2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
22 #include <cstring> // For std::memset
36 #include "syzygy/tbprobe.h"
43 namespace Tablebases {
51 namespace TB = Tablebases;
55 using namespace Search;
59 // Different node types, used as a template parameter
60 enum NodeType { NonPV, PV };
62 constexpr uint64_t TtHitAverageWindow = 4096;
63 constexpr uint64_t TtHitAverageResolution = 1024;
65 // Razor and futility margins
66 constexpr int RazorMargin = 510;
67 Value futility_margin(Depth d, bool improving) {
68 return Value(223 * (d - improving));
71 // Reductions lookup table, initialized at startup
72 int Reductions[MAX_MOVES]; // [depth or moveNumber]
74 Depth reduction(bool i, Depth d, int mn) {
75 int r = Reductions[d] * Reductions[mn];
76 return (r + 509) / 1024 + (!i && r > 894);
79 constexpr int futility_move_count(bool improving, Depth depth) {
80 return (3 + depth * depth) / (2 - improving);
83 // History and stats update bonus, based on depth
84 int stat_bonus(Depth d) {
85 return d > 13 ? 29 : 17 * d * d + 134 * d - 134;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Thread* thisThread) {
90 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
93 // Skill structure is used to implement strength limit
95 explicit Skill(int l) : level(l) {}
96 bool enabled() const { return level < 20; }
97 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
98 Move pick_best(size_t multiPV);
101 Move best = MOVE_NONE;
104 // Breadcrumbs are used to mark nodes as being searched by a given thread
106 std::atomic<Thread*> thread;
107 std::atomic<Key> key;
109 std::array<Breadcrumb, 1024> breadcrumbs;
111 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
112 // node for potential reductions. A free node will be marked upon entering the moves
113 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
114 struct ThreadHolding {
115 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
116 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
121 // See if another already marked this location, if not, mark it ourselves
122 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
125 (*location).thread.store(thisThread, std::memory_order_relaxed);
126 (*location).key.store(posKey, std::memory_order_relaxed);
129 else if ( tmp != thisThread
130 && (*location).key.load(std::memory_order_relaxed) == posKey)
136 if (owning) // Free the marked location
137 (*location).thread.store(nullptr, std::memory_order_relaxed);
140 bool marked() { return otherThread; }
143 Breadcrumb* location;
144 bool otherThread, owning;
147 template <NodeType NT>
148 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
150 template <NodeType NT>
151 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
153 Value value_to_tt(Value v, int ply);
154 Value value_from_tt(Value v, int ply, int r50c);
155 void update_pv(Move* pv, Move move, Move* childPv);
156 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
157 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
158 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
159 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
161 // perft() is our utility to verify move generation. All the leaf nodes up
162 // to the given depth are generated and counted, and the sum is returned.
164 uint64_t perft(Position& pos, Depth depth) {
167 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
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((22.0 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * 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 Eval::NNUE::verify();
232 if (rootMoves.empty())
234 rootMoves.emplace_back(MOVE_NONE);
235 sync_cout << "info depth 0 score "
236 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
241 Threads.start_searching(); // start non-main threads
242 Thread::search(); // main thread start searching
245 // When we reach the maximum depth, we can arrive here without a raise of
246 // Threads.stop. However, if we are pondering or in an infinite search,
247 // the UCI protocol states that we shouldn't print the best move before the
248 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
249 // until the GUI sends one of those commands.
251 while (!Threads.stop && (ponder || Limits.infinite))
252 {} // Busy wait for a stop or a ponder reset
254 // Stop the threads if not already stopped (also raise the stop if
255 // "ponderhit" just reset Threads.ponder).
258 // Wait until all threads have finished
259 Threads.wait_for_search_finished();
261 // When playing in 'nodes as time' mode, subtract the searched nodes from
262 // the available ones before exiting.
264 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
266 Thread* bestThread = this;
268 if ( int(Options["MultiPV"]) == 1
270 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
271 && rootMoves[0].pv[0] != MOVE_NONE)
272 bestThread = Threads.get_best_thread();
274 bestPreviousScore = bestThread->rootMoves[0].score;
276 // Send again PV info if we have a new best thread
277 if (bestThread != this)
278 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
280 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
282 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
283 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
285 std::cout << sync_endl;
289 /// Thread::search() is the main iterative deepening loop. It calls search()
290 /// repeatedly with increasing depth until the allocated thinking time has been
291 /// consumed, the user stops the search, or the maximum search depth is reached.
293 void Thread::search() {
295 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
296 // The former is needed to allow update_continuation_histories(ss-1, ...),
297 // which accesses its argument at ss-6, also near the root.
298 // The latter is needed for statScores and killer initialization.
299 Stack stack[MAX_PLY+10], *ss = stack+7;
301 Value bestValue, alpha, beta, delta;
302 Move lastBestMove = MOVE_NONE;
303 Depth lastBestMoveDepth = 0;
304 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
305 double timeReduction = 1, totBestMoveChanges = 0;
306 Color us = rootPos.side_to_move();
309 std::memset(ss-7, 0, 10 * sizeof(Stack));
310 for (int i = 7; i > 0; i--)
311 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
315 bestValue = delta = alpha = -VALUE_INFINITE;
316 beta = VALUE_INFINITE;
320 if (mainThread->bestPreviousScore == VALUE_INFINITE)
321 for (int i = 0; i < 4; ++i)
322 mainThread->iterValue[i] = VALUE_ZERO;
324 for (int i = 0; i < 4; ++i)
325 mainThread->iterValue[i] = mainThread->bestPreviousScore;
328 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
329 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
331 size_t multiPV = size_t(Options["MultiPV"]);
333 // Pick integer skill levels, but non-deterministically round up or down
334 // such that the average integer skill corresponds to the input floating point one.
