2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #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 };
64 // Razor and futility margins
65 constexpr int RazorMargin = 661;
66 Value futility_margin(Depth d, bool improving) {
67 return Value(198 * (d / ONE_PLY - improving));
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn];
75 return ((r + 520) / 1024 + (!i && r > 999)) * ONE_PLY;
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? -8 : 22 * d * d + 151 * d - 140;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : 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 / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 // Breadcrumbs are used to mark nodes as being searched by a given thread
107 std::atomic<Thread*> thread;
108 std::atomic<Key> key;
110 std::array<Breadcrumb, 1024> breadcrumbs;
112 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
113 // node for potential reductions. A free node will be marked upon entering the moves
114 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
115 struct ThreadHolding {
116 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
117 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
122 // See if another already marked this location, if not, mark it ourselves
123 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
126 (*location).thread.store(thisThread, std::memory_order_relaxed);
127 (*location).key.store(posKey, std::memory_order_relaxed);
130 else if ( tmp != thisThread
131 && (*location).key.load(std::memory_order_relaxed) == posKey)
137 if (owning) // Free the marked location
138 (*location).thread.store(nullptr, std::memory_order_relaxed);
141 bool marked() { return otherThread; }
144 Breadcrumb* location;
145 bool otherThread, owning;
148 template <NodeType NT>
149 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
151 template <NodeType NT>
152 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
154 Value value_to_tt(Value v, int ply);
155 Value value_from_tt(Value v, int ply);
156 void update_pv(Move* pv, Move move, Move* childPv);
157 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
158 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
159 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
161 // perft() is our utility to verify move generation. All the leaf nodes up
162 // to the given depth are generated and counted, and the sum is returned.
164 uint64_t perft(Position& pos, Depth depth) {
167 uint64_t cnt, nodes = 0;
168 const bool leaf = (depth == 2 * ONE_PLY);
170 for (const auto& m : MoveList<LEGAL>(pos))
172 if (Root && depth <= ONE_PLY)
177 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
182 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
190 /// Search::init() is called at startup to initialize various lookup tables
192 void Search::init() {
194 for (int i = 1; i < MAX_MOVES; ++i)
195 Reductions[i] = int(23.4 * std::log(i));
199 /// Search::clear() resets search state to its initial value
201 void Search::clear() {
203 Threads.main()->wait_for_search_finished();
205 Time.availableNodes = 0;
208 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
212 /// MainThread::search() is started when the program receives the UCI 'go'
213 /// command. It searches from the root position and outputs the "bestmove".
215 void MainThread::search() {
219 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
220 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
224 Color us = rootPos.side_to_move();
225 Time.init(Limits, us, rootPos.game_ply());
228 if (rootMoves.empty())
230 rootMoves.emplace_back(MOVE_NONE);
231 sync_cout << "info depth 0 score "
232 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
237 for (Thread* th : Threads)
239 th->bestMoveChanges = 0;
241 th->start_searching();
244 Thread::search(); // Let's start searching!
247 // When we reach the maximum depth, we can arrive here without a raise of
248 // Threads.stop. However, if we are pondering or in an infinite search,
249 // the UCI protocol states that we shouldn't print the best move before the
250 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
251 // until the GUI sends one of those commands.
253 while (!Threads.stop && (ponder || Limits.infinite))
254 {} // Busy wait for a stop or a ponder reset
256 // Stop the threads if not already stopped (also raise the stop if
257 // "ponderhit" just reset Threads.ponder).
260 // Wait until all threads have finished
261 for (Thread* th : Threads)
263 th->wait_for_search_finished();
265 // When playing in 'nodes as time' mode, subtract the searched nodes from
266 // the available ones before exiting.
268 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
270 Thread* bestThread = this;
272 // Check if there are threads with a better score than main thread
273 if ( Options["MultiPV"] == 1
275 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
276 && rootMoves[0].pv[0] != MOVE_NONE)
278 std::map<Move, int64_t> votes;
279 Value minScore = this->rootMoves[0].score;
281 // Find out minimum score
282 for (Thread* th: Threads)
283 minScore = std::min(minScore, th->rootMoves[0].score);
285 // Vote according to score and depth, and select the best thread
286 for (Thread* th : Threads)
288 votes[th->rootMoves[0].pv[0]] +=
289 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
291 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
293 // Make sure we pick the shortest mate
294 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
297 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
298 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
303 previousScore = bestThread->rootMoves[0].score;
305 // Send again PV info if we have a new best thread
306 if (bestThread != this)
307 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
309 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
311 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
312 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
314 std::cout << sync_endl;
318 /// Thread::search() is the main iterative deepening loop. It calls search()
319 /// repeatedly with increasing depth until the allocated thinking time has been
320 /// consumed, the user stops the search, or the maximum search depth is reached.
322 void Thread::search() {
324 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
325 // The former is needed to allow update_continuation_histories(ss-1, ...),
326 // which accesses its argument at ss-6, also near the root.
