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, SplitPointRoot, SplitPointPV, SplitPointNonPV };
54 // RootMove struct is used for moves at the root of the tree. For each root
55 // move, we store a score, a node count, and a PV (really a refutation
56 // in the case of moves which fail low). Score is normally set at
57 // -VALUE_INFINITE for all non-pv moves.
60 // RootMove::operator<() is the comparison function used when
61 // sorting the moves. A move m1 is considered to be better
62 // than a move m2 if it has an higher score
63 bool operator<(const RootMove& m) const { return score < m.score; }
65 void extract_pv_from_tt(Position& pos);
66 void insert_pv_in_tt(Position& pos);
74 // RootMoveList struct is mainly a std::vector of RootMove objects
75 struct RootMoveList : public std::vector<RootMove> {
77 void init(Position& pos, Move searchMoves[]);
78 RootMove* find(const Move& m, int startIndex = 0);
86 // Lookup table to check if a Piece is a slider and its access function
87 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
88 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
92 // Maximum depth for razoring
93 const Depth RazorDepth = 4 * ONE_PLY;
95 // Dynamic razoring margin based on depth
96 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
98 // Maximum depth for use of dynamic threat detection when null move fails low
99 const Depth ThreatDepth = 5 * ONE_PLY;
101 // Step 9. Internal iterative deepening
103 // Minimum depth for use of internal iterative deepening
104 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
106 // At Non-PV nodes we do an internal iterative deepening search
107 // when the static evaluation is bigger then beta - IIDMargin.
108 const Value IIDMargin = Value(0x100);
110 // Step 11. Decide the new search depth
112 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
113 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
114 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
115 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
116 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
118 // Minimum depth for use of singular extension
119 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
121 // Step 12. Futility pruning
123 // Futility margin for quiescence search
124 const Value FutilityMarginQS = Value(0x80);
126 // Futility lookup tables (initialized at startup) and their access functions
127 Value FutilityMargins[16][64]; // [depth][moveNumber]
128 int FutilityMoveCounts[32]; // [depth]
130 inline Value futility_margin(Depth d, int mn) {
132 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
133 : 2 * VALUE_INFINITE;
136 inline int futility_move_count(Depth d) {
138 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
141 // Step 14. Reduced search
143 // Reduction lookup tables (initialized at startup) and their access function
144 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
146 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
148 return (Depth) Reductions[PvNode][Min(d / ONE_PLY, 63)][Min(mn, 63)];
151 // Easy move margin. An easy move candidate must be at least this much
152 // better than the second best move.
153 const Value EasyMoveMargin = Value(0x200);
156 /// Namespace variables
162 int MultiPV, UCIMultiPV, MultiPVIteration;
164 // Time management variables
165 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
170 std::ofstream LogFile;
172 // Skill level adjustment
174 bool SkillLevelEnabled;
176 // Node counters, used only by thread[0] but try to keep in different cache
177 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
179 int NodesBetweenPolls = 30000;
187 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
189 template <NodeType NT>
190 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
192 template <NodeType NT>
193 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
195 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
196 bool connected_moves(const Position& pos, Move m1, Move m2);
197 Value value_to_tt(Value v, int ply);
198 Value value_from_tt(Value v, int ply);
199 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
200 bool connected_threat(const Position& pos, Move m, Move threat);
201 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
202 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
203 void update_gains(const Position& pos, Move move, Value before, Value after);
204 void do_skill_level(Move* best, Move* ponder);
206 int current_search_time(int set = 0);
207 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
208 string speed_to_uci(int64_t nodes);
209 string pv_to_uci(const Move pv[], int pvNum, bool chess960);
210 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
211 string depth_to_uci(Depth depth);
212 void poll(const Position& pos);
213 void wait_for_stop_or_ponderhit();
215 // MovePickerExt template class extends MovePicker and allows to choose at compile
216 // time the proper moves source according to the type of node. In the default case
217 // we simply create and use a standard MovePicker object.
218 template<bool SpNode> struct MovePickerExt : public MovePicker {
220 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
221 : MovePicker(p, ttm, d, h, ss, b) {}
224 // In case of a SpNode we use split point's shared MovePicker object as moves source
225 template<> struct MovePickerExt<true> : public MovePicker {
227 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
228 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
230 Move get_next_move() { return mp->get_next_move(); }
234 // Overload operator<<() to make it easier to print moves in a coordinate
235 // notation compatible with UCI protocol.
236 std::ostream& operator<<(std::ostream& os, Move m) {
238 bool chess960 = (os.iword(0) != 0); // See set960()
239 return os << move_to_uci(m, chess960);
242 // When formatting a move for std::cout we must know if we are in Chess960
243 // or not. To keep using the handy operator<<() on the move the trick is to
244 // embed this flag in the stream itself. Function-like named enum set960 is
245 // used as a custom manipulator and the stream internal general-purpose array,
246 // accessed through ios_base::iword(), is used to pass the flag to the move's
247 // operator<<() that will read it to properly format castling moves.
250 std::ostream& operator<< (std::ostream& os, const set960& f) {
252 os.iword(0) = int(f);
256 // extension() decides whether a move should be searched with normal depth,
257 // or with extended depth. Certain classes of moves (checking moves, in
258 // particular) are searched with bigger depth than ordinary moves and in
259 // any case are marked as 'dangerous'. Note that also if a move is not
260 // extended, as example because the corresponding UCI option is set to zero,
261 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
262 template <bool PvNode>
263 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
264 bool moveIsCheck, bool* dangerous) {
265 assert(m != MOVE_NONE);
267 Depth result = DEPTH_ZERO;
268 *dangerous = moveIsCheck;
270 if (moveIsCheck && pos.see_sign(m) >= 0)
271 result += CheckExtension[PvNode];
273 if (piece_type(pos.piece_on(move_from(m))) == PAWN)
275 Color c = pos.side_to_move();
276 if (relative_rank(c, move_to(m)) == RANK_7)
278 result += PawnPushTo7thExtension[PvNode];
281 if (pos.pawn_is_passed(c, move_to(m)))
283 result += PassedPawnExtension[PvNode];
288 if ( captureOrPromotion
289 && piece_type(pos.piece_on(move_to(m))) != PAWN
290 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
291 - piece_value_midgame(pos.piece_on(move_to(m))) == VALUE_ZERO)
292 && !move_is_special(m))
294 result += PawnEndgameExtension[PvNode];
298 return Min(result, ONE_PLY);
304 /// init_search() is called during startup to initialize various lookup tables
308 int d; // depth (ONE_PLY == 2)
309 int hd; // half depth (ONE_PLY == 1)
312 // Init reductions array
313 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
315 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
316 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
317 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
318 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
321 // Init futility margins array
322 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
323 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
325 // Init futility move count array
326 for (d = 0; d < 32; d++)
327 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
331 /// perft() is our utility to verify move generation. All the leaf nodes up to
332 /// the given depth are generated and counted and the sum returned.
