2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2022 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"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV, Root };
65 Value futility_margin(Depth d, bool improving) {
66 return Value(168 * (d - improving));
69 // Reductions lookup table, initialized at startup
70 int Reductions[MAX_MOVES]; // [depth or moveNumber]
72 Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {
73 int r = Reductions[d] * Reductions[mn];
74 return (r + 1463 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 1010);
77 constexpr int futility_move_count(bool improving, Depth depth) {
78 return (3 + depth * depth) / (2 - improving);
81 // History and stats update bonus, based on depth
82 int stat_bonus(Depth d) {
83 return std::min((9 * d + 270) * d - 311 , 2145);
86 // Add a small random component to draw evaluations to avoid 3-fold blindness
87 Value value_draw(Thread* thisThread) {
88 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
91 // Skill structure is used to implement strength limit. If we have an uci_elo then
92 // we convert it to a suitable fractional skill level using anchoring to CCRL Elo
93 // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
94 // results spanning a wide range of k values.
96 Skill(int skill_level, int uci_elo) {
98 level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0);
100 level = double(skill_level);
102 bool enabled() const { return level < 20.0; }
103 bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
104 Move pick_best(size_t multiPV);
107 Move best = MOVE_NONE;
110 template <NodeType nodeType>
111 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
113 template <NodeType nodeType>
114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
116 Value value_to_tt(Value v, int ply);
117 Value value_from_tt(Value v, int ply, int r50c);
118 void update_pv(Move* pv, Move move, Move* childPv);
119 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
120 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
121 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
122 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
124 // perft() is our utility to verify move generation. All the leaf nodes up
125 // to the given depth are generated and counted, and the sum is returned.
127 uint64_t perft(Position& pos, Depth depth) {
130 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
132 uint64_t cnt, nodes = 0;
133 const bool leaf = (depth == 2);
135 for (const auto& m : MoveList<LEGAL>(pos))
137 if (Root && depth <= 1)
142 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
147 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
155 /// Search::init() is called at startup to initialize various lookup tables
157 void Search::init() {
159 for (int i = 1; i < MAX_MOVES; ++i)
160 Reductions[i] = int((20.81 + std::log(Threads.size()) / 2) * std::log(i));
164 /// Search::clear() resets search state to its initial value
166 void Search::clear() {
168 Threads.main()->wait_for_search_finished();
170 Time.availableNodes = 0;
173 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
177 /// MainThread::search() is started when the program receives the UCI 'go'
178 /// command. It searches from the root position and outputs the "bestmove".
180 void MainThread::search() {
184 nodes = perft<true>(rootPos, Limits.perft);
185 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
189 Color us = rootPos.side_to_move();
190 Time.init(Limits, us, rootPos.game_ply());
193 Eval::NNUE::verify();
195 if (rootMoves.empty())
197 rootMoves.emplace_back(MOVE_NONE);
198 sync_cout << "info depth 0 score "
199 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
204 Threads.start_searching(); // start non-main threads
205 Thread::search(); // main thread start searching
208 // When we reach the maximum depth, we can arrive here without a raise of
209 // Threads.stop. However, if we are pondering or in an infinite search,
210 // the UCI protocol states that we shouldn't print the best move before the
211 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
212 // until the GUI sends one of those commands.
214 while (!Threads.stop && (ponder || Limits.infinite))
215 {} // Busy wait for a stop or a ponder reset
217 // Stop the threads if not already stopped (also raise the stop if
218 // "ponderhit" just reset Threads.ponder).
221 // Wait until all threads have finished
222 Threads.wait_for_search_finished();
224 // When playing in 'nodes as time' mode, subtract the searched nodes from
225 // the available ones before exiting.
227 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
229 Thread* bestThread = this;
230 Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
232 if ( int(Options["MultiPV"]) == 1
235 && rootMoves[0].pv[0] != MOVE_NONE)
236 bestThread = Threads.get_best_thread();
238 bestPreviousScore = bestThread->rootMoves[0].score;
239 bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
241 // Send again PV info if we have a new best thread
242 if (bestThread != this)
243 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
245 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
247 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
248 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
250 std::cout << sync_endl;
254 /// Thread::search() is the main iterative deepening loop. It calls search()
255 /// repeatedly with increasing depth until the allocated thinking time has been
256 /// consumed, the user stops the search, or the maximum search depth is reached.
258 void Thread::search() {
260 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
261 // The former is needed to allow update_continuation_histories(ss-1, ...),
262 // which accesses its argument at ss-6, also near the root.
263 // The latter is needed for statScore and killer initialization.
264 Stack stack[MAX_PLY+10], *ss = stack+7;
266 Value alpha, beta, delta;
267 Move lastBestMove = MOVE_NONE;
268 Depth lastBestMoveDepth = 0;
269 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
270 double timeReduction = 1, totBestMoveChanges = 0;
271 Color us = rootPos.side_to_move();
274 std::memset(ss-7, 0, 10 * sizeof(Stack));
275 for (int i = 7; i > 0; i--)
276 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
278 for (int i = 0; i <= MAX_PLY + 2; ++i)
283 bestValue = delta = alpha = -VALUE_INFINITE;
284 beta = VALUE_INFINITE;
288 if (mainThread->bestPreviousScore == VALUE_INFINITE)
289 for (int i = 0; i < 4; ++i)
290 mainThread->iterValue[i] = VALUE_ZERO;
292 for (int i = 0; i < 4; ++i)
293 mainThread->iterValue[i] = mainThread->bestPreviousScore;
296 size_t multiPV = size_t(Options["MultiPV"]);
297 Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
299 // When playing with strength handicap enable MultiPV search that we will
300 // use behind the scenes to retrieve a set of possible moves.
