2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 constexpr uint64_t TtHitAverageWindow = 4096;
65 constexpr uint64_t TtHitAverageResolution = 1024;
67 // Razor and futility margins
68 constexpr int RazorMargin = 527;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(227 * (d - improving));
73 // Reductions lookup table, initialized at startup
74 int Reductions[MAX_MOVES]; // [depth or moveNumber]
76 Depth reduction(bool i, Depth d, int mn) {
77 int r = Reductions[d] * Reductions[mn];
78 return (r + 570) / 1024 + (!i && r > 1018);
81 constexpr int futility_move_count(bool improving, Depth depth) {
82 return (3 + depth * depth) / (2 - improving);
85 // History and stats update bonus, based on depth
86 int stat_bonus(Depth d) {
87 return d > 15 ? 27 : 17 * d * d + 133 * d - 134;
90 // Add a small random component to draw evaluations to avoid 3fold-blindness
91 Value value_draw(Thread* thisThread) {
92 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 // Breadcrumbs are used to mark nodes as being searched by a given thread
108 std::atomic<Thread*> thread;
109 std::atomic<Key> key;
111 std::array<Breadcrumb, 1024> breadcrumbs;
113 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
114 // node for potential reductions. A free node will be marked upon entering the moves
115 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
116 struct ThreadHolding {
117 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
118 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
123 // See if another already marked this location, if not, mark it ourselves
124 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
127 (*location).thread.store(thisThread, std::memory_order_relaxed);
128 (*location).key.store(posKey, std::memory_order_relaxed);
131 else if ( tmp != thisThread
132 && (*location).key.load(std::memory_order_relaxed) == posKey)
138 if (owning) // Free the marked location
139 (*location).thread.store(nullptr, std::memory_order_relaxed);
142 bool marked() { return otherThread; }
145 Breadcrumb* location;
146 bool otherThread, owning;
149 template <NodeType NT>
150 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
152 template <NodeType NT>
153 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
155 Value value_to_tt(Value v, int ply);
156 Value value_from_tt(Value v, int ply, int r50c);
157 void update_pv(Move* pv, Move move, Move* childPv);
158 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
159 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
160 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
161 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
163 // perft() is our utility to verify move generation. All the leaf nodes up
164 // to the given depth are generated and counted, and the sum is returned.
166 uint64_t perft(Position& pos, Depth depth) {
169 uint64_t cnt, nodes = 0;
170 const bool leaf = (depth == 2);
172 for (const auto& m : MoveList<LEGAL>(pos))
174 if (Root && depth <= 1)
179 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
184 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
192 /// Search::init() is called at startup to initialize various lookup tables
194 void Search::init() {
196 for (int i = 1; i < MAX_MOVES; ++i)
197 Reductions[i] = int((24.8 + std::log(Threads.size())) * std::log(i));
201 /// Search::clear() resets search state to its initial value
203 void Search::clear() {
205 Threads.main()->wait_for_search_finished();
207 Time.availableNodes = 0;
210 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
214 /// MainThread::search() is started when the program receives the UCI 'go'
215 /// command. It searches from the root position and outputs the "bestmove".
217 void MainThread::search() {
221 nodes = perft<true>(rootPos, Limits.perft);
222 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
226 Color us = rootPos.side_to_move();
227 Time.init(Limits, us, rootPos.game_ply());
230 if (rootMoves.empty())
232 rootMoves.emplace_back(MOVE_NONE);
233 sync_cout << "info depth 0 score "
234 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
239 Threads.start_searching(); // start non-main threads
240 Thread::search(); // main thread start searching
243 // When we reach the maximum depth, we can arrive here without a raise of
244 // Threads.stop. However, if we are pondering or in an infinite search,
245 // the UCI protocol states that we shouldn't print the best move before the
246 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
247 // until the GUI sends one of those commands.
249 while (!Threads.stop && (ponder || Limits.infinite))
250 {} // Busy wait for a stop or a ponder reset
252 // Stop the threads if not already stopped (also raise the stop if
253 // "ponderhit" just reset Threads.ponder).
256 // Wait until all threads have finished
257 Threads.wait_for_search_finished();
259 // When playing in 'nodes as time' mode, subtract the searched nodes from
260 // the available ones before exiting.
262 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
264 Thread* bestThread = this;
266 if (int(Options["MultiPV"]) == 1 &&
268 !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"])) &&
269 rootMoves[0].pv[0] != MOVE_NONE)
270 bestThread = Threads.get_best_thread();
272 bestPreviousScore = bestThread->rootMoves[0].score;
274 // Send again PV info if we have a new best thread
275 if (bestThread != this)
276 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
278 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
280 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
281 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
283 std::cout << sync_endl;
287 /// Thread::search() is the main iterative deepening loop. It calls search()
288 /// repeatedly with increasing depth until the allocated thinking time has been
289 /// consumed, the user stops the search, or the maximum search depth is reached.
291 void Thread::search() {
293 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
294 // The former is needed to allow update_continuation_histories(ss-1, ...),
295 // which accesses its argument at ss-6, also near the root.
296 // The latter is needed for statScores and killer initialization.
