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);
312 optimism[us] = optimism[~us] = VALUE_ZERO;
314 int searchAgainCounter = 0;
316 // Iterative deepening loop until requested to stop or the target depth is reached
317 while ( ++rootDepth < MAX_PLY
319 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
321 // Age out PV variability metric
323 totBestMoveChanges /= 2;
325 // Save the last iteration's scores before first PV line is searched and
326 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
327 for (RootMove& rm : rootMoves)
328 rm.previousScore = rm.score;
333 if (!Threads.increaseDepth)
334 searchAgainCounter++;
336 // MultiPV loop. We perform a full root search for each PV line
337 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
342 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
343 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
347 // Reset UCI info selDepth for each depth and each PV line
350 // Reset aspiration window starting size
353 Value prev = rootMoves[pvIdx].averageScore;
354 delta = Value(10) + int(prev) * prev / 15620;
355 alpha = std::max(prev - delta,-VALUE_INFINITE);
356 beta = std::min(prev + delta, VALUE_INFINITE);
358 // Adjust optimism based on root move's previousScore
359 int opt = 118 * prev / (std::abs(prev) + 169);
360 optimism[ us] = Value(opt);
361 optimism[~us] = -optimism[us];
364 // Start with a small aspiration window and, in the case of a fail
365 // high/low, re-search with a bigger window until we don't fail
367 int failedHighCnt = 0;
370 // Adjust the effective depth searched, but ensuring at least one effective increment for every
371 // four searchAgain steps (see issue #2717).
372 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);
373 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
375 // Bring the best move to the front. It is critical that sorting
376 // is done with a stable algorithm because all the values but the
377 // first and eventually the new best one are set to -VALUE_INFINITE
378 // and we want to keep the same order for all the moves except the
379 // new PV that goes to the front. Note that in case of MultiPV
380 // search the already searched PV lines are preserved.
381 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
383 // If search has been stopped, we break immediately. Sorting is
384 // safe because RootMoves is still valid, although it refers to
385 // the previous iteration.
389 // When failing high/low give some update (without cluttering
390 // the UI) before a re-search.
393 && (bestValue <= alpha || bestValue >= beta)
394 && Time.elapsed() > 3000)
395 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
397 // In case of failing low/high increase aspiration window and
398 // re-search, otherwise exit the loop.
399 if (bestValue <= alpha)
401 beta = (alpha + beta) / 2;
402 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
406 mainThread->stopOnPonderhit = false;
408 else if (bestValue >= beta)
410 beta = std::min(bestValue + delta, VALUE_INFINITE);
416 delta += delta / 4 + 2;
418 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
421 // Sort the PV lines searched so far and update the GUI
422 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
425 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
426 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
430 completedDepth = rootDepth;
432 if (rootMoves[0].pv[0] != lastBestMove) {
433 lastBestMove = rootMoves[0].pv[0];
434 lastBestMoveDepth = rootDepth;
437 // Have we found a "mate in x"?
439 && bestValue >= VALUE_MATE_IN_MAX_PLY
440 && VALUE_MATE - bestValue <= 2 * Limits.mate)
446 // If skill level is enabled and time is up, pick a sub-optimal best move
447 if (skill.enabled() && skill.time_to_pick(rootDepth))
448 skill.pick_best(multiPV);
450 // Use part of the gained time from a previous stable move for the current move
451 for (Thread* th : Threads)
453 totBestMoveChanges += th->bestMoveChanges;
454 th->bestMoveChanges = 0;
457 // Do we have time for the next iteration? Can we stop searching now?
458 if ( Limits.use_time_management()
460 && !mainThread->stopOnPonderhit)
462 double fallingEval = (71 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
463 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 656.7;
464 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
466 // If the bestMove is stable over several iterations, reduce time accordingly
467 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.37 : 0.65;
468 double reduction = (1.4 + mainThread->previousTimeReduction) / (2.15 * timeReduction);
469 double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();
470 int complexity = mainThread->complexityAverage.value();
471 double complexPosition = std::min(1.0 + (complexity - 261) / 1738.7, 1.5);
473 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
475 // Cap used time in case of a single legal move for a better viewer experience in tournaments
476 // yielding correct scores and sufficiently fast moves.
477 if (rootMoves.size() == 1)
478 totalTime = std::min(500.0, totalTime);
480 // Stop the search if we have exceeded the totalTime
481 if (Time.elapsed() > totalTime)
483 // If we are allowed to ponder do not stop the search now but
484 // keep pondering until the GUI sends "ponderhit" or "stop".
