2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 constexpr uint64_t ttHitAverageWindow = 4096;
65 constexpr uint64_t ttHitAverageResolution = 1024;
67 // Razor and futility margins
68 constexpr int RazorMargin = 531;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(217 * (d - improving));
73 // Reductions lookup table, initialized at startup
74 int Reductions[MAX_MOVES]; // [depth or moveNumber]
76 Depth reduction(bool i, Depth d, int mn) {
77 int r = Reductions[d] * Reductions[mn];
78 return (r + 511) / 1024 + (!i && r > 1007);
81 constexpr int futility_move_count(bool improving, Depth depth) {
82 return (5 + depth * depth) * (1 + improving) / 2 - 1;
85 // History and stats update bonus, based on depth
86 int stat_bonus(Depth d) {
87 return d > 15 ? -8 : 19 * d * d + 155 * d - 132;
90 // Add a small random component to draw evaluations to avoid 3fold-blindness
91 Value value_draw(Thread* thisThread) {
92 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 // Breadcrumbs are used to mark nodes as being searched by a given thread
108 std::atomic<Thread*> thread;
109 std::atomic<Key> key;
111 std::array<Breadcrumb, 1024> breadcrumbs;
113 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
114 // node for potential reductions. A free node will be marked upon entering the moves
115 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
116 struct ThreadHolding {
117 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
118 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
123 // See if another already marked this location, if not, mark it ourselves
124 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
127 (*location).thread.store(thisThread, std::memory_order_relaxed);
128 (*location).key.store(posKey, std::memory_order_relaxed);
131 else if ( tmp != thisThread
132 && (*location).key.load(std::memory_order_relaxed) == posKey)
138 if (owning) // Free the marked location
139 (*location).thread.store(nullptr, std::memory_order_relaxed);
142 bool marked() { return otherThread; }
145 Breadcrumb* location;
146 bool otherThread, owning;
149 template <NodeType NT>
150 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
152 template <NodeType NT>
153 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
155 Value value_to_tt(Value v, int ply);
156 Value value_from_tt(Value v, int ply, int r50c);
157 void update_pv(Move* pv, Move move, Move* childPv);
158 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
159 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
160 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
161 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
163 // perft() is our utility to verify move generation. All the leaf nodes up
164 // to the given depth are generated and counted, and the sum is returned.
166 uint64_t perft(Position& pos, Depth depth) {
169 uint64_t cnt, nodes = 0;
170 const bool leaf = (depth == 2);
172 for (const auto& m : MoveList<LEGAL>(pos))
174 if (Root && depth <= 1)
179 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
184 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
192 /// Search::init() is called at startup to initialize various lookup tables
194 void Search::init() {
196 for (int i = 1; i < MAX_MOVES; ++i)
197 Reductions[i] = int((24.8 + std::log(Threads.size()) / 2) * std::log(i));
201 /// Search::clear() resets search state to its initial value
203 void Search::clear() {
205 Threads.main()->wait_for_search_finished();
207 Time.availableNodes = 0;
210 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
214 /// MainThread::search() is started when the program receives the UCI 'go'
215 /// command. It searches from the root position and outputs the "bestmove".
217 void MainThread::search() {
221 nodes = perft<true>(rootPos, Limits.perft);
222 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
226 Color us = rootPos.side_to_move();
227 Time.init(Limits, us, rootPos.game_ply());
230 if (rootMoves.empty())
232 rootMoves.emplace_back(MOVE_NONE);
233 sync_cout << "info depth 0 score "
234 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
239 for (Thread* th : Threads)
241 th->bestMoveChanges = 0;
243 th->start_searching();
246 Thread::search(); // Let's start searching!
249 // When we reach the maximum depth, we can arrive here without a raise of
250 // Threads.stop. However, if we are pondering or in an infinite search,
251 // the UCI protocol states that we shouldn't print the best move before the
252 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
253 // until the GUI sends one of those commands.
255 while (!Threads.stop && (ponder || Limits.infinite))
256 {} // Busy wait for a stop or a ponder reset
258 // Stop the threads if not already stopped (also raise the stop if
259 // "ponderhit" just reset Threads.ponder).
262 // Wait until all threads have finished
263 for (Thread* th : Threads)
265 th->wait_for_search_finished();
267 // When playing in 'nodes as time' mode, subtract the searched nodes from
268 // the available ones before exiting.
270 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
272 Thread* bestThread = this;
274 // Check if there are threads with a better score than main thread
275 if ( Options["MultiPV"] == 1
277 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
278 && rootMoves[0].pv[0] != MOVE_NONE)
280 std::map<Move, int64_t> votes;
281 Value minScore = this->rootMoves[0].score;
283 // Find out minimum score
284 for (Thread* th: Threads)
285 minScore = std::min(minScore, th->rootMoves[0].score);
287 // Vote according to score and depth, and select the best thread
288 for (Thread* th : Threads)
290 votes[th->rootMoves[0].pv[0]] +=
291 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
293 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
295 // Make sure we pick the shortest mate
296 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
299 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
300 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
305 previousScore = bestThread->rootMoves[0].score;
307 // Send again PV info if we have a new best thread
308 if (bestThread != this)
309 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
311 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
313 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
314 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
316 std::cout << sync_endl;
320 /// Thread::search() is the main iterative deepening loop. It calls search()
321 /// repeatedly with increasing depth until the allocated thinking time has been
322 /// consumed, the user stops the search, or the maximum search depth is reached.
