2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
22 #include <cstring> // For std::memset
37 #include "syzygy/tbprobe.h"
46 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 constexpr uint64_t TtHitAverageWindow = 4096;
66 constexpr uint64_t TtHitAverageResolution = 1024;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(234 * (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 + 503) / 1024 + (!i && r > 915);
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 > 14 ? 66 : 6 * d * d + 231 * d - 206;
90 // Add a small random component to draw evaluations to avoid 3-fold 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 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
171 uint64_t cnt, nodes = 0;
172 const bool leaf = (depth == 2);
174 for (const auto& m : MoveList<LEGAL>(pos))
176 if (Root && depth <= 1)
181 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
185 if (Root && Cluster::is_root())
186 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
194 /// Search::init() is called at startup to initialize various lookup tables
196 void Search::init() {
198 for (int i = 1; i < MAX_MOVES; ++i)
199 Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
203 /// Search::clear() resets search state to its initial value
205 void Search::clear() {
207 Threads.main()->wait_for_search_finished();
209 Time.availableNodes = 0;
212 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
216 /// MainThread::search() is started when the program receives the UCI 'go'
217 /// command. It searches from the root position and outputs the "bestmove".
219 void MainThread::search() {
223 nodes = perft<true>(rootPos, Limits.perft);
224 if (Cluster::is_root())
225 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
230 Color us = rootPos.side_to_move();
231 Time.init(Limits, us, rootPos.game_ply());
234 Eval::NNUE::verify();
236 if (rootMoves.empty())
238 rootMoves.emplace_back(MOVE_NONE);
239 if (Cluster::is_root())
240 sync_cout << "info depth 0 score "
241 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
246 Threads.start_searching(); // start non-main threads
247 Thread::search(); // main thread start searching
250 // When we reach the maximum depth, we can arrive here without a raise of
251 // Threads.stop. However, if we are pondering or in an infinite search,
252 // the UCI protocol states that we shouldn't print the best move before the
253 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
254 // until the GUI sends one of those commands.
256 while (!Threads.stop && (ponder || Limits.infinite))
257 { Cluster::signals_poll(); } // Busy wait for a stop or a ponder reset
259 // Stop the threads if not already stopped (also raise the stop if
260 // "ponderhit" just reset Threads.ponder).
263 // Signal and synchronize all other ranks
264 Cluster::signals_sync();
266 // Wait until all threads have finished
267 Threads.wait_for_search_finished();
269 // When playing in 'nodes as time' mode, subtract the searched nodes from
270 // the available ones before exiting.
272 Time.availableNodes += Limits.inc[us] - Cluster::nodes_searched();
274 Thread* bestThread = this;
276 if ( int(Options["MultiPV"]) == 1
278 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
279 && rootMoves[0].pv[0] != MOVE_NONE)
280 bestThread = Threads.get_best_thread();
282 // Prepare PVLine and ponder move
283 std::string PVLine = UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE);
285 Move bestMove = bestThread->rootMoves[0].pv[0];
286 Move ponderMove = MOVE_NONE;
287 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
288 ponderMove = bestThread->rootMoves[0].pv[1];
290 // Exchange info as needed
291 Cluster::MoveInfo mi{bestMove,
293 bestThread->completedDepth,
294 bestThread->rootMoves[0].score,
296 Cluster::pick_moves(mi, PVLine);
298 bestPreviousScore = static_cast<Value>(mi.score);
300 if (Cluster::is_root())
302 // Send again PV info if we have a new best thread/rank
303 if (bestThread != this || mi.rank != 0)
304 sync_cout << PVLine << sync_endl;
306 bestMove = static_cast<Move>(mi.move);
307 ponderMove = static_cast<Move>(mi.ponder);
309 if (ponderMove != MOVE_NONE)
310 sync_cout << "bestmove " << UCI::move(bestMove, rootPos.is_chess960())
311 << " ponder " << UCI::move(ponderMove, rootPos.is_chess960()) << sync_endl;
313 sync_cout << "bestmove " << UCI::move(bestMove, rootPos.is_chess960()) << sync_endl;
319 /// Thread::search() is the main iterative deepening loop. It calls search()
320 /// repeatedly with increasing depth until the allocated thinking time has been
321 /// consumed, the user stops the search, or the maximum search depth is reached.
323 void Thread::search() {
325 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
326 // The former is needed to allow update_continuation_histories(ss-1, ...),
327 // which accesses its argument at ss-6, also near the root.
328 // The latter is needed for statScores and killer initialization.
329 Stack stack[MAX_PLY+10], *ss = stack+7;
331 Value bestValue, alpha, beta, delta;
332 Move lastBestMove = MOVE_NONE;
333 Depth lastBestMoveDepth = 0;
334 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
335 double timeReduction = 1, totBestMoveChanges = 0;
336 Color us = rootPos.side_to_move();
339 std::memset(ss-7, 0, 10 * sizeof(Stack));
340 for (int i = 7; i > 0; i--)
341 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
345 bestValue = delta = alpha = -VALUE_INFINITE;
346 beta = VALUE_INFINITE;
350 if (mainThread->bestPreviousScore == VALUE_INFINITE)
351 for (int i = 0; i < 4; ++i)
352 mainThread->iterValue[i] = VALUE_ZERO;
354 for (int i = 0; i < 4; ++i)
355 mainThread->iterValue[i] = mainThread->bestPreviousScore;
358 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
359 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
361 size_t multiPV = size_t(Options["MultiPV"]);
363 // Pick integer skill levels, but non-deterministically round up or down
364 // such that the average integer skill corresponds to the input floating point one.
365 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
366 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
367 // for match (TC 60+0.6) results spanning a wide range of k values.
369 double floatLevel = Options["UCI_LimitStrength"] ?
