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 d > 14 ? 73 : 6 * d * d + 229 * d - 215;
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
117 explicit Skill(int l) : level(l) {}
118 bool enabled() const { return level < 20; }
119 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
120 Move pick_best(size_t multiPV);
123 Move best = MOVE_NONE;
126 template <NodeType nodeType>
127 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
129 template <NodeType nodeType>
130 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
132 Value value_to_tt(Value v, int ply);
133 Value value_from_tt(Value v, int ply, int r50c);
134 void update_pv(Move* pv, Move move, Move* childPv);
135 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
136 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
137 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
138 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
140 // perft() is our utility to verify move generation. All the leaf nodes up
141 // to the given depth are generated and counted, and the sum is returned.
143 uint64_t perft(Position& pos, Depth depth) {
146 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
148 uint64_t cnt, nodes = 0;
149 const bool leaf = (depth == 2);
151 for (const auto& m : MoveList<LEGAL>(pos))
153 if (Root && depth <= 1)
158 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
163 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
171 /// Search::init() is called at startup to initialize various lookup tables
173 void Search::init() {
175 for (int i = 1; i < MAX_MOVES; ++i)
176 Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i));
180 /// Search::clear() resets search state to its initial value
182 void Search::clear() {
184 Threads.main()->wait_for_search_finished();
186 Time.availableNodes = 0;
189 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
193 /// MainThread::search() is started when the program receives the UCI 'go'
194 /// command. It searches from the root position and outputs the "bestmove".
196 void MainThread::search() {
200 nodes = perft<true>(rootPos, Limits.perft);
201 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
205 Color us = rootPos.side_to_move();
206 Time.init(Limits, us, rootPos.game_ply());
209 Eval::NNUE::verify();
211 if (rootMoves.empty())
213 rootMoves.emplace_back(MOVE_NONE);
214 sync_cout << "info depth 0 score "
215 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
220 Threads.start_searching(); // start non-main threads
221 Thread::search(); // main thread start searching
224 // When we reach the maximum depth, we can arrive here without a raise of
225 // Threads.stop. However, if we are pondering or in an infinite search,
226 // the UCI protocol states that we shouldn't print the best move before the
227 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
228 // until the GUI sends one of those commands.
230 while (!Threads.stop && (ponder || Limits.infinite))
231 {} // Busy wait for a stop or a ponder reset
233 // Stop the threads if not already stopped (also raise the stop if
234 // "ponderhit" just reset Threads.ponder).
237 // Wait until all threads have finished
238 Threads.wait_for_search_finished();
240 // When playing in 'nodes as time' mode, subtract the searched nodes from
241 // the available ones before exiting.
243 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
245 Thread* bestThread = this;
247 if ( int(Options["MultiPV"]) == 1
249 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
250 && rootMoves[0].pv[0] != MOVE_NONE)
251 bestThread = Threads.get_best_thread();
253 bestPreviousScore = bestThread->rootMoves[0].score;
255 // Send again PV info if we have a new best thread
256 if (bestThread != this)
257 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
259 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
261 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
262 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
264 std::cout << sync_endl;
268 /// Thread::search() is the main iterative deepening loop. It calls search()
269 /// repeatedly with increasing depth until the allocated thinking time has been
270 /// consumed, the user stops the search, or the maximum search depth is reached.
272 void Thread::search() {
274 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
275 // The former is needed to allow update_continuation_histories(ss-1, ...),
276 // which accesses its argument at ss-6, also near the root.
277 // The latter is needed for statScore and killer initialization.
278 Stack stack[MAX_PLY+10], *ss = stack+7;
280 Value bestValue, alpha, beta, delta;
281 Move lastBestMove = MOVE_NONE;
282 Depth lastBestMoveDepth = 0;
283 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
284 double timeReduction = 1, totBestMoveChanges = 0;
285 Color us = rootPos.side_to_move();
288 std::memset(ss-7, 0, 10 * sizeof(Stack));
289 for (int i = 7; i > 0; i--)
290 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
292 for (int i = 0; i <= MAX_PLY + 2; ++i)
297 bestValue = delta = alpha = -VALUE_INFINITE;
298 beta = VALUE_INFINITE;
302 if (mainThread->bestPreviousScore == VALUE_INFINITE)
303 for (int i = 0; i < 4; ++i)
304 mainThread->iterValue[i] = VALUE_ZERO;
306 for (int i = 0; i < 4; ++i)
307 mainThread->iterValue[i] = mainThread->bestPreviousScore;
310 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
311 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
313 size_t multiPV = size_t(Options["MultiPV"]);
315 // Pick integer skill levels, but non-deterministically round up or down
316 // such that the average integer skill corresponds to the input floating point one.
317 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
318 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
319 // for match (TC 60+0.6) results spanning a wide range of k values.
321 double floatLevel = Options["UCI_LimitStrength"] ?
322 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
323 double(Options["Skill Level"]);
324 int intLevel = int(floatLevel) +
325 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
326 Skill skill(intLevel);
328 // When playing with strength handicap enable MultiPV search that we will
329 // use behind the scenes to retrieve a set of possible moves.
