2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2010 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/>.
40 #include "ucioption.h"
48 // Set to true to force running with one thread. Used for debugging
49 const bool FakeSplit = false;
51 // Different node types, used as template parameter
52 enum NodeType { Root, PV, NonPV, SplitPointPV, SplitPointNonPV };
54 // RootMove struct is used for moves at the root of the tree. For each root
55 // move, we store a pv_score, a node count, and a PV (really a refutation
56 // in the case of moves which fail low). Value pv_score is normally set at
57 // -VALUE_INFINITE for all non-pv moves.
61 RootMove(const RootMove& rm) { *this = rm; }
62 RootMove& operator=(const RootMove& rm);
64 // RootMove::operator<() is the comparison function used when
65 // sorting the moves. A move m1 is considered to be better
66 // than a move m2 if it has an higher pv_score
67 bool operator<(const RootMove& m) const { return pv_score < m.pv_score; }
69 void extract_pv_from_tt(Position& pos);
70 void insert_pv_in_tt(Position& pos);
74 Move pv[PLY_MAX_PLUS_2];
77 // RootMoveList struct is mainly a std::vector of RootMove objects
78 struct RootMoveList : public std::vector<RootMove> {
79 void init(Position& pos, Move searchMoves[]);
80 RootMove* find(const Move &m);
87 // Lookup table to check if a Piece is a slider and its access function
88 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
89 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 // Step 9. Internal iterative deepening
104 // Minimum depth for use of internal iterative deepening
105 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
107 // At Non-PV nodes we do an internal iterative deepening search
108 // when the static evaluation is bigger then beta - IIDMargin.
109 const Value IIDMargin = Value(0x100);
111 // Step 11. Decide the new search depth
113 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
114 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
115 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
116 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
117 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
119 // Minimum depth for use of singular extension
120 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
122 // Step 12. Futility pruning
124 // Futility margin for quiescence search
125 const Value FutilityMarginQS = Value(0x80);
127 // Futility lookup tables (initialized at startup) and their access functions
128 Value FutilityMargins[16][64]; // [depth][moveNumber]
129 int FutilityMoveCounts[32]; // [depth]
131 inline Value futility_margin(Depth d, int mn) {
133 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
134 : 2 * VALUE_INFINITE;
137 inline int futility_move_count(Depth d) {
139 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
142 // Step 14. Reduced search
144 // Reduction lookup tables (initialized at startup) and their access function
145 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
147 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
149 return (Depth) Reductions[PvNode][Min(d / ONE_PLY, 63)][Min(mn, 63)];
152 // Easy move margin. An easy move candidate must be at least this much
153 // better than the second best move.
154 const Value EasyMoveMargin = Value(0x200);
157 /// Namespace variables
163 int MultiPV, UCIMultiPV;
165 // Time management variables
166 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
171 std::ofstream LogFile;
173 // Skill level adjustment
175 bool SkillLevelEnabled;
177 // Node counters, used only by thread[0] but try to keep in different cache
178 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
179 bool SendSearchedNodes;
181 int NodesBetweenPolls = 30000;
189 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
191 template <NodeType NT>
192 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
194 template <NodeType NT>
195 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
197 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
198 bool connected_moves(const Position& pos, Move m1, Move m2);
199 Value value_to_tt(Value v, int ply);
200 Value value_from_tt(Value v, int ply);
201 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
202 bool connected_threat(const Position& pos, Move m, Move threat);
203 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
204 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
205 void update_gains(const Position& pos, Move move, Value before, Value after);
206 void do_skill_level(Move* best, Move* ponder);
208 int current_search_time(int set = 0);
209 string score_to_uci(Value v, Value alpha, Value beta);
210 string speed_to_uci(int64_t nodes);
211 string pv_to_uci(Move pv[], int pvNum, bool chess960);
212 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
213 string depth_to_uci(Depth depth);
214 void poll(const Position& pos);
215 void wait_for_stop_or_ponderhit();
217 // MovePickerExt template class extends MovePicker and allows to choose at compile
218 // time the proper moves source according to the type of node. In the default case
219 // we simply create and use a standard MovePicker object.
220 template<NodeType> struct MovePickerExt : public MovePicker {
222 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
223 : MovePicker(p, ttm, d, h, ss, b) {}
226 // In case of a SpNode we use split point's shared MovePicker object as moves source
227 template<> struct MovePickerExt<SplitPointNonPV> : public MovePickerExt<NonPV> {
229 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
230 : MovePickerExt<NonPV>(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
232 Move get_next_move() { return mp->get_next_move(); }
236 template<> struct MovePickerExt<SplitPointPV> : public MovePickerExt<SplitPointNonPV> {
238 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
239 : MovePickerExt<SplitPointNonPV>(p, ttm, d, h, ss, b) {}
242 // Overload operator<<() to make it easier to print moves in a coordinate
243 // notation compatible with UCI protocol.
244 std::ostream& operator<<(std::ostream& os, Move m) {
246 bool chess960 = (os.iword(0) != 0); // See set960()
247 return os << move_to_uci(m, chess960);
250 // When formatting a move for std::cout we must know if we are in Chess960
251 // or not. To keep using the handy operator<<() on the move the trick is to
252 // embed this flag in the stream itself. Function-like named enum set960 is
253 // used as a custom manipulator and the stream internal general-purpose array,
254 // accessed through ios_base::iword(), is used to pass the flag to the move's
255 // operator<<() that will read it to properly format castling moves.
258 std::ostream& operator<< (std::ostream& os, const set960& f) {
260 os.iword(0) = int(f);
264 // extension() decides whether a move should be searched with normal depth,
265 // or with extended depth. Certain classes of moves (checking moves, in
266 // particular) are searched with bigger depth than ordinary moves and in
267 // any case are marked as 'dangerous'. Note that also if a move is not
268 // extended, as example because the corresponding UCI option is set to zero,
269 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
270 template <bool PvNode>
271 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
272 bool moveIsCheck, bool* dangerous) {
273 assert(m != MOVE_NONE);
275 Depth result = DEPTH_ZERO;
276 *dangerous = moveIsCheck;
278 if (moveIsCheck && pos.see_sign(m) >= 0)
279 result += CheckExtension[PvNode];
281 if (piece_type(pos.piece_on(move_from(m))) == PAWN)
283 Color c = pos.side_to_move();
284 if (relative_rank(c, move_to(m)) == RANK_7)
286 result += PawnPushTo7thExtension[PvNode];
289 if (pos.pawn_is_passed(c, move_to(m)))
291 result += PassedPawnExtension[PvNode];
296 if ( captureOrPromotion
297 && piece_type(pos.piece_on(move_to(m))) != PAWN
298 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
299 - piece_value_midgame(pos.piece_on(move_to(m))) == VALUE_ZERO)
300 && !move_is_special(m))
302 result += PawnEndgameExtension[PvNode];
306 return Min(result, ONE_PLY);
312 /// init_search() is called during startup to initialize various lookup tables
316 int d; // depth (ONE_PLY == 2)
317 int hd; // half depth (ONE_PLY == 1)
320 // Init reductions array
321 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
323 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
324 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
325 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
326 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
329 // Init futility margins array
330 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
331 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
333 // Init futility move count array
334 for (d = 0; d < 32; d++)
335 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
339 /// perft() is our utility to verify move generation. All the leaf nodes up to
340 /// the given depth are generated and counted and the sum returned.
