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"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 constexpr uint64_t TtHitAverageWindow = 4096;
65 constexpr uint64_t TtHitAverageResolution = 1024;
68 Value futility_margin(Depth d, bool improving) {
69 return Value(234 * (d - improving));
72 // Reductions lookup table, initialized at startup
73 int Reductions[MAX_MOVES]; // [depth or moveNumber]
75 Depth reduction(bool i, Depth d, int mn) {
76 int r = Reductions[d] * Reductions[mn];
77 return (r + 503) / 1024 + (!i && r > 915);
80 constexpr int futility_move_count(bool improving, Depth depth) {
81 return (3 + depth * depth) / (2 - improving);
84 // History and stats update bonus, based on depth
85 int stat_bonus(Depth d) {
86 return d > 14 ? 66 : 6 * d * d + 231 * d - 206;
89 // Add a small random component to draw evaluations to avoid 3-fold blindness
90 Value value_draw(Thread* thisThread) {
91 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 // Breadcrumbs are used to mark nodes as being searched by a given thread
107 std::atomic<Thread*> thread;
108 std::atomic<Key> key;
110 std::array<Breadcrumb, 1024> breadcrumbs;
112 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
113 // node for potential reductions. A free node will be marked upon entering the moves
114 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
115 struct ThreadHolding {
116 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
117 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
122 // See if another already marked this location, if not, mark it ourselves
123 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
126 (*location).thread.store(thisThread, std::memory_order_relaxed);
127 (*location).key.store(posKey, std::memory_order_relaxed);
130 else if ( tmp != thisThread
131 && (*location).key.load(std::memory_order_relaxed) == posKey)
137 if (owning) // Free the marked location
138 (*location).thread.store(nullptr, std::memory_order_relaxed);
141 bool marked() { return otherThread; }
144 Breadcrumb* location;
145 bool otherThread, owning;
148 template <NodeType NT>
149 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
151 template <NodeType NT>
152 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
154 Value value_to_tt(Value v, int ply);
155 Value value_from_tt(Value v, int ply, int r50c);
156 void update_pv(Move* pv, Move move, Move* childPv);
157 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
158 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
159 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
160 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
162 // perft() is our utility to verify move generation. All the leaf nodes up
163 // to the given depth are generated and counted, and the sum is returned.
165 uint64_t perft(Position& pos, Depth depth) {
168 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
170 uint64_t cnt, nodes = 0;
171 const bool leaf = (depth == 2);
173 for (const auto& m : MoveList<LEGAL>(pos))
175 if (Root && depth <= 1)
180 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
185 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
193 /// Search::init() is called at startup to initialize various lookup tables
195 void Search::init() {
197 for (int i = 1; i < MAX_MOVES; ++i)
198 Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
202 /// Search::clear() resets search state to its initial value
204 void Search::clear() {
206 Threads.main()->wait_for_search_finished();
208 Time.availableNodes = 0;
211 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
215 /// MainThread::search() is started when the program receives the UCI 'go'
216 /// command. It searches from the root position and outputs the "bestmove".
218 void MainThread::search() {
222 nodes = perft<true>(rootPos, Limits.perft);
223 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
227 Color us = rootPos.side_to_move();
228 Time.init(Limits, us, rootPos.game_ply());
231 Eval::NNUE::verify();
233 if (rootMoves.empty())
235 rootMoves.emplace_back(MOVE_NONE);
236 sync_cout << "info depth 0 score "
237 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
242 Threads.start_searching(); // start non-main threads
243 Thread::search(); // main thread start searching
246 // When we reach the maximum depth, we can arrive here without a raise of
247 // Threads.stop. However, if we are pondering or in an infinite search,
248 // the UCI protocol states that we shouldn't print the best move before the
249 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
250 // until the GUI sends one of those commands.
252 while (!Threads.stop && (ponder || Limits.infinite))
253 {} // Busy wait for a stop or a ponder reset
255 // Stop the threads if not already stopped (also raise the stop if
256 // "ponderhit" just reset Threads.ponder).
259 // Wait until all threads have finished
260 Threads.wait_for_search_finished();
262 // When playing in 'nodes as time' mode, subtract the searched nodes from
263 // the available ones before exiting.
265 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
267 Thread* bestThread = this;
269 if ( int(Options["MultiPV"]) == 1
271 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
272 && rootMoves[0].pv[0] != MOVE_NONE)
273 bestThread = Threads.get_best_thread();
275 bestPreviousScore = bestThread->rootMoves[0].score;
277 // Send again PV info if we have a new best thread
278 if (bestThread != this)
279 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
281 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
283 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
284 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
286 std::cout << sync_endl;
290 /// Thread::search() is the main iterative deepening loop. It calls search()
291 /// repeatedly with increasing depth until the allocated thinking time has been
292 /// consumed, the user stops the search, or the maximum search depth is reached.
294 void Thread::search() {
296 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
297 // The former is needed to allow update_continuation_histories(ss-1, ...),
298 // which accesses its argument at ss-6, also near the root.
299 // The latter is needed for statScores and killer initialization.
300 Stack stack[MAX_PLY+10], *ss = stack+7;
302 Value bestValue, alpha, beta, delta;
303 Move lastBestMove = MOVE_NONE;
304 Depth lastBestMoveDepth = 0;
305 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
306 double timeReduction = 1, totBestMoveChanges = 0;
307 Color us = rootPos.side_to_move();
310 std::memset(ss-7, 0, 10 * sizeof(Stack));
311 for (int i = 7; i > 0; i--)
312 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
316 bestValue = delta = alpha = -VALUE_INFINITE;
317 beta = VALUE_INFINITE;
321 if (mainThread->bestPreviousScore == VALUE_INFINITE)
322 for (int i = 0; i < 4; ++i)
323 mainThread->iterValue[i] = VALUE_ZERO;
325 for (int i = 0; i < 4; ++i)
326 mainThread->iterValue[i] = mainThread->bestPreviousScore;
329 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
330 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
332 size_t multiPV = size_t(Options["MultiPV"]);
334 // Pick integer skill levels, but non-deterministically round up or down
335 // such that the average integer skill corresponds to the input floating point one.
