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 { NonPV, PV };
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);
75 std::string pv_info_to_uci(Position& pos, int depth, int selDepth,
76 Value alpha, Value beta, int pvIdx);
80 Move pv[PLY_MAX_PLUS_2];
83 // RootMoveList struct is just a vector of RootMove objects,
84 // with an handful of methods above the standard ones.
85 struct RootMoveList : public std::vector<RootMove> {
87 typedef std::vector<RootMove> Base;
89 void init(Position& pos, Move searchMoves[]);
90 void sort() { insertion_sort<RootMove, Base::iterator>(begin(), end()); }
91 void sort_multipv(int n) { insertion_sort<RootMove, Base::iterator>(begin(), begin() + n); }
96 // MovePickerExt template class extends MovePicker and allows to choose at compile
97 // time the proper moves source according to the type of node. In the default case
98 // we simply create and use a standard MovePicker object.
99 template<bool SpNode, bool Root> struct MovePickerExt : public MovePicker {
101 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
102 : MovePicker(p, ttm, d, h, ss, b) {}
104 RootMoveList::iterator rm; // Dummy, needed to compile
107 // In case of a SpNode we use split point's shared MovePicker object as moves source
108 template<> struct MovePickerExt<true, false> : public MovePicker {
110 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, SearchStack* ss, Value b)
111 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
113 Move get_next_move() { return mp->get_next_move(); }
115 RootMoveList::iterator rm; // Dummy, needed to compile
119 // In case of a Root node we use RootMoveList as moves source
120 template<> struct MovePickerExt<false, true> : public MovePicker {
122 MovePickerExt(const Position&, Move, Depth, const History&, SearchStack*, Value);
123 Move get_next_move();
125 RootMoveList::iterator rm;
132 // Lookup table to check if a Piece is a slider and its access function
133 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
134 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
138 // Maximum depth for razoring
139 const Depth RazorDepth = 4 * ONE_PLY;
141 // Dynamic razoring margin based on depth
142 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
144 // Maximum depth for use of dynamic threat detection when null move fails low
145 const Depth ThreatDepth = 5 * ONE_PLY;
147 // Step 9. Internal iterative deepening
149 // Minimum depth for use of internal iterative deepening
150 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
152 // At Non-PV nodes we do an internal iterative deepening search
153 // when the static evaluation is bigger then beta - IIDMargin.
154 const Value IIDMargin = Value(0x100);
156 // Step 11. Decide the new search depth
158 // Extensions. Array index 0 is used for non-PV nodes, index 1 for PV nodes
159 const Depth CheckExtension[] = { ONE_PLY / 2, ONE_PLY / 1 };
160 const Depth PawnEndgameExtension[] = { ONE_PLY / 1, ONE_PLY / 1 };
161 const Depth PawnPushTo7thExtension[] = { ONE_PLY / 2, ONE_PLY / 2 };
162 const Depth PassedPawnExtension[] = { DEPTH_ZERO, ONE_PLY / 2 };
164 // Minimum depth for use of singular extension
165 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
167 // Step 12. Futility pruning
169 // Futility margin for quiescence search
170 const Value FutilityMarginQS = Value(0x80);
172 // Futility lookup tables (initialized at startup) and their access functions
173 Value FutilityMargins[16][64]; // [depth][moveNumber]
174 int FutilityMoveCounts[32]; // [depth]
176 inline Value futility_margin(Depth d, int mn) {
178 return d < 7 * ONE_PLY ? FutilityMargins[Max(d, 1)][Min(mn, 63)]
179 : 2 * VALUE_INFINITE;
182 inline int futility_move_count(Depth d) {
184 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
187 // Step 14. Reduced search
189 // Reduction lookup tables (initialized at startup) and their access function
190 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
192 template <NodeType PV> inline Depth reduction(Depth d, int mn) {
194 return (Depth) Reductions[PV][Min(d / ONE_PLY, 63)][Min(mn, 63)];
197 // Easy move margin. An easy move candidate must be at least this much
198 // better than the second best move.
199 const Value EasyMoveMargin = Value(0x200);
202 /// Namespace variables
208 int MultiPV, UCIMultiPV;
210 // Time management variables
211 bool StopOnPonderhit, FirstRootMove, StopRequest, QuitRequest, AspirationFailLow;
216 std::ofstream LogFile;
218 // Skill level adjustment
220 bool SkillLevelEnabled;
222 // Node counters, used only by thread[0] but try to keep in different cache
223 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
224 bool SendSearchedNodes;
226 int NodesBetweenPolls = 30000;
234 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove);
236 template <NodeType PvNode, bool SpNode, bool Root>
237 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
239 template <NodeType PvNode>
240 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth);
242 template <NodeType PvNode>
243 inline Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
245 return depth < ONE_PLY ? qsearch<PvNode>(pos, ss, alpha, beta, DEPTH_ZERO)
246 : search<PvNode, false, false>(pos, ss, alpha, beta, depth);
249 template <NodeType PvNode>
250 Depth extension(const Position& pos, Move m, bool captureOrPromotion, bool moveIsCheck, bool* dangerous);
252 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
253 bool connected_moves(const Position& pos, Move m1, Move m2);
254 Value value_to_tt(Value v, int ply);
255 Value value_from_tt(Value v, int ply);
256 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
257 bool connected_threat(const Position& pos, Move m, Move threat);
258 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
259 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
260 void update_gains(const Position& pos, Move move, Value before, Value after);
261 void do_skill_level(Move* best, Move* ponder);
263 int current_search_time(int set = 0);
264 std::string value_to_uci(Value v);
265 std::string speed_to_uci(int64_t nodes);
266 void poll(const Position& pos);
267 void wait_for_stop_or_ponderhit();
269 // Overload operator<<() to make it easier to print moves in a coordinate
270 // notation compatible with UCI protocol.
271 std::ostream& operator<<(std::ostream& os, Move m) {
273 bool chess960 = (os.iword(0) != 0); // See set960()
274 return os << move_to_uci(m, chess960);
277 // When formatting a move for std::cout we must know if we are in Chess960
278 // or not. To keep using the handy operator<<() on the move the trick is to
279 // embed this flag in the stream itself. Function-like named enum set960 is
280 // used as a custom manipulator and the stream internal general-purpose array,
281 // accessed through ios_base::iword(), is used to pass the flag to the move's
282 // operator<<() that will read it to properly format castling moves.
285 std::ostream& operator<< (std::ostream& os, const set960& f) {
287 os.iword(0) = int(f);
294 /// init_search() is called during startup to initialize various lookup tables
298 int d; // depth (ONE_PLY == 2)
299 int hd; // half depth (ONE_PLY == 1)
302 // Init reductions array
303 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
305 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
306 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
307 Reductions[PV][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
308 Reductions[NonPV][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
311 // Init futility margins array
312 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
313 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
315 // Init futility move count array
316 for (d = 0; d < 32; d++)
317 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
321 /// perft() is our utility to verify move generation. All the leaf nodes up to
322 /// the given depth are generated and counted and the sum returned.
324 int64_t perft(Position& pos, Depth depth) {
326 MoveStack mlist[MAX_MOVES];
331 // Generate all legal moves
332 MoveStack* last = generate<MV_LEGAL>(pos, mlist);
334 // If we are at the last ply we don't need to do and undo
335 // the moves, just to count them.
