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"
43 volatile SignalsType Signals;
45 std::vector<Move> RootMoves;
46 Position RootPosition;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // Different node types, used as template parameter
60 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
62 // RootMove struct is used for moves at the root of the tree. For each root
63 // move, we store a score, a node count, and a PV (really a refutation
64 // in the case of moves which fail low). Score is normally set at
65 // -VALUE_INFINITE for all non-pv moves.
68 // RootMove::operator<() is the comparison function used when
69 // sorting the moves. A move m1 is considered to be better
70 // than a move m2 if it has an higher score
71 bool operator<(const RootMove& m) const { return score < m.score; }
73 void extract_pv_from_tt(Position& pos);
74 void insert_pv_in_tt(Position& pos);
82 // RootMoveList struct is mainly a std::vector of RootMove objects
83 struct RootMoveList : public std::vector<RootMove> {
85 void init(Position& pos, Move rootMoves[]);
86 RootMove* find(const Move& m, int startIndex = 0);
94 // Lookup table to check if a Piece is a slider and its access function
95 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
96 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
100 // Maximum depth for razoring
101 const Depth RazorDepth = 4 * ONE_PLY;
103 // Dynamic razoring margin based on depth
104 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
106 // Maximum depth for use of dynamic threat detection when null move fails low
107 const Depth ThreatDepth = 5 * ONE_PLY;
109 // Step 9. Internal iterative deepening
111 // Minimum depth for use of internal iterative deepening
112 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
114 // At Non-PV nodes we do an internal iterative deepening search
115 // when the static evaluation is bigger then beta - IIDMargin.
116 const Value IIDMargin = Value(0x100);
118 // Step 11. Decide the new search depth
120 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
121 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
122 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
123 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
124 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
126 // Minimum depth for use of singular extension
127 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
129 // Step 12. Futility pruning
131 // Futility margin for quiescence search
132 const Value FutilityMarginQS = Value(0x80);
134 // Futility lookup tables (initialized at startup) and their access functions
135 Value FutilityMargins[16][64]; // [depth][moveNumber]
136 int FutilityMoveCounts[32]; // [depth]
138 inline Value futility_margin(Depth d, int mn) {
140 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
141 : 2 * VALUE_INFINITE;
144 inline int futility_move_count(Depth d) {
146 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
149 // Step 14. Reduced search
151 // Reduction lookup tables (initialized at startup) and their access function
152 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
154 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
156 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
159 // Easy move margin. An easy move candidate must be at least this much
160 // better than the second best move.
161 const Value EasyMoveMargin = Value(0x150);
164 /// Namespace variables
170 int MultiPV, UCIMultiPV, MultiPVIdx;
172 // Time management variables
175 // Skill level adjustment
177 bool SkillLevelEnabled;
185 Move id_loop(Position& pos, Move rootMoves[], Move* ponderMove);
187 template <NodeType NT>
188 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
190 template <NodeType NT>
191 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
193 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
194 bool connected_moves(const Position& pos, Move m1, Move m2);
195 Value value_to_tt(Value v, int ply);
196 Value value_from_tt(Value v, int ply);
197 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
198 bool connected_threat(const Position& pos, Move m, Move threat);
199 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
200 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
201 void do_skill_level(Move* best, Move* ponder);
203 int elapsed_time(bool reset = false);
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);
210 // MovePickerExt template class extends MovePicker and allows to choose at compile
211 // time the proper moves source according to the type of node. In the default case
212 // we simply create and use a standard MovePicker object.
213 template<bool SpNode> struct MovePickerExt : public MovePicker {
215 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
216 : MovePicker(p, ttm, d, h, ss, b) {}
219 // In case of a SpNode we use split point's shared MovePicker object as moves source
220 template<> struct MovePickerExt<true> : public MovePicker {
222 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
223 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
225 Move get_next_move() { return mp->get_next_move(); }
229 // Overload operator<<() to make it easier to print moves in a coordinate
230 // notation compatible with UCI protocol.
231 std::ostream& operator<<(std::ostream& os, Move m) {
233 bool chess960 = (os.iword(0) != 0); // See set960()
234 return os << move_to_uci(m, chess960);
237 // When formatting a move for std::cout we must know if we are in Chess960
238 // or not. To keep using the handy operator<<() on the move the trick is to
239 // embed this flag in the stream itself. Function-like named enum set960 is
240 // used as a custom manipulator and the stream internal general-purpose array,
241 // accessed through ios_base::iword(), is used to pass the flag to the move's
242 // operator<<() that will read it to properly format castling moves.
245 std::ostream& operator<< (std::ostream& os, const set960& f) {
247 os.iword(0) = int(f);
251 // extension() decides whether a move should be searched with normal depth,
252 // or with extended depth. Certain classes of moves (checking moves, in
253 // particular) are searched with bigger depth than ordinary moves and in
254 // any case are marked as 'dangerous'. Note that also if a move is not
255 // extended, as example because the corresponding UCI option is set to zero,
256 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
257 template <bool PvNode>
258 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
259 bool moveIsCheck, bool* dangerous) {
260 assert(m != MOVE_NONE);
262 Depth result = DEPTH_ZERO;
263 *dangerous = moveIsCheck;
265 if (moveIsCheck && pos.see_sign(m) >= 0)
266 result += CheckExtension[PvNode];
268 if (type_of(pos.piece_on(move_from(m))) == PAWN)
270 Color c = pos.side_to_move();
271 if (relative_rank(c, move_to(m)) == RANK_7)
273 result += PawnPushTo7thExtension[PvNode];
276 if (pos.pawn_is_passed(c, move_to(m)))
278 result += PassedPawnExtension[PvNode];
283 if ( captureOrPromotion
284 && type_of(pos.piece_on(move_to(m))) != PAWN
285 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
286 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
289 result += PawnEndgameExtension[PvNode];
293 return std::min(result, ONE_PLY);
299 /// init_search() is called during startup to initialize various lookup tables
301 void Search::init() {
303 int d; // depth (ONE_PLY == 2)
304 int hd; // half depth (ONE_PLY == 1)
307 // Init reductions array
308 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
310 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
311 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
312 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
313 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
316 // Init futility margins array
317 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
318 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
320 // Init futility move count array
321 for (d = 0; d < 32; d++)
322 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
326 /// perft() is our utility to verify move generation. All the leaf nodes up to
327 /// the given depth are generated and counted and the sum returned.
