2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
22 #include <cstring> // For std::memset
37 #include "syzygy/tbprobe.h"
44 namespace Tablebases {
52 namespace TB = Tablebases;
56 using namespace Search;
60 // Different node types, used as a template parameter
61 enum NodeType { NonPV, PV };
63 constexpr uint64_t TtHitAverageWindow = 4096;
64 constexpr uint64_t TtHitAverageResolution = 1024;
66 // Razor and futility margins
67 constexpr int RazorMargin = 510;
68 Value futility_margin(Depth d, bool improving) {
69 return Value(223 * (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 + 509) / 1024 + (!i && r > 894);
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 > 13 ? 29 : 17 * d * d + 134 * d - 134;
89 // Add a small random component to draw evaluations to avoid 3fold-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 uint64_t cnt, nodes = 0;
169 const bool leaf = (depth == 2);
171 for (const auto& m : MoveList<LEGAL>(pos))
173 if (Root && depth <= 1)
178 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
182 if (Root && Cluster::is_root())
183 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
191 /// Search::init() is called at startup to initialize various lookup tables
193 void Search::init() {
195 for (int i = 1; i < MAX_MOVES; ++i)
196 Reductions[i] = int((22.0 + std::log(Threads.size())) * std::log(i));
200 /// Search::clear() resets search state to its initial value
202 void Search::clear() {
204 Threads.main()->wait_for_search_finished();
206 Time.availableNodes = 0;
209 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
213 /// MainThread::search() is started when the program receives the UCI 'go'
214 /// command. It searches from the root position and outputs the "bestmove".
216 void MainThread::search() {
220 nodes = perft<true>(rootPos, Limits.perft);
221 if (Cluster::is_root())
222 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
227 Color us = rootPos.side_to_move();
228 Time.init(Limits, us, rootPos.game_ply());
233 if (rootMoves.empty())
235 rootMoves.emplace_back(MOVE_NONE);
236 if (Cluster::is_root())
237 sync_cout << "info depth 0 score "
238 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
243 Threads.start_searching(); // start non-main threads
244 Thread::search(); // main thread start searching
247 // When we reach the maximum depth, we can arrive here without a raise of
248 // Threads.stop. However, if we are pondering or in an infinite search,
249 // the UCI protocol states that we shouldn't print the best move before the
250 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
251 // until the GUI sends one of those commands.
253 while (!Threads.stop && (ponder || Limits.infinite))
254 { Cluster::signals_poll(); } // Busy wait for a stop or a ponder reset
256 // Stop the threads if not already stopped (also raise the stop if
257 // "ponderhit" just reset Threads.ponder).
260 // Signal and synchronize all other ranks
261 Cluster::signals_sync();
263 // Wait until all threads have finished
264 Threads.wait_for_search_finished();
266 // When playing in 'nodes as time' mode, subtract the searched nodes from
267 // the available ones before exiting.
269 Time.availableNodes += Limits.inc[us] - Cluster::nodes_searched();
271 Thread* bestThread = this;
273 if ( int(Options["MultiPV"]) == 1
275 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
276 && rootMoves[0].pv[0] != MOVE_NONE)
277 bestThread = Threads.get_best_thread();
279 // Prepare PVLine and ponder move
280 std::string PVLine = UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE);
282 Move bestMove = bestThread->rootMoves[0].pv[0];
283 Move ponderMove = MOVE_NONE;
284 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
285 ponderMove = bestThread->rootMoves[0].pv[1];
287 // Exchange info as needed
288 Cluster::MoveInfo mi{bestMove,
290 bestThread->completedDepth,
291 bestThread->rootMoves[0].score,
293 Cluster::pick_moves(mi, PVLine);
295 bestPreviousScore = static_cast<Value>(mi.score);
297 if (Cluster::is_root())
299 // Send again PV info if we have a new best thread/rank
300 if (bestThread != this || mi.rank != 0)
301 sync_cout << PVLine << sync_endl;
303 bestMove = static_cast<Move>(mi.move);
304 ponderMove = static_cast<Move>(mi.ponder);
306 if (ponderMove != MOVE_NONE)
307 sync_cout << "bestmove " << UCI::move(bestMove, rootPos.is_chess960())
308 << " ponder " << UCI::move(ponderMove, rootPos.is_chess960()) << sync_endl;
310 sync_cout << "bestmove " << UCI::move(bestMove, rootPos.is_chess960()) << sync_endl;
316 /// Thread::search() is the main iterative deepening loop. It calls search()
317 /// repeatedly with increasing depth until the allocated thinking time has been
318 /// consumed, the user stops the search, or the maximum search depth is reached.
320 void Thread::search() {
322 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
323 // The former is needed to allow update_continuation_histories(ss-1, ...),
324 // which accesses its argument at ss-6, also near the root.
325 // The latter is needed for statScores and killer initialization.
326 Stack stack[MAX_PLY+10], *ss = stack+7;
328 Value bestValue, alpha, beta, delta;
329 Move lastBestMove = MOVE_NONE;
330 Depth lastBestMoveDepth = 0;
331 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
332 double timeReduction = 1, totBestMoveChanges = 0;
333 Color us = rootPos.side_to_move();
336 std::memset(ss-7, 0, 10 * sizeof(Stack));
337 for (int i = 7; i > 0; i--)
338 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
342 bestValue = delta = alpha = -VALUE_INFINITE;
343 beta = VALUE_INFINITE;
347 if (mainThread->bestPreviousScore == VALUE_INFINITE)
348 for (int i = 0; i < 4; ++i)
349 mainThread->iterValue[i] = VALUE_ZERO;
351 for (int i = 0; i < 4; ++i)
352 mainThread->iterValue[i] = mainThread->bestPreviousScore;
355 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
356 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
358 size_t multiPV = size_t(Options["MultiPV"]);
360 // Pick integer skill levels, but non-deterministically round up or down
361 // such that the average integer skill corresponds to the input floating point one.
