2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 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;
66 Value futility_margin(Depth d, bool improving) {
67 return Value(234 * (d - improving));
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d] * Reductions[mn];
75 return (r + 503) / 1024 + (!i && r > 915);
78 constexpr int futility_move_count(bool improving, Depth depth) {
79 return (3 + depth * depth) / (2 - improving);
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth d) {
84 return d > 14 ? 66 : 6 * d * d + 231 * d - 206;
87 // Add a small random component to draw evaluations to avoid 3-fold blindness
88 Value value_draw(Thread* thisThread) {
89 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
92 // Skill structure is used to implement strength limit
94 explicit Skill(int l) : level(l) {}
95 bool enabled() const { return level < 20; }
96 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
97 Move pick_best(size_t multiPV);
100 Move best = MOVE_NONE;
103 // Breadcrumbs are used to mark nodes as being searched by a given thread
105 std::atomic<Thread*> thread;
106 std::atomic<Key> key;
108 std::array<Breadcrumb, 1024> breadcrumbs;
110 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
111 // node for potential reductions. A free node will be marked upon entering the moves
112 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
113 struct ThreadHolding {
114 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
115 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
120 // See if another already marked this location, if not, mark it ourselves
121 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
124 (*location).thread.store(thisThread, std::memory_order_relaxed);
125 (*location).key.store(posKey, std::memory_order_relaxed);
128 else if ( tmp != thisThread
129 && (*location).key.load(std::memory_order_relaxed) == posKey)
135 if (owning) // Free the marked location
136 (*location).thread.store(nullptr, std::memory_order_relaxed);
139 bool marked() { return otherThread; }
142 Breadcrumb* location;
143 bool otherThread, owning;
146 template <NodeType NT>
147 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
149 template <NodeType NT>
150 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
152 Value value_to_tt(Value v, int ply);
153 Value value_from_tt(Value v, int ply, int r50c);
154 void update_pv(Move* pv, Move move, Move* childPv);
155 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
156 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
157 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
158 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
160 // perft() is our utility to verify move generation. All the leaf nodes up
161 // to the given depth are generated and counted, and the sum is returned.
163 uint64_t perft(Position& pos, Depth depth) {
166 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
168 uint64_t cnt, nodes = 0;
169 const bool leaf = (depth == 2);
171 for (const auto& m : MoveList<LEGAL>(pos))
173 if (Root && depth <= 1)
178 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
183 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
191 /// Search::init() is called at startup to initialize various lookup tables
193 void Search::init() {
195 for (int i = 1; i < MAX_MOVES; ++i)
196 Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
200 /// Search::clear() resets search state to its initial value
202 void Search::clear() {
204 Threads.main()->wait_for_search_finished();
206 Time.availableNodes = 0;
209 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
213 /// MainThread::search() is started when the program receives the UCI 'go'
214 /// command. It searches from the root position and outputs the "bestmove".
216 void MainThread::search() {
220 nodes = perft<true>(rootPos, Limits.perft);
221 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
225 Color us = rootPos.side_to_move();
226 Time.init(Limits, us, rootPos.game_ply());
229 Eval::NNUE::verify();
231 if (rootMoves.empty())
233 rootMoves.emplace_back(MOVE_NONE);
234 sync_cout << "info depth 0 score "
235 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
240 Threads.start_searching(); // start non-main threads
241 Thread::search(); // main thread start searching
244 // When we reach the maximum depth, we can arrive here without a raise of
245 // Threads.stop. However, if we are pondering or in an infinite search,
246 // the UCI protocol states that we shouldn't print the best move before the
247 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
248 // until the GUI sends one of those commands.
250 while (!Threads.stop && (ponder || Limits.infinite))
251 {} // Busy wait for a stop or a ponder reset
253 // Stop the threads if not already stopped (also raise the stop if
254 // "ponderhit" just reset Threads.ponder).
257 // Wait until all threads have finished
258 Threads.wait_for_search_finished();
260 // When playing in 'nodes as time' mode, subtract the searched nodes from
261 // the available ones before exiting.
263 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
265 Thread* bestThread = this;
267 if ( int(Options["MultiPV"]) == 1
269 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
270 && rootMoves[0].pv[0] != MOVE_NONE)
271 bestThread = Threads.get_best_thread();
273 bestPreviousScore = bestThread->rootMoves[0].score;
275 // Send again PV info if we have a new best thread
276 if (bestThread != this)
277 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
279 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
281 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
282 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
284 std::cout << sync_endl;
288 /// Thread::search() is the main iterative deepening loop. It calls search()
289 /// repeatedly with increasing depth until the allocated thinking time has been
290 /// consumed, the user stops the search, or the maximum search depth is reached.
292 void Thread::search() {
294 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
295 // The former is needed to allow update_continuation_histories(ss-1, ...),
296 // which accesses its argument at ss-6, also near the root.
297 // The latter is needed for statScores and killer initialization.
298 Stack stack[MAX_PLY+10], *ss = stack+7;
300 Value bestValue, alpha, beta, delta;
301 Move lastBestMove = MOVE_NONE;
302 Depth lastBestMoveDepth = 0;
303 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
304 double timeReduction = 1, totBestMoveChanges = 0;
305 Color us = rootPos.side_to_move();
308 std::memset(ss-7, 0, 10 * sizeof(Stack));
309 for (int i = 7; i > 0; i--)
310 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
314 bestValue = delta = alpha = -VALUE_INFINITE;
315 beta = VALUE_INFINITE;
319 if (mainThread->bestPreviousScore == VALUE_INFINITE)
320 for (int i = 0; i < 4; ++i)
321 mainThread->iterValue[i] = VALUE_ZERO;
323 for (int i = 0; i < 4; ++i)
324 mainThread->iterValue[i] = mainThread->bestPreviousScore;
327 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
328 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
330 size_t multiPV = size_t(Options["MultiPV"]);
332 // Pick integer skill levels, but non-deterministically round up or down
333 // such that the average integer skill corresponds to the input floating point one.