329 int64_t Search::perft(Position& pos, Depth depth) {
334 // Generate all legal moves
335 MoveList<MV_LEGAL> ml(pos);
337 // If we are at the last ply we don't need to do and undo
338 // the moves, just to count them.
339 if (depth <= ONE_PLY)
342 // Loop through all legal moves
344 for ( ; !ml.end(); ++ml)
346 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
347 sum += perft(pos, depth - ONE_PLY);
348 pos.undo_move(ml.move());
354 /// think() is the external interface to Stockfish's search, and is called by the
355 /// main thread when the program receives the UCI 'go' command. It searches from
356 /// RootPosition and at the end prints the "bestmove" to output.
358 void Search::think() {
360 static Book book; // Defined static to initialize the PRNG only once
362 Position& pos = RootPosition;
364 // Reset elapsed search time
367 // Set output stream mode: normal or chess960. Castling notation is different
368 cout << set960(pos.is_chess960());
370 // Look for a book move
371 if (Options["OwnBook"].value<bool>())
373 if (Options["Book File"].value<string>() != book.name())
374 book.open(Options["Book File"].value<string>());
376 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
377 if (bookMove != MOVE_NONE)
379 if (!Signals.stop && (Limits.ponder || Limits.infinite))
380 Threads.wait_for_stop_or_ponderhit();
382 cout << "bestmove " << bookMove << endl;
387 // Read UCI options: GUI could change UCI parameters during the game
388 read_evaluation_uci_options(pos.side_to_move());
389 Threads.read_uci_options();
391 // Set a new TT size if changed
392 TT.set_size(Options["Hash"].value<int>());
394 if (Options["Clear Hash"].value<bool>())
396 Options["Clear Hash"].set_value("false");
400 UCIMultiPV = Options["MultiPV"].value<int>();
401 SkillLevel = Options["Skill Level"].value<int>();
403 // Do we have to play with skill handicap? In this case enable MultiPV that
404 // we will use behind the scenes to retrieve a set of possible moves.
405 SkillLevelEnabled = (SkillLevel < 20);
406 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, 4) : UCIMultiPV);
408 // Write current search header to log file
409 if (Options["Use Search Log"].value<bool>())
411 Log log(Options["Search Log Filename"].value<string>());
412 log << "\nSearching: " << pos.to_fen()
413 << "\ninfinite: " << Limits.infinite
414 << " ponder: " << Limits.ponder
415 << " time: " << Limits.time
416 << " increment: " << Limits.increment
417 << " moves to go: " << Limits.movesToGo
421 // Wake up needed threads and reset maxPly counter
422 for (int i = 0; i < Threads.size(); i++)
424 Threads[i].maxPly = 0;
425 Threads[i].wake_up();
428 // Set best timer interval to avoid lagging under time pressure. Timer is
429 // used to check for remaining available thinking time.
430 TimeMgr.init(Limits, pos.startpos_ply_counter());
432 if (TimeMgr.available_time())
433 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
435 Threads.set_timer(100);
437 // We're ready to start thinking. Call the iterative deepening loop function
438 Move ponderMove = MOVE_NONE;
439 Move bestMove = id_loop(pos, &RootMoves[0], &ponderMove);
441 // Stop timer, no need to check for available time any more
442 Threads.set_timer(0);
444 // This makes all the slave threads to go to sleep, if not already sleeping
447 // Write current search final statistics to log file
448 if (Options["Use Search Log"].value<bool>())
450 int e = elapsed_time();
452 Log log(Options["Search Log Filename"].value<string>());
453 log << "Nodes: " << pos.nodes_searched()
454 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 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 // When we reach max depth we arrive here even without a StopRequest, but if
464 // we are pondering or in infinite search, we shouldn't print the best move
465 // before we are told to do so.
466 if (!Signals.stop && (Limits.ponder || Limits.infinite))
467 Threads.wait_for_stop_or_ponderhit();
469 // Could be MOVE_NONE when searching on a stalemate position
470 cout << "bestmove " << bestMove;
472 // UCI protol is not clear on allowing sending an empty ponder move, instead
473 // it is clear that ponder move is optional. So skip it if empty.
474 if (ponderMove != MOVE_NONE)
475 cout << " ponder " << ponderMove;
483 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
484 // with increasing depth until the allocated thinking time has been consumed,
485 // user stops the search, or the maximum search depth is reached.
487 Move id_loop(Position& pos, Move rootMoves[], Move* ponderMove) {
489 Stack ss[PLY_MAX_PLUS_2];
490 Value bestValues[PLY_MAX_PLUS_2];
491 int bestMoveChanges[PLY_MAX_PLUS_2];
492 int depth, aspirationDelta;
493 Value bestValue, alpha, beta;
494 Move bestMove, skillBest, skillPonder;
495 bool bestMoveNeverChanged = true;
497 // Initialize stuff before a new search
498 memset(ss, 0, 4 * sizeof(Stack));
501 *ponderMove = bestMove = skillBest = skillPonder = MOVE_NONE;
502 depth = aspirationDelta = 0;
503 bestValue = alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
504 ss->currentMove = MOVE_NULL; // Hack to skip update gains
506 // Moves to search are verified and copied
507 Rml.init(pos, rootMoves);
509 // Handle special case of searching on a mate/stalemate position
512 cout << "info" << depth_to_uci(DEPTH_ZERO)
513 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
518 // Iterative deepening loop until requested to stop or target depth reached
519 while (!Signals.stop && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
521 // Save now last iteration's scores, before Rml moves are reordered
522 for (size_t i = 0; i < Rml.size(); i++)
523 Rml[i].prevScore = Rml[i].score;
525 Rml.bestMoveChanges = 0;
527 // MultiPV loop. We perform a full root search for each PV line
528 for (MultiPVIdx = 0; MultiPVIdx < std::min(MultiPV, (int)Rml.size()); MultiPVIdx++)
530 // Calculate dynamic aspiration window based on previous iterations
531 if (depth >= 5 && abs(Rml[MultiPVIdx].prevScore) < VALUE_KNOWN_WIN)
533 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
534 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
536 aspirationDelta = std::min(std::max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
537 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
539 alpha = std::max(Rml[MultiPVIdx].prevScore - aspirationDelta, -VALUE_INFINITE);
540 beta = std::min(Rml[MultiPVIdx].prevScore + aspirationDelta, VALUE_INFINITE);
544 alpha = -VALUE_INFINITE;
545 beta = VALUE_INFINITE;
548 // Start with a small aspiration window and, in case of fail high/low,
549 // research with bigger window until not failing high/low anymore.
