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/>.
39 #include "ucioption.h"
47 // Set to true to force running with one thread. Used for debugging
48 const bool FakeSplit = false;
50 // Different node types, used as template parameter
51 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
53 // RootMove struct is used for moves at the root of the tree. For each root
54 // move, we store a score, a node count, and a PV (really a refutation
55 // in the case of moves which fail low). Score is normally set at
56 // -VALUE_INFINITE for all non-pv moves.
59 // RootMove::operator<() is the comparison function used when
60 // sorting the moves. A move m1 is considered to be better
61 // than a move m2 if it has an higher score
62 bool operator<(const RootMove& m) const { return score < m.score; }
64 void extract_pv_from_tt(Position& pos);
65 void insert_pv_in_tt(Position& pos);
73 // RootMoveList struct is mainly a std::vector of RootMove objects
74 struct RootMoveList : public std::vector<RootMove> {
76 void init(Position& pos, Move searchMoves[]);
77 RootMove* find(const Move& m, int startIndex = 0);
85 // Lookup table to check if a Piece is a slider and its access function
86 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
87 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
91 // Maximum depth for razoring
92 const Depth RazorDepth = 4 * ONE_PLY;
94 // Dynamic razoring margin based on depth
95 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
97 // Maximum depth for use of dynamic threat detection when null move fails low
98 const Depth ThreatDepth = 5 * ONE_PLY;
100 // Step 9. Internal iterative deepening
102 // Minimum depth for use of internal iterative deepening
103 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
105 // At Non-PV nodes we do an internal iterative deepening search
106 // when the static evaluation is bigger then beta - IIDMargin.
107 const Value IIDMargin = Value(0x100);
109 // Step 11. Decide the new search depth
111 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
112 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
113 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
114 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
115 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
117 // Minimum depth for use of singular extension
118 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
120 // Step 12. Futility pruning
122 // Futility margin for quiescence search
123 const Value FutilityMarginQS = Value(0x80);
125 // Futility lookup tables (initialized at startup) and their access functions
126 Value FutilityMargins[16][64]; // [depth][moveNumber]
127 int FutilityMoveCounts[32]; // [depth]
129 inline Value futility_margin(Depth d, int mn) {
131 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
132 : 2 * VALUE_INFINITE;
135 inline int futility_move_count(Depth d) {
137 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
140 // Step 14. Reduced search
142 // Reduction lookup tables (initialized at startup) and their access function
143 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
145 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
147 return (Depth) Reductions[PvNode][Min(d / ONE_PLY, 63)][Min(mn, 63)];
150 // Easy move margin. An easy move candidate must be at least this much
151 // better than the second best move.
152 const Value EasyMoveMargin = Value(0x200);
155 /// Namespace variables
161 int MultiPV, UCIMultiPV, MultiPVIdx;
163 // Time management variables
164 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
168 // Skill level adjustment
170 bool SkillLevelEnabled;
172 // Node counters, used only by thread[0] but try to keep in different cache
173 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
175 int NodesBetweenPolls = 30000;
183 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
185 template <NodeType NT>
186 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
188 template <NodeType NT>
189 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
191 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
192 bool connected_moves(const Position& pos, Move m1, Move m2);
193 Value value_to_tt(Value v, int ply);
194 Value value_from_tt(Value v, int ply);
195 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
196 bool connected_threat(const Position& pos, Move m, Move threat);
197 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
198 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
199 void update_gains(const Position& pos, Move move, Value before, Value after);
200 void do_skill_level(Move* best, Move* ponder);
202 int current_search_time(int set = 0);
203 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
204 string speed_to_uci(int64_t nodes);
205 string pv_to_uci(const Move pv[], int pvNum, bool chess960);
206 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
207 string depth_to_uci(Depth depth);
208 void poll(const Position& pos);
209 void wait_for_stop_or_ponderhit();
211 // MovePickerExt template class extends MovePicker and allows to choose at compile
212 // time the proper moves source according to the type of node. In the default case
213 // we simply create and use a standard MovePicker object.
214 template<bool SpNode> struct MovePickerExt : public MovePicker {
216 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
217 : MovePicker(p, ttm, d, h, ss, b) {}
220 // In case of a SpNode we use split point's shared MovePicker object as moves source
221 template<> struct MovePickerExt<true> : public MovePicker {
223 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
224 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
226 Move get_next_move() { return mp->get_next_move(); }
230 // Overload operator<<() to make it easier to print moves in a coordinate
231 // notation compatible with UCI protocol.
232 std::ostream& operator<<(std::ostream& os, Move m) {
234 bool chess960 = (os.iword(0) != 0); // See set960()
235 return os << move_to_uci(m, chess960);
238 // When formatting a move for std::cout we must know if we are in Chess960
239 // or not. To keep using the handy operator<<() on the move the trick is to
240 // embed this flag in the stream itself. Function-like named enum set960 is
241 // used as a custom manipulator and the stream internal general-purpose array,
242 // accessed through ios_base::iword(), is used to pass the flag to the move's
243 // operator<<() that will read it to properly format castling moves.
246 std::ostream& operator<< (std::ostream& os, const set960& f) {
248 os.iword(0) = int(f);
252 // extension() decides whether a move should be searched with normal depth,
253 // or with extended depth. Certain classes of moves (checking moves, in
254 // particular) are searched with bigger depth than ordinary moves and in
255 // any case are marked as 'dangerous'. Note that also if a move is not
256 // extended, as example because the corresponding UCI option is set to zero,
257 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
258 template <bool PvNode>
259 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
260 bool moveIsCheck, bool* dangerous) {
261 assert(m != MOVE_NONE);
263 Depth result = DEPTH_ZERO;
264 *dangerous = moveIsCheck;
266 if (moveIsCheck && pos.see_sign(m) >= 0)
267 result += CheckExtension[PvNode];
269 if (type_of(pos.piece_on(move_from(m))) == PAWN)
271 Color c = pos.side_to_move();
272 if (relative_rank(c, move_to(m)) == RANK_7)
274 result += PawnPushTo7thExtension[PvNode];
277 if (pos.pawn_is_passed(c, move_to(m)))
279 result += PassedPawnExtension[PvNode];
284 if ( captureOrPromotion
285 && type_of(pos.piece_on(move_to(m))) != PAWN
286 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
287 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
290 result += PawnEndgameExtension[PvNode];
294 return Min(result, ONE_PLY);
300 /// init_search() is called during startup to initialize various lookup tables
304 int d; // depth (ONE_PLY == 2)
305 int hd; // half depth (ONE_PLY == 1)
308 // Init reductions array
309 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
311 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
312 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
313 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
314 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
317 // Init futility margins array
318 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
319 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
321 // Init futility move count array
322 for (d = 0; d < 32; d++)
323 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
327 /// perft() is our utility to verify move generation. All the leaf nodes up to
328 /// the given depth are generated and counted and the sum returned.