336 if (depth <= ONE_PLY)
337 return int(last - mlist);
339 // Loop through all legal moves
341 for (MoveStack* cur = mlist; cur != last; cur++)
344 pos.do_move(m, st, ci, pos.move_gives_check(m, ci));
345 sum += perft(pos, depth - ONE_PLY);
352 /// think() is the external interface to Stockfish's search, and is called when
353 /// the program receives the UCI 'go' command. It initializes various global
354 /// variables, and calls id_loop(). It returns false when a "quit" command is
355 /// received during the search.
357 bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
361 // Initialize global search-related variables
362 StopOnPonderhit = StopRequest = QuitRequest = AspirationFailLow = SendSearchedNodes = false;
364 current_search_time(get_system_time());
366 TimeMgr.init(Limits, pos.startpos_ply_counter());
368 // Set best NodesBetweenPolls interval to avoid lagging under time pressure
370 NodesBetweenPolls = Min(Limits.maxNodes, 30000);
371 else if (Limits.time && Limits.time < 1000)
372 NodesBetweenPolls = 1000;
373 else if (Limits.time && Limits.time < 5000)
374 NodesBetweenPolls = 5000;
376 NodesBetweenPolls = 30000;
378 // Look for a book move
379 if (Options["OwnBook"].value<bool>())
381 if (Options["Book File"].value<std::string>() != book.name())
382 book.open(Options["Book File"].value<std::string>());
384 Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
385 if (bookMove != MOVE_NONE)
388 wait_for_stop_or_ponderhit();
390 cout << "bestmove " << bookMove << endl;
396 UCIMultiPV = Options["MultiPV"].value<int>();
397 SkillLevel = Options["Skill Level"].value<int>();
399 read_evaluation_uci_options(pos.side_to_move());
400 Threads.read_uci_options();
402 // If needed allocate pawn and material hash tables and adjust TT size
403 Threads.init_hash_tables();
404 TT.set_size(Options["Hash"].value<int>());
406 if (Options["Clear Hash"].value<bool>())
408 Options["Clear Hash"].set_value("false");
412 // Do we have to play with skill handicap? In this case enable MultiPV that
413 // we will use behind the scenes to retrieve a set of possible moves.
414 SkillLevelEnabled = (SkillLevel < 20);
415 MultiPV = (SkillLevelEnabled ? Max(UCIMultiPV, 4) : UCIMultiPV);
417 // Wake up needed threads and reset maxPly counter
418 for (int i = 0; i < Threads.size(); i++)
420 Threads[i].wake_up();
421 Threads[i].maxPly = 0;
424 // Write to log file and keep it open to be accessed during the search
425 if (Options["Use Search Log"].value<bool>())
427 std::string name = Options["Search Log Filename"].value<std::string>();
428 LogFile.open(name.c_str(), std::ios::out | std::ios::app);
430 if (LogFile.is_open())
431 LogFile << "\nSearching: " << pos.to_fen()
432 << "\ninfinite: " << Limits.infinite
433 << " ponder: " << Limits.ponder
434 << " time: " << Limits.time
435 << " increment: " << Limits.increment
436 << " moves to go: " << Limits.movesToGo
440 // We're ready to start thinking. Call the iterative deepening loop function
441 Move ponderMove = MOVE_NONE;
442 Move bestMove = id_loop(pos, searchMoves, &ponderMove);
444 cout << "info" << speed_to_uci(pos.nodes_searched()) << endl;
446 // Write final search statistics and close log file
447 if (LogFile.is_open())
449 int t = current_search_time();
451 LogFile << "Nodes: " << pos.nodes_searched()
452 << "\nNodes/second: " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
453 << "\nBest move: " << move_to_san(pos, bestMove);
456 pos.do_move(bestMove, st);
457 LogFile << "\nPonder move: " << move_to_san(pos, ponderMove) << endl;
458 pos.undo_move(bestMove); // Return from think() with unchanged position
462 // This makes all the threads to go to sleep
465 // If we are pondering or in infinite search, we shouldn't print the
466 // best move before we are told to do so.
467 if (!StopRequest && (Limits.ponder || Limits.infinite))
468 wait_for_stop_or_ponderhit();
470 // Could be MOVE_NONE when searching on a stalemate position
471 cout << "bestmove " << bestMove;
473 // UCI protol is not clear on allowing sending an empty ponder move, instead
474 // it is clear that ponder move is optional. So skip it if empty.
475 if (ponderMove != MOVE_NONE)
476 cout << " ponder " << ponderMove;
486 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
487 // with increasing depth until the allocated thinking time has been consumed,
488 // user stops the search, or the maximum search depth is reached.
490 Move id_loop(Position& pos, Move searchMoves[], Move* ponderMove) {
492 SearchStack ss[PLY_MAX_PLUS_2];
493 Value bestValues[PLY_MAX_PLUS_2];
494 int bestMoveChanges[PLY_MAX_PLUS_2];
495 int depth, selDepth, aspirationDelta;
496 Value value, alpha, beta;
497 Move bestMove, easyMove, skillBest, skillPonder;
499 // Initialize stuff before a new search
500 memset(ss, 0, 4 * sizeof(SearchStack));
503 *ponderMove = bestMove = easyMove = skillBest = skillPonder = MOVE_NONE;
504 depth = aspirationDelta = 0;
505 alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
506 ss->currentMove = MOVE_NULL; // Hack to skip update_gains()
508 // Moves to search are verified and copied
509 Rml.init(pos, searchMoves);
511 // Handle special case of searching on a mate/stalemate position
514 cout << "info depth 0 score "
515 << value_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW)
521 // Iterative deepening loop until requested to stop or target depth reached
522 while (!StopRequest && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
524 Rml.bestMoveChanges = 0;
525 cout << set960(pos.is_chess960()) << "info depth " << depth << endl;
527 // Calculate dynamic aspiration window based on previous iterations
528 if (MultiPV == 1 && depth >= 5 && abs(bestValues[depth - 1]) < VALUE_KNOWN_WIN)
530 int prevDelta1 = bestValues[depth - 1] - bestValues[depth - 2];
531 int prevDelta2 = bestValues[depth - 2] - bestValues[depth - 3];
533 aspirationDelta = Min(Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16), 24);
534 aspirationDelta = (aspirationDelta + 7) / 8 * 8; // Round to match grainSize
536 alpha = Max(bestValues[depth - 1] - aspirationDelta, -VALUE_INFINITE);
537 beta = Min(bestValues[depth - 1] + aspirationDelta, VALUE_INFINITE);
540 // Start with a small aspiration window and, in case of fail high/low,
541 // research with bigger window until not failing high/low anymore.
543 // Search starting from ss+1 to allow calling update_gains()
544 value = search<PV, false, true>(pos, ss+1, alpha, beta, depth * ONE_PLY);
546 // Write PV back to transposition table in case the relevant entries
547 // have been overwritten during the search.
548 for (int i = 0; i < Min(MultiPV, (int)Rml.size()); i++)
549 Rml[i].insert_pv_in_tt(pos);
551 // Value cannot be trusted. Break out immediately!
555 assert(value >= alpha);
557 // In case of failing high/low increase aspiration window and research,
558 // otherwise exit the fail high/low loop.
561 beta = Min(beta + aspirationDelta, VALUE_INFINITE);
562 aspirationDelta += aspirationDelta / 2;
564 else if (value <= alpha)
566 AspirationFailLow = true;
567 StopOnPonderhit = false;
569 alpha = Max(alpha - aspirationDelta, -VALUE_INFINITE);
570 aspirationDelta += aspirationDelta / 2;
575 } while (abs(value) < VALUE_KNOWN_WIN);
577 // Collect info about search result
578 bestMove = Rml[0].pv[0];
579 *ponderMove = Rml[0].pv[1];
580 bestValues[depth] = value;
581 bestMoveChanges[depth] = Rml.bestMoveChanges;
583 // Do we need to pick now the best and the ponder moves ?
