2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 constexpr uint64_t TtHitAverageWindow = 4096;
65 constexpr uint64_t TtHitAverageResolution = 1024;
67 // Razor and futility margins
68 constexpr int RazorMargin = 531;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(217 * (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 + 511) / 1024 + (!i && r > 1007);
81 constexpr int futility_move_count(bool improving, Depth depth) {
82 return (4 + depth * depth) / (2 - improving);
85 // History and stats update bonus, based on depth
86 int stat_bonus(Depth d) {
87 return d > 15 ? -8 : 19 * d * d + 155 * d - 132;
90 // Add a small random component to draw evaluations to avoid 3fold-blindness
91 Value value_draw(Thread* thisThread) {
92 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 // Breadcrumbs are used to mark nodes as being searched by a given thread
108 std::atomic<Thread*> thread;
109 std::atomic<Key> key;
111 std::array<Breadcrumb, 1024> breadcrumbs;
113 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
114 // node for potential reductions. A free node will be marked upon entering the moves
115 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
116 struct ThreadHolding {
117 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
118 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
123 // See if another already marked this location, if not, mark it ourselves
124 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
127 (*location).thread.store(thisThread, std::memory_order_relaxed);
128 (*location).key.store(posKey, std::memory_order_relaxed);
131 else if ( tmp != thisThread
132 && (*location).key.load(std::memory_order_relaxed) == posKey)
138 if (owning) // Free the marked location
139 (*location).thread.store(nullptr, std::memory_order_relaxed);
142 bool marked() { return otherThread; }
145 Breadcrumb* location;
146 bool otherThread, owning;
149 template <NodeType NT>
150 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
152 template <NodeType NT>
153 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
155 Value value_to_tt(Value v, int ply);
156 Value value_from_tt(Value v, int ply, int r50c);
157 void update_pv(Move* pv, Move move, Move* childPv);
158 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
159 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
160 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
161 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
163 // perft() is our utility to verify move generation. All the leaf nodes up
164 // to the given depth are generated and counted, and the sum is returned.
166 uint64_t perft(Position& pos, Depth depth) {
169 uint64_t cnt, nodes = 0;
170 const bool leaf = (depth == 2);
172 for (const auto& m : MoveList<LEGAL>(pos))
174 if (Root && depth <= 1)
179 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
184 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
192 /// Search::init() is called at startup to initialize various lookup tables
194 void Search::init() {
196 for (int i = 1; i < MAX_MOVES; ++i)
197 Reductions[i] = int((24.8 + std::log(Threads.size())) * std::log(i));
201 /// Search::clear() resets search state to its initial value
203 void Search::clear() {
205 Threads.main()->wait_for_search_finished();
207 Time.availableNodes = 0;
210 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
214 /// MainThread::search() is started when the program receives the UCI 'go'
215 /// command. It searches from the root position and outputs the "bestmove".
217 void MainThread::search() {
221 nodes = perft<true>(rootPos, Limits.perft);
222 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
226 Color us = rootPos.side_to_move();
227 Time.init(Limits, us, rootPos.game_ply());
230 if (rootMoves.empty())
232 rootMoves.emplace_back(MOVE_NONE);
233 sync_cout << "info depth 0 score "
234 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
239 for (Thread* th : Threads)
241 th->bestMoveChanges = 0;
243 th->start_searching();
246 Thread::search(); // Let's start searching!
249 // When we reach the maximum depth, we can arrive here without a raise of
250 // Threads.stop. However, if we are pondering or in an infinite search,
251 // the UCI protocol states that we shouldn't print the best move before the
252 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
253 // until the GUI sends one of those commands.
255 while (!Threads.stop && (ponder || Limits.infinite))
256 {} // Busy wait for a stop or a ponder reset
258 // Stop the threads if not already stopped (also raise the stop if
259 // "ponderhit" just reset Threads.ponder).
262 // Wait until all threads have finished
263 for (Thread* th : Threads)
265 th->wait_for_search_finished();
267 // When playing in 'nodes as time' mode, subtract the searched nodes from
268 // the available ones before exiting.
270 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
272 Thread* bestThread = this;
274 // Check if there are threads with a better score than main thread
275 if ( int(Options["MultiPV"]) == 1
277 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
278 && rootMoves[0].pv[0] != MOVE_NONE)
280 std::map<Move, int64_t> votes;
281 Value minScore = this->rootMoves[0].score;
283 // Find minimum score
284 for (Thread* th: Threads)
285 minScore = std::min(minScore, th->rootMoves[0].score);
287 // Vote according to score and depth, and select the best thread
288 for (Thread* th : Threads)
290 votes[th->rootMoves[0].pv[0]] +=
291 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
293 if (abs(bestThread->rootMoves[0].score) >= VALUE_TB_WIN_IN_MAX_PLY)
295 // Make sure we pick the shortest mate / TB conversion or stave off mate the longest
296 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
299 else if ( th->rootMoves[0].score >= VALUE_TB_WIN_IN_MAX_PLY
300 || ( th->rootMoves[0].score > VALUE_TB_LOSS_IN_MAX_PLY
301 && votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]]))
306 bestPreviousScore = bestThread->rootMoves[0].score;
308 // Send again PV info if we have a new best thread
309 if (bestThread != this)
310 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
312 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
314 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
315 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
317 std::cout << sync_endl;
321 /// Thread::search() is the main iterative deepening loop. It calls search()
322 /// repeatedly with increasing depth until the allocated thinking time has been
323 /// consumed, the user stops the search, or the maximum search depth is reached.