334 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
335 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
336 // for match (TC 60+0.6) results spanning a wide range of k values.
338 double floatLevel = Options["UCI_LimitStrength"] ?
339 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
340 double(Options["Skill Level"]);
341 int intLevel = int(floatLevel) +
342 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
343 Skill skill(intLevel);
345 // When playing with strength handicap enable MultiPV search that we will
346 // use behind the scenes to retrieve a set of possible moves.
348 multiPV = std::max(multiPV, (size_t)4);
350 multiPV = std::min(multiPV, rootMoves.size());
351 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
353 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
355 // In analysis mode, adjust contempt in accordance with user preference
356 if (Limits.infinite || Options["UCI_AnalyseMode"])
357 ct = Options["Analysis Contempt"] == "Off" ? 0
358 : Options["Analysis Contempt"] == "Both" ? ct
359 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
360 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
363 // Evaluation score is from the white point of view
364 contempt = (us == WHITE ? make_score(ct, ct / 2)
365 : -make_score(ct, ct / 2));
367 int searchAgainCounter = 0;
369 // Iterative deepening loop until requested to stop or the target depth is reached
370 while ( ++rootDepth < MAX_PLY
372 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
374 // Age out PV variability metric
376 totBestMoveChanges /= 2;
378 // Save the last iteration's scores before first PV line is searched and
379 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
380 for (RootMove& rm : rootMoves)
381 rm.previousScore = rm.score;
386 if (!Threads.increaseDepth)
387 searchAgainCounter++;
389 // MultiPV loop. We perform a full root search for each PV line
390 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
395 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
396 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
400 // Reset UCI info selDepth for each depth and each PV line
403 // Reset aspiration window starting size
406 Value prev = rootMoves[pvIdx].previousScore;
408 alpha = std::max(prev - delta,-VALUE_INFINITE);
409 beta = std::min(prev + delta, VALUE_INFINITE);
411 // Adjust contempt based on root move's previousScore (dynamic contempt)
412 int dct = ct + (113 - ct / 2) * prev / (abs(prev) + 147);
414 contempt = (us == WHITE ? make_score(dct, dct / 2)
415 : -make_score(dct, dct / 2));
418 // Start with a small aspiration window and, in the case of a fail
419 // high/low, re-search with a bigger window until we don't fail
424 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
425 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
427 // Bring the best move to the front. It is critical that sorting
428 // is done with a stable algorithm because all the values but the
429 // first and eventually the new best one are set to -VALUE_INFINITE
430 // and we want to keep the same order for all the moves except the
431 // new PV that goes to the front. Note that in case of MultiPV
432 // search the already searched PV lines are preserved.
433 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
435 // If search has been stopped, we break immediately. Sorting is
436 // safe because RootMoves is still valid, although it refers to
437 // the previous iteration.
441 // When failing high/low give some update (without cluttering
442 // the UI) before a re-search.
445 && (bestValue <= alpha || bestValue >= beta)
446 && Time.elapsed() > 3000)
447 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
449 // In case of failing low/high increase aspiration window and
450 // re-search, otherwise exit the loop.
451 if (bestValue <= alpha)
453 beta = (alpha + beta) / 2;
454 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
458 mainThread->stopOnPonderhit = false;
460 else if (bestValue >= beta)
462 beta = std::min(bestValue + delta, VALUE_INFINITE);
468 delta += delta / 4 + 5;
470 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
473 // Sort the PV lines searched so far and update the GUI
474 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
477 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
478 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
482 completedDepth = rootDepth;
484 if (rootMoves[0].pv[0] != lastBestMove) {
485 lastBestMove = rootMoves[0].pv[0];
486 lastBestMoveDepth = rootDepth;
489 // Have we found a "mate in x"?
491 && bestValue >= VALUE_MATE_IN_MAX_PLY
492 && VALUE_MATE - bestValue <= 2 * Limits.mate)
498 // If skill level is enabled and time is up, pick a sub-optimal best move
499 if (skill.enabled() && skill.time_to_pick(rootDepth))
500 skill.pick_best(multiPV);
502 // Do we have time for the next iteration? Can we stop searching now?
503 if ( Limits.use_time_management()
505 && !mainThread->stopOnPonderhit)
507 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
508 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
509 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
511 // If the bestMove is stable over several iterations, reduce time accordingly
512 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
513 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
515 // Use part of the gained time from a previous stable move for the current move
516 for (Thread* th : Threads)
518 totBestMoveChanges += th->bestMoveChanges;
519 th->bestMoveChanges = 0;
521 double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
523 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
525 // Cap used time in case of a single legal move for a better viewer experience in tournaments
526 // yielding correct scores and sufficiently fast moves.
527 if (rootMoves.size() == 1)
528 totalTime = std::min(500.0, totalTime);
530 // Stop the search if we have exceeded the totalTime
531 if (Time.elapsed() > totalTime)
533 // If we are allowed to ponder do not stop the search now but
534 // keep pondering until the GUI sends "ponderhit" or "stop".
