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 void id_loop(Position& pos);
160 template <NodeType NT>
161 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
163 template <NodeType NT>
164 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
166 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
167 bool connected_moves(const Position& pos, Move m1, Move m2);
168 Value value_to_tt(Value v, int ply);
169 Value value_from_tt(Value v, int ply);
170 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
171 bool connected_threat(const Position& pos, Move m, Move threat);
172 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
173 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
174 Move do_skill_level();
175 int elapsed_time(bool reset = false);
176 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
177 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
178 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
180 // MovePickerExt class template extends MovePicker and allows to choose at
181 // compile time the proper moves source according to the type of node. In the
182 // default case we simply create and use a standard MovePicker object.
183 template<bool SpNode> struct MovePickerExt : public MovePicker {
185 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
186 : MovePicker(p, ttm, d, h, ss, b) {}
189 // In case of a SpNode we use split point's shared MovePicker object as moves source
190 template<> struct MovePickerExt<true> : public MovePicker {
192 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
193 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
195 Move get_next_move() { return mp->get_next_move(); }
199 // Overload operator<<() to make it easier to print moves in a coordinate
200 // notation compatible with UCI protocol.
201 std::ostream& operator<<(std::ostream& os, Move m) {
203 bool chess960 = (os.iword(0) != 0); // See set960()
204 return os << move_to_uci(m, chess960);
207 // When formatting a move for std::cout we must know if we are in Chess960 or
208 // not. To keep using the handy operator<<() on the move the trick is to embed
209 // this flag in the stream itself. Function-like named enum set960 is used as
210 // a custom manipulator and the stream internal general-purpose array, accessed
211 // through ios_base::iword(), is used to pass the flag to the move's operator<<
212 // that will read it to properly format castling moves.
215 std::ostream& operator<<(std::ostream& os, const set960& f) {
221 // is_dangerous() checks whether a move belongs to some classes of known
222 // 'dangerous' moves so that we avoid to prune it.
223 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
225 // Test for a pawn pushed to 7th or a passed pawn move
226 if (type_of(pos.piece_on(move_from(m))) == PAWN)
228 Color c = pos.side_to_move();
229 if ( relative_rank(c, move_to(m)) == RANK_7
230 || pos.pawn_is_passed(c, move_to(m)))
234 // Test for a capture that triggers a pawn endgame
235 if ( captureOrPromotion
236 && type_of(pos.piece_on(move_to(m))) != PAWN
237 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
238 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
248 /// Search::init() is called during startup to initialize various lookup tables
250 void Search::init() {
252 int d; // depth (ONE_PLY == 2)
253 int hd; // half depth (ONE_PLY == 1)
256 // Init reductions array
257 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
259 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
260 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
261 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
262 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
265 // Init futility margins array
266 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
267 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
269 // Init futility move count array
270 for (d = 0; d < 32; d++)
271 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
275 /// Search::perft() is our utility to verify move generation. All the leaf nodes
276 /// up to the given depth are generated and counted and the sum returned.
278 int64_t Search::perft(Position& pos, Depth depth) {
283 MoveList<MV_LEGAL> ml(pos);
285 // At the last ply just return the number of moves (leaf nodes)
286 if (depth <= ONE_PLY)
290 for ( ; !ml.end(); ++ml)
292 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
293 sum += perft(pos, depth - ONE_PLY);
294 pos.undo_move(ml.move());
300 /// Search::think() is the external interface to Stockfish's search, and is
301 /// called by the main thread when the program receives the UCI 'go' command. It
302 /// searches from RootPosition and at the end prints the "bestmove" to output.
304 void Search::think() {
306 static Book book; // Defined static to initialize the PRNG only once
308 Position& pos = RootPosition;
310 TimeMgr.init(Limits, pos.startpos_ply_counter());
315 // Populate RootMoves with all the legal moves (default) or, if a SearchMoves
316 // is given, with the subset of legal moves to search.
317 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
318 if ( SearchMoves.empty()
319 || std::count(SearchMoves.begin(), SearchMoves.end(), ml.move()))
320 RootMoves.push_back(RootMove(ml.move()));
322 // Set output stream mode: normal or chess960. Castling notation is different
323 cout << set960(pos.is_chess960());
325 if (Options["OwnBook"].value<bool>())
327 if (Options["Book File"].value<string>() != book.name())
328 book.open(Options["Book File"].value<string>());
330 Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
331 if (bookMove != MOVE_NONE)
333 if (!Signals.stop && (Limits.ponder || Limits.infinite))
334 Threads.wait_for_stop_or_ponderhit();
336 cout << "bestmove " << bookMove << endl;
341 // Read UCI options: GUI could change UCI parameters during the game
342 read_evaluation_uci_options(pos.side_to_move());
343 Threads.read_uci_options();
345 TT.set_size(Options["Hash"].value<int>());
346 if (Options["Clear Hash"].value<bool>())
348 Options["Clear Hash"].set_value("false");
352 UCIMultiPV = Options["MultiPV"].value<size_t>();
353 SkillLevel = Options["Skill Level"].value<int>();
355 // Do we have to play with skill handicap? In this case enable MultiPV that
356 // we will use behind the scenes to retrieve a set of possible moves.
357 SkillLevelEnabled = (SkillLevel < 20);
358 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
360 if (Options["Use Search Log"].value<bool>())
362 Log log(Options["Search Log Filename"].value<string>());
363 log << "\nSearching: " << pos.to_fen()
364 << "\ninfinite: " << Limits.infinite
365 << " ponder: " << Limits.ponder
366 << " time: " << Limits.time
367 << " increment: " << Limits.increment
368 << " moves to go: " << Limits.movesToGo
372 for (int i = 0; i < Threads.size(); i++)
374 Threads[i].maxPly = 0;
375 Threads[i].wake_up();
378 // Set best timer interval to avoid lagging under time pressure. Timer is
379 // used to check for remaining available thinking time.