331 multiPV = std::max(multiPV, (size_t)4);
333 multiPV = std::min(multiPV, rootMoves.size());
335 ttHitAverage.set(50, 100); // initialize the running average at 50%
336 doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0%
337 doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0%
339 nodesLastExplosive = nodes;
340 nodesLastNormal = nodes;
341 state = EXPLOSION_NONE;
344 int searchAgainCounter = 0;
346 // Iterative deepening loop until requested to stop or the target depth is reached
347 while ( ++rootDepth < MAX_PLY
349 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
351 // Age out PV variability metric
353 totBestMoveChanges /= 2;
355 // Save the last iteration's scores before first PV line is searched and
356 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
357 for (RootMove& rm : rootMoves)
358 rm.previousScore = rm.score;
363 if (!Threads.increaseDepth)
364 searchAgainCounter++;
366 // MultiPV loop. We perform a full root search for each PV line
367 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
372 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
373 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
377 // Reset UCI info selDepth for each depth and each PV line
380 // Reset aspiration window starting size
383 Value prev = rootMoves[pvIdx].previousScore;
385 alpha = std::max(prev - delta,-VALUE_INFINITE);
386 beta = std::min(prev + delta, VALUE_INFINITE);
388 // Adjust trend based on root move's previousScore (dynamic contempt)
389 int tr = 113 * prev / (abs(prev) + 147);
391 trend = (us == WHITE ? make_score(tr, tr / 2)
392 : -make_score(tr, tr / 2));
395 // Start with a small aspiration window and, in the case of a fail
396 // high/low, re-search with a bigger window until we don't fail
398 int failedHighCnt = 0;
401 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
402 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
404 // Bring the best move to the front. It is critical that sorting
405 // is done with a stable algorithm because all the values but the
406 // first and eventually the new best one are set to -VALUE_INFINITE
407 // and we want to keep the same order for all the moves except the
408 // new PV that goes to the front. Note that in case of MultiPV
409 // search the already searched PV lines are preserved.
410 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
412 // If search has been stopped, we break immediately. Sorting is
413 // safe because RootMoves is still valid, although it refers to
414 // the previous iteration.
418 // When failing high/low give some update (without cluttering
419 // the UI) before a re-search.
422 && (bestValue <= alpha || bestValue >= beta)
423 && Time.elapsed() > 3000)
424 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
426 // In case of failing low/high increase aspiration window and
427 // re-search, otherwise exit the loop.
428 if (bestValue <= alpha)
430 beta = (alpha + beta) / 2;
431 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
435 mainThread->stopOnPonderhit = false;
437 else if (bestValue >= beta)
439 beta = std::min(bestValue + delta, VALUE_INFINITE);
445 delta += delta / 4 + 5;
447 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
450 // Sort the PV lines searched so far and update the GUI
451 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
454 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
455 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
459 completedDepth = rootDepth;
461 if (rootMoves[0].pv[0] != lastBestMove) {
462 lastBestMove = rootMoves[0].pv[0];
463 lastBestMoveDepth = rootDepth;
466 // Have we found a "mate in x"?
468 && bestValue >= VALUE_MATE_IN_MAX_PLY
469 && VALUE_MATE - bestValue <= 2 * Limits.mate)
475 // If skill level is enabled and time is up, pick a sub-optimal best move
476 if (skill.enabled() && skill.time_to_pick(rootDepth))
477 skill.pick_best(multiPV);
479 // Do we have time for the next iteration? Can we stop searching now?
480 if ( Limits.use_time_management()
482 && !mainThread->stopOnPonderhit)
484 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
485 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
486 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
488 // If the bestMove is stable over several iterations, reduce time accordingly
489 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
490 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
492 // Use part of the gained time from a previous stable move for the current move
493 for (Thread* th : Threads)
495 totBestMoveChanges += th->bestMoveChanges;
496 th->bestMoveChanges = 0;
498 double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth)
499 * totBestMoveChanges / Threads.size();
500 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
502 // Cap used time in case of a single legal move for a better viewer experience in tournaments
503 // yielding correct scores and sufficiently fast moves.
504 if (rootMoves.size() == 1)
505 totalTime = std::min(500.0, totalTime);
507 // Stop the search if we have exceeded the totalTime
508 if (Time.elapsed() > totalTime)
510 // If we are allowed to ponder do not stop the search now but
511 // keep pondering until the GUI sends "ponderhit" or "stop".
512 if (mainThread->ponder)
513 mainThread->stopOnPonderhit = true;
517 else if ( Threads.increaseDepth
518 && !mainThread->ponder
519 && Time.elapsed() > totalTime * 0.58)
520 Threads.increaseDepth = false;
522 Threads.increaseDepth = true;
525 mainThread->iterValue[iterIdx] = bestValue;
526 iterIdx = (iterIdx + 1) & 3;
532 mainThread->previousTimeReduction = timeReduction;
534 // If skill level is enabled, swap best PV line with the sub-optimal one
536 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
537 skill.best ? skill.best : skill.pick_best(multiPV)));
543 // search<>() is the main search function for both PV and non-PV nodes
545 template <NodeType nodeType>
546 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
548 Thread* thisThread = pos.this_thread();
550 // Step 0. Limit search explosion
552 && search_explosion(thisThread) == MUST_CALM_DOWN
553 && depth > (ss-1)->depth)
554 depth = (ss-1)->depth;
556 constexpr bool PvNode = nodeType != NonPV;
557 constexpr bool rootNode = nodeType == Root;
558 const Depth maxNextDepth = rootNode ? depth : depth + 1;
560 // Check if we have an upcoming move which draws by repetition, or
561 // if the opponent had an alternative move earlier to this position.
