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 improving ? (3 + depth * depth)
79 : (3 + depth * depth) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth d) {
84 return std::min((8 * d + 240) * d - 276 , 1907);
87 // Add a small random component to draw evaluations to avoid 3-fold blindness
88 Value value_draw(const Thread* thisThread) {
89 return VALUE_DRAW - 1 + Value(thisThread->nodes & 0x2);
92 // Skill structure is used to implement strength limit. If we have an uci_elo then
93 // we convert it to a suitable fractional skill level using anchoring to CCRL Elo
94 // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
95 // results spanning a wide range of k values.
97 Skill(int skill_level, int uci_elo) {
99 level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0);
101 level = double(skill_level);
103 bool enabled() const { return level < 20.0; }
104 bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
105 Move pick_best(size_t multiPV);
108 Move best = MOVE_NONE;
111 template <NodeType nodeType>
112 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
114 template <NodeType nodeType>
115 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
117 Value value_to_tt(Value v, int ply);
118 Value value_from_tt(Value v, int ply, int r50c);
119 void update_pv(Move* pv, Move move, const Move* childPv);
120 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
121 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
122 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
123 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
125 // perft() is our utility to verify move generation. All the leaf nodes up
126 // to the given depth are generated and counted, and the sum is returned.
128 uint64_t perft(Position& pos, Depth depth) {
131 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
133 uint64_t cnt, nodes = 0;
134 const bool leaf = (depth == 2);
136 for (const auto& m : MoveList<LEGAL>(pos))
138 if (Root && depth <= 1)
143 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
148 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
156 /// Search::init() is called at startup to initialize various lookup tables
158 void Search::init() {
160 for (int i = 1; i < MAX_MOVES; ++i)
161 Reductions[i] = int((20.81 + std::log(Threads.size()) / 2) * std::log(i));
165 /// Search::clear() resets search state to its initial value
167 void Search::clear() {
169 Threads.main()->wait_for_search_finished();
171 Time.availableNodes = 0;
174 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
178 /// MainThread::search() is started when the program receives the UCI 'go'
179 /// command. It searches from the root position and outputs the "bestmove".
181 void MainThread::search() {
185 nodes = perft<true>(rootPos, Limits.perft);
186 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
190 Color us = rootPos.side_to_move();
191 Time.init(Limits, us, rootPos.game_ply());
194 Eval::NNUE::verify();
196 if (rootMoves.empty())
198 rootMoves.emplace_back(MOVE_NONE);
199 sync_cout << "info depth 0 score "
200 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
205 Threads.start_searching(); // start non-main threads
206 Thread::search(); // main thread start searching
209 // When we reach the maximum depth, we can arrive here without a raise of
210 // Threads.stop. However, if we are pondering or in an infinite search,
211 // the UCI protocol states that we shouldn't print the best move before the
212 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
213 // until the GUI sends one of those commands.
215 while (!Threads.stop && (ponder || Limits.infinite))
216 {} // Busy wait for a stop or a ponder reset
218 // Stop the threads if not already stopped (also raise the stop if
219 // "ponderhit" just reset Threads.ponder).
222 // Wait until all threads have finished
223 Threads.wait_for_search_finished();
225 // When playing in 'nodes as time' mode, subtract the searched nodes from
226 // the available ones before exiting.
228 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
230 Thread* bestThread = this;
231 Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
233 if ( int(Options["MultiPV"]) == 1
236 && rootMoves[0].pv[0] != MOVE_NONE)
237 bestThread = Threads.get_best_thread();
239 bestPreviousScore = bestThread->rootMoves[0].score;
240 bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
242 for (Thread* th : Threads)
243 th->previousDepth = bestThread->completedDepth;
245 // Send again PV info if we have a new best thread
246 if (bestThread != this)
247 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
249 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
251 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
252 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
254 std::cout << sync_endl;
258 /// Thread::search() is the main iterative deepening loop. It calls search()
259 /// repeatedly with increasing depth until the allocated thinking time has been
260 /// consumed, the user stops the search, or the maximum search depth is reached.
262 void Thread::search() {
264 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
265 // The former is needed to allow update_continuation_histories(ss-1, ...),
266 // which accesses its argument at ss-6, also near the root.
267 // The latter is needed for statScore and killer initialization.
268 Stack stack[MAX_PLY+10], *ss = stack+7;
270 Value alpha, beta, delta;
271 Move lastBestMove = MOVE_NONE;
272 Depth lastBestMoveDepth = 0;
273 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
274 double timeReduction = 1, totBestMoveChanges = 0;
275 Color us = rootPos.side_to_move();
278 std::memset(ss-7, 0, 10 * sizeof(Stack));
279 for (int i = 7; i > 0; i--)
280 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
282 for (int i = 0; i <= MAX_PLY + 2; ++i)
287 bestValue = delta = alpha = -VALUE_INFINITE;
288 beta = VALUE_INFINITE;
292 if (mainThread->bestPreviousScore == VALUE_INFINITE)
293 for (int i = 0; i < 4; ++i)
294 mainThread->iterValue[i] = VALUE_ZERO;
296 for (int i = 0; i < 4; ++i)
297 mainThread->iterValue[i] = mainThread->bestPreviousScore;
300 size_t multiPV = size_t(Options["MultiPV"]);
301 Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
303 // When playing with strength handicap enable MultiPV search that we will
304 // use behind the scenes to retrieve a set of possible moves.