325 void Thread::search() {
327 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
328 // The former is needed to allow update_continuation_histories(ss-1, ...),
329 // which accesses its argument at ss-6, also near the root.
330 // The latter is needed for statScores and killer initialization.
331 Stack stack[MAX_PLY+10], *ss = stack+7;
333 Value bestValue, alpha, beta, delta;
334 Move lastBestMove = MOVE_NONE;
335 Depth lastBestMoveDepth = 0;
336 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
337 double timeReduction = 1, totBestMoveChanges = 0;
338 Color us = rootPos.side_to_move();
341 std::memset(ss-7, 0, 10 * sizeof(Stack));
342 for (int i = 7; i > 0; i--)
343 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
347 bestValue = delta = alpha = -VALUE_INFINITE;
348 beta = VALUE_INFINITE;
352 if (mainThread->bestPreviousScore == VALUE_INFINITE)
353 for (int i = 0; i < 4; ++i)
354 mainThread->iterValue[i] = VALUE_ZERO;
356 for (int i = 0; i < 4; ++i)
357 mainThread->iterValue[i] = mainThread->bestPreviousScore;
360 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
361 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
363 size_t multiPV = size_t(Options["MultiPV"]);
365 // Pick integer skill levels, but non-deterministically round up or down
366 // such that the average integer skill corresponds to the input floating point one.
367 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
368 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
369 // for match (TC 60+0.6) results spanning a wide range of k values.
371 double floatLevel = Options["UCI_LimitStrength"] ?
372 Utility::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
373 double(Options["Skill Level"]);
374 int intLevel = int(floatLevel) +
375 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
376 Skill skill(intLevel);
378 // When playing with strength handicap enable MultiPV search that we will
379 // use behind the scenes to retrieve a set of possible moves.
381 multiPV = std::max(multiPV, (size_t)4);
383 multiPV = std::min(multiPV, rootMoves.size());
384 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
386 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
388 // In analysis mode, adjust contempt in accordance with user preference
389 if (Limits.infinite || Options["UCI_AnalyseMode"])
390 ct = Options["Analysis Contempt"] == "Off" ? 0
391 : Options["Analysis Contempt"] == "Both" ? ct
392 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
393 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
396 // Evaluation score is from the white point of view
397 contempt = (us == WHITE ? make_score(ct, ct / 2)
398 : -make_score(ct, ct / 2));
400 int searchAgainCounter = 0;
402 // Iterative deepening loop until requested to stop or the target depth is reached
403 while ( ++rootDepth < MAX_PLY
405 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
407 // Age out PV variability metric
409 totBestMoveChanges /= 2;
411 // Save the last iteration's scores before first PV line is searched and
412 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
413 for (RootMove& rm : rootMoves)
414 rm.previousScore = rm.score;
419 if (!Threads.increaseDepth)
420 searchAgainCounter++;
422 // MultiPV loop. We perform a full root search for each PV line
423 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
428 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
429 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
433 // Reset UCI info selDepth for each depth and each PV line
436 // Reset aspiration window starting size
439 Value prev = rootMoves[pvIdx].previousScore;
441 alpha = std::max(prev - delta,-VALUE_INFINITE);
442 beta = std::min(prev + delta, VALUE_INFINITE);
444 // Adjust contempt based on root move's previousScore (dynamic contempt)
445 int dct = ct + (102 - ct / 2) * prev / (abs(prev) + 157);
447 contempt = (us == WHITE ? make_score(dct, dct / 2)
448 : -make_score(dct, dct / 2));
451 // Start with a small aspiration window and, in the case of a fail
452 // high/low, re-search with a bigger window until we don't fail
454 int failedHighCnt = 0;
457 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
458 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
460 // Bring the best move to the front. It is critical that sorting
461 // is done with a stable algorithm because all the values but the
462 // first and eventually the new best one are set to -VALUE_INFINITE
463 // and we want to keep the same order for all the moves except the
464 // new PV that goes to the front. Note that in case of MultiPV
465 // search the already searched PV lines are preserved.
466 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
468 // If search has been stopped, we break immediately. Sorting is
469 // safe because RootMoves is still valid, although it refers to
470 // the previous iteration.
474 // When failing high/low give some update (without cluttering
475 // the UI) before a re-search.
478 && (bestValue <= alpha || bestValue >= beta)
479 && Time.elapsed() > 3000)
480 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
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);
513 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
514 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
518 completedDepth = rootDepth;
520 if (rootMoves[0].pv[0] != lastBestMove) {
521 lastBestMove = rootMoves[0].pv[0];
522 lastBestMoveDepth = rootDepth;
525 // Have we found a "mate in x"?
527 && bestValue >= VALUE_MATE_IN_MAX_PLY
528 && VALUE_MATE - bestValue <= 2 * Limits.mate)
534 // If skill level is enabled and time is up, pick a sub-optimal best move
535 if (skill.enabled() && skill.time_to_pick(rootDepth))
536 skill.pick_best(multiPV);
538 // Do we have time for the next iteration? Can we stop searching now?
