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 two scores, 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, while non_pv_score is computed
58 // according to the order in which moves are returned by MovePicker.
62 RootMove(const RootMove& rm) { *this = rm; }
63 RootMove& operator=(const RootMove& rm);
65 // RootMove::operator<() is the comparison function used when
66 // sorting the moves. A move m1 is considered to be better
67 // than a move m2 if it has an higher pv_score, or if it has
68 // equal pv_score but m1 has the higher non_pv_score. In this way
69 // we are guaranteed that PV moves are always sorted as first.
70 bool operator<(const RootMove& m) const {
71 return pv_score != m.pv_score ? pv_score < m.pv_score
72 : non_pv_score < m.non_pv_score;
75 void extract_pv_from_tt(Position& pos);
76 void insert_pv_in_tt(Position& pos);
81 Move pv[PLY_MAX_PLUS_2];
84 // RootMoveList struct is mainly a std::vector of RootMove objects
85 struct RootMoveList : public std::vector<RootMove> {
86 void init(Position& pos, Move searchMoves[]);
87 RootMove* find(const Move &m);
94 // Lookup table to check if a Piece is a slider and its access function
95 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
96 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
100 // Maximum depth for razoring
101 const Depth RazorDepth = 4 * ONE_PLY;
103 // Dynamic razoring margin based on depth
104 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
106 // Maximum depth for use of dynamic threat detection when null move fails low
107 const Depth ThreatDepth = 5 * ONE_PLY;
109 // Step 9. Internal iterative deepening
111 // Minimum depth for use of internal iterative deepening
112 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
114 // At Non-PV nodes we do an internal iterative deepening search
115 // when the static evaluation is bigger then beta - IIDMargin.
116 const Value IIDMargin = Value(0x100);
118 // Step 11. Decide the new search depth
120 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
121 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
122 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
123 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
124 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
126 // Minimum depth for use of singular extension
127 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
129 // Step 12. Futility pruning
131 // Futility margin for quiescence search
132 const Value FutilityMarginQS = Value(0x80);
134 // Futility lookup tables (initialized at startup) and their access functions
135 Value FutilityMargins[16][64]; // [depth][moveNumber]
136 int FutilityMoveCounts[32]; // [depth]
138 inline Value futility_margin(Depth d, int mn) {
140 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
141 : 2 * VALUE_INFINITE;
144 inline int futility_move_count(Depth d) {
146 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
149 // Step 14. Reduced search
151 // Reduction lookup tables (initialized at startup) and their access function
152 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
154 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
156 return (Depth) Reductions[PvNode][Min(d / ONE_PLY, 63)][Min(mn, 63)];
159 // Easy move margin. An easy move candidate must be at least this much
160 // better than the second best move.
161 const Value EasyMoveMargin = Value(0x200);
164 /// Namespace variables
170 int MultiPV, UCIMultiPV;
172 // Time management variables
173 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
178 std::ofstream LogFile;
180 // Skill level adjustment
182 bool SkillLevelEnabled;
184 // Node counters, used only by thread[0] but try to keep in different cache
185 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
186 bool SendSearchedNodes;
188 int NodesBetweenPolls = 30000;
196 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
198 template <NodeType NT>
199 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
201 template <NodeType NT>
202 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
204 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
205 bool connected_moves(const Position& pos, Move m1, Move m2);
206 Value value_to_tt(Value v, int ply);
207 Value value_from_tt(Value v, int ply);
208 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
209 bool connected_threat(const Position& pos, Move m, Move threat);
210 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
211 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
212 void update_gains(const Position& pos, Move move, Value before, Value after);
213 void do_skill_level(Move* best, Move* ponder);
215 int current_search_time(int set = 0);
216 string score_to_uci(Value v, Value alpha, Value beta);
217 string speed_to_uci(int64_t nodes);
218 string pv_to_uci(Move pv[], int pvNum, bool chess960);
219 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
220 string depth_to_uci(Depth depth);
221 void poll(const Position& pos);
222 void wait_for_stop_or_ponderhit();
224 // MovePickerExt template class extends MovePicker and allows to choose at compile
225 // time the proper moves source according to the type of node. In the default case
226 // we simply create and use a standard MovePicker object.
227 template<NodeType> struct MovePickerExt : public MovePicker {
229 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
230 : MovePicker(p, ttm, d, h, ss, b) {}
233 // In case of a SpNode we use split point's shared MovePicker object as moves source
234 template<> struct MovePickerExt<SplitPointNonPV> : public MovePickerExt<NonPV> {
236 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
237 : MovePickerExt<NonPV>(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
239 Move get_next_move() { return mp->get_next_move(); }
243 template<> struct MovePickerExt<SplitPointPV> : public MovePickerExt<SplitPointNonPV> {
245 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
246 : MovePickerExt<SplitPointNonPV>(p, ttm, d, h, ss, b) {}
249 // Overload operator<<() to make it easier to print moves in a coordinate
250 // notation compatible with UCI protocol.
251 std::ostream& operator<<(std::ostream& os, Move m) {
253 bool chess960 = (os.iword(0) != 0); // See set960()
254 return os << move_to_uci(m, chess960);
257 // When formatting a move for std::cout we must know if we are in Chess960
258 // or not. To keep using the handy operator<<() on the move the trick is to
259 // embed this flag in the stream itself. Function-like named enum set960 is
260 // used as a custom manipulator and the stream internal general-purpose array,
261 // accessed through ios_base::iword(), is used to pass the flag to the move's
262 // operator<<() that will read it to properly format castling moves.
265 std::ostream& operator<< (std::ostream& os, const set960& f) {
267 os.iword(0) = int(f);
271 // extension() decides whether a move should be searched with normal depth,
272 // or with extended depth. Certain classes of moves (checking moves, in
273 // particular) are searched with bigger depth than ordinary moves and in
274 // any case are marked as 'dangerous'. Note that also if a move is not
275 // extended, as example because the corresponding UCI option is set to zero,
276 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
277 template <bool PvNode>
278 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
279 bool moveIsCheck, bool* dangerous) {
280 assert(m != MOVE_NONE);
282 Depth result = DEPTH_ZERO;
283 *dangerous = moveIsCheck;
285 if (moveIsCheck && pos.see_sign(m) >= 0)
286 result += CheckExtension[PvNode];
288 if (piece_type(pos.piece_on(move_from(m))) == PAWN)
290 Color c = pos.side_to_move();
291 if (relative_rank(c, move_to(m)) == RANK_7)
293 result += PawnPushTo7thExtension[PvNode];
296 if (pos.pawn_is_passed(c, move_to(m)))
298 result += PassedPawnExtension[PvNode];
303 if ( captureOrPromotion
304 && piece_type(pos.piece_on(move_to(m))) != PAWN
305 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
306 - piece_value_midgame(pos.piece_on(move_to(m))) == VALUE_ZERO)
307 && !move_is_special(m))
309 result += PawnEndgameExtension[PvNode];
313 return Min(result, ONE_PLY);
319 /// init_search() is called during startup to initialize various lookup tables
323 int d; // depth (ONE_PLY == 2)
324 int hd; // half depth (ONE_PLY == 1)
327 // Init reductions array
328 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
330 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
331 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
332 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
333 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
336 // Init futility margins array
337 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
338 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
340 // Init futility move count array
341 for (d = 0; d < 32; d++)
342 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
346 /// perft() is our utility to verify move generation. All the leaf nodes up to
347 /// the given depth are generated and counted and the sum returned.