362 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
363 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
364 // for match (TC 60+0.6) results spanning a wide range of k values.
366 double floatLevel = Options["UCI_LimitStrength"] ?
367 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
368 double(Options["Skill Level"]);
369 int intLevel = int(floatLevel) +
370 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
371 Skill skill(intLevel);
373 // When playing with strength handicap enable MultiPV search that we will
374 // use behind the scenes to retrieve a set of possible moves.
376 multiPV = std::max(multiPV, (size_t)4);
378 multiPV = std::min(multiPV, rootMoves.size());
379 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
381 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
383 // In analysis mode, adjust contempt in accordance with user preference
384 if (Limits.infinite || Options["UCI_AnalyseMode"])
385 ct = Options["Analysis Contempt"] == "Off" ? 0
386 : Options["Analysis Contempt"] == "Both" ? ct
387 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
388 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
391 // Evaluation score is from the white point of view
392 contempt = (us == WHITE ? make_score(ct, ct / 2)
393 : -make_score(ct, ct / 2));
395 int searchAgainCounter = 0;
397 // Iterative deepening loop until requested to stop or the target depth is reached
398 while ( ++rootDepth < MAX_PLY
400 && !(Limits.depth && mainThread && Cluster::is_root() && rootDepth > Limits.depth))
402 // Age out PV variability metric
404 totBestMoveChanges /= 2;
407 // Save the last iteration's scores before first PV line is searched and
408 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
409 for (RootMove& rm : rootMoves)
410 rm.previousScore = rm.score;
415 if (!Threads.increaseDepth)
416 searchAgainCounter++;
418 // MultiPV loop. We perform a full root search for each PV line
419 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
424 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
425 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
429 // Reset UCI info selDepth for each depth and each PV line
432 // Reset aspiration window starting size
435 Value prev = rootMoves[pvIdx].previousScore;
437 alpha = std::max(prev - delta,-VALUE_INFINITE);
438 beta = std::min(prev + delta, VALUE_INFINITE);
440 // Adjust contempt based on root move's previousScore (dynamic contempt)
441 int dct = ct + (105 - ct / 2) * prev / (abs(prev) + 149);
443 contempt = (us == WHITE ? make_score(dct, dct / 2)
444 : -make_score(dct, dct / 2));
447 // Start with a small aspiration window and, in the case of a fail
448 // high/low, re-search with a bigger window until we don't fail
450 int failedHighCnt = 0;
453 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
454 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
456 // Bring the best move to the front. It is critical that sorting
457 // is done with a stable algorithm because all the values but the
458 // first and eventually the new best one are set to -VALUE_INFINITE
459 // and we want to keep the same order for all the moves except the
460 // new PV that goes to the front. Note that in case of MultiPV
461 // search the already searched PV lines are preserved.
462 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
464 // If search has been stopped, we break immediately. Sorting is
465 // safe because RootMoves is still valid, although it refers to
466 // the previous iteration.
470 // When failing high/low give some update (without cluttering
471 // the UI) before a re-search.
472 if ( Cluster::is_root()
475 && (bestValue <= alpha || bestValue >= beta)
476 && Time.elapsed() > 3000)
478 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
479 Cluster::cluster_info(rootDepth);
482 // In case of failing low/high increase aspiration window and
483 // re-search, otherwise exit the loop.
484 if (bestValue <= alpha)
486 beta = (alpha + beta) / 2;
487 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
491 mainThread->stopOnPonderhit = false;
493 else if (bestValue >= beta)
495 beta = std::min(bestValue + delta, VALUE_INFINITE);
500 ++rootMoves[pvIdx].bestMoveCount;
504 delta += delta / 4 + 5;
506 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
509 // Sort the PV lines searched so far and update the GUI
510 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
512 if ( Cluster::is_root() && mainThread
513 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
515 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
516 Cluster::cluster_info(rootDepth);
521 completedDepth = rootDepth;
523 if (rootMoves[0].pv[0] != lastBestMove) {
524 lastBestMove = rootMoves[0].pv[0];
525 lastBestMoveDepth = rootDepth;
528 // Have we found a "mate in x"?
530 && bestValue >= VALUE_MATE_IN_MAX_PLY
531 && VALUE_MATE - bestValue <= 2 * Limits.mate)
537 // If skill level is enabled and time is up, pick a sub-optimal best move
538 if (skill.enabled() && skill.time_to_pick(rootDepth))
539 skill.pick_best(multiPV);
541 // Do we have time for the next iteration? Can we stop searching now?
542 if ( Limits.use_time_management()
544 && !mainThread->stopOnPonderhit)
546 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
547 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
548 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
550 // If the bestMove is stable over several iterations, reduce time accordingly
551 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
552 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
554 // Use part of the gained time from a previous stable move for the current move
555 for (Thread* th : Threads)
557 totBestMoveChanges += th->bestMoveChanges;
558 th->bestMoveChanges = 0;
560 double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
562 double totalTime = rootMoves.size() == 1 ? 0 :
563 Time.optimum() * fallingEval * reduction * bestMoveInstability;
565 // Stop the search if we have exceeded the totalTime, at least 1ms search
566 if (Time.elapsed() > totalTime)
568 // If we are allowed to ponder do not stop the search now but
569 // keep pondering until the GUI sends "ponderhit" or "stop".
