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 constexpr uint64_t ttHitAverageWindow = 4096;
65 constexpr uint64_t ttHitAverageResolution = 1024;
67 // Razor and futility margins
68 constexpr int RazorMargin = 531;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(217 * (d - improving));
73 // Reductions lookup table, initialized at startup
74 int Reductions[MAX_MOVES]; // [depth or moveNumber]
76 Depth reduction(bool i, Depth d, int mn) {
77 int r = Reductions[d] * Reductions[mn];
78 return (r + 511) / 1024 + (!i && r > 1007);
81 constexpr int futility_move_count(bool improving, Depth depth) {
82 return (5 + depth * depth) * (1 + improving) / 2 - 1;
85 // History and stats update bonus, based on depth
86 int stat_bonus(Depth d) {
87 return d > 15 ? -8 : 19 * d * d + 155 * d - 132;
90 // Add a small random component to draw evaluations to avoid 3fold-blindness
91 Value value_draw(Thread* thisThread) {
92 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 // Breadcrumbs are used to mark nodes as being searched by a given thread
108 std::atomic<Thread*> thread;
109 std::atomic<Key> key;
111 std::array<Breadcrumb, 1024> breadcrumbs;
113 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
114 // node for potential reductions. A free node will be marked upon entering the moves
115 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
116 struct ThreadHolding {
117 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
118 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
123 // See if another already marked this location, if not, mark it ourselves
124 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
127 (*location).thread.store(thisThread, std::memory_order_relaxed);
128 (*location).key.store(posKey, std::memory_order_relaxed);
131 else if ( tmp != thisThread
132 && (*location).key.load(std::memory_order_relaxed) == posKey)
138 if (owning) // Free the marked location
139 (*location).thread.store(nullptr, std::memory_order_relaxed);
142 bool marked() { return otherThread; }
145 Breadcrumb* location;
146 bool otherThread, owning;
149 template <NodeType NT>
150 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
152 template <NodeType NT>
153 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
155 Value value_to_tt(Value v, int ply);
156 Value value_from_tt(Value v, int ply, int r50c);
157 void update_pv(Move* pv, Move move, Move* childPv);
158 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
159 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
160 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
161 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
163 // perft() is our utility to verify move generation. All the leaf nodes up
164 // to the given depth are generated and counted, and the sum is returned.
166 uint64_t perft(Position& pos, Depth depth) {
169 uint64_t cnt, nodes = 0;
170 const bool leaf = (depth == 2);
172 for (const auto& m : MoveList<LEGAL>(pos))
174 if (Root && depth <= 1)
179 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
184 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
192 /// Search::init() is called at startup to initialize various lookup tables
194 void Search::init() {
196 for (int i = 1; i < MAX_MOVES; ++i)
197 Reductions[i] = int((24.8 + std::log(Threads.size()) / 2) * std::log(i));
201 /// Search::clear() resets search state to its initial value
203 void Search::clear() {
205 Threads.main()->wait_for_search_finished();
207 Time.availableNodes = 0;
210 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
214 /// MainThread::search() is started when the program receives the UCI 'go'
215 /// command. It searches from the root position and outputs the "bestmove".
217 void MainThread::search() {
221 nodes = perft<true>(rootPos, Limits.perft);
222 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
226 Color us = rootPos.side_to_move();
227 Time.init(Limits, us, rootPos.game_ply());
230 if (rootMoves.empty())
232 rootMoves.emplace_back(MOVE_NONE);
233 sync_cout << "info depth 0 score "
234 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
239 for (Thread* th : Threads)
241 th->bestMoveChanges = 0;
243 th->start_searching();
246 Thread::search(); // Let's start searching!
249 // When we reach the maximum depth, we can arrive here without a raise of
250 // Threads.stop. However, if we are pondering or in an infinite search,
251 // the UCI protocol states that we shouldn't print the best move before the
252 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
253 // until the GUI sends one of those commands.
255 while (!Threads.stop && (ponder || Limits.infinite))
256 {} // Busy wait for a stop or a ponder reset
258 // Stop the threads if not already stopped (also raise the stop if
259 // "ponderhit" just reset Threads.ponder).
262 // Wait until all threads have finished
263 for (Thread* th : Threads)
265 th->wait_for_search_finished();
267 // When playing in 'nodes as time' mode, subtract the searched nodes from
268 // the available ones before exiting.