342 int64_t perft(Position& pos, Depth depth) {
347 // Generate all legal moves
348 MoveList<MV_LEGAL> ml(pos);
350 // If we are at the last ply we don't need to do and undo
351 // the moves, just to count them.
352 if (depth <= ONE_PLY)
355 // Loop through all legal moves
357 for ( ; !ml.end(); ++ml)
359 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
360 sum += perft(pos, depth - ONE_PLY);
361 pos.undo_move(ml.move());
367 /// think() is the external interface to Stockfish's search, and is called when
368 /// the program receives the UCI 'go' command. It initializes various global
369 /// variables, and calls id_loop(). It returns false when a "quit" command is
370 /// received during the search.
372 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
376 // Initialize global search-related variables
377 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = SendSearchedNodes = false;
379 current_search_time(get_system_time());
381 TimeMgr.init(Limits, pos.startpos_ply_counter());
383 // Set output steram in normal or chess960 mode
384 cout << set960(pos.is_chess960());
386 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
388 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
389 else if (Limits.time && Limits.time < 1000)
390 NodesBetweenPolls = 1000;
391 else if (Limits.time && Limits.time < 5000)
392 NodesBetweenPolls = 5000;
394 NodesBetweenPolls = 30000;
396 // Look for a book move
397 if (Options["OwnBook"].value<bool>())
399 if (Options["Book File"].value<string>() != book.name())
400 book.open(Options["Book File"].value<string>());
402 Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
403 if (bookMove != MOVE_NONE)
406 wait_for_stop_or_ponderhit();
408 cout << "bestmove " << bookMove << endl;
414 UCIMultiPV = Options["MultiPV"].value<int>();
415 SkillLevel = Options["Skill Level"].value<int>();
417 read_evaluation_uci_options(pos.side_to_move());
418 Threads.read_uci_options();
420 // If needed allocate pawn and material hash tables and adjust TT size
421 Threads.init_hash_tables();
422 TT.set_size(Options["Hash"].value<int>());
424 if (Options["Clear Hash"].value<bool>())
426 Options["Clear Hash"].set_value("false");
430 // Do we have to play with skill handicap? In this case enable MultiPV that
431 // we will use behind the scenes to retrieve a set of possible moves.
432 SkillLevelEnabled = (SkillLevel < 20);
433 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
435 // Wake up needed threads and reset maxPly counter
436 for (int i = 0; i < Threads.size(); i++)
438 Threads[i].wake_up();
439 Threads[i].maxPly = 0;
442 // Write to log file and keep it open to be accessed during the search
443 if (Options["Use Search Log"].value<bool>())
445 string name = Options["Search Log Filename"].value<string>();
446 LogFile.open(name.c_str(), std::ios::out | std::ios::app);
448 if (LogFile.is_open())
449 LogFile << "\nSearching: " << pos.to_fen()
450 << "\ninfinite: " << Limits.infinite
451 << " ponder: " << Limits.ponder
452 << " time: " << Limits.time
453 << " increment: " << Limits.increment
454 << " moves to go: " << Limits.movesToGo
458 // We're ready to start thinking. Call the iterative deepening loop function
459 Move ponderMove = MOVE_NONE;
460 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
462 // Write final search statistics and close log file
463 if (LogFile.is_open())
465 int t = current_search_time();
467 LogFile << "Nodes: " << pos.nodes_searched()
468 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
469 << "\nBest move: " << move_to_san(pos, bestMove);
472 pos.do_move(bestMove, st);
473 LogFile << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
474 pos.undo_move(bestMove); // Return from think() with unchanged position
478 // This makes all the threads to go to sleep
481 // If we are pondering or in infinite search, we shouldn't print the
482 // best move before we are told to do so.
483 if (!StopRequest && (Limits.ponder || Limits.infinite))
484 wait_for_stop_or_ponderhit();
486 // Could be MOVE_NONE when searching on a stalemate position
487 cout << "bestmove " << bestMove;
489 // UCI protol is not clear on allowing sending an empty ponder move, instead
490 // it is clear that ponder move is optional. So skip it if empty.
491 if (ponderMove != MOVE_NONE)
492 cout << " ponder " << ponderMove;
502 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
503 // with increasing depth until the allocated thinking time has been consumed,
504 // user stops the search, or the maximum search depth is reached.
506 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
508 SearchStack ss[PLY_MAX_PLUS_2];
509 Value bestValues[PLY_MAX_PLUS_2];
510 int bestMoveChanges[PLY_MAX_PLUS_2];
511 int depth, aspirationDelta;
512 Value value, alpha, beta;
513 Move bestMove, easyMove, skillBest, skillPonder;
515 // Initialize stuff before a new search
516 memset(ss, 0, 4 * sizeof(SearchStack));
519 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
520 depth = aspirationDelta = 0;
521 alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
522 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
524 // Moves to search are verified and copied
525 Rml.init(pos, searchMoves);
527 // Handle special case of searching on a mate/stalemate position
530 cout << "info" << depth_to_uci(DEPTH_ZERO)
531 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
536 // Iterative deepening loop until requested to stop or target depth reached
537 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
539 Rml.bestMoveChanges = 0;
541 // Calculate dynamic aspiration window based on previous iterations
542 if (MultiPV == 1 && depth >= 5 && abs(bestValues[depth - 1]) < VALUE_KNOWN_WIN)
544 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
545 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
547 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
548 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
550 alpha = Max(bestValues[depth - 1] - aspirationDelta, -VALUE_INFINITE);
551 beta = Min(bestValues[depth - 1] + aspirationDelta, VALUE_INFINITE);
554 // Start with a small aspiration window and, in case of fail high/low,
555 // research with bigger window until not failing high/low anymore.
557 // Search starting from ss+1 to allow calling update_gains()
558 value = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
560 // It is critical that sorting is done with a stable algorithm
561 // because all the values but the first are usually set to
562 // -VALUE_INFINITE and we want to keep the same order for all
563 // the moves but the new PV that goes to head.
564 sort<RootMove>(Rml.begin(), Rml.end());
566 // Write PV back to transposition table in case the relevant entries
567 // have been overwritten during the search.
568 for (int i = 0; i < Min(MultiPV, (int)Rml.size()); i++)
569 Rml[i].insert_pv_in_tt(pos);
571 // Value cannot be trusted. Break out immediately!