570 if (mainThread->ponder)
571 mainThread->stopOnPonderhit = true;
575 else if ( Threads.increaseDepth
576 && !mainThread->ponder
577 && Time.elapsed() > totalTime * 0.58)
578 Threads.increaseDepth = false;
580 Threads.increaseDepth = true;
583 mainThread->iterValue[iterIdx] = bestValue;
584 iterIdx = (iterIdx + 1) & 3;
590 mainThread->previousTimeReduction = timeReduction;
592 // If skill level is enabled, swap best PV line with the sub-optimal one
594 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
595 skill.best ? skill.best : skill.pick_best(multiPV)));
601 // search<>() is the main search function for both PV and non-PV nodes
603 template <NodeType NT>
604 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
606 constexpr bool PvNode = NT == PV;
607 const bool rootNode = PvNode && ss->ply == 0;
609 // Check if we have an upcoming move which draws by repetition, or
610 // if the opponent had an alternative move earlier to this position.
611 if ( pos.rule50_count() >= 3
612 && alpha < VALUE_DRAW
614 && pos.has_game_cycle(ss->ply))
616 alpha = value_draw(pos.this_thread());
621 // Dive into quiescence search when the depth reaches zero
623 return qsearch<NT>(pos, ss, alpha, beta);
625 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
626 assert(PvNode || (alpha == beta - 1));
627 assert(0 < depth && depth < MAX_PLY);
628 assert(!(PvNode && cutNode));
630 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
634 Move ttMove, move, excludedMove, bestMove;
635 Depth extension, newDepth;
636 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
637 bool formerPv, givesCheck, improving, didLMR, priorCapture;
638 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
639 ttCapture, singularQuietLMR;
641 int moveCount, captureCount, quietCount;
643 // Step 1. Initialize node
644 Thread* thisThread = pos.this_thread();
645 ss->inCheck = pos.checkers();
646 priorCapture = pos.captured_piece();
647 Color us = pos.side_to_move();
648 moveCount = captureCount = quietCount = ss->moveCount = 0;
649 bestValue = -VALUE_INFINITE;
650 maxValue = VALUE_INFINITE;
652 // Check for the available remaining time
653 if (thisThread == Threads.main())
654 static_cast<MainThread*>(thisThread)->check_time();
656 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
657 if (PvNode && thisThread->selDepth < ss->ply + 1)
658 thisThread->selDepth = ss->ply + 1;
662 // Step 2. Check for aborted search and immediate draw
663 if ( Threads.stop.load(std::memory_order_relaxed)
664 || pos.is_draw(ss->ply)
665 || ss->ply >= MAX_PLY)
666 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
667 : value_draw(pos.this_thread());
669 // Step 3. Mate distance pruning. Even if we mate at the next move our score
670 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
671 // a shorter mate was found upward in the tree then there is no need to search
672 // because we will never beat the current alpha. Same logic but with reversed
673 // signs applies also in the opposite condition of being mated instead of giving
674 // mate. In this case return a fail-high score.
675 alpha = std::max(mated_in(ss->ply), alpha);
676 beta = std::min(mate_in(ss->ply+1), beta);
681 assert(0 <= ss->ply && ss->ply < MAX_PLY);
683 (ss+1)->ply = ss->ply + 1;
684 (ss+1)->ttPv = false;
685 (ss+1)->excludedMove = bestMove = MOVE_NONE;
686 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
687 Square prevSq = to_sq((ss-1)->currentMove);
689 // Initialize statScore to zero for the grandchildren of the current position.
690 // So statScore is shared between all grandchildren and only the first grandchild
691 // starts with statScore = 0. Later grandchildren start with the last calculated
692 // statScore of the previous grandchild. This influences the reduction rules in
693 // LMR which are based on the statScore of parent position.
695 (ss+2)->statScore = 0;
697 // Step 4. Transposition table lookup. We don't want the score of a partial
698 // search to overwrite a previous full search TT value, so we use a different
699 // position key in case of an excluded move.