335 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
336 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
337 // for match (TC 60+0.6) results spanning a wide range of k values.
339 double floatLevel = Options["UCI_LimitStrength"] ?
340 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
341 double(Options["Skill Level"]);
342 int intLevel = int(floatLevel) +
343 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
344 Skill skill(intLevel);
346 // When playing with strength handicap enable MultiPV search that we will
347 // use behind the scenes to retrieve a set of possible moves.
349 multiPV = std::max(multiPV, (size_t)4);
351 multiPV = std::min(multiPV, rootMoves.size());
352 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
354 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
356 // In analysis mode, adjust contempt in accordance with user preference
357 if (Limits.infinite || Options["UCI_AnalyseMode"])
358 ct = Options["Analysis Contempt"] == "Off" ? 0
359 : Options["Analysis Contempt"] == "Both" ? ct
360 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
361 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
364 // Evaluation score is from the white point of view
365 contempt = (us == WHITE ? make_score(ct, ct / 2)
366 : -make_score(ct, ct / 2));
368 int searchAgainCounter = 0;
370 // Iterative deepening loop until requested to stop or the target depth is reached
371 while ( ++rootDepth < MAX_PLY
373 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
375 // Age out PV variability metric
377 totBestMoveChanges /= 2;
379 // Save the last iteration's scores before first PV line is searched and
380 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
381 for (RootMove& rm : rootMoves)
382 rm.previousScore = rm.score;
387 if (!Threads.increaseDepth)
388 searchAgainCounter++;
390 // MultiPV loop. We perform a full root search for each PV line
391 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
396 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
397 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
401 // Reset UCI info selDepth for each depth and each PV line
404 // Reset aspiration window starting size
407 Value prev = rootMoves[pvIdx].previousScore;
409 alpha = std::max(prev - delta,-VALUE_INFINITE);
410 beta = std::min(prev + delta, VALUE_INFINITE);
412 // Adjust contempt based on root move's previousScore (dynamic contempt)
413 int dct = ct + (105 - ct / 2) * prev / (abs(prev) + 149);
415 contempt = (us == WHITE ? make_score(dct, dct / 2)
416 : -make_score(dct, dct / 2));
419 // Start with a small aspiration window and, in the case of a fail
420 // high/low, re-search with a bigger window until we don't fail
425 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
426 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
428 // Bring the best move to the front. It is critical that sorting
429 // is done with a stable algorithm because all the values but the
430 // first and eventually the new best one are set to -VALUE_INFINITE
431 // and we want to keep the same order for all the moves except the
432 // new PV that goes to the front. Note that in case of MultiPV
433 // search the already searched PV lines are preserved.
434 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
436 // If search has been stopped, we break immediately. Sorting is
437 // safe because RootMoves is still valid, although it refers to
438 // the previous iteration.
442 // When failing high/low give some update (without cluttering
443 // the UI) before a re-search.
446 && (bestValue <= alpha || bestValue >= beta)
447 && Time.elapsed() > 3000)
448 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
450 // In case of failing low/high increase aspiration window and
451 // re-search, otherwise exit the loop.
452 if (bestValue <= alpha)
454 beta = (alpha + beta) / 2;
455 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
459 mainThread->stopOnPonderhit = false;
461 else if (bestValue >= beta)
463 beta = std::min(bestValue + delta, VALUE_INFINITE);
469 delta += delta / 4 + 5;
471 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
474 // Sort the PV lines searched so far and update the GUI
475 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
478 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
479 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
483 completedDepth = rootDepth;
485 if (rootMoves[0].pv[0] != lastBestMove) {
486 lastBestMove = rootMoves[0].pv[0];
487 lastBestMoveDepth = rootDepth;
490 // Have we found a "mate in x"?
492 && bestValue >= VALUE_MATE_IN_MAX_PLY
493 && VALUE_MATE - bestValue <= 2 * Limits.mate)
499 // If skill level is enabled and time is up, pick a sub-optimal best move
500 if (skill.enabled() && skill.time_to_pick(rootDepth))
501 skill.pick_best(multiPV);
503 // Do we have time for the next iteration? Can we stop searching now?
504 if ( Limits.use_time_management()
506 && !mainThread->stopOnPonderhit)
508 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
509 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
510 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
512 // If the bestMove is stable over several iterations, reduce time accordingly
513 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
514 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
516 // Use part of the gained time from a previous stable move for the current move
517 for (Thread* th : Threads)
519 totBestMoveChanges += th->bestMoveChanges;
520 th->bestMoveChanges = 0;
522 double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
524 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
526 // Cap used time in case of a single legal move for a better viewer experience in tournaments
527 // yielding correct scores and sufficiently fast moves.
528 if (rootMoves.size() == 1)
529 totalTime = std::min(500.0, totalTime);
531 // Stop the search if we have exceeded the totalTime
532 if (Time.elapsed() > totalTime)
534 // If we are allowed to ponder do not stop the search now but
535 // keep pondering until the GUI sends "ponderhit" or "stop".