575 // Send full PV info to GUI if we are going to leave the loop or
576 // if we have a fail high/low and we are deep in the search.
577 if ((value > alpha && value < beta) || current_search_time() > 2000)
578 for (int i = 0; i < Min(UCIMultiPV, (int)Rml.size()); i++)
580 << depth_to_uci(depth * ONE_PLY)
581 << score_to_uci(Rml[i].pv_score, alpha, beta)
582 << speed_to_uci(pos.nodes_searched())
583 << pv_to_uci(Rml[i].pv, i + 1, pos.is_chess960()) << endl;
585 // In case of failing high/low increase aspiration window and research,
586 // otherwise exit the fail high/low loop.
589 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
590 aspirationDelta += aspirationDelta / 2;
592 else if (value <= alpha)
594 AspirationFailLow = true;
595 StopOnPonderhit = false;
597 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
598 aspirationDelta += aspirationDelta / 2;
603 } while (abs(value) < VALUE_KNOWN_WIN);
605 // Collect info about search result
606 bestMove = Rml[0].pv[0];
607 *ponderMove = Rml[0].pv[1];
608 bestValues[depth] = value;
609 bestMoveChanges[depth] = Rml.bestMoveChanges;
611 // Do we need to pick now the best and the ponder moves ?
612 if (SkillLevelEnabled && depth == 1 + SkillLevel)
613 do_skill_level(&skillBest, &skillPonder);
615 if (LogFile.is_open())
616 LogFile << pretty_pv(pos, depth, value, current_search_time(), Rml[0].pv) << endl;
618 // Init easyMove after first iteration or drop if differs from the best move
619 if (depth == 1 && (Rml.size() == 1 || Rml[0].pv_score > Rml[1].pv_score + EasyMoveMargin))
621 else if (bestMove != easyMove)
622 easyMove = MOVE_NONE;
624 // Check for some early stop condition
625 if (!StopRequest && Limits.useTimeManagement())
627 // Stop search early if one move seems to be much better than the
628 // others or if there is only a single legal move. Also in the latter
629 // case we search up to some depth anyway to get a proper score.
631 && easyMove == bestMove
633 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
634 && current_search_time() > TimeMgr.available_time() / 16)
635 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
636 && current_search_time() > TimeMgr.available_time() / 32)))
639 // Take in account some extra time if the best move has changed
640 if (depth > 4 && depth < 50)
641 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
643 // Stop search if most of available time is already consumed. We probably don't
644 // have enough time to search the first move at the next iteration anyway.
645 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
648 // If we are allowed to ponder do not stop the search now but keep pondering
649 if (StopRequest && Limits.ponder)
652 StopOnPonderhit = true;
657 // When using skills overwrite best and ponder moves with the sub-optimal ones
658 if (SkillLevelEnabled)
660 if (skillBest == MOVE_NONE) // Still unassigned ?
661 do_skill_level(&skillBest, &skillPonder);
663 bestMove = skillBest;
664 *ponderMove = skillPonder;
671 // search<>() is the main search function for both PV and non-PV nodes and for
672 // normal and SplitPoint nodes. When called just after a split point the search
673 // is simpler because we have already probed the hash table, done a null move
674 // search, and searched the first move before splitting, we don't have to repeat
675 // all this work again. We also don't need to store anything to the hash table
676 // here: This is taken care of after we return from the split point.
678 template <NodeType NT>
679 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
681 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV);
682 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV);
683 const bool RootNode = (NT == Root);
685 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
686 assert(beta > alpha && beta <= VALUE_INFINITE);
687 assert(PvNode || alpha == beta - 1);
688 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
690 Move movesSearched[MAX_MOVES];
695 Move ttMove, move, excludedMove, threatMove;
698 Value bestValue, value, oldAlpha;
699 Value refinedValue, nullValue, futilityBase, futilityValueScaled; // Non-PV specific
700 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
701 int moveCount = 0, playedMoveCount = 0;
702 Thread& thread = Threads[pos.thread()];
703 SplitPoint* sp = NULL;
705 refinedValue = bestValue = value = -VALUE_INFINITE;
707 inCheck = pos.in_check();
708 ss->ply = (ss-1)->ply + 1;
710 // Used to send selDepth info to GUI
711 if (PvNode && thread.maxPly < ss->ply)
712 thread.maxPly = ss->ply;
714 // Step 1. Initialize node and poll. Polling can abort search
717 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
718 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
719 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
725 ttMove = excludedMove = MOVE_NONE;
726 threatMove = sp->threatMove;
727 goto split_point_start;
730 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
736 // Step 2. Check for aborted search and immediate draw
738 || pos.is_draw<false>()
739 || ss->ply > PLY_MAX) && !RootNode)
742 // Step 3. Mate distance pruning
745 alpha = Max(value_mated_in(ss->ply), alpha);
746 beta = Min(value_mate_in(ss->ply+1), beta);
751 // Step 4. Transposition table lookup
752 // We don't want the score of a partial search to overwrite a previous full search
753 // TT value, so we use a different position key in case of an excluded move.
754 excludedMove = ss->excludedMove;
755 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
756 tte = TT.probe(posKey);
757 ttMove = tte ? tte->move() : MOVE_NONE;
759 // At PV nodes we check for exact scores, while at non-PV nodes we check for
760 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
761 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
762 // we should also update RootMoveList to avoid bogus output.
763 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
764 : ok_to_use_TT(tte, depth, beta, ss->ply)))
767 ss->bestMove = ttMove; // Can be MOVE_NONE
768 return value_from_tt(tte->value(), ss->ply);
771 // Step 5. Evaluate the position statically and update parent's gain statistics
773 ss->eval = ss->evalMargin = VALUE_NONE;
776 assert(tte->static_value() != VALUE_NONE);
778 ss->eval = tte->static_value();
779 ss->evalMargin = tte->static_value_margin();
780 refinedValue = refine_eval(tte, ss->eval, ss->ply);
784 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
785 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
788 // Save gain for the parent non-capture move
789 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
791 // Step 6. Razoring (is omitted in PV nodes)
793 && depth < RazorDepth
795 && refinedValue + razor_margin(depth) < beta
796 && ttMove == MOVE_NONE
797 && abs(beta) < VALUE_MATE_IN_PLY_MAX
798 && !pos.has_pawn_on_7th(pos.side_to_move()))
800 Value rbeta = beta - razor_margin(depth);
801 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
803 // Logically we should return (v + razor_margin(depth)), but
804 // surprisingly this did slightly weaker in tests.
808 // Step 7. Static null move pruning (is omitted in PV nodes)
809 // We're betting that the opponent doesn't have a move that will reduce
810 // the score by more than futility_margin(depth) if we do a null move.