551 // Search starts from ss+1 to allow referencing (ss-1). This is
552 // needed by update gains and ss copy when splitting at Root.
553 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
555 // Bring to front the best move. It is critical that sorting is
556 // done with a stable algorithm because all the values but the first
557 // and eventually the new best one are set to -VALUE_INFINITE and
558 // we want to keep the same order for all the moves but the new
559 // PV that goes to the front. Note that in case of MultiPV search
560 // the already searched PV lines are preserved.
561 sort<RootMove>(Rml.begin() + MultiPVIdx, Rml.end());
563 // In case we have found an exact score and we are going to leave
564 // the fail high/low loop then reorder the PV moves, otherwise
565 // leave the last PV move in its position so to be searched again.
566 // Of course this is needed only in MultiPV search.
567 if (MultiPVIdx && bestValue > alpha && bestValue < beta)
568 sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIdx);
570 // Write PV back to transposition table in case the relevant entries
571 // have been overwritten during the search.
572 for (int i = 0; i <= MultiPVIdx; i++)
573 Rml[i].insert_pv_in_tt(pos);
575 // If search has been stopped exit the aspiration window loop,
576 // note that sorting and writing PV back to TT is safe becuase
577 // Rml is still valid, although refers to the previous iteration.
581 // Send full PV info to GUI if we are going to leave the loop or
582 // if we have a fail high/low and we are deep in the search. UCI
583 // protocol requires to send all the PV lines also if are still
584 // to be searched and so refer to the previous search's score.
585 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
586 for (int i = 0; i < std::min(UCIMultiPV, (int)Rml.size()); i++)
588 bool updated = (i <= MultiPVIdx);
590 if (depth == 1 && !updated)
593 Depth d = (updated ? depth : depth - 1) * ONE_PLY;
594 Value s = (updated ? Rml[i].score : Rml[i].prevScore);
598 << (i == MultiPVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
599 << speed_to_uci(pos.nodes_searched())
600 << pv_to_uci(&Rml[i].pv[0], i + 1, pos.is_chess960())
604 // In case of failing high/low increase aspiration window and
605 // research, otherwise exit the fail high/low loop.
606 if (bestValue >= beta)
608 beta = std::min(beta + aspirationDelta, VALUE_INFINITE);
609 aspirationDelta += aspirationDelta / 2;
611 else if (bestValue <= alpha)
613 Signals.failedLowAtRoot = true;
614 Signals.stopOnPonderhit = false;
616 alpha = std::max(alpha - aspirationDelta, -VALUE_INFINITE);
617 aspirationDelta += aspirationDelta / 2;
622 } while (abs(bestValue) < VALUE_KNOWN_WIN);
625 // Collect info about search result
626 bestMove = Rml[0].pv[0];
627 *ponderMove = Rml[0].pv[1];
628 bestValues[depth] = bestValue;
629 bestMoveChanges[depth] = Rml.bestMoveChanges;
631 // Skills: Do we need to pick now the best and the ponder moves ?
632 if (SkillLevelEnabled && depth == 1 + SkillLevel)
633 do_skill_level(&skillBest, &skillPonder);
635 if (Options["Use Search Log"].value<bool>())
637 Log log(Options["Search Log Filename"].value<string>());
638 log << pretty_pv(pos, depth, bestValue, elapsed_time(), &Rml[0].pv[0]) << endl;
641 // Filter out startup noise when monitoring best move stability
642 if (depth > 2 && bestMoveChanges[depth])
643 bestMoveNeverChanged = false;
645 // Do we have time for the next iteration? Can we stop searching now?
646 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
648 bool stop = false; // Local variable instead of the volatile Signals.stop
650 // Take in account some extra time if the best move has changed
651 if (depth > 4 && depth < 50)
652 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
654 // Stop search if most of available time is already consumed. We probably don't
655 // have enough time to search the first move at the next iteration anyway.
656 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
659 // Stop search early if one move seems to be much better than others
662 && ( bestMoveNeverChanged
663 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
665 Value rBeta = bestValue - EasyMoveMargin;
666 (ss+1)->excludedMove = bestMove;
667 (ss+1)->skipNullMove = true;
668 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
669 (ss+1)->skipNullMove = false;
670 (ss+1)->excludedMove = MOVE_NONE;
678 // If we are allowed to ponder do not stop the search now but
679 // keep pondering until GUI sends "ponderhit" or "stop".
681 Signals.stopOnPonderhit = true;
688 // When using skills overwrite best and ponder moves with the sub-optimal ones
689 if (SkillLevelEnabled)
691 if (skillBest == MOVE_NONE) // Still unassigned ?
692 do_skill_level(&skillBest, &skillPonder);
694 bestMove = skillBest;
695 *ponderMove = skillPonder;
702 // search<>() is the main search function for both PV and non-PV nodes and for
703 // normal and SplitPoint nodes. When called just after a split point the search
704 // is simpler because we have already probed the hash table, done a null move
705 // search, and searched the first move before splitting, we don't have to repeat
706 // all this work again. We also don't need to store anything to the hash table
707 // here: This is taken care of after we return from the split point.
709 template <NodeType NT>
710 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
712 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
713 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
714 const bool RootNode = (NT == Root || NT == SplitPointRoot);
716 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
717 assert(beta > alpha && beta <= VALUE_INFINITE);
718 assert(PvNode || alpha == beta - 1);
719 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
721 Move movesSearched[MAX_MOVES];
726 Move ttMove, move, excludedMove, threatMove;
729 Value bestValue, value, oldAlpha;
730 Value refinedValue, nullValue, futilityBase, futilityValue;
731 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
732 bool captureOrPromotion, dangerous, doFullDepthSearch;
733 int moveCount = 0, playedMoveCount = 0;
734 Thread& thread = Threads[pos.thread()];
735 SplitPoint* sp = NULL;
737 refinedValue = bestValue = value = -VALUE_INFINITE;
739 inCheck = pos.in_check();
740 ss->ply = (ss-1)->ply + 1;
742 // Used to send selDepth info to GUI
743 if (PvNode && thread.maxPly < ss->ply)
744 thread.maxPly = ss->ply;
746 // Step 1. Initialize node
749 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
750 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
751 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
757 ttMove = excludedMove = MOVE_NONE;
758 threatMove = sp->threatMove;
759 goto split_point_start;
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 // Update gain for the parent non-capture move given the static position
825 // evaluation before and after the move.
