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>());
332 if ( bookMove != MOVE_NONE
333 && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
335 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
340 // Read UCI options: GUI could change UCI parameters during the game
341 read_evaluation_uci_options(pos.side_to_move());
342 Threads.read_uci_options();
344 TT.set_size(Options["Hash"].value<int>());
345 if (Options["Clear Hash"].value<bool>())
347 Options["Clear Hash"].set_value("false");
351 UCIMultiPV = Options["MultiPV"].value<size_t>();
352 SkillLevel = Options["Skill Level"].value<int>();
354 // Do we have to play with skill handicap? In this case enable MultiPV that
355 // we will use behind the scenes to retrieve a set of possible moves.
356 SkillLevelEnabled = (SkillLevel < 20);
357 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
359 if (Options["Use Search Log"].value<bool>())
361 Log log(Options["Search Log Filename"].value<string>());
362 log << "\nSearching: " << pos.to_fen()
363 << "\ninfinite: " << Limits.infinite
364 << " ponder: " << Limits.ponder
365 << " time: " << Limits.time
366 << " increment: " << Limits.increment
367 << " moves to go: " << Limits.movesToGo
371 for (int i = 0; i < Threads.size(); i++)
373 Threads[i].maxPly = 0;
374 Threads[i].wake_up();
377 // Set best timer interval to avoid lagging under time pressure. Timer is
378 // used to check for remaining available thinking time.
379 if (TimeMgr.available_time())
380 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
382 Threads.set_timer(100);
384 // We're ready to start searching. Call the iterative deepening loop function
387 // Stop timer and send all the slaves to sleep, if not already sleeping
388 Threads.set_timer(0);
391 if (Options["Use Search Log"].value<bool>())
393 int e = elapsed_time();
395 Log log(Options["Search Log Filename"].value<string>());
396 log << "Nodes: " << pos.nodes_searched()
397 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
398 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
401 pos.do_move(RootMoves[0].pv[0], st);
402 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
403 pos.undo_move(RootMoves[0].pv[0]);
408 // When we reach max depth we arrive here even without a StopRequest, but if
409 // we are pondering or in infinite search, we shouldn't print the best move
410 // before we are told to do so.
411 if (!Signals.stop && (Limits.ponder || Limits.infinite))
412 Threads.wait_for_stop_or_ponderhit();
414 // Could be MOVE_NONE when searching on a stalemate position
415 cout << "bestmove " << RootMoves[0].pv[0];
417 // UCI protol is not clear on allowing sending an empty ponder move, instead
418 // it is clear that ponder move is optional. So skip it if empty.
419 if (RootMoves[0].pv[1] != MOVE_NONE)
420 cout << " ponder " << RootMoves[0].pv[1];
428 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
429 // with increasing depth until the allocated thinking time has been consumed,
430 // user stops the search, or the maximum search depth is reached.
432 void id_loop(Position& pos) {
434 Stack ss[PLY_MAX_PLUS_2];
435 int depth, prevBestMoveChanges;
436 Value bestValue, alpha, beta, delta;
437 bool bestMoveNeverChanged = true;
438 Move skillBest = MOVE_NONE;
440 memset(ss, 0, 4 * sizeof(Stack));
441 depth = BestMoveChanges = 0;
442 bestValue = delta = -VALUE_INFINITE;
443 ss->currentMove = MOVE_NULL; // Hack to skip update gains
445 // Handle the special case of a mate/stalemate position
446 if (RootMoves.empty())
448 cout << "info depth 0"
449 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
451 RootMoves.push_back(MOVE_NONE);
455 // Iterative deepening loop until requested to stop or target depth reached
456 while (!Signals.stop && ++depth <= PLY_MAX && (!Limits.maxDepth || depth <= Limits.maxDepth))
458 // Save last iteration's scores before first PV line is searched and all
459 // the move scores but the (new) PV are set to -VALUE_INFINITE.
460 for (size_t i = 0; i < RootMoves.size(); i++)
461 RootMoves[i].prevScore = RootMoves[i].score;
463 prevBestMoveChanges = BestMoveChanges;
466 // MultiPV loop. We perform a full root search for each PV line
467 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
469 // Set aspiration window default width
470 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
473 alpha = RootMoves[PVIdx].prevScore - delta;
474 beta = RootMoves[PVIdx].prevScore + delta;
478 alpha = -VALUE_INFINITE;
479 beta = VALUE_INFINITE;
482 // Start with a small aspiration window and, in case of fail high/low,
483 // research with bigger window until not failing high/low anymore.
485 // Search starts from ss+1 to allow referencing (ss-1). This is
486 // needed by update gains and ss copy when splitting at Root.
487 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
489 // Bring to front the best move. It is critical that sorting is
490 // done with a stable algorithm because all the values but the first
491 // and eventually the new best one are set to -VALUE_INFINITE and
492 // we want to keep the same order for all the moves but the new
493 // PV that goes to the front. Note that in case of MultiPV search
494 // the already searched PV lines are preserved.
495 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
497 // In case we have found an exact score and we are going to leave
498 // the fail high/low loop then reorder the PV moves, otherwise
499 // leave the last PV move in its position so to be searched again.
500 // Of course this is needed only in MultiPV search.