536 if (mainThread->ponder)
537 mainThread->stopOnPonderhit = true;
541 else if ( Threads.increaseDepth
542 && !mainThread->ponder
543 && Time.elapsed() > totalTime * 0.58)
544 Threads.increaseDepth = false;
546 Threads.increaseDepth = true;
549 mainThread->iterValue[iterIdx] = bestValue;
550 iterIdx = (iterIdx + 1) & 3;
556 mainThread->previousTimeReduction = timeReduction;
558 // If skill level is enabled, swap best PV line with the sub-optimal one
560 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
561 skill.best ? skill.best : skill.pick_best(multiPV)));
567 // search<>() is the main search function for both PV and non-PV nodes
569 template <NodeType NT>
570 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
572 constexpr bool PvNode = NT == PV;
573 const bool rootNode = PvNode && ss->ply == 0;
574 const Depth maxNextDepth = rootNode ? depth : depth + 1;
576 // Check if we have an upcoming move which draws by repetition, or
577 // if the opponent had an alternative move earlier to this position.
578 if ( pos.rule50_count() >= 3
579 && alpha < VALUE_DRAW
581 && pos.has_game_cycle(ss->ply))
583 alpha = value_draw(pos.this_thread());
588 // Dive into quiescence search when the depth reaches zero
590 return qsearch<NT>(pos, ss, alpha, beta);
592 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
593 assert(PvNode || (alpha == beta - 1));
594 assert(0 < depth && depth < MAX_PLY);
595 assert(!(PvNode && cutNode));
597 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
599 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
603 Move ttMove, move, excludedMove, bestMove;
604 Depth extension, newDepth;
605 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
606 bool formerPv, givesCheck, improving, didLMR, priorCapture;
607 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
608 ttCapture, singularQuietLMR;
610 int moveCount, captureCount, quietCount;
612 // Step 1. Initialize node
613 Thread* thisThread = pos.this_thread();
614 ss->inCheck = pos.checkers();
615 priorCapture = pos.captured_piece();
616 Color us = pos.side_to_move();
617 moveCount = captureCount = quietCount = ss->moveCount = 0;
618 bestValue = -VALUE_INFINITE;
619 maxValue = VALUE_INFINITE;
621 // Check for the available remaining time
622 if (thisThread == Threads.main())
623 static_cast<MainThread*>(thisThread)->check_time();
625 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
626 if (PvNode && thisThread->selDepth < ss->ply + 1)
627 thisThread->selDepth = ss->ply + 1;
631 // Step 2. Check for aborted search and immediate draw
632 if ( Threads.stop.load(std::memory_order_relaxed)
633 || pos.is_draw(ss->ply)
634 || ss->ply >= MAX_PLY)
635 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
636 : value_draw(pos.this_thread());
638 // Step 3. Mate distance pruning. Even if we mate at the next move our score
639 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
640 // a shorter mate was found upward in the tree then there is no need to search
641 // because we will never beat the current alpha. Same logic but with reversed
642 // signs applies also in the opposite condition of being mated instead of giving
643 // mate. In this case return a fail-high score.
644 alpha = std::max(mated_in(ss->ply), alpha);
645 beta = std::min(mate_in(ss->ply+1), beta);
650 assert(0 <= ss->ply && ss->ply < MAX_PLY);
652 (ss+1)->ply = ss->ply + 1;
653 (ss+1)->ttPv = false;
654 (ss+1)->excludedMove = bestMove = MOVE_NONE;
655 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
656 Square prevSq = to_sq((ss-1)->currentMove);
658 // Initialize statScore to zero for the grandchildren of the current position.
659 // So statScore is shared between all grandchildren and only the first grandchild
660 // starts with statScore = 0. Later grandchildren start with the last calculated
661 // statScore of the previous grandchild. This influences the reduction rules in
662 // LMR which are based on the statScore of parent position.
664 (ss+2)->statScore = 0;
666 // Step 4. Transposition table lookup. We don't want the score of a partial
667 // search to overwrite a previous full search TT value, so we use a different
668 // position key in case of an excluded move.