826 if ( (move = (ss-1)->currentMove) != MOVE_NULL
827 && (ss-1)->eval != VALUE_NONE
828 && ss->eval != VALUE_NONE
829 && pos.captured_piece_type() == PIECE_TYPE_NONE
830 && !is_special(move))
832 Square to = move_to(move);
833 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
836 // Step 6. Razoring (is omitted in PV nodes)
838 && depth < RazorDepth
840 && refinedValue + razor_margin(depth) < beta
841 && ttMove == MOVE_NONE
842 && abs(beta) < VALUE_MATE_IN_PLY_MAX
843 && !pos.has_pawn_on_7th(pos.side_to_move()))
845 Value rbeta = beta - razor_margin(depth);
846 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
848 // Logically we should return (v + razor_margin(depth)), but
849 // surprisingly this did slightly weaker in tests.
853 // Step 7. Static null move pruning (is omitted in PV nodes)
854 // We're betting that the opponent doesn't have a move that will reduce
855 // the score by more than futility_margin(depth) if we do a null move.
858 && depth < RazorDepth
860 && refinedValue - futility_margin(depth, 0) >= beta
861 && abs(beta) < VALUE_MATE_IN_PLY_MAX
862 && pos.non_pawn_material(pos.side_to_move()))
863 return refinedValue - futility_margin(depth, 0);
865 // Step 8. Null move search with verification search (is omitted in PV nodes)
870 && refinedValue >= beta
871 && abs(beta) < VALUE_MATE_IN_PLY_MAX
872 && pos.non_pawn_material(pos.side_to_move()))
874 ss->currentMove = MOVE_NULL;
876 // Null move dynamic reduction based on depth
877 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
879 // Null move dynamic reduction based on value
880 if (refinedValue - PawnValueMidgame > beta)
883 pos.do_null_move<true>(st);
884 (ss+1)->skipNullMove = true;
885 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
886 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
887 (ss+1)->skipNullMove = false;
888 pos.do_null_move<false>(st);
890 if (nullValue >= beta)
892 // Do not return unproven mate scores
893 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
896 if (depth < 6 * ONE_PLY)
899 // Do verification search at high depths
900 ss->skipNullMove = true;
901 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
902 ss->skipNullMove = false;
909 // The null move failed low, which means that we may be faced with
910 // some kind of threat. If the previous move was reduced, check if
911 // the move that refuted the null move was somehow connected to the
912 // move which was reduced. If a connection is found, return a fail
913 // low score (which will cause the reduced move to fail high in the
914 // parent node, which will trigger a re-search with full depth).
915 threatMove = (ss+1)->bestMove;
917 if ( depth < ThreatDepth
919 && threatMove != MOVE_NONE
920 && connected_moves(pos, (ss-1)->currentMove, threatMove))
925 // Step 9. ProbCut (is omitted in PV nodes)
926 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
927 // and a reduced search returns a value much above beta, we can (almost) safely
928 // prune the previous move.
930 && depth >= RazorDepth + ONE_PLY
933 && excludedMove == MOVE_NONE
934 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
936 Value rbeta = beta + 200;
937 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
939 assert(rdepth >= ONE_PLY);
941 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
944 while ((move = mp.get_next_move()) != MOVE_NONE)
945 if (pos.pl_move_is_legal(move, ci.pinned))
947 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
948 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
955 // Step 10. Internal iterative deepening
956 if ( depth >= IIDDepth[PvNode]
957 && ttMove == MOVE_NONE
958 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
960 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
962 ss->skipNullMove = true;
963 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
964 ss->skipNullMove = false;
966 tte = TT.probe(posKey);
967 ttMove = tte ? tte->move() : MOVE_NONE;
970 split_point_start: // At split points actual search starts from here
972 // Initialize a MovePicker object for the current position
973 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
975 ss->bestMove = MOVE_NONE;
976 futilityBase = ss->eval + ss->evalMargin;
977 singularExtensionNode = !RootNode
979 && depth >= SingularExtensionDepth[PvNode]
980 && ttMove != MOVE_NONE
981 && !excludedMove // Do not allow recursive singular extension search
982 && (tte->type() & VALUE_TYPE_LOWER)
983 && tte->depth() >= depth - 3 * ONE_PLY;
986 lock_grab(&(sp->lock));
987 bestValue = sp->bestValue;
990 // Step 11. Loop through moves
991 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
992 while ( bestValue < beta
993 && (move = mp.get_next_move()) != MOVE_NONE
994 && !thread.cutoff_occurred())
998 if (move == excludedMove)
1001 // At root obey the "searchmoves" option and skip moves not listed in Root
1002 // Move List, as a consequence any illegal move is also skipped. In MultiPV
1003 // mode we also skip PV moves which have been already searched.
1004 if (RootNode && !Rml.find(move, MultiPVIdx))
1007 // At PV and SpNode nodes we want all moves to be legal since the beginning
1008 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
1013 moveCount = ++sp->moveCount;
1014 lock_release(&(sp->lock));
1021 // This is used by time management
1022 Signals.firstRootMove = (moveCount == 1);
1024 // Save the current node count before the move is searched
1025 nodes = pos.nodes_searched();
1027 // For long searches send current move info to GUI
1028 if (pos.thread() == 0 && elapsed_time() > 2000)
1029 cout << "info" << depth_to_uci(depth)
1030 << " currmove " << move
1031 << " currmovenumber " << moveCount + MultiPVIdx << endl;
1034 isPvMove = (PvNode && moveCount <= 1);
1035 givesCheck = pos.move_gives_check(move, ci);
1036 captureOrPromotion = pos.is_capture_or_promotion(move);
1038 // Step 12. Decide the new search depth
1039 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1041 // Singular extension search. If all moves but one fail low on a search of
1042 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1043 // is singular and should be extended. To verify this we do a reduced search
1044 // on all the other moves but the ttMove, if result is lower than ttValue minus
1045 // a margin then we extend ttMove.