501 if (PVIdx && bestValue > alpha && bestValue < beta)
502 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
504 // Write PV back to transposition table in case the relevant
505 // entries have been overwritten during the search.
506 for (size_t i = 0; i <= PVIdx; i++)
507 RootMoves[i].insert_pv_in_tt(pos);
509 // If search has been stopped exit the aspiration window loop.
510 // Sorting and writing PV back to TT is safe becuase RootMoves
511 // is still valid, although refers to previous iteration.
515 // Send full PV info to GUI if we are going to leave the loop or
516 // if we have a fail high/low and we are deep in the search.
517 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
518 pv_info_to_uci(pos, depth, alpha, beta);
520 // In case of failing high/low increase aspiration window and
521 // research, otherwise exit the fail high/low loop.
522 if (bestValue >= beta)
527 else if (bestValue <= alpha)
529 Signals.failedLowAtRoot = true;
530 Signals.stopOnPonderhit = false;
538 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
540 } while (abs(bestValue) < VALUE_KNOWN_WIN);
543 // Skills: Do we need to pick now the best move ?
544 if (SkillLevelEnabled && depth == 1 + SkillLevel)
545 skillBest = do_skill_level();
547 if (Options["Use Search Log"].value<bool>())
548 pv_info_to_log(pos, depth, bestValue, elapsed_time(), &RootMoves[0].pv[0]);
550 // Filter out startup noise when monitoring best move stability
551 if (depth > 2 && BestMoveChanges)
552 bestMoveNeverChanged = false;
554 // Do we have time for the next iteration? Can we stop searching now?
555 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
557 bool stop = false; // Local variable, not the volatile Signals.stop
559 // Take in account some extra time if the best move has changed
560 if (depth > 4 && depth < 50)
561 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
563 // Stop search if most of available time is already consumed. We
564 // probably don't have enough time to search the first move at the
565 // next iteration anyway.
566 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
569 // Stop search early if one move seems to be much better than others
572 && ( bestMoveNeverChanged
573 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
575 Value rBeta = bestValue - EasyMoveMargin;
576 (ss+1)->excludedMove = RootMoves[0].pv[0];
577 (ss+1)->skipNullMove = true;
578 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
579 (ss+1)->skipNullMove = false;
580 (ss+1)->excludedMove = MOVE_NONE;
588 // If we are allowed to ponder do not stop the search now but
589 // keep pondering until GUI sends "ponderhit" or "stop".
591 Signals.stopOnPonderhit = true;
598 // When using skills swap best PV line with the sub-optimal one
599 if (SkillLevelEnabled)
601 if (skillBest == MOVE_NONE) // Still unassigned ?
602 skillBest = do_skill_level();
604 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), skillBest));
609 // search<>() is the main search function for both PV and non-PV nodes and for
610 // normal and SplitPoint nodes. When called just after a split point the search
611 // is simpler because we have already probed the hash table, done a null move
612 // search, and searched the first move before splitting, we don't have to repeat
613 // all this work again. We also don't need to store anything to the hash table
614 // here: This is taken care of after we return from the split point.
616 template <NodeType NT>
617 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
619 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
620 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
621 const bool RootNode = (NT == Root || NT == SplitPointRoot);
623 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
624 assert(beta > alpha && beta <= VALUE_INFINITE);
625 assert(PvNode || alpha == beta - 1);
626 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
628 Move movesSearched[MAX_MOVES];
633 Move ttMove, move, excludedMove, threatMove;
636 Value bestValue, value, oldAlpha;
637 Value refinedValue, nullValue, futilityBase, futilityValue;
638 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
639 bool captureOrPromotion, dangerous, doFullDepthSearch;
640 int moveCount = 0, playedMoveCount = 0;
641 Thread& thread = Threads[pos.thread()];
642 SplitPoint* sp = NULL;
644 refinedValue = bestValue = value = -VALUE_INFINITE;
646 inCheck = pos.in_check();
647 ss->ply = (ss-1)->ply + 1;
649 // Used to send selDepth info to GUI
650 if (PvNode && thread.maxPly < ss->ply)
651 thread.maxPly = ss->ply;
653 // Step 1. Initialize node
656 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
657 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
658 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
664 ttMove = excludedMove = MOVE_NONE;
665 threatMove = sp->threatMove;
666 goto split_point_start;
669 // Step 2. Check for aborted search and immediate draw
671 || pos.is_draw<false>()
672 || ss->ply > PLY_MAX) && !RootNode)
675 // Step 3. Mate distance pruning
678 alpha = std::max(value_mated_in(ss->ply), alpha);
679 beta = std::min(value_mate_in(ss->ply+1), beta);
684 // Step 4. Transposition table lookup
685 // We don't want the score of a partial search to overwrite a previous full search
686 // TT value, so we use a different position key in case of an excluded move.
687 excludedMove = ss->excludedMove;
688 posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
689 tte = TT.probe(posKey);
690 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
692 // At PV nodes we check for exact scores, while at non-PV nodes we check for
693 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
694 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
695 // we should also update RootMoveList to avoid bogus output.