330 int64_t perft(Position& pos, Depth depth) {
335 // Generate all legal moves
336 MoveList<MV_LEGAL> ml(pos);
338 // If we are at the last ply we don't need to do and undo
339 // the moves, just to count them.
340 if (depth <= ONE_PLY)
343 // Loop through all legal moves
345 for ( ; !ml.end(); ++ml)
347 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
348 sum += perft(pos, depth - ONE_PLY);
349 pos.undo_move(ml.move());
355 /// think() is the external interface to Stockfish's search, and is called when
356 /// the program receives the UCI 'go' command. It initializes various global
357 /// variables, and calls id_loop(). It returns false when a "quit" command is
358 /// received during the search.
360 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
362 static Book book; // Define static to initialize the PRNG only once
364 // Initialize global search-related variables
365 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = false;
367 current_search_time(get_system_time());
369 TimeMgr.init(Limits, pos.startpos_ply_counter());
371 // Set output steram in normal or chess960 mode
372 cout << set960(pos.is_chess960());
374 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
376 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
377 else if (Limits.time && Limits.time < 1000)
378 NodesBetweenPolls = 1000;
379 else if (Limits.time && Limits.time < 5000)
380 NodesBetweenPolls = 5000;
382 NodesBetweenPolls = 30000;
384 // Look for a book move
385 if (Options["OwnBook"].value<bool>())
387 if (Options["Book File"].value<string>() != book.name())
388 book.open(Options["Book File"].value<string>());
390 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
391 if (bookMove != MOVE_NONE)
394 wait_for_stop_or_ponderhit();
396 cout << "bestmove " << bookMove << endl;
402 UCIMultiPV = Options["MultiPV"].value<int>();
403 SkillLevel = Options["Skill Level"].value<int>();
405 read_evaluation_uci_options(pos.side_to_move());
406 Threads.read_uci_options();
408 // Set a new TT size if changed
409 TT.set_size(Options["Hash"].value<int>());
411 if (Options["Clear Hash"].value<bool>())
413 Options["Clear Hash"].set_value("false");
417 // Do we have to play with skill handicap? In this case enable MultiPV that
418 // we will use behind the scenes to retrieve a set of possible moves.
419 SkillLevelEnabled = (SkillLevel < 20);
420 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
422 // Wake up needed threads and reset maxPly counter
423 for (int i = 0; i < Threads.size(); i++)
425 Threads[i].wake_up();
426 Threads[i].maxPly = 0;
429 // Write to log file and keep it open to be accessed during the search
430 if (Options["Use Search Log"].value<bool>())
432 Log log(Options["Search Log Filename"].value<string>());
433 log << "\nSearching: " << pos.to_fen()
434 << "\ninfinite: " << Limits.infinite
435 << " ponder: " << Limits.ponder
436 << " time: " << Limits.time
437 << " increment: " << Limits.increment
438 << " moves to go: " << Limits.movesToGo
442 // We're ready to start thinking. Call the iterative deepening loop function
443 Move ponderMove = MOVE_NONE;
444 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
446 // Write final search statistics and close log file
447 if (Options["Use Search Log"].value<bool>())
449 int t = current_search_time();
451 Log log(Options["Search Log Filename"].value<string>());
452 log << "Nodes: " << pos.nodes_searched()
453 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
454 << "\nBest move: " << move_to_san(pos, bestMove);
457 pos.do_move(bestMove, st);
458 log << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
459 pos.undo_move(bestMove); // Return from think() with unchanged position
462 // This makes all the threads to go to sleep
465 // If we are pondering or in infinite search, we shouldn't print the
466 // best move before we are told to do so.
467 if (!StopRequest && (Limits.ponder || Limits.infinite))
468 wait_for_stop_or_ponderhit();
470 // Could be MOVE_NONE when searching on a stalemate position
471 cout << "bestmove " << bestMove;
473 // UCI protol is not clear on allowing sending an empty ponder move, instead
474 // it is clear that ponder move is optional. So skip it if empty.
475 if (ponderMove != MOVE_NONE)
476 cout << " ponder " << ponderMove;
486 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
487 // with increasing depth until the allocated thinking time has been consumed,
488 // user stops the search, or the maximum search depth is reached.
490 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
492 SearchStack ss[PLY_MAX_PLUS_2];
493 Value bestValues[PLY_MAX_PLUS_2];
494 int bestMoveChanges[PLY_MAX_PLUS_2];
495 int depth, aspirationDelta;
496 Value value, alpha, beta;
497 Move bestMove, easyMove, skillBest, skillPonder;
499 // Initialize stuff before a new search
500 memset(ss, 0, 4 * sizeof(SearchStack));
503 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
504 depth = aspirationDelta = 0;
505 value = alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
506 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
508 // Moves to search are verified and copied
509 Rml.init(pos, searchMoves);
511 // Handle special case of searching on a mate/stalemate position
514 cout << "info" << depth_to_uci(DEPTH_ZERO)
515 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
520 // Iterative deepening loop until requested to stop or target depth reached
521 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
523 // Save now last iteration's scores, before Rml moves are reordered
524 for (size_t i = 0; i < Rml.size(); i++)
525 Rml[i].prevScore = Rml[i].score;
527 Rml.bestMoveChanges = 0;
529 // MultiPV loop. We perform a full root search for each PV line
530 for (MultiPVIdx = 0; MultiPVIdx < Min(MultiPV, (int)Rml.size()); MultiPVIdx++)
532 // Calculate dynamic aspiration window based on previous iterations
533 if (depth >= 5 && abs(Rml[MultiPVIdx].prevScore) < VALUE_KNOWN_WIN)
535 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
536 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
538 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
539 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
541 alpha = Max(Rml[MultiPVIdx].prevScore - aspirationDelta, -VALUE_INFINITE);
542 beta = Min(Rml[MultiPVIdx].prevScore + aspirationDelta, VALUE_INFINITE);
546 alpha = -VALUE_INFINITE;
547 beta = VALUE_INFINITE;
550 // Start with a small aspiration window and, in case of fail high/low,
551 // research with bigger window until not failing high/low anymore.
553 // Search starts from ss+1 to allow referencing (ss-1). This is
554 // needed by update_gains() and ss copy when splitting at Root.
555 value = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
557 // Bring to front the best move. It is critical that sorting is
558 // done with a stable algorithm because all the values but the first
559 // and eventually the new best one are set to -VALUE_INFINITE and
560 // we want to keep the same order for all the moves but the new
561 // PV that goes to the front. Note that in case of MultiPV search
562 // the already searched PV lines are preserved.
