2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2012 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
70 score = prevScore = -VALUE_INFINITE;
72 pv.push_back(MOVE_NONE);
75 bool operator<(const RootMove& m) const { return score < m.score; }
76 bool operator==(const Move& m) const { return pv[0] == m; }
78 void extract_pv_from_tt(Position& pos);
79 void insert_pv_in_tt(Position& pos);
89 // Lookup table to check if a Piece is a slider and its access function
90 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
91 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
93 // Maximum depth for razoring
94 const Depth RazorDepth = 4 * ONE_PLY;
96 // Dynamic razoring margin based on depth
97 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
99 // Maximum depth for use of dynamic threat detection when null move fails low
100 const Depth ThreatDepth = 5 * ONE_PLY;
102 // Minimum depth for use of internal iterative deepening
103 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
105 // At Non-PV nodes we do an internal iterative deepening search
106 // when the static evaluation is bigger then beta - IIDMargin.
107 const Value IIDMargin = Value(0x100);
109 // Minimum depth for use of singular extension
110 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
112 // Futility margin for quiescence search
113 const Value FutilityMarginQS = Value(0x80);
115 // Futility lookup tables (initialized at startup) and their access functions
116 Value FutilityMargins[16][64]; // [depth][moveNumber]
117 int FutilityMoveCounts[32]; // [depth]
119 inline Value futility_margin(Depth d, int mn) {
121 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
122 : 2 * VALUE_INFINITE;
125 inline int futility_move_count(Depth d) {
127 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
130 // Reduction lookup tables (initialized at startup) and their access function
131 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
133 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
135 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
138 // Easy move margin. An easy move candidate must be at least this much
139 // better than the second best move.
140 const Value EasyMoveMargin = Value(0x150);
143 /// Namespace variables
145 std::vector<RootMove> RootMoves;
146 size_t MultiPV, UCIMultiPV, PVIdx;
150 bool SkillLevelEnabled, Chess960;
156 template <NodeType NT>
157 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
159 template <NodeType NT>
160 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
162 void id_loop(Position& pos);
163 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bValue);
164 bool connected_moves(const Position& pos, Move m1, Move m2);
165 Value value_to_tt(Value v, int ply);
166 Value value_from_tt(Value v, int ply);
167 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply);
168 bool connected_threat(const Position& pos, Move m, Move threat);
169 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
170 Move do_skill_level();
171 int elapsed_time(bool reset = false);
172 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
173 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
174 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
176 // MovePickerExt class template extends MovePicker and allows to choose at
177 // compile time the proper moves source according to the type of node. In the
178 // default case we simply create and use a standard MovePicker object.
179 template<bool SpNode> struct MovePickerExt : public MovePicker {
181 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
182 : MovePicker(p, ttm, d, h, ss, b) {}
185 // In case of a SpNode we use split point's shared MovePicker object as moves source
186 template<> struct MovePickerExt<true> : public MovePicker {
188 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
189 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
191 Move next_move() { return mp->next_move(); }
195 // is_dangerous() checks whether a move belongs to some classes of known
196 // 'dangerous' moves so that we avoid to prune it.
197 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
199 // Test for a pawn pushed to 7th or a passed pawn move
200 if (type_of(pos.piece_on(move_from(m))) == PAWN)
202 Color c = pos.side_to_move();
203 if ( relative_rank(c, move_to(m)) == RANK_7
204 || pos.pawn_is_passed(c, move_to(m)))
208 // Test for a capture that triggers a pawn endgame
209 if ( captureOrPromotion
210 && type_of(pos.piece_on(move_to(m))) != PAWN
211 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
212 - PieceValueMidgame[pos.piece_on(move_to(m))] == VALUE_ZERO)
222 /// Search::init() is called during startup to initialize various lookup tables
224 void Search::init() {
226 int d; // depth (ONE_PLY == 2)
227 int hd; // half depth (ONE_PLY == 1)
230 // Init reductions array
231 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
233 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
234 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
235 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
236 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
239 // Init futility margins array
240 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
241 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
243 // Init futility move count array
244 for (d = 0; d < 32; d++)
245 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
249 /// Search::perft() is our utility to verify move generation. All the leaf nodes
250 /// up to the given depth are generated and counted and the sum returned.
252 int64_t Search::perft(Position& pos, Depth depth) {
257 MoveList<MV_LEGAL> ml(pos);
259 // At the last ply just return the number of moves (leaf nodes)
260 if (depth == ONE_PLY)
264 for ( ; !ml.end(); ++ml)
266 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
267 cnt += perft(pos, depth - ONE_PLY);
268 pos.undo_move(ml.move());
274 /// Search::think() is the external interface to Stockfish's search, and is
275 /// called by the main thread when the program receives the UCI 'go' command. It
276 /// searches from RootPosition and at the end prints the "bestmove" to output.
278 void Search::think() {
280 static Book book; // Defined static to initialize the PRNG only once
282 Position& pos = RootPosition;
283 Chess960 = pos.is_chess960();
285 TimeMgr.init(Limits, pos.startpos_ply_counter());
290 // Populate RootMoves with all the legal moves (default) or, if a SearchMoves
291 // is given, with the subset of legal moves to search.
292 for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
293 if ( SearchMoves.empty()
294 || count(SearchMoves.begin(), SearchMoves.end(), ml.move()))
295 RootMoves.push_back(RootMove(ml.move()));
297 if (Options["OwnBook"])
299 Move bookMove = book.probe(pos, Options["Book File"], Options["Best Book Move"]);
301 if ( bookMove != MOVE_NONE
302 && count(RootMoves.begin(), RootMoves.end(), bookMove))
304 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), bookMove));
309 // Read UCI options: GUI could change UCI parameters during the game
310 read_evaluation_uci_options(pos.side_to_move());
311 Threads.read_uci_options();
313 TT.set_size(Options["Hash"]);
314 if (Options["Clear Hash"])
316 Options["Clear Hash"] = false;
320 UCIMultiPV = Options["MultiPV"];
321 SkillLevel = Options["Skill Level"];
323 // Do we have to play with skill handicap? In this case enable MultiPV that
324 // we will use behind the scenes to retrieve a set of possible moves.