485 if (mainThread->ponder)
486 mainThread->stopOnPonderhit = true;
490 else if ( Threads.increaseDepth
491 && !mainThread->ponder
492 && Time.elapsed() > totalTime * 0.53)
493 Threads.increaseDepth = false;
495 Threads.increaseDepth = true;
498 mainThread->iterValue[iterIdx] = bestValue;
499 iterIdx = (iterIdx + 1) & 3;
505 mainThread->previousTimeReduction = timeReduction;
507 // If skill level is enabled, swap best PV line with the sub-optimal one
509 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
510 skill.best ? skill.best : skill.pick_best(multiPV)));
516 // search<>() is the main search function for both PV and non-PV nodes
518 template <NodeType nodeType>
519 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
521 constexpr bool PvNode = nodeType != NonPV;
522 constexpr bool rootNode = nodeType == Root;
523 const Depth maxNextDepth = rootNode ? depth : depth + 1;
525 // Check if we have an upcoming move which draws by repetition, or
526 // if the opponent had an alternative move earlier to this position.
528 && pos.rule50_count() >= 3
529 && alpha < VALUE_DRAW
530 && pos.has_game_cycle(ss->ply))
532 alpha = value_draw(pos.this_thread());
537 // Dive into quiescence search when the depth reaches zero
539 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
541 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
542 assert(PvNode || (alpha == beta - 1));
543 assert(0 < depth && depth < MAX_PLY);
544 assert(!(PvNode && cutNode));
546 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
548 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
552 Move ttMove, move, excludedMove, bestMove;
553 Depth extension, newDepth;
554 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
555 bool givesCheck, improving, priorCapture, singularQuietLMR;
556 bool capture, moveCountPruning, ttCapture;
558 int moveCount, captureCount, quietCount, improvement, complexity;
560 // Step 1. Initialize node
561 Thread* thisThread = pos.this_thread();
562 ss->inCheck = pos.checkers();
563 priorCapture = pos.captured_piece();
564 Color us = pos.side_to_move();
565 moveCount = captureCount = quietCount = ss->moveCount = 0;
566 bestValue = -VALUE_INFINITE;
567 maxValue = VALUE_INFINITE;
569 // Check for the available remaining time
570 if (thisThread == Threads.main())
571 static_cast<MainThread*>(thisThread)->check_time();
573 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
574 if (PvNode && thisThread->selDepth < ss->ply + 1)
575 thisThread->selDepth = ss->ply + 1;
579 // Step 2. Check for aborted search and immediate draw
580 if ( Threads.stop.load(std::memory_order_relaxed)
581 || pos.is_draw(ss->ply)
582 || ss->ply >= MAX_PLY)
583 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
584 : value_draw(pos.this_thread());
586 // Step 3. Mate distance pruning. Even if we mate at the next move our score
587 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
588 // a shorter mate was found upward in the tree then there is no need to search
589 // because we will never beat the current alpha. Same logic but with reversed
590 // signs applies also in the opposite condition of being mated instead of giving
591 // mate. In this case return a fail-high score.
592 alpha = std::max(mated_in(ss->ply), alpha);
593 beta = std::min(mate_in(ss->ply+1), beta);
598 thisThread->rootDelta = beta - alpha;
600 assert(0 <= ss->ply && ss->ply < MAX_PLY);
602 (ss+1)->ttPv = false;
603 (ss+1)->excludedMove = bestMove = MOVE_NONE;
604 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
605 (ss+2)->cutoffCnt = 0;
606 ss->doubleExtensions = (ss-1)->doubleExtensions;
607 Square prevSq = to_sq((ss-1)->currentMove);
609 // Initialize statScore to zero for the grandchildren of the current position.
610 // So statScore is shared between all grandchildren and only the first grandchild
611 // starts with statScore = 0. Later grandchildren start with the last calculated
612 // statScore of the previous grandchild. This influences the reduction rules in
613 // LMR which are based on the statScore of parent position.
615 (ss+2)->statScore = 0;
617 // Step 4. Transposition table lookup. We don't want the score of a partial
618 // search to overwrite a previous full search TT value, so we use a different
619 // position key in case of an excluded move.
620 excludedMove = ss->excludedMove;
621 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
622 tte = TT.probe(posKey, ss->ttHit);
623 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
624 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
625 : ss->ttHit ? tte->move() : MOVE_NONE;
626 ttCapture = ttMove && pos.capture(ttMove);
628 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
630 // At non-PV nodes we check for an early TT cutoff
633 && tte->depth() > depth - (tte->bound() == BOUND_EXACT)
634 && ttValue != VALUE_NONE // Possible in case of TT access race
635 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
637 // If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)
642 // Bonus for a quiet ttMove that fails high (~3 Elo)
644 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
646 // Extra penalty for early quiet moves of the previous ply (~0 Elo)
647 if ((ss-1)->moveCount <= 2 && !priorCapture)
648 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
650 // Penalty for a quiet ttMove that fails low (~1 Elo)
653 int penalty = -stat_bonus(depth);
654 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
655 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
659 // Partial workaround for the graph history interaction problem
660 // For high rule50 counts don't produce transposition table cutoffs.
