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]; }
98 // Maximum depth for razoring
99 const Depth RazorDepth = 4 * ONE_PLY;
101 // Dynamic razoring margin based on depth
102 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
104 // Maximum depth for use of dynamic threat detection when null move fails low
105 const Depth ThreatDepth = 5 * ONE_PLY;
107 // Minimum depth for use of internal iterative deepening
108 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
110 // At Non-PV nodes we do an internal iterative deepening search
111 // when the static evaluation is bigger then beta - IIDMargin.
112 const Value IIDMargin = Value(0x100);
114 // Minimum depth for use of singular extension
115 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
117 // Futility margin for quiescence search
118 const Value FutilityMarginQS = Value(0x80);
120 // Futility lookup tables (initialized at startup) and their access functions
121 Value FutilityMargins[16][64]; // [depth][moveNumber]
122 int FutilityMoveCounts[32]; // [depth]
124 inline Value futility_margin(Depth d, int mn) {
126 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
127 : 2 * VALUE_INFINITE;
130 inline int futility_move_count(Depth d) {
132 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
135 // Reduction lookup tables (initialized at startup) and their access function
136 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
138 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
140 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
143 // Easy move margin. An easy move candidate must be at least this much
144 // better than the second best move.
145 const Value EasyMoveMargin = Value(0x150);
148 /// Namespace variables
154 size_t MultiPV, UCIMultiPV, MultiPVIdx;
156 // Time management variables
159 // Skill level adjustment
161 bool SkillLevelEnabled;
169 Move id_loop(Position& pos, Move rootMoves[], Move* ponderMove);
171 template <NodeType NT>
172 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
174 template <NodeType NT>
175 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
177 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
178 bool connected_moves(const Position& pos, Move m1, Move m2);
179 Value value_to_tt(Value v, int ply);
180 Value value_from_tt(Value v, int ply);
181 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
182 bool connected_threat(const Position& pos, Move m, Move threat);
183 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
184 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
185 void do_skill_level(Move* best, Move* ponder);
187 int elapsed_time(bool reset = false);
188 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
189 string speed_to_uci(int64_t nodes);
190 string pv_to_uci(const Move pv[], int pvNum, bool chess960);
191 string pretty_pv(Position& pos, int depth, Value score, int time, Move pv[]);
192 string depth_to_uci(Depth depth);
194 // MovePickerExt template class extends MovePicker and allows to choose at compile
195 // time the proper moves source according to the type of node. In the default case
196 // we simply create and use a standard MovePicker object.
197 template<bool SpNode> struct MovePickerExt : public MovePicker {
199 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
200 : MovePicker(p, ttm, d, h, ss, b) {}
203 // In case of a SpNode we use split point's shared MovePicker object as moves source
204 template<> struct MovePickerExt<true> : public MovePicker {
206 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
207 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
209 Move get_next_move() { return mp->get_next_move(); }
213 // Overload operator<<() to make it easier to print moves in a coordinate
214 // notation compatible with UCI protocol.
215 std::ostream& operator<<(std::ostream& os, Move m) {
217 bool chess960 = (os.iword(0) != 0); // See set960()
218 return os << move_to_uci(m, chess960);
221 // When formatting a move for std::cout we must know if we are in Chess960
222 // or not. To keep using the handy operator<<() on the move the trick is to
223 // embed this flag in the stream itself. Function-like named enum set960 is
224 // used as a custom manipulator and the stream internal general-purpose array,
225 // accessed through ios_base::iword(), is used to pass the flag to the move's
226 // operator<<() that will read it to properly format castling moves.
229 std::ostream& operator<< (std::ostream& os, const set960& f) {
231 os.iword(0) = int(f);
235 // is_dangerous() checks whether a move belongs to some classes of known
236 // 'dangerous' moves so that we avoid to prune it.
237 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
239 // Test for a pawn pushed to 7th or a passed pawn move
240 if (type_of(pos.piece_on(move_from(m))) == PAWN)
242 Color c = pos.side_to_move();
243 if ( relative_rank(c, move_to(m)) == RANK_7
244 || pos.pawn_is_passed(c, move_to(m)))
248 // Test for a capture that triggers a pawn endgame
249 if ( captureOrPromotion
250 && type_of(pos.piece_on(move_to(m))) != PAWN
251 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
252 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
262 /// init_search() is called during startup to initialize various lookup tables
264 void Search::init() {
266 int d; // depth (ONE_PLY == 2)
267 int hd; // half depth (ONE_PLY == 1)
270 // Init reductions array
271 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
273 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
274 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
275 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
276 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
279 // Init futility margins array
280 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
281 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
283 // Init futility move count array
284 for (d = 0; d < 32; d++)
285 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
289 /// perft() is our utility to verify move generation. All the leaf nodes up to
290 /// the given depth are generated and counted and the sum returned.
292 int64_t Search::perft(Position& pos, Depth depth) {
297 // Generate all legal moves
298 MoveList<MV_LEGAL> ml(pos);
300 // If we are at the last ply we don't need to do and undo
301 // the moves, just to count them.
302 if (depth <= ONE_PLY)
305 // Loop through all legal moves
307 for ( ; !ml.end(); ++ml)
309 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
310 sum += perft(pos, depth - ONE_PLY);
311 pos.undo_move(ml.move());
317 /// think() is the external interface to Stockfish's search, and is called by the
318 /// main thread when the program receives the UCI 'go' command. It searches from
319 /// RootPosition and at the end prints the "bestmove" to output.
321 void Search::think() {
323 static Book book; // Defined static to initialize the PRNG only once
325 Position& pos = RootPosition;
327 // Reset elapsed search time
330 // Set output stream mode: normal or chess960. Castling notation is different
331 cout << set960(pos.is_chess960());
333 // Look for a book move
334 if (Options["OwnBook"].value<bool>())
336 if (Options["Book File"].value<string>() != book.name())
337 book.open(Options["Book File"].value<string>());
339 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
340 if (bookMove != MOVE_NONE)
342 if (!Signals.stop && (Limits.ponder || Limits.infinite))
343 Threads.wait_for_stop_or_ponderhit();
345 cout << "bestmove " << bookMove << endl;
350 // Read UCI options: GUI could change UCI parameters during the game
351 read_evaluation_uci_options(pos.side_to_move());
352 Threads.read_uci_options();
354 // Set a new TT size if changed
355 TT.set_size(Options["Hash"].value<int>());
357 if (Options["Clear Hash"].value<bool>())
359 Options["Clear Hash"].set_value("false");
363 UCIMultiPV = Options["MultiPV"].value<size_t>();
364 SkillLevel = Options["Skill Level"].value<size_t>();
366 // Do we have to play with skill handicap? In this case enable MultiPV that
367 // we will use behind the scenes to retrieve a set of possible moves.