302 multiPV = std::max(multiPV, (size_t)4);
304 multiPV = std::min(multiPV, rootMoves.size());
306 complexityAverage.set(202, 1);
309 optimism[ us] = Value(39);
310 optimism[~us] = -optimism[us];
312 int searchAgainCounter = 0;
314 // Iterative deepening loop until requested to stop or the target depth is reached
315 while ( ++rootDepth < MAX_PLY
317 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
319 // Age out PV variability metric
321 totBestMoveChanges /= 2;
323 // Save the last iteration's scores before first PV line is searched and
324 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
325 for (RootMove& rm : rootMoves)
326 rm.previousScore = rm.score;
331 if (!Threads.increaseDepth)
332 searchAgainCounter++;
334 // MultiPV loop. We perform a full root search for each PV line
335 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
340 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
341 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
345 // Reset UCI info selDepth for each depth and each PV line
348 // Reset aspiration window starting size
351 Value prev = rootMoves[pvIdx].averageScore;
352 delta = Value(16) + int(prev) * prev / 19178;
353 alpha = std::max(prev - delta,-VALUE_INFINITE);
354 beta = std::min(prev + delta, VALUE_INFINITE);
356 // Adjust trend and optimism based on root move's previousScore
357 int tr = sigmoid(prev, 3, 8, 90, 125, 1);
358 trend = (us == WHITE ? make_score(tr, tr / 2)
359 : -make_score(tr, tr / 2));
361 int opt = sigmoid(prev, 8, 17, 144, 13966, 183);
362 optimism[ us] = Value(opt);
363 optimism[~us] = -optimism[us];
366 // Start with a small aspiration window and, in the case of a fail
367 // high/low, re-search with a bigger window until we don't fail
369 int failedHighCnt = 0;
372 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
373 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
375 // Bring the best move to the front. It is critical that sorting
376 // is done with a stable algorithm because all the values but the
377 // first and eventually the new best one are set to -VALUE_INFINITE
378 // and we want to keep the same order for all the moves except the
379 // new PV that goes to the front. Note that in case of MultiPV
380 // search the already searched PV lines are preserved.
381 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
383 // If search has been stopped, we break immediately. Sorting is
384 // safe because RootMoves is still valid, although it refers to
385 // the previous iteration.
389 // When failing high/low give some update (without cluttering
390 // the UI) before a re-search.
393 && (bestValue <= alpha || bestValue >= beta)
394 && Time.elapsed() > 3000)
395 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
397 // In case of failing low/high increase aspiration window and
398 // re-search, otherwise exit the loop.
399 if (bestValue <= alpha)
401 beta = (alpha + beta) / 2;
402 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
406 mainThread->stopOnPonderhit = false;
408 else if (bestValue >= beta)
410 beta = std::min(bestValue + delta, VALUE_INFINITE);
416 delta += delta / 4 + 2;
418 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
421 // Sort the PV lines searched so far and update the GUI
422 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
425 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
426 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
430 completedDepth = rootDepth;
432 if (rootMoves[0].pv[0] != lastBestMove) {
433 lastBestMove = rootMoves[0].pv[0];
434 lastBestMoveDepth = rootDepth;
437 // Have we found a "mate in x"?
439 && bestValue >= VALUE_MATE_IN_MAX_PLY
440 && VALUE_MATE - bestValue <= 2 * Limits.mate)
446 // If skill level is enabled and time is up, pick a sub-optimal best move
447 if (skill.enabled() && skill.time_to_pick(rootDepth))
448 skill.pick_best(multiPV);
450 // Use part of the gained time from a previous stable move for the current move
451 for (Thread* th : Threads)
453 totBestMoveChanges += th->bestMoveChanges;
454 th->bestMoveChanges = 0;
457 // Do we have time for the next iteration? Can we stop searching now?
458 if ( Limits.use_time_management()
460 && !mainThread->stopOnPonderhit)
462 double fallingEval = (69 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
463 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 781.4;
464 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
466 // If the bestMove is stable over several iterations, reduce time accordingly
467 timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.63 : 0.73;
468 double reduction = (1.56 + mainThread->previousTimeReduction) / (2.20 * timeReduction);
469 double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth)
470 * totBestMoveChanges / Threads.size();
471 int complexity = mainThread->complexityAverage.value();
472 double complexPosition = std::clamp(1.0 + (complexity - 326) / 1618.1, 0.5, 1.5);
474 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
476 // Cap used time in case of a single legal move for a better viewer experience in tournaments
477 // yielding correct scores and sufficiently fast moves.
478 if (rootMoves.size() == 1)
479 totalTime = std::min(500.0, totalTime);
481 // Stop the search if we have exceeded the totalTime
482 if (Time.elapsed() > totalTime)
484 // If we are allowed to ponder do not stop the search now but
485 // keep pondering until the GUI sends "ponderhit" or "stop".
486 if (mainThread->ponder)
487 mainThread->stopOnPonderhit = true;
491 else if ( Threads.increaseDepth
492 && !mainThread->ponder
493 && Time.elapsed() > totalTime * 0.43)
494 Threads.increaseDepth = false;
496 Threads.increaseDepth = true;
499 mainThread->iterValue[iterIdx] = bestValue;
500 iterIdx = (iterIdx + 1) & 3;
506 mainThread->previousTimeReduction = timeReduction;
508 // If skill level is enabled, swap best PV line with the sub-optimal one
510 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
511 skill.best ? skill.best : skill.pick_best(multiPV)));
517 // search<>() is the main search function for both PV and non-PV nodes
519 template <NodeType nodeType>
520 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
522 constexpr bool PvNode = nodeType != NonPV;
523 constexpr bool rootNode = nodeType == Root;
524 const Depth maxNextDepth = rootNode ? depth : depth + 1;
526 // Check if we have an upcoming move which draws by repetition, or
527 // if the opponent had an alternative move earlier to this position.