297 Stack stack[MAX_PLY+10], *ss = stack+7;
299 Value bestValue, alpha, beta, delta;
300 Move lastBestMove = MOVE_NONE;
301 Depth lastBestMoveDepth = 0;
302 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
303 double timeReduction = 1, totBestMoveChanges = 0;
304 Color us = rootPos.side_to_move();
307 std::memset(ss-7, 0, 10 * sizeof(Stack));
308 for (int i = 7; i > 0; i--)
309 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
313 bestValue = delta = alpha = -VALUE_INFINITE;
314 beta = VALUE_INFINITE;
318 if (mainThread->bestPreviousScore == VALUE_INFINITE)
319 for (int i = 0; i < 4; ++i)
320 mainThread->iterValue[i] = VALUE_ZERO;
322 for (int i = 0; i < 4; ++i)
323 mainThread->iterValue[i] = mainThread->bestPreviousScore;
326 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
327 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
329 size_t multiPV = size_t(Options["MultiPV"]);
331 // Pick integer skill levels, but non-deterministically round up or down
332 // such that the average integer skill corresponds to the input floating point one.
333 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
334 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
335 // for match (TC 60+0.6) results spanning a wide range of k values.
337 double floatLevel = Options["UCI_LimitStrength"] ?
338 Utility::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
339 double(Options["Skill Level"]);
340 int intLevel = int(floatLevel) +
341 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
342 Skill skill(intLevel);
344 // When playing with strength handicap enable MultiPV search that we will
345 // use behind the scenes to retrieve a set of possible moves.
347 multiPV = std::max(multiPV, (size_t)4);
349 multiPV = std::min(multiPV, rootMoves.size());
350 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
352 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
354 // In analysis mode, adjust contempt in accordance with user preference
355 if (Limits.infinite || Options["UCI_AnalyseMode"])
356 ct = Options["Analysis Contempt"] == "Off" ? 0
357 : Options["Analysis Contempt"] == "Both" ? ct
358 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
359 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
362 // Evaluation score is from the white point of view
363 contempt = (us == WHITE ? make_score(ct, ct / 2)
364 : -make_score(ct, ct / 2));
366 int searchAgainCounter = 0;
368 // Iterative deepening loop until requested to stop or the target depth is reached
369 while ( ++rootDepth < MAX_PLY
371 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
373 // Age out PV variability metric
375 totBestMoveChanges /= 2;
377 // Save the last iteration's scores before first PV line is searched and
378 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
379 for (RootMove& rm : rootMoves)
380 rm.previousScore = rm.score;
385 if (!Threads.increaseDepth)
386 searchAgainCounter++;
388 // MultiPV loop. We perform a full root search for each PV line
389 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
394 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
395 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
399 // Reset UCI info selDepth for each depth and each PV line
402 // Reset aspiration window starting size
405 Value prev = rootMoves[pvIdx].previousScore;
407 alpha = std::max(prev - delta,-VALUE_INFINITE);
408 beta = std::min(prev + delta, VALUE_INFINITE);
410 // Adjust contempt based on root move's previousScore (dynamic contempt)
411 int dct = ct + (110 - ct / 2) * prev / (abs(prev) + 140);
413 contempt = (us == WHITE ? make_score(dct, dct / 2)
414 : -make_score(dct, dct / 2));
417 // Start with a small aspiration window and, in the case of a fail
418 // high/low, re-search with a bigger window until we don't fail
420 int failedHighCnt = 0;
423 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
424 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
426 // Bring the best move to the front. It is critical that sorting
427 // is done with a stable algorithm because all the values but the
428 // first and eventually the new best one are set to -VALUE_INFINITE
429 // and we want to keep the same order for all the moves except the
430 // new PV that goes to the front. Note that in case of MultiPV
431 // search the already searched PV lines are preserved.
432 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
434 // If search has been stopped, we break immediately. Sorting is
435 // safe because RootMoves is still valid, although it refers to
436 // the previous iteration.
440 // When failing high/low give some update (without cluttering
441 // the UI) before a re-search.
444 && (bestValue <= alpha || bestValue >= beta)
445 && Time.elapsed() > 3000)
446 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
448 // In case of failing low/high increase aspiration window and
449 // re-search, otherwise exit the loop.
450 if (bestValue <= alpha)
452 beta = (alpha + beta) / 2;
453 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
457 mainThread->stopOnPonderhit = false;
459 else if (bestValue >= beta)
461 beta = std::min(bestValue + delta, VALUE_INFINITE);
466 ++rootMoves[pvIdx].bestMoveCount;
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 = (296 + 6 * (mainThread->bestPreviousScore - bestValue)
510 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 725.0;
511 fallingEval = Utility::clamp(fallingEval, 0.5, 1.5);
513 // If the bestMove is stable over several iterations, reduce time accordingly
514 timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.92 : 0.95;
515 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.22 * 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 + totBestMoveChanges / Threads.size();
525 double totalTime = rootMoves.size() == 1 ? 0 :
526 Time.optimum() * fallingEval * reduction * bestMoveInstability;
528 // Stop the search if we have exceeded the totalTime, at least 1ms search
529 if (Time.elapsed() > totalTime)
531 // If we are allowed to ponder do not stop the search now but
532 // keep pondering until the GUI sends "ponderhit" or "stop".