563 sort<RootMove>(Rml.begin() + MultiPVIdx, Rml.end());
565 // In case we have found an exact score and we are going to leave
566 // the fail high/low loop then reorder the PV moves, otherwise
567 // leave the last PV move in its position so to be searched again.
568 // Of course this is needed only in MultiPV search.
569 if (MultiPVIdx && value > alpha && value < beta)
570 sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIdx);
572 // Write PV back to transposition table in case the relevant entries
573 // have been overwritten during the search.
574 for (int i = 0; i <= MultiPVIdx; i++)
575 Rml[i].insert_pv_in_tt(pos);
577 // If search has been stopped exit the aspiration window loop,
578 // note that sorting and writing PV back to TT is safe becuase
579 // Rml is still valid, although refers to the previous iteration.
583 // Send full PV info to GUI if we are going to leave the loop or
584 // if we have a fail high/low and we are deep in the search. UCI
585 // protocol requires to send all the PV lines also if are still
586 // to be searched and so refer to the previous search's score.
587 if ((value > alpha && value < beta) || current_search_time() > 2000)
588 for (int i = 0; i < Min(UCIMultiPV, (int)Rml.size()); i++)
590 bool updated = (i <= MultiPVIdx);
592 if (depth == 1 && !updated)
595 Depth d = (updated ? depth : depth - 1) * ONE_PLY;
596 Value s = (updated ? Rml[i].score : Rml[i].prevScore);
600 << (i == MultiPVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
601 << speed_to_uci(pos.nodes_searched())
602 << pv_to_uci(&Rml[i].pv[0], i + 1, pos.is_chess960())
606 // In case of failing high/low increase aspiration window and
607 // research, otherwise exit the fail high/low loop.
610 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
611 aspirationDelta += aspirationDelta / 2;
613 else if (value <= alpha)
615 AspirationFailLow = true;
616 StopOnPonderhit = false;
618 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
619 aspirationDelta += aspirationDelta / 2;
624 } while (abs(value) < VALUE_KNOWN_WIN);
627 // Collect info about search result
628 bestMove = Rml[0].pv[0];
629 *ponderMove = Rml[0].pv[1];
630 bestValues[depth] = value;
631 bestMoveChanges[depth] = Rml.bestMoveChanges;
633 // Skills: Do we need to pick now the best and the ponder moves ?
634 if (SkillLevelEnabled && depth == 1 + SkillLevel)
635 do_skill_level(&skillBest, &skillPonder);
637 if (Options["Use Search Log"].value<bool>())
639 Log log(Options["Search Log Filename"].value<string>());
640 log << pretty_pv(pos, depth, value, current_search_time(), &Rml[0].pv[0]) << endl;
643 // Init easyMove at first iteration or drop it if differs from the best move
644 if (depth == 1 && (Rml.size() == 1 || Rml[0].score > Rml[1].score + EasyMoveMargin))
646 else if (bestMove != easyMove)
647 easyMove = MOVE_NONE;
649 // Check for some early stop condition
650 if (!StopRequest && Limits.useTimeManagement())
652 // Easy move: Stop search early if one move seems to be much better
653 // than the others or if there is only a single legal move. Also in
654 // the latter case search to some depth anyway to get a proper score.
656 && easyMove == bestMove
658 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
659 && current_search_time() > TimeMgr.available_time() / 16)
660 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
661 && current_search_time() > TimeMgr.available_time() / 32)))
664 // Take in account some extra time if the best move has changed
665 if (depth > 4 && depth < 50)
666 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
668 // Stop search if most of available time is already consumed. We probably don't
669 // have enough time to search the first move at the next iteration anyway.
670 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
673 // If we are allowed to ponder do not stop the search now but keep pondering
674 if (StopRequest && Limits.ponder)
677 StopOnPonderhit = true;
682 // When using skills overwrite best and ponder moves with the sub-optimal ones
683 if (SkillLevelEnabled)
685 if (skillBest == MOVE_NONE) // Still unassigned ?
686 do_skill_level(&skillBest, &skillPonder);
688 bestMove = skillBest;
689 *ponderMove = skillPonder;
696 // search<>() is the main search function for both PV and non-PV nodes and for
697 // normal and SplitPoint nodes. When called just after a split point the search
698 // is simpler because we have already probed the hash table, done a null move
699 // search, and searched the first move before splitting, we don't have to repeat
700 // all this work again. We also don't need to store anything to the hash table
701 // here: This is taken care of after we return from the split point.
703 template <NodeType NT>
704 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
706 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
707 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
708 const bool RootNode = (NT == Root || NT == SplitPointRoot);
710 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
711 assert(beta > alpha && beta <= VALUE_INFINITE);
712 assert(PvNode || alpha == beta - 1);
713 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
715 Move movesSearched[MAX_MOVES];
720 Move ttMove, move, excludedMove, threatMove;
723 Value bestValue, value, oldAlpha;
724 Value refinedValue, nullValue, futilityBase, futilityValue;
725 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
726 int moveCount = 0, playedMoveCount = 0;
727 Thread& thread = Threads[pos.thread()];
728 SplitPoint* sp = NULL;
730 refinedValue = bestValue = value = -VALUE_INFINITE;
732 inCheck = pos.in_check();
733 ss->ply = (ss-1)->ply + 1;
735 // Used to send selDepth info to GUI
736 if (PvNode && thread.maxPly < ss->ply)
737 thread.maxPly = ss->ply;
739 // Step 1. Initialize node and poll. Polling can abort search
742 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
743 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
744 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
750 ttMove = excludedMove = MOVE_NONE;
751 threatMove = sp->threatMove;
752 goto split_point_start;
755 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
761 // Step 2. Check for aborted search and immediate draw
763 || pos.is_draw<false>()
764 || ss->ply > PLY_MAX) && !RootNode)
767 // Step 3. Mate distance pruning
770 alpha = Max(value_mated_in(ss->ply), alpha);
771 beta = Min(value_mate_in(ss->ply+1), beta);
776 // Step 4. Transposition table lookup
777 // We don't want the score of a partial search to overwrite a previous full search
778 // TT value, so we use a different position key in case of an excluded move.
779 excludedMove = ss->excludedMove;
780 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
781 tte = TT.probe(posKey);
782 ttMove = RootNode ? Rml[MultiPVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
784 // At PV nodes we check for exact scores, while at non-PV nodes we check for
785 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
786 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
787 // we should also update RootMoveList to avoid bogus output.
