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 };
64 constexpr uint64_t TtHitAverageWindow = 4096;
65 constexpr uint64_t TtHitAverageResolution = 1024;
68 Value futility_margin(Depth d, bool improving) {
69 return Value(214 * (d - improving));
72 // Reductions lookup table, initialized at startup
73 int Reductions[MAX_MOVES]; // [depth or moveNumber]
75 Depth reduction(bool i, Depth d, int mn) {
76 int r = Reductions[d] * Reductions[mn];
77 return (r + 534) / 1024 + (!i && r > 904);
80 constexpr int futility_move_count(bool improving, Depth depth) {
81 return (3 + depth * depth) / (2 - improving);
84 // History and stats update bonus, based on depth
85 int stat_bonus(Depth d) {
86 return d > 14 ? 73 : 6 * d * d + 229 * d - 215;
89 // Add a small random component to draw evaluations to avoid 3-fold blindness
90 Value value_draw(Thread* thisThread) {
91 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 template <NodeType nodeType>
106 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
108 template <NodeType nodeType>
109 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
111 Value value_to_tt(Value v, int ply);
112 Value value_from_tt(Value v, int ply, int r50c);
113 void update_pv(Move* pv, Move move, Move* childPv);
114 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
115 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth);
116 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
117 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
119 // perft() is our utility to verify move generation. All the leaf nodes up
120 // to the given depth are generated and counted, and the sum is returned.
122 uint64_t perft(Position& pos, Depth depth) {
125 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
127 uint64_t cnt, nodes = 0;
128 const bool leaf = (depth == 2);
130 for (const auto& m : MoveList<LEGAL>(pos))
132 if (Root && depth <= 1)
137 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
142 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
150 /// Search::init() is called at startup to initialize various lookup tables
152 void Search::init() {
154 for (int i = 1; i < MAX_MOVES; ++i)
155 Reductions[i] = int(21.9 * std::log(i));
159 /// Search::clear() resets search state to its initial value
161 void Search::clear() {
163 Threads.main()->wait_for_search_finished();
165 Time.availableNodes = 0;
168 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
172 /// MainThread::search() is started when the program receives the UCI 'go'
173 /// command. It searches from the root position and outputs the "bestmove".
175 void MainThread::search() {
179 nodes = perft<true>(rootPos, Limits.perft);
180 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
184 Color us = rootPos.side_to_move();
185 Time.init(Limits, us, rootPos.game_ply());
188 Eval::NNUE::verify();
190 if (rootMoves.empty())
192 rootMoves.emplace_back(MOVE_NONE);
193 sync_cout << "info depth 0 score "
194 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
199 Threads.start_searching(); // start non-main threads
200 Thread::search(); // main thread start searching
203 // When we reach the maximum depth, we can arrive here without a raise of
204 // Threads.stop. However, if we are pondering or in an infinite search,
205 // the UCI protocol states that we shouldn't print the best move before the
206 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
207 // until the GUI sends one of those commands.
209 while (!Threads.stop && (ponder || Limits.infinite))
210 {} // Busy wait for a stop or a ponder reset
212 // Stop the threads if not already stopped (also raise the stop if
213 // "ponderhit" just reset Threads.ponder).
216 // Wait until all threads have finished
217 Threads.wait_for_search_finished();
219 // When playing in 'nodes as time' mode, subtract the searched nodes from
220 // the available ones before exiting.
222 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
224 Thread* bestThread = this;
226 if ( int(Options["MultiPV"]) == 1
228 && !(Skill(Options["Skill Level"]).enabled() || int(Options["UCI_LimitStrength"]))
229 && rootMoves[0].pv[0] != MOVE_NONE)
230 bestThread = Threads.get_best_thread();
232 bestPreviousScore = bestThread->rootMoves[0].score;
234 // Send again PV info if we have a new best thread
235 if (bestThread != this)
236 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
238 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
240 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
241 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
243 std::cout << sync_endl;
247 /// Thread::search() is the main iterative deepening loop. It calls search()
248 /// repeatedly with increasing depth until the allocated thinking time has been
249 /// consumed, the user stops the search, or the maximum search depth is reached.
251 void Thread::search() {
253 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
254 // The former is needed to allow update_continuation_histories(ss-1, ...),
255 // which accesses its argument at ss-6, also near the root.
256 // The latter is needed for statScore and killer initialization.
257 Stack stack[MAX_PLY+10], *ss = stack+7;
259 Value bestValue, alpha, beta, delta;
260 Move lastBestMove = MOVE_NONE;
261 Depth lastBestMoveDepth = 0;
262 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
263 double timeReduction = 1, totBestMoveChanges = 0;
264 Color us = rootPos.side_to_move();
267 std::memset(ss-7, 0, 10 * sizeof(Stack));
268 for (int i = 7; i > 0; i--)
269 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
271 for (int i = 0; i <= MAX_PLY + 2; ++i)
276 bestValue = delta = alpha = -VALUE_INFINITE;
277 beta = VALUE_INFINITE;
281 if (mainThread->bestPreviousScore == VALUE_INFINITE)
282 for (int i = 0; i < 4; ++i)
283 mainThread->iterValue[i] = VALUE_ZERO;
285 for (int i = 0; i < 4; ++i)
286 mainThread->iterValue[i] = mainThread->bestPreviousScore;
289 std::copy(&lowPlyHistory[2][0], &lowPlyHistory.back().back() + 1, &lowPlyHistory[0][0]);
290 std::fill(&lowPlyHistory[MAX_LPH - 2][0], &lowPlyHistory.back().back() + 1, 0);
292 size_t multiPV = size_t(Options["MultiPV"]);
294 // Pick integer skill levels, but non-deterministically round up or down
295 // such that the average integer skill corresponds to the input floating point one.