584 if (SkillLevelEnabled && depth == 1 + SkillLevel)
585 do_skill_level(&skillBest, &skillPonder);
587 // Retrieve max searched depth among threads
589 for (int i = 0; i < Threads.size(); i++)
590 if (Threads[i].maxPly > selDepth)
591 selDepth = Threads[i].maxPly;
593 // Send PV line to GUI and to log file
594 for (int i = 0; i < Min(UCIMultiPV, (int)Rml.size()); i++)
595 cout << Rml[i].pv_info_to_uci(pos, depth, selDepth, alpha, beta, i) << endl;
597 if (LogFile.is_open())
598 LogFile << pretty_pv(pos, depth, value, current_search_time(), Rml[0].pv) << endl;
600 // Init easyMove after first iteration or drop if differs from the best move
601 if (depth == 1 && (Rml.size() == 1 || Rml[0].pv_score > Rml[1].pv_score + EasyMoveMargin))
603 else if (bestMove != easyMove)
604 easyMove = MOVE_NONE;
606 // Check for some early stop condition
607 if (!StopRequest && Limits.useTimeManagement())
609 // Stop search early when the last two iterations returned a mate score
611 && abs(bestValues[depth]) >= VALUE_MATE_IN_PLY_MAX
612 && abs(bestValues[depth - 1]) >= VALUE_MATE_IN_PLY_MAX)
615 // Stop search early if one move seems to be much better than the
616 // others or if there is only a single legal move. Also in the latter
617 // case we search up to some depth anyway to get a proper score.
619 && easyMove == bestMove
621 ||( Rml[0].nodes > (pos.nodes_searched() * 85) / 100
622 && current_search_time() > TimeMgr.available_time() / 16)
623 ||( Rml[0].nodes > (pos.nodes_searched() * 98) / 100
624 && current_search_time() > TimeMgr.available_time() / 32)))
627 // Take in account some extra time if the best move has changed
628 if (depth > 4 && depth < 50)
629 TimeMgr.pv_instability(bestMoveChanges[depth], bestMoveChanges[depth - 1]);
631 // Stop search if most of available time is already consumed. We probably don't
632 // have enough time to search the first move at the next iteration anyway.
633 if (current_search_time() > (TimeMgr.available_time() * 62) / 100)
636 // If we are allowed to ponder do not stop the search now but keep pondering
637 if (StopRequest && Limits.ponder)
640 StopOnPonderhit = true;
645 // When using skills overwrite best and ponder moves with the sub-optimal ones
646 if (SkillLevelEnabled)
648 if (skillBest == MOVE_NONE) // Still unassigned ?
649 do_skill_level(&skillBest, &skillPonder);
651 bestMove = skillBest;
652 *ponderMove = skillPonder;
659 // search<>() is the main search function for both PV and non-PV nodes and for
660 // normal and SplitPoint nodes. When called just after a split point the search
661 // is simpler because we have already probed the hash table, done a null move
662 // search, and searched the first move before splitting, we don't have to repeat
663 // all this work again. We also don't need to store anything to the hash table
664 // here: This is taken care of after we return from the split point.
666 template <NodeType PvNode, bool SpNode, bool Root>
667 Value search(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
669 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
670 assert(beta > alpha && beta <= VALUE_INFINITE);
671 assert(PvNode || alpha == beta - 1);
672 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
674 Move movesSearched[MAX_MOVES];
679 Move ttMove, move, excludedMove, threatMove;
682 Value bestValue, value, oldAlpha;
683 Value refinedValue, nullValue, futilityBase, futilityValueScaled; // Non-PV specific
684 bool isPvMove, inCheck, singularExtensionNode, givesCheck, captureOrPromotion, dangerous;
685 int moveCount = 0, playedMoveCount = 0;
686 int threadID = pos.thread();
687 SplitPoint* sp = NULL;
689 refinedValue = bestValue = value = -VALUE_INFINITE;
691 inCheck = pos.in_check();
692 ss->ply = (ss-1)->ply + 1;
694 // Used to send selDepth info to GUI
695 if (PvNode && Threads[threadID].maxPly < ss->ply)
696 Threads[threadID].maxPly = ss->ply;
702 ttMove = excludedMove = MOVE_NONE;
703 threatMove = sp->threatMove;
704 goto split_point_start;
709 // Step 1. Initialize node and poll. Polling can abort search
710 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
711 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
712 (ss+2)->killers[0] = (ss+2)->killers[1] = (ss+2)->mateKiller = MOVE_NONE;
714 if (threadID == 0 && ++NodesSincePoll > NodesBetweenPolls)
720 // Step 2. Check for aborted search and immediate draw
722 || Threads[threadID].cutoff_occurred()
724 || ss->ply > PLY_MAX) && !Root)
727 // Step 3. Mate distance pruning
728 alpha = Max(value_mated_in(ss->ply), alpha);
729 beta = Min(value_mate_in(ss->ply+1), beta);
733 // Step 4. Transposition table lookup
734 // We don't want the score of a partial search to overwrite a previous full search
735 // TT value, so we use a different position key in case of an excluded move.
736 excludedMove = ss->excludedMove;
737 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
739 tte = TT.probe(posKey);
740 ttMove = tte ? tte->move() : MOVE_NONE;
742 // At PV nodes we check for exact scores, while at non-PV nodes we check for
743 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
744 // smooth experience in analysis mode.
747 && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
748 : ok_to_use_TT(tte, depth, beta, ss->ply)))
751 ss->bestMove = ttMove; // Can be MOVE_NONE
752 return value_from_tt(tte->value(), ss->ply);
755 // Step 5. Evaluate the position statically and update parent's gain statistics
757 ss->eval = ss->evalMargin = VALUE_NONE;
760 assert(tte->static_value() != VALUE_NONE);
762 ss->eval = tte->static_value();
763 ss->evalMargin = tte->static_value_margin();
764 refinedValue = refine_eval(tte, ss->eval, ss->ply);
768 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
769 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
772 // Save gain for the parent non-capture move
773 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
775 // Step 6. Razoring (is omitted in PV nodes)
777 && depth < RazorDepth
779 && refinedValue + razor_margin(depth) < beta
780 && ttMove == MOVE_NONE
781 && abs(beta) < VALUE_MATE_IN_PLY_MAX
782 && !pos.has_pawn_on_7th(pos.side_to_move()))
784 Value rbeta = beta - razor_margin(depth);
785 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
787 // Logically we should return (v + razor_margin(depth)), but
788 // surprisingly this did slightly weaker in tests.
792 // Step 7. Static null move pruning (is omitted in PV nodes)
793 // We're betting that the opponent doesn't have a move that will reduce
794 // the score by more than futility_margin(depth) if we do a null move.
