1565 Value v = value_from_tt(tte->value(), ply);
1567 return ( tte->depth() >= depth
1568 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1569 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1571 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1572 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1576 // refine_eval() returns the transposition table score if possible, otherwise
1577 // falls back on static position evaluation.
1579 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1583 Value v = value_from_tt(tte->value(), ply);
1585 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1586 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1593 // current_search_time() returns the number of milliseconds which have passed
1594 // since the beginning of the current search.
1596 int elapsed_time(bool reset) {
1598 static int searchStartTime;
1601 searchStartTime = system_time();
1603 return system_time() - searchStartTime;
1607 // score_to_uci() converts a value to a string suitable for use with the UCI
1608 // protocol specifications:
1610 // cp <x> The score from the engine's point of view in centipawns.
1611 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1612 // use negative values for y.
1614 string score_to_uci(Value v, Value alpha, Value beta) {
1616 std::stringstream s;
1618 if (abs(v) < VALUE_MATE_IN_MAX_PLY)
1619 s << "cp " << v * 100 / int(PawnValueMidgame);
1621 s << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1623 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1629 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1630 // the PV lines also if are still to be searched and so refer to the previous
1633 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1635 int t = elapsed_time();
1638 for (int i = 0; i < Threads.size(); i++)
1639 if (Threads[i].maxPly > selDepth)
1640 selDepth = Threads[i].maxPly;
1642 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1644 bool updated = (i <= PVIdx);
1646 if (depth == 1 && !updated)
1649 int d = (updated ? depth : depth - 1);
1650 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1651 std::stringstream s;
1653 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1654 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1656 cout << "info depth " << d
1657 << " seldepth " << selDepth
1658 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1659 << " nodes " << pos.nodes_searched()
1660 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1662 << " multipv " << i + 1
1663 << " pv" << s.str() << endl;
1668 // pv_info_to_log() writes human-readable search information to the log file
1669 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1670 // uses the two below helpers to pretty format time and score respectively.
1672 string time_to_string(int millisecs) {
1674 const int MSecMinute = 1000 * 60;
1675 const int MSecHour = 1000 * 60 * 60;
1677 int hours = millisecs / MSecHour;
1678 int minutes = (millisecs % MSecHour) / MSecMinute;
1679 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1681 std::stringstream s;
1686 s << std::setfill('0') << std::setw(2) << minutes << ':'
1687 << std::setw(2) << seconds;
1691 string score_to_string(Value v) {
1693 std::stringstream s;
1695 if (v >= VALUE_MATE_IN_MAX_PLY)
1696 s << "#" << (VALUE_MATE - v + 1) / 2;
1697 else if (v <= VALUE_MATED_IN_MAX_PLY)
1698 s << "-#" << (VALUE_MATE + v) / 2;
1700 s << std::setprecision(2) << std::fixed << std::showpos
1701 << float(v) / PawnValueMidgame;
1706 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1708 const int64_t K = 1000;
1709 const int64_t M = 1000000;
1711 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1713 string san, padding;
1715 std::stringstream s;
1717 s << std::setw(2) << depth
1718 << std::setw(8) << score_to_string(value)
1719 << std::setw(8) << time_to_string(time);
1721 if (pos.nodes_searched() < M)
1722 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1724 else if (pos.nodes_searched() < K * M)
1725 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1728 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1730 padding = string(s.str().length(), ' ');
1731 length = padding.length();
1733 while (*m != MOVE_NONE)
1735 san = move_to_san(pos, *m);
1737 if (length + san.length() > 80)
1739 s << "\n" + padding;
1740 length = padding.length();
1744 length += san.length() + 1;
1746 pos.do_move(*m++, *st++);
1750 pos.undo_move(*--m);
1752 Log l(Options["Search Log Filename"]);
1753 l << s.str() << endl;
1757 // When playing with strength handicap choose best move among the MultiPV set
1758 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1760 Move do_skill_level() {
1762 assert(MultiPV > 1);
1766 // PRNG sequence should be not deterministic
1767 for (int i = abs(system_time() % 50); i > 0; i--)
1768 rk.rand<unsigned>();
1770 // RootMoves are already sorted by score in descending order
1771 size_t size = std::min(MultiPV, RootMoves.size());
1772 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1773 int weakness = 120 - 2 * SkillLevel;
1774 int max_s = -VALUE_INFINITE;
1775 Move best = MOVE_NONE;
1777 // Choose best move. For each move score we add two terms both dependent on
1778 // weakness, one deterministic and bigger for weaker moves, and one random,
1779 // then we choose the move with the resulting highest score.