306 multiPV = std::max(multiPV, (size_t)4);
308 multiPV = std::min(multiPV, rootMoves.size());
310 complexityAverage.set(174, 1);
313 optimism[ us] = Value(39);
314 optimism[~us] = -optimism[us];
316 int searchAgainCounter = 0;
318 // Iterative deepening loop until requested to stop or the target depth is reached
319 while ( ++rootDepth < MAX_PLY
321 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
323 // Age out PV variability metric
325 totBestMoveChanges /= 2;
327 // Save the last iteration's scores before first PV line is searched and
328 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
329 for (RootMove& rm : rootMoves)
330 rm.previousScore = rm.score;
335 if (!Threads.increaseDepth)
336 searchAgainCounter++;
338 // MultiPV loop. We perform a full root search for each PV line
339 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
344 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
345 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
349 // Reset UCI info selDepth for each depth and each PV line
352 // Reset aspiration window starting size
355 Value prev = rootMoves[pvIdx].averageScore;
356 delta = Value(16) + int(prev) * prev / 19178;
357 alpha = std::max(prev - delta,-VALUE_INFINITE);
358 beta = std::min(prev + delta, VALUE_INFINITE);
360 // Adjust trend and optimism based on root move's previousScore
361 int tr = sigmoid(prev, 3, 8, 90, 125, 1);
362 trend = (us == WHITE ? make_score(tr, tr / 2)
363 : -make_score(tr, tr / 2));
365 int opt = sigmoid(prev, 8, 17, 144, 13966, 183);
366 optimism[ us] = Value(opt);
367 optimism[~us] = -optimism[us];
370 // Start with a small aspiration window and, in the case of a fail
371 // high/low, re-search with a bigger window until we don't fail
373 int failedHighCnt = 0;
376 // Adjust the effective depth searched, but ensuring at least one effective increment for every
377 // four searchAgain steps (see issue #2717).
378 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);
379 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
381 // Bring the best move to the front. It is critical that sorting
382 // is done with a stable algorithm because all the values but the
383 // first and eventually the new best one are set to -VALUE_INFINITE
384 // and we want to keep the same order for all the moves except the
385 // new PV that goes to the front. Note that in case of MultiPV
386 // search the already searched PV lines are preserved.
387 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
389 // If search has been stopped, we break immediately. Sorting is
390 // safe because RootMoves is still valid, although it refers to
391 // the previous iteration.
395 // When failing high/low give some update (without cluttering
396 // the UI) before a re-search.
399 && (bestValue <= alpha || bestValue >= beta)
400 && Time.elapsed() > 3000)
401 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
403 // In case of failing low/high increase aspiration window and
404 // re-search, otherwise exit the loop.
405 if (bestValue <= alpha)
407 beta = (alpha + beta) / 2;
408 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
412 mainThread->stopOnPonderhit = false;
414 else if (bestValue >= beta)
416 beta = std::min(bestValue + delta, VALUE_INFINITE);
422 delta += delta / 4 + 2;
424 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
427 // Sort the PV lines searched so far and update the GUI
428 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
431 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
432 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
436 completedDepth = rootDepth;
438 if (rootMoves[0].pv[0] != lastBestMove) {
439 lastBestMove = rootMoves[0].pv[0];
440 lastBestMoveDepth = rootDepth;
443 // Have we found a "mate in x"?
445 && bestValue >= VALUE_MATE_IN_MAX_PLY
446 && VALUE_MATE - bestValue <= 2 * Limits.mate)
452 // If skill level is enabled and time is up, pick a sub-optimal best move
453 if (skill.enabled() && skill.time_to_pick(rootDepth))
454 skill.pick_best(multiPV);
456 // Use part of the gained time from a previous stable move for the current move
457 for (Thread* th : Threads)
459 totBestMoveChanges += th->bestMoveChanges;
460 th->bestMoveChanges = 0;
463 // Do we have time for the next iteration? Can we stop searching now?
464 if ( Limits.use_time_management()
466 && !mainThread->stopOnPonderhit)
468 double fallingEval = (69 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
469 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 781.4;
470 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
472 // If the bestMove is stable over several iterations, reduce time accordingly
473 timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.63 : 0.73;
474 double reduction = (1.56 + mainThread->previousTimeReduction) / (2.20 * timeReduction);
475 double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();
476 int complexity = mainThread->complexityAverage.value();
477 double complexPosition = std::min(1.0 + (complexity - 277) / 1819.1, 1.5);
479 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
481 // Cap used time in case of a single legal move for a better viewer experience in tournaments
482 // yielding correct scores and sufficiently fast moves.
483 if (rootMoves.size() == 1)
484 totalTime = std::min(500.0, totalTime);
486 // Stop the search if we have exceeded the totalTime
487 if (Time.elapsed() > totalTime)
489 // If we are allowed to ponder do not stop the search now but
490 // keep pondering until the GUI sends "ponderhit" or "stop".
491 if (mainThread->ponder)
492 mainThread->stopOnPonderhit = true;
496 else if ( Threads.increaseDepth
497 && !mainThread->ponder
498 && Time.elapsed() > totalTime * 0.43)
499 Threads.increaseDepth = false;
501 Threads.increaseDepth = true;
504 mainThread->iterValue[iterIdx] = bestValue;
505 iterIdx = (iterIdx + 1) & 3;
511 mainThread->previousTimeReduction = timeReduction;
513 // If skill level is enabled, swap best PV line with the sub-optimal one
515 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
516 skill.best ? skill.best : skill.pick_best(multiPV)));
522 // search<>() is the main search function for both PV and non-PV nodes
524 template <NodeType nodeType>
525 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
527 constexpr bool PvNode = nodeType != NonPV;
528 constexpr bool rootNode = nodeType == Root;
529 const Depth maxNextDepth = rootNode ? depth : depth + 1;
531 // Check if we have an upcoming move which draws by repetition, or
532 // if the opponent had an alternative move earlier to this position.