788 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
789 : can_return_tt(tte, depth, beta, ss->ply)))
792 ss->bestMove = move = ttMove; // Can be MOVE_NONE
793 value = value_from_tt(tte->value(), ss->ply);
797 && !pos.is_capture_or_promotion(move)
798 && move != ss->killers[0])
800 ss->killers[1] = ss->killers[0];
801 ss->killers[0] = move;
806 // Step 5. Evaluate the position statically and update parent's gain statistics
808 ss->eval = ss->evalMargin = VALUE_NONE;
811 assert(tte->static_value() != VALUE_NONE);
813 ss->eval = tte->static_value();
814 ss->evalMargin = tte->static_value_margin();
815 refinedValue = refine_eval(tte, ss->eval, ss->ply);
819 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
820 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
823 // Save gain for the parent non-capture move
824 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
826 // Step 6. Razoring (is omitted in PV nodes)
828 && depth < RazorDepth
830 && refinedValue + razor_margin(depth) < beta
831 && ttMove == MOVE_NONE
832 && abs(beta) < VALUE_MATE_IN_PLY_MAX
833 && !pos.has_pawn_on_7th(pos.side_to_move()))
835 Value rbeta = beta - razor_margin(depth);
836 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
838 // Logically we should return (v + razor_margin(depth)), but
839 // surprisingly this did slightly weaker in tests.
843 // Step 7. Static null move pruning (is omitted in PV nodes)
844 // We're betting that the opponent doesn't have a move that will reduce
845 // the score by more than futility_margin(depth) if we do a null move.
848 && depth < RazorDepth
850 && refinedValue - futility_margin(depth, 0) >= beta
851 && abs(beta) < VALUE_MATE_IN_PLY_MAX
852 && pos.non_pawn_material(pos.side_to_move()))
853 return refinedValue - futility_margin(depth, 0);
855 // Step 8. Null move search with verification search (is omitted in PV nodes)
860 && refinedValue >= beta
861 && abs(beta) < VALUE_MATE_IN_PLY_MAX
862 && pos.non_pawn_material(pos.side_to_move()))
864 ss->currentMove = MOVE_NULL;
866 // Null move dynamic reduction based on depth
867 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
869 // Null move dynamic reduction based on value
870 if (refinedValue - PawnValueMidgame > beta)
873 pos.do_null_move<true>(st);
874 (ss+1)->skipNullMove = true;
875 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
876 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
877 (ss+1)->skipNullMove = false;
878 pos.do_null_move<false>(st);
880 if (nullValue >= beta)
882 // Do not return unproven mate scores
883 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
886 if (depth < 6 * ONE_PLY)
889 // Do verification search at high depths
890 ss->skipNullMove = true;
891 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
892 ss->skipNullMove = false;
899 // The null move failed low, which means that we may be faced with
900 // some kind of threat. If the previous move was reduced, check if
901 // the move that refuted the null move was somehow connected to the
902 // move which was reduced. If a connection is found, return a fail
903 // low score (which will cause the reduced move to fail high in the
904 // parent node, which will trigger a re-search with full depth).
905 threatMove = (ss+1)->bestMove;
907 if ( depth < ThreatDepth
909 && threatMove != MOVE_NONE
910 && connected_moves(pos, (ss-1)->currentMove, threatMove))
915 // Step 9. ProbCut (is omitted in PV nodes)
916 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
917 // and a reduced search returns a value much above beta, we can (almost) safely
918 // prune the previous move.
920 && depth >= RazorDepth + ONE_PLY
923 && excludedMove == MOVE_NONE
924 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
926 Value rbeta = beta + 200;
927 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
929 assert(rdepth >= ONE_PLY);
931 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
934 while ((move = mp.get_next_move()) != MOVE_NONE)
935 if (pos.pl_move_is_legal(move, ci.pinned))
937 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
938 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
945 // Step 10. Internal iterative deepening
946 if ( depth >= IIDDepth[PvNode]
947 && ttMove == MOVE_NONE
948 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
950 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
952 ss->skipNullMove = true;
953 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
954 ss->skipNullMove = false;
956 tte = TT.probe(posKey);
957 ttMove = tte ? tte->move() : MOVE_NONE;
960 split_point_start: // At split points actual search starts from here
962 // Initialize a MovePicker object for the current position
963 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
965 ss->bestMove = MOVE_NONE;
966 futilityBase = ss->eval + ss->evalMargin;
967 singularExtensionNode = !RootNode
969 && depth >= SingularExtensionDepth[PvNode]
970 && ttMove != MOVE_NONE
971 && !excludedMove // Do not allow recursive singular extension search
972 && (tte->type() & VALUE_TYPE_LOWER)
973 && tte->depth() >= depth - 3 * ONE_PLY;
976 lock_grab(&(sp->lock));
977 bestValue = sp->bestValue;
980 // Step 11. Loop through moves
981 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
982 while ( bestValue < beta
983 && (move = mp.get_next_move()) != MOVE_NONE
984 && !thread.cutoff_occurred())
988 if (move == excludedMove)
991 // At root obey the "searchmoves" option and skip moves not listed in Root
992 // Move List, as a consequence any illegal move is also skipped. In MultiPV
993 // mode we also skip PV moves which have been already searched.
994 if (RootNode && !Rml.find(move, MultiPVIdx))
997 // At PV and SpNode nodes we want all moves to be legal since the beginning
998 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
1003 moveCount = ++sp->moveCount;
1004 lock_release(&(sp->lock));
1011 // This is used by time management
1012 FirstRootMove = (moveCount == 1);
1014 // Save the current node count before the move is searched
1015 nodes = pos.nodes_searched();
1017 // For long searches send current move info to GUI
1018 if (pos.thread() == 0 && current_search_time() > 2000)
1019 cout << "info" << depth_to_uci(depth)
1020 << " currmove " << move
1021 << " currmovenumber " << moveCount + MultiPVIdx << endl;
1024 // At Root and at first iteration do a PV search on all the moves to score root moves
1025 isPvMove = (PvNode && moveCount <= (RootNode && depth <= ONE_PLY ? MAX_MOVES : 1));
1026 givesCheck = pos.move_gives_check(move, ci);
1027 captureOrPromotion = pos.is_capture_or_promotion(move);
1029 // Step 12. Decide the new search depth
1030 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1032 // Singular extension search. If all moves but one fail low on a search of
1033 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1034 // is singular and should be extended. To verify this we do a reduced search
1035 // on all the other moves but the ttMove, if result is lower than ttValue minus
1036 // a margin then we extend ttMove.