563 && pos.rule50_count() >= 3
564 && alpha < VALUE_DRAW
565 && pos.has_game_cycle(ss->ply))
567 alpha = value_draw(pos.this_thread());
572 // Dive into quiescence search when the depth reaches zero
574 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
576 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
577 assert(PvNode || (alpha == beta - 1));
578 assert(0 < depth && depth < MAX_PLY);
579 assert(!(PvNode && cutNode));
581 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
583 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
587 Move ttMove, move, excludedMove, bestMove;
588 Depth extension, newDepth;
589 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
590 bool givesCheck, improving, didLMR, priorCapture;
591 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
592 ttCapture, singularQuietLMR, noLMRExtension;
594 int moveCount, captureCount, quietCount;
596 // Step 1. Initialize node
597 ss->inCheck = pos.checkers();
598 priorCapture = pos.captured_piece();
599 Color us = pos.side_to_move();
600 moveCount = captureCount = quietCount = ss->moveCount = 0;
601 bestValue = -VALUE_INFINITE;
602 maxValue = VALUE_INFINITE;
604 // Check for the available remaining time
605 if (thisThread == Threads.main())
606 static_cast<MainThread*>(thisThread)->check_time();
608 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
609 if (PvNode && thisThread->selDepth < ss->ply + 1)
610 thisThread->selDepth = ss->ply + 1;
614 // Step 2. Check for aborted search and immediate draw
615 if ( Threads.stop.load(std::memory_order_relaxed)
616 || pos.is_draw(ss->ply)
617 || ss->ply >= MAX_PLY)
618 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
619 : value_draw(pos.this_thread());
621 // Step 3. Mate distance pruning. Even if we mate at the next move our score
622 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
623 // a shorter mate was found upward in the tree then there is no need to search
624 // because we will never beat the current alpha. Same logic but with reversed
625 // signs applies also in the opposite condition of being mated instead of giving
626 // mate. In this case return a fail-high score.
627 alpha = std::max(mated_in(ss->ply), alpha);
628 beta = std::min(mate_in(ss->ply+1), beta);
633 assert(0 <= ss->ply && ss->ply < MAX_PLY);
635 (ss+1)->ttPv = false;
636 (ss+1)->excludedMove = bestMove = MOVE_NONE;
637 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
638 ss->doubleExtensions = (ss-1)->doubleExtensions;
640 Square prevSq = to_sq((ss-1)->currentMove);
642 // Update the running average statistics for double extensions
643 thisThread->doubleExtensionAverage[us].update(ss->depth > (ss-1)->depth);
645 // Initialize statScore to zero for the grandchildren of the current position.
646 // So statScore is shared between all grandchildren and only the first grandchild
647 // starts with statScore = 0. Later grandchildren start with the last calculated
648 // statScore of the previous grandchild. This influences the reduction rules in
649 // LMR which are based on the statScore of parent position.
651 (ss+2)->statScore = 0;
653 // Step 4. Transposition table lookup. We don't want the score of a partial
654 // search to overwrite a previous full search TT value, so we use a different
655 // position key in case of an excluded move.
656 excludedMove = ss->excludedMove;
657 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
658 tte = TT.probe(posKey, ss->ttHit);
659 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
660 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
661 : ss->ttHit ? tte->move() : MOVE_NONE;
663 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
665 // Update low ply history for previous move if we are near root and position is or has been in PV
668 && ss->ply - 1 < MAX_LPH
670 && is_ok((ss-1)->currentMove))
671 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
673 // running average of ttHit
674 thisThread->ttHitAverage.update(ss->ttHit);
676 // At non-PV nodes we check for an early TT cutoff
679 && tte->depth() >= depth
680 && ttValue != VALUE_NONE // Possible in case of TT access race
681 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
682 : (tte->bound() & BOUND_UPPER)))
684 // If ttMove is quiet, update move sorting heuristics on TT hit
689 // Bonus for a quiet ttMove that fails high
690 if (!pos.capture_or_promotion(ttMove))
691 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
693 // Extra penalty for early quiet moves of the previous ply
694 if ((ss-1)->moveCount <= 2 && !priorCapture)
695 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
697 // Penalty for a quiet ttMove that fails low
698 else if (!pos.capture_or_promotion(ttMove))
700 int penalty = -stat_bonus(depth);
701 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
702 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
706 // Partial workaround for the graph history interaction problem
707 // For high rule50 counts don't produce transposition table cutoffs.
708 if (pos.rule50_count() < 90)
712 // Step 5. Tablebases probe
713 if (!rootNode && TB::Cardinality)
715 int piecesCount = pos.count<ALL_PIECES>();
717 if ( piecesCount <= TB::Cardinality
718 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
719 && pos.rule50_count() == 0
720 && !pos.can_castle(ANY_CASTLING))
723 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
725 // Force check of time on the next occasion
726 if (thisThread == Threads.main())
727 static_cast<MainThread*>(thisThread)->callsCnt = 0;
729 if (err != TB::ProbeState::FAIL)
731 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
733 int drawScore = TB::UseRule50 ? 1 : 0;
735 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
736 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
737 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
738 : VALUE_DRAW + 2 * wdl * drawScore;
740 Bound b = wdl < -drawScore ? BOUND_UPPER
741 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
743 if ( b == BOUND_EXACT
744 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
746 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
747 std::min(MAX_PLY - 1, depth + 6),
748 MOVE_NONE, VALUE_NONE);
755 if (b == BOUND_LOWER)
756 bestValue = value, alpha = std::max(alpha, bestValue);
764 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
766 // Step 6. Static evaluation of the position
769 // Skip early pruning when in check
770 ss->staticEval = eval = VALUE_NONE;
776 // Never assume anything about values stored in TT
777 ss->staticEval = eval = tte->eval();
778 if (eval == VALUE_NONE)
779 ss->staticEval = eval = evaluate(pos);
781 // Randomize draw evaluation
782 if (eval == VALUE_DRAW)
783 eval = value_draw(thisThread);
785 // Can ttValue be used as a better position evaluation?