325 SkillLevelEnabled = (SkillLevel < 20);
326 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
328 if (Options["Use Search Log"])
330 Log log(Options["Search Log Filename"]);
331 log << "\nSearching: " << pos.to_fen()
332 << "\ninfinite: " << Limits.infinite
333 << " ponder: " << Limits.ponder
334 << " time: " << Limits.time
335 << " increment: " << Limits.increment
336 << " moves to go: " << Limits.movesToGo
340 for (int i = 0; i < Threads.size(); i++)
342 Threads[i].maxPly = 0;
343 Threads[i].wake_up();
346 // Set best timer interval to avoid lagging under time pressure. Timer is
347 // used to check for remaining available thinking time.
348 if (TimeMgr.available_time())
349 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 8, 20)));
351 Threads.set_timer(100);
353 // We're ready to start searching. Call the iterative deepening loop function
356 // Stop timer and send all the slaves to sleep, if not already sleeping
357 Threads.set_timer(0);
360 if (Options["Use Search Log"])
362 int e = elapsed_time();
364 Log log(Options["Search Log Filename"]);
365 log << "Nodes: " << pos.nodes_searched()
366 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
367 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
370 pos.do_move(RootMoves[0].pv[0], st);
371 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
372 pos.undo_move(RootMoves[0].pv[0]);
377 // When we reach max depth we arrive here even without a StopRequest, but if
378 // we are pondering or in infinite search, we shouldn't print the best move
379 // before we are told to do so.
380 if (!Signals.stop && (Limits.ponder || Limits.infinite))
381 Threads.wait_for_stop_or_ponderhit();
383 // Best move could be MOVE_NONE when searching on a stalemate position
384 cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], Chess960)
385 << " ponder " << move_to_uci(RootMoves[0].pv[1], Chess960) << endl;
391 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
392 // with increasing depth until the allocated thinking time has been consumed,
393 // user stops the search, or the maximum search depth is reached.
395 void id_loop(Position& pos) {
397 Stack ss[MAX_PLY_PLUS_2];
398 int depth, prevBestMoveChanges;
399 Value bestValue, alpha, beta, delta;
400 bool bestMoveNeverChanged = true;
401 Move skillBest = MOVE_NONE;
403 memset(ss, 0, 4 * sizeof(Stack));
404 depth = BestMoveChanges = 0;
405 bestValue = delta = -VALUE_INFINITE;
406 ss->currentMove = MOVE_NULL; // Hack to skip update gains
408 // Handle the special case of a mated/stalemate position
409 if (RootMoves.empty())
411 cout << "info depth 0 score "
412 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
414 RootMoves.push_back(MOVE_NONE);
418 // Iterative deepening loop until requested to stop or target depth reached
419 while (!Signals.stop && ++depth <= MAX_PLY && (!Limits.maxDepth || depth <= Limits.maxDepth))
421 // Save last iteration's scores before first PV line is searched and all
422 // the move scores but the (new) PV are set to -VALUE_INFINITE.
423 for (size_t i = 0; i < RootMoves.size(); i++)
424 RootMoves[i].prevScore = RootMoves[i].score;
426 prevBestMoveChanges = BestMoveChanges;
429 // MultiPV loop. We perform a full root search for each PV line
430 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
432 // Set aspiration window default width
433 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
436 alpha = RootMoves[PVIdx].prevScore - delta;
437 beta = RootMoves[PVIdx].prevScore + delta;
441 alpha = -VALUE_INFINITE;
442 beta = VALUE_INFINITE;
445 // Start with a small aspiration window and, in case of fail high/low,
446 // research with bigger window until not failing high/low anymore.
448 // Search starts from ss+1 to allow referencing (ss-1). This is
449 // needed by update gains and ss copy when splitting at Root.
450 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
452 // Bring to front the best move. It is critical that sorting is
453 // done with a stable algorithm because all the values but the first
454 // and eventually the new best one are set to -VALUE_INFINITE and
455 // we want to keep the same order for all the moves but the new
456 // PV that goes to the front. Note that in case of MultiPV search
457 // the already searched PV lines are preserved.
458 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
460 // In case we have found an exact score and we are going to leave
461 // the fail high/low loop then reorder the PV moves, otherwise
462 // leave the last PV move in its position so to be searched again.
463 // Of course this is needed only in MultiPV search.
464 if (PVIdx && bestValue > alpha && bestValue < beta)
465 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
467 // Write PV back to transposition table in case the relevant
468 // entries have been overwritten during the search.
469 for (size_t i = 0; i <= PVIdx; i++)
470 RootMoves[i].insert_pv_in_tt(pos);
472 // If search has been stopped exit the aspiration window loop.
473 // Sorting and writing PV back to TT is safe becuase RootMoves
474 // is still valid, although refers to previous iteration.
478 // Send full PV info to GUI if we are going to leave the loop or
479 // if we have a fail high/low and we are deep in the search.
480 if ((bestValue > alpha && bestValue < beta) || elapsed_time() > 2000)
481 pv_info_to_uci(pos, depth, alpha, beta);
483 // In case of failing high/low increase aspiration window and
484 // research, otherwise exit the fail high/low loop.
485 if (bestValue >= beta)
490 else if (bestValue <= alpha)
492 Signals.failedLowAtRoot = true;
493 Signals.stopOnPonderhit = false;
501 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
503 } while (abs(bestValue) < VALUE_KNOWN_WIN);
506 // Skills: Do we need to pick now the best move ?
507 if (SkillLevelEnabled && depth == 1 + SkillLevel)
508 skillBest = do_skill_level();
510 if (Options["Use Search Log"])
511 pv_info_to_log(pos, depth, bestValue, elapsed_time(), &RootMoves[0].pv[0]);
513 // Filter out startup noise when monitoring best move stability
514 if (depth > 2 && BestMoveChanges)
515 bestMoveNeverChanged = false;
517 // Do we have time for the next iteration? Can we stop searching now?
