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[std::max(int(d), 1)][std::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][std::min(int(d) / ONE_PLY, 63)][std::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, MultiPVIdx;
164 // Time management variables
165 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
169 // Skill level adjustment
171 bool SkillLevelEnabled;
173 // Node counters, used only by thread[0] but try to keep in different cache
174 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
176 int NodesBetweenPolls = 30000;
184 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
186 template <NodeType NT>
187 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
189 template <NodeType NT>
190 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
192 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
193 bool connected_moves(const Position& pos, Move m1, Move m2);
194 Value value_to_tt(Value v, int ply);
195 Value value_from_tt(Value v, int ply);
196 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
197 bool connected_threat(const Position& pos, Move m, Move threat);
198 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
199 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
200 void update_gains(const Position& pos, Move move, Value before, Value after);
201 void do_skill_level(Move* best, Move* ponder);
203 int current_search_time(int set = 0);
204 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
205 string speed_to_uci(int64_t nodes);
206 string pv_to_uci(const Move pv[], int pvNum, bool chess960);
207 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
208 string depth_to_uci(Depth depth);
209 void poll(const Position& pos);
210 void wait_for_stop_or_ponderhit();
212 // MovePickerExt template class extends MovePicker and allows to choose at compile
213 // time the proper moves source according to the type of node. In the default case
214 // we simply create and use a standard MovePicker object.
215 template<bool SpNode> struct MovePickerExt : public MovePicker {
217 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
218 : MovePicker(p, ttm, d, h, ss, b) {}
221 // In case of a SpNode we use split point's shared MovePicker object as moves source
222 template<> struct MovePickerExt<true> : public MovePicker {
224 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
225 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
227 Move get_next_move() { return mp->get_next_move(); }
231 // Overload operator<<() to make it easier to print moves in a coordinate
232 // notation compatible with UCI protocol.
233 std::ostream& operator<<(std::ostream& os, Move m) {
235 bool chess960 = (os.iword(0) != 0); // See set960()
236 return os << move_to_uci(m, chess960);
239 // When formatting a move for std::cout we must know if we are in Chess960
240 // or not. To keep using the handy operator<<() on the move the trick is to
241 // embed this flag in the stream itself. Function-like named enum set960 is
242 // used as a custom manipulator and the stream internal general-purpose array,
243 // accessed through ios_base::iword(), is used to pass the flag to the move's
244 // operator<<() that will read it to properly format castling moves.
247 std::ostream& operator<< (std::ostream& os, const set960& f) {
249 os.iword(0) = int(f);
253 // extension() decides whether a move should be searched with normal depth,
254 // or with extended depth. Certain classes of moves (checking moves, in
255 // particular) are searched with bigger depth than ordinary moves and in
256 // any case are marked as 'dangerous'. Note that also if a move is not
257 // extended, as example because the corresponding UCI option is set to zero,
258 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
259 template <bool PvNode>
260 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
261 bool moveIsCheck, bool* dangerous) {
262 assert(m != MOVE_NONE);
264 Depth result = DEPTH_ZERO;
265 *dangerous = moveIsCheck;
267 if (moveIsCheck && pos.see_sign(m) >= 0)
268 result += CheckExtension[PvNode];
270 if (type_of(pos.piece_on(move_from(m))) == PAWN)
272 Color c = pos.side_to_move();
273 if (relative_rank(c, move_to(m)) == RANK_7)
275 result += PawnPushTo7thExtension[PvNode];
278 if (pos.pawn_is_passed(c, move_to(m)))
280 result += PassedPawnExtension[PvNode];
285 if ( captureOrPromotion
286 && type_of(pos.piece_on(move_to(m))) != PAWN
287 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
288 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
291 result += PawnEndgameExtension[PvNode];
295 return std::min(result, ONE_PLY);
301 /// init_search() is called during startup to initialize various lookup tables
305 int d; // depth (ONE_PLY == 2)
306 int hd; // half depth (ONE_PLY == 1)
309 // Init reductions array
310 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
312 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
313 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
314 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
315 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
318 // Init futility margins array
319 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
320 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
322 // Init futility move count array
323 for (d = 0; d < 32; d++)
324 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
328 /// perft() is our utility to verify move generation. All the leaf nodes up to
329 /// the given depth are generated and counted and the sum returned.
331 int64_t perft(Position& pos, Depth depth) {
336 // Generate all legal moves
337 MoveList<MV_LEGAL> ml(pos);
339 // If we are at the last ply we don't need to do and undo
340 // the moves, just to count them.
341 if (depth <= ONE_PLY)
344 // Loop through all legal moves
346 for ( ; !ml.end(); ++ml)
348 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
349 sum += perft(pos, depth - ONE_PLY);
350 pos.undo_move(ml.move());
356 /// think() is the external interface to Stockfish's search, and is called when
357 /// the program receives the UCI 'go' command. It initializes various global
358 /// variables, and calls id_loop(). It returns false when a "quit" command is
359 /// received during the search.
361 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
363 static Book book; // Define static to initialize the PRNG only once
365 // Initialize global search-related variables
366 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = false;
368 current_search_time(get_system_time());
370 TimeMgr.init(Limits, pos.startpos_ply_counter());
372 // Set output steram in normal or chess960 mode
373 cout << set960(pos.is_chess960());
375 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
377 NodesBetweenPolls = std::min(Limits.maxNodes, 30000);
378 else if (Limits.time && Limits.time < 1000)
379 NodesBetweenPolls = 1000;
380 else if (Limits.time && Limits.time < 5000)
381 NodesBetweenPolls = 5000;
383 NodesBetweenPolls = 30000;
385 // Look for a book move
386 if (Options["OwnBook"].value<bool>())
388 if (Options["Book File"].value<string>() != book.name())
389 book.open(Options["Book File"].value<string>());
391 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
392 if (bookMove != MOVE_NONE)
395 wait_for_stop_or_ponderhit();
397 cout << "bestmove " << bookMove << endl;
403 UCIMultiPV = Options["MultiPV"].value<int>();
404 SkillLevel = Options["Skill Level"].value<int>();
406 read_evaluation_uci_options(pos.side_to_move());
407 Threads.read_uci_options();
409 // Set a new TT size if changed
410 TT.set_size(Options["Hash"].value<int>());
412 if (Options["Clear Hash"].value<bool>())
414 Options["Clear Hash"].set_value("false");
418 // Do we have to play with skill handicap? In this case enable MultiPV that
419 // we will use behind the scenes to retrieve a set of possible moves.