368 SkillLevelEnabled = (SkillLevel < 20);
369 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
371 // Write current search header to log file
372 if (Options["Use Search Log"].value<bool>())
374 Log log(Options["Search Log Filename"].value<string>());
375 log << "\nSearching: " << pos.to_fen()
376 << "\ninfinite: " << Limits.infinite
377 << " ponder: " << Limits.ponder
378 << " time: " << Limits.time
379 << " increment: " << Limits.increment
380 << " moves to go: " << Limits.movesToGo
384 // Wake up needed threads and reset maxPly counter
385 for (int i = 0; i < Threads.size(); i++)
387 Threads[i].maxPly = 0;
388 Threads[i].wake_up();
391 // Set best timer interval to avoid lagging under time pressure. Timer is
392 // used to check for remaining available thinking time.
393 TimeMgr.init(Limits, pos.startpos_ply_counter());
395 if (TimeMgr.available_time())
396 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
398 Threads.set_timer(100);
400 // We're ready to start thinking. Call the iterative deepening loop function
401 Move ponderMove = MOVE_NONE;
402 Move bestMove = id_loop(pos, &RootMoves[0], &ponderMove);
404 // Stop timer, no need to check for available time any more
405 Threads.set_timer(0);
407 // This makes all the slave threads to go to sleep, if not already sleeping
410 // Write current search final statistics to log file
411 if (Options["Use Search Log"].value<bool>())
413 int e = elapsed_time();
415 Log log(Options["Search Log Filename"].value<string>());
416 log << "Nodes: " << pos.nodes_searched()
417 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
418 << "\nBest move: " << move_to_san(pos, bestMove);
421 pos.do_move(bestMove, st);
422 log << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
423 pos.undo_move(bestMove); // Return from think() with unchanged position
426 // When we reach max depth we arrive here even without a StopRequest, but if
427 // we are pondering or in infinite search, we shouldn't print the best move
428 // before we are told to do so.
429 if (!Signals.stop && (Limits.ponder || Limits.infinite))
430 Threads.wait_for_stop_or_ponderhit();
432 // Could be MOVE_NONE when searching on a stalemate position
433 cout << "bestmove " << bestMove;
435 // UCI protol is not clear on allowing sending an empty ponder move, instead
436 // it is clear that ponder move is optional. So skip it if empty.
437 if (ponderMove != MOVE_NONE)
438 cout << " ponder " << ponderMove;
446 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
447 // with increasing depth until the allocated thinking time has been consumed,
448 // user stops the search, or the maximum search depth is reached.
450 Move id_loop(Position& pos, Move rootMoves[], Move* ponderMove) {
452 Stack ss[PLY_MAX_PLUS_2];
453 Value bestValues[PLY_MAX_PLUS_2];
454 int bestMoveChanges[PLY_MAX_PLUS_2];
455 int depth, aspirationDelta;
456 Value bestValue, alpha, beta;
457 Move bestMove, skillBest, skillPonder;
458 bool bestMoveNeverChanged = true;
460 // Initialize stuff before a new search
461 memset(ss, 0, 4 * sizeof(Stack));
464 *ponderMove = bestMove = skillBest = skillPonder = MOVE_NONE;
465 depth = aspirationDelta = 0;
466 bestValue = alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
467 ss->currentMove = MOVE_NULL; // Hack to skip update gains
469 // Moves to search are verified and copied
470 Rml.init(pos, rootMoves);
472 // Handle special case of searching on a mate/stalemate position
475 cout << "info" << depth_to_uci(DEPTH_ZERO)
476 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
481 // Iterative deepening loop until requested to stop or target depth reached
482 while (!Signals.stop && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
484 // Save now last iteration's scores, before Rml moves are reordered
485 for (size_t i = 0; i < Rml.size(); i++)
486 Rml[i].prevScore = Rml[i].score;
488 Rml.bestMoveChanges = 0;
490 // MultiPV loop. We perform a full root search for each PV line
491 for (MultiPVIdx = 0; MultiPVIdx < std::min(MultiPV, Rml.size()); MultiPVIdx++)
493 // Calculate dynamic aspiration window based on previous iterations
494 if (depth >= 5 && abs(Rml[MultiPVIdx].prevScore) < VALUE_KNOWN_WIN)
496 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
497 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
499 aspirationDelta = std::min(std::max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
500 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
502 alpha = std::max(Rml[MultiPVIdx].prevScore - aspirationDelta, -VALUE_INFINITE);
503 beta = std::min(Rml[MultiPVIdx].prevScore + aspirationDelta, VALUE_INFINITE);
507 alpha = -VALUE_INFINITE;
508 beta = VALUE_INFINITE;
511 // Start with a small aspiration window and, in case of fail high/low,
512 // research with bigger window until not failing high/low anymore.
514 // Search starts from ss+1 to allow referencing (ss-1). This is
515 // needed by update gains and ss copy when splitting at Root.
516 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
518 // Bring to front the best move. It is critical that sorting is
519 // done with a stable algorithm because all the values but the first
520 // and eventually the new best one are set to -VALUE_INFINITE and
521 // we want to keep the same order for all the moves but the new
522 // PV that goes to the front. Note that in case of MultiPV search
523 // the already searched PV lines are preserved.
524 sort<RootMove>(Rml.begin() + MultiPVIdx, Rml.end());
526 // In case we have found an exact score and we are going to leave
527 // the fail high/low loop then reorder the PV moves, otherwise
528 // leave the last PV move in its position so to be searched again.
529 // Of course this is needed only in MultiPV search.
530 if (MultiPVIdx && bestValue > alpha && bestValue < beta)
531 sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIdx);
533 // Write PV back to transposition table in case the relevant entries
534 // have been overwritten during the search.
535 for (size_t i = 0; i <= MultiPVIdx; i++)
536 Rml[i].insert_pv_in_tt(pos);
538 // If search has been stopped exit the aspiration window loop,
539 // note that sorting and writing PV back to TT is safe becuase
540 // Rml is still valid, although refers to the previous iteration.
