2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 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/>.
36 #include "ucioption.h"
40 volatile SignalsType Signals;
42 std::vector<RootMove> RootMoves;
45 Time::point SearchTime;
46 StateStackPtr SetupStates;
51 using namespace Search;
55 // Set to true to force running with one thread. Used for debugging
56 const bool FakeSplit = false;
58 // This is the minimum interval in msec between two check_time() calls
59 const int TimerResolution = 5;
61 // Different node types, used as template parameter
62 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
64 // Dynamic razoring margin based on depth
65 inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
67 // Futility lookup tables (initialized at startup) and their access functions
68 Value FutilityMargins[16][64]; // [depth][moveNumber]
69 int FutilityMoveCounts[32]; // [depth]
71 inline Value futility_margin(Depth d, int mn) {
73 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
77 // Reduction lookup tables (initialized at startup) and their access function
78 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
80 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
82 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
88 Value DrawValue[COLOR_NB];
91 CountermovesStats Countermoves;
93 template <NodeType NT>
94 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
96 template <NodeType NT, bool InCheck>
97 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
99 void id_loop(Position& pos);
100 Value value_to_tt(Value v, int ply);
101 Value value_from_tt(Value v, int ply);
102 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta);
103 bool allows(const Position& pos, Move first, Move second);
104 bool refutes(const Position& pos, Move first, Move second);
105 string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
108 Skill(int l) : level(l), best(MOVE_NONE) {}
110 if (enabled()) // Swap best PV line with the sub-optimal one
111 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
112 RootMoves.end(), best ? best : pick_move()));
115 bool enabled() const { return level < 20; }
116 bool time_to_pick(int depth) const { return depth == 1 + level; }
126 /// Search::init() is called during startup to initialize various lookup tables
128 void Search::init() {
130 int d; // depth (ONE_PLY == 2)
131 int hd; // half depth (ONE_PLY == 1)
134 // Init reductions array
135 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
137 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
138 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
139 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
140 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
143 // Init futility margins array
144 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
145 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
147 // Init futility move count array
148 for (d = 0; d < 32; d++)
149 FutilityMoveCounts[d] = int(3.001 + 0.3 * pow(double(d), 1.8));
153 /// Search::perft() is our utility to verify move generation. All the leaf nodes
154 /// up to the given depth are generated and counted and the sum returned.
156 size_t Search::perft(Position& pos, Depth depth) {
158 // At the last ply just return the number of legal moves (leaf nodes)
159 if (depth == ONE_PLY)
160 return MoveList<LEGAL>(pos).size();
166 for (MoveList<LEGAL> it(pos); *it; ++it)
168 pos.do_move(*it, st, ci, pos.move_gives_check(*it, ci));
169 cnt += perft(pos, depth - ONE_PLY);
177 /// Search::think() is the external interface to Stockfish's search, and is
178 /// called by the main thread when the program receives the UCI 'go' command. It
179 /// searches from RootPos and at the end prints the "bestmove" to output.
181 void Search::think() {
183 static PolyglotBook book; // Defined static to initialize the PRNG only once
185 RootColor = RootPos.side_to_move();
186 TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
188 if (RootMoves.empty())
190 RootMoves.push_back(MOVE_NONE);
191 sync_cout << "info depth 0 score "
192 << score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
198 if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
200 Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
202 if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
204 std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
209 if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
211 int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
212 cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
213 DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
214 DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
217 DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
219 if (Options["Use Search Log"])
221 Log log(Options["Search Log Filename"]);
222 log << "\nSearching: " << RootPos.fen()
223 << "\ninfinite: " << Limits.infinite
224 << " ponder: " << Limits.ponder
225 << " time: " << Limits.time[RootColor]
226 << " increment: " << Limits.inc[RootColor]
227 << " moves to go: " << Limits.movestogo
231 // Reset the threads, still sleeping: will be wake up at split time
232 for (size_t i = 0; i < Threads.size(); i++)
233 Threads[i]->maxPly = 0;
235 Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
237 // Set best timer interval to avoid lagging under time pressure. Timer is
238 // used to check for remaining available thinking time.
239 Threads.timer->msec =
240 Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
241 Limits.nodes ? 2 * TimerResolution
244 Threads.timer->notify_one(); // Wake up the recurring timer
246 id_loop(RootPos); // Let's start searching !
248 Threads.timer->msec = 0; // Stop the timer
249 Threads.sleepWhileIdle = true; // Send idle threads to sleep
251 if (Options["Use Search Log"])
253 Time::point elapsed = Time::now() - SearchTime + 1;
255 Log log(Options["Search Log Filename"]);
256 log << "Nodes: " << RootPos.nodes_searched()
257 << "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
258 << "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
261 RootPos.do_move(RootMoves[0].pv[0], st);
262 log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
263 RootPos.undo_move(RootMoves[0].pv[0]);
268 // When search is stopped this info is not printed
269 sync_cout << "info nodes " << RootPos.nodes_searched()
270 << " time " << Time::now() - SearchTime + 1 << sync_endl;
272 // When we reach max depth we arrive here even without Signals.stop is raised,
273 // but if we are pondering or in infinite search, according to UCI protocol,
274 // we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
275 // command. We simply wait here until GUI sends one of those commands (that
276 // raise Signals.stop).
