2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 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(214 * (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, bool rangeReduction) {
73 int r = Reductions[d] * Reductions[mn];
74 return (r + 534) / 1024 + (!i && r > 904) + rangeReduction;
77 constexpr int futility_move_count(bool improving, Depth depth) {
78 return (3 + depth * depth) / (2 - improving);
81 // History and stats update bonus, based on depth
82 int stat_bonus(Depth d) {
83 return std::min((6 * d + 229) * d - 215 , 2000);
86 // Add a small random component to draw evaluations to avoid 3-fold blindness
87 Value value_draw(Thread* thisThread) {
88 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
91 // Check if the current thread is in a search explosion
92 ExplosionState search_explosion(Thread* thisThread) {
94 uint64_t nodesNow = thisThread->nodes;
95 bool explosive = thisThread->doubleExtensionAverage[WHITE].is_greater(2, 100)
96 || thisThread->doubleExtensionAverage[BLACK].is_greater(2, 100);
99 thisThread->nodesLastExplosive = nodesNow;
101 thisThread->nodesLastNormal = nodesNow;
104 && thisThread->state == EXPLOSION_NONE
105 && nodesNow - thisThread->nodesLastNormal > 6000000)
106 thisThread->state = MUST_CALM_DOWN;
108 if ( thisThread->state == MUST_CALM_DOWN
109 && nodesNow - thisThread->nodesLastExplosive > 6000000)
110 thisThread->state = EXPLOSION_NONE;
112 return thisThread->state;
115 // Skill structure is used to implement strength limit. If we have an uci_elo then
116 // we convert it to a suitable fractional skill level using anchoring to CCRL Elo
117 // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
118 // results spanning a wide range of k values.
120 Skill(int skill_level, int uci_elo) {
122 level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0);
124 level = double(skill_level);
126 bool enabled() const { return level < 20.0; }
127 bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
128 Move pick_best(size_t multiPV);
131 Move best = MOVE_NONE;
134 template <NodeType nodeType>
135 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
137 template <NodeType nodeType>
138 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
140 Value value_to_tt(Value v, int ply);
141 Value value_from_tt(Value v, int ply, int r50c);
142 void update_pv(Move* pv, Move move, Move* childPv);
143 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
144 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
145 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
146 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
148 // perft() is our utility to verify move generation. All the leaf nodes up
149 // to the given depth are generated and counted, and the sum is returned.
151 uint64_t perft(Position& pos, Depth depth) {
154 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
156 uint64_t cnt, nodes = 0;
157 const bool leaf = (depth == 2);
159 for (const auto& m : MoveList<LEGAL>(pos))
161 if (Root && depth <= 1)
166 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
171 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
179 /// Search::init() is called at startup to initialize various lookup tables
181 void Search::init() {
183 for (int i = 1; i < MAX_MOVES; ++i)
184 Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i));
188 /// Search::clear() resets search state to its initial value
190 void Search::clear() {
192 Threads.main()->wait_for_search_finished();
194 Time.availableNodes = 0;
197 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
201 /// MainThread::search() is started when the program receives the UCI 'go'
202 /// command. It searches from the root position and outputs the "bestmove".
204 void MainThread::search() {
208 nodes = perft<true>(rootPos, Limits.perft);
209 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
213 Color us = rootPos.side_to_move();
214 Time.init(Limits, us, rootPos.game_ply());
217 Eval::NNUE::verify();
219 if (rootMoves.empty())
221 rootMoves.emplace_back(MOVE_NONE);
222 sync_cout << "info depth 0 score "
223 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
228 Threads.start_searching(); // start non-main threads
229 Thread::search(); // main thread start searching
232 // When we reach the maximum depth, we can arrive here without a raise of
233 // Threads.stop. However, if we are pondering or in an infinite search,
234 // the UCI protocol states that we shouldn't print the best move before the
235 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
236 // until the GUI sends one of those commands.
238 while (!Threads.stop && (ponder || Limits.infinite))
239 {} // Busy wait for a stop or a ponder reset
241 // Stop the threads if not already stopped (also raise the stop if
242 // "ponderhit" just reset Threads.ponder).
245 // Wait until all threads have finished
246 Threads.wait_for_search_finished();
248 // When playing in 'nodes as time' mode, subtract the searched nodes from
249 // the available ones before exiting.
251 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
253 Thread* bestThread = this;
254 Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
256 if ( int(Options["MultiPV"]) == 1
259 && rootMoves[0].pv[0] != MOVE_NONE)
260 bestThread = Threads.get_best_thread();
262 bestPreviousScore = bestThread->rootMoves[0].score;
264 // Send again PV info if we have a new best thread
265 if (bestThread != this)
266 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
268 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
270 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
271 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
273 std::cout << sync_endl;
277 /// Thread::search() is the main iterative deepening loop. It calls search()
278 /// repeatedly with increasing depth until the allocated thinking time has been
279 /// consumed, the user stops the search, or the maximum search depth is reached.
281 void Thread::search() {
283 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
284 // The former is needed to allow update_continuation_histories(ss-1, ...),
285 // which accesses its argument at ss-6, also near the root.
286 // The latter is needed for statScore and killer initialization.