544 // Send full PV info to GUI if we are going to leave the loop or
545 // if we have a fail high/low and we are deep in the search. UCI
546 // protocol requires to send all the PV lines also if are still
547 // to be searched and so refer to the previous search's score.
548 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
549 for (size_t i = 0; i < std::min(UCIMultiPV, Rml.size()); i++)
551 bool updated = (i <= MultiPVIdx);
553 if (depth == 1 && !updated)
556 Depth d = (updated ? depth : depth - 1) * ONE_PLY;
557 Value s = (updated ? Rml[i].score : Rml[i].prevScore);
561 << (i == MultiPVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
562 << speed_to_uci(pos.nodes_searched())
563 << pv_to_uci(&Rml[i].pv[0], i + 1, pos.is_chess960())
567 // In case of failing high/low increase aspiration window and
568 // research, otherwise exit the fail high/low loop.
569 if (bestValue >= beta)
571 beta = std::min(beta + aspirationDelta, VALUE_INFINITE);
572 aspirationDelta += aspirationDelta / 2;
574 else if (bestValue <= alpha)
576 Signals.failedLowAtRoot = true;
577 Signals.stopOnPonderhit = false;
579 alpha = std::max(alpha - aspirationDelta, -VALUE_INFINITE);
580 aspirationDelta += aspirationDelta / 2;
585 } while (abs(bestValue) < VALUE_KNOWN_WIN);
588 // Collect info about search result
589 bestMove = Rml[0].pv[0];
590 *ponderMove = Rml[0].pv[1];
591 bestValues[depth] = bestValue;
592 bestMoveChanges[depth] = Rml.bestMoveChanges;
594 // Skills: Do we need to pick now the best and the ponder moves ?
595 if (SkillLevelEnabled && depth == 1 + SkillLevel)
596 do_skill_level(&skillBest, &skillPonder);
598 if (Options["Use Search Log"].value<bool>())
600 Log log(Options["Search Log Filename"].value<string>());
601 log << pretty_pv(pos, depth, bestValue, elapsed_time(), &Rml[0].pv[0]) << endl;
604 // Filter out startup noise when monitoring best move stability
605 if (depth > 2 && bestMoveChanges[depth])
606 bestMoveNeverChanged = false;
608 // Do we have time for the next iteration? Can we stop searching now?
609 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
611 bool stop = false; // Local variable instead of the volatile Signals.stop
613 // Take in account some extra time if the best move has changed
614 if (depth > 4 && depth < 50)
615 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
617 // Stop search if most of available time is already consumed. We probably don't
618 // have enough time to search the first move at the next iteration anyway.
619 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
622 // Stop search early if one move seems to be much better than others
625 && ( bestMoveNeverChanged
626 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
628 Value rBeta = bestValue - EasyMoveMargin;
629 (ss+1)->excludedMove = bestMove;
630 (ss+1)->skipNullMove = true;
631 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
632 (ss+1)->skipNullMove = false;
633 (ss+1)->excludedMove = MOVE_NONE;
641 // If we are allowed to ponder do not stop the search now but
642 // keep pondering until GUI sends "ponderhit" or "stop".
644 Signals.stopOnPonderhit = true;
651 // When using skills overwrite best and ponder moves with the sub-optimal ones
652 if (SkillLevelEnabled)
654 if (skillBest == MOVE_NONE) // Still unassigned ?
655 do_skill_level(&skillBest, &skillPonder);
657 bestMove = skillBest;
658 *ponderMove = skillPonder;
665 // search<>() is the main search function for both PV and non-PV nodes and for
666 // normal and SplitPoint nodes. When called just after a split point the search
667 // is simpler because we have already probed the hash table, done a null move
668 // search, and searched the first move before splitting, we don't have to repeat
669 // all this work again. We also don't need to store anything to the hash table
670 // here: This is taken care of after we return from the split point.
672 template <NodeType NT>
673 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
675 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
676 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
677 const bool RootNode = (NT == Root || NT == SplitPointRoot);
679 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
680 assert(beta > alpha && beta <= VALUE_INFINITE);
681 assert(PvNode || alpha == beta - 1);
682 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
684 Move movesSearched[MAX_MOVES];
689 Move ttMove, move, excludedMove, threatMove;
692 Value bestValue, value, oldAlpha;
693 Value refinedValue, nullValue, futilityBase, futilityValue;
694 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
695 bool captureOrPromotion, dangerous, doFullDepthSearch;
696 int moveCount = 0, playedMoveCount = 0;
697 Thread& thread = Threads[pos.thread()];
698 SplitPoint* sp = NULL;
700 refinedValue = bestValue = value = -VALUE_INFINITE;
702 inCheck = pos.in_check();
703 ss->ply = (ss-1)->ply + 1;
705 // Used to send selDepth info to GUI
706 if (PvNode && thread.maxPly < ss->ply)
707 thread.maxPly = ss->ply;
709 // Step 1. Initialize node
712 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
713 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
714 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
720 ttMove = excludedMove = MOVE_NONE;
721 threatMove = sp->threatMove;
722 goto split_point_start;
725 // Step 2. Check for aborted search and immediate draw
727 || pos.is_draw<false>()
728 || ss->ply > PLY_MAX) && !RootNode)
731 // Step 3. Mate distance pruning
734 alpha = std::max(value_mated_in(ss->ply), alpha);
735 beta = std::min(value_mate_in(ss->ply+1), beta);
740 // Step 4. Transposition table lookup
741 // We don't want the score of a partial search to overwrite a previous full search
742 // TT value, so we use a different position key in case of an excluded move.
743 excludedMove = ss->excludedMove;
744 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
745 tte = TT.probe(posKey);
746 ttMove = RootNode ? Rml[MultiPVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
748 // At PV nodes we check for exact scores, while at non-PV nodes we check for
749 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
750 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
751 // we should also update RootMoveList to avoid bogus output.
752 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
753 : can_return_tt(tte, depth, beta, ss->ply)))
756 ss->bestMove = move = ttMove; // Can be MOVE_NONE
757 value = value_from_tt(tte->value(), ss->ply);
761 && !pos.is_capture_or_promotion(move)
762 && move != ss->killers[0])
764 ss->killers[1] = ss->killers[0];
765 ss->killers[0] = move;
770 // Step 5. Evaluate the position statically and update parent's gain statistics
772 ss->eval = ss->evalMargin = VALUE_NONE;
775 assert(tte->static_value() != VALUE_NONE);
777 ss->eval = tte->static_value();
778 ss->evalMargin = tte->static_value_margin();
779 refinedValue = refine_eval(tte, ss->eval, ss->ply);
783 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
784 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
787 // Update gain for the parent non-capture move given the static position
788 // evaluation before and after the move.