813 && depth < RazorDepth
815 && refinedValue - futility_margin(depth, 0) >= beta
816 && abs(beta) < VALUE_MATE_IN_PLY_MAX
817 && pos.non_pawn_material(pos.side_to_move()))
818 return refinedValue - futility_margin(depth, 0);
820 // Step 8. Null move search with verification search (is omitted in PV nodes)
825 && refinedValue >= beta
826 && abs(beta) < VALUE_MATE_IN_PLY_MAX
827 && pos.non_pawn_material(pos.side_to_move()))
829 ss->currentMove = MOVE_NULL;
831 // Null move dynamic reduction based on depth
832 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
834 // Null move dynamic reduction based on value
835 if (refinedValue - PawnValueMidgame > beta)
838 pos.do_null_move(st);
839 (ss+1)->skipNullMove = true;
840 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
841 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
842 (ss+1)->skipNullMove = false;
843 pos.undo_null_move();
845 if (nullValue >= beta)
847 // Do not return unproven mate scores
848 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
851 if (depth < 6 * ONE_PLY)
854 // Do verification search at high depths
855 ss->skipNullMove = true;
856 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
857 ss->skipNullMove = false;
864 // The null move failed low, which means that we may be faced with
865 // some kind of threat. If the previous move was reduced, check if
866 // the move that refuted the null move was somehow connected to the
867 // move which was reduced. If a connection is found, return a fail
868 // low score (which will cause the reduced move to fail high in the
869 // parent node, which will trigger a re-search with full depth).
870 threatMove = (ss+1)->bestMove;
872 if ( depth < ThreatDepth
874 && threatMove != MOVE_NONE
875 && connected_moves(pos, (ss-1)->currentMove, threatMove))
880 // Step 9. ProbCut (is omitted in PV nodes)
881 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
882 // and a reduced search returns a value much above beta, we can (almost) safely
883 // prune the previous move.
885 && depth >= RazorDepth + ONE_PLY
888 && excludedMove == MOVE_NONE
889 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
891 Value rbeta = beta + 200;
892 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
894 assert(rdepth >= ONE_PLY);
896 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
899 while ((move = mp.get_next_move()) != MOVE_NONE)
900 if (pos.pl_move_is_legal(move, ci.pinned))
902 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
903 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
910 // Step 10. Internal iterative deepening
911 if ( depth >= IIDDepth[PvNode]
912 && ttMove == MOVE_NONE
913 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
915 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
917 ss->skipNullMove = true;
918 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
919 ss->skipNullMove = false;
921 tte = TT.probe(posKey);
922 ttMove = tte ? tte->move() : MOVE_NONE;
925 split_point_start: // At split points actual search starts from here
927 // Initialize a MovePicker object for the current position
928 MovePickerExt<NT> mp(pos, RootNode ? Rml[0].pv[0] : ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
930 ss->bestMove = MOVE_NONE;
931 futilityBase = ss->eval + ss->evalMargin;
932 singularExtensionNode = !RootNode
934 && depth >= SingularExtensionDepth[PvNode]
935 && ttMove != MOVE_NONE
936 && !excludedMove // Do not allow recursive singular extension search
937 && (tte->type() & VALUE_TYPE_LOWER)
938 && tte->depth() >= depth - 3 * ONE_PLY;
941 lock_grab(&(sp->lock));
942 bestValue = sp->bestValue;
945 // Step 11. Loop through moves
946 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
947 while ( bestValue < beta
948 && (move = mp.get_next_move()) != MOVE_NONE
949 && !thread.cutoff_occurred())
951 assert(move_is_ok(move));
953 if (move == excludedMove)
956 // At root obey the "searchmoves" option and skip moves not listed in Root Move List
957 if (RootNode && !Rml.find(move))
960 // At PV and SpNode nodes we want all moves to be legal since the beginning
961 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
966 moveCount = ++sp->moveCount;
967 lock_release(&(sp->lock));
974 // This is used by time management
975 FirstRootMove = (moveCount == 1);
977 // Save the current node count before the move is searched
978 nodes = pos.nodes_searched();
980 // If it's time to send nodes info, do it here where we have the
981 // correct accumulated node counts searched by each thread.
982 if (SendSearchedNodes)
984 SendSearchedNodes = false;
985 cout << "info" << speed_to_uci(pos.nodes_searched()) << endl;
988 // For long searches send current move info to GUI
989 if (current_search_time() > 2000)
990 cout << "info" << depth_to_uci(depth)
991 << " currmove " << move << " currmovenumber " << moveCount << endl;
994 // At Root and at first iteration do a PV search on all the moves to score root moves
995 isPvMove = (PvNode && moveCount <= (!RootNode ? 1 : depth <= ONE_PLY ? MAX_MOVES : MultiPV));
996 givesCheck = pos.move_gives_check(move, ci);
997 captureOrPromotion = pos.move_is_capture_or_promotion(move);
999 // Step 12. Decide the new search depth
1000 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1002 // Singular extension search. If all moves but one fail low on a search of
1003 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1004 // is singular and should be extended. To verify this we do a reduced search
1005 // on all the other moves but the ttMove, if result is lower than ttValue minus
1006 // a margin then we extend ttMove.
1007 if ( singularExtensionNode
1009 && pos.pl_move_is_legal(move, ci.pinned)
1012 Value ttValue = value_from_tt(tte->value(), ss->ply);
1014 if (abs(ttValue) < VALUE_KNOWN_WIN)
1016 Value rBeta = ttValue - int(depth);
1017 ss->excludedMove = move;
1018 ss->skipNullMove = true;
1019 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1020 ss->skipNullMove = false;
1021 ss->excludedMove = MOVE_NONE;
1022 ss->bestMove = MOVE_NONE;
1028 // Update current move (this must be done after singular extension search)
1029 newDepth = depth - ONE_PLY + ext;
1031 // Step 13. Futility pruning (is omitted in PV nodes)
1033 && !captureOrPromotion
1037 && !move_is_castle(move))
1039 // Move count based pruning
1040 if ( moveCount >= futility_move_count(depth)
1041 && (!threatMove || !connected_threat(pos, move, threatMove))
1042 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1045 lock_grab(&(sp->lock));
1050 // Value based pruning
1051 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1052 // but fixing this made program slightly weaker.
1053 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1054 futilityValueScaled = futilityBase + futility_margin(predictedDepth, moveCount)
1055 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1057 if (futilityValueScaled < beta)
1061 lock_grab(&(sp->lock));
1062 if (futilityValueScaled > sp->bestValue)
1063 sp->bestValue = bestValue = futilityValueScaled;
1065 else if (futilityValueScaled > bestValue)
1066 bestValue = futilityValueScaled;
1071 // Prune moves with negative SEE at low depths
1072 if ( predictedDepth < 2 * ONE_PLY
1073 && bestValue > VALUE_MATED_IN_PLY_MAX
1074 && pos.see_sign(move) < 0)
1077 lock_grab(&(sp->lock));
1083 // Check for legality only before to do the move
1084 if (!pos.pl_move_is_legal(move, ci.pinned))
1090 ss->currentMove = move;
1091 if (!SpNode && !captureOrPromotion)
1092 movesSearched[playedMoveCount++] = move;
1094 // Step 14. Make the move
1095 pos.do_move(move, st, ci, givesCheck);
1097 // Step extra. pv search (only in PV nodes)
1098 // The first move in list is the expected PV
1100 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1101 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1104 // Step 15. Reduced depth search
1105 // If the move fails high will be re-searched at full depth.