661 if (pos.rule50_count() < 90)
665 // Step 5. Tablebases probe
666 if (!rootNode && TB::Cardinality)
668 int piecesCount = pos.count<ALL_PIECES>();
670 if ( piecesCount <= TB::Cardinality
671 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
672 && pos.rule50_count() == 0
673 && !pos.can_castle(ANY_CASTLING))
676 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
678 // Force check of time on the next occasion
679 if (thisThread == Threads.main())
680 static_cast<MainThread*>(thisThread)->callsCnt = 0;
682 if (err != TB::ProbeState::FAIL)
684 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
686 int drawScore = TB::UseRule50 ? 1 : 0;
688 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
689 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
690 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
691 : VALUE_DRAW + 2 * wdl * drawScore;
693 Bound b = wdl < -drawScore ? BOUND_UPPER
694 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
696 if ( b == BOUND_EXACT
697 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
699 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
700 std::min(MAX_PLY - 1, depth + 6),
701 MOVE_NONE, VALUE_NONE);
708 if (b == BOUND_LOWER)
709 bestValue = value, alpha = std::max(alpha, bestValue);
717 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
719 // Step 6. Static evaluation of the position
722 // Skip early pruning when in check
723 ss->staticEval = eval = VALUE_NONE;
731 // Never assume anything about values stored in TT
732 ss->staticEval = eval = tte->eval();
733 if (eval == VALUE_NONE)
734 ss->staticEval = eval = evaluate(pos, &complexity);
735 else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost
736 complexity = abs(ss->staticEval - pos.psq_eg_stm());
738 // ttValue can be used as a better position evaluation (~4 Elo)
739 if ( ttValue != VALUE_NONE
740 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
745 ss->staticEval = eval = evaluate(pos, &complexity);
747 // Save static evaluation into transposition table
749 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
752 thisThread->complexityAverage.update(complexity);
754 // Use static evaluation difference to improve quiet move ordering (~3 Elo)
755 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
757 int bonus = std::clamp(-19 * int((ss-1)->staticEval + ss->staticEval), -1914, 1914);
758 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
761 // Set up the improvement variable, which is the difference between the current
762 // static evaluation and the previous static evaluation at our turn (if we were
763 // in check at our previous move we look at the move prior to it). The improvement
764 // margin and the improving flag are used in various pruning heuristics.
765 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
766 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
768 improving = improvement > 0;
771 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
772 // return a fail low.
773 if (eval < alpha - 369 - 254 * depth * depth)
775 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
780 // Step 8. Futility pruning: child node (~25 Elo).
781 // The depth condition is important for mate finding.
784 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 303 >= beta
786 && eval < 28031) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
789 // Step 9. Null move search with verification search (~22 Elo)
791 && (ss-1)->currentMove != MOVE_NULL
792 && (ss-1)->statScore < 17139
794 && eval >= ss->staticEval
795 && ss->staticEval >= beta - 20 * depth - improvement / 13 + 233 + complexity / 25
797 && pos.non_pawn_material(us)
798 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
800 assert(eval - beta >= 0);
802 // Null move dynamic reduction based on depth, eval and complexity of position
803 Depth R = std::min(int(eval - beta) / 168, 7) + depth / 3 + 4 - (complexity > 861);
805 ss->currentMove = MOVE_NULL;
806 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
808 pos.do_null_move(st);
810 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
812 pos.undo_null_move();
814 if (nullValue >= beta)
816 // Do not return unproven mate or TB scores
817 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
820 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
823 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
825 // Do verification search at high depths, with null move pruning disabled
826 // for us, until ply exceeds nmpMinPly.
827 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
828 thisThread->nmpColor = us;
830 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
832 thisThread->nmpMinPly = 0;
839 probCutBeta = beta + 191 - 54 * improving;
841 // Step 10. ProbCut (~4 Elo)
842 // If we have a good enough capture and a reduced search returns a value
843 // much above beta, we can (almost) safely prune the previous move.
846 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
847 // if value from transposition table is lower than probCutBeta, don't attempt probCut
848 // there and in further interactions with transposition table cutoff depth is set to depth - 3
849 // because probCut search has depth set to depth - 4 but we also do a move before it
850 // so effective depth is equal to depth - 3
852 && tte->depth() >= depth - 3
853 && ttValue != VALUE_NONE
854 && ttValue < probCutBeta))
856 assert(probCutBeta < VALUE_INFINITE);
858 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);
860 while ((move = mp.next_move()) != MOVE_NONE)
861 if (move != excludedMove && pos.legal(move))
863 assert(pos.capture(move) || promotion_type(move) == QUEEN);
865 ss->currentMove = move;
866 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
868 [pos.moved_piece(move)]
871 pos.do_move(move, st);
873 // Perform a preliminary qsearch to verify that the move holds
874 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
876 // If the qsearch held, perform the regular search
877 if (value >= probCutBeta)
878 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
882 if (value >= probCutBeta)
884 // Save ProbCut data into transposition table
885 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3, move, ss->staticEval);
891 // Step 11. If the position is not in TT, decrease depth by 3.