336 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
337 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
338 // for match (TC 60+0.6) results spanning a wide range of k values.
340 double floatLevel = Options["UCI_LimitStrength"] ?
341 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
342 double(Options["Skill Level"]);
343 int intLevel = int(floatLevel) +
344 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
345 Skill skill(intLevel);
347 // When playing with strength handicap enable MultiPV search that we will
348 // use behind the scenes to retrieve a set of possible moves.
350 multiPV = std::max(multiPV, (size_t)4);
352 multiPV = std::min(multiPV, rootMoves.size());
353 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
355 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
357 // In analysis mode, adjust contempt in accordance with user preference
358 if (Limits.infinite || Options["UCI_AnalyseMode"])
359 ct = Options["Analysis Contempt"] == "Off" ? 0
360 : Options["Analysis Contempt"] == "Both" ? ct
361 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
362 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
365 // Evaluation score is from the white point of view
366 contempt = (us == WHITE ? make_score(ct, ct / 2)
367 : -make_score(ct, ct / 2));
369 int searchAgainCounter = 0;
371 // Iterative deepening loop until requested to stop or the target depth is reached
372 while ( ++rootDepth < MAX_PLY
374 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
376 // Age out PV variability metric
378 totBestMoveChanges /= 2;
380 // Save the last iteration's scores before first PV line is searched and
381 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
382 for (RootMove& rm : rootMoves)
383 rm.previousScore = rm.score;
388 if (!Threads.increaseDepth)
389 searchAgainCounter++;
391 // MultiPV loop. We perform a full root search for each PV line
392 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
397 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
398 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
402 // Reset UCI info selDepth for each depth and each PV line
405 // Reset aspiration window starting size
408 Value prev = rootMoves[pvIdx].previousScore;
410 alpha = std::max(prev - delta,-VALUE_INFINITE);
411 beta = std::min(prev + delta, VALUE_INFINITE);
413 // Adjust contempt based on root move's previousScore (dynamic contempt)
414 int dct = ct + (113 - ct / 2) * prev / (abs(prev) + 147);
416 contempt = (us == WHITE ? make_score(dct, dct / 2)
417 : -make_score(dct, dct / 2));
420 // Start with a small aspiration window and, in the case of a fail
421 // high/low, re-search with a bigger window until we don't fail
426 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
427 bestValue = Stockfish::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
429 // Bring the best move to the front. It is critical that sorting
430 // is done with a stable algorithm because all the values but the
431 // first and eventually the new best one are set to -VALUE_INFINITE
432 // and we want to keep the same order for all the moves except the
433 // new PV that goes to the front. Note that in case of MultiPV
434 // search the already searched PV lines are preserved.
435 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
437 // If search has been stopped, we break immediately. Sorting is
438 // safe because RootMoves is still valid, although it refers to
439 // the previous iteration.
443 // When failing high/low give some update (without cluttering
444 // the UI) before a re-search.
447 && (bestValue <= alpha || bestValue >= beta)
448 && Time.elapsed() > 3000)
449 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
451 // In case of failing low/high increase aspiration window and
452 // re-search, otherwise exit the loop.
453 if (bestValue <= alpha)
455 beta = (alpha + beta) / 2;
456 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
460 mainThread->stopOnPonderhit = false;
462 else if (bestValue >= beta)
464 beta = std::min(bestValue + delta, VALUE_INFINITE);
470 delta += delta / 4 + 5;
472 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
475 // Sort the PV lines searched so far and update the GUI
476 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
479 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
480 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
484 completedDepth = rootDepth;
486 if (rootMoves[0].pv[0] != lastBestMove) {
487 lastBestMove = rootMoves[0].pv[0];
488 lastBestMoveDepth = rootDepth;
491 // Have we found a "mate in x"?
493 && bestValue >= VALUE_MATE_IN_MAX_PLY
494 && VALUE_MATE - bestValue <= 2 * Limits.mate)
500 // If skill level is enabled and time is up, pick a sub-optimal best move
501 if (skill.enabled() && skill.time_to_pick(rootDepth))
502 skill.pick_best(multiPV);
504 // Do we have time for the next iteration? Can we stop searching now?
505 if ( Limits.use_time_management()
507 && !mainThread->stopOnPonderhit)
509 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
510 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
511 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
513 // If the bestMove is stable over several iterations, reduce time accordingly
514 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
515 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
517 // Use part of the gained time from a previous stable move for the current move
518 for (Thread* th : Threads)
520 totBestMoveChanges += th->bestMoveChanges;
521 th->bestMoveChanges = 0;
523 double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
525 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
527 // Cap used time in case of a single legal move for a better viewer experience in tournaments
528 // yielding correct scores and sufficiently fast moves.
529 if (rootMoves.size() == 1)
530 totalTime = std::min(500.0, totalTime);
532 // Stop the search if we have exceeded the totalTime
533 if (Time.elapsed() > totalTime)
535 // If we are allowed to ponder do not stop the search now but
536 // keep pondering until the GUI sends "ponderhit" or "stop".
