2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2010 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
39 #include "ucioption.h"
43 volatile SignalsType Signals;
45 std::vector<Move> SearchMoves;
46 Position RootPosition;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // Different node types, used as template parameter
60 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
62 // RootMove struct is used for moves at the root of the tree. For each root
63 // move we store a score, a node count, and a PV (really a refutation in the
64 // case of moves which fail low). Score is normally set at -VALUE_INFINITE for
71 score = prevScore = -VALUE_INFINITE;
73 pv.push_back(MOVE_NONE);
76 bool operator<(const RootMove& m) const { return score < m.score; }
77 bool operator==(const Move& m) const { return pv[0] == m; }
79 void extract_pv_from_tt(Position& pos);
80 void insert_pv_in_tt(Position& pos);
91 // Lookup table to check if a Piece is a slider and its access function
92 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
93 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
95 // Maximum depth for razoring
96 const Depth RazorDepth = 4 * ONE_PLY;
98 // Dynamic razoring margin based on depth
99 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
101 // Maximum depth for use of dynamic threat detection when null move fails low
102 const Depth ThreatDepth = 5 * ONE_PLY;
104 // Minimum depth for use of internal iterative deepening
105 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
107 // At Non-PV nodes we do an internal iterative deepening search
108 // when the static evaluation is bigger then beta - IIDMargin.
109 const Value IIDMargin = Value(0x100);
111 // Minimum depth for use of singular extension
112 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
114 // Futility margin for quiescence search
115 const Value FutilityMarginQS = Value(0x80);
117 // Futility lookup tables (initialized at startup) and their access functions
118 Value FutilityMargins[16][64]; // [depth][moveNumber]
119 int FutilityMoveCounts[32]; // [depth]
121 inline Value futility_margin(Depth d, int mn) {
123 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
124 : 2 * VALUE_INFINITE;
127 inline int futility_move_count(Depth d) {
129 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
132 // Reduction lookup tables (initialized at startup) and their access function
133 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
135 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
137 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
140 // Easy move margin. An easy move candidate must be at least this much
141 // better than the second best move.
142 const Value EasyMoveMargin = Value(0x150);
145 /// Namespace variables
147 std::vector<RootMove> RootMoves;
148 size_t MultiPV, UCIMultiPV, PVIdx;
152 bool SkillLevelEnabled;
158 template <NodeType NT>
159 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
161 template <NodeType NT>
162 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
164 void id_loop(Position& pos);
165 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
166 bool connected_moves(const Position& pos, Move m1, Move m2);
167 Value value_to_tt(Value v, int ply);
168 Value value_from_tt(Value v, int ply);
169 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
170 bool connected_threat(const Position& pos, Move m, Move threat);
171 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
172 Move do_skill_level();
173 int elapsed_time(bool reset = false);
174 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
175 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
176 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
178 // MovePickerExt class template extends MovePicker and allows to choose at
179 // compile time the proper moves source according to the type of node. In the
180 // default case we simply create and use a standard MovePicker object.
181 template<bool SpNode> struct MovePickerExt : public MovePicker {
183 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
184 : MovePicker(p, ttm, d, h, ss, b) {}
187 // In case of a SpNode we use split point's shared MovePicker object as moves source
188 template<> struct MovePickerExt<true> : public MovePicker {
190 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
191 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
193 Move get_next_move() { return mp->get_next_move(); }
197 // Overload operator<<() to make it easier to print moves in a coordinate
198 // notation compatible with UCI protocol.
199 std::ostream& operator<<(std::ostream& os, Move m) {
201 bool chess960 = (os.iword(0) != 0); // See set960()
202 return os << move_to_uci(m, chess960);
205 // When formatting a move for std::cout we must know if we are in Chess960 or
206 // not. To keep using the handy operator<<() on the move the trick is to embed
207 // this flag in the stream itself. Function-like named enum set960 is used as
208 // a custom manipulator and the stream internal general-purpose array, accessed
209 // through ios_base::iword(), is used to pass the flag to the move's operator<<
210 // that will read it to properly format castling moves.
213 std::ostream& operator<<(std::ostream& os, const set960& f) {
219 // is_dangerous() checks whether a move belongs to some classes of known
220 // 'dangerous' moves so that we avoid to prune it.
221 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
223 // Test for a pawn pushed to 7th or a passed pawn move
224 if (type_of(pos.piece_on(move_from(m))) == PAWN)
226 Color c = pos.side_to_move();
227 if ( relative_rank(c, move_to(m)) == RANK_7
228 || pos.pawn_is_passed(c, move_to(m)))
232 // Test for a capture that triggers a pawn endgame
233 if ( captureOrPromotion
234 && type_of(pos.piece_on(move_to(m))) != PAWN
235 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
236 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
246 /// Search::init() is called during startup to initialize various lookup tables
248 void Search::init() {
250 int d; // depth (ONE_PLY == 2)
251 int hd; // half depth (ONE_PLY == 1)
254 // Init reductions array
255 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
257 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
258 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
259 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
260 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
263 // Init futility margins array
264 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
265 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
267 // Init futility move count array
268 for (d = 0; d < 32; d++)
269 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
273 /// Search::perft() is our utility to verify move generation. All the leaf nodes
274 /// up to the given depth are generated and counted and the sum returned.
276 int64_t Search::perft(Position& pos, Depth depth) {
281 MoveList<MV_LEGAL> ml(pos);
283 // At the last ply just return the number of moves (leaf nodes)
284 if (depth <= ONE_PLY)
288 for ( ; !ml.end(); ++ml)
290 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
291 sum += perft(pos, depth - ONE_PLY);
292 pos.undo_move(ml.move());
298 /// Search::think() is the external interface to Stockfish's search, and is
299 /// called by the main thread when the program receives the UCI 'go' command. It
300 /// searches from RootPosition and at the end prints the "bestmove" to output.
