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"
46 // Set to true to force running with one thread. Used for debugging
47 const bool FakeSplit = false;
49 // Different node types, used as template parameter
50 enum NodeType { Root, PV, NonPV, SplitPointPV, SplitPointNonPV };
52 // RootMove struct is used for moves at the root of the tree. For each root
53 // move, we store two scores, a node count, and a PV (really a refutation
54 // in the case of moves which fail low). Value pv_score is normally set at
55 // -VALUE_INFINITE for all non-pv moves, while non_pv_score is computed
56 // according to the order in which moves are returned by MovePicker.
60 RootMove(const RootMove& rm) { *this = rm; }
61 RootMove& operator=(const RootMove& rm);
63 // RootMove::operator<() is the comparison function used when
64 // sorting the moves. A move m1 is considered to be better
65 // than a move m2 if it has an higher pv_score, or if it has
66 // equal pv_score but m1 has the higher non_pv_score. In this way
67 // we are guaranteed that PV moves are always sorted as first.
68 bool operator<(const RootMove& m) const {
69 return pv_score != m.pv_score ? pv_score < m.pv_score
70 : non_pv_score < m.non_pv_score;
73 void extract_pv_from_tt(Position& pos);
74 void insert_pv_in_tt(Position& pos);
79 Move pv[PLY_MAX_PLUS_2];
82 // RootMoveList struct is just a vector of RootMove objects,
83 // with an handful of methods above the standard ones.
84 struct RootMoveList : public std::vector<RootMove> {
86 typedef std::vector<RootMove> Base;
88 void init(Position& pos, Move searchMoves[]);
89 void sort() { insertion_sort<RootMove, Base::iterator>(begin(), end()); }
90 void sort_first(int n) { insertion_sort<RootMove, Base::iterator>(begin(), begin() + n); }
98 // Lookup table to check if a Piece is a slider and its access function
99 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
100 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
104 // Maximum depth for razoring
105 const Depth RazorDepth = 4 * ONE_PLY;
107 // Dynamic razoring margin based on depth
108 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
110 // Maximum depth for use of dynamic threat detection when null move fails low
111 const Depth ThreatDepth = 5 * ONE_PLY;
113 // Step 9. Internal iterative deepening
115 // Minimum depth for use of internal iterative deepening
116 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
118 // At Non-PV nodes we do an internal iterative deepening search
119 // when the static evaluation is bigger then beta - IIDMargin.
120 const Value IIDMargin = Value(0x100);
122 // Step 11. Decide the new search depth
124 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
125 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
126 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
127 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
128 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
130 // Minimum depth for use of singular extension
131 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
133 // Step 12. Futility pruning
135 // Futility margin for quiescence search
136 const Value FutilityMarginQS = Value(0x80);
138 // Futility lookup tables (initialized at startup) and their access functions
139 Value FutilityMargins[16][64]; // [depth][moveNumber]
140 int FutilityMoveCounts[32]; // [depth]
142 inline Value futility_margin(Depth d, int mn) {
144 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
145 : 2 * VALUE_INFINITE;
148 inline int futility_move_count(Depth d) {
150 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
153 // Step 14. Reduced search
155 // Reduction lookup tables (initialized at startup) and their access function
156 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
158 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
160 return (Depth) Reductions[PvNode][Min(d / ONE_PLY, 63)][Min(mn, 63)];
163 // Easy move margin. An easy move candidate must be at least this much
164 // better than the second best move.
165 const Value EasyMoveMargin = Value(0x200);
168 /// Namespace variables
174 int MultiPV, UCIMultiPV;
176 // Time management variables
177 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
182 std::ofstream LogFile;
184 // Skill level adjustment
186 bool SkillLevelEnabled;
188 // Node counters, used only by thread[0] but try to keep in different cache
189 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
190 bool SendSearchedNodes;
192 int NodesBetweenPolls = 30000;
200 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
202 template <NodeType NT>
203 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
205 template <NodeType NT>
206 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
208 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
209 bool connected_moves(const Position& pos, Move m1, Move m2);
210 Value value_to_tt(Value v, int ply);
211 Value value_from_tt(Value v, int ply);
212 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
213 bool connected_threat(const Position& pos, Move m, Move threat);
214 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
215 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
216 void update_gains(const Position& pos, Move move, Value before, Value after);
217 void do_skill_level(Move* best, Move* ponder);
219 int current_search_time(int set = 0);
220 std::string score_to_uci(Value v, Value alpha, Value beta);
221 std::string speed_to_uci(int64_t nodes);
222 std::string pv_to_uci(Move pv[], int pvNum);
223 std::string depth_to_uci(Depth depth);
224 void poll(const Position& pos);
225 void wait_for_stop_or_ponderhit();
227 // MovePickerExt template class extends MovePicker and allows to choose at compile
228 // time the proper moves source according to the type of node. In the default case
229 // we simply create and use a standard MovePicker object.
230 template<NodeType> struct MovePickerExt : public MovePicker {
232 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
233 : MovePicker(p, ttm, d, h, ss, b) {}
235 RootMove& current() { assert(false); return Rml[0]; } // Dummy, needed to compile
238 // In case of a SpNode we use split point's shared MovePicker object as moves source
239 template<> struct MovePickerExt<SplitPointNonPV> : public MovePickerExt<NonPV> {
241 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
242 : MovePickerExt<NonPV>(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
244 Move get_next_move() { return mp->get_next_move(); }
248 template<> struct MovePickerExt<SplitPointPV> : public MovePickerExt<SplitPointNonPV> {
250 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
251 : MovePickerExt<SplitPointNonPV>(p, ttm, d, h, ss, b) {}
254 // In case of a Root node we use RootMoveList as moves source
255 template<> struct MovePickerExt<Root> : public MovePicker {
257 MovePickerExt(const Position&, Move, Depth, const History&, SearchStack*, Value);
258 RootMove& current() { return Rml[cur]; }
259 Move get_next_move() { return ++cur < (int)Rml.size() ? Rml[cur].pv[0] : MOVE_NONE; }
264 // Overload operator<<() to make it easier to print moves in a coordinate
265 // notation compatible with UCI protocol.
266 std::ostream& operator<<(std::ostream& os, Move m) {
268 bool chess960 = (os.iword(0) != 0); // See set960()
269 return os << move_to_uci(m, chess960);
272 // When formatting a move for std::cout we must know if we are in Chess960
273 // or not. To keep using the handy operator<<() on the move the trick is to
274 // embed this flag in the stream itself. Function-like named enum set960 is
275 // used as a custom manipulator and the stream internal general-purpose array,
276 // accessed through ios_base::iword(), is used to pass the flag to the move's
277 // operator<<() that will read it to properly format castling moves.