537 if (mainThread->ponder)
538 mainThread->stopOnPonderhit = true;
542 else if ( Threads.increaseDepth
543 && !mainThread->ponder
544 && Time.elapsed() > totalTime * 0.58)
545 Threads.increaseDepth = false;
547 Threads.increaseDepth = true;
550 mainThread->iterValue[iterIdx] = bestValue;
551 iterIdx = (iterIdx + 1) & 3;
557 mainThread->previousTimeReduction = timeReduction;
559 // If skill level is enabled, swap best PV line with the sub-optimal one
561 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
562 skill.best ? skill.best : skill.pick_best(multiPV)));
568 // search<>() is the main search function for both PV and non-PV nodes
570 template <NodeType NT>
571 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
573 constexpr bool PvNode = NT == PV;
574 const bool rootNode = PvNode && ss->ply == 0;
575 const Depth maxNextDepth = rootNode ? depth : depth + 1;
577 // Check if we have an upcoming move which draws by repetition, or
578 // if the opponent had an alternative move earlier to this position.
579 if ( pos.rule50_count() >= 3
580 && alpha < VALUE_DRAW
582 && pos.has_game_cycle(ss->ply))
584 alpha = value_draw(pos.this_thread());
589 // Dive into quiescence search when the depth reaches zero
591 return qsearch<NT>(pos, ss, alpha, beta);
593 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
594 assert(PvNode || (alpha == beta - 1));
595 assert(0 < depth && depth < MAX_PLY);
596 assert(!(PvNode && cutNode));
598 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
600 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
604 Move ttMove, move, excludedMove, bestMove;
605 Depth extension, newDepth;
606 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
607 bool formerPv, givesCheck, improving, didLMR, priorCapture;
608 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
609 ttCapture, singularQuietLMR;
611 int moveCount, captureCount, quietCount;
613 // Step 1. Initialize node
614 Thread* thisThread = pos.this_thread();
615 ss->inCheck = pos.checkers();
616 priorCapture = pos.captured_piece();
617 Color us = pos.side_to_move();
618 moveCount = captureCount = quietCount = ss->moveCount = 0;
619 bestValue = -VALUE_INFINITE;
620 maxValue = VALUE_INFINITE;
622 // Check for the available remaining time
623 if (thisThread == Threads.main())
624 static_cast<MainThread*>(thisThread)->check_time();
626 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
627 if (PvNode && thisThread->selDepth < ss->ply + 1)
628 thisThread->selDepth = ss->ply + 1;
632 // Step 2. Check for aborted search and immediate draw
633 if ( Threads.stop.load(std::memory_order_relaxed)
634 || pos.is_draw(ss->ply)
635 || ss->ply >= MAX_PLY)
636 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
637 : value_draw(pos.this_thread());
639 // Step 3. Mate distance pruning. Even if we mate at the next move our score
640 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
641 // a shorter mate was found upward in the tree then there is no need to search
642 // because we will never beat the current alpha. Same logic but with reversed
643 // signs applies also in the opposite condition of being mated instead of giving
644 // mate. In this case return a fail-high score.
645 alpha = std::max(mated_in(ss->ply), alpha);
646 beta = std::min(mate_in(ss->ply+1), beta);
651 assert(0 <= ss->ply && ss->ply < MAX_PLY);
653 (ss+1)->ply = ss->ply + 1;
654 (ss+1)->ttPv = false;
655 (ss+1)->excludedMove = bestMove = MOVE_NONE;
656 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
657 Square prevSq = to_sq((ss-1)->currentMove);
659 // Initialize statScore to zero for the grandchildren of the current position.
660 // So statScore is shared between all grandchildren and only the first grandchild
661 // starts with statScore = 0. Later grandchildren start with the last calculated
662 // statScore of the previous grandchild. This influences the reduction rules in
663 // LMR which are based on the statScore of parent position.
665 (ss+2)->statScore = 0;
667 // Step 4. Transposition table lookup. We don't want the score of a partial
668 // search to overwrite a previous full search TT value, so we use a different
669 // position key in case of an excluded move.
670 excludedMove = ss->excludedMove;
671 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
672 tte = TT.probe(posKey, ss->ttHit);
673 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
674 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
675 : ss->ttHit ? tte->move() : MOVE_NONE;
677 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
678 formerPv = ss->ttPv && !PvNode;
680 // Update low ply history for previous move if we are near root and position is or has been in PV
683 && ss->ply - 1 < MAX_LPH
685 && is_ok((ss-1)->currentMove))
686 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
688 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
689 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
690 + TtHitAverageResolution * ss->ttHit;
692 // At non-PV nodes we check for an early TT cutoff
695 && tte->depth() >= depth
696 && ttValue != VALUE_NONE // Possible in case of TT access race
697 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
698 : (tte->bound() & BOUND_UPPER)))
700 // If ttMove is quiet, update move sorting heuristics on TT hit
705 // Bonus for a quiet ttMove that fails high
706 if (!pos.capture_or_promotion(ttMove))
707 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
709 // Extra penalty for early quiet moves of the previous ply
710 if ((ss-1)->moveCount <= 2 && !priorCapture)
711 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
713 // Penalty for a quiet ttMove that fails low
714 else if (!pos.capture_or_promotion(ttMove))
716 int penalty = -stat_bonus(depth);
717 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
718 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
722 // Partial workaround for the graph history interaction problem
723 // For high rule50 counts don't produce transposition table cutoffs.