669 excludedMove = ss->excludedMove;
670 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
671 tte = TT.probe(posKey, ss->ttHit);
672 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
673 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
674 : ss->ttHit ? tte->move() : MOVE_NONE;
676 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
677 formerPv = ss->ttPv && !PvNode;
681 && ss->ply - 1 < MAX_LPH
683 && is_ok((ss-1)->currentMove))
684 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
686 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
687 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
688 + TtHitAverageResolution * ss->ttHit;
690 // At non-PV nodes we check for an early TT cutoff
693 && tte->depth() >= depth
694 && ttValue != VALUE_NONE // Possible in case of TT access race
695 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
696 : (tte->bound() & BOUND_UPPER)))
698 // If ttMove is quiet, update move sorting heuristics on TT hit
703 if (!pos.capture_or_promotion(ttMove))
704 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
706 // Extra penalty for early quiet moves of the previous ply
707 if ((ss-1)->moveCount <= 2 && !priorCapture)
708 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
710 // Penalty for a quiet ttMove that fails low
711 else if (!pos.capture_or_promotion(ttMove))
713 int penalty = -stat_bonus(depth);
714 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
715 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
719 if (pos.rule50_count() < 90)
723 // Step 5. Tablebases probe
724 if (!rootNode && TB::Cardinality)
726 int piecesCount = pos.count<ALL_PIECES>();
728 if ( piecesCount <= TB::Cardinality
729 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
730 && pos.rule50_count() == 0
731 && !pos.can_castle(ANY_CASTLING))
734 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
736 // Force check of time on the next occasion
737 if (thisThread == Threads.main())
738 static_cast<MainThread*>(thisThread)->callsCnt = 0;
740 if (err != TB::ProbeState::FAIL)
742 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
744 int drawScore = TB::UseRule50 ? 1 : 0;
746 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
747 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
748 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
749 : VALUE_DRAW + 2 * wdl * drawScore;
751 Bound b = wdl < -drawScore ? BOUND_UPPER
752 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
754 if ( b == BOUND_EXACT
755 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
757 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
758 std::min(MAX_PLY - 1, depth + 6),
759 MOVE_NONE, VALUE_NONE);
766 if (b == BOUND_LOWER)
767 bestValue = value, alpha = std::max(alpha, bestValue);
775 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
777 // Step 6. Static evaluation of the position
780 // Skip early pruning when in check
781 ss->staticEval = eval = VALUE_NONE;
787 // Never assume anything about values stored in TT
788 ss->staticEval = eval = tte->eval();
789 if (eval == VALUE_NONE)
790 ss->staticEval = eval = evaluate(pos);
792 if (eval == VALUE_DRAW)
793 eval = value_draw(thisThread);
795 // Can ttValue be used as a better position evaluation?
796 if ( ttValue != VALUE_NONE
797 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
802 if ((ss-1)->currentMove != MOVE_NULL)
803 ss->staticEval = eval = evaluate(pos);
805 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
807 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
810 // Step 7. Razoring (~1 Elo)
811 if ( !rootNode // The required rootNode PV handling is not available in qsearch
813 && eval <= alpha - RazorMargin)
814 return qsearch<NT>(pos, ss, alpha, beta);
816 improving = (ss-2)->staticEval == VALUE_NONE
817 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
818 : ss->staticEval > (ss-2)->staticEval;
820 // Step 8. Futility pruning: child node (~50 Elo)
823 && eval - futility_margin(depth, improving) >= beta
824 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
827 // Step 9. Null move search with verification search (~40 Elo)
829 && (ss-1)->currentMove != MOVE_NULL
830 && (ss-1)->statScore < 22977
832 && eval >= ss->staticEval
833 && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 182
835 && pos.non_pawn_material(us)
836 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
838 assert(eval - beta >= 0);
840 // Null move dynamic reduction based on depth and value
841 Depth R = (982 + 85 * depth) / 256 + std::min(int(eval - beta) / 192, 3);
843 ss->currentMove = MOVE_NULL;
844 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
846 pos.do_null_move(st);
848 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
850 pos.undo_null_move();
852 if (nullValue >= beta)
854 // Do not return unproven mate or TB scores
855 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
858 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
861 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
863 // Do verification search at high depths, with null move pruning disabled
864 // for us, until ply exceeds nmpMinPly.
865 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
866 thisThread->nmpColor = us;
868 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
870 thisThread->nmpMinPly = 0;
877 probCutBeta = beta + 176 - 49 * improving;
879 // Step 10. ProbCut (~10 Elo)
880 // If we have a good enough capture and a reduced search returns a value
881 // much above beta, we can (almost) safely prune the previous move.
884 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
885 // if value from transposition table is lower than probCutBeta, don't attempt probCut
886 // there and in further interactions with transposition table cutoff depth is set to depth - 3
887 // because probCut search has depth set to depth - 4 but we also do a move before it
888 // so effective depth is equal to depth - 3
890 && tte->depth() >= depth - 3
891 && ttValue != VALUE_NONE
892 && ttValue < probCutBeta))
894 // if ttMove is a capture and value from transposition table is good enough produce probCut
895 // cutoff without digging into actual probCut search
897 && tte->depth() >= depth - 3
898 && ttValue != VALUE_NONE
899 && ttValue >= probCutBeta
901 && pos.capture_or_promotion(ttMove))
904 assert(probCutBeta < VALUE_INFINITE);
905 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
906 int probCutCount = 0;
907 bool ttPv = ss->ttPv;
910 while ( (move = mp.next_move()) != MOVE_NONE
911 && probCutCount < 2 + 2 * cutNode)
912 if (move != excludedMove && pos.legal(move))
914 assert(pos.capture_or_promotion(move));
917 captureOrPromotion = true;
920 ss->currentMove = move;
921 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
923 [pos.moved_piece(move)]
926 pos.do_move(move, st);
928 // Perform a preliminary qsearch to verify that the move holds
929 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
931 // If the qsearch held, perform the regular search
932 if (value >= probCutBeta)
933 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
937 if (value >= probCutBeta)
939 // if transposition table doesn't have equal or more deep info write probCut data into it
941 && tte->depth() >= depth - 3
942 && ttValue != VALUE_NONE))
943 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
945 depth - 3, move, ss->staticEval);
952 // Step 11. If the position is not in TT, decrease depth by 2
958 moves_loop: // When in check, search starts from here
960 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
961 nullptr , (ss-4)->continuationHistory,
962 nullptr , (ss-6)->continuationHistory };
964 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
966 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
967 &thisThread->lowPlyHistory,
975 singularQuietLMR = moveCountPruning = false;
976 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
978 // Mark this node as being searched
979 ThreadHolding th(thisThread, posKey, ss->ply);
981 // Step 12. Loop through all pseudo-legal moves until no moves remain
982 // or a beta cutoff occurs.