280 std::ostream& operator<< (std::ostream& os, const set960& f) {
282 os.iword(0) = int(f);
286 // extension() decides whether a move should be searched with normal depth,
287 // or with extended depth. Certain classes of moves (checking moves, in
288 // particular) are searched with bigger depth than ordinary moves and in
289 // any case are marked as 'dangerous'. Note that also if a move is not
290 // extended, as example because the corresponding UCI option is set to zero,
291 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
292 template <bool PvNode>
293 FORCE_INLINE Depth extension(const Position& pos, Move m, bool captureOrPromotion,
294 bool moveIsCheck, bool* dangerous) {
295 assert(m != MOVE_NONE);
297 Depth result = DEPTH_ZERO;
298 *dangerous = moveIsCheck;
300 if (moveIsCheck && pos.see_sign(m) >= 0)
301 result += CheckExtension[PvNode];
303 if (piece_type(pos.piece_on(move_from(m))) == PAWN)
305 Color c = pos.side_to_move();
306 if (relative_rank(c, move_to(m)) == RANK_7)
308 result += PawnPushTo7thExtension[PvNode];
311 if (pos.pawn_is_passed(c, move_to(m)))
313 result += PassedPawnExtension[PvNode];
318 if ( captureOrPromotion
319 && piece_type(pos.piece_on(move_to(m))) != PAWN
320 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
321 - piece_value_midgame(pos.piece_on(move_to(m))) == VALUE_ZERO)
322 && !move_is_special(m))
324 result += PawnEndgameExtension[PvNode];
328 return Min(result, ONE_PLY);
334 /// init_search() is called during startup to initialize various lookup tables
338 int d; // depth (ONE_PLY == 2)
339 int hd; // half depth (ONE_PLY == 1)
342 // Init reductions array
343 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
345 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
346 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
347 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
348 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
351 // Init futility margins array
352 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
353 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
355 // Init futility move count array
356 for (d = 0; d < 32; d++)
357 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
361 /// perft() is our utility to verify move generation. All the leaf nodes up to
362 /// the given depth are generated and counted and the sum returned.
364 int64_t perft(Position& pos, Depth depth) {
369 // Generate all legal moves
370 MoveList<MV_LEGAL> ml(pos);
372 // If we are at the last ply we don't need to do and undo
373 // the moves, just to count them.
374 if (depth <= ONE_PLY)
377 // Loop through all legal moves
379 for ( ; !ml.end(); ++ml)
381 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
382 sum += perft(pos, depth - ONE_PLY);
383 pos.undo_move(ml.move());
389 /// think() is the external interface to Stockfish's search, and is called when
390 /// the program receives the UCI 'go' command. It initializes various global
391 /// variables, and calls id_loop(). It returns false when a "quit" command is
392 /// received during the search.
394 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
398 // Initialize global search-related variables
399 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = SendSearchedNodes = false;
401 current_search_time(get_system_time());
403 TimeMgr.init(Limits, pos.startpos_ply_counter());
405 // Set output steram in normal or chess960 mode
406 cout << set960(pos.is_chess960());
408 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
410 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
411 else if (Limits.time && Limits.time < 1000)
412 NodesBetweenPolls = 1000;
413 else if (Limits.time && Limits.time < 5000)
414 NodesBetweenPolls = 5000;
416 NodesBetweenPolls = 30000;
418 // Look for a book move
419 if (Options["OwnBook"].value<bool>())
421 if (Options["Book File"].value<std::string>() != book.name())
422 book.open(Options["Book File"].value<std::string>());
424 Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
425 if (bookMove != MOVE_NONE)
428 wait_for_stop_or_ponderhit();
430 cout << "bestmove " << bookMove << endl;
436 UCIMultiPV = Options["MultiPV"].value<int>();
437 SkillLevel = Options["Skill Level"].value<int>();
439 read_evaluation_uci_options(pos.side_to_move());
440 Threads.read_uci_options();
442 // If needed allocate pawn and material hash tables and adjust TT size
443 Threads.init_hash_tables();
444 TT.set_size(Options["Hash"].value<int>());
446 if (Options["Clear Hash"].value<bool>())
448 Options["Clear Hash"].set_value("false");
452 // Do we have to play with skill handicap? In this case enable MultiPV that
453 // we will use behind the scenes to retrieve a set of possible moves.
454 SkillLevelEnabled = (SkillLevel < 20);
455 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
457 // Wake up needed threads and reset maxPly counter
458 for (int i = 0; i < Threads.size(); i++)
460 Threads[i].wake_up();
461 Threads[i].maxPly = 0;
464 // Write to log file and keep it open to be accessed during the search
465 if (Options["Use Search Log"].value<bool>())
467 std::string name = Options["Search Log Filename"].value<std::string>();
468 LogFile.open(name.c_str(), std::ios::out | std::ios::app);
470 if (LogFile.is_open())
471 LogFile << "\nSearching: " << pos.to_fen()
472 << "\ninfinite: " << Limits.infinite
473 << " ponder: " << Limits.ponder
474 << " time: " << Limits.time
475 << " increment: " << Limits.increment
476 << " moves to go: " << Limits.movesToGo
480 // We're ready to start thinking. Call the iterative deepening loop function
481 Move ponderMove = MOVE_NONE;
482 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
484 // Write final search statistics and close log file
485 if (LogFile.is_open())
487 int t = current_search_time();
489 LogFile << "Nodes: " << pos.nodes_searched()
490 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
491 << "\nBest move: " << move_to_san(pos, bestMove);
494 pos.do_move(bestMove, st);
495 LogFile << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
496 pos.undo_move(bestMove); // Return from think() with unchanged position
500 // This makes all the threads to go to sleep
503 // If we are pondering or in infinite search, we shouldn't print the
504 // best move before we are told to do so.
505 if (!StopRequest && (Limits.ponder || Limits.infinite))
506 wait_for_stop_or_ponderhit();
508 // Could be MOVE_NONE when searching on a stalemate position
509 cout << "bestmove " << bestMove;
511 // UCI protol is not clear on allowing sending an empty ponder move, instead
512 // it is clear that ponder move is optional. So skip it if empty.
513 if (ponderMove != MOVE_NONE)
514 cout << " ponder " << ponderMove;
524 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
525 // with increasing depth until the allocated thinking time has been consumed,
526 // user stops the search, or the maximum search depth is reached.