370 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
371 double(Options["Skill Level"]);
372 int intLevel = int(floatLevel) +
373 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
374 Skill skill(intLevel);
376 // When playing with strength handicap enable MultiPV search that we will
377 // use behind the scenes to retrieve a set of possible moves.
379 multiPV = std::max(multiPV, (size_t)4);
381 multiPV = std::min(multiPV, rootMoves.size());
382 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
384 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
386 // In analysis mode, adjust contempt in accordance with user preference
387 if (Limits.infinite || Options["UCI_AnalyseMode"])
388 ct = Options["Analysis Contempt"] == "Off" ? 0
389 : Options["Analysis Contempt"] == "Both" ? ct
390 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
391 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
394 // Evaluation score is from the white point of view
395 contempt = (us == WHITE ? make_score(ct, ct / 2)
396 : -make_score(ct, ct / 2));
398 int searchAgainCounter = 0;
400 // Iterative deepening loop until requested to stop or the target depth is reached
401 while ( ++rootDepth < MAX_PLY
403 && !(Limits.depth && mainThread && Cluster::is_root() && rootDepth > Limits.depth))
405 // Age out PV variability metric
407 totBestMoveChanges /= 2;
410 // Save the last iteration's scores before first PV line is searched and
411 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
412 for (RootMove& rm : rootMoves)
413 rm.previousScore = rm.score;
418 if (!Threads.increaseDepth)
419 searchAgainCounter++;
421 // MultiPV loop. We perform a full root search for each PV line
422 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
427 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
428 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
432 // Reset UCI info selDepth for each depth and each PV line
435 // Reset aspiration window starting size
438 Value prev = rootMoves[pvIdx].previousScore;
440 alpha = std::max(prev - delta,-VALUE_INFINITE);
441 beta = std::min(prev + delta, VALUE_INFINITE);
443 // Adjust contempt based on root move's previousScore (dynamic contempt)
444 int dct = ct + (113 - ct / 2) * prev / (abs(prev) + 147);
446 contempt = (us == WHITE ? make_score(dct, dct / 2)
447 : -make_score(dct, dct / 2));
450 // Start with a small aspiration window and, in the case of a fail
451 // high/low, re-search with a bigger window until we don't fail
456 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
457 bestValue = Stockfish::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
459 // Bring the best move to the front. It is critical that sorting
460 // is done with a stable algorithm because all the values but the
461 // first and eventually the new best one are set to -VALUE_INFINITE
462 // and we want to keep the same order for all the moves except the
463 // new PV that goes to the front. Note that in case of MultiPV
464 // search the already searched PV lines are preserved.
465 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
467 // If search has been stopped, we break immediately. Sorting is
468 // safe because RootMoves is still valid, although it refers to
469 // the previous iteration.
473 // When failing high/low give some update (without cluttering
474 // the UI) before a re-search.
475 if ( Cluster::is_root()
478 && (bestValue <= alpha || bestValue >= beta)
479 && Time.elapsed() > 3000)
481 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
482 Cluster::cluster_info(rootDepth);
485 // In case of failing low/high increase aspiration window and
486 // re-search, otherwise exit the loop.
487 if (bestValue <= alpha)
489 beta = (alpha + beta) / 2;
490 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
494 mainThread->stopOnPonderhit = false;
496 else if (bestValue >= beta)
498 beta = std::min(bestValue + delta, VALUE_INFINITE);
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 = Time.optimum() * fallingEval * reduction * bestMoveInstability;
564 // Cap used time in case of a single legal move for a better viewer experience in tournaments
565 // yielding correct scores and sufficiently fast moves.
566 if (rootMoves.size() == 1)
567 totalTime = std::min(500.0, totalTime);
569 // Stop the search if we have exceeded the totalTime
570 if (Time.elapsed() > totalTime)
572 // If we are allowed to ponder do not stop the search now but
573 // keep pondering until the GUI sends "ponderhit" or "stop".
574 if (mainThread->ponder)
575 mainThread->stopOnPonderhit = true;
579 else if ( Threads.increaseDepth
580 && !mainThread->ponder
581 && Time.elapsed() > totalTime * 0.58)
582 Threads.increaseDepth = false;
584 Threads.increaseDepth = true;
587 mainThread->iterValue[iterIdx] = bestValue;
588 iterIdx = (iterIdx + 1) & 3;
594 mainThread->previousTimeReduction = timeReduction;
596 // If skill level is enabled, swap best PV line with the sub-optimal one
598 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
599 skill.best ? skill.best : skill.pick_best(multiPV)));
605 // search<>() is the main search function for both PV and non-PV nodes
607 template <NodeType NT>
608 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
610 constexpr bool PvNode = NT == PV;
611 const bool rootNode = PvNode && ss->ply == 0;
612 const Depth maxNextDepth = rootNode ? depth : depth + 1;
614 // Check if we have an upcoming move which draws by repetition, or
615 // if the opponent had an alternative move earlier to this position.