892 // Use qsearch if depth is equal or below zero (~4 Elo)
898 return qsearch<PV>(pos, ss, alpha, beta);
905 moves_loop: // When in check, search starts here
907 // Step 12. A small Probcut idea, when we are in check (~0 Elo)
908 probCutBeta = beta + 417;
913 && (tte->bound() & BOUND_LOWER)
914 && tte->depth() >= depth - 3
915 && ttValue >= probCutBeta
916 && abs(ttValue) <= VALUE_KNOWN_WIN
917 && abs(beta) <= VALUE_KNOWN_WIN
922 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
923 nullptr , (ss-4)->continuationHistory,
924 nullptr , (ss-6)->continuationHistory };
926 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
928 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
935 moveCountPruning = singularQuietLMR = false;
937 // Indicate PvNodes that will probably fail low if the node was searched
938 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
939 bool likelyFailLow = PvNode
941 && (tte->bound() & BOUND_UPPER)
942 && tte->depth() >= depth;
944 // Step 13. Loop through all pseudo-legal moves until no moves remain
945 // or a beta cutoff occurs.
946 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
950 if (move == excludedMove)
953 // At root obey the "searchmoves" option and skip moves not listed in Root
954 // Move List. As a consequence any illegal move is also skipped. In MultiPV
955 // mode we also skip PV moves which have been already searched and those
956 // of lower "TB rank" if we are in a TB root position.
957 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
958 thisThread->rootMoves.begin() + thisThread->pvLast, move))
961 // Check for legality
962 if (!rootNode && !pos.legal(move))
965 ss->moveCount = ++moveCount;
967 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
968 sync_cout << "info depth " << depth
969 << " currmove " << UCI::move(move, pos.is_chess960())
970 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
972 (ss+1)->pv = nullptr;
975 capture = pos.capture(move);
976 movedPiece = pos.moved_piece(move);
977 givesCheck = pos.gives_check(move);
979 // Calculate new depth for this move
980 newDepth = depth - 1;
982 Value delta = beta - alpha;
984 // Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.
986 && pos.non_pawn_material(us)
987 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
989 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo)
990 moveCountPruning = moveCount >= futility_move_count(improving, depth);
992 // Reduced depth of the next LMR search
993 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0);
998 // Futility pruning for captures (~0 Elo)
1003 && ss->staticEval + 180 + 201 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1004 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1007 // SEE based pruning (~9 Elo)
1008 if (!pos.see_ge(move, Value(-222) * depth))
1013 int history = (*contHist[0])[movedPiece][to_sq(move)]
1014 + (*contHist[1])[movedPiece][to_sq(move)]
1015 + (*contHist[3])[movedPiece][to_sq(move)];
1017 // Continuation history based pruning (~2 Elo)
1019 && history < -3875 * (depth - 1))
1022 history += 2 * thisThread->mainHistory[us][from_to(move)];
1024 // Futility pruning: parent node (~9 Elo)
1027 && ss->staticEval + 106 + 145 * lmrDepth + history / 52 <= alpha)
1030 // Prune moves with negative SEE (~3 Elo)
1031 if (!pos.see_ge(move, Value(-24 * lmrDepth * lmrDepth - 15 * lmrDepth)))
1036 // Step 15. Extensions (~66 Elo)
1037 // We take care to not overdo to avoid search getting stuck.
1038 if (ss->ply < thisThread->rootDepth * 2)
1040 // Singular extension search (~58 Elo). If all moves but one fail low on a
1041 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1042 // then that move is singular and should be extended. To verify this we do
1043 // a reduced search on all the other moves but the ttMove and if the
1044 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1046 && depth >= 4 - (thisThread->previousDepth > 24) + 2 * (PvNode && tte->is_pv())
1048 && !excludedMove // Avoid recursive singular search
1049 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1050 && abs(ttValue) < VALUE_KNOWN_WIN
1051 && (tte->bound() & BOUND_LOWER)
1052 && tte->depth() >= depth - 3)
1054 Value singularBeta = ttValue - (3 + (ss->ttPv && !PvNode)) * depth;
1055 Depth singularDepth = (depth - 1) / 2;
1057 ss->excludedMove = move;
1058 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1059 ss->excludedMove = MOVE_NONE;
1061 if (value < singularBeta)
1064 singularQuietLMR = !ttCapture;
1066 // Avoid search explosion by limiting the number of double extensions
1068 && value < singularBeta - 25
1069 && ss->doubleExtensions <= 9)
1073 // Multi-cut pruning
1074 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1075 // search without the ttMove. So we assume this expected Cut-node is not singular,
1076 // that multiple moves fail high, and we can prune the whole subtree by returning
1078 else if (singularBeta >= beta)
1079 return singularBeta;
1081 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1082 else if (ttValue >= beta)
1085 // If the eval of ttMove is less than alpha and value, we reduce it (negative extension)
1086 else if (ttValue <= alpha && ttValue <= value)
1090 // Check extensions (~1 Elo)
1091 else if ( givesCheck
1093 && abs(ss->staticEval) > 82)
1096 // Quiet ttMove extensions (~0 Elo)
1099 && move == ss->killers[0]
1100 && (*contHist[0])[movedPiece][to_sq(move)] >= 5177)
1104 // Add extension to new depth
1105 newDepth += extension;
1106 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1108 // Speculative prefetch as early as possible
1109 prefetch(TT.first_entry(pos.key_after(move)));
1111 // Update the current move (this must be done after singular extension search)
1112 ss->currentMove = move;
1113 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1118 // Step 16. Make the move
1119 pos.do_move(move, st, givesCheck);
1121 // Step 17. Late moves reduction / extension (LMR, ~98 Elo)
1122 // We use various heuristics for the sons of a node after the first son has
1123 // been searched. In general we would like to reduce them, but there are many
1124 // cases where we extend a son if it has good chances to be "interesting".