327 // The latter is needed for statScores and killer initialization.
328 Stack stack[MAX_PLY+10], *ss = stack+7;
330 Value bestValue, alpha, beta, delta;
331 Move lastBestMove = MOVE_NONE;
332 Depth lastBestMoveDepth = DEPTH_ZERO;
333 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
334 double timeReduction = 1, totBestMoveChanges = 0;
335 Color us = rootPos.side_to_move();
337 std::memset(ss-7, 0, 10 * sizeof(Stack));
338 for (int i = 7; i > 0; i--)
339 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
342 bestValue = delta = alpha = -VALUE_INFINITE;
343 beta = VALUE_INFINITE;
345 size_t multiPV = Options["MultiPV"];
347 // Pick integer skill levels, but non-deterministically round up or down
348 // such that the average integer skill corresponds to the input floating point one.
349 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
350 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
351 // for match (TC 60+0.6) results spanning a wide range of k values.
353 double floatLevel = Options["UCI_LimitStrength"] ?
354 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
355 double(Options["Skill Level"]);
356 int intLevel = int(floatLevel) +
357 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
358 Skill skill(intLevel);
360 // When playing with strength handicap enable MultiPV search that we will
361 // use behind the scenes to retrieve a set of possible moves.
363 multiPV = std::max(multiPV, (size_t)4);
365 multiPV = std::min(multiPV, rootMoves.size());
367 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
369 // In analysis mode, adjust contempt in accordance with user preference
370 if (Limits.infinite || Options["UCI_AnalyseMode"])
371 ct = Options["Analysis Contempt"] == "Off" ? 0
372 : Options["Analysis Contempt"] == "Both" ? ct
373 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
374 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
377 // Evaluation score is from the white point of view
378 contempt = (us == WHITE ? make_score(ct, ct / 2)
379 : -make_score(ct, ct / 2));
381 // Iterative deepening loop until requested to stop or the target depth is reached
382 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
384 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
386 // Age out PV variability metric
388 totBestMoveChanges /= 2;
390 // Save the last iteration's scores before first PV line is searched and
391 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
392 for (RootMove& rm : rootMoves)
393 rm.previousScore = rm.score;
398 // MultiPV loop. We perform a full root search for each PV line
399 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
404 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
405 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
409 // Reset UCI info selDepth for each depth and each PV line
412 // Reset aspiration window starting size
413 if (rootDepth >= 4 * ONE_PLY)
415 Value previousScore = rootMoves[pvIdx].previousScore;
417 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
418 beta = std::min(previousScore + delta, VALUE_INFINITE);
420 // Adjust contempt based on root move's previousScore (dynamic contempt)
421 int dct = ct + 86 * previousScore / (abs(previousScore) + 176);
423 contempt = (us == WHITE ? make_score(dct, dct / 2)
424 : -make_score(dct, dct / 2));
427 // Start with a small aspiration window and, in the case of a fail
428 // high/low, re-search with a bigger window until we don't fail
430 int failedHighCnt = 0;
433 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
434 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
436 // Bring the best move to the front. It is critical that sorting
437 // is done with a stable algorithm because all the values but the
438 // first and eventually the new best one are set to -VALUE_INFINITE
439 // and we want to keep the same order for all the moves except the
440 // new PV that goes to the front. Note that in case of MultiPV
441 // search the already searched PV lines are preserved.
442 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
444 // If search has been stopped, we break immediately. Sorting is
445 // safe because RootMoves is still valid, although it refers to
446 // the previous iteration.
450 // When failing high/low give some update (without cluttering
451 // the UI) before a re-search.
454 && (bestValue <= alpha || bestValue >= beta)
455 && Time.elapsed() > 3000)
456 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
458 // In case of failing low/high increase aspiration window and
459 // re-search, otherwise exit the loop.
460 if (bestValue <= alpha)
462 beta = (alpha + beta) / 2;
463 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
467 mainThread->stopOnPonderhit = false;
469 else if (bestValue >= beta)
471 beta = std::min(bestValue + delta, VALUE_INFINITE);
477 delta += delta / 4 + 5;
479 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
482 // Sort the PV lines searched so far and update the GUI
483 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
486 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
487 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
491 completedDepth = rootDepth;
493 if (rootMoves[0].pv[0] != lastBestMove) {
494 lastBestMove = rootMoves[0].pv[0];
495 lastBestMoveDepth = rootDepth;
498 // Have we found a "mate in x"?
500 && bestValue >= VALUE_MATE_IN_MAX_PLY
501 && VALUE_MATE - bestValue <= 2 * Limits.mate)
507 // If skill level is enabled and time is up, pick a sub-optimal best move
508 if (skill.enabled() && skill.time_to_pick(rootDepth))
509 skill.pick_best(multiPV);
511 // Do we have time for the next iteration? Can we stop searching now?