334 int64_t perft(Position& pos, Depth depth) {
339 // Generate all legal moves
340 MoveList<MV_LEGAL> ml(pos);
342 // If we are at the last ply we don't need to do and undo
343 // the moves, just to count them.
344 if (depth <= ONE_PLY)
347 // Loop through all legal moves
349 for ( ; !ml.end(); ++ml)
351 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
352 sum += perft(pos, depth - ONE_PLY);
353 pos.undo_move(ml.move());
359 /// think() is the external interface to Stockfish's search, and is called when
360 /// the program receives the UCI 'go' command. It initializes various global
361 /// variables, and calls id_loop(). It returns false when a "quit" command is
362 /// received during the search.
364 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
368 // Initialize global search-related variables
369 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = false;
371 current_search_time(get_system_time());
373 TimeMgr.init(Limits, pos.startpos_ply_counter());
375 // Set output steram in normal or chess960 mode
376 cout << set960(pos.is_chess960());
378 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
380 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
381 else if (Limits.time && Limits.time < 1000)
382 NodesBetweenPolls = 1000;
383 else if (Limits.time && Limits.time < 5000)
384 NodesBetweenPolls = 5000;
386 NodesBetweenPolls = 30000;
388 // Look for a book move
389 if (Options["OwnBook"].value<bool>())
391 if (Options["Book File"].value<string>() != book.name())
392 book.open(Options["Book File"].value<string>());
394 Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
395 if (bookMove != MOVE_NONE)
398 wait_for_stop_or_ponderhit();
400 cout << "bestmove " << bookMove << endl;
406 UCIMultiPV = Options["MultiPV"].value<int>();
407 SkillLevel = Options["Skill Level"].value<int>();
409 read_evaluation_uci_options(pos.side_to_move());
410 Threads.read_uci_options();
412 // Set a new TT size if changed
413 TT.set_size(Options["Hash"].value<int>());
415 if (Options["Clear Hash"].value<bool>())
417 Options["Clear Hash"].set_value("false");
421 // Do we have to play with skill handicap? In this case enable MultiPV that
422 // we will use behind the scenes to retrieve a set of possible moves.
423 SkillLevelEnabled = (SkillLevel < 20);
424 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
426 // Wake up needed threads and reset maxPly counter
427 for (int i = 0; i < Threads.size(); i++)
429 Threads[i].wake_up();
430 Threads[i].maxPly = 0;
433 // Write to log file and keep it open to be accessed during the search
434 if (Options["Use Search Log"].value<bool>())
436 string name = Options["Search Log Filename"].value<string>();
437 LogFile.open(name.c_str(), std::ios::out | std::ios::app);
439 if (LogFile.is_open())
440 LogFile << "\nSearching: " << pos.to_fen()
441 << "\ninfinite: " << Limits.infinite
442 << " ponder: " << Limits.ponder
443 << " time: " << Limits.time
444 << " increment: " << Limits.increment
445 << " moves to go: " << Limits.movesToGo
449 // We're ready to start thinking. Call the iterative deepening loop function
450 Move ponderMove = MOVE_NONE;
451 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
453 // Write final search statistics and close log file
454 if (LogFile.is_open())
456 int t = current_search_time();
458 LogFile << "Nodes: " << pos.nodes_searched()
459 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
460 << "\nBest move: " << move_to_san(pos, bestMove);
463 pos.do_move(bestMove, st);
464 LogFile << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
465 pos.undo_move(bestMove); // Return from think() with unchanged position
469 // This makes all the threads to go to sleep
472 // If we are pondering or in infinite search, we shouldn't print the
473 // best move before we are told to do so.
474 if (!StopRequest && (Limits.ponder || Limits.infinite))
475 wait_for_stop_or_ponderhit();
477 // Could be MOVE_NONE when searching on a stalemate position
478 cout << "bestmove " << bestMove;
480 // UCI protol is not clear on allowing sending an empty ponder move, instead
481 // it is clear that ponder move is optional. So skip it if empty.
482 if (ponderMove != MOVE_NONE)
483 cout << " ponder " << ponderMove;
493 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
494 // with increasing depth until the allocated thinking time has been consumed,
495 // user stops the search, or the maximum search depth is reached.
497 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
499 SearchStack ss[PLY_MAX_PLUS_2];
500 Value bestValues[PLY_MAX_PLUS_2];
501 int bestMoveChanges[PLY_MAX_PLUS_2];
502 int depth, aspirationDelta;
503 Value value, alpha, beta;
504 Move bestMove, easyMove, skillBest, skillPonder;
506 // Initialize stuff before a new search
507 memset(ss, 0, 4 * sizeof(SearchStack));
510 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
511 depth = aspirationDelta = 0;
512 value = alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
513 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
515 // Moves to search are verified and copied
516 Rml.init(pos, searchMoves);
518 // Handle special case of searching on a mate/stalemate position
521 cout << "info" << depth_to_uci(DEPTH_ZERO)
522 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
527 // Iterative deepening loop until requested to stop or target depth reached
528 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
530 // Save last iteration's scores, this needs to be done now, because in
531 // the following MultiPV loop Rml moves could be reordered.
532 for (size_t i = 0; i < Rml.size(); i++)
533 Rml[i].prevScore = Rml[i].score;
535 Rml.bestMoveChanges = 0;
537 // MultiPV iteration loop. At depth 1 perform at least 2 iterations to
538 // get a score of the second best move for easy move detection.
539 int e = Min(Max(MultiPV, 2 * int(depth == 1)), (int)Rml.size());
540 for (MultiPVIteration = 0; MultiPVIteration < e; MultiPVIteration++)
542 // Calculate dynamic aspiration window based on previous iterations
543 if (depth >= 5 && abs(Rml[MultiPVIteration].prevScore) < VALUE_KNOWN_WIN)
545 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
546 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
548 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
549 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
551 alpha = Max(Rml[MultiPVIteration].prevScore - aspirationDelta, -VALUE_INFINITE);
552 beta = Min(Rml[MultiPVIteration].prevScore + aspirationDelta, VALUE_INFINITE);
556 alpha = -VALUE_INFINITE;
557 beta = VALUE_INFINITE;
560 // Start with a small aspiration window and, in case of fail high/low,
561 // research with bigger window until not failing high/low anymore.