349 int64_t perft(Position& pos, Depth depth) {
354 // Generate all legal moves
355 MoveList<MV_LEGAL> ml(pos);
357 // If we are at the last ply we don't need to do and undo
358 // the moves, just to count them.
359 if (depth <= ONE_PLY)
362 // Loop through all legal moves
364 for ( ; !ml.end(); ++ml)
366 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
367 sum += perft(pos, depth - ONE_PLY);
368 pos.undo_move(ml.move());
374 /// think() is the external interface to Stockfish's search, and is called when
375 /// the program receives the UCI 'go' command. It initializes various global
376 /// variables, and calls id_loop(). It returns false when a "quit" command is
377 /// received during the search.
379 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
383 // Initialize global search-related variables
384 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = SendSearchedNodes = false;
386 current_search_time(get_system_time());
388 TimeMgr.init(Limits, pos.startpos_ply_counter());
390 // Set output steram in normal or chess960 mode
391 cout << set960(pos.is_chess960());
393 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
395 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
396 else if (Limits.time && Limits.time < 1000)
397 NodesBetweenPolls = 1000;
398 else if (Limits.time && Limits.time < 5000)
399 NodesBetweenPolls = 5000;
401 NodesBetweenPolls = 30000;
403 // Look for a book move
404 if (Options["OwnBook"].value<bool>())
406 if (Options["Book File"].value<string>() != book.name())
407 book.open(Options["Book File"].value<string>());
409 Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
410 if (bookMove != MOVE_NONE)
413 wait_for_stop_or_ponderhit();
415 cout << "bestmove " << bookMove << endl;
421 UCIMultiPV = Options["MultiPV"].value<int>();
422 SkillLevel = Options["Skill Level"].value<int>();
424 read_evaluation_uci_options(pos.side_to_move());
425 Threads.read_uci_options();
427 // If needed allocate pawn and material hash tables and adjust TT size
428 Threads.init_hash_tables();
429 TT.set_size(Options["Hash"].value<int>());
431 if (Options["Clear Hash"].value<bool>())
433 Options["Clear Hash"].set_value("false");
437 // Do we have to play with skill handicap? In this case enable MultiPV that
438 // we will use behind the scenes to retrieve a set of possible moves.
439 SkillLevelEnabled = (SkillLevel < 20);
440 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
442 // Wake up needed threads and reset maxPly counter
443 for (int i = 0; i < Threads.size(); i++)
445 Threads[i].wake_up();
446 Threads[i].maxPly = 0;
449 // Write to log file and keep it open to be accessed during the search
450 if (Options["Use Search Log"].value<bool>())
452 string name = Options["Search Log Filename"].value<string>();
453 LogFile.open(name.c_str(), std::ios::out | std::ios::app);
455 if (LogFile.is_open())
456 LogFile << "\nSearching: " << pos.to_fen()
457 << "\ninfinite: " << Limits.infinite
458 << " ponder: " << Limits.ponder
459 << " time: " << Limits.time
460 << " increment: " << Limits.increment
461 << " moves to go: " << Limits.movesToGo
465 // We're ready to start thinking. Call the iterative deepening loop function
466 Move ponderMove = MOVE_NONE;
467 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
469 // Write final search statistics and close log file
470 if (LogFile.is_open())
472 int t = current_search_time();
474 LogFile << "Nodes: " << pos.nodes_searched()
475 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
476 << "\nBest move: " << move_to_san(pos, bestMove);
479 pos.do_move(bestMove, st);
480 LogFile << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
481 pos.undo_move(bestMove); // Return from think() with unchanged position
485 // This makes all the threads to go to sleep
488 // If we are pondering or in infinite search, we shouldn't print the
489 // best move before we are told to do so.
490 if (!StopRequest && (Limits.ponder || Limits.infinite))
491 wait_for_stop_or_ponderhit();
493 // Could be MOVE_NONE when searching on a stalemate position
494 cout << "bestmove " << bestMove;
496 // UCI protol is not clear on allowing sending an empty ponder move, instead
497 // it is clear that ponder move is optional. So skip it if empty.
498 if (ponderMove != MOVE_NONE)
499 cout << " ponder " << ponderMove;
509 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
510 // with increasing depth until the allocated thinking time has been consumed,
511 // user stops the search, or the maximum search depth is reached.
513 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
515 SearchStack ss[PLY_MAX_PLUS_2];
516 Value bestValues[PLY_MAX_PLUS_2];
517 int bestMoveChanges[PLY_MAX_PLUS_2];
518 int depth, aspirationDelta;
519 Value value, alpha, beta;
520 Move bestMove, easyMove, skillBest, skillPonder;
522 // Initialize stuff before a new search
523 memset(ss, 0, 4 * sizeof(SearchStack));
526 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
527 depth = aspirationDelta = 0;
528 alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
529 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
531 // Moves to search are verified and copied
532 Rml.init(pos, searchMoves);
534 // Handle special case of searching on a mate/stalemate position
537 cout << "info" << depth_to_uci(DEPTH_ZERO)
538 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
543 // Iterative deepening loop until requested to stop or target depth reached
544 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
546 Rml.bestMoveChanges = 0;
548 // Calculate dynamic aspiration window based on previous iterations
549 if (MultiPV == 1 && depth >= 5 && abs(bestValues[depth - 1]) < VALUE_KNOWN_WIN)
551 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
552 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
554 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
555 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
557 alpha = Max(bestValues[depth - 1] - aspirationDelta, -VALUE_INFINITE);
558 beta = Min(bestValues[depth - 1] + aspirationDelta, VALUE_INFINITE);
561 // Start with a small aspiration window and, in case of fail high/low,
562 // research with bigger window until not failing high/low anymore.
564 // Search starting from ss+1 to allow calling update_gains()
565 value = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
567 // It is critical that sorting is done with a stable algorithm
568 // because all the values but the first are usually set to
569 // -VALUE_INFINITE and we want to keep the same order for all
570 // the moves but the new PV that goes to head.
571 sort<RootMove>(Rml.begin(), Rml.end());
573 // Write PV back to transposition table in case the relevant entries
574 // have been overwritten during the search.
575 for (int i = 0; i < Min(MultiPV, (int)Rml.size()); i++)
576 Rml[i].insert_pv_in_tt(pos);
578 // Value cannot be trusted. Break out immediately!