302 void Search::think() {
304 static Book book; // Defined static to initialize the PRNG only once
306 Position& pos = RootPosition;
308 TimeMgr.init(Limits, pos.startpos_ply_counter());
313 // Populate RootMoves with all the legal moves (default) or, if a SearchMoves
314 // is given, with the subset of legal moves to search.
315 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
316 if ( SearchMoves.empty()
317 || std::count(SearchMoves.begin(), SearchMoves.end(), ml.move()))
318 RootMoves.push_back(RootMove(ml.move()));
320 // Set output stream mode: normal or chess960. Castling notation is different
321 cout << set960(pos.is_chess960());
323 if (Options["OwnBook"].value<bool>())
325 if (Options["Book File"].value<string>() != book.name())
326 book.open(Options["Book File"].value<string>());
328 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
330 if ( bookMove != MOVE_NONE
331 && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
333 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
338 // Read UCI options: GUI could change UCI parameters during the game
339 read_evaluation_uci_options(pos.side_to_move());
340 Threads.read_uci_options();
342 TT.set_size(Options["Hash"].value<int>());
343 if (Options["Clear Hash"].value<bool>())
345 Options["Clear Hash"].set_value("false");
349 UCIMultiPV = Options["MultiPV"].value<size_t>();
350 SkillLevel = Options["Skill Level"].value<int>();
352 // Do we have to play with skill handicap? In this case enable MultiPV that
353 // we will use behind the scenes to retrieve a set of possible moves.
354 SkillLevelEnabled = (SkillLevel < 20);
355 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
357 if (Options["Use Search Log"].value<bool>())
359 Log log(Options["Search Log Filename"].value<string>());
360 log << "\nSearching: " << pos.to_fen()
361 << "\ninfinite: " << Limits.infinite
362 << " ponder: " << Limits.ponder
363 << " time: " << Limits.time
364 << " increment: " << Limits.increment
365 << " moves to go: " << Limits.movesToGo
369 for (int i = 0; i < Threads.size(); i++)
371 Threads[i].maxPly = 0;
372 Threads[i].wake_up();
375 // Set best timer interval to avoid lagging under time pressure. Timer is
376 // used to check for remaining available thinking time.
377 if (TimeMgr.available_time())
378 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
380 Threads.set_timer(100);
382 // We're ready to start searching. Call the iterative deepening loop function
385 // Stop timer and send all the slaves to sleep, if not already sleeping
386 Threads.set_timer(0);
389 if (Options["Use Search Log"].value<bool>())
391 int e = elapsed_time();
393 Log log(Options["Search Log Filename"].value<string>());
394 log << "Nodes: " << pos.nodes_searched()
395 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
396 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
399 pos.do_move(RootMoves[0].pv[0], st);
400 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
401 pos.undo_move(RootMoves[0].pv[0]);
406 // When we reach max depth we arrive here even without a StopRequest, but if
407 // we are pondering or in infinite search, we shouldn't print the best move
408 // before we are told to do so.
409 if (!Signals.stop && (Limits.ponder || Limits.infinite))
410 Threads.wait_for_stop_or_ponderhit();
412 // Could be MOVE_NONE when searching on a stalemate position
413 cout << "bestmove " << RootMoves[0].pv[0];
415 // UCI protol is not clear on allowing sending an empty ponder move, instead
416 // it is clear that ponder move is optional. So skip it if empty.
417 if (RootMoves[0].pv[1] != MOVE_NONE)
418 cout << " ponder " << RootMoves[0].pv[1];
426 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
427 // with increasing depth until the allocated thinking time has been consumed,
428 // user stops the search, or the maximum search depth is reached.
430 void id_loop(Position& pos) {
432 Stack ss[PLY_MAX_PLUS_2];
433 int depth, prevBestMoveChanges;
434 Value bestValue, alpha, beta, delta;
435 bool bestMoveNeverChanged = true;
436 Move skillBest = MOVE_NONE;
438 memset(ss, 0, 4 * sizeof(Stack));
439 depth = BestMoveChanges = 0;
440 bestValue = delta = -VALUE_INFINITE;
441 ss->currentMove = MOVE_NULL; // Hack to skip update gains
443 // Handle the special case of a mate/stalemate position
444 if (RootMoves.empty())
446 cout << "info depth 0"
447 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
449 RootMoves.push_back(MOVE_NONE);
453 // Iterative deepening loop until requested to stop or target depth reached
454 while (!Signals.stop && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
456 // Save last iteration's scores before first PV line is searched and all
457 // the move scores but the (new) PV are set to -VALUE_INFINITE.
458 for (size_t i = 0; i < RootMoves.size(); i++)
459 RootMoves[i].prevScore = RootMoves[i].score;
461 prevBestMoveChanges = BestMoveChanges;
464 // MultiPV loop. We perform a full root search for each PV line
465 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
467 // Set aspiration window default width
468 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
471 alpha = RootMoves[PVIdx].prevScore - delta;
472 beta = RootMoves[PVIdx].prevScore + delta;
476 alpha = -VALUE_INFINITE;
477 beta = VALUE_INFINITE;
480 // Start with a small aspiration window and, in case of fail high/low,
481 // research with bigger window until not failing high/low anymore.
483 // Search starts from ss+1 to allow referencing (ss-1). This is
484 // needed by update gains and ss copy when splitting at Root.
485 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
487 // Bring to front the best move. It is critical that sorting is
488 // done with a stable algorithm because all the values but the first
489 // and eventually the new best one are set to -VALUE_INFINITE and
490 // we want to keep the same order for all the moves but the new
491 // PV that goes to the front. Note that in case of MultiPV search
492 // the already searched PV lines are preserved.
493 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
495 // In case we have found an exact score and we are going to leave
496 // the fail high/low loop then reorder the PV moves, otherwise
497 // leave the last PV move in its position so to be searched again.
498 // Of course this is needed only in MultiPV search.
499 if (PVIdx && bestValue > alpha && bestValue < beta)
500 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
502 // Write PV back to transposition table in case the relevant
503 // entries have been overwritten during the search.