324 void Thread::search() {
326 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
327 // The former is needed to allow update_continuation_histories(ss-1, ...),
328 // which accesses its argument at ss-6, also near the root.
329 // The latter is needed for statScores and killer initialization.
330 Stack stack[MAX_PLY+10], *ss = stack+7;
332 Value bestValue, alpha, beta, delta;
333 Move lastBestMove = MOVE_NONE;
334 Depth lastBestMoveDepth = 0;
335 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
336 double timeReduction = 1, totBestMoveChanges = 0;
337 Color us = rootPos.side_to_move();
340 std::memset(ss-7, 0, 10 * sizeof(Stack));
341 for (int i = 7; i > 0; i--)
342 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
346 bestValue = delta = alpha = -VALUE_INFINITE;
347 beta = VALUE_INFINITE;
351 if (mainThread->previousScore == VALUE_INFINITE)
352 for (int i=0; i<4; ++i)
353 mainThread->iterValue[i] = VALUE_ZERO;
355 for (int i=0; i<4; ++i)
356 mainThread->iterValue[i] = mainThread->previousScore;
359 size_t multiPV = Options["MultiPV"];
361 // Pick integer skill levels, but non-deterministically round up or down
362 // such that the average integer skill corresponds to the input floating point one.
363 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
364 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
365 // for match (TC 60+0.6) results spanning a wide range of k values.
367 double floatLevel = Options["UCI_LimitStrength"] ?
368 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
369 double(Options["Skill Level"]);
370 int intLevel = int(floatLevel) +
371 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
372 Skill skill(intLevel);
374 // When playing with strength handicap enable MultiPV search that we will
375 // use behind the scenes to retrieve a set of possible moves.
377 multiPV = std::max(multiPV, (size_t)4);
379 multiPV = std::min(multiPV, rootMoves.size());
380 ttHitAverage = ttHitAverageWindow * ttHitAverageResolution / 2;
382 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
384 // In analysis mode, adjust contempt in accordance with user preference
385 if (Limits.infinite || Options["UCI_AnalyseMode"])
386 ct = Options["Analysis Contempt"] == "Off" ? 0
387 : Options["Analysis Contempt"] == "Both" ? ct
388 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
389 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
392 // Evaluation score is from the white point of view
393 contempt = (us == WHITE ? make_score(ct, ct / 2)
394 : -make_score(ct, ct / 2));
396 int searchAgainCounter = 0;
398 // Iterative deepening loop until requested to stop or the target depth is reached
399 while ( ++rootDepth < MAX_PLY
401 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
403 // Age out PV variability metric
405 totBestMoveChanges /= 2;
407 // Save the last iteration's scores before first PV line is searched and
408 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
409 for (RootMove& rm : rootMoves)
410 rm.previousScore = rm.score;
415 if (!Threads.increaseDepth)
416 searchAgainCounter++;
418 // MultiPV loop. We perform a full root search for each PV line
419 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
424 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
425 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
429 // Reset UCI info selDepth for each depth and each PV line
432 // Reset aspiration window starting size
435 Value previousScore = rootMoves[pvIdx].previousScore;
436 delta = Value(21 + abs(previousScore) / 256);
437 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
438 beta = std::min(previousScore + delta, VALUE_INFINITE);
440 // Adjust contempt based on root move's previousScore (dynamic contempt)
441 int dct = ct + (102 - ct / 2) * previousScore / (abs(previousScore) + 157);
443 contempt = (us == WHITE ? make_score(dct, dct / 2)
444 : -make_score(dct, dct / 2));
447 // Start with a small aspiration window and, in the case of a fail
448 // high/low, re-search with a bigger window until we don't fail
450 int failedHighCnt = 0;
453 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
454 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
456 // Bring the best move to the front. It is critical that sorting
457 // is done with a stable algorithm because all the values but the
458 // first and eventually the new best one are set to -VALUE_INFINITE
459 // and we want to keep the same order for all the moves except the
460 // new PV that goes to the front. Note that in case of MultiPV
461 // search the already searched PV lines are preserved.
462 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
464 // If search has been stopped, we break immediately. Sorting is
465 // safe because RootMoves is still valid, although it refers to
466 // the previous iteration.
470 // When failing high/low give some update (without cluttering
471 // the UI) before a re-search.
474 && (bestValue <= alpha || bestValue >= beta)
475 && Time.elapsed() > 3000)
476 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
478 // In case of failing low/high increase aspiration window and
479 // re-search, otherwise exit the loop.
480 if (bestValue <= alpha)
482 beta = (alpha + beta) / 2;
483 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
487 mainThread->stopOnPonderhit = false;
489 else if (bestValue >= beta)
491 beta = std::min(bestValue + delta, VALUE_INFINITE);
496 ++rootMoves[pvIdx].bestMoveCount;
500 delta += delta / 4 + 5;
502 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
505 // Sort the PV lines searched so far and update the GUI
506 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
509 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
510 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
514 completedDepth = rootDepth;
516 if (rootMoves[0].pv[0] != lastBestMove) {
517 lastBestMove = rootMoves[0].pv[0];
518 lastBestMoveDepth = rootDepth;
521 // Have we found a "mate in x"?
