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(165 * (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 + 1642 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 916);
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((12 * d + 282) * d - 349 , 1594);
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((20.26 + 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(155, 1);
313 optimism[us] = optimism[~us] = VALUE_ZERO;
315 int searchAgainCounter = 0;
317 // Iterative deepening loop until requested to stop or the target depth is reached
318 while ( ++rootDepth < MAX_PLY
320 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
322 // Age out PV variability metric
324 totBestMoveChanges /= 2;
326 // Save the last iteration's scores before first PV line is searched and
327 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
328 for (RootMove& rm : rootMoves)
329 rm.previousScore = rm.score;
334 if (!Threads.increaseDepth)
335 searchAgainCounter++;
337 // MultiPV loop. We perform a full root search for each PV line
338 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
343 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
344 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
348 // Reset UCI info selDepth for each depth and each PV line
351 // Reset aspiration window starting size
354 Value prev = rootMoves[pvIdx].averageScore;
355 delta = Value(10) + int(prev) * prev / 15620;
356 alpha = std::max(prev - delta,-VALUE_INFINITE);
357 beta = std::min(prev + delta, VALUE_INFINITE);
359 // Adjust trend and optimism based on root move's previousScore
360 int tr = 116 * prev / (std::abs(prev) + 89);
361 trend = (us == WHITE ? make_score(tr, tr / 2)
362 : -make_score(tr, tr / 2));
364 int opt = 118 * prev / (std::abs(prev) + 169);
365 optimism[ us] = Value(opt);
366 optimism[~us] = -optimism[us];
369 // Start with a small aspiration window and, in the case of a fail
370 // high/low, re-search with a bigger window until we don't fail
372 int failedHighCnt = 0;
375 // Adjust the effective depth searched, but ensuring at least one effective increment for every
376 // four searchAgain steps (see issue #2717).
377 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);
378 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
380 // Bring the best move to the front. It is critical that sorting
381 // is done with a stable algorithm because all the values but the
382 // first and eventually the new best one are set to -VALUE_INFINITE
383 // and we want to keep the same order for all the moves except the
384 // new PV that goes to the front. Note that in case of MultiPV
385 // search the already searched PV lines are preserved.
386 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
388 // If search has been stopped, we break immediately. Sorting is
389 // safe because RootMoves is still valid, although it refers to
390 // the previous iteration.
394 // When failing high/low give some update (without cluttering
395 // the UI) before a re-search.
398 && (bestValue <= alpha || bestValue >= beta)
399 && Time.elapsed() > 3000)
400 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
402 // In case of failing low/high increase aspiration window and
403 // re-search, otherwise exit the loop.
404 if (bestValue <= alpha)
406 beta = (alpha + beta) / 2;
407 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
411 mainThread->stopOnPonderhit = false;
413 else if (bestValue >= beta)
415 beta = std::min(bestValue + delta, VALUE_INFINITE);
421 delta += delta / 4 + 2;
423 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
426 // Sort the PV lines searched so far and update the GUI
427 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
430 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
431 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
435 completedDepth = rootDepth;
437 if (rootMoves[0].pv[0] != lastBestMove) {
438 lastBestMove = rootMoves[0].pv[0];
439 lastBestMoveDepth = rootDepth;
442 // Have we found a "mate in x"?
444 && bestValue >= VALUE_MATE_IN_MAX_PLY
445 && VALUE_MATE - bestValue <= 2 * Limits.mate)
451 // If skill level is enabled and time is up, pick a sub-optimal best move
452 if (skill.enabled() && skill.time_to_pick(rootDepth))
453 skill.pick_best(multiPV);
455 // Use part of the gained time from a previous stable move for the current move
456 for (Thread* th : Threads)
458 totBestMoveChanges += th->bestMoveChanges;
459 th->bestMoveChanges = 0;
462 // Do we have time for the next iteration? Can we stop searching now?
463 if ( Limits.use_time_management()
465 && !mainThread->stopOnPonderhit)
467 double fallingEval = (71 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
468 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 656.7;
469 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
471 // If the bestMove is stable over several iterations, reduce time accordingly
472 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.37 : 0.65;
473 double reduction = (1.4 + mainThread->previousTimeReduction) / (2.15 * timeReduction);
474 double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();
475 int complexity = mainThread->complexityAverage.value();
476 double complexPosition = std::min(1.0 + (complexity - 261) / 1738.7, 1.5);
478 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
480 // Cap used time in case of a single legal move for a better viewer experience in tournaments
481 // yielding correct scores and sufficiently fast moves.
482 if (rootMoves.size() == 1)
483 totalTime = std::min(500.0, totalTime);
485 // Stop the search if we have exceeded the totalTime
486 if (Time.elapsed() > totalTime)
488 // If we are allowed to ponder do not stop the search now but
489 // keep pondering until the GUI sends "ponderhit" or "stop".