277 if (!Signals.stop && (Limits.ponder || Limits.infinite))
279 Signals.stopOnPonderhit = true;
280 RootPos.this_thread()->wait_for(Signals.stop);
283 // Best move could be MOVE_NONE when searching on a stalemate position
284 sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
285 << " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
292 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
293 // with increasing depth until the allocated thinking time has been consumed,
294 // user stops the search, or the maximum search depth is reached.
296 void id_loop(Position& pos) {
298 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
299 int depth, prevBestMoveChanges;
300 Value bestValue, alpha, beta, delta;
302 memset(ss-1, 0, 4 * sizeof(Stack));
303 depth = BestMoveChanges = 0;
304 bestValue = delta = -VALUE_INFINITE;
305 (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
309 Countermoves.clear();
311 PVSize = Options["MultiPV"];
312 Skill skill(Options["Skill Level"]);
314 // Do we have to play with skill handicap? In this case enable MultiPV search
315 // that we will use behind the scenes to retrieve a set of possible moves.
316 if (skill.enabled() && PVSize < 4)
319 PVSize = std::min(PVSize, RootMoves.size());
321 // Iterative deepening loop until requested to stop or target depth reached
322 while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
324 // Save last iteration's scores before first PV line is searched and all
325 // the move scores but the (new) PV are set to -VALUE_INFINITE.
326 for (size_t i = 0; i < RootMoves.size(); i++)
327 RootMoves[i].prevScore = RootMoves[i].score;
329 prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
332 // MultiPV loop. We perform a full root search for each PV line
333 for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
335 // Set aspiration window default width
336 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
339 alpha = RootMoves[PVIdx].prevScore - delta;
340 beta = RootMoves[PVIdx].prevScore + delta;
344 alpha = -VALUE_INFINITE;
345 beta = VALUE_INFINITE;
348 // Start with a small aspiration window and, in case of fail high/low,
349 // research with bigger window until not failing high/low anymore.
352 bestValue = search<Root>(pos, ss, alpha, beta, depth * ONE_PLY, false);
354 // Bring to front the best move. It is critical that sorting is
355 // done with a stable algorithm because all the values but the first
356 // and eventually the new best one are set to -VALUE_INFINITE and
357 // we want to keep the same order for all the moves but the new
358 // PV that goes to the front. Note that in case of MultiPV search
359 // the already searched PV lines are preserved.
360 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
362 // Write PV back to transposition table in case the relevant
363 // entries have been overwritten during the search.
364 for (size_t i = 0; i <= PVIdx; i++)
365 RootMoves[i].insert_pv_in_tt(pos);
367 // If search has been stopped return immediately. Sorting and
368 // writing PV back to TT is safe becuase RootMoves is still
369 // valid, although refers to previous iteration.
373 // In case of failing high/low increase aspiration window and
374 // research, otherwise exit the loop.
375 if (bestValue > alpha && bestValue < beta)
378 // Give some update (without cluttering the UI) before to research
379 if (Time::now() - SearchTime > 3000)
380 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
382 if (abs(bestValue) >= VALUE_KNOWN_WIN)
384 alpha = -VALUE_INFINITE;
385 beta = VALUE_INFINITE;
387 else if (bestValue >= beta)
394 Signals.failedLowAtRoot = true;
395 Signals.stopOnPonderhit = false;
401 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
404 // Sort the PV lines searched so far and update the GUI
405 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
407 if (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
408 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
411 // Do we need to pick now the sub-optimal best move ?
412 if (skill.enabled() && skill.time_to_pick(depth))
415 if (Options["Use Search Log"])
417 RootMove& rm = RootMoves[0];
418 if (skill.best != MOVE_NONE)
419 rm = *std::find(RootMoves.begin(), RootMoves.end(), skill.best);
421 Log log(Options["Search Log Filename"]);
422 log << pretty_pv(pos, depth, rm.score, Time::now() - SearchTime, &rm.pv[0])
426 // Do we have found a "mate in x"?
428 && bestValue >= VALUE_MATE_IN_MAX_PLY
429 && VALUE_MATE - bestValue <= 2 * Limits.mate)
432 // Do we have time for the next iteration? Can we stop searching now?
433 if (Limits.use_time_management() && !Signals.stopOnPonderhit)
435 bool stop = false; // Local variable, not the volatile Signals.stop
437 // Take in account some extra time if the best move has changed
438 if (depth > 4 && depth < 50 && PVSize == 1)
439 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
441 // Stop search if most of available time is already consumed. We
442 // probably don't have enough time to search the first move at the
443 // next iteration anyway.
444 if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
447 // Stop search early if one move seems to be much better than others
451 && bestValue > VALUE_MATED_IN_MAX_PLY
452 && ( RootMoves.size() == 1
453 || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
455 Value rBeta = bestValue - 2 * PawnValueMg;
456 ss->excludedMove = RootMoves[0].pv[0];
457 ss->skipNullMove = true;
458 Value v = search<NonPV>(pos, ss, rBeta - 1, rBeta, (depth - 3) * ONE_PLY, true);
459 ss->skipNullMove = false;
460 ss->excludedMove = MOVE_NONE;
468 // If we are allowed to ponder do not stop the search now but
469 // keep pondering until GUI sends "ponderhit" or "stop".
471 Signals.stopOnPonderhit = true;
480 // search<>() is the main search function for both PV and non-PV nodes and for
481 // normal and SplitPoint nodes. When called just after a split point the search
482 // is simpler because we have already probed the hash table, done a null move
483 // search, and searched the first move before splitting, we don't have to repeat
484 // all this work again. We also don't need to store anything to the hash table
485 // here: This is taken care of after we return from the split point.