512 if ( Limits.use_time_management()
514 && !mainThread->stopOnPonderhit)
516 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
517 fallingEval = clamp(fallingEval, 0.5, 1.5);
519 // If the bestMove is stable over several iterations, reduce time accordingly
520 timeReduction = lastBestMoveDepth + 9 * ONE_PLY < completedDepth ? 1.97 : 0.98;
521 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
523 // Use part of the gained time from a previous stable move for the current move
524 for (Thread* th : Threads)
526 totBestMoveChanges += th->bestMoveChanges;
527 th->bestMoveChanges = 0;
529 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
531 // Stop the search if we have only one legal move, or if available time elapsed
532 if ( rootMoves.size() == 1
533 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
535 // If we are allowed to ponder do not stop the search now but
536 // keep pondering until the GUI sends "ponderhit" or "stop".
537 if (mainThread->ponder)
538 mainThread->stopOnPonderhit = true;
548 mainThread->previousTimeReduction = timeReduction;
550 // If skill level is enabled, swap best PV line with the sub-optimal one
552 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
553 skill.best ? skill.best : skill.pick_best(multiPV)));
559 // search<>() is the main search function for both PV and non-PV nodes
561 template <NodeType NT>
562 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
564 constexpr bool PvNode = NT == PV;
565 const bool rootNode = PvNode && ss->ply == 0;
567 // Check if we have an upcoming move which draws by repetition, or
568 // if the opponent had an alternative move earlier to this position.
569 if ( pos.rule50_count() >= 3
570 && alpha < VALUE_DRAW
572 && pos.has_game_cycle(ss->ply))
574 alpha = value_draw(depth, pos.this_thread());
579 // Dive into quiescence search when the depth reaches zero
581 return qsearch<NT>(pos, ss, alpha, beta);
583 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
584 assert(PvNode || (alpha == beta - 1));
585 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
586 assert(!(PvNode && cutNode));
587 assert(depth / ONE_PLY * ONE_PLY == depth);
589 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
593 Move ttMove, move, excludedMove, bestMove;
594 Depth extension, newDepth;
595 Value bestValue, value, ttValue, eval, maxValue;
596 bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR;
597 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
599 int moveCount, captureCount, quietCount, singularLMR;
601 // Step 1. Initialize node
602 Thread* thisThread = pos.this_thread();
603 inCheck = pos.checkers();
604 Color us = pos.side_to_move();
605 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
606 bestValue = -VALUE_INFINITE;
607 maxValue = VALUE_INFINITE;
609 // Check for the available remaining time
610 if (thisThread == Threads.main())
611 static_cast<MainThread*>(thisThread)->check_time();
613 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
614 if (PvNode && thisThread->selDepth < ss->ply + 1)
615 thisThread->selDepth = ss->ply + 1;
619 // Step 2. Check for aborted search and immediate draw
620 if ( Threads.stop.load(std::memory_order_relaxed)
621 || pos.is_draw(ss->ply)
622 || ss->ply >= MAX_PLY)
623 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
624 : value_draw(depth, pos.this_thread());
626 // Step 3. Mate distance pruning. Even if we mate at the next move our score
627 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
628 // a shorter mate was found upward in the tree then there is no need to search
629 // because we will never beat the current alpha. Same logic but with reversed
630 // signs applies also in the opposite condition of being mated instead of giving
631 // mate. In this case return a fail-high score.
632 alpha = std::max(mated_in(ss->ply), alpha);
633 beta = std::min(mate_in(ss->ply+1), beta);
638 assert(0 <= ss->ply && ss->ply < MAX_PLY);
640 (ss+1)->ply = ss->ply + 1;
641 (ss+1)->excludedMove = bestMove = MOVE_NONE;
642 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
643 Square prevSq = to_sq((ss-1)->currentMove);
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+4)->statScore = 0;
653 (ss+2)->statScore = 0;
655 // Step 4. Transposition table lookup. We don't want the score of a partial
656 // search to overwrite a previous full search TT value, so we use a different
657 // position key in case of an excluded move.