539 if ( Limits.use_time_management()
541 && !mainThread->stopOnPonderhit)
543 double fallingEval = (332 + 6 * (mainThread->bestPreviousScore - bestValue)
544 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 704.0;
545 fallingEval = Utility::clamp(fallingEval, 0.5, 1.5);
547 // If the bestMove is stable over several iterations, reduce time accordingly
548 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.94 : 0.91;
549 double reduction = (1.41 + mainThread->previousTimeReduction) / (2.27 * timeReduction);
551 // Use part of the gained time from a previous stable move for the current move
552 for (Thread* th : Threads)
554 totBestMoveChanges += th->bestMoveChanges;
555 th->bestMoveChanges = 0;
557 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
559 double totalTime = rootMoves.size() == 1 ? 0 :
560 Time.optimum() * fallingEval * reduction * bestMoveInstability;
562 // Stop the search if we have exceeded the totalTime, at least 1ms search.
563 if (Time.elapsed() > totalTime)
565 // If we are allowed to ponder do not stop the search now but
566 // keep pondering until the GUI sends "ponderhit" or "stop".
567 if (mainThread->ponder)
568 mainThread->stopOnPonderhit = true;
572 else if ( Threads.increaseDepth
573 && !mainThread->ponder
574 && Time.elapsed() > totalTime * 0.6)
575 Threads.increaseDepth = false;
577 Threads.increaseDepth = true;
580 mainThread->iterValue[iterIdx] = bestValue;
581 iterIdx = (iterIdx + 1) & 3;
587 mainThread->previousTimeReduction = timeReduction;
589 // If skill level is enabled, swap best PV line with the sub-optimal one
591 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
592 skill.best ? skill.best : skill.pick_best(multiPV)));
598 // search<>() is the main search function for both PV and non-PV nodes
600 template <NodeType NT>
601 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
603 constexpr bool PvNode = NT == PV;
604 const bool rootNode = PvNode && ss->ply == 0;
606 // Check if we have an upcoming move which draws by repetition, or
607 // if the opponent had an alternative move earlier to this position.
608 if ( pos.rule50_count() >= 3
609 && alpha < VALUE_DRAW
611 && pos.has_game_cycle(ss->ply))
613 alpha = value_draw(pos.this_thread());
618 // Dive into quiescence search when the depth reaches zero
620 return qsearch<NT>(pos, ss, alpha, beta);
622 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
623 assert(PvNode || (alpha == beta - 1));
624 assert(0 < depth && depth < MAX_PLY);
625 assert(!(PvNode && cutNode));
627 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
631 Move ttMove, move, excludedMove, bestMove;
632 Depth extension, newDepth;
633 Value bestValue, value, ttValue, eval, maxValue;
634 bool ttHit, ttPv, formerPv, givesCheck, improving, didLMR, priorCapture;
635 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
636 ttCapture, singularQuietLMR;
638 int moveCount, captureCount, quietCount;
640 // Step 1. Initialize node
641 Thread* thisThread = pos.this_thread();
642 ss->inCheck = pos.checkers();
643 priorCapture = pos.captured_piece();
644 Color us = pos.side_to_move();
645 moveCount = captureCount = quietCount = ss->moveCount = 0;
646 bestValue = -VALUE_INFINITE;
647 maxValue = VALUE_INFINITE;
649 // Check for the available remaining time
650 if (thisThread == Threads.main())
651 static_cast<MainThread*>(thisThread)->check_time();
653 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
654 if (PvNode && thisThread->selDepth < ss->ply + 1)
655 thisThread->selDepth = ss->ply + 1;
659 // Step 2. Check for aborted search and immediate draw
660 if ( Threads.stop.load(std::memory_order_relaxed)
661 || pos.is_draw(ss->ply)
662 || ss->ply >= MAX_PLY)
663 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
664 : value_draw(pos.this_thread());
666 // Step 3. Mate distance pruning. Even if we mate at the next move our score
667 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
668 // a shorter mate was found upward in the tree then there is no need to search
669 // because we will never beat the current alpha. Same logic but with reversed
670 // signs applies also in the opposite condition of being mated instead of giving
671 // mate. In this case return a fail-high score.
672 alpha = std::max(mated_in(ss->ply), alpha);
673 beta = std::min(mate_in(ss->ply+1), beta);
678 assert(0 <= ss->ply && ss->ply < MAX_PLY);
680 (ss+1)->ply = ss->ply + 1;
681 (ss+1)->excludedMove = bestMove = MOVE_NONE;
682 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
683 Square prevSq = to_sq((ss-1)->currentMove);
685 // Initialize statScore to zero for the grandchildren of the current position.
686 // So statScore is shared between all grandchildren and only the first grandchild
687 // starts with statScore = 0. Later grandchildren start with the last calculated
688 // statScore of the previous grandchild. This influences the reduction rules in
689 // LMR which are based on the statScore of parent position.
691 (ss+4)->statScore = 0;
693 (ss+2)->statScore = 0;
695 // Step 4. Transposition table lookup. We don't want the score of a partial
696 // search to overwrite a previous full search TT value, so we use a different
697 // position key in case of an excluded move.