724 if (pos.rule50_count() < 90)
728 // Step 5. Tablebases probe
729 if (!rootNode && TB::Cardinality)
731 int piecesCount = pos.count<ALL_PIECES>();
733 if ( piecesCount <= TB::Cardinality
734 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
735 && pos.rule50_count() == 0
736 && !pos.can_castle(ANY_CASTLING))
739 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
741 // Force check of time on the next occasion
742 if (thisThread == Threads.main())
743 static_cast<MainThread*>(thisThread)->callsCnt = 0;
745 if (err != TB::ProbeState::FAIL)
747 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
749 int drawScore = TB::UseRule50 ? 1 : 0;
751 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
752 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
753 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
754 : VALUE_DRAW + 2 * wdl * drawScore;
756 Bound b = wdl < -drawScore ? BOUND_UPPER
757 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
759 if ( b == BOUND_EXACT
760 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
762 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
763 std::min(MAX_PLY - 1, depth + 6),
764 MOVE_NONE, VALUE_NONE);
771 if (b == BOUND_LOWER)
772 bestValue = value, alpha = std::max(alpha, bestValue);
780 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
782 // Step 6. Static evaluation of the position
785 // Skip early pruning when in check
786 ss->staticEval = eval = VALUE_NONE;
792 // Never assume anything about values stored in TT
793 ss->staticEval = eval = tte->eval();
794 if (eval == VALUE_NONE)
795 ss->staticEval = eval = evaluate(pos);
797 // Randomize draw evaluation
798 if (eval == VALUE_DRAW)
799 eval = value_draw(thisThread);
801 // Can ttValue be used as a better position evaluation?
802 if ( ttValue != VALUE_NONE
803 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
808 // In case of null move search use previous static eval with a different sign
809 // and addition of two tempos
810 if ((ss-1)->currentMove != MOVE_NULL)
811 ss->staticEval = eval = evaluate(pos);
813 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
815 // Save static evaluation into transposition table
816 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
819 // Use static evaluation difference to improve quiet move ordering
820 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
822 int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
823 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
826 // Set up improving flag that is used in various pruning heuristics
827 // We define position as improving if static evaluation of position is better
828 // Than the previous static evaluation at our turn
829 // In case of us being in check at our previous move we look at move prior to it
830 improving = (ss-2)->staticEval == VALUE_NONE
831 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
832 : ss->staticEval > (ss-2)->staticEval;
834 // Step 7. Futility pruning: child node (~50 Elo)
837 && eval - futility_margin(depth, improving) >= beta
838 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
841 // Step 8. Null move search with verification search (~40 Elo)
843 && (ss-1)->currentMove != MOVE_NULL
844 && (ss-1)->statScore < 24185
846 && eval >= ss->staticEval
847 && ss->staticEval >= beta - 24 * depth - 34 * improving + 162 * ss->ttPv + 159
849 && pos.non_pawn_material(us)
850 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
852 assert(eval - beta >= 0);
854 // Null move dynamic reduction based on depth and value
855 Depth R = (1062 + 68 * depth) / 256 + std::min(int(eval - beta) / 190, 3);
857 ss->currentMove = MOVE_NULL;
858 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
860 pos.do_null_move(st);
862 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
864 pos.undo_null_move();
866 if (nullValue >= beta)
868 // Do not return unproven mate or TB scores
869 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
872 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
875 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
877 // Do verification search at high depths, with null move pruning disabled
878 // for us, until ply exceeds nmpMinPly.
879 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
880 thisThread->nmpColor = us;
882 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
884 thisThread->nmpMinPly = 0;
891 probCutBeta = beta + 209 - 44 * improving;
893 // Step 9. ProbCut (~10 Elo)
894 // If we have a good enough capture and a reduced search returns a value
895 // much above beta, we can (almost) safely prune the previous move.
898 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
899 // if value from transposition table is lower than probCutBeta, don't attempt probCut
900 // there and in further interactions with transposition table cutoff depth is set to depth - 3
901 // because probCut search has depth set to depth - 4 but we also do a move before it
902 // so effective depth is equal to depth - 3
904 && tte->depth() >= depth - 3
905 && ttValue != VALUE_NONE
906 && ttValue < probCutBeta))
908 // if ttMove is a capture and value from transposition table is good enough produce probCut
909 // cutoff without digging into actual probCut search
911 && tte->depth() >= depth - 3
912 && ttValue != VALUE_NONE
913 && ttValue >= probCutBeta
915 && pos.capture_or_promotion(ttMove))
918 assert(probCutBeta < VALUE_INFINITE);
920 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
921 int probCutCount = 0;
922 bool ttPv = ss->ttPv;
925 while ( (move = mp.next_move()) != MOVE_NONE
926 && probCutCount < 2 + 2 * cutNode)
927 if (move != excludedMove && pos.legal(move))
929 assert(pos.capture_or_promotion(move));
932 captureOrPromotion = true;
935 ss->currentMove = move;
936 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
938 [pos.moved_piece(move)]
941 pos.do_move(move, st);
943 // Perform a preliminary qsearch to verify that the move holds
944 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
946 // If the qsearch held, perform the regular search
947 if (value >= probCutBeta)
948 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
952 if (value >= probCutBeta)
954 // if transposition table doesn't have equal or more deep info write probCut data into it
956 && tte->depth() >= depth - 3
957 && ttValue != VALUE_NONE))
958 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
960 depth - 3, move, ss->staticEval);
967 // Step 10. If the position is not in TT, decrease depth by 2
973 moves_loop: // When in check, search starts from here
975 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
977 // Step 11. A small Probcut idea, when we are in check
978 probCutBeta = beta + 400;
983 && (tte->bound() & BOUND_LOWER)
984 && tte->depth() >= depth - 3
985 && ttValue >= probCutBeta
986 && abs(ttValue) <= VALUE_KNOWN_WIN
987 && abs(beta) <= VALUE_KNOWN_WIN
992 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
993 nullptr , (ss-4)->continuationHistory,
994 nullptr , (ss-6)->continuationHistory };
996 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
998 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
999 &thisThread->lowPlyHistory,
1007 singularQuietLMR = moveCountPruning = false;
1009 // Mark this node as being searched
1010 ThreadHolding th(thisThread, posKey, ss->ply);
1012 // Step 12. Loop through all pseudo-legal moves until no moves remain
1013 // or a beta cutoff occurs.