287 Stack stack[MAX_PLY+10], *ss = stack+7;
289 Value alpha, beta, delta;
290 Move lastBestMove = MOVE_NONE;
291 Depth lastBestMoveDepth = 0;
292 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
293 double timeReduction = 1, totBestMoveChanges = 0;
294 Color us = rootPos.side_to_move();
297 std::memset(ss-7, 0, 10 * sizeof(Stack));
298 for (int i = 7; i > 0; i--)
299 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
301 for (int i = 0; i <= MAX_PLY + 2; ++i)
306 bestValue = delta = alpha = -VALUE_INFINITE;
307 beta = VALUE_INFINITE;
311 if (mainThread->bestPreviousScore == VALUE_INFINITE)
312 for (int i = 0; i < 4; ++i)
313 mainThread->iterValue[i] = VALUE_ZERO;
315 for (int i = 0; i < 4; ++i)
316 mainThread->iterValue[i] = mainThread->bestPreviousScore;
319 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
320 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
322 size_t multiPV = size_t(Options["MultiPV"]);
323 Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
325 // When playing with strength handicap enable MultiPV search that we will
326 // use behind the scenes to retrieve a set of possible moves.
328 multiPV = std::max(multiPV, (size_t)4);
330 multiPV = std::min(multiPV, rootMoves.size());
332 doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0%
333 doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0%
335 nodesLastExplosive = nodes;
336 nodesLastNormal = nodes;
337 state = EXPLOSION_NONE;
339 optimism[ us] = Value(25);
340 optimism[~us] = -optimism[us];
342 int searchAgainCounter = 0;
344 // Iterative deepening loop until requested to stop or the target depth is reached
345 while ( ++rootDepth < MAX_PLY
347 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
349 // Age out PV variability metric
351 totBestMoveChanges /= 2;
353 // Save the last iteration's scores before first PV line is searched and
354 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
355 for (RootMove& rm : rootMoves)
356 rm.previousScore = rm.score;
361 if (!Threads.increaseDepth)
362 searchAgainCounter++;
364 // MultiPV loop. We perform a full root search for each PV line
365 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
370 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
371 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
375 // Reset UCI info selDepth for each depth and each PV line
378 // Reset aspiration window starting size
381 Value prev = rootMoves[pvIdx].averageScore;
382 delta = Value(17) + int(prev) * prev / 16384;
383 alpha = std::max(prev - delta,-VALUE_INFINITE);
384 beta = std::min(prev + delta, VALUE_INFINITE);
386 // Adjust trend and optimism based on root move's previousScore
387 int tr = sigmoid(prev, 0, 0, 147, 113, 1);
388 trend = (us == WHITE ? make_score(tr, tr / 2)
389 : -make_score(tr, tr / 2));
391 int opt = sigmoid(prev, 0, 25, 147, 14464, 256);
392 optimism[ us] = Value(opt);
393 optimism[~us] = -optimism[us];
396 // Start with a small aspiration window and, in the case of a fail
397 // high/low, re-search with a bigger window until we don't fail
399 int failedHighCnt = 0;
402 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
403 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
405 // Bring the best move to the front. It is critical that sorting
406 // is done with a stable algorithm because all the values but the
407 // first and eventually the new best one are set to -VALUE_INFINITE
408 // and we want to keep the same order for all the moves except the
409 // new PV that goes to the front. Note that in case of MultiPV
410 // search the already searched PV lines are preserved.
411 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
413 // If search has been stopped, we break immediately. Sorting is
414 // safe because RootMoves is still valid, although it refers to
415 // the previous iteration.
419 // When failing high/low give some update (without cluttering
420 // the UI) before a re-search.
423 && (bestValue <= alpha || bestValue >= beta)
424 && Time.elapsed() > 3000)
425 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
427 // In case of failing low/high increase aspiration window and
428 // re-search, otherwise exit the loop.
429 if (bestValue <= alpha)
431 beta = (alpha + beta) / 2;
432 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
436 mainThread->stopOnPonderhit = false;
438 else if (bestValue >= beta)
440 beta = std::min(bestValue + delta, VALUE_INFINITE);
446 delta += delta / 4 + 5;
448 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
451 // Sort the PV lines searched so far and update the GUI
452 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
455 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
456 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
460 completedDepth = rootDepth;
462 if (rootMoves[0].pv[0] != lastBestMove) {
463 lastBestMove = rootMoves[0].pv[0];
464 lastBestMoveDepth = rootDepth;
467 // Have we found a "mate in x"?
469 && bestValue >= VALUE_MATE_IN_MAX_PLY
470 && VALUE_MATE - bestValue <= 2 * Limits.mate)
476 // If skill level is enabled and time is up, pick a sub-optimal best move
477 if (skill.enabled() && skill.time_to_pick(rootDepth))
478 skill.pick_best(multiPV);
480 // Do we have time for the next iteration? Can we stop searching now?
481 if ( Limits.use_time_management()
483 && !mainThread->stopOnPonderhit)
485 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
486 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
487 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
489 // If the bestMove is stable over several iterations, reduce time accordingly
490 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
491 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
493 // Use part of the gained time from a previous stable move for the current move
494 for (Thread* th : Threads)
496 totBestMoveChanges += th->bestMoveChanges;
497 th->bestMoveChanges = 0;
499 double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth)
500 * totBestMoveChanges / Threads.size();
501 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
503 // Cap used time in case of a single legal move for a better viewer experience in tournaments
504 // yielding correct scores and sufficiently fast moves.
505 if (rootMoves.size() == 1)
506 totalTime = std::min(500.0, totalTime);
508 // Stop the search if we have exceeded the totalTime
509 if (Time.elapsed() > totalTime)
511 // If we are allowed to ponder do not stop the search now but
512 // keep pondering until the GUI sends "ponderhit" or "stop".