786 if ( ttValue != VALUE_NONE
787 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
792 // In case of null move search use previous static eval with a different sign
793 // and addition of two tempos
794 if ((ss-1)->currentMove != MOVE_NULL)
795 ss->staticEval = eval = evaluate(pos);
797 ss->staticEval = eval = -(ss-1)->staticEval;
799 // Save static evaluation into transposition table
801 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
804 // Use static evaluation difference to improve quiet move ordering
805 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
807 int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval), -1000, 1000);
808 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
811 // Set up improving flag that is used in various pruning heuristics
812 // We define position as improving if static evaluation of position is better
813 // Than the previous static evaluation at our turn
814 // In case of us being in check at our previous move we look at move prior to it
815 improving = (ss-2)->staticEval == VALUE_NONE
816 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
817 : ss->staticEval > (ss-2)->staticEval;
819 // Step 7. Futility pruning: child node (~50 Elo).
820 // The depth condition is important for mate finding.
823 && eval - futility_margin(depth, improving) >= beta
824 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
827 // Step 8. Null move search with verification search (~40 Elo)
829 && (ss-1)->currentMove != MOVE_NULL
830 && (ss-1)->statScore < 23767
832 && eval >= ss->staticEval
833 && ss->staticEval >= beta - 20 * depth - 22 * improving + 168 * ss->ttPv + 177
835 && pos.non_pawn_material(us)
836 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
838 assert(eval - beta >= 0);
840 // Null move dynamic reduction based on depth and value
841 Depth R = std::min(int(eval - beta) / 205, 3) + depth / 3 + 4;
843 ss->currentMove = MOVE_NULL;
844 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
846 pos.do_null_move(st);
848 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
850 pos.undo_null_move();
852 if (nullValue >= beta)
854 // Do not return unproven mate or TB scores
855 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
858 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
861 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
863 // Do verification search at high depths, with null move pruning disabled
864 // for us, until ply exceeds nmpMinPly.
865 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
866 thisThread->nmpColor = us;
868 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
870 thisThread->nmpMinPly = 0;
877 probCutBeta = beta + 209 - 44 * improving;
879 // Step 9. ProbCut (~4 Elo)
880 // If we have a good enough capture and a reduced search returns a value
881 // much above beta, we can (almost) safely prune the previous move.
884 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
885 // if value from transposition table is lower than probCutBeta, don't attempt probCut
886 // there and in further interactions with transposition table cutoff depth is set to depth - 3
887 // because probCut search has depth set to depth - 4 but we also do a move before it
888 // so effective depth is equal to depth - 3
890 && tte->depth() >= depth - 3
891 && ttValue != VALUE_NONE
892 && ttValue < probCutBeta))
894 assert(probCutBeta < VALUE_INFINITE);
896 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
897 int probCutCount = 0;
898 bool ttPv = ss->ttPv;
901 while ( (move = mp.next_move()) != MOVE_NONE
902 && probCutCount < 2 + 2 * cutNode)
903 if (move != excludedMove && pos.legal(move))
905 assert(pos.capture_or_promotion(move));
908 captureOrPromotion = true;
911 ss->currentMove = move;
912 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
914 [pos.moved_piece(move)]
917 pos.do_move(move, st);
919 // Perform a preliminary qsearch to verify that the move holds
920 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
922 // If the qsearch held, perform the regular search
923 if (value >= probCutBeta)
924 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
928 if (value >= probCutBeta)
930 // if transposition table doesn't have equal or more deep info write probCut data into it
932 && tte->depth() >= depth - 3
933 && ttValue != VALUE_NONE))
934 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
936 depth - 3, move, ss->staticEval);
943 // Step 10. If the position is not in TT, decrease depth by 2 or 1 depending on node type
954 moves_loop: // When in check, search starts here
956 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
957 int rangeReduction = 0;
959 // Step 11. A small Probcut idea, when we are in check
960 probCutBeta = beta + 409;
965 && (tte->bound() & BOUND_LOWER)
966 && tte->depth() >= depth - 3
967 && ttValue >= probCutBeta
968 && abs(ttValue) <= VALUE_KNOWN_WIN
969 && abs(beta) <= VALUE_KNOWN_WIN
974 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
975 nullptr , (ss-4)->continuationHistory,
976 nullptr , (ss-6)->continuationHistory };
978 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
980 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
981 &thisThread->lowPlyHistory,
989 singularQuietLMR = moveCountPruning = noLMRExtension = false;
991 // Indicate PvNodes that will probably fail low if the node was searched
992 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
993 bool likelyFailLow = PvNode
995 && (tte->bound() & BOUND_UPPER)
996 && tte->depth() >= depth;
998 // Step 12. Loop through all pseudo-legal moves until no moves remain
999 // or a beta cutoff occurs.
1000 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
1002 assert(is_ok(move));
1004 if (move == excludedMove)
1007 // At root obey the "searchmoves" option and skip moves not listed in Root
1008 // Move List. As a consequence any illegal move is also skipped. In MultiPV
1009 // mode we also skip PV moves which have been already searched and those
1010 // of lower "TB rank" if we are in a TB root position.