380 if (TimeMgr.available_time())
381 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
383 Threads.set_timer(100);
385 // We're ready to start searching. Call the iterative deepening loop function
388 // Stop timer and send all the slaves to sleep, if not already sleeping
389 Threads.set_timer(0);
392 if (Options["Use Search Log"].value<bool>())
394 int e = elapsed_time();
396 Log log(Options["Search Log Filename"].value<string>());
397 log << "Nodes: " << pos.nodes_searched()
398 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
399 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
402 pos.do_move(RootMoves[0].pv[0], st);
403 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
404 pos.undo_move(RootMoves[0].pv[0]);
407 // When we reach max depth we arrive here even without a StopRequest, but if
408 // we are pondering or in infinite search, we shouldn't print the best move
409 // before we are told to do so.
410 if (!Signals.stop && (Limits.ponder || Limits.infinite))
411 Threads.wait_for_stop_or_ponderhit();
413 // Could be MOVE_NONE when searching on a stalemate position
414 cout << "bestmove " << RootMoves[0].pv[0];
416 // UCI protol is not clear on allowing sending an empty ponder move, instead
417 // it is clear that ponder move is optional. So skip it if empty.
418 if (RootMoves[0].pv[1] != MOVE_NONE)
419 cout << " ponder " << RootMoves[0].pv[1];
427 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
428 // with increasing depth until the allocated thinking time has been consumed,
429 // user stops the search, or the maximum search depth is reached.
431 void id_loop(Position& pos) {
433 Stack ss[PLY_MAX_PLUS_2];
434 int depth, prevBestMoveChanges;
435 Value bestValue, alpha, beta, delta;
436 bool bestMoveNeverChanged = true;
437 Move skillBest = MOVE_NONE;
439 memset(ss, 0, 4 * sizeof(Stack));
440 depth = BestMoveChanges = 0;
441 bestValue = delta = -VALUE_INFINITE;
442 ss->currentMove = MOVE_NULL; // Hack to skip update gains
444 // Handle the special case of a mate/stalemate position
445 if (RootMoves.empty())
447 cout << "info depth 0"
448 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
450 RootMoves.push_back(MOVE_NONE);
454 // Iterative deepening loop until requested to stop or target depth reached
455 while (!Signals.stop && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
457 // Save last iteration's scores before first PV line is searched and all
458 // the move scores but the (new) PV are set to -VALUE_INFINITE.
459 for (size_t i = 0; i < RootMoves.size(); i++)
460 RootMoves[i].prevScore = RootMoves[i].score;
462 prevBestMoveChanges = BestMoveChanges;
465 // MultiPV loop. We perform a full root search for each PV line
466 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
468 // Set aspiration window default width
469 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
472 alpha = RootMoves[PVIdx].prevScore - delta;
473 beta = RootMoves[PVIdx].prevScore + delta;
477 alpha = -VALUE_INFINITE;
478 beta = VALUE_INFINITE;
481 // Start with a small aspiration window and, in case of fail high/low,
482 // research with bigger window until not failing high/low anymore.
484 // Search starts from ss+1 to allow referencing (ss-1). This is
485 // needed by update gains and ss copy when splitting at Root.
486 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
488 // Bring to front the best move. It is critical that sorting is
489 // done with a stable algorithm because all the values but the first
490 // and eventually the new best one are set to -VALUE_INFINITE and
491 // we want to keep the same order for all the moves but the new
492 // PV that goes to the front. Note that in case of MultiPV search
493 // the already searched PV lines are preserved.
494 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
496 // In case we have found an exact score and we are going to leave
497 // the fail high/low loop then reorder the PV moves, otherwise
498 // leave the last PV move in its position so to be searched again.
499 // Of course this is needed only in MultiPV search.
500 if (PVIdx && bestValue > alpha && bestValue < beta)
501 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
503 // Write PV back to transposition table in case the relevant
504 // entries have been overwritten during the search.
505 for (size_t i = 0; i <= PVIdx; i++)
506 RootMoves[i].insert_pv_in_tt(pos);
508 // If search has been stopped exit the aspiration window loop.
509 // Sorting and writing PV back to TT is safe becuase RootMoves
510 // is still valid, although refers to previous iteration.
514 // Send full PV info to GUI if we are going to leave the loop or
515 // if we have a fail high/low and we are deep in the search.
516 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
517 pv_info_to_uci(pos, depth, alpha, beta);
519 // In case of failing high/low increase aspiration window and
520 // research, otherwise exit the fail high/low loop.
521 if (bestValue >= beta)
526 else if (bestValue <= alpha)
528 Signals.failedLowAtRoot = true;
529 Signals.stopOnPonderhit = false;
537 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
539 } while (abs(bestValue) < VALUE_KNOWN_WIN);
542 // Skills: Do we need to pick now the best move ?
543 if (SkillLevelEnabled && depth == 1 + SkillLevel)
544 skillBest = do_skill_level();
546 if (Options["Use Search Log"].value<bool>())
547 pv_info_to_log(pos, depth, bestValue, elapsed_time(), &RootMoves[0].pv[0]);
549 // Filter out startup noise when monitoring best move stability
550 if (depth > 2 && BestMoveChanges)
551 bestMoveNeverChanged = false;
553 // Do we have time for the next iteration? Can we stop searching now?