518 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.useTimeManagement())
520 bool stop = false; // Local variable, not the volatile Signals.stop
522 // Take in account some extra time if the best move has changed
523 if (depth > 4 && depth < 50)
524 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
526 // Stop search if most of available time is already consumed. We
527 // probably don't have enough time to search the first move at the
528 // next iteration anyway.
529 if (elapsed_time() > (TimeMgr.available_time() * 62) / 100)
532 // Stop search early if one move seems to be much better than others
535 && ( bestMoveNeverChanged
536 || elapsed_time() > (TimeMgr.available_time() * 40) / 100))
538 Value rBeta = bestValue - EasyMoveMargin;
539 (ss+1)->excludedMove = RootMoves[0].pv[0];
540 (ss+1)->skipNullMove = true;
541 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth * ONE_PLY) / 2);
542 (ss+1)->skipNullMove = false;
543 (ss+1)->excludedMove = MOVE_NONE;
551 // If we are allowed to ponder do not stop the search now but
552 // keep pondering until GUI sends "ponderhit" or "stop".
554 Signals.stopOnPonderhit = true;
561 // When using skills swap best PV line with the sub-optimal one
562 if (SkillLevelEnabled)
564 if (skillBest == MOVE_NONE) // Still unassigned ?
565 skillBest = do_skill_level();
567 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), skillBest));
572 // search<>() is the main search function for both PV and non-PV nodes and for
573 // normal and SplitPoint nodes. When called just after a split point the search
574 // is simpler because we have already probed the hash table, done a null move
575 // search, and searched the first move before splitting, we don't have to repeat
576 // all this work again. We also don't need to store anything to the hash table
577 // here: This is taken care of after we return from the split point.
579 template <NodeType NT>
580 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
582 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
583 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
584 const bool RootNode = (NT == Root || NT == SplitPointRoot);
586 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
587 assert(PvNode == (alpha != beta - 1));
588 assert(depth > DEPTH_ZERO);
589 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
591 Move movesSearched[MAX_MOVES];
595 Move ttMove, move, excludedMove, threatMove;
598 Value bestValue, value, oldAlpha;
599 Value refinedValue, nullValue, futilityBase, futilityValue;
600 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
601 bool captureOrPromotion, dangerous, doFullDepthSearch;
602 int moveCount = 0, playedMoveCount = 0;
603 Thread& thread = Threads[pos.thread()];
604 SplitPoint* sp = NULL;
606 refinedValue = bestValue = value = -VALUE_INFINITE;
608 inCheck = pos.in_check();
609 ss->ply = (ss-1)->ply + 1;
611 // Used to send selDepth info to GUI
612 if (PvNode && thread.maxPly < ss->ply)
613 thread.maxPly = ss->ply;
615 // Step 1. Initialize node
618 ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
619 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
620 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
626 ttMove = excludedMove = MOVE_NONE;
627 threatMove = sp->threatMove;
628 goto split_point_start;
631 // Step 2. Check for aborted search and immediate draw
633 || pos.is_draw<false>()
634 || ss->ply > MAX_PLY) && !RootNode)
637 // Step 3. Mate distance pruning. Even if we mate at the next move our score
638 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
639 // a shorter mate was found upward in the tree then there is no need to search
640 // further, we will never beat current alpha. Same logic but with reversed signs
641 // applies also in the opposite condition of being mated instead of giving mate,
642 // in this case return a fail-high score.
645 alpha = std::max(mated_in(ss->ply), alpha);
646 beta = std::min(mate_in(ss->ply+1), beta);
651 // Step 4. Transposition table lookup
652 // We don't want the score of a partial search to overwrite a previous full search
653 // TT value, so we use a different position key in case of an excluded move.
654 excludedMove = ss->excludedMove;
655 posKey = excludedMove ? pos.exclusion_key() : pos.key();
656 tte = TT.probe(posKey);
657 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
659 // At PV nodes we check for exact scores, while at non-PV nodes we check for
660 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
661 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
662 // we should also update RootMoveList to avoid bogus output.
663 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == VALUE_TYPE_EXACT
664 : can_return_tt(tte, depth, beta, ss->ply)))
667 ss->bestMove = move = ttMove; // Can be MOVE_NONE
668 value = value_from_tt(tte->value(), ss->ply);
672 && !pos.is_capture_or_promotion(move)
673 && move != ss->killers[0])
675 ss->killers[1] = ss->killers[0];
676 ss->killers[0] = move;
681 // Step 5. Evaluate the position statically and update parent's gain statistics
683 ss->eval = ss->evalMargin = VALUE_NONE;
686 assert(tte->static_value() != VALUE_NONE);
688 ss->eval = tte->static_value();
689 ss->evalMargin = tte->static_value_margin();
690 refinedValue = refine_eval(tte, ss->eval, ss->ply);
694 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
695 TT.store(posKey, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
698 // Update gain for the parent non-capture move given the static position
699 // evaluation before and after the move.
700 if ( (move = (ss-1)->currentMove) != MOVE_NULL
701 && (ss-1)->eval != VALUE_NONE
702 && ss->eval != VALUE_NONE
703 && pos.captured_piece_type() == NO_PIECE_TYPE
704 && !is_special(move))
706 Square to = move_to(move);
707 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
710 // Step 6. Razoring (is omitted in PV nodes)
712 && depth < RazorDepth
714 && refinedValue + razor_margin(depth) < beta
715 && ttMove == MOVE_NONE
716 && abs(beta) < VALUE_MATE_IN_MAX_PLY
717 && !pos.has_pawn_on_7th(pos.side_to_move()))
719 Value rbeta = beta - razor_margin(depth);
720 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
722 // Logically we should return (v + razor_margin(depth)), but
723 // surprisingly this did slightly weaker in tests.
727 // Step 7. Static null move pruning (is omitted in PV nodes)
728 // We're betting that the opponent doesn't have a move that will reduce
729 // the score by more than futility_margin(depth) if we do a null move.