523 && bestValue >= VALUE_MATE_IN_MAX_PLY
524 && VALUE_MATE - bestValue <= 2 * Limits.mate)
530 // If skill level is enabled and time is up, pick a sub-optimal best move
531 if (skill.enabled() && skill.time_to_pick(rootDepth))
532 skill.pick_best(multiPV);
534 // Do we have time for the next iteration? Can we stop searching now?
535 if ( Limits.use_time_management()
537 && !mainThread->stopOnPonderhit)
539 double fallingEval = (332 + 6 * (mainThread->previousScore - bestValue)
540 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 704.0;
541 fallingEval = clamp(fallingEval, 0.5, 1.5);
543 // If the bestMove is stable over several iterations, reduce time accordingly
544 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.94 : 0.91;
545 double reduction = (1.41 + mainThread->previousTimeReduction) / (2.27 * timeReduction);
547 // Use part of the gained time from a previous stable move for the current move
548 for (Thread* th : Threads)
550 totBestMoveChanges += th->bestMoveChanges;
551 th->bestMoveChanges = 0;
553 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
555 // Stop the search if we have only one legal move, or if available time elapsed
556 if ( rootMoves.size() == 1
557 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
559 // If we are allowed to ponder do not stop the search now but
560 // keep pondering until the GUI sends "ponderhit" or "stop".
561 if (mainThread->ponder)
562 mainThread->stopOnPonderhit = true;
566 else if ( Threads.increaseDepth
567 && !mainThread->ponder
568 && Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability * 0.6)
569 Threads.increaseDepth = false;
571 Threads.increaseDepth = true;
574 mainThread->iterValue[iterIdx] = bestValue;
575 iterIdx = (iterIdx + 1) & 3;
581 mainThread->previousTimeReduction = timeReduction;
583 // If skill level is enabled, swap best PV line with the sub-optimal one
585 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
586 skill.best ? skill.best : skill.pick_best(multiPV)));
592 // search<>() is the main search function for both PV and non-PV nodes
594 template <NodeType NT>
595 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
597 constexpr bool PvNode = NT == PV;
598 const bool rootNode = PvNode && ss->ply == 0;
600 // Check if we have an upcoming move which draws by repetition, or
601 // if the opponent had an alternative move earlier to this position.
602 if ( pos.rule50_count() >= 3
603 && alpha < VALUE_DRAW
605 && pos.has_game_cycle(ss->ply))
607 alpha = value_draw(pos.this_thread());
612 // Dive into quiescence search when the depth reaches zero
614 return qsearch<NT>(pos, ss, alpha, beta);
616 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
617 assert(PvNode || (alpha == beta - 1));
618 assert(0 < depth && depth < MAX_PLY);
619 assert(!(PvNode && cutNode));
621 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
625 Move ttMove, move, excludedMove, bestMove;
626 Depth extension, newDepth;
627 Value bestValue, value, ttValue, eval, maxValue;
628 bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
629 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
631 int moveCount, captureCount, quietCount;
633 // Step 1. Initialize node
634 Thread* thisThread = pos.this_thread();
635 inCheck = pos.checkers();
636 priorCapture = pos.captured_piece();
637 Color us = pos.side_to_move();
638 moveCount = captureCount = quietCount = ss->moveCount = 0;
639 bestValue = -VALUE_INFINITE;
640 maxValue = VALUE_INFINITE;
642 // Check for the available remaining time
643 if (thisThread == Threads.main())
644 static_cast<MainThread*>(thisThread)->check_time();
646 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
647 if (PvNode && thisThread->selDepth < ss->ply + 1)
648 thisThread->selDepth = ss->ply + 1;
652 // Step 2. Check for aborted search and immediate draw
653 if ( Threads.stop.load(std::memory_order_relaxed)
654 || pos.is_draw(ss->ply)
655 || ss->ply >= MAX_PLY)
656 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
657 : value_draw(pos.this_thread());
659 // Step 3. Mate distance pruning. Even if we mate at the next move our score
660 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
661 // a shorter mate was found upward in the tree then there is no need to search
662 // because we will never beat the current alpha. Same logic but with reversed
663 // signs applies also in the opposite condition of being mated instead of giving
664 // mate. In this case return a fail-high score.
665 alpha = std::max(mated_in(ss->ply), alpha);
666 beta = std::min(mate_in(ss->ply+1), beta);
671 assert(0 <= ss->ply && ss->ply < MAX_PLY);
673 (ss+1)->ply = ss->ply + 1;
674 (ss+1)->excludedMove = bestMove = MOVE_NONE;
675 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
676 Square prevSq = to_sq((ss-1)->currentMove);
678 // Initialize statScore to zero for the grandchildren of the current position.
679 // So statScore is shared between all grandchildren and only the first grandchild
680 // starts with statScore = 0. Later grandchildren start with the last calculated
681 // statScore of the previous grandchild. This influences the reduction rules in
682 // LMR which are based on the statScore of parent position.
684 (ss+4)->statScore = 0;
686 (ss+2)->statScore = 0;
688 // Step 4. Transposition table lookup. We don't want the score of a partial
689 // search to overwrite a previous full search TT value, so we use a different
690 // position key in case of an excluded move.