797 && depth < RazorDepth
799 && refinedValue - futility_margin(depth, 0) >= beta
800 && abs(beta) < VALUE_MATE_IN_PLY_MAX
801 && pos.non_pawn_material(pos.side_to_move()))
802 return refinedValue - futility_margin(depth, 0);
804 // Step 8. Null move search with verification search (is omitted in PV nodes)
809 && refinedValue >= beta
810 && abs(beta) < VALUE_MATE_IN_PLY_MAX
811 && pos.non_pawn_material(pos.side_to_move()))
813 ss->currentMove = MOVE_NULL;
815 // Null move dynamic reduction based on depth
816 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
818 // Null move dynamic reduction based on value
819 if (refinedValue - PawnValueMidgame > beta)
822 pos.do_null_move(st);
823 (ss+1)->skipNullMove = true;
824 nullValue = -search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
825 (ss+1)->skipNullMove = false;
826 pos.undo_null_move();
828 if (nullValue >= beta)
830 // Do not return unproven mate scores
831 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
834 if (depth < 6 * ONE_PLY)
837 // Do verification search at high depths
838 ss->skipNullMove = true;
839 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
840 ss->skipNullMove = false;
847 // The null move failed low, which means that we may be faced with
848 // some kind of threat. If the previous move was reduced, check if
849 // the move that refuted the null move was somehow connected to the
850 // move which was reduced. If a connection is found, return a fail
851 // low score (which will cause the reduced move to fail high in the
852 // parent node, which will trigger a re-search with full depth).
853 threatMove = (ss+1)->bestMove;
855 if ( depth < ThreatDepth
857 && threatMove != MOVE_NONE
858 && connected_moves(pos, (ss-1)->currentMove, threatMove))
863 // Step 9. Internal iterative deepening
864 if ( depth >= IIDDepth[PvNode]
865 && ttMove == MOVE_NONE
866 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
868 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
870 ss->skipNullMove = true;
871 search<PvNode>(pos, ss, alpha, beta, d);
872 ss->skipNullMove = false;
874 tte = TT.probe(posKey);
875 ttMove = tte ? tte->move() : MOVE_NONE;
878 split_point_start: // At split points actual search starts from here
880 // Initialize a MovePicker object for the current position
881 MovePickerExt<SpNode, Root> mp(pos, ttMove, depth, H, ss, (PvNode ? -VALUE_INFINITE : beta));
883 Bitboard pinned = pos.pinned_pieces(pos.side_to_move());
884 ss->bestMove = MOVE_NONE;
885 futilityBase = ss->eval + ss->evalMargin;
886 singularExtensionNode = !Root
888 && depth >= SingularExtensionDepth[PvNode]
889 && ttMove != MOVE_NONE
890 && !excludedMove // Do not allow recursive singular extension search
891 && (tte->type() & VALUE_TYPE_LOWER)
892 && tte->depth() >= depth - 3 * ONE_PLY;
895 lock_grab(&(sp->lock));
896 bestValue = sp->bestValue;
899 // Step 10. Loop through moves
900 // Loop through all legal moves until no moves remain or a beta cutoff occurs
901 while ( bestValue < beta
902 && (move = mp.get_next_move()) != MOVE_NONE
903 && !Threads[threadID].cutoff_occurred())
905 assert(move_is_ok(move));
907 if (move == excludedMove)
910 // At PV and SpNode nodes we want the moves to be legal
911 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, pinned))
916 moveCount = ++sp->moveCount;
917 lock_release(&(sp->lock));
924 // This is used by time management
925 FirstRootMove = (moveCount == 1);
927 // Save the current node count before the move is searched
928 nodes = pos.nodes_searched();
930 // If it's time to send nodes info, do it here where we have the
931 // correct accumulated node counts searched by each thread.
932 if (SendSearchedNodes)
934 SendSearchedNodes = false;
935 cout << "info" << speed_to_uci(pos.nodes_searched()) << endl;
938 if (current_search_time() > 2000)
939 cout << "info currmove " << move
940 << " currmovenumber " << moveCount << endl;
943 // At Root and at first iteration do a PV search on all the moves to score root moves
944 isPvMove = (PvNode && moveCount <= (Root ? depth <= ONE_PLY ? 1000 : MultiPV : 1));
945 givesCheck = pos.move_gives_check(move, ci);
946 captureOrPromotion = pos.move_is_capture(move) || move_is_promotion(move);
948 // Step 11. Decide the new search depth
949 ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
951 // Singular extension search. If all moves but one fail low on a search of
952 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
953 // is singular and should be extended. To verify this we do a reduced search
954 // on all the other moves but the ttMove, if result is lower than ttValue minus
955 // a margin then we extend ttMove.
956 if ( singularExtensionNode
958 && pos.pl_move_is_legal(move, pinned)
961 Value ttValue = value_from_tt(tte->value(), ss->ply);
963 if (abs(ttValue) < VALUE_KNOWN_WIN)
965 Value rBeta = ttValue - int(depth);
966 ss->excludedMove = move;
967 ss->skipNullMove = true;
968 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
969 ss->skipNullMove = false;
970 ss->excludedMove = MOVE_NONE;
971 ss->bestMove = MOVE_NONE;
977 // Update current move (this must be done after singular extension search)
978 newDepth = depth - ONE_PLY + ext;
980 // Step 12. Futility pruning (is omitted in PV nodes)
982 && !captureOrPromotion
986 && !move_is_castle(move))
988 // Move count based pruning
989 if ( moveCount >= futility_move_count(depth)
990 && (!threatMove || !connected_threat(pos, move, threatMove))
991 && bestValue > VALUE_MATED_IN_PLY_MAX) // FIXME bestValue is racy
994 lock_grab(&(sp->lock));
999 // Value based pruning
1000 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1001 // but fixing this made program slightly weaker.
1002 Depth predictedDepth = newDepth - reduction<NonPV>(depth, moveCount);
1003 futilityValueScaled = futilityBase + futility_margin(predictedDepth, moveCount)
1004 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1006 if (futilityValueScaled < beta)
1010 lock_grab(&(sp->lock));
1011 if (futilityValueScaled > sp->bestValue)
1012 sp->bestValue = bestValue = futilityValueScaled;
1014 else if (futilityValueScaled > bestValue)
1015 bestValue = futilityValueScaled;
1020 // Prune moves with negative SEE at low depths
1021 if ( predictedDepth < 2 * ONE_PLY
1022 && bestValue > VALUE_MATED_IN_PLY_MAX
1023 && pos.see_sign(move) < 0)
1026 lock_grab(&(sp->lock));
1032 // Check for legality only before to do the move
1033 if (!pos.pl_move_is_legal(move, pinned))
1039 ss->currentMove = move;
1041 // Step 13. Make the move
1042 pos.do_move(move, st, ci, givesCheck);
1044 if (!SpNode && !captureOrPromotion)
1045 movesSearched[playedMoveCount++] = move;
1047 // Step extra. pv search (only in PV nodes)
1048 // The first move in list is the expected PV
1051 // Aspiration window is disabled in multi-pv case
1052 if (Root && MultiPV > 1)
1053 alpha = -VALUE_INFINITE;
1055 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1059 // Step 14. Reduced depth search
1060 // If the move fails high will be re-searched at full depth.
1061 bool doFullDepthSearch = true;
1062 alpha = SpNode ? sp->alpha : alpha;
1064 if ( depth >= 3 * ONE_PLY
1065 && !captureOrPromotion
1067 && !move_is_castle(move)
1068 && ss->killers[0] != move
1069 && ss->killers[1] != move)
1071 ss->reduction = reduction<PvNode>(depth, moveCount);
1074 Depth d = newDepth - ss->reduction;
1075 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1077 doFullDepthSearch = (value > alpha);
1079 ss->reduction = DEPTH_ZERO; // Restore original reduction
1082 // Probcut search for bad captures. If a reduced search returns a value
1083 // very below beta then we can (almost) safely prune the bad capture.