487 template <NodeType NT>
488 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
490 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
491 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
492 const bool RootNode = (NT == Root || NT == SplitPointRoot);
494 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
495 assert(PvNode || (alpha == beta - 1));
496 assert(depth > DEPTH_ZERO);
498 Move movesSearched[64];
501 SplitPoint* splitPoint;
503 Move ttMove, move, excludedMove, bestMove, threatMove;
505 Value bestValue, value, ttValue;
506 Value eval, nullValue, futilityValue;
507 bool inCheck, givesCheck, pvMove, singularExtensionNode;
508 bool captureOrPromotion, dangerous, doFullDepthSearch;
509 int moveCount, playedMoveCount;
511 // Step 1. Initialize node
512 Thread* thisThread = pos.this_thread();
513 moveCount = playedMoveCount = 0;
514 inCheck = pos.checkers();
518 splitPoint = ss->splitPoint;
519 bestMove = splitPoint->bestMove;
520 threatMove = splitPoint->threatMove;
521 bestValue = splitPoint->bestValue;
523 ttMove = excludedMove = MOVE_NONE;
524 ttValue = VALUE_NONE;
526 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
528 goto split_point_start;
531 bestValue = -VALUE_INFINITE;
532 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
533 ss->ply = (ss-1)->ply + 1;
534 ss->futilityMoveCount = 0;
535 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
536 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
538 // Used to send selDepth info to GUI
539 if (PvNode && thisThread->maxPly < ss->ply)
540 thisThread->maxPly = ss->ply;
544 // Step 2. Check for aborted search and immediate draw
545 if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
546 return DrawValue[pos.side_to_move()];
548 // Step 3. Mate distance pruning. Even if we mate at the next move our score
549 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
550 // a shorter mate was found upward in the tree then there is no need to search
551 // further, we will never beat current alpha. Same logic but with reversed signs
552 // applies also in the opposite condition of being mated instead of giving mate,
553 // in this case return a fail-high score.
554 alpha = std::max(mated_in(ss->ply), alpha);
555 beta = std::min(mate_in(ss->ply+1), beta);
560 // Step 4. Transposition table lookup
561 // We don't want the score of a partial search to overwrite a previous full search
562 // TT value, so we use a different position key in case of an excluded move.
563 excludedMove = ss->excludedMove;
564 posKey = excludedMove ? pos.exclusion_key() : pos.key();
565 tte = TT.probe(posKey);
566 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
567 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
569 // At PV nodes we check for exact scores, while at non-PV nodes we check for
570 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
571 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
572 // we should also update RootMoveList to avoid bogus output.
575 && tte->depth() >= depth
576 && ttValue != VALUE_NONE // Only in case of TT access race
577 && ( PvNode ? tte->type() == BOUND_EXACT
578 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
579 : (tte->type() & BOUND_UPPER)))
582 ss->currentMove = ttMove; // Can be MOVE_NONE
586 && !pos.is_capture_or_promotion(ttMove)
587 && ttMove != ss->killers[0])
589 ss->killers[1] = ss->killers[0];
590 ss->killers[0] = ttMove;
595 // Step 5. Evaluate the position statically and update parent's gain statistics
597 ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
601 // Never assume anything on values stored in TT
602 if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
603 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
604 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
606 // Can ttValue be used as a better position evaluation?
607 if (ttValue != VALUE_NONE)
608 if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
609 || ((tte->type() & BOUND_UPPER) && ttValue < eval))
614 eval = ss->staticEval = evaluate(pos, ss->evalMargin);
615 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
616 ss->staticEval, ss->evalMargin);
619 // Update gain for the parent non-capture move given the static position
620 // evaluation before and after the move.
621 if ( (move = (ss-1)->currentMove) != MOVE_NULL
622 && (ss-1)->staticEval != VALUE_NONE
623 && ss->staticEval != VALUE_NONE
624 && !pos.captured_piece_type()
625 && type_of(move) == NORMAL)
627 Square to = to_sq(move);
628 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
631 // Step 6. Razoring (is omitted in PV nodes)
633 && depth < 4 * ONE_PLY
635 && eval + razor_margin(depth) < beta
636 && ttMove == MOVE_NONE
637 && abs(beta) < VALUE_MATE_IN_MAX_PLY
638 && !pos.pawn_on_7th(pos.side_to_move()))
640 Value rbeta = beta - razor_margin(depth);
641 Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
643 // Logically we should return (v + razor_margin(depth)), but
644 // surprisingly this did slightly weaker in tests.
648 // Step 7. Static null move pruning (is omitted in PV nodes)
649 // We're betting that the opponent doesn't have a move that will reduce
650 // the score by more than futility_margin(depth) if we do a null move.