533 if (mainThread->ponder)
534 mainThread->stopOnPonderhit = true;
538 else if ( Threads.increaseDepth
539 && !mainThread->ponder
540 && Time.elapsed() > totalTime * 0.56)
541 Threads.increaseDepth = false;
543 Threads.increaseDepth = true;
546 mainThread->iterValue[iterIdx] = bestValue;
547 iterIdx = (iterIdx + 1) & 3;
553 mainThread->previousTimeReduction = timeReduction;
555 // If skill level is enabled, swap best PV line with the sub-optimal one
557 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
558 skill.best ? skill.best : skill.pick_best(multiPV)));
564 // search<>() is the main search function for both PV and non-PV nodes
566 template <NodeType NT>
567 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
569 constexpr bool PvNode = NT == PV;
570 const bool rootNode = PvNode && ss->ply == 0;
572 // Check if we have an upcoming move which draws by repetition, or
573 // if the opponent had an alternative move earlier to this position.
574 if ( pos.rule50_count() >= 3
575 && alpha < VALUE_DRAW
577 && pos.has_game_cycle(ss->ply))
579 alpha = value_draw(pos.this_thread());
584 // Dive into quiescence search when the depth reaches zero
586 return qsearch<NT>(pos, ss, alpha, beta);
588 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
589 assert(PvNode || (alpha == beta - 1));
590 assert(0 < depth && depth < MAX_PLY);
591 assert(!(PvNode && cutNode));
593 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
597 Move ttMove, move, excludedMove, bestMove;
598 Depth extension, newDepth;
599 Value bestValue, value, ttValue, eval, maxValue;
600 bool ttHit, ttPv, formerPv, givesCheck, improving, didLMR, priorCapture;
601 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
602 ttCapture, singularQuietLMR;
604 int moveCount, captureCount, quietCount;
606 // Step 1. Initialize node
607 Thread* thisThread = pos.this_thread();
608 ss->inCheck = pos.checkers();
609 priorCapture = pos.captured_piece();
610 Color us = pos.side_to_move();
611 moveCount = captureCount = quietCount = ss->moveCount = 0;
612 bestValue = -VALUE_INFINITE;
613 maxValue = VALUE_INFINITE;
615 // Check for the available remaining time
616 if (thisThread == Threads.main())
617 static_cast<MainThread*>(thisThread)->check_time();
619 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
620 if (PvNode && thisThread->selDepth < ss->ply + 1)
621 thisThread->selDepth = ss->ply + 1;
625 // Step 2. Check for aborted search and immediate draw
626 if ( Threads.stop.load(std::memory_order_relaxed)
627 || pos.is_draw(ss->ply)
628 || ss->ply >= MAX_PLY)
629 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
630 : value_draw(pos.this_thread());
632 // Step 3. Mate distance pruning. Even if we mate at the next move our score
633 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
634 // a shorter mate was found upward in the tree then there is no need to search
635 // because we will never beat the current alpha. Same logic but with reversed
636 // signs applies also in the opposite condition of being mated instead of giving
637 // mate. In this case return a fail-high score.
638 alpha = std::max(mated_in(ss->ply), alpha);
639 beta = std::min(mate_in(ss->ply+1), beta);
644 assert(0 <= ss->ply && ss->ply < MAX_PLY);
646 (ss+1)->ply = ss->ply + 1;
647 (ss+1)->excludedMove = bestMove = MOVE_NONE;
648 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
649 Square prevSq = to_sq((ss-1)->currentMove);
651 // Initialize statScore to zero for the grandchildren of the current position.
652 // So statScore is shared between all grandchildren and only the first grandchild
653 // starts with statScore = 0. Later grandchildren start with the last calculated
654 // statScore of the previous grandchild. This influences the reduction rules in
655 // LMR which are based on the statScore of parent position.
657 (ss+4)->statScore = 0;
659 (ss+2)->statScore = 0;
661 // Step 4. Transposition table lookup. We don't want the score of a partial
662 // search to overwrite a previous full search TT value, so we use a different
663 // position key in case of an excluded move.