691 excludedMove = ss->excludedMove;
692 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
693 tte = TT.probe(posKey, ttHit);
694 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
695 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
696 : ttHit ? tte->move() : MOVE_NONE;
697 ttPv = PvNode || (ttHit && tte->is_pv());
698 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
699 thisThread->ttHitAverage = (ttHitAverageWindow - 1) * thisThread->ttHitAverage / ttHitAverageWindow
700 + ttHitAverageResolution * ttHit;
702 // At non-PV nodes we check for an early TT cutoff
705 && tte->depth() >= depth
706 && ttValue != VALUE_NONE // Possible in case of TT access race
707 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
708 : (tte->bound() & BOUND_UPPER)))
710 // If ttMove is quiet, update move sorting heuristics on TT hit
715 if (!pos.capture_or_promotion(ttMove))
716 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
718 // Extra penalty for early quiet moves of the previous ply
719 if ((ss-1)->moveCount <= 2 && !priorCapture)
720 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
722 // Penalty for a quiet ttMove that fails low
723 else if (!pos.capture_or_promotion(ttMove))
725 int penalty = -stat_bonus(depth);
726 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
727 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
731 if (pos.rule50_count() < 90)
735 // Step 5. Tablebases probe
736 if (!rootNode && TB::Cardinality)
738 int piecesCount = pos.count<ALL_PIECES>();
740 if ( piecesCount <= TB::Cardinality
741 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
742 && pos.rule50_count() == 0
743 && !pos.can_castle(ANY_CASTLING))
746 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
748 // Force check of time on the next occasion
749 if (thisThread == Threads.main())
750 static_cast<MainThread*>(thisThread)->callsCnt = 0;
752 if (err != TB::ProbeState::FAIL)
754 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
756 int drawScore = TB::UseRule50 ? 1 : 0;
758 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
759 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
760 : VALUE_DRAW + 2 * wdl * drawScore;
762 Bound b = wdl < -drawScore ? BOUND_UPPER
763 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
765 if ( b == BOUND_EXACT
766 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
768 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
769 std::min(MAX_PLY - 1, depth + 6),
770 MOVE_NONE, VALUE_NONE);
777 if (b == BOUND_LOWER)
778 bestValue = value, alpha = std::max(alpha, bestValue);
786 // Step 6. Static evaluation of the position
789 ss->staticEval = eval = VALUE_NONE;
791 goto moves_loop; // Skip early pruning when in check
795 // Never assume anything about values stored in TT
796 ss->staticEval = eval = tte->eval();
797 if (eval == VALUE_NONE)
798 ss->staticEval = eval = evaluate(pos);
800 if (eval == VALUE_DRAW)
801 eval = value_draw(thisThread);
803 // Can ttValue be used as a better position evaluation?
804 if ( ttValue != VALUE_NONE
805 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
810 if ((ss-1)->currentMove != MOVE_NULL)
812 int bonus = -(ss-1)->statScore / 512;
814 ss->staticEval = eval = evaluate(pos) + bonus;
817 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
819 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
822 // Step 7. Razoring (~1 Elo)
823 if ( !rootNode // The required rootNode PV handling is not available in qsearch
825 && eval <= alpha - RazorMargin)
826 return qsearch<NT>(pos, ss, alpha, beta);
828 improving = (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval > (ss-4)->staticEval
829 || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval > (ss-2)->staticEval;
831 // Step 8. Futility pruning: child node (~50 Elo)
834 && eval - futility_margin(depth, improving) >= beta
835 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
838 // Step 9. Null move search with verification search (~40 Elo)
840 && (ss-1)->currentMove != MOVE_NULL
841 && (ss-1)->statScore < 23397
843 && eval >= ss->staticEval
844 && ss->staticEval >= beta - 32 * depth - 30 * improving + 120 * ttPv + 292
846 && pos.non_pawn_material(us)
847 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
849 assert(eval - beta >= 0);
851 // Null move dynamic reduction based on depth and value
852 Depth R = (854 + 68 * depth) / 258 + std::min(int(eval - beta) / 192, 3);
854 ss->currentMove = MOVE_NULL;
855 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
857 pos.do_null_move(st);
859 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
861 pos.undo_null_move();
863 if (nullValue >= beta)
865 // Do not return unproven mate scores
866 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
869 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
872 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
874 // Do verification search at high depths, with null move pruning disabled
875 // for us, until ply exceeds nmpMinPly.
876 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
877 thisThread->nmpColor = us;
879 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
881 thisThread->nmpMinPly = 0;
888 // Step 10. ProbCut (~10 Elo)
889 // If we have a good enough capture and a reduced search returns a value
890 // much above beta, we can (almost) safely prune the previous move.
893 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
895 Value raisedBeta = std::min(beta + 189 - 45 * improving, VALUE_INFINITE);
896 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
897 int probCutCount = 0;
899 while ( (move = mp.next_move()) != MOVE_NONE
900 && probCutCount < 2 + 2 * cutNode)
901 if (move != excludedMove && pos.legal(move))
903 assert(pos.capture_or_promotion(move));
906 captureOrPromotion = true;
909 ss->currentMove = move;
910 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
912 [pos.moved_piece(move)]
915 pos.do_move(move, st);
917 // Perform a preliminary qsearch to verify that the move holds
918 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
920 // If the qsearch held, perform the regular search
921 if (value >= raisedBeta)
922 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
926 if (value >= raisedBeta)
931 // Step 11. Internal iterative deepening (~1 Elo)
932 if (depth >= 7 && !ttMove)
934 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
936 tte = TT.probe(posKey, ttHit);
937 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
938 ttMove = ttHit ? tte->move() : MOVE_NONE;
941 moves_loop: // When in check, search starts from here
943 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
944 nullptr , (ss-4)->continuationHistory,
945 nullptr , (ss-6)->continuationHistory };
947 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
949 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
950 &thisThread->captureHistory,
956 singularLMR = moveCountPruning = false;
957 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
959 // Mark this node as being searched
960 ThreadHolding th(thisThread, posKey, ss->ply);
962 // Step 12. Loop through all pseudo-legal moves until no moves remain
963 // or a beta cutoff occurs.