582 // Send full PV info to GUI if we are going to leave the loop or
583 // if we have a fail high/low and we are deep in the search.
584 if ((value > alpha && value < beta) || current_search_time() > 2000)
585 for (int i = 0; i < Min(UCIMultiPV, (int)Rml.size()); i++)
587 << depth_to_uci(depth * ONE_PLY)
588 << score_to_uci(Rml[i].pv_score, alpha, beta)
589 << speed_to_uci(pos.nodes_searched())
590 << pv_to_uci(Rml[i].pv, i + 1, pos.is_chess960()) << endl;
592 // In case of failing high/low increase aspiration window and research,
593 // otherwise exit the fail high/low loop.
596 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
597 aspirationDelta += aspirationDelta / 2;
599 else if (value <= alpha)
601 AspirationFailLow = true;
602 StopOnPonderhit = false;
604 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
605 aspirationDelta += aspirationDelta / 2;
610 } while (abs(value) < VALUE_KNOWN_WIN);
612 // Collect info about search result
613 bestMove = Rml[0].pv[0];
614 *ponderMove = Rml[0].pv[1];
615 bestValues[depth] = value;
616 bestMoveChanges[depth] = Rml.bestMoveChanges;
618 // Do we need to pick now the best and the ponder moves ?
619 if (SkillLevelEnabled && depth == 1 + SkillLevel)
620 do_skill_level(&skillBest, &skillPonder);
622 if (LogFile.is_open())
623 LogFile << pretty_pv(pos, depth, value, current_search_time(), Rml[0].pv) << endl;
625 // Init easyMove after first iteration or drop if differs from the best move
626 if (depth == 1 && (Rml.size() == 1 || Rml[0].pv_score > Rml[1].pv_score + EasyMoveMargin))
628 else if (bestMove != easyMove)
629 easyMove = MOVE_NONE;
631 // Check for some early stop condition
632 if (!StopRequest && Limits.useTimeManagement())
634 // Stop search early if one move seems to be much better than the
635 // others or if there is only a single legal move. Also in the latter
636 // case we search up to some depth anyway to get a proper score.
638 && easyMove == bestMove
640 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
641 && current_search_time() > TimeMgr.available_time() / 16)
642 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
643 && current_search_time() > TimeMgr.available_time() / 32)))
646 // Take in account some extra time if the best move has changed
647 if (depth > 4 && depth < 50)
648 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
650 // Stop search if most of available time is already consumed. We probably don't
651 // have enough time to search the first move at the next iteration anyway.
652 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
655 // If we are allowed to ponder do not stop the search now but keep pondering
656 if (StopRequest && Limits.ponder)
659 StopOnPonderhit = true;
664 // When using skills overwrite best and ponder moves with the sub-optimal ones
665 if (SkillLevelEnabled)
667 if (skillBest == MOVE_NONE) // Still unassigned ?
668 do_skill_level(&skillBest, &skillPonder);
670 bestMove = skillBest;
671 *ponderMove = skillPonder;
678 // search<>() is the main search function for both PV and non-PV nodes and for
679 // normal and SplitPoint nodes. When called just after a split point the search
680 // is simpler because we have already probed the hash table, done a null move
681 // search, and searched the first move before splitting, we don't have to repeat
682 // all this work again. We also don't need to store anything to the hash table
683 // here: This is taken care of after we return from the split point.
685 template <NodeType NT>
686 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
688 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV);
689 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV);
690 const bool RootNode = (NT == Root);
692 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
693 assert(beta > alpha && beta <= VALUE_INFINITE);
694 assert(PvNode || alpha == beta - 1);
695 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
697 Move movesSearched[MAX_MOVES];
702 Move ttMove, move, excludedMove, threatMove;
705 Value bestValue, value, oldAlpha;
706 Value refinedValue, nullValue, futilityBase, futilityValueScaled; // Non-PV specific
707 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
708 int moveCount = 0, playedMoveCount = 0;
709 Thread& thread = Threads[pos.thread()];
710 SplitPoint* sp = NULL;
712 refinedValue = bestValue = value = -VALUE_INFINITE;
714 inCheck = pos.in_check();
715 ss->ply = (ss-1)->ply + 1;
717 // Used to send selDepth info to GUI
718 if (PvNode && thread.maxPly < ss->ply)
719 thread.maxPly = ss->ply;
721 // Step 1. Initialize node and poll. Polling can abort search
724 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
725 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
726 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
732 ttMove = excludedMove = MOVE_NONE;
733 threatMove = sp->threatMove;
734 goto split_point_start;
737 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
743 // Step 2. Check for aborted search and immediate draw
745 || pos.is_draw<false>()
746 || ss->ply > PLY_MAX) && !RootNode)
749 // Step 3. Mate distance pruning
752 alpha = Max(value_mated_in(ss->ply), alpha);
753 beta = Min(value_mate_in(ss->ply+1), beta);
758 // Step 4. Transposition table lookup
759 // We don't want the score of a partial search to overwrite a previous full search
760 // TT value, so we use a different position key in case of an excluded move.
761 excludedMove = ss->excludedMove;
762 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
763 tte = TT.probe(posKey);
764 ttMove = tte ? tte->move() : MOVE_NONE;
766 // At PV nodes we check for exact scores, while at non-PV nodes we check for
767 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
768 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
769 // we should also update RootMoveList to avoid bogus output.
770 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
771 : ok_to_use_TT(tte, depth, beta, ss->ply)))
774 ss->bestMove = ttMove; // Can be MOVE_NONE
775 return value_from_tt(tte->value(), ss->ply);
778 // Step 5. Evaluate the position statically and update parent's gain statistics
780 ss->eval = ss->evalMargin = VALUE_NONE;
783 assert(tte->static_value() != VALUE_NONE);
785 ss->eval = tte->static_value();
786 ss->evalMargin = tte->static_value_margin();
787 refinedValue = refine_eval(tte, ss->eval, ss->ply);
791 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
792 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
795 // Save gain for the parent non-capture move
796 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
798 // Step 6. Razoring (is omitted in PV nodes)
800 && depth < RazorDepth
802 && refinedValue + razor_margin(depth) < beta
803 && ttMove == MOVE_NONE
804 && abs(beta) < VALUE_MATE_IN_PLY_MAX
805 && !pos.has_pawn_on_7th(pos.side_to_move()))
807 Value rbeta = beta - razor_margin(depth);
808 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
810 // Logically we should return (v + razor_margin(depth)), but
811 // surprisingly this did slightly weaker in tests.
815 // Step 7. Static null move pruning (is omitted in PV nodes)
816 // We're betting that the opponent doesn't have a move that will reduce
817 // the score by more than futility_margin(depth) if we do a null move.