270 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
272 Thread* bestThread = this;
274 // Check if there are threads with a better score than main thread
275 if ( Options["MultiPV"] == 1
277 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
278 && rootMoves[0].pv[0] != MOVE_NONE)
280 std::map<Move, int64_t> votes;
281 Value minScore = this->rootMoves[0].score;
283 // Find out minimum score
284 for (Thread* th: Threads)
285 minScore = std::min(minScore, th->rootMoves[0].score);
287 // Vote according to score and depth, and select the best thread
288 for (Thread* th : Threads)
290 votes[th->rootMoves[0].pv[0]] +=
291 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
293 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
295 // Make sure we pick the shortest mate
296 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
299 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
300 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
305 previousScore = bestThread->rootMoves[0].score;
307 // Send again PV info if we have a new best thread
308 if (bestThread != this)
309 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
311 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
313 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
314 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
316 std::cout << sync_endl;
320 /// Thread::search() is the main iterative deepening loop. It calls search()
321 /// repeatedly with increasing depth until the allocated thinking time has been
322 /// consumed, the user stops the search, or the maximum search depth is reached.
324 void Thread::search() {
326 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
327 // The former is needed to allow update_continuation_histories(ss-1, ...),
328 // which accesses its argument at ss-6, also near the root.
329 // The latter is needed for statScores and killer initialization.
330 Stack stack[MAX_PLY+10], *ss = stack+7;
332 Value bestValue, alpha, beta, delta;
333 Move lastBestMove = MOVE_NONE;
334 Depth lastBestMoveDepth = 0;
335 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
336 double timeReduction = 1, totBestMoveChanges = 0;
337 Color us = rootPos.side_to_move();
340 std::memset(ss-7, 0, 10 * sizeof(Stack));
341 for (int i = 7; i > 0; i--)
342 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
346 bestValue = delta = alpha = -VALUE_INFINITE;
347 beta = VALUE_INFINITE;
351 if (mainThread->previousScore == VALUE_INFINITE)
352 for (int i=0; i<4; ++i)
353 mainThread->iterValue[i] = VALUE_ZERO;
355 for (int i=0; i<4; ++i)
356 mainThread->iterValue[i] = mainThread->previousScore;
359 size_t multiPV = Options["MultiPV"];
361 // Pick integer skill levels, but non-deterministically round up or down
362 // such that the average integer skill corresponds to the input floating point one.
363 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
364 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
365 // for match (TC 60+0.6) results spanning a wide range of k values.
367 double floatLevel = Options["UCI_LimitStrength"] ?
368 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
369 double(Options["Skill Level"]);
370 int intLevel = int(floatLevel) +
371 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
372 Skill skill(intLevel);
374 // When playing with strength handicap enable MultiPV search that we will
375 // use behind the scenes to retrieve a set of possible moves.
377 multiPV = std::max(multiPV, (size_t)4);
379 multiPV = std::min(multiPV, rootMoves.size());
380 ttHitAverage = ttHitAverageWindow * ttHitAverageResolution / 2;
382 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
384 // In analysis mode, adjust contempt in accordance with user preference
385 if (Limits.infinite || Options["UCI_AnalyseMode"])
386 ct = Options["Analysis Contempt"] == "Off" ? 0
387 : Options["Analysis Contempt"] == "Both" ? ct
388 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
389 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
392 // Evaluation score is from the white point of view
393 contempt = (us == WHITE ? make_score(ct, ct / 2)
394 : -make_score(ct, ct / 2));
396 // Iterative deepening loop until requested to stop or the target depth is reached
397 while ( ++rootDepth < MAX_PLY
399 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
401 // Age out PV variability metric
403 totBestMoveChanges /= 2;
405 // Save the last iteration's scores before first PV line is searched and
406 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
407 for (RootMove& rm : rootMoves)
408 rm.previousScore = rm.score;
413 // MultiPV loop. We perform a full root search for each PV line
414 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
419 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
420 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
424 // Reset UCI info selDepth for each depth and each PV line
427 // Reset aspiration window starting size
430 Value previousScore = rootMoves[pvIdx].previousScore;
431 delta = Value(21 + abs(previousScore) / 256);
432 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
433 beta = std::min(previousScore + delta, VALUE_INFINITE);
435 // Adjust contempt based on root move's previousScore (dynamic contempt)
436 int dct = ct + (102 - ct / 2) * previousScore / (abs(previousScore) + 157);
438 contempt = (us == WHITE ? make_score(dct, dct / 2)
439 : -make_score(dct, dct / 2));
442 // Start with a small aspiration window and, in the case of a fail
443 // high/low, re-search with a bigger window until we don't fail
445 int failedHighCnt = 0;
448 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt);
449 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
451 // Bring the best move to the front. It is critical that sorting
452 // is done with a stable algorithm because all the values but the
453 // first and eventually the new best one are set to -VALUE_INFINITE
454 // and we want to keep the same order for all the moves except the
455 // new PV that goes to the front. Note that in case of MultiPV
456 // search the already searched PV lines are preserved.
457 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
459 // If search has been stopped, we break immediately. Sorting is
460 // safe because RootMoves is still valid, although it refers to
461 // the previous iteration.