964 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
968 if (move == excludedMove)
971 // At root obey the "searchmoves" option and skip moves not listed in Root
972 // Move List. As a consequence any illegal move is also skipped. In MultiPV
973 // mode we also skip PV moves which have been already searched and those
974 // of lower "TB rank" if we are in a TB root position.
975 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
976 thisThread->rootMoves.begin() + thisThread->pvLast, move))
979 ss->moveCount = ++moveCount;
981 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
982 sync_cout << "info depth " << depth
983 << " currmove " << UCI::move(move, pos.is_chess960())
984 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
986 (ss+1)->pv = nullptr;
989 captureOrPromotion = pos.capture_or_promotion(move);
990 movedPiece = pos.moved_piece(move);
991 givesCheck = pos.gives_check(move);
993 // Calculate new depth for this move
994 newDepth = depth - 1;
996 // Step 13. Pruning at shallow depth (~200 Elo)
998 && pos.non_pawn_material(us)
999 && bestValue > VALUE_MATED_IN_MAX_PLY)
1001 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1002 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1004 if ( !captureOrPromotion
1007 // Reduced depth of the next LMR search
1008 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1010 // Countermoves based pruning (~20 Elo)
1011 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1012 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1013 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1016 // Futility pruning: parent node (~5 Elo)
1019 && ss->staticEval + 235 + 172 * lmrDepth <= alpha
1020 && thisThread->mainHistory[us][from_to(move)]
1021 + (*contHist[0])[movedPiece][to_sq(move)]
1022 + (*contHist[1])[movedPiece][to_sq(move)]
1023 + (*contHist[3])[movedPiece][to_sq(move)] < 25000)
1026 // Prune moves with negative SEE (~20 Elo)
1027 if (!pos.see_ge(move, Value(-(32 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1030 else if (!pos.see_ge(move, Value(-194) * depth)) // (~25 Elo)
1034 // Step 14. Extensions (~75 Elo)
1036 // Singular extension search (~70 Elo). If all moves but one fail low on a
1037 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1038 // then that move is singular and should be extended. To verify this we do
1039 // a reduced search on all the other moves but the ttMove and if the
1040 // result is lower than ttValue minus a margin then we will extend the ttMove.
1044 && !excludedMove // Avoid recursive singular search
1045 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1046 && abs(ttValue) < VALUE_KNOWN_WIN
1047 && (tte->bound() & BOUND_LOWER)
1048 && tte->depth() >= depth - 3
1051 Value singularBeta = ttValue - (((ttPv && !PvNode) + 4) * depth) / 2;
1052 Depth halfDepth = depth / 2;
1053 ss->excludedMove = move;
1054 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
1055 ss->excludedMove = MOVE_NONE;
1057 if (value < singularBeta)
1063 // Multi-cut pruning
1064 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1065 // search without the ttMove. So we assume this expected Cut-node is not singular,
1066 // that multiple moves fail high, and we can prune the whole subtree by returning
1068 else if (singularBeta >= beta)
1069 return singularBeta;
1072 // Check extension (~2 Elo)
1073 else if ( givesCheck
1074 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1077 // Passed pawn extension
1078 else if ( move == ss->killers[0]
1079 && pos.advanced_pawn_push(move)
1080 && pos.pawn_passed(us, to_sq(move)))
1083 // Last captures extension
1084 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1085 && pos.non_pawn_material() <= 2 * RookValueMg)
1088 // Castling extension
1089 if (type_of(move) == CASTLING)
1092 // Add extension to new depth
1093 newDepth += extension;
1095 // Speculative prefetch as early as possible
1096 prefetch(TT.first_entry(pos.key_after(move)));
1098 // Check for legality just before making the move
1099 if (!rootNode && !pos.legal(move))
1101 ss->moveCount = --moveCount;
1105 // Update the current move (this must be done after singular extension search)
1106 ss->currentMove = move;
1107 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1108 [captureOrPromotion]
1112 // Step 15. Make the move
1113 pos.do_move(move, st, givesCheck);
1115 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1116 // re-searched at full depth.
1118 && moveCount > 1 + 2 * rootNode
1119 && (!rootNode || thisThread->best_move_count(move) == 0)
1120 && ( !captureOrPromotion
1122 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1124 || thisThread->ttHitAverage < 375 * ttHitAverageResolution * ttHitAverageWindow / 1024))
1126 Depth r = reduction(improving, depth, moveCount);
1128 // Decrease reduction if the ttHit running average is large
1129 if (thisThread->ttHitAverage > 500 * ttHitAverageResolution * ttHitAverageWindow / 1024)
1132 // Reduction if other threads are searching this position.