732 && depth < RazorDepth
734 && refinedValue - futility_margin(depth, 0) >= beta
735 && abs(beta) < VALUE_MATE_IN_MAX_PLY
736 && pos.non_pawn_material(pos.side_to_move()))
737 return refinedValue - futility_margin(depth, 0);
739 // Step 8. Null move search with verification search (is omitted in PV nodes)
744 && refinedValue >= beta
745 && abs(beta) < VALUE_MATE_IN_MAX_PLY
746 && pos.non_pawn_material(pos.side_to_move()))
748 ss->currentMove = MOVE_NULL;
750 // Null move dynamic reduction based on depth
751 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
753 // Null move dynamic reduction based on value
754 if (refinedValue - PawnValueMidgame > beta)
757 pos.do_null_move<true>(st);
758 (ss+1)->skipNullMove = true;
759 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
760 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
761 (ss+1)->skipNullMove = false;
762 pos.do_null_move<false>(st);
764 if (nullValue >= beta)
766 // Do not return unproven mate scores
767 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
770 if (depth < 6 * ONE_PLY)
773 // Do verification search at high depths
774 ss->skipNullMove = true;
775 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
776 ss->skipNullMove = false;
783 // The null move failed low, which means that we may be faced with
784 // some kind of threat. If the previous move was reduced, check if
785 // the move that refuted the null move was somehow connected to the
786 // move which was reduced. If a connection is found, return a fail
787 // low score (which will cause the reduced move to fail high in the
788 // parent node, which will trigger a re-search with full depth).
789 threatMove = (ss+1)->bestMove;
791 if ( depth < ThreatDepth
793 && threatMove != MOVE_NONE
794 && connected_moves(pos, (ss-1)->currentMove, threatMove))
799 // Step 9. ProbCut (is omitted in PV nodes)
800 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
801 // and a reduced search returns a value much above beta, we can (almost) safely
802 // prune the previous move.
804 && depth >= RazorDepth + ONE_PLY
807 && excludedMove == MOVE_NONE
808 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
810 Value rbeta = beta + 200;
811 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
813 assert(rdepth >= ONE_PLY);
815 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
818 while ((move = mp.next_move()) != MOVE_NONE)
819 if (pos.pl_move_is_legal(move, ci.pinned))
821 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
822 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
829 // Step 10. Internal iterative deepening
830 if ( depth >= IIDDepth[PvNode]
831 && ttMove == MOVE_NONE
832 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
834 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
836 ss->skipNullMove = true;
837 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
838 ss->skipNullMove = false;
840 tte = TT.probe(posKey);
841 ttMove = tte ? tte->move() : MOVE_NONE;
844 split_point_start: // At split points actual search starts from here
846 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
848 ss->bestMove = MOVE_NONE;
849 futilityBase = ss->eval + ss->evalMargin;
850 singularExtensionNode = !RootNode
852 && depth >= SingularExtensionDepth[PvNode]
853 && ttMove != MOVE_NONE
854 && !excludedMove // Recursive singular search is not allowed
855 && (tte->type() & VALUE_TYPE_LOWER)
856 && tte->depth() >= depth - 3 * ONE_PLY;
859 lock_grab(&(sp->lock));
860 bestValue = sp->bestValue;
861 moveCount = sp->moveCount;
863 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
866 // Step 11. Loop through moves
867 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
868 while ( bestValue < beta
869 && (move = mp.next_move()) != MOVE_NONE
870 && !thread.cutoff_occurred())
874 if (move == excludedMove)
877 // At root obey the "searchmoves" option and skip moves not listed in Root
878 // Move List, as a consequence any illegal move is also skipped. In MultiPV
879 // mode we also skip PV moves which have been already searched.
880 if (RootNode && !count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
883 // At PV and SpNode nodes we want all moves to be legal since the beginning
884 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
889 moveCount = ++sp->moveCount;
890 lock_release(&(sp->lock));
897 Signals.firstRootMove = (moveCount == 1);
899 if (pos.thread() == 0 && elapsed_time() > 2000)
900 cout << "info depth " << depth / ONE_PLY
901 << " currmove " << move_to_uci(move, Chess960)
902 << " currmovenumber " << moveCount + PVIdx << endl;
905 isPvMove = (PvNode && moveCount <= 1);
906 captureOrPromotion = pos.is_capture_or_promotion(move);
907 givesCheck = pos.move_gives_check(move, ci);
908 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
911 // Step 12. Extend checks and, in PV nodes, also dangerous moves
912 if (PvNode && dangerous)
915 else if (givesCheck && pos.see_sign(move) >= 0)
916 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
918 // Singular extension search. If all moves but one fail low on a search of
919 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
920 // is singular and should be extended. To verify this we do a reduced search
921 // on all the other moves but the ttMove, if result is lower than ttValue minus
922 // a margin then we extend ttMove.
923 if ( singularExtensionNode
926 && pos.pl_move_is_legal(move, ci.pinned))
928 Value ttValue = value_from_tt(tte->value(), ss->ply);
930 if (abs(ttValue) < VALUE_KNOWN_WIN)
932 Value rBeta = ttValue - int(depth);
933 ss->excludedMove = move;
934 ss->skipNullMove = true;
935 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
936 ss->skipNullMove = false;
937 ss->excludedMove = MOVE_NONE;
938 ss->bestMove = MOVE_NONE;
944 // Update current move (this must be done after singular extension search)
945 newDepth = depth - ONE_PLY + ext;
947 // Step 13. Futility pruning (is omitted in PV nodes)
949 && !captureOrPromotion
954 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
956 // Move count based pruning
957 if ( moveCount >= futility_move_count(depth)
958 && (!threatMove || !connected_threat(pos, move, threatMove)))
961 lock_grab(&(sp->lock));
966 // Value based pruning
967 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
968 // but fixing this made program slightly weaker.
969 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
970 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
971 + H.gain(pos.piece_on(move_from(move)), move_to(move));
973 if (futilityValue < beta)
976 lock_grab(&(sp->lock));
981 // Prune moves with negative SEE at low depths
982 if ( predictedDepth < 2 * ONE_PLY
983 && pos.see_sign(move) < 0)
986 lock_grab(&(sp->lock));
992 // Check for legality only before to do the move
993 if (!pos.pl_move_is_legal(move, ci.pinned))
999 ss->currentMove = move;
1000 if (!SpNode && !captureOrPromotion)
1001 movesSearched[playedMoveCount++] = move;
1003 // Step 14. Make the move
1004 pos.do_move(move, st, ci, givesCheck);
1006 // Step 15. Reduced depth search (LMR). If the move fails high will be
1007 // re-searched at full depth.