504 for (size_t i = 0; i <= PVIdx; i++)
505 RootMoves[i].insert_pv_in_tt(pos);
507 // If search has been stopped exit the aspiration window loop.
508 // Sorting and writing PV back to TT is safe becuase RootMoves
509 // is still valid, although refers to previous iteration.
513 // Send full PV info to GUI if we are going to leave the loop or
514 // if we have a fail high/low and we are deep in the search.
515 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
516 pv_info_to_uci(pos, depth, alpha, beta);
518 // In case of failing high/low increase aspiration window and
519 // research, otherwise exit the fail high/low loop.
520 if (bestValue >= beta)
525 else if (bestValue <= alpha)
527 Signals.failedLowAtRoot = true;
528 Signals.stopOnPonderhit = false;
536 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
538 } while (abs(bestValue) < VALUE_KNOWN_WIN);
541 // Skills: Do we need to pick now the best move ?
542 if (SkillLevelEnabled && depth == 1 + SkillLevel)
543 skillBest = do_skill_level();
545 if (Options["Use Search Log"].value<bool>())
546 pv_info_to_log(pos, depth, bestValue, elapsed_time(), &RootMoves[0].pv[0]);
548 // Filter out startup noise when monitoring best move stability
549 if (depth > 2 && BestMoveChanges)
550 bestMoveNeverChanged = false;
552 // Do we have time for the next iteration? Can we stop searching now?
553 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
555 bool stop = false; // Local variable, not the volatile Signals.stop
557 // Take in account some extra time if the best move has changed
558 if (depth > 4 && depth < 50)
559 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
561 // Stop search if most of available time is already consumed. We
562 // probably don't have enough time to search the first move at the
563 // next iteration anyway.
564 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
567 // Stop search early if one move seems to be much better than others
570 && ( bestMoveNeverChanged
571 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
573 Value rBeta = bestValue - EasyMoveMargin;
574 (ss+1)->excludedMove = RootMoves[0].pv[0];
575 (ss+1)->skipNullMove = true;
576 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
577 (ss+1)->skipNullMove = false;
578 (ss+1)->excludedMove = MOVE_NONE;
586 // If we are allowed to ponder do not stop the search now but
587 // keep pondering until GUI sends "ponderhit" or "stop".
589 Signals.stopOnPonderhit = true;
596 // When using skills swap best PV line with the sub-optimal one
597 if (SkillLevelEnabled)
599 if (skillBest == MOVE_NONE) // Still unassigned ?
600 skillBest = do_skill_level();
602 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), skillBest));
607 // search<>() is the main search function for both PV and non-PV nodes and for
608 // normal and SplitPoint nodes. When called just after a split point the search
609 // is simpler because we have already probed the hash table, done a null move
610 // search, and searched the first move before splitting, we don't have to repeat
611 // all this work again. We also don't need to store anything to the hash table
612 // here: This is taken care of after we return from the split point.
614 template <NodeType NT>
615 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
617 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
618 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
619 const bool RootNode = (NT == Root || NT == SplitPointRoot);
621 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
622 assert(beta > alpha && beta <= VALUE_INFINITE);
623 assert(PvNode || alpha == beta - 1);
624 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
626 Move movesSearched[MAX_MOVES];
631 Move ttMove, move, excludedMove, threatMove;
634 Value bestValue, value, oldAlpha;
635 Value refinedValue, nullValue, futilityBase, futilityValue;
636 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
637 bool captureOrPromotion, dangerous, doFullDepthSearch;
638 int moveCount = 0, playedMoveCount = 0;
639 Thread& thread = Threads[pos.thread()];
640 SplitPoint* sp = NULL;
642 refinedValue = bestValue = value = -VALUE_INFINITE;
644 inCheck = pos.in_check();
645 ss->ply = (ss-1)->ply + 1;
647 // Used to send selDepth info to GUI
648 if (PvNode && thread.maxPly < ss->ply)
649 thread.maxPly = ss->ply;
651 // Step 1. Initialize node
654 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
655 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
656 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
662 ttMove = excludedMove = MOVE_NONE;
663 threatMove = sp->threatMove;
664 goto split_point_start;
667 // Step 2. Check for aborted search and immediate draw
669 || pos.is_draw<false>()
670 || ss->ply > PLY_MAX) && !RootNode)
673 // Step 3. Mate distance pruning
676 alpha = std::max(value_mated_in(ss->ply), alpha);
677 beta = std::min(value_mate_in(ss->ply+1), beta);
682 // Step 4. Transposition table lookup
683 // We don't want the score of a partial search to overwrite a previous full search
684 // TT value, so we use a different position key in case of an excluded move.
685 excludedMove = ss->excludedMove;
686 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
687 tte = TT.probe(posKey);
688 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
690 // At PV nodes we check for exact scores, while at non-PV nodes we check for
691 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
692 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
693 // we should also update RootMoveList to avoid bogus output.
694 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
695 : can_return_tt(tte, depth, beta, ss->ply)))
698 ss->bestMove = move = ttMove; // Can be MOVE_NONE
699 value = value_from_tt(tte->value(), ss->ply);
703 && !pos.is_capture_or_promotion(move)
704 && move != ss->killers[0])
706 ss->killers[1] = ss->killers[0];
707 ss->killers[0] = move;
712 // Step 5. Evaluate the position statically and update parent's gain statistics
714 ss->eval = ss->evalMargin = VALUE_NONE;
717 assert(tte->static_value() != VALUE_NONE);
719 ss->eval = tte->static_value();
720 ss->evalMargin = tte->static_value_margin();
721 refinedValue = refine_eval(tte, ss->eval, ss->ply);
725 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
726 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
729 // Update gain for the parent non-capture move given the static position
730 // evaluation before and after the move.