534 && pos.rule50_count() >= 3
535 && alpha < VALUE_DRAW
536 && pos.has_game_cycle(ss->ply))
538 alpha = value_draw(pos.this_thread());
543 // Dive into quiescence search when the depth reaches zero
545 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
547 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
548 assert(PvNode || (alpha == beta - 1));
549 assert(0 < depth && depth < MAX_PLY);
550 assert(!(PvNode && cutNode));
552 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
554 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
558 Move ttMove, move, excludedMove, bestMove;
559 Depth extension, newDepth;
560 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
561 bool givesCheck, improving, didLMR, priorCapture, singularQuietLMR;
562 bool capture, doFullDepthSearch, moveCountPruning, ttCapture;
564 int moveCount, captureCount, quietCount, improvement, complexity;
566 // Step 1. Initialize node
567 Thread* thisThread = pos.this_thread();
568 thisThread->depth = depth;
569 ss->inCheck = pos.checkers();
570 priorCapture = pos.captured_piece();
571 Color us = pos.side_to_move();
572 moveCount = captureCount = quietCount = ss->moveCount = 0;
573 bestValue = -VALUE_INFINITE;
574 maxValue = VALUE_INFINITE;
576 // Check for the available remaining time
577 if (thisThread == Threads.main())
578 static_cast<MainThread*>(thisThread)->check_time();
580 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
581 if (PvNode && thisThread->selDepth < ss->ply + 1)
582 thisThread->selDepth = ss->ply + 1;
586 // Step 2. Check for aborted search and immediate draw
587 if ( Threads.stop.load(std::memory_order_relaxed)
588 || pos.is_draw(ss->ply)
589 || ss->ply >= MAX_PLY)
590 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
591 : value_draw(pos.this_thread());
593 // Step 3. Mate distance pruning. Even if we mate at the next move our score
594 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
595 // a shorter mate was found upward in the tree then there is no need to search
596 // because we will never beat the current alpha. Same logic but with reversed
597 // signs applies also in the opposite condition of being mated instead of giving
598 // mate. In this case return a fail-high score.
599 alpha = std::max(mated_in(ss->ply), alpha);
600 beta = std::min(mate_in(ss->ply+1), beta);
605 thisThread->rootDelta = beta - alpha;
607 assert(0 <= ss->ply && ss->ply < MAX_PLY);
609 (ss+1)->ttPv = false;
610 (ss+1)->excludedMove = bestMove = MOVE_NONE;
611 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
612 (ss+2)->cutoffCnt = 0;
613 ss->doubleExtensions = (ss-1)->doubleExtensions;
614 Square prevSq = to_sq((ss-1)->currentMove);
616 // Initialize statScore to zero for the grandchildren of the current position.
617 // So statScore is shared between all grandchildren and only the first grandchild
618 // starts with statScore = 0. Later grandchildren start with the last calculated
619 // statScore of the previous grandchild. This influences the reduction rules in
620 // LMR which are based on the statScore of parent position.
622 (ss+2)->statScore = 0;
624 // Step 4. Transposition table lookup. We don't want the score of a partial
625 // search to overwrite a previous full search TT value, so we use a different
626 // position key in case of an excluded move.
627 excludedMove = ss->excludedMove;
628 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
629 tte = TT.probe(posKey, ss->ttHit);
630 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
631 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
632 : ss->ttHit ? tte->move() : MOVE_NONE;
633 ttCapture = ttMove && pos.capture(ttMove);
635 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
637 // At non-PV nodes we check for an early TT cutoff
640 && tte->depth() > depth - ((int)thisThread->id() & 0x1) - (tte->bound() == BOUND_EXACT)
641 && ttValue != VALUE_NONE // Possible in case of TT access race
642 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
644 // If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)
649 // Bonus for a quiet ttMove that fails high (~3 Elo)
651 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
653 // Extra penalty for early quiet moves of the previous ply (~0 Elo)
654 if ((ss-1)->moveCount <= 2 && !priorCapture)
655 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
657 // Penalty for a quiet ttMove that fails low (~1 Elo)
660 int penalty = -stat_bonus(depth);
661 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
662 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
666 // Partial workaround for the graph history interaction problem
667 // For high rule50 counts don't produce transposition table cutoffs.