465 // When failing high/low give some update (without cluttering
466 // the UI) before a re-search.
469 && (bestValue <= alpha || bestValue >= beta)
470 && Time.elapsed() > 3000)
471 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
473 // In case of failing low/high increase aspiration window and
474 // re-search, otherwise exit the loop.
475 if (bestValue <= alpha)
477 beta = (alpha + beta) / 2;
478 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
482 mainThread->stopOnPonderhit = false;
484 else if (bestValue >= beta)
486 beta = std::min(bestValue + delta, VALUE_INFINITE);
491 ++rootMoves[pvIdx].bestMoveCount;
495 delta += delta / 4 + 5;
497 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
500 // Sort the PV lines searched so far and update the GUI
501 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
504 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
505 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
509 completedDepth = rootDepth;
511 if (rootMoves[0].pv[0] != lastBestMove) {
512 lastBestMove = rootMoves[0].pv[0];
513 lastBestMoveDepth = rootDepth;
516 // Have we found a "mate in x"?
518 && bestValue >= VALUE_MATE_IN_MAX_PLY
519 && VALUE_MATE - bestValue <= 2 * Limits.mate)
525 // If skill level is enabled and time is up, pick a sub-optimal best move
526 if (skill.enabled() && skill.time_to_pick(rootDepth))
527 skill.pick_best(multiPV);
529 // Do we have time for the next iteration? Can we stop searching now?
530 if ( Limits.use_time_management()
532 && !mainThread->stopOnPonderhit)
534 double fallingEval = (332 + 6 * (mainThread->previousScore - bestValue)
535 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 704.0;
536 fallingEval = clamp(fallingEval, 0.5, 1.5);
538 // If the bestMove is stable over several iterations, reduce time accordingly
539 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.94 : 0.91;
540 double reduction = (1.41 + mainThread->previousTimeReduction) / (2.27 * timeReduction);
542 // Use part of the gained time from a previous stable move for the current move
543 for (Thread* th : Threads)
545 totBestMoveChanges += th->bestMoveChanges;
546 th->bestMoveChanges = 0;
548 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
550 // Stop the search if we have only one legal move, or if available time elapsed
551 if ( rootMoves.size() == 1
552 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
554 // If we are allowed to ponder do not stop the search now but
555 // keep pondering until the GUI sends "ponderhit" or "stop".
556 if (mainThread->ponder)
557 mainThread->stopOnPonderhit = true;
563 mainThread->iterValue[iterIdx] = bestValue;
564 iterIdx = (iterIdx + 1) & 3;
570 mainThread->previousTimeReduction = timeReduction;
572 // If skill level is enabled, swap best PV line with the sub-optimal one
574 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
575 skill.best ? skill.best : skill.pick_best(multiPV)));
581 // search<>() is the main search function for both PV and non-PV nodes
583 template <NodeType NT>
584 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
586 constexpr bool PvNode = NT == PV;
587 const bool rootNode = PvNode && ss->ply == 0;
589 // Check if we have an upcoming move which draws by repetition, or
590 // if the opponent had an alternative move earlier to this position.
591 if ( pos.rule50_count() >= 3
592 && alpha < VALUE_DRAW
594 && pos.has_game_cycle(ss->ply))
596 alpha = value_draw(pos.this_thread());
601 // Dive into quiescence search when the depth reaches zero
603 return qsearch<NT>(pos, ss, alpha, beta);
605 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
606 assert(PvNode || (alpha == beta - 1));
607 assert(0 < depth && depth < MAX_PLY);
608 assert(!(PvNode && cutNode));
610 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
614 Move ttMove, move, excludedMove, bestMove;
615 Depth extension, newDepth;
616 Value bestValue, value, ttValue, eval, maxValue;
617 bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
618 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
620 int moveCount, captureCount, quietCount;
622 // Step 1. Initialize node
623 Thread* thisThread = pos.this_thread();
624 inCheck = pos.checkers();
625 priorCapture = pos.captured_piece();
626 Color us = pos.side_to_move();
627 moveCount = captureCount = quietCount = ss->moveCount = 0;
628 bestValue = -VALUE_INFINITE;
629 maxValue = VALUE_INFINITE;
631 // Check for the available remaining time
632 if (thisThread == Threads.main())
633 static_cast<MainThread*>(thisThread)->check_time();
635 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
636 if (PvNode && thisThread->selDepth < ss->ply + 1)
637 thisThread->selDepth = ss->ply + 1;
641 // Step 2. Check for aborted search and immediate draw
642 if ( Threads.stop.load(std::memory_order_relaxed)
643 || pos.is_draw(ss->ply)
644 || ss->ply >= MAX_PLY)
645 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
646 : value_draw(pos.this_thread());
648 // Step 3. Mate distance pruning. Even if we mate at the next move our score
649 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
650 // a shorter mate was found upward in the tree then there is no need to search
651 // because we will never beat the current alpha. Same logic but with reversed
652 // signs applies also in the opposite condition of being mated instead of giving
653 // mate. In this case return a fail-high score.