1106 bool doFullDepthSearch = true;
1108 if ( depth > 3 * ONE_PLY
1109 && !captureOrPromotion
1111 && !move_is_castle(move)
1112 && ss->killers[0] != move
1113 && ss->killers[1] != move
1114 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1116 Depth d = newDepth - ss->reduction;
1117 alpha = SpNode ? sp->alpha : alpha;
1119 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1120 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1122 ss->reduction = DEPTH_ZERO;
1123 doFullDepthSearch = (value > alpha);
1126 // Step 16. Full depth search
1127 if (doFullDepthSearch)
1129 alpha = SpNode ? sp->alpha : alpha;
1130 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1131 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1133 // Step extra. pv search (only in PV nodes)
1134 // Search only for possible new PV nodes, if instead value >= beta then
1135 // parent node fails low with value <= alpha and tries another move.
1136 if (PvNode && value > alpha && (RootNode || value < beta))
1137 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1138 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1142 // Step 17. Undo move
1143 pos.undo_move(move);
1145 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1147 // Step 18. Check for new best move
1150 lock_grab(&(sp->lock));
1151 bestValue = sp->bestValue;
1155 if (value > bestValue)
1158 ss->bestMove = move;
1163 && value < beta) // We want always alpha < beta
1166 if (SpNode && !thread.cutoff_occurred())
1168 sp->bestValue = value;
1169 sp->ss->bestMove = move;
1171 sp->is_betaCutoff = (value >= beta);
1177 // Finished searching the move. If StopRequest is true, the search
1178 // was aborted because the user interrupted the search or because we
1179 // ran out of time. In this case, the return value of the search cannot
1180 // be trusted, and we break out of the loop without updating the best
1185 // Remember searched nodes counts for this move
1186 Rml.find(move)->nodes += pos.nodes_searched() - nodes;
1188 // PV move or new best move ?
1189 if (isPvMove || value > alpha)
1192 Rml.find(move)->pv_score = value;
1193 Rml.find(move)->extract_pv_from_tt(pos);
1195 // We record how often the best move has been changed in each
1196 // iteration. This information is used for time management: When
1197 // the best move changes frequently, we allocate some more time.
1198 if (!isPvMove && MultiPV == 1)
1199 Rml.bestMoveChanges++;
1206 // All other moves but the PV are set to the lowest value, this
1207 // is not a problem when sorting becuase sort is stable and move
1208 // position in the list is preserved, just the PV is pushed up.
1209 Rml.find(move)->pv_score = -VALUE_INFINITE;
1213 // Step 19. Check for split
1216 && depth >= Threads.min_split_depth()
1218 && Threads.available_slave_exists(pos.thread())
1220 && !thread.cutoff_occurred())
1221 Threads.split<FakeSplit>(pos, ss, &alpha, beta, &bestValue, depth,
1222 threatMove, moveCount, &mp, PvNode);
1225 // Step 20. Check for mate and stalemate
1226 // All legal moves have been searched and if there are
1227 // no legal moves, it must be mate or stalemate.
1228 // If one move was excluded return fail low score.
1229 if (!SpNode && !moveCount)
1230 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1232 // Step 21. Update tables
1233 // If the search is not aborted, update the transposition table,
1234 // history counters, and killer moves.
1235 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1237 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1238 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1239 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1241 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1243 // Update killers and history only for non capture moves that fails high
1244 if ( bestValue >= beta
1245 && !pos.move_is_capture_or_promotion(move))
1247 if (move != ss->killers[0])
1249 ss->killers[1] = ss->killers[0];
1250 ss->killers[0] = move;
1252 update_history(pos, move, depth, movesSearched, playedMoveCount);
1258 // Here we have the lock still grabbed
1259 sp->is_slave[pos.thread()] = false;
1260 sp->nodes += pos.nodes_searched();
1261 lock_release(&(sp->lock));
1264 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1269 // qsearch() is the quiescence search function, which is called by the main
1270 // search function when the remaining depth is zero (or, to be more precise,
1271 // less than ONE_PLY).
1273 template <NodeType NT>
1274 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1276 const bool PvNode = (NT == PV);
1278 assert(NT == PV || NT == NonPV);
1279 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1280 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1281 assert(PvNode || alpha == beta - 1);
1283 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1287 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1288 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1291 Value oldAlpha = alpha;
1293 ss->bestMove = ss->currentMove = MOVE_NONE;
1294 ss->ply = (ss-1)->ply + 1;
1296 // Check for an instant draw or maximum ply reached
1297 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1300 // Decide whether or not to include checks, this fixes also the type of
1301 // TT entry depth that we are going to use. Note that in qsearch we use
1302 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1303 inCheck = pos.in_check();
1304 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1306 // Transposition table lookup. At PV nodes, we don't use the TT for
1307 // pruning, but only for move ordering.