698 excludedMove = ss->excludedMove;
699 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
700 tte = TT.probe(posKey, ttHit);
701 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
702 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
703 : ttHit ? tte->move() : MOVE_NONE;
704 ttPv = PvNode || (ttHit && tte->is_pv());
705 formerPv = ttPv && !PvNode;
707 if (ttPv && depth > 12 && ss->ply - 1 < MAX_LPH && !pos.captured_piece() && is_ok((ss-1)->currentMove))
708 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
710 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
711 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
712 + TtHitAverageResolution * ttHit;
714 // At non-PV nodes we check for an early TT cutoff
717 && tte->depth() >= depth
718 && ttValue != VALUE_NONE // Possible in case of TT access race
719 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
720 : (tte->bound() & BOUND_UPPER)))
722 // If ttMove is quiet, update move sorting heuristics on TT hit
727 if (!pos.capture_or_promotion(ttMove))
728 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
730 // Extra penalty for early quiet moves of the previous ply
731 if ((ss-1)->moveCount <= 2 && !priorCapture)
732 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
734 // Penalty for a quiet ttMove that fails low
735 else if (!pos.capture_or_promotion(ttMove))
737 int penalty = -stat_bonus(depth);
738 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
739 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
743 if (pos.rule50_count() < 90)
747 // Step 5. Tablebases probe
748 if (!rootNode && TB::Cardinality)
750 int piecesCount = pos.count<ALL_PIECES>();
752 if ( piecesCount <= TB::Cardinality
753 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
754 && pos.rule50_count() == 0
755 && !pos.can_castle(ANY_CASTLING))
758 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
760 // Force check of time on the next occasion
761 if (thisThread == Threads.main())
762 static_cast<MainThread*>(thisThread)->callsCnt = 0;
764 if (err != TB::ProbeState::FAIL)
766 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
768 int drawScore = TB::UseRule50 ? 1 : 0;
770 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
771 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
772 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
773 : VALUE_DRAW + 2 * wdl * drawScore;
775 Bound b = wdl < -drawScore ? BOUND_UPPER
776 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
778 if ( b == BOUND_EXACT
779 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
781 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
782 std::min(MAX_PLY - 1, depth + 6),
783 MOVE_NONE, VALUE_NONE);
790 if (b == BOUND_LOWER)
791 bestValue = value, alpha = std::max(alpha, bestValue);
799 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
801 // Step 6. Static evaluation of the position
804 ss->staticEval = eval = VALUE_NONE;
806 goto moves_loop; // Skip early pruning when in check
810 // Never assume anything about values stored in TT
811 ss->staticEval = eval = tte->eval();
812 if (eval == VALUE_NONE)
813 ss->staticEval = eval = evaluate(pos);
815 if (eval == VALUE_DRAW)
816 eval = value_draw(thisThread);
818 // Can ttValue be used as a better position evaluation?
819 if ( ttValue != VALUE_NONE
820 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
825 if ((ss-1)->currentMove != MOVE_NULL)
827 int bonus = -(ss-1)->statScore / 512;
829 ss->staticEval = eval = evaluate(pos) + bonus;
832 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
834 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
837 // Step 7. Razoring (~1 Elo)
838 if ( !rootNode // The required rootNode PV handling is not available in qsearch
840 && eval <= alpha - RazorMargin)
841 return qsearch<NT>(pos, ss, alpha, beta);
843 improving = (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval > (ss-4)->staticEval
844 || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval > (ss-2)->staticEval;
846 // Step 8. Futility pruning: child node (~50 Elo)
849 && eval - futility_margin(depth, improving) >= beta
850 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
853 // Step 9. Null move search with verification search (~40 Elo)
855 && (ss-1)->currentMove != MOVE_NULL
856 && (ss-1)->statScore < 23397
858 && eval >= ss->staticEval
859 && ss->staticEval >= beta - 32 * depth - 30 * improving + 120 * ttPv + 292
861 && pos.non_pawn_material(us)
862 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
864 assert(eval - beta >= 0);
866 // Null move dynamic reduction based on depth and value
867 Depth R = (854 + 68 * depth) / 258 + std::min(int(eval - beta) / 192, 3);
869 ss->currentMove = MOVE_NULL;
870 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
872 pos.do_null_move(st);
874 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
876 pos.undo_null_move();
878 if (nullValue >= beta)
880 // Do not return unproven mate or TB scores
881 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
884 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
887 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
889 // Do verification search at high depths, with null move pruning disabled
890 // for us, until ply exceeds nmpMinPly.
891 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
892 thisThread->nmpColor = us;
894 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
896 thisThread->nmpMinPly = 0;
903 // Step 10. ProbCut (~10 Elo)
904 // If we have a good enough capture and a reduced search returns a value
905 // much above beta, we can (almost) safely prune the previous move.
908 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY)
910 Value raisedBeta = beta + 189 - 45 * improving;
911 assert(raisedBeta < VALUE_INFINITE);
912 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &captureHistory);
913 int probCutCount = 0;
915 while ( (move = mp.next_move()) != MOVE_NONE
916 && probCutCount < 2 + 2 * cutNode
918 && tte->depth() >= depth - 4
919 && ttValue < raisedBeta))
920 if (move != excludedMove && pos.legal(move))
922 assert(pos.capture_or_promotion(move));
925 captureOrPromotion = true;
928 ss->currentMove = move;
929 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
931 [pos.moved_piece(move)]
934 pos.do_move(move, st);
936 // Perform a preliminary qsearch to verify that the move holds
937 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
939 // If the qsearch held, perform the regular search
940 if (value >= raisedBeta)
941 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
945 if (value >= raisedBeta)
950 // Step 11. Internal iterative deepening (~1 Elo)
951 if (depth >= 7 && !ttMove)
953 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
955 tte = TT.probe(posKey, ttHit);
956 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
957 ttMove = ttHit ? tte->move() : MOVE_NONE;
960 moves_loop: // When in check, search starts from here
962 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
963 nullptr , (ss-4)->continuationHistory,
964 nullptr , (ss-6)->continuationHistory };
966 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
968 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
969 &thisThread->lowPlyHistory,
977 singularQuietLMR = moveCountPruning = false;
978 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
980 // Mark this node as being searched
981 ThreadHolding th(thisThread, posKey, ss->ply);
983 // Step 12. Loop through all pseudo-legal moves until no moves remain
984 // or a beta cutoff occurs.