983 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
987 if (move == excludedMove)
990 // At root obey the "searchmoves" option and skip moves not listed in Root
991 // Move List. As a consequence any illegal move is also skipped. In MultiPV
992 // mode we also skip PV moves which have been already searched and those
993 // of lower "TB rank" if we are in a TB root position.
994 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
995 thisThread->rootMoves.begin() + thisThread->pvLast, move))
998 // Check for legality
999 if (!rootNode && !pos.legal(move))
1002 ss->moveCount = ++moveCount;
1004 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1005 sync_cout << "info depth " << depth
1006 << " currmove " << UCI::move(move, pos.is_chess960())
1007 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1009 (ss+1)->pv = nullptr;
1012 captureOrPromotion = pos.capture_or_promotion(move);
1013 movedPiece = pos.moved_piece(move);
1014 givesCheck = pos.gives_check(move);
1016 // Calculate new depth for this move
1017 newDepth = depth - 1;
1019 // Step 13. Pruning at shallow depth (~200 Elo)
1021 && pos.non_pawn_material(us)
1022 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1024 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1025 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1027 // Reduced depth of the next LMR search
1028 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1030 if ( !captureOrPromotion
1033 // Countermoves based pruning (~20 Elo)
1034 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1035 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1036 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1039 // Futility pruning: parent node (~5 Elo)
1042 && ss->staticEval + 283 + 170 * lmrDepth <= alpha
1043 && (*contHist[0])[movedPiece][to_sq(move)]
1044 + (*contHist[1])[movedPiece][to_sq(move)]
1045 + (*contHist[3])[movedPiece][to_sq(move)]
1046 + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
1049 // Prune moves with negative SEE (~20 Elo)
1050 if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1055 // Capture history based pruning when the move doesn't give check
1058 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1061 // See based pruning
1062 if (!pos.see_ge(move, Value(-221) * depth)) // (~25 Elo)
1067 // Step 14. Extensions (~75 Elo)
1069 // Singular extension search (~70 Elo). If all moves but one fail low on a
1070 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1071 // then that move is singular and should be extended. To verify this we do
1072 // a reduced search on all the other moves but the ttMove and if the
1073 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1077 && !excludedMove // Avoid recursive singular search
1078 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1079 && abs(ttValue) < VALUE_KNOWN_WIN
1080 && (tte->bound() & BOUND_LOWER)
1081 && tte->depth() >= depth - 3)
1083 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1084 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1085 ss->excludedMove = move;
1086 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1087 ss->excludedMove = MOVE_NONE;
1089 if (value < singularBeta)
1092 singularQuietLMR = !ttCapture;
1095 // Multi-cut pruning
1096 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1097 // search without the ttMove. So we assume this expected Cut-node is not singular,
1098 // that multiple moves fail high, and we can prune the whole subtree by returning
1100 else if (singularBeta >= beta)
1101 return singularBeta;
1103 // If the eval of ttMove is greater than beta we try also if there is another
1104 // move that pushes it over beta, if so also produce a cutoff.
1105 else if (ttValue >= beta)
1107 ss->excludedMove = move;
1108 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1109 ss->excludedMove = MOVE_NONE;
1116 // Check extension (~2 Elo)
1117 else if ( givesCheck
1118 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1121 // Last captures extension
1122 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1123 && pos.non_pawn_material() <= 2 * RookValueMg)
1126 // Late irreversible move extension
1128 && pos.rule50_count() > 80
1129 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1132 // Add extension to new depth
1133 newDepth += extension;
1135 // Speculative prefetch as early as possible
1136 prefetch(TT.first_entry(pos.key_after(move)));
1138 // Update the current move (this must be done after singular extension search)
1139 ss->currentMove = move;
1140 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1141 [captureOrPromotion]
1145 // Step 15. Make the move
1146 pos.do_move(move, st, givesCheck);
1148 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1149 // re-searched at full depth.