296 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
297 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
298 // for match (TC 60+0.6) results spanning a wide range of k values.
300 double floatLevel = Options["UCI_LimitStrength"] ?
301 std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
302 double(Options["Skill Level"]);
303 int intLevel = int(floatLevel) +
304 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
305 Skill skill(intLevel);
307 // When playing with strength handicap enable MultiPV search that we will
308 // use behind the scenes to retrieve a set of possible moves.
310 multiPV = std::max(multiPV, (size_t)4);
312 multiPV = std::min(multiPV, rootMoves.size());
313 ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
317 int searchAgainCounter = 0;
319 // Iterative deepening loop until requested to stop or the target depth is reached
320 while ( ++rootDepth < MAX_PLY
322 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
324 // Age out PV variability metric
326 totBestMoveChanges /= 2;
328 // Save the last iteration's scores before first PV line is searched and
329 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
330 for (RootMove& rm : rootMoves)
331 rm.previousScore = rm.score;
336 if (!Threads.increaseDepth)
337 searchAgainCounter++;
339 // MultiPV loop. We perform a full root search for each PV line
340 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
345 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
346 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
350 // Reset UCI info selDepth for each depth and each PV line
353 // Reset aspiration window starting size
356 Value prev = rootMoves[pvIdx].previousScore;
358 alpha = std::max(prev - delta,-VALUE_INFINITE);
359 beta = std::min(prev + delta, VALUE_INFINITE);
361 // Adjust trend based on root move's previousScore (dynamic contempt)
362 int tr = 113 * prev / (abs(prev) + 147);
364 trend = (us == WHITE ? make_score(tr, tr / 2)
365 : -make_score(tr, tr / 2));
368 // Start with a small aspiration window and, in the case of a fail
369 // high/low, re-search with a bigger window until we don't fail
371 int failedHighCnt = 0;
374 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
375 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
377 // Bring the best move to the front. It is critical that sorting
378 // is done with a stable algorithm because all the values but the
379 // first and eventually the new best one are set to -VALUE_INFINITE
380 // and we want to keep the same order for all the moves except the
381 // new PV that goes to the front. Note that in case of MultiPV
382 // search the already searched PV lines are preserved.
383 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
385 // If search has been stopped, we break immediately. Sorting is
386 // safe because RootMoves is still valid, although it refers to
387 // the previous iteration.
391 // When failing high/low give some update (without cluttering
392 // the UI) before a re-search.
395 && (bestValue <= alpha || bestValue >= beta)
396 && Time.elapsed() > 3000)
397 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
399 // In case of failing low/high increase aspiration window and
400 // re-search, otherwise exit the loop.
401 if (bestValue <= alpha)
403 beta = (alpha + beta) / 2;
404 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
408 mainThread->stopOnPonderhit = false;
410 else if (bestValue >= beta)
412 beta = std::min(bestValue + delta, VALUE_INFINITE);
418 delta += delta / 4 + 5;
420 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
423 // Sort the PV lines searched so far and update the GUI
424 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
427 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
428 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
432 completedDepth = rootDepth;
434 if (rootMoves[0].pv[0] != lastBestMove) {
435 lastBestMove = rootMoves[0].pv[0];
436 lastBestMoveDepth = rootDepth;
439 // Have we found a "mate in x"?
441 && bestValue >= VALUE_MATE_IN_MAX_PLY
442 && VALUE_MATE - bestValue <= 2 * Limits.mate)
448 // If skill level is enabled and time is up, pick a sub-optimal best move
449 if (skill.enabled() && skill.time_to_pick(rootDepth))
450 skill.pick_best(multiPV);
452 // Do we have time for the next iteration? Can we stop searching now?
453 if ( Limits.use_time_management()
455 && !mainThread->stopOnPonderhit)
457 double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue)
458 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0;
459 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
461 // If the bestMove is stable over several iterations, reduce time accordingly
462 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95;
463 double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction);
465 // Use part of the gained time from a previous stable move for the current move
466 for (Thread* th : Threads)
468 totBestMoveChanges += th->bestMoveChanges;
469 th->bestMoveChanges = 0;
471 double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth)
472 * totBestMoveChanges / Threads.size();
473 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
475 // Cap used time in case of a single legal move for a better viewer experience in tournaments
476 // yielding correct scores and sufficiently fast moves.
477 if (rootMoves.size() == 1)
478 totalTime = std::min(500.0, totalTime);
480 // Stop the search if we have exceeded the totalTime
481 if (Time.elapsed() > totalTime)
483 // If we are allowed to ponder do not stop the search now but
484 // keep pondering until the GUI sends "ponderhit" or "stop".
485 if (mainThread->ponder)
486 mainThread->stopOnPonderhit = true;
490 else if ( Threads.increaseDepth
491 && !mainThread->ponder
492 && Time.elapsed() > totalTime * 0.58)
493 Threads.increaseDepth = false;
495 Threads.increaseDepth = true;
498 mainThread->iterValue[iterIdx] = bestValue;
499 iterIdx = (iterIdx + 1) & 3;
505 mainThread->previousTimeReduction = timeReduction;
507 // If skill level is enabled, swap best PV line with the sub-optimal one
509 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
510 skill.best ? skill.best : skill.pick_best(multiPV)));
516 // search<>() is the main search function for both PV and non-PV nodes
518 template <NodeType nodeType>
519 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
521 constexpr bool PvNode = nodeType != NonPV;
522 constexpr bool rootNode = nodeType == Root;
523 const Depth maxNextDepth = rootNode ? depth : depth + 1;
525 // Check if we have an upcoming move which draws by repetition, or
526 // if the opponent had an alternative move earlier to this position.