1037 if ( singularExtensionNode
1039 && pos.pl_move_is_legal(move, ci.pinned)
1042 Value ttValue = value_from_tt(tte->value(), ss->ply);
1044 if (abs(ttValue) < VALUE_KNOWN_WIN)
1046 Value rBeta = ttValue - int(depth);
1047 ss->excludedMove = move;
1048 ss->skipNullMove = true;
1049 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1050 ss->skipNullMove = false;
1051 ss->excludedMove = MOVE_NONE;
1052 ss->bestMove = MOVE_NONE;
1058 // Update current move (this must be done after singular extension search)
1059 newDepth = depth - ONE_PLY + ext;
1061 // Step 13. Futility pruning (is omitted in PV nodes)
1063 && !captureOrPromotion
1067 && !is_castle(move))
1069 // Move count based pruning
1070 if ( moveCount >= futility_move_count(depth)
1071 && (!threatMove || !connected_threat(pos, move, threatMove))
1072 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1075 lock_grab(&(sp->lock));
1080 // Value based pruning
1081 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1082 // but fixing this made program slightly weaker.
1083 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1084 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1085 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1087 if (futilityValue < beta)
1091 lock_grab(&(sp->lock));
1092 if (futilityValue > sp->bestValue)
1093 sp->bestValue = bestValue = futilityValue;
1095 else if (futilityValue > bestValue)
1096 bestValue = futilityValue;
1101 // Prune moves with negative SEE at low depths
1102 if ( predictedDepth < 2 * ONE_PLY
1103 && bestValue > VALUE_MATED_IN_PLY_MAX
1104 && pos.see_sign(move) < 0)
1107 lock_grab(&(sp->lock));
1113 // Check for legality only before to do the move
1114 if (!pos.pl_move_is_legal(move, ci.pinned))
1120 ss->currentMove = move;
1121 if (!SpNode && !captureOrPromotion)
1122 movesSearched[playedMoveCount++] = move;
1124 // Step 14. Make the move
1125 pos.do_move(move, st, ci, givesCheck);
1127 // Step extra. pv search (only in PV nodes)
1128 // The first move in list is the expected PV
1130 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1131 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1134 // Step 15. Reduced depth search
1135 // If the move fails high will be re-searched at full depth.
1136 bool doFullDepthSearch = true;
1138 if ( depth > 3 * ONE_PLY
1139 && !captureOrPromotion
1142 && ss->killers[0] != move
1143 && ss->killers[1] != move
1144 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1146 Depth d = newDepth - ss->reduction;
1147 alpha = SpNode ? sp->alpha : alpha;
1149 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1150 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1152 ss->reduction = DEPTH_ZERO;
1153 doFullDepthSearch = (value > alpha);
1156 // Step 16. Full depth search
1157 if (doFullDepthSearch)
1159 alpha = SpNode ? sp->alpha : alpha;
1160 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1161 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1163 // Step extra. pv search (only in PV nodes)
1164 // Search only for possible new PV nodes, if instead value >= beta then
1165 // parent node fails low with value <= alpha and tries another move.
1166 if (PvNode && value > alpha && (RootNode || value < beta))
1167 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1168 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1172 // Step 17. Undo move
1173 pos.undo_move(move);
1175 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1177 // Step 18. Check for new best move
1180 lock_grab(&(sp->lock));
1181 bestValue = sp->bestValue;
1185 // Finished searching the move. If StopRequest is true, the search
1186 // was aborted because the user interrupted the search or because we
1187 // ran out of time. In this case, the return value of the search cannot
1188 // be trusted, and we don't update the best move and/or PV.
1189 if (RootNode && !StopRequest)
1191 // Remember searched nodes counts for this move
1192 RootMove* rm = Rml.find(move);
1193 rm->nodes += pos.nodes_searched() - nodes;
1195 // PV move or new best move ?
1196 if (isPvMove || value > alpha)
1200 rm->extract_pv_from_tt(pos);
1202 // We record how often the best move has been changed in each
1203 // iteration. This information is used for time management: When
1204 // the best move changes frequently, we allocate some more time.
1205 if (!isPvMove && MultiPV == 1)
1206 Rml.bestMoveChanges++;
1209 // All other moves but the PV are set to the lowest value, this
1210 // is not a problem when sorting becuase sort is stable and move
1211 // position in the list is preserved, just the PV is pushed up.
1212 rm->score = -VALUE_INFINITE;
1216 if (value > bestValue)
1219 ss->bestMove = move;
1223 && value < beta) // We want always alpha < beta
1226 if (SpNode && !thread.cutoff_occurred())
1228 sp->bestValue = value;
1229 sp->ss->bestMove = move;
1231 sp->is_betaCutoff = (value >= beta);
1235 // Step 19. Check for split
1237 && depth >= Threads.min_split_depth()
1239 && Threads.available_slave_exists(pos.thread())
1241 && !thread.cutoff_occurred())
1242 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1243 threatMove, moveCount, &mp, NT);
1246 // Step 20. Check for mate and stalemate
1247 // All legal moves have been searched and if there are no legal moves, it
1248 // must be mate or stalemate. Note that we can have a false positive in
1249 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1250 // harmless because return value is discarded anyhow in the parent nodes.
1251 // If we are in a singular extension search then return a fail low score.
1252 if (!SpNode && !moveCount)
1253 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1255 // Step 21. Update tables
1256 // If the search is not aborted, update the transposition table,
1257 // history counters, and killer moves.
1258 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1260 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1261 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1262 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1264 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1266 // Update killers and history only for non capture moves that fails high
1267 if ( bestValue >= beta
1268 && !pos.is_capture_or_promotion(move))
1270 if (move != ss->killers[0])
1272 ss->killers[1] = ss->killers[0];
1273 ss->killers[0] = move;
1275 update_history(pos, move, depth, movesSearched, playedMoveCount);
1281 // Here we have the lock still grabbed
1282 sp->is_slave[pos.thread()] = false;
1283 sp->nodes += pos.nodes_searched();
1284 lock_release(&(sp->lock));
1287 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1292 // qsearch() is the quiescence search function, which is called by the main
1293 // search function when the remaining depth is zero (or, to be more precise,
1294 // less than ONE_PLY).
1296 template <NodeType NT>
1297 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1299 const bool PvNode = (NT == PV);
1301 assert(NT == PV || NT == NonPV);
1302 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1303 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1304 assert(PvNode || alpha == beta - 1);
1306 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1310 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1311 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1315 Value oldAlpha = alpha;
1317 ss->bestMove = ss->currentMove = MOVE_NONE;
1318 ss->ply = (ss-1)->ply + 1;
1320 // Check for an instant draw or maximum ply reached
1321 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1324 // Decide whether or not to include checks, this fixes also the type of
1325 // TT entry depth that we are going to use. Note that in qsearch we use
1326 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1327 inCheck = pos.in_check();
1328 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1330 // Transposition table lookup. At PV nodes, we don't use the TT for
1331 // pruning, but only for move ordering.