1008 if ( depth > 3 * ONE_PLY
1010 && !captureOrPromotion
1013 && ss->killers[0] != move
1014 && ss->killers[1] != move)
1016 ss->reduction = reduction<PvNode>(depth, moveCount);
1017 Depth d = newDepth - ss->reduction;
1018 alpha = SpNode ? sp->alpha : alpha;
1020 value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1021 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
1023 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1024 ss->reduction = DEPTH_ZERO;
1027 doFullDepthSearch = !isPvMove;
1029 // Step 16. Full depth search, when LMR is skipped or fails high
1030 if (doFullDepthSearch)
1032 alpha = SpNode ? sp->alpha : alpha;
1033 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1034 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
1037 // Only for PV nodes do a full PV search on the first move or after a fail
1038 // high, in the latter case search only if value < beta, otherwise let the
1039 // parent node to fail low with value <= alpha and to try another move.
1040 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
1041 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1042 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
1044 // Step 17. Undo move
1045 pos.undo_move(move);
1047 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1049 // Step 18. Check for new best move
1052 lock_grab(&(sp->lock));
1053 bestValue = sp->bestValue;
1057 // Finished searching the move. If StopRequest is true, the search
1058 // was aborted because the user interrupted the search or because we
1059 // ran out of time. In this case, the return value of the search cannot
1060 // be trusted, and we don't update the best move and/or PV.
1061 if (RootNode && !Signals.stop)
1063 RootMove& rm = *find(RootMoves.begin(), RootMoves.end(), move);
1065 // PV move or new best move ?
1066 if (isPvMove || value > alpha)
1069 rm.extract_pv_from_tt(pos);
1071 // We record how often the best move has been changed in each
1072 // iteration. This information is used for time management: When
1073 // the best move changes frequently, we allocate some more time.
1074 if (!isPvMove && MultiPV == 1)
1078 // All other moves but the PV are set to the lowest value, this
1079 // is not a problem when sorting becuase sort is stable and move
1080 // position in the list is preserved, just the PV is pushed up.
1081 rm.score = -VALUE_INFINITE;
1085 if (value > bestValue)
1088 ss->bestMove = move;
1092 && value < beta) // We want always alpha < beta
1095 if (SpNode && !thread.cutoff_occurred())
1097 sp->bestValue = value;
1098 sp->ss->bestMove = move;
1100 sp->is_betaCutoff = (value >= beta);
1104 // Step 19. Check for split
1106 && depth >= Threads.min_split_depth()
1108 && Threads.available_slave_exists(pos.thread())
1110 && !thread.cutoff_occurred())
1111 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
1112 threatMove, moveCount, &mp, NT);
1115 // Step 20. Check for mate and stalemate
1116 // All legal moves have been searched and if there are no legal moves, it
1117 // must be mate or stalemate. Note that we can have a false positive in
1118 // case of StopRequest or thread.cutoff_occurred() are set, but this is
1119 // harmless because return value is discarded anyhow in the parent nodes.
1120 // If we are in a singular extension search then return a fail low score.
1122 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1124 // If we have pruned all the moves without searching return a fail-low score
1125 if (bestValue == -VALUE_INFINITE)
1127 assert(!playedMoveCount);
1132 // Step 21. Update tables
1133 // Update transposition table entry, killers and history
1134 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1136 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1137 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1138 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1140 TT.store(posKey, value_to_tt(bestValue, ss->ply), vt, depth, move, ss->eval, ss->evalMargin);
1142 // Update killers and history for non capture cut-off moves
1143 if ( bestValue >= beta
1144 && !pos.is_capture_or_promotion(move)
1147 if (move != ss->killers[0])
1149 ss->killers[1] = ss->killers[0];
1150 ss->killers[0] = move;
1153 // Increase history value of the cut-off move
1154 Value bonus = Value(int(depth) * int(depth));
1155 H.add(pos.piece_on(move_from(move)), move_to(move), bonus);
1157 // Decrease history of all the other played non-capture moves
1158 for (int i = 0; i < playedMoveCount - 1; i++)
1160 Move m = movesSearched[i];
1161 H.add(pos.piece_on(move_from(m)), move_to(m), -bonus);
1168 // Here we have the lock still grabbed
1169 sp->is_slave[pos.thread()] = false;
1170 sp->nodes += pos.nodes_searched();
1171 lock_release(&(sp->lock));
1174 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1180 // qsearch() is the quiescence search function, which is called by the main
1181 // search function when the remaining depth is zero (or, to be more precise,
1182 // less than ONE_PLY).
1184 template <NodeType NT>
1185 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1187 const bool PvNode = (NT == PV);
1189 assert(NT == PV || NT == NonPV);
1190 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1191 assert(PvNode == (alpha != beta - 1));
1192 assert(depth <= DEPTH_ZERO);
1193 assert(pos.thread() >= 0 && pos.thread() < Threads.size());
1197 Value bestValue, value, evalMargin, futilityValue, futilityBase;
1198 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1202 Value oldAlpha = alpha;
1204 ss->bestMove = ss->currentMove = MOVE_NONE;
1205 ss->ply = (ss-1)->ply + 1;
1207 // Check for an instant draw or maximum ply reached
1208 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1211 // Decide whether or not to include checks, this fixes also the type of
1212 // TT entry depth that we are going to use. Note that in qsearch we use
1213 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1214 inCheck = pos.in_check();
1215 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1217 // Transposition table lookup. At PV nodes, we don't use the TT for
1218 // pruning, but only for move ordering.