513 if (mainThread->ponder)
514 mainThread->stopOnPonderhit = true;
518 else if ( Threads.increaseDepth
519 && !mainThread->ponder
520 && Time.elapsed() > totalTime * 0.58)
521 Threads.increaseDepth = false;
523 Threads.increaseDepth = true;
526 mainThread->iterValue[iterIdx] = bestValue;
527 iterIdx = (iterIdx + 1) & 3;
533 mainThread->previousTimeReduction = timeReduction;
535 // If skill level is enabled, swap best PV line with the sub-optimal one
537 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
538 skill.best ? skill.best : skill.pick_best(multiPV)));
544 // search<>() is the main search function for both PV and non-PV nodes
546 template <NodeType nodeType>
547 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
549 Thread* thisThread = pos.this_thread();
551 // Step 0. Limit search explosion
553 && search_explosion(thisThread) == MUST_CALM_DOWN
554 && depth > (ss-1)->depth)
555 depth = (ss-1)->depth;
557 constexpr bool PvNode = nodeType != NonPV;
558 constexpr bool rootNode = nodeType == Root;
559 const Depth maxNextDepth = rootNode ? depth : depth + 1;
561 // Check if we have an upcoming move which draws by repetition, or
562 // if the opponent had an alternative move earlier to this position.
564 && pos.rule50_count() >= 3
565 && alpha < VALUE_DRAW
566 && pos.has_game_cycle(ss->ply))
568 alpha = value_draw(pos.this_thread());
573 // Dive into quiescence search when the depth reaches zero
575 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
577 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
578 assert(PvNode || (alpha == beta - 1));
579 assert(0 < depth && depth < MAX_PLY);
580 assert(!(PvNode && cutNode));
582 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
584 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
588 Move ttMove, move, excludedMove, bestMove;
589 Depth extension, newDepth;
590 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
591 bool givesCheck, improving, didLMR, priorCapture;
592 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
593 ttCapture, singularQuietLMR;
595 int moveCount, captureCount, quietCount, bestMoveCount, improvement;
597 // Step 1. Initialize node
598 ss->inCheck = pos.checkers();
599 priorCapture = pos.captured_piece();
600 Color us = pos.side_to_move();
601 moveCount = bestMoveCount = captureCount = quietCount = ss->moveCount = 0;
602 bestValue = -VALUE_INFINITE;
603 maxValue = VALUE_INFINITE;
605 // Check for the available remaining time
606 if (thisThread == Threads.main())
607 static_cast<MainThread*>(thisThread)->check_time();
609 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
610 if (PvNode && thisThread->selDepth < ss->ply + 1)
611 thisThread->selDepth = ss->ply + 1;
615 // Step 2. Check for aborted search and immediate draw
616 if ( Threads.stop.load(std::memory_order_relaxed)
617 || pos.is_draw(ss->ply)
618 || ss->ply >= MAX_PLY)
619 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
620 : value_draw(pos.this_thread());
622 // Step 3. Mate distance pruning. Even if we mate at the next move our score
623 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
624 // a shorter mate was found upward in the tree then there is no need to search
625 // because we will never beat the current alpha. Same logic but with reversed
626 // signs applies also in the opposite condition of being mated instead of giving
627 // mate. In this case return a fail-high score.
628 alpha = std::max(mated_in(ss->ply), alpha);
629 beta = std::min(mate_in(ss->ply+1), beta);
634 thisThread->rootDelta = beta - alpha;
636 assert(0 <= ss->ply && ss->ply < MAX_PLY);
638 (ss+1)->ttPv = false;
639 (ss+1)->excludedMove = bestMove = MOVE_NONE;
640 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
641 ss->doubleExtensions = (ss-1)->doubleExtensions;
643 Square prevSq = to_sq((ss-1)->currentMove);
645 // Update the running average statistics for double extensions
646 thisThread->doubleExtensionAverage[us].update(ss->depth > (ss-1)->depth);
648 // Initialize statScore to zero for the grandchildren of the current position.
649 // So statScore is shared between all grandchildren and only the first grandchild
650 // starts with statScore = 0. Later grandchildren start with the last calculated
651 // statScore of the previous grandchild. This influences the reduction rules in
652 // LMR which are based on the statScore of parent position.
654 (ss+2)->statScore = 0;
656 // Step 4. Transposition table lookup. We don't want the score of a partial
657 // search to overwrite a previous full search TT value, so we use a different
658 // position key in case of an excluded move.
659 excludedMove = ss->excludedMove;
660 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
661 tte = TT.probe(posKey, ss->ttHit);
662 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
663 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
664 : ss->ttHit ? tte->move() : MOVE_NONE;
665 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
667 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
669 // Update low ply history for previous move if we are near root and position is or has been in PV
672 && ss->ply - 1 < MAX_LPH
674 && is_ok((ss-1)->currentMove))
675 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
677 // At non-PV nodes we check for an early TT cutoff
680 && tte->depth() > depth - (thisThread->id() % 2 == 1)
681 && ttValue != VALUE_NONE // Possible in case of TT access race
682 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
683 : (tte->bound() & BOUND_UPPER)))
685 // If ttMove is quiet, update move sorting heuristics on TT hit
690 // Bonus for a quiet ttMove that fails high
692 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
694 // Extra penalty for early quiet moves of the previous ply
695 if ((ss-1)->moveCount <= 2 && !priorCapture)
696 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
698 // Penalty for a quiet ttMove that fails low
701 int penalty = -stat_bonus(depth);
702 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
703 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
707 // Partial workaround for the graph history interaction problem
708 // For high rule50 counts don't produce transposition table cutoffs.