528 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
530 SearchStack ss[PLY_MAX_PLUS_2];
531 Value bestValues[PLY_MAX_PLUS_2];
532 int bestMoveChanges[PLY_MAX_PLUS_2];
533 int depth, aspirationDelta;
534 Value value, alpha, beta;
535 Move bestMove, easyMove, skillBest, skillPonder;
537 // Initialize stuff before a new search
538 memset(ss, 0, 4 * sizeof(SearchStack));
541 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
542 depth = aspirationDelta = 0;
543 alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
544 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
546 // Moves to search are verified and copied
547 Rml.init(pos, searchMoves);
549 // Handle special case of searching on a mate/stalemate position
552 cout << "info" << depth_to_uci(DEPTH_ZERO)
553 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW, alpha, beta) << endl;
558 // Iterative deepening loop until requested to stop or target depth reached
559 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
561 Rml.bestMoveChanges = 0;
563 // Calculate dynamic aspiration window based on previous iterations
564 if (MultiPV == 1 && depth >= 5 && abs(bestValues[depth - 1]) < VALUE_KNOWN_WIN)
566 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
567 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
569 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
570 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
572 alpha = Max(bestValues[depth - 1] - aspirationDelta, -VALUE_INFINITE);
573 beta = Min(bestValues[depth - 1] + aspirationDelta, VALUE_INFINITE);
576 // Start with a small aspiration window and, in case of fail high/low,
577 // research with bigger window until not failing high/low anymore.
579 // Search starting from ss+1 to allow calling update_gains()
580 value = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
582 // Write PV back to transposition table in case the relevant entries
583 // have been overwritten during the search.
584 for (int i = 0; i < Min(MultiPV, (int)Rml.size()); i++)
585 Rml[i].insert_pv_in_tt(pos);
587 // Value cannot be trusted. Break out immediately!
591 // Send full PV info to GUI if we are going to leave the loop or
592 // if we have a fail high/low and we are deep in the search.
593 if ((value > alpha && value < beta) || current_search_time() > 2000)
594 for (int i = 0; i < Min(UCIMultiPV, (int)Rml.size()); i++)
596 << depth_to_uci(depth * ONE_PLY)
597 << score_to_uci(Rml[i].pv_score, alpha, beta)
598 << speed_to_uci(pos.nodes_searched())
599 << pv_to_uci(Rml[i].pv, i + 1) << endl;
601 // In case of failing high/low increase aspiration window and research,
602 // otherwise exit the fail high/low loop.
605 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
606 aspirationDelta += aspirationDelta / 2;
608 else if (value <= alpha)
610 AspirationFailLow = true;
611 StopOnPonderhit = false;
613 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
614 aspirationDelta += aspirationDelta / 2;
619 } while (abs(value) < VALUE_KNOWN_WIN);
621 // Collect info about search result
622 bestMove = Rml[0].pv[0];
623 *ponderMove = Rml[0].pv[1];
624 bestValues[depth] = value;
625 bestMoveChanges[depth] = Rml.bestMoveChanges;
627 // Do we need to pick now the best and the ponder moves ?
628 if (SkillLevelEnabled && depth == 1 + SkillLevel)
629 do_skill_level(&skillBest, &skillPonder);
631 if (LogFile.is_open())
632 LogFile << pretty_pv(pos, depth, value, current_search_time(), Rml[0].pv) << endl;
634 // Init easyMove after first iteration or drop if differs from the best move
635 if (depth == 1 && (Rml.size() == 1 || Rml[0].pv_score > Rml[1].pv_score + EasyMoveMargin))
637 else if (bestMove != easyMove)
638 easyMove = MOVE_NONE;
640 // Check for some early stop condition
641 if (!StopRequest && Limits.useTimeManagement())
643 // Stop search early when the last two iterations returned a mate score
645 && abs(bestValues[depth]) >= VALUE_MATE_IN_PLY_MAX
646 && abs(bestValues[depth - 1]) >= VALUE_MATE_IN_PLY_MAX)
649 // Stop search early if one move seems to be much better than the
650 // others or if there is only a single legal move. Also in the latter
651 // case we search up to some depth anyway to get a proper score.
653 && easyMove == bestMove
655 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
656 && current_search_time() > TimeMgr.available_time() / 16)
657 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
658 && current_search_time() > TimeMgr.available_time() / 32)))
661 // Take in account some extra time if the best move has changed
662 if (depth > 4 && depth < 50)
663 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
665 // Stop search if most of available time is already consumed. We probably don't
666 // have enough time to search the first move at the next iteration anyway.
667 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
670 // If we are allowed to ponder do not stop the search now but keep pondering
671 if (StopRequest && Limits.ponder)
674 StopOnPonderhit = true;
679 // When using skills overwrite best and ponder moves with the sub-optimal ones
680 if (SkillLevelEnabled)
682 if (skillBest == MOVE_NONE) // Still unassigned ?
683 do_skill_level(&skillBest, &skillPonder);
685 bestMove = skillBest;
686 *ponderMove = skillPonder;
693 // search<>() is the main search function for both PV and non-PV nodes and for
694 // normal and SplitPoint nodes. When called just after a split point the search
695 // is simpler because we have already probed the hash table, done a null move
696 // search, and searched the first move before splitting, we don't have to repeat
697 // all this work again. We also don't need to store anything to the hash table
698 // here: This is taken care of after we return from the split point.
700 template <NodeType NT>
701 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
703 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV);
704 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV);
705 const bool RootNode = (NT == Root);
707 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
708 assert(beta > alpha && beta <= VALUE_INFINITE);
709 assert(PvNode || alpha == beta - 1);
710 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
712 Move movesSearched[MAX_MOVES];
717 Move ttMove, move, excludedMove, threatMove;
720 Value bestValue, value, oldAlpha;
721 Value refinedValue, nullValue, futilityBase, futilityValueScaled; // Non-PV specific
722 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
723 int moveCount = 0, playedMoveCount = 0;
724 Thread& thread = Threads[pos.thread()];
725 SplitPoint* sp = NULL;
727 refinedValue = bestValue = value = -VALUE_INFINITE;
729 inCheck = pos.in_check();
730 ss->ply = (ss-1)->ply + 1;
732 // Used to send selDepth info to GUI
733 if (PvNode && thread.maxPly < ss->ply)
734 thread.maxPly = ss->ply;
736 // Step 1. Initialize node and poll. Polling can abort search
739 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
740 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
741 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
747 ttMove = excludedMove = MOVE_NONE;
748 threatMove = sp->threatMove;
749 goto split_point_start;
752 if (pos.thread() == 0 && ++NodesSincePoll > NodesBetweenPolls)
758 // Step 2. Check for aborted search and immediate draw
760 || pos.is_draw<false>()
761 || ss->ply > PLY_MAX) && !RootNode)
764 // Step 3. Mate distance pruning
767 alpha = Max(value_mated_in(ss->ply), alpha);
768 beta = Min(value_mate_in(ss->ply+1), beta);
773 // Step 4. Transposition table lookup
774 // We don't want the score of a partial search to overwrite a previous full search
775 // TT value, so we use a different position key in case of an excluded move.
776 excludedMove = ss->excludedMove;
777 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
778 tte = TT.probe(posKey);
779 ttMove = tte ? tte->move() : MOVE_NONE;
781 // At PV nodes we check for exact scores, while at non-PV nodes we check for
782 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
783 // smooth experience in analysis mode.