1046 if ( singularExtensionNode
1048 && pos.pl_move_is_legal(move, ci.pinned)
1051 Value ttValue = value_from_tt(tte->value(), ss->ply);
1053 if (abs(ttValue) < VALUE_KNOWN_WIN)
1055 Value rBeta = ttValue - int(depth);
1056 ss->excludedMove = move;
1057 ss->skipNullMove = true;
1058 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1059 ss->skipNullMove = false;
1060 ss->excludedMove = MOVE_NONE;
1061 ss->bestMove = MOVE_NONE;
1067 // Update current move (this must be done after singular extension search)
1068 newDepth = depth - ONE_PLY + ext;
1070 // Step 13. Futility pruning (is omitted in PV nodes)
1072 && !captureOrPromotion
1076 && !is_castle(move))
1078 // Move count based pruning
1079 if ( moveCount >= futility_move_count(depth)
1080 && (!threatMove || !connected_threat(pos, move, threatMove))
1081 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1084 lock_grab(&(sp->lock));
1089 // Value based pruning
1090 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1091 // but fixing this made program slightly weaker.
1092 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1093 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1094 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1096 if (futilityValue < beta)
1100 lock_grab(&(sp->lock));
1101 if (futilityValue > sp->bestValue)
1102 sp->bestValue = bestValue = futilityValue;
1104 else if (futilityValue > bestValue)
1105 bestValue = futilityValue;
1110 // Prune moves with negative SEE at low depths
1111 if ( predictedDepth < 2 * ONE_PLY
1112 && bestValue > VALUE_MATED_IN_PLY_MAX
1113 && pos.see_sign(move) < 0)
1116 lock_grab(&(sp->lock));
1122 // Check for legality only before to do the move
1123 if (!pos.pl_move_is_legal(move, ci.pinned))
1129 ss->currentMove = move;
1130 if (!SpNode && !captureOrPromotion)
1131 movesSearched[playedMoveCount++] = move;
1133 // Step 14. Make the move
1134 pos.do_move(move, st, ci, givesCheck);
1136 // Step 15. Reduced depth search (LMR). If the move fails high will be
1137 // re-searched at full depth.
1138 if ( depth > 3 * ONE_PLY
1140 && !captureOrPromotion
1143 && ss->killers[0] != move
1144 && ss->killers[1] != move)
1146 ss->reduction = reduction<PvNode>(depth, moveCount);
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 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1154 ss->reduction = DEPTH_ZERO;
1157 doFullDepthSearch = !isPvMove;
1159 // Step 16. Full depth search, when LMR is skipped or fails high
1160 if (doFullDepthSearch)
1162 alpha = SpNode ? sp->alpha : alpha;
1163 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1164 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1167 // Only for PV nodes do a full PV search on the first move or after a fail
1168 // high, in the latter case search only if value < beta, otherwise let the
1169 // parent node to fail low with value <= alpha and to try another move.
1170 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1171 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1172 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1174 // Step 17. Undo move
1175 pos.undo_move(move);
1177 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1179 // Step 18. Check for new best move
1182 lock_grab(&(sp->lock));
1183 bestValue = sp->bestValue;
1187 // Finished searching the move. If StopRequest is true, the search
1188 // was aborted because the user interrupted the search or because we
1189 // ran out of time. In this case, the return value of the search cannot
1190 // be trusted, and we don't update the best move and/or PV.
1191 if (RootNode && !Signals.stop)
1193 // Remember searched nodes counts for this move
1194 RootMove* rm = Rml.find(move);
1195 rm->nodes += pos.nodes_searched() - nodes;
1197 // PV move or new best move ?
1198 if (isPvMove || value > alpha)
1202 rm->extract_pv_from_tt(pos);
1204 // We record how often the best move has been changed in each
1205 // iteration. This information is used for time management: When
1206 // the best move changes frequently, we allocate some more time.
1207 if (!isPvMove && MultiPV == 1)
1208 Rml.bestMoveChanges++;
1211 // All other moves but the PV are set to the lowest value, this
1212 // is not a problem when sorting becuase sort is stable and move
1213 // position in the list is preserved, just the PV is pushed up.
1214 rm->score = -VALUE_INFINITE;
1218 if (value > bestValue)
1221 ss->bestMove = move;
1225 && value < beta) // We want always alpha < beta
1228 if (SpNode && !thread.cutoff_occurred())
1230 sp->bestValue = value;
1231 sp->ss->bestMove = move;
1233 sp->is_betaCutoff = (value >= beta);
1237 // Step 19. Check for split
1239 && depth >= Threads.min_split_depth()
1241 && Threads.available_slave_exists(pos.thread())
1243 && !thread.cutoff_occurred())
1244 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1245 threatMove, moveCount, &mp, NT);
1248 // Step 20. Check for mate and stalemate
1249 // All legal moves have been searched and if there are no legal moves, it
1250 // must be mate or stalemate. Note that we can have a false positive in
1251 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1252 // harmless because return value is discarded anyhow in the parent nodes.
1253 // If we are in a singular extension search then return a fail low score.
1254 if (!SpNode && !moveCount)
1255 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1257 // Step 21. Update tables
1258 // If the search is not aborted, update the transposition table,
1259 // history counters, and killer moves.
1260 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1262 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1263 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1264 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1266 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1268 // Update killers and history only for non capture moves that fails high
1269 if ( bestValue >= beta
1270 && !pos.is_capture_or_promotion(move))
1272 if (move != ss->killers[0])
1274 ss->killers[1] = ss->killers[0];
1275 ss->killers[0] = move;
1277 update_history(pos, move, depth, movesSearched, playedMoveCount);
1283 // Here we have the lock still grabbed
1284 sp->is_slave[pos.thread()] = false;
1285 sp->nodes += pos.nodes_searched();
1286 lock_release(&(sp->lock));
1289 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1294 // qsearch() is the quiescence search function, which is called by the main
1295 // search function when the remaining depth is zero (or, to be more precise,
1296 // less than ONE_PLY).
1298 template <NodeType NT>
1299 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1301 const bool PvNode = (NT == PV);
1303 assert(NT == PV || NT == NonPV);
1304 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1305 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1306 assert(PvNode || alpha == beta - 1);
1308 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1312 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1313 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1317 Value oldAlpha = alpha;
1319 ss->bestMove = ss->currentMove = MOVE_NONE;
1320 ss->ply = (ss-1)->ply + 1;
1322 // Check for an instant draw or maximum ply reached
1323 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1326 // Decide whether or not to include checks, this fixes also the type of
1327 // TT entry depth that we are going to use. Note that in qsearch we use
1328 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1329 inCheck = pos.in_check();
1330 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1332 // Transposition table lookup. At PV nodes, we don't use the TT for
1333 // pruning, but only for move ordering.