528 && pos.rule50_count() >= 3
529 && alpha < VALUE_DRAW
530 && pos.has_game_cycle(ss->ply))
532 alpha = value_draw(pos.this_thread());
537 // Dive into quiescence search when the depth reaches zero
539 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
541 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
542 assert(PvNode || (alpha == beta - 1));
543 assert(0 < depth && depth < MAX_PLY);
544 assert(!(PvNode && cutNode));
546 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
548 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
552 Move ttMove, move, excludedMove, bestMove;
553 Depth extension, newDepth;
554 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
555 bool givesCheck, improving, didLMR, priorCapture;
556 bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
557 ttCapture, singularQuietLMR;
559 int moveCount, captureCount, quietCount;
561 // Step 1. Initialize node
562 Thread* thisThread = pos.this_thread();
563 ss->inCheck = pos.checkers();
564 priorCapture = pos.captured_piece();
565 Color us = pos.side_to_move();
566 moveCount = captureCount = quietCount = ss->moveCount = 0;
567 bestValue = -VALUE_INFINITE;
568 maxValue = VALUE_INFINITE;
570 // Check for the available remaining time
571 if (thisThread == Threads.main())
572 static_cast<MainThread*>(thisThread)->check_time();
574 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
575 if (PvNode && thisThread->selDepth < ss->ply + 1)
576 thisThread->selDepth = ss->ply + 1;
580 // Step 2. Check for aborted search and immediate draw
581 if ( Threads.stop.load(std::memory_order_relaxed)
582 || pos.is_draw(ss->ply)
583 || ss->ply >= MAX_PLY)
584 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
585 : value_draw(pos.this_thread());
587 // Step 3. Mate distance pruning. Even if we mate at the next move our score
588 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
589 // a shorter mate was found upward in the tree then there is no need to search
590 // because we will never beat the current alpha. Same logic but with reversed
591 // signs applies also in the opposite condition of being mated instead of giving
592 // mate. In this case return a fail-high score.
593 alpha = std::max(mated_in(ss->ply), alpha);
594 beta = std::min(mate_in(ss->ply+1), beta);
599 assert(0 <= ss->ply && ss->ply < MAX_PLY);
601 (ss+1)->ttPv = false;
602 (ss+1)->excludedMove = bestMove = MOVE_NONE;
603 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
604 ss->doubleExtensions = (ss-1)->doubleExtensions;
605 Square prevSq = to_sq((ss-1)->currentMove);
607 // Initialize statScore to zero for the grandchildren of the current position.
608 // So statScore is shared between all grandchildren and only the first grandchild
609 // starts with statScore = 0. Later grandchildren start with the last calculated
610 // statScore of the previous grandchild. This influences the reduction rules in
611 // LMR which are based on the statScore of parent position.
613 (ss+2)->statScore = 0;
615 // Step 4. Transposition table lookup. We don't want the score of a partial
616 // search to overwrite a previous full search TT value, so we use a different
617 // position key in case of an excluded move.
618 excludedMove = ss->excludedMove;
619 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
620 tte = TT.probe(posKey, ss->ttHit);
621 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
622 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
623 : ss->ttHit ? tte->move() : MOVE_NONE;
625 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
627 // Update low ply history for previous move if we are near root and position is or has been in PV
630 && ss->ply - 1 < MAX_LPH
632 && is_ok((ss-1)->currentMove))
633 thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5);
635 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
636 thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow
637 + TtHitAverageResolution * ss->ttHit;
639 // At non-PV nodes we check for an early TT cutoff
642 && tte->depth() >= depth
643 && ttValue != VALUE_NONE // Possible in case of TT access race
644 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
645 : (tte->bound() & BOUND_UPPER)))
647 // If ttMove is quiet, update move sorting heuristics on TT hit
652 // Bonus for a quiet ttMove that fails high
653 if (!pos.capture_or_promotion(ttMove))
654 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
656 // Extra penalty for early quiet moves of the previous ply
657 if ((ss-1)->moveCount <= 2 && !priorCapture)
658 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
660 // Penalty for a quiet ttMove that fails low
661 else if (!pos.capture_or_promotion(ttMove))
663 int penalty = -stat_bonus(depth);
664 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
665 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
669 // Partial workaround for the graph history interaction problem
670 // For high rule50 counts don't produce transposition table cutoffs.