420 SkillLevelEnabled = (SkillLevel < 20);
421 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, 4) : UCIMultiPV);
423 // Wake up needed threads and reset maxPly counter
424 for (int i = 0; i < Threads.size(); i++)
426 Threads[i].wake_up();
427 Threads[i].maxPly = 0;
430 // Write to log file and keep it open to be accessed during the search
431 if (Options["Use Search Log"].value<bool>())
433 Log log(Options["Search Log Filename"].value<string>());
434 log << "\nSearching: " << pos.to_fen()
435 << "\ninfinite: " << Limits.infinite
436 << " ponder: " << Limits.ponder
437 << " time: " << Limits.time
438 << " increment: " << Limits.increment
439 << " moves to go: " << Limits.movesToGo
443 // We're ready to start thinking. Call the iterative deepening loop function
444 Move ponderMove = MOVE_NONE;
445 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
447 // Write final search statistics and close log file
448 if (Options["Use Search Log"].value<bool>())
450 int t = current_search_time();
452 Log log(Options["Search Log Filename"].value<string>());
453 log << "Nodes: " << pos.nodes_searched()
454 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
455 << "\nBest move: " << move_to_san(pos, bestMove);
458 pos.do_move(bestMove, st);
459 log << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
460 pos.undo_move(bestMove); // Return from think() with unchanged position
463 // This makes all the threads to go to sleep
466 // If we are pondering or in infinite search, we shouldn't print the
467 // best move before we are told to do so.
468 if (!StopRequest && (Limits.ponder || Limits.infinite))
469 wait_for_stop_or_ponderhit();
471 // Could be MOVE_NONE when searching on a stalemate position
472 cout << "bestmove " << bestMove;
474 // UCI protol is not clear on allowing sending an empty ponder move, instead
475 // it is clear that ponder move is optional. So skip it if empty.
476 if (ponderMove != MOVE_NONE)
477 cout << " ponder " << ponderMove;
487 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
488 // with increasing depth until the allocated thinking time has been consumed,
489 // user stops the search, or the maximum search depth is reached.
491 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
493 SearchStack ss[PLY_MAX_PLUS_2];
494 Value bestValues[PLY_MAX_PLUS_2];
495 int bestMoveChanges[PLY_MAX_PLUS_2];
496 int depth, aspirationDelta;
497 Value value, alpha, beta;
498 Move bestMove, easyMove, skillBest, skillPonder;
500 // Initialize stuff before a new search
501 memset(ss, 0, 4 * sizeof(SearchStack));
504 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
505 depth = aspirationDelta = 0;
506 value = alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
507 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
509 // Moves to search are verified and copied
510 Rml.init(pos, searchMoves);
512 // Handle special case of searching on a mate/stalemate position
515 cout << "info" << depth_to_uci(DEPTH_ZERO)
516 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
521 // Iterative deepening loop until requested to stop or target depth reached
522 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
524 // Save now last iteration's scores, before Rml moves are reordered
525 for (size_t i = 0; i < Rml.size(); i++)
526 Rml[i].prevScore = Rml[i].score;
528 Rml.bestMoveChanges = 0;
530 // MultiPV loop. We perform a full root search for each PV line
531 for (MultiPVIdx = 0; MultiPVIdx < std::min(MultiPV, (int)Rml.size()); MultiPVIdx++)
533 // Calculate dynamic aspiration window based on previous iterations
534 if (depth >= 5 && abs(Rml[MultiPVIdx].prevScore) < VALUE_KNOWN_WIN)
536 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
537 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
539 aspirationDelta = std::min(std::max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
540 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
542 alpha = std::max(Rml[MultiPVIdx].prevScore - aspirationDelta, -VALUE_INFINITE);
543 beta = std::min(Rml[MultiPVIdx].prevScore + aspirationDelta, VALUE_INFINITE);
547 alpha = -VALUE_INFINITE;
548 beta = VALUE_INFINITE;
551 // Start with a small aspiration window and, in case of fail high/low,
552 // research with bigger window until not failing high/low anymore.
554 // Search starts from ss+1 to allow referencing (ss-1). This is
555 // needed by update_gains() and ss copy when splitting at Root.
556 value = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
558 // Bring to front the best move. It is critical that sorting is
559 // done with a stable algorithm because all the values but the first
560 // and eventually the new best one are set to -VALUE_INFINITE and
561 // we want to keep the same order for all the moves but the new
562 // PV that goes to the front. Note that in case of MultiPV search
563 // the already searched PV lines are preserved.
564 sort<RootMove>(Rml.begin() + MultiPVIdx, Rml.end());
566 // In case we have found an exact score and we are going to leave
567 // the fail high/low loop then reorder the PV moves, otherwise
568 // leave the last PV move in its position so to be searched again.
569 // Of course this is needed only in MultiPV search.
570 if (MultiPVIdx && value > alpha && value < beta)
571 sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIdx);
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 <= MultiPVIdx; i++)
576 Rml[i].insert_pv_in_tt(pos);
578 // If search has been stopped exit the aspiration window loop,
579 // note that sorting and writing PV back to TT is safe becuase
580 // Rml is still valid, although refers to the previous iteration.
584 // Send full PV info to GUI if we are going to leave the loop or
585 // if we have a fail high/low and we are deep in the search. UCI
586 // protocol requires to send all the PV lines also if are still
587 // to be searched and so refer to the previous search's score.
588 if ((value > alpha && value < beta) || current_search_time() > 2000)
589 for (int i = 0; i < std::min(UCIMultiPV, (int)Rml.size()); i++)
591 bool updated = (i <= MultiPVIdx);
593 if (depth == 1 && !updated)
596 Depth d = (updated ? depth : depth - 1) * ONE_PLY;
597 Value s = (updated ? Rml[i].score : Rml[i].prevScore);
601 << (i == MultiPVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
602 << speed_to_uci(pos.nodes_searched())
603 << pv_to_uci(&Rml[i].pv[0], i + 1, pos.is_chess960())
607 // In case of failing high/low increase aspiration window and
608 // research, otherwise exit the fail high/low loop.