529 && pos.rule50_count() >= 3
530 && alpha < VALUE_DRAW
531 && pos.has_game_cycle(ss->ply))
533 alpha = value_draw(pos.this_thread());
538 // Dive into quiescence search when the depth reaches zero
540 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
542 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
543 assert(PvNode || (alpha == beta - 1));
544 assert(0 < depth && depth < MAX_PLY);
545 assert(!(PvNode && cutNode));
547 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
549 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
553 Move ttMove, move, excludedMove, bestMove;
554 Depth extension, newDepth;
555 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
556 bool givesCheck, improving, didLMR, priorCapture;
557 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
559 int moveCount, captureCount, quietCount, bestMoveCount, improvement, complexity;
561 // Step 1. Initialize node
562 Thread* thisThread = pos.this_thread();
563 ss->inCheck = pos.checkers();
564 priorCapture = pos.captured_piece();
565 Color us = pos.side_to_move();
566 moveCount = bestMoveCount = captureCount = quietCount = ss->moveCount = 0;
567 bestValue = -VALUE_INFINITE;
568 maxValue = VALUE_INFINITE;
570 // Check for the available remaining time
571 if (thisThread == Threads.main())
572 static_cast<MainThread*>(thisThread)->check_time();
574 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
575 if (PvNode && thisThread->selDepth < ss->ply + 1)
576 thisThread->selDepth = ss->ply + 1;
580 // Step 2. Check for aborted search and immediate draw
581 if ( Threads.stop.load(std::memory_order_relaxed)
582 || pos.is_draw(ss->ply)
583 || ss->ply >= MAX_PLY)
584 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
585 : value_draw(pos.this_thread());
587 // Step 3. Mate distance pruning. Even if we mate at the next move our score
588 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
589 // a shorter mate was found upward in the tree then there is no need to search
590 // because we will never beat the current alpha. Same logic but with reversed
591 // signs applies also in the opposite condition of being mated instead of giving
592 // mate. In this case return a fail-high score.
593 alpha = std::max(mated_in(ss->ply), alpha);
594 beta = std::min(mate_in(ss->ply+1), beta);
599 thisThread->rootDelta = beta - alpha;
601 assert(0 <= ss->ply && ss->ply < MAX_PLY);
603 (ss+1)->ttPv = false;
604 (ss+1)->excludedMove = bestMove = MOVE_NONE;
605 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
606 ss->doubleExtensions = (ss-1)->doubleExtensions;
608 Square prevSq = to_sq((ss-1)->currentMove);
610 // Initialize statScore to zero for the grandchildren of the current position.
611 // So statScore is shared between all grandchildren and only the first grandchild
612 // starts with statScore = 0. Later grandchildren start with the last calculated
613 // statScore of the previous grandchild. This influences the reduction rules in
614 // LMR which are based on the statScore of parent position.
616 (ss+2)->statScore = 0;
618 // Step 4. Transposition table lookup. We don't want the score of a partial
619 // search to overwrite a previous full search TT value, so we use a different
620 // position key in case of an excluded move.
621 excludedMove = ss->excludedMove;
622 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
623 tte = TT.probe(posKey, ss->ttHit);
624 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
625 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
626 : ss->ttHit ? tte->move() : MOVE_NONE;
627 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
629 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
631 // At non-PV nodes we check for an early TT cutoff
634 && tte->depth() > depth - (thisThread->id() % 2 == 1)
635 && ttValue != VALUE_NONE // Possible in case of TT access race
636 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
637 : (tte->bound() & BOUND_UPPER)))
639 // If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)
644 // Bonus for a quiet ttMove that fails high (~3 Elo)
646 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
648 // Extra penalty for early quiet moves of the previous ply (~0 Elo)
649 if ((ss-1)->moveCount <= 2 && !priorCapture)
650 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
652 // Penalty for a quiet ttMove that fails low (~1 Elo)
655 int penalty = -stat_bonus(depth);
656 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
657 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
661 // Partial workaround for the graph history interaction problem
662 // For high rule50 counts don't produce transposition table cutoffs.
663 if (pos.rule50_count() < 90)
667 // Step 5. Tablebases probe
668 if (!rootNode && TB::Cardinality)
670 int piecesCount = pos.count<ALL_PIECES>();
672 if ( piecesCount <= TB::Cardinality
673 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
674 && pos.rule50_count() == 0
675 && !pos.can_castle(ANY_CASTLING))
678 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
680 // Force check of time on the next occasion
681 if (thisThread == Threads.main())
682 static_cast<MainThread*>(thisThread)->callsCnt = 0;
684 if (err != TB::ProbeState::FAIL)
686 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
688 int drawScore = TB::UseRule50 ? 1 : 0;
690 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
691 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
692 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
693 : VALUE_DRAW + 2 * wdl * drawScore;
695 Bound b = wdl < -drawScore ? BOUND_UPPER
696 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
698 if ( b == BOUND_EXACT
699 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
701 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
702 std::min(MAX_PLY - 1, depth + 6),
703 MOVE_NONE, VALUE_NONE);
710 if (b == BOUND_LOWER)
711 bestValue = value, alpha = std::max(alpha, bestValue);
719 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
721 // Step 6. Static evaluation of the position
724 // Skip early pruning when in check
725 ss->staticEval = eval = VALUE_NONE;
733 // Never assume anything about values stored in TT
734 ss->staticEval = eval = tte->eval();
735 if (eval == VALUE_NONE)
736 ss->staticEval = eval = evaluate(pos);
738 // Randomize draw evaluation
739 if (eval == VALUE_DRAW)
740 eval = value_draw(thisThread);
742 // ttValue can be used as a better position evaluation (~4 Elo)
743 if ( ttValue != VALUE_NONE
744 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
749 ss->staticEval = eval = evaluate(pos);
751 // Save static evaluation into transposition table
753 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
756 // Use static evaluation difference to improve quiet move ordering (~3 Elo)
757 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
759 int bonus = std::clamp(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000);
760 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
763 // Set up the improvement variable, which is the difference between the current
764 // static evaluation and the previous static evaluation at our turn (if we were
765 // in check at our previous move we look at the move prior to it). The improvement
766 // margin and the improving flag are used in various pruning heuristics.