1011 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
1012 thisThread->rootMoves.begin() + thisThread->pvLast, move))
1015 // Check for legality
1016 if (!rootNode && !pos.legal(move))
1019 ss->moveCount = ++moveCount;
1021 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
1022 sync_cout << "info depth " << depth
1023 << " currmove " << UCI::move(move, pos.is_chess960())
1024 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
1026 (ss+1)->pv = nullptr;
1029 captureOrPromotion = pos.capture_or_promotion(move);
1030 movedPiece = pos.moved_piece(move);
1031 givesCheck = pos.gives_check(move);
1033 // Calculate new depth for this move
1034 newDepth = depth - 1;
1036 // Step 13. Pruning at shallow depth (~200 Elo). Depth conditions are important for mate finding.
1038 && pos.non_pawn_material(us)
1039 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1041 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1042 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1044 // Reduced depth of the next LMR search
1045 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0);
1047 if ( captureOrPromotion
1050 // Capture history based pruning when the move doesn't give check
1053 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1056 // SEE based pruning
1057 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1062 // Continuation history based pruning (~20 Elo)
1064 && (*contHist[0])[movedPiece][to_sq(move)]
1065 + (*contHist[1])[movedPiece][to_sq(move)]
1066 + (*contHist[3])[movedPiece][to_sq(move)] < -3000 * depth + 3000)
1069 // Futility pruning: parent node (~5 Elo)
1072 && ss->staticEval + 172 + 145 * lmrDepth <= alpha)
1075 // Prune moves with negative SEE (~20 Elo)
1076 if (!pos.see_ge(move, Value(-21 * lmrDepth * lmrDepth - 21 * lmrDepth)))
1081 // Step 14. Extensions (~75 Elo)
1083 // Singular extension search (~70 Elo). If all moves but one fail low on a
1084 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1085 // then that move is singular and should be extended. To verify this we do
1086 // a reduced search on all the other moves but the ttMove and if the
1087 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1091 && !excludedMove // Avoid recursive singular search
1092 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1093 && abs(ttValue) < VALUE_KNOWN_WIN
1094 && (tte->bound() & BOUND_LOWER)
1095 && tte->depth() >= depth - 3)
1097 Value singularBeta = ttValue - 3 * depth;
1098 Depth singularDepth = (depth - 1) / 2;
1100 ss->excludedMove = move;
1101 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1102 ss->excludedMove = MOVE_NONE;
1104 if (value < singularBeta)
1107 singularQuietLMR = !ttCapture;
1109 // Avoid search explosion by limiting the number of double extensions
1111 && value < singularBeta - 75
1112 && ss->doubleExtensions <= 6)
1115 noLMRExtension = true;
1119 // Multi-cut pruning
1120 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1121 // search without the ttMove. So we assume this expected Cut-node is not singular,
1122 // that multiple moves fail high, and we can prune the whole subtree by returning
1124 else if (singularBeta >= beta)
1125 return singularBeta;
1127 // If the eval of ttMove is greater than beta we try also if there is another
1128 // move that pushes it over beta, if so the position also has probably multiple
1129 // moves giving fail highs. We will then reduce the ttMove (negative extension).
1130 else if (ttValue >= beta)
1132 ss->excludedMove = move;
1133 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1134 ss->excludedMove = MOVE_NONE;
1141 // Capture extensions for PvNodes and cutNodes
1142 else if ( (PvNode || cutNode)
1143 && captureOrPromotion
1148 else if ( givesCheck
1150 && abs(ss->staticEval) > 100)
1153 // Quiet ttMove extensions
1156 && move == ss->killers[0]
1157 && (*contHist[0])[movedPiece][to_sq(move)] >= 10000)
1160 // Add extension to new depth
1161 newDepth += extension;
1162 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1164 // Speculative prefetch as early as possible
1165 prefetch(TT.first_entry(pos.key_after(move)));
1167 // Update the current move (this must be done after singular extension search)
1168 ss->currentMove = move;
1169 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1170 [captureOrPromotion]
1174 // Step 15. Make the move
1175 pos.do_move(move, st, givesCheck);
1177 // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
1178 // We use various heuristics for the sons of a node after the first son has
1179 // been searched. In general we would like to reduce them, but there are many
1180 // cases where we extend a son if it has good chances to be "interesting".
1182 && moveCount > 1 + 2 * rootNode
1183 && ( !captureOrPromotion
1184 || (cutNode && (ss-1)->moveCount > 1)
1186 && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
1188 Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
1193 // Decrease reduction if the ttHit running average is large (~0 Elo)
1194 if (thisThread->ttHitAverage.is_greater(537, 1024))
1197 // Decrease reduction if position is or has been on the PV
1198 // and node is not likely to fail low. (~3 Elo)
1203 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1204 if ( (rootNode || !PvNode)
1205 && thisThread->bestMoveChanges <= 2)
1208 // Decrease reduction if opponent's move count is high (~1 Elo)
1209 if ((ss-1)->moveCount > 13)
1212 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1213 if (singularQuietLMR)
1216 // Increase reduction for cut nodes (~3 Elo)
1217 if (cutNode && move != ss->killers[0])
1220 // Increase reduction if ttMove is a capture (~3 Elo)
1224 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1225 + (*contHist[0])[movedPiece][to_sq(move)]
1226 + (*contHist[1])[movedPiece][to_sq(move)]
1227 + (*contHist[3])[movedPiece][to_sq(move)]
1230 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1231 r -= ss->statScore / 14721;
1233 // In general we want to cap the LMR depth search at newDepth. But if reductions
1234 // are really negative and movecount is low, we allow this move to be searched
1235 // deeper than the first move (this may lead to hidden double extensions if
1236 // newDepth got its own extension before).
