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 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * 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());
228 Eval::NNUE::verify();
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);
467 delta += delta / 4 + 5;
469 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
472 // Sort the PV lines searched so far and update the GUI
473 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
476 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
477 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
481 completedDepth = rootDepth;
483 if (rootMoves[0].pv[0] != lastBestMove) {
484 lastBestMove = rootMoves[0].pv[0];
485 lastBestMoveDepth = rootDepth;
488 // Have we found a "mate in x"?
490 && bestValue >= VALUE_MATE_IN_MAX_PLY
491 && VALUE_MATE - bestValue <= 2 * Limits.mate)
497 // If skill level is enabled and time is up, pick a sub-optimal best move
498 if (skill.enabled() && skill.time_to_pick(rootDepth))
499 skill.pick_best(multiPV);
501 // Do we have time for the next iteration? Can we stop searching now?
502 if ( Limits.use_time_management()
504 && !mainThread->stopOnPonderhit)
506 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
507 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
508 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
510 // If the bestMove is stable over several iterations, reduce time accordingly
511 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
512 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
514 // Use part of the gained time from a previous stable move for the current move
515 for (Thread* th : Threads)
517 totBestMoveChanges += th->bestMoveChanges;
518 th->bestMoveChanges = 0;
520 double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
522 double totalTime = rootMoves.size() == 1 ? 0 :
523 Time.optimum() * fallingEval * reduction * bestMoveInstability;
525 // Stop the search if we have exceeded the totalTime, at least 1ms search
526 if (Time.elapsed() > totalTime)
528 // If we are allowed to ponder do not stop the search now but
529 // keep pondering until the GUI sends "ponderhit" or "stop".
530 if (mainThread->ponder)
531 mainThread->stopOnPonderhit = true;
535 else if ( Threads.increaseDepth
536 && !mainThread->ponder
537 && Time.elapsed() > totalTime * 0.58)
538 Threads.increaseDepth = false;
540 Threads.increaseDepth = true;
543 mainThread->iterValue[iterIdx] = bestValue;
544 iterIdx = (iterIdx + 1) & 3;
550 mainThread->previousTimeReduction = timeReduction;
552 // If skill level is enabled, swap best PV line with the sub-optimal one
554 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
555 skill.best ? skill.best : skill.pick_best(multiPV)));
561 // search<>() is the main search function for both PV and non-PV nodes
563 template <NodeType NT>
564 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
566 constexpr bool PvNode = NT == PV;
567 const bool rootNode = PvNode && ss->ply == 0;
569 // Check if we have an upcoming move which draws by repetition, or
570 // if the opponent had an alternative move earlier to this position.
571 if ( pos.rule50_count() >= 3
572 && alpha < VALUE_DRAW
574 && pos.has_game_cycle(ss->ply))
576 alpha = value_draw(pos.this_thread());
581 // Dive into quiescence search when the depth reaches zero
583 return qsearch<NT>(pos, ss, alpha, beta);
585 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
586 assert(PvNode || (alpha == beta - 1));
587 assert(0 < depth && depth < MAX_PLY);
588 assert(!(PvNode && cutNode));
590 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
594 Move ttMove, move, excludedMove, bestMove;
595 Depth extension, newDepth;
596 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
597 bool formerPv, givesCheck, improving, didLMR, priorCapture;
598 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
599 ttCapture, singularQuietLMR;
601 int moveCount, captureCount, quietCount;
603 // Step 1. Initialize node
604 Thread* thisThread = pos.this_thread();
605 ss->inCheck = pos.checkers();
606 priorCapture = pos.captured_piece();
607 Color us = pos.side_to_move();
608 moveCount = captureCount = quietCount = ss->moveCount = 0;
609 bestValue = -VALUE_INFINITE;
610 maxValue = VALUE_INFINITE;
612 // Check for the available remaining time
613 if (thisThread == Threads.main())
614 static_cast<MainThread*>(thisThread)->check_time();
616 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
617 if (PvNode && thisThread->selDepth < ss->ply + 1)
618 thisThread->selDepth = ss->ply + 1;
622 // Step 2. Check for aborted search and immediate draw
623 if ( Threads.stop.load(std::memory_order_relaxed)
624 || pos.is_draw(ss->ply)
625 || ss->ply >= MAX_PLY)
626 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
627 : value_draw(pos.this_thread());
629 // Step 3. Mate distance pruning. Even if we mate at the next move our score
630 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
631 // a shorter mate was found upward in the tree then there is no need to search
632 // because we will never beat the current alpha. Same logic but with reversed
633 // signs applies also in the opposite condition of being mated instead of giving
634 // mate. In this case return a fail-high score.