668 if (pos.rule50_count() < 90)
672 // Step 5. Tablebases probe
673 if (!rootNode && TB::Cardinality)
675 int piecesCount = pos.count<ALL_PIECES>();
677 if ( piecesCount <= TB::Cardinality
678 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
679 && pos.rule50_count() == 0
680 && !pos.can_castle(ANY_CASTLING))
683 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
685 // Force check of time on the next occasion
686 if (thisThread == Threads.main())
687 static_cast<MainThread*>(thisThread)->callsCnt = 0;
689 if (err != TB::ProbeState::FAIL)
691 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
693 int drawScore = TB::UseRule50 ? 1 : 0;
695 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
696 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
697 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
698 : VALUE_DRAW + 2 * wdl * drawScore;
700 Bound b = wdl < -drawScore ? BOUND_UPPER
701 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
703 if ( b == BOUND_EXACT
704 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
706 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
707 std::min(MAX_PLY - 1, depth + 6),
708 MOVE_NONE, VALUE_NONE);
715 if (b == BOUND_LOWER)
716 bestValue = value, alpha = std::max(alpha, bestValue);
724 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
726 // Step 6. Static evaluation of the position
729 // Skip early pruning when in check
730 ss->staticEval = eval = VALUE_NONE;
738 // Never assume anything about values stored in TT
739 ss->staticEval = eval = tte->eval();
740 if (eval == VALUE_NONE)
741 ss->staticEval = eval = evaluate(pos, &complexity);
742 else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost
743 complexity = abs(ss->staticEval - pos.psq_eg_stm());
745 // Randomize draw evaluation
746 if (eval == VALUE_DRAW)
747 eval = value_draw(thisThread);
749 // ttValue can be used as a better position evaluation (~4 Elo)
750 if ( ttValue != VALUE_NONE
751 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
756 ss->staticEval = eval = evaluate(pos, &complexity);
758 // Save static evaluation into transposition table
760 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
763 thisThread->complexityAverage.update(complexity);
765 // Use static evaluation difference to improve quiet move ordering (~3 Elo)
766 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
768 int bonus = std::clamp(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000);
769 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
772 // Set up the improvement variable, which is the difference between the current
773 // static evaluation and the previous static evaluation at our turn (if we were
774 // in check at our previous move we look at the move prior to it). The improvement
775 // margin and the improving flag are used in various pruning heuristics.
776 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
777 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
779 improving = improvement > 0;
782 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
783 // return a fail low.
786 && eval < alpha - 348 - 258 * depth * depth)
788 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
793 // Step 8. Futility pruning: child node (~25 Elo).
794 // The depth condition is important for mate finding.
797 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta
799 && eval < 26305) // larger than VALUE_KNOWN_WIN, but smaller than TB wins.
802 // Step 9. Null move search with verification search (~22 Elo)
804 && (ss-1)->currentMove != MOVE_NULL
805 && (ss-1)->statScore < 14695
807 && eval >= ss->staticEval
808 && ss->staticEval >= beta - 15 * depth - improvement / 15 + 201 + complexity / 24
810 && pos.non_pawn_material(us)
811 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
813 assert(eval - beta >= 0);
815 // Null move dynamic reduction based on depth, eval and complexity of position
816 Depth R = std::min(int(eval - beta) / 147, 5) + depth / 3 + 4 - (complexity > 650);
818 ss->currentMove = MOVE_NULL;
819 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
821 pos.do_null_move(st);
823 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
825 pos.undo_null_move();
827 if (nullValue >= beta)
829 // Do not return unproven mate or TB scores
830 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
833 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
836 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
838 // Do verification search at high depths, with null move pruning disabled
839 // for us, until ply exceeds nmpMinPly.
840 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
841 thisThread->nmpColor = us;
843 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
845 thisThread->nmpMinPly = 0;
852 probCutBeta = beta + 179 - 46 * improving;
854 // Step 10. ProbCut (~4 Elo)
855 // If we have a good enough capture and a reduced search returns a value
856 // much above beta, we can (almost) safely prune the previous move.
859 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
860 // if value from transposition table is lower than probCutBeta, don't attempt probCut
861 // there and in further interactions with transposition table cutoff depth is set to depth - 3
862 // because probCut search has depth set to depth - 4 but we also do a move before it
863 // so effective depth is equal to depth - 3
865 && tte->depth() >= depth - 3
866 && ttValue != VALUE_NONE
867 && ttValue < probCutBeta))
869 assert(probCutBeta < VALUE_INFINITE);
871 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);
873 while ((move = mp.next_move()) != MOVE_NONE)
874 if (move != excludedMove && pos.legal(move))
876 assert(pos.capture(move) || promotion_type(move) == QUEEN);
878 ss->currentMove = move;
879 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
881 [pos.moved_piece(move)]
884 pos.do_move(move, st);
886 // Perform a preliminary qsearch to verify that the move holds
887 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
889 // If the qsearch held, perform the regular search
890 if (value >= probCutBeta)
891 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
895 if (value >= probCutBeta)
897 // Save ProbCut data into transposition table
898 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3, move, ss->staticEval);
904 // Step 11. If the position is not in TT, decrease depth by 3.
905 // Use qsearch if depth is equal or below zero (~4 Elo)
911 return qsearch<PV>(pos, ss, alpha, beta);
918 moves_loop: // When in check, search starts here
920 // Step 12. A small Probcut idea, when we are in check (~0 Elo)
921 probCutBeta = beta + 481;
926 && (tte->bound() & BOUND_LOWER)
927 && tte->depth() >= depth - 3
928 && ttValue >= probCutBeta
929 && abs(ttValue) <= VALUE_KNOWN_WIN
930 && abs(beta) <= VALUE_KNOWN_WIN
935 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
936 nullptr , (ss-4)->continuationHistory,
937 nullptr , (ss-6)->continuationHistory };
939 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
941 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
948 moveCountPruning = singularQuietLMR = false;
950 // Indicate PvNodes that will probably fail low if the node was searched
951 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
952 bool likelyFailLow = PvNode
954 && (tte->bound() & BOUND_UPPER)
955 && tte->depth() >= depth;
957 // Step 13. Loop through all pseudo-legal moves until no moves remain
958 // or a beta cutoff occurs.
959 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
963 if (move == excludedMove)
966 // At root obey the "searchmoves" option and skip moves not listed in Root
967 // Move List. As a consequence any illegal move is also skipped. In MultiPV
968 // mode we also skip PV moves which have been already searched and those
969 // of lower "TB rank" if we are in a TB root position.