664 excludedMove = ss->excludedMove;
665 posKey = pos.key() ^ (Key(excludedMove) << 48); // Isn't a very good hash
666 tte = TT.probe(posKey, ttHit);
667 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
668 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
669 : ttHit ? tte->move() : MOVE_NONE;
670 ttPv = PvNode || (ttHit && tte->is_pv());
671 formerPv = ttPv && !PvNode;
673 if (ttPv && depth > 12 && ss->ply - 1 < MAX_LPH && !priorCapture && is_ok((ss-1)->currentMove))
674 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
676 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
677 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
678 + TtHitAverageResolution * ttHit;
680 // At non-PV nodes we check for an early TT cutoff
683 && tte->depth() >= depth
684 && ttValue != VALUE_NONE // Possible in case of TT access race
685 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
686 : (tte->bound() & BOUND_UPPER)))
688 // If ttMove is quiet, update move sorting heuristics on TT hit
693 if (!pos.capture_or_promotion(ttMove))
694 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
696 // Extra penalty for early quiet moves of the previous ply
697 if ((ss-1)->moveCount <= 2 && !priorCapture)
698 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
700 // Penalty for a quiet ttMove that fails low
701 else if (!pos.capture_or_promotion(ttMove))
703 int penalty = -stat_bonus(depth);
704 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
705 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
709 if (pos.rule50_count() < 90)
713 // Step 5. Tablebases probe
714 if (!rootNode && TB::Cardinality)
716 int piecesCount = pos.count<ALL_PIECES>();
718 if ( piecesCount <= TB::Cardinality
719 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
720 && pos.rule50_count() == 0
721 && !pos.can_castle(ANY_CASTLING))
724 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
726 // Force check of time on the next occasion
727 if (thisThread == Threads.main())
728 static_cast<MainThread*>(thisThread)->callsCnt = 0;
730 if (err != TB::ProbeState::FAIL)
732 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
734 int drawScore = TB::UseRule50 ? 1 : 0;
736 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
737 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
738 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
739 : VALUE_DRAW + 2 * wdl * drawScore;
741 Bound b = wdl < -drawScore ? BOUND_UPPER
742 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
744 if ( b == BOUND_EXACT
745 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
747 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
748 std::min(MAX_PLY - 1, depth + 6),
749 MOVE_NONE, VALUE_NONE);
756 if (b == BOUND_LOWER)
757 bestValue = value, alpha = std::max(alpha, bestValue);
765 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
767 // Step 6. Static evaluation of the position
770 // Skip early pruning when in check
771 ss->staticEval = eval = VALUE_NONE;
777 // Never assume anything about values stored in TT
778 ss->staticEval = eval = tte->eval();
779 if (eval == VALUE_NONE)
780 ss->staticEval = eval = evaluate(pos);
782 if (eval == VALUE_DRAW)
783 eval = value_draw(thisThread);
785 // Can ttValue be used as a better position evaluation?
786 if ( ttValue != VALUE_NONE
787 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
792 if ((ss-1)->currentMove != MOVE_NULL)
794 int bonus = -(ss-1)->statScore / 512;
796 ss->staticEval = eval = evaluate(pos) + bonus;
799 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
801 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
804 // Step 7. Razoring (~1 Elo)
805 if ( !rootNode // The required rootNode PV handling is not available in qsearch
807 && eval <= alpha - RazorMargin)
808 return qsearch<NT>(pos, ss, alpha, beta);
810 improving = (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval > (ss-4)->staticEval
811 || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval > (ss-2)->staticEval;
813 // Step 8. Futility pruning: child node (~50 Elo)
816 && eval - futility_margin(depth, improving) >= beta
817 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
820 // Step 9. Null move search with verification search (~40 Elo)
822 && (ss-1)->currentMove != MOVE_NULL
823 && (ss-1)->statScore < 23824
825 && eval >= ss->staticEval
826 && ss->staticEval >= beta - 33 * depth - 33 * improving + 112 * ttPv + 311
828 && pos.non_pawn_material(us)
829 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
831 assert(eval - beta >= 0);
833 // Null move dynamic reduction based on depth and value
834 Depth R = (737 + 77 * depth) / 246 + std::min(int(eval - beta) / 192, 3);
836 ss->currentMove = MOVE_NULL;
837 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
839 pos.do_null_move(st);
841 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
843 pos.undo_null_move();
845 if (nullValue >= beta)
847 // Do not return unproven mate or TB scores
848 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
851 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
854 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
856 // Do verification search at high depths, with null move pruning disabled
857 // for us, until ply exceeds nmpMinPly.
858 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
859 thisThread->nmpColor = us;
861 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
863 thisThread->nmpMinPly = 0;
870 // Step 10. ProbCut (~10 Elo)
871 // If we have a good enough capture and a reduced search returns a value
872 // much above beta, we can (almost) safely prune the previous move.
875 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY)
877 Value raisedBeta = beta + 176 - 49 * improving;
878 assert(raisedBeta < VALUE_INFINITE);
879 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &captureHistory);
880 int probCutCount = 0;
882 while ( (move = mp.next_move()) != MOVE_NONE
883 && probCutCount < 2 + 2 * cutNode
885 && tte->depth() >= depth - 4
886 && ttValue < raisedBeta))
887 if (move != excludedMove && pos.legal(move))
889 assert(pos.capture_or_promotion(move));
892 captureOrPromotion = true;
895 ss->currentMove = move;
896 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
898 [pos.moved_piece(move)]
901 pos.do_move(move, st);
903 // Perform a preliminary qsearch to verify that the move holds
904 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
906 // If the qsearch held, perform the regular search
907 if (value >= raisedBeta)
908 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
912 if (value >= raisedBeta)
917 // Step 11. Internal iterative deepening (~1 Elo)
918 if (depth >= 7 && !ttMove)
920 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
922 tte = TT.probe(posKey, ttHit);
923 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
924 ttMove = ttHit ? tte->move() : MOVE_NONE;
927 moves_loop: // When in check, search starts from here
929 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
930 nullptr , (ss-4)->continuationHistory,
931 nullptr , (ss-6)->continuationHistory };
933 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
935 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
936 &thisThread->lowPlyHistory,
944 singularQuietLMR = moveCountPruning = false;
945 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
947 // Mark this node as being searched
948 ThreadHolding th(thisThread, posKey, ss->ply);
950 // Step 12. Loop through all pseudo-legal moves until no moves remain
951 // or a beta cutoff occurs.