696 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
697 : can_return_tt(tte, depth, beta, ss->ply)))
700 ss->bestMove = move = ttMove; // Can be MOVE_NONE
701 value = value_from_tt(tte->value(), ss->ply);
705 && !pos.is_capture_or_promotion(move)
706 && move != ss->killers[0])
708 ss->killers[1] = ss->killers[0];
709 ss->killers[0] = move;
714 // Step 5. Evaluate the position statically and update parent's gain statistics
716 ss->eval = ss->evalMargin = VALUE_NONE;
719 assert(tte->static_value() != VALUE_NONE);
721 ss->eval = tte->static_value();
722 ss->evalMargin = tte->static_value_margin();
723 refinedValue = refine_eval(tte, ss->eval, ss->ply);
727 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
728 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
731 // Update gain for the parent non-capture move given the static position
732 // evaluation before and after the move.
733 if ( (move = (ss-1)->currentMove) != MOVE_NULL
734 && (ss-1)->eval != VALUE_NONE
735 && ss->eval != VALUE_NONE
736 && pos.captured_piece_type() == PIECE_TYPE_NONE
737 && !is_special(move))
739 Square to = move_to(move);
740 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
743 // Step 6. Razoring (is omitted in PV nodes)
745 && depth < RazorDepth
747 && refinedValue + razor_margin(depth) < beta
748 && ttMove == MOVE_NONE
749 && abs(beta) < VALUE_MATE_IN_PLY_MAX
750 && !pos.has_pawn_on_7th(pos.side_to_move()))
752 Value rbeta = beta - razor_margin(depth);
753 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
755 // Logically we should return (v + razor_margin(depth)), but
756 // surprisingly this did slightly weaker in tests.
760 // Step 7. Static null move pruning (is omitted in PV nodes)
761 // We're betting that the opponent doesn't have a move that will reduce
762 // the score by more than futility_margin(depth) if we do a null move.
765 && depth < RazorDepth
767 && refinedValue - futility_margin(depth, 0) >= beta
768 && abs(beta) < VALUE_MATE_IN_PLY_MAX
769 && pos.non_pawn_material(pos.side_to_move()))
770 return refinedValue - futility_margin(depth, 0);
772 // Step 8. Null move search with verification search (is omitted in PV nodes)
777 && refinedValue >= beta
778 && abs(beta) < VALUE_MATE_IN_PLY_MAX
779 && pos.non_pawn_material(pos.side_to_move()))
781 ss->currentMove = MOVE_NULL;
783 // Null move dynamic reduction based on depth
784 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
786 // Null move dynamic reduction based on value
787 if (refinedValue - PawnValueMidgame > beta)
790 pos.do_null_move<true>(st);
791 (ss+1)->skipNullMove = true;
792 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
793 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
794 (ss+1)->skipNullMove = false;
795 pos.do_null_move<false>(st);
797 if (nullValue >= beta)
799 // Do not return unproven mate scores
800 if (nullValue >= VALUE_MATE_IN_PLY_MAX)
803 if (depth < 6 * ONE_PLY)
806 // Do verification search at high depths
807 ss->skipNullMove = true;
808 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
809 ss->skipNullMove = false;
816 // The null move failed low, which means that we may be faced with
817 // some kind of threat. If the previous move was reduced, check if
818 // the move that refuted the null move was somehow connected to the
819 // move which was reduced. If a connection is found, return a fail
820 // low score (which will cause the reduced move to fail high in the
821 // parent node, which will trigger a re-search with full depth).
822 threatMove = (ss+1)->bestMove;
824 if ( depth < ThreatDepth
826 && threatMove != MOVE_NONE
827 && connected_moves(pos, (ss-1)->currentMove, threatMove))
832 // Step 9. ProbCut (is omitted in PV nodes)
833 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
834 // and a reduced search returns a value much above beta, we can (almost) safely
835 // prune the previous move.
837 && depth >= RazorDepth + ONE_PLY
840 && excludedMove == MOVE_NONE
841 && abs(beta) < VALUE_MATE_IN_PLY_MAX)
843 Value rbeta = beta + 200;
844 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
846 assert(rdepth >= ONE_PLY);
848 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
851 while ((move = mp.get_next_move()) != MOVE_NONE)
852 if (pos.pl_move_is_legal(move, ci.pinned))
854 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
855 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
862 // Step 10. Internal iterative deepening
863 if ( depth >= IIDDepth[PvNode]
864 && ttMove == MOVE_NONE
865 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
867 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
869 ss->skipNullMove = true;
870 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
871 ss->skipNullMove = false;
873 tte = TT.probe(posKey);
874 ttMove = tte ? tte->move() : MOVE_NONE;
877 split_point_start: // At split points actual search starts from here
879 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
881 ss->bestMove = MOVE_NONE;
882 futilityBase = ss->eval + ss->evalMargin;
883 singularExtensionNode = !RootNode
885 && depth >= SingularExtensionDepth[PvNode]
886 && ttMove != MOVE_NONE
887 && !excludedMove // Recursive singular search is not allowed
888 && (tte->type() & VALUE_TYPE_LOWER)
889 && tte->depth() >= depth - 3 * ONE_PLY;
892 lock_grab(&(sp->lock));
893 bestValue = sp->bestValue;
894 moveCount = sp->moveCount;
896 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
899 // Step 11. Loop through moves
900 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
901 while ( bestValue < beta
902 && (move = mp.get_next_move()) != MOVE_NONE
903 && !thread.cutoff_occurred())
907 if (move == excludedMove)
910 // At root obey the "searchmoves" option and skip moves not listed in Root
911 // Move List, as a consequence any illegal move is also skipped. In MultiPV
912 // mode we also skip PV moves which have been already searched.