709 if (pos.rule50_count() < 90)
713 // Step 5. Tablebases probe
714 if (!rootNode && TB::Cardinality)
716 int piecesCount = pos.count<ALL_PIECES>();
718 if ( piecesCount <= TB::Cardinality
719 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
720 && pos.rule50_count() == 0
721 && !pos.can_castle(ANY_CASTLING))
724 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
726 // Force check of time on the next occasion
727 if (thisThread == Threads.main())
728 static_cast<MainThread*>(thisThread)->callsCnt = 0;
730 if (err != TB::ProbeState::FAIL)
732 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
734 int drawScore = TB::UseRule50 ? 1 : 0;
736 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
737 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
738 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
739 : VALUE_DRAW + 2 * wdl * drawScore;
741 Bound b = wdl < -drawScore ? BOUND_UPPER
742 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
744 if ( b == BOUND_EXACT
745 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
747 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
748 std::min(MAX_PLY - 1, depth + 6),
749 MOVE_NONE, VALUE_NONE);
756 if (b == BOUND_LOWER)
757 bestValue = value, alpha = std::max(alpha, bestValue);
765 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
767 // Step 6. Static evaluation of the position
770 // Skip early pruning when in check
771 ss->staticEval = eval = VALUE_NONE;
778 // Never assume anything about values stored in TT
779 ss->staticEval = eval = tte->eval();
780 if (eval == VALUE_NONE)
781 ss->staticEval = eval = evaluate(pos);
783 // Randomize draw evaluation
784 if (eval == VALUE_DRAW)
785 eval = value_draw(thisThread);
787 // Can ttValue be used as a better position evaluation?
788 if ( ttValue != VALUE_NONE
789 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
794 ss->staticEval = eval = evaluate(pos);
796 // Save static evaluation into transposition table
798 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
801 // Use static evaluation difference to improve quiet move ordering
802 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
804 int bonus = std::clamp(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000);
805 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
808 // Set up the improvement variable, which is the difference between the current
809 // static evaluation and the previous static evaluation at our turn (if we were
810 // in check at our previous move we look at the move prior to it). The improvement
811 // margin and the improving flag are used in various pruning heuristics.
812 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
813 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
816 improving = improvement > 0;
818 // Step 7. Futility pruning: child node (~50 Elo).
819 // The depth condition is important for mate finding.
822 && eval - futility_margin(depth, improving) >= beta
823 && eval < 15000) // 50% larger than VALUE_KNOWN_WIN, but smaller than TB wins.
826 // Step 8. Null move search with verification search (~40 Elo)
828 && (ss-1)->currentMove != MOVE_NULL
829 && (ss-1)->statScore < 23767
831 && eval >= ss->staticEval
832 && ss->staticEval >= beta - 20 * depth - improvement / 15 + 204
834 && pos.non_pawn_material(us)
835 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
837 assert(eval - beta >= 0);
839 // Null move dynamic reduction based on depth and value
840 Depth R = std::min(int(eval - beta) / 205, 3) + depth / 3 + 4;
842 ss->currentMove = MOVE_NULL;
843 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
845 pos.do_null_move(st);
847 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
849 pos.undo_null_move();
851 if (nullValue >= beta)
853 // Do not return unproven mate or TB scores
854 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
857 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
860 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
862 // Do verification search at high depths, with null move pruning disabled
863 // for us, until ply exceeds nmpMinPly.
864 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
865 thisThread->nmpColor = us;
867 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
869 thisThread->nmpMinPly = 0;
876 probCutBeta = beta + 209 - 44 * improving;
878 // Step 9. ProbCut (~4 Elo)
879 // If we have a good enough capture and a reduced search returns a value
880 // much above beta, we can (almost) safely prune the previous move.
883 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
884 // if value from transposition table is lower than probCutBeta, don't attempt probCut
885 // there and in further interactions with transposition table cutoff depth is set to depth - 3
886 // because probCut search has depth set to depth - 4 but we also do a move before it
887 // so effective depth is equal to depth - 3
889 && tte->depth() >= depth - 3
890 && ttValue != VALUE_NONE
891 && ttValue < probCutBeta))
893 assert(probCutBeta < VALUE_INFINITE);
895 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
896 bool ttPv = ss->ttPv;
899 while ((move = mp.next_move()) != MOVE_NONE)
900 if (move != excludedMove && pos.legal(move))
902 assert(pos.capture_or_promotion(move));
905 captureOrPromotion = true;
907 ss->currentMove = move;
908 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
910 [pos.moved_piece(move)]
913 pos.do_move(move, st);
915 // Perform a preliminary qsearch to verify that the move holds
916 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
918 // If the qsearch held, perform the regular search
919 if (value >= probCutBeta)
920 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
924 if (value >= probCutBeta)
926 // if transposition table doesn't have equal or more deep info write probCut data into it
928 && tte->depth() >= depth - 3
929 && ttValue != VALUE_NONE))
930 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
932 depth - 3, move, ss->staticEval);
939 // Step 10. If the position is not in TT, decrease depth by 2 or 1 depending on node type
950 moves_loop: // When in check, search starts here
952 int rangeReduction = 0;
954 // Step 11. A small Probcut idea, when we are in check
955 probCutBeta = beta + 409;
960 && (tte->bound() & BOUND_LOWER)
961 && tte->depth() >= depth - 3
962 && ttValue >= probCutBeta
963 && abs(ttValue) <= VALUE_KNOWN_WIN
964 && abs(beta) <= VALUE_KNOWN_WIN
969 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
970 nullptr , (ss-4)->continuationHistory,
971 nullptr , (ss-6)->continuationHistory };
973 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
975 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
976 &thisThread->lowPlyHistory,
984 singularQuietLMR = moveCountPruning = false;
986 // Indicate PvNodes that will probably fail low if the node was searched
987 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
988 bool likelyFailLow = PvNode
990 && (tte->bound() & BOUND_UPPER)
991 && tte->depth() >= depth;
993 // Step 12. Loop through all pseudo-legal moves until no moves remain
994 // or a beta cutoff occurs.