784 if (tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
785 : ok_to_use_TT(tte, depth, beta, ss->ply)))
788 ss->bestMove = ttMove; // Can be MOVE_NONE
789 return value_from_tt(tte->value(), ss->ply);
792 // Step 5. Evaluate the position statically and update parent's gain statistics
794 ss->eval = ss->evalMargin = VALUE_NONE;
797 assert(tte->static_value() != VALUE_NONE);
799 ss->eval = tte->static_value();
800 ss->evalMargin = tte->static_value_margin();
801 refinedValue = refine_eval(tte, ss->eval, ss->ply);
805 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
806 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
809 // Save gain for the parent non-capture move
810 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
812 // Step 6. Razoring (is omitted in PV nodes)
814 && depth < RazorDepth
816 && refinedValue + razor_margin(depth) < beta
817 && ttMove == MOVE_NONE
818 && abs(beta) < VALUE_MATE_IN_PLY_MAX
819 && !pos.has_pawn_on_7th(pos.side_to_move()))
821 Value rbeta = beta - razor_margin(depth);
822 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
824 // Logically we should return (v + razor_margin(depth)), but
825 // surprisingly this did slightly weaker in tests.
829 // Step 7. Static null move pruning (is omitted in PV nodes)
830 // We're betting that the opponent doesn't have a move that will reduce
831 // the score by more than futility_margin(depth) if we do a null move.
834 && depth < RazorDepth
836 && refinedValue - futility_margin(depth, 0) >= beta
837 && abs(beta) < VALUE_MATE_IN_PLY_MAX
838 && pos.non_pawn_material(pos.side_to_move()))
839 return refinedValue - futility_margin(depth, 0);
841 // Step 8. Null move search with verification search (is omitted in PV nodes)
846 && refinedValue >= beta
847 && abs(beta) < VALUE_MATE_IN_PLY_MAX
848 && pos.non_pawn_material(pos.side_to_move()))
850 ss->currentMove = MOVE_NULL;
852 // Null move dynamic reduction based on depth
853 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
855 // Null move dynamic reduction based on value
856 if (refinedValue - PawnValueMidgame > beta)
859 pos.do_null_move(st);
860 (ss+1)->skipNullMove = true;
861 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
862 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
863 (ss+1)->skipNullMove = false;
864 pos.undo_null_move();
866 if (nullValue >= beta)
868 // Do not return unproven mate scores
869 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
872 if (depth < 6 * ONE_PLY)
875 // Do verification search at high depths
876 ss->skipNullMove = true;
877 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
878 ss->skipNullMove = false;
885 // The null move failed low, which means that we may be faced with
886 // some kind of threat. If the previous move was reduced, check if
887 // the move that refuted the null move was somehow connected to the
888 // move which was reduced. If a connection is found, return a fail
889 // low score (which will cause the reduced move to fail high in the
890 // parent node, which will trigger a re-search with full depth).
891 threatMove = (ss+1)->bestMove;
893 if ( depth < ThreatDepth
895 && threatMove != MOVE_NONE
896 && connected_moves(pos, (ss-1)->currentMove, threatMove))
901 // Step 9. ProbCut (is omitted in PV nodes)
902 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
903 // and a reduced search returns a value much above beta, we can (almost) safely
904 // prune the previous move.
906 && depth >= RazorDepth + ONE_PLY
909 && excludedMove == MOVE_NONE
910 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
912 Value rbeta = beta + 200;
913 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
915 assert(rdepth >= ONE_PLY);
917 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
920 while ((move = mp.get_next_move()) != MOVE_NONE)
921 if (pos.pl_move_is_legal(move, ci.pinned))
923 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
924 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
931 // Step 10. Internal iterative deepening
932 if ( depth >= IIDDepth[PvNode]
933 && ttMove == MOVE_NONE
934 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
936 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
938 ss->skipNullMove = true;
939 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
940 ss->skipNullMove = false;
942 tte = TT.probe(posKey);
943 ttMove = tte ? tte->move() : MOVE_NONE;
946 split_point_start: // At split points actual search starts from here
948 // Initialize a MovePicker object for the current position
949 MovePickerExt<NT> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
951 ss->bestMove = MOVE_NONE;
952 futilityBase = ss->eval + ss->evalMargin;
953 singularExtensionNode = !RootNode
955 && depth >= SingularExtensionDepth[PvNode]
956 && ttMove != MOVE_NONE
957 && !excludedMove // Do not allow recursive singular extension search
958 && (tte->type() & VALUE_TYPE_LOWER)
959 && tte->depth() >= depth - 3 * ONE_PLY;
962 lock_grab(&(sp->lock));
963 bestValue = sp->bestValue;
966 // Step 11. Loop through moves
967 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
968 while ( bestValue < beta
969 && (move = mp.get_next_move()) != MOVE_NONE
970 && !thread.cutoff_occurred())
972 assert(move_is_ok(move));
974 if (move == excludedMove)
977 // At PV and SpNode nodes we want all moves to be legal since the beginning
978 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
983 moveCount = ++sp->moveCount;
984 lock_release(&(sp->lock));
991 // This is used by time management
992 FirstRootMove = (moveCount == 1);
994 // Save the current node count before the move is searched
995 nodes = pos.nodes_searched();
997 // If it's time to send nodes info, do it here where we have the
998 // correct accumulated node counts searched by each thread.
999 if (SendSearchedNodes)
1001 SendSearchedNodes = false;
1002 cout << "info" << speed_to_uci(pos.nodes_searched()) << endl;
1005 // For long searches send current move info to GUI
1006 if (current_search_time() > 2000)
1007 cout << "info" << depth_to_uci(depth)
1008 << " currmove " << move << " currmovenumber " << moveCount << endl;
1011 // At Root and at first iteration do a PV search on all the moves to score root moves
1012 isPvMove = (PvNode && moveCount <= (!RootNode ? 1 : depth <= ONE_PLY ? MAX_MOVES : MultiPV));
1013 givesCheck = pos.move_gives_check(move, ci);
1014 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1016 // Step 12. Decide the new search depth
1017 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
1019 // Singular extension search. If all moves but one fail low on a search of
1020 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
1021 // is singular and should be extended. To verify this we do a reduced search
1022 // on all the other moves but the ttMove, if result is lower than ttValue minus
1023 // a margin then we extend ttMove.