913 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
916 // At PV and SpNode nodes we want all moves to be legal since the beginning
917 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
922 moveCount = ++sp->moveCount;
923 lock_release(&(sp->lock));
930 // This is used by time management
931 Signals.firstRootMove = (moveCount == 1);
933 nodes = pos.nodes_searched();
935 if (pos.thread() == 0 && elapsed_time() > 2000)
936 cout << "info depth " << depth / ONE_PLY
937 << " currmove " << move
938 << " currmovenumber " << moveCount + PVIdx << endl;
941 isPvMove = (PvNode && moveCount <= 1);
942 captureOrPromotion = pos.is_capture_or_promotion(move);
943 givesCheck = pos.move_gives_check(move, ci);
944 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
947 // Step 12. Extend checks and, in PV nodes, also dangerous moves
948 if (PvNode && dangerous)
951 else if (givesCheck && pos.see_sign(move) >= 0)
952 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
954 // Singular extension search. If all moves but one fail low on a search of
955 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
956 // is singular and should be extended. To verify this we do a reduced search
957 // on all the other moves but the ttMove, if result is lower than ttValue minus
958 // a margin then we extend ttMove.
959 if ( singularExtensionNode
962 && pos.pl_move_is_legal(move, ci.pinned))
964 Value ttValue = value_from_tt(tte->value(), ss->ply);
966 if (abs(ttValue) < VALUE_KNOWN_WIN)
968 Value rBeta = ttValue - int(depth);
969 ss->excludedMove = move;
970 ss->skipNullMove = true;
971 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
972 ss->skipNullMove = false;
973 ss->excludedMove = MOVE_NONE;
974 ss->bestMove = MOVE_NONE;
980 // Update current move (this must be done after singular extension search)
981 newDepth = depth - ONE_PLY + ext;
983 // Step 13. Futility pruning (is omitted in PV nodes)
985 && !captureOrPromotion
990 && (bestValue > VALUE_MATED_IN_PLY_MAX || bestValue == -VALUE_INFINITE))
992 // Move count based pruning
993 if ( moveCount >= futility_move_count(depth)
994 && (!threatMove || !connected_threat(pos, move, threatMove)))
997 lock_grab(&(sp->lock));
1002 // Value based pruning
1003 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
1004 // but fixing this made program slightly weaker.
1005 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
1006 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
1007 + H.gain(pos.piece_on(move_from(move)), move_to(move));
1009 if (futilityValue < beta)
1012 lock_grab(&(sp->lock));
1017 // Prune moves with negative SEE at low depths
1018 if ( predictedDepth < 2 * ONE_PLY
1019 && pos.see_sign(move) < 0)
1022 lock_grab(&(sp->lock));
1028 // Check for legality only before to do the move
1029 if (!pos.pl_move_is_legal(move, ci.pinned))
1035 ss->currentMove = move;
1036 if (!SpNode && !captureOrPromotion)
1037 movesSearched[playedMoveCount++] = move;
1039 // Step 14. Make the move
1040 pos.do_move(move, st, ci, givesCheck);
1042 // Step 15. Reduced depth search (LMR). If the move fails high will be
1043 // re-searched at full depth.
1044 if ( depth > 3 * ONE_PLY
1046 && !captureOrPromotion
1049 && ss->killers[0] != move
1050 && ss->killers[1] != move)
1052 ss->reduction = reduction<PvNode>(depth, moveCount);
1053 Depth d = newDepth - ss->reduction;
1054 alpha = SpNode ? sp->alpha : alpha;
1056 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1057 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1059 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1060 ss->reduction = DEPTH_ZERO;
1063 doFullDepthSearch = !isPvMove;
1065 // Step 16. Full depth search, when LMR is skipped or fails high
1066 if (doFullDepthSearch)
1068 alpha = SpNode ? sp->alpha : alpha;
1069 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1070 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1073 // Only for PV nodes do a full PV search on the first move or after a fail
1074 // high, in the latter case search only if value < beta, otherwise let the
1075 // parent node to fail low with value <= alpha and to try another move.
1076 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1077 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1078 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1080 // Step 17. Undo move
1081 pos.undo_move(move);
1083 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1085 // Step 18. Check for new best move
1088 lock_grab(&(sp->lock));
1089 bestValue = sp->bestValue;
1093 // Finished searching the move. If StopRequest is true, the search
1094 // was aborted because the user interrupted the search or because we
1095 // ran out of time. In this case, the return value of the search cannot
1096 // be trusted, and we don't update the best move and/or PV.
1097 if (RootNode && !Signals.stop)
1099 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1100 rm.nodes += pos.nodes_searched() - nodes;
1102 // PV move or new best move ?
1103 if (isPvMove || value > alpha)
1106 rm.extract_pv_from_tt(pos);
1108 // We record how often the best move has been changed in each
1109 // iteration. This information is used for time management: When
1110 // the best move changes frequently, we allocate some more time.
