2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
22 #include <cstring> // For std::memset
36 #include "syzygy/tbprobe.h"
43 namespace Tablebases {
51 namespace TB = Tablebases;
55 using namespace Search;
59 // Different node types, used as a template parameter
60 enum NodeType { NonPV, PV };
62 constexpr uint64_t TtHitAverageWindow = 4096;
63 constexpr uint64_t TtHitAverageResolution = 1024;
65 // Razor and futility margins
66 constexpr int RazorMargin = 510;
67 Value futility_margin(Depth d, bool improving) {
68 return Value(223 * (d - improving));
71 // Reductions lookup table, initialized at startup
72 int Reductions[MAX_MOVES]; // [depth or moveNumber]
74 Depth reduction(bool i, Depth d, int mn) {
75 int r = Reductions[d] * Reductions[mn];
76 return (r + 509) / 1024 + (!i && r > 894);
79 constexpr int futility_move_count(bool improving, Depth depth) {
80 return (3 + depth * depth) / (2 - improving);
83 // History and stats update bonus, based on depth
84 int stat_bonus(Depth d) {
85 return d > 13 ? 29 : 17 * d * d + 134 * d - 134;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Thread* thisThread) {
90 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
93 // Skill structure is used to implement strength limit
95 explicit Skill(int l) : level(l) {}
96 bool enabled() const { return level < 20; }
97 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
98 Move pick_best(size_t multiPV);
101 Move best = MOVE_NONE;
104 // Breadcrumbs are used to mark nodes as being searched by a given thread
106 std::atomic<Thread*> thread;
107 std::atomic<Key> key;
109 std::array<Breadcrumb, 1024> breadcrumbs;
111 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
112 // node for potential reductions. A free node will be marked upon entering the moves
113 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
114 struct ThreadHolding {
115 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
116 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
121 // See if another already marked this location, if not, mark it ourselves
122 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
125 (*location).thread.store(thisThread, std::memory_order_relaxed);
126 (*location).key.store(posKey, std::memory_order_relaxed);
129 else if ( tmp != thisThread
130 && (*location).key.load(std::memory_order_relaxed) == posKey)
136 if (owning) // Free the marked location
137 (*location).thread.store(nullptr, std::memory_order_relaxed);
140 bool marked() { return otherThread; }
143 Breadcrumb* location;
144 bool otherThread, owning;
147 template <NodeType NT>
148 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
150 template <NodeType NT>
151 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
153 Value value_to_tt(Value v, int ply);
154 Value value_from_tt(Value v, int ply, int r50c);
155 void update_pv(Move* pv, Move move, Move* childPv);
156 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
157 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
158 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
159 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
161 // perft() is our utility to verify move generation. All the leaf nodes up
162 // to the given depth are generated and counted, and the sum is returned.
164 uint64_t perft(Position& pos, Depth depth) {
167 uint64_t cnt, nodes = 0;
168 const bool leaf = (depth == 2);
170 for (const auto& m : MoveList<LEGAL>(pos))
172 if (Root && depth <= 1)
177 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
182 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
190 /// Search::init() is called at startup to initialize various lookup tables
192 void Search::init() {
194 for (int i = 1; i < MAX_MOVES; ++i)
195 Reductions[i] = int((22.0 + std::log(Threads.size())) * std::log(i));
199 /// Search::clear() resets search state to its initial value
201 void Search::clear() {
203 Threads.main()->wait_for_search_finished();
205 Time.availableNodes = 0;
208 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
212 /// MainThread::search() is started when the program receives the UCI 'go'
213 /// command. It searches from the root position and outputs the "bestmove".
215 void MainThread::search() {
219 nodes = perft<true>(rootPos, Limits.perft);
220 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
224 Color us = rootPos.side_to_move();
225 Time.init(Limits, us, rootPos.game_ply());
230 if (rootMoves.empty())
232 rootMoves.emplace_back(MOVE_NONE);
233 sync_cout << "info depth 0 score "
234 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
239 Threads.start_searching(); // start non-main threads
240 Thread::search(); // main thread start searching
243 // When we reach the maximum depth, we can arrive here without a raise of
244 // Threads.stop. However, if we are pondering or in an infinite search,
245 // the UCI protocol states that we shouldn't print the best move before the
246 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
247 // until the GUI sends one of those commands.
249 while (!Threads.stop && (ponder || Limits.infinite))
250 {} // Busy wait for a stop or a ponder reset
252 // Stop the threads if not already stopped (also raise the stop if
253 // "ponderhit" just reset Threads.ponder).
256 // Wait until all threads have finished
257 Threads.wait_for_search_finished();
259 // When playing in 'nodes as time' mode, subtract the searched nodes from
260 // the available ones before exiting.
262 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
264 Thread* bestThread = this;
266 if ( int(Options["MultiPV"]) == 1
268 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
269 && rootMoves[0].pv[0] != MOVE_NONE)
270 bestThread = Threads.get_best_thread();
272 bestPreviousScore = bestThread->rootMoves[0].score;
274 // Send again PV info if we have a new best thread
275 if (bestThread != this)
276 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
278 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
280 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
281 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
283 std::cout << sync_endl;
287 /// Thread::search() is the main iterative deepening loop. It calls search()
288 /// repeatedly with increasing depth until the allocated thinking time has been
289 /// consumed, the user stops the search, or the maximum search depth is reached.
291 void Thread::search() {
293 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
294 // The former is needed to allow update_continuation_histories(ss-1, ...),
295 // which accesses its argument at ss-6, also near the root.