1780 for (size_t i = 0; i < size; i++)
1782 int s = RootMoves[i].score;
1784 // Don't allow crazy blunders even at very low skills
1785 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1788 // This is our magic formula
1789 s += ( weakness * int(RootMoves[0].score - s)
1790 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1795 best = RootMoves[i].pv[0];
1802 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1803 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1804 // allow to always have a ponder move even when we fail high at root and also a
1805 // long PV to print that is important for position analysis.
1807 void RootMove::extract_pv_from_tt(Position& pos) {
1809 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1814 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1818 pos.do_move(m, *st++);
1820 while ( (tte = TT.probe(pos.key())) != NULL
1821 && tte->move() != MOVE_NONE
1822 && pos.is_pseudo_legal(tte->move())
1823 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1825 && (!pos.is_draw<false>() || ply < 2))
1827 pv.push_back(tte->move());
1828 pos.do_move(tte->move(), *st++);
1831 pv.push_back(MOVE_NONE);
1833 do pos.undo_move(pv[--ply]); while (ply);
1837 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1838 // the PV back into the TT. This makes sure the old PV moves are searched
1839 // first, even if the old TT entries have been overwritten.
1841 void RootMove::insert_pv_in_tt(Position& pos) {
1843 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1846 Value v, m = VALUE_NONE;
1849 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1855 // Don't overwrite existing correct entries
1856 if (!tte || tte->move() != pv[ply])
1858 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1859 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
1861 pos.do_move(pv[ply], *st++);
1863 } while (pv[++ply] != MOVE_NONE);
1865 do pos.undo_move(pv[--ply]); while (ply);
1871 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1872 /// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
1873 /// for which the thread is the master.
1875 void Thread::idle_loop(SplitPoint* sp) {
1879 // If we are not searching, wait for a condition to be signaled
1880 // instead of wasting CPU time polling for work.
1883 || (Threads.use_sleeping_threads() && !is_searching))
1885 assert((!sp && threadID) || Threads.use_sleeping_threads());
1893 // Grab the lock to avoid races with Thread::wake_up()
1894 lock_grab(&sleepLock);
1896 // If we are master and all slaves have finished don't go to sleep
1897 if (sp && Threads.split_point_finished(sp))
1899 lock_release(&sleepLock);
1903 // Do sleep after retesting sleep conditions under lock protection, in
1904 // particular we need to avoid a deadlock in case a master thread has,
1905 // in the meanwhile, allocated us and sent the wake_up() call before we
1906 // had the chance to grab the lock.
1907 if (do_sleep || !is_searching)
1908 cond_wait(&sleepCond, &sleepLock);
1910 lock_release(&sleepLock);
1913 // If this thread has been assigned work, launch a search
1916 assert(!do_terminate);
1918 // Copy split point position and search stack and call search()
1919 Stack ss[MAX_PLY_PLUS_2];
1920 SplitPoint* tsp = splitPoint;
1921 Position pos(*tsp->pos, threadID);
1923 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
1926 if (tsp->nodeType == Root)
1927 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1928 else if (tsp->nodeType == PV)
1929 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1930 else if (tsp->nodeType == NonPV)
1931 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1935 assert(is_searching);
1937 is_searching = false;
1939 // Wake up master thread so to allow it to return from the idle loop in
1940 // case we are the last slave of the split point.
1941 if ( Threads.use_sleeping_threads()
1942 && threadID != tsp->master
1943 && !Threads[tsp->master].is_searching)
1944 Threads[tsp->master].wake_up();
1947 // If this thread is the master of a split point and all slaves have
1948 // finished their work at this split point, return from the idle loop.
1949 if (sp && Threads.split_point_finished(sp))
1951 // Because sp->is_slave[] is reset under lock protection,
1952 // be sure sp->lock has been released before to return.
1953 lock_grab(&(sp->lock));
1954 lock_release(&(sp->lock));
1961 /// check_time() is called by the timer thread when the timer triggers. It is
1962 /// used to print debug info and, more important, to detect when we are out of
1963 /// available time and so stop the search.
1967 static int lastInfoTime;
1968 int e = elapsed_time();
1970 if (system_time() - lastInfoTime >= 1000 || !lastInfoTime)
1972 lastInfoTime = system_time();
1979 bool stillAtFirstMove = Signals.firstRootMove
1980 && !Signals.failedLowAtRoot
1981 && e > TimeMgr.available_time();
1983 bool noMoreTime = e > TimeMgr.maximum_time() - 2 * TimerResolution
1984 || stillAtFirstMove;
1986 if ( (Limits.use_time_management() && noMoreTime)
1987 || (Limits.maxTime && e >= Limits.maxTime)
1988 /* missing nodes limit */ ) // FIXME
1989 Signals.stop = true;