490 if (mainThread->ponder)
491 mainThread->stopOnPonderhit = true;
495 else if ( Threads.increaseDepth
496 && !mainThread->ponder
497 && Time.elapsed() > totalTime * 0.53)
498 Threads.increaseDepth = false;
500 Threads.increaseDepth = true;
503 mainThread->iterValue[iterIdx] = bestValue;
504 iterIdx = (iterIdx + 1) & 3;
510 mainThread->previousTimeReduction = timeReduction;
512 // If skill level is enabled, swap best PV line with the sub-optimal one
514 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
515 skill.best ? skill.best : skill.pick_best(multiPV)));
521 // search<>() is the main search function for both PV and non-PV nodes
523 template <NodeType nodeType>
524 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
526 constexpr bool PvNode = nodeType != NonPV;
527 constexpr bool rootNode = nodeType == Root;
528 const Depth maxNextDepth = rootNode ? depth : depth + 1;
530 // Check if we have an upcoming move which draws by repetition, or
531 // if the opponent had an alternative move earlier to this position.
533 && pos.rule50_count() >= 3
534 && alpha < VALUE_DRAW
535 && pos.has_game_cycle(ss->ply))
537 alpha = value_draw(pos.this_thread());
542 // Dive into quiescence search when the depth reaches zero
544 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
546 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
547 assert(PvNode || (alpha == beta - 1));
548 assert(0 < depth && depth < MAX_PLY);
549 assert(!(PvNode && cutNode));
551 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
553 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
557 Move ttMove, move, excludedMove, bestMove;
558 Depth extension, newDepth;
559 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
560 bool givesCheck, improving, priorCapture, singularQuietLMR;
561 bool capture, moveCountPruning, ttCapture;
563 int moveCount, captureCount, quietCount, improvement, complexity;
565 // Step 1. Initialize node
566 Thread* thisThread = pos.this_thread();
567 ss->inCheck = pos.checkers();
568 priorCapture = pos.captured_piece();
569 Color us = pos.side_to_move();
570 moveCount = captureCount = quietCount = ss->moveCount = 0;
571 bestValue = -VALUE_INFINITE;
572 maxValue = VALUE_INFINITE;
574 // Check for the available remaining time
575 if (thisThread == Threads.main())
576 static_cast<MainThread*>(thisThread)->check_time();
578 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
579 if (PvNode && thisThread->selDepth < ss->ply + 1)
580 thisThread->selDepth = ss->ply + 1;
584 // Step 2. Check for aborted search and immediate draw
585 if ( Threads.stop.load(std::memory_order_relaxed)
586 || pos.is_draw(ss->ply)
587 || ss->ply >= MAX_PLY)
588 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
589 : value_draw(pos.this_thread());
591 // Step 3. Mate distance pruning. Even if we mate at the next move our score
592 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
593 // a shorter mate was found upward in the tree then there is no need to search
594 // because we will never beat the current alpha. Same logic but with reversed
595 // signs applies also in the opposite condition of being mated instead of giving
596 // mate. In this case return a fail-high score.
597 alpha = std::max(mated_in(ss->ply), alpha);
598 beta = std::min(mate_in(ss->ply+1), beta);
603 thisThread->rootDelta = beta - alpha;
605 assert(0 <= ss->ply && ss->ply < MAX_PLY);
607 (ss+1)->ttPv = false;
608 (ss+1)->excludedMove = bestMove = MOVE_NONE;
609 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
610 (ss+2)->cutoffCnt = 0;
611 ss->doubleExtensions = (ss-1)->doubleExtensions;
612 Square prevSq = to_sq((ss-1)->currentMove);
614 // Initialize statScore to zero for the grandchildren of the current position.
615 // So statScore is shared between all grandchildren and only the first grandchild
616 // starts with statScore = 0. Later grandchildren start with the last calculated
617 // statScore of the previous grandchild. This influences the reduction rules in
618 // LMR which are based on the statScore of parent position.
620 (ss+2)->statScore = 0;
622 // Step 4. Transposition table lookup. We don't want the score of a partial
623 // search to overwrite a previous full search TT value, so we use a different
624 // position key in case of an excluded move.