554 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
556 bool stop = false; // Local variable, not the volatile Signals.stop
558 // Take in account some extra time if the best move has changed
559 if (depth > 4 && depth < 50)
560 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
562 // Stop search if most of available time is already consumed. We
563 // probably don't have enough time to search the first move at the
564 // next iteration anyway.
565 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
568 // Stop search early if one move seems to be much better than others
571 && ( bestMoveNeverChanged
572 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
574 Value rBeta = bestValue - EasyMoveMargin;
575 (ss+1)->excludedMove = RootMoves[0].pv[0];
576 (ss+1)->skipNullMove = true;
577 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
578 (ss+1)->skipNullMove = false;
579 (ss+1)->excludedMove = MOVE_NONE;
587 // If we are allowed to ponder do not stop the search now but
588 // keep pondering until GUI sends "ponderhit" or "stop".
590 Signals.stopOnPonderhit = true;
597 // When using skills swap best PV line with the sub-optimal one
598 if (SkillLevelEnabled)
600 if (skillBest == MOVE_NONE) // Still unassigned ?
601 skillBest = do_skill_level();
603 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), skillBest));
608 // search<>() is the main search function for both PV and non-PV nodes and for
609 // normal and SplitPoint nodes. When called just after a split point the search
610 // is simpler because we have already probed the hash table, done a null move
611 // search, and searched the first move before splitting, we don't have to repeat
612 // all this work again. We also don't need to store anything to the hash table
613 // here: This is taken care of after we return from the split point.
615 template <NodeType NT>
616 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
618 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
619 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
620 const bool RootNode = (NT == Root || NT == SplitPointRoot);
622 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
623 assert(beta > alpha && beta <= VALUE_INFINITE);
624 assert(PvNode || alpha == beta - 1);
625 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
627 Move movesSearched[MAX_MOVES];
632 Move ttMove, move, excludedMove, threatMove;
635 Value bestValue, value, oldAlpha;
636 Value refinedValue, nullValue, futilityBase, futilityValue;
637 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
638 bool captureOrPromotion, dangerous, doFullDepthSearch;
639 int moveCount = 0, playedMoveCount = 0;
640 Thread& thread = Threads[pos.thread()];
641 SplitPoint* sp = NULL;
643 refinedValue = bestValue = value = -VALUE_INFINITE;
645 inCheck = pos.in_check();
646 ss->ply = (ss-1)->ply + 1;
648 // Used to send selDepth info to GUI
649 if (PvNode && thread.maxPly < ss->ply)
650 thread.maxPly = ss->ply;
652 // Step 1. Initialize node
655 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
656 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
657 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
663 ttMove = excludedMove = MOVE_NONE;
664 threatMove = sp->threatMove;
665 goto split_point_start;
668 // Step 2. Check for aborted search and immediate draw
670 || pos.is_draw<false>()
671 || ss->ply > PLY_MAX) && !RootNode)
674 // Step 3. Mate distance pruning
677 alpha = std::max(value_mated_in(ss->ply), alpha);
678 beta = std::min(value_mate_in(ss->ply+1), beta);
683 // Step 4. Transposition table lookup
684 // We don't want the score of a partial search to overwrite a previous full search
685 // TT value, so we use a different position key in case of an excluded move.
686 excludedMove = ss->excludedMove;
687 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
688 tte = TT.probe(posKey);
689 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
691 // At PV nodes we check for exact scores, while at non-PV nodes we check for
692 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
693 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
694 // we should also update RootMoveList to avoid bogus output.
695 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
696 : can_return_tt(tte, depth, beta, ss->ply)))
699 ss->bestMove = move = ttMove; // Can be MOVE_NONE
700 value = value_from_tt(tte->value(), ss->ply);
704 && !pos.is_capture_or_promotion(move)
705 && move != ss->killers[0])
707 ss->killers[1] = ss->killers[0];
708 ss->killers[0] = move;
713 // Step 5. Evaluate the position statically and update parent's gain statistics
715 ss->eval = ss->evalMargin = VALUE_NONE;
718 assert(tte->static_value() != VALUE_NONE);
720 ss->eval = tte->static_value();
721 ss->evalMargin = tte->static_value_margin();
722 refinedValue = refine_eval(tte, ss->eval, ss->ply);
726 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
727 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
730 // Update gain for the parent non-capture move given the static position
731 // evaluation before and after the move.
732 if ( (move = (ss-1)->currentMove) != MOVE_NULL
733 && (ss-1)->eval != VALUE_NONE
734 && ss->eval != VALUE_NONE
735 && pos.captured_piece_type() == PIECE_TYPE_NONE
736 && !is_special(move))
738 Square to = move_to(move);
739 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
742 // Step 6. Razoring (is omitted in PV nodes)
744 && depth < RazorDepth
746 && refinedValue + razor_margin(depth) < beta
747 && ttMove == MOVE_NONE
748 && abs(beta) < VALUE_MATE_IN_PLY_MAX
749 && !pos.has_pawn_on_7th(pos.side_to_move()))
751 Value rbeta = beta - razor_margin(depth);
752 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
754 // Logically we should return (v + razor_margin(depth)), but
755 // surprisingly this did slightly weaker in tests.
759 // Step 7. Static null move pruning (is omitted in PV nodes)
760 // We're betting that the opponent doesn't have a move that will reduce
761 // the score by more than futility_margin(depth) if we do a null move.