535 if (mainThread->ponder)
536 mainThread->stopOnPonderhit = true;
540 else if ( Threads.increaseDepth
541 && !mainThread->ponder
542 && Time.elapsed() > totalTime * 0.58)
543 Threads.increaseDepth = false;
545 Threads.increaseDepth = true;
548 mainThread->iterValue[iterIdx] = bestValue;
549 iterIdx = (iterIdx + 1) & 3;
555 mainThread->previousTimeReduction = timeReduction;
557 // If skill level is enabled, swap best PV line with the sub-optimal one
559 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
560 skill.best ? skill.best : skill.pick_best(multiPV)));
566 // search<>() is the main search function for both PV and non-PV nodes
568 template <NodeType NT>
569 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
571 constexpr bool PvNode = NT == PV;
572 const bool rootNode = PvNode && ss->ply == 0;
573 const Depth maxNextDepth = rootNode ? depth : depth + 1;
575 // Check if we have an upcoming move which draws by repetition, or
576 // if the opponent had an alternative move earlier to this position.
577 if ( pos.rule50_count() >= 3
578 && alpha < VALUE_DRAW
580 && pos.has_game_cycle(ss->ply))
582 alpha = value_draw(pos.this_thread());
587 // Dive into quiescence search when the depth reaches zero
589 return qsearch<NT>(pos, ss, alpha, beta);
591 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
592 assert(PvNode || (alpha == beta - 1));
593 assert(0 < depth && depth < MAX_PLY);
594 assert(!(PvNode && cutNode));
596 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
598 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
602 Move ttMove, move, excludedMove, bestMove;
603 Depth extension, newDepth;
604 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
605 bool formerPv, givesCheck, improving, didLMR, priorCapture;
606 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
607 ttCapture, singularQuietLMR;
609 int moveCount, captureCount, quietCount;
611 // Step 1. Initialize node
612 Thread* thisThread = pos.this_thread();
613 ss->inCheck = pos.checkers();
614 priorCapture = pos.captured_piece();
615 Color us = pos.side_to_move();
616 moveCount = captureCount = quietCount = ss->moveCount = 0;
617 bestValue = -VALUE_INFINITE;
618 maxValue = VALUE_INFINITE;
620 // Check for the available remaining time
621 if (thisThread == Threads.main())
622 static_cast<MainThread*>(thisThread)->check_time();
624 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
625 if (PvNode && thisThread->selDepth < ss->ply + 1)
626 thisThread->selDepth = ss->ply + 1;
630 // Step 2. Check for aborted search and immediate draw
631 if ( Threads.stop.load(std::memory_order_relaxed)
632 || pos.is_draw(ss->ply)
633 || ss->ply >= MAX_PLY)
634 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
635 : value_draw(pos.this_thread());
637 // Step 3. Mate distance pruning. Even if we mate at the next move our score
638 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
639 // a shorter mate was found upward in the tree then there is no need to search
640 // because we will never beat the current alpha. Same logic but with reversed
641 // signs applies also in the opposite condition of being mated instead of giving
642 // mate. In this case return a fail-high score.
643 alpha = std::max(mated_in(ss->ply), alpha);
644 beta = std::min(mate_in(ss->ply+1), beta);
649 assert(0 <= ss->ply && ss->ply < MAX_PLY);
651 (ss+1)->ply = ss->ply + 1;
652 (ss+1)->ttPv = false;
653 (ss+1)->excludedMove = bestMove = MOVE_NONE;
654 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
655 Square prevSq = to_sq((ss-1)->currentMove);
657 // Initialize statScore to zero for the grandchildren of the current position.
658 // So statScore is shared between all grandchildren and only the first grandchild
659 // starts with statScore = 0. Later grandchildren start with the last calculated
660 // statScore of the previous grandchild. This influences the reduction rules in
661 // LMR which are based on the statScore of parent position.
663 (ss+2)->statScore = 0;
665 // Step 4. Transposition table lookup. We don't want the score of a partial
666 // search to overwrite a previous full search TT value, so we use a different
667 // position key in case of an excluded move.
668 excludedMove = ss->excludedMove;
669 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
670 tte = TT.probe(posKey, ss->ttHit);
671 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
672 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
673 : ss->ttHit ? tte->move() : MOVE_NONE;
675 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
676 formerPv = ss->ttPv && !PvNode;
678 // Update low ply history for previous move if we are near root and position is or has been in PV
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 // Bonus for a quiet ttMove that fails high
704 if (!pos.capture_or_promotion(ttMove))
705 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
707 // Extra penalty for early quiet moves of the previous ply
708 if ((ss-1)->moveCount <= 2 && !priorCapture)
709 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
711 // Penalty for a quiet ttMove that fails low
712 else if (!pos.capture_or_promotion(ttMove))
714 int penalty = -stat_bonus(depth);
715 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
716 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
720 // Partial workaround for the graph history interaction problem
721 // For high rule50 counts don't produce transposition table cutoffs.