658 excludedMove = ss->excludedMove;
659 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
660 tte = TT.probe(posKey, ttHit);
661 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
662 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
663 : ttHit ? tte->move() : MOVE_NONE;
664 ttPv = PvNode || (ttHit && tte->is_pv());
666 // At non-PV nodes we check for an early TT cutoff
669 && tte->depth() >= depth
670 && ttValue != VALUE_NONE // Possible in case of TT access race
671 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
672 : (tte->bound() & BOUND_UPPER)))
674 // If ttMove is quiet, update move sorting heuristics on TT hit
679 if (!pos.capture_or_promotion(ttMove))
680 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
682 // Extra penalty for early quiet moves of the previous ply
683 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
684 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
686 // Penalty for a quiet ttMove that fails low
687 else if (!pos.capture_or_promotion(ttMove))
689 int penalty = -stat_bonus(depth);
690 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
691 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
697 // Step 5. Tablebases probe
698 if (!rootNode && TB::Cardinality)
700 int piecesCount = pos.count<ALL_PIECES>();
702 if ( piecesCount <= TB::Cardinality
703 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
704 && pos.rule50_count() == 0
705 && !pos.can_castle(ANY_CASTLING))
708 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
710 // Force check of time on the next occasion
711 if (thisThread == Threads.main())
712 static_cast<MainThread*>(thisThread)->callsCnt = 0;
714 if (err != TB::ProbeState::FAIL)
716 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
718 int drawScore = TB::UseRule50 ? 1 : 0;
720 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
721 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
722 : VALUE_DRAW + 2 * wdl * drawScore;
724 Bound b = wdl < -drawScore ? BOUND_UPPER
725 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
727 if ( b == BOUND_EXACT
728 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
730 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
731 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
732 MOVE_NONE, VALUE_NONE);
739 if (b == BOUND_LOWER)
740 bestValue = value, alpha = std::max(alpha, bestValue);
748 // Step 6. Static evaluation of the position
751 ss->staticEval = eval = VALUE_NONE;
753 goto moves_loop; // Skip early pruning when in check
757 // Never assume anything about values stored in TT
758 ss->staticEval = eval = tte->eval();
759 if (eval == VALUE_NONE)
760 ss->staticEval = eval = evaluate(pos);
762 // Can ttValue be used as a better position evaluation?
763 if ( ttValue != VALUE_NONE
764 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
769 if ((ss-1)->currentMove != MOVE_NULL)
771 int bonus = -(ss-1)->statScore / 512;
773 ss->staticEval = eval = evaluate(pos) + bonus;
776 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
778 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
781 // Step 7. Razoring (~2 Elo)
782 if ( !rootNode // The required rootNode PV handling is not available in qsearch
783 && depth < 2 * ONE_PLY
784 && eval <= alpha - RazorMargin)
785 return qsearch<NT>(pos, ss, alpha, beta);
787 improving = ss->staticEval >= (ss-2)->staticEval
788 || (ss-2)->staticEval == VALUE_NONE;
790 // Step 8. Futility pruning: child node (~30 Elo)
792 && depth < 7 * ONE_PLY
793 && eval - futility_margin(depth, improving) >= beta
794 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
797 // Step 9. Null move search with verification search (~40 Elo)
799 && (ss-1)->currentMove != MOVE_NULL
800 && (ss-1)->statScore < 22661
802 && eval >= ss->staticEval
803 && ss->staticEval >= beta - 33 * depth / ONE_PLY + 299 - improving * 30
805 && pos.non_pawn_material(us)
806 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
808 assert(eval - beta >= 0);
810 // Null move dynamic reduction based on depth and value
811 Depth R = ((835 + 70 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 185, 3)) * ONE_PLY;
813 ss->currentMove = MOVE_NULL;
814 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
816 pos.do_null_move(st);
818 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
820 pos.undo_null_move();
822 if (nullValue >= beta)
824 // Do not return unproven mate scores
825 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
828 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13 * ONE_PLY))
831 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
833 // Do verification search at high depths, with null move pruning disabled
834 // for us, until ply exceeds nmpMinPly.
835 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
836 thisThread->nmpColor = us;
838 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
840 thisThread->nmpMinPly = 0;
847 // Step 10. ProbCut (~10 Elo)
848 // If we have a good enough capture and a reduced search returns a value
849 // much above beta, we can (almost) safely prune the previous move.
851 && depth >= 5 * ONE_PLY
852 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
854 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
855 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
856 int probCutCount = 0;
858 while ( (move = mp.next_move()) != MOVE_NONE
859 && probCutCount < 2 + 2 * cutNode)
860 if (move != excludedMove && pos.legal(move))
864 ss->currentMove = move;
865 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
867 assert(depth >= 5 * ONE_PLY);
869 pos.do_move(move, st);
871 // Perform a preliminary qsearch to verify that the move holds
872 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
874 // If the qsearch held, perform the regular search
875 if (value >= raisedBeta)
876 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
880 if (value >= raisedBeta)
885 // Step 11. Internal iterative deepening (~2 Elo)
886 if (depth >= 7 * ONE_PLY && !ttMove)
888 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
890 tte = TT.probe(posKey, ttHit);
891 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
892 ttMove = ttHit ? tte->move() : MOVE_NONE;
895 moves_loop: // When in check, search starts from here
897 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
898 nullptr, (ss-4)->continuationHistory,
899 nullptr, (ss-6)->continuationHistory };
901 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
903 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
904 &thisThread->captureHistory,
909 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
910 moveCountPruning = false;
911 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
913 // Mark this node as being searched
914 ThreadHolding th(thisThread, posKey, ss->ply);
916 // Step 12. Loop through all pseudo-legal moves until no moves remain
917 // or a beta cutoff occurs.
918 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
922 if (move == excludedMove)
925 // At root obey the "searchmoves" option and skip moves not listed in Root
926 // Move List. As a consequence any illegal move is also skipped. In MultiPV
927 // mode we also skip PV moves which have been already searched and those
928 // of lower "TB rank" if we are in a TB root position.