635 alpha = std::max(mated_in(ss->ply), alpha);
636 beta = std::min(mate_in(ss->ply+1), beta);
641 assert(0 <= ss->ply && ss->ply < MAX_PLY);
643 (ss+1)->ply = ss->ply + 1;
644 (ss+1)->ttPv = false;
645 (ss+1)->excludedMove = bestMove = MOVE_NONE;
646 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
647 Square prevSq = to_sq((ss-1)->currentMove);
649 // Initialize statScore to zero for the grandchildren of the current position.
650 // So statScore is shared between all grandchildren and only the first grandchild
651 // starts with statScore = 0. Later grandchildren start with the last calculated
652 // statScore of the previous grandchild. This influences the reduction rules in
653 // LMR which are based on the statScore of parent position.
655 (ss+2)->statScore = 0;
657 // Step 4. Transposition table lookup. We don't want the score of a partial
658 // search to overwrite a previous full search TT value, so we use a different
659 // position key in case of an excluded move.
660 excludedMove = ss->excludedMove;
661 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
662 tte = TT.probe(posKey, ss->ttHit);
663 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
664 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
665 : ss->ttHit ? tte->move() : MOVE_NONE;
667 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
668 formerPv = ss->ttPv && !PvNode;
672 && ss->ply - 1 < MAX_LPH
674 && is_ok((ss-1)->currentMove))
675 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
677 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
678 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
679 + TtHitAverageResolution * ss->ttHit;
681 // At non-PV nodes we check for an early TT cutoff
684 && tte->depth() >= depth
685 && ttValue != VALUE_NONE // Possible in case of TT access race
686 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
687 : (tte->bound() & BOUND_UPPER)))
689 // If ttMove is quiet, update move sorting heuristics on TT hit
694 if (!pos.capture_or_promotion(ttMove))
695 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
697 // Extra penalty for early quiet moves of the previous ply
698 if ((ss-1)->moveCount <= 2 && !priorCapture)
699 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
701 // Penalty for a quiet ttMove that fails low
702 else if (!pos.capture_or_promotion(ttMove))
704 int penalty = -stat_bonus(depth);
705 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
706 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
710 if (pos.rule50_count() < 90)
714 // Step 5. Tablebases probe
715 if (!rootNode && TB::Cardinality)
717 int piecesCount = pos.count<ALL_PIECES>();
719 if ( piecesCount <= TB::Cardinality
720 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
721 && pos.rule50_count() == 0
722 && !pos.can_castle(ANY_CASTLING))
725 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
727 // Force check of time on the next occasion
728 if (thisThread == Threads.main())
729 static_cast<MainThread*>(thisThread)->callsCnt = 0;
731 if (err != TB::ProbeState::FAIL)
733 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
735 int drawScore = TB::UseRule50 ? 1 : 0;
737 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
738 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
739 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
740 : VALUE_DRAW + 2 * wdl * drawScore;
742 Bound b = wdl < -drawScore ? BOUND_UPPER
743 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
745 if ( b == BOUND_EXACT
746 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
748 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
749 std::min(MAX_PLY - 1, depth + 6),
750 MOVE_NONE, VALUE_NONE);
757 if (b == BOUND_LOWER)
758 bestValue = value, alpha = std::max(alpha, bestValue);
766 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
768 // Step 6. Static evaluation of the position
771 // Skip early pruning when in check
772 ss->staticEval = eval = VALUE_NONE;
778 // Never assume anything about values stored in TT
779 ss->staticEval = eval = tte->eval();
780 if (eval == VALUE_NONE)
781 ss->staticEval = eval = evaluate(pos);
783 if (eval == VALUE_DRAW)
784 eval = value_draw(thisThread);
786 // Can ttValue be used as a better position evaluation?