1308 tte = TT.probe(pos.get_key());
1309 ttMove = (tte ? tte->move() : MOVE_NONE);
1311 if (!PvNode && tte && ok_to_use_TT(tte, ttDepth, beta, ss->ply))
1313 ss->bestMove = ttMove; // Can be MOVE_NONE
1314 return value_from_tt(tte->value(), ss->ply);
1317 // Evaluate the position statically
1320 bestValue = futilityBase = -VALUE_INFINITE;
1321 ss->eval = evalMargin = VALUE_NONE;
1322 enoughMaterial = false;
1328 assert(tte->static_value() != VALUE_NONE);
1330 evalMargin = tte->static_value_margin();
1331 ss->eval = bestValue = tte->static_value();
1334 ss->eval = bestValue = evaluate(pos, evalMargin);
1336 // Stand pat. Return immediately if static value is at least beta
1337 if (bestValue >= beta)
1340 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1345 if (PvNode && bestValue > alpha)
1348 // Futility pruning parameters, not needed when in check
1349 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1350 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1353 // Initialize a MovePicker object for the current position, and prepare
1354 // to search the moves. Because the depth is <= 0 here, only captures,
1355 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1357 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1360 // Loop through the moves until no moves remain or a beta cutoff occurs
1361 while ( alpha < beta
1362 && (move = mp.get_next_move()) != MOVE_NONE)
1364 assert(move_is_ok(move));
1366 givesCheck = pos.move_gives_check(move, ci);
1374 && !move_is_promotion(move)
1375 && !pos.move_is_passed_pawn_push(move))
1377 futilityValue = futilityBase
1378 + piece_value_endgame(pos.piece_on(move_to(move)))
1379 + (move_is_ep(move) ? PawnValueEndgame : VALUE_ZERO);
1381 if (futilityValue < alpha)
1383 if (futilityValue > bestValue)
1384 bestValue = futilityValue;
1388 // Prune moves with negative or equal SEE
1389 if ( futilityBase < beta
1390 && depth < DEPTH_ZERO
1391 && pos.see(move) <= 0)
1395 // Detect non-capture evasions that are candidate to be pruned
1396 evasionPrunable = !PvNode
1398 && bestValue > VALUE_MATED_IN_PLY_MAX
1399 && !pos.move_is_capture(move)
1400 && !pos.can_castle(pos.side_to_move());
1402 // Don't search moves with negative SEE values
1404 && (!inCheck || evasionPrunable)
1406 && !move_is_promotion(move)
1407 && pos.see_sign(move) < 0)
1410 // Don't search useless checks
1415 && !pos.move_is_capture_or_promotion(move)
1416 && ss->eval + PawnValueMidgame / 4 < beta
1417 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1419 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1420 bestValue = ss->eval + PawnValueMidgame / 4;
1425 // Check for legality only before to do the move
1426 if (!pos.pl_move_is_legal(move, ci.pinned))
1429 // Update current move
1430 ss->currentMove = move;
1432 // Make and search the move
1433 pos.do_move(move, st, ci, givesCheck);
1434 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1435 pos.undo_move(move);
1437 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1440 if (value > bestValue)
1446 ss->bestMove = move;
1451 // All legal moves have been searched. A special case: If we're in check
1452 // and no legal moves were found, it is checkmate.
1453 if (inCheck && bestValue == -VALUE_INFINITE)
1454 return value_mated_in(ss->ply);
1456 // Update transposition table
1457 ValueType vt = (bestValue <= oldAlpha ? VALUE_TYPE_UPPER : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT);
1458 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, ss->bestMove, ss->eval, evalMargin);
1460 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1466 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1467 // bestValue is updated only when returning false because in that case move
1470 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1472 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1473 Square from, to, ksq, victimSq;
1476 Value futilityValue, bv = *bestValue;
1478 from = move_from(move);
1480 them = opposite_color(pos.side_to_move());
1481 ksq = pos.king_square(them);
1482 kingAtt = pos.attacks_from<KING>(ksq);
1483 pc = pos.piece_on(from);
1485 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1486 oldAtt = pos.attacks_from(pc, from, occ);
1487 newAtt = pos.attacks_from(pc, to, occ);
1489 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1490 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1492 if (!(b && (b & (b - 1))))
1495 // Rule 2. Queen contact check is very dangerous
1496 if ( piece_type(pc) == QUEEN
1497 && bit_is_set(kingAtt, to))
1500 // Rule 3. Creating new double threats with checks
1501 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1505 victimSq = pop_1st_bit(&b);
1506 futilityValue = futilityBase + piece_value_endgame(pos.piece_on(victimSq));
1508 // Note that here we generate illegal "double move"!
1509 if ( futilityValue >= beta
1510 && pos.see_sign(make_move(from, victimSq)) >= 0)
1513 if (futilityValue > bv)
1517 // Update bestValue only if check is not dangerous (because we will prune the move)
1523 // connected_moves() tests whether two moves are 'connected' in the sense
1524 // that the first move somehow made the second move possible (for instance
1525 // if the moving piece is the same in both moves). The first move is assumed
1526 // to be the move that was made to reach the current position, while the
1527 // second move is assumed to be a move from the current position.
1529 bool connected_moves(const Position& pos, Move m1, Move m2) {
1531 Square f1, t1, f2, t2;
1535 assert(m1 && move_is_ok(m1));
1536 assert(m2 && move_is_ok(m2));
1538 // Case 1: The moving piece is the same in both moves
1544 // Case 2: The destination square for m2 was vacated by m1
1550 // Case 3: Moving through the vacated square
1551 p2 = pos.piece_on(f2);
1552 if ( piece_is_slider(p2)
1553 && bit_is_set(squares_between(f2, t2), f1))
1556 // Case 4: The destination square for m2 is defended by the moving piece in m1
1557 p1 = pos.piece_on(t1);
1558 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1561 // Case 5: Discovered check, checking piece is the piece moved in m1
1562 ksq = pos.king_square(pos.side_to_move());
1563 if ( piece_is_slider(p1)
1564 && bit_is_set(squares_between(t1, ksq), f2))
1566 Bitboard occ = pos.occupied_squares();
1567 clear_bit(&occ, f2);
1568 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1575 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1576 // "plies to mate from the current ply". Non-mate scores are unchanged.
1577 // The function is called before storing a value to the transposition table.
1579 Value value_to_tt(Value v, int ply) {
1581 if (v >= VALUE_MATE_IN_PLY_MAX)
1584 if (v <= VALUE_MATED_IN_PLY_MAX)
1591 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1592 // the transposition table to a mate score corrected for the current ply.
1594 Value value_from_tt(Value v, int ply) {
1596 if (v >= VALUE_MATE_IN_PLY_MAX)
1599 if (v <= VALUE_MATED_IN_PLY_MAX)
1606 // connected_threat() tests whether it is safe to forward prune a move or if
1607 // is somehow connected to the threat move returned by null search.
1609 bool connected_threat(const Position& pos, Move m, Move threat) {
1611 assert(move_is_ok(m));
1612 assert(threat && move_is_ok(threat));
1613 assert(!pos.move_is_capture_or_promotion(m));
1614 assert(!pos.move_is_passed_pawn_push(m));
1616 Square mfrom, mto, tfrom, tto;
1618 mfrom = move_from(m);
1620 tfrom = move_from(threat);
1621 tto = move_to(threat);
1623 // Case 1: Don't prune moves which move the threatened piece
1627 // Case 2: If the threatened piece has value less than or equal to the
1628 // value of the threatening piece, don't prune moves which defend it.
1629 if ( pos.move_is_capture(threat)
1630 && ( piece_value_midgame(pos.piece_on(tfrom)) >= piece_value_midgame(pos.piece_on(tto))
1631 || piece_type(pos.piece_on(tfrom)) == KING)
1632 && pos.move_attacks_square(m, tto))
1635 // Case 3: If the moving piece in the threatened move is a slider, don't
1636 // prune safe moves which block its ray.
1637 if ( piece_is_slider(pos.piece_on(tfrom))
1638 && bit_is_set(squares_between(tfrom, tto), mto)
1639 && pos.see_sign(m) >= 0)
1646 // ok_to_use_TT() returns true if a transposition table score
1647 // can be used at a given point in search.
1649 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
1651 Value v = value_from_tt(tte->value(), ply);
1653 return ( tte->depth() >= depth
1654 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1655 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1657 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1658 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1662 // refine_eval() returns the transposition table score if
1663 // possible otherwise falls back on static position evaluation.