1237 int deeper = r >= -1 ? 0
1238 : noLMRExtension ? 0
1239 : moveCount <= 5 ? 1
1240 : (depth > 6 && PvNode) ? 1
1243 Depth d = std::clamp(newDepth - r, 1, newDepth + deeper);
1245 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1247 // Range reductions (~3 Elo)
1248 if (ss->staticEval - value < 30 && depth > 7)
1251 // If the son is reduced and fails high it will be re-searched at full depth
1252 doFullDepthSearch = value > alpha && d < newDepth;
1257 doFullDepthSearch = !PvNode || moveCount > 1;
1261 // Step 17. Full depth search when LMR is skipped or fails high
1262 if (doFullDepthSearch)
1264 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1266 // If the move passed LMR update its stats
1267 if (didLMR && !captureOrPromotion)
1269 int bonus = value > alpha ? stat_bonus(newDepth)
1270 : -stat_bonus(newDepth);
1272 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1276 // For PV nodes only, do a full PV search on the first move or after a fail
1277 // high (in the latter case search only if value < beta), otherwise let the
1278 // parent node fail low with value <= alpha and try another move.
1279 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1282 (ss+1)->pv[0] = MOVE_NONE;
1284 value = -search<PV>(pos, ss+1, -beta, -alpha,
1285 std::min(maxNextDepth, newDepth), false);
1288 // Step 18. Undo move
1289 pos.undo_move(move);
1291 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1293 // Step 19. Check for a new best move
1294 // Finished searching the move. If a stop occurred, the return value of
1295 // the search cannot be trusted, and we return immediately without
1296 // updating best move, PV and TT.
1297 if (Threads.stop.load(std::memory_order_relaxed))
1302 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1303 thisThread->rootMoves.end(), move);
1305 // PV move or new best move?
1306 if (moveCount == 1 || value > alpha)
1309 rm.selDepth = thisThread->selDepth;
1314 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1315 rm.pv.push_back(*m);
1317 // We record how often the best move has been changed in each
1318 // iteration. This information is used for time management and LMR
1320 ++thisThread->bestMoveChanges;
1323 // All other moves but the PV are set to the lowest value: this
1324 // is not a problem when sorting because the sort is stable and the
1325 // move position in the list is preserved - just the PV is pushed up.
1326 rm.score = -VALUE_INFINITE;
1329 if (value > bestValue)
1337 if (PvNode && !rootNode) // Update pv even in fail-high case
1338 update_pv(ss->pv, move, (ss+1)->pv);
1340 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1344 assert(value >= beta); // Fail high
1350 // If the move is worse than some previously searched move, remember it to update its stats later
1351 if (move != bestMove)
1353 if (captureOrPromotion && captureCount < 32)
1354 capturesSearched[captureCount++] = move;
1356 else if (!captureOrPromotion && quietCount < 64)
1357 quietsSearched[quietCount++] = move;
1361 // The following condition would detect a stop only after move loop has been
1362 // completed. But in this case bestValue is valid because we have fully
1363 // searched our subtree, and we can anyhow save the result in TT.
1369 // Step 20. Check for mate and stalemate
1370 // All legal moves have been searched and if there are no legal moves, it
1371 // must be a mate or a stalemate. If we are in a singular extension search then
1372 // return a fail low score.
1374 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1377 bestValue = excludedMove ? alpha :
1378 ss->inCheck ? mated_in(ss->ply)
1381 // If there is a move which produces search value greater than alpha we update stats of searched moves
1383 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1384 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1386 // Bonus for prior countermove that caused the fail low
1387 else if ( (depth >= 3 || PvNode)
1389 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + (PvNode || cutNode)));
1392 bestValue = std::min(bestValue, maxValue);
1394 // If no good move is found and the previous position was ttPv, then the previous
1395 // opponent move is probably good and the new position is added to the search tree.
1396 if (bestValue <= alpha)
1397 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1398 // Otherwise, a counter move has been found and if the position is the last leaf
1399 // in the search tree, remove the position from the search tree.
1401 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1403 // Write gathered information in transposition table
1404 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1405 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1406 bestValue >= beta ? BOUND_LOWER :
1407 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1408 depth, bestMove, ss->staticEval);
1410 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1416 // qsearch() is the quiescence search function, which is called by the main search
1417 // function with zero depth, or recursively with further decreasing depth per call.
1418 template <NodeType nodeType>
1419 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1421 static_assert(nodeType != Root);
1422 constexpr bool PvNode = nodeType == PV;
1424 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1425 assert(PvNode || (alpha == beta - 1));
1430 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1434 Move ttMove, move, bestMove;
1436 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1437 bool pvHit, givesCheck, captureOrPromotion;
1442 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1444 ss->pv[0] = MOVE_NONE;
1447 Thread* thisThread = pos.this_thread();
1448 bestMove = MOVE_NONE;
1449 ss->inCheck = pos.checkers();
1452 // Check for an immediate draw or maximum ply reached
1453 if ( pos.is_draw(ss->ply)
1454 || ss->ply >= MAX_PLY)
1455 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1457 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1459 // Decide whether or not to include checks: this fixes also the type of
1460 // TT entry depth that we are going to use. Note that in qsearch we use
1461 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1462 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1463 : DEPTH_QS_NO_CHECKS;
1464 // Transposition table lookup
1466 tte = TT.probe(posKey, ss->ttHit);
1467 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1468 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1469 pvHit = ss->ttHit && tte->is_pv();
1473 && tte->depth() >= ttDepth
1474 && ttValue != VALUE_NONE // Only in case of TT access race
1475 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1476 : (tte->bound() & BOUND_UPPER)))
1479 // Evaluate the position statically
1482 ss->staticEval = VALUE_NONE;
1483 bestValue = futilityBase = -VALUE_INFINITE;
1489 // Never assume anything about values stored in TT
1490 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1491 ss->staticEval = bestValue = evaluate(pos);
1493 // Can ttValue be used as a better position evaluation?