820 && depth < RazorDepth
822 && refinedValue - futility_margin(depth, 0) >= beta
823 && abs(beta) < VALUE_MATE_IN_PLY_MAX
824 && pos.non_pawn_material(pos.side_to_move()))
825 return refinedValue - futility_margin(depth, 0);
827 // Step 8. Null move search with verification search (is omitted in PV nodes)
832 && refinedValue >= beta
833 && abs(beta) < VALUE_MATE_IN_PLY_MAX
834 && pos.non_pawn_material(pos.side_to_move()))
836 ss->currentMove = MOVE_NULL;
838 // Null move dynamic reduction based on depth
839 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
841 // Null move dynamic reduction based on value
842 if (refinedValue - PawnValueMidgame > beta)
845 pos.do_null_move(st);
846 (ss+1)->skipNullMove = true;
847 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
848 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
849 (ss+1)->skipNullMove = false;
850 pos.undo_null_move();
852 if (nullValue >= beta)
854 // Do not return unproven mate scores
855 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
858 if (depth < 6 * ONE_PLY)
861 // Do verification search at high depths
862 ss->skipNullMove = true;
863 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
864 ss->skipNullMove = false;
871 // The null move failed low, which means that we may be faced with
872 // some kind of threat. If the previous move was reduced, check if
873 // the move that refuted the null move was somehow connected to the
874 // move which was reduced. If a connection is found, return a fail
875 // low score (which will cause the reduced move to fail high in the
876 // parent node, which will trigger a re-search with full depth).
877 threatMove = (ss+1)->bestMove;
879 if ( depth < ThreatDepth
881 && threatMove != MOVE_NONE
882 && connected_moves(pos, (ss-1)->currentMove, threatMove))
887 // Step 9. ProbCut (is omitted in PV nodes)
888 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
889 // and a reduced search returns a value much above beta, we can (almost) safely
890 // prune the previous move.
892 && depth >= RazorDepth + ONE_PLY
895 && excludedMove == MOVE_NONE
896 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
898 Value rbeta = beta + 200;
899 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
901 assert(rdepth >= ONE_PLY);
903 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
906 while ((move = mp.get_next_move()) != MOVE_NONE)
907 if (pos.pl_move_is_legal(move, ci.pinned))
909 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
910 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
917 // Step 10. Internal iterative deepening
918 if ( depth >= IIDDepth[PvNode]
919 && ttMove == MOVE_NONE
920 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
922 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
924 ss->skipNullMove = true;
925 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
926 ss->skipNullMove = false;
928 tte = TT.probe(posKey);
929 ttMove = tte ? tte->move() : MOVE_NONE;
932 split_point_start: // At split points actual search starts from here
934 // Initialize a MovePicker object for the current position
935 MovePickerExt<NT> mp(pos, RootNode ? Rml[0].pv[0] : ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
937 ss->bestMove = MOVE_NONE;
938 futilityBase = ss->eval + ss->evalMargin;
939 singularExtensionNode = !RootNode
941 && depth >= SingularExtensionDepth[PvNode]
942 && ttMove != MOVE_NONE
943 && !excludedMove // Do not allow recursive singular extension search
944 && (tte->type() & VALUE_TYPE_LOWER)
945 && tte->depth() >= depth - 3 * ONE_PLY;
948 lock_grab(&(sp->lock));
949 bestValue = sp->bestValue;
952 // Step 11. Loop through moves
953 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
954 while ( bestValue < beta
955 && (move = mp.get_next_move()) != MOVE_NONE
956 && !thread.cutoff_occurred())
958 assert(move_is_ok(move));
960 if (move == excludedMove)
963 // At PV and SpNode nodes we want all moves to be legal since the beginning
964 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
969 moveCount = ++sp->moveCount;
970 lock_release(&(sp->lock));
977 // This is used by time management
978 FirstRootMove = (moveCount == 1);
980 // Save the current node count before the move is searched
981 nodes = pos.nodes_searched();
983 // If it's time to send nodes info, do it here where we have the
984 // correct accumulated node counts searched by each thread.
985 if (SendSearchedNodes)
987 SendSearchedNodes = false;
988 cout << "info" << speed_to_uci(pos.nodes_searched()) << endl;
991 // For long searches send current move info to GUI
992 if (current_search_time() > 2000)
993 cout << "info" << depth_to_uci(depth)
994 << " currmove " << move << " currmovenumber " << moveCount << endl;
997 // At Root and at first iteration do a PV search on all the moves to score root moves
998 isPvMove = (PvNode && moveCount <= (!RootNode ? 1 : depth <= ONE_PLY ? MAX_MOVES : MultiPV));
999 givesCheck = pos.move_gives_check(move, ci);
1000 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1002 // Step 12. Decide the new search depth
1003 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1005 // Singular extension search. If all moves but one fail low on a search of
1006 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1007 // is singular and should be extended. To verify this we do a reduced search
1008 // on all the other moves but the ttMove, if result is lower than ttValue minus
1009 // a margin then we extend ttMove.
1010 if ( singularExtensionNode
1012 && pos.pl_move_is_legal(move, ci.pinned)
1015 Value ttValue = value_from_tt(tte->value(), ss->ply);
1017 if (abs(ttValue) < VALUE_KNOWN_WIN)
1019 Value rBeta = ttValue - int(depth);
1020 ss->excludedMove = move;
1021 ss->skipNullMove = true;
1022 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1023 ss->skipNullMove = false;
1024 ss->excludedMove = MOVE_NONE;
1025 ss->bestMove = MOVE_NONE;
1031 // Update current move (this must be done after singular extension search)
1032 newDepth = depth - ONE_PLY + ext;
1034 // Step 13. Futility pruning (is omitted in PV nodes)
1036 && !captureOrPromotion
1040 && !move_is_castle(move))
1042 // Move count based pruning
1043 if ( moveCount >= futility_move_count(depth)
1044 && (!threatMove || !connected_threat(pos, move, threatMove))
1045 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1048 lock_grab(&(sp->lock));
1053 // Value based pruning
1054 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1055 // but fixing this made program slightly weaker.
1056 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1057 futilityValueScaled = futilityBase + futility_margin(predictedDepth, moveCount)
1058 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1060 if (futilityValueScaled < beta)
1064 lock_grab(&(sp->lock));
1065 if (futilityValueScaled > sp->bestValue)
1066 sp->bestValue = bestValue = futilityValueScaled;
1068 else if (futilityValueScaled > bestValue)
1069 bestValue = futilityValueScaled;
1074 // Prune moves with negative SEE at low depths
1075 if ( predictedDepth < 2 * ONE_PLY
1076 && bestValue > VALUE_MATED_IN_PLY_MAX
1077 && pos.see_sign(move) < 0)
1080 lock_grab(&(sp->lock));
1086 // Check for legality only before to do the move
1087 if (!pos.pl_move_is_legal(move, ci.pinned))
1093 ss->currentMove = move;
1094 if (!SpNode && !captureOrPromotion)
1095 movesSearched[playedMoveCount++] = move;
1097 // Step 14. Make the move
1098 pos.do_move(move, st, ci, givesCheck);
1100 // Step extra. pv search (only in PV nodes)
1101 // The first move in list is the expected PV
1103 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1104 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1107 // Step 15. Reduced depth search
1108 // If the move fails high will be re-searched at full depth.