611 beta = std::min(beta + aspirationDelta, VALUE_INFINITE);
612 aspirationDelta += aspirationDelta / 2;
614 else if (value <= alpha)
616 AspirationFailLow = true;
617 StopOnPonderhit = false;
619 alpha = std::max(alpha - aspirationDelta, -VALUE_INFINITE);
620 aspirationDelta += aspirationDelta / 2;
625 } while (abs(value) < VALUE_KNOWN_WIN);
628 // Collect info about search result
629 bestMove = Rml[0].pv[0];
630 *ponderMove = Rml[0].pv[1];
631 bestValues[depth] = value;
632 bestMoveChanges[depth] = Rml.bestMoveChanges;
634 // Skills: Do we need to pick now the best and the ponder moves ?
635 if (SkillLevelEnabled && depth == 1 + SkillLevel)
636 do_skill_level(&skillBest, &skillPonder);
638 if (Options["Use Search Log"].value<bool>())
640 Log log(Options["Search Log Filename"].value<string>());
641 log << pretty_pv(pos, depth, value, current_search_time(), &Rml[0].pv[0]) << endl;
644 // Init easyMove at first iteration or drop it if differs from the best move
645 if (depth == 1 && (Rml.size() == 1 || Rml[0].score > Rml[1].score + EasyMoveMargin))
647 else if (bestMove != easyMove)
648 easyMove = MOVE_NONE;
650 // Check for some early stop condition
651 if (!StopRequest && Limits.useTimeManagement())
653 // Easy move: Stop search early if one move seems to be much better
654 // than the others or if there is only a single legal move. Also in
655 // the latter case search to some depth anyway to get a proper score.
657 && easyMove == bestMove
659 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
660 && current_search_time() > TimeMgr.available_time() / 16)
661 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
662 && current_search_time() > TimeMgr.available_time() / 32)))
665 // Take in account some extra time if the best move has changed
666 if (depth > 4 && depth < 50)
667 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
669 // Stop search if most of available time is already consumed. We probably don't
670 // have enough time to search the first move at the next iteration anyway.
671 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
674 // If we are allowed to ponder do not stop the search now but keep pondering
675 if (StopRequest && Limits.ponder)
678 StopOnPonderhit = true;
683 // When using skills overwrite best and ponder moves with the sub-optimal ones
684 if (SkillLevelEnabled)
686 if (skillBest == MOVE_NONE) // Still unassigned ?
687 do_skill_level(&skillBest, &skillPonder);
689 bestMove = skillBest;
690 *ponderMove = skillPonder;
697 // search<>() is the main search function for both PV and non-PV nodes and for
698 // normal and SplitPoint nodes. When called just after a split point the search
699 // is simpler because we have already probed the hash table, done a null move
700 // search, and searched the first move before splitting, we don't have to repeat
701 // all this work again. We also don't need to store anything to the hash table
702 // here: This is taken care of after we return from the split point.
704 template <NodeType NT>
705 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
707 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
708 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
709 const bool RootNode = (NT == Root || NT == SplitPointRoot);
711 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
712 assert(beta > alpha && beta <= VALUE_INFINITE);
713 assert(PvNode || alpha == beta - 1);
714 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
716 Move movesSearched[MAX_MOVES];
721 Move ttMove, move, excludedMove, threatMove;
724 Value bestValue, value, oldAlpha;
725 Value refinedValue, nullValue, futilityBase, futilityValue;
726 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
727 int moveCount = 0, playedMoveCount = 0;
728 Thread& thread = Threads[pos.thread()];
729 SplitPoint* sp = NULL;
731 refinedValue = bestValue = value = -VALUE_INFINITE;
733 inCheck = pos.in_check();
734 ss->ply = (ss-1)->ply + 1;
736 // Used to send selDepth info to GUI
737 if (PvNode && thread.maxPly < ss->ply)
738 thread.maxPly = ss->ply;
740 // Step 1. Initialize node and poll. Polling can abort search
743 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
744 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
745 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
751 ttMove = excludedMove = MOVE_NONE;
752 threatMove = sp->threatMove;
753 goto split_point_start;
756 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
762 // Step 2. Check for aborted search and immediate draw
764 || pos.is_draw<false>()
765 || ss->ply > PLY_MAX) && !RootNode)
768 // Step 3. Mate distance pruning
771 alpha = std::max(value_mated_in(ss->ply), alpha);
772 beta = std::min(value_mate_in(ss->ply+1), beta);
777 // Step 4. Transposition table lookup
778 // We don't want the score of a partial search to overwrite a previous full search
779 // TT value, so we use a different position key in case of an excluded move.
780 excludedMove = ss->excludedMove;
781 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
782 tte = TT.probe(posKey);
783 ttMove = RootNode ? Rml[MultiPVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
785 // At PV nodes we check for exact scores, while at non-PV nodes we check for
786 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
787 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
788 // we should also update RootMoveList to avoid bogus output.
789 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
790 : can_return_tt(tte, depth, beta, ss->ply)))
793 ss->bestMove = move = ttMove; // Can be MOVE_NONE
794 value = value_from_tt(tte->value(), ss->ply);
798 && !pos.is_capture_or_promotion(move)
799 && move != ss->killers[0])
801 ss->killers[1] = ss->killers[0];
802 ss->killers[0] = move;
807 // Step 5. Evaluate the position statically and update parent's gain statistics
809 ss->eval = ss->evalMargin = VALUE_NONE;
812 assert(tte->static_value() != VALUE_NONE);
814 ss->eval = tte->static_value();
815 ss->evalMargin = tte->static_value_margin();
816 refinedValue = refine_eval(tte, ss->eval, ss->ply);
820 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
821 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
824 // Save gain for the parent non-capture move
825 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
827 // Step 6. Razoring (is omitted in PV nodes)
829 && depth < RazorDepth
831 && refinedValue + razor_margin(depth) < beta
832 && ttMove == MOVE_NONE
833 && abs(beta) < VALUE_MATE_IN_PLY_MAX
834 && !pos.has_pawn_on_7th(pos.side_to_move()))
836 Value rbeta = beta - razor_margin(depth);
837 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
839 // Logically we should return (v + razor_margin(depth)), but
840 // surprisingly this did slightly weaker in tests.
844 // Step 7. Static null move pruning (is omitted in PV nodes)
845 // We're betting that the opponent doesn't have a move that will reduce
846 // the score by more than futility_margin(depth) if we do a null move.