985 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
989 if (move == excludedMove)
992 // At root obey the "searchmoves" option and skip moves not listed in Root
993 // Move List. As a consequence any illegal move is also skipped. In MultiPV
994 // mode we also skip PV moves which have been already searched and those
995 // of lower "TB rank" if we are in a TB root position.
996 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
997 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1000 ss->moveCount = ++moveCount;
1002 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1003 sync_cout << "info depth " << depth
1004 << " currmove " << UCI::move(move, pos.is_chess960())
1005 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1007 (ss+1)->pv = nullptr;
1010 captureOrPromotion = pos.capture_or_promotion(move);
1011 movedPiece = pos.moved_piece(move);
1012 givesCheck = pos.gives_check(move);
1014 // Calculate new depth for this move
1015 newDepth = depth - 1;
1017 // Step 13. Pruning at shallow depth (~200 Elo)
1019 && pos.non_pawn_material(us)
1020 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1022 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1023 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1025 // Reduced depth of the next LMR search
1026 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1028 if ( !captureOrPromotion
1031 // Countermoves based pruning (~20 Elo)
1032 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1033 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1034 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1037 // Futility pruning: parent node (~5 Elo)
1040 && ss->staticEval + 235 + 172 * lmrDepth <= alpha
1041 && (*contHist[0])[movedPiece][to_sq(move)]
1042 + (*contHist[1])[movedPiece][to_sq(move)]
1043 + (*contHist[3])[movedPiece][to_sq(move)] < 27400)
1046 // Prune moves with negative SEE (~20 Elo)
1047 if (!pos.see_ge(move, Value(-(32 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1052 // Capture history based pruning when the move doesn't give check
1055 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1058 // Futility pruning for captures
1061 && !(PvNode && abs(bestValue) < 2)
1063 && ss->staticEval + 270 + 384 * lmrDepth + PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] <= alpha)
1066 // See based pruning
1067 if (!pos.see_ge(move, Value(-194) * depth)) // (~25 Elo)
1072 // Step 14. Extensions (~75 Elo)
1074 // Singular extension search (~70 Elo). If all moves but one fail low on a
1075 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1076 // then that move is singular and should be extended. To verify this we do
1077 // a reduced search on all the other moves but the ttMove and if the
1078 // result is lower than ttValue minus a margin then we will extend the ttMove.
1082 && !excludedMove // Avoid recursive singular search
1083 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1084 && abs(ttValue) < VALUE_KNOWN_WIN
1085 && (tte->bound() & BOUND_LOWER)
1086 && tte->depth() >= depth - 3
1089 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1090 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1091 ss->excludedMove = move;
1092 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1093 ss->excludedMove = MOVE_NONE;
1095 if (value < singularBeta)
1098 singularQuietLMR = !ttCapture;
1101 // Multi-cut pruning
1102 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1103 // search without the ttMove. So we assume this expected Cut-node is not singular,
1104 // that multiple moves fail high, and we can prune the whole subtree by returning
1106 else if (singularBeta >= beta)
1107 return singularBeta;
1109 // If the eval of ttMove is greater than beta we try also if there is an other move that
1110 // pushes it over beta, if so also produce a cutoff
1111 else if (ttValue >= beta)
1113 ss->excludedMove = move;
1114 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1115 ss->excludedMove = MOVE_NONE;
1122 // Check extension (~2 Elo)
1123 else if ( givesCheck
1124 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1127 // Passed pawn extension
1128 else if ( move == ss->killers[0]
1129 && pos.advanced_pawn_push(move)
1130 && pos.pawn_passed(us, to_sq(move)))
1133 // Last captures extension
1134 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1135 && pos.non_pawn_material() <= 2 * RookValueMg)
1138 // Castling extension
1139 if (type_of(move) == CASTLING)
1142 // Late irreversible move extension
1144 && pos.rule50_count() > 80
1145 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1148 // Add extension to new depth
1149 newDepth += extension;
1151 // Speculative prefetch as early as possible
1152 prefetch(TT.first_entry(pos.key_after(move)));
1154 // Check for legality just before making the move
1155 if (!rootNode && !pos.legal(move))
1157 ss->moveCount = --moveCount;
1161 // Update the current move (this must be done after singular extension search)
1162 ss->currentMove = move;
1163 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1164 [captureOrPromotion]
1168 // Step 15. Make the move
1169 pos.do_move(move, st, givesCheck);
1171 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1172 // re-searched at full depth.
1174 && moveCount > 1 + 2 * rootNode
1175 && (!rootNode || thisThread->best_move_count(move) == 0)
1176 && ( !captureOrPromotion
1178 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1180 || thisThread->ttHitAverage < 375 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1182 Depth r = reduction(improving, depth, moveCount);
1184 // Decrease reduction if the ttHit running average is large
1185 if (thisThread->ttHitAverage > 500 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1188 // Reduction if other threads are searching this position.