767 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
768 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
771 improving = improvement > 0;
772 complexity = abs(ss->staticEval - (us == WHITE ? eg_value(pos.psq_score()) : -eg_value(pos.psq_score())));
774 thisThread->complexityAverage.update(complexity);
777 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
778 // return a fail low.
781 && eval < alpha - 348 - 258 * depth * depth)
783 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
788 // Step 8. Futility pruning: child node (~25 Elo).
789 // The depth condition is important for mate finding.
792 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta
794 && eval < 26305) // larger than VALUE_KNOWN_WIN, but smaller than TB wins.
797 // Step 9. Null move search with verification search (~22 Elo)
799 && (ss-1)->currentMove != MOVE_NULL
800 && (ss-1)->statScore < 14695
802 && eval >= ss->staticEval
803 && ss->staticEval >= beta - 15 * depth - improvement / 15 + 198 + complexity / 28
805 && pos.non_pawn_material(us)
806 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
808 assert(eval - beta >= 0);
810 // Null move dynamic reduction based on depth, eval and complexity of position
811 Depth R = std::min(int(eval - beta) / 147, 5) + depth / 3 + 4 - (complexity > 753);
813 ss->currentMove = MOVE_NULL;
814 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
816 pos.do_null_move(st);
818 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
820 pos.undo_null_move();
822 if (nullValue >= beta)
824 // Do not return unproven mate or TB scores
825 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
828 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
831 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
833 // Do verification search at high depths, with null move pruning disabled
834 // for us, until ply exceeds nmpMinPly.
835 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
836 thisThread->nmpColor = us;
838 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
840 thisThread->nmpMinPly = 0;
847 probCutBeta = beta + 179 - 46 * improving;
849 // Step 10. ProbCut (~4 Elo)
850 // If we have a good enough capture and a reduced search returns a value
851 // much above beta, we can (almost) safely prune the previous move.
854 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
855 // if value from transposition table is lower than probCutBeta, don't attempt probCut
856 // there and in further interactions with transposition table cutoff depth is set to depth - 3
857 // because probCut search has depth set to depth - 4 but we also do a move before it
858 // so effective depth is equal to depth - 3
860 && tte->depth() >= depth - 3
861 && ttValue != VALUE_NONE
862 && ttValue < probCutBeta))
864 assert(probCutBeta < VALUE_INFINITE);
866 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);
867 bool ttPv = ss->ttPv;
870 while ((move = mp.next_move()) != MOVE_NONE)
871 if (move != excludedMove && pos.legal(move))
873 assert(pos.capture_or_promotion(move));
875 captureOrPromotion = true;
877 ss->currentMove = move;
878 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
880 [pos.moved_piece(move)]
883 pos.do_move(move, st);
885 // Perform a preliminary qsearch to verify that the move holds
886 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
888 // If the qsearch held, perform the regular search
889 if (value >= probCutBeta)
890 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
894 if (value >= probCutBeta)
896 // if transposition table doesn't have equal or more deep info write probCut data into it
898 && tte->depth() >= depth - 3
899 && ttValue != VALUE_NONE))
900 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
902 depth - 3, move, ss->staticEval);
909 // Step 11. If the position is not in TT, decrease depth by 2 or 1 depending on node type (~3 Elo)
920 moves_loop: // When in check, search starts here
922 // Step 12. A small Probcut idea, when we are in check (~0 Elo)
923 probCutBeta = beta + 481;
928 && (tte->bound() & BOUND_LOWER)
929 && tte->depth() >= depth - 3
930 && ttValue >= probCutBeta
931 && abs(ttValue) <= VALUE_KNOWN_WIN
932 && abs(beta) <= VALUE_KNOWN_WIN
937 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
938 nullptr , (ss-4)->continuationHistory,
939 nullptr , (ss-6)->continuationHistory };
941 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
943 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
950 moveCountPruning = false;
952 // Indicate PvNodes that will probably fail low if the node was searched
953 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
954 bool likelyFailLow = PvNode
956 && (tte->bound() & BOUND_UPPER)
957 && tte->depth() >= depth;
959 // Step 13. Loop through all pseudo-legal moves until no moves remain
960 // or a beta cutoff occurs.
961 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
965 if (move == excludedMove)
968 // At root obey the "searchmoves" option and skip moves not listed in Root
969 // Move List. As a consequence any illegal move is also skipped. In MultiPV
970 // mode we also skip PV moves which have been already searched and those
971 // of lower "TB rank" if we are in a TB root position.