563 // Search starting from ss+1 to allow referencing (ss-1). This is
564 // needed by update_gains() and ss copy when splitting at Root.
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() + MultiPVIteration, Rml.end());
573 // In case we have found an exact score reorder the PV moves
574 // before leaving the fail high/low loop, otherwise leave the
575 // last PV move in its position so to be searched again.
576 if (value > alpha && value < beta)
577 sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIteration);
579 // Write PV back to transposition table in case the relevant entries
580 // have been overwritten during the search.
581 for (int i = 0; i <= MultiPVIteration; i++)
582 Rml[i].insert_pv_in_tt(pos);
584 // Value cannot be trusted. Break out immediately!
588 // Send full PV info to GUI if we are going to leave the loop or
589 // if we have a fail high/low and we are deep in the search.
590 if ((value > alpha && value < beta) || current_search_time() > 2000)
591 for (int i = 0; i < Min(UCIMultiPV, MultiPVIteration + 1); i++)
593 << depth_to_uci(depth * ONE_PLY)
594 << (i == MultiPVIteration ? score_to_uci(Rml[i].score, alpha, beta) :
595 score_to_uci(Rml[i].score))
596 << speed_to_uci(pos.nodes_searched())
597 << pv_to_uci(&Rml[i].pv[0], i + 1, pos.is_chess960())
600 // In case of failing high/low increase aspiration window and research,
601 // otherwise exit the fail high/low loop.
604 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
605 aspirationDelta += aspirationDelta / 2;
607 else if (value <= alpha)
609 AspirationFailLow = true;
610 StopOnPonderhit = false;
612 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
613 aspirationDelta += aspirationDelta / 2;
618 } while (abs(value) < VALUE_KNOWN_WIN);
621 // Collect info about search result
622 bestMove = Rml[0].pv[0];
623 *ponderMove = Rml[0].pv[1];
624 bestValues[depth] = value;
625 bestMoveChanges[depth] = Rml.bestMoveChanges;
627 // Do we need to pick now the best and the ponder moves ?
628 if (SkillLevelEnabled && depth == 1 + SkillLevel)
629 do_skill_level(&skillBest, &skillPonder);
631 if (LogFile.is_open())
632 LogFile << pretty_pv(pos, depth, value, current_search_time(), &Rml[0].pv[0]) << endl;
634 // Init easyMove after first iteration or drop if differs from the best move
635 if (depth == 1 && (Rml.size() == 1 || Rml[0].score > Rml[1].score + EasyMoveMargin))
637 else if (bestMove != easyMove)
638 easyMove = MOVE_NONE;
640 // Check for some early stop condition
641 if (!StopRequest && Limits.useTimeManagement())
643 // Stop search early if one move seems to be much better than the
644 // others or if there is only a single legal move. Also in the latter
645 // case we search up to some depth anyway to get a proper score.
647 && easyMove == bestMove
649 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
650 && current_search_time() > TimeMgr.available_time() / 16)
651 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
652 && current_search_time() > TimeMgr.available_time() / 32)))
655 // Take in account some extra time if the best move has changed
656 if (depth > 4 && depth < 50)
657 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
659 // Stop search if most of available time is already consumed. We probably don't
660 // have enough time to search the first move at the next iteration anyway.
661 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
664 // If we are allowed to ponder do not stop the search now but keep pondering
665 if (StopRequest && Limits.ponder)
668 StopOnPonderhit = true;
673 // When using skills overwrite best and ponder moves with the sub-optimal ones
674 if (SkillLevelEnabled)
676 if (skillBest == MOVE_NONE) // Still unassigned ?
677 do_skill_level(&skillBest, &skillPonder);
679 bestMove = skillBest;
680 *ponderMove = skillPonder;
687 // search<>() is the main search function for both PV and non-PV nodes and for
688 // normal and SplitPoint nodes. When called just after a split point the search
689 // is simpler because we have already probed the hash table, done a null move
690 // search, and searched the first move before splitting, we don't have to repeat
691 // all this work again. We also don't need to store anything to the hash table
692 // here: This is taken care of after we return from the split point.
694 template <NodeType NT>
695 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
697 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
698 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
699 const bool RootNode = (NT == Root || NT == SplitPointRoot);
701 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
702 assert(beta > alpha && beta <= VALUE_INFINITE);
703 assert(PvNode || alpha == beta - 1);
704 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
706 Move movesSearched[MAX_MOVES];
711 Move ttMove, move, excludedMove, threatMove;
714 Value bestValue, value, oldAlpha;
715 Value refinedValue, nullValue, futilityBase, futilityValue;
716 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
717 int moveCount = 0, playedMoveCount = 0;
718 Thread& thread = Threads[pos.thread()];
719 SplitPoint* sp = NULL;
721 refinedValue = bestValue = value = -VALUE_INFINITE;
723 inCheck = pos.in_check();
724 ss->ply = (ss-1)->ply + 1;
726 // Used to send selDepth info to GUI
727 if (PvNode && thread.maxPly < ss->ply)
728 thread.maxPly = ss->ply;
730 // Step 1. Initialize node and poll. Polling can abort search
733 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
734 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
735 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
741 ttMove = excludedMove = MOVE_NONE;
742 threatMove = sp->threatMove;
743 goto split_point_start;
746 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
752 // Step 2. Check for aborted search and immediate draw
754 || pos.is_draw<false>()
755 || ss->ply > PLY_MAX) && !RootNode)
758 // Step 3. Mate distance pruning
761 alpha = Max(value_mated_in(ss->ply), alpha);
762 beta = Min(value_mate_in(ss->ply+1), beta);
767 // Step 4. Transposition table lookup
768 // We don't want the score of a partial search to overwrite a previous full search
769 // TT value, so we use a different position key in case of an excluded move.
770 excludedMove = ss->excludedMove;
771 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
772 tte = TT.probe(posKey);
773 ttMove = RootNode ? Rml[MultiPVIteration].pv[0] : tte ? tte->move() : MOVE_NONE;
775 // At PV nodes we check for exact scores, while at non-PV nodes we check for
776 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
777 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
778 // we should also update RootMoveList to avoid bogus output.