296 // The latter is needed for statScores and killer initialization.
297 Stack stack[MAX_PLY+10], *ss = stack+7;
299 Value bestValue, alpha, beta, delta;
300 Move lastBestMove = MOVE_NONE;
301 Depth lastBestMoveDepth = 0;
302 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
303 double timeReduction = 1, totBestMoveChanges = 0;
304 Color us = rootPos.side_to_move();
307 std::memset(ss-7, 0, 10 * sizeof(Stack));
308 for (int i = 7; i > 0; i--)
309 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
313 bestValue = delta = alpha = -VALUE_INFINITE;
314 beta = VALUE_INFINITE;
318 if (mainThread->bestPreviousScore == VALUE_INFINITE)
319 for (int i = 0; i < 4; ++i)
320 mainThread->iterValue[i] = VALUE_ZERO;
322 for (int i = 0; i < 4; ++i)
323 mainThread->iterValue[i] = mainThread->bestPreviousScore;
326 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
327 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
329 size_t multiPV = size_t(Options["MultiPV"]);
331 // Pick integer skill levels, but non-deterministically round up or down
332 // such that the average integer skill corresponds to the input floating point one.
333 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
334 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
335 // for match (TC 60+0.6) results spanning a wide range of k values.
337 double floatLevel = Options["UCI_LimitStrength"] ?
338 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
339 double(Options["Skill Level"]);
340 int intLevel = int(floatLevel) +
341 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
342 Skill skill(intLevel);
344 // When playing with strength handicap enable MultiPV search that we will
345 // use behind the scenes to retrieve a set of possible moves.
347 multiPV = std::max(multiPV, (size_t)4);
349 multiPV = std::min(multiPV, rootMoves.size());
350 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
352 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
354 // In analysis mode, adjust contempt in accordance with user preference
355 if (Limits.infinite || Options["UCI_AnalyseMode"])
356 ct = Options["Analysis Contempt"] == "Off" ? 0
357 : Options["Analysis Contempt"] == "Both" ? ct
358 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
359 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
362 // Evaluation score is from the white point of view
363 contempt = (us == WHITE ? make_score(ct, ct / 2)
364 : -make_score(ct, ct / 2));
366 int searchAgainCounter = 0;
368 // Iterative deepening loop until requested to stop or the target depth is reached
369 while ( ++rootDepth < MAX_PLY
371 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
373 // Age out PV variability metric
375 totBestMoveChanges /= 2;
377 // Save the last iteration's scores before first PV line is searched and
378 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
379 for (RootMove& rm : rootMoves)
380 rm.previousScore = rm.score;
385 if (!Threads.increaseDepth)
386 searchAgainCounter++;
388 // MultiPV loop. We perform a full root search for each PV line
389 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
394 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
395 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
399 // Reset UCI info selDepth for each depth and each PV line
402 // Reset aspiration window starting size
405 Value prev = rootMoves[pvIdx].previousScore;
407 alpha = std::max(prev - delta,-VALUE_INFINITE);
408 beta = std::min(prev + delta, VALUE_INFINITE);
410 // Adjust contempt based on root move's previousScore (dynamic contempt)
411 int dct = ct + (105 - ct / 2) * prev / (abs(prev) + 149);
413 contempt = (us == WHITE ? make_score(dct, dct / 2)
414 : -make_score(dct, dct / 2));
417 // Start with a small aspiration window and, in the case of a fail
418 // high/low, re-search with a bigger window until we don't fail
420 int failedHighCnt = 0;
423 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
424 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
426 // Bring the best move to the front. It is critical that sorting
427 // is done with a stable algorithm because all the values but the
428 // first and eventually the new best one are set to -VALUE_INFINITE
429 // and we want to keep the same order for all the moves except the
430 // new PV that goes to the front. Note that in case of MultiPV
431 // search the already searched PV lines are preserved.
432 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
434 // If search has been stopped, we break immediately. Sorting is
435 // safe because RootMoves is still valid, although it refers to
436 // the previous iteration.
440 // When failing high/low give some update (without cluttering
441 // the UI) before a re-search.
444 && (bestValue <= alpha || bestValue >= beta)
445 && Time.elapsed() > 3000)
446 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
448 // In case of failing low/high increase aspiration window and
449 // re-search, otherwise exit the loop.
450 if (bestValue <= alpha)
452 beta = (alpha + beta) / 2;
453 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
457 mainThread->stopOnPonderhit = false;
459 else if (bestValue >= beta)
461 beta = std::min(bestValue + delta, VALUE_INFINITE);
466 ++rootMoves[pvIdx].bestMoveCount;
470 delta += delta / 4 + 5;
472 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
475 // Sort the PV lines searched so far and update the GUI
476 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
479 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
480 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
484 completedDepth = rootDepth;
486 if (rootMoves[0].pv[0] != lastBestMove) {
487 lastBestMove = rootMoves[0].pv[0];
488 lastBestMoveDepth = rootDepth;
491 // Have we found a "mate in x"?
493 && bestValue >= VALUE_MATE_IN_MAX_PLY
494 && VALUE_MATE - bestValue <= 2 * Limits.mate)
500 // If skill level is enabled and time is up, pick a sub-optimal best move
501 if (skill.enabled() && skill.time_to_pick(rootDepth))
502 skill.pick_best(multiPV);
504 // Do we have time for the next iteration? Can we stop searching now?