995 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
999 if (move == excludedMove)
1002 // At root obey the "searchmoves" option and skip moves not listed in Root
1003 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1004 // mode we also skip PV moves which have been already searched and those
1005 // of lower "TB rank" if we are in a TB root position.
1006 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1007 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1010 // Check for legality
1011 if (!rootNode && !pos.legal(move))
1014 ss->moveCount = ++moveCount;
1016 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1017 sync_cout << "info depth " << depth
1018 << " currmove " << UCI::move(move, pos.is_chess960())
1019 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1021 (ss+1)->pv = nullptr;
1024 captureOrPromotion = pos.capture_or_promotion(move);
1025 movedPiece = pos.moved_piece(move);
1026 givesCheck = pos.gives_check(move);
1028 // Calculate new depth for this move
1029 newDepth = depth - 1;
1031 // Step 13. Pruning at shallow depth (~200 Elo). Depth conditions are important for mate finding.
1033 && pos.non_pawn_material(us)
1034 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1036 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1037 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1039 // Reduced depth of the next LMR search
1040 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0);
1042 if ( captureOrPromotion
1045 // Capture history based pruning when the move doesn't give check
1048 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1051 // SEE based pruning
1052 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1057 int history = (*contHist[0])[movedPiece][to_sq(move)]
1058 + (*contHist[1])[movedPiece][to_sq(move)]
1059 + (*contHist[3])[movedPiece][to_sq(move)];
1061 // Continuation history based pruning (~20 Elo)
1063 && history < -3000 * depth + 3000)
1066 history += thisThread->mainHistory[us][from_to(move)];
1068 // Futility pruning: parent node (~5 Elo)
1071 && ss->staticEval + 142 + 139 * lmrDepth + history / 64 <= alpha)
1074 // Prune moves with negative SEE (~20 Elo)
1075 if (!pos.see_ge(move, Value(-21 * lmrDepth * lmrDepth - 21 * lmrDepth)))
1080 // Step 14. Extensions (~75 Elo)
1082 // Singular extension search (~70 Elo). If all moves but one fail low on a
1083 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1084 // then that move is singular and should be extended. To verify this we do
1085 // a reduced search on all the other moves but the ttMove and if the
1086 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1090 && !excludedMove // Avoid recursive singular search
1091 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1092 && abs(ttValue) < VALUE_KNOWN_WIN
1093 && (tte->bound() & BOUND_LOWER)
1094 && tte->depth() >= depth - 3)
1096 Value singularBeta = ttValue - 3 * depth;
1097 Depth singularDepth = (depth - 1) / 2;
1099 ss->excludedMove = move;
1100 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1101 ss->excludedMove = MOVE_NONE;
1103 if (value < singularBeta)
1106 singularQuietLMR = !ttCapture;
1108 // Avoid search explosion by limiting the number of double extensions
1110 && value < singularBeta - 75
1111 && ss->doubleExtensions <= 6)
1115 // Multi-cut pruning
1116 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1117 // search without the ttMove. So we assume this expected Cut-node is not singular,
1118 // that multiple moves fail high, and we can prune the whole subtree by returning
1120 else if (singularBeta >= beta)
1121 return singularBeta;
1123 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1124 else if (ttValue >= beta)
1128 // Capture extensions for PvNodes and cutNodes
1129 else if ( (PvNode || cutNode)
1130 && captureOrPromotion
1135 else if ( givesCheck
1137 && abs(ss->staticEval) > 100)
1140 // Quiet ttMove extensions
1143 && move == ss->killers[0]
1144 && (*contHist[0])[movedPiece][to_sq(move)] >= 10000)
1147 // Add extension to new depth
1148 newDepth += extension;
1149 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1151 // Speculative prefetch as early as possible
1152 prefetch(TT.first_entry(pos.key_after(move)));
1154 // Update the current move (this must be done after singular extension search)
1155 ss->currentMove = move;
1156 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1157 [captureOrPromotion]
1161 // Step 15. Make the move
1162 pos.do_move(move, st, givesCheck);
1164 // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
1165 // We use various heuristics for the sons of a node after the first son has
1166 // been searched. In general we would like to reduce them, but there are many
1167 // cases where we extend a son if it has good chances to be "interesting".
1169 && moveCount > 1 + 2 * rootNode
1171 || !captureOrPromotion
1172 || (cutNode && (ss-1)->moveCount > 1)))
1174 Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
1176 // Decrease reduction at some PvNodes (~2 Elo)
1178 && bestMoveCount <= 3
1179 && beta - alpha >= thisThread->rootDelta / 4)
1182 // Decrease reduction if position is or has been on the PV
1183 // and node is not likely to fail low. (~3 Elo)
1188 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1189 if ( (rootNode || !PvNode)
1190 && thisThread->bestMoveChanges <= 2)
1193 // Decrease reduction if opponent's move count is high (~1 Elo)
1194 if ((ss-1)->moveCount > 13)
1197 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1198 if (singularQuietLMR)
1201 // Increase reduction for cut nodes (~3 Elo)
1202 if (cutNode && move != ss->killers[0])
1205 // Increase reduction if ttMove is a capture (~3 Elo)
1209 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1210 + (*contHist[0])[movedPiece][to_sq(move)]
1211 + (*contHist[1])[movedPiece][to_sq(move)]
1212 + (*contHist[3])[movedPiece][to_sq(move)]
1215 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1216 r -= ss->statScore / 14721;
1218 // In general we want to cap the LMR depth search at newDepth. But if reductions
1219 // are really negative and movecount is low, we allow this move to be searched
1220 // deeper than the first move (this may lead to hidden double extensions).