1332 tte = TT.probe(pos.get_key());
1333 ttMove = (tte ? tte->move() : MOVE_NONE);
1335 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1337 ss->bestMove = ttMove; // Can be MOVE_NONE
1338 return value_from_tt(tte->value(), ss->ply);
1341 // Evaluate the position statically
1344 bestValue = futilityBase = -VALUE_INFINITE;
1345 ss->eval = evalMargin = VALUE_NONE;
1346 enoughMaterial = false;
1352 assert(tte->static_value() != VALUE_NONE);
1354 evalMargin = tte->static_value_margin();
1355 ss->eval = bestValue = tte->static_value();
1358 ss->eval = bestValue = evaluate(pos, evalMargin);
1360 // Stand pat. Return immediately if static value is at least beta
1361 if (bestValue >= beta)
1364 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1369 if (PvNode && bestValue > alpha)
1372 // Futility pruning parameters, not needed when in check
1373 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1374 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1377 // Initialize a MovePicker object for the current position, and prepare
1378 // to search the moves. Because the depth is <= 0 here, only captures,
1379 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1381 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1384 // Loop through the moves until no moves remain or a beta cutoff occurs
1385 while ( bestValue < beta
1386 && (move = mp.get_next_move()) != MOVE_NONE)
1388 assert(is_ok(move));
1390 givesCheck = pos.move_gives_check(move, ci);
1398 && !is_promotion(move)
1399 && !pos.is_passed_pawn_push(move))
1401 futilityValue = futilityBase
1402 + PieceValueEndgame[pos.piece_on(move_to(move))]
1403 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1405 if (futilityValue < beta)
1407 if (futilityValue > bestValue)
1408 bestValue = futilityValue;
1413 // Prune moves with negative or equal SEE
1414 if ( futilityBase < beta
1415 && depth < DEPTH_ZERO
1416 && pos.see(move) <= 0)
1420 // Detect non-capture evasions that are candidate to be pruned
1421 evasionPrunable = !PvNode
1423 && bestValue > VALUE_MATED_IN_PLY_MAX
1424 && !pos.is_capture(move)
1425 && !pos.can_castle(pos.side_to_move());
1427 // Don't search moves with negative SEE values
1429 && (!inCheck || evasionPrunable)
1431 && !is_promotion(move)
1432 && pos.see_sign(move) < 0)
1435 // Don't search useless checks
1440 && !pos.is_capture_or_promotion(move)
1441 && ss->eval + PawnValueMidgame / 4 < beta
1442 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1444 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1445 bestValue = ss->eval + PawnValueMidgame / 4;
1450 // Check for legality only before to do the move
1451 if (!pos.pl_move_is_legal(move, ci.pinned))
1454 // Update current move
1455 ss->currentMove = move;
1457 // Make and search the move
1458 pos.do_move(move, st, ci, givesCheck);
1459 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1460 pos.undo_move(move);
1462 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1465 if (value > bestValue)
1468 ss->bestMove = move;
1472 && value < beta) // We want always alpha < beta
1477 // All legal moves have been searched. A special case: If we're in check
1478 // and no legal moves were found, it is checkmate.
1479 if (inCheck && bestValue == -VALUE_INFINITE)
1480 return value_mated_in(ss->ply);
1482 // Update transposition table
1483 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1484 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1485 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1487 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1489 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1495 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1496 // bestValue is updated only when returning false because in that case move
1499 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1501 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1502 Square from, to, ksq, victimSq;
1505 Value futilityValue, bv = *bestValue;
1507 from = move_from(move);
1509 them = flip(pos.side_to_move());
1510 ksq = pos.king_square(them);
1511 kingAtt = pos.attacks_from<KING>(ksq);
1512 pc = pos.piece_on(from);
1514 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1515 oldAtt = pos.attacks_from(pc, from, occ);
1516 newAtt = pos.attacks_from(pc, to, occ);
1518 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1519 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1521 if (!(b && (b & (b - 1))))
1524 // Rule 2. Queen contact check is very dangerous
1525 if ( type_of(pc) == QUEEN
1526 && bit_is_set(kingAtt, to))
1529 // Rule 3. Creating new double threats with checks
1530 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1534 victimSq = pop_1st_bit(&b);
1535 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1537 // Note that here we generate illegal "double move"!
1538 if ( futilityValue >= beta
1539 && pos.see_sign(make_move(from, victimSq)) >= 0)
1542 if (futilityValue > bv)
1546 // Update bestValue only if check is not dangerous (because we will prune the move)
1552 // connected_moves() tests whether two moves are 'connected' in the sense
1553 // that the first move somehow made the second move possible (for instance
1554 // if the moving piece is the same in both moves). The first move is assumed
1555 // to be the move that was made to reach the current position, while the
1556 // second move is assumed to be a move from the current position.
1558 bool connected_moves(const Position& pos, Move m1, Move m2) {
1560 Square f1, t1, f2, t2;
1567 // Case 1: The moving piece is the same in both moves
1573 // Case 2: The destination square for m2 was vacated by m1
1579 // Case 3: Moving through the vacated square
1580 p2 = pos.piece_on(f2);
1581 if ( piece_is_slider(p2)
1582 && bit_is_set(squares_between(f2, t2), f1))
1585 // Case 4: The destination square for m2 is defended by the moving piece in m1
1586 p1 = pos.piece_on(t1);
1587 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1590 // Case 5: Discovered check, checking piece is the piece moved in m1
1591 ksq = pos.king_square(pos.side_to_move());
1592 if ( piece_is_slider(p1)
1593 && bit_is_set(squares_between(t1, ksq), f2))
1595 Bitboard occ = pos.occupied_squares();
1596 clear_bit(&occ, f2);
1597 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1604 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1605 // "plies to mate from the current ply". Non-mate scores are unchanged.
1606 // The function is called before storing a value to the transposition table.
1608 Value value_to_tt(Value v, int ply) {
1610 if (v >= VALUE_MATE_IN_PLY_MAX)
1613 if (v <= VALUE_MATED_IN_PLY_MAX)
1620 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1621 // the transposition table to a mate score corrected for the current ply.
1623 Value value_from_tt(Value v, int ply) {
1625 if (v >= VALUE_MATE_IN_PLY_MAX)
1628 if (v <= VALUE_MATED_IN_PLY_MAX)
1635 // connected_threat() tests whether it is safe to forward prune a move or if
1636 // is somehow connected to the threat move returned by null search.
1638 bool connected_threat(const Position& pos, Move m, Move threat) {
1641 assert(is_ok(threat));
1642 assert(!pos.is_capture_or_promotion(m));
1643 assert(!pos.is_passed_pawn_push(m));
1645 Square mfrom, mto, tfrom, tto;
1647 mfrom = move_from(m);
1649 tfrom = move_from(threat);
1650 tto = move_to(threat);
1652 // Case 1: Don't prune moves which move the threatened piece
1656 // Case 2: If the threatened piece has value less than or equal to the
1657 // value of the threatening piece, don't prune moves which defend it.