616 if ( pos.rule50_count() >= 3
617 && alpha < VALUE_DRAW
619 && pos.has_game_cycle(ss->ply))
621 alpha = value_draw(pos.this_thread());
626 // Dive into quiescence search when the depth reaches zero
628 return qsearch<NT>(pos, ss, alpha, beta);
630 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
631 assert(PvNode || (alpha == beta - 1));
632 assert(0 < depth && depth < MAX_PLY);
633 assert(!(PvNode && cutNode));
635 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
637 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
641 Move ttMove, move, excludedMove, bestMove;
642 Depth extension, newDepth;
643 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
644 bool formerPv, givesCheck, improving, didLMR, priorCapture;
645 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
646 ttCapture, singularQuietLMR;
648 int moveCount, captureCount, quietCount;
650 // Step 1. Initialize node
651 Thread* thisThread = pos.this_thread();
652 ss->inCheck = pos.checkers();
653 priorCapture = pos.captured_piece();
654 Color us = pos.side_to_move();
655 moveCount = captureCount = quietCount = ss->moveCount = 0;
656 bestValue = -VALUE_INFINITE;
657 maxValue = VALUE_INFINITE;
658 ss->distanceFromPv = (PvNode ? 0 : ss->distanceFromPv);
660 // Check for the available remaining time
661 if (thisThread == Threads.main())
662 static_cast<MainThread*>(thisThread)->check_time();
664 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
665 if (PvNode && thisThread->selDepth < ss->ply + 1)
666 thisThread->selDepth = ss->ply + 1;
670 // Step 2. Check for aborted search and immediate draw
671 if ( Threads.stop.load(std::memory_order_relaxed)
672 || pos.is_draw(ss->ply)
673 || ss->ply >= MAX_PLY)
674 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
675 : value_draw(pos.this_thread());
677 // Step 3. Mate distance pruning. Even if we mate at the next move our score
678 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
679 // a shorter mate was found upward in the tree then there is no need to search
680 // because we will never beat the current alpha. Same logic but with reversed
681 // signs applies also in the opposite condition of being mated instead of giving
682 // mate. In this case return a fail-high score.
683 alpha = std::max(mated_in(ss->ply), alpha);
684 beta = std::min(mate_in(ss->ply+1), beta);
689 assert(0 <= ss->ply && ss->ply < MAX_PLY);
691 (ss+1)->ply = ss->ply + 1;
692 (ss+1)->ttPv = false;
693 (ss+1)->excludedMove = bestMove = MOVE_NONE;
694 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
695 Square prevSq = to_sq((ss-1)->currentMove);
697 // Initialize statScore to zero for the grandchildren of the current position.
698 // So statScore is shared between all grandchildren and only the first grandchild
699 // starts with statScore = 0. Later grandchildren start with the last calculated
700 // statScore of the previous grandchild. This influences the reduction rules in
701 // LMR which are based on the statScore of parent position.
703 (ss+2)->statScore = 0;
705 // Step 4. Transposition table lookup. We don't want the score of a partial
706 // search to overwrite a previous full search TT value, so we use a different
707 // position key in case of an excluded move.
708 excludedMove = ss->excludedMove;
709 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
710 tte = TT.probe(posKey, ss->ttHit);
711 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
712 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
713 : ss->ttHit ? tte->move() : MOVE_NONE;
715 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
716 formerPv = ss->ttPv && !PvNode;
718 // Update low ply history for previous move if we are near root and position is or has been in PV
721 && ss->ply - 1 < MAX_LPH
723 && is_ok((ss-1)->currentMove))
724 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
726 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
727 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
728 + TtHitAverageResolution * ss->ttHit;
730 // At non-PV nodes we check for an early TT cutoff
733 && tte->depth() >= depth
734 && ttValue != VALUE_NONE // Possible in case of TT access race
735 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
736 : (tte->bound() & BOUND_UPPER)))
738 // If ttMove is quiet, update move sorting heuristics on TT hit
743 // Bonus for a quiet ttMove that fails high
744 if (!pos.capture_or_promotion(ttMove))
745 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
747 // Extra penalty for early quiet moves of the previous ply
748 if ((ss-1)->moveCount <= 2 && !priorCapture)
749 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
751 // Penalty for a quiet ttMove that fails low
752 else if (!pos.capture_or_promotion(ttMove))
754 int penalty = -stat_bonus(depth);
755 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
756 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
760 // Partial workaround for the graph history interaction problem
761 // For high rule50 counts don't produce transposition table cutoffs.
762 if (pos.rule50_count() < 90)
766 // Step 5. Tablebases probe
767 if (!rootNode && TB::Cardinality)
769 int piecesCount = pos.count<ALL_PIECES>();
771 if ( piecesCount <= TB::Cardinality
772 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
773 && pos.rule50_count() == 0
774 && !pos.can_castle(ANY_CASTLING))
777 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
779 // Force check of time on the next occasion
780 if (thisThread == Threads.main())
781 static_cast<MainThread*>(thisThread)->callsCnt = 0;
783 if (err != TB::ProbeState::FAIL)
785 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
787 int drawScore = TB::UseRule50 ? 1 : 0;
789 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
790 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
791 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
792 : VALUE_DRAW + 2 * wdl * drawScore;
794 Bound b = wdl < -drawScore ? BOUND_UPPER
795 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
797 if ( b == BOUND_EXACT
798 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
800 Cluster::save(thisThread, tte,
801 posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
802 std::min(MAX_PLY - 1, depth + 6),
803 MOVE_NONE, VALUE_NONE);
810 if (b == BOUND_LOWER)
811 bestValue = value, alpha = std::max(alpha, bestValue);
819 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
821 // Step 6. Static evaluation of the position
824 // Skip early pruning when in check
825 ss->staticEval = eval = VALUE_NONE;
831 // Never assume anything about values stored in TT
832 ss->staticEval = eval = tte->eval();
833 if (eval == VALUE_NONE)
834 ss->staticEval = eval = evaluate(pos);
836 // Randomize draw evaluation
837 if (eval == VALUE_DRAW)
838 eval = value_draw(thisThread);
840 // Can ttValue be used as a better position evaluation?