1126 && moveCount > 1 + (PvNode && ss->ply <= 1)
1129 || (cutNode && (ss-1)->moveCount > 1)))
1131 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
1133 // Decrease reduction if position is or has been on the PV
1134 // and node is not likely to fail low. (~3 Elo)
1139 // Decrease reduction if opponent's move count is high (~1 Elo)
1140 if ((ss-1)->moveCount > 7)
1143 // Increase reduction for cut nodes (~3 Elo)
1147 // Increase reduction if ttMove is a capture (~3 Elo)
1151 // Decrease reduction for PvNodes based on depth
1153 r -= 1 + 11 / (3 + depth);
1155 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1156 if (singularQuietLMR)
1159 // Dicrease reduction if we move a threatened piece (~1 Elo)
1161 && (mp.threatenedPieces & from_sq(move)))
1164 // Increase reduction if next ply has a lot of fail high
1165 if ((ss+1)->cutoffCnt > 3 && !PvNode)
1168 ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
1169 + (*contHist[0])[movedPiece][to_sq(move)]
1170 + (*contHist[1])[movedPiece][to_sq(move)]
1171 + (*contHist[3])[movedPiece][to_sq(move)]
1174 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1175 r -= ss->statScore / (13628 + 4000 * (depth > 7 && depth < 19));
1177 // In general we want to cap the LMR depth search at newDepth, but when
1178 // reduction is negative, we allow this move a limited search extension
1179 // beyond the first move depth. This may lead to hidden double extensions.
1180 Depth d = std::clamp(newDepth - r, 1, newDepth + 1);
1182 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1184 // Do full depth search when reduced LMR search fails high
1185 if (value > alpha && d < newDepth)
1187 // Adjust full depth search based on LMR results - if result
1188 // was good enough search deeper, if it was bad enough search shallower
1189 const bool doDeeperSearch = value > (alpha + 64 + 11 * (newDepth - d));
1190 const bool doShallowerSearch = value < bestValue + newDepth;
1191 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch - doShallowerSearch, !cutNode);
1193 int bonus = value > alpha ? stat_bonus(newDepth)
1194 : -stat_bonus(newDepth);
1199 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1203 // Step 18. Full depth search when LMR is skipped
1204 else if (!PvNode || moveCount > 1)
1206 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1209 // For PV nodes only, do a full PV search on the first move or after a fail
1210 // high (in the latter case search only if value < beta), otherwise let the
1211 // parent node fail low with value <= alpha and try another move.
1212 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1215 (ss+1)->pv[0] = MOVE_NONE;
1217 value = -search<PV>(pos, ss+1, -beta, -alpha,
1218 std::min(maxNextDepth, newDepth), false);
1221 // Step 19. Undo move
1222 pos.undo_move(move);
1224 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1226 // Step 20. Check for a new best move
1227 // Finished searching the move. If a stop occurred, the return value of
1228 // the search cannot be trusted, and we return immediately without
1229 // updating best move, PV and TT.
1230 if (Threads.stop.load(std::memory_order_relaxed))
1235 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1236 thisThread->rootMoves.end(), move);
1238 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1240 // PV move or new best move?
1241 if (moveCount == 1 || value > alpha)
1244 rm.selDepth = thisThread->selDepth;
1249 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1250 rm.pv.push_back(*m);
1252 // We record how often the best move has been changed in each iteration.
1253 // This information is used for time management. In MultiPV mode,
1254 // we must take care to only do this for the first PV line.