722 if (pos.rule50_count() < 90)
726 // Step 5. Tablebases probe
727 if (!rootNode && TB::Cardinality)
729 int piecesCount = pos.count<ALL_PIECES>();
731 if ( piecesCount <= TB::Cardinality
732 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
733 && pos.rule50_count() == 0
734 && !pos.can_castle(ANY_CASTLING))
737 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
739 // Force check of time on the next occasion
740 if (thisThread == Threads.main())
741 static_cast<MainThread*>(thisThread)->callsCnt = 0;
743 if (err != TB::ProbeState::FAIL)
745 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
747 int drawScore = TB::UseRule50 ? 1 : 0;
749 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
750 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
751 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
752 : VALUE_DRAW + 2 * wdl * drawScore;
754 Bound b = wdl < -drawScore ? BOUND_UPPER
755 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
757 if ( b == BOUND_EXACT
758 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
760 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
761 std::min(MAX_PLY - 1, depth + 6),
762 MOVE_NONE, VALUE_NONE);
769 if (b == BOUND_LOWER)
770 bestValue = value, alpha = std::max(alpha, bestValue);
778 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
780 // Step 6. Static evaluation of the position
783 // Skip early pruning when in check
784 ss->staticEval = eval = VALUE_NONE;
790 // Never assume anything about values stored in TT
791 ss->staticEval = eval = tte->eval();
792 if (eval == VALUE_NONE)
793 ss->staticEval = eval = evaluate(pos);
795 // Randomize draw evaluation
796 if (eval == VALUE_DRAW)
797 eval = value_draw(thisThread);
799 // Can ttValue be used as a better position evaluation?
800 if ( ttValue != VALUE_NONE
801 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
806 // In case of null move search use previous static eval with a different sign
807 // and addition of two tempos
808 if ((ss-1)->currentMove != MOVE_NULL)
809 ss->staticEval = eval = evaluate(pos);
811 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
813 // Save static evaluation into transposition table
814 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
817 // Use static evaluation difference to improve quiet move ordering
818 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
820 int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
821 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
824 // Set up improving flag that is used in various pruning heuristics
825 // We define position as improving if static evaluation of position is better
826 // Than the previous static evaluation at our turn
827 // In case of us being in check at our previous move we look at move prior to it
828 improving = (ss-2)->staticEval == VALUE_NONE
829 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
830 : ss->staticEval > (ss-2)->staticEval;
832 // Step 7. Futility pruning: child node (~50 Elo)
835 && eval - futility_margin(depth, improving) >= beta
836 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
839 // Step 8. Null move search with verification search (~40 Elo)
841 && (ss-1)->currentMove != MOVE_NULL
842 && (ss-1)->statScore < 22661
844 && eval >= ss->staticEval
845 && ss->staticEval >= beta - 24 * depth - 34 * improving + 162 * ss->ttPv + 159
847 && pos.non_pawn_material(us)
848 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
850 assert(eval - beta >= 0);
852 // Null move dynamic reduction based on depth and value
853 Depth R = (1062 + 68 * depth) / 256 + std::min(int(eval - beta) / 190, 3);
855 ss->currentMove = MOVE_NULL;
856 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
858 pos.do_null_move(st);
860 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
862 pos.undo_null_move();
864 if (nullValue >= beta)
866 // Do not return unproven mate or TB scores
867 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
870 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
873 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
875 // Do verification search at high depths, with null move pruning disabled
876 // for us, until ply exceeds nmpMinPly.
877 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
878 thisThread->nmpColor = us;
880 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
882 thisThread->nmpMinPly = 0;
889 probCutBeta = beta + 209 - 44 * improving;
891 // Step 9. ProbCut (~10 Elo)
892 // If we have a good enough capture and a reduced search returns a value
893 // much above beta, we can (almost) safely prune the previous move.
896 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
897 // if value from transposition table is lower than probCutBeta, don't attempt probCut
898 // there and in further interactions with transposition table cutoff depth is set to depth - 3
899 // because probCut search has depth set to depth - 4 but we also do a move before it
900 // so effective depth is equal to depth - 3
902 && tte->depth() >= depth - 3
903 && ttValue != VALUE_NONE
904 && ttValue < probCutBeta))
906 // if ttMove is a capture and value from transposition table is good enough produce probCut
907 // cutoff without digging into actual probCut search
909 && tte->depth() >= depth - 3
910 && ttValue != VALUE_NONE
911 && ttValue >= probCutBeta
913 && pos.capture_or_promotion(ttMove))
916 assert(probCutBeta < VALUE_INFINITE);
917 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
918 int probCutCount = 0;
919 bool ttPv = ss->ttPv;
922 while ( (move = mp.next_move()) != MOVE_NONE
923 && probCutCount < 2 + 2 * cutNode)
924 if (move != excludedMove && pos.legal(move))
926 assert(pos.capture_or_promotion(move));
929 captureOrPromotion = true;
932 ss->currentMove = move;
933 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
935 [pos.moved_piece(move)]
938 pos.do_move(move, st);
940 // Perform a preliminary qsearch to verify that the move holds
941 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
943 // If the qsearch held, perform the regular search
944 if (value >= probCutBeta)
945 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
949 if (value >= probCutBeta)
951 // if transposition table doesn't have equal or more deep info write probCut data into it
953 && tte->depth() >= depth - 3
954 && ttValue != VALUE_NONE))
955 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
957 depth - 3, move, ss->staticEval);
964 // Step 10. If the position is not in TT, decrease depth by 2
970 moves_loop: // When in check, search starts from here
972 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
973 nullptr , (ss-4)->continuationHistory,
974 nullptr , (ss-6)->continuationHistory };
976 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
978 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
979 &thisThread->lowPlyHistory,
987 singularQuietLMR = moveCountPruning = false;
988 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
990 // Mark this node as being searched
991 ThreadHolding th(thisThread, posKey, ss->ply);
993 // Step 11. Loop through all pseudo-legal moves until no moves remain
994 // or a beta cutoff occurs.
995 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
999 if (move == excludedMove)
1002 // At root obey the "searchmoves" option and skip moves not listed in Root
1003 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1004 // mode we also skip PV moves which have been already searched and those
1005 // of lower "TB rank" if we are in a TB root position.