789 if ( (move = (ss-1)->currentMove) != MOVE_NULL
790 && (ss-1)->eval != VALUE_NONE
791 && ss->eval != VALUE_NONE
792 && pos.captured_piece_type() == PIECE_TYPE_NONE
793 && !is_special(move))
795 Square to = move_to(move);
796 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
799 // Step 6. Razoring (is omitted in PV nodes)
801 && depth < RazorDepth
803 && refinedValue + razor_margin(depth) < beta
804 && ttMove == MOVE_NONE
805 && abs(beta) < VALUE_MATE_IN_PLY_MAX
806 && !pos.has_pawn_on_7th(pos.side_to_move()))
808 Value rbeta = beta - razor_margin(depth);
809 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
811 // Logically we should return (v + razor_margin(depth)), but
812 // surprisingly this did slightly weaker in tests.
816 // Step 7. Static null move pruning (is omitted in PV nodes)
817 // We're betting that the opponent doesn't have a move that will reduce
818 // the score by more than futility_margin(depth) if we do a null move.
821 && depth < RazorDepth
823 && refinedValue - futility_margin(depth, 0) >= beta
824 && abs(beta) < VALUE_MATE_IN_PLY_MAX
825 && pos.non_pawn_material(pos.side_to_move()))
826 return refinedValue - futility_margin(depth, 0);
828 // Step 8. Null move search with verification search (is omitted in PV nodes)
833 && refinedValue >= beta
834 && abs(beta) < VALUE_MATE_IN_PLY_MAX
835 && pos.non_pawn_material(pos.side_to_move()))
837 ss->currentMove = MOVE_NULL;
839 // Null move dynamic reduction based on depth
840 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
842 // Null move dynamic reduction based on value
843 if (refinedValue - PawnValueMidgame > beta)
846 pos.do_null_move<true>(st);
847 (ss+1)->skipNullMove = true;
848 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
849 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
850 (ss+1)->skipNullMove = false;
851 pos.do_null_move<false>(st);
853 if (nullValue >= beta)
855 // Do not return unproven mate scores
856 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
859 if (depth < 6 * ONE_PLY)
862 // Do verification search at high depths
863 ss->skipNullMove = true;
864 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
865 ss->skipNullMove = false;
872 // The null move failed low, which means that we may be faced with
873 // some kind of threat. If the previous move was reduced, check if
874 // the move that refuted the null move was somehow connected to the
875 // move which was reduced. If a connection is found, return a fail
876 // low score (which will cause the reduced move to fail high in the
877 // parent node, which will trigger a re-search with full depth).
878 threatMove = (ss+1)->bestMove;
880 if ( depth < ThreatDepth
882 && threatMove != MOVE_NONE
883 && connected_moves(pos, (ss-1)->currentMove, threatMove))
888 // Step 9. ProbCut (is omitted in PV nodes)
889 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
890 // and a reduced search returns a value much above beta, we can (almost) safely
891 // prune the previous move.
893 && depth >= RazorDepth + ONE_PLY
896 && excludedMove == MOVE_NONE
897 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
899 Value rbeta = beta + 200;
900 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
902 assert(rdepth >= ONE_PLY);
904 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
907 while ((move = mp.get_next_move()) != MOVE_NONE)
908 if (pos.pl_move_is_legal(move, ci.pinned))
910 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
911 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
918 // Step 10. Internal iterative deepening
919 if ( depth >= IIDDepth[PvNode]
920 && ttMove == MOVE_NONE
921 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
923 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
925 ss->skipNullMove = true;
926 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
927 ss->skipNullMove = false;
929 tte = TT.probe(posKey);
930 ttMove = tte ? tte->move() : MOVE_NONE;
933 split_point_start: // At split points actual search starts from here
935 // Initialize a MovePicker object for the current position
936 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
938 ss->bestMove = MOVE_NONE;
939 futilityBase = ss->eval + ss->evalMargin;
940 singularExtensionNode = !RootNode
942 && depth >= SingularExtensionDepth[PvNode]
943 && ttMove != MOVE_NONE
944 && !excludedMove // Do not allow recursive singular extension search
945 && (tte->type() & VALUE_TYPE_LOWER)
946 && tte->depth() >= depth - 3 * ONE_PLY;
949 lock_grab(&(sp->lock));
950 bestValue = sp->bestValue;
953 // Step 11. Loop through moves
954 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
955 while ( bestValue < beta
956 && (move = mp.get_next_move()) != MOVE_NONE
957 && !thread.cutoff_occurred())
961 if (move == excludedMove)
964 // At root obey the "searchmoves" option and skip moves not listed in Root
965 // Move List, as a consequence any illegal move is also skipped. In MultiPV
966 // mode we also skip PV moves which have been already searched.
967 if (RootNode && !Rml.find(move, MultiPVIdx))
970 // At PV and SpNode nodes we want all moves to be legal since the beginning
971 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
976 moveCount = ++sp->moveCount;
977 lock_release(&(sp->lock));
984 // This is used by time management
985 Signals.firstRootMove = (moveCount == 1);
987 // Save the current node count before the move is searched
988 nodes = pos.nodes_searched();
990 // For long searches send current move info to GUI
991 if (pos.thread() == 0 && elapsed_time() > 2000)
992 cout << "info" << depth_to_uci(depth)
993 << " currmove " << move
994 << " currmovenumber " << moveCount + MultiPVIdx << endl;
997 isPvMove = (PvNode && moveCount <= 1);
998 captureOrPromotion = pos.is_capture_or_promotion(move);
999 givesCheck = pos.move_gives_check(move, ci);
1000 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
1003 // Step 12. Extend checks and, in PV nodes, also dangerous moves
1004 if (PvNode && dangerous)
1007 else if (givesCheck && pos.see_sign(move) >= 0)
1008 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
1010 // Singular extension search. If all moves but one fail low on a search of
1011 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1012 // is singular and should be extended. To verify this we do a reduced search
1013 // on all the other moves but the ttMove, if result is lower than ttValue minus
1014 // a margin then we extend ttMove.