779 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
780 : can_return_tt(tte, depth, beta, ss->ply)))
783 ss->bestMove = ttMove; // Can be MOVE_NONE
784 return value_from_tt(tte->value(), ss->ply);
787 // Step 5. Evaluate the position statically and update parent's gain statistics
789 ss->eval = ss->evalMargin = VALUE_NONE;
792 assert(tte->static_value() != VALUE_NONE);
794 ss->eval = tte->static_value();
795 ss->evalMargin = tte->static_value_margin();
796 refinedValue = refine_eval(tte, ss->eval, ss->ply);
800 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
801 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
804 // Save gain for the parent non-capture move
805 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
807 // Step 6. Razoring (is omitted in PV nodes)
809 && depth < RazorDepth
811 && refinedValue + razor_margin(depth) < beta
812 && ttMove == MOVE_NONE
813 && abs(beta) < VALUE_MATE_IN_PLY_MAX
814 && !pos.has_pawn_on_7th(pos.side_to_move()))
816 Value rbeta = beta - razor_margin(depth);
817 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
819 // Logically we should return (v + razor_margin(depth)), but
820 // surprisingly this did slightly weaker in tests.
824 // Step 7. Static null move pruning (is omitted in PV nodes)
825 // We're betting that the opponent doesn't have a move that will reduce
826 // the score by more than futility_margin(depth) if we do a null move.
829 && depth < RazorDepth
831 && refinedValue - futility_margin(depth, 0) >= beta
832 && abs(beta) < VALUE_MATE_IN_PLY_MAX
833 && pos.non_pawn_material(pos.side_to_move()))
834 return refinedValue - futility_margin(depth, 0);
836 // Step 8. Null move search with verification search (is omitted in PV nodes)
841 && refinedValue >= beta
842 && abs(beta) < VALUE_MATE_IN_PLY_MAX
843 && pos.non_pawn_material(pos.side_to_move()))
845 ss->currentMove = MOVE_NULL;
847 // Null move dynamic reduction based on depth
848 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
850 // Null move dynamic reduction based on value
851 if (refinedValue - PawnValueMidgame > beta)
854 pos.do_null_move(st);
855 (ss+1)->skipNullMove = true;
856 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
857 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
858 (ss+1)->skipNullMove = false;
859 pos.undo_null_move();
861 if (nullValue >= beta)
863 // Do not return unproven mate scores
864 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
867 if (depth < 6 * ONE_PLY)
870 // Do verification search at high depths
871 ss->skipNullMove = true;
872 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
873 ss->skipNullMove = false;
880 // The null move failed low, which means that we may be faced with
881 // some kind of threat. If the previous move was reduced, check if
882 // the move that refuted the null move was somehow connected to the
883 // move which was reduced. If a connection is found, return a fail
884 // low score (which will cause the reduced move to fail high in the
885 // parent node, which will trigger a re-search with full depth).
886 threatMove = (ss+1)->bestMove;
888 if ( depth < ThreatDepth
890 && threatMove != MOVE_NONE
891 && connected_moves(pos, (ss-1)->currentMove, threatMove))
896 // Step 9. ProbCut (is omitted in PV nodes)
897 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
898 // and a reduced search returns a value much above beta, we can (almost) safely
899 // prune the previous move.
901 && depth >= RazorDepth + ONE_PLY
904 && excludedMove == MOVE_NONE
905 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
907 Value rbeta = beta + 200;
908 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
910 assert(rdepth >= ONE_PLY);
912 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
915 while ((move = mp.get_next_move()) != MOVE_NONE)
916 if (pos.pl_move_is_legal(move, ci.pinned))
918 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
919 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
926 // Step 10. Internal iterative deepening
927 if ( depth >= IIDDepth[PvNode]
928 && ttMove == MOVE_NONE
929 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
931 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
933 ss->skipNullMove = true;
934 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
935 ss->skipNullMove = false;
937 tte = TT.probe(posKey);
938 ttMove = tte ? tte->move() : MOVE_NONE;
941 split_point_start: // At split points actual search starts from here
943 // Initialize a MovePicker object for the current position
944 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
946 ss->bestMove = MOVE_NONE;
947 futilityBase = ss->eval + ss->evalMargin;
948 singularExtensionNode = !RootNode
950 && depth >= SingularExtensionDepth[PvNode]
951 && ttMove != MOVE_NONE
952 && !excludedMove // Do not allow recursive singular extension search
953 && (tte->type() & VALUE_TYPE_LOWER)
954 && tte->depth() >= depth - 3 * ONE_PLY;
957 lock_grab(&(sp->lock));
958 bestValue = sp->bestValue;
961 // Step 11. Loop through moves
962 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
963 while ( bestValue < beta
964 && (move = mp.get_next_move()) != MOVE_NONE
965 && !thread.cutoff_occurred())
967 assert(move_is_ok(move));
969 if (move == excludedMove)
972 // At root obey the "searchmoves" option and skip moves not listed in Root Move List.
973 // Also in MultiPV mode we skip moves which already have got an exact score
974 // in previous MultiPV Iteration. Finally any illegal move is skipped here.
975 if (RootNode && !Rml.find(move, MultiPVIteration))
978 // At PV and SpNode nodes we want all moves to be legal since the beginning
979 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
984 moveCount = ++sp->moveCount;
985 lock_release(&(sp->lock));
992 // This is used by time management
993 FirstRootMove = (moveCount == 1);
995 // Save the current node count before the move is searched
996 nodes = pos.nodes_searched();
998 // For long searches send current move info to GUI
999 if (pos.thread() == 0 && current_search_time() > 2000)
1000 cout << "info" << depth_to_uci(depth)
1001 << " currmove " << move
1002 << " currmovenumber " << moveCount + MultiPVIteration << endl;
1005 isPvMove = (PvNode && moveCount == 1);
1006 givesCheck = pos.move_gives_check(move, ci);
1007 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1009 // Step 12. Decide the new search depth
1010 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1012 // Singular extension search. If all moves but one fail low on a search of
1013 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1014 // is singular and should be extended. To verify this we do a reduced search
1015 // on all the other moves but the ttMove, if result is lower than ttValue minus
1016 // a margin then we extend ttMove.
1017 if ( singularExtensionNode
1019 && pos.pl_move_is_legal(move, ci.pinned)
1022 Value ttValue = value_from_tt(tte->value(), ss->ply);
1024 if (abs(ttValue) < VALUE_KNOWN_WIN)
1026 Value rBeta = ttValue - int(depth);
1027 ss->excludedMove = move;
1028 ss->skipNullMove = true;
1029 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1030 ss->skipNullMove = false;
1031 ss->excludedMove = MOVE_NONE;
1032 ss->bestMove = MOVE_NONE;
1038 // Update current move (this must be done after singular extension search)
1039 newDepth = depth - ONE_PLY + ext;
1041 // Step 13. Futility pruning (is omitted in PV nodes)
1043 && !captureOrPromotion
1047 && !move_is_castle(move))
1049 // Move count based pruning
1050 if ( moveCount >= futility_move_count(depth)
1051 && (!threatMove || !connected_threat(pos, move, threatMove))
1052 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1055 lock_grab(&(sp->lock));
1060 // Value based pruning
1061 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1062 // but fixing this made program slightly weaker.