625 excludedMove = ss->excludedMove;
626 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
627 tte = TT.probe(posKey, ss->ttHit);
628 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
629 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
630 : ss->ttHit ? tte->move() : MOVE_NONE;
631 ttCapture = ttMove && pos.capture(ttMove);
633 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
635 // At non-PV nodes we check for an early TT cutoff
638 && tte->depth() > depth - (tte->bound() == BOUND_EXACT)
639 && ttValue != VALUE_NONE // Possible in case of TT access race
640 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
642 // If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)
647 // Bonus for a quiet ttMove that fails high (~3 Elo)
649 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
651 // Extra penalty for early quiet moves of the previous ply (~0 Elo)
652 if ((ss-1)->moveCount <= 2 && !priorCapture)
653 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
655 // Penalty for a quiet ttMove that fails low (~1 Elo)
658 int penalty = -stat_bonus(depth);
659 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
660 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
664 // Partial workaround for the graph history interaction problem
665 // For high rule50 counts don't produce transposition table cutoffs.
666 if (pos.rule50_count() < 90)
670 // Step 5. Tablebases probe
671 if (!rootNode && TB::Cardinality)
673 int piecesCount = pos.count<ALL_PIECES>();
675 if ( piecesCount <= TB::Cardinality
676 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
677 && pos.rule50_count() == 0
678 && !pos.can_castle(ANY_CASTLING))
681 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
683 // Force check of time on the next occasion
684 if (thisThread == Threads.main())
685 static_cast<MainThread*>(thisThread)->callsCnt = 0;
687 if (err != TB::ProbeState::FAIL)
689 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
691 int drawScore = TB::UseRule50 ? 1 : 0;
693 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
694 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
695 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
696 : VALUE_DRAW + 2 * wdl * drawScore;
698 Bound b = wdl < -drawScore ? BOUND_UPPER
699 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
701 if ( b == BOUND_EXACT
702 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
704 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
705 std::min(MAX_PLY - 1, depth + 6),
706 MOVE_NONE, VALUE_NONE);
713 if (b == BOUND_LOWER)
714 bestValue = value, alpha = std::max(alpha, bestValue);
722 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
724 // Step 6. Static evaluation of the position
727 // Skip early pruning when in check
728 ss->staticEval = eval = VALUE_NONE;
736 // Never assume anything about values stored in TT
737 ss->staticEval = eval = tte->eval();
738 if (eval == VALUE_NONE)
739 ss->staticEval = eval = evaluate(pos, &complexity);
740 else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost
741 complexity = abs(ss->staticEval - pos.psq_eg_stm());
743 // ttValue can be used as a better position evaluation (~4 Elo)
744 if ( ttValue != VALUE_NONE
745 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
750 ss->staticEval = eval = evaluate(pos, &complexity);
752 // Save static evaluation into transposition table
754 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
757 thisThread->complexityAverage.update(complexity);
759 // Use static evaluation difference to improve quiet move ordering (~3 Elo)
760 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
762 int bonus = std::clamp(-19 * int((ss-1)->staticEval + ss->staticEval), -1914, 1914);
763 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
766 // Set up the improvement variable, which is the difference between the current
767 // static evaluation and the previous static evaluation at our turn (if we were
768 // in check at our previous move we look at the move prior to it). The improvement
769 // margin and the improving flag are used in various pruning heuristics.
770 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
771 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
773 improving = improvement > 0;
776 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
777 // return a fail low.
779 && eval < alpha - 369 - 254 * depth * depth)
781 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
786 // Step 8. Futility pruning: child node (~25 Elo).
787 // The depth condition is important for mate finding.
790 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 301 >= beta
792 && eval < 28692) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
795 // Step 9. Null move search with verification search (~22 Elo)
797 && (ss-1)->currentMove != MOVE_NULL
798 && (ss-1)->statScore < 17139
800 && eval >= ss->staticEval
801 && ss->staticEval >= beta - 20 * depth - improvement / 13 + 233 + complexity / 25
803 && pos.non_pawn_material(us)
804 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
806 assert(eval - beta >= 0);
808 // Null move dynamic reduction based on depth, eval and complexity of position
809 Depth R = std::min(int(eval - beta) / 168, 7) + depth / 3 + 4 - (complexity > 861);
811 ss->currentMove = MOVE_NULL;
812 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
814 pos.do_null_move(st);
816 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
818 pos.undo_null_move();
820 if (nullValue >= beta)
822 // Do not return unproven mate or TB scores
823 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
826 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
829 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
831 // Do verification search at high depths, with null move pruning disabled
832 // for us, until ply exceeds nmpMinPly.
833 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
834 thisThread->nmpColor = us;
836 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
838 thisThread->nmpMinPly = 0;
845 probCutBeta = beta + 191 - 54 * improving;
847 // Step 10. ProbCut (~4 Elo)
848 // If we have a good enough capture and a reduced search returns a value
849 // much above beta, we can (almost) safely prune the previous move.