787 if ( ttValue != VALUE_NONE
788 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
793 if ((ss-1)->currentMove != MOVE_NULL)
794 ss->staticEval = eval = evaluate(pos);
796 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
798 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
801 // Step 7. Razoring (~1 Elo)
802 if ( !rootNode // The required rootNode PV handling is not available in qsearch
804 && eval <= alpha - RazorMargin)
805 return qsearch<NT>(pos, ss, alpha, beta);
807 improving = (ss-2)->staticEval == VALUE_NONE
808 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
809 : ss->staticEval > (ss-2)->staticEval;
811 // Step 8. Futility pruning: child node (~50 Elo)
814 && eval - futility_margin(depth, improving) >= beta
815 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
818 // Step 9. Null move search with verification search (~40 Elo)
820 && (ss-1)->currentMove != MOVE_NULL
821 && (ss-1)->statScore < 22977
823 && eval >= ss->staticEval
824 && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 182
826 && pos.non_pawn_material(us)
827 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
829 assert(eval - beta >= 0);
831 // Null move dynamic reduction based on depth and value
832 Depth R = (982 + 85 * depth) / 256 + std::min(int(eval - beta) / 192, 3);
834 ss->currentMove = MOVE_NULL;
835 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
837 pos.do_null_move(st);
839 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
841 pos.undo_null_move();
843 if (nullValue >= beta)
845 // Do not return unproven mate or TB scores
846 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
849 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
852 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
854 // Do verification search at high depths, with null move pruning disabled
855 // for us, until ply exceeds nmpMinPly.
856 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
857 thisThread->nmpColor = us;
859 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
861 thisThread->nmpMinPly = 0;
868 probCutBeta = beta + 176 - 49 * improving;
870 // Step 10. ProbCut (~10 Elo)
871 // If we have a good enough capture and a reduced search returns a value
872 // much above beta, we can (almost) safely prune the previous move.
875 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
876 // if value from transposition table is lower than probCutBeta, don't attempt probCut
877 // there and in further interactions with transposition table cutoff depth is set to depth - 3
878 // because probCut search has depth set to depth - 4 but we also do a move before it
879 // so effective depth is equal to depth - 3
881 && tte->depth() >= depth - 3
882 && ttValue != VALUE_NONE
883 && ttValue < probCutBeta))
885 // if ttMove is a capture and value from transposition table is good enough produce probCut
886 // cutoff without digging into actual probCut search
888 && tte->depth() >= depth - 3
889 && ttValue != VALUE_NONE
890 && ttValue >= probCutBeta
892 && pos.capture_or_promotion(ttMove))
895 assert(probCutBeta < VALUE_INFINITE);
896 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
897 int probCutCount = 0;
898 bool ttPv = ss->ttPv;
901 while ( (move = mp.next_move()) != MOVE_NONE
902 && probCutCount < 2 + 2 * cutNode)
903 if (move != excludedMove && pos.legal(move))
905 assert(pos.capture_or_promotion(move));
908 captureOrPromotion = true;
911 ss->currentMove = move;
912 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
914 [pos.moved_piece(move)]
917 pos.do_move(move, st);
919 // Perform a preliminary qsearch to verify that the move holds
920 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
922 // If the qsearch held, perform the regular search
923 if (value >= probCutBeta)
924 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
928 if (value >= probCutBeta)
930 // if transposition table doesn't have equal or more deep info write probCut data into it
932 && tte->depth() >= depth - 3
933 && ttValue != VALUE_NONE))
934 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
936 depth - 3, move, ss->staticEval);
943 // Step 11. If the position is not in TT, decrease depth by 2
949 moves_loop: // When in check, search starts from here
951 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
952 nullptr , (ss-4)->continuationHistory,
953 nullptr , (ss-6)->continuationHistory };
955 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
957 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
958 &thisThread->lowPlyHistory,
966 singularQuietLMR = moveCountPruning = false;
967 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
969 // Mark this node as being searched
970 ThreadHolding th(thisThread, posKey, ss->ply);
972 // Step 12. Loop through all pseudo-legal moves until no moves remain
973 // or a beta cutoff occurs.