849 && depth < RazorDepth
851 && refinedValue - futility_margin(depth, 0) >= beta
852 && abs(beta) < VALUE_MATE_IN_PLY_MAX
853 && pos.non_pawn_material(pos.side_to_move()))
854 return refinedValue - futility_margin(depth, 0);
856 // Step 8. Null move search with verification search (is omitted in PV nodes)
861 && refinedValue >= beta
862 && abs(beta) < VALUE_MATE_IN_PLY_MAX
863 && pos.non_pawn_material(pos.side_to_move()))
865 ss->currentMove = MOVE_NULL;
867 // Null move dynamic reduction based on depth
868 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
870 // Null move dynamic reduction based on value
871 if (refinedValue - PawnValueMidgame > beta)
874 pos.do_null_move<true>(st);
875 (ss+1)->skipNullMove = true;
876 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
877 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
878 (ss+1)->skipNullMove = false;
879 pos.do_null_move<false>(st);
881 if (nullValue >= beta)
883 // Do not return unproven mate scores
884 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
887 if (depth < 6 * ONE_PLY)
890 // Do verification search at high depths
891 ss->skipNullMove = true;
892 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
893 ss->skipNullMove = false;
900 // The null move failed low, which means that we may be faced with
901 // some kind of threat. If the previous move was reduced, check if
902 // the move that refuted the null move was somehow connected to the
903 // move which was reduced. If a connection is found, return a fail
904 // low score (which will cause the reduced move to fail high in the
905 // parent node, which will trigger a re-search with full depth).
906 threatMove = (ss+1)->bestMove;
908 if ( depth < ThreatDepth
910 && threatMove != MOVE_NONE
911 && connected_moves(pos, (ss-1)->currentMove, threatMove))
916 // Step 9. ProbCut (is omitted in PV nodes)
917 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
918 // and a reduced search returns a value much above beta, we can (almost) safely
919 // prune the previous move.
921 && depth >= RazorDepth + ONE_PLY
924 && excludedMove == MOVE_NONE
925 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
927 Value rbeta = beta + 200;
928 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
930 assert(rdepth >= ONE_PLY);
932 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
935 while ((move = mp.get_next_move()) != MOVE_NONE)
936 if (pos.pl_move_is_legal(move, ci.pinned))
938 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
939 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
946 // Step 10. Internal iterative deepening
947 if ( depth >= IIDDepth[PvNode]
948 && ttMove == MOVE_NONE
949 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
951 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
953 ss->skipNullMove = true;
954 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
955 ss->skipNullMove = false;
957 tte = TT.probe(posKey);
958 ttMove = tte ? tte->move() : MOVE_NONE;
961 split_point_start: // At split points actual search starts from here
963 // Initialize a MovePicker object for the current position
964 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
966 ss->bestMove = MOVE_NONE;
967 futilityBase = ss->eval + ss->evalMargin;
968 singularExtensionNode = !RootNode
970 && depth >= SingularExtensionDepth[PvNode]
971 && ttMove != MOVE_NONE
972 && !excludedMove // Do not allow recursive singular extension search
973 && (tte->type() & VALUE_TYPE_LOWER)
974 && tte->depth() >= depth - 3 * ONE_PLY;
977 lock_grab(&(sp->lock));
978 bestValue = sp->bestValue;
981 // Step 11. Loop through moves
982 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
983 while ( bestValue < beta
984 && (move = mp.get_next_move()) != MOVE_NONE
985 && !thread.cutoff_occurred())
989 if (move == excludedMove)
992 // At root obey the "searchmoves" option and skip moves not listed in Root
993 // Move List, as a consequence any illegal move is also skipped. In MultiPV
994 // mode we also skip PV moves which have been already searched.
995 if (RootNode && !Rml.find(move, MultiPVIdx))
998 // At PV and SpNode nodes we want all moves to be legal since the beginning
999 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
1004 moveCount = ++sp->moveCount;
1005 lock_release(&(sp->lock));
1012 // This is used by time management
1013 FirstRootMove = (moveCount == 1);
1015 // Save the current node count before the move is searched
1016 nodes = pos.nodes_searched();
1018 // For long searches send current move info to GUI
1019 if (pos.thread() == 0 && current_search_time() > 2000)
1020 cout << "info" << depth_to_uci(depth)
1021 << " currmove " << move
1022 << " currmovenumber " << moveCount + MultiPVIdx << endl;
1025 // At Root and at first iteration do a PV search on all the moves to score root moves
1026 isPvMove = (PvNode && moveCount <= (RootNode && depth <= ONE_PLY ? MAX_MOVES : 1));
1027 givesCheck = pos.move_gives_check(move, ci);
1028 captureOrPromotion = pos.is_capture_or_promotion(move);
1030 // Step 12. Decide the new search depth
1031 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1033 // Singular extension search. If all moves but one fail low on a search of
1034 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1035 // is singular and should be extended. To verify this we do a reduced search
1036 // on all the other moves but the ttMove, if result is lower than ttValue minus
1037 // a margin then we extend ttMove.
1038 if ( singularExtensionNode
1040 && pos.pl_move_is_legal(move, ci.pinned)
1043 Value ttValue = value_from_tt(tte->value(), ss->ply);
1045 if (abs(ttValue) < VALUE_KNOWN_WIN)
1047 Value rBeta = ttValue - int(depth);
1048 ss->excludedMove = move;
1049 ss->skipNullMove = true;
1050 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1051 ss->skipNullMove = false;
1052 ss->excludedMove = MOVE_NONE;
1053 ss->bestMove = MOVE_NONE;
1059 // Update current move (this must be done after singular extension search)
1060 newDepth = depth - ONE_PLY + ext;
1062 // Step 13. Futility pruning (is omitted in PV nodes)
1064 && !captureOrPromotion
1068 && !is_castle(move))
1070 // Move count based pruning
1071 if ( moveCount >= futility_move_count(depth)
1072 && (!threatMove || !connected_threat(pos, move, threatMove))
1073 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1076 lock_grab(&(sp->lock));
1081 // Value based pruning
1082 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1083 // but fixing this made program slightly weaker.
1084 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1085 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1086 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1088 if (futilityValue < beta)
1092 lock_grab(&(sp->lock));
1093 if (futilityValue > sp->bestValue)
1094 sp->bestValue = bestValue = futilityValue;
1096 else if (futilityValue > bestValue)
1097 bestValue = futilityValue;
1102 // Prune moves with negative SEE at low depths
1103 if ( predictedDepth < 2 * ONE_PLY
1104 && bestValue > VALUE_MATED_IN_PLY_MAX
1105 && pos.see_sign(move) < 0)
1108 lock_grab(&(sp->lock));
1114 // Check for legality only before to do the move
1115 if (!pos.pl_move_is_legal(move, ci.pinned))
1121 ss->currentMove = move;
1122 if (!SpNode && !captureOrPromotion)
1123 movesSearched[playedMoveCount++] = move;
1125 // Step 14. Make the move
1126 pos.do_move(move, st, ci, givesCheck);
1128 // Step extra. pv search (only in PV nodes)
1129 // The first move in list is the expected PV
1131 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1132 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1135 // Step 15. Reduced depth search
1136 // If the move fails high will be re-searched at full depth.