1109 bool doFullDepthSearch = true;
1111 if ( depth > 3 * ONE_PLY
1112 && !captureOrPromotion
1114 && !move_is_castle(move)
1115 && ss->killers[0] != move
1116 && ss->killers[1] != move
1117 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1119 Depth d = newDepth - ss->reduction;
1120 alpha = SpNode ? sp->alpha : alpha;
1122 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1123 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1125 ss->reduction = DEPTH_ZERO;
1126 doFullDepthSearch = (value > alpha);
1129 // Step 16. Full depth search
1130 if (doFullDepthSearch)
1132 alpha = SpNode ? sp->alpha : alpha;
1133 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1134 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1136 // Step extra. pv search (only in PV nodes)
1137 // Search only for possible new PV nodes, if instead value >= beta then
1138 // parent node fails low with value <= alpha and tries another move.
1139 if (PvNode && value > alpha && (RootNode || value < beta))
1140 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1141 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1145 // Step 17. Undo move
1146 pos.undo_move(move);
1148 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1150 // Step 18. Check for new best move
1153 lock_grab(&(sp->lock));
1154 bestValue = sp->bestValue;
1158 if (value > bestValue)
1161 ss->bestMove = move;
1166 && value < beta) // We want always alpha < beta
1169 if (SpNode && !thread.cutoff_occurred())
1171 sp->bestValue = value;
1172 sp->ss->bestMove = move;
1174 sp->is_betaCutoff = (value >= beta);
1180 // Finished searching the move. If StopRequest is true, the search
1181 // was aborted because the user interrupted the search or because we
1182 // ran out of time. In this case, the return value of the search cannot
1183 // be trusted, and we break out of the loop without updating the best
1188 // Remember searched nodes counts for this move
1189 Rml.find(move)->nodes += pos.nodes_searched() - nodes;
1191 // PV move or new best move ?
1192 if (isPvMove || value > alpha)
1195 Rml.find(move)->pv_score = value;
1196 Rml.find(move)->extract_pv_from_tt(pos);
1198 // We record how often the best move has been changed in each
1199 // iteration. This information is used for time management: When
1200 // the best move changes frequently, we allocate some more time.
1201 if (!isPvMove && MultiPV == 1)
1202 Rml.bestMoveChanges++;
1209 // All other moves but the PV are set to the lowest value, this
1210 // is not a problem when sorting becuase sort is stable and move
1211 // position in the list is preserved, just the PV is pushed up.
1212 Rml.find(move)->pv_score = -VALUE_INFINITE;
1216 // Step 19. Check for split
1219 && depth >= Threads.min_split_depth()
1221 && Threads.available_slave_exists(pos.thread())
1223 && !thread.cutoff_occurred())
1224 Threads.split<FakeSplit>(pos, ss, &alpha, beta, &bestValue, depth,
1225 threatMove, moveCount, &mp, PvNode);
1228 // Step 20. Check for mate and stalemate
1229 // All legal moves have been searched and if there are
1230 // no legal moves, it must be mate or stalemate.
1231 // If one move was excluded return fail low score.
1232 if (!SpNode && !moveCount)
1233 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1235 // Step 21. Update tables
1236 // If the search is not aborted, update the transposition table,
1237 // history counters, and killer moves.
1238 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1240 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1241 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1242 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1244 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1246 // Update killers and history only for non capture moves that fails high
1247 if ( bestValue >= beta
1248 && !pos.move_is_capture_or_promotion(move))
1250 if (move != ss->killers[0])
1252 ss->killers[1] = ss->killers[0];
1253 ss->killers[0] = move;
1255 update_history(pos, move, depth, movesSearched, playedMoveCount);
1261 // Here we have the lock still grabbed
1262 sp->is_slave[pos.thread()] = false;
1263 sp->nodes += pos.nodes_searched();
1264 lock_release(&(sp->lock));
1267 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1272 // qsearch() is the quiescence search function, which is called by the main
1273 // search function when the remaining depth is zero (or, to be more precise,
1274 // less than ONE_PLY).
1276 template <NodeType NT>
1277 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1279 const bool PvNode = (NT == PV);
1281 assert(NT == PV || NT == NonPV);
1282 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1283 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1284 assert(PvNode || alpha == beta - 1);
1286 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1290 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1291 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1294 Value oldAlpha = alpha;
1296 ss->bestMove = ss->currentMove = MOVE_NONE;
1297 ss->ply = (ss-1)->ply + 1;
1299 // Check for an instant draw or maximum ply reached
1300 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1303 // Decide whether or not to include checks, this fixes also the type of
1304 // TT entry depth that we are going to use. Note that in qsearch we use
1305 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1306 inCheck = pos.in_check();
1307 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1309 // Transposition table lookup. At PV nodes, we don't use the TT for
1310 // pruning, but only for move ordering.
1311 tte = TT.probe(pos.get_key());
1312 ttMove = (tte ? tte->move() : MOVE_NONE);
1314 if (!PvNode && tte && ok_to_use_TT(tte, ttDepth, beta, ss->ply))
1316 ss->bestMove = ttMove; // Can be MOVE_NONE
1317 return value_from_tt(tte->value(), ss->ply);
1320 // Evaluate the position statically
1323 bestValue = futilityBase = -VALUE_INFINITE;
1324 ss->eval = evalMargin = VALUE_NONE;
1325 enoughMaterial = false;
1331 assert(tte->static_value() != VALUE_NONE);
1333 evalMargin = tte->static_value_margin();
1334 ss->eval = bestValue = tte->static_value();
1337 ss->eval = bestValue = evaluate(pos, evalMargin);
1339 // Stand pat. Return immediately if static value is at least beta
1340 if (bestValue >= beta)
1343 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1348 if (PvNode && bestValue > alpha)
1351 // Futility pruning parameters, not needed when in check
1352 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1353 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1356 // Initialize a MovePicker object for the current position, and prepare
1357 // to search the moves. Because the depth is <= 0 here, only captures,
1358 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1360 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1363 // Loop through the moves until no moves remain or a beta cutoff occurs
1364 while ( alpha < beta
1365 && (move = mp.get_next_move()) != MOVE_NONE)
1367 assert(move_is_ok(move));
1369 givesCheck = pos.move_gives_check(move, ci);
1377 && !move_is_promotion(move)
1378 && !pos.move_is_passed_pawn_push(move))
1380 futilityValue = futilityBase
1381 + piece_value_endgame(pos.piece_on(move_to(move)))
1382 + (move_is_ep(move) ? PawnValueEndgame : VALUE_ZERO);
1384 if (futilityValue < alpha)
1386 if (futilityValue > bestValue)
1387 bestValue = futilityValue;
1391 // Prune moves with negative or equal SEE
1392 if ( futilityBase < beta
1393 && depth < DEPTH_ZERO
1394 && pos.see(move) <= 0)
1398 // Detect non-capture evasions that are candidate to be pruned
1399 evasionPrunable = !PvNode
1401 && bestValue > VALUE_MATED_IN_PLY_MAX
1402 && !pos.move_is_capture(move)
1403 && !pos.can_castle(pos.side_to_move());
1405 // Don't search moves with negative SEE values
1407 && (!inCheck || evasionPrunable)
1409 && !move_is_promotion(move)
1410 && pos.see_sign(move) < 0)
1413 // Don't search useless checks
1418 && !pos.move_is_capture_or_promotion(move)
1419 && ss->eval + PawnValueMidgame / 4 < beta
1420 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1422 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1423 bestValue = ss->eval + PawnValueMidgame / 4;
1428 // Check for legality only before to do the move
1429 if (!pos.pl_move_is_legal(move, ci.pinned))
1432 // Update current move
1433 ss->currentMove = move;
1435 // Make and search the move
1436 pos.do_move(move, st, ci, givesCheck);
1437 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1438 pos.undo_move(move);
1440 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1443 if (value > bestValue)
1449 ss->bestMove = move;
1454 // All legal moves have been searched. A special case: If we're in check
1455 // and no legal moves were found, it is checkmate.