1015 if ( singularExtensionNode
1018 && pos.pl_move_is_legal(move, ci.pinned))
1020 Value ttValue = value_from_tt(tte->value(), ss->ply);
1022 if (abs(ttValue) < VALUE_KNOWN_WIN)
1024 Value rBeta = ttValue - int(depth);
1025 ss->excludedMove = move;
1026 ss->skipNullMove = true;
1027 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1028 ss->skipNullMove = false;
1029 ss->excludedMove = MOVE_NONE;
1030 ss->bestMove = MOVE_NONE;
1036 // Update current move (this must be done after singular extension search)
1037 newDepth = depth - ONE_PLY + ext;
1039 // Step 13. Futility pruning (is omitted in PV nodes)
1041 && !captureOrPromotion
1045 && !is_castle(move))
1047 // Move count based pruning
1048 if ( moveCount >= futility_move_count(depth)
1049 && (!threatMove || !connected_threat(pos, move, threatMove))
1050 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1053 lock_grab(&(sp->lock));
1058 // Value based pruning
1059 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1060 // but fixing this made program slightly weaker.
1061 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1062 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1063 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1065 if (futilityValue < beta)
1069 lock_grab(&(sp->lock));
1070 if (futilityValue > sp->bestValue)
1071 sp->bestValue = bestValue = futilityValue;
1073 else if (futilityValue > bestValue)
1074 bestValue = futilityValue;
1079 // Prune moves with negative SEE at low depths
1080 if ( predictedDepth < 2 * ONE_PLY
1081 && bestValue > VALUE_MATED_IN_PLY_MAX
1082 && pos.see_sign(move) < 0)
1085 lock_grab(&(sp->lock));
1091 // Check for legality only before to do the move
1092 if (!pos.pl_move_is_legal(move, ci.pinned))
1098 ss->currentMove = move;
1099 if (!SpNode && !captureOrPromotion)
1100 movesSearched[playedMoveCount++] = move;
1102 // Step 14. Make the move
1103 pos.do_move(move, st, ci, givesCheck);
1105 // Step 15. Reduced depth search (LMR). If the move fails high will be
1106 // re-searched at full depth.
1107 if ( depth > 3 * ONE_PLY
1109 && !captureOrPromotion
1112 && ss->killers[0] != move
1113 && ss->killers[1] != move)
1115 ss->reduction = reduction<PvNode>(depth, moveCount);
1116 Depth d = newDepth - ss->reduction;
1117 alpha = SpNode ? sp->alpha : alpha;
1119 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1120 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1122 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1123 ss->reduction = DEPTH_ZERO;
1126 doFullDepthSearch = !isPvMove;
1128 // Step 16. Full depth search, when LMR is skipped or fails high
1129 if (doFullDepthSearch)
1131 alpha = SpNode ? sp->alpha : alpha;
1132 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1133 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1136 // Only for PV nodes do a full PV search on the first move or after a fail
1137 // high, in the latter case search only if value < beta, otherwise let the
1138 // parent node to fail low with value <= alpha and to try another move.
1139 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1140 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1141 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1143 // Step 17. Undo move
1144 pos.undo_move(move);
1146 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1148 // Step 18. Check for new best move
1151 lock_grab(&(sp->lock));
1152 bestValue = sp->bestValue;
1156 // Finished searching the move. If StopRequest is true, the search
1157 // was aborted because the user interrupted the search or because we
1158 // ran out of time. In this case, the return value of the search cannot
1159 // be trusted, and we don't update the best move and/or PV.
1160 if (RootNode && !Signals.stop)
1162 // Remember searched nodes counts for this move
1163 RootMove* rm = Rml.find(move);
1164 rm->nodes += pos.nodes_searched() - nodes;
1166 // PV move or new best move ?
1167 if (isPvMove || value > alpha)
1171 rm->extract_pv_from_tt(pos);
1173 // We record how often the best move has been changed in each
1174 // iteration. This information is used for time management: When
1175 // the best move changes frequently, we allocate some more time.
1176 if (!isPvMove && MultiPV == 1)
1177 Rml.bestMoveChanges++;
1180 // All other moves but the PV are set to the lowest value, this
1181 // is not a problem when sorting becuase sort is stable and move
1182 // position in the list is preserved, just the PV is pushed up.
1183 rm->score = -VALUE_INFINITE;
1187 if (value > bestValue)
1190 ss->bestMove = move;
1194 && value < beta) // We want always alpha < beta
1197 if (SpNode && !thread.cutoff_occurred())
1199 sp->bestValue = value;
1200 sp->ss->bestMove = move;
1202 sp->is_betaCutoff = (value >= beta);
1206 // Step 19. Check for split
1208 && depth >= Threads.min_split_depth()
1210 && Threads.available_slave_exists(pos.thread())
1212 && !thread.cutoff_occurred())
1213 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1214 threatMove, moveCount, &mp, NT);
1217 // Step 20. Check for mate and stalemate
1218 // All legal moves have been searched and if there are no legal moves, it
1219 // must be mate or stalemate. Note that we can have a false positive in
1220 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1221 // harmless because return value is discarded anyhow in the parent nodes.
1222 // If we are in a singular extension search then return a fail low score.
1223 if (!SpNode && !moveCount)
1224 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1226 // Step 21. Update tables
1227 // If the search is not aborted, update the transposition table,
1228 // history counters, and killer moves.
1229 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1231 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1232 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1233 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1235 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1237 // Update killers and history only for non capture moves that fails high
1238 if ( bestValue >= beta
1239 && !pos.is_capture_or_promotion(move))
1241 if (move != ss->killers[0])
1243 ss->killers[1] = ss->killers[0];
1244 ss->killers[0] = move;
1246 update_history(pos, move, depth, movesSearched, playedMoveCount);
1252 // Here we have the lock still grabbed
1253 sp->is_slave[pos.thread()] = false;
1254 sp->nodes += pos.nodes_searched();
1255 lock_release(&(sp->lock));
1258 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1263 // qsearch() is the quiescence search function, which is called by the main
1264 // search function when the remaining depth is zero (or, to be more precise,
1265 // less than ONE_PLY).
1267 template <NodeType NT>
1268 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1270 const bool PvNode = (NT == PV);
1272 assert(NT == PV || NT == NonPV);
1273 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1274 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1275 assert(PvNode || alpha == beta - 1);
1277 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1281 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1282 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1286 Value oldAlpha = alpha;
1288 ss->bestMove = ss->currentMove = MOVE_NONE;
1289 ss->ply = (ss-1)->ply + 1;
1291 // Check for an instant draw or maximum ply reached
1292 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1295 // Decide whether or not to include checks, this fixes also the type of
1296 // TT entry depth that we are going to use. Note that in qsearch we use
1297 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1298 inCheck = pos.in_check();
1299 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1301 // Transposition table lookup. At PV nodes, we don't use the TT for
1302 // pruning, but only for move ordering.