671 if (pos.rule50_count() < 90)
675 // Step 5. Tablebases probe
676 if (!rootNode && TB::Cardinality)
678 int piecesCount = pos.count<ALL_PIECES>();
680 if ( piecesCount <= TB::Cardinality
681 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
682 && pos.rule50_count() == 0
683 && !pos.can_castle(ANY_CASTLING))
686 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
688 // Force check of time on the next occasion
689 if (thisThread == Threads.main())
690 static_cast<MainThread*>(thisThread)->callsCnt = 0;
692 if (err != TB::ProbeState::FAIL)
694 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
696 int drawScore = TB::UseRule50 ? 1 : 0;
698 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
699 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
700 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
701 : VALUE_DRAW + 2 * wdl * drawScore;
703 Bound b = wdl < -drawScore ? BOUND_UPPER
704 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
706 if ( b == BOUND_EXACT
707 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
709 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
710 std::min(MAX_PLY - 1, depth + 6),
711 MOVE_NONE, VALUE_NONE);
718 if (b == BOUND_LOWER)
719 bestValue = value, alpha = std::max(alpha, bestValue);
727 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
729 // Step 6. Static evaluation of the position
732 // Skip early pruning when in check
733 ss->staticEval = eval = VALUE_NONE;
739 // Never assume anything about values stored in TT
740 ss->staticEval = eval = tte->eval();
741 if (eval == VALUE_NONE)
742 ss->staticEval = eval = evaluate(pos);
744 // Randomize draw evaluation
745 if (eval == VALUE_DRAW)
746 eval = value_draw(thisThread);
748 // Can ttValue be used as a better position evaluation?
749 if ( ttValue != VALUE_NONE
750 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
755 // In case of null move search use previous static eval with a different sign
756 // and addition of two tempos
757 if ((ss-1)->currentMove != MOVE_NULL)
758 ss->staticEval = eval = evaluate(pos);
760 ss->staticEval = eval = -(ss-1)->staticEval;
762 // Save static evaluation into transposition table
764 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
767 // Use static evaluation difference to improve quiet move ordering
768 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
770 int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval), -1000, 1000);
771 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
774 // Set up improving flag that is used in various pruning heuristics
775 // We define position as improving if static evaluation of position is better
776 // Than the previous static evaluation at our turn
777 // In case of us being in check at our previous move we look at move prior to it
778 improving = (ss-2)->staticEval == VALUE_NONE
779 ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
780 : ss->staticEval > (ss-2)->staticEval;
782 // Step 7. Futility pruning: child node (~50 Elo)
784 && eval - futility_margin(depth, improving) >= beta
785 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
788 // Step 8. Null move search with verification search (~40 Elo)
790 && (ss-1)->currentMove != MOVE_NULL
791 && (ss-1)->statScore < 23767
793 && eval >= ss->staticEval
794 && ss->staticEval >= beta - 20 * depth - 22 * improving + 168 * ss->ttPv + 159
796 && pos.non_pawn_material(us)
797 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
799 assert(eval - beta >= 0);
801 // Null move dynamic reduction based on depth and value
802 Depth R = (1090 + 81 * depth) / 256 + std::min(int(eval - beta) / 205, 3);
804 ss->currentMove = MOVE_NULL;
805 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
807 pos.do_null_move(st);
809 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
811 pos.undo_null_move();
813 if (nullValue >= beta)
815 // Do not return unproven mate or TB scores
816 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
819 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
822 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
824 // Do verification search at high depths, with null move pruning disabled
825 // for us, until ply exceeds nmpMinPly.
826 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
827 thisThread->nmpColor = us;
829 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
831 thisThread->nmpMinPly = 0;
838 probCutBeta = beta + 209 - 44 * improving;
840 // Step 9. ProbCut (~4 Elo)
841 // If we have a good enough capture and a reduced search returns a value
842 // much above beta, we can (almost) safely prune the previous move.
845 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
846 // if value from transposition table is lower than probCutBeta, don't attempt probCut
847 // there and in further interactions with transposition table cutoff depth is set to depth - 3
848 // because probCut search has depth set to depth - 4 but we also do a move before it
849 // so effective depth is equal to depth - 3
851 && tte->depth() >= depth - 3
852 && ttValue != VALUE_NONE
853 && ttValue < probCutBeta))
855 assert(probCutBeta < VALUE_INFINITE);
857 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
858 int probCutCount = 0;
859 bool ttPv = ss->ttPv;
862 while ( (move = mp.next_move()) != MOVE_NONE
863 && probCutCount < 2 + 2 * cutNode)
864 if (move != excludedMove && pos.legal(move))
866 assert(pos.capture_or_promotion(move));
869 captureOrPromotion = true;
872 ss->currentMove = move;
873 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
875 [pos.moved_piece(move)]
878 pos.do_move(move, st);
880 // Perform a preliminary qsearch to verify that the move holds
881 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
883 // If the qsearch held, perform the regular search
884 if (value >= probCutBeta)
885 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
889 if (value >= probCutBeta)
891 // if transposition table doesn't have equal or more deep info write probCut data into it
893 && tte->depth() >= depth - 3
894 && ttValue != VALUE_NONE))
895 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
897 depth - 3, move, ss->staticEval);
904 // Step 10. If the position is not in TT, decrease depth by 2
910 moves_loop: // When in check, search starts from here
912 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
914 // Step 11. A small Probcut idea, when we are in check
915 probCutBeta = beta + 409;
920 && (tte->bound() & BOUND_LOWER)
921 && tte->depth() >= depth - 3
922 && ttValue >= probCutBeta
923 && abs(ttValue) <= VALUE_KNOWN_WIN
924 && abs(beta) <= VALUE_KNOWN_WIN
929 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
930 nullptr , (ss-4)->continuationHistory,
931 nullptr , (ss-6)->continuationHistory };
933 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
935 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
936 &thisThread->lowPlyHistory,
944 singularQuietLMR = moveCountPruning = false;
945 bool doubleExtension = false;
947 // Indicate PvNodes that will probably fail low if the node was searched
948 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
949 bool likelyFailLow = PvNode
951 && (tte->bound() & BOUND_UPPER)
952 && tte->depth() >= depth;
954 // Step 12. Loop through all pseudo-legal moves until no moves remain
955 // or a beta cutoff occurs.