1111 if (!isPvMove && MultiPV == 1)
1115 // All other moves but the PV are set to the lowest value, this
1116 // is not a problem when sorting becuase sort is stable and move
1117 // position in the list is preserved, just the PV is pushed up.
1118 rm.score = -VALUE_INFINITE;
1122 if (value > bestValue)
1125 ss->bestMove = move;
1129 && value < beta) // We want always alpha < beta
1132 if (SpNode && !thread.cutoff_occurred())
1134 sp->bestValue = value;
1135 sp->ss->bestMove = move;
1137 sp->is_betaCutoff = (value >= beta);
1141 // Step 19. Check for split
1143 && depth >= Threads.min_split_depth()
1145 && Threads.available_slave_exists(pos.thread())
1147 && !thread.cutoff_occurred())
1148 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1149 threatMove, moveCount, &mp, NT);
1152 // Step 20. Check for mate and stalemate
1153 // All legal moves have been searched and if there are no legal moves, it
1154 // must be mate or stalemate. Note that we can have a false positive in
1155 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1156 // harmless because return value is discarded anyhow in the parent nodes.
1157 // If we are in a singular extension search then return a fail low score.
1159 return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
1161 // If we have pruned all the moves without searching return a fail-low score
1162 if (bestValue == -VALUE_INFINITE)
1164 assert(!playedMoveCount);
1169 // Step 21. Update tables
1170 // Update transposition table entry, history and killers
1171 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1173 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1174 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1175 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1177 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1179 // Update killers and history only for non capture moves that fails high
1180 if ( bestValue >= beta
1181 && !pos.is_capture_or_promotion(move))
1183 if (move != ss->killers[0])
1185 ss->killers[1] = ss->killers[0];
1186 ss->killers[0] = move;
1188 update_history(pos, move, depth, movesSearched, playedMoveCount);
1194 // Here we have the lock still grabbed
1195 sp->is_slave[pos.thread()] = false;
1196 sp->nodes += pos.nodes_searched();
1197 lock_release(&(sp->lock));
1200 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1206 // qsearch() is the quiescence search function, which is called by the main
1207 // search function when the remaining depth is zero (or, to be more precise,
1208 // less than ONE_PLY).
1210 template <NodeType NT>
1211 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1213 const bool PvNode = (NT == PV);
1215 assert(NT == PV || NT == NonPV);
1216 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1217 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1218 assert(PvNode || alpha == beta - 1);
1220 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1224 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1225 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1229 Value oldAlpha = alpha;
1231 ss->bestMove = ss->currentMove = MOVE_NONE;
1232 ss->ply = (ss-1)->ply + 1;
1234 // Check for an instant draw or maximum ply reached
1235 if (pos.is_draw<true>() || ss->ply > PLY_MAX)
1238 // Decide whether or not to include checks, this fixes also the type of
1239 // TT entry depth that we are going to use. Note that in qsearch we use
1240 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1241 inCheck = pos.in_check();
1242 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1244 // Transposition table lookup. At PV nodes, we don't use the TT for
1245 // pruning, but only for move ordering.
1246 tte = TT.probe(pos.get_key());
1247 ttMove = (tte ? tte->move() : MOVE_NONE);
1249 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1251 ss->bestMove = ttMove; // Can be MOVE_NONE
1252 return value_from_tt(tte->value(), ss->ply);
1255 // Evaluate the position statically
1258 bestValue = futilityBase = -VALUE_INFINITE;
1259 ss->eval = evalMargin = VALUE_NONE;
1260 enoughMaterial = false;
1266 assert(tte->static_value() != VALUE_NONE);
1268 evalMargin = tte->static_value_margin();
1269 ss->eval = bestValue = tte->static_value();
1272 ss->eval = bestValue = evaluate(pos, evalMargin);
1274 // Stand pat. Return immediately if static value is at least beta
1275 if (bestValue >= beta)
1278 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1283 if (PvNode && bestValue > alpha)
1286 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1287 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1290 // Initialize a MovePicker object for the current position, and prepare
1291 // to search the moves. Because the depth is <= 0 here, only captures,
1292 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1294 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1297 // Loop through the moves until no moves remain or a beta cutoff occurs
1298 while ( bestValue < beta
1299 && (move = mp.get_next_move()) != MOVE_NONE)
1301 assert(is_ok(move));
1303 givesCheck = pos.move_gives_check(move, ci);
1311 && !is_promotion(move)
1312 && !pos.is_passed_pawn_push(move))
1314 futilityValue = futilityBase
1315 + PieceValueEndgame[pos.piece_on(move_to(move))]
1316 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1318 if (futilityValue < beta)
1320 if (futilityValue > bestValue)
1321 bestValue = futilityValue;
1326 // Prune moves with negative or equal SEE
1327 if ( futilityBase < beta
1328 && depth < DEPTH_ZERO
1329 && pos.see(move) <= 0)
1333 // Detect non-capture evasions that are candidate to be pruned
1334 evasionPrunable = !PvNode
1336 && bestValue > VALUE_MATED_IN_PLY_MAX
1337 && !pos.is_capture(move)
1338 && !pos.can_castle(pos.side_to_move());
1340 // Don't search moves with negative SEE values
1342 && (!inCheck || evasionPrunable)
1344 && !is_promotion(move)
1345 && pos.see_sign(move) < 0)
1348 // Don't search useless checks
1353 && !pos.is_capture_or_promotion(move)
1354 && ss->eval + PawnValueMidgame / 4 < beta
1355 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1357 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1358 bestValue = ss->eval + PawnValueMidgame / 4;
1363 // Check for legality only before to do the move
1364 if (!pos.pl_move_is_legal(move, ci.pinned))
1367 ss->currentMove = move;
1369 // Make and search the move
1370 pos.do_move(move, st, ci, givesCheck);
1371 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1372 pos.undo_move(move);
1374 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1377 if (value > bestValue)
1380 ss->bestMove = move;
1384 && value < beta) // We want always alpha < beta
1389 // All legal moves have been searched. A special case: If we're in check
1390 // and no legal moves were found, it is checkmate.