1136 // Decrease reduction if position is or has been on the PV (~10 Elo)
1140 // Decrease reduction if opponent's move count is high (~5 Elo)
1141 if ((ss-1)->moveCount > 14)
1144 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1148 if (!captureOrPromotion)
1150 // Increase reduction if ttMove is a capture (~5 Elo)
1154 // Increase reduction for cut nodes (~10 Elo)
1158 // Decrease reduction for moves that escape a capture. Filter out
1159 // castling moves, because they are coded as "king captures rook" and
1160 // hence break make_move(). (~2 Elo)
1161 else if ( type_of(move) == NORMAL
1162 && !pos.see_ge(reverse_move(move)))
1165 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1166 + (*contHist[0])[movedPiece][to_sq(move)]
1167 + (*contHist[1])[movedPiece][to_sq(move)]
1168 + (*contHist[3])[movedPiece][to_sq(move)]
1171 // Reset statScore to zero if negative and most stats shows >= 0
1172 if ( ss->statScore < 0
1173 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1174 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1175 && thisThread->mainHistory[us][from_to(move)] >= 0)
1178 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1179 if (ss->statScore >= -102 && (ss-1)->statScore < -114)
1182 else if ((ss-1)->statScore >= -116 && ss->statScore < -154)
1185 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1186 r -= ss->statScore / 16384;
1189 // Increase reduction for captures/promotions if late move and at low depth
1190 else if (depth < 8 && moveCount > 2)
1193 Depth d = clamp(newDepth - r, 1, newDepth);
1195 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1197 doFullDepthSearch = (value > alpha && d != newDepth), didLMR = true;
1200 doFullDepthSearch = !PvNode || moveCount > 1, didLMR = false;
1202 // Step 17. Full depth search when LMR is skipped or fails high
1203 if (doFullDepthSearch)
1205 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1207 if (didLMR && !captureOrPromotion)
1209 int bonus = value > alpha ? stat_bonus(newDepth)
1210 : -stat_bonus(newDepth);
1212 if (move == ss->killers[0])
1215 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1219 // For PV nodes only, do a full PV search on the first move or after a fail
1220 // high (in the latter case search only if value < beta), otherwise let the
1221 // parent node fail low with value <= alpha and try another move.
1222 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1225 (ss+1)->pv[0] = MOVE_NONE;
1227 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1230 // Step 18. Undo move
1231 pos.undo_move(move);
1233 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1235 // Step 19. Check for a new best move
1236 // Finished searching the move. If a stop occurred, the return value of
1237 // the search cannot be trusted, and we return immediately without
1238 // updating best move, PV and TT.
1239 if (Threads.stop.load(std::memory_order_relaxed))
1244 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1245 thisThread->rootMoves.end(), move);
1247 // PV move or new best move?
1248 if (moveCount == 1 || value > alpha)
1251 rm.selDepth = thisThread->selDepth;
1256 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1257 rm.pv.push_back(*m);
1259 // We record how often the best move has been changed in each
1260 // iteration. This information is used for time management: When
1261 // the best move changes frequently, we allocate some more time.
1263 ++thisThread->bestMoveChanges;
1266 // All other moves but the PV are set to the lowest value: this
1267 // is not a problem when sorting because the sort is stable and the
1268 // move position in the list is preserved - just the PV is pushed up.
1269 rm.score = -VALUE_INFINITE;
1272 if (value > bestValue)
1280 if (PvNode && !rootNode) // Update pv even in fail-high case
1281 update_pv(ss->pv, move, (ss+1)->pv);
1283 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1287 assert(value >= beta); // Fail high
1294 if (move != bestMove)
1296 if (captureOrPromotion && captureCount < 32)
1297 capturesSearched[captureCount++] = move;
1299 else if (!captureOrPromotion && quietCount < 64)
1300 quietsSearched[quietCount++] = move;
1304 // The following condition would detect a stop only after move loop has been
1305 // completed. But in this case bestValue is valid because we have fully
1306 // searched our subtree, and we can anyhow save the result in TT.
1312 // Step 20. Check for mate and stalemate
1313 // All legal moves have been searched and if there are no legal moves, it
1314 // must be a mate or a stalemate. If we are in a singular extension search then
1315 // return a fail low score.
1317 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1320 bestValue = excludedMove ? alpha
1321 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1324 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1325 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1327 // Bonus for prior countermove that caused the fail low
1328 else if ( (depth >= 3 || PvNode)
1330 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1333 bestValue = std::min(bestValue, maxValue);
1336 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1337 bestValue >= beta ? BOUND_LOWER :
1338 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1339 depth, bestMove, ss->staticEval);
1341 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1347 // qsearch() is the quiescence search function, which is called by the main search
1348 // function with zero depth, or recursively with further decreasing depth per call.
1349 template <NodeType NT>
1350 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1352 constexpr bool PvNode = NT == PV;
1354 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1355 assert(PvNode || (alpha == beta - 1));
1362 Move ttMove, move, bestMove;
1364 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1365 bool ttHit, pvHit, inCheck, givesCheck, captureOrPromotion, evasionPrunable;
1370 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1372 ss->pv[0] = MOVE_NONE;
1375 Thread* thisThread = pos.this_thread();
1376 (ss+1)->ply = ss->ply + 1;
1377 bestMove = MOVE_NONE;
1378 inCheck = pos.checkers();
1381 // Check for an immediate draw or maximum ply reached
1382 if ( pos.is_draw(ss->ply)
1383 || ss->ply >= MAX_PLY)
1384 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1386 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1388 // Decide whether or not to include checks: this fixes also the type of
1389 // TT entry depth that we are going to use. Note that in qsearch we use
1390 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1391 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1392 : DEPTH_QS_NO_CHECKS;
1393 // Transposition table lookup
1395 tte = TT.probe(posKey, ttHit);
1396 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1397 ttMove = ttHit ? tte->move() : MOVE_NONE;
1398 pvHit = ttHit && tte->is_pv();
1402 && tte->depth() >= ttDepth
1403 && ttValue != VALUE_NONE // Only in case of TT access race
1404 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1405 : (tte->bound() & BOUND_UPPER)))
1408 // Evaluate the position statically
1411 ss->staticEval = VALUE_NONE;
1412 bestValue = futilityBase = -VALUE_INFINITE;
1418 // Never assume anything about values stored in TT
1419 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1420 ss->staticEval = bestValue = evaluate(pos);
1422 // Can ttValue be used as a better position evaluation?