1456 if (inCheck && bestValue == -VALUE_INFINITE)
1457 return value_mated_in(ss->ply);
1459 // Update transposition table
1460 ValueType vt = (bestValue <= oldAlpha ? VALUE_TYPE_UPPER : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT);
1461 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, ss->bestMove, ss->eval, evalMargin);
1463 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1469 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1470 // bestValue is updated only when returning false because in that case move
1473 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1475 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1476 Square from, to, ksq, victimSq;
1479 Value futilityValue, bv = *bestValue;
1481 from = move_from(move);
1483 them = opposite_color(pos.side_to_move());
1484 ksq = pos.king_square(them);
1485 kingAtt = pos.attacks_from<KING>(ksq);
1486 pc = pos.piece_on(from);
1488 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1489 oldAtt = pos.attacks_from(pc, from, occ);
1490 newAtt = pos.attacks_from(pc, to, occ);
1492 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1493 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1495 if (!(b && (b & (b - 1))))
1498 // Rule 2. Queen contact check is very dangerous
1499 if ( piece_type(pc) == QUEEN
1500 && bit_is_set(kingAtt, to))
1503 // Rule 3. Creating new double threats with checks
1504 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1508 victimSq = pop_1st_bit(&b);
1509 futilityValue = futilityBase + piece_value_endgame(pos.piece_on(victimSq));
1511 // Note that here we generate illegal "double move"!
1512 if ( futilityValue >= beta
1513 && pos.see_sign(make_move(from, victimSq)) >= 0)
1516 if (futilityValue > bv)
1520 // Update bestValue only if check is not dangerous (because we will prune the move)
1526 // connected_moves() tests whether two moves are 'connected' in the sense
1527 // that the first move somehow made the second move possible (for instance
1528 // if the moving piece is the same in both moves). The first move is assumed
1529 // to be the move that was made to reach the current position, while the
1530 // second move is assumed to be a move from the current position.
1532 bool connected_moves(const Position& pos, Move m1, Move m2) {
1534 Square f1, t1, f2, t2;
1538 assert(m1 && move_is_ok(m1));
1539 assert(m2 && move_is_ok(m2));
1541 // Case 1: The moving piece is the same in both moves
1547 // Case 2: The destination square for m2 was vacated by m1
1553 // Case 3: Moving through the vacated square
1554 p2 = pos.piece_on(f2);
1555 if ( piece_is_slider(p2)
1556 && bit_is_set(squares_between(f2, t2), f1))
1559 // Case 4: The destination square for m2 is defended by the moving piece in m1
1560 p1 = pos.piece_on(t1);
1561 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1564 // Case 5: Discovered check, checking piece is the piece moved in m1
1565 ksq = pos.king_square(pos.side_to_move());
1566 if ( piece_is_slider(p1)
1567 && bit_is_set(squares_between(t1, ksq), f2))
1569 Bitboard occ = pos.occupied_squares();
1570 clear_bit(&occ, f2);
1571 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1578 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1579 // "plies to mate from the current ply". Non-mate scores are unchanged.
1580 // The function is called before storing a value to the transposition table.
1582 Value value_to_tt(Value v, int ply) {
1584 if (v >= VALUE_MATE_IN_PLY_MAX)
1587 if (v <= VALUE_MATED_IN_PLY_MAX)
1594 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1595 // the transposition table to a mate score corrected for the current ply.
1597 Value value_from_tt(Value v, int ply) {
1599 if (v >= VALUE_MATE_IN_PLY_MAX)
1602 if (v <= VALUE_MATED_IN_PLY_MAX)
1609 // connected_threat() tests whether it is safe to forward prune a move or if
1610 // is somehow connected to the threat move returned by null search.
1612 bool connected_threat(const Position& pos, Move m, Move threat) {
1614 assert(move_is_ok(m));
1615 assert(threat && move_is_ok(threat));
1616 assert(!pos.move_is_capture_or_promotion(m));
1617 assert(!pos.move_is_passed_pawn_push(m));
1619 Square mfrom, mto, tfrom, tto;
1621 mfrom = move_from(m);
1623 tfrom = move_from(threat);
1624 tto = move_to(threat);
1626 // Case 1: Don't prune moves which move the threatened piece
1630 // Case 2: If the threatened piece has value less than or equal to the
1631 // value of the threatening piece, don't prune moves which defend it.
1632 if ( pos.move_is_capture(threat)
1633 && ( piece_value_midgame(pos.piece_on(tfrom)) >= piece_value_midgame(pos.piece_on(tto))
1634 || piece_type(pos.piece_on(tfrom)) == KING)
1635 && pos.move_attacks_square(m, tto))
1638 // Case 3: If the moving piece in the threatened move is a slider, don't
1639 // prune safe moves which block its ray.
1640 if ( piece_is_slider(pos.piece_on(tfrom))
1641 && bit_is_set(squares_between(tfrom, tto), mto)
1642 && pos.see_sign(m) >= 0)
1649 // ok_to_use_TT() returns true if a transposition table score
1650 // can be used at a given point in search.
1652 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
1654 Value v = value_from_tt(tte->value(), ply);
1656 return ( tte->depth() >= depth
1657 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1658 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1660 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1661 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1665 // refine_eval() returns the transposition table score if
1666 // possible otherwise falls back on static position evaluation.