1137 bool doFullDepthSearch = true;
1139 if ( depth > 3 * ONE_PLY
1140 && !captureOrPromotion
1143 && ss->killers[0] != move
1144 && ss->killers[1] != move
1145 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1147 Depth d = newDepth - ss->reduction;
1148 alpha = SpNode ? sp->alpha : alpha;
1150 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1151 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1153 ss->reduction = DEPTH_ZERO;
1154 doFullDepthSearch = (value > alpha);
1157 // Step 16. Full depth search
1158 if (doFullDepthSearch)
1160 alpha = SpNode ? sp->alpha : alpha;
1161 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1162 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1164 // Step extra. pv search (only in PV nodes)
1165 // Search only for possible new PV nodes, if instead value >= beta then
1166 // parent node fails low with value <= alpha and tries another move.
1167 if (PvNode && value > alpha && (RootNode || value < beta))
1168 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1169 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1173 // Step 17. Undo move
1174 pos.undo_move(move);
1176 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1178 // Step 18. Check for new best move
1181 lock_grab(&(sp->lock));
1182 bestValue = sp->bestValue;
1186 // Finished searching the move. If StopRequest is true, the search
1187 // was aborted because the user interrupted the search or because we
1188 // ran out of time. In this case, the return value of the search cannot
1189 // be trusted, and we don't update the best move and/or PV.
1190 if (RootNode && !StopRequest)
1192 // Remember searched nodes counts for this move
1193 RootMove* rm = Rml.find(move);
1194 rm->nodes += pos.nodes_searched() - nodes;
1196 // PV move or new best move ?
1197 if (isPvMove || value > alpha)
1201 rm->extract_pv_from_tt(pos);
1203 // We record how often the best move has been changed in each
1204 // iteration. This information is used for time management: When
1205 // the best move changes frequently, we allocate some more time.
1206 if (!isPvMove && MultiPV == 1)
1207 Rml.bestMoveChanges++;
1210 // All other moves but the PV are set to the lowest value, this
1211 // is not a problem when sorting becuase sort is stable and move
1212 // position in the list is preserved, just the PV is pushed up.
1213 rm->score = -VALUE_INFINITE;
1217 if (value > bestValue)
1220 ss->bestMove = move;
1224 && value < beta) // We want always alpha < beta
1227 if (SpNode && !thread.cutoff_occurred())
1229 sp->bestValue = value;
1230 sp->ss->bestMove = move;
1232 sp->is_betaCutoff = (value >= beta);
1236 // Step 19. Check for split
1238 && depth >= Threads.min_split_depth()
1240 && Threads.available_slave_exists(pos.thread())
1242 && !thread.cutoff_occurred())
1243 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1244 threatMove, moveCount, &mp, NT);
1247 // Step 20. Check for mate and stalemate
1248 // All legal moves have been searched and if there are no legal moves, it
1249 // must be mate or stalemate. Note that we can have a false positive in
1250 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1251 // harmless because return value is discarded anyhow in the parent nodes.
1252 // If we are in a singular extension search then return a fail low score.
1253 if (!SpNode && !moveCount)
1254 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1256 // Step 21. Update tables
1257 // If the search is not aborted, update the transposition table,
1258 // history counters, and killer moves.
1259 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1261 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1262 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1263 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1265 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1267 // Update killers and history only for non capture moves that fails high
1268 if ( bestValue >= beta
1269 && !pos.is_capture_or_promotion(move))
1271 if (move != ss->killers[0])
1273 ss->killers[1] = ss->killers[0];
1274 ss->killers[0] = move;
1276 update_history(pos, move, depth, movesSearched, playedMoveCount);
1282 // Here we have the lock still grabbed
1283 sp->is_slave[pos.thread()] = false;
1284 sp->nodes += pos.nodes_searched();
1285 lock_release(&(sp->lock));
1288 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1293 // qsearch() is the quiescence search function, which is called by the main
1294 // search function when the remaining depth is zero (or, to be more precise,
1295 // less than ONE_PLY).
1297 template <NodeType NT>
1298 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1300 const bool PvNode = (NT == PV);
1302 assert(NT == PV || NT == NonPV);
1303 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1304 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1305 assert(PvNode || alpha == beta - 1);
1307 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1311 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1312 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1316 Value oldAlpha = alpha;
1318 ss->bestMove = ss->currentMove = MOVE_NONE;
1319 ss->ply = (ss-1)->ply + 1;
1321 // Check for an instant draw or maximum ply reached
1322 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1325 // Decide whether or not to include checks, this fixes also the type of
1326 // TT entry depth that we are going to use. Note that in qsearch we use
1327 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1328 inCheck = pos.in_check();
1329 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1331 // Transposition table lookup. At PV nodes, we don't use the TT for
1332 // pruning, but only for move ordering.