1256 && !thisThread->pvIdx)
1257 ++thisThread->bestMoveChanges;
1260 // All other moves but the PV are set to the lowest value: this
1261 // is not a problem when sorting because the sort is stable and the
1262 // move position in the list is preserved - just the PV is pushed up.
1263 rm.score = -VALUE_INFINITE;
1266 if (value > bestValue)
1274 if (PvNode && !rootNode) // Update pv even in fail-high case
1275 update_pv(ss->pv, move, (ss+1)->pv);
1277 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1281 // Reduce other moves if we have found at least one score improvement
1284 && beta < VALUE_KNOWN_WIN
1285 && alpha > -VALUE_KNOWN_WIN)
1293 assert(value >= beta); // Fail high
1300 // If the move is worse than some previously searched move, remember it to update its stats later
1301 if (move != bestMove)
1303 if (capture && captureCount < 32)
1304 capturesSearched[captureCount++] = move;
1306 else if (!capture && quietCount < 64)
1307 quietsSearched[quietCount++] = move;
1311 // The following condition would detect a stop only after move loop has been
1312 // completed. But in this case bestValue is valid because we have fully
1313 // searched our subtree, and we can anyhow save the result in TT.
1319 // Step 21. Check for mate and stalemate
1320 // All legal moves have been searched and if there are no legal moves, it
1321 // must be a mate or a stalemate. If we are in a singular extension search then
1322 // return a fail low score.
1324 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1327 bestValue = excludedMove ? alpha :
1328 ss->inCheck ? mated_in(ss->ply)
1331 // If there is a move which produces search value greater than alpha we update stats of searched moves
1333 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1334 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1336 // Bonus for prior countermove that caused the fail low
1337 else if ( (depth >= 5 || PvNode)
1340 //Assign extra bonus if current node is PvNode or cutNode
1341 //or fail low was really bad
1342 bool extraBonus = PvNode
1344 || bestValue < alpha - 62 * depth;
1346 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + extraBonus));
1350 bestValue = std::min(bestValue, maxValue);
1352 // If no good move is found and the previous position was ttPv, then the previous
1353 // opponent move is probably good and the new position is added to the search tree.
1354 if (bestValue <= alpha)
1355 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1357 // Write gathered information in transposition table
1358 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1359 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1360 bestValue >= beta ? BOUND_LOWER :
1361 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1362 depth, bestMove, ss->staticEval);
1364 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1370 // qsearch() is the quiescence search function, which is called by the main search
1371 // function with zero depth, or recursively with further decreasing depth per call.
1373 template <NodeType nodeType>
1374 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1376 static_assert(nodeType != Root);
1377 constexpr bool PvNode = nodeType == PV;
1379 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1380 assert(PvNode || (alpha == beta - 1));
1385 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1389 Move ttMove, move, bestMove;
1391 Value bestValue, value, ttValue, futilityValue, futilityBase;
1392 bool pvHit, givesCheck, capture;
1398 ss->pv[0] = MOVE_NONE;
1401 Thread* thisThread = pos.this_thread();
1402 bestMove = MOVE_NONE;
1403 ss->inCheck = pos.checkers();
1406 // Check for an immediate draw or maximum ply reached
1407 if ( pos.is_draw(ss->ply)
1408 || ss->ply >= MAX_PLY)
1409 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1411 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1413 // Decide whether or not to include checks: this fixes also the type of
1414 // TT entry depth that we are going to use. Note that in qsearch we use
1415 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1416 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1417 : DEPTH_QS_NO_CHECKS;
1418 // Transposition table lookup
1420 tte = TT.probe(posKey, ss->ttHit);
1421 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1422 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1423 pvHit = ss->ttHit && tte->is_pv();
1427 && tte->depth() >= ttDepth
1428 && ttValue != VALUE_NONE // Only in case of TT access race
1429 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
1432 // Evaluate the position statically
1435 ss->staticEval = VALUE_NONE;
1436 bestValue = futilityBase = -VALUE_INFINITE;
1442 // Never assume anything about values stored in TT
1443 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1444 ss->staticEval = bestValue = evaluate(pos);
1446 // ttValue can be used as a better position evaluation (~7 Elo)
1447 if ( ttValue != VALUE_NONE
1448 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1449 bestValue = ttValue;
1452 // In case of null move search use previous static eval with a different sign
1453 ss->staticEval = bestValue =
1454 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1455 : -(ss-1)->staticEval;
1457 // Stand pat. Return immediately if static value is at least beta
1458 if (bestValue >= beta)
1460 // Save gathered info in transposition table
1462 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1463 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1468 if (PvNode && bestValue > alpha)
1471 futilityBase = bestValue + 153;
1474 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1475 nullptr , (ss-4)->continuationHistory,
1476 nullptr , (ss-6)->continuationHistory };
1478 // Initialize a MovePicker object for the current position, and prepare
1479 // to search the moves. Because the depth is <= 0 here, only captures,
1480 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1481 // will be generated.