700 excludedMove = ss->excludedMove;
701 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
702 tte = TT.probe(posKey, ss->ttHit);
703 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
704 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
705 : ss->ttHit ? tte->move() : MOVE_NONE;
707 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
708 formerPv = ss->ttPv && !PvNode;
712 && ss->ply - 1 < MAX_LPH
714 && is_ok((ss-1)->currentMove))
715 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
717 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
718 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
719 + TtHitAverageResolution * ss->ttHit;
721 // At non-PV nodes we check for an early TT cutoff
724 && tte->depth() >= depth
725 && ttValue != VALUE_NONE // Possible in case of TT access race
726 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
727 : (tte->bound() & BOUND_UPPER)))
729 // If ttMove is quiet, update move sorting heuristics on TT hit
734 if (!pos.capture_or_promotion(ttMove))
735 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
737 // Extra penalty for early quiet moves of the previous ply
738 if ((ss-1)->moveCount <= 2 && !priorCapture)
739 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
741 // Penalty for a quiet ttMove that fails low
742 else if (!pos.capture_or_promotion(ttMove))
744 int penalty = -stat_bonus(depth);
745 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
746 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
750 if (pos.rule50_count() < 90)
754 // Step 5. Tablebases probe
755 if (!rootNode && TB::Cardinality)
757 int piecesCount = pos.count<ALL_PIECES>();
759 if ( piecesCount <= TB::Cardinality
760 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
761 && pos.rule50_count() == 0
762 && !pos.can_castle(ANY_CASTLING))
765 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
767 // Force check of time on the next occasion
768 if (thisThread == Threads.main())
769 static_cast<MainThread*>(thisThread)->callsCnt = 0;
771 if (err != TB::ProbeState::FAIL)
773 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
775 int drawScore = TB::UseRule50 ? 1 : 0;
777 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
778 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
779 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
780 : VALUE_DRAW + 2 * wdl * drawScore;
782 Bound b = wdl < -drawScore ? BOUND_UPPER
783 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
785 if ( b == BOUND_EXACT
786 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
788 Cluster::save(thisThread, tte,
789 posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
790 std::min(MAX_PLY - 1, depth + 6),
791 MOVE_NONE, VALUE_NONE);
798 if (b == BOUND_LOWER)
799 bestValue = value, alpha = std::max(alpha, bestValue);
807 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
809 // Step 6. Static evaluation of the position
812 // Skip early pruning when in check
813 ss->staticEval = eval = VALUE_NONE;
819 // Never assume anything about values stored in TT
820 ss->staticEval = eval = tte->eval();
821 if (eval == VALUE_NONE)
822 ss->staticEval = eval = evaluate(pos);
824 if (eval == VALUE_DRAW)
825 eval = value_draw(thisThread);
827 // Can ttValue be used as a better position evaluation?
828 if ( ttValue != VALUE_NONE
829 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
834 if ((ss-1)->currentMove != MOVE_NULL)
835 ss->staticEval = eval = evaluate(pos);
837 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
839 Cluster::save(thisThread, tte,
840 posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
844 // Step 7. Razoring (~1 Elo)
845 if ( !rootNode // The required rootNode PV handling is not available in qsearch
847 && eval <= alpha - RazorMargin)
848 return qsearch<NT>(pos, ss, alpha, beta);
850 improving = (ss-2)->staticEval == VALUE_NONE
851 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
852 : ss->staticEval > (ss-2)->staticEval;
854 // Step 8. Futility pruning: child node (~50 Elo)
857 && eval - futility_margin(depth, improving) >= beta
858 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
861 // Step 9. Null move search with verification search (~40 Elo)
863 && (ss-1)->currentMove != MOVE_NULL
864 && (ss-1)->statScore < 22977
866 && eval >= ss->staticEval
867 && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 182
869 && pos.non_pawn_material(us)
870 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
872 assert(eval - beta >= 0);
874 // Null move dynamic reduction based on depth and value
875 Depth R = (817 + 71 * depth) / 213 + std::min(int(eval - beta) / 192, 3);
877 ss->currentMove = MOVE_NULL;
878 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
880 pos.do_null_move(st);
882 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
884 pos.undo_null_move();
886 if (nullValue >= beta)
888 // Do not return unproven mate or TB scores
889 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
892 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
895 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
897 // Do verification search at high depths, with null move pruning disabled
898 // for us, until ply exceeds nmpMinPly.
899 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
900 thisThread->nmpColor = us;
902 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
904 thisThread->nmpMinPly = 0;
911 probCutBeta = beta + 176 - 49 * improving;
913 // Step 10. ProbCut (~10 Elo)
914 // If we have a good enough capture and a reduced search returns a value
915 // much above beta, we can (almost) safely prune the previous move.
918 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
919 // if value from transposition table is lower than probCutBeta, don't attempt probCut
920 // there and in further interactions with transposition table cutoff depth is set to depth - 3
921 // because probCut search has depth set to depth - 4 but we also do a move before it
922 // so effective depth is equal to depth - 3
924 && tte->depth() >= depth - 3
925 && ttValue != VALUE_NONE
926 && ttValue < probCutBeta))
928 // if ttMove is a capture and value from transposition table is good enough produce probCut
929 // cutoff without digging into actual probCut search
931 && tte->depth() >= depth - 3
932 && ttValue != VALUE_NONE
933 && ttValue >= probCutBeta
935 && pos.capture_or_promotion(ttMove))
938 assert(probCutBeta < VALUE_INFINITE);
939 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
940 int probCutCount = 0;
941 bool ttPv = ss->ttPv;
944 while ( (move = mp.next_move()) != MOVE_NONE
945 && probCutCount < 2 + 2 * cutNode)
946 if (move != excludedMove && pos.legal(move))
948 assert(pos.capture_or_promotion(move));
951 captureOrPromotion = true;
954 ss->currentMove = move;
955 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
957 [pos.moved_piece(move)]
960 pos.do_move(move, st);
962 // Perform a preliminary qsearch to verify that the move holds
963 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
965 // If the qsearch held, perform the regular search
966 if (value >= probCutBeta)
967 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
971 if (value >= probCutBeta)
973 // if transposition table doesn't have equal or more deep info write probCut data into it
975 && tte->depth() >= depth - 3
976 && ttValue != VALUE_NONE))
977 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
979 depth - 3, move, ss->staticEval);
986 // Step 11. If the position is not in TT, decrease depth by 2
992 moves_loop: // When in check, search starts from here
994 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
995 nullptr , (ss-4)->continuationHistory,
996 nullptr , (ss-6)->continuationHistory };
998 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
1000 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1001 &thisThread->lowPlyHistory,
1009 singularQuietLMR = moveCountPruning = false;
1010 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
1012 // Mark this node as being searched
1013 ThreadHolding th(thisThread, posKey, ss->ply);
1015 // Step 12. Loop through all pseudo-legal moves until no moves remain
1016 // or a beta cutoff occurs.