974 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
978 if (move == excludedMove)
981 // At root obey the "searchmoves" option and skip moves not listed in Root
982 // Move List. As a consequence any illegal move is also skipped. In MultiPV
983 // mode we also skip PV moves which have been already searched and those
984 // of lower "TB rank" if we are in a TB root position.
985 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
986 thisThread->rootMoves.begin() + thisThread->pvLast, move))
989 // Check for legality
990 if (!rootNode && !pos.legal(move))
993 ss->moveCount = ++moveCount;
995 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
996 sync_cout << "info depth " << depth
997 << " currmove " << UCI::move(move, pos.is_chess960())
998 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1000 (ss+1)->pv = nullptr;
1003 captureOrPromotion = pos.capture_or_promotion(move);
1004 movedPiece = pos.moved_piece(move);
1005 givesCheck = pos.gives_check(move);
1007 // Calculate new depth for this move
1008 newDepth = depth - 1;
1010 // Step 13. Pruning at shallow depth (~200 Elo)
1012 && pos.non_pawn_material(us)
1013 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1015 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1016 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1018 // Reduced depth of the next LMR search
1019 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1021 if ( !captureOrPromotion
1024 // Countermoves based pruning (~20 Elo)
1025 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1026 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1027 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1030 // Futility pruning: parent node (~5 Elo)
1033 && ss->staticEval + 283 + 170 * lmrDepth <= alpha
1034 && (*contHist[0])[movedPiece][to_sq(move)]
1035 + (*contHist[1])[movedPiece][to_sq(move)]
1036 + (*contHist[3])[movedPiece][to_sq(move)]
1037 + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
1040 // Prune moves with negative SEE (~20 Elo)
1041 if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1046 // Capture history based pruning when the move doesn't give check
1049 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1052 // See based pruning
1053 if (!pos.see_ge(move, Value(-221) * depth)) // (~25 Elo)
1058 // Step 14. Extensions (~75 Elo)
1060 // Singular extension search (~70 Elo). If all moves but one fail low on a
1061 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1062 // then that move is singular and should be extended. To verify this we do
1063 // a reduced search on all the other moves but the ttMove and if the
1064 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1068 && !excludedMove // Avoid recursive singular search
1069 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1070 && abs(ttValue) < VALUE_KNOWN_WIN
1071 && (tte->bound() & BOUND_LOWER)
1072 && tte->depth() >= depth - 3)
1074 Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
1075 Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
1076 ss->excludedMove = move;
1077 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1078 ss->excludedMove = MOVE_NONE;
1080 if (value < singularBeta)
1083 singularQuietLMR = !ttCapture;
1086 // Multi-cut pruning
1087 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1088 // search without the ttMove. So we assume this expected Cut-node is not singular,
1089 // that multiple moves fail high, and we can prune the whole subtree by returning
1091 else if (singularBeta >= beta)
1092 return singularBeta;
1094 // If the eval of ttMove is greater than beta we try also if there is another
1095 // move that pushes it over beta, if so also produce a cutoff.
1096 else if (ttValue >= beta)
1098 ss->excludedMove = move;
1099 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1100 ss->excludedMove = MOVE_NONE;
1107 // Check extension (~2 Elo)
1108 else if ( givesCheck
1109 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1112 // Last captures extension
1113 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1114 && pos.non_pawn_material() <= 2 * RookValueMg)
1117 // Late irreversible move extension
1119 && pos.rule50_count() > 80
1120 && (captureOrPromotion || type_of(movedPiece) == PAWN))
1123 // Add extension to new depth
1124 newDepth += extension;
1126 // Speculative prefetch as early as possible
1127 prefetch(TT.first_entry(pos.key_after(move)));
1129 // Update the current move (this must be done after singular extension search)
1130 ss->currentMove = move;
1131 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1132 [captureOrPromotion]
1136 // Step 15. Make the move
1137 pos.do_move(move, st, givesCheck);
1139 // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
1140 // re-searched at full depth.