1219 tte = TT.probe(pos.key());
1220 ttMove = (tte ? tte->move() : MOVE_NONE);
1222 if (!PvNode && tte && can_return_tt(tte, ttDepth, beta, ss->ply))
1224 ss->bestMove = ttMove; // Can be MOVE_NONE
1225 return value_from_tt(tte->value(), ss->ply);
1228 // Evaluate the position statically
1231 bestValue = futilityBase = -VALUE_INFINITE;
1232 ss->eval = evalMargin = VALUE_NONE;
1233 enoughMaterial = false;
1239 assert(tte->static_value() != VALUE_NONE);
1241 evalMargin = tte->static_value_margin();
1242 ss->eval = bestValue = tte->static_value();
1245 ss->eval = bestValue = evaluate(pos, evalMargin);
1247 // Stand pat. Return immediately if static value is at least beta
1248 if (bestValue >= beta)
1251 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), VALUE_TYPE_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1256 if (PvNode && bestValue > alpha)
1259 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1260 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1263 // Initialize a MovePicker object for the current position, and prepare
1264 // to search the moves. Because the depth is <= 0 here, only captures,
1265 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1267 MovePicker mp(pos, ttMove, depth, H, move_to((ss-1)->currentMove));
1270 // Loop through the moves until no moves remain or a beta cutoff occurs
1271 while ( bestValue < beta
1272 && (move = mp.next_move()) != MOVE_NONE)
1274 assert(is_ok(move));
1276 givesCheck = pos.move_gives_check(move, ci);
1284 && !is_promotion(move)
1285 && !pos.is_passed_pawn_push(move))
1287 futilityValue = futilityBase
1288 + PieceValueEndgame[pos.piece_on(move_to(move))]
1289 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1291 if (futilityValue < beta)
1293 if (futilityValue > bestValue)
1294 bestValue = futilityValue;
1299 // Prune moves with negative or equal SEE
1300 if ( futilityBase < beta
1301 && depth < DEPTH_ZERO
1302 && pos.see(move) <= 0)
1306 // Detect non-capture evasions that are candidate to be pruned
1307 evasionPrunable = !PvNode
1309 && bestValue > VALUE_MATED_IN_MAX_PLY
1310 && !pos.is_capture(move)
1311 && !pos.can_castle(pos.side_to_move());
1313 // Don't search moves with negative SEE values
1315 && (!inCheck || evasionPrunable)
1317 && !is_promotion(move)
1318 && pos.see_sign(move) < 0)
1321 // Don't search useless checks
1326 && !pos.is_capture_or_promotion(move)
1327 && ss->eval + PawnValueMidgame / 4 < beta
1328 && !check_is_dangerous(pos, move, futilityBase, beta, &bestValue))
1330 if (ss->eval + PawnValueMidgame / 4 > bestValue)
1331 bestValue = ss->eval + PawnValueMidgame / 4;
1336 // Check for legality only before to do the move
1337 if (!pos.pl_move_is_legal(move, ci.pinned))
1340 ss->currentMove = move;
1342 // Make and search the move
1343 pos.do_move(move, st, ci, givesCheck);
1344 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1345 pos.undo_move(move);
1347 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1350 if (value > bestValue)
1353 ss->bestMove = move;
1357 && value < beta) // We want always alpha < beta
1362 // All legal moves have been searched. A special case: If we're in check
1363 // and no legal moves were found, it is checkmate.
1364 if (inCheck && bestValue == -VALUE_INFINITE)
1365 return mated_in(ss->ply); // Plies to mate from the root
1367 // Update transposition table
1368 move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;
1369 vt = bestValue <= oldAlpha ? VALUE_TYPE_UPPER
1370 : bestValue >= beta ? VALUE_TYPE_LOWER : VALUE_TYPE_EXACT;
1372 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), vt, ttDepth, move, ss->eval, evalMargin);
1374 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1380 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1381 // bestValue is updated only when returning false because in that case move
1384 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta, Value *bestValue)
1386 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1387 Square from, to, ksq, victimSq;
1390 Value futilityValue, bv = *bestValue;
1392 from = move_from(move);
1394 them = flip(pos.side_to_move());
1395 ksq = pos.king_square(them);
1396 kingAtt = pos.attacks_from<KING>(ksq);
1397 pc = pos.piece_on(from);
1399 occ = pos.occupied_squares() & ~(1ULL << from) & ~(1ULL << ksq);
1400 oldAtt = pos.attacks_from(pc, from, occ);
1401 newAtt = pos.attacks_from(pc, to, occ);
1403 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1404 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1406 if (!(b && (b & (b - 1))))
1409 // Rule 2. Queen contact check is very dangerous
1410 if ( type_of(pc) == QUEEN
1411 && bit_is_set(kingAtt, to))
1414 // Rule 3. Creating new double threats with checks
1415 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1419 victimSq = pop_1st_bit(&b);
1420 futilityValue = futilityBase + PieceValueEndgame[pos.piece_on(victimSq)];
1422 // Note that here we generate illegal "double move"!
1423 if ( futilityValue >= beta
1424 && pos.see_sign(make_move(from, victimSq)) >= 0)
1427 if (futilityValue > bv)
1431 // Update bestValue only if check is not dangerous (because we will prune the move)
1437 // connected_moves() tests whether two moves are 'connected' in the sense
1438 // that the first move somehow made the second move possible (for instance
1439 // if the moving piece is the same in both moves). The first move is assumed
1440 // to be the move that was made to reach the current position, while the
1441 // second move is assumed to be a move from the current position.
1443 bool connected_moves(const Position& pos, Move m1, Move m2) {
1445 Square f1, t1, f2, t2;
1452 // Case 1: The moving piece is the same in both moves
1458 // Case 2: The destination square for m2 was vacated by m1
1464 // Case 3: Moving through the vacated square
1465 p2 = pos.piece_on(f2);
1466 if ( piece_is_slider(p2)
1467 && bit_is_set(squares_between(f2, t2), f1))
1470 // Case 4: The destination square for m2 is defended by the moving piece in m1
1471 p1 = pos.piece_on(t1);
1472 if (bit_is_set(pos.attacks_from(p1, t1), t2))
1475 // Case 5: Discovered check, checking piece is the piece moved in m1
1476 ksq = pos.king_square(pos.side_to_move());
1477 if ( piece_is_slider(p1)
1478 && bit_is_set(squares_between(t1, ksq), f2))
1480 Bitboard occ = pos.occupied_squares();
1481 clear_bit(&occ, f2);
1482 if (bit_is_set(pos.attacks_from(p1, t1, occ), ksq))
1489 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1490 // "plies to mate from the current position". Non-mate scores are unchanged.