1014 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
1016 assert(is_ok(move));
1018 if (move == excludedMove)
1021 // At root obey the "searchmoves" option and skip moves not listed in Root
1022 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1023 // mode we also skip PV moves which have been already searched and those
1024 // of lower "TB rank" if we are in a TB root position.
1025 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1026 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1029 // Check for legality
1030 if (!rootNode && !pos.legal(move))
1033 ss->moveCount = ++moveCount;
1035 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1036 sync_cout << "info depth " << depth
1037 << " currmove " << UCI::move(move, pos.is_chess960())
1038 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1040 (ss+1)->pv = nullptr;
1043 captureOrPromotion = pos.capture_or_promotion(move);
1044 movedPiece = pos.moved_piece(move);
1045 givesCheck = pos.gives_check(move);
1047 // Indicate PvNodes that will probably fail low if node was searched with non-PV search
1048 // at depth equal or greater to current depth and result of this search was far below alpha
1049 bool likelyFailLow = PvNode
1051 && (tte->bound() & BOUND_UPPER)
1052 && ttValue < alpha + 200 + 100 * depth
1053 && tte->depth() >= depth;
1055 // Calculate new depth for this move
1056 newDepth = depth - 1;
1058 // Step 13. Pruning at shallow depth (~200 Elo)
1060 && pos.non_pawn_material(us)
1061 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1063 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1064 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1066 // Reduced depth of the next LMR search
1067 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1069 if ( captureOrPromotion
1072 // Capture history based pruning when the move doesn't give check
1075 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1078 // SEE based pruning
1079 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1084 // Countermoves based pruning (~20 Elo)
1085 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1086 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1087 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1090 // Futility pruning: parent node (~5 Elo)
1093 && ss->staticEval + 174 + 157 * lmrDepth <= alpha
1094 && (*contHist[0])[movedPiece][to_sq(move)]
1095 + (*contHist[1])[movedPiece][to_sq(move)]
1096 + (*contHist[3])[movedPiece][to_sq(move)]
1097 + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 28255)
1100 // Prune moves with negative SEE (~20 Elo)
1101 if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1106 // Step 14. Extensions (~75 Elo)
1108 // Singular extension search (~70 Elo). If all moves but one fail low on a
1109 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1110 // then that move is singular and should be extended. To verify this we do
1111 // a reduced search on all the other moves but the ttMove and if the
1112 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1116 && !excludedMove // Avoid recursive singular search
1117 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1118 && abs(ttValue) < VALUE_KNOWN_WIN
1119 && (tte->bound() & BOUND_LOWER)
1120 && tte->depth() >= depth - 3)
1122 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1123 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1125 ss->excludedMove = move;
1126 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1127 ss->excludedMove = MOVE_NONE;
1129 if (value < singularBeta)
1132 singularQuietLMR = !ttCapture;
1133 if (!PvNode && value < singularBeta - 140)
1137 // Multi-cut pruning
1138 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1139 // search without the ttMove. So we assume this expected Cut-node is not singular,
1140 // that multiple moves fail high, and we can prune the whole subtree by returning
1142 else if (singularBeta >= beta)
1143 return singularBeta;
1145 // If the eval of ttMove is greater than beta we try also if there is another
1146 // move that pushes it over beta, if so also produce a cutoff.
1147 else if (ttValue >= beta)
1149 ss->excludedMove = move;
1150 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1151 ss->excludedMove = MOVE_NONE;
1158 // Add extension to new depth
1159 newDepth += extension;
1161 // Speculative prefetch as early as possible
1162 prefetch(TT.first_entry(pos.key_after(move)));
1164 // Update the current move (this must be done after singular extension search)
1165 ss->currentMove = move;
1166 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1167 [captureOrPromotion]
1171 // Step 15. Make the move
1172 pos.do_move(move, st, givesCheck);
1174 // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
1175 // We use various heuristics for the sons of a node after the first son has
1176 // been searched. In general we would like to reduce them, but there are many
1177 // cases where we extend a son if it has good chances to be "interesting".
1179 && moveCount > 1 + 2 * rootNode
1180 && ( !captureOrPromotion
1182 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1184 || (!PvNode && !formerPv && captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 3678)
1185 || thisThread->ttHitAverage < 432 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1186 && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
1188 Depth r = reduction(improving, depth, moveCount);
1190 // Decrease reduction if the ttHit running average is large
1191 if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1194 // Increase reduction if other threads are searching this position
1198 // Decrease reduction if position is or has been on the PV
1199 // and node is not likely to fail low. (~10 Elo)
1204 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1205 if ( (rootNode || !PvNode)
1206 && thisThread->rootDepth > 10
1207 && thisThread->bestMoveChanges <= 2)
1210 // More reductions for late moves if position was not in previous PV
1211 if ( moveCountPruning
1215 // Decrease reduction if opponent's move count is high (~5 Elo)
1216 if ((ss-1)->moveCount > 13)
1219 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1220 if (singularQuietLMR)
1223 if (captureOrPromotion)
1225 // Increase reduction for non-checking captures likely to be bad
1227 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
1232 // Increase reduction if ttMove is a capture (~5 Elo)
1236 // Increase reduction at root if failing high
1237 r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0;
1239 // Increase reduction for cut nodes (~10 Elo)
1243 // Decrease reduction for moves that escape a capture. Filter out
1244 // castling moves, because they are coded as "king captures rook" and
1245 // hence break reverse_move() (~2 Elo)
1246 else if ( type_of(move) == NORMAL
1247 && !pos.see_ge(reverse_move(move)))
1248 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1250 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1251 + (*contHist[0])[movedPiece][to_sq(move)]
1252 + (*contHist[1])[movedPiece][to_sq(move)]
1253 + (*contHist[3])[movedPiece][to_sq(move)]
1256 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1257 if (ss->statScore >= -89 && (ss-1)->statScore < -116)
1260 else if ((ss-1)->statScore >= -112 && ss->statScore < -100)
1263 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1264 // If we are not in check use statScore, but if we are in check we use
1265 // the sum of main history and first continuation history with an offset.