1024 if ( singularExtensionNode
1026 && pos.pl_move_is_legal(move, ci.pinned)
1029 Value ttValue = value_from_tt(tte->value(), ss->ply);
1031 if (abs(ttValue) < VALUE_KNOWN_WIN)
1033 Value rBeta = ttValue - int(depth);
1034 ss->excludedMove = move;
1035 ss->skipNullMove = true;
1036 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
1037 ss->skipNullMove = false;
1038 ss->excludedMove = MOVE_NONE;
1039 ss->bestMove = MOVE_NONE;
1045 // Update current move (this must be done after singular extension search)
1046 newDepth = depth - ONE_PLY + ext;
1048 // Step 13. Futility pruning (is omitted in PV nodes)
1050 && !captureOrPromotion
1054 && !move_is_castle(move))
1056 // Move count based pruning
1057 if ( moveCount >= futility_move_count(depth)
1058 && (!threatMove || !connected_threat(pos, move, threatMove))
1059 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
1062 lock_grab(&(sp->lock));
1067 // Value based pruning
1068 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1069 // but fixing this made program slightly weaker.
1070 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1071 futilityValueScaled = futilityBase + futility_margin(predictedDepth, moveCount)
1072 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1074 if (futilityValueScaled < beta)
1078 lock_grab(&(sp->lock));
1079 if (futilityValueScaled > sp->bestValue)
1080 sp->bestValue = bestValue = futilityValueScaled;
1082 else if (futilityValueScaled > bestValue)
1083 bestValue = futilityValueScaled;
1088 // Prune moves with negative SEE at low depths
1089 if ( predictedDepth < 2 * ONE_PLY
1090 && bestValue > VALUE_MATED_IN_PLY_MAX
1091 && pos.see_sign(move) < 0)
1094 lock_grab(&(sp->lock));
1100 // Check for legality only before to do the move
1101 if (!pos.pl_move_is_legal(move, ci.pinned))
1107 ss->currentMove = move;
1108 if (!SpNode && !captureOrPromotion)
1109 movesSearched[playedMoveCount++] = move;
1111 // Step 14. Make the move
1112 pos.do_move(move, st, ci, givesCheck);
1114 // Step extra. pv search (only in PV nodes)
1115 // The first move in list is the expected PV
1117 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1118 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1121 // Step 15. Reduced depth search
1122 // If the move fails high will be re-searched at full depth.
1123 bool doFullDepthSearch = true;
1125 if ( depth > 3 * ONE_PLY
1126 && !captureOrPromotion
1128 && !move_is_castle(move)
1129 && ss->killers[0] != move
1130 && ss->killers[1] != move
1131 && (ss->reduction = reduction<PvNode>(depth, moveCount)) != DEPTH_ZERO)
1133 Depth d = newDepth - ss->reduction;
1134 alpha = SpNode ? sp->alpha : alpha;
1136 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1137 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1139 ss->reduction = DEPTH_ZERO;
1140 doFullDepthSearch = (value > alpha);
1143 // Step 16. Full depth search
1144 if (doFullDepthSearch)
1146 alpha = SpNode ? sp->alpha : alpha;
1147 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1148 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1150 // Step extra. pv search (only in PV nodes)
1151 // Search only for possible new PV nodes, if instead value >= beta then
1152 // parent node fails low with value <= alpha and tries another move.
1153 if (PvNode && value > alpha && (RootNode || value < beta))
1154 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1155 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1159 // Step 17. Undo move
1160 pos.undo_move(move);
1162 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1164 // Step 18. Check for new best move
1167 lock_grab(&(sp->lock));
1168 bestValue = sp->bestValue;
1172 if (value > bestValue)
1175 ss->bestMove = move;
1180 && value < beta) // We want always alpha < beta
1183 if (SpNode && !thread.cutoff_occurred())
1185 sp->bestValue = value;
1186 sp->ss->bestMove = move;
1188 sp->is_betaCutoff = (value >= beta);
1194 // Finished searching the move. If StopRequest is true, the search
1195 // was aborted because the user interrupted the search or because we
1196 // ran out of time. In this case, the return value of the search cannot
1197 // be trusted, and we break out of the loop without updating the best
1202 // Remember searched nodes counts for this move
1203 mp.current().nodes += pos.nodes_searched() - nodes;
1205 // PV move or new best move ?
1206 if (isPvMove || value > alpha)
1209 mp.current().pv_score = value;
1210 mp.current().extract_pv_from_tt(pos);
1212 // We record how often the best move has been changed in each
1213 // iteration. This information is used for time management: When
1214 // the best move changes frequently, we allocate some more time.
1215 if (!isPvMove && MultiPV == 1)
1216 Rml.bestMoveChanges++;
1218 // It is critical that sorting is done with a stable algorithm
1219 // because all the values but the first are usually set to
1220 // -VALUE_INFINITE and we want to keep the same order for all
1221 // the moves but the new PV that goes to head.
1222 Rml.sort_first(moveCount);
1224 // Update alpha. In multi-pv we don't use aspiration window, so set
1225 // alpha equal to minimum score among the PV lines searched so far.
1227 alpha = Rml[Min(moveCount, MultiPV) - 1].pv_score;
1228 else if (value > alpha)
1232 // All other moves but the PV are set to the lowest value, this
1233 // is not a problem when sorting becuase sort is stable and move
1234 // position in the list is preserved, just the PV is pushed up.
1235 mp.current().pv_score = -VALUE_INFINITE;
1239 // Step 19. Check for split
1242 && depth >= Threads.min_split_depth()
1244 && Threads.available_slave_exists(pos.thread())
1246 && !thread.cutoff_occurred())
1247 Threads.split<FakeSplit>(pos, ss, &alpha, beta, &bestValue, depth,
1248 threatMove, moveCount, &mp, PvNode);
1251 // Step 20. Check for mate and stalemate
1252 // All legal moves have been searched and if there are
1253 // no legal moves, it must be mate or stalemate.
1254 // If one move was excluded return fail low score.
1255 if (!SpNode && !moveCount)
1256 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1258 // Step 21. Update tables
1259 // If the search is not aborted, update the transposition table,
1260 // history counters, and killer moves.
1261 if (!SpNode && !StopRequest && !thread.cutoff_occurred())
1263 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1264 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1265 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1267 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1269 // Update killers and history only for non capture moves that fails high
1270 if ( bestValue >= beta
1271 && !pos.move_is_capture_or_promotion(move))
1273 if (move != ss->killers[0])
1275 ss->killers[1] = ss->killers[0];
1276 ss->killers[0] = move;
1278 update_history(pos, move, depth, movesSearched, playedMoveCount);
1284 // Here we have the lock still grabbed
1285 sp->is_slave[pos.thread()] = false;
1286 sp->nodes += pos.nodes_searched();
1287 lock_release(&(sp->lock));
1290 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1295 // qsearch() is the quiescence search function, which is called by the main
1296 // search function when the remaining depth is zero (or, to be more precise,
1297 // less than ONE_PLY).