1006 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1007 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1010 // Check for legality
1011 if (!rootNode && !pos.legal(move))
1014 ss->moveCount = ++moveCount;
1016 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1017 sync_cout << "info depth " << depth
1018 << " currmove " << UCI::move(move, pos.is_chess960())
1019 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1021 (ss+1)->pv = nullptr;
1024 captureOrPromotion = pos.capture_or_promotion(move);
1025 movedPiece = pos.moved_piece(move);
1026 givesCheck = pos.gives_check(move);
1028 // Indicate PvNodes that will probably fail low if node was searched with non-PV search
1029 // at depth equal or greater to current depth and result of this search was far below alpha
1030 bool likelyFailLow = PvNode
1032 && (tte->bound() & BOUND_UPPER)
1033 && ttValue < alpha + 200 + 100 * depth
1034 && tte->depth() >= depth;
1036 // Calculate new depth for this move
1037 newDepth = depth - 1;
1039 // Step 12. Pruning at shallow depth (~200 Elo)
1041 && pos.non_pawn_material(us)
1042 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1044 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1045 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1047 // Reduced depth of the next LMR search
1048 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1050 if ( captureOrPromotion
1053 // Capture history based pruning when the move doesn't give check
1056 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1059 // SEE based pruning
1060 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1065 // Countermoves based pruning (~20 Elo)
1066 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1067 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1068 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1071 // Futility pruning: parent node (~5 Elo)
1074 && ss->staticEval + 174 + 157 * lmrDepth <= alpha
1075 && (*contHist[0])[movedPiece][to_sq(move)]
1076 + (*contHist[1])[movedPiece][to_sq(move)]
1077 + (*contHist[3])[movedPiece][to_sq(move)]
1078 + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 26237)
1081 // Prune moves with negative SEE (~20 Elo)
1082 if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1087 // Step 13. Extensions (~75 Elo)
1089 // Singular extension search (~70 Elo). If all moves but one fail low on a
1090 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1091 // then that move is singular and should be extended. To verify this we do
1092 // a reduced search on all the other moves but the ttMove and if the
1093 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1097 && !excludedMove // Avoid recursive singular search
1098 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1099 && abs(ttValue) < VALUE_KNOWN_WIN
1100 && (tte->bound() & BOUND_LOWER)
1101 && tte->depth() >= depth - 3)
1103 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1104 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1105 ss->excludedMove = move;
1106 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1107 ss->excludedMove = MOVE_NONE;
1109 if (value < singularBeta)
1112 singularQuietLMR = !ttCapture;
1115 // Multi-cut pruning
1116 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1117 // search without the ttMove. So we assume this expected Cut-node is not singular,
1118 // that multiple moves fail high, and we can prune the whole subtree by returning
1120 else if (singularBeta >= beta)
1121 return singularBeta;
1123 // If the eval of ttMove is greater than beta we try also if there is another
1124 // move that pushes it over beta, if so also produce a cutoff.
1125 else if (ttValue >= beta)
1127 ss->excludedMove = move;
1128 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1129 ss->excludedMove = MOVE_NONE;
1136 // Check extension (~2 Elo)
1137 else if ( givesCheck
1138 && (pos.is_discovered_check_on_king(~us, move) || pos.see_ge(move)))
1141 // Last captures extension
1142 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1143 && pos.non_pawn_material() <= 2 * RookValueMg)
1146 // Add extension to new depth
1147 newDepth += extension;
1149 // Speculative prefetch as early as possible
1150 prefetch(TT.first_entry(pos.key_after(move)));
1152 // Update the current move (this must be done after singular extension search)
1153 ss->currentMove = move;
1154 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1155 [captureOrPromotion]
1159 // Step 14. Make the move
1160 pos.do_move(move, st, givesCheck);
1162 // Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1163 // re-searched at full depth.
1165 && moveCount > 1 + 2 * rootNode
1166 && ( !captureOrPromotion
1168 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1170 || (!PvNode && !formerPv && captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 4506)
1171 || thisThread->ttHitAverage < 432 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1173 Depth r = reduction(improving, depth, moveCount);
1175 // Decrease reduction if the ttHit running average is large
1176 if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1179 // Increase reduction if other threads are searching this position
1183 // Decrease reduction if position is or has been on the PV
1184 // and node is not likely to fail low. (~10 Elo)
1185 if (ss->ttPv && !likelyFailLow)
1188 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1189 if ((rootNode || !PvNode) && thisThread->rootDepth > 10 && thisThread->bestMoveChanges <= 2)
1192 // More reductions for late moves if position was not in previous PV
1193 if (moveCountPruning && !formerPv)
1196 // Decrease reduction if opponent's move count is high (~5 Elo)
1197 if ((ss-1)->moveCount > 13)
1200 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1201 if (singularQuietLMR)
1204 if (captureOrPromotion)
1206 // Unless giving check, this capture is likely bad
1208 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
1213 // Increase reduction if ttMove is a capture (~5 Elo)
1217 // Increase reduction at root if failing high
1218 r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0;
1220 // Increase reduction for cut nodes (~10 Elo)
1224 // Decrease reduction for moves that escape a capture. Filter out
1225 // castling moves, because they are coded as "king captures rook" and
1226 // hence break make_move(). (~2 Elo)
1227 else if ( type_of(move) == NORMAL
1228 && !pos.see_ge(reverse_move(move)))
1229 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1231 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1232 + (*contHist[0])[movedPiece][to_sq(move)]
1233 + (*contHist[1])[movedPiece][to_sq(move)]
1234 + (*contHist[3])[movedPiece][to_sq(move)]
1237 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1238 if (ss->statScore >= -89 && (ss-1)->statScore < -116)
1241 else if ((ss-1)->statScore >= -112 && ss->statScore < -100)
1244 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1245 // If we are not in check use statScore, if we are in check
1246 // use sum of main history and first continuation history with an offset
1248 r -= (thisThread->mainHistory[us][from_to(move)]
1249 + (*contHist[0])[movedPiece][to_sq(move)] - 4341) / 16384;
1251 r -= ss->statScore / 14382;
1254 Depth d = std::clamp(newDepth - r, 1, newDepth);
1256 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1258 doFullDepthSearch = value > alpha && d != newDepth;
1264 doFullDepthSearch = !PvNode || moveCount > 1;
1269 // Step 16. Full depth search when LMR is skipped or fails high
1270 if (doFullDepthSearch)
1272 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1274 // If the move passed LMR update its stats
1275 if (didLMR && !captureOrPromotion)
1277 int bonus = value > alpha ? stat_bonus(newDepth)
1278 : -stat_bonus(newDepth);
1280 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1284 // For PV nodes only, do a full PV search on the first move or after a fail
1285 // high (in the latter case search only if value < beta), otherwise let the
1286 // parent node fail low with value <= alpha and try another move.