1491 // The function is called before storing a value to the transposition table.
1493 Value value_to_tt(Value v, int ply) {
1495 if (v >= VALUE_MATE_IN_MAX_PLY)
1498 if (v <= VALUE_MATED_IN_MAX_PLY)
1505 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1506 // from the transposition table (where refers to the plies to mate/be mated
1507 // from current position) to "plies to mate/be mated from the root".
1509 Value value_from_tt(Value v, int ply) {
1511 if (v >= VALUE_MATE_IN_MAX_PLY)
1514 if (v <= VALUE_MATED_IN_MAX_PLY)
1521 // connected_threat() tests whether it is safe to forward prune a move or if
1522 // is somehow connected to the threat move returned by null search.
1524 bool connected_threat(const Position& pos, Move m, Move threat) {
1527 assert(is_ok(threat));
1528 assert(!pos.is_capture_or_promotion(m));
1529 assert(!pos.is_passed_pawn_push(m));
1531 Square mfrom, mto, tfrom, tto;
1533 mfrom = move_from(m);
1535 tfrom = move_from(threat);
1536 tto = move_to(threat);
1538 // Case 1: Don't prune moves which move the threatened piece
1542 // Case 2: If the threatened piece has value less than or equal to the
1543 // value of the threatening piece, don't prune moves which defend it.
1544 if ( pos.is_capture(threat)
1545 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1546 || type_of(pos.piece_on(tfrom)) == KING)
1547 && pos.move_attacks_square(m, tto))
1550 // Case 3: If the moving piece in the threatened move is a slider, don't
1551 // prune safe moves which block its ray.
1552 if ( piece_is_slider(pos.piece_on(tfrom))
1553 && bit_is_set(squares_between(tfrom, tto), mto)
1554 && pos.see_sign(m) >= 0)
1561 // can_return_tt() returns true if a transposition table score can be used to
1562 // cut-off at a given point in search.
1564 bool can_return_tt(const TTEntry* tte, Depth depth, Value beta, int ply) {
1566 Value v = value_from_tt(tte->value(), ply);
1568 return ( tte->depth() >= depth
1569 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1570 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1572 && ( ((tte->type() & VALUE_TYPE_LOWER) && v >= beta)
1573 || ((tte->type() & VALUE_TYPE_UPPER) && v < beta));
1577 // refine_eval() returns the transposition table score if possible, otherwise
1578 // falls back on static position evaluation.
1580 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
1584 Value v = value_from_tt(tte->value(), ply);
1586 if ( ((tte->type() & VALUE_TYPE_LOWER) && v >= defaultEval)
1587 || ((tte->type() & VALUE_TYPE_UPPER) && v < defaultEval))
1594 // current_search_time() returns the number of milliseconds which have passed
1595 // since the beginning of the current search.
1597 int elapsed_time(bool reset) {
1599 static int searchStartTime;
1602 searchStartTime = system_time();
1604 return system_time() - searchStartTime;
1608 // score_to_uci() converts a value to a string suitable for use with the UCI
1609 // protocol specifications:
1611 // cp <x> The score from the engine's point of view in centipawns.
1612 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1613 // use negative values for y.
1615 string score_to_uci(Value v, Value alpha, Value beta) {
1617 std::stringstream s;
1619 if (abs(v) < VALUE_MATE_IN_MAX_PLY)
1620 s << "cp " << v * 100 / int(PawnValueMidgame);
1622 s << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1624 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1630 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1631 // the PV lines also if are still to be searched and so refer to the previous
1634 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1636 int t = elapsed_time();
1639 for (int i = 0; i < Threads.size(); i++)
1640 if (Threads[i].maxPly > selDepth)
1641 selDepth = Threads[i].maxPly;
1643 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1645 bool updated = (i <= PVIdx);
1647 if (depth == 1 && !updated)
1650 int d = (updated ? depth : depth - 1);
1651 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1652 std::stringstream s;
1654 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1655 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1657 cout << "info depth " << d
1658 << " seldepth " << selDepth
1659 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1660 << " nodes " << pos.nodes_searched()
1661 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1663 << " multipv " << i + 1
1664 << " pv" << s.str() << endl;
1669 // pv_info_to_log() writes human-readable search information to the log file
1670 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1671 // uses the two below helpers to pretty format time and score respectively.
1673 string time_to_string(int millisecs) {
1675 const int MSecMinute = 1000 * 60;
1676 const int MSecHour = 1000 * 60 * 60;
1678 int hours = millisecs / MSecHour;
1679 int minutes = (millisecs % MSecHour) / MSecMinute;
1680 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1682 std::stringstream s;
1687 s << std::setfill('0') << std::setw(2) << minutes << ':'
1688 << std::setw(2) << seconds;
1692 string score_to_string(Value v) {
1694 std::stringstream s;
1696 if (v >= VALUE_MATE_IN_MAX_PLY)
1697 s << "#" << (VALUE_MATE - v + 1) / 2;
1698 else if (v <= VALUE_MATED_IN_MAX_PLY)
1699 s << "-#" << (VALUE_MATE + v) / 2;
1701 s << std::setprecision(2) << std::fixed << std::showpos
1702 << float(v) / PawnValueMidgame;
1707 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1709 const int64_t K = 1000;
1710 const int64_t M = 1000000;
1712 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1714 string san, padding;
1716 std::stringstream s;
1718 s << std::setw(2) << depth
1719 << std::setw(8) << score_to_string(value)
1720 << std::setw(8) << time_to_string(time);
1722 if (pos.nodes_searched() < M)
1723 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1725 else if (pos.nodes_searched() < K * M)
1726 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1729 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1731 padding = string(s.str().length(), ' ');
1732 length = padding.length();
1734 while (*m != MOVE_NONE)
1736 san = move_to_san(pos, *m);
1738 if (length + san.length() > 80)
1740 s << "\n" + padding;
1741 length = padding.length();
1745 length += san.length() + 1;
1747 pos.do_move(*m++, *st++);
1751 pos.undo_move(*--m);
1753 Log l(Options["Search Log Filename"]);
1754 l << s.str() << endl;
1758 // When playing with strength handicap choose best move among the MultiPV set
1759 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1761 Move do_skill_level() {
1763 assert(MultiPV > 1);
1767 // PRNG sequence should be not deterministic
1768 for (int i = abs(system_time() % 50); i > 0; i--)
1769 rk.rand<unsigned>();
1771 // RootMoves are already sorted by score in descending order
1772 size_t size = std::min(MultiPV, RootMoves.size());
1773 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1774 int weakness = 120 - 2 * SkillLevel;
1775 int max_s = -VALUE_INFINITE;
1776 Move best = MOVE_NONE;
1778 // Choose best move. For each move score we add two terms both dependent on
1779 // weakness, one deterministic and bigger for weaker moves, and one random,
1780 // then we choose the move with the resulting highest score.