1084 if ( depth >= 3 * ONE_PLY
1085 && depth < 8 * ONE_PLY
1086 && mp.isBadCapture()
1089 && !move_is_promotion(move)
1090 && abs(alpha) < VALUE_MATE_IN_PLY_MAX)
1092 ss->reduction = 3 * ONE_PLY;
1093 Value rAlpha = alpha - 300;
1094 Depth d = newDepth - ss->reduction;
1095 value = -search<NonPV>(pos, ss+1, -(rAlpha+1), -rAlpha, d);
1096 doFullDepthSearch = (value > rAlpha);
1097 ss->reduction = DEPTH_ZERO; // Restore original reduction
1100 // Step 15. Full depth search
1101 if (doFullDepthSearch)
1103 alpha = SpNode ? sp->alpha : alpha;
1104 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1106 // Step extra. pv search (only in PV nodes)
1107 // Search only for possible new PV nodes, if instead value >= beta then
1108 // parent node fails low with value <= alpha and tries another move.
1109 if (PvNode && value > alpha && (Root || value < beta))
1110 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1114 // Step 16. Undo move
1115 pos.undo_move(move);
1117 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1119 // Step 17. Check for new best move
1122 lock_grab(&(sp->lock));
1123 bestValue = sp->bestValue;
1127 if (value > bestValue && !(SpNode && Threads[threadID].cutoff_occurred()))
1132 sp->bestValue = value;
1134 if (!Root && value > alpha)
1136 if (PvNode && value < beta) // We want always alpha < beta
1144 sp->is_betaCutoff = true;
1146 if (value == value_mate_in(ss->ply + 1))
1147 ss->mateKiller = move;
1149 ss->bestMove = move;
1152 sp->ss->bestMove = move;
1158 // Finished searching the move. If StopRequest is true, the search
1159 // was aborted because the user interrupted the search or because we
1160 // ran out of time. In this case, the return value of the search cannot
1161 // be trusted, and we break out of the loop without updating the best
1166 // Remember searched nodes counts for this move
1167 mp.rm->nodes += pos.nodes_searched() - nodes;
1169 // PV move or new best move ?
1170 if (isPvMove || value > alpha)
1173 ss->bestMove = move;
1174 mp.rm->pv_score = value;
1175 mp.rm->extract_pv_from_tt(pos);
1177 // We record how often the best move has been changed in each
1178 // iteration. This information is used for time management: When
1179 // the best move changes frequently, we allocate some more time.
1180 if (!isPvMove && MultiPV == 1)
1181 Rml.bestMoveChanges++;
1183 Rml.sort_multipv(moveCount);
1185 // Update alpha. In multi-pv we don't use aspiration window, so
1186 // set alpha equal to minimum score among the PV lines.
1188 alpha = Rml[Min(moveCount, MultiPV) - 1].pv_score; // FIXME why moveCount?
1189 else if (value > alpha)
1193 mp.rm->pv_score = -VALUE_INFINITE;
1197 // Step 18. Check for split
1200 && depth >= Threads.min_split_depth()
1202 && Threads.available_slave_exists(threadID)
1204 && !Threads[threadID].cutoff_occurred())
1205 Threads.split<FakeSplit>(pos, ss, &alpha, beta, &bestValue, depth,
1206 threatMove, moveCount, &mp, PvNode);
1209 // Step 19. Check for mate and stalemate
1210 // All legal moves have been searched and if there are
1211 // no legal moves, it must be mate or stalemate.
1212 // If one move was excluded return fail low score.
1213 if (!SpNode && !moveCount)
1214 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1216 // Step 20. Update tables
1217 // If the search is not aborted, update the transposition table,
1218 // history counters, and killer moves.
1219 if (!SpNode && !StopRequest && !Threads[threadID].cutoff_occurred())
1221 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1222 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1223 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1225 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1227 // Update killers and history only for non capture moves that fails high
1228 if ( bestValue >= beta
1229 && !pos.move_is_capture(move)
1230 && !move_is_promotion(move))
1232 if (move != ss->killers[0])
1234 ss->killers[1] = ss->killers[0];
1235 ss->killers[0] = move;
1237 update_history(pos, move, depth, movesSearched, playedMoveCount);
1243 // Here we have the lock still grabbed
1244 sp->is_slave[threadID] = false;
1245 sp->nodes += pos.nodes_searched();
1246 lock_release(&(sp->lock));
1249 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1254 // qsearch() is the quiescence search function, which is called by the main
1255 // search function when the remaining depth is zero (or, to be more precise,
1256 // less than ONE_PLY).
1258 template <NodeType PvNode>
1259 Value qsearch(Position& pos, SearchStack* ss, Value alpha, Value beta, Depth depth) {
1261 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1262 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1263 assert(PvNode || alpha == beta - 1);
1265 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1269 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1270 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1273 Value oldAlpha = alpha;
1275 ss->bestMove = ss->currentMove = MOVE_NONE;
1276 ss->ply = (ss-1)->ply + 1;
1278 // Check for an instant draw or maximum ply reached
1279 if (ss->ply > PLY_MAX || pos.is_draw())
1282 // Decide whether or not to include checks, this fixes also the type of
1283 // TT entry depth that we are going to use. Note that in qsearch we use
1284 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1285 inCheck = pos.in_check();
1286 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1288 // Transposition table lookup. At PV nodes, we don't use the TT for
1289 // pruning, but only for move ordering.
1290 tte = TT.probe(pos.get_key());
1291 ttMove = (tte ? tte->move() : MOVE_NONE);
1293 if (!PvNode && tte && ok_to_use_TT(tte, ttDepth, beta, ss->ply))
1295 ss->bestMove = ttMove; // Can be MOVE_NONE
1296 return value_from_tt(tte->value(), ss->ply);
1299 // Evaluate the position statically
1302 bestValue = futilityBase = -VALUE_INFINITE;
1303 ss->eval = evalMargin = VALUE_NONE;
1304 enoughMaterial = false;
1310 assert(tte->static_value() != VALUE_NONE);
1312 evalMargin = tte->static_value_margin();
1313 ss->eval = bestValue = tte->static_value();
1316 ss->eval = bestValue = evaluate(pos, evalMargin);
1318 update_gains(pos, (ss-1)->currentMove, (ss-1)->eval, ss->eval);
1320 // Stand pat. Return immediately if static value is at least beta
1321 if (bestValue >= beta)
1324 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1329 if (PvNode && bestValue > alpha)
1332 // Futility pruning parameters, not needed when in check
1333 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1334 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1337 // Initialize a MovePicker object for the current position, and prepare
1338 // to search the moves. Because the depth is <= 0 here, only captures,
1339 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1341 MovePicker mp(pos, ttMove, depth, H);
1343 Bitboard pinned = pos.pinned_pieces(pos.side_to_move());
1345 // Loop through the moves until no moves remain or a beta cutoff occurs
1346 while ( alpha < beta
1347 && (move = mp.get_next_move()) != MOVE_NONE)
1349 assert(move_is_ok(move));
1351 givesCheck = pos.move_gives_check(move, ci);
1359 && !move_is_promotion(move)
1360 && !pos.move_is_passed_pawn_push(move))
1362 futilityValue = futilityBase
1363 + pos.endgame_value_of_piece_on(move_to(move))
1364 + (move_is_ep(move) ? PawnValueEndgame : VALUE_ZERO);
1366 if (futilityValue < alpha)
1368 if (futilityValue > bestValue)
1369 bestValue = futilityValue;
1373 // Prune moves with negative or equal SEE
1374 if ( futilityBase < beta
1375 && depth < DEPTH_ZERO
1376 && pos.see(move) <= 0)
1380 // Detect non-capture evasions that are candidate to be pruned
1381 evasionPrunable = !PvNode
1383 && bestValue > VALUE_MATED_IN_PLY_MAX
1384 && !pos.move_is_capture(move)
1385 && !pos.can_castle(pos.side_to_move());
1387 // Don't search moves with negative SEE values
1389 && (!inCheck || evasionPrunable)
1391 && !move_is_promotion(move)
1392 && pos.see_sign(move) < 0)
1395 // Don't search useless checks
1400 && !pos.move_is_capture(move)
1401 && !move_is_promotion(move)
1402 && ss->eval + PawnValueMidgame / 4 < beta
1403 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1405 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1406 bestValue = ss->eval + PawnValueMidgame / 4;
1411 // Check for legality only before to do the move
1412 if (!pos.pl_move_is_legal(move, pinned))
1415 // Update current move
1416 ss->currentMove = move;
1418 // Make and search the move
1419 pos.do_move(move, st, ci, givesCheck);
1420 value = -qsearch<PvNode>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1421 pos.undo_move(move);
1423 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1426 if (value > bestValue)
1432 ss->bestMove = move;
1437 // All legal moves have been searched. A special case: If we're in check
1438 // and no legal moves were found, it is checkmate.