852 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
853 // if value from transposition table is lower than probCutBeta, don't attempt probCut
854 // there and in further interactions with transposition table cutoff depth is set to depth - 3
855 // because probCut search has depth set to depth - 4 but we also do a move before it
856 // so effective depth is equal to depth - 3
858 && tte->depth() >= depth - 3
859 && ttValue != VALUE_NONE
860 && ttValue < probCutBeta))
862 assert(probCutBeta < VALUE_INFINITE);
864 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);
866 while ((move = mp.next_move()) != MOVE_NONE)
867 if (move != excludedMove && pos.legal(move))
869 assert(pos.capture(move) || promotion_type(move) == QUEEN);
871 ss->currentMove = move;
872 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
874 [pos.moved_piece(move)]
877 pos.do_move(move, st);
879 // Perform a preliminary qsearch to verify that the move holds
880 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
882 // If the qsearch held, perform the regular search
883 if (value >= probCutBeta)
884 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
888 if (value >= probCutBeta)
890 // Save ProbCut data into transposition table
891 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3, move, ss->staticEval);
897 // Step 11. If the position is not in TT, decrease depth by 3.
898 // Use qsearch if depth is equal or below zero (~4 Elo)
904 return qsearch<PV>(pos, ss, alpha, beta);
911 moves_loop: // When in check, search starts here
913 // Step 12. A small Probcut idea, when we are in check (~0 Elo)
914 probCutBeta = beta + 417;
919 && (tte->bound() & BOUND_LOWER)
920 && tte->depth() >= depth - 3
921 && ttValue >= probCutBeta
922 && abs(ttValue) <= VALUE_KNOWN_WIN
923 && abs(beta) <= VALUE_KNOWN_WIN
928 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
929 nullptr , (ss-4)->continuationHistory,
930 nullptr , (ss-6)->continuationHistory };
932 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
934 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
941 moveCountPruning = singularQuietLMR = false;
943 // Indicate PvNodes that will probably fail low if the node was searched
944 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
945 bool likelyFailLow = PvNode
947 && (tte->bound() & BOUND_UPPER)
948 && tte->depth() >= depth;
950 // Step 13. Loop through all pseudo-legal moves until no moves remain
951 // or a beta cutoff occurs.
952 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
956 if (move == excludedMove)
959 // At root obey the "searchmoves" option and skip moves not listed in Root
960 // Move List. As a consequence any illegal move is also skipped. In MultiPV
961 // mode we also skip PV moves which have been already searched and those
962 // of lower "TB rank" if we are in a TB root position.
963 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
964 thisThread->rootMoves.begin() + thisThread->pvLast, move))
967 // Check for legality
968 if (!rootNode && !pos.legal(move))
971 ss->moveCount = ++moveCount;
973 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
974 sync_cout << "info depth " << depth
975 << " currmove " << UCI::move(move, pos.is_chess960())
976 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
978 (ss+1)->pv = nullptr;
981 capture = pos.capture(move);
982 movedPiece = pos.moved_piece(move);
983 givesCheck = pos.gives_check(move);
985 // Calculate new depth for this move
986 newDepth = depth - 1;
988 Value delta = beta - alpha;
990 // Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.
992 && pos.non_pawn_material(us)
993 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
995 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo)
996 moveCountPruning = moveCount >= futility_move_count(improving, depth);
998 // Reduced depth of the next LMR search
999 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0);
1004 // Futility pruning for captures (~0 Elo)
1005 if ( !pos.empty(to_sq(move))
1010 && ss->staticEval + 180 + 201 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1011 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1014 // SEE based pruning (~9 Elo)
1015 if (!pos.see_ge(move, Value(-222) * depth))
1020 int history = (*contHist[0])[movedPiece][to_sq(move)]
1021 + (*contHist[1])[movedPiece][to_sq(move)]
1022 + (*contHist[3])[movedPiece][to_sq(move)];
1024 // Continuation history based pruning (~2 Elo)
1026 && history < -3875 * (depth - 1))
1029 history += 2 * thisThread->mainHistory[us][from_to(move)];
1031 // Futility pruning: parent node (~9 Elo)
1034 && ss->staticEval + 106 + 145 * lmrDepth + history / 52 <= alpha)
1037 // Prune moves with negative SEE (~3 Elo)
1038 if (!pos.see_ge(move, Value(-24 * lmrDepth * lmrDepth - 15 * lmrDepth)))
1043 // Step 15. Extensions (~66 Elo)
1044 // We take care to not overdo to avoid search getting stuck.