1192 // Decrease reduction if position is or has been on the PV (~10 Elo)
1196 if (moveCountPruning && !formerPv)
1199 // Decrease reduction if opponent's move count is high (~5 Elo)
1200 if ((ss-1)->moveCount > 14)
1203 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1204 if (singularQuietLMR)
1207 if (!captureOrPromotion)
1209 // Increase reduction if ttMove is a capture (~5 Elo)
1213 // Increase reduction for cut nodes (~10 Elo)
1217 // Decrease reduction for moves that escape a capture. Filter out
1218 // castling moves, because they are coded as "king captures rook" and
1219 // hence break make_move(). (~2 Elo)
1220 else if ( type_of(move) == NORMAL
1221 && !pos.see_ge(reverse_move(move)))
1224 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1225 + (*contHist[0])[movedPiece][to_sq(move)]
1226 + (*contHist[1])[movedPiece][to_sq(move)]
1227 + (*contHist[3])[movedPiece][to_sq(move)]
1230 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1231 if (ss->statScore >= -102 && (ss-1)->statScore < -114)
1234 else if ((ss-1)->statScore >= -116 && ss->statScore < -154)
1237 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1238 r -= ss->statScore / 16434;
1242 // Increase reduction for captures/promotions if late move and at low depth
1243 if (depth < 8 && moveCount > 2)
1246 // Unless giving check, this capture is likely bad
1248 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 200 * depth <= alpha)
1252 Depth d = Utility::clamp(newDepth - r, 1, newDepth);
1254 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1256 doFullDepthSearch = value > alpha && d != newDepth;
1262 doFullDepthSearch = !PvNode || moveCount > 1;
1267 // Step 17. Full depth search when LMR is skipped or fails high
1268 if (doFullDepthSearch)
1270 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1272 if (didLMR && !captureOrPromotion)
1274 int bonus = value > alpha ? stat_bonus(newDepth)
1275 : -stat_bonus(newDepth);
1277 if (move == ss->killers[0])
1280 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1284 // For PV nodes only, do a full PV search on the first move or after a fail
1285 // high (in the latter case search only if value < beta), otherwise let the
1286 // parent node fail low with value <= alpha and try another move.
1287 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1290 (ss+1)->pv[0] = MOVE_NONE;
1292 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1295 // Step 18. Undo move
1296 pos.undo_move(move);
1298 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1300 // Step 19. Check for a new best move
1301 // Finished searching the move. If a stop occurred, the return value of
1302 // the search cannot be trusted, and we return immediately without
1303 // updating best move, PV and TT.
1304 if (Threads.stop.load(std::memory_order_relaxed))
1309 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1310 thisThread->rootMoves.end(), move);
1312 // PV move or new best move?
1313 if (moveCount == 1 || value > alpha)
1316 rm.selDepth = thisThread->selDepth;
1321 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1322 rm.pv.push_back(*m);
1324 // We record how often the best move has been changed in each
1325 // iteration. This information is used for time management: When
1326 // the best move changes frequently, we allocate some more time.
1328 ++thisThread->bestMoveChanges;
1331 // All other moves but the PV are set to the lowest value: this
1332 // is not a problem when sorting because the sort is stable and the
1333 // move position in the list is preserved - just the PV is pushed up.
1334 rm.score = -VALUE_INFINITE;
1337 if (value > bestValue)
1345 if (PvNode && !rootNode) // Update pv even in fail-high case
1346 update_pv(ss->pv, move, (ss+1)->pv);
1348 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1352 assert(value >= beta); // Fail high
1359 if (move != bestMove)
1361 if (captureOrPromotion && captureCount < 32)
1362 capturesSearched[captureCount++] = move;
1364 else if (!captureOrPromotion && quietCount < 64)
1365 quietsSearched[quietCount++] = move;
1369 // The following condition would detect a stop only after move loop has been
1370 // completed. But in this case bestValue is valid because we have fully
1371 // searched our subtree, and we can anyhow save the result in TT.
1377 // Step 20. Check for mate and stalemate
1378 // All legal moves have been searched and if there are no legal moves, it
1379 // must be a mate or a stalemate. If we are in a singular extension search then
1380 // return a fail low score.
1382 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1385 bestValue = excludedMove ? alpha
1386 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1389 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1390 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1392 // Bonus for prior countermove that caused the fail low
1393 else if ( (depth >= 3 || PvNode)
1395 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1398 bestValue = std::min(bestValue, maxValue);
1400 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1401 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1402 bestValue >= beta ? BOUND_LOWER :
1403 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1404 depth, bestMove, ss->staticEval);
1406 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1412 // qsearch() is the quiescence search function, which is called by the main search
1413 // function with zero depth, or recursively with further decreasing depth per call.