1665 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1669 Value v = value_from_tt(tte->value(), ply);
1671 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1672 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1679 // update_history() registers a good move that produced a beta-cutoff
1680 // in history and marks as failures all the other moves of that ply.
1682 void update_history(const Position& pos, Move move, Depth depth,
1683 Move movesSearched[], int moveCount) {
1685 Value bonus = Value(int(depth) * int(depth));
1687 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1689 for (int i = 0; i < moveCount - 1; i++)
1691 m = movesSearched[i];
1695 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1700 // update_gains() updates the gains table of a non-capture move given
1701 // the static position evaluation before and after the move.
1703 void update_gains(const Position& pos, Move m, Value before, Value after) {
1706 && before != VALUE_NONE
1707 && after != VALUE_NONE
1708 && pos.captured_piece_type() == PIECE_TYPE_NONE
1709 && !move_is_special(m))
1710 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1714 // current_search_time() returns the number of milliseconds which have passed
1715 // since the beginning of the current search.
1717 int current_search_time(int set) {
1719 static int searchStartTime;
1722 searchStartTime = set;
1724 return get_system_time() - searchStartTime;
1728 // score_to_uci() converts a value to a string suitable for use with the UCI
1729 // protocol specifications:
1731 // cp <x> The score from the engine's point of view in centipawns.
1732 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1733 // use negative values for y.
1735 string score_to_uci(Value v, Value alpha, Value beta) {
1737 std::stringstream s;
1739 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1740 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1742 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1744 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1750 // speed_to_uci() returns a string with time stats of current search suitable
1751 // to be sent to UCI gui.
1753 string speed_to_uci(int64_t nodes) {
1755 std::stringstream s;
1756 int t = current_search_time();
1758 s << " nodes " << nodes
1759 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1765 // pv_to_uci() returns a string with information on the current PV line
1766 // formatted according to UCI specification.
1768 string pv_to_uci(Move pv[], int pvNum, bool chess960) {
1770 std::stringstream s;
1772 s << " multipv " << pvNum << " pv " << set960(chess960);
1774 for ( ; *pv != MOVE_NONE; pv++)
1780 // depth_to_uci() returns a string with information on the current depth and
1781 // seldepth formatted according to UCI specification.
1783 string depth_to_uci(Depth depth) {
1785 std::stringstream s;
1787 // Retrieve max searched depth among threads
1789 for (int i = 0; i < Threads.size(); i++)
1790 if (Threads[i].maxPly > selDepth)
1791 selDepth = Threads[i].maxPly;
1793 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1798 string time_to_string(int millisecs) {
1800 const int MSecMinute = 1000 * 60;
1801 const int MSecHour = 1000 * 60 * 60;
1803 int hours = millisecs / MSecHour;
1804 int minutes = (millisecs % MSecHour) / MSecMinute;
1805 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1807 std::stringstream s;
1812 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1816 string score_to_string(Value v) {
1818 std::stringstream s;
1820 if (v >= VALUE_MATE_IN_PLY_MAX)
1821 s << "#" << (VALUE_MATE - v + 1) / 2;
1822 else if (v <= VALUE_MATED_IN_PLY_MAX)
1823 s << "-#" << (VALUE_MATE + v) / 2;
1825 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1830 // pretty_pv() creates a human-readable string from a position and a PV.
1831 // It is used to write search information to the log file (which is created
1832 // when the UCI parameter "Use Search Log" is "true").
1834 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1836 const int64_t K = 1000;
1837 const int64_t M = 1000000;
1838 const int startColumn = 28;
1839 const size_t maxLength = 80 - startColumn;
1841 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1844 std::stringstream s;
1847 // First print depth, score, time and searched nodes...
1848 s << set960(pos.is_chess960())
1849 << std::setw(2) << depth
1850 << std::setw(8) << score_to_string(value)
1851 << std::setw(8) << time_to_string(time);
1853 if (pos.nodes_searched() < M)
1854 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1855 else if (pos.nodes_searched() < K * M)
1856 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1858 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1860 // ...then print the full PV line in short algebraic notation
1861 while (*m != MOVE_NONE)
1863 san = move_to_san(pos, *m);
1864 length += san.length() + 1;
1866 if (length > maxLength)
1868 length = san.length() + 1;
1869 s << "\n" + string(startColumn, ' ');
1873 pos.do_move(*m++, *st++);
1876 // Restore original position before to leave
1877 while (m != pv) pos.undo_move(*--m);
1882 // poll() performs two different functions: It polls for user input, and it
1883 // looks at the time consumed so far and decides if it's time to abort the
1886 void poll(const Position& pos) {
1888 static int lastInfoTime;
1889 int t = current_search_time();
1892 if (input_available())
1894 // We are line oriented, don't read single chars
1897 if (!std::getline(std::cin, command) || command == "quit")
1899 // Quit the program as soon as possible
1900 Limits.ponder = false;
1901 QuitRequest = StopRequest = true;
1904 else if (command == "stop")
1906 // Stop calculating as soon as possible, but still send the "bestmove"
1907 // and possibly the "ponder" token when finishing the search.
1908 Limits.ponder = false;
1911 else if (command == "ponderhit")
1913 // The opponent has played the expected move. GUI sends "ponderhit" if
1914 // we were told to ponder on the same move the opponent has played. We
1915 // should continue searching but switching from pondering to normal search.
1916 Limits.ponder = false;
1918 if (StopOnPonderhit)
1923 // Print search information
1927 else if (lastInfoTime > t)
1928 // HACK: Must be a new search where we searched less than
1929 // NodesBetweenPolls nodes during the first second of search.
1932 else if (t - lastInfoTime >= 1000)
1937 dbg_print_hit_rate();
1939 // Send info on searched nodes as soon as we return to root
1940 SendSearchedNodes = true;
1943 // Should we stop the search?
1947 bool stillAtFirstMove = FirstRootMove
1948 && !AspirationFailLow
1949 && t > TimeMgr.available_time();
1951 bool noMoreTime = t > TimeMgr.maximum_time()
1952 || stillAtFirstMove;
1954 if ( (Limits.useTimeManagement() && noMoreTime)
1955 || (Limits.maxTime && t >= Limits.maxTime)
1956 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1961 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1962 // while the program is pondering. The point is to work around a wrinkle in
1963 // the UCI protocol: When pondering, the engine is not allowed to give a
1964 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1965 // We simply wait here until one of these commands is sent, and return,
1966 // after which the bestmove and pondermove will be printed.