1142 && moveCount > 1 + 2 * rootNode
1143 && ( !captureOrPromotion
1145 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1147 || thisThread->ttHitAverage < 427 * TtHitAverageResolution * TtHitAverageWindow / 1024))
1149 Depth r = reduction(improving, depth, moveCount);
1151 // Decrease reduction if the ttHit running average is large
1152 if (thisThread->ttHitAverage > 509 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1155 // Reduction if other threads are searching this position
1159 // Decrease reduction if position is or has been on the PV (~10 Elo)
1163 if (moveCountPruning && !formerPv)
1166 // Decrease reduction if opponent's move count is high (~5 Elo)
1167 if ((ss-1)->moveCount > 13)
1170 // Decrease reduction if ttMove has been singularly extended (~3 Elo)
1171 if (singularQuietLMR)
1174 if (!captureOrPromotion)
1176 // Increase reduction if ttMove is a capture (~5 Elo)
1180 // Increase reduction for cut nodes (~10 Elo)
1184 // Decrease reduction for moves that escape a capture. Filter out
1185 // castling moves, because they are coded as "king captures rook" and
1186 // hence break make_move(). (~2 Elo)
1187 else if ( type_of(move) == NORMAL
1188 && !pos.see_ge(reverse_move(move)))
1189 r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
1191 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1192 + (*contHist[0])[movedPiece][to_sq(move)]
1193 + (*contHist[1])[movedPiece][to_sq(move)]
1194 + (*contHist[3])[movedPiece][to_sq(move)]
1197 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1198 if (ss->statScore >= -106 && (ss-1)->statScore < -104)
1201 else if ((ss-1)->statScore >= -119 && ss->statScore < -140)
1204 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1205 r -= ss->statScore / 14884;
1209 // Increase reduction for captures/promotions if late move and at low depth
1210 if (depth < 8 && moveCount > 2)
1213 // Unless giving check, this capture is likely bad
1215 && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 213 * depth <= alpha)
1219 Depth d = std::clamp(newDepth - r, 1, newDepth);
1221 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1223 doFullDepthSearch = value > alpha && d != newDepth;
1229 doFullDepthSearch = !PvNode || moveCount > 1;
1234 // Step 17. Full depth search when LMR is skipped or fails high
1235 if (doFullDepthSearch)
1237 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1239 if (didLMR && !captureOrPromotion)
1241 int bonus = value > alpha ? stat_bonus(newDepth)
1242 : -stat_bonus(newDepth);
1244 if (move == ss->killers[0])
1247 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1251 // For PV nodes only, do a full PV search on the first move or after a fail
1252 // high (in the latter case search only if value < beta), otherwise let the
1253 // parent node fail low with value <= alpha and try another move.
1254 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1257 (ss+1)->pv[0] = MOVE_NONE;
1259 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1262 // Step 18. Undo move
1263 pos.undo_move(move);
1265 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1267 // Step 19. Check for a new best move
1268 // Finished searching the move. If a stop occurred, the return value of
1269 // the search cannot be trusted, and we return immediately without
1270 // updating best move, PV and TT.
1271 if (Threads.stop.load(std::memory_order_relaxed))
1276 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1277 thisThread->rootMoves.end(), move);
1279 // PV move or new best move?
1280 if (moveCount == 1 || value > alpha)
1283 rm.selDepth = thisThread->selDepth;
1288 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1289 rm.pv.push_back(*m);
1291 // We record how often the best move has been changed in each
1292 // iteration. This information is used for time management: when
1293 // the best move changes frequently, we allocate some more time.
1295 ++thisThread->bestMoveChanges;
1298 // All other moves but the PV are set to the lowest value: this
1299 // is not a problem when sorting because the sort is stable and the
1300 // move position in the list is preserved - just the PV is pushed up.
1301 rm.score = -VALUE_INFINITE;
1304 if (value > bestValue)
1312 if (PvNode && !rootNode) // Update pv even in fail-high case
1313 update_pv(ss->pv, move, (ss+1)->pv);
1315 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1319 assert(value >= beta); // Fail high
1326 if (move != bestMove)
1328 if (captureOrPromotion && captureCount < 32)
1329 capturesSearched[captureCount++] = move;
1331 else if (!captureOrPromotion && quietCount < 64)
1332 quietsSearched[quietCount++] = move;
1336 // The following condition would detect a stop only after move loop has been
1337 // completed. But in this case bestValue is valid because we have fully
1338 // searched our subtree, and we can anyhow save the result in TT.