1482 Square prevSq = to_sq((ss-1)->currentMove);
1483 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1484 &thisThread->captureHistory,
1488 int quietCheckEvasions = 0;
1490 // Loop through the moves until no moves remain or a beta cutoff occurs
1491 while ((move = mp.next_move()) != MOVE_NONE)
1493 assert(is_ok(move));
1495 // Check for legality
1496 if (!pos.legal(move))
1499 givesCheck = pos.gives_check(move);
1500 capture = pos.capture(move);
1504 // Futility pruning and moveCount pruning (~5 Elo)
1505 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1507 && to_sq(move) != prevSq
1508 && futilityBase > -VALUE_KNOWN_WIN
1509 && type_of(move) != PROMOTION)
1515 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1517 if (futilityValue <= alpha)
1519 bestValue = std::max(bestValue, futilityValue);
1523 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1525 bestValue = std::max(bestValue, futilityBase);
1530 // Do not search moves with negative SEE values (~5 Elo)
1531 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1532 && !pos.see_ge(move))
1535 // Speculative prefetch as early as possible
1536 prefetch(TT.first_entry(pos.key_after(move)));
1538 ss->currentMove = move;
1539 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1541 [pos.moved_piece(move)]
1544 // Continuation history based pruning (~2 Elo)
1546 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1547 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < 0
1548 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < 0)
1551 // movecount pruning for quiet check evasions
1552 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1553 && quietCheckEvasions > 1
1558 quietCheckEvasions += !capture && ss->inCheck;
1560 // Make and search the move
1561 pos.do_move(move, st, givesCheck);
1562 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1563 pos.undo_move(move);
1565 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1567 // Check for a new best move
1568 if (value > bestValue)
1576 if (PvNode) // Update pv even in fail-high case
1577 update_pv(ss->pv, move, (ss+1)->pv);
1579 if (PvNode && value < beta) // Update alpha here!
1587 // All legal moves have been searched. A special case: if we're in check
1588 // and no legal moves were found, it is checkmate.
1589 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1591 assert(!MoveList<LEGAL>(pos).size());
1593 return mated_in(ss->ply); // Plies to mate from the root
1596 // Save gathered info in transposition table
1597 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1598 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1599 ttDepth, bestMove, ss->staticEval);
1601 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1607 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1608 // "plies to mate from the current position". Standard scores are unchanged.
1609 // The function is called before storing a value in the transposition table.
1611 Value value_to_tt(Value v, int ply) {
1613 assert(v != VALUE_NONE);
1615 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1616 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1620 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1621 // from the transposition table (which refers to the plies to mate/be mated from
1622 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1623 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1624 // and the graph history interaction, we return an optimal TB score instead.
1626 Value value_from_tt(Value v, int ply, int r50c) {
1628 if (v == VALUE_NONE)
1631 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1633 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1634 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1639 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1641 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1642 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1651 // update_pv() adds current move and appends child pv[]
1653 void update_pv(Move* pv, Move move, const Move* childPv) {
1655 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1661 // update_all_stats() updates stats at the end of search() when a bestMove is found
1663 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1664 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1666 Color us = pos.side_to_move();
1667 Thread* thisThread = pos.this_thread();
1668 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1669 Piece moved_piece = pos.moved_piece(bestMove);
1670 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1671 int bonus1 = stat_bonus(depth + 1);
1673 if (!pos.capture(bestMove))
1675 int bonus2 = bestValue > beta + 137 ? bonus1 // larger bonus
1676 : stat_bonus(depth); // smaller bonus
1678 // Increase stats for the best move in case it was a quiet move
1679 update_quiet_stats(pos, ss, bestMove, bonus2);
1681 // Decrease stats for all non-best quiet moves
1682 for (int i = 0; i < quietCount; ++i)
1684 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1685 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1689 // Increase stats for the best move in case it was a capture move
1690 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1692 // Extra penalty for a quiet early move that was not a TT move or
1693 // main killer move in previous ply when it gets refuted.
1694 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1695 && !pos.captured_piece())
1696 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1698 // Decrease stats for all non-best capture moves
1699 for (int i = 0; i < captureCount; ++i)
1701 moved_piece = pos.moved_piece(capturesSearched[i]);
1702 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1703 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1708 // update_continuation_histories() updates histories of the move pairs formed
1709 // by moves at ply -1, -2, -4, and -6 with current move.