956 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
960 if (move == excludedMove)
963 // At root obey the "searchmoves" option and skip moves not listed in Root
964 // Move List. As a consequence any illegal move is also skipped. In MultiPV
965 // mode we also skip PV moves which have been already searched and those
966 // of lower "TB rank" if we are in a TB root position.
967 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
968 thisThread->rootMoves.begin() + thisThread->pvLast, move))
971 // Check for legality
972 if (!rootNode && !pos.legal(move))
975 ss->moveCount = ++moveCount;
977 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
978 sync_cout << "info depth " << depth
979 << " currmove " << UCI::move(move, pos.is_chess960())
980 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
982 (ss+1)->pv = nullptr;
985 captureOrPromotion = pos.capture_or_promotion(move);
986 movedPiece = pos.moved_piece(move);
987 givesCheck = pos.gives_check(move);
989 // Calculate new depth for this move
990 newDepth = depth - 1;
992 // Step 13. Pruning at shallow depth (~200 Elo)
994 && pos.non_pawn_material(us)
995 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
997 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
998 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1000 // Reduced depth of the next LMR search
1001 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1003 if ( captureOrPromotion
1006 // Capture history based pruning when the move doesn't give check
1009 && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
1012 // SEE based pruning
1013 if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
1018 // Continuation history based pruning (~20 Elo)
1020 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1021 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1024 // Futility pruning: parent node (~5 Elo)
1026 && ss->staticEval + 174 + 157 * lmrDepth <= alpha
1027 && (*contHist[0])[movedPiece][to_sq(move)]
1028 + (*contHist[1])[movedPiece][to_sq(move)]
1029 + (*contHist[3])[movedPiece][to_sq(move)]
1030 + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 28255)
1033 // Prune moves with negative SEE (~20 Elo)
1034 if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1039 // Step 14. Extensions (~75 Elo)
1041 // Singular extension search (~70 Elo). If all moves but one fail low on a
1042 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1043 // then that move is singular and should be extended. To verify this we do
1044 // a reduced search on all the other moves but the ttMove and if the
1045 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1049 && !excludedMove // Avoid recursive singular search
1050 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1051 && abs(ttValue) < VALUE_KNOWN_WIN
1052 && (tte->bound() & BOUND_LOWER)
1053 && tte->depth() >= depth - 3)
1055 Value singularBeta = ttValue - 2 * depth;
1056 Depth singularDepth = (depth - 1) / 2;
1058 ss->excludedMove = move;
1059 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1060 ss->excludedMove = MOVE_NONE;
1062 if (value < singularBeta)
1065 singularQuietLMR = !ttCapture;
1067 // Avoid search explosion by limiting the number of double extensions to at most 3
1069 && value < singularBeta - 93
1070 && ss->doubleExtensions < 3)
1073 doubleExtension = true;
1077 // Multi-cut pruning
1078 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1079 // search without the ttMove. So we assume this expected Cut-node is not singular,
1080 // that multiple moves fail high, and we can prune the whole subtree by returning
1082 else if (singularBeta >= beta)
1083 return singularBeta;
1085 // If the eval of ttMove is greater than beta we try also if there is another
1086 // move that pushes it over beta, if so also produce a cutoff.
1087 else if (ttValue >= beta)
1089 ss->excludedMove = move;
1090 value = search<NonPV>(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode);
1091 ss->excludedMove = MOVE_NONE;
1097 else if ( givesCheck
1099 && abs(ss->staticEval) > Value(100))
1102 // Add extension to new depth
1103 newDepth += extension;
1104 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1106 // Speculative prefetch as early as possible
1107 prefetch(TT.first_entry(pos.key_after(move)));
1109 // Update the current move (this must be done after singular extension search)
1110 ss->currentMove = move;
1111 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1112 [captureOrPromotion]
1116 // Step 15. Make the move
1117 pos.do_move(move, st, givesCheck);
1119 // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
1120 // We use various heuristics for the sons of a node after the first son has
1121 // been searched. In general we would like to reduce them, but there are many
1122 // cases where we extend a son if it has good chances to be "interesting".
1124 && moveCount > 1 + 2 * rootNode
1125 && ( !captureOrPromotion
1126 || (cutNode && (ss-1)->moveCount > 1)
1128 && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
1130 Depth r = reduction(improving, depth, moveCount);
1135 // Decrease reduction if the ttHit running average is large (~0 Elo)
1136 if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
1139 // Decrease reduction if position is or has been on the PV
1140 // and node is not likely to fail low. (~3 Elo)
1145 // Increase reduction at root and non-PV nodes when the best move does not change frequently
1146 if ( (rootNode || !PvNode)
1147 && thisThread->bestMoveChanges <= 2)
1150 // Decrease reduction if opponent's move count is high (~1 Elo)
1151 if ((ss-1)->moveCount > 13)
1154 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1155 if (singularQuietLMR)
1158 // Increase reduction for cut nodes (~3 Elo)
1159 if (cutNode && move != ss->killers[0])
1162 // Increase reduction if ttMove is a capture (~3 Elo)
1166 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1167 + (*contHist[0])[movedPiece][to_sq(move)]
1168 + (*contHist[1])[movedPiece][to_sq(move)]
1169 + (*contHist[3])[movedPiece][to_sq(move)]
1172 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1173 r -= ss->statScore / 14721;
1175 // In general we want to cap the LMR depth search at newDepth. But if
1176 // reductions are really negative and movecount is low, we allow this move
1177 // to be searched deeper than the first move, unless ttMove was extended by 2.