1494 if ( ttValue != VALUE_NONE
1495 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1496 bestValue = ttValue;
1499 // In case of null move search use previous static eval with a different sign
1500 // and addition of two tempos
1501 ss->staticEval = bestValue =
1502 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1503 : -(ss-1)->staticEval;
1505 // Stand pat. Return immediately if static value is at least beta
1506 if (bestValue >= beta)
1508 // Save gathered info in transposition table
1510 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1511 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1516 if (PvNode && bestValue > alpha)
1519 futilityBase = bestValue + 155;
1522 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1523 nullptr , (ss-4)->continuationHistory,
1524 nullptr , (ss-6)->continuationHistory };
1526 // Initialize a MovePicker object for the current position, and prepare
1527 // to search the moves. Because the depth is <= 0 here, only captures,
1528 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1529 // will be generated.
1530 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1531 &thisThread->captureHistory,
1533 to_sq((ss-1)->currentMove));
1535 // Loop through the moves until no moves remain or a beta cutoff occurs
1536 while ((move = mp.next_move()) != MOVE_NONE)
1538 assert(is_ok(move));
1540 // Check for legality
1541 if (!pos.legal(move))
1544 givesCheck = pos.gives_check(move);
1545 captureOrPromotion = pos.capture_or_promotion(move);
1549 // Futility pruning and moveCount pruning
1550 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1552 && futilityBase > -VALUE_KNOWN_WIN
1553 && type_of(move) != PROMOTION)
1559 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1561 if (futilityValue <= alpha)
1563 bestValue = std::max(bestValue, futilityValue);
1567 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1569 bestValue = std::max(bestValue, futilityBase);
1574 // Do not search moves with negative SEE values
1575 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1576 && !pos.see_ge(move))
1579 // Speculative prefetch as early as possible
1580 prefetch(TT.first_entry(pos.key_after(move)));
1582 ss->currentMove = move;
1583 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1584 [captureOrPromotion]
1585 [pos.moved_piece(move)]
1588 // Continuation history based pruning
1589 if ( !captureOrPromotion
1590 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1591 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1592 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1595 // Make and search the move
1596 pos.do_move(move, st, givesCheck);
1597 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1598 pos.undo_move(move);
1600 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1602 // Check for a new best move
1603 if (value > bestValue)
1611 if (PvNode) // Update pv even in fail-high case
1612 update_pv(ss->pv, move, (ss+1)->pv);
1614 if (PvNode && value < beta) // Update alpha here!
1622 // All legal moves have been searched. A special case: if we're in check
1623 // and no legal moves were found, it is checkmate.
1624 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1626 assert(!MoveList<LEGAL>(pos).size());
1628 return mated_in(ss->ply); // Plies to mate from the root
1631 // Save gathered info in transposition table
1632 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1633 bestValue >= beta ? BOUND_LOWER :
1634 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1635 ttDepth, bestMove, ss->staticEval);
1637 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1643 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1644 // "plies to mate from the current position". Standard scores are unchanged.
1645 // The function is called before storing a value in the transposition table.
1647 Value value_to_tt(Value v, int ply) {
1649 assert(v != VALUE_NONE);
1651 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1652 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1656 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1657 // from the transposition table (which refers to the plies to mate/be mated from
1658 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1659 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1660 // and the graph history interaction, we return an optimal TB score instead.
1662 Value value_from_tt(Value v, int ply, int r50c) {
1664 if (v == VALUE_NONE)
1667 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1669 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1670 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1675 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1677 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1678 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1687 // update_pv() adds current move and appends child pv[]
1689 void update_pv(Move* pv, Move move, Move* childPv) {
1691 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1697 // update_all_stats() updates stats at the end of search() when a bestMove is found
1699 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1700 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1703 Color us = pos.side_to_move();
1704 Thread* thisThread = pos.this_thread();
1705 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1706 Piece moved_piece = pos.moved_piece(bestMove);
1707 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1709 bonus1 = stat_bonus(depth + 1);
1710 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1711 : std::min(bonus1, stat_bonus(depth)); // smaller bonus
1713 if (!pos.capture_or_promotion(bestMove))
1715 // Increase stats for the best move in case it was a quiet move
1716 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1718 // Decrease stats for all non-best quiet moves
1719 for (int i = 0; i < quietCount; ++i)
1721 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1722 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1726 // Increase stats for the best move in case it was a capture move
1727 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1729 // Extra penalty for a quiet early move that was not a TT move or
1730 // main killer move in previous ply when it gets refuted.
1731 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1732 && !pos.captured_piece())
1733 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1735 // Decrease stats for all non-best capture moves
1736 for (int i = 0; i < captureCount; ++i)
1738 moved_piece = pos.moved_piece(capturesSearched[i]);
1739 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1740 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1745 // update_continuation_histories() updates histories of the move pairs formed
1746 // by moves at ply -1, -2, -4, and -6 with current move.