1151 && moveCount > 1 + 2 * rootNode
1152 && ( !captureOrPromotion
1154 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1156 || thisThread->ttHitAverage < 427 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1158 Depth r = reduction(improving, depth, moveCount);
1160 // Decrease reduction if the ttHit running average is large
1161 if (thisThread->ttHitAverage > 509 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1164 // Reduction if other threads are searching this position
1168 // Decrease reduction if position is or has been on the PV (~10 Elo)
1172 if (!PvNode && depth > 10 && thisThread->bestMoveChanges <= 2)
1175 if (moveCountPruning && !formerPv)
1178 // Decrease reduction if opponent's move count is high (~5 Elo)
1179 if ((ss-1)->moveCount > 13)
1182 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1183 if (singularQuietLMR)
1186 if (!captureOrPromotion)
1188 // Increase reduction if ttMove is a capture (~5 Elo)
1192 // Increase reduction at root if failing high
1193 r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0;
1195 // Increase reduction for cut nodes (~10 Elo)
1199 // Decrease reduction for moves that escape a capture. Filter out
1200 // castling moves, because they are coded as "king captures rook" and
1201 // hence break make_move(). (~2 Elo)
1202 else if ( type_of(move) == NORMAL
1203 && !pos.see_ge(reverse_move(move)))
1204 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1206 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1207 + (*contHist[0])[movedPiece][to_sq(move)]
1208 + (*contHist[1])[movedPiece][to_sq(move)]
1209 + (*contHist[3])[movedPiece][to_sq(move)]
1212 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1213 if (ss->statScore >= -106 && (ss-1)->statScore < -104)
1216 else if ((ss-1)->statScore >= -119 && ss->statScore < -140)
1219 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1220 r -= ss->statScore / 14884;
1224 // Increase reduction for captures/promotions if late move and at low depth
1225 if (depth < 8 && moveCount > 2)
1228 // Unless giving check, this capture is likely bad
1230 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 213 * depth <= alpha)
1234 Depth d = std::clamp(newDepth - r, 1, newDepth);
1236 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1238 doFullDepthSearch = value > alpha && d != newDepth;
1244 doFullDepthSearch = !PvNode || moveCount > 1;
1249 // Step 17. Full depth search when LMR is skipped or fails high
1250 if (doFullDepthSearch)
1252 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1254 if (didLMR && !captureOrPromotion)
1256 int bonus = value > alpha ? stat_bonus(newDepth)
1257 : -stat_bonus(newDepth);
1259 if (move == ss->killers[0])
1262 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1266 // For PV nodes only, do a full PV search on the first move or after a fail
1267 // high (in the latter case search only if value < beta), otherwise let the
1268 // parent node fail low with value <= alpha and try another move.
1269 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1272 (ss+1)->pv[0] = MOVE_NONE;
1274 value = -search<PV>(pos, ss+1, -beta, -alpha,
1275 std::min(maxNextDepth, newDepth), false);
1278 // Step 18. Undo move
1279 pos.undo_move(move);
1281 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1283 // Step 19. Check for a new best move
1284 // Finished searching the move. If a stop occurred, the return value of
1285 // the search cannot be trusted, and we return immediately without
1286 // updating best move, PV and TT.
1287 if (Threads.stop.load(std::memory_order_relaxed))
1292 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1293 thisThread->rootMoves.end(), move);
1295 // PV move or new best move?
1296 if (moveCount == 1 || value > alpha)
1299 rm.selDepth = thisThread->selDepth;
1304 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1305 rm.pv.push_back(*m);
1307 // We record how often the best move has been changed in each
1308 // iteration. This information is used for time management: when
1309 // the best move changes frequently, we allocate some more time.
1311 ++thisThread->bestMoveChanges;
1314 // All other moves but the PV are set to the lowest value: this
1315 // is not a problem when sorting because the sort is stable and the
1316 // move position in the list is preserved - just the PV is pushed up.
1317 rm.score = -VALUE_INFINITE;
1320 if (value > bestValue)
1328 if (PvNode && !rootNode) // Update pv even in fail-high case
1329 update_pv(ss->pv, move, (ss+1)->pv);
1331 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1335 assert(value >= beta); // Fail high
1342 if (move != bestMove)
1344 if (captureOrPromotion && captureCount < 32)
1345 capturesSearched[captureCount++] = move;
1347 else if (!captureOrPromotion && quietCount < 64)
1348 quietsSearched[quietCount++] = move;
1352 // The following condition would detect a stop only after move loop has been
1353 // completed. But in this case bestValue is valid because we have fully
1354 // searched our subtree, and we can anyhow save the result in TT.
1360 // Step 20. Check for mate and stalemate
1361 // All legal moves have been searched and if there are no legal moves, it
1362 // must be a mate or a stalemate. If we are in a singular extension search then
1363 // return a fail low score.
1365 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1368 bestValue = excludedMove ? alpha
1369 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1372 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1373 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1375 // Bonus for prior countermove that caused the fail low
1376 else if ( (depth >= 3 || PvNode)
1378 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1381 bestValue = std::min(bestValue, maxValue);
1383 // If no good move is found and the previous position was ttPv, then the previous
1384 // opponent move is probably good and the new position is added to the search tree.
1385 if (bestValue <= alpha)
1386 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1387 // Otherwise, a counter move has been found and if the position is the last leaf
1388 // in the search tree, remove the position from the search tree.
1390 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1392 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1393 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1394 bestValue >= beta ? BOUND_LOWER :
1395 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1396 depth, bestMove, ss->staticEval);
1398 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1404 // qsearch() is the quiescence search function, which is called by the main search
1405 // function with zero depth, or recursively with further decreasing depth per call.
1406 template <NodeType NT>
1407 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1409 constexpr bool PvNode = NT == PV;
1411 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1412 assert(PvNode || (alpha == beta - 1));
1417 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
1421 Move ttMove, move, bestMove;
1423 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1424 bool pvHit, givesCheck, captureOrPromotion;
1429 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1431 ss->pv[0] = MOVE_NONE;
1434 Thread* thisThread = pos.this_thread();
1435 (ss+1)->ply = ss->ply + 1;
1436 bestMove = MOVE_NONE;
1437 ss->inCheck = pos.checkers();
1440 // Check for an immediate draw or maximum ply reached
1441 if ( pos.is_draw(ss->ply)
1442 || ss->ply >= MAX_PLY)
1443 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1445 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1447 // Decide whether or not to include checks: this fixes also the type of
1448 // TT entry depth that we are going to use. Note that in qsearch we use
1449 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1450 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1451 : DEPTH_QS_NO_CHECKS;
1452 // Transposition table lookup
1454 tte = TT.probe(posKey, ss->ttHit);
1455 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1456 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1457 pvHit = ss->ttHit && tte->is_pv();
1461 && tte->depth() >= ttDepth
1462 && ttValue != VALUE_NONE // Only in case of TT access race
1463 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1464 : (tte->bound() & BOUND_UPPER)))
1467 // Evaluate the position statically
1470 ss->staticEval = VALUE_NONE;
1471 bestValue = futilityBase = -VALUE_INFINITE;
1477 // Never assume anything about values stored in TT
1478 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1479 ss->staticEval = bestValue = evaluate(pos);
1481 // Can ttValue be used as a better position evaluation?