970 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
971 thisThread->rootMoves.begin() + thisThread->pvLast, move))
974 // Check for legality
975 if (!rootNode && !pos.legal(move))
978 ss->moveCount = ++moveCount;
980 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
981 sync_cout << "info depth " << depth
982 << " currmove " << UCI::move(move, pos.is_chess960())
983 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
985 (ss+1)->pv = nullptr;
988 capture = pos.capture(move);
989 movedPiece = pos.moved_piece(move);
990 givesCheck = pos.gives_check(move);
992 // Calculate new depth for this move
993 newDepth = depth - 1;
995 Value delta = beta - alpha;
997 // Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.
999 && pos.non_pawn_material(us)
1000 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1002 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo)
1003 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1005 // Reduced depth of the next LMR search
1006 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0);
1011 // Futility pruning for captures (~0 Elo)
1012 if ( !pos.empty(to_sq(move))
1017 && ss->staticEval + 281 + 179 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1018 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1021 // SEE based pruning (~9 Elo)
1022 if (!pos.see_ge(move, Value(-203) * depth))
1027 int history = (*contHist[0])[movedPiece][to_sq(move)]
1028 + (*contHist[1])[movedPiece][to_sq(move)]
1029 + (*contHist[3])[movedPiece][to_sq(move)];
1031 // Continuation history based pruning (~2 Elo)
1033 && history < -3875 * (depth - 1))
1036 history += 2 * thisThread->mainHistory[us][from_to(move)];
1038 // Futility pruning: parent node (~9 Elo)
1041 && ss->staticEval + 122 + 138 * lmrDepth + history / 60 <= alpha)
1044 // Prune moves with negative SEE (~3 Elo)
1045 if (!pos.see_ge(move, Value(-25 * lmrDepth * lmrDepth - 20 * lmrDepth)))
1050 // Step 15. Extensions (~66 Elo)
1051 // We take care to not overdo to avoid search getting stuck.
1052 if (ss->ply < thisThread->rootDepth * 2)
1054 // Singular extension search (~58 Elo). If all moves but one fail low on a
1055 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1056 // then that move is singular and should be extended. To verify this we do
1057 // a reduced search on all the other moves but the ttMove and if the
1058 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1060 && depth >= 4 - (thisThread->previousDepth > 27) + 2 * (PvNode && tte->is_pv())
1062 && !excludedMove // Avoid recursive singular search
1063 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1064 && abs(ttValue) < VALUE_KNOWN_WIN
1065 && (tte->bound() & BOUND_LOWER)
1066 && tte->depth() >= depth - 3)
1068 Value singularBeta = ttValue - 3 * depth;
1069 Depth singularDepth = (depth - 1) / 2;
1071 ss->excludedMove = move;
1072 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1073 ss->excludedMove = MOVE_NONE;
1075 if (value < singularBeta)
1078 singularQuietLMR = !ttCapture;
1080 // Avoid search explosion by limiting the number of double extensions
1082 && value < singularBeta - 26
1083 && ss->doubleExtensions <= 8)
1087 // Multi-cut pruning
1088 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1089 // search without the ttMove. So we assume this expected Cut-node is not singular,
1090 // that multiple moves fail high, and we can prune the whole subtree by returning
1092 else if (singularBeta >= beta)
1093 return singularBeta;
1095 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1096 else if (ttValue >= beta)
1099 // If the eval of ttMove is less than alpha and value, we reduce it (negative extension)
1100 else if (ttValue <= alpha && ttValue <= value)
1104 // Check extensions (~1 Elo)
1105 else if ( givesCheck
1107 && abs(ss->staticEval) > 71)
1110 // Quiet ttMove extensions (~0 Elo)
1113 && move == ss->killers[0]
1114 && (*contHist[0])[movedPiece][to_sq(move)] >= 5491)
1118 // Add extension to new depth
1119 newDepth += extension;
1120 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1122 // Speculative prefetch as early as possible
1123 prefetch(TT.first_entry(pos.key_after(move)));
1125 // Update the current move (this must be done after singular extension search)
1126 ss->currentMove = move;
1127 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1132 // Step 16. Make the move
1133 pos.do_move(move, st, givesCheck);
1135 bool doDeeperSearch = false;
1137 // Step 17. Late moves reduction / extension (LMR, ~98 Elo)
1138 // We use various heuristics for the sons of a node after the first son has
1139 // been searched. In general we would like to reduce them, but there are many
1140 // cases where we extend a son if it has good chances to be "interesting".
1142 && moveCount > 1 + (PvNode && ss->ply <= 1)
1145 || (cutNode && (ss-1)->moveCount > 1)))
1147 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
1149 // Decrease reduction if position is or has been on the PV
1150 // and node is not likely to fail low. (~3 Elo)
1155 // Decrease reduction if opponent's move count is high (~1 Elo)
1156 if ((ss-1)->moveCount > 7)
1159 // Increase reduction for cut nodes (~3 Elo)
1163 // Increase reduction if ttMove is a capture (~3 Elo)
1167 // Decrease reduction for PvNodes based on depth
1169 r -= 1 + 15 / (3 + depth);
1171 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1172 if (singularQuietLMR)
1175 // Increase reduction if next ply has a lot of fail high else reset count to 0
1176 if ((ss+1)->cutoffCnt > 3 && !PvNode)
1179 ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
1180 + (*contHist[0])[movedPiece][to_sq(move)]
1181 + (*contHist[1])[movedPiece][to_sq(move)]
1182 + (*contHist[3])[movedPiece][to_sq(move)]
1185 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1186 r -= ss->statScore / 15914;
1188 // In general we want to cap the LMR depth search at newDepth, but when
1189 // reduction is negative, we allow this move a limited search extension
1190 // beyond the first move depth. This may lead to hidden double extensions.