1658 if ( pos.is_capture(threat)
1659 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1660 || type_of(pos.piece_on(tfrom)) == KING)
1661 && pos.move_attacks_square(m, tto))
1664 // Case 3: If the moving piece in the threatened move is a slider, don't
1665 // prune safe moves which block its ray.
1666 if ( piece_is_slider(pos.piece_on(tfrom))
1667 && bit_is_set(squares_between(tfrom, tto), mto)
1668 && pos.see_sign(m) >= 0)
1675 // can_return_tt() returns true if a transposition table score
1676 // can be used to cut-off at a given point in search.
1678 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1680 Value v = value_from_tt(tte->value(), ply);
1682 return ( tte->depth() >= depth
1683 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1684 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1686 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1687 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1691 // refine_eval() returns the transposition table score if
1692 // possible otherwise falls back on static position evaluation.
1694 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1698 Value v = value_from_tt(tte->value(), ply);
1700 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1701 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1708 // update_history() registers a good move that produced a beta-cutoff
1709 // in history and marks as failures all the other moves of that ply.
1711 void update_history(const Position& pos, Move move, Depth depth,
1712 Move movesSearched[], int moveCount) {
1714 Value bonus = Value(int(depth) * int(depth));
1716 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1718 for (int i = 0; i < moveCount - 1; i++)
1720 m = movesSearched[i];
1724 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1729 // update_gains() updates the gains table of a non-capture move given
1730 // the static position evaluation before and after the move.
1732 void update_gains(const Position& pos, Move m, Value before, Value after) {
1735 && before != VALUE_NONE
1736 && after != VALUE_NONE
1737 && pos.captured_piece_type() == PIECE_TYPE_NONE
1739 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1743 // current_search_time() returns the number of milliseconds which have passed
1744 // since the beginning of the current search.
1746 int current_search_time(int set) {
1748 static int searchStartTime;
1751 searchStartTime = set;
1753 return get_system_time() - searchStartTime;
1757 // score_to_uci() converts a value to a string suitable for use with the UCI
1758 // protocol specifications:
1760 // cp <x> The score from the engine's point of view in centipawns.
1761 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1762 // use negative values for y.
1764 string score_to_uci(Value v, Value alpha, Value beta) {
1766 std::stringstream s;
1768 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1769 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1771 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1773 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1779 // speed_to_uci() returns a string with time stats of current search suitable
1780 // to be sent to UCI gui.
1782 string speed_to_uci(int64_t nodes) {
1784 std::stringstream s;
1785 int t = current_search_time();
1787 s << " nodes " << nodes
1788 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1794 // pv_to_uci() returns a string with information on the current PV line
1795 // formatted according to UCI specification.
1797 string pv_to_uci(const Move pv[], int pvNum, bool chess960) {
1799 std::stringstream s;
1801 s << " multipv " << pvNum << " pv " << set960(chess960);
1803 for ( ; *pv != MOVE_NONE; pv++)
1809 // depth_to_uci() returns a string with information on the current depth and
1810 // seldepth formatted according to UCI specification.
1812 string depth_to_uci(Depth depth) {
1814 std::stringstream s;
1816 // Retrieve max searched depth among threads
1818 for (int i = 0; i < Threads.size(); i++)
1819 if (Threads[i].maxPly > selDepth)
1820 selDepth = Threads[i].maxPly;
1822 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1827 string time_to_string(int millisecs) {
1829 const int MSecMinute = 1000 * 60;
1830 const int MSecHour = 1000 * 60 * 60;
1832 int hours = millisecs / MSecHour;
1833 int minutes = (millisecs % MSecHour) / MSecMinute;
1834 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1836 std::stringstream s;
1841 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1845 string score_to_string(Value v) {
1847 std::stringstream s;
1849 if (v >= VALUE_MATE_IN_PLY_MAX)
1850 s << "#" << (VALUE_MATE - v + 1) / 2;
1851 else if (v <= VALUE_MATED_IN_PLY_MAX)
1852 s << "-#" << (VALUE_MATE + v) / 2;
1854 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1859 // pretty_pv() creates a human-readable string from a position and a PV.
1860 // It is used to write search information to the log file (which is created
1861 // when the UCI parameter "Use Search Log" is "true").
1863 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1865 const int64_t K = 1000;
1866 const int64_t M = 1000000;
1867 const int startColumn = 28;
1868 const size_t maxLength = 80 - startColumn;
1870 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1873 std::stringstream s;
1876 // First print depth, score, time and searched nodes...
1877 s << set960(pos.is_chess960())
1878 << std::setw(2) << depth
1879 << std::setw(8) << score_to_string(value)
1880 << std::setw(8) << time_to_string(time);
1882 if (pos.nodes_searched() < M)
1883 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1884 else if (pos.nodes_searched() < K * M)
1885 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1887 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1889 // ...then print the full PV line in short algebraic notation
1890 while (*m != MOVE_NONE)
1892 san = move_to_san(pos, *m);
1893 length += san.length() + 1;
1895 if (length > maxLength)
1897 length = san.length() + 1;
1898 s << "\n" + string(startColumn, ' ');
1902 pos.do_move(*m++, *st++);
1905 // Restore original position before to leave
1906 while (m != pv) pos.undo_move(*--m);
1911 // poll() performs two different functions: It polls for user input, and it
1912 // looks at the time consumed so far and decides if it's time to abort the
1915 void poll(const Position& pos) {
1917 static int lastInfoTime;
1918 int t = current_search_time();
1921 if (input_available())
1923 // We are line oriented, don't read single chars
1926 if (!std::getline(std::cin, command) || command == "quit")
1928 // Quit the program as soon as possible
1929 Limits.ponder = false;
1930 QuitRequest = StopRequest = true;
1933 else if (command == "stop")
1935 // Stop calculating as soon as possible, but still send the "bestmove"
1936 // and possibly the "ponder" token when finishing the search.
1937 Limits.ponder = false;
1940 else if (command == "ponderhit")
1942 // The opponent has played the expected move. GUI sends "ponderhit" if
1943 // we were told to ponder on the same move the opponent has played. We
1944 // should continue searching but switching from pondering to normal search.
1945 Limits.ponder = false;
1947 if (StopOnPonderhit)
1952 // Print search information
1956 else if (lastInfoTime > t)
1957 // HACK: Must be a new search where we searched less than
1958 // NodesBetweenPolls nodes during the first second of search.
1961 else if (t - lastInfoTime >= 1000)
1966 dbg_print_hit_rate();
1969 // Should we stop the search?