1017 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
1019 assert(is_ok(move));
1021 if (move == excludedMove)
1024 // At root obey the "searchmoves" option and skip moves not listed in Root
1025 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1026 // mode we also skip PV moves which have been already searched and those
1027 // of lower "TB rank" if we are in a TB root position.
1028 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1029 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1032 // Check for legality
1033 if (!rootNode && !pos.legal(move))
1036 ss->moveCount = ++moveCount;
1038 if (rootNode && Cluster::is_root() && thisThread == Threads.main() && Time.elapsed() > 3000)
1039 sync_cout << "info depth " << depth
1040 << " currmove " << UCI::move(move, pos.is_chess960())
1041 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1043 (ss+1)->pv = nullptr;
1046 captureOrPromotion = pos.capture_or_promotion(move);
1047 movedPiece = pos.moved_piece(move);
1048 givesCheck = pos.gives_check(move);
1050 // Calculate new depth for this move
1051 newDepth = depth - 1;
1053 // Step 13. Pruning at shallow depth (~200 Elo)
1055 && pos.non_pawn_material(us)
1056 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1058 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1059 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1061 // Reduced depth of the next LMR search
1062 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1064 if ( !captureOrPromotion
1067 // Countermoves based pruning (~20 Elo)
1068 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1069 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1070 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1073 // Futility pruning: parent node (~5 Elo)
1076 && ss->staticEval + 283 + 170 * lmrDepth <= alpha
1077 && (*contHist[0])[movedPiece][to_sq(move)]
1078 + (*contHist[1])[movedPiece][to_sq(move)]
1079 + (*contHist[3])[movedPiece][to_sq(move)]
1080 + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
1083 // Prune moves with negative SEE (~20 Elo)
1084 if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1089 // Capture history based pruning when the move doesn't give check
1092 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1095 // Futility pruning for captures
1098 && !(PvNode && abs(bestValue) < 2)
1100 && ss->staticEval + 169 + 244 * lmrDepth
1101 + PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] <= alpha)
1104 // See based pruning
1105 if (!pos.see_ge(move, Value(-221) * depth)) // (~25 Elo)
1110 // Step 14. Extensions (~75 Elo)
1112 // Singular extension search (~70 Elo). If all moves but one fail low on a
1113 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1114 // then that move is singular and should be extended. To verify this we do
1115 // a reduced search on all the other moves but the ttMove and if the
1116 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1120 && !excludedMove // Avoid recursive singular search
1121 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1122 && abs(ttValue) < VALUE_KNOWN_WIN
1123 && (tte->bound() & BOUND_LOWER)
1124 && tte->depth() >= depth - 3)
1126 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1127 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1128 ss->excludedMove = move;
1129 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1130 ss->excludedMove = MOVE_NONE;
1132 if (value < singularBeta)
1135 singularQuietLMR = !ttCapture;
1138 // Multi-cut pruning
1139 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1140 // search without the ttMove. So we assume this expected Cut-node is not singular,
1141 // that multiple moves fail high, and we can prune the whole subtree by returning
1143 else if (singularBeta >= beta)
1144 return singularBeta;
1146 // If the eval of ttMove is greater than beta we try also if there is another
1147 // move that pushes it over beta, if so also produce a cutoff.
1148 else if (ttValue >= beta)
1150 ss->excludedMove = move;
1151 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1152 ss->excludedMove = MOVE_NONE;
1159 // Check extension (~2 Elo)
1160 else if ( givesCheck
1161 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1164 // Last captures extension
1165 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1166 && pos.non_pawn_material() <= 2 * RookValueMg)
1169 // Late irreversible move extension
1171 && pos.rule50_count() > 80
1172 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1175 // Add extension to new depth
1176 newDepth += extension;
1178 // Speculative prefetch as early as possible
1179 prefetch(TT.first_entry(pos.key_after(move)));
1181 // Update the current move (this must be done after singular extension search)
1182 ss->currentMove = move;
1183 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1184 [captureOrPromotion]
1188 // Step 15. Make the move
1189 pos.do_move(move, st, givesCheck);
1191 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1192 // re-searched at full depth.