1439 if (inCheck && bestValue == -VALUE_INFINITE)
1440 return value_mated_in(ss->ply);
1442 // Update transposition table
1443 ValueType vt = (bestValue <= oldAlpha ? VALUE_TYPE_UPPER : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT);
1444 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, ss->bestMove, ss->eval, evalMargin);
1446 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1452 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1453 // bestValue is updated only when returning false because in that case move
1456 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1458 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1459 Square from, to, ksq, victimSq;
1462 Value futilityValue, bv = *bestValue;
1464 from = move_from(move);
1466 them = opposite_color(pos.side_to_move());
1467 ksq = pos.king_square(them);
1468 kingAtt = pos.attacks_from<KING>(ksq);
1469 pc = pos.piece_on(from);
1471 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1472 oldAtt = pos.attacks_from(pc, from, occ);
1473 newAtt = pos.attacks_from(pc, to, occ);
1475 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1476 b = kingAtt & ~pos.pieces_of_color(them) & ~newAtt & ~(1ULL << to);
1478 if (!(b && (b & (b - 1))))
1481 // Rule 2. Queen contact check is very dangerous
1482 if ( type_of_piece(pc) == QUEEN
1483 && bit_is_set(kingAtt, to))
1486 // Rule 3. Creating new double threats with checks
1487 b = pos.pieces_of_color(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1491 victimSq = pop_1st_bit(&b);
1492 futilityValue = futilityBase + pos.endgame_value_of_piece_on(victimSq);
1494 // Note that here we generate illegal "double move"!
1495 if ( futilityValue >= beta
1496 && pos.see_sign(make_move(from, victimSq)) >= 0)
1499 if (futilityValue > bv)
1503 // Update bestValue only if check is not dangerous (because we will prune the move)
1509 // connected_moves() tests whether two moves are 'connected' in the sense
1510 // that the first move somehow made the second move possible (for instance
1511 // if the moving piece is the same in both moves). The first move is assumed
1512 // to be the move that was made to reach the current position, while the
1513 // second move is assumed to be a move from the current position.
1515 bool connected_moves(const Position& pos, Move m1, Move m2) {
1517 Square f1, t1, f2, t2;
1520 assert(m1 && move_is_ok(m1));
1521 assert(m2 && move_is_ok(m2));
1523 // Case 1: The moving piece is the same in both moves
1529 // Case 2: The destination square for m2 was vacated by m1
1535 // Case 3: Moving through the vacated square
1536 if ( piece_is_slider(pos.piece_on(f2))
1537 && bit_is_set(squares_between(f2, t2), f1))
1540 // Case 4: The destination square for m2 is defended by the moving piece in m1
1541 p = pos.piece_on(t1);
1542 if (bit_is_set(pos.attacks_from(p, t1), t2))
1545 // Case 5: Discovered check, checking piece is the piece moved in m1
1546 if ( piece_is_slider(p)
1547 && bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), f2)
1548 && !bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), t2))
1550 // discovered_check_candidates() works also if the Position's side to
1551 // move is the opposite of the checking piece.
1552 Color them = opposite_color(pos.side_to_move());
1553 Bitboard dcCandidates = pos.discovered_check_candidates(them);
1555 if (bit_is_set(dcCandidates, f2))
1562 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1563 // "plies to mate from the current ply". Non-mate scores are unchanged.
1564 // The function is called before storing a value to the transposition table.
1566 Value value_to_tt(Value v, int ply) {
1568 if (v >= VALUE_MATE_IN_PLY_MAX)
1571 if (v <= VALUE_MATED_IN_PLY_MAX)
1578 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1579 // the transposition table to a mate score corrected for the current ply.
1581 Value value_from_tt(Value v, int ply) {
1583 if (v >= VALUE_MATE_IN_PLY_MAX)
1586 if (v <= VALUE_MATED_IN_PLY_MAX)
1593 // extension() decides whether a move should be searched with normal depth,
1594 // or with extended depth. Certain classes of moves (checking moves, in
1595 // particular) are searched with bigger depth than ordinary moves and in
1596 // any case are marked as 'dangerous'. Note that also if a move is not
1597 // extended, as example because the corresponding UCI option is set to zero,
1598 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
1599 template <NodeType PvNode>
1600 Depth extension(const Position& pos, Move m, bool captureOrPromotion,
1601 bool moveIsCheck, bool* dangerous) {
1603 assert(m != MOVE_NONE);
1605 Depth result = DEPTH_ZERO;
1606 *dangerous = moveIsCheck;
1608 if (moveIsCheck && pos.see_sign(m) >= 0)
1609 result += CheckExtension[PvNode];
1611 if (pos.type_of_piece_on(move_from(m)) == PAWN)
1613 Color c = pos.side_to_move();
1614 if (relative_rank(c, move_to(m)) == RANK_7)
1616 result += PawnPushTo7thExtension[PvNode];
1619 if (pos.pawn_is_passed(c, move_to(m)))
1621 result += PassedPawnExtension[PvNode];
1626 if ( captureOrPromotion
1627 && pos.type_of_piece_on(move_to(m)) != PAWN
1628 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
1629 - pos.midgame_value_of_piece_on(move_to(m)) == VALUE_ZERO)
1630 && !move_is_special(m))
1632 result += PawnEndgameExtension[PvNode];
1636 return Min(result, ONE_PLY);
1640 // connected_threat() tests whether it is safe to forward prune a move or if
1641 // is somehow connected to the threat move returned by null search.
1643 bool connected_threat(const Position& pos, Move m, Move threat) {
1645 assert(move_is_ok(m));
1646 assert(threat && move_is_ok(threat));
1647 assert(!pos.move_gives_check(m));
1648 assert(!pos.move_is_capture(m) && !move_is_promotion(m));
1649 assert(!pos.move_is_passed_pawn_push(m));
1651 Square mfrom, mto, tfrom, tto;
1653 mfrom = move_from(m);
1655 tfrom = move_from(threat);
1656 tto = move_to(threat);
1658 // Case 1: Don't prune moves which move the threatened piece
1662 // Case 2: If the threatened piece has value less than or equal to the
1663 // value of the threatening piece, don't prune moves which defend it.