1299 template <NodeType NT>
1300 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1302 const bool PvNode = (NT == PV);
1304 assert(NT == PV || NT == NonPV);
1305 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1306 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1307 assert(PvNode || alpha == beta - 1);
1309 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1313 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1314 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 && ok_to_use_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 ( alpha < beta
1388 && (move = mp.get_next_move()) != MOVE_NONE)
1390 assert(move_is_ok(move));
1392 givesCheck = pos.move_gives_check(move, ci);
1400 && !move_is_promotion(move)
1401 && !pos.move_is_passed_pawn_push(move))
1403 futilityValue = futilityBase
1404 + piece_value_endgame(pos.piece_on(move_to(move)))
1405 + (move_is_ep(move) ? PawnValueEndgame : VALUE_ZERO);
1407 if (futilityValue < alpha)
1409 if (futilityValue > bestValue)
1410 bestValue = futilityValue;
1414 // Prune moves with negative or equal SEE
1415 if ( futilityBase < beta
1416 && depth < DEPTH_ZERO
1417 && pos.see(move) <= 0)
1421 // Detect non-capture evasions that are candidate to be pruned
1422 evasionPrunable = !PvNode
1424 && bestValue > VALUE_MATED_IN_PLY_MAX
1425 && !pos.move_is_capture(move)
1426 && !pos.can_castle(pos.side_to_move());
1428 // Don't search moves with negative SEE values
1430 && (!inCheck || evasionPrunable)
1432 && !move_is_promotion(move)
1433 && pos.see_sign(move) < 0)
1436 // Don't search useless checks
1441 && !pos.move_is_capture_or_promotion(move)
1442 && ss->eval + PawnValueMidgame / 4 < beta
1443 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1445 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1446 bestValue = ss->eval + PawnValueMidgame / 4;
1451 // Check for legality only before to do the move
1452 if (!pos.pl_move_is_legal(move, ci.pinned))
1455 // Update current move
1456 ss->currentMove = move;
1458 // Make and search the move
1459 pos.do_move(move, st, ci, givesCheck);
1460 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1461 pos.undo_move(move);
1463 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1466 if (value > bestValue)
1472 ss->bestMove = move;
1477 // All legal moves have been searched. A special case: If we're in check
1478 // and no legal moves were found, it is checkmate.
1479 if (inCheck && bestValue == -VALUE_INFINITE)
1480 return value_mated_in(ss->ply);
1482 // Update transposition table
1483 ValueType vt = (bestValue <= oldAlpha ? VALUE_TYPE_UPPER : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT);
1484 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, ss->bestMove, ss->eval, evalMargin);
1486 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1492 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1493 // bestValue is updated only when returning false because in that case move
1496 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1498 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1499 Square from, to, ksq, victimSq;
1502 Value futilityValue, bv = *bestValue;
1504 from = move_from(move);
1506 them = opposite_color(pos.side_to_move());
1507 ksq = pos.king_square(them);
1508 kingAtt = pos.attacks_from<KING>(ksq);
1509 pc = pos.piece_on(from);
1511 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1512 oldAtt = pos.attacks_from(pc, from, occ);
1513 newAtt = pos.attacks_from(pc, to, occ);
1515 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1516 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1518 if (!(b && (b & (b - 1))))
1521 // Rule 2. Queen contact check is very dangerous
1522 if ( piece_type(pc) == QUEEN
1523 && bit_is_set(kingAtt, to))
1526 // Rule 3. Creating new double threats with checks
1527 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1531 victimSq = pop_1st_bit(&b);
1532 futilityValue = futilityBase + piece_value_endgame(pos.piece_on(victimSq));
1534 // Note that here we generate illegal "double move"!
1535 if ( futilityValue >= beta
1536 && pos.see_sign(make_move(from, victimSq)) >= 0)
1539 if (futilityValue > bv)
1543 // Update bestValue only if check is not dangerous (because we will prune the move)
1549 // connected_moves() tests whether two moves are 'connected' in the sense
1550 // that the first move somehow made the second move possible (for instance
1551 // if the moving piece is the same in both moves). The first move is assumed
1552 // to be the move that was made to reach the current position, while the
1553 // second move is assumed to be a move from the current position.
1555 bool connected_moves(const Position& pos, Move m1, Move m2) {
1557 Square f1, t1, f2, t2;
1560 assert(m1 && move_is_ok(m1));
1561 assert(m2 && move_is_ok(m2));
1563 // Case 1: The moving piece is the same in both moves
1569 // Case 2: The destination square for m2 was vacated by m1
1575 // Case 3: Moving through the vacated square
1576 if ( piece_is_slider(pos.piece_on(f2))
1577 && bit_is_set(squares_between(f2, t2), f1))
1580 // Case 4: The destination square for m2 is defended by the moving piece in m1
1581 p = pos.piece_on(t1);
1582 if (bit_is_set(pos.attacks_from(p, t1), t2))
1585 // Case 5: Discovered check, checking piece is the piece moved in m1
1586 if ( piece_is_slider(p)
1587 && bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), f2)
1588 && !bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), t2))
1590 // discovered_check_candidates() works also if the Position's side to
1591 // move is the opposite of the checking piece.
1592 Color them = opposite_color(pos.side_to_move());
1593 Bitboard dcCandidates = pos.discovered_check_candidates(them);
1595 if (bit_is_set(dcCandidates, f2))
1602 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1603 // "plies to mate from the current ply". Non-mate scores are unchanged.
1604 // The function is called before storing a value to the transposition table.
1606 Value value_to_tt(Value v, int ply) {
1608 if (v >= VALUE_MATE_IN_PLY_MAX)
1611 if (v <= VALUE_MATED_IN_PLY_MAX)
1618 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1619 // the transposition table to a mate score corrected for the current ply.
1621 Value value_from_tt(Value v, int ply) {
1623 if (v >= VALUE_MATE_IN_PLY_MAX)
1626 if (v <= VALUE_MATED_IN_PLY_MAX)
1633 // connected_threat() tests whether it is safe to forward prune a move or if
1634 // is somehow connected to the threat move returned by null search.
1636 bool connected_threat(const Position& pos, Move m, Move threat) {
1638 assert(move_is_ok(m));
1639 assert(threat && move_is_ok(threat));
1640 assert(!pos.move_is_capture_or_promotion(m));
1641 assert(!pos.move_is_passed_pawn_push(m));
1643 Square mfrom, mto, tfrom, tto;
1645 mfrom = move_from(m);
1647 tfrom = move_from(threat);
1648 tto = move_to(threat);
1650 // Case 1: Don't prune moves which move the threatened piece
1654 // Case 2: If the threatened piece has value less than or equal to the
1655 // value of the threatening piece, don't prune moves which defend it.