1194 && moveCount > 1 + 2 * rootNode + 2 * (PvNode && abs(bestValue) < 2)
1195 && ( !captureOrPromotion
1197 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1199 || thisThread->ttHitAverage < 427 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1201 Depth r = reduction(improving, depth, moveCount);
1203 // Decrease reduction if the ttHit running average is large
1204 if (thisThread->ttHitAverage > 509 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1207 // Reduction if other threads are searching this position
1211 // Decrease reduction if position is or has been on the PV (~10 Elo)
1215 if (moveCountPruning && !formerPv)
1218 // Decrease reduction if opponent's move count is high (~5 Elo)
1219 if ((ss-1)->moveCount > 13)
1222 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1223 if (singularQuietLMR)
1226 if (!captureOrPromotion)
1228 // Increase reduction if ttMove is a capture (~5 Elo)
1232 // Increase reduction for cut nodes (~10 Elo)
1236 // Decrease reduction for moves that escape a capture. Filter out
1237 // castling moves, because they are coded as "king captures rook" and
1238 // hence break make_move(). (~2 Elo)
1239 else if ( type_of(move) == NORMAL
1240 && !pos.see_ge(reverse_move(move)))
1241 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1243 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1244 + (*contHist[0])[movedPiece][to_sq(move)]
1245 + (*contHist[1])[movedPiece][to_sq(move)]
1246 + (*contHist[3])[movedPiece][to_sq(move)]
1249 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1250 if (ss->statScore >= -106 && (ss-1)->statScore < -104)
1253 else if ((ss-1)->statScore >= -119 && ss->statScore < -140)
1256 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1257 r -= ss->statScore / 14884;
1261 // Increase reduction for captures/promotions if late move and at low depth
1262 if (depth < 8 && moveCount > 2)
1265 // Unless giving check, this capture is likely bad
1267 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 213 * depth <= alpha)
1271 Depth d = std::clamp(newDepth - r, 1, newDepth);
1273 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1275 doFullDepthSearch = value > alpha && d != newDepth;
1281 doFullDepthSearch = !PvNode || moveCount > 1;
1286 // Step 17. Full depth search when LMR is skipped or fails high
1287 if (doFullDepthSearch)
1289 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1291 if (didLMR && !captureOrPromotion)
1293 int bonus = value > alpha ? stat_bonus(newDepth)
1294 : -stat_bonus(newDepth);
1296 if (move == ss->killers[0])
1299 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1303 // For PV nodes only, do a full PV search on the first move or after a fail
1304 // high (in the latter case search only if value < beta), otherwise let the
1305 // parent node fail low with value <= alpha and try another move.
1306 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1309 (ss+1)->pv[0] = MOVE_NONE;
1311 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1314 // Step 18. Undo move
1315 pos.undo_move(move);
1317 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1319 // Step 19. Check for a new best move
1320 // Finished searching the move. If a stop occurred, the return value of
1321 // the search cannot be trusted, and we return immediately without
1322 // updating best move, PV and TT.
1323 if (Threads.stop.load(std::memory_order_relaxed))
1328 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1329 thisThread->rootMoves.end(), move);
1331 // PV move or new best move?
1332 if (moveCount == 1 || value > alpha)
1335 rm.selDepth = thisThread->selDepth;
1340 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1341 rm.pv.push_back(*m);
1343 // We record how often the best move has been changed in each
1344 // iteration. This information is used for time management: when
1345 // the best move changes frequently, we allocate some more time.
1347 ++thisThread->bestMoveChanges;
1350 // All other moves but the PV are set to the lowest value: this
1351 // is not a problem when sorting because the sort is stable and the
1352 // move position in the list is preserved - just the PV is pushed up.
1353 rm.score = -VALUE_INFINITE;
1356 if (value > bestValue)
1364 if (PvNode && !rootNode) // Update pv even in fail-high case
1365 update_pv(ss->pv, move, (ss+1)->pv);
1367 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1371 assert(value >= beta); // Fail high
1378 if (move != bestMove)
1380 if (captureOrPromotion && captureCount < 32)
1381 capturesSearched[captureCount++] = move;
1383 else if (!captureOrPromotion && quietCount < 64)
1384 quietsSearched[quietCount++] = move;
1388 // The following condition would detect a stop only after move loop has been
1389 // completed. But in this case bestValue is valid because we have fully
1390 // searched our subtree, and we can anyhow save the result in TT.
1396 // Step 20. Check for mate and stalemate
1397 // All legal moves have been searched and if there are no legal moves, it
1398 // must be a mate or a stalemate. If we are in a singular extension search then
1399 // return a fail low score.
1401 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1404 bestValue = excludedMove ? alpha
1405 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1408 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1409 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1411 // Bonus for prior countermove that caused the fail low
1412 else if ( (depth >= 3 || PvNode)
1414 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1417 bestValue = std::min(bestValue, maxValue);
1419 // If no good move is found and the previous position was ttPv, then the previous
1420 // opponent move is probably good and the new position is added to the search tree.
1421 if (bestValue <= alpha)
1422 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1423 // Otherwise, a counter move has been found and if the position is the last leaf
1424 // in the search tree, remove the position from the search tree.
1426 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1428 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1429 Cluster::save(thisThread, tte,
1430 posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1431 bestValue >= beta ? BOUND_LOWER :
1432 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1433 depth, bestMove, ss->staticEval);
1435 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1441 // qsearch() is the quiescence search function, which is called by the main search
1442 // function with zero depth, or recursively with further decreasing depth per call.
1443 template <NodeType NT>
1444 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1446 constexpr bool PvNode = NT == PV;
1448 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1449 assert(PvNode || (alpha == beta - 1));
1456 Move ttMove, move, bestMove;
1458 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1459 bool pvHit, givesCheck, captureOrPromotion;
1464 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1466 ss->pv[0] = MOVE_NONE;
1469 Thread* thisThread = pos.this_thread();
1470 (ss+1)->ply = ss->ply + 1;
1471 bestMove = MOVE_NONE;
1472 ss->inCheck = pos.checkers();
1475 // Check for an immediate draw or maximum ply reached
1476 if ( pos.is_draw(ss->ply)
1477 || ss->ply >= MAX_PLY)
1478 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1480 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1482 // Decide whether or not to include checks: this fixes also the type of
1483 // TT entry depth that we are going to use. Note that in qsearch we use
1484 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1485 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1486 : DEPTH_QS_NO_CHECKS;
1487 // Transposition table lookup
1489 tte = TT.probe(posKey, ss->ttHit);
1490 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1491 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1492 pvHit = ss->ttHit && tte->is_pv();
1496 && tte->depth() >= ttDepth
1497 && ttValue != VALUE_NONE // Only in case of TT access race
1498 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1499 : (tte->bound() & BOUND_UPPER)))
1502 // Evaluate the position statically
1505 ss->staticEval = VALUE_NONE;
1506 bestValue = futilityBase = -VALUE_INFINITE;
1512 // Never assume anything about values stored in TT
1513 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1514 ss->staticEval = bestValue = evaluate(pos);
1516 // Can ttValue be used as a better position evaluation?