1333 tte = TT.probe(pos.get_key());
1334 ttMove = (tte ? tte->move() : MOVE_NONE);
1336 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1338 ss->bestMove = ttMove; // Can be MOVE_NONE
1339 return value_from_tt(tte->value(), ss->ply);
1342 // Evaluate the position statically
1345 bestValue = futilityBase = -VALUE_INFINITE;
1346 ss->eval = evalMargin = VALUE_NONE;
1347 enoughMaterial = false;
1353 assert(tte->static_value() != VALUE_NONE);
1355 evalMargin = tte->static_value_margin();
1356 ss->eval = bestValue = tte->static_value();
1359 ss->eval = bestValue = evaluate(pos, evalMargin);
1361 // Stand pat. Return immediately if static value is at least beta
1362 if (bestValue >= beta)
1365 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1370 if (PvNode && bestValue > alpha)
1373 // Futility pruning parameters, not needed when in check
1374 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1375 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1378 // Initialize a MovePicker object for the current position, and prepare
1379 // to search the moves. Because the depth is <= 0 here, only captures,
1380 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1382 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1385 // Loop through the moves until no moves remain or a beta cutoff occurs
1386 while ( bestValue < beta
1387 && (move = mp.get_next_move()) != MOVE_NONE)
1389 assert(is_ok(move));
1391 givesCheck = pos.move_gives_check(move, ci);
1399 && !is_promotion(move)
1400 && !pos.is_passed_pawn_push(move))
1402 futilityValue = futilityBase
1403 + PieceValueEndgame[pos.piece_on(move_to(move))]
1404 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1406 if (futilityValue < beta)
1408 if (futilityValue > bestValue)
1409 bestValue = futilityValue;
1414 // Prune moves with negative or equal SEE
1415 if ( futilityBase < beta
1416 && depth < DEPTH_ZERO
1417 && pos.see(move) <= 0)
1421 // Detect non-capture evasions that are candidate to be pruned
1422 evasionPrunable = !PvNode
1424 && bestValue > VALUE_MATED_IN_PLY_MAX
1425 && !pos.is_capture(move)
1426 && !pos.can_castle(pos.side_to_move());
1428 // Don't search moves with negative SEE values
1430 && (!inCheck || evasionPrunable)
1432 && !is_promotion(move)
1433 && pos.see_sign(move) < 0)
1436 // Don't search useless checks
1441 && !pos.is_capture_or_promotion(move)
1442 && ss->eval + PawnValueMidgame / 4 < beta
1443 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1445 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1446 bestValue = ss->eval + PawnValueMidgame / 4;
1451 // Check for legality only before to do the move
1452 if (!pos.pl_move_is_legal(move, ci.pinned))
1455 // Update current move
1456 ss->currentMove = move;
1458 // Make and search the move
1459 pos.do_move(move, st, ci, givesCheck);
1460 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1461 pos.undo_move(move);
1463 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1466 if (value > bestValue)
1469 ss->bestMove = move;
1473 && value < beta) // We want always alpha < beta
1478 // All legal moves have been searched. A special case: If we're in check
1479 // and no legal moves were found, it is checkmate.
1480 if (inCheck && bestValue == -VALUE_INFINITE)
1481 return value_mated_in(ss->ply);
1483 // Update transposition table
1484 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1485 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1486 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1488 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1490 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1496 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1497 // bestValue is updated only when returning false because in that case move
1500 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1502 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1503 Square from, to, ksq, victimSq;
1506 Value futilityValue, bv = *bestValue;
1508 from = move_from(move);
1510 them = flip(pos.side_to_move());
1511 ksq = pos.king_square(them);
1512 kingAtt = pos.attacks_from<KING>(ksq);
1513 pc = pos.piece_on(from);
1515 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1516 oldAtt = pos.attacks_from(pc, from, occ);
1517 newAtt = pos.attacks_from(pc, to, occ);
1519 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1520 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1522 if (!(b && (b & (b - 1))))
1525 // Rule 2. Queen contact check is very dangerous
1526 if ( type_of(pc) == QUEEN
1527 && bit_is_set(kingAtt, to))
1530 // Rule 3. Creating new double threats with checks
1531 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1535 victimSq = pop_1st_bit(&b);
1536 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1538 // Note that here we generate illegal "double move"!
1539 if ( futilityValue >= beta
1540 && pos.see_sign(make_move(from, victimSq)) >= 0)
1543 if (futilityValue > bv)
1547 // Update bestValue only if check is not dangerous (because we will prune the move)
1553 // connected_moves() tests whether two moves are 'connected' in the sense
1554 // that the first move somehow made the second move possible (for instance
1555 // if the moving piece is the same in both moves). The first move is assumed
1556 // to be the move that was made to reach the current position, while the
1557 // second move is assumed to be a move from the current position.
1559 bool connected_moves(const Position& pos, Move m1, Move m2) {
1561 Square f1, t1, f2, t2;
1568 // Case 1: The moving piece is the same in both moves
1574 // Case 2: The destination square for m2 was vacated by m1
1580 // Case 3: Moving through the vacated square
1581 p2 = pos.piece_on(f2);
1582 if ( piece_is_slider(p2)
1583 && bit_is_set(squares_between(f2, t2), f1))
1586 // Case 4: The destination square for m2 is defended by the moving piece in m1
1587 p1 = pos.piece_on(t1);
1588 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1591 // Case 5: Discovered check, checking piece is the piece moved in m1
1592 ksq = pos.king_square(pos.side_to_move());
1593 if ( piece_is_slider(p1)
1594 && bit_is_set(squares_between(t1, ksq), f2))
1596 Bitboard occ = pos.occupied_squares();
1597 clear_bit(&occ, f2);
1598 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1605 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1606 // "plies to mate from the current ply". Non-mate scores are unchanged.
1607 // The function is called before storing a value to the transposition table.
1609 Value value_to_tt(Value v, int ply) {
1611 if (v >= VALUE_MATE_IN_PLY_MAX)
1614 if (v <= VALUE_MATED_IN_PLY_MAX)
1621 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1622 // the transposition table to a mate score corrected for the current ply.
1624 Value value_from_tt(Value v, int ply) {
1626 if (v >= VALUE_MATE_IN_PLY_MAX)
1629 if (v <= VALUE_MATED_IN_PLY_MAX)
1636 // connected_threat() tests whether it is safe to forward prune a move or if
1637 // is somehow connected to the threat move returned by null search.
1639 bool connected_threat(const Position& pos, Move m, Move threat) {
1642 assert(is_ok(threat));
1643 assert(!pos.is_capture_or_promotion(m));
1644 assert(!pos.is_passed_pawn_push(m));
1646 Square mfrom, mto, tfrom, tto;
1648 mfrom = move_from(m);
1650 tfrom = move_from(threat);
1651 tto = move_to(threat);
1653 // Case 1: Don't prune moves which move the threatened piece
1657 // Case 2: If the threatened piece has value less than or equal to the
1658 // value of the threatening piece, don't prune moves which defend it.
1659 if ( pos.is_capture(threat)
1660 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1661 || type_of(pos.piece_on(tfrom)) == KING)
1662 && pos.move_attacks_square(m, tto))
1665 // Case 3: If the moving piece in the threatened move is a slider, don't
1666 // prune safe moves which block its ray.
1667 if ( piece_is_slider(pos.piece_on(tfrom))
1668 && bit_is_set(squares_between(tfrom, tto), mto)
1669 && pos.see_sign(m) >= 0)
1676 // can_return_tt() returns true if a transposition table score
1677 // can be used to cut-off at a given point in search.