952 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
956 if (move == excludedMove)
959 // At root obey the "searchmoves" option and skip moves not listed in Root
960 // Move List. As a consequence any illegal move is also skipped. In MultiPV
961 // mode we also skip PV moves which have been already searched and those
962 // of lower "TB rank" if we are in a TB root position.
963 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
964 thisThread->rootMoves.begin() + thisThread->pvLast, move))
967 ss->moveCount = ++moveCount;
969 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
970 sync_cout << "info depth " << depth
971 << " currmove " << UCI::move(move, pos.is_chess960())
972 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
974 (ss+1)->pv = nullptr;
977 captureOrPromotion = pos.capture_or_promotion(move);
978 movedPiece = pos.moved_piece(move);
979 givesCheck = pos.gives_check(move);
981 // Calculate new depth for this move
982 newDepth = depth - 1;
984 // Step 13. Pruning at shallow depth (~200 Elo)
986 && pos.non_pawn_material(us)
987 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
989 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
990 moveCountPruning = moveCount >= futility_move_count(improving, depth);
992 // Reduced depth of the next LMR search
993 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
995 if ( !captureOrPromotion
998 // Countermoves based pruning (~20 Elo)
999 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1000 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1001 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1004 // Futility pruning: parent node (~5 Elo)
1007 && ss->staticEval + 284 + 188 * lmrDepth <= alpha
1008 && (*contHist[0])[movedPiece][to_sq(move)]
1009 + (*contHist[1])[movedPiece][to_sq(move)]
1010 + (*contHist[3])[movedPiece][to_sq(move)]
1011 + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 28388)
1014 // Prune moves with negative SEE (~20 Elo)
1015 if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 17)) * lmrDepth * lmrDepth)))
1020 // Capture history based pruning when the move doesn't give check
1023 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1026 // Futility pruning for captures
1029 && !(PvNode && abs(bestValue) < 2)
1030 && PieceValue[MG][type_of(movedPiece)] >= PieceValue[MG][type_of(pos.piece_on(to_sq(move)))]
1032 && ss->staticEval + 267 + 391 * lmrDepth
1033 + PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] <= alpha)
1036 // See based pruning
1037 if (!pos.see_ge(move, Value(-202) * depth)) // (~25 Elo)
1042 // Step 14. Extensions (~75 Elo)
1044 // Singular extension search (~70 Elo). If all moves but one fail low on a
1045 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1046 // then that move is singular and should be extended. To verify this we do
1047 // a reduced search on all the other moves but the ttMove and if the
1048 // result is lower than ttValue minus a margin then we will extend the ttMove.
1052 && !excludedMove // Avoid recursive singular search
1053 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1054 && abs(ttValue) < VALUE_KNOWN_WIN
1055 && (tte->bound() & BOUND_LOWER)
1056 && tte->depth() >= depth - 3
1059 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1060 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1061 ss->excludedMove = move;
1062 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1063 ss->excludedMove = MOVE_NONE;
1065 if (value < singularBeta)
1068 singularQuietLMR = !ttCapture;
1071 // Multi-cut pruning
1072 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1073 // search without the ttMove. So we assume this expected Cut-node is not singular,
1074 // that multiple moves fail high, and we can prune the whole subtree by returning
1076 else if (singularBeta >= beta)
1077 return singularBeta;
1079 // If the eval of ttMove is greater than beta we try also if there is another
1080 // move that pushes it over beta, if so also produce a cutoff.
1081 else if (ttValue >= beta)
1083 ss->excludedMove = move;
1084 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1085 ss->excludedMove = MOVE_NONE;
1092 // Check extension (~2 Elo)
1093 else if ( givesCheck
1094 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1097 // Passed pawn extension
1098 else if ( move == ss->killers[0]
1099 && pos.advanced_pawn_push(move)
1100 && pos.pawn_passed(us, to_sq(move)))
1103 // Last captures extension
1104 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1105 && pos.non_pawn_material() <= 2 * RookValueMg)
1108 // Castling extension
1109 if (type_of(move) == CASTLING)
1112 // Late irreversible move extension
1114 && pos.rule50_count() > 80
1115 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1118 // Add extension to new depth
1119 newDepth += extension;
1121 // Speculative prefetch as early as possible
1122 prefetch(TT.first_entry(pos.key_after(move)));
1124 // Check for legality just before making the move
1125 if (!rootNode && !pos.legal(move))
1127 ss->moveCount = --moveCount;
1131 // Update the current move (this must be done after singular extension search)
1132 ss->currentMove = move;
1133 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1134 [captureOrPromotion]
1138 // Step 15. Make the move
1139 pos.do_move(move, st, givesCheck);
1141 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1142 // re-searched at full depth.