764 && depth < RazorDepth
766 && refinedValue - futility_margin(depth, 0) >= beta
767 && abs(beta) < VALUE_MATE_IN_PLY_MAX
768 && pos.non_pawn_material(pos.side_to_move()))
769 return refinedValue - futility_margin(depth, 0);
771 // Step 8. Null move search with verification search (is omitted in PV nodes)
776 && refinedValue >= beta
777 && abs(beta) < VALUE_MATE_IN_PLY_MAX
778 && pos.non_pawn_material(pos.side_to_move()))
780 ss->currentMove = MOVE_NULL;
782 // Null move dynamic reduction based on depth
783 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
785 // Null move dynamic reduction based on value
786 if (refinedValue - PawnValueMidgame > beta)
789 pos.do_null_move<true>(st);
790 (ss+1)->skipNullMove = true;
791 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
792 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
793 (ss+1)->skipNullMove = false;
794 pos.do_null_move<false>(st);
796 if (nullValue >= beta)
798 // Do not return unproven mate scores
799 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
802 if (depth < 6 * ONE_PLY)
805 // Do verification search at high depths
806 ss->skipNullMove = true;
807 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
808 ss->skipNullMove = false;
815 // The null move failed low, which means that we may be faced with
816 // some kind of threat. If the previous move was reduced, check if
817 // the move that refuted the null move was somehow connected to the
818 // move which was reduced. If a connection is found, return a fail
819 // low score (which will cause the reduced move to fail high in the
820 // parent node, which will trigger a re-search with full depth).
821 threatMove = (ss+1)->bestMove;
823 if ( depth < ThreatDepth
825 && threatMove != MOVE_NONE
826 && connected_moves(pos, (ss-1)->currentMove, threatMove))
831 // Step 9. ProbCut (is omitted in PV nodes)
832 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
833 // and a reduced search returns a value much above beta, we can (almost) safely
834 // prune the previous move.
836 && depth >= RazorDepth + ONE_PLY
839 && excludedMove == MOVE_NONE
840 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
842 Value rbeta = beta + 200;
843 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
845 assert(rdepth >= ONE_PLY);
847 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
850 while ((move = mp.get_next_move()) != MOVE_NONE)
851 if (pos.pl_move_is_legal(move, ci.pinned))
853 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
854 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
861 // Step 10. Internal iterative deepening
862 if ( depth >= IIDDepth[PvNode]
863 && ttMove == MOVE_NONE
864 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
866 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
868 ss->skipNullMove = true;
869 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
870 ss->skipNullMove = false;
872 tte = TT.probe(posKey);
873 ttMove = tte ? tte->move() : MOVE_NONE;
876 split_point_start: // At split points actual search starts from here
878 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
880 ss->bestMove = MOVE_NONE;
881 futilityBase = ss->eval + ss->evalMargin;
882 singularExtensionNode = !RootNode
884 && depth >= SingularExtensionDepth[PvNode]
885 && ttMove != MOVE_NONE
886 && !excludedMove // Recursive singular search is not allowed
887 && (tte->type() & VALUE_TYPE_LOWER)
888 && tte->depth() >= depth - 3 * ONE_PLY;
891 lock_grab(&(sp->lock));
892 bestValue = sp->bestValue;
893 moveCount = sp->moveCount;
895 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
898 // Step 11. Loop through moves
899 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
900 while ( bestValue < beta
901 && (move = mp.get_next_move()) != MOVE_NONE
902 && !thread.cutoff_occurred())
906 if (move == excludedMove)
909 // At root obey the "searchmoves" option and skip moves not listed in Root
910 // Move List, as a consequence any illegal move is also skipped. In MultiPV
911 // mode we also skip PV moves which have been already searched.
912 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
915 // At PV and SpNode nodes we want all moves to be legal since the beginning
916 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
921 moveCount = ++sp->moveCount;
922 lock_release(&(sp->lock));
929 // This is used by time management
930 Signals.firstRootMove = (moveCount == 1);
932 nodes = pos.nodes_searched();
934 if (pos.thread() == 0 && elapsed_time() > 2000)
935 cout << "info depth " << depth / ONE_PLY
936 << " currmove " << move
937 << " currmovenumber " << moveCount + PVIdx << endl;
940 isPvMove = (PvNode && moveCount <= 1);
941 captureOrPromotion = pos.is_capture_or_promotion(move);
942 givesCheck = pos.move_gives_check(move, ci);
943 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
946 // Step 12. Extend checks and, in PV nodes, also dangerous moves
947 if (PvNode && dangerous)
950 else if (givesCheck && pos.see_sign(move) >= 0)
951 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
953 // Singular extension search. If all moves but one fail low on a search of
954 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
955 // is singular and should be extended. To verify this we do a reduced search
956 // on all the other moves but the ttMove, if result is lower than ttValue minus
957 // a margin then we extend ttMove.
958 if ( singularExtensionNode
961 && pos.pl_move_is_legal(move, ci.pinned))
963 Value ttValue = value_from_tt(tte->value(), ss->ply);
965 if (abs(ttValue) < VALUE_KNOWN_WIN)
967 Value rBeta = ttValue - int(depth);
968 ss->excludedMove = move;
969 ss->skipNullMove = true;
970 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
971 ss->skipNullMove = false;
972 ss->excludedMove = MOVE_NONE;
973 ss->bestMove = MOVE_NONE;
979 // Update current move (this must be done after singular extension search)
980 newDepth = depth - ONE_PLY + ext;
982 // Step 13. Futility pruning (is omitted in PV nodes)
984 && !captureOrPromotion
989 && (bestValue > VALUE_MATED_IN_PLY_MAX || bestValue == -VALUE_INFINITE))
991 // Move count based pruning
992 if ( moveCount >= futility_move_count(depth)
993 && (!threatMove || !connected_threat(pos, move, threatMove)))
996 lock_grab(&(sp->lock));
1001 // Value based pruning
1002 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1003 // but fixing this made program slightly weaker.