1267 r -= (thisThread->mainHistory[us][from_to(move)]
1268 + (*contHist[0])[movedPiece][to_sq(move)] - 3833) / 16384;
1270 r -= ss->statScore / 14790;
1273 // In general we want to cap the LMR depth search at newDepth. But if
1274 // reductions are really negative and movecount is low, we allow this move
1275 // to be searched deeper than the first move.
1276 Depth d = std::clamp(newDepth - r, 1, newDepth + (r < -1 && moveCount <= 5));
1278 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1280 // If the son is reduced and fails high it will be re-searched at full depth
1281 doFullDepthSearch = value > alpha && d < newDepth;
1286 doFullDepthSearch = !PvNode || moveCount > 1;
1290 // Step 17. Full depth search when LMR is skipped or fails high
1291 if (doFullDepthSearch)
1293 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1295 // If the move passed LMR update its stats
1296 if (didLMR && !captureOrPromotion)
1298 int bonus = value > alpha ? stat_bonus(newDepth)
1299 : -stat_bonus(newDepth);
1301 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1305 // For PV nodes only, do a full PV search on the first move or after a fail
1306 // high (in the latter case search only if value < beta), otherwise let the
1307 // parent node fail low with value <= alpha and try another move.
1308 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1311 (ss+1)->pv[0] = MOVE_NONE;
1313 value = -search<PV>(pos, ss+1, -beta, -alpha,
1314 std::min(maxNextDepth, newDepth), false);
1317 // Step 18. Undo move
1318 pos.undo_move(move);
1320 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1322 // Step 19. Check for a new best move
1323 // Finished searching the move. If a stop occurred, the return value of
1324 // the search cannot be trusted, and we return immediately without
1325 // updating best move, PV and TT.
1326 if (Threads.stop.load(std::memory_order_relaxed))
1331 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1332 thisThread->rootMoves.end(), move);
1334 // PV move or new best move?
1335 if (moveCount == 1 || value > alpha)
1338 rm.selDepth = thisThread->selDepth;
1343 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1344 rm.pv.push_back(*m);
1346 // We record how often the best move has been changed in each
1347 // iteration. This information is used for time management and LMR
1349 ++thisThread->bestMoveChanges;
1352 // All other moves but the PV are set to the lowest value: this
1353 // is not a problem when sorting because the sort is stable and the
1354 // move position in the list is preserved - just the PV is pushed up.
1355 rm.score = -VALUE_INFINITE;
1358 if (value > bestValue)
1366 if (PvNode && !rootNode) // Update pv even in fail-high case
1367 update_pv(ss->pv, move, (ss+1)->pv);
1369 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1373 assert(value >= beta); // Fail high
1380 // If the move is worse than some previously searched move, remember it to update its stats later
1381 if (move != bestMove)
1383 if (captureOrPromotion && captureCount < 32)
1384 capturesSearched[captureCount++] = move;
1386 else if (!captureOrPromotion && quietCount < 64)
1387 quietsSearched[quietCount++] = move;
1391 // The following condition would detect a stop only after move loop has been
1392 // completed. But in this case bestValue is valid because we have fully
1393 // searched our subtree, and we can anyhow save the result in TT.
1399 // Step 20. Check for mate and stalemate
1400 // All legal moves have been searched and if there are no legal moves, it
1401 // must be a mate or a stalemate. If we are in a singular extension search then
1402 // return a fail low score.
1404 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1407 bestValue = excludedMove ? alpha :
1408 ss->inCheck ? mated_in(ss->ply)
1411 // If there is a move which produces search value greater than alpha we update stats of searched moves
1413 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1414 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1416 // Bonus for prior countermove that caused the fail low
1417 else if ( (depth >= 3 || PvNode)
1419 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1422 bestValue = std::min(bestValue, maxValue);
1424 // If no good move is found and the previous position was ttPv, then the previous
1425 // opponent move is probably good and the new position is added to the search tree.
1426 if (bestValue <= alpha)
1427 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1428 // Otherwise, a counter move has been found and if the position is the last leaf
1429 // in the search tree, remove the position from the search tree.
1431 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1433 // Write gathered information in transposition table
1434 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1435 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1436 bestValue >= beta ? BOUND_LOWER :
1437 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1438 depth, bestMove, ss->staticEval);
1440 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1446 // qsearch() is the quiescence search function, which is called by the main search
1447 // function with zero depth, or recursively with further decreasing depth per call.
1448 template <NodeType NT>
1449 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1451 constexpr bool PvNode = NT == PV;
1453 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1454 assert(PvNode || (alpha == beta - 1));
1459 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1463 Move ttMove, move, bestMove;
1465 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1466 bool pvHit, givesCheck, captureOrPromotion;
1471 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1473 ss->pv[0] = MOVE_NONE;
1476 Thread* thisThread = pos.this_thread();
1477 (ss+1)->ply = ss->ply + 1;
1478 bestMove = MOVE_NONE;
1479 ss->inCheck = pos.checkers();
1482 // Check for an immediate draw or maximum ply reached
1483 if ( pos.is_draw(ss->ply)
1484 || ss->ply >= MAX_PLY)
1485 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1487 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1489 // Decide whether or not to include checks: this fixes also the type of
1490 // TT entry depth that we are going to use. Note that in qsearch we use
1491 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1492 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1493 : DEPTH_QS_NO_CHECKS;
1494 // Transposition table lookup
1496 tte = TT.probe(posKey, ss->ttHit);
1497 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1498 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1499 pvHit = ss->ttHit && tte->is_pv();
1503 && tte->depth() >= ttDepth
1504 && ttValue != VALUE_NONE // Only in case of TT access race
1505 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1506 : (tte->bound() & BOUND_UPPER)))
1509 // Evaluate the position statically
1512 ss->staticEval = VALUE_NONE;
1513 bestValue = futilityBase = -VALUE_INFINITE;
1519 // Never assume anything about values stored in TT
1520 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1521 ss->staticEval = bestValue = evaluate(pos);
1523 // Can ttValue be used as a better position evaluation?