1221 int deeper = r >= -1 ? 0
1222 : moveCount <= 5 ? 2
1223 : PvNode && depth > 6 ? 1
1224 : cutNode && moveCount <= 7 ? 1
1227 Depth d = std::clamp(newDepth - r, 1, newDepth + deeper);
1229 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1231 // Range reductions (~3 Elo)
1232 if (ss->staticEval - value < 30 && depth > 7)
1235 // If the son is reduced and fails high it will be re-searched at full depth
1236 doFullDepthSearch = value > alpha && d < newDepth;
1241 doFullDepthSearch = !PvNode || moveCount > 1;
1245 // Step 17. Full depth search when LMR is skipped or fails high
1246 if (doFullDepthSearch)
1248 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1250 // If the move passed LMR update its stats
1251 if (didLMR && !captureOrPromotion)
1253 int bonus = value > alpha ? stat_bonus(newDepth)
1254 : -stat_bonus(newDepth);
1256 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1260 // For PV nodes only, do a full PV search on the first move or after a fail
1261 // high (in the latter case search only if value < beta), otherwise let the
1262 // parent node fail low with value <= alpha and try another move.
1263 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1266 (ss+1)->pv[0] = MOVE_NONE;
1268 value = -search<PV>(pos, ss+1, -beta, -alpha,
1269 std::min(maxNextDepth, newDepth), false);
1272 // Step 18. Undo move
1273 pos.undo_move(move);
1275 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1277 // Step 19. Check for a new best move
1278 // Finished searching the move. If a stop occurred, the return value of
1279 // the search cannot be trusted, and we return immediately without
1280 // updating best move, PV and TT.
1281 if (Threads.stop.load(std::memory_order_relaxed))
1286 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1287 thisThread->rootMoves.end(), move);
1289 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1291 // PV move or new best move?
1292 if (moveCount == 1 || value > alpha)
1295 rm.selDepth = thisThread->selDepth;
1300 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1301 rm.pv.push_back(*m);
1303 // We record how often the best move has been changed in each iteration.
1304 // This information is used for time management and LMR. In MultiPV mode,
1305 // we must take care to only do this for the first PV line.
1307 && !thisThread->pvIdx)
1308 ++thisThread->bestMoveChanges;
1311 // All other moves but the PV are set to the lowest value: this
1312 // is not a problem when sorting because the sort is stable and the
1313 // move position in the list is preserved - just the PV is pushed up.
1314 rm.score = -VALUE_INFINITE;
1317 if (value > bestValue)
1325 if (PvNode && !rootNode) // Update pv even in fail-high case
1326 update_pv(ss->pv, move, (ss+1)->pv);
1328 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1335 assert(value >= beta); // Fail high
1341 // If the move is worse than some previously searched move, remember it to update its stats later
1342 if (move != bestMove)
1344 if (captureOrPromotion && captureCount < 32)
1345 capturesSearched[captureCount++] = move;
1347 else if (!captureOrPromotion && quietCount < 64)
1348 quietsSearched[quietCount++] = move;
1352 // The following condition would detect a stop only after move loop has been
1353 // completed. But in this case bestValue is valid because we have fully
1354 // searched our subtree, and we can anyhow save the result in TT.
1360 // Step 20. Check for mate and stalemate
1361 // All legal moves have been searched and if there are no legal moves, it
1362 // must be a mate or a stalemate. If we are in a singular extension search then
1363 // return a fail low score.
1365 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1368 bestValue = excludedMove ? alpha :
1369 ss->inCheck ? mated_in(ss->ply)
1372 // If there is a move which produces search value greater than alpha we update stats of searched moves
1374 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1375 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1377 // Bonus for prior countermove that caused the fail low
1378 else if ( (depth >= 3 || PvNode)
1380 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + (PvNode || cutNode)));
1383 bestValue = std::min(bestValue, maxValue);
1385 // If no good move is found and the previous position was ttPv, then the previous
1386 // opponent move is probably good and the new position is added to the search tree.
1387 if (bestValue <= alpha)
1388 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1389 // Otherwise, a counter move has been found and if the position is the last leaf
1390 // in the search tree, remove the position from the search tree.
1392 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1394 // Write gathered information in transposition table
1395 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1396 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1397 bestValue >= beta ? BOUND_LOWER :
1398 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1399 depth, bestMove, ss->staticEval);
1401 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1407 // qsearch() is the quiescence search function, which is called by the main search
1408 // function with zero depth, or recursively with further decreasing depth per call.