1679 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1681 Value v = value_from_tt(tte->value(), ply);
1683 return ( tte->depth() >= depth
1684 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1685 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1687 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1688 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1692 // refine_eval() returns the transposition table score if
1693 // possible otherwise falls back on static position evaluation.
1695 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1699 Value v = value_from_tt(tte->value(), ply);
1701 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1702 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1709 // update_history() registers a good move that produced a beta-cutoff
1710 // in history and marks as failures all the other moves of that ply.
1712 void update_history(const Position& pos, Move move, Depth depth,
1713 Move movesSearched[], int moveCount) {
1715 Value bonus = Value(int(depth) * int(depth));
1717 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1719 for (int i = 0; i < moveCount - 1; i++)
1721 m = movesSearched[i];
1725 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1730 // update_gains() updates the gains table of a non-capture move given
1731 // the static position evaluation before and after the move.
1733 void update_gains(const Position& pos, Move m, Value before, Value after) {
1736 && before != VALUE_NONE
1737 && after != VALUE_NONE
1738 && pos.captured_piece_type() == PIECE_TYPE_NONE
1740 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1744 // current_search_time() returns the number of milliseconds which have passed
1745 // since the beginning of the current search.
1747 int current_search_time(int set) {
1749 static int searchStartTime;
1752 searchStartTime = set;
1754 return get_system_time() - searchStartTime;
1758 // score_to_uci() converts a value to a string suitable for use with the UCI
1759 // protocol specifications:
1761 // cp <x> The score from the engine's point of view in centipawns.
1762 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1763 // use negative values for y.
1765 string score_to_uci(Value v, Value alpha, Value beta) {
1767 std::stringstream s;
1769 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1770 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1772 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1774 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1780 // speed_to_uci() returns a string with time stats of current search suitable
1781 // to be sent to UCI gui.
1783 string speed_to_uci(int64_t nodes) {
1785 std::stringstream s;
1786 int t = current_search_time();
1788 s << " nodes " << nodes
1789 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1795 // pv_to_uci() returns a string with information on the current PV line
1796 // formatted according to UCI specification.
1798 string pv_to_uci(const Move pv[], int pvNum, bool chess960) {
1800 std::stringstream s;
1802 s << " multipv " << pvNum << " pv " << set960(chess960);
1804 for ( ; *pv != MOVE_NONE; pv++)
1810 // depth_to_uci() returns a string with information on the current depth and
1811 // seldepth formatted according to UCI specification.
1813 string depth_to_uci(Depth depth) {
1815 std::stringstream s;
1817 // Retrieve max searched depth among threads
1819 for (int i = 0; i < Threads.size(); i++)
1820 if (Threads[i].maxPly > selDepth)
1821 selDepth = Threads[i].maxPly;
1823 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1828 string time_to_string(int millisecs) {
1830 const int MSecMinute = 1000 * 60;
1831 const int MSecHour = 1000 * 60 * 60;
1833 int hours = millisecs / MSecHour;
1834 int minutes = (millisecs % MSecHour) / MSecMinute;
1835 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1837 std::stringstream s;
1842 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1846 string score_to_string(Value v) {
1848 std::stringstream s;
1850 if (v >= VALUE_MATE_IN_PLY_MAX)
1851 s << "#" << (VALUE_MATE - v + 1) / 2;
1852 else if (v <= VALUE_MATED_IN_PLY_MAX)
1853 s << "-#" << (VALUE_MATE + v) / 2;
1855 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1860 // pretty_pv() creates a human-readable string from a position and a PV.
1861 // It is used to write search information to the log file (which is created
1862 // when the UCI parameter "Use Search Log" is "true").
1864 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1866 const int64_t K = 1000;
1867 const int64_t M = 1000000;
1868 const int startColumn = 28;
1869 const size_t maxLength = 80 - startColumn;
1871 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1874 std::stringstream s;
1877 // First print depth, score, time and searched nodes...
1878 s << set960(pos.is_chess960())
1879 << std::setw(2) << depth
1880 << std::setw(8) << score_to_string(value)
1881 << std::setw(8) << time_to_string(time);
1883 if (pos.nodes_searched() < M)
1884 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1885 else if (pos.nodes_searched() < K * M)
1886 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1888 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1890 // ...then print the full PV line in short algebraic notation
1891 while (*m != MOVE_NONE)
1893 san = move_to_san(pos, *m);
1894 length += san.length() + 1;
1896 if (length > maxLength)
1898 length = san.length() + 1;
1899 s << "\n" + string(startColumn, ' ');
1903 pos.do_move(*m++, *st++);
1906 // Restore original position before to leave
1907 while (m != pv) pos.undo_move(*--m);
1912 // poll() performs two different functions: It polls for user input, and it
1913 // looks at the time consumed so far and decides if it's time to abort the
1916 void poll(const Position& pos) {
1918 static int lastInfoTime;
1919 int t = current_search_time();
1922 if (input_available())
1924 // We are line oriented, don't read single chars
1927 if (!std::getline(std::cin, command) || command == "quit")
1929 // Quit the program as soon as possible
1930 Limits.ponder = false;
1931 QuitRequest = StopRequest = true;
1934 else if (command == "stop")
1936 // Stop calculating as soon as possible, but still send the "bestmove"
1937 // and possibly the "ponder" token when finishing the search.
1938 Limits.ponder = false;
1941 else if (command == "ponderhit")
1943 // The opponent has played the expected move. GUI sends "ponderhit" if
1944 // we were told to ponder on the same move the opponent has played. We
1945 // should continue searching but switching from pondering to normal search.
1946 Limits.ponder = false;
1948 if (StopOnPonderhit)
1953 // Print search information
1957 else if (lastInfoTime > t)
1958 // HACK: Must be a new search where we searched less than
1959 // NodesBetweenPolls nodes during the first second of search.
1962 else if (t - lastInfoTime >= 1000)
1967 dbg_print_hit_rate();
1970 // Should we stop the search?
1974 bool stillAtFirstMove = FirstRootMove
1975 && !AspirationFailLow
1976 && t > TimeMgr.available_time();
1978 bool noMoreTime = t > TimeMgr.maximum_time()
1979 || stillAtFirstMove;
1981 if ( (Limits.useTimeManagement() && noMoreTime)
1982 || (Limits.maxTime && t >= Limits.maxTime)
1983 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1988 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1989 // while the program is pondering. The point is to work around a wrinkle in
1990 // the UCI protocol: When pondering, the engine is not allowed to give a
1991 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1992 // We simply wait here until one of these commands is sent, and return,
1993 // after which the bestmove and pondermove will be printed.