1668 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1672 Value v = value_from_tt(tte->value(), ply);
1674 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1675 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1682 // update_history() registers a good move that produced a beta-cutoff
1683 // in history and marks as failures all the other moves of that ply.
1685 void update_history(const Position& pos, Move move, Depth depth,
1686 Move movesSearched[], int moveCount) {
1688 Value bonus = Value(int(depth) * int(depth));
1690 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1692 for (int i = 0; i < moveCount - 1; i++)
1694 m = movesSearched[i];
1698 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1703 // update_gains() updates the gains table of a non-capture move given
1704 // the static position evaluation before and after the move.
1706 void update_gains(const Position& pos, Move m, Value before, Value after) {
1709 && before != VALUE_NONE
1710 && after != VALUE_NONE
1711 && pos.captured_piece_type() == PIECE_TYPE_NONE
1712 && !move_is_special(m))
1713 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1717 // current_search_time() returns the number of milliseconds which have passed
1718 // since the beginning of the current search.
1720 int current_search_time(int set) {
1722 static int searchStartTime;
1725 searchStartTime = set;
1727 return get_system_time() - searchStartTime;
1731 // score_to_uci() converts a value to a string suitable for use with the UCI
1732 // protocol specifications:
1734 // cp <x> The score from the engine's point of view in centipawns.
1735 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1736 // use negative values for y.
1738 string score_to_uci(Value v, Value alpha, Value beta) {
1740 std::stringstream s;
1742 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1743 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1745 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1747 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1753 // speed_to_uci() returns a string with time stats of current search suitable
1754 // to be sent to UCI gui.
1756 string speed_to_uci(int64_t nodes) {
1758 std::stringstream s;
1759 int t = current_search_time();
1761 s << " nodes " << nodes
1762 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1768 // pv_to_uci() returns a string with information on the current PV line
1769 // formatted according to UCI specification.
1771 string pv_to_uci(Move pv[], int pvNum, bool chess960) {
1773 std::stringstream s;
1775 s << " multipv " << pvNum << " pv " << set960(chess960);
1777 for ( ; *pv != MOVE_NONE; pv++)
1783 // depth_to_uci() returns a string with information on the current depth and
1784 // seldepth formatted according to UCI specification.
1786 string depth_to_uci(Depth depth) {
1788 std::stringstream s;
1790 // Retrieve max searched depth among threads
1792 for (int i = 0; i < Threads.size(); i++)
1793 if (Threads[i].maxPly > selDepth)
1794 selDepth = Threads[i].maxPly;
1796 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1801 string time_to_string(int millisecs) {
1803 const int MSecMinute = 1000 * 60;
1804 const int MSecHour = 1000 * 60 * 60;
1806 int hours = millisecs / MSecHour;
1807 int minutes = (millisecs % MSecHour) / MSecMinute;
1808 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1810 std::stringstream s;
1815 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1819 string score_to_string(Value v) {
1821 std::stringstream s;
1823 if (v >= VALUE_MATE_IN_PLY_MAX)
1824 s << "#" << (VALUE_MATE - v + 1) / 2;
1825 else if (v <= VALUE_MATED_IN_PLY_MAX)
1826 s << "-#" << (VALUE_MATE + v) / 2;
1828 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1833 // pretty_pv() creates a human-readable string from a position and a PV.
1834 // It is used to write search information to the log file (which is created
1835 // when the UCI parameter "Use Search Log" is "true").
1837 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1839 const int64_t K = 1000;
1840 const int64_t M = 1000000;
1841 const int startColumn = 28;
1842 const size_t maxLength = 80 - startColumn;
1844 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1847 std::stringstream s;
1850 // First print depth, score, time and searched nodes...
1851 s << set960(pos.is_chess960())
1852 << std::setw(2) << depth
1853 << std::setw(8) << score_to_string(value)
1854 << std::setw(8) << time_to_string(time);
1856 if (pos.nodes_searched() < M)
1857 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1858 else if (pos.nodes_searched() < K * M)
1859 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1861 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1863 // ...then print the full PV line in short algebraic notation
1864 while (*m != MOVE_NONE)
1866 san = move_to_san(pos, *m);
1867 length += san.length() + 1;
1869 if (length > maxLength)
1871 length = san.length() + 1;
1872 s << "\n" + string(startColumn, ' ');
1876 pos.do_move(*m++, *st++);
1879 // Restore original position before to leave
1880 while (m != pv) pos.undo_move(*--m);
1885 // poll() performs two different functions: It polls for user input, and it
1886 // looks at the time consumed so far and decides if it's time to abort the
1889 void poll(const Position& pos) {
1891 static int lastInfoTime;
1892 int t = current_search_time();
1895 if (input_available())
1897 // We are line oriented, don't read single chars
1900 if (!std::getline(std::cin, command) || command == "quit")
1902 // Quit the program as soon as possible
1903 Limits.ponder = false;
1904 QuitRequest = StopRequest = true;
1907 else if (command == "stop")
1909 // Stop calculating as soon as possible, but still send the "bestmove"
1910 // and possibly the "ponder" token when finishing the search.
1911 Limits.ponder = false;
1914 else if (command == "ponderhit")
1916 // The opponent has played the expected move. GUI sends "ponderhit" if
1917 // we were told to ponder on the same move the opponent has played. We
1918 // should continue searching but switching from pondering to normal search.
1919 Limits.ponder = false;
1921 if (StopOnPonderhit)
1926 // Print search information
1930 else if (lastInfoTime > t)
1931 // HACK: Must be a new search where we searched less than
1932 // NodesBetweenPolls nodes during the first second of search.
1935 else if (t - lastInfoTime >= 1000)
1940 dbg_print_hit_rate();
1942 // Send info on searched nodes as soon as we return to root
1943 SendSearchedNodes = true;
1946 // Should we stop the search?
1950 bool stillAtFirstMove = FirstRootMove
1951 && !AspirationFailLow
1952 && t > TimeMgr.available_time();
1954 bool noMoreTime = t > TimeMgr.maximum_time()
1955 || stillAtFirstMove;
1957 if ( (Limits.useTimeManagement() && noMoreTime)
1958 || (Limits.maxTime && t >= Limits.maxTime)
1959 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1964 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1965 // while the program is pondering. The point is to work around a wrinkle in
1966 // the UCI protocol: When pondering, the engine is not allowed to give a
1967 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1968 // We simply wait here until one of these commands is sent, and return,
1969 // after which the bestmove and pondermove will be printed.