972 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
973 thisThread->rootMoves.begin() + thisThread->pvLast, move))
976 // Check for legality
977 if (!rootNode && !pos.legal(move))
980 ss->moveCount = ++moveCount;
982 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
983 sync_cout << "info depth " << depth
984 << " currmove " << UCI::move(move, pos.is_chess960())
985 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
987 (ss+1)->pv = nullptr;
990 captureOrPromotion = pos.capture_or_promotion(move);
991 movedPiece = pos.moved_piece(move);
992 givesCheck = pos.gives_check(move);
994 // Calculate new depth for this move
995 newDepth = depth - 1;
997 Value delta = beta - alpha;
999 // Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.
1001 && pos.non_pawn_material(us)
1002 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1004 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo)
1005 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1007 // Reduced depth of the next LMR search
1008 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0);
1010 if ( captureOrPromotion
1013 // Futility pruning for captures (~0 Elo)
1014 if ( !pos.empty(to_sq(move))
1019 && ss->staticEval + 281 + 179 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1020 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1023 // SEE based pruning (~9 Elo)
1024 if (!pos.see_ge(move, Value(-203) * depth))
1029 int history = (*contHist[0])[movedPiece][to_sq(move)]
1030 + (*contHist[1])[movedPiece][to_sq(move)]
1031 + (*contHist[3])[movedPiece][to_sq(move)];
1033 // Continuation history based pruning (~2 Elo)
1035 && history < -3875 * (depth - 1))
1038 history += thisThread->mainHistory[us][from_to(move)];
1040 // Futility pruning: parent node (~9 Elo)
1043 && ss->staticEval + 122 + 138 * lmrDepth + history / 60 <= alpha)
1046 // Prune moves with negative SEE (~3 Elo)
1047 if (!pos.see_ge(move, Value(-25 * lmrDepth * lmrDepth - 20 * lmrDepth)))
1052 // Step 15. Extensions (~66 Elo)
1053 // We take care to not overdo to avoid search getting stuck.
1054 if (ss->ply < thisThread->rootDepth * 2)
1056 // Singular extension search (~58 Elo). If all moves but one fail low on a
1057 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1058 // then that move is singular and should be extended. To verify this we do
1059 // a reduced search on all the other moves but the ttMove and if the
1060 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1062 && depth >= 4 + 2 * (PvNode && tte->is_pv())
1064 && !excludedMove // Avoid recursive singular search
1065 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1066 && abs(ttValue) < VALUE_KNOWN_WIN
1067 && (tte->bound() & BOUND_LOWER)
1068 && tte->depth() >= depth - 3)
1070 Value singularBeta = ttValue - 3 * depth;
1071 Depth singularDepth = (depth - 1) / 2;
1073 ss->excludedMove = move;
1074 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1075 ss->excludedMove = MOVE_NONE;
1077 if (value < singularBeta)
1081 // Avoid search explosion by limiting the number of double extensions
1083 && value < singularBeta - 26
1084 && ss->doubleExtensions <= 8)
1088 // Multi-cut pruning
1089 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1090 // search without the ttMove. So we assume this expected Cut-node is not singular,
1091 // that multiple moves fail high, and we can prune the whole subtree by returning
1093 else if (singularBeta >= beta)
1094 return singularBeta;
1096 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1097 else if (ttValue >= beta)
1101 // Check extensions (~1 Elo)
1102 else if ( givesCheck
1104 && abs(ss->staticEval) > 71)
1107 // Quiet ttMove extensions (~0 Elo)
1110 && move == ss->killers[0]
1111 && (*contHist[0])[movedPiece][to_sq(move)] >= 5491)
1115 // Add extension to new depth
1116 newDepth += extension;
1117 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1119 // Speculative prefetch as early as possible
1120 prefetch(TT.first_entry(pos.key_after(move)));
1122 // Update the current move (this must be done after singular extension search)
1123 ss->currentMove = move;
1124 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1125 [captureOrPromotion]
1129 // Step 16. Make the move
1130 pos.do_move(move, st, givesCheck);
1132 bool doDeeperSearch = false;
1134 // Step 17. Late moves reduction / extension (LMR, ~98 Elo)
1135 // We use various heuristics for the sons of a node after the first son has
1136 // been searched. In general we would like to reduce them, but there are many
1137 // cases where we extend a son if it has good chances to be "interesting".
1139 && moveCount > 1 + (PvNode && ss->ply <= 1)
1141 || !captureOrPromotion
1142 || (cutNode && (ss-1)->moveCount > 1)))
1144 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
1146 // Decrease reduction at some PvNodes (~2 Elo)
1148 && bestMoveCount <= 3)
1151 // Decrease reduction if position is or has been on the PV
1152 // and node is not likely to fail low. (~3 Elo)
1157 // Decrease reduction if opponent's move count is high (~1 Elo)
1158 if ((ss-1)->moveCount > 7)
1161 // Increase reduction for cut nodes (~3 Elo)
1162 if (cutNode && move != ss->killers[0])
1165 // Increase reduction if ttMove is a capture (~3 Elo)
1169 // Decrease reduction at PvNodes if bestvalue
1170 // is vastly different from static evaluation
1171 if (PvNode && !ss->inCheck && abs(ss->staticEval - bestValue) > 250)
1174 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1175 + (*contHist[0])[movedPiece][to_sq(move)]
1176 + (*contHist[1])[movedPiece][to_sq(move)]
1177 + (*contHist[3])[movedPiece][to_sq(move)]
1180 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1181 r -= ss->statScore / 15914;
1183 // In general we want to cap the LMR depth search at newDepth. But if reductions
1184 // are really negative and movecount is low, we allow this move to be searched
1185 // deeper than the first move (this may lead to hidden double extensions).