1004 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1005 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1006 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1008 if (futilityValue < beta)
1011 lock_grab(&(sp->lock));
1016 // Prune moves with negative SEE at low depths
1017 if ( predictedDepth < 2 * ONE_PLY
1018 && pos.see_sign(move) < 0)
1021 lock_grab(&(sp->lock));
1027 // Check for legality only before to do the move
1028 if (!pos.pl_move_is_legal(move, ci.pinned))
1034 ss->currentMove = move;
1035 if (!SpNode && !captureOrPromotion)
1036 movesSearched[playedMoveCount++] = move;
1038 // Step 14. Make the move
1039 pos.do_move(move, st, ci, givesCheck);
1041 // Step 15. Reduced depth search (LMR). If the move fails high will be
1042 // re-searched at full depth.
1043 if ( depth > 3 * ONE_PLY
1045 && !captureOrPromotion
1048 && ss->killers[0] != move
1049 && ss->killers[1] != move)
1051 ss->reduction = reduction<PvNode>(depth, moveCount);
1052 Depth d = newDepth - ss->reduction;
1053 alpha = SpNode ? sp->alpha : alpha;
1055 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1056 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1058 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1059 ss->reduction = DEPTH_ZERO;
1062 doFullDepthSearch = !isPvMove;
1064 // Step 16. Full depth search, when LMR is skipped or fails high
1065 if (doFullDepthSearch)
1067 alpha = SpNode ? sp->alpha : alpha;
1068 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1069 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1072 // Only for PV nodes do a full PV search on the first move or after a fail
1073 // high, in the latter case search only if value < beta, otherwise let the
1074 // parent node to fail low with value <= alpha and to try another move.
1075 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1076 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1077 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1079 // Step 17. Undo move
1080 pos.undo_move(move);
1082 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1084 // Step 18. Check for new best move
1087 lock_grab(&(sp->lock));
1088 bestValue = sp->bestValue;
1092 // Finished searching the move. If StopRequest is true, the search
1093 // was aborted because the user interrupted the search or because we
1094 // ran out of time. In this case, the return value of the search cannot
1095 // be trusted, and we don't update the best move and/or PV.
1096 if (RootNode && !Signals.stop)
1098 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1099 rm.nodes += pos.nodes_searched() - nodes;
1101 // PV move or new best move ?
1102 if (isPvMove || value > alpha)
1105 rm.extract_pv_from_tt(pos);
1107 // We record how often the best move has been changed in each
1108 // iteration. This information is used for time management: When
1109 // the best move changes frequently, we allocate some more time.
1110 if (!isPvMove && MultiPV == 1)
1114 // All other moves but the PV are set to the lowest value, this
1115 // is not a problem when sorting becuase sort is stable and move
1116 // position in the list is preserved, just the PV is pushed up.
1117 rm.score = -VALUE_INFINITE;
1121 if (value > bestValue)
1124 ss->bestMove = move;
1128 && value < beta) // We want always alpha < beta
1131 if (SpNode && !thread.cutoff_occurred())
1133 sp->bestValue = value;
1134 sp->ss->bestMove = move;
1136 sp->is_betaCutoff = (value >= beta);
1140 // Step 19. Check for split
1142 && depth >= Threads.min_split_depth()
1144 && Threads.available_slave_exists(pos.thread())
1146 && !thread.cutoff_occurred())
1147 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1148 threatMove, moveCount, &mp, NT);
1151 // Step 20. Check for mate and stalemate
1152 // All legal moves have been searched and if there are no legal moves, it
1153 // must be mate or stalemate. Note that we can have a false positive in
1154 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1155 // harmless because return value is discarded anyhow in the parent nodes.
1156 // If we are in a singular extension search then return a fail low score.
1158 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1160 // If we have pruned all the moves without searching return a fail-low score
1161 if (bestValue == -VALUE_INFINITE)
1163 assert(!playedMoveCount);
1168 // Step 21. Update tables
1169 // Update transposition table entry, history and killers
1170 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1172 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1173 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1174 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1176 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1178 // Update killers and history only for non capture moves that fails high
1179 if ( bestValue >= beta
1180 && !pos.is_capture_or_promotion(move))
1182 if (move != ss->killers[0])
1184 ss->killers[1] = ss->killers[0];
1185 ss->killers[0] = move;
1187 update_history(pos, move, depth, movesSearched, playedMoveCount);
1193 // Here we have the lock still grabbed
1194 sp->is_slave[pos.thread()] = false;
1195 sp->nodes += pos.nodes_searched();
1196 lock_release(&(sp->lock));
1199 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1205 // qsearch() is the quiescence search function, which is called by the main
1206 // search function when the remaining depth is zero (or, to be more precise,
1207 // less than ONE_PLY).
1209 template <NodeType NT>
1210 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1212 const bool PvNode = (NT == PV);
1214 assert(NT == PV || NT == NonPV);
1215 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1216 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1217 assert(PvNode || alpha == beta - 1);
1219 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1223 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1224 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1228 Value oldAlpha = alpha;
1230 ss->bestMove = ss->currentMove = MOVE_NONE;
1231 ss->ply = (ss-1)->ply + 1;
1233 // Check for an instant draw or maximum ply reached
1234 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1237 // Decide whether or not to include checks, this fixes also the type of
1238 // TT entry depth that we are going to use. Note that in qsearch we use
1239 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1240 inCheck = pos.in_check();
1241 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1243 // Transposition table lookup. At PV nodes, we don't use the TT for
1244 // pruning, but only for move ordering.