1191 Depth d = std::clamp(newDepth - r, 1, newDepth + 1);
1193 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1195 // If the son is reduced and fails high it will be re-searched at full depth
1196 doFullDepthSearch = value > alpha && d < newDepth;
1197 doDeeperSearch = value > (alpha + 78 + 11 * (newDepth - d));
1202 doFullDepthSearch = !PvNode || moveCount > 1;
1206 // Step 18. Full depth search when LMR is skipped or fails high
1207 if (doFullDepthSearch)
1209 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode);
1211 // If the move passed LMR update its stats
1214 int bonus = value > alpha ? stat_bonus(newDepth)
1215 : -stat_bonus(newDepth);
1220 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1224 // For PV nodes only, do a full PV search on the first move or after a fail
1225 // high (in the latter case search only if value < beta), otherwise let the
1226 // parent node fail low with value <= alpha and try another move.
1227 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1230 (ss+1)->pv[0] = MOVE_NONE;
1232 value = -search<PV>(pos, ss+1, -beta, -alpha,
1233 std::min(maxNextDepth, newDepth), false);
1236 // Step 19. Undo move
1237 pos.undo_move(move);
1239 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1241 // Step 20. Check for a new best move
1242 // Finished searching the move. If a stop occurred, the return value of
1243 // the search cannot be trusted, and we return immediately without
1244 // updating best move, PV and TT.
1245 if (Threads.stop.load(std::memory_order_relaxed))
1250 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1251 thisThread->rootMoves.end(), move);
1253 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1255 // PV move or new best move?
1256 if (moveCount == 1 || value > alpha)
1259 rm.selDepth = thisThread->selDepth;
1264 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1265 rm.pv.push_back(*m);
1267 // We record how often the best move has been changed in each iteration.
1268 // This information is used for time management. In MultiPV mode,
1269 // we must take care to only do this for the first PV line.
1271 && !thisThread->pvIdx)
1272 ++thisThread->bestMoveChanges;
1275 // All other moves but the PV are set to the lowest value: this
1276 // is not a problem when sorting because the sort is stable and the
1277 // move position in the list is preserved - just the PV is pushed up.
1278 rm.score = -VALUE_INFINITE;
1281 if (value > bestValue)
1289 if (PvNode && !rootNode) // Update pv even in fail-high case
1290 update_pv(ss->pv, move, (ss+1)->pv);
1292 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1296 // Reduce other moves if we have found at least one score improvement
1299 && beta < VALUE_KNOWN_WIN
1300 && alpha > -VALUE_KNOWN_WIN)
1308 assert(value >= beta); // Fail high
1317 // If the move is worse than some previously searched move, remember it to update its stats later
1318 if (move != bestMove)
1320 if (capture && captureCount < 32)
1321 capturesSearched[captureCount++] = move;
1323 else if (!capture && quietCount < 64)
1324 quietsSearched[quietCount++] = move;
1328 // The following condition would detect a stop only after move loop has been
1329 // completed. But in this case bestValue is valid because we have fully
1330 // searched our subtree, and we can anyhow save the result in TT.
1336 // Step 21. Check for mate and stalemate
1337 // All legal moves have been searched and if there are no legal moves, it
1338 // must be a mate or a stalemate. If we are in a singular extension search then
1339 // return a fail low score.
1341 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1344 bestValue = excludedMove ? alpha :
1345 ss->inCheck ? mated_in(ss->ply)
1348 // If there is a move which produces search value greater than alpha we update stats of searched moves
1350 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1351 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1353 // Bonus for prior countermove that caused the fail low
1354 else if ( (depth >= 4 || PvNode)
1357 //Assign extra bonus if current node is PvNode or cutNode
1358 //or fail low was really bad
1359 bool extraBonus = PvNode
1361 || bestValue < alpha - 70 * depth;
1363 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + extraBonus));
1367 bestValue = std::min(bestValue, maxValue);
1369 // If no good move is found and the previous position was ttPv, then the previous
1370 // opponent move is probably good and the new position is added to the search tree.
1371 if (bestValue <= alpha)
1372 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1374 // Write gathered information in transposition table
1375 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1376 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1377 bestValue >= beta ? BOUND_LOWER :
1378 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1379 depth, bestMove, ss->staticEval);
1381 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1387 // qsearch() is the quiescence search function, which is called by the main search
1388 // function with zero depth, or recursively with further decreasing depth per call.