1524 if ( ttValue != VALUE_NONE
1525 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1526 bestValue = ttValue;
1529 // In case of null move search use previous static eval with a different sign
1530 // and addition of two tempos
1531 ss->staticEval = bestValue =
1532 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1533 : -(ss-1)->staticEval + 2 * Tempo;
1535 // Stand pat. Return immediately if static value is at least beta
1536 if (bestValue >= beta)
1538 // Save gathered info in transposition table
1540 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1541 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1546 if (PvNode && bestValue > alpha)
1549 futilityBase = bestValue + 155;
1552 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1553 nullptr , (ss-4)->continuationHistory,
1554 nullptr , (ss-6)->continuationHistory };
1556 // Initialize a MovePicker object for the current position, and prepare
1557 // to search the moves. Because the depth is <= 0 here, only captures,
1558 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1559 // will be generated.
1560 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1561 &thisThread->captureHistory,
1563 to_sq((ss-1)->currentMove));
1565 // Loop through the moves until no moves remain or a beta cutoff occurs
1566 while ((move = mp.next_move()) != MOVE_NONE)
1568 assert(is_ok(move));
1570 givesCheck = pos.gives_check(move);
1571 captureOrPromotion = pos.capture_or_promotion(move);
1575 // Futility pruning and moveCount pruning
1576 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1578 && futilityBase > -VALUE_KNOWN_WIN
1579 && type_of(move) != PROMOTION)
1585 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1587 if (futilityValue <= alpha)
1589 bestValue = std::max(bestValue, futilityValue);
1593 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1595 bestValue = std::max(bestValue, futilityBase);
1600 // Do not search moves with negative SEE values
1601 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1602 && !pos.see_ge(move))
1605 // Speculative prefetch as early as possible
1606 prefetch(TT.first_entry(pos.key_after(move)));
1608 // Check for legality just before making the move
1609 if (!pos.legal(move))
1615 ss->currentMove = move;
1616 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1617 [captureOrPromotion]
1618 [pos.moved_piece(move)]
1621 // CounterMove based pruning
1622 if ( !captureOrPromotion
1623 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1624 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1625 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1628 // Make and search the move
1629 pos.do_move(move, st, givesCheck);
1630 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1631 pos.undo_move(move);
1633 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1635 // Check for a new best move
1636 if (value > bestValue)
1644 if (PvNode) // Update pv even in fail-high case
1645 update_pv(ss->pv, move, (ss+1)->pv);
1647 if (PvNode && value < beta) // Update alpha here!
1655 // All legal moves have been searched. A special case: if we're in check
1656 // and no legal moves were found, it is checkmate.
1657 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1659 assert(!MoveList<LEGAL>(pos).size());
1661 return mated_in(ss->ply); // Plies to mate from the root
1664 // Save gathered info in transposition table
1665 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1666 bestValue >= beta ? BOUND_LOWER :
1667 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1668 ttDepth, bestMove, ss->staticEval);
1670 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1676 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1677 // "plies to mate from the current position". Standard scores are unchanged.
1678 // The function is called before storing a value in the transposition table.
1680 Value value_to_tt(Value v, int ply) {
1682 assert(v != VALUE_NONE);
1684 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1685 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1689 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1690 // from the transposition table (which refers to the plies to mate/be mated from
1691 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1692 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1693 // and the graph history interaction, we return an optimal TB score instead.
1695 Value value_from_tt(Value v, int ply, int r50c) {
1697 if (v == VALUE_NONE)
1700 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1702 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1703 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1708 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1710 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1711 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1720 // update_pv() adds current move and appends child pv[]
1722 void update_pv(Move* pv, Move move, Move* childPv) {
1724 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1730 // update_all_stats() updates stats at the end of search() when a bestMove is found
1732 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1733 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1736 Color us = pos.side_to_move();
1737 Thread* thisThread = pos.this_thread();
1738 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1739 Piece moved_piece = pos.moved_piece(bestMove);
1740 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1742 bonus1 = stat_bonus(depth + 1);
1743 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1744 : std::min(bonus1, stat_bonus(depth)); // smaller bonus
1746 if (!pos.capture_or_promotion(bestMove))
1748 // Increase stats for the best move in case it was a quiet move
1749 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1751 // Decrease stats for all non-best quiet moves
1752 for (int i = 0; i < quietCount; ++i)
1754 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1755 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1759 // Increase stats for the best move in case it was a capture move
1760 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1762 // Extra penalty for a quiet early move that was not a TT move or
1763 // main killer move in previous ply when it gets refuted.
1764 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1765 && !pos.captured_piece())
1766 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1768 // Decrease stats for all non-best capture moves
1769 for (int i = 0; i < captureCount; ++i)
1771 moved_piece = pos.moved_piece(capturesSearched[i]);
1772 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1773 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1778 // update_continuation_histories() updates histories of the move pairs formed
1779 // by moves at ply -1, -2, -4, and -6 with current move.