1144 && moveCount > 1 + 2 * rootNode
1145 && (!rootNode || thisThread->best_move_count(move) == 0)
1146 && ( !captureOrPromotion
1148 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1150 || thisThread->ttHitAverage < 415 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1152 Depth r = reduction(improving, depth, moveCount);
1154 // Decrease reduction if the ttHit running average is large
1155 if (thisThread->ttHitAverage > 473 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1158 // Reduction if other threads are searching this position
1162 // Decrease reduction if position is or has been on the PV (~10 Elo)
1166 if (moveCountPruning && !formerPv)
1169 // Decrease reduction if opponent's move count is high (~5 Elo)
1170 if ((ss-1)->moveCount > 13)
1173 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1174 if (singularQuietLMR)
1177 if (!captureOrPromotion)
1179 // Increase reduction if ttMove is a capture (~5 Elo)
1183 // Increase reduction for cut nodes (~10 Elo)
1187 // Decrease reduction for moves that escape a capture. Filter out
1188 // castling moves, because they are coded as "king captures rook" and
1189 // hence break make_move(). (~2 Elo)
1190 else if ( type_of(move) == NORMAL
1191 && !pos.see_ge(reverse_move(move)))
1192 r -= 2 + ttPv - (type_of(movedPiece) == PAWN);
1194 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1195 + (*contHist[0])[movedPiece][to_sq(move)]
1196 + (*contHist[1])[movedPiece][to_sq(move)]
1197 + (*contHist[3])[movedPiece][to_sq(move)]
1200 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1201 if (ss->statScore >= -100 && (ss-1)->statScore < -112)
1204 else if ((ss-1)->statScore >= -125 && ss->statScore < -138)
1207 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1208 r -= ss->statScore / 14615;
1212 // Increase reduction for captures/promotions if late move and at low depth
1213 if (depth < 8 && moveCount > 2)
1216 // Unless giving check, this capture is likely bad
1218 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 211 * depth <= alpha)
1222 Depth d = Utility::clamp(newDepth - r, 1, newDepth);
1224 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1226 doFullDepthSearch = value > alpha && d != newDepth;
1232 doFullDepthSearch = !PvNode || moveCount > 1;
1237 // Step 17. Full depth search when LMR is skipped or fails high
1238 if (doFullDepthSearch)
1240 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1242 if (didLMR && !captureOrPromotion)
1244 int bonus = value > alpha ? stat_bonus(newDepth)
1245 : -stat_bonus(newDepth);
1247 if (move == ss->killers[0])
1250 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1254 // For PV nodes only, do a full PV search on the first move or after a fail
1255 // high (in the latter case search only if value < beta), otherwise let the
1256 // parent node fail low with value <= alpha and try another move.
1257 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1260 (ss+1)->pv[0] = MOVE_NONE;
1262 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1265 // Step 18. Undo move
1266 pos.undo_move(move);
1268 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1270 // Step 19. Check for a new best move
1271 // Finished searching the move. If a stop occurred, the return value of
1272 // the search cannot be trusted, and we return immediately without
1273 // updating best move, PV and TT.
1274 if (Threads.stop.load(std::memory_order_relaxed))
1279 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1280 thisThread->rootMoves.end(), move);
1282 // PV move or new best move?
1283 if (moveCount == 1 || value > alpha)
1286 rm.selDepth = thisThread->selDepth;
1291 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1292 rm.pv.push_back(*m);
1294 // We record how often the best move has been changed in each
1295 // iteration. This information is used for time management: when
1296 // the best move changes frequently, we allocate some more time.
1298 ++thisThread->bestMoveChanges;
1301 // All other moves but the PV are set to the lowest value: this
1302 // is not a problem when sorting because the sort is stable and the
1303 // move position in the list is preserved - just the PV is pushed up.
1304 rm.score = -VALUE_INFINITE;
1307 if (value > bestValue)
1315 if (PvNode && !rootNode) // Update pv even in fail-high case
1316 update_pv(ss->pv, move, (ss+1)->pv);
1318 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1322 assert(value >= beta); // Fail high
1329 if (move != bestMove)
1331 if (captureOrPromotion && captureCount < 32)
1332 capturesSearched[captureCount++] = move;
1334 else if (!captureOrPromotion && quietCount < 64)
1335 quietsSearched[quietCount++] = move;
1339 // The following condition would detect a stop only after move loop has been
1340 // completed. But in this case bestValue is valid because we have fully
1341 // searched our subtree, and we can anyhow save the result in TT.
1347 // Step 20. Check for mate and stalemate
1348 // All legal moves have been searched and if there are no legal moves, it
1349 // must be a mate or a stalemate. If we are in a singular extension search then
1350 // return a fail low score.
1352 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1355 bestValue = excludedMove ? alpha
1356 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1359 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1360 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1362 // Bonus for prior countermove that caused the fail low
1363 else if ( (depth >= 3 || PvNode)
1365 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1368 bestValue = std::min(bestValue, maxValue);
1370 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1371 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1372 bestValue >= beta ? BOUND_LOWER :
1373 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1374 depth, bestMove, ss->staticEval);
1376 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1382 // qsearch() is the quiescence search function, which is called by the main search
1383 // function with zero depth, or recursively with further decreasing depth per call.