1409 template <NodeType nodeType>
1410 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1412 static_assert(nodeType != Root);
1413 constexpr bool PvNode = nodeType == PV;
1415 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1416 assert(PvNode || (alpha == beta - 1));
1421 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1425 Move ttMove, move, bestMove;
1427 Value bestValue, value, ttValue, futilityValue, futilityBase;
1428 bool pvHit, givesCheck, captureOrPromotion;
1434 ss->pv[0] = MOVE_NONE;
1437 Thread* thisThread = pos.this_thread();
1438 bestMove = MOVE_NONE;
1439 ss->inCheck = pos.checkers();
1442 // Check for an immediate draw or maximum ply reached
1443 if ( pos.is_draw(ss->ply)
1444 || ss->ply >= MAX_PLY)
1445 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1447 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1449 // Decide whether or not to include checks: this fixes also the type of
1450 // TT entry depth that we are going to use. Note that in qsearch we use
1451 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1452 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1453 : DEPTH_QS_NO_CHECKS;
1454 // Transposition table lookup
1456 tte = TT.probe(posKey, ss->ttHit);
1457 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1458 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1459 pvHit = ss->ttHit && tte->is_pv();
1463 && tte->depth() >= ttDepth
1464 && ttValue != VALUE_NONE // Only in case of TT access race
1465 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1466 : (tte->bound() & BOUND_UPPER)))
1469 // Evaluate the position statically
1472 ss->staticEval = VALUE_NONE;
1473 bestValue = futilityBase = -VALUE_INFINITE;
1479 // Never assume anything about values stored in TT
1480 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1481 ss->staticEval = bestValue = evaluate(pos);
1483 // Can ttValue be used as a better position evaluation?
1484 if ( ttValue != VALUE_NONE
1485 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1486 bestValue = ttValue;
1489 // In case of null move search use previous static eval with a different sign
1490 ss->staticEval = bestValue =
1491 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1492 : -(ss-1)->staticEval;
1494 // Stand pat. Return immediately if static value is at least beta
1495 if (bestValue >= beta)
1497 // Save gathered info in transposition table
1499 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1500 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1505 if (PvNode && bestValue > alpha)
1508 futilityBase = bestValue + 155;
1511 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1512 nullptr , (ss-4)->continuationHistory,
1513 nullptr , (ss-6)->continuationHistory };
1515 // Initialize a MovePicker object for the current position, and prepare
1516 // to search the moves. Because the depth is <= 0 here, only captures,
1517 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1518 // will be generated.
1519 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1520 &thisThread->captureHistory,
1522 to_sq((ss-1)->currentMove));
1524 // Loop through the moves until no moves remain or a beta cutoff occurs
1525 while ((move = mp.next_move()) != MOVE_NONE)
1527 assert(is_ok(move));
1529 // Check for legality
1530 if (!pos.legal(move))
1533 givesCheck = pos.gives_check(move);
1534 captureOrPromotion = pos.capture_or_promotion(move);
1538 // Futility pruning and moveCount pruning
1539 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1541 && futilityBase > -VALUE_KNOWN_WIN
1542 && type_of(move) != PROMOTION)
1548 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1550 if (futilityValue <= alpha)
1552 bestValue = std::max(bestValue, futilityValue);
1556 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1558 bestValue = std::max(bestValue, futilityBase);
1563 // Do not search moves with negative SEE values
1564 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1565 && !pos.see_ge(move))
1568 // Speculative prefetch as early as possible
1569 prefetch(TT.first_entry(pos.key_after(move)));
1571 ss->currentMove = move;
1572 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1573 [captureOrPromotion]
1574 [pos.moved_piece(move)]
1577 // Continuation history based pruning
1578 if ( !captureOrPromotion
1579 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1580 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1581 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1584 // Make and search the move
1585 pos.do_move(move, st, givesCheck);
1586 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1587 pos.undo_move(move);
1589 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1591 // Check for a new best move
1592 if (value > bestValue)
1600 if (PvNode) // Update pv even in fail-high case
1601 update_pv(ss->pv, move, (ss+1)->pv);
1603 if (PvNode && value < beta) // Update alpha here!
1611 // All legal moves have been searched. A special case: if we're in check
1612 // and no legal moves were found, it is checkmate.
1613 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1615 assert(!MoveList<LEGAL>(pos).size());
1617 return mated_in(ss->ply); // Plies to mate from the root
1620 // Save gathered info in transposition table
1621 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1622 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1623 ttDepth, bestMove, ss->staticEval);
1625 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1631 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1632 // "plies to mate from the current position". Standard scores are unchanged.
1633 // The function is called before storing a value in the transposition table.
1635 Value value_to_tt(Value v, int ply) {
1637 assert(v != VALUE_NONE);
1639 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1640 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1644 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1645 // from the transposition table (which refers to the plies to mate/be mated from
1646 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1647 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1648 // and the graph history interaction, we return an optimal TB score instead.
1650 Value value_from_tt(Value v, int ply, int r50c) {
1652 if (v == VALUE_NONE)
1655 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1657 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1658 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1663 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1665 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1666 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1675 // update_pv() adds current move and appends child pv[]
1677 void update_pv(Move* pv, Move move, Move* childPv) {
1679 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1685 // update_all_stats() updates stats at the end of search() when a bestMove is found
1687 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1688 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1691 Color us = pos.side_to_move();
1692 Thread* thisThread = pos.this_thread();
1693 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1694 Piece moved_piece = pos.moved_piece(bestMove);
1695 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1697 bonus1 = stat_bonus(depth + 1);
1698 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1699 : stat_bonus(depth); // smaller bonus
1701 if (!pos.capture_or_promotion(bestMove))
1703 // Increase stats for the best move in case it was a quiet move
1704 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1706 // Decrease stats for all non-best quiet moves
1707 for (int i = 0; i < quietCount; ++i)
1709 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1710 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1714 // Increase stats for the best move in case it was a capture move
1715 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1717 // Extra penalty for a quiet early move that was not a TT move or
1718 // main killer move in previous ply when it gets refuted.
1719 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1720 && !pos.captured_piece())
1721 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1723 // Decrease stats for all non-best capture moves
1724 for (int i = 0; i < captureCount; ++i)
1726 moved_piece = pos.moved_piece(capturesSearched[i]);
1727 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1728 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1733 // update_continuation_histories() updates histories of the move pairs formed
1734 // by moves at ply -1, -2, -4, and -6 with current move.