1482 if ( ttValue != VALUE_NONE
1483 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1484 bestValue = ttValue;
1487 ss->staticEval = bestValue =
1488 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1489 : -(ss-1)->staticEval + 2 * Tempo;
1491 // Stand pat. Return immediately if static value is at least beta
1492 if (bestValue >= beta)
1495 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1496 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1501 if (PvNode && bestValue > alpha)
1504 futilityBase = bestValue + 145;
1507 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1508 nullptr , (ss-4)->continuationHistory,
1509 nullptr , (ss-6)->continuationHistory };
1511 // Initialize a MovePicker object for the current position, and prepare
1512 // to search the moves. Because the depth is <= 0 here, only captures,
1513 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1514 // will be generated.
1515 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1516 &thisThread->captureHistory,
1518 to_sq((ss-1)->currentMove));
1520 // Loop through the moves until no moves remain or a beta cutoff occurs
1521 while ((move = mp.next_move()) != MOVE_NONE)
1523 assert(is_ok(move));
1525 givesCheck = pos.gives_check(move);
1526 captureOrPromotion = pos.capture_or_promotion(move);
1531 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1533 && futilityBase > -VALUE_KNOWN_WIN
1534 && !pos.advanced_pawn_push(move))
1536 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1538 // moveCount pruning
1542 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1544 if (futilityValue <= alpha)
1546 bestValue = std::max(bestValue, futilityValue);
1550 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1552 bestValue = std::max(bestValue, futilityBase);
1557 // Do not search moves with negative SEE values
1558 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1559 && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
1560 && !pos.see_ge(move))
1563 // Speculative prefetch as early as possible
1564 prefetch(TT.first_entry(pos.key_after(move)));
1566 // Check for legality just before making the move
1567 if (!pos.legal(move))
1573 ss->currentMove = move;
1574 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1575 [captureOrPromotion]
1576 [pos.moved_piece(move)]
1579 // CounterMove based pruning
1580 if ( !captureOrPromotion
1581 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1582 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1583 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1586 // Make and search the move
1587 pos.do_move(move, st, givesCheck);
1588 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1589 pos.undo_move(move);
1591 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1593 // Check for a new best move
1594 if (value > bestValue)
1602 if (PvNode) // Update pv even in fail-high case
1603 update_pv(ss->pv, move, (ss+1)->pv);
1605 if (PvNode && value < beta) // Update alpha here!
1613 // All legal moves have been searched. A special case: if we're in check
1614 // and no legal moves were found, it is checkmate.
1615 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1617 assert(!MoveList<LEGAL>(pos).size());
1619 return mated_in(ss->ply); // Plies to mate from the root
1622 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1623 bestValue >= beta ? BOUND_LOWER :
1624 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1625 ttDepth, bestMove, ss->staticEval);
1627 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1633 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1634 // "plies to mate from the current position". Standard scores are unchanged.
1635 // The function is called before storing a value in the transposition table.
1637 Value value_to_tt(Value v, int ply) {
1639 assert(v != VALUE_NONE);
1641 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1642 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1646 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1647 // from the transposition table (which refers to the plies to mate/be mated from
1648 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1649 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1650 // and the graph history interaction, we return an optimal TB score instead.
1652 Value value_from_tt(Value v, int ply, int r50c) {
1654 if (v == VALUE_NONE)
1657 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1659 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1660 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1665 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1667 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1668 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1677 // update_pv() adds current move and appends child pv[]
1679 void update_pv(Move* pv, Move move, Move* childPv) {
1681 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1687 // update_all_stats() updates stats at the end of search() when a bestMove is found
1689 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1690 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1693 Color us = pos.side_to_move();
1694 Thread* thisThread = pos.this_thread();
1695 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1696 Piece moved_piece = pos.moved_piece(bestMove);
1697 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1699 bonus1 = stat_bonus(depth + 1);
1700 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1701 : stat_bonus(depth); // smaller bonus
1703 if (!pos.capture_or_promotion(bestMove))
1705 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1707 // Decrease all the non-best quiet moves
1708 for (int i = 0; i < quietCount; ++i)
1710 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1711 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1715 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1717 // Extra penalty for a quiet early move that was not a TT move or main killer move in previous ply when it gets refuted
1718 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1719 && !pos.captured_piece())
1720 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1722 // Decrease all the non-best capture moves
1723 for (int i = 0; i < captureCount; ++i)
1725 moved_piece = pos.moved_piece(capturesSearched[i]);
1726 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1727 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1732 // update_continuation_histories() updates histories of the move pairs formed
1733 // by moves at ply -1, -2, -4, and -6 with current move.