1063 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1064 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1065 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1067 if (futilityValue < beta)
1071 lock_grab(&(sp->lock));
1072 if (futilityValue > sp->bestValue)
1073 sp->bestValue = bestValue = futilityValue;
1075 else if (futilityValue > bestValue)
1076 bestValue = futilityValue;
1081 // Prune moves with negative SEE at low depths
1082 if ( predictedDepth < 2 * ONE_PLY
1083 && bestValue > VALUE_MATED_IN_PLY_MAX
1084 && pos.see_sign(move) < 0)
1087 lock_grab(&(sp->lock));
1093 // Check for legality only before to do the move
1094 if (!pos.pl_move_is_legal(move, ci.pinned))
1100 ss->currentMove = move;
1101 if (!SpNode && !captureOrPromotion)
1102 movesSearched[playedMoveCount++] = move;
1104 // Step 14. Make the move
1105 pos.do_move(move, st, ci, givesCheck);
1107 // Step extra. pv search (only in PV nodes)
1108 // The first move in list is the expected PV
1110 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1111 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1114 // Step 15. Reduced depth search
1115 // If the move fails high will be re-searched at full depth.
1116 bool doFullDepthSearch = true;
1118 if ( depth > 3 * ONE_PLY
1119 && !captureOrPromotion
1121 && !move_is_castle(move)
1122 && ss->killers[0] != move
1123 && ss->killers[1] != move
1124 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1126 Depth d = newDepth - ss->reduction;
1127 alpha = SpNode ? sp->alpha : alpha;
1129 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1130 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1132 ss->reduction = DEPTH_ZERO;
1133 doFullDepthSearch = (value > alpha);
1136 // Step 16. Full depth search
1137 if (doFullDepthSearch)
1139 alpha = SpNode ? sp->alpha : alpha;
1140 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1141 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1143 // Step extra. pv search (only in PV nodes)
1144 // Search only for possible new PV nodes, if instead value >= beta then
1145 // parent node fails low with value <= alpha and tries another move.
1146 if (PvNode && value > alpha && (RootNode || value < beta))
1147 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1148 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1152 // Step 17. Undo move
1153 pos.undo_move(move);
1155 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1157 // Step 18. Check for new best move
1160 lock_grab(&(sp->lock));
1161 bestValue = sp->bestValue;
1165 // Finished searching the move. If StopRequest is true, the search
1166 // was aborted because the user interrupted the search or because we
1167 // ran out of time. In this case, the return value of the search cannot
1168 // be trusted, and we don't update the best move and/or PV.
1169 if (RootNode && !StopRequest)
1171 // Remember searched nodes counts for this move
1172 RootMove* rm = Rml.find(move);
1173 rm->nodes += pos.nodes_searched() - nodes;
1175 // PV move or new best move ?
1176 if (isPvMove || value > alpha)
1180 rm->extract_pv_from_tt(pos);
1182 // We record how often the best move has been changed in each
1183 // iteration. This information is used for time management: When
1184 // the best move changes frequently, we allocate some more time.
1185 if (!isPvMove && MultiPV == 1)
1186 Rml.bestMoveChanges++;
1189 // All other moves but the PV are set to the lowest value, this
1190 // is not a problem when sorting becuase sort is stable and move
1191 // position in the list is preserved, just the PV is pushed up.
1192 rm->score = -VALUE_INFINITE;
1196 if (value > bestValue)
1199 ss->bestMove = move;
1203 && value < beta) // We want always alpha < beta
1206 if (SpNode && !thread.cutoff_occurred())
1208 sp->bestValue = value;
1209 sp->ss->bestMove = move;
1211 sp->is_betaCutoff = (value >= beta);
1215 // Step 19. Check for split
1217 && depth >= Threads.min_split_depth()
1219 && Threads.available_slave_exists(pos.thread())
1221 && !thread.cutoff_occurred())
1222 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1223 threatMove, moveCount, &mp, NT);
1226 // Step 20. Check for mate and stalemate
1227 // All legal moves have been searched and if there are
1228 // no legal moves, it must be mate or stalemate.
1229 // If one move was excluded return fail low score.
1230 if (!SpNode && !moveCount)
1231 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1233 // Step 21. Update tables
1234 // If the search is not aborted, update the transposition table,
1235 // history counters, and killer moves.
1236 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1238 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1239 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1240 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1242 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1244 // Update killers and history only for non capture moves that fails high
1245 if ( bestValue >= beta
1246 && !pos.move_is_capture_or_promotion(move))
1248 if (move != ss->killers[0])
1250 ss->killers[1] = ss->killers[0];
1251 ss->killers[0] = move;
1253 update_history(pos, move, depth, movesSearched, playedMoveCount);
1259 // Here we have the lock still grabbed
1260 sp->is_slave[pos.thread()] = false;
1261 sp->nodes += pos.nodes_searched();
1262 lock_release(&(sp->lock));
1265 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1270 // qsearch() is the quiescence search function, which is called by the main
1271 // search function when the remaining depth is zero (or, to be more precise,
1272 // less than ONE_PLY).
1274 template <NodeType NT>
1275 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1277 const bool PvNode = (NT == PV);
1279 assert(NT == PV || NT == NonPV);
1280 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1281 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1282 assert(PvNode || alpha == beta - 1);
1284 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1288 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1289 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1293 Value oldAlpha = alpha;
1295 ss->bestMove = ss->currentMove = MOVE_NONE;
1296 ss->ply = (ss-1)->ply + 1;
1298 // Check for an instant draw or maximum ply reached
1299 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1302 // Decide whether or not to include checks, this fixes also the type of
1303 // TT entry depth that we are going to use. Note that in qsearch we use
1304 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1305 inCheck = pos.in_check();
1306 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1308 // Transposition table lookup. At PV nodes, we don't use the TT for
1309 // pruning, but only for move ordering.