1736 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1738 for (int i : {1, 2, 4, 6})
1740 // Only update first 2 continuation histories if we are in check
1741 if (ss->inCheck && i > 2)
1743 if (is_ok((ss-i)->currentMove))
1744 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1749 // update_quiet_stats() updates move sorting heuristics
1751 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1754 if (ss->killers[0] != move)
1756 ss->killers[1] = ss->killers[0];
1757 ss->killers[0] = move;
1760 Color us = pos.side_to_move();
1761 Thread* thisThread = pos.this_thread();
1762 thisThread->mainHistory[us][from_to(move)] << bonus;
1763 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1765 // Update countermove history
1766 if (is_ok((ss-1)->currentMove))
1768 Square prevSq = to_sq((ss-1)->currentMove);
1769 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1772 // Update low ply history
1773 if (depth > 11 && ss->ply < MAX_LPH)
1774 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1777 // When playing with strength handicap, choose best move among a set of RootMoves
1778 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1780 Move Skill::pick_best(size_t multiPV) {
1782 const RootMoves& rootMoves = Threads.main()->rootMoves;
1783 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1785 // RootMoves are already sorted by score in descending order
1786 Value topScore = rootMoves[0].score;
1787 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1788 int maxScore = -VALUE_INFINITE;
1789 double weakness = 120 - 2 * level;
1791 // Choose best move. For each move score we add two terms, both dependent on
1792 // weakness. One is deterministic and bigger for weaker levels, and one is
1793 // random. Then we choose the move with the resulting highest score.
1794 for (size_t i = 0; i < multiPV; ++i)
1796 // This is our magic formula
1797 int push = int(( weakness * int(topScore - rootMoves[i].score)
1798 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1800 if (rootMoves[i].score + push >= maxScore)
1802 maxScore = rootMoves[i].score + push;
1803 best = rootMoves[i].pv[0];
1813 /// MainThread::check_time() is used to print debug info and, more importantly,
1814 /// to detect when we are out of available time and thus stop the search.
1816 void MainThread::check_time() {
1821 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1822 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1824 static TimePoint lastInfoTime = now();
1826 TimePoint elapsed = Time.elapsed();
1827 TimePoint tick = Limits.startTime + elapsed;
1829 if (tick - lastInfoTime >= 1000)
1831 lastInfoTime = tick;
1835 // We should not stop pondering until told so by the GUI
1839 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1840 || (Limits.movetime && elapsed >= Limits.movetime)
1841 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1842 Threads.stop = true;
1846 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1847 /// that all (if any) unsearched PV lines are sent using a previous search score.
1849 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1851 std::stringstream ss;
1852 TimePoint elapsed = Time.elapsed() + 1;
1853 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1854 size_t pvIdx = pos.this_thread()->pvIdx;
1855 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1856 uint64_t nodesSearched = Threads.nodes_searched();
1857 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1859 for (size_t i = 0; i < multiPV; ++i)
1861 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1863 if (depth == 1 && !updated && i > 0)
1866 Depth d = updated ? depth : std::max(1, depth - 1);
1867 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1869 if (v == -VALUE_INFINITE)
1872 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1873 v = tb ? rootMoves[i].tbScore : v;
1875 if (ss.rdbuf()->in_avail()) // Not at first line
1880 << " seldepth " << rootMoves[i].selDepth
1881 << " multipv " << i + 1
1882 << " score " << UCI::value(v);
1884 if (Options["UCI_ShowWDL"])
1885 ss << UCI::wdl(v, pos.game_ply());
1887 if (!tb && i == pvIdx)
1888 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1890 ss << " nodes " << nodesSearched
1891 << " nps " << nodesSearched * 1000 / elapsed;
1893 if (elapsed > 1000) // Earlier makes little sense
1894 ss << " hashfull " << TT.hashfull();
1896 ss << " tbhits " << tbHits
1897 << " time " << elapsed
1900 for (Move m : rootMoves[i].pv)
1901 ss << " " << UCI::move(m, pos.is_chess960());
1908 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1909 /// before exiting the search, for instance, in case we stop the search during a
1910 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1911 /// otherwise in case of 'ponder on' we have nothing to think on.
1913 bool RootMove::extract_ponder_from_tt(Position& pos) {
1916 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1920 assert(pv.size() == 1);
1922 if (pv[0] == MOVE_NONE)
1925 pos.do_move(pv[0], st);
1926 TTEntry* tte = TT.probe(pos.key(), ttHit);
1930 Move m = tte->move(); // Local copy to be SMP safe
1931 if (MoveList<LEGAL>(pos).contains(m))
1935 pos.undo_move(pv[0]);
1936 return pv.size() > 1;
1939 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1942 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1943 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1944 Cardinality = int(Options["SyzygyProbeLimit"]);
1945 bool dtz_available = true;
1947 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1948 // ProbeDepth == DEPTH_ZERO
1949 if (Cardinality > MaxCardinality)
1951 Cardinality = MaxCardinality;
1955 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1957 // Rank moves using DTZ tables
1958 RootInTB = root_probe(pos, rootMoves);
1962 // DTZ tables are missing; try to rank moves using WDL tables
1963 dtz_available = false;
1964 RootInTB = root_probe_wdl(pos, rootMoves);
1970 // Sort moves according to TB rank
1971 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1972 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1974 // Probe during search only if DTZ is not available and we are winning
1975 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1980 // Clean up if root_probe() and root_probe_wdl() have failed
1981 for (auto& m : rootMoves)
1986 } // namespace Stockfish