1045 if (ss->ply < thisThread->rootDepth * 2)
1047 // Singular extension search (~58 Elo). If all moves but one fail low on a
1048 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1049 // then that move is singular and should be extended. To verify this we do
1050 // a reduced search on all the other moves but the ttMove and if the
1051 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1053 && depth >= 4 - (thisThread->previousDepth > 24) + 2 * (PvNode && tte->is_pv())
1055 && !excludedMove // Avoid recursive singular search
1056 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1057 && abs(ttValue) < VALUE_KNOWN_WIN
1058 && (tte->bound() & BOUND_LOWER)
1059 && tte->depth() >= depth - 3)
1061 Value singularBeta = ttValue - (3 + (ss->ttPv && !PvNode)) * depth;
1062 Depth singularDepth = (depth - 1) / 2;
1064 ss->excludedMove = move;
1065 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1066 ss->excludedMove = MOVE_NONE;
1068 if (value < singularBeta)
1071 singularQuietLMR = !ttCapture;
1073 // Avoid search explosion by limiting the number of double extensions
1075 && value < singularBeta - 25
1076 && ss->doubleExtensions <= 9)
1080 // Multi-cut pruning
1081 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1082 // search without the ttMove. So we assume this expected Cut-node is not singular,
1083 // that multiple moves fail high, and we can prune the whole subtree by returning
1085 else if (singularBeta >= beta)
1086 return singularBeta;
1088 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1089 else if (ttValue >= beta)
1092 // If the eval of ttMove is less than alpha and value, we reduce it (negative extension)
1093 else if (ttValue <= alpha && ttValue <= value)
1097 // Check extensions (~1 Elo)
1098 else if ( givesCheck
1100 && abs(ss->staticEval) > 82)
1103 // Quiet ttMove extensions (~0 Elo)
1106 && move == ss->killers[0]
1107 && (*contHist[0])[movedPiece][to_sq(move)] >= 5177)
1111 // Add extension to new depth
1112 newDepth += extension;
1113 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1115 // Speculative prefetch as early as possible
1116 prefetch(TT.first_entry(pos.key_after(move)));
1118 // Update the current move (this must be done after singular extension search)
1119 ss->currentMove = move;
1120 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1125 // Step 16. Make the move
1126 pos.do_move(move, st, givesCheck);
1128 // Step 17. Late moves reduction / extension (LMR, ~98 Elo)
1129 // We use various heuristics for the sons of a node after the first son has
1130 // been searched. In general we would like to reduce them, but there are many
1131 // cases where we extend a son if it has good chances to be "interesting".
1133 && moveCount > 1 + (PvNode && ss->ply <= 1)
1136 || (cutNode && (ss-1)->moveCount > 1)))
1138 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
1140 // Decrease reduction if position is or has been on the PV
1141 // and node is not likely to fail low. (~3 Elo)
1146 // Decrease reduction if opponent's move count is high (~1 Elo)
1147 if ((ss-1)->moveCount > 7)
1150 // Increase reduction for cut nodes (~3 Elo)
1154 // Increase reduction if ttMove is a capture (~3 Elo)
1158 // Decrease reduction for PvNodes based on depth
1160 r -= 1 + 11 / (3 + depth);
1162 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1163 if (singularQuietLMR)
1166 // Dicrease reduction if we move a threatened piece (~1 Elo)
1168 && (mp.threatenedPieces & from_sq(move)))
1171 // Increase reduction if next ply has a lot of fail high
1172 if ((ss+1)->cutoffCnt > 3 && !PvNode)
1175 ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
1176 + (*contHist[0])[movedPiece][to_sq(move)]
1177 + (*contHist[1])[movedPiece][to_sq(move)]
1178 + (*contHist[3])[movedPiece][to_sq(move)]
1181 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1182 r -= (ss->statScore + 5 * alpha) / 15448;
1184 // In general we want to cap the LMR depth search at newDepth, but when
1185 // reduction is negative, we allow this move a limited search extension
1186 // beyond the first move depth. This may lead to hidden double extensions.
1187 Depth d = std::clamp(newDepth - r, 1, newDepth + 1);
1189 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1191 // Do full depth search when reduced LMR search fails high
1192 if (value > alpha && d < newDepth)
1194 const bool doDeeperSearch = value > (alpha + 64 + 11 * (newDepth - d));
1195 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode);
1197 int bonus = value > alpha ? stat_bonus(newDepth)
1198 : -stat_bonus(newDepth);
1203 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1207 // Step 18. Full depth search when LMR is skipped
1208 else if (!PvNode || moveCount > 1)
1210 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1213 // For PV nodes only, do a full PV search on the first move or after a fail
1214 // high (in the latter case search only if value < beta), otherwise let the
1215 // parent node fail low with value <= alpha and try another move.