1390 template <NodeType nodeType>
1391 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1393 static_assert(nodeType != Root);
1394 constexpr bool PvNode = nodeType == PV;
1396 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1397 assert(PvNode || (alpha == beta - 1));
1402 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1406 Move ttMove, move, bestMove;
1408 Value bestValue, value, ttValue, futilityValue, futilityBase;
1409 bool pvHit, givesCheck, capture;
1415 ss->pv[0] = MOVE_NONE;
1418 Thread* thisThread = pos.this_thread();
1419 bestMove = MOVE_NONE;
1420 ss->inCheck = pos.checkers();
1423 // Check for an immediate draw or maximum ply reached
1424 if ( pos.is_draw(ss->ply)
1425 || ss->ply >= MAX_PLY)
1426 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1428 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1430 // Decide whether or not to include checks: this fixes also the type of
1431 // TT entry depth that we are going to use. Note that in qsearch we use
1432 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1433 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1434 : DEPTH_QS_NO_CHECKS;
1435 // Transposition table lookup
1437 tte = TT.probe(posKey, ss->ttHit);
1438 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1439 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1440 pvHit = ss->ttHit && tte->is_pv();
1444 && tte->depth() >= ttDepth
1445 && ttValue != VALUE_NONE // Only in case of TT access race
1446 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : 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 // ttValue can be used as a better position evaluation (~7 Elo)
1464 if ( ttValue != VALUE_NONE
1465 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1466 bestValue = ttValue;
1469 // In case of null move search use previous static eval with a different sign
1470 ss->staticEval = bestValue =
1471 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1472 : -(ss-1)->staticEval;
1474 // Stand pat. Return immediately if static value is at least beta
1475 if (bestValue >= beta)
1477 // Save gathered info in transposition table
1479 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1480 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1485 if (PvNode && bestValue > alpha)
1488 futilityBase = bestValue + 118;
1491 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1492 nullptr , (ss-4)->continuationHistory,
1493 nullptr , (ss-6)->continuationHistory };
1495 // Initialize a MovePicker object for the current position, and prepare
1496 // to search the moves. Because the depth is <= 0 here, only captures,
1497 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1498 // will be generated.
1499 Square prevSq = to_sq((ss-1)->currentMove);
1500 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1501 &thisThread->captureHistory,
1505 int quietCheckEvasions = 0;
1507 // Loop through the moves until no moves remain or a beta cutoff occurs
1508 while ((move = mp.next_move()) != MOVE_NONE)
1510 assert(is_ok(move));
1512 // Check for legality
1513 if (!pos.legal(move))
1516 givesCheck = pos.gives_check(move);
1517 capture = pos.capture(move);
1521 // Futility pruning and moveCount pruning (~5 Elo)
1522 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1524 && to_sq(move) != prevSq
1525 && futilityBase > -VALUE_KNOWN_WIN
1526 && type_of(move) != PROMOTION)
1532 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1534 if (futilityValue <= alpha)
1536 bestValue = std::max(bestValue, futilityValue);
1540 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1542 bestValue = std::max(bestValue, futilityBase);
1547 // Do not search moves with negative SEE values (~5 Elo)
1548 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1549 && !pos.see_ge(move))
1552 // Speculative prefetch as early as possible
1553 prefetch(TT.first_entry(pos.key_after(move)));
1555 ss->currentMove = move;
1556 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1558 [pos.moved_piece(move)]
1561 // Continuation history based pruning (~2 Elo)
1563 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1564 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1565 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1568 // movecount pruning for quiet check evasions
1569 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1570 && quietCheckEvasions > 1
1575 quietCheckEvasions += !capture && ss->inCheck;
1577 // Make and search the move
1578 pos.do_move(move, st, givesCheck);
1579 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1580 pos.undo_move(move);
1582 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1584 // Check for a new best move
1585 if (value > bestValue)
1593 if (PvNode) // Update pv even in fail-high case
1594 update_pv(ss->pv, move, (ss+1)->pv);
1596 if (PvNode && value < beta) // Update alpha here!
1604 // All legal moves have been searched. A special case: if we're in check
1605 // and no legal moves were found, it is checkmate.
1606 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1608 assert(!MoveList<LEGAL>(pos).size());
1610 return mated_in(ss->ply); // Plies to mate from the root
1613 // Save gathered info in transposition table
1614 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1615 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1616 ttDepth, bestMove, ss->staticEval);
1618 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1624 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1625 // "plies to mate from the current position". Standard scores are unchanged.
1626 // The function is called before storing a value in the transposition table.
1628 Value value_to_tt(Value v, int ply) {
1630 assert(v != VALUE_NONE);
1632 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1633 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1637 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1638 // from the transposition table (which refers to the plies to mate/be mated from
1639 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1640 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1641 // and the graph history interaction, we return an optimal TB score instead.
1643 Value value_from_tt(Value v, int ply, int r50c) {
1645 if (v == VALUE_NONE)
1648 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1650 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1651 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1656 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1658 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1659 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1668 // update_pv() adds current move and appends child pv[]
1670 void update_pv(Move* pv, Move move, const Move* childPv) {
1672 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1678 // update_all_stats() updates stats at the end of search() when a bestMove is found
1680 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1681 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1683 Color us = pos.side_to_move();
1684 Thread* thisThread = pos.this_thread();
1685 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1686 Piece moved_piece = pos.moved_piece(bestMove);
1687 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1688 int bonus1 = stat_bonus(depth + 1);
1690 if (!pos.capture(bestMove))
1692 int bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1693 : stat_bonus(depth); // smaller bonus
1695 // Increase stats for the best move in case it was a quiet move
1696 update_quiet_stats(pos, ss, bestMove, bonus2);
1698 // Decrease stats for all non-best quiet moves
1699 for (int i = 0; i < quietCount; ++i)
1701 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1702 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1706 // Increase stats for the best move in case it was a capture move
1707 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1709 // Extra penalty for a quiet early move that was not a TT move or
1710 // main killer move in previous ply when it gets refuted.
1711 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1712 && !pos.captured_piece())
1713 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1715 // Decrease stats for all non-best capture moves
1716 for (int i = 0; i < captureCount; ++i)
1718 moved_piece = pos.moved_piece(capturesSearched[i]);
1719 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1720 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1725 // update_continuation_histories() updates histories of the move pairs formed
1726 // by moves at ply -1, -2, -4, and -6 with current move.