653 && depth < 4 * ONE_PLY
655 && eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
656 && abs(beta) < VALUE_MATE_IN_MAX_PLY
657 && abs(eval) < VALUE_KNOWN_WIN
658 && pos.non_pawn_material(pos.side_to_move()))
659 return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
661 // Step 8. Null move search with verification search (is omitted in PV nodes)
667 && abs(beta) < VALUE_MATE_IN_MAX_PLY
668 && pos.non_pawn_material(pos.side_to_move()))
670 ss->currentMove = MOVE_NULL;
672 // Null move dynamic reduction based on depth
673 Depth R = 3 * ONE_PLY + depth / 4;
675 // Null move dynamic reduction based on value
676 if (eval - PawnValueMg > beta)
679 pos.do_null_move(st);
680 (ss+1)->skipNullMove = true;
681 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
682 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
683 (ss+1)->skipNullMove = false;
684 pos.undo_null_move();
686 if (nullValue >= beta)
688 // Do not return unproven mate scores
689 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
692 if (depth < 12 * ONE_PLY)
695 // Do verification search at high depths
696 ss->skipNullMove = true;
697 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
698 ss->skipNullMove = false;
705 // The null move failed low, which means that we may be faced with
706 // some kind of threat. If the previous move was reduced, check if
707 // the move that refuted the null move was somehow connected to the
708 // move which was reduced. If a connection is found, return a fail
709 // low score (which will cause the reduced move to fail high in the
710 // parent node, which will trigger a re-search with full depth).
711 threatMove = (ss+1)->currentMove;
713 if ( depth < 5 * ONE_PLY
715 && threatMove != MOVE_NONE
716 && allows(pos, (ss-1)->currentMove, threatMove))
721 // Step 9. ProbCut (is omitted in PV nodes)
722 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
723 // and a reduced search returns a value much above beta, we can (almost) safely
724 // prune the previous move.
726 && depth >= 5 * ONE_PLY
729 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
731 Value rbeta = beta + 200;
732 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
734 assert(rdepth >= ONE_PLY);
735 assert((ss-1)->currentMove != MOVE_NONE);
736 assert((ss-1)->currentMove != MOVE_NULL);
738 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
741 while ((move = mp.next_move<false>()) != MOVE_NONE)
742 if (pos.pl_move_is_legal(move, ci.pinned))
744 ss->currentMove = move;
745 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
746 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
753 // Step 10. Internal iterative deepening
754 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
755 && ttMove == MOVE_NONE
756 && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
758 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
760 ss->skipNullMove = true;
761 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
762 ss->skipNullMove = false;
764 tte = TT.probe(posKey);
765 ttMove = tte ? tte->move() : MOVE_NONE;
768 split_point_start: // At split points actual search starts from here
770 Square prevMoveSq = to_sq((ss-1)->currentMove);
771 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
772 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
774 MovePicker mp(pos, ttMove, depth, History, countermoves, ss, PvNode ? -VALUE_INFINITE : beta);
776 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
777 singularExtensionNode = !RootNode
779 && depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
780 && ttMove != MOVE_NONE
781 && !excludedMove // Recursive singular search is not allowed
782 && (tte->type() & BOUND_LOWER)
783 && tte->depth() >= depth - 3 * ONE_PLY;
785 // Step 11. Loop through moves
786 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
787 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
791 if (move == excludedMove)
794 // At root obey the "searchmoves" option and skip moves not listed in Root
795 // Move List, as a consequence any illegal move is also skipped. In MultiPV
796 // mode we also skip PV moves which have been already searched.
797 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
802 // Shared counter cannot be decremented later if move turns out to be illegal
803 if (!pos.pl_move_is_legal(move, ci.pinned))
806 moveCount = ++splitPoint->moveCount;
807 splitPoint->mutex.unlock();
814 Signals.firstRootMove = (moveCount == 1);
816 if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
817 sync_cout << "info depth " << depth / ONE_PLY
818 << " currmove " << move_to_uci(move, pos.is_chess960())
819 << " currmovenumber " << moveCount + PVIdx << sync_endl;
823 captureOrPromotion = pos.is_capture_or_promotion(move);
824 givesCheck = pos.move_gives_check(move, ci);
825 dangerous = givesCheck
826 || pos.is_passed_pawn_push(move)
827 || type_of(move) == CASTLE
828 || ( captureOrPromotion // Entering a pawn endgame?
829 && type_of(pos.piece_on(to_sq(move))) != PAWN
830 && type_of(move) == NORMAL
831 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
832 - PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
834 // Step 12. Extend checks and, in PV nodes, also dangerous moves
835 if (PvNode && dangerous)
838 else if (givesCheck && pos.see_sign(move) >= 0)
841 // Singular extension search. If all moves but one fail low on a search of
842 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
843 // is singular and should be extended. To verify this we do a reduced search
844 // on all the other moves but the ttMove, if result is lower than ttValue minus
845 // a margin then we extend ttMove.
846 if ( singularExtensionNode
849 && pos.pl_move_is_legal(move, ci.pinned)
850 && abs(ttValue) < VALUE_KNOWN_WIN)
852 assert(ttValue != VALUE_NONE);
854 Value rBeta = ttValue - int(depth);
855 ss->excludedMove = move;
856 ss->skipNullMove = true;
857 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
858 ss->skipNullMove = false;
859 ss->excludedMove = MOVE_NONE;
865 // Update current move (this must be done after singular extension search)
866 newDepth = depth - ONE_PLY + ext;
868 // Step 13. Futility pruning (is omitted in PV nodes)
870 && !captureOrPromotion
873 /* && move != ttMove Already implicit in the next condition */
874 && bestValue > VALUE_MATED_IN_MAX_PLY)
876 // Move count based pruning
877 if ( depth < 16 * ONE_PLY
878 && moveCount >= FutilityMoveCounts[depth]
879 && (!threatMove || !refutes(pos, move, threatMove)))
882 splitPoint->mutex.lock();
887 // Value based pruning
888 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
889 // but fixing this made program slightly weaker.