505 if ( Limits.use_time_management()
507 && !mainThread->stopOnPonderhit)
509 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
510 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
511 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
513 // If the bestMove is stable over several iterations, reduce time accordingly
514 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
515 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
517 // Use part of the gained time from a previous stable move for the current move
518 for (Thread* th : Threads)
520 totBestMoveChanges += th->bestMoveChanges;
521 th->bestMoveChanges = 0;
523 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
525 double totalTime = rootMoves.size() == 1 ? 0 :
526 Time.optimum() * fallingEval * reduction * bestMoveInstability;
528 // Stop the search if we have exceeded the totalTime, at least 1ms search
529 if (Time.elapsed() > totalTime)
531 // If we are allowed to ponder do not stop the search now but
532 // keep pondering until the GUI sends "ponderhit" or "stop".
533 if (mainThread->ponder)
534 mainThread->stopOnPonderhit = true;
538 else if ( Threads.increaseDepth
539 && !mainThread->ponder
540 && Time.elapsed() > totalTime * 0.58)
541 Threads.increaseDepth = false;
543 Threads.increaseDepth = true;
546 mainThread->iterValue[iterIdx] = bestValue;
547 iterIdx = (iterIdx + 1) & 3;
553 mainThread->previousTimeReduction = timeReduction;
555 // If skill level is enabled, swap best PV line with the sub-optimal one
557 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
558 skill.best ? skill.best : skill.pick_best(multiPV)));
564 // search<>() is the main search function for both PV and non-PV nodes
566 template <NodeType NT>
567 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
569 constexpr bool PvNode = NT == PV;
570 const bool rootNode = PvNode && ss->ply == 0;
572 // Check if we have an upcoming move which draws by repetition, or
573 // if the opponent had an alternative move earlier to this position.
574 if ( pos.rule50_count() >= 3
575 && alpha < VALUE_DRAW
577 && pos.has_game_cycle(ss->ply))
579 alpha = value_draw(pos.this_thread());
584 // Dive into quiescence search when the depth reaches zero
586 return qsearch<NT>(pos, ss, alpha, beta);
588 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
589 assert(PvNode || (alpha == beta - 1));
590 assert(0 < depth && depth < MAX_PLY);
591 assert(!(PvNode && cutNode));
593 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
597 Move ttMove, move, excludedMove, bestMove;
598 Depth extension, newDepth;
599 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
600 bool ttHit, ttPv, formerPv, givesCheck, improving, didLMR, priorCapture;
601 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
602 ttCapture, singularQuietLMR;
604 int moveCount, captureCount, quietCount;
606 // Step 1. Initialize node
607 Thread* thisThread = pos.this_thread();
608 ss->inCheck = pos.checkers();
609 priorCapture = pos.captured_piece();
610 Color us = pos.side_to_move();
611 moveCount = captureCount = quietCount = ss->moveCount = 0;
612 bestValue = -VALUE_INFINITE;
613 maxValue = VALUE_INFINITE;
615 // Check for the available remaining time
616 if (thisThread == Threads.main())
617 static_cast<MainThread*>(thisThread)->check_time();
619 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
620 if (PvNode && thisThread->selDepth < ss->ply + 1)
621 thisThread->selDepth = ss->ply + 1;
625 // Step 2. Check for aborted search and immediate draw
626 if ( Threads.stop.load(std::memory_order_relaxed)
627 || pos.is_draw(ss->ply)
628 || ss->ply >= MAX_PLY)
629 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
630 : value_draw(pos.this_thread());
632 // Step 3. Mate distance pruning. Even if we mate at the next move our score
633 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
634 // a shorter mate was found upward in the tree then there is no need to search
635 // because we will never beat the current alpha. Same logic but with reversed
636 // signs applies also in the opposite condition of being mated instead of giving
637 // mate. In this case return a fail-high score.
638 alpha = std::max(mated_in(ss->ply), alpha);
639 beta = std::min(mate_in(ss->ply+1), beta);
644 assert(0 <= ss->ply && ss->ply < MAX_PLY);
646 (ss+1)->ply = ss->ply + 1;
647 (ss+1)->excludedMove = bestMove = MOVE_NONE;
648 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
649 Square prevSq = to_sq((ss-1)->currentMove);
651 // Initialize statScore to zero for the grandchildren of the current position.
652 // So statScore is shared between all grandchildren and only the first grandchild
653 // starts with statScore = 0. Later grandchildren start with the last calculated
654 // statScore of the previous grandchild. This influences the reduction rules in
655 // LMR which are based on the statScore of parent position.
657 (ss+4)->statScore = 0;
659 (ss+2)->statScore = 0;
661 // Step 4. Transposition table lookup. We don't want the score of a partial
662 // search to overwrite a previous full search TT value, so we use a different
663 // position key in case of an excluded move.