1344 // Step 20. Check for mate and stalemate
1345 // All legal moves have been searched and if there are no legal moves, it
1346 // must be a mate or a stalemate. If we are in a singular extension search then
1347 // return a fail low score.
1349 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1352 bestValue = excludedMove ? alpha
1353 : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1356 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1357 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1359 // Bonus for prior countermove that caused the fail low
1360 else if ( (depth >= 3 || PvNode)
1362 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1365 bestValue = std::min(bestValue, maxValue);
1367 // If no good move is found and the previous position was ttPv, then the previous
1368 // opponent move is probably good and the new position is added to the search tree.
1369 if (bestValue <= alpha)
1370 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1371 // Otherwise, a counter move has been found and if the position is the last leaf
1372 // in the search tree, remove the position from the search tree.
1374 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1376 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1377 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1378 bestValue >= beta ? BOUND_LOWER :
1379 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1380 depth, bestMove, ss->staticEval);
1382 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1388 // qsearch() is the quiescence search function, which is called by the main search
1389 // function with zero depth, or recursively with further decreasing depth per call.
1390 template <NodeType NT>
1391 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1393 constexpr bool PvNode = NT == PV;
1395 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1396 assert(PvNode || (alpha == beta - 1));
1403 Move ttMove, move, bestMove;
1405 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1406 bool pvHit, givesCheck, captureOrPromotion;
1411 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1413 ss->pv[0] = MOVE_NONE;
1416 Thread* thisThread = pos.this_thread();
1417 (ss+1)->ply = ss->ply + 1;
1418 bestMove = MOVE_NONE;
1419 ss->inCheck = pos.checkers();
1422 // Check for an immediate draw or maximum ply reached
1423 if ( pos.is_draw(ss->ply)
1424 || ss->ply >= MAX_PLY)
1425 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1427 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1429 // Decide whether or not to include checks: this fixes also the type of
1430 // TT entry depth that we are going to use. Note that in qsearch we use
1431 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1432 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1433 : DEPTH_QS_NO_CHECKS;
1434 // Transposition table lookup
1436 tte = TT.probe(posKey, ss->ttHit);
1437 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1438 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1439 pvHit = ss->ttHit && tte->is_pv();
1443 && tte->depth() >= ttDepth
1444 && ttValue != VALUE_NONE // Only in case of TT access race
1445 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1446 : (tte->bound() & BOUND_UPPER)))
1449 // Evaluate the position statically
1452 ss->staticEval = VALUE_NONE;
1453 bestValue = futilityBase = -VALUE_INFINITE;
1459 // Never assume anything about values stored in TT
1460 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1461 ss->staticEval = bestValue = evaluate(pos);
1463 // Can ttValue be used as a better position evaluation?
1464 if ( ttValue != VALUE_NONE
1465 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1466 bestValue = ttValue;
1469 ss->staticEval = bestValue =
1470 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1471 : -(ss-1)->staticEval + 2 * Tempo;
1473 // Stand pat. Return immediately if static value is at least beta
1474 if (bestValue >= beta)
1477 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1478 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1483 if (PvNode && bestValue > alpha)
1486 futilityBase = bestValue + 145;
1489 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1490 nullptr , (ss-4)->continuationHistory,
1491 nullptr , (ss-6)->continuationHistory };
1493 // Initialize a MovePicker object for the current position, and prepare
1494 // to search the moves. Because the depth is <= 0 here, only captures,
1495 // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
1496 // will be generated.
1497 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1498 &thisThread->captureHistory,
1500 to_sq((ss-1)->currentMove));
1502 // Loop through the moves until no moves remain or a beta cutoff occurs
1503 while ((move = mp.next_move()) != MOVE_NONE)
1505 assert(is_ok(move));
1507 givesCheck = pos.gives_check(move);
1508 captureOrPromotion = pos.capture_or_promotion(move);
1515 && futilityBase > -VALUE_KNOWN_WIN
1516 && !pos.advanced_pawn_push(move))
1518 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1520 // moveCount pruning
1524 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1526 if (futilityValue <= alpha)
1528 bestValue = std::max(bestValue, futilityValue);
1532 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1534 bestValue = std::max(bestValue, futilityBase);
1539 // Do not search moves with negative SEE values
1541 && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
1542 && !pos.see_ge(move))
1545 // Speculative prefetch as early as possible
1546 prefetch(TT.first_entry(pos.key_after(move)));
1548 // Check for legality just before making the move
1549 if (!pos.legal(move))
1555 ss->currentMove = move;
1556 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1557 [captureOrPromotion]
1558 [pos.moved_piece(move)]
1561 // CounterMove based pruning
1562 if ( !captureOrPromotion
1564 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1565 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1568 // Make and search the move
1569 pos.do_move(move, st, givesCheck);
1570 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1571 pos.undo_move(move);
1573 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1575 // Check for a new best move
1576 if (value > bestValue)
1584 if (PvNode) // Update pv even in fail-high case
1585 update_pv(ss->pv, move, (ss+1)->pv);
1587 if (PvNode && value < beta) // Update alpha here!