731 if ( (move = (ss-1)->currentMove) != MOVE_NULL
732 && (ss-1)->eval != VALUE_NONE
733 && ss->eval != VALUE_NONE
734 && pos.captured_piece_type() == PIECE_TYPE_NONE
735 && !is_special(move))
737 Square to = move_to(move);
738 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
741 // Step 6. Razoring (is omitted in PV nodes)
743 && depth < RazorDepth
745 && refinedValue + razor_margin(depth) < beta
746 && ttMove == MOVE_NONE
747 && abs(beta) < VALUE_MATE_IN_PLY_MAX
748 && !pos.has_pawn_on_7th(pos.side_to_move()))
750 Value rbeta = beta - razor_margin(depth);
751 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
753 // Logically we should return (v + razor_margin(depth)), but
754 // surprisingly this did slightly weaker in tests.
758 // Step 7. Static null move pruning (is omitted in PV nodes)
759 // We're betting that the opponent doesn't have a move that will reduce
760 // the score by more than futility_margin(depth) if we do a null move.
763 && depth < RazorDepth
765 && refinedValue - futility_margin(depth, 0) >= beta
766 && abs(beta) < VALUE_MATE_IN_PLY_MAX
767 && pos.non_pawn_material(pos.side_to_move()))
768 return refinedValue - futility_margin(depth, 0);
770 // Step 8. Null move search with verification search (is omitted in PV nodes)
775 && refinedValue >= beta
776 && abs(beta) < VALUE_MATE_IN_PLY_MAX
777 && pos.non_pawn_material(pos.side_to_move()))
779 ss->currentMove = MOVE_NULL;
781 // Null move dynamic reduction based on depth
782 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
784 // Null move dynamic reduction based on value
785 if (refinedValue - PawnValueMidgame > beta)
788 pos.do_null_move<true>(st);
789 (ss+1)->skipNullMove = true;
790 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
791 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
792 (ss+1)->skipNullMove = false;
793 pos.do_null_move<false>(st);
795 if (nullValue >= beta)
797 // Do not return unproven mate scores
798 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
801 if (depth < 6 * ONE_PLY)
804 // Do verification search at high depths
805 ss->skipNullMove = true;
806 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
807 ss->skipNullMove = false;
814 // The null move failed low, which means that we may be faced with
815 // some kind of threat. If the previous move was reduced, check if
816 // the move that refuted the null move was somehow connected to the
817 // move which was reduced. If a connection is found, return a fail
818 // low score (which will cause the reduced move to fail high in the
819 // parent node, which will trigger a re-search with full depth).
820 threatMove = (ss+1)->bestMove;
822 if ( depth < ThreatDepth
824 && threatMove != MOVE_NONE
825 && connected_moves(pos, (ss-1)->currentMove, threatMove))
830 // Step 9. ProbCut (is omitted in PV nodes)
831 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
832 // and a reduced search returns a value much above beta, we can (almost) safely
833 // prune the previous move.
835 && depth >= RazorDepth + ONE_PLY
838 && excludedMove == MOVE_NONE
839 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
841 Value rbeta = beta + 200;
842 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
844 assert(rdepth >= ONE_PLY);
846 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
849 while ((move = mp.get_next_move()) != MOVE_NONE)
850 if (pos.pl_move_is_legal(move, ci.pinned))
852 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
853 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
860 // Step 10. Internal iterative deepening
861 if ( depth >= IIDDepth[PvNode]
862 && ttMove == MOVE_NONE
863 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
865 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
867 ss->skipNullMove = true;
868 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
869 ss->skipNullMove = false;
871 tte = TT.probe(posKey);
872 ttMove = tte ? tte->move() : MOVE_NONE;
875 split_point_start: // At split points actual search starts from here
877 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
879 ss->bestMove = MOVE_NONE;
880 futilityBase = ss->eval + ss->evalMargin;
881 singularExtensionNode = !RootNode
883 && depth >= SingularExtensionDepth[PvNode]
884 && ttMove != MOVE_NONE
885 && !excludedMove // Recursive singular search is not allowed
886 && (tte->type() & VALUE_TYPE_LOWER)
887 && tte->depth() >= depth - 3 * ONE_PLY;
890 lock_grab(&(sp->lock));
891 bestValue = sp->bestValue;
892 moveCount = sp->moveCount;
894 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
897 // Step 11. Loop through moves
898 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
899 while ( bestValue < beta
900 && (move = mp.get_next_move()) != MOVE_NONE
901 && !thread.cutoff_occurred())
905 if (move == excludedMove)
908 // At root obey the "searchmoves" option and skip moves not listed in Root
909 // Move List, as a consequence any illegal move is also skipped. In MultiPV
910 // mode we also skip PV moves which have been already searched.
911 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
914 // At PV and SpNode nodes we want all moves to be legal since the beginning
915 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
920 moveCount = ++sp->moveCount;
921 lock_release(&(sp->lock));
928 // This is used by time management
929 Signals.firstRootMove = (moveCount == 1);
931 nodes = pos.nodes_searched();
933 if (pos.thread() == 0 && elapsed_time() > 2000)
934 cout << "info depth " << depth / ONE_PLY
935 << " currmove " << move
936 << " currmovenumber " << moveCount + PVIdx << endl;
939 isPvMove = (PvNode && moveCount <= 1);
940 captureOrPromotion = pos.is_capture_or_promotion(move);
941 givesCheck = pos.move_gives_check(move, ci);
942 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
945 // Step 12. Extend checks and, in PV nodes, also dangerous moves
946 if (PvNode && dangerous)
949 else if (givesCheck && pos.see_sign(move) >= 0)
950 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
952 // Singular extension search. If all moves but one fail low on a search of
953 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
954 // is singular and should be extended. To verify this we do a reduced search
955 // on all the other moves but the ttMove, if result is lower than ttValue minus
956 // a margin then we extend ttMove.