664 excludedMove = ss->excludedMove;
665 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
666 tte = TT.probe(posKey, ttHit);
667 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
668 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
669 : ttHit ? tte->move() : MOVE_NONE;
670 ttPv = PvNode || (ttHit && tte->is_pv());
671 formerPv = ttPv && !PvNode;
675 && ss->ply - 1 < MAX_LPH
677 && is_ok((ss-1)->currentMove))
678 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
680 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
681 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
682 + TtHitAverageResolution * ttHit;
684 // At non-PV nodes we check for an early TT cutoff
687 && tte->depth() >= depth
688 && ttValue != VALUE_NONE // Possible in case of TT access race
689 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
690 : (tte->bound() & BOUND_UPPER)))
692 // If ttMove is quiet, update move sorting heuristics on TT hit
697 if (!pos.capture_or_promotion(ttMove))
698 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
700 // Extra penalty for early quiet moves of the previous ply
701 if ((ss-1)->moveCount <= 2 && !priorCapture)
702 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
704 // Penalty for a quiet ttMove that fails low
705 else if (!pos.capture_or_promotion(ttMove))
707 int penalty = -stat_bonus(depth);
708 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
709 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
713 if (pos.rule50_count() < 90)
717 // Step 5. Tablebases probe
718 if (!rootNode && TB::Cardinality)
720 int piecesCount = pos.count<ALL_PIECES>();
722 if ( piecesCount <= TB::Cardinality
723 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
724 && pos.rule50_count() == 0
725 && !pos.can_castle(ANY_CASTLING))
728 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
730 // Force check of time on the next occasion
731 if (thisThread == Threads.main())
732 static_cast<MainThread*>(thisThread)->callsCnt = 0;
734 if (err != TB::ProbeState::FAIL)
736 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
738 int drawScore = TB::UseRule50 ? 1 : 0;
740 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
741 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
742 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
743 : VALUE_DRAW + 2 * wdl * drawScore;
745 Bound b = wdl < -drawScore ? BOUND_UPPER
746 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
748 if ( b == BOUND_EXACT
749 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
751 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
752 std::min(MAX_PLY - 1, depth + 6),
753 MOVE_NONE, VALUE_NONE);
760 if (b == BOUND_LOWER)
761 bestValue = value, alpha = std::max(alpha, bestValue);
769 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
771 // Step 6. Static evaluation of the position
774 // Skip early pruning when in check
775 ss->staticEval = eval = VALUE_NONE;
781 // Never assume anything about values stored in TT
782 ss->staticEval = eval = tte->eval();
783 if (eval == VALUE_NONE)
784 ss->staticEval = eval = evaluate(pos);
786 if (eval == VALUE_DRAW)
787 eval = value_draw(thisThread);
789 // Can ttValue be used as a better position evaluation?
790 if ( ttValue != VALUE_NONE
791 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
796 if ((ss-1)->currentMove != MOVE_NULL)
797 ss->staticEval = eval = evaluate(pos);
799 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
801 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
804 // Step 7. Razoring (~1 Elo)
805 if ( !rootNode // The required rootNode PV handling is not available in qsearch
807 && eval <= alpha - RazorMargin)
808 return qsearch<NT>(pos, ss, alpha, beta);
810 improving = (ss-2)->staticEval == VALUE_NONE
811 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
812 : ss->staticEval > (ss-2)->staticEval;
814 // Step 8. Futility pruning: child node (~50 Elo)
817 && eval - futility_margin(depth, improving) >= beta
818 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
821 // Step 9. Null move search with verification search (~40 Elo)
823 && (ss-1)->currentMove != MOVE_NULL
824 && (ss-1)->statScore < 22977
826 && eval >= ss->staticEval
827 && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ttPv + 182
829 && pos.non_pawn_material(us)
830 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
832 assert(eval - beta >= 0);
834 // Null move dynamic reduction based on depth and value
835 Depth R = (817 + 71 * depth) / 213 + std::min(int(eval - beta) / 192, 3);
837 ss->currentMove = MOVE_NULL;
838 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
840 pos.do_null_move(st);
842 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
844 pos.undo_null_move();
846 if (nullValue >= beta)
848 // Do not return unproven mate or TB scores
849 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
852 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
855 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
857 // Do verification search at high depths, with null move pruning disabled
858 // for us, until ply exceeds nmpMinPly.
859 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
860 thisThread->nmpColor = us;
862 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
864 thisThread->nmpMinPly = 0;
871 probCutBeta = beta + 176 - 49 * improving;
873 // Step 10. ProbCut (~10 Elo)
874 // If we have a good enough capture and a reduced search returns a value
875 // much above beta, we can (almost) safely prune the previous move.
878 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
879 // if value from transposition table is lower than probCutBeta, don't attempt probCut
880 // there and in further interactions with transposition table cutoff depth is set to depth - 3
881 // because probCut search has depth set to depth - 4 but we also do a move before it
882 // so effective depth is equal to depth - 3
884 && tte->depth() >= depth - 3
885 && ttValue != VALUE_NONE
886 && ttValue < probCutBeta))
888 // if ttMove is a capture and value from transposition table is good enough produce probCut
889 // cutoff without digging into actual probCut search
891 && tte->depth() >= depth - 3
892 && ttValue != VALUE_NONE
893 && ttValue >= probCutBeta
895 && pos.capture_or_promotion(ttMove))
898 assert(probCutBeta < VALUE_INFINITE);
899 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
900 int probCutCount = 0;
902 while ( (move = mp.next_move()) != MOVE_NONE
903 && probCutCount < 2 + 2 * cutNode)
904 if (move != excludedMove && pos.legal(move))
906 assert(pos.capture_or_promotion(move));
909 captureOrPromotion = true;
912 ss->currentMove = move;
913 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
915 [pos.moved_piece(move)]
918 pos.do_move(move, st);
920 // Perform a preliminary qsearch to verify that the move holds
921 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
923 // If the qsearch held, perform the regular search
924 if (value >= probCutBeta)
925 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
929 if (value >= probCutBeta)
931 // if transposition table doesn't have equal or more deep info write probCut data into it
933 && tte->depth() >= depth - 3
934 && ttValue != VALUE_NONE))
935 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
937 depth - 3, move, ss->staticEval);
943 moves_loop: // When in check, search starts from here
945 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
946 nullptr , (ss-4)->continuationHistory,
947 nullptr , (ss-6)->continuationHistory };
949 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
951 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
952 &thisThread->lowPlyHistory,
960 singularQuietLMR = moveCountPruning = false;
961 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
963 // Mark this node as being searched
964 ThreadHolding th(thisThread, posKey, ss->ply);
966 // Step 11. Loop through all pseudo-legal moves until no moves remain
967 // or a beta cutoff occurs.