1656 if ( pos.move_is_capture(threat)
1657 && ( piece_value_midgame(pos.piece_on(tfrom)) >= piece_value_midgame(pos.piece_on(tto))
1658 || piece_type(pos.piece_on(tfrom)) == KING)
1659 && pos.move_attacks_square(m, tto))
1662 // Case 3: If the moving piece in the threatened move is a slider, don't
1663 // prune safe moves which block its ray.
1664 if ( piece_is_slider(pos.piece_on(tfrom))
1665 && bit_is_set(squares_between(tfrom, tto), mto)
1666 && pos.see_sign(m) >= 0)
1673 // ok_to_use_TT() returns true if a transposition table score
1674 // can be used at a given point in search.
1676 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
1678 Value v = value_from_tt(tte->value(), ply);
1680 return ( tte->depth() >= depth
1681 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1682 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1684 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1685 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1689 // refine_eval() returns the transposition table score if
1690 // possible otherwise falls back on static position evaluation.
1692 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1696 Value v = value_from_tt(tte->value(), ply);
1698 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1699 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1706 // update_history() registers a good move that produced a beta-cutoff
1707 // in history and marks as failures all the other moves of that ply.
1709 void update_history(const Position& pos, Move move, Depth depth,
1710 Move movesSearched[], int moveCount) {
1712 Value bonus = Value(int(depth) * int(depth));
1714 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1716 for (int i = 0; i < moveCount - 1; i++)
1718 m = movesSearched[i];
1722 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1727 // update_gains() updates the gains table of a non-capture move given
1728 // the static position evaluation before and after the move.
1730 void update_gains(const Position& pos, Move m, Value before, Value after) {
1733 && before != VALUE_NONE
1734 && after != VALUE_NONE
1735 && pos.captured_piece_type() == PIECE_TYPE_NONE
1736 && !move_is_special(m))
1737 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1741 // current_search_time() returns the number of milliseconds which have passed
1742 // since the beginning of the current search.
1744 int current_search_time(int set) {
1746 static int searchStartTime;
1749 searchStartTime = set;
1751 return get_system_time() - searchStartTime;
1755 // score_to_uci() converts a value to a string suitable for use with the UCI
1756 // protocol specifications:
1758 // cp <x> The score from the engine's point of view in centipawns.
1759 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1760 // use negative values for y.
1762 std::string score_to_uci(Value v, Value alpha, Value beta) {
1764 std::stringstream s;
1766 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1767 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1769 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1771 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1777 // speed_to_uci() returns a string with time stats of current search suitable
1778 // to be sent to UCI gui.
1780 std::string speed_to_uci(int64_t nodes) {
1782 std::stringstream s;
1783 int t = current_search_time();
1785 s << " nodes " << nodes
1786 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1792 // pv_to_uci() returns a string with information on the current PV line
1793 // formatted according to UCI specification.
1795 std::string pv_to_uci(Move pv[], int pvNum) {
1797 std::stringstream s;
1799 s << " multipv " << pvNum << " pv ";
1801 for ( ; *pv != MOVE_NONE; pv++)
1807 // depth_to_uci() returns a string with information on the current depth and
1808 // seldepth formatted according to UCI specification.
1810 std::string depth_to_uci(Depth depth) {
1812 std::stringstream s;
1814 // Retrieve max searched depth among threads
1816 for (int i = 0; i < Threads.size(); i++)
1817 if (Threads[i].maxPly > selDepth)
1818 selDepth = Threads[i].maxPly;
1820 s << " depth " << depth / ONE_PLY << " seldepth " << selDepth;
1826 // poll() performs two different functions: It polls for user input, and it
1827 // looks at the time consumed so far and decides if it's time to abort the
1830 void poll(const Position& pos) {
1832 static int lastInfoTime;
1833 int t = current_search_time();
1836 if (input_available())
1838 // We are line oriented, don't read single chars
1839 std::string command;
1841 if (!std::getline(std::cin, command) || command == "quit")
1843 // Quit the program as soon as possible
1844 Limits.ponder = false;
1845 QuitRequest = StopRequest = true;
1848 else if (command == "stop")
1850 // Stop calculating as soon as possible, but still send the "bestmove"
1851 // and possibly the "ponder" token when finishing the search.
1852 Limits.ponder = false;
1855 else if (command == "ponderhit")
1857 // The opponent has played the expected move. GUI sends "ponderhit" if
1858 // we were told to ponder on the same move the opponent has played. We
1859 // should continue searching but switching from pondering to normal search.
1860 Limits.ponder = false;
1862 if (StopOnPonderhit)
1867 // Print search information
1871 else if (lastInfoTime > t)
1872 // HACK: Must be a new search where we searched less than
1873 // NodesBetweenPolls nodes during the first second of search.
1876 else if (t - lastInfoTime >= 1000)
1881 dbg_print_hit_rate();
1883 // Send info on searched nodes as soon as we return to root
1884 SendSearchedNodes = true;
1887 // Should we stop the search?
1891 bool stillAtFirstMove = FirstRootMove
1892 && !AspirationFailLow
1893 && t > TimeMgr.available_time();
1895 bool noMoreTime = t > TimeMgr.maximum_time()
1896 || stillAtFirstMove;
1898 if ( (Limits.useTimeManagement() && noMoreTime)
1899 || (Limits.maxTime && t >= Limits.maxTime)
1900 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1905 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1906 // while the program is pondering. The point is to work around a wrinkle in
1907 // the UCI protocol: When pondering, the engine is not allowed to give a
1908 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1909 // We simply wait here until one of these commands is sent, and return,
1910 // after which the bestmove and pondermove will be printed.
1912 void wait_for_stop_or_ponderhit() {
1914 std::string command;
1916 // Wait for a command from stdin
1917 while ( std::getline(std::cin, command)
1918 && command != "ponderhit" && command != "stop" && command != "quit") {};
1920 if (command != "ponderhit" && command != "stop")
1921 QuitRequest = true; // Must be "quit" or getline() returned false
1925 // When playing with strength handicap choose best move among the MultiPV set
1926 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1927 void do_skill_level(Move* best, Move* ponder) {
1929 assert(MultiPV > 1);
1933 // Rml list is already sorted by pv_score in descending order
1935 int max_s = -VALUE_INFINITE;
1936 int size = Min(MultiPV, (int)Rml.size());
1937 int max = Rml[0].pv_score;
1938 int var = Min(max - Rml[size - 1].pv_score, PawnValueMidgame);
1939 int wk = 120 - 2 * SkillLevel;
1941 // PRNG sequence should be non deterministic
1942 for (int i = abs(get_system_time() % 50); i > 0; i--)
1943 rk.rand<unsigned>();
1945 // Choose best move. For each move's score we add two terms both dependent
1946 // on wk, one deterministic and bigger for weaker moves, and one random,
1947 // then we choose the move with the resulting highest score.