957 if ( singularExtensionNode
960 && pos.pl_move_is_legal(move, ci.pinned))
962 Value ttValue = value_from_tt(tte->value(), ss->ply);
964 if (abs(ttValue) < VALUE_KNOWN_WIN)
966 Value rBeta = ttValue - int(depth);
967 ss->excludedMove = move;
968 ss->skipNullMove = true;
969 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
970 ss->skipNullMove = false;
971 ss->excludedMove = MOVE_NONE;
972 ss->bestMove = MOVE_NONE;
978 // Update current move (this must be done after singular extension search)
979 newDepth = depth - ONE_PLY + ext;
981 // Step 13. Futility pruning (is omitted in PV nodes)
983 && !captureOrPromotion
988 && (bestValue > VALUE_MATED_IN_PLY_MAX || bestValue == -VALUE_INFINITE))
990 // Move count based pruning
991 if ( moveCount >= futility_move_count(depth)
992 && (!threatMove || !connected_threat(pos, move, threatMove)))
995 lock_grab(&(sp->lock));
1000 // Value based pruning
1001 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1002 // but fixing this made program slightly weaker.
1003 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1004 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1005 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1007 if (futilityValue < beta)
1010 lock_grab(&(sp->lock));
1015 // Prune moves with negative SEE at low depths
1016 if ( predictedDepth < 2 * ONE_PLY
1017 && pos.see_sign(move) < 0)
1020 lock_grab(&(sp->lock));
1026 // Check for legality only before to do the move
1027 if (!pos.pl_move_is_legal(move, ci.pinned))
1033 ss->currentMove = move;
1034 if (!SpNode && !captureOrPromotion)
1035 movesSearched[playedMoveCount++] = move;
1037 // Step 14. Make the move
1038 pos.do_move(move, st, ci, givesCheck);
1040 // Step 15. Reduced depth search (LMR). If the move fails high will be
1041 // re-searched at full depth.
1042 if ( depth > 3 * ONE_PLY
1044 && !captureOrPromotion
1047 && ss->killers[0] != move
1048 && ss->killers[1] != move)
1050 ss->reduction = reduction<PvNode>(depth, moveCount);
1051 Depth d = newDepth - ss->reduction;
1052 alpha = SpNode ? sp->alpha : alpha;
1054 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1055 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1057 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1058 ss->reduction = DEPTH_ZERO;
1061 doFullDepthSearch = !isPvMove;
1063 // Step 16. Full depth search, when LMR is skipped or fails high
1064 if (doFullDepthSearch)
1066 alpha = SpNode ? sp->alpha : alpha;
1067 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1068 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1071 // Only for PV nodes do a full PV search on the first move or after a fail
1072 // high, in the latter case search only if value < beta, otherwise let the
1073 // parent node to fail low with value <= alpha and to try another move.
1074 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1075 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1076 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1078 // Step 17. Undo move
1079 pos.undo_move(move);
1081 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1083 // Step 18. Check for new best move
1086 lock_grab(&(sp->lock));
1087 bestValue = sp->bestValue;
1091 // Finished searching the move. If StopRequest is true, the search
1092 // was aborted because the user interrupted the search or because we
1093 // ran out of time. In this case, the return value of the search cannot
1094 // be trusted, and we don't update the best move and/or PV.
1095 if (RootNode && !Signals.stop)
1097 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1098 rm.nodes += pos.nodes_searched() - nodes;
1100 // PV move or new best move ?
1101 if (isPvMove || value > alpha)
1104 rm.extract_pv_from_tt(pos);
1106 // We record how often the best move has been changed in each
1107 // iteration. This information is used for time management: When
1108 // the best move changes frequently, we allocate some more time.
1109 if (!isPvMove && MultiPV == 1)
1113 // All other moves but the PV are set to the lowest value, this
1114 // is not a problem when sorting becuase sort is stable and move
1115 // position in the list is preserved, just the PV is pushed up.
1116 rm.score = -VALUE_INFINITE;
1120 if (value > bestValue)
1123 ss->bestMove = move;
1127 && value < beta) // We want always alpha < beta
1130 if (SpNode && !thread.cutoff_occurred())
1132 sp->bestValue = value;
1133 sp->ss->bestMove = move;
1135 sp->is_betaCutoff = (value >= beta);
1139 // Step 19. Check for split
1141 && depth >= Threads.min_split_depth()
1143 && Threads.available_slave_exists(pos.thread())
1145 && !thread.cutoff_occurred())
1146 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1147 threatMove, moveCount, &mp, NT);
1150 // Step 20. Check for mate and stalemate
1151 // All legal moves have been searched and if there are no legal moves, it
1152 // must be mate or stalemate. Note that we can have a false positive in
1153 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1154 // harmless because return value is discarded anyhow in the parent nodes.
1155 // If we are in a singular extension search then return a fail low score.
1157 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1159 // If we have pruned all the moves without searching return a fail-low score
1160 if (bestValue == -VALUE_INFINITE)
1162 assert(!playedMoveCount);
1167 // Step 21. Update tables
1168 // Update transposition table entry, killers and history
1169 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1171 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1172 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1173 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1175 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1177 // Update killers and history for non capture cut-off moves
1178 if (bestValue >= beta && !pos.is_capture_or_promotion(move))
1180 if (move != ss->killers[0])
1182 ss->killers[1] = ss->killers[0];
1183 ss->killers[0] = move;
1186 // Increase history value of the cut-off move
1187 Value bonus = Value(int(depth) * int(depth));
1188 H.add(pos.piece_on(move_from(move)), move_to(move), bonus);
1190 // Decrease history of all the other played non-capture moves
1191 for (int i = 0; i < playedMoveCount - 1; i++)
1193 Move m = movesSearched[i];
1194 H.add(pos.piece_on(move_from(m)), move_to(m), -bonus);
1201 // Here we have the lock still grabbed
1202 sp->is_slave[pos.thread()] = false;
1203 sp->nodes += pos.nodes_searched();
1204 lock_release(&(sp->lock));
1207 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1213 // qsearch() is the quiescence search function, which is called by the main
1214 // search function when the remaining depth is zero (or, to be more precise,
1215 // less than ONE_PLY).