1517 if ( ttValue != VALUE_NONE
1518 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1519 bestValue = ttValue;
1522 ss->staticEval = bestValue =
1523 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1524 : -(ss-1)->staticEval + 2 * Tempo;
1526 // Stand pat. Return immediately if static value is at least beta
1527 if (bestValue >= beta)
1530 Cluster::save(thisThread, tte,
1531 posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1532 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1537 if (PvNode && bestValue > alpha)
1540 futilityBase = bestValue + 145;
1543 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1544 nullptr , (ss-4)->continuationHistory,
1545 nullptr , (ss-6)->continuationHistory };
1547 // Initialize a MovePicker object for the current position, and prepare
1548 // to search the moves. Because the depth is <= 0 here, only captures,
1549 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1550 // will be generated.
1551 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1552 &thisThread->captureHistory,
1554 to_sq((ss-1)->currentMove));
1556 // Loop through the moves until no moves remain or a beta cutoff occurs
1557 while ((move = mp.next_move()) != MOVE_NONE)
1559 assert(is_ok(move));
1561 givesCheck = pos.gives_check(move);
1562 captureOrPromotion = pos.capture_or_promotion(move);
1569 && futilityBase > -VALUE_KNOWN_WIN
1570 && !pos.advanced_pawn_push(move))
1572 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1574 // moveCount pruning
1578 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1580 if (futilityValue <= alpha)
1582 bestValue = std::max(bestValue, futilityValue);
1586 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1588 bestValue = std::max(bestValue, futilityBase);
1593 // Do not search moves with negative SEE values
1595 && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
1596 && !pos.see_ge(move))
1599 // Speculative prefetch as early as possible
1600 prefetch(TT.first_entry(pos.key_after(move)));
1602 // Check for legality just before making the move
1603 if (!pos.legal(move))
1609 ss->currentMove = move;
1610 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1611 [captureOrPromotion]
1612 [pos.moved_piece(move)]
1615 if ( !captureOrPromotion
1617 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1618 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1621 // Make and search the move
1622 pos.do_move(move, st, givesCheck);
1623 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1624 pos.undo_move(move);
1626 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1628 // Check for a new best move
1629 if (value > bestValue)
1637 if (PvNode) // Update pv even in fail-high case
1638 update_pv(ss->pv, move, (ss+1)->pv);
1640 if (PvNode && value < beta) // Update alpha here!
1648 // All legal moves have been searched. A special case: if we're in check
1649 // and no legal moves were found, it is checkmate.
1650 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1651 return mated_in(ss->ply); // Plies to mate from the root
1653 Cluster::save(thisThread, tte,
1654 posKey, value_to_tt(bestValue, ss->ply), pvHit,
1655 bestValue >= beta ? BOUND_LOWER :
1656 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1657 ttDepth, bestMove, ss->staticEval);
1659 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1665 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1666 // "plies to mate from the current position". Standard scores are unchanged.
1667 // The function is called before storing a value in the transposition table.
1669 Value value_to_tt(Value v, int ply) {
1671 assert(v != VALUE_NONE);
1673 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1674 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1678 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1679 // from the transposition table (which refers to the plies to mate/be mated from
1680 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1681 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1682 // and the graph history interaction, we return an optimal TB score instead.
1684 Value value_from_tt(Value v, int ply, int r50c) {
1686 if (v == VALUE_NONE)
1689 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1691 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1692 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1697 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1699 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1700 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1709 // update_pv() adds current move and appends child pv[]
1711 void update_pv(Move* pv, Move move, Move* childPv) {
1713 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1719 // update_all_stats() updates stats at the end of search() when a bestMove is found
1721 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1722 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1725 Color us = pos.side_to_move();
1726 Thread* thisThread = pos.this_thread();
1727 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1728 Piece moved_piece = pos.moved_piece(bestMove);
1729 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1731 bonus1 = stat_bonus(depth + 1);
1732 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1733 : stat_bonus(depth); // smaller bonus
1735 if (!pos.capture_or_promotion(bestMove))
1737 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1739 // Decrease all the non-best quiet moves
1740 for (int i = 0; i < quietCount; ++i)
1742 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1743 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1747 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1749 // Extra penalty for a quiet early move that was not a TT move or main killer move in previous ply when it gets refuted
1750 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1751 && !pos.captured_piece())
1752 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1754 // Decrease all the non-best capture moves
1755 for (int i = 0; i < captureCount; ++i)
1757 moved_piece = pos.moved_piece(capturesSearched[i]);
1758 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1759 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1764 // update_continuation_histories() updates histories of the move pairs formed
1765 // by moves at ply -1, -2, -4, and -6 with current move.