1287 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1290 (ss+1)->pv[0] = MOVE_NONE;
1292 value = -search<PV>(pos, ss+1, -beta, -alpha,
1293 std::min(maxNextDepth, newDepth), false);
1296 // Step 17. Undo move
1297 pos.undo_move(move);
1299 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1301 // Step 18. Check for a new best move
1302 // Finished searching the move. If a stop occurred, the return value of
1303 // the search cannot be trusted, and we return immediately without
1304 // updating best move, PV and TT.
1305 if (Threads.stop.load(std::memory_order_relaxed))
1310 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1311 thisThread->rootMoves.end(), move);
1313 // PV move or new best move?
1314 if (moveCount == 1 || value > alpha)
1317 rm.selDepth = thisThread->selDepth;
1322 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1323 rm.pv.push_back(*m);
1325 // We record how often the best move has been changed in each
1326 // iteration. This information is used for time management and LMR
1328 ++thisThread->bestMoveChanges;
1331 // All other moves but the PV are set to the lowest value: this
1332 // is not a problem when sorting because the sort is stable and the
1333 // move position in the list is preserved - just the PV is pushed up.
1334 rm.score = -VALUE_INFINITE;
1337 if (value > bestValue)
1345 if (PvNode && !rootNode) // Update pv even in fail-high case
1346 update_pv(ss->pv, move, (ss+1)->pv);
1348 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1352 assert(value >= beta); // Fail high
1359 // If the move is worse than some previously searched move, remember it to update its stats later
1360 if (move != bestMove)
1362 if (captureOrPromotion && captureCount < 32)
1363 capturesSearched[captureCount++] = move;
1365 else if (!captureOrPromotion && quietCount < 64)
1366 quietsSearched[quietCount++] = move;
1370 // The following condition would detect a stop only after move loop has been
1371 // completed. But in this case bestValue is valid because we have fully
1372 // searched our subtree, and we can anyhow save the result in TT.
1378 // Step 19. Check for mate and stalemate
1379 // All legal moves have been searched and if there are no legal moves, it
1380 // must be a mate or a stalemate. If we are in a singular extension search then
1381 // return a fail low score.
1383 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1386 bestValue = excludedMove ? alpha
1387 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1389 // If there is a move which produces search value greater than alpha we update stats of searched moves
1391 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1392 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1394 // Bonus for prior countermove that caused the fail low
1395 else if ( (depth >= 3 || PvNode)
1397 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1400 bestValue = std::min(bestValue, maxValue);
1402 // If no good move is found and the previous position was ttPv, then the previous
1403 // opponent move is probably good and the new position is added to the search tree.
1404 if (bestValue <= alpha)
1405 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1406 // Otherwise, a counter move has been found and if the position is the last leaf
1407 // in the search tree, remove the position from the search tree.
1409 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1411 // Write gathered information in transposition table
1412 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1413 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1414 bestValue >= beta ? BOUND_LOWER :
1415 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1416 depth, bestMove, ss->staticEval);
1418 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1424 // qsearch() is the quiescence search function, which is called by the main search
1425 // function with zero depth, or recursively with further decreasing depth per call.
1426 template <NodeType NT>
1427 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1429 constexpr bool PvNode = NT == PV;
1431 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1432 assert(PvNode || (alpha == beta - 1));
1437 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
1441 Move ttMove, move, bestMove;
1443 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1444 bool pvHit, givesCheck, captureOrPromotion;
1449 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1451 ss->pv[0] = MOVE_NONE;
1454 Thread* thisThread = pos.this_thread();
1455 (ss+1)->ply = ss->ply + 1;
1456 bestMove = MOVE_NONE;
1457 ss->inCheck = pos.checkers();
1460 // Check for an immediate draw or maximum ply reached
1461 if ( pos.is_draw(ss->ply)
1462 || ss->ply >= MAX_PLY)
1463 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1465 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1467 // Decide whether or not to include checks: this fixes also the type of
1468 // TT entry depth that we are going to use. Note that in qsearch we use
1469 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1470 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1471 : DEPTH_QS_NO_CHECKS;
1472 // Transposition table lookup
1474 tte = TT.probe(posKey, ss->ttHit);
1475 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1476 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1477 pvHit = ss->ttHit && tte->is_pv();
1481 && tte->depth() >= ttDepth
1482 && ttValue != VALUE_NONE // Only in case of TT access race
1483 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1484 : (tte->bound() & BOUND_UPPER)))
1487 // Evaluate the position statically
1490 ss->staticEval = VALUE_NONE;
1491 bestValue = futilityBase = -VALUE_INFINITE;
1497 // Never assume anything about values stored in TT
1498 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1499 ss->staticEval = bestValue = evaluate(pos);
1501 // Can ttValue be used as a better position evaluation?