1423 if ( ttValue != VALUE_NONE
1424 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1425 bestValue = ttValue;
1428 ss->staticEval = bestValue =
1429 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1430 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1432 // Stand pat. Return immediately if static value is at least beta
1433 if (bestValue >= beta)
1436 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1437 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1442 if (PvNode && bestValue > alpha)
1445 futilityBase = bestValue + 154;
1448 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1449 nullptr , (ss-4)->continuationHistory,
1450 nullptr , (ss-6)->continuationHistory };
1452 // Initialize a MovePicker object for the current position, and prepare
1453 // to search the moves. Because the depth is <= 0 here, only captures,
1454 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1456 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1457 &thisThread->captureHistory,
1459 to_sq((ss-1)->currentMove));
1461 // Loop through the moves until no moves remain or a beta cutoff occurs
1462 while ((move = mp.next_move()) != MOVE_NONE)
1464 assert(is_ok(move));
1466 givesCheck = pos.gives_check(move);
1467 captureOrPromotion = pos.capture_or_promotion(move);
1474 && futilityBase > -VALUE_KNOWN_WIN
1475 && !pos.advanced_pawn_push(move))
1477 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1479 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1481 if (futilityValue <= alpha)
1483 bestValue = std::max(bestValue, futilityValue);
1487 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1489 bestValue = std::max(bestValue, futilityBase);
1494 // Detect non-capture evasions that are candidates to be pruned
1495 evasionPrunable = inCheck
1496 && (depth != 0 || moveCount > 2)
1497 && bestValue > VALUE_MATED_IN_MAX_PLY
1498 && !pos.capture(move);
1500 // Don't search moves with negative SEE values
1501 if ( (!inCheck || evasionPrunable) && !pos.see_ge(move))
1504 // Speculative prefetch as early as possible
1505 prefetch(TT.first_entry(pos.key_after(move)));
1507 // Check for legality just before making the move
1508 if (!pos.legal(move))
1514 ss->currentMove = move;
1515 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1516 [captureOrPromotion]
1517 [pos.moved_piece(move)]
1520 // Make and search the move
1521 pos.do_move(move, st, givesCheck);
1522 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1523 pos.undo_move(move);
1525 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1527 // Check for a new best move
1528 if (value > bestValue)
1536 if (PvNode) // Update pv even in fail-high case
1537 update_pv(ss->pv, move, (ss+1)->pv);
1539 if (PvNode && value < beta) // Update alpha here!
1547 // All legal moves have been searched. A special case: If we're in check
1548 // and no legal moves were found, it is checkmate.
1549 if (inCheck && bestValue == -VALUE_INFINITE)
1550 return mated_in(ss->ply); // Plies to mate from the root
1552 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1553 bestValue >= beta ? BOUND_LOWER :
1554 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1555 ttDepth, bestMove, ss->staticEval);
1557 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1563 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1564 // "plies to mate from the current position". Non-mate scores are unchanged.
1565 // The function is called before storing a value in the transposition table.
1567 Value value_to_tt(Value v, int ply) {
1569 assert(v != VALUE_NONE);
1571 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1572 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1576 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1577 // from the transposition table (which refers to the plies to mate/be mated
1578 // from current position) to "plies to mate/be mated from the root".
1580 Value value_from_tt(Value v, int ply, int r50c) {
1582 return v == VALUE_NONE ? VALUE_NONE
1583 : v >= VALUE_MATE_IN_MAX_PLY ? VALUE_MATE - v > 99 - r50c ? VALUE_MATE_IN_MAX_PLY : v - ply
1584 : v <= VALUE_MATED_IN_MAX_PLY ? VALUE_MATE + v > 99 - r50c ? VALUE_MATED_IN_MAX_PLY : v + ply : v;
1588 // update_pv() adds current move and appends child pv[]
1590 void update_pv(Move* pv, Move move, Move* childPv) {
1592 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1598 // update_all_stats() updates stats at the end of search() when a bestMove is found
1600 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1601 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1604 Color us = pos.side_to_move();
1605 Thread* thisThread = pos.this_thread();
1606 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1607 Piece moved_piece = pos.moved_piece(bestMove);
1608 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1610 bonus1 = stat_bonus(depth + 1);
1611 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1612 : stat_bonus(depth); // smaller bonus
1614 if (!pos.capture_or_promotion(bestMove))
1616 update_quiet_stats(pos, ss, bestMove, bonus2);
1618 // Decrease all the non-best quiet moves
1619 for (int i = 0; i < quietCount; ++i)
1621 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1622 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1626 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1628 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1629 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1630 && !pos.captured_piece())
1631 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1633 // Decrease all the non-best capture moves
1634 for (int i = 0; i < captureCount; ++i)
1636 moved_piece = pos.moved_piece(capturesSearched[i]);
1637 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1638 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1643 // update_continuation_histories() updates histories of the move pairs formed
1644 // by moves at ply -1, -2, and -4 with current move.