1391 if (inCheck && bestValue == -VALUE_INFINITE)
1392 return value_mated_in(ss->ply);
1394 // Update transposition table
1395 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1396 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1397 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1399 TT.store(pos.get_key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1401 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1407 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1408 // bestValue is updated only when returning false because in that case move
1411 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1413 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1414 Square from, to, ksq, victimSq;
1417 Value futilityValue, bv = *bestValue;
1419 from = move_from(move);
1421 them = flip(pos.side_to_move());
1422 ksq = pos.king_square(them);
1423 kingAtt = pos.attacks_from<KING>(ksq);
1424 pc = pos.piece_on(from);
1426 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1427 oldAtt = pos.attacks_from(pc, from, occ);
1428 newAtt = pos.attacks_from(pc, to, occ);
1430 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1431 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1433 if (!(b && (b & (b - 1))))
1436 // Rule 2. Queen contact check is very dangerous
1437 if ( type_of(pc) == QUEEN
1438 && bit_is_set(kingAtt, to))
1441 // Rule 3. Creating new double threats with checks
1442 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1446 victimSq = pop_1st_bit(&b);
1447 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1449 // Note that here we generate illegal "double move"!
1450 if ( futilityValue >= beta
1451 && pos.see_sign(make_move(from, victimSq)) >= 0)
1454 if (futilityValue > bv)
1458 // Update bestValue only if check is not dangerous (because we will prune the move)
1464 // connected_moves() tests whether two moves are 'connected' in the sense
1465 // that the first move somehow made the second move possible (for instance
1466 // if the moving piece is the same in both moves). The first move is assumed
1467 // to be the move that was made to reach the current position, while the
1468 // second move is assumed to be a move from the current position.
1470 bool connected_moves(const Position& pos, Move m1, Move m2) {
1472 Square f1, t1, f2, t2;
1479 // Case 1: The moving piece is the same in both moves
1485 // Case 2: The destination square for m2 was vacated by m1
1491 // Case 3: Moving through the vacated square
1492 p2 = pos.piece_on(f2);
1493 if ( piece_is_slider(p2)
1494 && bit_is_set(squares_between(f2, t2), f1))
1497 // Case 4: The destination square for m2 is defended by the moving piece in m1
1498 p1 = pos.piece_on(t1);
1499 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1502 // Case 5: Discovered check, checking piece is the piece moved in m1
1503 ksq = pos.king_square(pos.side_to_move());
1504 if ( piece_is_slider(p1)
1505 && bit_is_set(squares_between(t1, ksq), f2))
1507 Bitboard occ = pos.occupied_squares();
1508 clear_bit(&occ, f2);
1509 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1516 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1517 // "plies to mate from the current ply". Non-mate scores are unchanged.
1518 // The function is called before storing a value to the transposition table.
1520 Value value_to_tt(Value v, int ply) {
1522 if (v >= VALUE_MATE_IN_PLY_MAX)
1525 if (v <= VALUE_MATED_IN_PLY_MAX)
1532 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score from
1533 // the transposition table to a mate score corrected for the current ply.
1535 Value value_from_tt(Value v, int ply) {
1537 if (v >= VALUE_MATE_IN_PLY_MAX)
1540 if (v <= VALUE_MATED_IN_PLY_MAX)
1547 // connected_threat() tests whether it is safe to forward prune a move or if
1548 // is somehow connected to the threat move returned by null search.
1550 bool connected_threat(const Position& pos, Move m, Move threat) {
1553 assert(is_ok(threat));
1554 assert(!pos.is_capture_or_promotion(m));
1555 assert(!pos.is_passed_pawn_push(m));
1557 Square mfrom, mto, tfrom, tto;
1559 mfrom = move_from(m);
1561 tfrom = move_from(threat);
1562 tto = move_to(threat);
1564 // Case 1: Don't prune moves which move the threatened piece
1568 // Case 2: If the threatened piece has value less than or equal to the
1569 // value of the threatening piece, don't prune moves which defend it.
1570 if ( pos.is_capture(threat)
1571 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1572 || type_of(pos.piece_on(tfrom)) == KING)
1573 && pos.move_attacks_square(m, tto))
1576 // Case 3: If the moving piece in the threatened move is a slider, don't
1577 // prune safe moves which block its ray.
1578 if ( piece_is_slider(pos.piece_on(tfrom))
1579 && bit_is_set(squares_between(tfrom, tto), mto)
1580 && pos.see_sign(m) >= 0)
1587 // can_return_tt() returns true if a transposition table score can be used to
1588 // cut-off at a given point in search.