1186 int deeper = r >= -1 ? 0
1187 : moveCount <= 4 ? 2
1188 : PvNode && depth > 4 ? 1
1189 : cutNode && moveCount <= 8 ? 1
1192 Depth d = std::clamp(newDepth - r, 1, newDepth + deeper);
1194 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1196 // If the son is reduced and fails high it will be re-searched at full depth
1197 doFullDepthSearch = value > alpha && d < newDepth;
1198 doDeeperSearch = value > (alpha + 78 + 11 * (newDepth - d));
1203 doFullDepthSearch = !PvNode || moveCount > 1;
1207 // Step 18. Full depth search when LMR is skipped or fails high
1208 if (doFullDepthSearch)
1210 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode);
1212 // If the move passed LMR update its stats
1215 int bonus = value > alpha ? stat_bonus(newDepth)
1216 : -stat_bonus(newDepth);
1218 if (captureOrPromotion)
1221 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1225 // For PV nodes only, do a full PV search on the first move or after a fail
1226 // high (in the latter case search only if value < beta), otherwise let the
1227 // parent node fail low with value <= alpha and try another move.
1228 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1231 (ss+1)->pv[0] = MOVE_NONE;
1233 value = -search<PV>(pos, ss+1, -beta, -alpha,
1234 std::min(maxNextDepth, newDepth), false);
1237 // Step 19. Undo move
1238 pos.undo_move(move);
1240 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1242 // Step 20. Check for a new best move
1243 // Finished searching the move. If a stop occurred, the return value of
1244 // the search cannot be trusted, and we return immediately without
1245 // updating best move, PV and TT.
1246 if (Threads.stop.load(std::memory_order_relaxed))
1251 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1252 thisThread->rootMoves.end(), move);
1254 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1256 // PV move or new best move?
1257 if (moveCount == 1 || value > alpha)
1260 rm.selDepth = thisThread->selDepth;
1265 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1266 rm.pv.push_back(*m);
1268 // We record how often the best move has been changed in each iteration.
1269 // This information is used for time management. In MultiPV mode,
1270 // we must take care to only do this for the first PV line.
1272 && !thisThread->pvIdx)
1273 ++thisThread->bestMoveChanges;
1276 // All other moves but the PV are set to the lowest value: this
1277 // is not a problem when sorting because the sort is stable and the
1278 // move position in the list is preserved - just the PV is pushed up.
1279 rm.score = -VALUE_INFINITE;
1282 if (value > bestValue)
1290 if (PvNode && !rootNode) // Update pv even in fail-high case
1291 update_pv(ss->pv, move, (ss+1)->pv);
1293 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1300 assert(value >= beta); // Fail high
1306 // If the move is worse than some previously searched move, remember it to update its stats later
1307 if (move != bestMove)
1309 if (captureOrPromotion && captureCount < 32)
1310 capturesSearched[captureCount++] = move;
1312 else if (!captureOrPromotion && quietCount < 64)
1313 quietsSearched[quietCount++] = move;
1317 // The following condition would detect a stop only after move loop has been
1318 // completed. But in this case bestValue is valid because we have fully
1319 // searched our subtree, and we can anyhow save the result in TT.
1325 // Step 21. Check for mate and stalemate
1326 // All legal moves have been searched and if there are no legal moves, it
1327 // must be a mate or a stalemate. If we are in a singular extension search then
1328 // return a fail low score.
1330 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1333 bestValue = excludedMove ? alpha :
1334 ss->inCheck ? mated_in(ss->ply)
1337 // If there is a move which produces search value greater than alpha we update stats of searched moves
1339 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1340 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1342 // Bonus for prior countermove that caused the fail low
1343 else if ( (depth >= 4 || PvNode)
1346 //Assign extra bonus if current node is PvNode or cutNode
1347 //or fail low was really bad
1348 bool extraBonus = PvNode
1350 || bestValue < alpha - 70 * depth;
1352 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + extraBonus));
1356 bestValue = std::min(bestValue, maxValue);
1358 // If no good move is found and the previous position was ttPv, then the previous
1359 // opponent move is probably good and the new position is added to the search tree.
1360 if (bestValue <= alpha)
1361 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1362 // Otherwise, a counter move has been found and if the position is the last leaf
1363 // in the search tree, remove the position from the search tree.
1365 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1367 // Write gathered information in transposition table
1368 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1369 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1370 bestValue >= beta ? BOUND_LOWER :
1371 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1372 depth, bestMove, ss->staticEval);
1374 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1380 // qsearch() is the quiescence search function, which is called by the main search
1381 // function with zero depth, or recursively with further decreasing depth per call.