890 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
891 futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
892 + Gains[pos.piece_moved(move)][to_sq(move)];
894 if (futilityValue < beta)
896 bestValue = std::max(bestValue, futilityValue);
900 splitPoint->mutex.lock();
901 if (bestValue > splitPoint->bestValue)
902 splitPoint->bestValue = bestValue;
907 // Prune moves with negative SEE at low depths
908 if ( predictedDepth < 4 * ONE_PLY
909 && pos.see_sign(move) < 0)
912 splitPoint->mutex.lock();
917 // We have not pruned the move that will be searched, but remember how
918 // far in the move list we are to be more aggressive in the child node.
919 ss->futilityMoveCount = moveCount;
922 ss->futilityMoveCount = 0;
924 // Check for legality only before to do the move
925 if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
931 pvMove = PvNode && moveCount == 1;
932 ss->currentMove = move;
933 if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
934 movesSearched[playedMoveCount++] = move;
936 // Step 14. Make the move
937 pos.do_move(move, st, ci, givesCheck);
939 // Step 15. Reduced depth search (LMR). If the move fails high will be
940 // re-searched at full depth.
941 if ( depth > 3 * ONE_PLY
943 && !captureOrPromotion
946 && move != ss->killers[0]
947 && move != ss->killers[1])
949 ss->reduction = reduction<PvNode>(depth, moveCount);
951 if (!PvNode && cutNode)
952 ss->reduction += ONE_PLY;
954 if (move == countermoves[0] || move == countermoves[1])
955 ss->reduction = std::max(DEPTH_ZERO, ss->reduction-ONE_PLY);
957 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
959 alpha = splitPoint->alpha;
961 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
963 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
964 ss->reduction = DEPTH_ZERO;
967 doFullDepthSearch = !pvMove;
969 // Step 16. Full depth search, when LMR is skipped or fails high
970 if (doFullDepthSearch)
973 alpha = splitPoint->alpha;
975 value = newDepth < ONE_PLY ?
976 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
977 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
978 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
981 // Only for PV nodes do a full PV search on the first move or after a fail
982 // high, in the latter case search only if value < beta, otherwise let the
983 // parent node to fail low with value <= alpha and to try another move.
984 if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
985 value = newDepth < ONE_PLY ?
986 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
987 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
988 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
989 // Step 17. Undo move
992 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
994 // Step 18. Check for new best move
997 splitPoint->mutex.lock();
998 bestValue = splitPoint->bestValue;
999 alpha = splitPoint->alpha;
1002 // Finished searching the move. If Signals.stop is true, the search
1003 // was aborted because the user interrupted the search or because we
1004 // ran out of time. In this case, the return value of the search cannot
1005 // be trusted, and we don't update the best move and/or PV.
1006 if (Signals.stop || thisThread->cutoff_occurred())
1007 return value; // To avoid returning VALUE_INFINITE
1011 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1013 // PV move or new best move ?
1014 if (pvMove || value > alpha)
1017 rm.extract_pv_from_tt(pos);
1019 // We record how often the best move has been changed in each
1020 // iteration. This information is used for time management: When
1021 // the best move changes frequently, we allocate some more time.
1026 // All other moves but the PV are set to the lowest value, this
1027 // is not a problem when sorting becuase sort is stable and move
1028 // position in the list is preserved, just the PV is pushed up.
1029 rm.score = -VALUE_INFINITE;
1032 if (value > bestValue)
1034 bestValue = SpNode ? splitPoint->bestValue = value : value;
1038 bestMove = SpNode ? splitPoint->bestMove = move : move;
1040 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1041 alpha = SpNode ? splitPoint->alpha = value : value;
1044 assert(value >= beta); // Fail high
1047 splitPoint->cutoff = true;
1054 // Step 19. Check for splitting the search
1056 && depth >= Threads.minimumSplitDepth
1057 && Threads.available_slave(thisThread)
1058 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1060 assert(bestValue < beta);
1062 thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
1063 depth, threatMove, moveCount, &mp, NT, cutNode);
1064 if (bestValue >= beta)
1072 // Step 20. Check for mate and stalemate
1073 // All legal moves have been searched and if there are no legal moves, it
1074 // must be mate or stalemate. Note that we can have a false positive in
1075 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1076 // harmless because return value is discarded anyhow in the parent nodes.
1077 // If we are in a singular extension search then return a fail low score.
1078 // A split node has at least one move, the one tried before to be splitted.
1080 return excludedMove ? alpha
1081 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1083 // If we have pruned all the moves without searching return a fail-low score
1084 if (bestValue == -VALUE_INFINITE)
1086 assert(!playedMoveCount);
1091 if (bestValue >= beta) // Failed high
1093 TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
1094 bestMove, ss->staticEval, ss->evalMargin);
1096 if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
1098 if (bestMove != ss->killers[0])
1100 ss->killers[1] = ss->killers[0];
1101 ss->killers[0] = bestMove;
1104 // Increase history value of the cut-off move
1105 Value bonus = Value(int(depth) * int(depth));
1106 History.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
1107 if (is_ok((ss-1)->currentMove))
1108 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, bestMove);
1110 // Decrease history of all the other played non-capture moves
1111 for (int i = 0; i < playedMoveCount - 1; i++)
1113 Move m = movesSearched[i];
1114 History.update(pos.piece_moved(m), to_sq(m), -bonus);
1118 else // Failed low or PV search
1119 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1120 PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
1121 depth, bestMove, ss->staticEval, ss->evalMargin);
1123 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1129 // qsearch() is the quiescence search function, which is called by the main
1130 // search function when the remaining depth is zero (or, to be more precise,
1131 // less than ONE_PLY).