1595 // All legal moves have been searched. A special case: if we're in check
1596 // and no legal moves were found, it is checkmate.
1597 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1598 return mated_in(ss->ply); // Plies to mate from the root
1600 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1601 bestValue >= beta ? BOUND_LOWER :
1602 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1603 ttDepth, bestMove, ss->staticEval);
1605 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1611 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1612 // "plies to mate from the current position". Standard scores are unchanged.
1613 // The function is called before storing a value in the transposition table.
1615 Value value_to_tt(Value v, int ply) {
1617 assert(v != VALUE_NONE);
1619 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1620 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1624 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1625 // from the transposition table (which refers to the plies to mate/be mated from
1626 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1627 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1628 // and the graph history interaction, we return an optimal TB score instead.
1630 Value value_from_tt(Value v, int ply, int r50c) {
1632 if (v == VALUE_NONE)
1635 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1637 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1638 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1643 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1645 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1646 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1655 // update_pv() adds current move and appends child pv[]
1657 void update_pv(Move* pv, Move move, Move* childPv) {
1659 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1665 // update_all_stats() updates stats at the end of search() when a bestMove is found
1667 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1668 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1671 Color us = pos.side_to_move();
1672 Thread* thisThread = pos.this_thread();
1673 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1674 Piece moved_piece = pos.moved_piece(bestMove);
1675 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1677 bonus1 = stat_bonus(depth + 1);
1678 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1679 : stat_bonus(depth); // smaller bonus
1681 if (!pos.capture_or_promotion(bestMove))
1683 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1685 // Decrease all the non-best quiet moves
1686 for (int i = 0; i < quietCount; ++i)
1688 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1689 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1693 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1695 // Extra penalty for a quiet early move that was not a TT move or main killer move in previous ply when it gets refuted
1696 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1697 && !pos.captured_piece())
1698 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1700 // Decrease all the non-best capture moves
1701 for (int i = 0; i < captureCount; ++i)
1703 moved_piece = pos.moved_piece(capturesSearched[i]);
1704 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1705 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1710 // update_continuation_histories() updates histories of the move pairs formed
1711 // by moves at ply -1, -2, -4, and -6 with current move.
1713 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1715 for (int i : {1, 2, 4, 6})
1717 if (ss->inCheck && i > 2)
1719 if (is_ok((ss-i)->currentMove))
1720 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1725 // update_quiet_stats() updates move sorting heuristics
1727 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1729 if (ss->killers[0] != move)
1731 ss->killers[1] = ss->killers[0];
1732 ss->killers[0] = move;
1735 Color us = pos.side_to_move();
1736 Thread* thisThread = pos.this_thread();
1737 thisThread->mainHistory[us][from_to(move)] << bonus;
1738 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1740 if (type_of(pos.moved_piece(move)) != PAWN)
1741 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1743 if (is_ok((ss-1)->currentMove))
1745 Square prevSq = to_sq((ss-1)->currentMove);
1746 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1749 if (depth > 11 && ss->ply < MAX_LPH)
1750 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1753 // When playing with strength handicap, choose best move among a set of RootMoves
1754 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1756 Move Skill::pick_best(size_t multiPV) {
1758 const RootMoves& rootMoves = Threads.main()->rootMoves;
1759 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1761 // RootMoves are already sorted by score in descending order
1762 Value topScore = rootMoves[0].score;
1763 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1764 int weakness = 120 - 2 * level;
1765 int maxScore = -VALUE_INFINITE;
1767 // Choose best move. For each move score we add two terms, both dependent on
1768 // weakness. One is deterministic and bigger for weaker levels, and one is
1769 // random. Then we choose the move with the resulting highest score.