1781 for (size_t i = 0; i < size; i++)
1783 int s = RootMoves[i].score;
1785 // Don't allow crazy blunders even at very low skills
1786 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1789 // This is our magic formula
1790 s += ( weakness * int(RootMoves[0].score - s)
1791 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1796 best = RootMoves[i].pv[0];
1803 // extract_pv_from_tt() builds a PV by adding moves from the transposition table.
1804 // We consider also failing high nodes and not only VALUE_TYPE_EXACT nodes. This
1805 // allow to always have a ponder move even when we fail high at root and also a
1806 // long PV to print that is important for position analysis.
1808 void RootMove::extract_pv_from_tt(Position& pos) {
1810 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1815 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1819 pos.do_move(m, *st++);
1821 while ( (tte = TT.probe(pos.key())) != NULL
1822 && tte->move() != MOVE_NONE
1823 && pos.is_pseudo_legal(tte->move())
1824 && pos.pl_move_is_legal(tte->move(), pos.pinned_pieces())
1826 && (!pos.is_draw<false>() || ply < 2))
1828 pv.push_back(tte->move());
1829 pos.do_move(tte->move(), *st++);
1832 pv.push_back(MOVE_NONE);
1834 do pos.undo_move(pv[--ply]); while (ply);
1838 // insert_pv_in_tt() is called at the end of a search iteration, and inserts
1839 // the PV back into the TT. This makes sure the old PV moves are searched
1840 // first, even if the old TT entries have been overwritten.
1842 void RootMove::insert_pv_in_tt(Position& pos) {
1844 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1847 Value v, m = VALUE_NONE;
1850 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1856 // Don't overwrite existing correct entries
1857 if (!tte || tte->move() != pv[ply])
1859 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1860 TT.store(k, VALUE_NONE, VALUE_TYPE_NONE, DEPTH_NONE, pv[ply], v, m);
1862 pos.do_move(pv[ply], *st++);
1864 } while (pv[++ply] != MOVE_NONE);
1866 do pos.undo_move(pv[--ply]); while (ply);
1872 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1873 /// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
1874 /// for which the thread is the master.
1876 void Thread::idle_loop(SplitPoint* sp) {
1880 // If we are not searching, wait for a condition to be signaled
1881 // instead of wasting CPU time polling for work.
1884 || (Threads.use_sleeping_threads() && !is_searching))
1886 assert((!sp && threadID) || Threads.use_sleeping_threads());
1894 // Grab the lock to avoid races with Thread::wake_up()
1895 lock_grab(&sleepLock);
1897 // If we are master and all slaves have finished don't go to sleep
1898 if (sp && Threads.split_point_finished(sp))
1900 lock_release(&sleepLock);
1904 // Do sleep after retesting sleep conditions under lock protection, in
1905 // particular we need to avoid a deadlock in case a master thread has,
1906 // in the meanwhile, allocated us and sent the wake_up() call before we
1907 // had the chance to grab the lock.
1908 if (do_sleep || !is_searching)
1909 cond_wait(&sleepCond, &sleepLock);
1911 lock_release(&sleepLock);
1914 // If this thread has been assigned work, launch a search
1917 assert(!do_terminate);
1919 // Copy split point position and search stack and call search()
1920 Stack ss[MAX_PLY_PLUS_2];
1921 SplitPoint* tsp = splitPoint;
1922 Position pos(*tsp->pos, threadID);
1924 memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
1927 if (tsp->nodeType == Root)
1928 search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1929 else if (tsp->nodeType == PV)
1930 search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1931 else if (tsp->nodeType == NonPV)
1932 search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
1936 assert(is_searching);
1938 is_searching = false;
1940 // Wake up master thread so to allow it to return from the idle loop in
1941 // case we are the last slave of the split point.
1942 if ( Threads.use_sleeping_threads()
1943 && threadID != tsp->master
1944 && !Threads[tsp->master].is_searching)
1945 Threads[tsp->master].wake_up();
1948 // If this thread is the master of a split point and all slaves have
1949 // finished their work at this split point, return from the idle loop.
1950 if (sp && Threads.split_point_finished(sp))
1952 // Because sp->is_slave[] is reset under lock protection,
1953 // be sure sp->lock has been released before to return.
1954 lock_grab(&(sp->lock));
1955 lock_release(&(sp->lock));
1962 /// do_timer_event() is called by the timer thread when the timer triggers. It
1963 /// is used to print debug info and, more important, to detect when we are out of
1964 /// available time and so stop the search.
1966 void do_timer_event() {
1968 static int lastInfoTime;
1969 int e = elapsed_time();
1971 if (system_time() - lastInfoTime >= 1000 || !lastInfoTime)
1973 lastInfoTime = system_time();
1980 bool stillAtFirstMove = Signals.firstRootMove
1981 && !Signals.failedLowAtRoot
1982 && e > TimeMgr.available_time();
1984 bool noMoreTime = e > TimeMgr.maximum_time()
1985 || stillAtFirstMove;
1987 if ( (Limits.useTimeManagement() && noMoreTime)
1988 || (Limits.maxTime && e >= Limits.maxTime)
1989 /* missing nodes limit */ ) // FIXME
1990 Signals.stop = true;