1216 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1219 (ss+1)->pv[0] = MOVE_NONE;
1221 value = -search<PV>(pos, ss+1, -beta, -alpha,
1222 std::min(maxNextDepth, newDepth), false);
1225 // Step 19. Undo move
1226 pos.undo_move(move);
1228 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1230 // Step 20. Check for a new best move
1231 // Finished searching the move. If a stop occurred, the return value of
1232 // the search cannot be trusted, and we return immediately without
1233 // updating best move, PV and TT.
1234 if (Threads.stop.load(std::memory_order_relaxed))
1239 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1240 thisThread->rootMoves.end(), move);
1242 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1244 // PV move or new best move?
1245 if (moveCount == 1 || value > alpha)
1248 rm.selDepth = thisThread->selDepth;
1253 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1254 rm.pv.push_back(*m);
1256 // We record how often the best move has been changed in each iteration.
1257 // This information is used for time management. In MultiPV mode,
1258 // we must take care to only do this for the first PV line.
1260 && !thisThread->pvIdx)
1261 ++thisThread->bestMoveChanges;
1264 // All other moves but the PV are set to the lowest value: this
1265 // is not a problem when sorting because the sort is stable and the
1266 // move position in the list is preserved - just the PV is pushed up.
1267 rm.score = -VALUE_INFINITE;
1270 if (value > bestValue)
1278 if (PvNode && !rootNode) // Update pv even in fail-high case
1279 update_pv(ss->pv, move, (ss+1)->pv);
1281 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1285 // Reduce other moves if we have found at least one score improvement
1288 && beta < VALUE_KNOWN_WIN
1289 && alpha > -VALUE_KNOWN_WIN)
1297 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 (capture && captureCount < 32)
1310 capturesSearched[captureCount++] = move;
1312 else if (!capture && 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 >= 5 || 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 - 62 * 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);
1363 // Write gathered information in transposition table
1364 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1365 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1366 bestValue >= beta ? BOUND_LOWER :
1367 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1368 depth, bestMove, ss->staticEval);
1370 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1376 // qsearch() is the quiescence search function, which is called by the main search
1377 // function with zero depth, or recursively with further decreasing depth per call.
1379 template <NodeType nodeType>
1380 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1382 static_assert(nodeType != Root);
1383 constexpr bool PvNode = nodeType == PV;
1385 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1386 assert(PvNode || (alpha == beta - 1));
1391 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1395 Move ttMove, move, bestMove;
1397 Value bestValue, value, ttValue, futilityValue, futilityBase;
1398 bool pvHit, givesCheck, capture;
1404 ss->pv[0] = MOVE_NONE;
1407 Thread* thisThread = pos.this_thread();
1408 bestMove = MOVE_NONE;
1409 ss->inCheck = pos.checkers();
1412 // Check for an immediate draw or maximum ply reached
1413 if ( pos.is_draw(ss->ply)
1414 || ss->ply >= MAX_PLY)
1415 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1417 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1419 // Decide whether or not to include checks: this fixes also the type of
1420 // TT entry depth that we are going to use. Note that in qsearch we use
1421 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1422 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1423 : DEPTH_QS_NO_CHECKS;
1424 // Transposition table lookup
1426 tte = TT.probe(posKey, ss->ttHit);
1427 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1428 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1429 pvHit = ss->ttHit && tte->is_pv();
1433 && tte->depth() >= ttDepth
1434 && ttValue != VALUE_NONE // Only in case of TT access race
1435 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
1438 // Evaluate the position statically
1441 ss->staticEval = VALUE_NONE;
1442 bestValue = futilityBase = -VALUE_INFINITE;
1448 // Never assume anything about values stored in TT
1449 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1450 ss->staticEval = bestValue = evaluate(pos);
1452 // ttValue can be used as a better position evaluation (~7 Elo)
1453 if ( ttValue != VALUE_NONE
1454 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1455 bestValue = ttValue;
1458 // In case of null move search use previous static eval with a different sign
1459 ss->staticEval = bestValue =
1460 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1461 : -(ss-1)->staticEval;
1463 // Stand pat. Return immediately if static value is at least beta
1464 if (bestValue >= beta)
1466 // Save gathered info in transposition table
1468 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1469 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1474 if (PvNode && bestValue > alpha)
1477 futilityBase = bestValue + 153;
1480 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1481 nullptr , (ss-4)->continuationHistory,
1482 nullptr , (ss-6)->continuationHistory };
1484 // Initialize a MovePicker object for the current position, and prepare
1485 // to search the moves. Because the depth is <= 0 here, only captures,
1486 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1487 // will be generated.