1334 tte = TT.probe(pos.get_key());
1335 ttMove = (tte ? tte->move() : MOVE_NONE);
1337 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1339 ss->bestMove = ttMove; // Can be MOVE_NONE
1340 return value_from_tt(tte->value(), ss->ply);
1343 // Evaluate the position statically
1346 bestValue = futilityBase = -VALUE_INFINITE;
1347 ss->eval = evalMargin = VALUE_NONE;
1348 enoughMaterial = false;
1354 assert(tte->static_value() != VALUE_NONE);
1356 evalMargin = tte->static_value_margin();
1357 ss->eval = bestValue = tte->static_value();
1360 ss->eval = bestValue = evaluate(pos, evalMargin);
1362 // Stand pat. Return immediately if static value is at least beta
1363 if (bestValue >= beta)
1366 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1371 if (PvNode && bestValue > alpha)
1374 // Futility pruning parameters, not needed when in check
1375 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1376 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1379 // Initialize a MovePicker object for the current position, and prepare
1380 // to search the moves. Because the depth is <= 0 here, only captures,
1381 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1383 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1386 // Loop through the moves until no moves remain or a beta cutoff occurs
1387 while ( bestValue < beta
1388 && (move = mp.get_next_move()) != MOVE_NONE)
1390 assert(is_ok(move));
1392 givesCheck = pos.move_gives_check(move, ci);
1400 && !is_promotion(move)
1401 && !pos.is_passed_pawn_push(move))
1403 futilityValue = futilityBase
1404 + PieceValueEndgame[pos.piece_on(move_to(move))]
1405 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1407 if (futilityValue < beta)
1409 if (futilityValue > bestValue)
1410 bestValue = futilityValue;
1415 // Prune moves with negative or equal SEE
1416 if ( futilityBase < beta
1417 && depth < DEPTH_ZERO
1418 && pos.see(move) <= 0)
1422 // Detect non-capture evasions that are candidate to be pruned
1423 evasionPrunable = !PvNode
1425 && bestValue > VALUE_MATED_IN_PLY_MAX
1426 && !pos.is_capture(move)
1427 && !pos.can_castle(pos.side_to_move());
1429 // Don't search moves with negative SEE values
1431 && (!inCheck || evasionPrunable)
1433 && !is_promotion(move)
1434 && pos.see_sign(move) < 0)
1437 // Don't search useless checks
1442 && !pos.is_capture_or_promotion(move)
1443 && ss->eval + PawnValueMidgame / 4 < beta
1444 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1446 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1447 bestValue = ss->eval + PawnValueMidgame / 4;
1452 // Check for legality only before to do the move
1453 if (!pos.pl_move_is_legal(move, ci.pinned))
1456 // Update current move
1457 ss->currentMove = move;
1459 // Make and search the move
1460 pos.do_move(move, st, ci, givesCheck);
1461 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1462 pos.undo_move(move);
1464 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1467 if (value > bestValue)
1470 ss->bestMove = move;
1474 && value < beta) // We want always alpha < beta
1479 // All legal moves have been searched. A special case: If we're in check
1480 // and no legal moves were found, it is checkmate.
1481 if (inCheck && bestValue == -VALUE_INFINITE)
1482 return value_mated_in(ss->ply);
1484 // Update transposition table
1485 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1486 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1487 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1489 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1491 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1497 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1498 // bestValue is updated only when returning false because in that case move
1501 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1503 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1504 Square from, to, ksq, victimSq;
1507 Value futilityValue, bv = *bestValue;
1509 from = move_from(move);
1511 them = flip(pos.side_to_move());
1512 ksq = pos.king_square(them);
1513 kingAtt = pos.attacks_from<KING>(ksq);
1514 pc = pos.piece_on(from);
1516 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1517 oldAtt = pos.attacks_from(pc, from, occ);
1518 newAtt = pos.attacks_from(pc, to, occ);
1520 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1521 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1523 if (!(b && (b & (b - 1))))
1526 // Rule 2. Queen contact check is very dangerous
1527 if ( type_of(pc) == QUEEN
1528 && bit_is_set(kingAtt, to))
1531 // Rule 3. Creating new double threats with checks
1532 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1536 victimSq = pop_1st_bit(&b);
1537 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1539 // Note that here we generate illegal "double move"!
1540 if ( futilityValue >= beta
1541 && pos.see_sign(make_move(from, victimSq)) >= 0)
1544 if (futilityValue > bv)
1548 // Update bestValue only if check is not dangerous (because we will prune the move)
1554 // connected_moves() tests whether two moves are 'connected' in the sense
1555 // that the first move somehow made the second move possible (for instance
1556 // if the moving piece is the same in both moves). The first move is assumed
1557 // to be the move that was made to reach the current position, while the
1558 // second move is assumed to be a move from the current position.
1560 bool connected_moves(const Position& pos, Move m1, Move m2) {
1562 Square f1, t1, f2, t2;
1569 // Case 1: The moving piece is the same in both moves
1575 // Case 2: The destination square for m2 was vacated by m1
1581 // Case 3: Moving through the vacated square
1582 p2 = pos.piece_on(f2);
1583 if ( piece_is_slider(p2)
1584 && bit_is_set(squares_between(f2, t2), f1))
1587 // Case 4: The destination square for m2 is defended by the moving piece in m1
1588 p1 = pos.piece_on(t1);
1589 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1592 // Case 5: Discovered check, checking piece is the piece moved in m1
1593 ksq = pos.king_square(pos.side_to_move());
1594 if ( piece_is_slider(p1)
1595 && bit_is_set(squares_between(t1, ksq), f2))
1597 Bitboard occ = pos.occupied_squares();
1598 clear_bit(&occ, f2);
1599 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1606 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1607 // "plies to mate from the current ply". Non-mate scores are unchanged.
1608 // The function is called before storing a value to the transposition table.