1735 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1737 for (int i : {1, 2, 4, 6})
1739 if (ss->inCheck && i > 2)
1741 if (is_ok((ss-i)->currentMove))
1742 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1747 // update_quiet_stats() updates move sorting heuristics
1749 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1751 if (ss->killers[0] != move)
1753 ss->killers[1] = ss->killers[0];
1754 ss->killers[0] = move;
1757 Color us = pos.side_to_move();
1758 Thread* thisThread = pos.this_thread();
1759 thisThread->mainHistory[us][from_to(move)] << bonus;
1760 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1762 if (type_of(pos.moved_piece(move)) != PAWN)
1763 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1765 if (is_ok((ss-1)->currentMove))
1767 Square prevSq = to_sq((ss-1)->currentMove);
1768 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1771 if (depth > 11 && ss->ply < MAX_LPH)
1772 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1775 // When playing with strength handicap, choose best move among a set of RootMoves
1776 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1778 Move Skill::pick_best(size_t multiPV) {
1780 const RootMoves& rootMoves = Threads.main()->rootMoves;
1781 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1783 // RootMoves are already sorted by score in descending order
1784 Value topScore = rootMoves[0].score;
1785 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1786 int weakness = 120 - 2 * level;
1787 int maxScore = -VALUE_INFINITE;
1789 // Choose best move. For each move score we add two terms, both dependent on
1790 // weakness. One is deterministic and bigger for weaker levels, and one is
1791 // random. Then we choose the move with the resulting highest score.
1792 for (size_t i = 0; i < multiPV; ++i)
1794 // This is our magic formula
1795 int push = ( weakness * int(topScore - rootMoves[i].score)
1796 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1798 if (rootMoves[i].score + push >= maxScore)
1800 maxScore = rootMoves[i].score + push;
1801 best = rootMoves[i].pv[0];
1811 /// MainThread::check_time() is used to print debug info and, more importantly,
1812 /// to detect when we are out of available time and thus stop the search.
1814 void MainThread::check_time() {
1819 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1820 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1822 static TimePoint lastInfoTime = now();
1824 TimePoint elapsed = Time.elapsed();
1825 TimePoint tick = Limits.startTime + elapsed;
1827 if (tick - lastInfoTime >= 1000)
1829 lastInfoTime = tick;
1833 // We should not stop pondering until told so by the GUI
1837 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1838 || (Limits.movetime && elapsed >= Limits.movetime)
1839 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1840 Threads.stop = true;
1844 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1845 /// that all (if any) unsearched PV lines are sent using a previous search score.
1847 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1849 std::stringstream ss;
1850 TimePoint elapsed = Time.elapsed() + 1;
1851 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1852 size_t pvIdx = pos.this_thread()->pvIdx;
1853 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1854 uint64_t nodesSearched = Threads.nodes_searched();
1855 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1857 for (size_t i = 0; i < multiPV; ++i)
1859 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1861 if (depth == 1 && !updated && i > 0)
1864 Depth d = updated ? depth : std::max(1, depth - 1);
1865 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1867 if (v == -VALUE_INFINITE)
1870 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1871 v = tb ? rootMoves[i].tbScore : v;
1873 if (ss.rdbuf()->in_avail()) // Not at first line
1878 << " seldepth " << rootMoves[i].selDepth
1879 << " multipv " << i + 1
1880 << " score " << UCI::value(v);
1882 if (Options["UCI_ShowWDL"])
1883 ss << UCI::wdl(v, pos.game_ply());
1885 if (!tb && i == pvIdx)
1886 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1888 ss << " nodes " << nodesSearched
1889 << " nps " << nodesSearched * 1000 / elapsed;
1891 if (elapsed > 1000) // Earlier makes little sense
1892 ss << " hashfull " << TT.hashfull();
1894 ss << " tbhits " << tbHits
1895 << " time " << elapsed
1898 for (Move m : rootMoves[i].pv)
1899 ss << " " << UCI::move(m, pos.is_chess960());
1906 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1907 /// before exiting the search, for instance, in case we stop the search during a
1908 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1909 /// otherwise in case of 'ponder on' we have nothing to think on.
1911 bool RootMove::extract_ponder_from_tt(Position& pos) {
1914 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
1918 assert(pv.size() == 1);
1920 if (pv[0] == MOVE_NONE)
1923 pos.do_move(pv[0], st);
1924 TTEntry* tte = TT.probe(pos.key(), ttHit);
1928 Move m = tte->move(); // Local copy to be SMP safe
1929 if (MoveList<LEGAL>(pos).contains(m))
1933 pos.undo_move(pv[0]);
1934 return pv.size() > 1;
1937 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1940 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1941 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1942 Cardinality = int(Options["SyzygyProbeLimit"]);
1943 bool dtz_available = true;
1945 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1946 // ProbeDepth == DEPTH_ZERO
1947 if (Cardinality > MaxCardinality)
1949 Cardinality = MaxCardinality;
1953 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1955 // Rank moves using DTZ tables
1956 RootInTB = root_probe(pos, rootMoves);
1960 // DTZ tables are missing; try to rank moves using WDL tables
1961 dtz_available = false;
1962 RootInTB = root_probe_wdl(pos, rootMoves);
1968 // Sort moves according to TB rank
1969 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1970 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1972 // Probe during search only if DTZ is not available and we are winning
1973 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1978 // Clean up if root_probe() and root_probe_wdl() have failed
1979 for (auto& m : rootMoves)