1384 template <NodeType NT>
1385 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1387 constexpr bool PvNode = NT == PV;
1389 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1390 assert(PvNode || (alpha == beta - 1));
1397 Move ttMove, move, bestMove;
1399 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1400 bool ttHit, pvHit, givesCheck, captureOrPromotion;
1405 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1407 ss->pv[0] = MOVE_NONE;
1410 Thread* thisThread = pos.this_thread();
1411 (ss+1)->ply = ss->ply + 1;
1412 bestMove = MOVE_NONE;
1413 ss->inCheck = pos.checkers();
1416 // Check for an immediate draw or maximum ply reached
1417 if ( pos.is_draw(ss->ply)
1418 || ss->ply >= MAX_PLY)
1419 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1421 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1423 // Decide whether or not to include checks: this fixes also the type of
1424 // TT entry depth that we are going to use. Note that in qsearch we use
1425 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1426 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1427 : DEPTH_QS_NO_CHECKS;
1428 // Transposition table lookup
1430 tte = TT.probe(posKey, ttHit);
1431 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1432 ttMove = ttHit ? tte->move() : MOVE_NONE;
1433 pvHit = ttHit && tte->is_pv();
1437 && tte->depth() >= ttDepth
1438 && ttValue != VALUE_NONE // Only in case of TT access race
1439 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1440 : (tte->bound() & BOUND_UPPER)))
1443 // Evaluate the position statically
1446 ss->staticEval = VALUE_NONE;
1447 bestValue = futilityBase = -VALUE_INFINITE;
1453 // Never assume anything about values stored in TT
1454 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1455 ss->staticEval = bestValue = evaluate(pos);
1457 // Can ttValue be used as a better position evaluation?
1458 if ( ttValue != VALUE_NONE
1459 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1460 bestValue = ttValue;
1463 ss->staticEval = bestValue =
1464 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1465 : -(ss-1)->staticEval + 2 * Tempo;
1467 // Stand pat. Return immediately if static value is at least beta
1468 if (bestValue >= beta)
1471 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1472 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1477 if (PvNode && bestValue > alpha)
1480 futilityBase = bestValue + 141;
1483 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1484 nullptr , (ss-4)->continuationHistory,
1485 nullptr , (ss-6)->continuationHistory };
1487 // Initialize a MovePicker object for the current position, and prepare
1488 // to search the moves. Because the depth is <= 0 here, only captures,
1489 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1491 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1492 &thisThread->captureHistory,
1494 to_sq((ss-1)->currentMove));
1496 // Loop through the moves until no moves remain or a beta cutoff occurs
1497 while ((move = mp.next_move()) != MOVE_NONE)
1499 assert(is_ok(move));
1501 givesCheck = pos.gives_check(move);
1502 captureOrPromotion = pos.capture_or_promotion(move);
1509 && futilityBase > -VALUE_KNOWN_WIN
1510 && !pos.advanced_pawn_push(move))
1512 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1514 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1516 if (futilityValue <= alpha)
1518 bestValue = std::max(bestValue, futilityValue);
1522 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1524 bestValue = std::max(bestValue, futilityBase);
1529 // Do not search moves with negative SEE values
1530 if ( !ss->inCheck && !pos.see_ge(move))
1533 // Speculative prefetch as early as possible
1534 prefetch(TT.first_entry(pos.key_after(move)));
1536 // Check for legality just before making the move
1537 if (!pos.legal(move))
1543 ss->currentMove = move;
1544 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1545 [captureOrPromotion]
1546 [pos.moved_piece(move)]
1549 // Make and search the move
1550 pos.do_move(move, st, givesCheck);
1551 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1552 pos.undo_move(move);
1554 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1556 // Check for a new best move
1557 if (value > bestValue)
1565 if (PvNode) // Update pv even in fail-high case
1566 update_pv(ss->pv, move, (ss+1)->pv);
1568 if (PvNode && value < beta) // Update alpha here!
1576 // All legal moves have been searched. A special case: if we're in check
1577 // and no legal moves were found, it is checkmate.
1578 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1579 return mated_in(ss->ply); // Plies to mate from the root
1581 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1582 bestValue >= beta ? BOUND_LOWER :
1583 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1584 ttDepth, bestMove, ss->staticEval);
1586 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1592 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1593 // "plies to mate from the current position". Standard scores are unchanged.
1594 // The function is called before storing a value in the transposition table.
1596 Value value_to_tt(Value v, int ply) {
1598 assert(v != VALUE_NONE);
1600 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1601 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1605 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1606 // from the transposition table (which refers to the plies to mate/be mated from
1607 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1608 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1609 // and the graph history interaction, we return an optimal TB score instead.
1611 Value value_from_tt(Value v, int ply, int r50c) {
1613 if (v == VALUE_NONE)
1616 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1618 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1619 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1624 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1626 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1627 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1636 // update_pv() adds current move and appends child pv[]
1638 void update_pv(Move* pv, Move move, Move* childPv) {
1640 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1646 // update_all_stats() updates stats at the end of search() when a bestMove is found
1648 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1649 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1652 Color us = pos.side_to_move();
1653 Thread* thisThread = pos.this_thread();
1654 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1655 Piece moved_piece = pos.moved_piece(bestMove);
1656 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1658 bonus1 = stat_bonus(depth + 1);
1659 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1660 : stat_bonus(depth); // smaller bonus
1662 if (!pos.capture_or_promotion(bestMove))
1664 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1666 // Decrease all the non-best quiet moves
1667 for (int i = 0; i < quietCount; ++i)
1669 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1670 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1674 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1676 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1677 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1678 && !pos.captured_piece())
1679 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1681 // Decrease all the non-best capture moves
1682 for (int i = 0; i < captureCount; ++i)
1684 moved_piece = pos.moved_piece(capturesSearched[i]);
1685 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1686 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1691 // update_continuation_histories() updates histories of the move pairs formed
1692 // by moves at ply -1, -2, -4, and -6 with current move.