1303 tte = TT.probe(pos.get_key());
1304 ttMove = (tte ? tte->move() : MOVE_NONE);
1306 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1308 ss->bestMove = ttMove; // Can be MOVE_NONE
1309 return value_from_tt(tte->value(), ss->ply);
1312 // Evaluate the position statically
1315 bestValue = futilityBase = -VALUE_INFINITE;
1316 ss->eval = evalMargin = VALUE_NONE;
1317 enoughMaterial = false;
1323 assert(tte->static_value() != VALUE_NONE);
1325 evalMargin = tte->static_value_margin();
1326 ss->eval = bestValue = tte->static_value();
1329 ss->eval = bestValue = evaluate(pos, evalMargin);
1331 // Stand pat. Return immediately if static value is at least beta
1332 if (bestValue >= beta)
1335 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1340 if (PvNode && bestValue > alpha)
1343 // Futility pruning parameters, not needed when in check
1344 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1345 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1348 // Initialize a MovePicker object for the current position, and prepare
1349 // to search the moves. Because the depth is <= 0 here, only captures,
1350 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1352 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1355 // Loop through the moves until no moves remain or a beta cutoff occurs
1356 while ( bestValue < beta
1357 && (move = mp.get_next_move()) != MOVE_NONE)
1359 assert(is_ok(move));
1361 givesCheck = pos.move_gives_check(move, ci);
1369 && !is_promotion(move)
1370 && !pos.is_passed_pawn_push(move))
1372 futilityValue = futilityBase
1373 + PieceValueEndgame[pos.piece_on(move_to(move))]
1374 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1376 if (futilityValue < beta)
1378 if (futilityValue > bestValue)
1379 bestValue = futilityValue;
1384 // Prune moves with negative or equal SEE
1385 if ( futilityBase < beta
1386 && depth < DEPTH_ZERO
1387 && pos.see(move) <= 0)
1391 // Detect non-capture evasions that are candidate to be pruned
1392 evasionPrunable = !PvNode
1394 && bestValue > VALUE_MATED_IN_PLY_MAX
1395 && !pos.is_capture(move)
1396 && !pos.can_castle(pos.side_to_move());
1398 // Don't search moves with negative SEE values
1400 && (!inCheck || evasionPrunable)
1402 && !is_promotion(move)
1403 && pos.see_sign(move) < 0)
1406 // Don't search useless checks
1411 && !pos.is_capture_or_promotion(move)
1412 && ss->eval + PawnValueMidgame / 4 < beta
1413 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1415 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1416 bestValue = ss->eval + PawnValueMidgame / 4;
1421 // Check for legality only before to do the move
1422 if (!pos.pl_move_is_legal(move, ci.pinned))
1425 // Update current move
1426 ss->currentMove = move;
1428 // Make and search the move
1429 pos.do_move(move, st, ci, givesCheck);
1430 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1431 pos.undo_move(move);
1433 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1436 if (value > bestValue)
1439 ss->bestMove = move;
1443 && value < beta) // We want always alpha < beta
1448 // All legal moves have been searched. A special case: If we're in check
1449 // and no legal moves were found, it is checkmate.
1450 if (inCheck && bestValue == -VALUE_INFINITE)
1451 return value_mated_in(ss->ply);
1453 // Update transposition table
1454 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1455 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1456 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1458 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1460 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1466 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1467 // bestValue is updated only when returning false because in that case move
1470 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1472 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1473 Square from, to, ksq, victimSq;
1476 Value futilityValue, bv = *bestValue;
1478 from = move_from(move);
1480 them = flip(pos.side_to_move());
1481 ksq = pos.king_square(them);
1482 kingAtt = pos.attacks_from<KING>(ksq);
1483 pc = pos.piece_on(from);
1485 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1486 oldAtt = pos.attacks_from(pc, from, occ);
1487 newAtt = pos.attacks_from(pc, to, occ);
1489 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1490 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1492 if (!(b && (b & (b - 1))))
1495 // Rule 2. Queen contact check is very dangerous
1496 if ( type_of(pc) == QUEEN
1497 && bit_is_set(kingAtt, to))
1500 // Rule 3. Creating new double threats with checks
1501 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1505 victimSq = pop_1st_bit(&b);
1506 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1508 // Note that here we generate illegal "double move"!
1509 if ( futilityValue >= beta
1510 && pos.see_sign(make_move(from, victimSq)) >= 0)
1513 if (futilityValue > bv)
1517 // Update bestValue only if check is not dangerous (because we will prune the move)
1523 // connected_moves() tests whether two moves are 'connected' in the sense
1524 // that the first move somehow made the second move possible (for instance
1525 // if the moving piece is the same in both moves). The first move is assumed
1526 // to be the move that was made to reach the current position, while the
1527 // second move is assumed to be a move from the current position.
1529 bool connected_moves(const Position& pos, Move m1, Move m2) {
1531 Square f1, t1, f2, t2;
1538 // Case 1: The moving piece is the same in both moves
1544 // Case 2: The destination square for m2 was vacated by m1
1550 // Case 3: Moving through the vacated square
1551 p2 = pos.piece_on(f2);
1552 if ( piece_is_slider(p2)
1553 && bit_is_set(squares_between(f2, t2), f1))
1556 // Case 4: The destination square for m2 is defended by the moving piece in m1
1557 p1 = pos.piece_on(t1);
1558 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1561 // Case 5: Discovered check, checking piece is the piece moved in m1
1562 ksq = pos.king_square(pos.side_to_move());
1563 if ( piece_is_slider(p1)
1564 && bit_is_set(squares_between(t1, ksq), f2))
1566 Bitboard occ = pos.occupied_squares();
1567 clear_bit(&occ, f2);
1568 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1575 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1576 // "plies to mate from the current ply". Non-mate scores are unchanged.
1577 // The function is called before storing a value to the transposition table.
1579 Value value_to_tt(Value v, int ply) {
1581 if (v >= VALUE_MATE_IN_PLY_MAX)
1584 if (v <= VALUE_MATED_IN_PLY_MAX)
1591 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1592 // the transposition table to a mate score corrected for the current ply.