841 if ( ttValue != VALUE_NONE
842 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
847 // In case of null move search use previous static eval with a different sign
848 // and addition of two tempos
849 if ((ss-1)->currentMove != MOVE_NULL)
850 ss->staticEval = eval = evaluate(pos);
852 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
854 // Save static evaluation into transposition table
855 Cluster::save(thisThread, tte,
856 posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
860 // Use static evaluation difference to improve quiet move ordering
861 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
863 int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
864 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
867 // Set up improving flag that is used in various pruning heuristics
868 // We define position as improving if static evaluation of position is better
869 // Than the previous static evaluation at our turn
870 // In case of us being in check at our previous move we look at move prior to it
871 improving = (ss-2)->staticEval == VALUE_NONE
872 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
873 : ss->staticEval > (ss-2)->staticEval;
875 // Step 7. Futility pruning: child node (~50 Elo)
878 && eval - futility_margin(depth, improving) >= beta
879 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
882 // Step 8. Null move search with verification search (~40 Elo)
884 && (ss-1)->currentMove != MOVE_NULL
885 && (ss-1)->statScore < 24185
887 && eval >= ss->staticEval
888 && ss->staticEval >= beta - 24 * depth - 34 * improving + 162 * ss->ttPv + 159
890 && pos.non_pawn_material(us)
891 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
893 assert(eval - beta >= 0);
895 // Null move dynamic reduction based on depth and value
896 Depth R = (1062 + 68 * depth) / 256 + std::min(int(eval - beta) / 190, 3);
898 ss->currentMove = MOVE_NULL;
899 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
901 pos.do_null_move(st);
903 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
905 pos.undo_null_move();
907 if (nullValue >= beta)
909 // Do not return unproven mate or TB scores
910 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
913 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
916 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
918 // Do verification search at high depths, with null move pruning disabled
919 // for us, until ply exceeds nmpMinPly.
920 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
921 thisThread->nmpColor = us;
923 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
925 thisThread->nmpMinPly = 0;
932 probCutBeta = beta + 209 - 44 * improving;
934 // Step 9. ProbCut (~10 Elo)
935 // If we have a good enough capture and a reduced search returns a value
936 // much above beta, we can (almost) safely prune the previous move.
939 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
940 // if value from transposition table is lower than probCutBeta, don't attempt probCut
941 // there and in further interactions with transposition table cutoff depth is set to depth - 3
942 // because probCut search has depth set to depth - 4 but we also do a move before it
943 // so effective depth is equal to depth - 3
945 && tte->depth() >= depth - 3
946 && ttValue != VALUE_NONE
947 && ttValue < probCutBeta))
949 // if ttMove is a capture and value from transposition table is good enough produce probCut
950 // cutoff without digging into actual probCut search
952 && tte->depth() >= depth - 3
953 && ttValue != VALUE_NONE
954 && ttValue >= probCutBeta
956 && pos.capture_or_promotion(ttMove))
959 assert(probCutBeta < VALUE_INFINITE);
960 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
961 int probCutCount = 0;
962 bool ttPv = ss->ttPv;
965 while ( (move = mp.next_move()) != MOVE_NONE
966 && probCutCount < 2 + 2 * cutNode)
967 if (move != excludedMove && pos.legal(move))
969 assert(pos.capture_or_promotion(move));
972 captureOrPromotion = true;
975 ss->currentMove = move;
976 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
978 [pos.moved_piece(move)]
981 pos.do_move(move, st);
983 // Perform a preliminary qsearch to verify that the move holds
984 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
986 // If the qsearch held, perform the regular search
987 if (value >= probCutBeta)
988 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
992 if (value >= probCutBeta)
994 // if transposition table doesn't have equal or more deep info write probCut data into it
996 && tte->depth() >= depth - 3
997 && ttValue != VALUE_NONE))
998 Cluster::save(thisThread, tte, posKey, value_to_tt(value, ss->ply), ttPv,
1000 depth - 3, move, ss->staticEval);
1007 // Step 10. If the position is not in TT, decrease depth by 2
1013 moves_loop: // When in check, search starts from here
1015 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
1017 // Step 11. A small Probcut idea, when we are in check
1018 probCutBeta = beta + 400;
1023 && (tte->bound() & BOUND_LOWER)
1024 && tte->depth() >= depth - 3
1025 && ttValue >= probCutBeta
1026 && abs(ttValue) <= VALUE_KNOWN_WIN
1027 && abs(beta) <= VALUE_KNOWN_WIN
1032 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1033 nullptr , (ss-4)->continuationHistory,
1034 nullptr , (ss-6)->continuationHistory };
1036 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
1038 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1039 &thisThread->lowPlyHistory,
1047 singularQuietLMR = moveCountPruning = false;
1049 // Mark this node as being searched
1050 ThreadHolding th(thisThread, posKey, ss->ply);
1052 // Step 12. Loop through all pseudo-legal moves until no moves remain
1053 // or a beta cutoff occurs.
1054 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
1056 assert(is_ok(move));
1058 if (move == excludedMove)
1061 // At root obey the "searchmoves" option and skip moves not listed in Root
1062 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1063 // mode we also skip PV moves which have been already searched and those
1064 // of lower "TB rank" if we are in a TB root position.