1995 void wait_for_stop_or_ponderhit() {
1999 // Wait for a command from stdin
2000 while ( std::getline(std::cin, command)
2001 && command != "ponderhit" && command != "stop" && command != "quit") {};
2003 if (command != "ponderhit" && command != "stop")
2004 QuitRequest = true; // Must be "quit" or getline() returned false
2008 // When playing with strength handicap choose best move among the MultiPV set
2009 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
2010 void do_skill_level(Move* best, Move* ponder) {
2012 assert(MultiPV > 1);
2016 // Rml list is already sorted by score in descending order
2018 int max_s = -VALUE_INFINITE;
2019 int size = std::min(MultiPV, (int)Rml.size());
2020 int max = Rml[0].score;
2021 int var = std::min(max - Rml[size - 1].score, int(PawnValueMidgame));
2022 int wk = 120 - 2 * SkillLevel;
2024 // PRNG sequence should be non deterministic
2025 for (int i = abs(get_system_time() % 50); i > 0; i--)
2026 rk.rand<unsigned>();
2028 // Choose best move. For each move's score we add two terms both dependent
2029 // on wk, one deterministic and bigger for weaker moves, and one random,
2030 // then we choose the move with the resulting highest score.
2031 for (int i = 0; i < size; i++)
2035 // Don't allow crazy blunders even at very low skills
2036 if (i > 0 && Rml[i-1].score > s + EasyMoveMargin)
2039 // This is our magical formula
2040 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
2045 *best = Rml[i].pv[0];
2046 *ponder = Rml[i].pv[1];
2052 /// RootMove and RootMoveList method's definitions
2054 void RootMoveList::init(Position& pos, Move searchMoves[]) {
2057 bestMoveChanges = 0;
2060 // Generate all legal moves and add them to RootMoveList
2061 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2063 // If we have a searchMoves[] list then verify the move
2064 // is in the list before to add it.
2065 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2067 if (sm != searchMoves && *sm != ml.move())
2071 rm.pv.push_back(ml.move());
2072 rm.pv.push_back(MOVE_NONE);
2073 rm.score = rm.prevScore = -VALUE_INFINITE;
2079 RootMove* RootMoveList::find(const Move& m, int startIndex) {
2081 for (size_t i = startIndex; i < size(); i++)
2082 if ((*this)[i].pv[0] == m)
2088 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2089 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2090 // allow to always have a ponder move even when we fail high at root and also a
2091 // long PV to print that is important for position analysis.
2093 void RootMove::extract_pv_from_tt(Position& pos) {
2095 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2100 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
2104 pos.do_move(m, *st++);
2106 while ( (tte = TT.probe(pos.get_key())) != NULL
2107 && tte->move() != MOVE_NONE
2108 && pos.is_pseudo_legal(tte->move())
2109 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2111 && (!pos.is_draw<false>() || ply < 2))
2113 pv.push_back(tte->move());
2114 pos.do_move(tte->move(), *st++);
2117 pv.push_back(MOVE_NONE);
2119 do pos.undo_move(pv[--ply]); while (ply);
2122 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2123 // the PV back into the TT. This makes sure the old PV moves are searched
2124 // first, even if the old TT entries have been overwritten.
2126 void RootMove::insert_pv_in_tt(Position& pos) {
2128 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2131 Value v, m = VALUE_NONE;
2134 assert(pv[0] != MOVE_NONE && pos.is_pseudo_legal(pv[0]));
2140 // Don't overwrite existing correct entries
2141 if (!tte || tte->move() != pv[ply])
2143 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2144 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2146 pos.do_move(pv[ply], *st++);
2148 } while (pv[++ply] != MOVE_NONE);
2150 do pos.undo_move(pv[--ply]); while (ply);
2155 // Little helper used by idle_loop() to check that all the slave threads of a
2156 // split point have finished searching.
2158 static bool all_slaves_finished(SplitPoint* sp) {
2160 for (int i = 0; i < Threads.size(); i++)
2161 if (sp->is_slave[i])
2168 // Thread::idle_loop() is where the thread is parked when it has no work to do.
2169 // The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
2170 // for which the thread is the master.
2172 void Thread::idle_loop(SplitPoint* sp) {
2176 // If we are not searching, wait for a condition to be signaled
2177 // instead of wasting CPU time polling for work.
2180 || (Threads.use_sleeping_threads() && !is_searching))
2182 assert((!sp && threadID) || Threads.use_sleeping_threads());
2184 // Slave thread should exit as soon as do_terminate flag raises
2191 // Grab the lock to avoid races with Thread::wake_up()
2192 lock_grab(&sleepLock);
2194 // If we are master and all slaves have finished don't go to sleep
2195 if (sp && all_slaves_finished(sp))
2197 lock_release(&sleepLock);
2201 // Do sleep after retesting sleep conditions under lock protection, in
2202 // particular we need to avoid a deadlock in case a master thread has,
2203 // in the meanwhile, allocated us and sent the wake_up() call before we
2204 // had the chance to grab the lock.
2205 if (do_sleep || !is_searching)
2206 cond_wait(&sleepCond, &sleepLock);
2208 lock_release(&sleepLock);
2211 // If this thread has been assigned work, launch a search
2214 assert(!do_terminate);
2216 // Copy split point position and search stack and call search()
2217 SearchStack ss[PLY_MAX_PLUS_2];
2218 SplitPoint* tsp = splitPoint;
2219 Position pos(*tsp->pos, threadID);
2221 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2224 if (tsp->nodeType == Root)
2225 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2226 else if (tsp->nodeType == PV)
2227 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2228 else if (tsp->nodeType == NonPV)
2229 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2233 assert(is_searching);
2235 is_searching = false;
2237 // Wake up master thread so to allow it to return from the idle loop in
2238 // case we are the last slave of the split point.
2239 if ( Threads.use_sleeping_threads()
2240 && threadID != tsp->master
2241 && !Threads[tsp->master].is_searching)
2242 Threads[tsp->master].wake_up();
2245 // If this thread is the master of a split point and all slaves have
2246 // finished their work at this split point, return from the idle loop.
2247 if (sp && all_slaves_finished(sp))
2249 // Because sp->is_slave[] is reset under lock protection,
2250 // be sure sp->lock has been released before to return.
2251 lock_grab(&(sp->lock));
2252 lock_release(&(sp->lock));