1310 tte = TT.probe(pos.get_key());
1311 ttMove = (tte ? tte->move() : MOVE_NONE);
1313 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1315 ss->bestMove = ttMove; // Can be MOVE_NONE
1316 return value_from_tt(tte->value(), ss->ply);
1319 // Evaluate the position statically
1322 bestValue = futilityBase = -VALUE_INFINITE;
1323 ss->eval = evalMargin = VALUE_NONE;
1324 enoughMaterial = false;
1330 assert(tte->static_value() != VALUE_NONE);
1332 evalMargin = tte->static_value_margin();
1333 ss->eval = bestValue = tte->static_value();
1336 ss->eval = bestValue = evaluate(pos, evalMargin);
1338 // Stand pat. Return immediately if static value is at least beta
1339 if (bestValue >= beta)
1342 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1347 if (PvNode && bestValue > alpha)
1350 // Futility pruning parameters, not needed when in check
1351 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1352 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1355 // Initialize a MovePicker object for the current position, and prepare
1356 // to search the moves. Because the depth is <= 0 here, only captures,
1357 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1359 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1362 // Loop through the moves until no moves remain or a beta cutoff occurs
1363 while ( bestValue < beta
1364 && (move = mp.get_next_move()) != MOVE_NONE)
1366 assert(move_is_ok(move));
1368 givesCheck = pos.move_gives_check(move, ci);
1376 && !move_is_promotion(move)
1377 && !pos.move_is_passed_pawn_push(move))
1379 futilityValue = futilityBase
1380 + piece_value_endgame(pos.piece_on(move_to(move)))
1381 + (move_is_ep(move) ? PawnValueEndgame : VALUE_ZERO);
1383 if (futilityValue < beta)
1385 if (futilityValue > bestValue)
1386 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)
1446 ss->bestMove = move;
1450 && value < beta) // We want always alpha < beta
1455 // All legal moves have been searched. A special case: If we're in check
1456 // and no legal moves were found, it is checkmate.
1457 if (inCheck && bestValue == -VALUE_INFINITE)
1458 return value_mated_in(ss->ply);
1460 // Update transposition table
1461 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1462 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1463 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1465 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1467 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1473 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1474 // bestValue is updated only when returning false because in that case move
1477 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1479 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1480 Square from, to, ksq, victimSq;
1483 Value futilityValue, bv = *bestValue;
1485 from = move_from(move);
1487 them = opposite_color(pos.side_to_move());
1488 ksq = pos.king_square(them);
1489 kingAtt = pos.attacks_from<KING>(ksq);
1490 pc = pos.piece_on(from);
1492 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1493 oldAtt = pos.attacks_from(pc, from, occ);
1494 newAtt = pos.attacks_from(pc, to, occ);
1496 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1497 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1499 if (!(b && (b & (b - 1))))
1502 // Rule 2. Queen contact check is very dangerous
1503 if ( piece_type(pc) == QUEEN
1504 && bit_is_set(kingAtt, to))
1507 // Rule 3. Creating new double threats with checks
1508 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1512 victimSq = pop_1st_bit(&b);
1513 futilityValue = futilityBase + piece_value_endgame(pos.piece_on(victimSq));
1515 // Note that here we generate illegal "double move"!
1516 if ( futilityValue >= beta
1517 && pos.see_sign(make_move(from, victimSq)) >= 0)
1520 if (futilityValue > bv)
1524 // Update bestValue only if check is not dangerous (because we will prune the move)
1530 // connected_moves() tests whether two moves are 'connected' in the sense
1531 // that the first move somehow made the second move possible (for instance
1532 // if the moving piece is the same in both moves). The first move is assumed
1533 // to be the move that was made to reach the current position, while the
1534 // second move is assumed to be a move from the current position.
1536 bool connected_moves(const Position& pos, Move m1, Move m2) {
1538 Square f1, t1, f2, t2;
1542 assert(m1 && move_is_ok(m1));
1543 assert(m2 && move_is_ok(m2));
1545 // Case 1: The moving piece is the same in both moves
1551 // Case 2: The destination square for m2 was vacated by m1
1557 // Case 3: Moving through the vacated square
1558 p2 = pos.piece_on(f2);
1559 if ( piece_is_slider(p2)
1560 && bit_is_set(squares_between(f2, t2), f1))
1563 // Case 4: The destination square for m2 is defended by the moving piece in m1
1564 p1 = pos.piece_on(t1);
1565 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1568 // Case 5: Discovered check, checking piece is the piece moved in m1
1569 ksq = pos.king_square(pos.side_to_move());
1570 if ( piece_is_slider(p1)
1571 && bit_is_set(squares_between(t1, ksq), f2))
1573 Bitboard occ = pos.occupied_squares();
1574 clear_bit(&occ, f2);
1575 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1582 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1583 // "plies to mate from the current ply". Non-mate scores are unchanged.
1584 // The function is called before storing a value to the transposition table.
1586 Value value_to_tt(Value v, int ply) {
1588 if (v >= VALUE_MATE_IN_PLY_MAX)
1591 if (v <= VALUE_MATED_IN_PLY_MAX)
1598 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1599 // the transposition table to a mate score corrected for the current ply.
1601 Value value_from_tt(Value v, int ply) {
1603 if (v >= VALUE_MATE_IN_PLY_MAX)
1606 if (v <= VALUE_MATED_IN_PLY_MAX)
1613 // connected_threat() tests whether it is safe to forward prune a move or if
1614 // is somehow connected to the threat move returned by null search.
1616 bool connected_threat(const Position& pos, Move m, Move threat) {
1618 assert(move_is_ok(m));
1619 assert(threat && move_is_ok(threat));
1620 assert(!pos.move_is_capture_or_promotion(m));
1621 assert(!pos.move_is_passed_pawn_push(m));
1623 Square mfrom, mto, tfrom, tto;
1625 mfrom = move_from(m);
1627 tfrom = move_from(threat);
1628 tto = move_to(threat);
1630 // Case 1: Don't prune moves which move the threatened piece
1634 // Case 2: If the threatened piece has value less than or equal to the
1635 // value of the threatening piece, don't prune moves which defend it.
1636 if ( pos.move_is_capture(threat)
1637 && ( piece_value_midgame(pos.piece_on(tfrom)) >= piece_value_midgame(pos.piece_on(tto))
1638 || piece_type(pos.piece_on(tfrom)) == KING)
1639 && pos.move_attacks_square(m, tto))
1642 // Case 3: If the moving piece in the threatened move is a slider, don't
1643 // prune safe moves which block its ray.
1644 if ( piece_is_slider(pos.piece_on(tfrom))
1645 && bit_is_set(squares_between(tfrom, tto), mto)
1646 && pos.see_sign(m) >= 0)
1653 // can_return_tt() returns true if a transposition table score
1654 // can be used to cut-off at a given point in search.