1590 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1592 Value v = value_from_tt(tte->value(), ply);
1594 return ( tte->depth() >= depth
1595 || v >= std::max(VALUE_MATE_IN_PLY_MAX, beta)
1596 || v < std::min(VALUE_MATED_IN_PLY_MAX, beta))
1598 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1599 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1603 // refine_eval() returns the transposition table score if possible, otherwise
1604 // falls back on static position evaluation.
1606 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1610 Value v = value_from_tt(tte->value(), ply);
1612 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1613 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1620 // update_history() registers a good move that produced a beta-cutoff in
1621 // history and marks as failures all the other moves of that ply.
1623 void update_history(const Position& pos, Move move, Depth depth,
1624 Move movesSearched[], int moveCount) {
1626 Value bonus = Value(int(depth) * int(depth));
1628 H.update(pos.piece_on(move_from(move)), move_to(move), bonus);
1630 for (int i = 0; i < moveCount - 1; i++)
1632 m = movesSearched[i];
1636 H.update(pos.piece_on(move_from(m)), move_to(m), -bonus);
1641 // current_search_time() returns the number of milliseconds which have passed
1642 // since the beginning of the current search.
1644 int elapsed_time(bool reset) {
1646 static int searchStartTime;
1649 searchStartTime = get_system_time();
1651 return get_system_time() - searchStartTime;
1655 // score_to_uci() converts a value to a string suitable for use with the UCI
1656 // protocol specifications:
1658 // cp <x> The score from the engine's point of view in centipawns.
1659 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1660 // use negative values for y.
1662 string score_to_uci(Value v, Value alpha, Value beta) {
1664 std::stringstream s;
1666 if (abs(v) < VALUE_MATE - PLY_MAX * ONE_PLY)
1667 s << " score cp " << int(v) * 100 / int(PawnValueMidgame); // Scale to centipawns
1669 s << " score mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1671 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1677 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1678 // the PV lines also if are still to be searched and so refer to the previous
1681 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1683 int t = elapsed_time();
1686 for (int i = 0; i < Threads.size(); i++)
1687 if (Threads[i].maxPly > selDepth)
1688 selDepth = Threads[i].maxPly;
1690 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1692 bool updated = (i <= PVIdx);
1694 if (depth == 1 && !updated)
1697 int d = (updated ? depth : depth - 1);
1698 Value s = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1702 << " seldepth " << selDepth
1703 << (i == PVIdx ? score_to_uci(s, alpha, beta) : score_to_uci(s))
1704 << " nodes " << pos.nodes_searched()
1705 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1707 << " multipv " << i + 1 << " pv";
1709 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1710 cout << " " << RootMoves[i].pv[j];
1717 // pv_info_to_log() writes human-readable search information to the log file
1718 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1719 // uses the two below helpers to pretty format time and score respectively.
1721 string time_to_string(int millisecs) {
1723 const int MSecMinute = 1000 * 60;
1724 const int MSecHour = 1000 * 60 * 60;
1726 int hours = millisecs / MSecHour;
1727 int minutes = (millisecs % MSecHour) / MSecMinute;
1728 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1730 std::stringstream s;
1735 s << std::setfill('0') << std::setw(2) << minutes << ':'
1736 << std::setw(2) << seconds;
1740 string score_to_string(Value v) {
1742 std::stringstream s;
1744 if (v >= VALUE_MATE_IN_PLY_MAX)
1745 s << "#" << (VALUE_MATE - v + 1) / 2;
1746 else if (v <= VALUE_MATED_IN_PLY_MAX)
1747 s << "-#" << (VALUE_MATE + v) / 2;
1749 s << std::setprecision(2) << std::fixed << std::showpos
1750 << float(v) / PawnValueMidgame;
1755 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1757 const int64_t K = 1000;
1758 const int64_t M = 1000000;
1760 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1762 string san, padding;
1764 std::stringstream s;
1766 s << set960(pos.is_chess960())
1767 << std::setw(2) << depth
1768 << std::setw(8) << score_to_string(value)
1769 << std::setw(8) << time_to_string(time);
1771 if (pos.nodes_searched() < M)
1772 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1774 else if (pos.nodes_searched() < K * M)
1775 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1778 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1780 padding = string(s.str().length(), ' ');
1781 length = padding.length();
1783 while (*m != MOVE_NONE)
1785 san = move_to_san(pos, *m);
1787 if (length + san.length() > 80)
1789 s << "\n" + padding;
1790 length = padding.length();
1794 length += san.length() + 1;
1796 pos.do_move(*m++, *st++);
1800 pos.undo_move(*--m);
1802 Log l(Options["Search Log Filename"].value<string>());
1803 l << s.str() << endl;
1807 // When playing with strength handicap choose best move among the MultiPV set
1808 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1810 Move do_skill_level() {
1812 assert(MultiPV > 1);
1816 // PRNG sequence should be not deterministic
1817 for (int i = abs(get_system_time() % 50); i > 0; i--)
1818 rk.rand<unsigned>();
1820 // Rml list is already sorted by score in descending order
1821 size_t size = std::min(MultiPV, RootMoves.size());
1822 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1823 int weakness = 120 - 2 * SkillLevel;
1824 int max_s = -VALUE_INFINITE;
1825 Move best = MOVE_NONE;
1827 // Choose best move. For each move score we add two terms both dependent on
1828 // weakness, one deterministic and bigger for weaker moves, and one random,
1829 // then we choose the move with the resulting highest score.