1770 for (size_t i = 0; i < multiPV; ++i)
1772 // This is our magic formula
1773 int push = ( weakness * int(topScore - rootMoves[i].score)
1774 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1776 if (rootMoves[i].score + push >= maxScore)
1778 maxScore = rootMoves[i].score + push;
1779 best = rootMoves[i].pv[0];
1789 /// MainThread::check_time() is used to print debug info and, more importantly,
1790 /// to detect when we are out of available time and thus stop the search.
1792 void MainThread::check_time() {
1797 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1798 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1800 static TimePoint lastInfoTime = now();
1802 TimePoint elapsed = Time.elapsed();
1803 TimePoint tick = Limits.startTime + elapsed;
1805 if (tick - lastInfoTime >= 1000)
1807 lastInfoTime = tick;
1811 // We should not stop pondering until told so by the GUI
1815 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1816 || (Limits.movetime && elapsed >= Limits.movetime)
1817 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1818 Threads.stop = true;
1822 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1823 /// that all (if any) unsearched PV lines are sent using a previous search score.
1825 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1827 std::stringstream ss;
1828 TimePoint elapsed = Time.elapsed() + 1;
1829 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1830 size_t pvIdx = pos.this_thread()->pvIdx;
1831 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1832 uint64_t nodesSearched = Threads.nodes_searched();
1833 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1835 for (size_t i = 0; i < multiPV; ++i)
1837 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1839 if (depth == 1 && !updated && i > 0)
1842 Depth d = updated ? depth : std::max(1, depth - 1);
1843 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1845 if (v == -VALUE_INFINITE)
1848 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1849 v = tb ? rootMoves[i].tbScore : v;
1851 if (ss.rdbuf()->in_avail()) // Not at first line
1856 << " seldepth " << rootMoves[i].selDepth
1857 << " multipv " << i + 1
1858 << " score " << UCI::value(v);
1860 if (Options["UCI_ShowWDL"])
1861 ss << UCI::wdl(v, pos.game_ply());
1863 if (!tb && i == pvIdx)
1864 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1866 ss << " nodes " << nodesSearched
1867 << " nps " << nodesSearched * 1000 / elapsed;
1869 if (elapsed > 1000) // Earlier makes little sense
1870 ss << " hashfull " << TT.hashfull();
1872 ss << " tbhits " << tbHits
1873 << " time " << elapsed
1876 for (Move m : rootMoves[i].pv)
1877 ss << " " << UCI::move(m, pos.is_chess960());
1884 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1885 /// before exiting the search, for instance, in case we stop the search during a
1886 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1887 /// otherwise in case of 'ponder on' we have nothing to think on.
1889 bool RootMove::extract_ponder_from_tt(Position& pos) {
1894 assert(pv.size() == 1);
1896 if (pv[0] == MOVE_NONE)
1899 pos.do_move(pv[0], st);
1900 TTEntry* tte = TT.probe(pos.key(), ttHit);
1904 Move m = tte->move(); // Local copy to be SMP safe
1905 if (MoveList<LEGAL>(pos).contains(m))
1909 pos.undo_move(pv[0]);
1910 return pv.size() > 1;
1913 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1916 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1917 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1918 Cardinality = int(Options["SyzygyProbeLimit"]);
1919 bool dtz_available = true;
1921 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1922 // ProbeDepth == DEPTH_ZERO
1923 if (Cardinality > MaxCardinality)
1925 Cardinality = MaxCardinality;
1929 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1931 // Rank moves using DTZ tables
1932 RootInTB = root_probe(pos, rootMoves);
1936 // DTZ tables are missing; try to rank moves using WDL tables
1937 dtz_available = false;
1938 RootInTB = root_probe_wdl(pos, rootMoves);
1944 // Sort moves according to TB rank
1945 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1946 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1948 // Probe during search only if DTZ is not available and we are winning
1949 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1954 // Clean up if root_probe() and root_probe_wdl() have failed
1955 for (auto& m : rootMoves)