1502 if ( ttValue != VALUE_NONE
1503 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1504 bestValue = ttValue;
1507 // In case of null move search use previous static eval with a different sign
1508 // and addition of two tempos
1509 ss->staticEval = bestValue =
1510 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1511 : -(ss-1)->staticEval + 2 * Tempo;
1513 // Stand pat. Return immediately if static value is at least beta
1514 if (bestValue >= beta)
1516 // Save gathered info in transposition table
1518 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1519 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1524 if (PvNode && bestValue > alpha)
1527 futilityBase = bestValue + 155;
1530 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1531 nullptr , (ss-4)->continuationHistory,
1532 nullptr , (ss-6)->continuationHistory };
1534 // Initialize a MovePicker object for the current position, and prepare
1535 // to search the moves. Because the depth is <= 0 here, only captures,
1536 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1537 // will be generated.
1538 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1539 &thisThread->captureHistory,
1541 to_sq((ss-1)->currentMove));
1543 // Loop through the moves until no moves remain or a beta cutoff occurs
1544 while ((move = mp.next_move()) != MOVE_NONE)
1546 assert(is_ok(move));
1548 givesCheck = pos.gives_check(move);
1549 captureOrPromotion = pos.capture_or_promotion(move);
1554 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1556 && futilityBase > -VALUE_KNOWN_WIN
1557 && !pos.advanced_pawn_push(move))
1559 assert(type_of(move) != EN_PASSANT); // Due to !pos.advanced_pawn_push
1561 // moveCount pruning
1565 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1567 if (futilityValue <= alpha)
1569 bestValue = std::max(bestValue, futilityValue);
1573 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1575 bestValue = std::max(bestValue, futilityBase);
1580 // Do not search moves with negative SEE values
1581 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1582 && !pos.see_ge(move))
1585 // Speculative prefetch as early as possible
1586 prefetch(TT.first_entry(pos.key_after(move)));
1588 // Check for legality just before making the move
1589 if (!pos.legal(move))
1595 ss->currentMove = move;
1596 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1597 [captureOrPromotion]
1598 [pos.moved_piece(move)]
1601 // CounterMove based pruning
1602 if ( !captureOrPromotion
1603 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1604 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1605 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1608 // Make and search the move
1609 pos.do_move(move, st, givesCheck);
1610 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1611 pos.undo_move(move);
1613 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1615 // Check for a new best move
1616 if (value > bestValue)
1624 if (PvNode) // Update pv even in fail-high case
1625 update_pv(ss->pv, move, (ss+1)->pv);
1627 if (PvNode && value < beta) // Update alpha here!
1635 // All legal moves have been searched. A special case: if we're in check
1636 // and no legal moves were found, it is checkmate.
1637 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1639 assert(!MoveList<LEGAL>(pos).size());
1641 return mated_in(ss->ply); // Plies to mate from the root
1644 // Save gathered info in transposition table
1645 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1646 bestValue >= beta ? BOUND_LOWER :
1647 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1648 ttDepth, bestMove, ss->staticEval);
1650 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1656 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1657 // "plies to mate from the current position". Standard scores are unchanged.
1658 // The function is called before storing a value in the transposition table.
1660 Value value_to_tt(Value v, int ply) {
1662 assert(v != VALUE_NONE);
1664 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1665 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1669 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1670 // from the transposition table (which refers to the plies to mate/be mated from
1671 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1672 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1673 // and the graph history interaction, we return an optimal TB score instead.
1675 Value value_from_tt(Value v, int ply, int r50c) {
1677 if (v == VALUE_NONE)
1680 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1682 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1683 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1688 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1690 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1691 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1700 // update_pv() adds current move and appends child pv[]
1702 void update_pv(Move* pv, Move move, Move* childPv) {
1704 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1710 // update_all_stats() updates stats at the end of search() when a bestMove is found
1712 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1713 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1716 Color us = pos.side_to_move();
1717 Thread* thisThread = pos.this_thread();
1718 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1719 Piece moved_piece = pos.moved_piece(bestMove);
1720 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1722 bonus1 = stat_bonus(depth + 1);
1723 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1724 : std::min(bonus1, stat_bonus(depth)); // smaller bonus
1726 if (!pos.capture_or_promotion(bestMove))
1728 // Increase stats for the best move in case it was a quiet move
1729 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1731 // Decrease stats for all non-best quiet moves
1732 for (int i = 0; i < quietCount; ++i)
1734 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1735 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1739 // Increase stats for the best move in case it was a capture move
1740 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1742 // Extra penalty for a quiet early move that was not a TT move or
1743 // main killer move in previous ply when it gets refuted.
1744 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1745 && !pos.captured_piece())
1746 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1748 // Decrease stats for all non-best capture moves
1749 for (int i = 0; i < captureCount; ++i)
1751 moved_piece = pos.moved_piece(capturesSearched[i]);
1752 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1753 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1758 // update_continuation_histories() updates histories of the move pairs formed
1759 // by moves at ply -1, -2, -4, and -6 with current move.