1382 template <NodeType nodeType>
1383 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1385 static_assert(nodeType != Root);
1386 constexpr bool PvNode = nodeType == PV;
1388 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1389 assert(PvNode || (alpha == beta - 1));
1394 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1398 Move ttMove, move, bestMove;
1400 Value bestValue, value, ttValue, futilityValue, futilityBase;
1401 bool pvHit, givesCheck, captureOrPromotion;
1407 ss->pv[0] = MOVE_NONE;
1410 Thread* thisThread = pos.this_thread();
1411 bestMove = MOVE_NONE;
1412 ss->inCheck = pos.checkers();
1415 // Check for an immediate draw or maximum ply reached
1416 if ( pos.is_draw(ss->ply)
1417 || ss->ply >= MAX_PLY)
1418 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1420 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1422 // Decide whether or not to include checks: this fixes also the type of
1423 // TT entry depth that we are going to use. Note that in qsearch we use
1424 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1425 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1426 : DEPTH_QS_NO_CHECKS;
1427 // Transposition table lookup
1429 tte = TT.probe(posKey, ss->ttHit);
1430 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1431 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1432 pvHit = ss->ttHit && tte->is_pv();
1436 && tte->depth() >= ttDepth
1437 && ttValue != VALUE_NONE // Only in case of TT access race
1438 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1439 : (tte->bound() & BOUND_UPPER)))
1442 // Evaluate the position statically
1445 ss->staticEval = VALUE_NONE;
1446 bestValue = futilityBase = -VALUE_INFINITE;
1452 // Never assume anything about values stored in TT
1453 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1454 ss->staticEval = bestValue = evaluate(pos);
1456 // ttValue can be used as a better position evaluation (~7 Elo)
1457 if ( ttValue != VALUE_NONE
1458 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1459 bestValue = ttValue;
1462 // In case of null move search use previous static eval with a different sign
1463 ss->staticEval = bestValue =
1464 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1465 : -(ss-1)->staticEval;
1467 // Stand pat. Return immediately if static value is at least beta
1468 if (bestValue >= beta)
1470 // Save gathered info in transposition table
1472 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1473 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1478 if (PvNode && bestValue > alpha)
1481 futilityBase = bestValue + 118;
1484 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1485 nullptr , (ss-4)->continuationHistory,
1486 nullptr , (ss-6)->continuationHistory };
1488 // Initialize a MovePicker object for the current position, and prepare
1489 // to search the moves. Because the depth is <= 0 here, only captures,
1490 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1491 // will be generated.
1492 Square prevSq = to_sq((ss-1)->currentMove);
1493 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1494 &thisThread->captureHistory,
1498 int quietCheckEvasions = 0;
1500 // Loop through the moves until no moves remain or a beta cutoff occurs
1501 while ((move = mp.next_move()) != MOVE_NONE)
1503 assert(is_ok(move));
1505 // Check for legality
1506 if (!pos.legal(move))
1509 givesCheck = pos.gives_check(move);
1510 captureOrPromotion = pos.capture_or_promotion(move);
1514 // Futility pruning and moveCount pruning (~5 Elo)
1515 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1517 && to_sq(move) != prevSq
1518 && futilityBase > -VALUE_KNOWN_WIN
1519 && type_of(move) != PROMOTION)
1525 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1527 if (futilityValue <= alpha)
1529 bestValue = std::max(bestValue, futilityValue);
1533 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1535 bestValue = std::max(bestValue, futilityBase);
1540 // Do not search moves with negative SEE values (~5 Elo)
1541 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1542 && !pos.see_ge(move))
1545 // Speculative prefetch as early as possible
1546 prefetch(TT.first_entry(pos.key_after(move)));
1548 ss->currentMove = move;
1549 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1550 [captureOrPromotion]
1551 [pos.moved_piece(move)]
1554 // Continuation history based pruning (~2 Elo)
1555 if ( !captureOrPromotion
1556 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1557 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1558 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1561 // movecount pruning for quiet check evasions
1562 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1563 && quietCheckEvasions > 1
1564 && !captureOrPromotion
1568 quietCheckEvasions += !captureOrPromotion && ss->inCheck;
1570 // Make and search the move
1571 pos.do_move(move, st, givesCheck);
1572 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1573 pos.undo_move(move);
1575 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1577 // Check for a new best move
1578 if (value > bestValue)
1586 if (PvNode) // Update pv even in fail-high case
1587 update_pv(ss->pv, move, (ss+1)->pv);
1589 if (PvNode && value < beta) // Update alpha here!
1597 // All legal moves have been searched. A special case: if we're in check
1598 // and no legal moves were found, it is checkmate.
1599 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1601 assert(!MoveList<LEGAL>(pos).size());
1603 return mated_in(ss->ply); // Plies to mate from the root
1606 // Save gathered info in transposition table
1607 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1608 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1609 ttDepth, bestMove, ss->staticEval);
1611 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1617 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1618 // "plies to mate from the current position". Standard scores are unchanged.
1619 // The function is called before storing a value in the transposition table.
1621 Value value_to_tt(Value v, int ply) {
1623 assert(v != VALUE_NONE);
1625 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1626 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1630 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1631 // from the transposition table (which refers to the plies to mate/be mated from
1632 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1633 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1634 // and the graph history interaction, we return an optimal TB score instead.
1636 Value value_from_tt(Value v, int ply, int r50c) {
1638 if (v == VALUE_NONE)
1641 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1643 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1644 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1649 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1651 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1652 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1661 // update_pv() adds current move and appends child pv[]
1663 void update_pv(Move* pv, Move move, Move* childPv) {
1665 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1671 // update_all_stats() updates stats at the end of search() when a bestMove is found
1673 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1674 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1677 Color us = pos.side_to_move();
1678 Thread* thisThread = pos.this_thread();
1679 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1680 Piece moved_piece = pos.moved_piece(bestMove);
1681 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1683 bonus1 = stat_bonus(depth + 1);
1684 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1685 : stat_bonus(depth); // smaller bonus
1687 if (!pos.capture_or_promotion(bestMove))
1689 // Increase stats for the best move in case it was a quiet move
1690 update_quiet_stats(pos, ss, bestMove, bonus2);
1692 // Decrease stats for all non-best quiet moves
1693 for (int i = 0; i < quietCount; ++i)
1695 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1696 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1700 // Increase stats for the best move in case it was a capture move
1701 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1703 // Extra penalty for a quiet early move that was not a TT move or
1704 // main killer move in previous ply when it gets refuted.