1610 Value value_to_tt(Value v, int ply) {
1612 if (v >= VALUE_MATE_IN_PLY_MAX)
1615 if (v <= VALUE_MATED_IN_PLY_MAX)
1622 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1623 // the transposition table to a mate score corrected for the current ply.
1625 Value value_from_tt(Value v, int ply) {
1627 if (v >= VALUE_MATE_IN_PLY_MAX)
1630 if (v <= VALUE_MATED_IN_PLY_MAX)
1637 // connected_threat() tests whether it is safe to forward prune a move or if
1638 // is somehow connected to the threat move returned by null search.
1640 bool connected_threat(const Position& pos, Move m, Move threat) {
1643 assert(is_ok(threat));
1644 assert(!pos.is_capture_or_promotion(m));
1645 assert(!pos.is_passed_pawn_push(m));
1647 Square mfrom, mto, tfrom, tto;
1649 mfrom = move_from(m);
1651 tfrom = move_from(threat);
1652 tto = move_to(threat);
1654 // Case 1: Don't prune moves which move the threatened piece
1658 // Case 2: If the threatened piece has value less than or equal to the
1659 // value of the threatening piece, don't prune moves which defend it.
1660 if ( pos.is_capture(threat)
1661 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1662 || type_of(pos.piece_on(tfrom)) == KING)
1663 && pos.move_attacks_square(m, tto))
1666 // Case 3: If the moving piece in the threatened move is a slider, don't
1667 // prune safe moves which block its ray.
1668 if ( piece_is_slider(pos.piece_on(tfrom))
1669 && bit_is_set(squares_between(tfrom, tto), mto)
1670 && pos.see_sign(m) >= 0)
1677 // can_return_tt() returns true if a transposition table score
1678 // can be used to cut-off at a given point in search.
1680 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1682 Value v = value_from_tt(tte->value(), ply);
1684 return ( tte->depth() >= depth
1685 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1686 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1688 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1689 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1693 // refine_eval() returns the transposition table score if
1694 // possible otherwise falls back on static position evaluation.
1696 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1700 Value v = value_from_tt(tte->value(), ply);
1702 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1703 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1710 // update_history() registers a good move that produced a beta-cutoff
1711 // in history and marks as failures all the other moves of that ply.
1713 void update_history(const Position& pos, Move move, Depth depth,
1714 Move movesSearched[], int moveCount) {
1716 Value bonus = Value(int(depth) * int(depth));
1718 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1720 for (int i = 0; i < moveCount - 1; i++)
1722 m = movesSearched[i];
1726 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1731 // current_search_time() returns the number of milliseconds which have passed
1732 // since the beginning of the current search.
1734 int elapsed_time(bool reset) {
1736 static int searchStartTime;
1739 searchStartTime = get_system_time();
1741 return get_system_time() - searchStartTime;
1745 // score_to_uci() converts a value to a string suitable for use with the UCI
1746 // protocol specifications:
1748 // cp <x> The score from the engine's point of view in centipawns.
1749 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1750 // use negative values for y.
1752 string score_to_uci(Value v, Value alpha, Value beta) {
1754 std::stringstream s;
1756 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1757 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1759 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1761 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1767 // speed_to_uci() returns a string with time stats of current search suitable
1768 // to be sent to UCI gui.
1770 string speed_to_uci(int64_t nodes) {
1772 std::stringstream s;
1773 int t = elapsed_time();
1775 s << " nodes " << nodes
1776 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1783 // pv_to_uci() returns a string with information on the current PV line
1784 // formatted according to UCI specification.
1786 string pv_to_uci(const Move pv[], int pvNum, bool chess960) {
1788 std::stringstream s;
1790 s << " multipv " << pvNum << " pv " << set960(chess960);
1792 for ( ; *pv != MOVE_NONE; pv++)
1799 // depth_to_uci() returns a string with information on the current depth and
1800 // seldepth formatted according to UCI specification.
1802 string depth_to_uci(Depth depth) {
1804 std::stringstream s;
1806 // Retrieve max searched depth among threads
1808 for (int i = 0; i < Threads.size(); i++)
1809 if (Threads[i].maxPly > selDepth)
1810 selDepth = Threads[i].maxPly;
1812 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1817 string time_to_string(int millisecs) {
1819 const int MSecMinute = 1000 * 60;
1820 const int MSecHour = 1000 * 60 * 60;
1822 int hours = millisecs / MSecHour;
1823 int minutes = (millisecs % MSecHour) / MSecMinute;
1824 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1826 std::stringstream s;
1831 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1835 string score_to_string(Value v) {
1837 std::stringstream s;
1839 if (v >= VALUE_MATE_IN_PLY_MAX)
1840 s << "#" << (VALUE_MATE - v + 1) / 2;
1841 else if (v <= VALUE_MATED_IN_PLY_MAX)
1842 s << "-#" << (VALUE_MATE + v) / 2;
1844 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1850 // pretty_pv() creates a human-readable string from a position and a PV.
1851 // It is used to write search information to the log file (which is created
1852 // when the UCI parameter "Use Search Log" is "true").
1854 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1856 const int64_t K = 1000;
1857 const int64_t M = 1000000;
1858 const int startColumn = 28;
1859 const size_t maxLength = 80 - startColumn;
1861 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1864 std::stringstream s;
1867 // First print depth, score, time and searched nodes...
1868 s << set960(pos.is_chess960())
1869 << std::setw(2) << depth
1870 << std::setw(8) << score_to_string(value)
1871 << std::setw(8) << time_to_string(time);
1873 if (pos.nodes_searched() < M)
1874 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1875 else if (pos.nodes_searched() < K * M)
1876 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1878 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1880 // ...then print the full PV line in short algebraic notation
1881 while (*m != MOVE_NONE)
1883 san = move_to_san(pos, *m);
1884 length += san.length() + 1;
1886 if (length > maxLength)
1888 length = san.length() + 1;
1889 s << "\n" + string(startColumn, ' ');
1893 pos.do_move(*m++, *st++);
1896 // Restore original position before to leave
1897 while (m != pv) pos.undo_move(*--m);
1903 // When playing with strength handicap choose best move among the MultiPV set
1904 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1906 void do_skill_level(Move* best, Move* ponder) {
1908 assert(MultiPV > 1);
1912 // Rml list is already sorted by score in descending order
1914 int max_s = -VALUE_INFINITE;
1915 int size = std::min(MultiPV, (int)Rml.size());
1916 int max = Rml[0].score;
1917 int var = std::min(max - Rml[size - 1].score, int(PawnValueMidgame));
1918 int wk = 120 - 2 * SkillLevel;
1920 // PRNG sequence should be non deterministic
1921 for (int i = abs(get_system_time() % 50); i > 0; i--)
1922 rk.rand<unsigned>();
1924 // Choose best move. For each move's score we add two terms both dependent
1925 // on wk, one deterministic and bigger for weaker moves, and one random,
1926 // then we choose the move with the resulting highest score.