929 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
930 thisThread->rootMoves.begin() + thisThread->pvLast, move))
933 ss->moveCount = ++moveCount;
935 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
936 sync_cout << "info depth " << depth / ONE_PLY
937 << " currmove " << UCI::move(move, pos.is_chess960())
938 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
940 (ss+1)->pv = nullptr;
942 extension = DEPTH_ZERO;
943 captureOrPromotion = pos.capture_or_promotion(move);
944 movedPiece = pos.moved_piece(move);
945 givesCheck = pos.gives_check(move);
947 // Step 13. Extensions (~70 Elo)
949 // Singular extension search (~60 Elo). If all moves but one fail low on a
950 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
951 // then that move is singular and should be extended. To verify this we do
952 // a reduced search on all the other moves but the ttMove and if the
953 // result is lower than ttValue minus a margin then we will extend the ttMove.
954 if ( depth >= 6 * ONE_PLY
957 && !excludedMove // Avoid recursive singular search
958 /* && ttValue != VALUE_NONE Already implicit in the next condition */
959 && abs(ttValue) < VALUE_KNOWN_WIN
960 && (tte->bound() & BOUND_LOWER)
961 && tte->depth() >= depth - 3 * ONE_PLY
964 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
965 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
966 ss->excludedMove = move;
967 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
968 ss->excludedMove = MOVE_NONE;
970 if (value < singularBeta)
975 if (value < singularBeta - std::min(4 * depth / ONE_PLY, 36))
980 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
981 // search without the ttMove. So we assume this expected Cut-node is not singular,
982 // that multiple moves fail high, and we can prune the whole subtree by returning
984 else if ( eval >= beta
985 && singularBeta >= beta)
989 // Check extension (~2 Elo)
991 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
994 // Castling extension
995 else if (type_of(move) == CASTLING)
1000 && pos.rule50_count() > 18
1001 && depth < 3 * ONE_PLY
1002 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
1003 extension = ONE_PLY;
1005 // Passed pawn extension
1006 else if ( move == ss->killers[0]
1007 && pos.advanced_pawn_push(move)
1008 && pos.pawn_passed(us, to_sq(move)))
1009 extension = ONE_PLY;
1011 // Calculate new depth for this move
1012 newDepth = depth - ONE_PLY + extension;
1014 // Step 14. Pruning at shallow depth (~170 Elo)
1016 && pos.non_pawn_material(us)
1017 && bestValue > VALUE_MATED_IN_MAX_PLY)
1019 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1020 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
1022 if ( !captureOrPromotion
1024 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1026 // Move count based pruning
1027 if (moveCountPruning)
1030 // Reduced depth of the next LMR search
1031 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
1032 lmrDepth /= ONE_PLY;
1034 // Countermoves based pruning (~20 Elo)
1035 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1036 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1037 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1040 // Futility pruning: parent node (~2 Elo)
1043 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
1046 // Prune moves with negative SEE (~10 Elo)
1047 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1050 else if ( (!givesCheck || !extension)
1051 && !pos.see_ge(move, Value(-199) * (depth / ONE_PLY))) // (~20 Elo)
1055 // Speculative prefetch as early as possible
1056 prefetch(TT.first_entry(pos.key_after(move)));
1058 // Check for legality just before making the move
1059 if (!rootNode && !pos.legal(move))
1061 ss->moveCount = --moveCount;
1065 // Update the current move (this must be done after singular extension search)
1066 ss->currentMove = move;
1067 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1069 // Step 15. Make the move
1070 pos.do_move(move, st, givesCheck);
1072 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1073 // re-searched at full depth.
1074 if ( depth >= 3 * ONE_PLY
1075 && moveCount > 1 + 3 * rootNode
1076 && ( !captureOrPromotion
1078 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1081 Depth r = reduction(improving, depth, moveCount);
1083 // Reduction if other threads are searching this position.
1087 // Decrease reduction if position is or has been on the PV
1091 // Decrease reduction if opponent's move count is high (~10 Elo)
1092 if ((ss-1)->moveCount > 15)
1095 // Decrease reduction if move has been singularly extended
1096 r -= singularLMR * ONE_PLY;
1098 if (!captureOrPromotion)
1100 // Increase reduction if ttMove is a capture (~0 Elo)
1104 // Increase reduction for cut nodes (~5 Elo)
1108 // Decrease reduction for moves that escape a capture. Filter out
1109 // castling moves, because they are coded as "king captures rook" and
1110 // hence break make_move(). (~5 Elo)
1111 else if ( type_of(move) == NORMAL
1112 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1115 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1116 + (*contHist[0])[movedPiece][to_sq(move)]
1117 + (*contHist[1])[movedPiece][to_sq(move)]
1118 + (*contHist[3])[movedPiece][to_sq(move)]
1121 // Reset statScore to zero if negative and most stats shows >= 0
1122 if ( ss->statScore < 0
1123 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1124 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1125 && thisThread->mainHistory[us][from_to(move)] >= 0)
1128 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1129 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1132 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1135 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1136 r -= ss->statScore / 16384 * ONE_PLY;
1139 Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
1141 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1143 doFullDepthSearch = (value > alpha && d != newDepth), doLMR = true;
1146 doFullDepthSearch = !PvNode || moveCount > 1, doLMR = false;
1148 // Step 17. Full depth search when LMR is skipped or fails high
1149 if (doFullDepthSearch)
1151 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1153 if (doLMR && !captureOrPromotion)
1155 int bonus = value > alpha ? stat_bonus(newDepth)
1156 : -stat_bonus(newDepth);
1158 if (move == ss->killers[0])
1161 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1165 // For PV nodes only, do a full PV search on the first move or after a fail
1166 // high (in the latter case search only if value < beta), otherwise let the
1167 // parent node fail low with value <= alpha and try another move.