1971 void wait_for_stop_or_ponderhit() {
1975 // Wait for a command from stdin
1976 while ( std::getline(std::cin, command)
1977 && command != "ponderhit" && command != "stop" && command != "quit") {};
1979 if (command != "ponderhit" && command != "stop")
1980 QuitRequest = true; // Must be "quit" or getline() returned false
1984 // When playing with strength handicap choose best move among the MultiPV set
1985 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1986 void do_skill_level(Move* best, Move* ponder) {
1988 assert(MultiPV > 1);
1992 // Rml list is already sorted by pv_score in descending order
1994 int max_s = -VALUE_INFINITE;
1995 int size = Min(MultiPV, (int)Rml.size());
1996 int max = Rml[0].pv_score;
1997 int var = Min(max - Rml[size - 1].pv_score, PawnValueMidgame);
1998 int wk = 120 - 2 * SkillLevel;
2000 // PRNG sequence should be non deterministic
2001 for (int i = abs(get_system_time() % 50); i > 0; i--)
2002 rk.rand<unsigned>();
2004 // Choose best move. For each move's score we add two terms both dependent
2005 // on wk, one deterministic and bigger for weaker moves, and one random,
2006 // then we choose the move with the resulting highest score.
2007 for (int i = 0; i < size; i++)
2009 s = Rml[i].pv_score;
2011 // Don't allow crazy blunders even at very low skills
2012 if (i > 0 && Rml[i-1].pv_score > s + EasyMoveMargin)
2015 // This is our magical formula
2016 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
2021 *best = Rml[i].pv[0];
2022 *ponder = Rml[i].pv[1];
2028 /// RootMove and RootMoveList method's definitions
2030 RootMove::RootMove() {
2033 pv_score = non_pv_score = -VALUE_INFINITE;
2037 RootMove& RootMove::operator=(const RootMove& rm) {
2039 const Move* src = rm.pv;
2042 // Avoid a costly full rm.pv[] copy
2043 do *dst++ = *src; while (*src++ != MOVE_NONE);
2046 pv_score = rm.pv_score;
2047 non_pv_score = rm.non_pv_score;
2051 void RootMoveList::init(Position& pos, Move searchMoves[]) {
2054 bestMoveChanges = 0;
2057 // Generate all legal moves and add them to RootMoveList
2058 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2060 // If we have a searchMoves[] list then verify the move
2061 // is in the list before to add it.
2062 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2064 if (sm != searchMoves && *sm != ml.move())
2068 rm.pv[0] = ml.move();
2069 rm.pv[1] = MOVE_NONE;
2070 rm.pv_score = -VALUE_INFINITE;
2075 RootMove* RootMoveList::find(const Move &m) {
2077 for (int i = 0; i < int(size()); i++)
2079 if ((*this)[i].pv[0] == m)
2086 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2087 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2088 // allow to always have a ponder move even when we fail high at root and also a
2089 // long PV to print that is important for position analysis.
2091 void RootMove::extract_pv_from_tt(Position& pos) {
2093 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2097 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2099 pos.do_move(pv[0], *st++);
2101 while ( (tte = TT.probe(pos.get_key())) != NULL
2102 && tte->move() != MOVE_NONE
2103 && pos.move_is_pl(tte->move())
2104 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2106 && (!pos.is_draw<false>() || ply < 2))
2108 pv[ply] = tte->move();
2109 pos.do_move(pv[ply++], *st++);
2111 pv[ply] = MOVE_NONE;
2113 do pos.undo_move(pv[--ply]); while (ply);
2116 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2117 // the PV back into the TT. This makes sure the old PV moves are searched
2118 // first, even if the old TT entries have been overwritten.
2120 void RootMove::insert_pv_in_tt(Position& pos) {
2122 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2125 Value v, m = VALUE_NONE;
2128 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2134 // Don't overwrite existing correct entries
2135 if (!tte || tte->move() != pv[ply])
2137 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2138 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2140 pos.do_move(pv[ply], *st++);
2142 } while (pv[++ply] != MOVE_NONE);
2144 do pos.undo_move(pv[--ply]); while (ply);
2149 // ThreadsManager::idle_loop() is where the threads are parked when they have no work
2150 // to do. The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint
2151 // object for which the current thread is the master.
2153 void ThreadsManager::idle_loop(int threadID, SplitPoint* sp) {
2155 assert(threadID >= 0 && threadID < MAX_THREADS);
2162 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2163 // master should exit as last one.
2164 if (allThreadsShouldExit)
2167 threads[threadID].state = Thread::TERMINATED;
2171 // If we are not thinking, wait for a condition to be signaled
2172 // instead of wasting CPU time polling for work.
2173 while ( threadID >= activeThreads
2174 || threads[threadID].state == Thread::INITIALIZING
2175 || (useSleepingThreads && threads[threadID].state == Thread::AVAILABLE))
2177 assert(!sp || useSleepingThreads);
2178 assert(threadID != 0 || useSleepingThreads);
2180 if (threads[threadID].state == Thread::INITIALIZING)
2181 threads[threadID].state = Thread::AVAILABLE;
2183 // Grab the lock to avoid races with Thread::wake_up()
2184 lock_grab(&threads[threadID].sleepLock);
2186 // If we are master and all slaves have finished do not go to sleep
2187 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2188 allFinished = (i == activeThreads);
2190 if (allFinished || allThreadsShouldExit)
2192 lock_release(&threads[threadID].sleepLock);
2196 // Do sleep here after retesting sleep conditions
2197 if (threadID >= activeThreads || threads[threadID].state == Thread::AVAILABLE)
2198 cond_wait(&threads[threadID].sleepCond, &threads[threadID].sleepLock);
2200 lock_release(&threads[threadID].sleepLock);
2203 // If this thread has been assigned work, launch a search
2204 if (threads[threadID].state == Thread::WORKISWAITING)
2206 assert(!allThreadsShouldExit);
2208 threads[threadID].state = Thread::SEARCHING;
2210 // Copy split point position and search stack and call search()
2211 // with SplitPoint template parameter set to true.
2212 SearchStack ss[PLY_MAX_PLUS_2];
2213 SplitPoint* tsp = threads[threadID].splitPoint;
2214 Position pos(*tsp->pos, threadID);
2216 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2220 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2222 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2224 assert(threads[threadID].state == Thread::SEARCHING);
2226 threads[threadID].state = Thread::AVAILABLE;
2228 // Wake up master thread so to allow it to return from the idle loop in
2229 // case we are the last slave of the split point.
2230 if ( useSleepingThreads
2231 && threadID != tsp->master
2232 && threads[tsp->master].state == Thread::AVAILABLE)
2233 threads[tsp->master].wake_up();
2236 // If this thread is the master of a split point and all slaves have
2237 // finished their work at this split point, return from the idle loop.
2238 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2239 allFinished = (i == activeThreads);
2243 // Because sp->slaves[] is reset under lock protection,
2244 // be sure sp->lock has been released before to return.
2245 lock_grab(&(sp->lock));
2246 lock_release(&(sp->lock));
2248 // In helpful master concept a master can help only a sub-tree, and
2249 // because here is all finished is not possible master is booked.
2250 assert(threads[threadID].state == Thread::AVAILABLE);
2252 threads[threadID].state = Thread::SEARCHING;