1488 Square prevSq = to_sq((ss-1)->currentMove);
1489 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1490 &thisThread->captureHistory,
1494 int quietCheckEvasions = 0;
1496 // Loop through the moves until no moves remain or a beta cutoff occurs
1497 while ((move = mp.next_move()) != MOVE_NONE)
1499 assert(is_ok(move));
1501 // Check for legality
1502 if (!pos.legal(move))
1505 givesCheck = pos.gives_check(move);
1506 capture = pos.capture(move);
1510 // Futility pruning and moveCount pruning (~5 Elo)
1511 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1513 && to_sq(move) != prevSq
1514 && futilityBase > -VALUE_KNOWN_WIN
1515 && type_of(move) != PROMOTION)
1521 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1523 if (futilityValue <= alpha)
1525 bestValue = std::max(bestValue, futilityValue);
1529 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1531 bestValue = std::max(bestValue, futilityBase);
1536 // Do not search moves with negative SEE values (~5 Elo)
1537 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1538 && !pos.see_ge(move))
1541 // Speculative prefetch as early as possible
1542 prefetch(TT.first_entry(pos.key_after(move)));
1544 ss->currentMove = move;
1545 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1547 [pos.moved_piece(move)]
1550 // Continuation history based pruning (~2 Elo)
1552 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1553 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1554 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1557 // movecount pruning for quiet check evasions
1558 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1559 && quietCheckEvasions > 1
1564 quietCheckEvasions += !capture && ss->inCheck;
1566 // Make and search the move
1567 pos.do_move(move, st, givesCheck);
1568 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1569 pos.undo_move(move);
1571 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1573 // Check for a new best move
1574 if (value > bestValue)
1582 if (PvNode) // Update pv even in fail-high case
1583 update_pv(ss->pv, move, (ss+1)->pv);
1585 if (PvNode && value < beta) // Update alpha here!
1593 // All legal moves have been searched. A special case: if we're in check
1594 // and no legal moves were found, it is checkmate.
1595 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1597 assert(!MoveList<LEGAL>(pos).size());
1599 return mated_in(ss->ply); // Plies to mate from the root
1602 // Save gathered info in transposition table
1603 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1604 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1605 ttDepth, bestMove, ss->staticEval);
1607 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1613 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1614 // "plies to mate from the current position". Standard scores are unchanged.
1615 // The function is called before storing a value in the transposition table.
1617 Value value_to_tt(Value v, int ply) {
1619 assert(v != VALUE_NONE);
1621 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1622 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1626 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1627 // from the transposition table (which refers to the plies to mate/be mated from
1628 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1629 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1630 // and the graph history interaction, we return an optimal TB score instead.
1632 Value value_from_tt(Value v, int ply, int r50c) {
1634 if (v == VALUE_NONE)
1637 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1639 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1640 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1645 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1647 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1648 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1657 // update_pv() adds current move and appends child pv[]
1659 void update_pv(Move* pv, Move move, const Move* childPv) {
1661 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1667 // update_all_stats() updates stats at the end of search() when a bestMove is found
1669 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1670 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1672 Color us = pos.side_to_move();
1673 Thread* thisThread = pos.this_thread();
1674 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1675 Piece moved_piece = pos.moved_piece(bestMove);
1676 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1677 int bonus1 = stat_bonus(depth + 1);
1679 if (!pos.capture(bestMove))
1681 int bonus2 = bestValue > beta + 137 ? bonus1 // larger bonus
1682 : stat_bonus(depth); // smaller bonus
1684 // Increase stats for the best move in case it was a quiet move
1685 update_quiet_stats(pos, ss, bestMove, bonus2);
1687 // Decrease stats for all non-best quiet moves
1688 for (int i = 0; i < quietCount; ++i)
1690 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1691 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1695 // Increase stats for the best move in case it was a capture move
1696 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1698 // Extra penalty for a quiet early move that was not a TT move or
1699 // main killer move in previous ply when it gets refuted.
1700 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1701 && !pos.captured_piece())
1702 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1704 // Decrease stats for all non-best capture moves
1705 for (int i = 0; i < captureCount; ++i)
1707 moved_piece = pos.moved_piece(capturesSearched[i]);
1708 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1709 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1714 // update_continuation_histories() updates histories of the move pairs formed
1715 // by moves at ply -1, -2, -4, and -6 with current move.