1968 void wait_for_stop_or_ponderhit() {
1972 // Wait for a command from stdin
1973 while ( std::getline(std::cin, command)
1974 && command != "ponderhit" && command != "stop" && command != "quit") {};
1976 if (command != "ponderhit" && command != "stop")
1977 QuitRequest = true; // Must be "quit" or getline() returned false
1981 // When playing with strength handicap choose best move among the MultiPV set
1982 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1983 void do_skill_level(Move* best, Move* ponder) {
1985 assert(MultiPV > 1);
1989 // Rml list is already sorted by pv_score in descending order
1991 int max_s = -VALUE_INFINITE;
1992 int size = Min(MultiPV, (int)Rml.size());
1993 int max = Rml[0].pv_score;
1994 int var = Min(max - Rml[size - 1].pv_score, PawnValueMidgame);
1995 int wk = 120 - 2 * SkillLevel;
1997 // PRNG sequence should be non deterministic
1998 for (int i = abs(get_system_time() % 50); i > 0; i--)
1999 rk.rand<unsigned>();
2001 // Choose best move. For each move's score we add two terms both dependent
2002 // on wk, one deterministic and bigger for weaker moves, and one random,
2003 // then we choose the move with the resulting highest score.
2004 for (int i = 0; i < size; i++)
2006 s = Rml[i].pv_score;
2008 // Don't allow crazy blunders even at very low skills
2009 if (i > 0 && Rml[i-1].pv_score > s + EasyMoveMargin)
2012 // This is our magical formula
2013 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
2018 *best = Rml[i].pv[0];
2019 *ponder = Rml[i].pv[1];
2025 /// RootMove and RootMoveList method's definitions
2027 RootMove::RootMove() {
2030 pv_score = -VALUE_INFINITE;
2034 RootMove& RootMove::operator=(const RootMove& rm) {
2036 const Move* src = rm.pv;
2039 // Avoid a costly full rm.pv[] copy
2040 do *dst++ = *src; while (*src++ != MOVE_NONE);
2043 pv_score = rm.pv_score;
2047 void RootMoveList::init(Position& pos, Move searchMoves[]) {
2050 bestMoveChanges = 0;
2053 // Generate all legal moves and add them to RootMoveList
2054 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2056 // If we have a searchMoves[] list then verify the move
2057 // is in the list before to add it.
2058 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2060 if (sm != searchMoves && *sm != ml.move())
2064 rm.pv[0] = ml.move();
2065 rm.pv[1] = MOVE_NONE;
2066 rm.pv_score = -VALUE_INFINITE;
2071 RootMove* RootMoveList::find(const Move &m) {
2073 for (int i = 0; i < int(size()); i++)
2075 if ((*this)[i].pv[0] == m)
2082 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2083 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2084 // allow to always have a ponder move even when we fail high at root and also a
2085 // long PV to print that is important for position analysis.
2087 void RootMove::extract_pv_from_tt(Position& pos) {
2089 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2093 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2095 pos.do_move(pv[0], *st++);
2097 while ( (tte = TT.probe(pos.get_key())) != NULL
2098 && tte->move() != MOVE_NONE
2099 && pos.move_is_pl(tte->move())
2100 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2102 && (!pos.is_draw<false>() || ply < 2))
2104 pv[ply] = tte->move();
2105 pos.do_move(pv[ply++], *st++);
2107 pv[ply] = MOVE_NONE;
2109 do pos.undo_move(pv[--ply]); while (ply);
2112 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2113 // the PV back into the TT. This makes sure the old PV moves are searched
2114 // first, even if the old TT entries have been overwritten.
2116 void RootMove::insert_pv_in_tt(Position& pos) {
2118 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2121 Value v, m = VALUE_NONE;
2124 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2130 // Don't overwrite existing correct entries
2131 if (!tte || tte->move() != pv[ply])
2133 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2134 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2136 pos.do_move(pv[ply], *st++);
2138 } while (pv[++ply] != MOVE_NONE);
2140 do pos.undo_move(pv[--ply]); while (ply);
2145 // ThreadsManager::idle_loop() is where the threads are parked when they have no work
2146 // to do. The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint
2147 // object for which the current thread is the master.
2149 void ThreadsManager::idle_loop(int threadID, SplitPoint* sp) {
2151 assert(threadID >= 0 && threadID < MAX_THREADS);
2158 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2159 // master should exit as last one.
2160 if (allThreadsShouldExit)
2163 threads[threadID].state = Thread::TERMINATED;
2167 // If we are not thinking, wait for a condition to be signaled
2168 // instead of wasting CPU time polling for work.
2169 while ( threadID >= activeThreads
2170 || threads[threadID].state == Thread::INITIALIZING
2171 || (useSleepingThreads && threads[threadID].state == Thread::AVAILABLE))
2173 assert(!sp || useSleepingThreads);
2174 assert(threadID != 0 || useSleepingThreads);
2176 if (threads[threadID].state == Thread::INITIALIZING)
2177 threads[threadID].state = Thread::AVAILABLE;
2179 // Grab the lock to avoid races with Thread::wake_up()
2180 lock_grab(&threads[threadID].sleepLock);
2182 // If we are master and all slaves have finished do not go to sleep
2183 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2184 allFinished = (i == activeThreads);
2186 if (allFinished || allThreadsShouldExit)
2188 lock_release(&threads[threadID].sleepLock);
2192 // Do sleep here after retesting sleep conditions
2193 if (threadID >= activeThreads || threads[threadID].state == Thread::AVAILABLE)
2194 cond_wait(&threads[threadID].sleepCond, &threads[threadID].sleepLock);
2196 lock_release(&threads[threadID].sleepLock);
2199 // If this thread has been assigned work, launch a search
2200 if (threads[threadID].state == Thread::WORKISWAITING)
2202 assert(!allThreadsShouldExit);
2204 threads[threadID].state = Thread::SEARCHING;
2206 // Copy split point position and search stack and call search()
2207 // with SplitPoint template parameter set to true.
2208 SearchStack ss[PLY_MAX_PLUS_2];
2209 SplitPoint* tsp = threads[threadID].splitPoint;
2210 Position pos(*tsp->pos, threadID);
2212 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2216 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2218 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2220 assert(threads[threadID].state == Thread::SEARCHING);
2222 threads[threadID].state = Thread::AVAILABLE;
2224 // Wake up master thread so to allow it to return from the idle loop in
2225 // case we are the last slave of the split point.
2226 if ( useSleepingThreads
2227 && threadID != tsp->master
2228 && threads[tsp->master].state == Thread::AVAILABLE)
2229 threads[tsp->master].wake_up();
2232 // If this thread is the master of a split point and all slaves have
2233 // finished their work at this split point, return from the idle loop.
2234 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2235 allFinished = (i == activeThreads);
2239 // Because sp->slaves[] is reset under lock protection,
2240 // be sure sp->lock has been released before to return.
2241 lock_grab(&(sp->lock));
2242 lock_release(&(sp->lock));
2244 // In helpful master concept a master can help only a sub-tree, and
2245 // because here is all finished is not possible master is booked.
2246 assert(threads[threadID].state == Thread::AVAILABLE);
2248 threads[threadID].state = Thread::SEARCHING;