1168 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1171 (ss+1)->pv[0] = MOVE_NONE;
1173 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1176 // Step 18. Undo move
1177 pos.undo_move(move);
1179 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1181 // Step 19. Check for a new best move
1182 // Finished searching the move. If a stop occurred, the return value of
1183 // the search cannot be trusted, and we return immediately without
1184 // updating best move, PV and TT.
1185 if (Threads.stop.load(std::memory_order_relaxed))
1190 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1191 thisThread->rootMoves.end(), move);
1193 // PV move or new best move?
1194 if (moveCount == 1 || value > alpha)
1197 rm.selDepth = thisThread->selDepth;
1202 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1203 rm.pv.push_back(*m);
1205 // We record how often the best move has been changed in each
1206 // iteration. This information is used for time management: When
1207 // the best move changes frequently, we allocate some more time.
1209 ++thisThread->bestMoveChanges;
1212 // All other moves but the PV are set to the lowest value: this
1213 // is not a problem when sorting because the sort is stable and the
1214 // move position in the list is preserved - just the PV is pushed up.
1215 rm.score = -VALUE_INFINITE;
1218 if (value > bestValue)
1226 if (PvNode && !rootNode) // Update pv even in fail-high case
1227 update_pv(ss->pv, move, (ss+1)->pv);
1229 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1233 assert(value >= beta); // Fail high
1240 if (move != bestMove)
1242 if (captureOrPromotion && captureCount < 32)
1243 capturesSearched[captureCount++] = move;
1245 else if (!captureOrPromotion && quietCount < 64)
1246 quietsSearched[quietCount++] = move;
1250 // The following condition would detect a stop only after move loop has been
1251 // completed. But in this case bestValue is valid because we have fully
1252 // searched our subtree, and we can anyhow save the result in TT.
1258 // Step 20. Check for mate and stalemate
1259 // All legal moves have been searched and if there are no legal moves, it
1260 // must be a mate or a stalemate. If we are in a singular extension search then
1261 // return a fail low score.
1263 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1266 bestValue = excludedMove ? alpha
1267 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1270 // Quiet best move: update move sorting heuristics
1271 if (!pos.capture_or_promotion(bestMove))
1272 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1273 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1275 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1277 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1278 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1279 && !pos.captured_piece())
1280 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1283 // Bonus for prior countermove that caused the fail low
1284 else if ( (depth >= 3 * ONE_PLY || PvNode)
1285 && !pos.captured_piece())
1286 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1289 bestValue = std::min(bestValue, maxValue);
1292 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1293 bestValue >= beta ? BOUND_LOWER :
1294 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1295 depth, bestMove, ss->staticEval);
1297 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1303 // qsearch() is the quiescence search function, which is called by the main search
1304 // function with zero depth, or recursively with further decreasing depth per call.
1305 template <NodeType NT>
1306 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1308 constexpr bool PvNode = NT == PV;
1310 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1311 assert(PvNode || (alpha == beta - 1));
1312 assert(depth <= DEPTH_ZERO);
1313 assert(depth / ONE_PLY * ONE_PLY == depth);
1319 Move ttMove, move, bestMove;
1321 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1322 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1327 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1329 ss->pv[0] = MOVE_NONE;
1332 Thread* thisThread = pos.this_thread();
1333 (ss+1)->ply = ss->ply + 1;
1334 bestMove = MOVE_NONE;
1335 inCheck = pos.checkers();
1338 // Check for an immediate draw or maximum ply reached
1339 if ( pos.is_draw(ss->ply)
1340 || ss->ply >= MAX_PLY)
1341 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1343 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1345 // Decide whether or not to include checks: this fixes also the type of
1346 // TT entry depth that we are going to use. Note that in qsearch we use
1347 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1348 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1349 : DEPTH_QS_NO_CHECKS;
1350 // Transposition table lookup
1352 tte = TT.probe(posKey, ttHit);
1353 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1354 ttMove = ttHit ? tte->move() : MOVE_NONE;
1355 pvHit = ttHit && tte->is_pv();
1359 && tte->depth() >= ttDepth
1360 && ttValue != VALUE_NONE // Only in case of TT access race
1361 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1362 : (tte->bound() & BOUND_UPPER)))
1365 // Evaluate the position statically
1368 ss->staticEval = VALUE_NONE;
1369 bestValue = futilityBase = -VALUE_INFINITE;
1375 // Never assume anything about values stored in TT
1376 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1377 ss->staticEval = bestValue = evaluate(pos);
1379 // Can ttValue be used as a better position evaluation?