1767 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1769 for (int i : {1, 2, 4, 6})
1771 if (ss->inCheck && i > 2)
1773 if (is_ok((ss-i)->currentMove))
1774 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1779 // update_quiet_stats() updates move sorting heuristics
1781 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1783 if (ss->killers[0] != move)
1785 ss->killers[1] = ss->killers[0];
1786 ss->killers[0] = move;
1789 Color us = pos.side_to_move();
1790 Thread* thisThread = pos.this_thread();
1791 thisThread->mainHistory[us][from_to(move)] << bonus;
1792 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1794 if (type_of(pos.moved_piece(move)) != PAWN)
1795 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1797 if (is_ok((ss-1)->currentMove))
1799 Square prevSq = to_sq((ss-1)->currentMove);
1800 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1803 if (depth > 11 && ss->ply < MAX_LPH)
1804 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1807 // When playing with strength handicap, choose best move among a set of RootMoves
1808 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1810 Move Skill::pick_best(size_t multiPV) {
1812 const RootMoves& rootMoves = Threads.main()->rootMoves;
1813 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1815 // RootMoves are already sorted by score in descending order
1816 Value topScore = rootMoves[0].score;
1817 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1818 int weakness = 120 - 2 * level;
1819 int maxScore = -VALUE_INFINITE;
1821 // Choose best move. For each move score we add two terms, both dependent on
1822 // weakness. One is deterministic and bigger for weaker levels, and one is
1823 // random. Then we choose the move with the resulting highest score.
1824 for (size_t i = 0; i < multiPV; ++i)
1826 // This is our magic formula
1827 int push = ( weakness * int(topScore - rootMoves[i].score)
1828 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1830 if (rootMoves[i].score + push >= maxScore)
1832 maxScore = rootMoves[i].score + push;
1833 best = rootMoves[i].pv[0];
1843 /// MainThread::check_time() is used to print debug info and, more importantly,
1844 /// to detect when we are out of available time and thus stop the search.
1846 void MainThread::check_time() {
1851 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1852 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1854 static TimePoint lastInfoTime = now();
1856 TimePoint elapsed = Time.elapsed();
1857 TimePoint tick = Limits.startTime + elapsed;
1859 if (tick - lastInfoTime >= 1000)
1861 lastInfoTime = tick;
1865 // poll on MPI signals
1866 Cluster::signals_poll();
1868 // We should not stop pondering until told so by the GUI
1872 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1873 || (Limits.movetime && elapsed >= Limits.movetime)
1874 || (Limits.nodes && Cluster::nodes_searched() >= (uint64_t)Limits.nodes))
1875 Threads.stop = true;
1879 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1880 /// that all (if any) unsearched PV lines are sent using a previous search score.
1882 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1884 std::stringstream ss;
1885 TimePoint elapsed = Time.elapsed() + 1;
1886 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1887 size_t pvIdx = pos.this_thread()->pvIdx;
1888 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1889 uint64_t nodesSearched = Cluster::nodes_searched();
1890 uint64_t tbHits = Cluster::tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1892 for (size_t i = 0; i < multiPV; ++i)
1894 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1896 if (depth == 1 && !updated && i > 0)
1899 Depth d = updated ? depth : std::max(1, depth - 1);
1900 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1902 if (v == -VALUE_INFINITE)
1905 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1906 v = tb ? rootMoves[i].tbScore : v;
1908 if (ss.rdbuf()->in_avail()) // Not at first line
1913 << " seldepth " << rootMoves[i].selDepth
1914 << " multipv " << i + 1
1915 << " score " << UCI::value(v);
1917 if (Options["UCI_ShowWDL"])
1918 ss << UCI::wdl(v, pos.game_ply());
1920 if (!tb && i == pvIdx)
1921 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1923 ss << " nodes " << nodesSearched
1924 << " nps " << nodesSearched * 1000 / elapsed;
1926 if (elapsed > 1000) // Earlier makes little sense
1927 ss << " hashfull " << TT.hashfull();
1929 ss << " tbhits " << tbHits
1930 << " time " << elapsed
1933 for (Move m : rootMoves[i].pv)
1934 ss << " " << UCI::move(m, pos.is_chess960());
1941 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1942 /// before exiting the search, for instance, in case we stop the search during a
1943 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1944 /// otherwise in case of 'ponder on' we have nothing to think on.
1946 bool RootMove::extract_ponder_from_tt(Position& pos) {
1951 assert(pv.size() == 1);
1953 if (pv[0] == MOVE_NONE)
1956 pos.do_move(pv[0], st);
1957 TTEntry* tte = TT.probe(pos.key(), ttHit);
1961 Move m = tte->move(); // Local copy to be SMP safe
1962 if (MoveList<LEGAL>(pos).contains(m))
1966 pos.undo_move(pv[0]);
1967 return pv.size() > 1;
1970 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1973 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1974 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1975 Cardinality = int(Options["SyzygyProbeLimit"]);
1976 bool dtz_available = true;
1978 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1979 // ProbeDepth == DEPTH_ZERO
1980 if (Cardinality > MaxCardinality)
1982 Cardinality = MaxCardinality;
1986 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1988 // Rank moves using DTZ tables
1989 RootInTB = root_probe(pos, rootMoves);
1993 // DTZ tables are missing; try to rank moves using WDL tables
1994 dtz_available = false;
1995 RootInTB = root_probe_wdl(pos, rootMoves);
2001 // Sort moves according to TB rank
2002 std::stable_sort(rootMoves.begin(), rootMoves.end(),
2003 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
2005 // Probe during search only if DTZ is not available and we are winning
2006 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
2011 // Clean up if root_probe() and root_probe_wdl() have failed
2012 for (auto& m : rootMoves)