1065 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1066 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1069 // Check for legality
1070 if (!rootNode && !pos.legal(move))
1073 ss->moveCount = ++moveCount;
1075 if (rootNode && Cluster::is_root() && thisThread == Threads.main() && Time.elapsed() > 3000)
1076 sync_cout << "info depth " << depth
1077 << " currmove " << UCI::move(move, pos.is_chess960())
1078 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1080 (ss+1)->pv = nullptr;
1083 captureOrPromotion = pos.capture_or_promotion(move);
1084 movedPiece = pos.moved_piece(move);
1085 givesCheck = pos.gives_check(move);
1087 // Indicate PvNodes that will probably fail low if node was searched with non-PV search
1088 // at depth equal or greater to current depth and result of this search was far below alpha
1089 bool likelyFailLow = PvNode
1091 && (tte->bound() & BOUND_UPPER)
1092 && ttValue < alpha + 200 + 100 * depth
1093 && tte->depth() >= depth;
1095 // Calculate new depth for this move
1096 newDepth = depth - 1;
1098 // Step 13. Pruning at shallow depth (~200 Elo)
1100 && pos.non_pawn_material(us)
1101 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1103 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1104 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1106 // Reduced depth of the next LMR search
1107 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1109 if ( captureOrPromotion
1112 // Capture history based pruning when the move doesn't give check
1115 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1118 // SEE based pruning
1119 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1124 // Countermoves based pruning (~20 Elo)
1125 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1126 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1127 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1130 // Futility pruning: parent node (~5 Elo)
1133 && ss->staticEval + 174 + 157 * lmrDepth <= alpha
1134 && (*contHist[0])[movedPiece][to_sq(move)]
1135 + (*contHist[1])[movedPiece][to_sq(move)]
1136 + (*contHist[3])[movedPiece][to_sq(move)]
1137 + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 28255)
1140 // Prune moves with negative SEE (~20 Elo)
1141 if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1146 // Step 14. Extensions (~75 Elo)
1148 // Singular extension search (~70 Elo). If all moves but one fail low on a
1149 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1150 // then that move is singular and should be extended. To verify this we do
1151 // a reduced search on all the other moves but the ttMove and if the
1152 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1156 && !excludedMove // Avoid recursive singular search
1157 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1158 && abs(ttValue) < VALUE_KNOWN_WIN
1159 && (tte->bound() & BOUND_LOWER)
1160 && tte->depth() >= depth - 3)
1162 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1163 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1164 ss->excludedMove = move;
1165 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1166 ss->excludedMove = MOVE_NONE;
1168 if (value < singularBeta)
1171 singularQuietLMR = !ttCapture;
1174 // Multi-cut pruning
1175 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1176 // search without the ttMove. So we assume this expected Cut-node is not singular,
1177 // that multiple moves fail high, and we can prune the whole subtree by returning
1179 else if (singularBeta >= beta)
1180 return singularBeta;
1182 // If the eval of ttMove is greater than beta we try also if there is another
1183 // move that pushes it over beta, if so also produce a cutoff.
1184 else if (ttValue >= beta)
1186 ss->excludedMove = move;
1187 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1188 ss->excludedMove = MOVE_NONE;
1195 // Check extension (~2 Elo)
1196 else if ( givesCheck
1197 && (pos.is_discovered_check_on_king(~us, move) || pos.see_ge(move)))
1200 // Last captures extension
1201 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1202 && pos.non_pawn_material() <= 2 * RookValueMg)
1205 // Add extension to new depth
1206 newDepth += extension;
1208 // Speculative prefetch as early as possible
1209 prefetch(TT.first_entry(pos.key_after(move)));
1211 // Update the current move (this must be done after singular extension search)
1212 ss->currentMove = move;
1213 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1214 [captureOrPromotion]
1218 // Step 15. Make the move
1219 pos.do_move(move, st, givesCheck);
1221 (ss+1)->distanceFromPv = ss->distanceFromPv + moveCount - 1;
1223 // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
1224 // We use various heuristics for the sons of a node after the first son has
1225 // been searched. In general we would like to reduce them, but there are many
1226 // cases where we extend a son if it has good chances to be "interesting".
1228 && moveCount > 1 + 2 * rootNode
1229 && ( !captureOrPromotion
1231 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1233 || (!PvNode && !formerPv && captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 3678)
1234 || thisThread->ttHitAverage < 432 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1236 Depth r = reduction(improving, depth, moveCount);
1238 // Decrease reduction if the ttHit running average is large
1239 if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1242 // Increase reduction if other threads are searching this position
1246 // Decrease reduction if position is or has been on the PV
1247 // and node is not likely to fail low. (~10 Elo)
1248 if (ss->ttPv && !likelyFailLow)
1251 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1252 if ((rootNode || !PvNode) && thisThread->rootDepth > 10 && thisThread->bestMoveChanges <= 2)
1255 // More reductions for late moves if position was not in previous PV
1256 if (moveCountPruning && !formerPv)
1259 // Decrease reduction if opponent's move count is high (~5 Elo)
1260 if ((ss-1)->moveCount > 13)
1263 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1264 if (singularQuietLMR)
1267 if (captureOrPromotion)
1269 // Unless giving check, this capture is likely bad
1271 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
1276 // Increase reduction if ttMove is a capture (~5 Elo)
1280 // Increase reduction at root if failing high
1281 r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0;
1283 // Increase reduction for cut nodes (~10 Elo)
1287 // Decrease reduction for moves that escape a capture. Filter out
1288 // castling moves, because they are coded as "king captures rook" and
1289 // hence break make_move(). (~2 Elo)
1290 else if ( type_of(move) == NORMAL
1291 && !pos.see_ge(reverse_move(move)))
1292 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1294 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1295 + (*contHist[0])[movedPiece][to_sq(move)]
1296 + (*contHist[1])[movedPiece][to_sq(move)]
1297 + (*contHist[3])[movedPiece][to_sq(move)]
1300 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1301 if (ss->statScore >= -89 && (ss-1)->statScore < -116)
1304 else if ((ss-1)->statScore >= -112 && ss->statScore < -100)
1307 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1308 // If we are not in check use statScore, but if we are in check we use
1309 // the sum of main history and first continuation history with an offset.
1311 r -= (thisThread->mainHistory[us][from_to(move)]
1312 + (*contHist[0])[movedPiece][to_sq(move)] - 3833) / 16384;
1314 r -= ss->statScore / 14790;
1317 // In general we want to cap the LMR depth search at newDepth. But for nodes
1318 // close to the principal variation the cap is at (newDepth + 1), which will
1319 // allow these nodes to be searched deeper than the pv (up to 4 plies deeper).