1414 template <NodeType NT>
1415 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1417 constexpr bool PvNode = NT == PV;
1419 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1420 assert(PvNode || (alpha == beta - 1));
1427 Move ttMove, move, bestMove;
1429 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1430 bool ttHit, pvHit, givesCheck, captureOrPromotion;
1435 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1437 ss->pv[0] = MOVE_NONE;
1440 Thread* thisThread = pos.this_thread();
1441 (ss+1)->ply = ss->ply + 1;
1442 bestMove = MOVE_NONE;
1443 ss->inCheck = pos.checkers();
1446 // Check for an immediate draw or maximum ply reached
1447 if ( pos.is_draw(ss->ply)
1448 || ss->ply >= MAX_PLY)
1449 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1451 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1453 // Decide whether or not to include checks: this fixes also the type of
1454 // TT entry depth that we are going to use. Note that in qsearch we use
1455 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1456 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1457 : DEPTH_QS_NO_CHECKS;
1458 // Transposition table lookup
1460 tte = TT.probe(posKey, ttHit);
1461 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1462 ttMove = ttHit ? tte->move() : MOVE_NONE;
1463 pvHit = ttHit && tte->is_pv();
1467 && tte->depth() >= ttDepth
1468 && ttValue != VALUE_NONE // Only in case of TT access race
1469 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1470 : (tte->bound() & BOUND_UPPER)))
1473 // Evaluate the position statically
1476 ss->staticEval = VALUE_NONE;
1477 bestValue = futilityBase = -VALUE_INFINITE;
1483 // Never assume anything about values stored in TT
1484 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1485 ss->staticEval = bestValue = evaluate(pos);
1487 // Can ttValue be used as a better position evaluation?
1488 if ( ttValue != VALUE_NONE
1489 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1490 bestValue = ttValue;
1493 ss->staticEval = bestValue =
1494 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1495 : -(ss-1)->staticEval + 2 * Tempo;
1497 // Stand pat. Return immediately if static value is at least beta
1498 if (bestValue >= beta)
1501 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1502 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1507 if (PvNode && bestValue > alpha)
1510 futilityBase = bestValue + 154;
1513 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1514 nullptr , (ss-4)->continuationHistory,
1515 nullptr , (ss-6)->continuationHistory };
1517 // Initialize a MovePicker object for the current position, and prepare
1518 // to search the moves. Because the depth is <= 0 here, only captures,
1519 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1521 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1522 &thisThread->captureHistory,
1524 to_sq((ss-1)->currentMove));
1526 // Loop through the moves until no moves remain or a beta cutoff occurs
1527 while ((move = mp.next_move()) != MOVE_NONE)
1529 assert(is_ok(move));
1531 givesCheck = pos.gives_check(move);
1532 captureOrPromotion = pos.capture_or_promotion(move);
1539 && futilityBase > -VALUE_KNOWN_WIN
1540 && !pos.advanced_pawn_push(move))
1542 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1544 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1546 if (futilityValue <= alpha)
1548 bestValue = std::max(bestValue, futilityValue);
1552 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1554 bestValue = std::max(bestValue, futilityBase);
1559 // Don't search moves with negative SEE values
1560 if ( !ss->inCheck && !pos.see_ge(move))
1563 // Speculative prefetch as early as possible
1564 prefetch(TT.first_entry(pos.key_after(move)));
1566 // Check for legality just before making the move
1567 if (!pos.legal(move))
1573 ss->currentMove = move;
1574 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1575 [captureOrPromotion]
1576 [pos.moved_piece(move)]
1579 // Make and search the move
1580 pos.do_move(move, st, givesCheck);
1581 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1582 pos.undo_move(move);
1584 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1586 // Check for a new best move
1587 if (value > bestValue)
1595 if (PvNode) // Update pv even in fail-high case
1596 update_pv(ss->pv, move, (ss+1)->pv);
1598 if (PvNode && value < beta) // Update alpha here!
1606 // All legal moves have been searched. A special case: If we're in check
1607 // and no legal moves were found, it is checkmate.
1608 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1609 return mated_in(ss->ply); // Plies to mate from the root
1611 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1612 bestValue >= beta ? BOUND_LOWER :
1613 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1614 ttDepth, bestMove, ss->staticEval);
1616 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1622 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1623 // "plies to mate from the current position". standard scores are unchanged.
1624 // The function is called before storing a value in the transposition table.
1626 Value value_to_tt(Value v, int ply) {
1628 assert(v != VALUE_NONE);
1630 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1631 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1635 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate or TB score
1636 // from the transposition table (which refers to the plies to mate/be mated
1637 // from current position) to "plies to mate/be mated (TB win/loss) from the root".
1638 // However, for mate scores, to avoid potentially false mate scores related to the 50 moves rule,
1639 // and the graph history interaction, return an optimal TB score instead.
1641 Value value_from_tt(Value v, int ply, int r50c) {
1643 if (v == VALUE_NONE)
1646 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1648 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1649 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1654 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1656 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1657 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1666 // update_pv() adds current move and appends child pv[]
1668 void update_pv(Move* pv, Move move, Move* childPv) {
1670 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1676 // update_all_stats() updates stats at the end of search() when a bestMove is found
1678 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1679 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1682 Color us = pos.side_to_move();
1683 Thread* thisThread = pos.this_thread();
1684 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1685 Piece moved_piece = pos.moved_piece(bestMove);
1686 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1688 bonus1 = stat_bonus(depth + 1);
1689 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1690 : stat_bonus(depth); // smaller bonus
1692 if (!pos.capture_or_promotion(bestMove))
1694 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1696 // Decrease all the non-best quiet moves
1697 for (int i = 0; i < quietCount; ++i)
1699 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1700 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1704 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1706 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1707 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1708 && !pos.captured_piece())
1709 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1711 // Decrease all the non-best capture moves
1712 for (int i = 0; i < captureCount; ++i)
1714 moved_piece = pos.moved_piece(capturesSearched[i]);
1715 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1716 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1721 // update_continuation_histories() updates histories of the move pairs formed
1722 // by moves at ply -1, -2, -4, and -6 with current move.