1664 if ( pos.move_is_capture(threat)
1665 && ( pos.midgame_value_of_piece_on(tfrom) >= pos.midgame_value_of_piece_on(tto)
1666 || pos.type_of_piece_on(tfrom) == KING)
1667 && pos.move_attacks_square(m, tto))
1670 // Case 3: If the moving piece in the threatened move is a slider, don't
1671 // prune safe moves which block its ray.
1672 if ( piece_is_slider(pos.piece_on(tfrom))
1673 && bit_is_set(squares_between(tfrom, tto), mto)
1674 && pos.see_sign(m) >= 0)
1681 // ok_to_use_TT() returns true if a transposition table score
1682 // can be used at a given point in search.
1684 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
1686 Value v = value_from_tt(tte->value(), ply);
1688 return ( tte->depth() >= depth
1689 || v >= Max(VALUE_MATE_IN_PLY_MAX, beta)
1690 || v < Min(VALUE_MATED_IN_PLY_MAX, beta))
1692 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1693 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1697 // refine_eval() returns the transposition table score if
1698 // possible otherwise falls back on static position evaluation.
1700 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1704 Value v = value_from_tt(tte->value(), ply);
1706 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1707 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1714 // update_history() registers a good move that produced a beta-cutoff
1715 // in history and marks as failures all the other moves of that ply.
1717 void update_history(const Position& pos, Move move, Depth depth,
1718 Move movesSearched[], int moveCount) {
1720 Value bonus = Value(int(depth) * int(depth));
1722 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1724 for (int i = 0; i < moveCount - 1; i++)
1726 m = movesSearched[i];
1730 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1735 // update_gains() updates the gains table of a non-capture move given
1736 // the static position evaluation before and after the move.
1738 void update_gains(const Position& pos, Move m, Value before, Value after) {
1741 && before != VALUE_NONE
1742 && after != VALUE_NONE
1743 && pos.captured_piece_type() == PIECE_TYPE_NONE
1744 && !move_is_special(m))
1745 H.update_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
1749 // current_search_time() returns the number of milliseconds which have passed
1750 // since the beginning of the current search.
1752 int current_search_time(int set) {
1754 static int searchStartTime;
1757 searchStartTime = set;
1759 return get_system_time() - searchStartTime;
1763 // value_to_uci() converts a value to a string suitable for use with the UCI
1764 // protocol specifications:
1766 // cp <x> The score from the engine's point of view in centipawns.
1767 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1768 // use negative values for y.
1770 std::string value_to_uci(Value v) {
1772 std::stringstream s;
1774 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1775 s << "cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1777 s << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1783 // speed_to_uci() returns a string with time stats of current search suitable
1784 // to be sent to UCI gui.
1786 std::string speed_to_uci(int64_t nodes) {
1788 std::stringstream s;
1789 int t = current_search_time();
1791 s << " nodes " << nodes
1792 << " nps " << (t > 0 ? int(nodes * 1000 / t) : 0)
1799 // poll() performs two different functions: It polls for user input, and it
1800 // looks at the time consumed so far and decides if it's time to abort the
1803 void poll(const Position& pos) {
1805 static int lastInfoTime;
1806 int t = current_search_time();
1809 if (input_available())
1811 // We are line oriented, don't read single chars
1812 std::string command;
1814 if (!std::getline(std::cin, command) || command == "quit")
1816 // Quit the program as soon as possible
1817 Limits.ponder = false;
1818 QuitRequest = StopRequest = true;
1821 else if (command == "stop")
1823 // Stop calculating as soon as possible, but still send the "bestmove"
1824 // and possibly the "ponder" token when finishing the search.
1825 Limits.ponder = false;
1828 else if (command == "ponderhit")
1830 // The opponent has played the expected move. GUI sends "ponderhit" if
1831 // we were told to ponder on the same move the opponent has played. We
1832 // should continue searching but switching from pondering to normal search.
1833 Limits.ponder = false;
1835 if (StopOnPonderhit)
1840 // Print search information
1844 else if (lastInfoTime > t)
1845 // HACK: Must be a new search where we searched less than
1846 // NodesBetweenPolls nodes during the first second of search.
1849 else if (t - lastInfoTime >= 1000)
1854 dbg_print_hit_rate();
1856 // Send info on searched nodes as soon as we return to root
1857 SendSearchedNodes = true;
1860 // Should we stop the search?
1864 bool stillAtFirstMove = FirstRootMove
1865 && !AspirationFailLow
1866 && t > TimeMgr.available_time();
1868 bool noMoreTime = t > TimeMgr.maximum_time()
1869 || stillAtFirstMove;
1871 if ( (Limits.useTimeManagement() && noMoreTime)
1872 || (Limits.maxTime && t >= Limits.maxTime)
1873 || (Limits.maxNodes && pos.nodes_searched() >= Limits.maxNodes)) // FIXME
1878 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
1879 // while the program is pondering. The point is to work around a wrinkle in
1880 // the UCI protocol: When pondering, the engine is not allowed to give a
1881 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
1882 // We simply wait here until one of these commands is sent, and return,
1883 // after which the bestmove and pondermove will be printed.
1885 void wait_for_stop_or_ponderhit() {
1887 std::string command;
1889 // Wait for a command from stdin
1890 while ( std::getline(std::cin, command)
1891 && command != "ponderhit" && command != "stop" && command != "quit") {};
1893 if (command != "ponderhit" && command != "stop")
1894 QuitRequest = true; // Must be "quit" or getline() returned false
1898 // When playing with strength handicap choose best move among the MultiPV set
1899 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1900 void do_skill_level(Move* best, Move* ponder) {
1902 assert(MultiPV > 1);
1906 // Rml list is already sorted by pv_score in descending order
1908 int max_s = -VALUE_INFINITE;
1909 int size = Min(MultiPV, (int)Rml.size());
1910 int max = Rml[0].pv_score;
1911 int var = Min(max - Rml[size - 1].pv_score, PawnValueMidgame);
1912 int wk = 120 - 2 * SkillLevel;
1914 // PRNG sequence should be non deterministic
1915 for (int i = abs(get_system_time() % 50); i > 0; i--)
1916 rk.rand<unsigned>();
1918 // Choose best move. For each move's score we add two terms both dependent
1919 // on wk, one deterministic and bigger for weaker moves, and one random,
1920 // then we choose the move with the resulting highest score.
1921 for (int i = 0; i < size; i++)
1923 s = Rml[i].pv_score;
1925 // Don't allow crazy blunders even at very low skills
1926 if (i > 0 && Rml[i-1].pv_score > s + EasyMoveMargin)
1929 // This is our magical formula
1930 s += ((max - s) * wk + var * (rk.rand<unsigned>() % wk)) / 128;
1935 *best = Rml[i].pv[0];
1936 *ponder = Rml[i].pv[1];
1942 /// RootMove and RootMoveList method's definitions
1944 RootMove::RootMove() {
1947 pv_score = non_pv_score = -VALUE_INFINITE;
1951 RootMove& RootMove::operator=(const RootMove& rm) {
1953 const Move* src = rm.pv;
1956 // Avoid a costly full rm.pv[] copy
1957 do *dst++ = *src; while (*src++ != MOVE_NONE);
1960 pv_score = rm.pv_score;
1961 non_pv_score = rm.non_pv_score;
1965 void RootMoveList::init(Position& pos, Move searchMoves[]) {
1967 MoveStack mlist[MAX_MOVES];
1971 bestMoveChanges = 0;
1973 // Generate all legal moves and add them to RootMoveList
1974 MoveStack* last = generate<MV_LEGAL>(pos, mlist);
1975 for (MoveStack* cur = mlist; cur != last; cur++)
1977 // If we have a searchMoves[] list then verify cur->move
1978 // is in the list before to add it.