1594 Value value_from_tt(Value v, int ply) {
1596 if (v >= VALUE_MATE_IN_PLY_MAX)
1599 if (v <= VALUE_MATED_IN_PLY_MAX)
1606 // connected_threat() tests whether it is safe to forward prune a move or if
1607 // is somehow connected to the threat move returned by null search.
1609 bool connected_threat(const Position& pos, Move m, Move threat) {
1612 assert(is_ok(threat));
1613 assert(!pos.is_capture_or_promotion(m));
1614 assert(!pos.is_passed_pawn_push(m));
1616 Square mfrom, mto, tfrom, tto;
1618 mfrom = move_from(m);
1620 tfrom = move_from(threat);
1621 tto = move_to(threat);
1623 // Case 1: Don't prune moves which move the threatened piece
1627 // Case 2: If the threatened piece has value less than or equal to the
1628 // value of the threatening piece, don't prune moves which defend it.
1629 if ( pos.is_capture(threat)
1630 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1631 || type_of(pos.piece_on(tfrom)) == KING)
1632 && pos.move_attacks_square(m, tto))
1635 // Case 3: If the moving piece in the threatened move is a slider, don't
1636 // prune safe moves which block its ray.
1637 if ( piece_is_slider(pos.piece_on(tfrom))
1638 && bit_is_set(squares_between(tfrom, tto), mto)
1639 && pos.see_sign(m) >= 0)
1646 // can_return_tt() returns true if a transposition table score
1647 // can be used to cut-off at a given point in search.
1649 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1651 Value v = value_from_tt(tte->value(), ply);
1653 return ( tte->depth() >= depth
1654 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1655 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1657 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1658 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1662 // refine_eval() returns the transposition table score if
1663 // possible otherwise falls back on static position evaluation.
1665 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1669 Value v = value_from_tt(tte->value(), ply);
1671 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1672 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1679 // update_history() registers a good move that produced a beta-cutoff
1680 // in history and marks as failures all the other moves of that ply.
1682 void update_history(const Position& pos, Move move, Depth depth,
1683 Move movesSearched[], int moveCount) {
1685 Value bonus = Value(int(depth) * int(depth));
1687 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1689 for (int i = 0; i < moveCount - 1; i++)
1691 m = movesSearched[i];
1695 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1700 // current_search_time() returns the number of milliseconds which have passed
1701 // since the beginning of the current search.
1703 int elapsed_time(bool reset) {
1705 static int searchStartTime;
1708 searchStartTime = get_system_time();
1710 return get_system_time() - searchStartTime;
1714 // score_to_uci() converts a value to a string suitable for use with the UCI
1715 // protocol specifications:
1717 // cp <x> The score from the engine's point of view in centipawns.
1718 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1719 // use negative values for y.
1721 string score_to_uci(Value v, Value alpha, Value beta) {
1723 std::stringstream s;
1725 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1726 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1728 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1730 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1736 // speed_to_uci() returns a string with time stats of current search suitable
1737 // to be sent to UCI gui.
1739 string speed_to_uci(int64_t nodes) {
1741 std::stringstream s;
1742 int t = elapsed_time();
1744 s << " nodes " << nodes
1745 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1752 // pv_to_uci() returns a string with information on the current PV line
1753 // formatted according to UCI specification.
1755 string pv_to_uci(const Move pv[], int pvNum, bool chess960) {
1757 std::stringstream s;
1759 s << " multipv " << pvNum << " pv " << set960(chess960);
1761 for ( ; *pv != MOVE_NONE; pv++)
1768 // depth_to_uci() returns a string with information on the current depth and
1769 // seldepth formatted according to UCI specification.
1771 string depth_to_uci(Depth depth) {
1773 std::stringstream s;
1775 // Retrieve max searched depth among threads
1777 for (int i = 0; i < Threads.size(); i++)
1778 if (Threads[i].maxPly > selDepth)
1779 selDepth = Threads[i].maxPly;
1781 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1786 string time_to_string(int millisecs) {
1788 const int MSecMinute = 1000 * 60;
1789 const int MSecHour = 1000 * 60 * 60;
1791 int hours = millisecs / MSecHour;
1792 int minutes = (millisecs % MSecHour) / MSecMinute;
1793 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1795 std::stringstream s;
1800 s << std::setfill('0') << std::setw(2) << minutes << ':' << std::setw(2) << seconds;
1804 string score_to_string(Value v) {
1806 std::stringstream s;
1808 if (v >= VALUE_MATE_IN_PLY_MAX)
1809 s << "#" << (VALUE_MATE - v + 1) / 2;
1810 else if (v <= VALUE_MATED_IN_PLY_MAX)
1811 s << "-#" << (VALUE_MATE + v) / 2;
1813 s << std::setprecision(2) << std::fixed << std::showpos << float(v) / PawnValueMidgame;
1819 // pretty_pv() creates a human-readable string from a position and a PV.
1820 // It is used to write search information to the log file (which is created
1821 // when the UCI parameter "Use Search Log" is "true").
1823 string pretty_pv(Position& pos, int depth, Value value, int time, Move pv[]) {
1825 const int64_t K = 1000;
1826 const int64_t M = 1000000;
1827 const int startColumn = 28;
1828 const size_t maxLength = 80 - startColumn;
1830 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1833 std::stringstream s;
1836 // First print depth, score, time and searched nodes...
1837 s << set960(pos.is_chess960())
1838 << std::setw(2) << depth
1839 << std::setw(8) << score_to_string(value)
1840 << std::setw(8) << time_to_string(time);
1842 if (pos.nodes_searched() < M)
1843 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1844 else if (pos.nodes_searched() < K * M)
1845 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1847 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1849 // ...then print the full PV line in short algebraic notation
1850 while (*m != MOVE_NONE)
1852 san = move_to_san(pos, *m);
1853 length += san.length() + 1;
1855 if (length > maxLength)
1857 length = san.length() + 1;
1858 s << "\n" + string(startColumn, ' ');
1862 pos.do_move(*m++, *st++);
1865 // Restore original position before to leave
1866 while (m != pv) pos.undo_move(*--m);
1872 // When playing with strength handicap choose best move among the MultiPV set
1873 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1875 void do_skill_level(Move* best, Move* ponder) {
1877 assert(MultiPV > 1);
1881 // Rml list is already sorted by score in descending order
1883 size_t size = std::min(MultiPV, Rml.size());
1884 int max_s = -VALUE_INFINITE;
1885 int max = Rml[0].score;
1886 int var = std::min(max - Rml[size - 1].score, int(PawnValueMidgame));
1887 int wk = 120 - 2 * SkillLevel;
1889 // PRNG sequence should be non deterministic
1890 for (int i = abs(get_system_time() % 50); i > 0; i--)
1891 rk.rand<unsigned>();
1893 // Choose best move. For each move's score we add two terms both dependent
1894 // on wk, one deterministic and bigger for weaker moves, and one random,
1895 // then we choose the move with the resulting highest score.