1178 Depth d = std::clamp(newDepth - r, 1, newDepth + (r < -1 && moveCount <= 5 && !doubleExtension));
1180 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1182 // If the son is reduced and fails high it will be re-searched at full depth
1183 doFullDepthSearch = value > alpha && d < newDepth;
1188 doFullDepthSearch = !PvNode || moveCount > 1;
1192 // Step 17. Full depth search when LMR is skipped or fails high
1193 if (doFullDepthSearch)
1195 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1197 // If the move passed LMR update its stats
1198 if (didLMR && !captureOrPromotion)
1200 int bonus = value > alpha ? stat_bonus(newDepth)
1201 : -stat_bonus(newDepth);
1203 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1207 // For PV nodes only, do a full PV search on the first move or after a fail
1208 // high (in the latter case search only if value < beta), otherwise let the
1209 // parent node fail low with value <= alpha and try another move.
1210 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1213 (ss+1)->pv[0] = MOVE_NONE;
1215 value = -search<PV>(pos, ss+1, -beta, -alpha,
1216 std::min(maxNextDepth, newDepth), false);
1219 // Step 18. Undo move
1220 pos.undo_move(move);
1222 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1224 // Step 19. Check for a new best move
1225 // Finished searching the move. If a stop occurred, the return value of
1226 // the search cannot be trusted, and we return immediately without
1227 // updating best move, PV and TT.
1228 if (Threads.stop.load(std::memory_order_relaxed))
1233 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1234 thisThread->rootMoves.end(), move);
1236 // PV move or new best move?
1237 if (moveCount == 1 || value > alpha)
1240 rm.selDepth = thisThread->selDepth;
1245 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1246 rm.pv.push_back(*m);
1248 // We record how often the best move has been changed in each
1249 // iteration. This information is used for time management and LMR
1251 ++thisThread->bestMoveChanges;
1254 // All other moves but the PV are set to the lowest value: this
1255 // is not a problem when sorting because the sort is stable and the
1256 // move position in the list is preserved - just the PV is pushed up.
1257 rm.score = -VALUE_INFINITE;
1260 if (value > bestValue)
1268 if (PvNode && !rootNode) // Update pv even in fail-high case
1269 update_pv(ss->pv, move, (ss+1)->pv);
1271 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1275 assert(value >= beta); // Fail high
1281 // If the move is worse than some previously searched move, remember it to update its stats later
1282 if (move != bestMove)
1284 if (captureOrPromotion && captureCount < 32)
1285 capturesSearched[captureCount++] = move;
1287 else if (!captureOrPromotion && quietCount < 64)
1288 quietsSearched[quietCount++] = move;
1292 // The following condition would detect a stop only after move loop has been
1293 // completed. But in this case bestValue is valid because we have fully
1294 // searched our subtree, and we can anyhow save the result in TT.
1300 // Step 20. Check for mate and stalemate
1301 // All legal moves have been searched and if there are no legal moves, it
1302 // must be a mate or a stalemate. If we are in a singular extension search then
1303 // return a fail low score.
1305 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1308 bestValue = excludedMove ? alpha :
1309 ss->inCheck ? mated_in(ss->ply)
1312 // If there is a move which produces search value greater than alpha we update stats of searched moves
1314 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1315 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1317 // Bonus for prior countermove that caused the fail low
1318 else if ( (depth >= 3 || PvNode)
1320 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1323 bestValue = std::min(bestValue, maxValue);
1325 // If no good move is found and the previous position was ttPv, then the previous
1326 // opponent move is probably good and the new position is added to the search tree.
1327 if (bestValue <= alpha)
1328 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1329 // Otherwise, a counter move has been found and if the position is the last leaf
1330 // in the search tree, remove the position from the search tree.
1332 ss->ttPv = ss->ttPv && (ss+1)->ttPv;
1334 // Write gathered information in transposition table
1335 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1336 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1337 bestValue >= beta ? BOUND_LOWER :
1338 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1339 depth, bestMove, ss->staticEval);
1341 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1347 // qsearch() is the quiescence search function, which is called by the main search
1348 // function with zero depth, or recursively with further decreasing depth per call.
1349 template <NodeType nodeType>
1350 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1352 static_assert(nodeType != Root);
1353 constexpr bool PvNode = nodeType == PV;
1355 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1356 assert(PvNode || (alpha == beta - 1));
1361 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1365 Move ttMove, move, bestMove;
1367 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1368 bool pvHit, givesCheck, captureOrPromotion;
1373 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1375 ss->pv[0] = MOVE_NONE;
1378 Thread* thisThread = pos.this_thread();
1379 bestMove = MOVE_NONE;
1380 ss->inCheck = pos.checkers();
1383 // Check for an immediate draw or maximum ply reached
1384 if ( pos.is_draw(ss->ply)
1385 || ss->ply >= MAX_PLY)
1386 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1388 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1390 // Decide whether or not to include checks: this fixes also the type of
1391 // TT entry depth that we are going to use. Note that in qsearch we use
1392 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1393 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1394 : DEPTH_QS_NO_CHECKS;
1395 // Transposition table lookup
1397 tte = TT.probe(posKey, ss->ttHit);
1398 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1399 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1400 pvHit = ss->ttHit && tte->is_pv();
1404 && tte->depth() >= ttDepth
1405 && ttValue != VALUE_NONE // Only in case of TT access race
1406 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1407 : (tte->bound() & BOUND_UPPER)))
1410 // Evaluate the position statically
1413 ss->staticEval = VALUE_NONE;
1414 bestValue = futilityBase = -VALUE_INFINITE;
1420 // Never assume anything about values stored in TT
1421 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1422 ss->staticEval = bestValue = evaluate(pos);
1424 // Can ttValue be used as a better position evaluation?