1979 for (sm = searchMoves; *sm && *sm != cur->move; sm++) {}
1981 if (searchMoves[0] && *sm != cur->move)
1985 rm.pv[0] = cur->move;
1986 rm.pv[1] = MOVE_NONE;
1987 rm.pv_score = -VALUE_INFINITE;
1992 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1993 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1994 // allow to always have a ponder move even when we fail high at root and also a
1995 // long PV to print that is important for position analysis.
1997 void RootMove::extract_pv_from_tt(Position& pos) {
1999 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2003 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2005 pos.do_move(pv[0], *st++);
2007 while ( (tte = TT.probe(pos.get_key())) != NULL
2008 && tte->move() != MOVE_NONE
2009 && pos.move_is_pl(tte->move())
2010 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces(pos.side_to_move()))
2012 && (!pos.is_draw() || ply < 2))
2014 pv[ply] = tte->move();
2015 pos.do_move(pv[ply++], *st++);
2017 pv[ply] = MOVE_NONE;
2019 do pos.undo_move(pv[--ply]); while (ply);
2022 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
2023 // the PV back into the TT. This makes sure the old PV moves are searched
2024 // first, even if the old TT entries have been overwritten.
2026 void RootMove::insert_pv_in_tt(Position& pos) {
2028 StateInfo state[PLY_MAX_PLUS_2], *st = state;
2031 Value v, m = VALUE_NONE;
2034 assert(pv[0] != MOVE_NONE && pos.move_is_pl(pv[0]));
2040 // Don't overwrite existing correct entries
2041 if (!tte || tte->move() != pv[ply])
2043 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
2044 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
2046 pos.do_move(pv[ply], *st++);
2048 } while (pv[++ply] != MOVE_NONE);
2050 do pos.undo_move(pv[--ply]); while (ply);
2053 // pv_info_to_uci() returns a string with information on the current PV line
2054 // formatted according to UCI specification.
2056 std::string RootMove::pv_info_to_uci(Position& pos, int depth, int selDepth, Value alpha,
2057 Value beta, int pvIdx) {
2058 std::stringstream s;
2060 s << "info depth " << depth
2061 << " seldepth " << selDepth
2062 << " multipv " << pvIdx + 1
2063 << " score " << value_to_uci(pv_score)
2064 << (pv_score >= beta ? " lowerbound" : pv_score <= alpha ? " upperbound" : "")
2065 << speed_to_uci(pos.nodes_searched())
2068 for (Move* m = pv; *m != MOVE_NONE; m++)
2074 // Specializations for MovePickerExt in case of Root node
2075 MovePickerExt<false, true>::MovePickerExt(const Position& p, Move ttm, Depth d,
2076 const History& h, SearchStack* ss, Value b)
2077 : MovePicker(p, ttm, d, h, ss, b), firstCall(true) {
2079 Value score = VALUE_ZERO;
2081 // Score root moves using standard ordering used in main search, the moves
2082 // are scored according to the order in which they are returned by MovePicker.
2083 // This is the second order score that is used to compare the moves when
2084 // the first orders pv_score of both moves are equal.
2085 while ((move = MovePicker::get_next_move()) != MOVE_NONE)
2086 for (rm = Rml.begin(); rm != Rml.end(); ++rm)
2087 if (rm->pv[0] == move)
2089 rm->non_pv_score = score--;
2097 Move MovePickerExt<false, true>::get_next_move() {
2104 return rm != Rml.end() ? rm->pv[0] : MOVE_NONE;
2110 // ThreadsManager::idle_loop() is where the threads are parked when they have no work
2111 // to do. The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint
2112 // object for which the current thread is the master.
2114 void ThreadsManager::idle_loop(int threadID, SplitPoint* sp) {
2116 assert(threadID >= 0 && threadID < MAX_THREADS);
2123 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2124 // master should exit as last one.
2125 if (allThreadsShouldExit)
2128 threads[threadID].state = Thread::TERMINATED;
2132 // If we are not thinking, wait for a condition to be signaled
2133 // instead of wasting CPU time polling for work.
2134 while ( threadID >= activeThreads
2135 || threads[threadID].state == Thread::INITIALIZING
2136 || (useSleepingThreads && threads[threadID].state == Thread::AVAILABLE))
2138 assert(!sp || useSleepingThreads);
2139 assert(threadID != 0 || useSleepingThreads);
2141 if (threads[threadID].state == Thread::INITIALIZING)
2142 threads[threadID].state = Thread::AVAILABLE;
2144 // Grab the lock to avoid races with Thread::wake_up()
2145 lock_grab(&threads[threadID].sleepLock);
2147 // If we are master and all slaves have finished do not go to sleep
2148 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2149 allFinished = (i == activeThreads);
2151 if (allFinished || allThreadsShouldExit)
2153 lock_release(&threads[threadID].sleepLock);
2157 // Do sleep here after retesting sleep conditions
2158 if (threadID >= activeThreads || threads[threadID].state == Thread::AVAILABLE)
2159 cond_wait(&threads[threadID].sleepCond, &threads[threadID].sleepLock);
2161 lock_release(&threads[threadID].sleepLock);
2164 // If this thread has been assigned work, launch a search
2165 if (threads[threadID].state == Thread::WORKISWAITING)
2167 assert(!allThreadsShouldExit);
2169 threads[threadID].state = Thread::SEARCHING;
2171 // Copy split point position and search stack and call search()
2172 // with SplitPoint template parameter set to true.
2173 SearchStack ss[PLY_MAX_PLUS_2];
2174 SplitPoint* tsp = threads[threadID].splitPoint;
2175 Position pos(*tsp->pos, threadID);
2177 memcpy(ss, tsp->ss - 1, 4 * sizeof(SearchStack));
2181 search<PV, true, false>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2183 search<NonPV, true, false>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
2185 assert(threads[threadID].state == Thread::SEARCHING);
2187 threads[threadID].state = Thread::AVAILABLE;
2189 // Wake up master thread so to allow it to return from the idle loop in
2190 // case we are the last slave of the split point.
2191 if ( useSleepingThreads
2192 && threadID != tsp->master
2193 && threads[tsp->master].state == Thread::AVAILABLE)
2194 threads[tsp->master].wake_up();
2197 // If this thread is the master of a split point and all slaves have
2198 // finished their work at this split point, return from the idle loop.
2199 for (i = 0; sp && i < activeThreads && !sp->is_slave[i]; i++) {}
2200 allFinished = (i == activeThreads);
2204 // Because sp->slaves[] is reset under lock protection,
2205 // be sure sp->lock has been released before to return.
2206 lock_grab(&(sp->lock));
2207 lock_release(&(sp->lock));
2209 // In helpful master concept a master can help only a sub-tree, and
2210 // because here is all finished is not possible master is booked.
2211 assert(threads[threadID].state == Thread::AVAILABLE);
2213 threads[threadID].state = Thread::SEARCHING;