1245 tte = TT.probe(pos.get_key());
1246 ttMove = (tte ? tte->move() : MOVE_NONE);
1248 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1250 ss->bestMove = ttMove; // Can be MOVE_NONE
1251 return value_from_tt(tte->value(), ss->ply);
1254 // Evaluate the position statically
1257 bestValue = futilityBase = -VALUE_INFINITE;
1258 ss->eval = evalMargin = VALUE_NONE;
1259 enoughMaterial = false;
1265 assert(tte->static_value() != VALUE_NONE);
1267 evalMargin = tte->static_value_margin();
1268 ss->eval = bestValue = tte->static_value();
1271 ss->eval = bestValue = evaluate(pos, evalMargin);
1273 // Stand pat. Return immediately if static value is at least beta
1274 if (bestValue >= beta)
1277 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1282 if (PvNode && bestValue > alpha)
1285 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1286 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1289 // Initialize a MovePicker object for the current position, and prepare
1290 // to search the moves. Because the depth is <= 0 here, only captures,
1291 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1293 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1296 // Loop through the moves until no moves remain or a beta cutoff occurs
1297 while ( bestValue < beta
1298 && (move = mp.get_next_move()) != MOVE_NONE)
1300 assert(is_ok(move));
1302 givesCheck = pos.move_gives_check(move, ci);
1310 && !is_promotion(move)
1311 && !pos.is_passed_pawn_push(move))
1313 futilityValue = futilityBase
1314 + PieceValueEndgame[pos.piece_on(move_to(move))]
1315 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1317 if (futilityValue < beta)
1319 if (futilityValue > bestValue)
1320 bestValue = futilityValue;
1325 // Prune moves with negative or equal SEE
1326 if ( futilityBase < beta
1327 && depth < DEPTH_ZERO
1328 && pos.see(move) <= 0)
1332 // Detect non-capture evasions that are candidate to be pruned
1333 evasionPrunable = !PvNode
1335 && bestValue > VALUE_MATED_IN_PLY_MAX
1336 && !pos.is_capture(move)
1337 && !pos.can_castle(pos.side_to_move());
1339 // Don't search moves with negative SEE values
1341 && (!inCheck || evasionPrunable)
1343 && !is_promotion(move)
1344 && pos.see_sign(move) < 0)
1347 // Don't search useless checks
1352 && !pos.is_capture_or_promotion(move)
1353 && ss->eval + PawnValueMidgame / 4 < beta
1354 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1356 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1357 bestValue = ss->eval + PawnValueMidgame / 4;
1362 // Check for legality only before to do the move
1363 if (!pos.pl_move_is_legal(move, ci.pinned))
1366 ss->currentMove = move;
1368 // Make and search the move
1369 pos.do_move(move, st, ci, givesCheck);
1370 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1371 pos.undo_move(move);
1373 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1376 if (value > bestValue)
1379 ss->bestMove = move;
1383 && value < beta) // We want always alpha < beta
1388 // All legal moves have been searched. A special case: If we're in check
1389 // and no legal moves were found, it is checkmate.
1390 if (inCheck && bestValue == -VALUE_INFINITE)
1391 return value_mated_in(ss->ply);
1393 // Update transposition table
1394 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1395 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1396 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1398 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1400 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1406 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1407 // bestValue is updated only when returning false because in that case move
1410 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1412 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1413 Square from, to, ksq, victimSq;
1416 Value futilityValue, bv = *bestValue;
1418 from = move_from(move);
1420 them = flip(pos.side_to_move());
1421 ksq = pos.king_square(them);
1422 kingAtt = pos.attacks_from<KING>(ksq);
1423 pc = pos.piece_on(from);
1425 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1426 oldAtt = pos.attacks_from(pc, from, occ);
1427 newAtt = pos.attacks_from(pc, to, occ);
1429 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1430 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1432 if (!(b && (b & (b - 1))))
1435 // Rule 2. Queen contact check is very dangerous
1436 if ( type_of(pc) == QUEEN
1437 && bit_is_set(kingAtt, to))
1440 // Rule 3. Creating new double threats with checks
1441 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1445 victimSq = pop_1st_bit(&b);
1446 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1448 // Note that here we generate illegal "double move"!
1449 if ( futilityValue >= beta
1450 && pos.see_sign(make_move(from, victimSq)) >= 0)
1453 if (futilityValue > bv)
1457 // Update bestValue only if check is not dangerous (because we will prune the move)
1463 // connected_moves() tests whether two moves are 'connected' in the sense
1464 // that the first move somehow made the second move possible (for instance
1465 // if the moving piece is the same in both moves). The first move is assumed
1466 // to be the move that was made to reach the current position, while the
1467 // second move is assumed to be a move from the current position.
1469 bool connected_moves(const Position& pos, Move m1, Move m2) {
1471 Square f1, t1, f2, t2;
1478 // Case 1: The moving piece is the same in both moves
1484 // Case 2: The destination square for m2 was vacated by m1
1490 // Case 3: Moving through the vacated square
1491 p2 = pos.piece_on(f2);
1492 if ( piece_is_slider(p2)
1493 && bit_is_set(squares_between(f2, t2), f1))
1496 // Case 4: The destination square for m2 is defended by the moving piece in m1
1497 p1 = pos.piece_on(t1);
1498 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1501 // Case 5: Discovered check, checking piece is the piece moved in m1
1502 ksq = pos.king_square(pos.side_to_move());
1503 if ( piece_is_slider(p1)
1504 && bit_is_set(squares_between(t1, ksq), f2))
1506 Bitboard occ = pos.occupied_squares();
1507 clear_bit(&occ, f2);
1508 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1515 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1516 // "plies to mate from the current ply". Non-mate scores are unchanged.
1517 // The function is called before storing a value to the transposition table.
1519 Value value_to_tt(Value v, int ply) {
1521 if (v >= VALUE_MATE_IN_PLY_MAX)
1524 if (v <= VALUE_MATED_IN_PLY_MAX)
1531 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1532 // the transposition table to a mate score corrected for the current ply.