1217 template <NodeType NT>
1218 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1220 const bool PvNode = (NT == PV);
1222 assert(NT == PV || NT == NonPV);
1223 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1224 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1225 assert(PvNode || alpha == beta - 1);
1227 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1231 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1232 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1236 Value oldAlpha = alpha;
1238 ss->bestMove = ss->currentMove = MOVE_NONE;
1239 ss->ply = (ss-1)->ply + 1;
1241 // Check for an instant draw or maximum ply reached
1242 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1245 // Decide whether or not to include checks, this fixes also the type of
1246 // TT entry depth that we are going to use. Note that in qsearch we use
1247 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1248 inCheck = pos.in_check();
1249 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1251 // Transposition table lookup. At PV nodes, we don't use the TT for
1252 // pruning, but only for move ordering.
1253 tte = TT.probe(pos.get_key());
1254 ttMove = (tte ? tte->move() : MOVE_NONE);
1256 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1258 ss->bestMove = ttMove; // Can be MOVE_NONE
1259 return value_from_tt(tte->value(), ss->ply);
1262 // Evaluate the position statically
1265 bestValue = futilityBase = -VALUE_INFINITE;
1266 ss->eval = evalMargin = VALUE_NONE;
1267 enoughMaterial = false;
1273 assert(tte->static_value() != VALUE_NONE);
1275 evalMargin = tte->static_value_margin();
1276 ss->eval = bestValue = tte->static_value();
1279 ss->eval = bestValue = evaluate(pos, evalMargin);
1281 // Stand pat. Return immediately if static value is at least beta
1282 if (bestValue >= beta)
1285 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1290 if (PvNode && bestValue > alpha)
1293 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1294 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1297 // Initialize a MovePicker object for the current position, and prepare
1298 // to search the moves. Because the depth is <= 0 here, only captures,
1299 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1301 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1304 // Loop through the moves until no moves remain or a beta cutoff occurs
1305 while ( bestValue < beta
1306 && (move = mp.get_next_move()) != MOVE_NONE)
1308 assert(is_ok(move));
1310 givesCheck = pos.move_gives_check(move, ci);
1318 && !is_promotion(move)
1319 && !pos.is_passed_pawn_push(move))
1321 futilityValue = futilityBase
1322 + PieceValueEndgame[pos.piece_on(move_to(move))]
1323 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1325 if (futilityValue < beta)
1327 if (futilityValue > bestValue)
1328 bestValue = futilityValue;
1333 // Prune moves with negative or equal SEE
1334 if ( futilityBase < beta
1335 && depth < DEPTH_ZERO
1336 && pos.see(move) <= 0)
1340 // Detect non-capture evasions that are candidate to be pruned
1341 evasionPrunable = !PvNode
1343 && bestValue > VALUE_MATED_IN_PLY_MAX
1344 && !pos.is_capture(move)
1345 && !pos.can_castle(pos.side_to_move());
1347 // Don't search moves with negative SEE values
1349 && (!inCheck || evasionPrunable)
1351 && !is_promotion(move)
1352 && pos.see_sign(move) < 0)
1355 // Don't search useless checks
1360 && !pos.is_capture_or_promotion(move)
1361 && ss->eval + PawnValueMidgame / 4 < beta
1362 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1364 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1365 bestValue = ss->eval + PawnValueMidgame / 4;
1370 // Check for legality only before to do the move
1371 if (!pos.pl_move_is_legal(move, ci.pinned))
1374 ss->currentMove = move;
1376 // Make and search the move
1377 pos.do_move(move, st, ci, givesCheck);
1378 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1379 pos.undo_move(move);
1381 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1384 if (value > bestValue)
1387 ss->bestMove = move;
1391 && value < beta) // We want always alpha < beta
1396 // All legal moves have been searched. A special case: If we're in check
1397 // and no legal moves were found, it is checkmate.
1398 if (inCheck && bestValue == -VALUE_INFINITE)
1399 return value_mated_in(ss->ply);
1401 // Update transposition table
1402 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1403 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1404 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1406 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1408 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1414 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1415 // bestValue is updated only when returning false because in that case move
1418 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1420 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1421 Square from, to, ksq, victimSq;
1424 Value futilityValue, bv = *bestValue;
1426 from = move_from(move);
1428 them = flip(pos.side_to_move());
1429 ksq = pos.king_square(them);
1430 kingAtt = pos.attacks_from<KING>(ksq);
1431 pc = pos.piece_on(from);
1433 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1434 oldAtt = pos.attacks_from(pc, from, occ);
1435 newAtt = pos.attacks_from(pc, to, occ);
1437 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1438 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1440 if (!(b && (b & (b - 1))))
1443 // Rule 2. Queen contact check is very dangerous
1444 if ( type_of(pc) == QUEEN
1445 && bit_is_set(kingAtt, to))
1448 // Rule 3. Creating new double threats with checks
1449 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1453 victimSq = pop_1st_bit(&b);
1454 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1456 // Note that here we generate illegal "double move"!
1457 if ( futilityValue >= beta
1458 && pos.see_sign(make_move(from, victimSq)) >= 0)
1461 if (futilityValue > bv)
1465 // Update bestValue only if check is not dangerous (because we will prune the move)
1471 // connected_moves() tests whether two moves are 'connected' in the sense
1472 // that the first move somehow made the second move possible (for instance
1473 // if the moving piece is the same in both moves). The first move is assumed
1474 // to be the move that was made to reach the current position, while the
1475 // second move is assumed to be a move from the current position.