1425 if ( ttValue != VALUE_NONE
1426 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1427 bestValue = ttValue;
1430 // In case of null move search use previous static eval with a different sign
1431 // and addition of two tempos
1432 ss->staticEval = bestValue =
1433 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1434 : -(ss-1)->staticEval;
1436 // Stand pat. Return immediately if static value is at least beta
1437 if (bestValue >= beta)
1439 // Save gathered info in transposition table
1441 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1442 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1447 if (PvNode && bestValue > alpha)
1450 futilityBase = bestValue + 155;
1453 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1454 nullptr , (ss-4)->continuationHistory,
1455 nullptr , (ss-6)->continuationHistory };
1457 // Initialize a MovePicker object for the current position, and prepare
1458 // to search the moves. Because the depth is <= 0 here, only captures,
1459 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1460 // will be generated.
1461 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1462 &thisThread->captureHistory,
1464 to_sq((ss-1)->currentMove));
1466 // Loop through the moves until no moves remain or a beta cutoff occurs
1467 while ((move = mp.next_move()) != MOVE_NONE)
1469 assert(is_ok(move));
1471 // Check for legality
1472 if (!pos.legal(move))
1475 givesCheck = pos.gives_check(move);
1476 captureOrPromotion = pos.capture_or_promotion(move);
1480 // Futility pruning and moveCount pruning
1481 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1483 && futilityBase > -VALUE_KNOWN_WIN
1484 && type_of(move) != PROMOTION)
1490 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1492 if (futilityValue <= alpha)
1494 bestValue = std::max(bestValue, futilityValue);
1498 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1500 bestValue = std::max(bestValue, futilityBase);
1505 // Do not search moves with negative SEE values
1506 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1507 && !pos.see_ge(move))
1510 // Speculative prefetch as early as possible
1511 prefetch(TT.first_entry(pos.key_after(move)));
1513 ss->currentMove = move;
1514 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1515 [captureOrPromotion]
1516 [pos.moved_piece(move)]
1519 // Continuation history based pruning
1520 if ( !captureOrPromotion
1521 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1522 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1523 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1526 // Make and search the move
1527 pos.do_move(move, st, givesCheck);
1528 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1529 pos.undo_move(move);
1531 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1533 // Check for a new best move
1534 if (value > bestValue)
1542 if (PvNode) // Update pv even in fail-high case
1543 update_pv(ss->pv, move, (ss+1)->pv);
1545 if (PvNode && value < beta) // Update alpha here!
1553 // All legal moves have been searched. A special case: if we're in check
1554 // and no legal moves were found, it is checkmate.
1555 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1557 assert(!MoveList<LEGAL>(pos).size());
1559 return mated_in(ss->ply); // Plies to mate from the root
1562 // Save gathered info in transposition table
1563 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1564 bestValue >= beta ? BOUND_LOWER :
1565 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1566 ttDepth, bestMove, ss->staticEval);
1568 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1574 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1575 // "plies to mate from the current position". Standard scores are unchanged.
1576 // The function is called before storing a value in the transposition table.
1578 Value value_to_tt(Value v, int ply) {
1580 assert(v != VALUE_NONE);
1582 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1583 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1587 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1588 // from the transposition table (which refers to the plies to mate/be mated from
1589 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1590 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1591 // and the graph history interaction, we return an optimal TB score instead.
1593 Value value_from_tt(Value v, int ply, int r50c) {
1595 if (v == VALUE_NONE)
1598 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1600 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1601 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1606 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1608 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1609 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1618 // update_pv() adds current move and appends child pv[]
1620 void update_pv(Move* pv, Move move, Move* childPv) {
1622 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1628 // update_all_stats() updates stats at the end of search() when a bestMove is found
1630 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1631 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1634 Color us = pos.side_to_move();
1635 Thread* thisThread = pos.this_thread();
1636 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1637 Piece moved_piece = pos.moved_piece(bestMove);
1638 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1640 bonus1 = stat_bonus(depth + 1);
1641 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1642 : std::min(bonus1, stat_bonus(depth)); // smaller bonus
1644 if (!pos.capture_or_promotion(bestMove))
1646 // Increase stats for the best move in case it was a quiet move
1647 update_quiet_stats(pos, ss, bestMove, bonus2, depth);
1649 // Decrease stats for all non-best quiet moves
1650 for (int i = 0; i < quietCount; ++i)
1652 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1653 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1657 // Increase stats for the best move in case it was a capture move
1658 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1660 // Extra penalty for a quiet early move that was not a TT move or
1661 // main killer move in previous ply when it gets refuted.
1662 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1663 && !pos.captured_piece())
1664 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1666 // Decrease stats for all non-best capture moves
1667 for (int i = 0; i < captureCount; ++i)
1669 moved_piece = pos.moved_piece(capturesSearched[i]);
1670 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1671 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1676 // update_continuation_histories() updates histories of the move pairs formed
1677 // by moves at ply -1, -2, -4, and -6 with current move.