1781 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1783 for (int i : {1, 2, 4, 6})
1785 // Only update first 2 continuation histories if we are in check
1786 if (ss->inCheck && i > 2)
1788 if (is_ok((ss-i)->currentMove))
1789 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1794 // update_quiet_stats() updates move sorting heuristics
1796 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1799 if (ss->killers[0] != move)
1801 ss->killers[1] = ss->killers[0];
1802 ss->killers[0] = move;
1805 Color us = pos.side_to_move();
1806 Thread* thisThread = pos.this_thread();
1807 thisThread->mainHistory[us][from_to(move)] << bonus;
1808 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1810 // Penalty for reversed move in case of moved piece not being a pawn
1811 if (type_of(pos.moved_piece(move)) != PAWN)
1812 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1814 // Update countermove history
1815 if (is_ok((ss-1)->currentMove))
1817 Square prevSq = to_sq((ss-1)->currentMove);
1818 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1821 // Update low ply history
1822 if (depth > 11 && ss->ply < MAX_LPH)
1823 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1826 // When playing with strength handicap, choose best move among a set of RootMoves
1827 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1829 Move Skill::pick_best(size_t multiPV) {
1831 const RootMoves& rootMoves = Threads.main()->rootMoves;
1832 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1834 // RootMoves are already sorted by score in descending order
1835 Value topScore = rootMoves[0].score;
1836 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1837 int weakness = 120 - 2 * level;
1838 int maxScore = -VALUE_INFINITE;
1840 // Choose best move. For each move score we add two terms, both dependent on
1841 // weakness. One is deterministic and bigger for weaker levels, and one is
1842 // random. Then we choose the move with the resulting highest score.
1843 for (size_t i = 0; i < multiPV; ++i)
1845 // This is our magic formula
1846 int push = ( weakness * int(topScore - rootMoves[i].score)
1847 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1849 if (rootMoves[i].score + push >= maxScore)
1851 maxScore = rootMoves[i].score + push;
1852 best = rootMoves[i].pv[0];
1862 /// MainThread::check_time() is used to print debug info and, more importantly,
1863 /// to detect when we are out of available time and thus stop the search.
1865 void MainThread::check_time() {
1870 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1871 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1873 static TimePoint lastInfoTime = now();
1875 TimePoint elapsed = Time.elapsed();
1876 TimePoint tick = Limits.startTime + elapsed;
1878 if (tick - lastInfoTime >= 1000)
1880 lastInfoTime = tick;
1884 // We should not stop pondering until told so by the GUI
1888 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1889 || (Limits.movetime && elapsed >= Limits.movetime)
1890 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1891 Threads.stop = true;
1895 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1896 /// that all (if any) unsearched PV lines are sent using a previous search score.
1898 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1900 std::stringstream ss;
1901 TimePoint elapsed = Time.elapsed() + 1;
1902 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1903 size_t pvIdx = pos.this_thread()->pvIdx;
1904 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1905 uint64_t nodesSearched = Threads.nodes_searched();
1906 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1908 for (size_t i = 0; i < multiPV; ++i)
1910 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1912 if (depth == 1 && !updated && i > 0)
1915 Depth d = updated ? depth : std::max(1, depth - 1);
1916 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1918 if (v == -VALUE_INFINITE)
1921 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1922 v = tb ? rootMoves[i].tbScore : v;
1924 if (ss.rdbuf()->in_avail()) // Not at first line
1929 << " seldepth " << rootMoves[i].selDepth
1930 << " multipv " << i + 1
1931 << " score " << UCI::value(v);
1933 if (Options["UCI_ShowWDL"])
1934 ss << UCI::wdl(v, pos.game_ply());
1936 if (!tb && i == pvIdx)
1937 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1939 ss << " nodes " << nodesSearched
1940 << " nps " << nodesSearched * 1000 / elapsed;
1942 if (elapsed > 1000) // Earlier makes little sense
1943 ss << " hashfull " << TT.hashfull();
1945 ss << " tbhits " << tbHits
1946 << " time " << elapsed
1949 for (Move m : rootMoves[i].pv)
1950 ss << " " << UCI::move(m, pos.is_chess960());
1957 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1958 /// before exiting the search, for instance, in case we stop the search during a
1959 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1960 /// otherwise in case of 'ponder on' we have nothing to think on.
1962 bool RootMove::extract_ponder_from_tt(Position& pos) {
1965 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1969 assert(pv.size() == 1);
1971 if (pv[0] == MOVE_NONE)
1974 pos.do_move(pv[0], st);
1975 TTEntry* tte = TT.probe(pos.key(), ttHit);
1979 Move m = tte->move(); // Local copy to be SMP safe
1980 if (MoveList<LEGAL>(pos).contains(m))
1984 pos.undo_move(pv[0]);
1985 return pv.size() > 1;
1988 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1991 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1992 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1993 Cardinality = int(Options["SyzygyProbeLimit"]);
1994 bool dtz_available = true;
1996 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1997 // ProbeDepth == DEPTH_ZERO
1998 if (Cardinality > MaxCardinality)
2000 Cardinality = MaxCardinality;
2004 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
2006 // Rank moves using DTZ tables
2007 RootInTB = root_probe(pos, rootMoves);
2011 // DTZ tables are missing; try to rank moves using WDL tables
2012 dtz_available = false;
2013 RootInTB = root_probe_wdl(pos, rootMoves);
2019 // Sort moves according to TB rank
2020 std::stable_sort(rootMoves.begin(), rootMoves.end(),
2021 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
2023 // Probe during search only if DTZ is not available and we are winning
2024 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
2029 // Clean up if root_probe() and root_probe_wdl() have failed
2030 for (auto& m : rootMoves)
2035 } // namespace Stockfish