1748 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1750 for (int i : {1, 2, 4, 6})
1752 // Only update first 2 continuation histories if we are in check
1753 if (ss->inCheck && i > 2)
1755 if (is_ok((ss-i)->currentMove))
1756 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1761 // update_quiet_stats() updates move sorting heuristics
1763 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1766 if (ss->killers[0] != move)
1768 ss->killers[1] = ss->killers[0];
1769 ss->killers[0] = move;
1772 Color us = pos.side_to_move();
1773 Thread* thisThread = pos.this_thread();
1774 thisThread->mainHistory[us][from_to(move)] << bonus;
1775 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1777 // Penalty for reversed move in case of moved piece not being a pawn
1778 if (type_of(pos.moved_piece(move)) != PAWN)
1779 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1781 // Update countermove history
1782 if (is_ok((ss-1)->currentMove))
1784 Square prevSq = to_sq((ss-1)->currentMove);
1785 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1788 // Update low ply history
1789 if (depth > 11 && ss->ply < MAX_LPH)
1790 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1793 // When playing with strength handicap, choose best move among a set of RootMoves
1794 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1796 Move Skill::pick_best(size_t multiPV) {
1798 const RootMoves& rootMoves = Threads.main()->rootMoves;
1799 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1801 // RootMoves are already sorted by score in descending order
1802 Value topScore = rootMoves[0].score;
1803 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1804 int weakness = 120 - 2 * level;
1805 int maxScore = -VALUE_INFINITE;
1807 // Choose best move. For each move score we add two terms, both dependent on
1808 // weakness. One is deterministic and bigger for weaker levels, and one is
1809 // random. Then we choose the move with the resulting highest score.
1810 for (size_t i = 0; i < multiPV; ++i)
1812 // This is our magic formula
1813 int push = ( weakness * int(topScore - rootMoves[i].score)
1814 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1816 if (rootMoves[i].score + push >= maxScore)
1818 maxScore = rootMoves[i].score + push;
1819 best = rootMoves[i].pv[0];
1829 /// MainThread::check_time() is used to print debug info and, more importantly,
1830 /// to detect when we are out of available time and thus stop the search.
1832 void MainThread::check_time() {
1837 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1838 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1840 static TimePoint lastInfoTime = now();
1842 TimePoint elapsed = Time.elapsed();
1843 TimePoint tick = Limits.startTime + elapsed;
1845 if (tick - lastInfoTime >= 1000)
1847 lastInfoTime = tick;
1851 // We should not stop pondering until told so by the GUI
1855 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1856 || (Limits.movetime && elapsed >= Limits.movetime)
1857 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1858 Threads.stop = true;
1862 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1863 /// that all (if any) unsearched PV lines are sent using a previous search score.
1865 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1867 std::stringstream ss;
1868 TimePoint elapsed = Time.elapsed() + 1;
1869 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1870 size_t pvIdx = pos.this_thread()->pvIdx;
1871 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1872 uint64_t nodesSearched = Threads.nodes_searched();
1873 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1875 for (size_t i = 0; i < multiPV; ++i)
1877 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1879 if (depth == 1 && !updated && i > 0)
1882 Depth d = updated ? depth : std::max(1, depth - 1);
1883 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1885 if (v == -VALUE_INFINITE)
1888 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1889 v = tb ? rootMoves[i].tbScore : v;
1891 if (ss.rdbuf()->in_avail()) // Not at first line
1896 << " seldepth " << rootMoves[i].selDepth
1897 << " multipv " << i + 1
1898 << " score " << UCI::value(v);
1900 if (Options["UCI_ShowWDL"])
1901 ss << UCI::wdl(v, pos.game_ply());
1903 if (!tb && i == pvIdx)
1904 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1906 ss << " nodes " << nodesSearched
1907 << " nps " << nodesSearched * 1000 / elapsed;
1909 if (elapsed > 1000) // Earlier makes little sense
1910 ss << " hashfull " << TT.hashfull();
1912 ss << " tbhits " << tbHits
1913 << " time " << elapsed
1916 for (Move m : rootMoves[i].pv)
1917 ss << " " << UCI::move(m, pos.is_chess960());
1924 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1925 /// before exiting the search, for instance, in case we stop the search during a
1926 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1927 /// otherwise in case of 'ponder on' we have nothing to think on.
1929 bool RootMove::extract_ponder_from_tt(Position& pos) {
1932 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1936 assert(pv.size() == 1);
1938 if (pv[0] == MOVE_NONE)
1941 pos.do_move(pv[0], st);
1942 TTEntry* tte = TT.probe(pos.key(), ttHit);
1946 Move m = tte->move(); // Local copy to be SMP safe
1947 if (MoveList<LEGAL>(pos).contains(m))
1951 pos.undo_move(pv[0]);
1952 return pv.size() > 1;
1955 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1958 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1959 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1960 Cardinality = int(Options["SyzygyProbeLimit"]);
1961 bool dtz_available = true;
1963 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1964 // ProbeDepth == DEPTH_ZERO
1965 if (Cardinality > MaxCardinality)
1967 Cardinality = MaxCardinality;
1971 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1973 // Rank moves using DTZ tables
1974 RootInTB = root_probe(pos, rootMoves);
1978 // DTZ tables are missing; try to rank moves using WDL tables
1979 dtz_available = false;
1980 RootInTB = root_probe_wdl(pos, rootMoves);
1986 // Sort moves according to TB rank
1987 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1988 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1990 // Probe during search only if DTZ is not available and we are winning
1991 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1996 // Clean up if root_probe() and root_probe_wdl() have failed
1997 for (auto& m : rootMoves)
2002 } // namespace Stockfish