1694 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1696 for (int i : {1, 2, 4, 6})
1698 if (ss->inCheck && i > 2)
1700 if (is_ok((ss-i)->currentMove))
1701 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1706 // update_quiet_stats() updates move sorting heuristics
1708 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1710 if (ss->killers[0] != move)
1712 ss->killers[1] = ss->killers[0];
1713 ss->killers[0] = move;
1716 Color us = pos.side_to_move();
1717 Thread* thisThread = pos.this_thread();
1718 thisThread->mainHistory[us][from_to(move)] << bonus;
1719 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1721 if (type_of(pos.moved_piece(move)) != PAWN)
1722 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1724 if (is_ok((ss-1)->currentMove))
1726 Square prevSq = to_sq((ss-1)->currentMove);
1727 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1730 if (depth > 11 && ss->ply < MAX_LPH)
1731 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 6);
1734 // When playing with strength handicap, choose best move among a set of RootMoves
1735 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1737 Move Skill::pick_best(size_t multiPV) {
1739 const RootMoves& rootMoves = Threads.main()->rootMoves;
1740 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1742 // RootMoves are already sorted by score in descending order
1743 Value topScore = rootMoves[0].score;
1744 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1745 int weakness = 120 - 2 * level;
1746 int maxScore = -VALUE_INFINITE;
1748 // Choose best move. For each move score we add two terms, both dependent on
1749 // weakness. One is deterministic and bigger for weaker levels, and one is
1750 // random. Then we choose the move with the resulting highest score.
1751 for (size_t i = 0; i < multiPV; ++i)
1753 // This is our magic formula
1754 int push = ( weakness * int(topScore - rootMoves[i].score)
1755 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1757 if (rootMoves[i].score + push >= maxScore)
1759 maxScore = rootMoves[i].score + push;
1760 best = rootMoves[i].pv[0];
1770 /// MainThread::check_time() is used to print debug info and, more importantly,
1771 /// to detect when we are out of available time and thus stop the search.
1773 void MainThread::check_time() {
1778 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1779 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1781 static TimePoint lastInfoTime = now();
1783 TimePoint elapsed = Time.elapsed();
1784 TimePoint tick = Limits.startTime + elapsed;
1786 if (tick - lastInfoTime >= 1000)
1788 lastInfoTime = tick;
1792 // We should not stop pondering until told so by the GUI
1796 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1797 || (Limits.movetime && elapsed >= Limits.movetime)
1798 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1799 Threads.stop = true;
1803 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1804 /// that all (if any) unsearched PV lines are sent using a previous search score.
1806 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1808 std::stringstream ss;
1809 TimePoint elapsed = Time.elapsed() + 1;
1810 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1811 size_t pvIdx = pos.this_thread()->pvIdx;
1812 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1813 uint64_t nodesSearched = Threads.nodes_searched();
1814 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1816 for (size_t i = 0; i < multiPV; ++i)
1818 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1820 if (depth == 1 && !updated)
1823 Depth d = updated ? depth : depth - 1;
1824 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1826 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1827 v = tb ? rootMoves[i].tbScore : v;
1829 if (ss.rdbuf()->in_avail()) // Not at first line
1834 << " seldepth " << rootMoves[i].selDepth
1835 << " multipv " << i + 1
1836 << " score " << UCI::value(v);
1838 if (!tb && i == pvIdx)
1839 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1841 ss << " nodes " << nodesSearched
1842 << " nps " << nodesSearched * 1000 / elapsed;
1844 if (elapsed > 1000) // Earlier makes little sense
1845 ss << " hashfull " << TT.hashfull();
1847 ss << " tbhits " << tbHits
1848 << " time " << elapsed
1851 for (Move m : rootMoves[i].pv)
1852 ss << " " << UCI::move(m, pos.is_chess960());
1859 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1860 /// before exiting the search, for instance, in case we stop the search during a
1861 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1862 /// otherwise in case of 'ponder on' we have nothing to think on.
1864 bool RootMove::extract_ponder_from_tt(Position& pos) {
1869 assert(pv.size() == 1);
1871 if (pv[0] == MOVE_NONE)
1874 pos.do_move(pv[0], st);
1875 TTEntry* tte = TT.probe(pos.key(), ttHit);
1879 Move m = tte->move(); // Local copy to be SMP safe
1880 if (MoveList<LEGAL>(pos).contains(m))
1884 pos.undo_move(pv[0]);
1885 return pv.size() > 1;
1888 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1891 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1892 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1893 Cardinality = int(Options["SyzygyProbeLimit"]);
1894 bool dtz_available = true;
1896 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1897 // ProbeDepth == DEPTH_ZERO
1898 if (Cardinality > MaxCardinality)
1900 Cardinality = MaxCardinality;
1904 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1906 // Rank moves using DTZ tables
1907 RootInTB = root_probe(pos, rootMoves);
1911 // DTZ tables are missing; try to rank moves using WDL tables
1912 dtz_available = false;
1913 RootInTB = root_probe_wdl(pos, rootMoves);
1919 // Sort moves according to TB rank
1920 std::sort(rootMoves.begin(), rootMoves.end(),
1921 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1923 // Probe during search only if DTZ is not available and we are winning
1924 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1929 // Clean up if root_probe() and root_probe_wdl() have failed
1930 for (auto& m : rootMoves)