1320 Depth d = std::clamp(newDepth - r, 1, newDepth + ((ss+1)->distanceFromPv <= 4));
1322 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1324 // If the son is reduced and fails high it will be re-searched at full depth
1325 doFullDepthSearch = value > alpha && d < newDepth;
1330 doFullDepthSearch = !PvNode || moveCount > 1;
1334 // Step 17. Full depth search when LMR is skipped or fails high
1335 if (doFullDepthSearch)
1337 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1339 // If the move passed LMR update its stats
1340 if (didLMR && !captureOrPromotion)
1342 int bonus = value > alpha ? stat_bonus(newDepth)
1343 : -stat_bonus(newDepth);
1345 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1349 // For PV nodes only, do a full PV search on the first move or after a fail
1350 // high (in the latter case search only if value < beta), otherwise let the
1351 // parent node fail low with value <= alpha and try another move.
1352 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1355 (ss+1)->pv[0] = MOVE_NONE;
1357 value = -search<PV>(pos, ss+1, -beta, -alpha,
1358 std::min(maxNextDepth, newDepth), false);
1361 // Step 18. Undo move
1362 pos.undo_move(move);
1364 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1366 // Step 19. Check for a new best move
1367 // Finished searching the move. If a stop occurred, the return value of
1368 // the search cannot be trusted, and we return immediately without
1369 // updating best move, PV and TT.
1370 if (Threads.stop.load(std::memory_order_relaxed))
1375 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1376 thisThread->rootMoves.end(), move);
1378 // PV move or new best move?
1379 if (moveCount == 1 || value > alpha)
1382 rm.selDepth = thisThread->selDepth;
1387 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1388 rm.pv.push_back(*m);
1390 // We record how often the best move has been changed in each
1391 // iteration. This information is used for time management and LMR
1393 ++thisThread->bestMoveChanges;
1396 // All other moves but the PV are set to the lowest value: this
1397 // is not a problem when sorting because the sort is stable and the
1398 // move position in the list is preserved - just the PV is pushed up.
1399 rm.score = -VALUE_INFINITE;
1402 if (value > bestValue)
1410 if (PvNode && !rootNode) // Update pv even in fail-high case
1411 update_pv(ss->pv, move, (ss+1)->pv);
1413 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1417 assert(value >= beta); // Fail high
1424 // If the move is worse than some previously searched move, remember it to update its stats later
1425 if (move != bestMove)
1427 if (captureOrPromotion && captureCount < 32)
1428 capturesSearched[captureCount++] = move;
1430 else if (!captureOrPromotion && quietCount < 64)
1431 quietsSearched[quietCount++] = move;
1435 // The following condition would detect a stop only after move loop has been
1436 // completed. But in this case bestValue is valid because we have fully
1437 // searched our subtree, and we can anyhow save the result in TT.
1443 // Step 20. Check for mate and stalemate
1444 // All legal moves have been searched and if there are no legal moves, it
1445 // must be a mate or a stalemate. If we are in a singular extension search then
1446 // return a fail low score.
1448 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1451 bestValue = excludedMove ? alpha
1452 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1454 // If there is a move which produces search value greater than alpha we update stats of searched moves
1456 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1457 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1459 // Bonus for prior countermove that caused the fail low
1460 else if ( (depth >= 3 || PvNode)
1462 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1465 bestValue = std::min(bestValue, maxValue);
1467 // If no good move is found and the previous position was ttPv, then the previous
1468 // opponent move is probably good and the new position is added to the search tree.
1469 if (bestValue <= alpha)
1470 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1471 // Otherwise, a counter move has been found and if the position is the last leaf
1472 // in the search tree, remove the position from the search tree.
1474 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1476 // Write gathered information in transposition table
1477 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1478 Cluster::save(thisThread, tte,
1479 posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1480 bestValue >= beta ? BOUND_LOWER :
1481 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1482 depth, bestMove, ss->staticEval);
1484 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1490 // qsearch() is the quiescence search function, which is called by the main search
1491 // function with zero depth, or recursively with further decreasing depth per call.
1492 template <NodeType NT>
1493 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1495 constexpr bool PvNode = NT == PV;
1497 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1498 assert(PvNode || (alpha == beta - 1));
1503 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
1507 Move ttMove, move, bestMove;
1509 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1510 bool pvHit, givesCheck, captureOrPromotion;
1515 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1517 ss->pv[0] = MOVE_NONE;
1520 Thread* thisThread = pos.this_thread();
1521 (ss+1)->ply = ss->ply + 1;
1522 bestMove = MOVE_NONE;
1523 ss->inCheck = pos.checkers();
1526 // Check for an immediate draw or maximum ply reached
1527 if ( pos.is_draw(ss->ply)
1528 || ss->ply >= MAX_PLY)
1529 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1531 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1533 // Decide whether or not to include checks: this fixes also the type of
1534 // TT entry depth that we are going to use. Note that in qsearch we use
1535 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1536 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1537 : DEPTH_QS_NO_CHECKS;
1538 // Transposition table lookup
1540 tte = TT.probe(posKey, ss->ttHit);
1541 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1542 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1543 pvHit = ss->ttHit && tte->is_pv();
1547 && tte->depth() >= ttDepth
1548 && ttValue != VALUE_NONE // Only in case of TT access race
1549 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1550 : (tte->bound() & BOUND_UPPER)))
1553 // Evaluate the position statically
1556 ss->staticEval = VALUE_NONE;
1557 bestValue = futilityBase = -VALUE_INFINITE;
1563 // Never assume anything about values stored in TT
1564 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1565 ss->staticEval = bestValue = evaluate(pos);
1567 // Can ttValue be used as a better position evaluation?