1656 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1658 Value v = value_from_tt(tte->value(), ply);
1660 return ( tte->depth() >= depth
1661 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1662 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1664 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1665 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1669 // refine_eval() returns the transposition table score if
1670 // possible otherwise falls back on static position evaluation.
1672 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1676 Value v = value_from_tt(tte->value(), ply);
1678 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1679 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1686 // update_history() registers a good move that produced a beta-cutoff
1687 // in history and marks as failures all the other moves of that ply.
1689 void update_history(const Position& pos, Move move, Depth depth,
1690 Move movesSearched[], int moveCount) {
1692 Value bonus = Value(int(depth) * int(depth));
1694 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1696 for (int i = 0; i < moveCount - 1; i++)
1698 m = movesSearched[i];
1702 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1707 // update_gains() updates the gains table of a non-capture move given
1708 // the static position evaluation before and after the move.
1710 void update_gains(const Position& pos, Move m, Value before, Value after) {
1713 && before != VALUE_NONE
1714 && after != VALUE_NONE
1715 && pos.captured_piece_type() == PIECE_TYPE_NONE
1716 && !move_is_special(m))
1717 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1721 // current_search_time() returns the number of milliseconds which have passed
1722 // since the beginning of the current search.
1724 int current_search_time(int set) {
1726 static int searchStartTime;
1729 searchStartTime = set;
1731 return get_system_time() - searchStartTime;
1735 // score_to_uci() converts a value to a string suitable for use with the UCI
1736 // protocol specifications:
1738 // cp <x> The score from the engine's point of view in centipawns.
1739 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1740 // use negative values for y.
1742 string score_to_uci(Value v, Value alpha, Value beta) {
1744 std::stringstream s;
1746 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1747 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1749 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1751 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1757 // speed_to_uci() returns a string with time stats of current search suitable
1758 // to be sent to UCI gui.
1760 string speed_to_uci(int64_t nodes) {
1762 std::stringstream s;
1763 int t = current_search_time();
1765 s << " nodes " << nodes
1766 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1772 // pv_to_uci() returns a string with information on the current PV line
1773 // formatted according to UCI specification.
1775 string pv_to_uci(const Move pv[], int pvNum, bool chess960) {
1777 std::stringstream s;
1779 s << " multipv " << pvNum << " pv " << set960(chess960);
1781 for ( ; *pv != MOVE_NONE; pv++)
1787 // depth_to_uci() returns a string with information on the current depth and
1788 // seldepth formatted according to UCI specification.
1790 string depth_to_uci(Depth depth) {
1792 std::stringstream s;
1794 // Retrieve max searched depth among threads
1796 for (int i = 0; i < Threads.size(); i++)
1797 if (Threads[i].maxPly > selDepth)
1798 selDepth = Threads[i].maxPly;
1800 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1805 string time_to_string(int millisecs) {
1807 const int MSecMinute = 1000 * 60;
1808 const int MSecHour = 1000 * 60 * 60;
1810 int hours = millisecs / MSecHour;
1811 int minutes = (millisecs % MSecHour) / MSecMinute;
1812 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1814 std::stringstream s;
1819 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1823 string score_to_string(Value v) {
1825 std::stringstream s;
1827 if (v >= VALUE_MATE_IN_PLY_MAX)
1828 s << "#" << (VALUE_MATE - v + 1) / 2;
1829 else if (v <= VALUE_MATED_IN_PLY_MAX)
1830 s << "-#" << (VALUE_MATE + v) / 2;
1832 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1837 // pretty_pv() creates a human-readable string from a position and a PV.
1838 // It is used to write search information to the log file (which is created
1839 // when the UCI parameter "Use Search Log" is "true").
1841 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1843 const int64_t K = 1000;
1844 const int64_t M = 1000000;
1845 const int startColumn = 28;
1846 const size_t maxLength = 80 - startColumn;
1848 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1851 std::stringstream s;
1854 // First print depth, score, time and searched nodes...
1855 s << set960(pos.is_chess960())
1856 << std::setw(2) << depth
1857 << std::setw(8) << score_to_string(value)
1858 << std::setw(8) << time_to_string(time);
1860 if (pos.nodes_searched() < M)
1861 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1862 else if (pos.nodes_searched() < K * M)
1863 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1865 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1867 // ...then print the full PV line in short algebraic notation
1868 while (*m != MOVE_NONE)
1870 san = move_to_san(pos, *m);
1871 length += san.length() + 1;
1873 if (length > maxLength)
1875 length = san.length() + 1;
1876 s << "\n" + string(startColumn, ' ');
1880 pos.do_move(*m++, *st++);
1883 // Restore original position before to leave
1884 while (m != pv) pos.undo_move(*--m);
1889 // poll() performs two different functions: It polls for user input, and it
1890 // looks at the time consumed so far and decides if it's time to abort the
1893 void poll(const Position& pos) {
1895 static int lastInfoTime;
1896 int t = current_search_time();
1899 if (input_available())
1901 // We are line oriented, don't read single chars
1904 if (!std::getline(std::cin, command) || command == "quit")
1906 // Quit the program as soon as possible
1907 Limits.ponder = false;
1908 QuitRequest = StopRequest = true;
1911 else if (command == "stop")
1913 // Stop calculating as soon as possible, but still send the "bestmove"
1914 // and possibly the "ponder" token when finishing the search.
1915 Limits.ponder = false;
1918 else if (command == "ponderhit")
1920 // The opponent has played the expected move. GUI sends "ponderhit" if
1921 // we were told to ponder on the same move the opponent has played. We
1922 // should continue searching but switching from pondering to normal search.
1923 Limits.ponder = false;
1925 if (StopOnPonderhit)
1930 // Print search information
1934 else if (lastInfoTime > t)
1935 // HACK: Must be a new search where we searched less than
1936 // NodesBetweenPolls nodes during the first second of search.
1939 else if (t - lastInfoTime >= 1000)
1944 dbg_print_hit_rate();
1947 // Should we stop the search?
1951 bool stillAtFirstMove = FirstRootMove
1952 && !AspirationFailLow
1953 && t > TimeMgr.available_time();
1955 bool noMoreTime = t > TimeMgr.maximum_time()
1956 || stillAtFirstMove;
1958 if ( (Limits.useTimeManagement() && noMoreTime)
1959 || (Limits.maxTime && t >= Limits.maxTime)
1960 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1965 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1966 // while the program is pondering. The point is to work around a wrinkle in
1967 // the UCI protocol: When pondering, the engine is not allowed to give a
1968 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1969 // We simply wait here until one of these commands is sent, and return,
1970 // after which the bestmove and pondermove will be printed.