1728 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1730 for (int i : {1, 2, 4, 6})
1732 // Only update first 2 continuation histories if we are in check
1733 if (ss->inCheck && i > 2)
1735 if (is_ok((ss-i)->currentMove))
1736 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1741 // update_quiet_stats() updates move sorting heuristics
1743 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1746 if (ss->killers[0] != move)
1748 ss->killers[1] = ss->killers[0];
1749 ss->killers[0] = move;
1752 Color us = pos.side_to_move();
1753 Thread* thisThread = pos.this_thread();
1754 thisThread->mainHistory[us][from_to(move)] << bonus;
1755 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1757 // Update countermove history
1758 if (is_ok((ss-1)->currentMove))
1760 Square prevSq = to_sq((ss-1)->currentMove);
1761 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1765 // When playing with strength handicap, choose best move among a set of RootMoves
1766 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1768 Move Skill::pick_best(size_t multiPV) {
1770 const RootMoves& rootMoves = Threads.main()->rootMoves;
1771 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1773 // RootMoves are already sorted by score in descending order
1774 Value topScore = rootMoves[0].score;
1775 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1776 int maxScore = -VALUE_INFINITE;
1777 double weakness = 120 - 2 * level;
1779 // Choose best move. For each move score we add two terms, both dependent on
1780 // weakness. One is deterministic and bigger for weaker levels, and one is
1781 // random. Then we choose the move with the resulting highest score.
1782 for (size_t i = 0; i < multiPV; ++i)
1784 // This is our magic formula
1785 int push = int(( weakness * int(topScore - rootMoves[i].score)
1786 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1788 if (rootMoves[i].score + push >= maxScore)
1790 maxScore = rootMoves[i].score + push;
1791 best = rootMoves[i].pv[0];
1801 /// MainThread::check_time() is used to print debug info and, more importantly,
1802 /// to detect when we are out of available time and thus stop the search.
1804 void MainThread::check_time() {
1809 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1810 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1812 static TimePoint lastInfoTime = now();
1814 TimePoint elapsed = Time.elapsed();
1815 TimePoint tick = Limits.startTime + elapsed;
1817 if (tick - lastInfoTime >= 1000)
1819 lastInfoTime = tick;
1823 // We should not stop pondering until told so by the GUI
1827 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1828 || (Limits.movetime && elapsed >= Limits.movetime)
1829 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1830 Threads.stop = true;
1834 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1835 /// that all (if any) unsearched PV lines are sent using a previous search score.
1837 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1839 std::stringstream ss;
1840 TimePoint elapsed = Time.elapsed() + 1;
1841 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1842 size_t pvIdx = pos.this_thread()->pvIdx;
1843 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1844 uint64_t nodesSearched = Threads.nodes_searched();
1845 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1847 for (size_t i = 0; i < multiPV; ++i)
1849 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1851 if (depth == 1 && !updated && i > 0)
1854 Depth d = updated ? depth : std::max(1, depth - 1);
1855 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1857 if (v == -VALUE_INFINITE)
1860 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1861 v = tb ? rootMoves[i].tbScore : v;
1863 if (ss.rdbuf()->in_avail()) // Not at first line
1868 << " seldepth " << rootMoves[i].selDepth
1869 << " multipv " << i + 1
1870 << " score " << UCI::value(v);
1872 if (Options["UCI_ShowWDL"])
1873 ss << UCI::wdl(v, pos.game_ply());
1875 if (!tb && i == pvIdx)
1876 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1878 ss << " nodes " << nodesSearched
1879 << " nps " << nodesSearched * 1000 / elapsed;
1881 if (elapsed > 1000) // Earlier makes little sense
1882 ss << " hashfull " << TT.hashfull();
1884 ss << " tbhits " << tbHits
1885 << " time " << elapsed
1888 for (Move m : rootMoves[i].pv)
1889 ss << " " << UCI::move(m, pos.is_chess960());
1896 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1897 /// before exiting the search, for instance, in case we stop the search during a
1898 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1899 /// otherwise in case of 'ponder on' we have nothing to think on.
1901 bool RootMove::extract_ponder_from_tt(Position& pos) {
1904 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1908 assert(pv.size() == 1);
1910 if (pv[0] == MOVE_NONE)
1913 pos.do_move(pv[0], st);
1914 TTEntry* tte = TT.probe(pos.key(), ttHit);
1918 Move m = tte->move(); // Local copy to be SMP safe
1919 if (MoveList<LEGAL>(pos).contains(m))
1923 pos.undo_move(pv[0]);
1924 return pv.size() > 1;
1927 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1930 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1931 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1932 Cardinality = int(Options["SyzygyProbeLimit"]);
1933 bool dtz_available = true;
1935 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1936 // ProbeDepth == DEPTH_ZERO
1937 if (Cardinality > MaxCardinality)
1939 Cardinality = MaxCardinality;
1943 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1945 // Rank moves using DTZ tables
1946 RootInTB = root_probe(pos, rootMoves);
1950 // DTZ tables are missing; try to rank moves using WDL tables
1951 dtz_available = false;
1952 RootInTB = root_probe_wdl(pos, rootMoves);
1958 // Sort moves according to TB rank
1959 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1960 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1962 // Probe during search only if DTZ is not available and we are winning
1963 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1968 // Clean up if root_probe() and root_probe_wdl() have failed
1969 for (auto& m : rootMoves)
1974 } // namespace Stockfish