968 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
972 if (move == excludedMove)
975 // At root obey the "searchmoves" option and skip moves not listed in Root
976 // Move List. As a consequence any illegal move is also skipped. In MultiPV
977 // mode we also skip PV moves which have been already searched and those
978 // of lower "TB rank" if we are in a TB root position.
979 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
980 thisThread->rootMoves.begin() + thisThread->pvLast, move))
983 // Check for legality
984 if (!rootNode && !pos.legal(move))
987 ss->moveCount = ++moveCount;
989 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
990 sync_cout << "info depth " << depth
991 << " currmove " << UCI::move(move, pos.is_chess960())
992 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
994 (ss+1)->pv = nullptr;
997 captureOrPromotion = pos.capture_or_promotion(move);
998 movedPiece = pos.moved_piece(move);
999 givesCheck = pos.gives_check(move);
1001 // Calculate new depth for this move
1002 newDepth = depth - 1;
1004 // Step 12. Pruning at shallow depth (~200 Elo)
1006 && pos.non_pawn_material(us)
1007 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1009 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1010 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1012 // Reduced depth of the next LMR search
1013 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1015 if ( !captureOrPromotion
1018 // Countermoves based pruning (~20 Elo)
1019 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1020 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1021 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1024 // Futility pruning: parent node (~5 Elo)
1027 && ss->staticEval + 283 + 170 * lmrDepth <= alpha
1028 && (*contHist[0])[movedPiece][to_sq(move)]
1029 + (*contHist[1])[movedPiece][to_sq(move)]
1030 + (*contHist[3])[movedPiece][to_sq(move)]
1031 + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
1034 // Prune moves with negative SEE (~20 Elo)
1035 if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1040 // Capture history based pruning when the move doesn't give check
1043 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1046 // Futility pruning for captures
1049 && !(PvNode && abs(bestValue) < 2)
1050 && PieceValue[MG][type_of(movedPiece)] >= PieceValue[MG][type_of(pos.piece_on(to_sq(move)))]
1052 && ss->staticEval + 169 + 244 * lmrDepth
1053 + PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] <= alpha)
1056 // See based pruning
1057 if (!pos.see_ge(move, Value(-221) * depth)) // (~25 Elo)
1062 // Step 13. Extensions (~75 Elo)
1064 // Singular extension search (~70 Elo). If all moves but one fail low on a
1065 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1066 // then that move is singular and should be extended. To verify this we do
1067 // a reduced search on all the other moves but the ttMove and if the
1068 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1072 && !excludedMove // Avoid recursive singular search
1073 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1074 && abs(ttValue) < VALUE_KNOWN_WIN
1075 && (tte->bound() & BOUND_LOWER)
1076 && tte->depth() >= depth - 3)
1078 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1079 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1080 ss->excludedMove = move;
1081 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1082 ss->excludedMove = MOVE_NONE;
1084 if (value < singularBeta)
1087 singularQuietLMR = !ttCapture;
1090 // Multi-cut pruning
1091 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1092 // search without the ttMove. So we assume this expected Cut-node is not singular,
1093 // that multiple moves fail high, and we can prune the whole subtree by returning
1095 else if (singularBeta >= beta)
1096 return singularBeta;
1098 // If the eval of ttMove is greater than beta we try also if there is another
1099 // move that pushes it over beta, if so also produce a cutoff.
1100 else if (ttValue >= beta)
1102 ss->excludedMove = move;
1103 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1104 ss->excludedMove = MOVE_NONE;
1111 // Check extension (~2 Elo)
1112 else if ( givesCheck
1113 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1116 // Last captures extension
1117 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1118 && pos.non_pawn_material() <= 2 * RookValueMg)
1121 // Castling extension
1122 if ( type_of(move) == CASTLING
1123 && popcount(pos.pieces(us) & ~pos.pieces(PAWN) & (to_sq(move) & KingSide ? KingSide : QueenSide)) <= 2)
1126 // Late irreversible move extension
1128 && pos.rule50_count() > 80
1129 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1132 // Add extension to new depth
1133 newDepth += extension;
1135 // Speculative prefetch as early as possible
1136 prefetch(TT.first_entry(pos.key_after(move)));
1138 // Update the current move (this must be done after singular extension search)
1139 ss->currentMove = move;
1140 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1141 [captureOrPromotion]
1145 // Step 14. Make the move
1146 pos.do_move(move, st, givesCheck);
1148 // Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1149 // re-searched at full depth.