1972 void wait_for_stop_or_ponderhit() {
1976 // Wait for a command from stdin
1977 while ( std::getline(std::cin, command)
1978 && command != "ponderhit" && command != "stop" && command != "quit") {};
1980 if (command != "ponderhit" && command != "stop")
1981 QuitRequest = true; // Must be "quit" or getline() returned false
1985 // When playing with strength handicap choose best move among the MultiPV set
1986 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1987 void do_skill_level(Move* best, Move* ponder) {
1989 assert(MultiPV > 1);
1993 // Rml list is already sorted by score in descending order
1995 int max_s = -VALUE_INFINITE;
1996 int size = Min(MultiPV, (int)Rml.size());
1997 int max = Rml[0].score;
1998 int var = Min(max - Rml[size - 1].score, PawnValueMidgame);
1999 int wk = 120 - 2 * SkillLevel;
2001 // PRNG sequence should be non deterministic
2002 for (int i = abs(get_system_time() % 50); i > 0; i--)
2003 rk.rand<unsigned>();
2005 // Choose best move. For each move's score we add two terms both dependent
2006 // on wk, one deterministic and bigger for weaker moves, and one random,
2007 // then we choose the move with the resulting highest score.
2008 for (int i = 0; i < size; i++)
2012 // Don't allow crazy blunders even at very low skills
2013 if (i > 0 && Rml[i-1].score > s + EasyMoveMargin)
2016 // This is our magical formula
2017 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
2022 *best = Rml[i].pv[0];
2023 *ponder = Rml[i].pv[1];
2029 /// RootMove and RootMoveList method's definitions
2031 void RootMoveList::init(Position& pos, Move searchMoves[]) {
2034 bestMoveChanges = 0;
2037 // Generate all legal moves and add them to RootMoveList
2038 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2040 // If we have a searchMoves[] list then verify the move
2041 // is in the list before to add it.
2042 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2044 if (sm != searchMoves && *sm != ml.move())
2048 rm.pv.push_back(ml.move());
2049 rm.pv.push_back(MOVE_NONE);
2050 rm.score = rm.prevScore = -VALUE_INFINITE;
2056 RootMove* RootMoveList::find(const Move& m, int startIndex) {
2058 for (size_t i = startIndex; i < size(); i++)
2059 if ((*this)[i].pv[0] == m)
2065 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2066 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2067 // allow to always have a ponder move even when we fail high at root and also a
2068 // long PV to print that is important for position analysis.
2070 void RootMove::extract_pv_from_tt(Position& pos) {
2072 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2077 assert(m != MOVE_NONE && pos.move_is_pl(m));
2081 pos.do_move(m, *st++);
2083 while ( (tte = TT.probe(pos.get_key())) != NULL
2084 && tte->move() != MOVE_NONE
2085 && pos.move_is_pl(tte->move())
2086 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2088 && (!pos.is_draw<false>() || ply < 2))
2090 pv.push_back(tte->move());
2091 pos.do_move(tte->move(), *st++);
2094 pv.push_back(MOVE_NONE);
2096 do pos.undo_move(pv[--ply]); while (ply);
2099 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2100 // the PV back into the TT. This makes sure the old PV moves are searched
2101 // first, even if the old TT entries have been overwritten.
2103 void RootMove::insert_pv_in_tt(Position& pos) {
2105 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2108 Value v, m = VALUE_NONE;
2111 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2117 // Don't overwrite existing correct entries
2118 if (!tte || tte->move() != pv[ply])
2120 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2121 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2123 pos.do_move(pv[ply], *st++);
2125 } while (pv[++ply] != MOVE_NONE);
2127 do pos.undo_move(pv[--ply]); while (ply);
2132 // Little helper used by idle_loop() to check that all the slave threads of a
2133 // split point have finished searching.
2135 static bool all_slaves_finished(SplitPoint* sp) {
2137 for (int i = 0; i < Threads.size(); i++)
2138 if (sp->is_slave[i])
2145 // Thread::idle_loop() is where the thread is parked when it has no work to do.
2146 // The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
2147 // for which the thread is the master.
2149 void Thread::idle_loop(SplitPoint* sp) {
2153 // If we are not searching, wait for a condition to be signaled
2154 // instead of wasting CPU time polling for work.
2157 || (Threads.use_sleeping_threads() && !is_searching))
2159 assert((!sp && threadID) || Threads.use_sleeping_threads());
2161 // Slave thread should exit as soon as do_terminate flag raises
2168 // Grab the lock to avoid races with Thread::wake_up()
2169 lock_grab(&sleepLock);
2171 // If we are master and all slaves have finished don't go to sleep
2172 if (sp && all_slaves_finished(sp))
2174 lock_release(&sleepLock);
2178 // Do sleep after retesting sleep conditions under lock protection, in
2179 // particular we need to avoid a deadlock in case a master thread has,
2180 // in the meanwhile, allocated us and sent the wake_up() call before we
2181 // had the chance to grab the lock.
2182 if (do_sleep || !is_searching)
2183 cond_wait(&sleepCond, &sleepLock);
2185 lock_release(&sleepLock);
2188 // If this thread has been assigned work, launch a search
2191 assert(!do_terminate);
2193 // Copy split point position and search stack and call search()
2194 SearchStack ss[PLY_MAX_PLUS_2];
2195 SplitPoint* tsp = splitPoint;
2196 Position pos(*tsp->pos, threadID);
2198 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2201 if (tsp->nodeType == Root)
2202 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2203 else if (tsp->nodeType == PV)
2204 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2205 else if (tsp->nodeType == NonPV)
2206 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2210 assert(is_searching);
2212 is_searching = false;
2214 // Wake up master thread so to allow it to return from the idle loop in
2215 // case we are the last slave of the split point.
2216 if ( Threads.use_sleeping_threads()
2217 && threadID != tsp->master
2218 && !Threads[tsp->master].is_searching)
2219 Threads[tsp->master].wake_up();
2222 // If this thread is the master of a split point and all slaves have
2223 // finished their work at this split point, return from the idle loop.
2224 if (sp && all_slaves_finished(sp))
2226 // Because sp->is_slave[] is reset under lock protection,
2227 // be sure sp->lock has been released before to return.
2228 lock_grab(&(sp->lock));
2229 lock_release(&(sp->lock));