1717 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1719 for (int i : {1, 2, 4, 6})
1721 // Only update first 2 continuation histories if we are in check
1722 if (ss->inCheck && i > 2)
1724 if (is_ok((ss-i)->currentMove))
1725 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1730 // update_quiet_stats() updates move sorting heuristics
1732 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1735 if (ss->killers[0] != move)
1737 ss->killers[1] = ss->killers[0];
1738 ss->killers[0] = move;
1741 Color us = pos.side_to_move();
1742 Thread* thisThread = pos.this_thread();
1743 thisThread->mainHistory[us][from_to(move)] << bonus;
1744 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1746 // Update countermove history
1747 if (is_ok((ss-1)->currentMove))
1749 Square prevSq = to_sq((ss-1)->currentMove);
1750 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1754 // When playing with strength handicap, choose best move among a set of RootMoves
1755 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1757 Move Skill::pick_best(size_t multiPV) {
1759 const RootMoves& rootMoves = Threads.main()->rootMoves;
1760 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1762 // RootMoves are already sorted by score in descending order
1763 Value topScore = rootMoves[0].score;
1764 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1765 int maxScore = -VALUE_INFINITE;
1766 double weakness = 120 - 2 * level;
1768 // Choose best move. For each move score we add two terms, both dependent on
1769 // weakness. One is deterministic and bigger for weaker levels, and one is
1770 // random. Then we choose the move with the resulting highest score.
1771 for (size_t i = 0; i < multiPV; ++i)
1773 // This is our magic formula
1774 int push = int(( weakness * int(topScore - rootMoves[i].score)
1775 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1777 if (rootMoves[i].score + push >= maxScore)
1779 maxScore = rootMoves[i].score + push;
1780 best = rootMoves[i].pv[0];
1790 /// MainThread::check_time() is used to print debug info and, more importantly,
1791 /// to detect when we are out of available time and thus stop the search.
1793 void MainThread::check_time() {
1798 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1799 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1801 static TimePoint lastInfoTime = now();
1803 TimePoint elapsed = Time.elapsed();
1804 TimePoint tick = Limits.startTime + elapsed;
1806 if (tick - lastInfoTime >= 1000)
1808 lastInfoTime = tick;
1812 // We should not stop pondering until told so by the GUI
1816 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1817 || (Limits.movetime && elapsed >= Limits.movetime)
1818 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1819 Threads.stop = true;
1823 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1824 /// that all (if any) unsearched PV lines are sent using a previous search score.
1826 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1828 std::stringstream ss;
1829 TimePoint elapsed = Time.elapsed() + 1;
1830 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1831 size_t pvIdx = pos.this_thread()->pvIdx;
1832 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1833 uint64_t nodesSearched = Threads.nodes_searched();
1834 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1836 for (size_t i = 0; i < multiPV; ++i)
1838 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1840 if (depth == 1 && !updated && i > 0)
1843 Depth d = updated ? depth : std::max(1, depth - 1);
1844 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1846 if (v == -VALUE_INFINITE)
1849 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1850 v = tb ? rootMoves[i].tbScore : v;
1852 if (ss.rdbuf()->in_avail()) // Not at first line
1857 << " seldepth " << rootMoves[i].selDepth
1858 << " multipv " << i + 1
1859 << " score " << UCI::value(v);
1861 if (Options["UCI_ShowWDL"])
1862 ss << UCI::wdl(v, pos.game_ply());
1864 if (!tb && i == pvIdx)
1865 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1867 ss << " nodes " << nodesSearched
1868 << " nps " << nodesSearched * 1000 / elapsed
1869 << " hashfull " << TT.hashfull()
1870 << " tbhits " << tbHits
1871 << " time " << elapsed
1874 for (Move m : rootMoves[i].pv)
1875 ss << " " << UCI::move(m, pos.is_chess960());
1882 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1883 /// before exiting the search, for instance, in case we stop the search during a
1884 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1885 /// otherwise in case of 'ponder on' we have nothing to think on.
1887 bool RootMove::extract_ponder_from_tt(Position& pos) {
1890 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1894 assert(pv.size() == 1);
1896 if (pv[0] == MOVE_NONE)
1899 pos.do_move(pv[0], st);
1900 TTEntry* tte = TT.probe(pos.key(), ttHit);
1904 Move m = tte->move(); // Local copy to be SMP safe
1905 if (MoveList<LEGAL>(pos).contains(m))
1909 pos.undo_move(pv[0]);
1910 return pv.size() > 1;
1913 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1916 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1917 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1918 Cardinality = int(Options["SyzygyProbeLimit"]);
1919 bool dtz_available = true;
1921 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1922 // ProbeDepth == DEPTH_ZERO
1923 if (Cardinality > MaxCardinality)
1925 Cardinality = MaxCardinality;
1929 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1931 // Rank moves using DTZ tables
1932 RootInTB = root_probe(pos, rootMoves);
1936 // DTZ tables are missing; try to rank moves using WDL tables
1937 dtz_available = false;
1938 RootInTB = root_probe_wdl(pos, rootMoves);
1944 // Sort moves according to TB rank
1945 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1946 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1948 // Probe during search only if DTZ is not available and we are winning
1949 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1954 // Clean up if root_probe() and root_probe_wdl() have failed
1955 for (auto& m : rootMoves)
1960 } // namespace Stockfish