1568 if ( ttValue != VALUE_NONE
1569 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1570 bestValue = ttValue;
1573 // In case of null move search use previous static eval with a different sign
1574 // and addition of two tempos
1575 ss->staticEval = bestValue =
1576 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1577 : -(ss-1)->staticEval + 2 * Tempo;
1579 // Stand pat. Return immediately if static value is at least beta
1580 if (bestValue >= beta)
1582 // Save gathered info in transposition table
1584 Cluster::save(thisThread, tte,
1585 posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1586 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1591 if (PvNode && bestValue > alpha)
1594 futilityBase = bestValue + 155;
1597 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1598 nullptr , (ss-4)->continuationHistory,
1599 nullptr , (ss-6)->continuationHistory };
1601 // Initialize a MovePicker object for the current position, and prepare
1602 // to search the moves. Because the depth is <= 0 here, only captures,
1603 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1604 // will be generated.
1605 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1606 &thisThread->captureHistory,
1608 to_sq((ss-1)->currentMove));
1610 // Loop through the moves until no moves remain or a beta cutoff occurs
1611 while ((move = mp.next_move()) != MOVE_NONE)
1613 assert(is_ok(move));
1615 givesCheck = pos.gives_check(move);
1616 captureOrPromotion = pos.capture_or_promotion(move);
1620 // Futility pruning and moveCount pruning
1621 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1623 && futilityBase > -VALUE_KNOWN_WIN
1624 && !pos.advanced_pawn_push(move))
1630 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1632 if (futilityValue <= alpha)
1634 bestValue = std::max(bestValue, futilityValue);
1638 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1640 bestValue = std::max(bestValue, futilityBase);
1645 // Do not search moves with negative SEE values
1646 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1647 && !pos.see_ge(move))
1650 // Speculative prefetch as early as possible
1651 prefetch(TT.first_entry(pos.key_after(move)));
1653 // Check for legality just before making the move
1654 if (!pos.legal(move))
1660 ss->currentMove = move;
1661 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1662 [captureOrPromotion]
1663 [pos.moved_piece(move)]
1666 // CounterMove based pruning
1667 if ( !captureOrPromotion
1668 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1669 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1670 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1673 // Make and search the move
1674 pos.do_move(move, st, givesCheck);
1675 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1676 pos.undo_move(move);
1678 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1680 // Check for a new best move
1681 if (value > bestValue)
1689 if (PvNode) // Update pv even in fail-high case
1690 update_pv(ss->pv, move, (ss+1)->pv);
1692 if (PvNode && value < beta) // Update alpha here!
1700 // All legal moves have been searched. A special case: if we're in check
1701 // and no legal moves were found, it is checkmate.
1702 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1704 assert(!MoveList<LEGAL>(pos).size());
1706 return mated_in(ss->ply); // Plies to mate from the root
1709 // Save gathered info in transposition table
1710 Cluster::save(thisThread, tte,
1711 posKey, value_to_tt(bestValue, ss->ply), pvHit,
1712 bestValue >= beta ? BOUND_LOWER :
1713 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1714 ttDepth, bestMove, ss->staticEval);
1716 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1722 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1723 // "plies to mate from the current position". Standard scores are unchanged.
1724 // The function is called before storing a value in the transposition table.
1726 Value value_to_tt(Value v, int ply) {
1728 assert(v != VALUE_NONE);
1730 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1731 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1735 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1736 // from the transposition table (which refers to the plies to mate/be mated from
1737 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1738 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1739 // and the graph history interaction, we return an optimal TB score instead.
1741 Value value_from_tt(Value v, int ply, int r50c) {
1743 if (v == VALUE_NONE)
1746 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1748 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1749 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1754 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1756 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1757 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1766 // update_pv() adds current move and appends child pv[]
1768 void update_pv(Move* pv, Move move, Move* childPv) {
1770 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1776 // update_all_stats() updates stats at the end of search() when a bestMove is found
1778 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1779 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1782 Color us = pos.side_to_move();
1783 Thread* thisThread = pos.this_thread();
1784 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1785 Piece moved_piece = pos.moved_piece(bestMove);
1786 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1788 bonus1 = stat_bonus(depth + 1);
1789 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1790 : std::min(bonus1, stat_bonus(depth)); // smaller bonus
1792 if (!pos.capture_or_promotion(bestMove))
1794 // Increase stats for the best move in case it was a quiet move
1795 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1797 // Decrease stats for all non-best quiet moves
1798 for (int i = 0; i < quietCount; ++i)
1800 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1801 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1805 // Increase stats for the best move in case it was a capture move
1806 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1808 // Extra penalty for a quiet early move that was not a TT move or
1809 // main killer move in previous ply when it gets refuted.
1810 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1811 && !pos.captured_piece())
1812 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1814 // Decrease stats for all non-best capture moves
1815 for (int i = 0; i < captureCount; ++i)
1817 moved_piece = pos.moved_piece(capturesSearched[i]);
1818 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1819 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1824 // update_continuation_histories() updates histories of the move pairs formed
1825 // by moves at ply -1, -2, -4, and -6 with current move.