1973 bool stillAtFirstMove = FirstRootMove
1974 && !AspirationFailLow
1975 && t > TimeMgr.available_time();
1977 bool noMoreTime = t > TimeMgr.maximum_time()
1978 || stillAtFirstMove;
1980 if ( (Limits.useTimeManagement() && noMoreTime)
1981 || (Limits.maxTime && t >= Limits.maxTime)
1982 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1987 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1988 // while the program is pondering. The point is to work around a wrinkle in
1989 // the UCI protocol: When pondering, the engine is not allowed to give a
1990 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1991 // We simply wait here until one of these commands is sent, and return,
1992 // after which the bestmove and pondermove will be printed.
1994 void wait_for_stop_or_ponderhit() {
1998 // Wait for a command from stdin
1999 while ( std::getline(std::cin, command)
2000 && command != "ponderhit" && command != "stop" && command != "quit") {};
2002 if (command != "ponderhit" && command != "stop")
2003 QuitRequest = true; // Must be "quit" or getline() returned false
2007 // When playing with strength handicap choose best move among the MultiPV set
2008 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
2009 void do_skill_level(Move* best, Move* ponder) {
2011 assert(MultiPV > 1);
2015 // Rml list is already sorted by score in descending order
2017 int max_s = -VALUE_INFINITE;
2018 int size = Min(MultiPV, (int)Rml.size());
2019 int max = Rml[0].score;
2020 int var = Min(max - Rml[size - 1].score, PawnValueMidgame);
2021 int wk = 120 - 2 * SkillLevel;
2023 // PRNG sequence should be non deterministic
2024 for (int i = abs(get_system_time() % 50); i > 0; i--)
2025 rk.rand<unsigned>();
2027 // Choose best move. For each move's score we add two terms both dependent
2028 // on wk, one deterministic and bigger for weaker moves, and one random,
2029 // then we choose the move with the resulting highest score.
2030 for (int i = 0; i < size; i++)
2034 // Don't allow crazy blunders even at very low skills
2035 if (i > 0 && Rml[i-1].score > s + EasyMoveMargin)
2038 // This is our magical formula
2039 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
2044 *best = Rml[i].pv[0];
2045 *ponder = Rml[i].pv[1];
2051 /// RootMove and RootMoveList method's definitions
2053 void RootMoveList::init(Position& pos, Move searchMoves[]) {
2056 bestMoveChanges = 0;
2059 // Generate all legal moves and add them to RootMoveList
2060 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2062 // If we have a searchMoves[] list then verify the move
2063 // is in the list before to add it.
2064 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2066 if (sm != searchMoves && *sm != ml.move())
2070 rm.pv.push_back(ml.move());
2071 rm.pv.push_back(MOVE_NONE);
2072 rm.score = rm.prevScore = -VALUE_INFINITE;
2078 RootMove* RootMoveList::find(const Move& m, int startIndex) {
2080 for (size_t i = startIndex; i < size(); i++)
2081 if ((*this)[i].pv[0] == m)
2087 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2088 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2089 // allow to always have a ponder move even when we fail high at root and also a
2090 // long PV to print that is important for position analysis.
2092 void RootMove::extract_pv_from_tt(Position& pos) {
2094 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2099 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
2103 pos.do_move(m, *st++);
2105 while ( (tte = TT.probe(pos.get_key())) != NULL
2106 && tte->move() != MOVE_NONE
2107 && pos.is_pseudo_legal(tte->move())
2108 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2110 && (!pos.is_draw<false>() || ply < 2))
2112 pv.push_back(tte->move());
2113 pos.do_move(tte->move(), *st++);
2116 pv.push_back(MOVE_NONE);
2118 do pos.undo_move(pv[--ply]); while (ply);
2121 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2122 // the PV back into the TT. This makes sure the old PV moves are searched
2123 // first, even if the old TT entries have been overwritten.
2125 void RootMove::insert_pv_in_tt(Position& pos) {
2127 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2130 Value v, m = VALUE_NONE;
2133 assert(pv[0] != MOVE_NONE && pos.is_pseudo_legal(pv[0]));
2139 // Don't overwrite existing correct entries
2140 if (!tte || tte->move() != pv[ply])
2142 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2143 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2145 pos.do_move(pv[ply], *st++);
2147 } while (pv[++ply] != MOVE_NONE);
2149 do pos.undo_move(pv[--ply]); while (ply);
2154 // Little helper used by idle_loop() to check that all the slave threads of a
2155 // split point have finished searching.
2157 static bool all_slaves_finished(SplitPoint* sp) {
2159 for (int i = 0; i < Threads.size(); i++)
2160 if (sp->is_slave[i])
2167 // Thread::idle_loop() is where the thread is parked when it has no work to do.
2168 // The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
2169 // for which the thread is the master.
2171 void Thread::idle_loop(SplitPoint* sp) {
2175 // If we are not searching, wait for a condition to be signaled
2176 // instead of wasting CPU time polling for work.
2179 || (Threads.use_sleeping_threads() && !is_searching))
2181 assert((!sp && threadID) || Threads.use_sleeping_threads());
2183 // Slave thread should exit as soon as do_terminate flag raises
2190 // Grab the lock to avoid races with Thread::wake_up()
2191 lock_grab(&sleepLock);
2193 // If we are master and all slaves have finished don't go to sleep
2194 if (sp && all_slaves_finished(sp))
2196 lock_release(&sleepLock);
2200 // Do sleep after retesting sleep conditions under lock protection, in
2201 // particular we need to avoid a deadlock in case a master thread has,
2202 // in the meanwhile, allocated us and sent the wake_up() call before we
2203 // had the chance to grab the lock.
2204 if (do_sleep || !is_searching)
2205 cond_wait(&sleepCond, &sleepLock);
2207 lock_release(&sleepLock);
2210 // If this thread has been assigned work, launch a search
2213 assert(!do_terminate);
2215 // Copy split point position and search stack and call search()
2216 SearchStack ss[PLY_MAX_PLUS_2];
2217 SplitPoint* tsp = splitPoint;
2218 Position pos(*tsp->pos, threadID);
2220 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2223 if (tsp->nodeType == Root)
2224 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2225 else if (tsp->nodeType == PV)
2226 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2227 else if (tsp->nodeType == NonPV)
2228 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2232 assert(is_searching);
2234 is_searching = false;
2236 // Wake up master thread so to allow it to return from the idle loop in
2237 // case we are the last slave of the split point.
2238 if ( Threads.use_sleeping_threads()
2239 && threadID != tsp->master
2240 && !Threads[tsp->master].is_searching)
2241 Threads[tsp->master].wake_up();
2244 // If this thread is the master of a split point and all slaves have
2245 // finished their work at this split point, return from the idle loop.
2246 if (sp && all_slaves_finished(sp))
2248 // Because sp->is_slave[] is reset under lock protection,
2249 // be sure sp->lock has been released before to return.
2250 lock_grab(&(sp->lock));
2251 lock_release(&(sp->lock));