1477 bool connected_moves(const Position& pos, Move m1, Move m2) {
1479 Square f1, t1, f2, t2;
1486 // Case 1: The moving piece is the same in both moves
1492 // Case 2: The destination square for m2 was vacated by m1
1498 // Case 3: Moving through the vacated square
1499 p2 = pos.piece_on(f2);
1500 if ( piece_is_slider(p2)
1501 && bit_is_set(squares_between(f2, t2), f1))
1504 // Case 4: The destination square for m2 is defended by the moving piece in m1
1505 p1 = pos.piece_on(t1);
1506 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1509 // Case 5: Discovered check, checking piece is the piece moved in m1
1510 ksq = pos.king_square(pos.side_to_move());
1511 if ( piece_is_slider(p1)
1512 && bit_is_set(squares_between(t1, ksq), f2))
1514 Bitboard occ = pos.occupied_squares();
1515 clear_bit(&occ, f2);
1516 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1523 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1524 // "plies to mate from the current ply". Non-mate scores are unchanged.
1525 // The function is called before storing a value to the transposition table.
1527 Value value_to_tt(Value v, int ply) {
1529 if (v >= VALUE_MATE_IN_PLY_MAX)
1532 if (v <= VALUE_MATED_IN_PLY_MAX)
1539 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1540 // the transposition table to a mate score corrected for the current ply.
1542 Value value_from_tt(Value v, int ply) {
1544 if (v >= VALUE_MATE_IN_PLY_MAX)
1547 if (v <= VALUE_MATED_IN_PLY_MAX)
1554 // connected_threat() tests whether it is safe to forward prune a move or if
1555 // is somehow connected to the threat move returned by null search.
1557 bool connected_threat(const Position& pos, Move m, Move threat) {
1560 assert(is_ok(threat));
1561 assert(!pos.is_capture_or_promotion(m));
1562 assert(!pos.is_passed_pawn_push(m));
1564 Square mfrom, mto, tfrom, tto;
1566 mfrom = move_from(m);
1568 tfrom = move_from(threat);
1569 tto = move_to(threat);
1571 // Case 1: Don't prune moves which move the threatened piece
1575 // Case 2: If the threatened piece has value less than or equal to the
1576 // value of the threatening piece, don't prune moves which defend it.
1577 if ( pos.is_capture(threat)
1578 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1579 || type_of(pos.piece_on(tfrom)) == KING)
1580 && pos.move_attacks_square(m, tto))
1583 // Case 3: If the moving piece in the threatened move is a slider, don't
1584 // prune safe moves which block its ray.
1585 if ( piece_is_slider(pos.piece_on(tfrom))
1586 && bit_is_set(squares_between(tfrom, tto), mto)
1587 && pos.see_sign(m) >= 0)
1594 // can_return_tt() returns true if a transposition table score can be used to
1595 // cut-off at a given point in search.
1597 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1599 Value v = value_from_tt(tte->value(), ply);
1601 return ( tte->depth() >= depth
1602 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1603 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1605 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1606 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1610 // refine_eval() returns the transposition table score if possible, otherwise
1611 // falls back on static position evaluation.
1613 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1617 Value v = value_from_tt(tte->value(), ply);
1619 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1620 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1627 // current_search_time() returns the number of milliseconds which have passed
1628 // since the beginning of the current search.
1630 int elapsed_time(bool reset) {
1632 static int searchStartTime;
1635 searchStartTime = get_system_time();
1637 return get_system_time() - searchStartTime;
1641 // score_to_uci() converts a value to a string suitable for use with the UCI
1642 // protocol specifications:
1644 // cp <x> The score from the engine's point of view in centipawns.
1645 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1646 // use negative values for y.
1648 string score_to_uci(Value v, Value alpha, Value beta) {
1650 std::stringstream s;
1652 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1653 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1655 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1657 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1663 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1664 // the PV lines also if are still to be searched and so refer to the previous
1667 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1669 int t = elapsed_time();
1672 for (int i = 0; i < Threads.size(); i++)
1673 if (Threads[i].maxPly > selDepth)
1674 selDepth = Threads[i].maxPly;
1676 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1678 bool updated = (i <= PVIdx);
1680 if (depth == 1 && !updated)
1683 int d = (updated ? depth : depth - 1);
1684 Value s = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1688 << " seldepth " << selDepth
1689 << (i == PVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
1690 << " nodes " << pos.nodes_searched()
1691 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1693 << " multipv " << i + 1 << " pv";
1695 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1696 cout << " " << RootMoves[i].pv[j];
1703 // pv_info_to_log() writes human-readable search information to the log file
1704 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1705 // uses the two below helpers to pretty format time and score respectively.
1707 string time_to_string(int millisecs) {
1709 const int MSecMinute = 1000 * 60;
1710 const int MSecHour = 1000 * 60 * 60;
1712 int hours = millisecs / MSecHour;
1713 int minutes = (millisecs % MSecHour) / MSecMinute;
1714 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1716 std::stringstream s;
1721 s << std::setfill('0') << std::setw(2) << minutes << ':'
1722 << std::setw(2) << seconds;
1726 string score_to_string(Value v) {
1728 std::stringstream s;
1730 if (v >= VALUE_MATE_IN_PLY_MAX)
1731 s << "#" << (VALUE_MATE - v + 1) / 2;
1732 else if (v <= VALUE_MATED_IN_PLY_MAX)
1733 s << "-#" << (VALUE_MATE + v) / 2;
1735 s << std::setprecision(2) << std::fixed << std::showpos
1736 << float(v) / PawnValueMidgame;
1741 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1743 const int64_t K = 1000;
1744 const int64_t M = 1000000;
1746 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1748 string san, padding;
1750 std::stringstream s;
1752 s << set960(pos.is_chess960())
1753 << std::setw(2) << depth
1754 << std::setw(8) << score_to_string(value)
1755 << std::setw(8) << time_to_string(time);
1757 if (pos.nodes_searched() < M)
1758 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1760 else if (pos.nodes_searched() < K * M)
1761 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1764 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1766 padding = string(s.str().length(), ' ');
1767 length = padding.length();
1769 while (*m != MOVE_NONE)
1771 san = move_to_san(pos, *m);
1773 if (length + san.length() > 80)
1775 s << "\n" + padding;
1776 length = padding.length();
1780 length += san.length() + 1;
1782 pos.do_move(*m++, *st++);
1786 pos.undo_move(*--m);
1788 Log l(Options["Search Log Filename"].value<string>());
1789 l << s.str() << endl;
1793 // When playing with strength handicap choose best move among the MultiPV set
1794 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1796 Move do_skill_level() {
1798 assert(MultiPV > 1);
1802 // PRNG sequence should be not deterministic
1803 for (int i = abs(get_system_time() % 50); i > 0; i--)
1804 rk.rand<unsigned>();
1806 // RootMoves are already sorted by score in descending order
1807 size_t size = std::min(MultiPV, RootMoves.size());
1808 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1809 int weakness = 120 - 2 * SkillLevel;
1810 int max_s = -VALUE_INFINITE;
1811 Move best = MOVE_NONE;
1813 // Choose best move. For each move score we add two terms both dependent on
1814 // weakness, one deterministic and bigger for weaker moves, and one random,
1815 // then we choose the move with the resulting highest score.