1646 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1648 for (int i : {1, 2, 4, 6})
1649 if (is_ok((ss-i)->currentMove))
1650 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1654 // update_quiet_stats() updates move sorting heuristics
1656 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1658 if (ss->killers[0] != move)
1660 ss->killers[1] = ss->killers[0];
1661 ss->killers[0] = move;
1664 Color us = pos.side_to_move();
1665 Thread* thisThread = pos.this_thread();
1666 thisThread->mainHistory[us][from_to(move)] << bonus;
1667 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1669 if (type_of(pos.moved_piece(move)) != PAWN)
1670 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1672 if (is_ok((ss-1)->currentMove))
1674 Square prevSq = to_sq((ss-1)->currentMove);
1675 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1679 // When playing with strength handicap, choose best move among a set of RootMoves
1680 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1682 Move Skill::pick_best(size_t multiPV) {
1684 const RootMoves& rootMoves = Threads.main()->rootMoves;
1685 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1687 // RootMoves are already sorted by score in descending order
1688 Value topScore = rootMoves[0].score;
1689 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1690 int weakness = 120 - 2 * level;
1691 int maxScore = -VALUE_INFINITE;
1693 // Choose best move. For each move score we add two terms, both dependent on
1694 // weakness. One is deterministic and bigger for weaker levels, and one is
1695 // random. Then we choose the move with the resulting highest score.
1696 for (size_t i = 0; i < multiPV; ++i)
1698 // This is our magic formula
1699 int push = ( weakness * int(topScore - rootMoves[i].score)
1700 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1702 if (rootMoves[i].score + push >= maxScore)
1704 maxScore = rootMoves[i].score + push;
1705 best = rootMoves[i].pv[0];
1714 /// MainThread::check_time() is used to print debug info and, more importantly,
1715 /// to detect when we are out of available time and thus stop the search.
1717 void MainThread::check_time() {
1722 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1723 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1725 static TimePoint lastInfoTime = now();
1727 TimePoint elapsed = Time.elapsed();
1728 TimePoint tick = Limits.startTime + elapsed;
1730 if (tick - lastInfoTime >= 1000)
1732 lastInfoTime = tick;
1736 // We should not stop pondering until told so by the GUI
1740 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1741 || (Limits.movetime && elapsed >= Limits.movetime)
1742 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1743 Threads.stop = true;
1747 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1748 /// that all (if any) unsearched PV lines are sent using a previous search score.
1750 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1752 std::stringstream ss;
1753 TimePoint elapsed = Time.elapsed() + 1;
1754 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1755 size_t pvIdx = pos.this_thread()->pvIdx;
1756 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1757 uint64_t nodesSearched = Threads.nodes_searched();
1758 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1760 for (size_t i = 0; i < multiPV; ++i)
1762 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1764 if (depth == 1 && !updated)
1767 Depth d = updated ? depth : depth - 1;
1768 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1770 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1771 v = tb ? rootMoves[i].tbScore : v;
1773 if (ss.rdbuf()->in_avail()) // Not at first line
1778 << " seldepth " << rootMoves[i].selDepth
1779 << " multipv " << i + 1
1780 << " score " << UCI::value(v);
1782 if (!tb && i == pvIdx)
1783 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1785 ss << " nodes " << nodesSearched
1786 << " nps " << nodesSearched * 1000 / elapsed;
1788 if (elapsed > 1000) // Earlier makes little sense
1789 ss << " hashfull " << TT.hashfull();
1791 ss << " tbhits " << tbHits
1792 << " time " << elapsed
1795 for (Move m : rootMoves[i].pv)
1796 ss << " " << UCI::move(m, pos.is_chess960());
1803 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1804 /// before exiting the search, for instance, in case we stop the search during a
1805 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1806 /// otherwise in case of 'ponder on' we have nothing to think on.
1808 bool RootMove::extract_ponder_from_tt(Position& pos) {
1813 assert(pv.size() == 1);
1815 if (pv[0] == MOVE_NONE)
1818 pos.do_move(pv[0], st);
1819 TTEntry* tte = TT.probe(pos.key(), ttHit);
1823 Move m = tte->move(); // Local copy to be SMP safe
1824 if (MoveList<LEGAL>(pos).contains(m))
1828 pos.undo_move(pv[0]);
1829 return pv.size() > 1;
1832 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1835 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1836 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1837 Cardinality = int(Options["SyzygyProbeLimit"]);
1838 bool dtz_available = true;
1840 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1841 // ProbeDepth == DEPTH_ZERO
1842 if (Cardinality > MaxCardinality)
1844 Cardinality = MaxCardinality;
1848 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1850 // Rank moves using DTZ tables
1851 RootInTB = root_probe(pos, rootMoves);
1855 // DTZ tables are missing; try to rank moves using WDL tables
1856 dtz_available = false;
1857 RootInTB = root_probe_wdl(pos, rootMoves);
1863 // Sort moves according to TB rank
1864 std::sort(rootMoves.begin(), rootMoves.end(),
1865 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1867 // Probe during search only if DTZ is not available and we are winning
1868 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1873 // Clean up if root_probe() and root_probe_wdl() have failed
1874 for (auto& m : rootMoves)