1827 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1829 for (int i : {1, 2, 4, 6})
1831 // Only update first 2 continuation histories if we are in check
1832 if (ss->inCheck && i > 2)
1834 if (is_ok((ss-i)->currentMove))
1835 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1840 // update_quiet_stats() updates move sorting heuristics
1842 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1845 if (ss->killers[0] != move)
1847 ss->killers[1] = ss->killers[0];
1848 ss->killers[0] = move;
1851 Color us = pos.side_to_move();
1852 Thread* thisThread = pos.this_thread();
1853 thisThread->mainHistory[us][from_to(move)] << bonus;
1854 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1856 // Penalty for reversed move in case of moved piece not being a pawn
1857 if (type_of(pos.moved_piece(move)) != PAWN)
1858 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1860 // Update countermove history
1861 if (is_ok((ss-1)->currentMove))
1863 Square prevSq = to_sq((ss-1)->currentMove);
1864 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1867 // Update low ply history
1868 if (depth > 11 && ss->ply < MAX_LPH)
1869 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1872 // When playing with strength handicap, choose best move among a set of RootMoves
1873 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1875 Move Skill::pick_best(size_t multiPV) {
1877 const RootMoves& rootMoves = Threads.main()->rootMoves;
1878 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1880 // RootMoves are already sorted by score in descending order
1881 Value topScore = rootMoves[0].score;
1882 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1883 int weakness = 120 - 2 * level;
1884 int maxScore = -VALUE_INFINITE;
1886 // Choose best move. For each move score we add two terms, both dependent on
1887 // weakness. One is deterministic and bigger for weaker levels, and one is
1888 // random. Then we choose the move with the resulting highest score.
1889 for (size_t i = 0; i < multiPV; ++i)
1891 // This is our magic formula
1892 int push = ( weakness * int(topScore - rootMoves[i].score)
1893 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1895 if (rootMoves[i].score + push >= maxScore)
1897 maxScore = rootMoves[i].score + push;
1898 best = rootMoves[i].pv[0];
1908 /// MainThread::check_time() is used to print debug info and, more importantly,
1909 /// to detect when we are out of available time and thus stop the search.
1911 void MainThread::check_time() {
1916 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1917 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1919 static TimePoint lastInfoTime = now();
1921 TimePoint elapsed = Time.elapsed();
1922 TimePoint tick = Limits.startTime + elapsed;
1924 if (tick - lastInfoTime >= 1000)
1926 lastInfoTime = tick;
1930 // poll on MPI signals
1931 Cluster::signals_poll();
1933 // We should not stop pondering until told so by the GUI
1937 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1938 || (Limits.movetime && elapsed >= Limits.movetime)
1939 || (Limits.nodes && Cluster::nodes_searched() >= (uint64_t)Limits.nodes))
1940 Threads.stop = true;
1944 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1945 /// that all (if any) unsearched PV lines are sent using a previous search score.
1947 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1949 std::stringstream ss;
1950 TimePoint elapsed = Time.elapsed() + 1;
1951 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1952 size_t pvIdx = pos.this_thread()->pvIdx;
1953 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1954 uint64_t nodesSearched = Cluster::nodes_searched();
1955 uint64_t tbHits = Cluster::tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1957 for (size_t i = 0; i < multiPV; ++i)
1959 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1961 if (depth == 1 && !updated && i > 0)
1964 Depth d = updated ? depth : std::max(1, depth - 1);
1965 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1967 if (v == -VALUE_INFINITE)
1970 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1971 v = tb ? rootMoves[i].tbScore : v;
1973 if (ss.rdbuf()->in_avail()) // Not at first line
1978 << " seldepth " << rootMoves[i].selDepth
1979 << " multipv " << i + 1
1980 << " score " << UCI::value(v);
1982 if (Options["UCI_ShowWDL"])
1983 ss << UCI::wdl(v, pos.game_ply());
1985 if (!tb && i == pvIdx)
1986 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1988 ss << " nodes " << nodesSearched
1989 << " nps " << nodesSearched * 1000 / elapsed;
1991 if (elapsed > 1000) // Earlier makes little sense
1992 ss << " hashfull " << TT.hashfull();
1994 ss << " tbhits " << tbHits
1995 << " time " << elapsed
1998 for (Move m : rootMoves[i].pv)
1999 ss << " " << UCI::move(m, pos.is_chess960());
2006 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
2007 /// before exiting the search, for instance, in case we stop the search during a
2008 /// fail high at root. We try hard to have a ponder move to return to the GUI,
2009 /// otherwise in case of 'ponder on' we have nothing to think on.
2011 bool RootMove::extract_ponder_from_tt(Position& pos) {
2014 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
2018 assert(pv.size() == 1);
2020 if (pv[0] == MOVE_NONE)
2023 pos.do_move(pv[0], st);
2024 TTEntry* tte = TT.probe(pos.key(), ttHit);
2028 Move m = tte->move(); // Local copy to be SMP safe
2029 if (MoveList<LEGAL>(pos).contains(m))
2033 pos.undo_move(pv[0]);
2034 return pv.size() > 1;
2037 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
2040 UseRule50 = bool(Options["Syzygy50MoveRule"]);
2041 ProbeDepth = int(Options["SyzygyProbeDepth"]);
2042 Cardinality = int(Options["SyzygyProbeLimit"]);
2043 bool dtz_available = true;
2045 // Tables with fewer pieces than SyzygyProbeLimit are searched with
2046 // ProbeDepth == DEPTH_ZERO
2047 if (Cardinality > MaxCardinality)
2049 Cardinality = MaxCardinality;
2053 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
2055 // Rank moves using DTZ tables
2056 RootInTB = root_probe(pos, rootMoves);
2060 // DTZ tables are missing; try to rank moves using WDL tables
2061 dtz_available = false;
2062 RootInTB = root_probe_wdl(pos, rootMoves);
2068 // Sort moves according to TB rank
2069 std::stable_sort(rootMoves.begin(), rootMoves.end(),
2070 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
2072 // Probe during search only if DTZ is not available and we are winning
2073 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
2078 // Clean up if root_probe() and root_probe_wdl() have failed
2079 for (auto& m : rootMoves)
2084 } // namespace Stockfish