1534 Value value_from_tt(Value v, int ply) {
1536 if (v >= VALUE_MATE_IN_PLY_MAX)
1539 if (v <= VALUE_MATED_IN_PLY_MAX)
1546 // connected_threat() tests whether it is safe to forward prune a move or if
1547 // is somehow connected to the threat move returned by null search.
1549 bool connected_threat(const Position& pos, Move m, Move threat) {
1552 assert(is_ok(threat));
1553 assert(!pos.is_capture_or_promotion(m));
1554 assert(!pos.is_passed_pawn_push(m));
1556 Square mfrom, mto, tfrom, tto;
1558 mfrom = move_from(m);
1560 tfrom = move_from(threat);
1561 tto = move_to(threat);
1563 // Case 1: Don't prune moves which move the threatened piece
1567 // Case 2: If the threatened piece has value less than or equal to the
1568 // value of the threatening piece, don't prune moves which defend it.
1569 if ( pos.is_capture(threat)
1570 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1571 || type_of(pos.piece_on(tfrom)) == KING)
1572 && pos.move_attacks_square(m, tto))
1575 // Case 3: If the moving piece in the threatened move is a slider, don't
1576 // prune safe moves which block its ray.
1577 if ( piece_is_slider(pos.piece_on(tfrom))
1578 && bit_is_set(squares_between(tfrom, tto), mto)
1579 && pos.see_sign(m) >= 0)
1586 // can_return_tt() returns true if a transposition table score can be used to
1587 // cut-off at a given point in search.
1589 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1591 Value v = value_from_tt(tte->value(), ply);
1593 return ( tte->depth() >= depth
1594 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1595 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1597 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1598 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1602 // refine_eval() returns the transposition table score if possible, otherwise
1603 // falls back on static position evaluation.
1605 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1609 Value v = value_from_tt(tte->value(), ply);
1611 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1612 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1619 // update_history() registers a good move that produced a beta-cutoff in
1620 // history and marks as failures all the other moves of that ply.
1622 void update_history(const Position& pos, Move move, Depth depth,
1623 Move movesSearched[], int moveCount) {
1625 Value bonus = Value(int(depth) * int(depth));
1627 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1629 for (int i = 0; i < moveCount - 1; i++)
1631 m = movesSearched[i];
1635 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1640 // current_search_time() returns the number of milliseconds which have passed
1641 // since the beginning of the current search.
1643 int elapsed_time(bool reset) {
1645 static int searchStartTime;
1648 searchStartTime = get_system_time();
1650 return get_system_time() - searchStartTime;
1654 // score_to_uci() converts a value to a string suitable for use with the UCI
1655 // protocol specifications:
1657 // cp <x> The score from the engine's point of view in centipawns.
1658 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1659 // use negative values for y.
1661 string score_to_uci(Value v, Value alpha, Value beta) {
1663 std::stringstream s;
1665 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1666 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1668 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1670 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1676 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1677 // the PV lines also if are still to be searched and so refer to the previous
1680 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1682 int t = elapsed_time();
1685 for (int i = 0; i < Threads.size(); i++)
1686 if (Threads[i].maxPly > selDepth)
1687 selDepth = Threads[i].maxPly;
1689 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1691 bool updated = (i <= PVIdx);
1693 if (depth == 1 && !updated)
1696 int d = (updated ? depth : depth - 1);
1697 Value s = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1701 << " seldepth " << selDepth
1702 << (i == PVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
1703 << " nodes " << pos.nodes_searched()
1704 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1706 << " multipv " << i + 1 << " pv";
1708 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1709 cout << " " << RootMoves[i].pv[j];
1716 // pv_info_to_log() writes human-readable search information to the log file
1717 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1718 // uses the two below helpers to pretty format time and score respectively.
1720 string time_to_string(int millisecs) {
1722 const int MSecMinute = 1000 * 60;
1723 const int MSecHour = 1000 * 60 * 60;
1725 int hours = millisecs / MSecHour;
1726 int minutes = (millisecs % MSecHour) / MSecMinute;
1727 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1729 std::stringstream s;
1734 s << std::setfill('0') << std::setw(2) << minutes << ':'
1735 << std::setw(2) << seconds;
1739 string score_to_string(Value v) {
1741 std::stringstream s;
1743 if (v >= VALUE_MATE_IN_PLY_MAX)
1744 s << "#" << (VALUE_MATE - v + 1) / 2;
1745 else if (v <= VALUE_MATED_IN_PLY_MAX)
1746 s << "-#" << (VALUE_MATE + v) / 2;
1748 s << std::setprecision(2) << std::fixed << std::showpos
1749 << float(v) / PawnValueMidgame;
1754 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1756 const int64_t K = 1000;
1757 const int64_t M = 1000000;
1759 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1761 string san, padding;
1763 std::stringstream s;
1765 s << set960(pos.is_chess960())
1766 << std::setw(2) << depth
1767 << std::setw(8) << score_to_string(value)
1768 << std::setw(8) << time_to_string(time);
1770 if (pos.nodes_searched() < M)
1771 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1773 else if (pos.nodes_searched() < K * M)
1774 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1777 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1779 padding = string(s.str().length(), ' ');
1780 length = padding.length();
1782 while (*m != MOVE_NONE)
1784 san = move_to_san(pos, *m);
1786 if (length + san.length() > 80)
1788 s << "\n" + padding;
1789 length = padding.length();
1793 length += san.length() + 1;
1795 pos.do_move(*m++, *st++);
1799 pos.undo_move(*--m);
1801 Log l(Options["Search Log Filename"].value<string>());
1802 l << s.str() << endl;
1806 // When playing with strength handicap choose best move among the MultiPV set
1807 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1809 Move do_skill_level() {
1811 assert(MultiPV > 1);
1815 // PRNG sequence should be not deterministic
1816 for (int i = abs(get_system_time() % 50); i > 0; i--)
1817 rk.rand<unsigned>();
1819 // Rml list is already sorted by score in descending order
1820 size_t size = std::min(MultiPV, RootMoves.size());
1821 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1822 int weakness = 120 - 2 * SkillLevel;
1823 int max_s = -VALUE_INFINITE;
1824 Move best = MOVE_NONE;
1826 // Choose best move. For each move score we add two terms both dependent on
1827 // weakness, one deterministic and bigger for weaker moves, and one random,
1828 // then we choose the move with the resulting highest score.