1133 template <NodeType NT, bool InCheck>
1134 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1136 const bool PvNode = (NT == PV);
1138 assert(NT == PV || NT == NonPV);
1139 assert(InCheck == !!pos.checkers());
1140 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1141 assert(PvNode || (alpha == beta - 1));
1142 assert(depth <= DEPTH_ZERO);
1147 Move ttMove, move, bestMove;
1148 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1149 bool givesCheck, enoughMaterial, evasionPrunable;
1152 // To flag BOUND_EXACT a node with eval above alpha and no available moves
1156 ss->currentMove = bestMove = MOVE_NONE;
1157 ss->ply = (ss-1)->ply + 1;
1159 // Check for an instant draw or maximum ply reached
1160 if (pos.is_draw() || ss->ply > MAX_PLY)
1161 return DrawValue[pos.side_to_move()];
1163 // Decide whether or not to include checks, this fixes also the type of
1164 // TT entry depth that we are going to use. Note that in qsearch we use
1165 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1166 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1167 : DEPTH_QS_NO_CHECKS;
1169 // Transposition table lookup. At PV nodes, we don't use the TT for
1170 // pruning, but only for move ordering.
1172 tte = TT.probe(posKey);
1173 ttMove = tte ? tte->move() : MOVE_NONE;
1174 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1177 && tte->depth() >= ttDepth
1178 && ttValue != VALUE_NONE // Only in case of TT access race
1179 && ( PvNode ? tte->type() == BOUND_EXACT
1180 : ttValue >= beta ? (tte->type() & BOUND_LOWER)
1181 : (tte->type() & BOUND_UPPER)))
1183 ss->currentMove = ttMove; // Can be MOVE_NONE
1187 // Evaluate the position statically
1190 ss->staticEval = ss->evalMargin = VALUE_NONE;
1191 bestValue = futilityBase = -VALUE_INFINITE;
1192 enoughMaterial = false;
1198 // Never assume anything on values stored in TT
1199 if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
1200 ||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
1201 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1204 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
1206 // Stand pat. Return immediately if static value is at least beta
1207 if (bestValue >= beta)
1210 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1211 DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
1216 if (PvNode && bestValue > alpha)
1219 futilityBase = ss->staticEval + ss->evalMargin + Value(128);
1220 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
1223 // Initialize a MovePicker object for the current position, and prepare
1224 // to search the moves. Because the depth is <= 0 here, only captures,
1225 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1227 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1230 // Loop through the moves until no moves remain or a beta cutoff occurs
1231 while ((move = mp.next_move<false>()) != MOVE_NONE)
1233 assert(is_ok(move));
1235 givesCheck = pos.move_gives_check(move, ci);
1243 && type_of(move) != PROMOTION
1244 && !pos.is_passed_pawn_push(move))
1246 futilityValue = futilityBase
1247 + PieceValue[EG][pos.piece_on(to_sq(move))]
1248 + (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
1250 if (futilityValue < beta)
1252 bestValue = std::max(bestValue, futilityValue);
1256 // Prune moves with negative or equal SEE and also moves with positive
1257 // SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
1258 if ( futilityBase < beta
1259 && pos.see(move, beta - futilityBase) <= 0)
1261 bestValue = std::max(bestValue, futilityBase);
1266 // Detect non-capture evasions that are candidate to be pruned
1267 evasionPrunable = !PvNode
1269 && bestValue > VALUE_MATED_IN_MAX_PLY
1270 && !pos.is_capture(move)
1271 && !pos.can_castle(pos.side_to_move());
1273 // Don't search moves with negative SEE values
1275 && (!InCheck || evasionPrunable)
1277 && type_of(move) != PROMOTION
1278 && pos.see_sign(move) < 0)
1281 // Don't search useless checks
1286 && !pos.is_capture_or_promotion(move)
1287 && ss->staticEval + PawnValueMg / 4 < beta
1288 && !check_is_dangerous(pos, move, futilityBase, beta))
1291 // Check for legality only before to do the move
1292 if (!pos.pl_move_is_legal(move, ci.pinned))
1295 ss->currentMove = move;
1297 // Make and search the move
1298 pos.do_move(move, st, ci, givesCheck);
1299 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1300 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1301 pos.undo_move(move);
1303 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1305 // Check for new best move
1306 if (value > bestValue)
1312 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1319 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1320 ttDepth, move, ss->staticEval, ss->evalMargin);
1328 // All legal moves have been searched. A special case: If we're in check
1329 // and no legal moves were found, it is checkmate.
1330 if (InCheck && bestValue == -VALUE_INFINITE)
1331 return mated_in(ss->ply); // Plies to mate from the root
1333 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1334 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1335 ttDepth, bestMove, ss->staticEval, ss->evalMargin);
1337 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1343 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1344 // "plies to mate from the current position". Non-mate scores are unchanged.
1345 // The function is called before storing a value to the transposition table.