1816 for (size_t i = 0; i < size; i++)
1818 int s = RootMoves[i].score;
1820 // Don't allow crazy blunders even at very low skills
1821 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1824 // This is our magic formula
1825 s += ( weakness * int(RootMoves[0].score - s)
1826 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1831 best = RootMoves[i].pv[0];
1838 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1839 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1840 // allow to always have a ponder move even when we fail high at root and also a
1841 // long PV to print that is important for position analysis.
1843 void RootMove::extract_pv_from_tt(Position& pos) {
1845 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1850 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1854 pos.do_move(m, *st++);
1856 while ( (tte = TT.probe(pos.get_key())) != NULL
1857 && tte->move() != MOVE_NONE
1858 && pos.is_pseudo_legal(tte->move())
1859 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1861 && (!pos.is_draw<false>() || ply < 2))
1863 pv.push_back(tte->move());
1864 pos.do_move(tte->move(), *st++);
1867 pv.push_back(MOVE_NONE);
1869 do pos.undo_move(pv[--ply]); while (ply);
1873 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1874 // the PV back into the TT. This makes sure the old PV moves are searched
1875 // first, even if the old TT entries have been overwritten.
1877 void RootMove::insert_pv_in_tt(Position& pos) {
1879 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1882 Value v, m = VALUE_NONE;
1885 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1891 // Don't overwrite existing correct entries
1892 if (!tte || tte->move() != pv[ply])
1894 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1895 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
1897 pos.do_move(pv[ply], *st++);
1899 } while (pv[++ply] != MOVE_NONE);
1901 do pos.undo_move(pv[--ply]); while (ply);
1907 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1908 /// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
1909 /// for which the thread is the master.
1911 void Thread::idle_loop(SplitPoint* sp) {
1915 // If we are not searching, wait for a condition to be signaled
1916 // instead of wasting CPU time polling for work.
1919 || (Threads.use_sleeping_threads() && !is_searching))
1921 assert((!sp && threadID) || Threads.use_sleeping_threads());
1929 // Grab the lock to avoid races with Thread::wake_up()
1930 lock_grab(&sleepLock);
1932 // If we are master and all slaves have finished don't go to sleep
1933 if (sp && Threads.split_point_finished(sp))
1935 lock_release(&sleepLock);
1939 // Do sleep after retesting sleep conditions under lock protection, in
1940 // particular we need to avoid a deadlock in case a master thread has,
1941 // in the meanwhile, allocated us and sent the wake_up() call before we
1942 // had the chance to grab the lock.
1943 if (do_sleep || !is_searching)
1944 cond_wait(&sleepCond, &sleepLock);
1946 lock_release(&sleepLock);
1949 // If this thread has been assigned work, launch a search
1952 assert(!do_terminate);
1954 // Copy split point position and search stack and call search()
1955 Stack ss[PLY_MAX_PLUS_2];
1956 SplitPoint* tsp = splitPoint;
1957 Position pos(*tsp->pos, threadID);
1959 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
1962 if (tsp->nodeType == Root)
1963 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1964 else if (tsp->nodeType == PV)
1965 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1966 else if (tsp->nodeType == NonPV)
1967 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1971 assert(is_searching);
1973 is_searching = false;
1975 // Wake up master thread so to allow it to return from the idle loop in
1976 // case we are the last slave of the split point.
1977 if ( Threads.use_sleeping_threads()
1978 && threadID != tsp->master
1979 && !Threads[tsp->master].is_searching)
1980 Threads[tsp->master].wake_up();
1983 // If this thread is the master of a split point and all slaves have
1984 // finished their work at this split point, return from the idle loop.
1985 if (sp && Threads.split_point_finished(sp))
1987 // Because sp->is_slave[] is reset under lock protection,
1988 // be sure sp->lock has been released before to return.
1989 lock_grab(&(sp->lock));
1990 lock_release(&(sp->lock));
1997 /// do_timer_event() is called by the timer thread when the timer triggers. It
1998 /// is used to print debug info and, more important, to detect when we are out of
1999 /// available time and so stop the search.
2001 void do_timer_event() {
2003 static int lastInfoTime;
2004 int e = elapsed_time();
2006 if (get_system_time() - lastInfoTime >= 1000 || !lastInfoTime)
2008 lastInfoTime = get_system_time();
2011 dbg_print_hit_rate();
2017 bool stillAtFirstMove = Signals.firstRootMove
2018 && !Signals.failedLowAtRoot
2019 && e > TimeMgr.available_time();
2021 bool noMoreTime = e > TimeMgr.maximum_time()
2022 || stillAtFirstMove;
2024 if ( (Limits.useTimeManagement() && noMoreTime)
2025 || (Limits.maxTime && e >= Limits.maxTime)
2026 /* missing nodes limit */ ) // FIXME
2027 Signals.stop = true;