1151 && moveCount > 1 + 2 * rootNode + 2 * (PvNode && abs(bestValue) < 2)
1152 && (!rootNode || thisThread->best_move_count(move) == 0)
1153 && ( !captureOrPromotion
1155 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1157 || thisThread->ttHitAverage < 427 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1159 Depth r = reduction(improving, depth, moveCount);
1161 // Decrease reduction at non-check cut nodes for second move at low depths
1168 // Decrease reduction if the ttHit running average is large
1169 if (thisThread->ttHitAverage > 509 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1172 // Reduction if other threads are searching this position
1176 // Decrease reduction if position is or has been on the PV (~10 Elo)
1180 if (moveCountPruning && !formerPv)
1183 // Decrease reduction if opponent's move count is high (~5 Elo)
1184 if ((ss-1)->moveCount > 13)
1187 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1188 if (singularQuietLMR)
1191 if (!captureOrPromotion)
1193 // Increase reduction if ttMove is a capture (~5 Elo)
1197 // Increase reduction for cut nodes (~10 Elo)
1201 // Decrease reduction for moves that escape a capture. Filter out
1202 // castling moves, because they are coded as "king captures rook" and
1203 // hence break make_move(). (~2 Elo)
1204 else if ( type_of(move) == NORMAL
1205 && !pos.see_ge(reverse_move(move)))
1206 r -= 2 + ttPv - (type_of(movedPiece) == PAWN);
1208 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1209 + (*contHist[0])[movedPiece][to_sq(move)]
1210 + (*contHist[1])[movedPiece][to_sq(move)]
1211 + (*contHist[3])[movedPiece][to_sq(move)]
1214 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1215 if (ss->statScore >= -106 && (ss-1)->statScore < -104)
1218 else if ((ss-1)->statScore >= -119 && ss->statScore < -140)
1221 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1222 r -= ss->statScore / 14884;
1226 // Increase reduction for captures/promotions if late move and at low depth
1227 if (depth < 8 && moveCount > 2)
1230 // Unless giving check, this capture is likely bad
1232 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 213 * depth <= alpha)
1236 Depth d = std::clamp(newDepth - r, 1, newDepth);
1238 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1240 doFullDepthSearch = value > alpha && d != newDepth;
1246 doFullDepthSearch = !PvNode || moveCount > 1;
1251 // Step 16. Full depth search when LMR is skipped or fails high
1252 if (doFullDepthSearch)
1254 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1256 if (didLMR && !captureOrPromotion)
1258 int bonus = value > alpha ? stat_bonus(newDepth)
1259 : -stat_bonus(newDepth);
1261 if (move == ss->killers[0])
1264 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1268 // For PV nodes only, do a full PV search on the first move or after a fail
1269 // high (in the latter case search only if value < beta), otherwise let the
1270 // parent node fail low with value <= alpha and try another move.
1271 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1274 (ss+1)->pv[0] = MOVE_NONE;
1276 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1279 // Step 17. Undo move
1280 pos.undo_move(move);
1282 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1284 // Step 18. Check for a new best move
1285 // Finished searching the move. If a stop occurred, the return value of
1286 // the search cannot be trusted, and we return immediately without
1287 // updating best move, PV and TT.
1288 if (Threads.stop.load(std::memory_order_relaxed))
1293 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1294 thisThread->rootMoves.end(), move);
1296 // PV move or new best move?
1297 if (moveCount == 1 || value > alpha)
1300 rm.selDepth = thisThread->selDepth;
1305 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1306 rm.pv.push_back(*m);
1308 // We record how often the best move has been changed in each
1309 // iteration. This information is used for time management: when
1310 // the best move changes frequently, we allocate some more time.
1312 ++thisThread->bestMoveChanges;
1315 // All other moves but the PV are set to the lowest value: this
1316 // is not a problem when sorting because the sort is stable and the
1317 // move position in the list is preserved - just the PV is pushed up.
1318 rm.score = -VALUE_INFINITE;
1321 if (value > bestValue)
1329 if (PvNode && !rootNode) // Update pv even in fail-high case
1330 update_pv(ss->pv, move, (ss+1)->pv);
1332 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1336 assert(value >= beta); // Fail high
1343 if (move != bestMove)
1345 if (captureOrPromotion && captureCount < 32)
1346 capturesSearched[captureCount++] = move;
1348 else if (!captureOrPromotion && quietCount < 64)
1349 quietsSearched[quietCount++] = move;
1353 // The following condition would detect a stop only after move loop has been
1354 // completed. But in this case bestValue is valid because we have fully
1355 // searched our subtree, and we can anyhow save the result in TT.
1361 // Step 19. Check for mate and stalemate
1362 // All legal moves have been searched and if there are no legal moves, it
1363 // must be a mate or a stalemate. If we are in a singular extension search then
1364 // return a fail low score.
1366 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1369 bestValue = excludedMove ? alpha
1370 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1373 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1374 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1376 // Bonus for prior countermove that caused the fail low
1377 else if ( (depth >= 3 || PvNode)
1379 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1382 bestValue = std::min(bestValue, maxValue);
1384 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1385 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1386 bestValue >= beta ? BOUND_LOWER :
1387 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1388 depth, bestMove, ss->staticEval);
1390 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1396 // qsearch() is the quiescence search function, which is called by the main search
1397 // function with zero depth, or recursively with further decreasing depth per call.