1347 Value value_to_tt(Value v, int ply) {
1349 assert(v != VALUE_NONE);
1351 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1352 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1356 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1357 // from the transposition table (where refers to the plies to mate/be mated
1358 // from current position) to "plies to mate/be mated from the root".
1360 Value value_from_tt(Value v, int ply) {
1362 return v == VALUE_NONE ? VALUE_NONE
1363 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1364 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1368 // check_is_dangerous() tests if a checking move can be pruned in qsearch()
1370 bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
1372 Piece pc = pos.piece_moved(move);
1373 Square from = from_sq(move);
1374 Square to = to_sq(move);
1375 Color them = ~pos.side_to_move();
1376 Square ksq = pos.king_square(them);
1377 Bitboard enemies = pos.pieces(them);
1378 Bitboard kingAtt = pos.attacks_from<KING>(ksq);
1379 Bitboard occ = pos.pieces() ^ from ^ ksq;
1380 Bitboard oldAtt = pos.attacks_from(pc, from, occ);
1381 Bitboard newAtt = pos.attacks_from(pc, to, occ);
1383 // Checks which give opponent's king at most one escape square are dangerous
1384 if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
1387 // Queen contact check is very dangerous
1388 if (type_of(pc) == QUEEN && (kingAtt & to))
1391 // Creating new double threats with checks is dangerous
1392 Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
1395 // Note that here we generate illegal "double move"!
1396 if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
1404 // allows() tests whether the 'first' move at previous ply somehow makes the
1405 // 'second' move possible, for instance if the moving piece is the same in
1406 // both moves. Normally the second move is the threat (the best move returned
1407 // from a null search that fails low).
1409 bool allows(const Position& pos, Move first, Move second) {
1411 assert(is_ok(first));
1412 assert(is_ok(second));
1413 assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
1414 assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
1416 Square m1from = from_sq(first);
1417 Square m2from = from_sq(second);
1418 Square m1to = to_sq(first);
1419 Square m2to = to_sq(second);
1421 // The piece is the same or second's destination was vacated by the first move
1422 if (m1to == m2from || m2to == m1from)
1425 // Second one moves through the square vacated by first one
1426 if (between_bb(m2from, m2to) & m1from)
1429 // Second's destination is defended by the first move's piece
1430 Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
1434 // Second move gives a discovered check through the first's checking piece
1435 if (m1att & pos.king_square(pos.side_to_move()))
1437 assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
1445 // refutes() tests whether a 'first' move is able to defend against a 'second'
1446 // opponent's move. In this case will not be pruned. Normally the second move
1447 // is the threat (the best move returned from a null search that fails low).
1449 bool refutes(const Position& pos, Move first, Move second) {
1451 assert(is_ok(first));
1452 assert(is_ok(second));
1454 Square m1from = from_sq(first);
1455 Square m2from = from_sq(second);
1456 Square m1to = to_sq(first);
1457 Square m2to = to_sq(second);
1459 // Don't prune moves of the threatened piece
1463 // If the threatened piece has value less than or equal to the value of the
1464 // threat piece, don't prune moves which defend it.
1465 if ( pos.is_capture(second)
1466 && ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
1467 || type_of(pos.piece_on(m2from)) == KING))
1469 // Update occupancy as if the piece and the threat are moving
1470 Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
1471 Piece pc = pos.piece_on(m1from);
1473 // The moved piece attacks the square 'tto' ?
1474 if (pos.attacks_from(pc, m1to, occ) & m2to)
1477 // Scan for possible X-ray attackers behind the moved piece
1478 Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
1479 | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
1481 // Verify attackers are triggered by our move and not already existing
1482 if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
1486 // Don't prune safe moves which block the threat path
1487 if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
1494 // When playing with strength handicap choose best move among the MultiPV set
1495 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1497 Move Skill::pick_move() {
1501 // PRNG sequence should be not deterministic
1502 for (int i = Time::now() % 50; i > 0; i--)
1503 rk.rand<unsigned>();
1505 // RootMoves are already sorted by score in descending order
1506 int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
1507 int weakness = 120 - 2 * level;
1508 int max_s = -VALUE_INFINITE;
1511 // Choose best move. For each move score we add two terms both dependent on
1512 // weakness, one deterministic and bigger for weaker moves, and one random,
1513 // then we choose the move with the resulting highest score.
1514 for (size_t i = 0; i < PVSize; i++)
1516 int s = RootMoves[i].score;
1518 // Don't allow crazy blunders even at very low skills
1519 if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
1522 // This is our magic formula
1523 s += ( weakness * int(RootMoves[0].score - s)
1524 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1529 best = RootMoves[i].pv[0];
1536 // uci_pv() formats PV information according to UCI protocol. UCI requires
1537 // to send all the PV lines also if are still to be searched and so refer to
1538 // the previous search score.
1540 string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
1542 std::stringstream s;
1543 Time::point elapsed = Time::now() - SearchTime + 1;
1544 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1547 for (size_t i = 0; i < Threads.size(); i++)
1548 if (Threads[i]->maxPly > selDepth)
1549 selDepth = Threads[i]->maxPly;
1551 for (size_t i = 0; i < uciPVSize; i++)
1553 bool updated = (i <= PVIdx);
1555 if (depth == 1 && !updated)
1558 int d = updated ? depth : depth - 1;
1559 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1561 if (s.rdbuf()->in_avail()) // Not at first line
1564 s << "info depth " << d
1565 << " seldepth " << selDepth
1566 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1567 << " nodes " << pos.nodes_searched()
1568 << " nps " << pos.nodes_searched() * 1000 / elapsed
1569 << " time " << elapsed
1570 << " multipv " << i + 1
1573 for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1574 s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
1583 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1584 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1585 /// allow to always have a ponder move even when we fail high at root, and a
1586 /// long PV to print that is important for position analysis.