1927 for (int i = 0; i < size; i++)
1931 // Don't allow crazy blunders even at very low skills
1932 if (i > 0 && Rml[i-1].score > s + EasyMoveMargin)
1935 // This is our magical formula
1936 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
1941 *best = Rml[i].pv[0];
1942 *ponder = Rml[i].pv[1];
1948 /// RootMove and RootMoveList method's definitions
1950 void RootMoveList::init(Position& pos, Move rootMoves[]) {
1953 bestMoveChanges = 0;
1956 // Generate all legal moves and add them to RootMoveList
1957 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
1959 // If we have a rootMoves[] list then verify the move
1960 // is in the list before to add it.
1961 for (sm = rootMoves; *sm && *sm != ml.move(); sm++) {}
1963 if (sm != rootMoves && *sm != ml.move())
1967 rm.pv.push_back(ml.move());
1968 rm.pv.push_back(MOVE_NONE);
1969 rm.score = rm.prevScore = -VALUE_INFINITE;
1975 RootMove* RootMoveList::find(const Move& m, int startIndex) {
1977 for (size_t i = startIndex; i < size(); i++)
1978 if ((*this)[i].pv[0] == m)
1985 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1986 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1987 // allow to always have a ponder move even when we fail high at root and also a
1988 // long PV to print that is important for position analysis.
1990 void RootMove::extract_pv_from_tt(Position& pos) {
1992 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1997 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
2001 pos.do_move(m, *st++);
2003 while ( (tte = TT.probe(pos.get_key())) != NULL
2004 && tte->move() != MOVE_NONE
2005 && pos.is_pseudo_legal(tte->move())
2006 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
2008 && (!pos.is_draw<false>() || ply < 2))
2010 pv.push_back(tte->move());
2011 pos.do_move(tte->move(), *st++);
2014 pv.push_back(MOVE_NONE);
2016 do pos.undo_move(pv[--ply]); while (ply);
2020 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2021 // the PV back into the TT. This makes sure the old PV moves are searched
2022 // first, even if the old TT entries have been overwritten.
2024 void RootMove::insert_pv_in_tt(Position& pos) {
2026 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2029 Value v, m = VALUE_NONE;
2032 assert(pv[0] != MOVE_NONE && pos.is_pseudo_legal(pv[0]));
2038 // Don't overwrite existing correct entries
2039 if (!tte || tte->move() != pv[ply])
2041 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2042 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2044 pos.do_move(pv[ply], *st++);
2046 } while (pv[++ply] != MOVE_NONE);
2048 do pos.undo_move(pv[--ply]); while (ply);
2054 // Thread::idle_loop() is where the thread is parked when it has no work to do.
2055 // The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
2056 // for which the thread is the master.
2058 void Thread::idle_loop(SplitPoint* sp) {
2062 // If we are not searching, wait for a condition to be signaled
2063 // instead of wasting CPU time polling for work.
2066 || (Threads.use_sleeping_threads() && !is_searching))
2068 assert((!sp && threadID) || Threads.use_sleeping_threads());
2070 // Slave thread should exit as soon as do_terminate flag raises
2077 // Grab the lock to avoid races with Thread::wake_up()
2078 lock_grab(&sleepLock);
2080 // If we are master and all slaves have finished don't go to sleep
2081 if (sp && Threads.split_point_finished(sp))
2083 lock_release(&sleepLock);
2087 // Do sleep after retesting sleep conditions under lock protection, in
2088 // particular we need to avoid a deadlock in case a master thread has,
2089 // in the meanwhile, allocated us and sent the wake_up() call before we
2090 // had the chance to grab the lock.
2091 if (do_sleep || !is_searching)
2092 cond_wait(&sleepCond, &sleepLock);
2094 lock_release(&sleepLock);
2097 // If this thread has been assigned work, launch a search
2100 assert(!do_terminate);
2102 // Copy split point position and search stack and call search()
2103 Stack ss[PLY_MAX_PLUS_2];
2104 SplitPoint* tsp = splitPoint;
2105 Position pos(*tsp->pos, threadID);
2107 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
2110 if (tsp->nodeType == Root)
2111 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2112 else if (tsp->nodeType == PV)
2113 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2114 else if (tsp->nodeType == NonPV)
2115 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2119 assert(is_searching);
2121 is_searching = false;
2123 // Wake up master thread so to allow it to return from the idle loop in
2124 // case we are the last slave of the split point.
2125 if ( Threads.use_sleeping_threads()
2126 && threadID != tsp->master
2127 && !Threads[tsp->master].is_searching)
2128 Threads[tsp->master].wake_up();
2131 // If this thread is the master of a split point and all slaves have
2132 // finished their work at this split point, return from the idle loop.
2133 if (sp && Threads.split_point_finished(sp))
2135 // Because sp->is_slave[] is reset under lock protection,
2136 // be sure sp->lock has been released before to return.
2137 lock_grab(&(sp->lock));
2138 lock_release(&(sp->lock));
2145 // do_timer_event() is called by the timer thread when the timer triggers
2147 void do_timer_event() {
2149 static int lastInfoTime;
2150 int e = elapsed_time();
2152 // Print debug information every one second
2153 if (!lastInfoTime || get_system_time() - lastInfoTime >= 1000)
2155 lastInfoTime = get_system_time();
2158 dbg_print_hit_rate();
2161 // Should we stop the search?
2165 bool stillAtFirstMove = Signals.firstRootMove
2166 && !Signals.failedLowAtRoot
2167 && e > TimeMgr.available_time();
2169 bool noMoreTime = e > TimeMgr.maximum_time()
2170 || stillAtFirstMove;
2172 if ( (Limits.useTimeManagement() && noMoreTime)
2173 || (Limits.maxTime && e >= Limits.maxTime)
2174 /* missing nodes limit */ ) // FIXME
2175 Signals.stop = true;