1948 for (int i = 0; i < size; i++)
1950 s = Rml[i].pv_score;
1952 // Don't allow crazy blunders even at very low skills
1953 if (i > 0 && Rml[i-1].pv_score > s + EasyMoveMargin)
1956 // This is our magical formula
1957 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
1962 *best = Rml[i].pv[0];
1963 *ponder = Rml[i].pv[1];
1969 /// RootMove and RootMoveList method's definitions
1971 RootMove::RootMove() {
1974 pv_score = non_pv_score = -VALUE_INFINITE;
1978 RootMove& RootMove::operator=(const RootMove& rm) {
1980 const Move* src = rm.pv;
1983 // Avoid a costly full rm.pv[] copy
1984 do *dst++ = *src; while (*src++ != MOVE_NONE);
1987 pv_score = rm.pv_score;
1988 non_pv_score = rm.non_pv_score;
1992 void RootMoveList::init(Position& pos, Move searchMoves[]) {
1995 bestMoveChanges = 0;
1998 // Generate all legal moves and add them to RootMoveList
1999 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
2001 // If we have a searchMoves[] list then verify the move
2002 // is in the list before to add it.
2003 for (sm = searchMoves; *sm && *sm != ml.move(); sm++) {}
2005 if (sm != searchMoves && *sm != ml.move())
2009 rm.pv[0] = ml.move();
2010 rm.pv[1] = MOVE_NONE;
2011 rm.pv_score = -VALUE_INFINITE;
2016 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
2017 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
2018 // allow to always have a ponder move even when we fail high at root and also a
2019 // long PV to print that is important for position analysis.
2021 void RootMove::extract_pv_from_tt(Position& pos) {
2023 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2027 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2029 pos.do_move(pv[0], *st++);
2031 while ( (tte = TT.probe(pos.get_key())) != NULL
2032 && tte->move() != MOVE_NONE
2033 && pos.move_is_pl(tte->move())
2034 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces(pos.side_to_move()))
2036 && (!pos.is_draw<false>() || ply < 2))
2038 pv[ply] = tte->move();
2039 pos.do_move(pv[ply++], *st++);
2041 pv[ply] = MOVE_NONE;
2043 do pos.undo_move(pv[--ply]); while (ply);
2046 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2047 // the PV back into the TT. This makes sure the old PV moves are searched
2048 // first, even if the old TT entries have been overwritten.
2050 void RootMove::insert_pv_in_tt(Position& pos) {
2052 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2055 Value v, m = VALUE_NONE;
2058 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2064 // Don't overwrite existing correct entries
2065 if (!tte || tte->move() != pv[ply])
2067 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2068 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2070 pos.do_move(pv[ply], *st++);
2072 } while (pv[++ply] != MOVE_NONE);
2074 do pos.undo_move(pv[--ply]); while (ply);
2077 // Specializations for MovePickerExt in case of Root node
2078 MovePickerExt<Root>::MovePickerExt(const Position& p, Move ttm, Depth d,
2079 const History& h, SearchStack* ss, Value b)
2080 : MovePicker(p, ttm, d, h, ss, b), cur(-1) {
2082 Value score = VALUE_ZERO;
2084 // Score root moves using standard ordering used in main search, the moves
2085 // are scored according to the order in which they are returned by MovePicker.
2086 // This is the second order score that is used to compare the moves when
2087 // the first orders pv_score of both moves are equal.
2088 while ((move = MovePicker::get_next_move()) != MOVE_NONE)
2089 for (RootMoveList::iterator rm = Rml.begin(); rm != Rml.end(); ++rm)
2090 if (rm->pv[0] == move)
2092 rm->non_pv_score = score--;
2102 // ThreadsManager::idle_loop() is where the threads are parked when they have no work
2103 // to do. The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint
2104 // object for which the current thread is the master.
2106 void ThreadsManager::idle_loop(int threadID, SplitPoint* sp) {
2108 assert(threadID >= 0 && threadID < MAX_THREADS);
2115 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2116 // master should exit as last one.
2117 if (allThreadsShouldExit)
2120 threads[threadID].state = Thread::TERMINATED;
2124 // If we are not thinking, wait for a condition to be signaled
2125 // instead of wasting CPU time polling for work.
2126 while ( threadID >= activeThreads
2127 || threads[threadID].state == Thread::INITIALIZING
2128 || (useSleepingThreads && threads[threadID].state == Thread::AVAILABLE))
2130 assert(!sp || useSleepingThreads);
2131 assert(threadID != 0 || useSleepingThreads);
2133 if (threads[threadID].state == Thread::INITIALIZING)
2134 threads[threadID].state = Thread::AVAILABLE;
2136 // Grab the lock to avoid races with Thread::wake_up()
2137 lock_grab(&threads[threadID].sleepLock);
2139 // If we are master and all slaves have finished do not go to sleep
2140 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2141 allFinished = (i == activeThreads);
2143 if (allFinished || allThreadsShouldExit)
2145 lock_release(&threads[threadID].sleepLock);
2149 // Do sleep here after retesting sleep conditions
2150 if (threadID >= activeThreads || threads[threadID].state == Thread::AVAILABLE)
2151 cond_wait(&threads[threadID].sleepCond, &threads[threadID].sleepLock);
2153 lock_release(&threads[threadID].sleepLock);
2156 // If this thread has been assigned work, launch a search
2157 if (threads[threadID].state == Thread::WORKISWAITING)
2159 assert(!allThreadsShouldExit);
2161 threads[threadID].state = Thread::SEARCHING;
2163 // Copy split point position and search stack and call search()
2164 // with SplitPoint template parameter set to true.
2165 SearchStack ss[PLY_MAX_PLUS_2];
2166 SplitPoint* tsp = threads[threadID].splitPoint;
2167 Position pos(*tsp->pos, threadID);
2169 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2173 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2175 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2177 assert(threads[threadID].state == Thread::SEARCHING);
2179 threads[threadID].state = Thread::AVAILABLE;
2181 // Wake up master thread so to allow it to return from the idle loop in
2182 // case we are the last slave of the split point.
2183 if ( useSleepingThreads
2184 && threadID != tsp->master
2185 && threads[tsp->master].state == Thread::AVAILABLE)
2186 threads[tsp->master].wake_up();
2189 // If this thread is the master of a split point and all slaves have
2190 // finished their work at this split point, return from the idle loop.
2191 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2192 allFinished = (i == activeThreads);
2196 // Because sp->slaves[] is reset under lock protection,
2197 // be sure sp->lock has been released before to return.
2198 lock_grab(&(sp->lock));
2199 lock_release(&(sp->lock));
2201 // In helpful master concept a master can help only a sub-tree, and
2202 // because here is all finished is not possible master is booked.
2203 assert(threads[threadID].state == Thread::AVAILABLE);
2205 threads[threadID].state = Thread::SEARCHING;