1380 if ( ttValue != VALUE_NONE
1381 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1382 bestValue = ttValue;
1385 ss->staticEval = bestValue =
1386 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1387 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1389 // Stand pat. Return immediately if static value is at least beta
1390 if (bestValue >= beta)
1393 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1394 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1399 if (PvNode && bestValue > alpha)
1402 futilityBase = bestValue + 153;
1405 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1406 nullptr, (ss-4)->continuationHistory,
1407 nullptr, (ss-6)->continuationHistory };
1409 // Initialize a MovePicker object for the current position, and prepare
1410 // to search the moves. Because the depth is <= 0 here, only captures,
1411 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1413 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1414 &thisThread->captureHistory,
1416 to_sq((ss-1)->currentMove));
1418 // Loop through the moves until no moves remain or a beta cutoff occurs
1419 while ((move = mp.next_move()) != MOVE_NONE)
1421 assert(is_ok(move));
1423 givesCheck = pos.gives_check(move);
1430 && futilityBase > -VALUE_KNOWN_WIN
1431 && !pos.advanced_pawn_push(move))
1433 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1435 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1437 if (futilityValue <= alpha)
1439 bestValue = std::max(bestValue, futilityValue);
1443 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1445 bestValue = std::max(bestValue, futilityBase);
1450 // Detect non-capture evasions that are candidates to be pruned
1451 evasionPrunable = inCheck
1452 && (depth != DEPTH_ZERO || moveCount > 2)
1453 && bestValue > VALUE_MATED_IN_MAX_PLY
1454 && !pos.capture(move);
1456 // Don't search moves with negative SEE values
1457 if ( (!inCheck || evasionPrunable)
1458 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1459 && !pos.see_ge(move))
1462 // Speculative prefetch as early as possible
1463 prefetch(TT.first_entry(pos.key_after(move)));
1465 // Check for legality just before making the move
1466 if (!pos.legal(move))
1472 ss->currentMove = move;
1473 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1475 // Make and search the move
1476 pos.do_move(move, st, givesCheck);
1477 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1478 pos.undo_move(move);
1480 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1482 // Check for a new best move
1483 if (value > bestValue)
1491 if (PvNode) // Update pv even in fail-high case
1492 update_pv(ss->pv, move, (ss+1)->pv);
1494 if (PvNode && value < beta) // Update alpha here!
1502 // All legal moves have been searched. A special case: If we're in check
1503 // and no legal moves were found, it is checkmate.
1504 if (inCheck && bestValue == -VALUE_INFINITE)
1505 return mated_in(ss->ply); // Plies to mate from the root
1507 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1508 bestValue >= beta ? BOUND_LOWER :
1509 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1510 ttDepth, bestMove, ss->staticEval);
1512 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1518 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1519 // "plies to mate from the current position". Non-mate scores are unchanged.
1520 // The function is called before storing a value in the transposition table.
1522 Value value_to_tt(Value v, int ply) {
1524 assert(v != VALUE_NONE);
1526 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1527 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1531 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1532 // from the transposition table (which refers to the plies to mate/be mated
1533 // from current position) to "plies to mate/be mated from the root".
1535 Value value_from_tt(Value v, int ply) {
1537 return v == VALUE_NONE ? VALUE_NONE
1538 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1539 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1543 // update_pv() adds current move and appends child pv[]
1545 void update_pv(Move* pv, Move move, Move* childPv) {
1547 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1553 // update_continuation_histories() updates histories of the move pairs formed
1554 // by moves at ply -1, -2, and -4 with current move.