1398 template <NodeType NT>
1399 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1401 constexpr bool PvNode = NT == PV;
1403 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1404 assert(PvNode || (alpha == beta - 1));
1411 Move ttMove, move, bestMove;
1413 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1414 bool ttHit, pvHit, givesCheck, captureOrPromotion;
1419 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1421 ss->pv[0] = MOVE_NONE;
1424 Thread* thisThread = pos.this_thread();
1425 (ss+1)->ply = ss->ply + 1;
1426 bestMove = MOVE_NONE;
1427 ss->inCheck = pos.checkers();
1430 // Check for an immediate draw or maximum ply reached
1431 if ( pos.is_draw(ss->ply)
1432 || ss->ply >= MAX_PLY)
1433 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1435 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1437 // Decide whether or not to include checks: this fixes also the type of
1438 // TT entry depth that we are going to use. Note that in qsearch we use
1439 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1440 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1441 : DEPTH_QS_NO_CHECKS;
1442 // Transposition table lookup
1444 tte = TT.probe(posKey, ttHit);
1445 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1446 ttMove = ttHit ? tte->move() : MOVE_NONE;
1447 pvHit = ttHit && tte->is_pv();
1451 && tte->depth() >= ttDepth
1452 && ttValue != VALUE_NONE // Only in case of TT access race
1453 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1454 : (tte->bound() & BOUND_UPPER)))
1457 // Evaluate the position statically
1460 ss->staticEval = VALUE_NONE;
1461 bestValue = futilityBase = -VALUE_INFINITE;
1467 // Never assume anything about values stored in TT
1468 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1469 ss->staticEval = bestValue = evaluate(pos);
1471 // Can ttValue be used as a better position evaluation?
1472 if ( ttValue != VALUE_NONE
1473 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1474 bestValue = ttValue;
1477 ss->staticEval = bestValue =
1478 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1479 : -(ss-1)->staticEval + 2 * Tempo;
1481 // Stand pat. Return immediately if static value is at least beta
1482 if (bestValue >= beta)
1485 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1486 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1491 if (PvNode && bestValue > alpha)
1494 futilityBase = bestValue + 145;
1497 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1498 nullptr , (ss-4)->continuationHistory,
1499 nullptr , (ss-6)->continuationHistory };
1501 // Initialize a MovePicker object for the current position, and prepare
1502 // to search the moves. Because the depth is <= 0 here, only captures,
1503 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1504 // will be generated.
1505 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1506 &thisThread->captureHistory,
1508 to_sq((ss-1)->currentMove));
1510 // Loop through the moves until no moves remain or a beta cutoff occurs
1511 while ((move = mp.next_move()) != MOVE_NONE)
1513 assert(is_ok(move));
1515 givesCheck = pos.gives_check(move);
1516 captureOrPromotion = pos.capture_or_promotion(move);
1523 && futilityBase > -VALUE_KNOWN_WIN
1524 && !pos.advanced_pawn_push(move))
1526 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1528 // moveCount pruning
1532 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1534 if (futilityValue <= alpha)
1536 bestValue = std::max(bestValue, futilityValue);
1540 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1542 bestValue = std::max(bestValue, futilityBase);
1547 // Do not search moves with negative SEE values
1548 if (!ss->inCheck && !pos.see_ge(move))
1551 // Speculative prefetch as early as possible
1552 prefetch(TT.first_entry(pos.key_after(move)));
1554 // Check for legality just before making the move
1555 if (!pos.legal(move))
1561 ss->currentMove = move;
1562 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1563 [captureOrPromotion]
1564 [pos.moved_piece(move)]
1567 if ( !captureOrPromotion
1568 && moveCount >= abs(depth) + 1
1569 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1570 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1573 // Make and search the move
1574 pos.do_move(move, st, givesCheck);
1575 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1576 pos.undo_move(move);
1578 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1580 // Check for a new best move
1581 if (value > bestValue)
1589 if (PvNode) // Update pv even in fail-high case
1590 update_pv(ss->pv, move, (ss+1)->pv);
1592 if (PvNode && value < beta) // Update alpha here!
1600 // All legal moves have been searched. A special case: if we're in check
1601 // and no legal moves were found, it is checkmate.
1602 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1603 return mated_in(ss->ply); // Plies to mate from the root
1605 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1606 bestValue >= beta ? BOUND_LOWER :
1607 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1608 ttDepth, bestMove, ss->staticEval);
1610 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1616 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1617 // "plies to mate from the current position". Standard scores are unchanged.
1618 // The function is called before storing a value in the transposition table.
1620 Value value_to_tt(Value v, int ply) {
1622 assert(v != VALUE_NONE);
1624 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1625 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1629 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1630 // from the transposition table (which refers to the plies to mate/be mated from
1631 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1632 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1633 // and the graph history interaction, we return an optimal TB score instead.
1635 Value value_from_tt(Value v, int ply, int r50c) {
1637 if (v == VALUE_NONE)
1640 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1642 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1643 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1648 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1650 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1651 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1660 // update_pv() adds current move and appends child pv[]
1662 void update_pv(Move* pv, Move move, Move* childPv) {
1664 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1670 // update_all_stats() updates stats at the end of search() when a bestMove is found
1672 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1673 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1676 Color us = pos.side_to_move();
1677 Thread* thisThread = pos.this_thread();
1678 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1679 Piece moved_piece = pos.moved_piece(bestMove);
1680 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1682 bonus1 = stat_bonus(depth + 1);
1683 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1684 : stat_bonus(depth); // smaller bonus
1686 if (!pos.capture_or_promotion(bestMove))
1688 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1690 // Decrease all the non-best quiet moves
1691 for (int i = 0; i < quietCount; ++i)
1693 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1694 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1698 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1700 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1701 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1702 && !pos.captured_piece())
1703 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1705 // Decrease all the non-best capture moves
1706 for (int i = 0; i < captureCount; ++i)
1708 moved_piece = pos.moved_piece(capturesSearched[i]);
1709 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1710 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1715 // update_continuation_histories() updates histories of the move pairs formed
1716 // by moves at ply -1, -2, -4, and -6 with current move.