1724 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1726 for (int i : {1, 2, 4, 6})
1728 if (ss->inCheck && i > 2)
1730 if (is_ok((ss-i)->currentMove))
1731 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1736 // update_quiet_stats() updates move sorting heuristics
1738 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1740 if (ss->killers[0] != move)
1742 ss->killers[1] = ss->killers[0];
1743 ss->killers[0] = move;
1746 Color us = pos.side_to_move();
1747 Thread* thisThread = pos.this_thread();
1748 thisThread->mainHistory[us][from_to(move)] << bonus;
1749 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1751 if (type_of(pos.moved_piece(move)) != PAWN)
1752 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1754 if (is_ok((ss-1)->currentMove))
1756 Square prevSq = to_sq((ss-1)->currentMove);
1757 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1760 if (depth > 12 && ss->ply < MAX_LPH)
1761 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1764 // When playing with strength handicap, choose best move among a set of RootMoves
1765 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1767 Move Skill::pick_best(size_t multiPV) {
1769 const RootMoves& rootMoves = Threads.main()->rootMoves;
1770 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1772 // RootMoves are already sorted by score in descending order
1773 Value topScore = rootMoves[0].score;
1774 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1775 int weakness = 120 - 2 * level;
1776 int maxScore = -VALUE_INFINITE;
1778 // Choose best move. For each move score we add two terms, both dependent on
1779 // weakness. One is deterministic and bigger for weaker levels, and one is
1780 // random. Then we choose the move with the resulting highest score.
1781 for (size_t i = 0; i < multiPV; ++i)
1783 // This is our magic formula
1784 int push = ( weakness * int(topScore - rootMoves[i].score)
1785 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1787 if (rootMoves[i].score + push >= maxScore)
1789 maxScore = rootMoves[i].score + push;
1790 best = rootMoves[i].pv[0];
1799 /// MainThread::check_time() is used to print debug info and, more importantly,
1800 /// to detect when we are out of available time and thus stop the search.
1802 void MainThread::check_time() {
1807 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1808 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1810 static TimePoint lastInfoTime = now();
1812 TimePoint elapsed = Time.elapsed();
1813 TimePoint tick = Limits.startTime + elapsed;
1815 if (tick - lastInfoTime >= 1000)
1817 lastInfoTime = tick;
1821 // We should not stop pondering until told so by the GUI
1825 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1826 || (Limits.movetime && elapsed >= Limits.movetime)
1827 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1828 Threads.stop = true;
1832 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1833 /// that all (if any) unsearched PV lines are sent using a previous search score.
1835 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1837 std::stringstream ss;
1838 TimePoint elapsed = Time.elapsed() + 1;
1839 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1840 size_t pvIdx = pos.this_thread()->pvIdx;
1841 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1842 uint64_t nodesSearched = Threads.nodes_searched();
1843 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1845 for (size_t i = 0; i < multiPV; ++i)
1847 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1849 if (depth == 1 && !updated)
1852 Depth d = updated ? depth : depth - 1;
1853 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1855 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1856 v = tb ? rootMoves[i].tbScore : v;
1858 if (ss.rdbuf()->in_avail()) // Not at first line
1863 << " seldepth " << rootMoves[i].selDepth
1864 << " multipv " << i + 1
1865 << " score " << UCI::value(v);
1867 if (!tb && i == pvIdx)
1868 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1870 ss << " nodes " << nodesSearched
1871 << " nps " << nodesSearched * 1000 / elapsed;
1873 if (elapsed > 1000) // Earlier makes little sense
1874 ss << " hashfull " << TT.hashfull();
1876 ss << " tbhits " << tbHits
1877 << " time " << elapsed
1880 for (Move m : rootMoves[i].pv)
1881 ss << " " << UCI::move(m, pos.is_chess960());
1888 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1889 /// before exiting the search, for instance, in case we stop the search during a
1890 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1891 /// otherwise in case of 'ponder on' we have nothing to think on.
1893 bool RootMove::extract_ponder_from_tt(Position& pos) {
1898 assert(pv.size() == 1);
1900 if (pv[0] == MOVE_NONE)
1903 pos.do_move(pv[0], st);
1904 TTEntry* tte = TT.probe(pos.key(), ttHit);
1908 Move m = tte->move(); // Local copy to be SMP safe
1909 if (MoveList<LEGAL>(pos).contains(m))
1913 pos.undo_move(pv[0]);
1914 return pv.size() > 1;
1917 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1920 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1921 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1922 Cardinality = int(Options["SyzygyProbeLimit"]);
1923 bool dtz_available = true;
1925 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1926 // ProbeDepth == DEPTH_ZERO
1927 if (Cardinality > MaxCardinality)
1929 Cardinality = MaxCardinality;
1933 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1935 // Rank moves using DTZ tables
1936 RootInTB = root_probe(pos, rootMoves);
1940 // DTZ tables are missing; try to rank moves using WDL tables
1941 dtz_available = false;
1942 RootInTB = root_probe_wdl(pos, rootMoves);
1948 // Sort moves according to TB rank
1949 std::sort(rootMoves.begin(), rootMoves.end(),
1950 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1952 // Probe during search only if DTZ is not available and we are winning
1953 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1958 // Clean up if root_probe() and root_probe_wdl() have failed
1959 for (auto& m : rootMoves)