1556 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1558 for (int i : {1, 2, 4, 6})
1559 if (is_ok((ss-i)->currentMove))
1560 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1564 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1566 void update_capture_stats(const Position& pos, Move move,
1567 Move* captures, int captureCount, int bonus) {
1569 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1570 Piece moved_piece = pos.moved_piece(move);
1571 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1573 if (pos.capture_or_promotion(move))
1574 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1576 // Decrease all the other played capture moves
1577 for (int i = 0; i < captureCount; ++i)
1579 moved_piece = pos.moved_piece(captures[i]);
1580 captured = type_of(pos.piece_on(to_sq(captures[i])));
1581 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1586 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1588 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1589 Move* quiets, int quietCount, int bonus) {
1591 if (ss->killers[0] != move)
1593 ss->killers[1] = ss->killers[0];
1594 ss->killers[0] = move;
1597 Color us = pos.side_to_move();
1598 Thread* thisThread = pos.this_thread();
1599 thisThread->mainHistory[us][from_to(move)] << bonus;
1600 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1602 if (is_ok((ss-1)->currentMove))
1604 Square prevSq = to_sq((ss-1)->currentMove);
1605 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1608 // Decrease all the other played quiet moves
1609 for (int i = 0; i < quietCount; ++i)
1611 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1612 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1616 // When playing with strength handicap, choose best move among a set of RootMoves
1617 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1619 Move Skill::pick_best(size_t multiPV) {
1621 const RootMoves& rootMoves = Threads.main()->rootMoves;
1622 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1624 // RootMoves are already sorted by score in descending order
1625 Value topScore = rootMoves[0].score;
1626 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1627 int weakness = 120 - 2 * level;
1628 int maxScore = -VALUE_INFINITE;
1630 // Choose best move. For each move score we add two terms, both dependent on
1631 // weakness. One is deterministic and bigger for weaker levels, and one is
1632 // random. Then we choose the move with the resulting highest score.
1633 for (size_t i = 0; i < multiPV; ++i)
1635 // This is our magic formula
1636 int push = ( weakness * int(topScore - rootMoves[i].score)
1637 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1639 if (rootMoves[i].score + push >= maxScore)
1641 maxScore = rootMoves[i].score + push;
1642 best = rootMoves[i].pv[0];
1651 /// MainThread::check_time() is used to print debug info and, more importantly,
1652 /// to detect when we are out of available time and thus stop the search.
1654 void MainThread::check_time() {
1659 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1660 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1662 static TimePoint lastInfoTime = now();
1664 TimePoint elapsed = Time.elapsed();
1665 TimePoint tick = Limits.startTime + elapsed;
1667 if (tick - lastInfoTime >= 1000)
1669 lastInfoTime = tick;
1673 // We should not stop pondering until told so by the GUI
1677 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1678 || (Limits.movetime && elapsed >= Limits.movetime)
1679 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1680 Threads.stop = true;
1684 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1685 /// that all (if any) unsearched PV lines are sent using a previous search score.
1687 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1689 std::stringstream ss;
1690 TimePoint elapsed = Time.elapsed() + 1;
1691 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1692 size_t pvIdx = pos.this_thread()->pvIdx;
1693 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1694 uint64_t nodesSearched = Threads.nodes_searched();
1695 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1697 for (size_t i = 0; i < multiPV; ++i)
1699 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1701 if (depth == ONE_PLY && !updated)
1704 Depth d = updated ? depth : depth - ONE_PLY;
1705 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1707 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1708 v = tb ? rootMoves[i].tbScore : v;
1710 if (ss.rdbuf()->in_avail()) // Not at first line
1714 << " depth " << d / ONE_PLY
1715 << " seldepth " << rootMoves[i].selDepth
1716 << " multipv " << i + 1
1717 << " score " << UCI::value(v);
1719 if (!tb && i == pvIdx)
1720 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1722 ss << " nodes " << nodesSearched
1723 << " nps " << nodesSearched * 1000 / elapsed;
1725 if (elapsed > 1000) // Earlier makes little sense
1726 ss << " hashfull " << TT.hashfull();
1728 ss << " tbhits " << tbHits
1729 << " time " << elapsed
1732 for (Move m : rootMoves[i].pv)
1733 ss << " " << UCI::move(m, pos.is_chess960());
1740 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1741 /// before exiting the search, for instance, in case we stop the search during a
1742 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1743 /// otherwise in case of 'ponder on' we have nothing to think on.
1745 bool RootMove::extract_ponder_from_tt(Position& pos) {
1750 assert(pv.size() == 1);
1752 if (pv[0] == MOVE_NONE)
1755 pos.do_move(pv[0], st);
1756 TTEntry* tte = TT.probe(pos.key(), ttHit);
1760 Move m = tte->move(); // Local copy to be SMP safe
1761 if (MoveList<LEGAL>(pos).contains(m))
1765 pos.undo_move(pv[0]);
1766 return pv.size() > 1;
1769 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1772 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1773 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1774 Cardinality = int(Options["SyzygyProbeLimit"]);
1775 bool dtz_available = true;
1777 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1778 // ProbeDepth == DEPTH_ZERO
1779 if (Cardinality > MaxCardinality)
1781 Cardinality = MaxCardinality;
1782 ProbeDepth = DEPTH_ZERO;
1785 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1787 // Rank moves using DTZ tables
1788 RootInTB = root_probe(pos, rootMoves);
1792 // DTZ tables are missing; try to rank moves using WDL tables
1793 dtz_available = false;
1794 RootInTB = root_probe_wdl(pos, rootMoves);
1800 // Sort moves according to TB rank
1801 std::sort(rootMoves.begin(), rootMoves.end(),
1802 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1804 // Probe during search only if DTZ is not available and we are winning
1805 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1810 // Clean up if root_probe() and root_probe_wdl() have failed
1811 for (auto& m : rootMoves)