1718 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1720 for (int i : {1, 2, 4, 6})
1722 if (ss->inCheck && i > 2)
1724 if (is_ok((ss-i)->currentMove))
1725 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1730 // update_quiet_stats() updates move sorting heuristics
1732 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1734 if (ss->killers[0] != move)
1736 ss->killers[1] = ss->killers[0];
1737 ss->killers[0] = move;
1740 Color us = pos.side_to_move();
1741 Thread* thisThread = pos.this_thread();
1742 thisThread->mainHistory[us][from_to(move)] << bonus;
1743 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1745 if (type_of(pos.moved_piece(move)) != PAWN)
1746 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1748 if (is_ok((ss-1)->currentMove))
1750 Square prevSq = to_sq((ss-1)->currentMove);
1751 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1754 if (depth > 11 && ss->ply < MAX_LPH)
1755 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1758 // When playing with strength handicap, choose best move among a set of RootMoves
1759 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1761 Move Skill::pick_best(size_t multiPV) {
1763 const RootMoves& rootMoves = Threads.main()->rootMoves;
1764 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1766 // RootMoves are already sorted by score in descending order
1767 Value topScore = rootMoves[0].score;
1768 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1769 int weakness = 120 - 2 * level;
1770 int maxScore = -VALUE_INFINITE;
1772 // Choose best move. For each move score we add two terms, both dependent on
1773 // weakness. One is deterministic and bigger for weaker levels, and one is
1774 // random. Then we choose the move with the resulting highest score.
1775 for (size_t i = 0; i < multiPV; ++i)
1777 // This is our magic formula
1778 int push = ( weakness * int(topScore - rootMoves[i].score)
1779 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1781 if (rootMoves[i].score + push >= maxScore)
1783 maxScore = rootMoves[i].score + push;
1784 best = rootMoves[i].pv[0];
1794 /// MainThread::check_time() is used to print debug info and, more importantly,
1795 /// to detect when we are out of available time and thus stop the search.
1797 void MainThread::check_time() {
1802 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1803 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1805 static TimePoint lastInfoTime = now();
1807 TimePoint elapsed = Time.elapsed();
1808 TimePoint tick = Limits.startTime + elapsed;
1810 if (tick - lastInfoTime >= 1000)
1812 lastInfoTime = tick;
1816 // We should not stop pondering until told so by the GUI
1820 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1821 || (Limits.movetime && elapsed >= Limits.movetime)
1822 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1823 Threads.stop = true;
1827 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1828 /// that all (if any) unsearched PV lines are sent using a previous search score.
1830 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1832 std::stringstream ss;
1833 TimePoint elapsed = Time.elapsed() + 1;
1834 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1835 size_t pvIdx = pos.this_thread()->pvIdx;
1836 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1837 uint64_t nodesSearched = Threads.nodes_searched();
1838 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1840 for (size_t i = 0; i < multiPV; ++i)
1842 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1844 if (depth == 1 && !updated)
1847 Depth d = updated ? depth : depth - 1;
1848 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1850 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1851 v = tb ? rootMoves[i].tbScore : v;
1853 if (ss.rdbuf()->in_avail()) // Not at first line
1858 << " seldepth " << rootMoves[i].selDepth
1859 << " multipv " << i + 1
1860 << " score " << UCI::value(v);
1862 if (Options["UCI_ShowWDL"])
1863 ss << UCI::wdl(v, pos.game_ply());
1865 if (!tb && i == pvIdx)
1866 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1868 ss << " nodes " << nodesSearched
1869 << " nps " << nodesSearched * 1000 / elapsed;
1871 if (elapsed > 1000) // Earlier makes little sense
1872 ss << " hashfull " << TT.hashfull();
1874 ss << " tbhits " << tbHits
1875 << " time " << elapsed
1878 for (Move m : rootMoves[i].pv)
1879 ss << " " << UCI::move(m, pos.is_chess960());
1886 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1887 /// before exiting the search, for instance, in case we stop the search during a
1888 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1889 /// otherwise in case of 'ponder on' we have nothing to think on.
1891 bool RootMove::extract_ponder_from_tt(Position& pos) {
1896 assert(pv.size() == 1);
1898 if (pv[0] == MOVE_NONE)
1901 pos.do_move(pv[0], st);
1902 TTEntry* tte = TT.probe(pos.key(), ttHit);
1906 Move m = tte->move(); // Local copy to be SMP safe
1907 if (MoveList<LEGAL>(pos).contains(m))
1911 pos.undo_move(pv[0]);
1912 return pv.size() > 1;
1915 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1918 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1919 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1920 Cardinality = int(Options["SyzygyProbeLimit"]);
1921 bool dtz_available = true;
1923 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1924 // ProbeDepth == DEPTH_ZERO
1925 if (Cardinality > MaxCardinality)
1927 Cardinality = MaxCardinality;
1931 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1933 // Rank moves using DTZ tables
1934 RootInTB = root_probe(pos, rootMoves);
1938 // DTZ tables are missing; try to rank moves using WDL tables
1939 dtz_available = false;
1940 RootInTB = root_probe_wdl(pos, rootMoves);
1946 // Sort moves according to TB rank
1947 std::sort(rootMoves.begin(), rootMoves.end(),
1948 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1950 // Probe during search only if DTZ is not available and we are winning
1951 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1956 // Clean up if root_probe() and root_probe_wdl() have failed
1957 for (auto& m : rootMoves)