1588 void RootMove::extract_pv_from_tt(Position& pos) {
1590 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1600 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1602 pos.do_move(pv[ply++], *st++);
1603 tte = TT.probe(pos.key());
1606 && pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
1607 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1609 && (!pos.is_draw() || ply < 2));
1611 pv.push_back(MOVE_NONE); // Must be zero-terminating
1613 while (ply) pos.undo_move(pv[--ply]);
1617 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1618 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1619 /// first, even if the old TT entries have been overwritten.
1621 void RootMove::insert_pv_in_tt(Position& pos) {
1623 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1628 tte = TT.probe(pos.key());
1630 if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
1631 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
1633 assert(MoveList<LEGAL>(pos).contains(pv[ply]));
1635 pos.do_move(pv[ply++], *st++);
1637 } while (pv[ply] != MOVE_NONE);
1639 while (ply) pos.undo_move(pv[--ply]);
1643 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1645 void Thread::idle_loop() {
1647 // Pointer 'this_sp' is not null only if we are called from split(), and not
1648 // at the thread creation. So it means we are the split point's master.
1649 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1651 assert(!this_sp || (this_sp->masterThread == this && searching));
1655 // If we are not searching, wait for a condition to be signaled instead of
1656 // wasting CPU time polling for work.
1657 while ((!searching && Threads.sleepWhileIdle) || exit)
1665 // Grab the lock to avoid races with Thread::notify_one()
1668 // If we are master and all slaves have finished then exit idle_loop
1669 if (this_sp && !this_sp->slavesMask)
1675 // Do sleep after retesting sleep conditions under lock protection, in
1676 // particular we need to avoid a deadlock in case a master thread has,
1677 // in the meanwhile, allocated us and sent the notify_one() call before
1678 // we had the chance to grab the lock.
1679 if (!searching && !exit)
1680 sleepCondition.wait(mutex);
1685 // If this thread has been assigned work, launch a search
1690 Threads.mutex.lock();
1693 SplitPoint* sp = activeSplitPoint;
1695 Threads.mutex.unlock();
1697 Stack stack[MAX_PLY_PLUS_2], *ss = stack+1; // To allow referencing (ss-1)
1698 Position pos(*sp->pos, this);
1700 memcpy(ss-1, sp->ss-1, 4 * sizeof(Stack));
1701 ss->splitPoint = sp;
1705 assert(activePosition == NULL);
1707 activePosition = &pos;
1709 switch (sp->nodeType) {
1711 search<SplitPointRoot>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1714 search<SplitPointPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1717 search<SplitPointNonPV>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1726 activePosition = NULL;
1727 sp->slavesMask &= ~(1ULL << idx);
1728 sp->nodes += pos.nodes_searched();
1730 // Wake up master thread so to allow it to return from the idle loop
1731 // in case we are the last slave of the split point.
1732 if ( Threads.sleepWhileIdle
1733 && this != sp->masterThread
1736 assert(!sp->masterThread->searching);
1737 sp->masterThread->notify_one();
1740 // After releasing the lock we cannot access anymore any SplitPoint
1741 // related data in a safe way becuase it could have been released under
1742 // our feet by the sp master. Also accessing other Thread objects is
1743 // unsafe because if we are exiting there is a chance are already freed.
1747 // If this thread is the master of a split point and all slaves have finished
1748 // their work at this split point, return from the idle loop.
1749 if (this_sp && !this_sp->slavesMask)
1751 this_sp->mutex.lock();
1752 bool finished = !this_sp->slavesMask; // Retest under lock protection
1753 this_sp->mutex.unlock();
1761 /// check_time() is called by the timer thread when the timer triggers. It is
1762 /// used to print debug info and, more important, to detect when we are out of
1763 /// available time and so stop the search.
1767 static Time::point lastInfoTime = Time::now();
1768 int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
1770 if (Time::now() - lastInfoTime >= 1000)
1772 lastInfoTime = Time::now();
1781 Threads.mutex.lock();
1783 nodes = RootPos.nodes_searched();
1785 // Loop across all split points and sum accumulated SplitPoint nodes plus
1786 // all the currently active positions nodes.
1787 for (size_t i = 0; i < Threads.size(); i++)
1788 for (int j = 0; j < Threads[i]->splitPointsSize; j++)
1790 SplitPoint& sp = Threads[i]->splitPoints[j];
1795 Bitboard sm = sp.slavesMask;
1798 Position* pos = Threads[pop_lsb(&sm)]->activePosition;
1800 nodes += pos->nodes_searched();
1806 Threads.mutex.unlock();
1809 Time::point elapsed = Time::now() - SearchTime;
1810 bool stillAtFirstMove = Signals.firstRootMove
1811 && !Signals.failedLowAtRoot
1812 && elapsed > TimeMgr.available_time();
1814 bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
1815 || stillAtFirstMove;
1817 if ( (Limits.use_time_management() && noMoreTime)
1818 || (Limits.movetime && elapsed >= Limits.movetime)
1819 || (Limits.nodes && nodes >= Limits.nodes))
1820 Signals.stop = true;