1679 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1681 for (int i : {1, 2, 4, 6})
1683 // Only update first 2 continuation histories if we are in check
1684 if (ss->inCheck && i > 2)
1686 if (is_ok((ss-i)->currentMove))
1687 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1692 // update_quiet_stats() updates move sorting heuristics
1694 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
1697 if (ss->killers[0] != move)
1699 ss->killers[1] = ss->killers[0];
1700 ss->killers[0] = move;
1703 Color us = pos.side_to_move();
1704 Thread* thisThread = pos.this_thread();
1705 thisThread->mainHistory[us][from_to(move)] << bonus;
1706 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1708 // Penalty for reversed move in case of moved piece not being a pawn
1709 if (type_of(pos.moved_piece(move)) != PAWN)
1710 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1712 // Update countermove history
1713 if (is_ok((ss-1)->currentMove))
1715 Square prevSq = to_sq((ss-1)->currentMove);
1716 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1719 // Update low ply history
1720 if (depth > 11 && ss->ply < MAX_LPH)
1721 thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
1724 // When playing with strength handicap, choose best move among a set of RootMoves
1725 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1727 Move Skill::pick_best(size_t multiPV) {
1729 const RootMoves& rootMoves = Threads.main()->rootMoves;
1730 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1732 // RootMoves are already sorted by score in descending order
1733 Value topScore = rootMoves[0].score;
1734 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1735 int weakness = 120 - 2 * level;
1736 int maxScore = -VALUE_INFINITE;
1738 // Choose best move. For each move score we add two terms, both dependent on
1739 // weakness. One is deterministic and bigger for weaker levels, and one is
1740 // random. Then we choose the move with the resulting highest score.
1741 for (size_t i = 0; i < multiPV; ++i)
1743 // This is our magic formula
1744 int push = ( weakness * int(topScore - rootMoves[i].score)
1745 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1747 if (rootMoves[i].score + push >= maxScore)
1749 maxScore = rootMoves[i].score + push;
1750 best = rootMoves[i].pv[0];
1760 /// MainThread::check_time() is used to print debug info and, more importantly,
1761 /// to detect when we are out of available time and thus stop the search.
1763 void MainThread::check_time() {
1768 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1769 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1771 static TimePoint lastInfoTime = now();
1773 TimePoint elapsed = Time.elapsed();
1774 TimePoint tick = Limits.startTime + elapsed;
1776 if (tick - lastInfoTime >= 1000)
1778 lastInfoTime = tick;
1782 // We should not stop pondering until told so by the GUI
1786 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1787 || (Limits.movetime && elapsed >= Limits.movetime)
1788 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1789 Threads.stop = true;
1793 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1794 /// that all (if any) unsearched PV lines are sent using a previous search score.
1796 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1798 std::stringstream ss;
1799 TimePoint elapsed = Time.elapsed() + 1;
1800 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1801 size_t pvIdx = pos.this_thread()->pvIdx;
1802 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1803 uint64_t nodesSearched = Threads.nodes_searched();
1804 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1806 for (size_t i = 0; i < multiPV; ++i)
1808 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1810 if (depth == 1 && !updated && i > 0)
1813 Depth d = updated ? depth : std::max(1, depth - 1);
1814 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1816 if (v == -VALUE_INFINITE)
1819 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1820 v = tb ? rootMoves[i].tbScore : v;
1822 if (ss.rdbuf()->in_avail()) // Not at first line
1827 << " seldepth " << rootMoves[i].selDepth
1828 << " multipv " << i + 1
1829 << " score " << UCI::value(v);
1831 if (Options["UCI_ShowWDL"])
1832 ss << UCI::wdl(v, pos.game_ply());
1834 if (!tb && i == pvIdx)
1835 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1837 ss << " nodes " << nodesSearched
1838 << " nps " << nodesSearched * 1000 / elapsed;
1840 if (elapsed > 1000) // Earlier makes little sense
1841 ss << " hashfull " << TT.hashfull();
1843 ss << " tbhits " << tbHits
1844 << " time " << elapsed
1847 for (Move m : rootMoves[i].pv)
1848 ss << " " << UCI::move(m, pos.is_chess960());
1855 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1856 /// before exiting the search, for instance, in case we stop the search during a
1857 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1858 /// otherwise in case of 'ponder on' we have nothing to think on.
1860 bool RootMove::extract_ponder_from_tt(Position& pos) {
1863 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1867 assert(pv.size() == 1);
1869 if (pv[0] == MOVE_NONE)
1872 pos.do_move(pv[0], st);
1873 TTEntry* tte = TT.probe(pos.key(), ttHit);
1877 Move m = tte->move(); // Local copy to be SMP safe
1878 if (MoveList<LEGAL>(pos).contains(m))
1882 pos.undo_move(pv[0]);
1883 return pv.size() > 1;
1886 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1889 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1890 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1891 Cardinality = int(Options["SyzygyProbeLimit"]);
1892 bool dtz_available = true;
1894 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1895 // ProbeDepth == DEPTH_ZERO
1896 if (Cardinality > MaxCardinality)
1898 Cardinality = MaxCardinality;
1902 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1904 // Rank moves using DTZ tables
1905 RootInTB = root_probe(pos, rootMoves);
1909 // DTZ tables are missing; try to rank moves using WDL tables
1910 dtz_available = false;
1911 RootInTB = root_probe_wdl(pos, rootMoves);
1917 // Sort moves according to TB rank
1918 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1919 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1921 // Probe during search only if DTZ is not available and we are winning
1922 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1927 // Clean up if root_probe() and root_probe_wdl() have failed
1928 for (auto& m : rootMoves)
1933 } // namespace Stockfish