1896 for (size_t i = 0; i < size; i++)
1900 // Don't allow crazy blunders even at very low skills
1901 if (i > 0 && Rml[i-1].score > s + EasyMoveMargin)
1904 // This is our magical formula
1905 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
1910 *best = Rml[i].pv[0];
1911 *ponder = Rml[i].pv[1];
1917 /// RootMove and RootMoveList method's definitions
1919 void RootMoveList::init(Position& pos, Move rootMoves[]) {
1922 bestMoveChanges = 0;
1925 // Generate all legal moves and add them to RootMoveList
1926 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
1928 // If we have a rootMoves[] list then verify the move
1929 // is in the list before to add it.
1930 for (sm = rootMoves; *sm && *sm != ml.move(); sm++) {}
1932 if (sm != rootMoves && *sm != ml.move())
1936 rm.pv.push_back(ml.move());
1937 rm.pv.push_back(MOVE_NONE);
1938 rm.score = rm.prevScore = -VALUE_INFINITE;
1944 RootMove* RootMoveList::find(const Move& m, int startIndex) {
1946 for (size_t i = startIndex; i < size(); i++)
1947 if ((*this)[i].pv[0] == m)
1954 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1955 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1956 // allow to always have a ponder move even when we fail high at root and also a
1957 // long PV to print that is important for position analysis.
1959 void RootMove::extract_pv_from_tt(Position& pos) {
1961 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1966 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1970 pos.do_move(m, *st++);
1972 while ( (tte = TT.probe(pos.get_key())) != NULL
1973 && tte->move() != MOVE_NONE
1974 && pos.is_pseudo_legal(tte->move())
1975 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1977 && (!pos.is_draw<false>() || ply < 2))
1979 pv.push_back(tte->move());
1980 pos.do_move(tte->move(), *st++);
1983 pv.push_back(MOVE_NONE);
1985 do pos.undo_move(pv[--ply]); while (ply);
1989 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1990 // the PV back into the TT. This makes sure the old PV moves are searched
1991 // first, even if the old TT entries have been overwritten.
1993 void RootMove::insert_pv_in_tt(Position& pos) {
1995 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1998 Value v, m = VALUE_NONE;
2001 assert(pv[0] != MOVE_NONE && pos.is_pseudo_legal(pv[0]));
2007 // Don't overwrite existing correct entries
2008 if (!tte || tte->move() != pv[ply])
2010 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2011 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2013 pos.do_move(pv[ply], *st++);
2015 } while (pv[++ply] != MOVE_NONE);
2017 do pos.undo_move(pv[--ply]); while (ply);
2023 // Thread::idle_loop() is where the thread is parked when it has no work to do.
2024 // The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
2025 // for which the thread is the master.
2027 void Thread::idle_loop(SplitPoint* sp) {
2031 // If we are not searching, wait for a condition to be signaled
2032 // instead of wasting CPU time polling for work.
2035 || (Threads.use_sleeping_threads() && !is_searching))
2037 assert((!sp && threadID) || Threads.use_sleeping_threads());
2039 // Slave thread should exit as soon as do_terminate flag raises
2046 // Grab the lock to avoid races with Thread::wake_up()
2047 lock_grab(&sleepLock);
2049 // If we are master and all slaves have finished don't go to sleep
2050 if (sp && Threads.split_point_finished(sp))
2052 lock_release(&sleepLock);
2056 // Do sleep after retesting sleep conditions under lock protection, in
2057 // particular we need to avoid a deadlock in case a master thread has,
2058 // in the meanwhile, allocated us and sent the wake_up() call before we
2059 // had the chance to grab the lock.
2060 if (do_sleep || !is_searching)
2061 cond_wait(&sleepCond, &sleepLock);
2063 lock_release(&sleepLock);
2066 // If this thread has been assigned work, launch a search
2069 assert(!do_terminate);
2071 // Copy split point position and search stack and call search()
2072 Stack ss[PLY_MAX_PLUS_2];
2073 SplitPoint* tsp = splitPoint;
2074 Position pos(*tsp->pos, threadID);
2076 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
2079 if (tsp->nodeType == Root)
2080 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2081 else if (tsp->nodeType == PV)
2082 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2083 else if (tsp->nodeType == NonPV)
2084 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2088 assert(is_searching);
2090 is_searching = false;
2092 // Wake up master thread so to allow it to return from the idle loop in
2093 // case we are the last slave of the split point.
2094 if ( Threads.use_sleeping_threads()
2095 && threadID != tsp->master
2096 && !Threads[tsp->master].is_searching)
2097 Threads[tsp->master].wake_up();
2100 // If this thread is the master of a split point and all slaves have
2101 // finished their work at this split point, return from the idle loop.
2102 if (sp && Threads.split_point_finished(sp))
2104 // Because sp->is_slave[] is reset under lock protection,
2105 // be sure sp->lock has been released before to return.
2106 lock_grab(&(sp->lock));
2107 lock_release(&(sp->lock));
2114 // do_timer_event() is called by the timer thread when the timer triggers
2116 void do_timer_event() {
2118 static int lastInfoTime;
2119 int e = elapsed_time();
2121 // Print debug information every one second
2122 if (!lastInfoTime || get_system_time() - lastInfoTime >= 1000)
2124 lastInfoTime = get_system_time();
2127 dbg_print_hit_rate();
2130 // Should we stop the search?
2134 bool stillAtFirstMove = Signals.firstRootMove
2135 && !Signals.failedLowAtRoot
2136 && e > TimeMgr.available_time();
2138 bool noMoreTime = e > TimeMgr.maximum_time()
2139 || stillAtFirstMove;
2141 if ( (Limits.useTimeManagement() && noMoreTime)
2142 || (Limits.maxTime && e >= Limits.maxTime)
2143 /* missing nodes limit */ ) // FIXME
2144 Signals.stop = true;