1829 for (size_t i = 0; i < size; i++)
1831 int s = RootMoves[i].score;
1833 // Don't allow crazy blunders even at very low skills
1834 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1837 // This is our magic formula
1838 s += ( weakness * int(RootMoves[0].score - s)
1839 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1844 best = RootMoves[i].pv[0];
1851 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1852 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1853 // allow to always have a ponder move even when we fail high at root and also a
1854 // long PV to print that is important for position analysis.
1856 void RootMove::extract_pv_from_tt(Position& pos) {
1858 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1863 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1867 pos.do_move(m, *st++);
1869 while ( (tte = TT.probe(pos.get_key())) != NULL
1870 && tte->move() != MOVE_NONE
1871 && pos.is_pseudo_legal(tte->move())
1872 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1874 && (!pos.is_draw<false>() || ply < 2))
1876 pv.push_back(tte->move());
1877 pos.do_move(tte->move(), *st++);
1880 pv.push_back(MOVE_NONE);
1882 do pos.undo_move(pv[--ply]); while (ply);
1886 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1887 // the PV back into the TT. This makes sure the old PV moves are searched
1888 // first, even if the old TT entries have been overwritten.
1890 void RootMove::insert_pv_in_tt(Position& pos) {
1892 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1895 Value v, m = VALUE_NONE;
1898 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1904 // Don't overwrite existing correct entries
1905 if (!tte || tte->move() != pv[ply])
1907 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1908 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
1910 pos.do_move(pv[ply], *st++);
1912 } while (pv[++ply] != MOVE_NONE);
1914 do pos.undo_move(pv[--ply]); while (ply);
1920 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1921 /// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
1922 /// for which the thread is the master.
1924 void Thread::idle_loop(SplitPoint* sp) {
1928 // If we are not searching, wait for a condition to be signaled
1929 // instead of wasting CPU time polling for work.
1932 || (Threads.use_sleeping_threads() && !is_searching))
1934 assert((!sp && threadID) || Threads.use_sleeping_threads());
1942 // Grab the lock to avoid races with Thread::wake_up()
1943 lock_grab(&sleepLock);
1945 // If we are master and all slaves have finished don't go to sleep
1946 if (sp && Threads.split_point_finished(sp))
1948 lock_release(&sleepLock);
1952 // Do sleep after retesting sleep conditions under lock protection, in
1953 // particular we need to avoid a deadlock in case a master thread has,
1954 // in the meanwhile, allocated us and sent the wake_up() call before we
1955 // had the chance to grab the lock.
1956 if (do_sleep || !is_searching)
1957 cond_wait(&sleepCond, &sleepLock);
1959 lock_release(&sleepLock);
1962 // If this thread has been assigned work, launch a search
1965 assert(!do_terminate);
1967 // Copy split point position and search stack and call search()
1968 Stack ss[PLY_MAX_PLUS_2];
1969 SplitPoint* tsp = splitPoint;
1970 Position pos(*tsp->pos, threadID);
1972 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
1975 if (tsp->nodeType == Root)
1976 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1977 else if (tsp->nodeType == PV)
1978 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1979 else if (tsp->nodeType == NonPV)
1980 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1984 assert(is_searching);
1986 is_searching = false;
1988 // Wake up master thread so to allow it to return from the idle loop in
1989 // case we are the last slave of the split point.
1990 if ( Threads.use_sleeping_threads()
1991 && threadID != tsp->master
1992 && !Threads[tsp->master].is_searching)
1993 Threads[tsp->master].wake_up();
1996 // If this thread is the master of a split point and all slaves have
1997 // finished their work at this split point, return from the idle loop.
1998 if (sp && Threads.split_point_finished(sp))
2000 // Because sp->is_slave[] is reset under lock protection,
2001 // be sure sp->lock has been released before to return.
2002 lock_grab(&(sp->lock));
2003 lock_release(&(sp->lock));
2010 /// do_timer_event() is called by the timer thread when the timer triggers. It
2011 /// is used to print debug info and, more important, to detect when we are out of
2012 /// available time and so stop the search.
2014 void do_timer_event() {
2016 static int lastInfoTime;
2017 int e = elapsed_time();
2019 if (get_system_time() - lastInfoTime >= 1000 || !lastInfoTime)
2021 lastInfoTime = get_system_time();
2024 dbg_print_hit_rate();
2030 bool stillAtFirstMove = Signals.firstRootMove
2031 && !Signals.failedLowAtRoot
2032 && e > TimeMgr.available_time();
2034 bool noMoreTime = e > TimeMgr.maximum_time()
2035 || stillAtFirstMove;
2037 if ( (Limits.useTimeManagement() && noMoreTime)
2038 || (Limits.maxTime && e >= Limits.maxTime)
2039 /* missing nodes limit */ ) // FIXME
2040 Signals.stop = true;