1761 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1763 for (int i : {1, 2, 4, 6})
1765 // Only update first 2 continuation histories if we are in check
1766 if (ss->inCheck && i > 2)
1768 if (is_ok((ss-i)->currentMove))
1769 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1774 // update_quiet_stats() updates move sorting heuristics
1776 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1779 if (ss->killers[0] != move)
1781 ss->killers[1] = ss->killers[0];
1782 ss->killers[0] = move;
1785 Color us = pos.side_to_move();
1786 Thread* thisThread = pos.this_thread();
1787 thisThread->mainHistory[us][from_to(move)] << bonus;
1788 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1790 // Penalty for reversed move in case of moved piece not being a pawn
1791 if (type_of(pos.moved_piece(move)) != PAWN)
1792 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1794 // Update countermove history
1795 if (is_ok((ss-1)->currentMove))
1797 Square prevSq = to_sq((ss-1)->currentMove);
1798 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1801 // Update low ply history
1802 if (depth > 11 && ss->ply < MAX_LPH)
1803 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1806 // When playing with strength handicap, choose best move among a set of RootMoves
1807 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1809 Move Skill::pick_best(size_t multiPV) {
1811 const RootMoves& rootMoves = Threads.main()->rootMoves;
1812 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1814 // RootMoves are already sorted by score in descending order
1815 Value topScore = rootMoves[0].score;
1816 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1817 int weakness = 120 - 2 * level;
1818 int maxScore = -VALUE_INFINITE;
1820 // Choose best move. For each move score we add two terms, both dependent on
1821 // weakness. One is deterministic and bigger for weaker levels, and one is
1822 // random. Then we choose the move with the resulting highest score.
1823 for (size_t i = 0; i < multiPV; ++i)
1825 // This is our magic formula
1826 int push = ( weakness * int(topScore - rootMoves[i].score)
1827 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1829 if (rootMoves[i].score + push >= maxScore)
1831 maxScore = rootMoves[i].score + push;
1832 best = rootMoves[i].pv[0];
1842 /// MainThread::check_time() is used to print debug info and, more importantly,
1843 /// to detect when we are out of available time and thus stop the search.
1845 void MainThread::check_time() {
1850 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1851 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1853 static TimePoint lastInfoTime = now();
1855 TimePoint elapsed = Time.elapsed();
1856 TimePoint tick = Limits.startTime + elapsed;
1858 if (tick - lastInfoTime >= 1000)
1860 lastInfoTime = tick;
1864 // We should not stop pondering until told so by the GUI
1868 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1869 || (Limits.movetime && elapsed >= Limits.movetime)
1870 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1871 Threads.stop = true;
1875 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1876 /// that all (if any) unsearched PV lines are sent using a previous search score.
1878 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1880 std::stringstream ss;
1881 TimePoint elapsed = Time.elapsed() + 1;
1882 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1883 size_t pvIdx = pos.this_thread()->pvIdx;
1884 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1885 uint64_t nodesSearched = Threads.nodes_searched();
1886 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1888 for (size_t i = 0; i < multiPV; ++i)
1890 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1892 if (depth == 1 && !updated && i > 0)
1895 Depth d = updated ? depth : std::max(1, depth - 1);
1896 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1898 if (v == -VALUE_INFINITE)
1901 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1902 v = tb ? rootMoves[i].tbScore : v;
1904 if (ss.rdbuf()->in_avail()) // Not at first line
1909 << " seldepth " << rootMoves[i].selDepth
1910 << " multipv " << i + 1
1911 << " score " << UCI::value(v);
1913 if (Options["UCI_ShowWDL"])
1914 ss << UCI::wdl(v, pos.game_ply());
1916 if (!tb && i == pvIdx)
1917 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1919 ss << " nodes " << nodesSearched
1920 << " nps " << nodesSearched * 1000 / elapsed;
1922 if (elapsed > 1000) // Earlier makes little sense
1923 ss << " hashfull " << TT.hashfull();
1925 ss << " tbhits " << tbHits
1926 << " time " << elapsed
1929 for (Move m : rootMoves[i].pv)
1930 ss << " " << UCI::move(m, pos.is_chess960());
1937 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1938 /// before exiting the search, for instance, in case we stop the search during a
1939 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1940 /// otherwise in case of 'ponder on' we have nothing to think on.
1942 bool RootMove::extract_ponder_from_tt(Position& pos) {
1945 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
1949 assert(pv.size() == 1);
1951 if (pv[0] == MOVE_NONE)
1954 pos.do_move(pv[0], st);
1955 TTEntry* tte = TT.probe(pos.key(), ttHit);
1959 Move m = tte->move(); // Local copy to be SMP safe
1960 if (MoveList<LEGAL>(pos).contains(m))
1964 pos.undo_move(pv[0]);
1965 return pv.size() > 1;
1968 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1971 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1972 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1973 Cardinality = int(Options["SyzygyProbeLimit"]);
1974 bool dtz_available = true;
1976 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1977 // ProbeDepth == DEPTH_ZERO
1978 if (Cardinality > MaxCardinality)
1980 Cardinality = MaxCardinality;
1984 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1986 // Rank moves using DTZ tables
1987 RootInTB = root_probe(pos, rootMoves);
1991 // DTZ tables are missing; try to rank moves using WDL tables
1992 dtz_available = false;
1993 RootInTB = root_probe_wdl(pos, rootMoves);
1999 // Sort moves according to TB rank
2000 std::stable_sort(rootMoves.begin(), rootMoves.end(),
2001 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
2003 // Probe during search only if DTZ is not available and we are winning
2004 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
2009 // Clean up if root_probe() and root_probe_wdl() have failed
2010 for (auto& m : rootMoves)