1705 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1706 && !pos.captured_piece())
1707 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1709 // Decrease stats for all non-best capture moves
1710 for (int i = 0; i < captureCount; ++i)
1712 moved_piece = pos.moved_piece(capturesSearched[i]);
1713 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1714 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1719 // update_continuation_histories() updates histories of the move pairs formed
1720 // by moves at ply -1, -2, -4, and -6 with current move.
1722 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1724 for (int i : {1, 2, 4, 6})
1726 // Only update first 2 continuation histories if we are in check
1727 if (ss->inCheck && i > 2)
1729 if (is_ok((ss-i)->currentMove))
1730 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1735 // update_quiet_stats() updates move sorting heuristics
1737 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1740 if (ss->killers[0] != move)
1742 ss->killers[1] = ss->killers[0];
1743 ss->killers[0] = move;
1746 Color us = pos.side_to_move();
1747 Thread* thisThread = pos.this_thread();
1748 thisThread->mainHistory[us][from_to(move)] << bonus;
1749 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1751 // Update countermove history
1752 if (is_ok((ss-1)->currentMove))
1754 Square prevSq = to_sq((ss-1)->currentMove);
1755 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1759 // When playing with strength handicap, choose best move among a set of RootMoves
1760 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1762 Move Skill::pick_best(size_t multiPV) {
1764 const RootMoves& rootMoves = Threads.main()->rootMoves;
1765 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1767 // RootMoves are already sorted by score in descending order
1768 Value topScore = rootMoves[0].score;
1769 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1770 int maxScore = -VALUE_INFINITE;
1771 double weakness = 120 - 2 * level;
1773 // Choose best move. For each move score we add two terms, both dependent on
1774 // weakness. One is deterministic and bigger for weaker levels, and one is
1775 // random. Then we choose the move with the resulting highest score.
1776 for (size_t i = 0; i < multiPV; ++i)
1778 // This is our magic formula
1779 int push = int(( weakness * int(topScore - rootMoves[i].score)
1780 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1782 if (rootMoves[i].score + push >= maxScore)
1784 maxScore = rootMoves[i].score + push;
1785 best = rootMoves[i].pv[0];
1795 /// MainThread::check_time() is used to print debug info and, more importantly,
1796 /// to detect when we are out of available time and thus stop the search.
1798 void MainThread::check_time() {
1803 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1804 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1806 static TimePoint lastInfoTime = now();
1808 TimePoint elapsed = Time.elapsed();
1809 TimePoint tick = Limits.startTime + elapsed;
1811 if (tick - lastInfoTime >= 1000)
1813 lastInfoTime = tick;
1817 // We should not stop pondering until told so by the GUI
1821 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1822 || (Limits.movetime && elapsed >= Limits.movetime)
1823 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1824 Threads.stop = true;
1828 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1829 /// that all (if any) unsearched PV lines are sent using a previous search score.
1831 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1833 std::stringstream ss;
1834 TimePoint elapsed = Time.elapsed() + 1;
1835 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1836 size_t pvIdx = pos.this_thread()->pvIdx;
1837 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1838 uint64_t nodesSearched = Threads.nodes_searched();
1839 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1841 for (size_t i = 0; i < multiPV; ++i)
1843 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1845 if (depth == 1 && !updated && i > 0)
1848 Depth d = updated ? depth : std::max(1, depth - 1);
1849 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1851 if (v == -VALUE_INFINITE)
1854 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1855 v = tb ? rootMoves[i].tbScore : v;
1857 if (ss.rdbuf()->in_avail()) // Not at first line
1862 << " seldepth " << rootMoves[i].selDepth
1863 << " multipv " << i + 1
1864 << " score " << UCI::value(v);
1866 if (Options["UCI_ShowWDL"])
1867 ss << UCI::wdl(v, pos.game_ply());
1869 if (!tb && i == pvIdx)
1870 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1872 ss << " nodes " << nodesSearched
1873 << " nps " << nodesSearched * 1000 / elapsed;
1875 if (elapsed > 1000) // Earlier makes little sense
1876 ss << " hashfull " << TT.hashfull();
1878 ss << " tbhits " << tbHits
1879 << " time " << elapsed
1882 for (Move m : rootMoves[i].pv)
1883 ss << " " << UCI::move(m, pos.is_chess960());
1890 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1891 /// before exiting the search, for instance, in case we stop the search during a
1892 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1893 /// otherwise in case of 'ponder on' we have nothing to think on.
1895 bool RootMove::extract_ponder_from_tt(Position& pos) {
1898 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1902 assert(pv.size() == 1);
1904 if (pv[0] == MOVE_NONE)
1907 pos.do_move(pv[0], st);
1908 TTEntry* tte = TT.probe(pos.key(), ttHit);
1912 Move m = tte->move(); // Local copy to be SMP safe
1913 if (MoveList<LEGAL>(pos).contains(m))
1917 pos.undo_move(pv[0]);
1918 return pv.size() > 1;
1921 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1924 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1925 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1926 Cardinality = int(Options["SyzygyProbeLimit"]);
1927 bool dtz_available = true;
1929 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1930 // ProbeDepth == DEPTH_ZERO
1931 if (Cardinality > MaxCardinality)
1933 Cardinality = MaxCardinality;
1937 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1939 // Rank moves using DTZ tables
1940 RootInTB = root_probe(pos, rootMoves);
1944 // DTZ tables are missing; try to rank moves using WDL tables
1945 dtz_available = false;
1946 RootInTB = root_probe_wdl(pos, rootMoves);
1952 // Sort moves according to TB rank
1953 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1954 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1956 // Probe during search only if DTZ is not available and we are winning
1957 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1962 // Clean up if root_probe() and root_probe_wdl() have failed
1963 for (auto& m : rootMoves)
1968 } // namespace Stockfish