1830 for (size_t i = 0; i < size; i++)
1832 int s = RootMoves[i].score;
1834 // Don't allow crazy blunders even at very low skills
1835 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1838 // This is our magic formula
1839 s += ( weakness * int(RootMoves[0].score - s)
1840 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1845 best = RootMoves[i].pv[0];
1852 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1853 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1854 // allow to always have a ponder move even when we fail high at root and also a
1855 // long PV to print that is important for position analysis.
1857 void RootMove::extract_pv_from_tt(Position& pos) {
1859 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1864 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1868 pos.do_move(m, *st++);
1870 while ( (tte = TT.probe(pos.get_key())) != NULL
1871 && tte->move() != MOVE_NONE
1872 && pos.is_pseudo_legal(tte->move())
1873 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1875 && (!pos.is_draw<false>() || ply < 2))
1877 pv.push_back(tte->move());
1878 pos.do_move(tte->move(), *st++);
1881 pv.push_back(MOVE_NONE);
1883 do pos.undo_move(pv[--ply]); while (ply);
1887 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1888 // the PV back into the TT. This makes sure the old PV moves are searched
1889 // first, even if the old TT entries have been overwritten.
1891 void RootMove::insert_pv_in_tt(Position& pos) {
1893 StateInfo state[PLY_MAX_PLUS_2], *st = state;
1896 Value v, m = VALUE_NONE;
1899 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1905 // Don't overwrite existing correct entries
1906 if (!tte || tte->move() != pv[ply])
1908 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1909 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
1911 pos.do_move(pv[ply], *st++);
1913 } while (pv[++ply] != MOVE_NONE);
1915 do pos.undo_move(pv[--ply]); while (ply);
1921 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1922 /// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
1923 /// for which the thread is the master.
1925 void Thread::idle_loop(SplitPoint* sp) {
1929 // If we are not searching, wait for a condition to be signaled
1930 // instead of wasting CPU time polling for work.
1933 || (Threads.use_sleeping_threads() && !is_searching))
1935 assert((!sp && threadID) || Threads.use_sleeping_threads());
1943 // Grab the lock to avoid races with Thread::wake_up()
1944 lock_grab(&sleepLock);
1946 // If we are master and all slaves have finished don't go to sleep
1947 if (sp && Threads.split_point_finished(sp))
1949 lock_release(&sleepLock);
1953 // Do sleep after retesting sleep conditions under lock protection, in
1954 // particular we need to avoid a deadlock in case a master thread has,
1955 // in the meanwhile, allocated us and sent the wake_up() call before we
1956 // had the chance to grab the lock.
1957 if (do_sleep || !is_searching)
1958 cond_wait(&sleepCond, &sleepLock);
1960 lock_release(&sleepLock);
1963 // If this thread has been assigned work, launch a search
1966 assert(!do_terminate);
1968 // Copy split point position and search stack and call search()
1969 Stack ss[PLY_MAX_PLUS_2];
1970 SplitPoint* tsp = splitPoint;
1971 Position pos(*tsp->pos, threadID);
1973 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
1976 if (tsp->nodeType == Root)
1977 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1978 else if (tsp->nodeType == PV)
1979 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1980 else if (tsp->nodeType == NonPV)
1981 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1985 assert(is_searching);
1987 is_searching = false;
1989 // Wake up master thread so to allow it to return from the idle loop in
1990 // case we are the last slave of the split point.
1991 if ( Threads.use_sleeping_threads()
1992 && threadID != tsp->master
1993 && !Threads[tsp->master].is_searching)
1994 Threads[tsp->master].wake_up();
1997 // If this thread is the master of a split point and all slaves have
1998 // finished their work at this split point, return from the idle loop.
1999 if (sp && Threads.split_point_finished(sp))
2001 // Because sp->is_slave[] is reset under lock protection,
2002 // be sure sp->lock has been released before to return.
2003 lock_grab(&(sp->lock));
2004 lock_release(&(sp->lock));
2011 /// do_timer_event() is called by the timer thread when the timer triggers. It
2012 /// is used to print debug info and, more important, to detect when we are out of
2013 /// available time and so stop the search.
2015 void do_timer_event() {
2017 static int lastInfoTime;
2018 int e = elapsed_time();
2020 if (get_system_time() - lastInfoTime >= 1000 || !lastInfoTime)
2022 lastInfoTime = get_system_time();
2025 dbg_print_hit_rate();
2031 bool stillAtFirstMove = Signals.firstRootMove
2032 && !Signals.failedLowAtRoot
2033 && e > TimeMgr.available_time();
2035 bool noMoreTime = e > TimeMgr.maximum_time()
2036 || stillAtFirstMove;
2038 if ( (Limits.useTimeManagement() && noMoreTime)
2039 || (Limits.maxTime && e >= Limits.maxTime)
2040 /* missing nodes limit */ ) // FIXME
2041 Signals.stop = true;