1711 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1713 for (int i : {1, 2, 4, 6})
1715 // Only update first 2 continuation histories if we are in check
1716 if (ss->inCheck && i > 2)
1718 if (is_ok((ss-i)->currentMove))
1719 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1724 // update_quiet_stats() updates move sorting heuristics
1726 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1729 if (ss->killers[0] != move)
1731 ss->killers[1] = ss->killers[0];
1732 ss->killers[0] = move;
1735 Color us = pos.side_to_move();
1736 Thread* thisThread = pos.this_thread();
1737 thisThread->mainHistory[us][from_to(move)] << bonus;
1738 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1740 // Update countermove history
1741 if (is_ok((ss-1)->currentMove))
1743 Square prevSq = to_sq((ss-1)->currentMove);
1744 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1748 // When playing with strength handicap, choose best move among a set of RootMoves
1749 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1751 Move Skill::pick_best(size_t multiPV) {
1753 const RootMoves& rootMoves = Threads.main()->rootMoves;
1754 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1756 // RootMoves are already sorted by score in descending order
1757 Value topScore = rootMoves[0].score;
1758 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1759 int maxScore = -VALUE_INFINITE;
1760 double weakness = 120 - 2 * level;
1762 // Choose best move. For each move score we add two terms, both dependent on
1763 // weakness. One is deterministic and bigger for weaker levels, and one is
1764 // random. Then we choose the move with the resulting highest score.
1765 for (size_t i = 0; i < multiPV; ++i)
1767 // This is our magic formula
1768 int push = int(( weakness * int(topScore - rootMoves[i].score)
1769 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1771 if (rootMoves[i].score + push >= maxScore)
1773 maxScore = rootMoves[i].score + push;
1774 best = rootMoves[i].pv[0];
1784 /// MainThread::check_time() is used to print debug info and, more importantly,
1785 /// to detect when we are out of available time and thus stop the search.
1787 void MainThread::check_time() {
1792 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1793 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1795 static TimePoint lastInfoTime = now();
1797 TimePoint elapsed = Time.elapsed();
1798 TimePoint tick = Limits.startTime + elapsed;
1800 if (tick - lastInfoTime >= 1000)
1802 lastInfoTime = tick;
1806 // We should not stop pondering until told so by the GUI
1810 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1811 || (Limits.movetime && elapsed >= Limits.movetime)
1812 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1813 Threads.stop = true;
1817 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1818 /// that all (if any) unsearched PV lines are sent using a previous search score.
1820 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1822 std::stringstream ss;
1823 TimePoint elapsed = Time.elapsed() + 1;
1824 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1825 size_t pvIdx = pos.this_thread()->pvIdx;
1826 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1827 uint64_t nodesSearched = Threads.nodes_searched();
1828 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1830 for (size_t i = 0; i < multiPV; ++i)
1832 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1834 if (depth == 1 && !updated && i > 0)
1837 Depth d = updated ? depth : std::max(1, depth - 1);
1838 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1840 if (v == -VALUE_INFINITE)
1843 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1844 v = tb ? rootMoves[i].tbScore : v;
1846 if (ss.rdbuf()->in_avail()) // Not at first line
1851 << " seldepth " << rootMoves[i].selDepth
1852 << " multipv " << i + 1
1853 << " score " << UCI::value(v);
1855 if (Options["UCI_ShowWDL"])
1856 ss << UCI::wdl(v, pos.game_ply());
1858 if (!tb && i == pvIdx)
1859 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1861 ss << " nodes " << nodesSearched
1862 << " nps " << nodesSearched * 1000 / elapsed
1863 << " hashfull " << TT.hashfull()
1864 << " tbhits " << tbHits
1865 << " time " << elapsed
1868 for (Move m : rootMoves[i].pv)
1869 ss << " " << UCI::move(m, pos.is_chess960());
1876 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1877 /// before exiting the search, for instance, in case we stop the search during a
1878 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1879 /// otherwise in case of 'ponder on' we have nothing to think on.
1881 bool RootMove::extract_ponder_from_tt(Position& pos) {
1884 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1888 assert(pv.size() == 1);
1890 if (pv[0] == MOVE_NONE)
1893 pos.do_move(pv[0], st);
1894 TTEntry* tte = TT.probe(pos.key(), ttHit);
1898 Move m = tte->move(); // Local copy to be SMP safe
1899 if (MoveList<LEGAL>(pos).contains(m))
1903 pos.undo_move(pv[0]);
1904 return pv.size() > 1;
1907 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1910 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1911 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1912 Cardinality = int(Options["SyzygyProbeLimit"]);
1913 bool dtz_available = true;
1915 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1916 // ProbeDepth == DEPTH_ZERO
1917 if (Cardinality > MaxCardinality)
1919 Cardinality = MaxCardinality;
1923 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1925 // Rank moves using DTZ tables
1926 RootInTB = root_probe(pos, rootMoves);
1930 // DTZ tables are missing; try to rank moves using WDL tables
1931 dtz_available = false;
1932 RootInTB = root_probe_wdl(pos, rootMoves);
1938 // Sort moves according to TB rank
1939 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1940 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1942 // Probe during search only if DTZ is not available and we are winning
1943 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1948 // Clean up if root_probe() and root_probe_wdl() have failed
1949 for (auto& m : rootMoves)
1954 } // namespace Stockfish