654 alpha = std::max(mated_in(ss->ply), alpha);
655 beta = std::min(mate_in(ss->ply+1), beta);
660 assert(0 <= ss->ply && ss->ply < MAX_PLY);
662 (ss+1)->ply = ss->ply + 1;
663 (ss+1)->excludedMove = bestMove = MOVE_NONE;
664 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
665 Square prevSq = to_sq((ss-1)->currentMove);
667 // Initialize statScore to zero for the grandchildren of the current position.
668 // So statScore is shared between all grandchildren and only the first grandchild
669 // starts with statScore = 0. Later grandchildren start with the last calculated
670 // statScore of the previous grandchild. This influences the reduction rules in
671 // LMR which are based on the statScore of parent position.
673 (ss+4)->statScore = 0;
675 (ss+2)->statScore = 0;
677 // Step 4. Transposition table lookup. We don't want the score of a partial
678 // search to overwrite a previous full search TT value, so we use a different
679 // position key in case of an excluded move.
680 excludedMove = ss->excludedMove;
681 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
682 tte = TT.probe(posKey, ttHit);
683 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
684 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
685 : ttHit ? tte->move() : MOVE_NONE;
686 ttPv = PvNode || (ttHit && tte->is_pv());
687 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
688 thisThread->ttHitAverage = (ttHitAverageWindow - 1) * thisThread->ttHitAverage / ttHitAverageWindow
689 + ttHitAverageResolution * ttHit;
691 // At non-PV nodes we check for an early TT cutoff
694 && tte->depth() >= depth
695 && ttValue != VALUE_NONE // Possible in case of TT access race
696 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
697 : (tte->bound() & BOUND_UPPER)))
699 // If ttMove is quiet, update move sorting heuristics on TT hit
704 if (!pos.capture_or_promotion(ttMove))
705 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
707 // Extra penalty for early quiet moves of the previous ply
708 if ((ss-1)->moveCount <= 2 && !priorCapture)
709 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
711 // Penalty for a quiet ttMove that fails low
712 else if (!pos.capture_or_promotion(ttMove))
714 int penalty = -stat_bonus(depth);
715 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
716 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
722 // Step 5. Tablebases probe
723 if (!rootNode && TB::Cardinality)
725 int piecesCount = pos.count<ALL_PIECES>();
727 if ( piecesCount <= TB::Cardinality
728 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
729 && pos.rule50_count() == 0
730 && !pos.can_castle(ANY_CASTLING))
733 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
735 // Force check of time on the next occasion
736 if (thisThread == Threads.main())
737 static_cast<MainThread*>(thisThread)->callsCnt = 0;
739 if (err != TB::ProbeState::FAIL)
741 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
743 int drawScore = TB::UseRule50 ? 1 : 0;
745 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
746 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
747 : VALUE_DRAW + 2 * wdl * drawScore;
749 Bound b = wdl < -drawScore ? BOUND_UPPER
750 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
752 if ( b == BOUND_EXACT
753 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
755 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
756 std::min(MAX_PLY - 1, depth + 6),
757 MOVE_NONE, VALUE_NONE);
764 if (b == BOUND_LOWER)
765 bestValue = value, alpha = std::max(alpha, bestValue);
773 // Step 6. Static evaluation of the position
776 ss->staticEval = eval = VALUE_NONE;
778 goto moves_loop; // Skip early pruning when in check
782 // Never assume anything about values stored in TT
783 ss->staticEval = eval = tte->eval();
784 if (eval == VALUE_NONE)
785 ss->staticEval = eval = evaluate(pos);
787 if (eval == VALUE_DRAW)
788 eval = value_draw(thisThread);
790 // Can ttValue be used as a better position evaluation?
791 if ( ttValue != VALUE_NONE
792 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
797 if ((ss-1)->currentMove != MOVE_NULL)
799 int bonus = -(ss-1)->statScore / 512;
801 ss->staticEval = eval = evaluate(pos) + bonus;
804 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
806 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
809 // Step 7. Razoring (~2 Elo)
810 if ( !rootNode // The required rootNode PV handling is not available in qsearch
812 && eval <= alpha - RazorMargin)
813 return qsearch<NT>(pos, ss, alpha, beta);
815 improving = (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval >= (ss-4)->staticEval
816 || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval >= (ss-2)->staticEval;
818 // Step 8. Futility pruning: child node (~30 Elo)
821 && eval - futility_margin(depth, improving) >= beta
822 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
825 // Step 9. Null move search with verification search (~40 Elo)
827 && (ss-1)->currentMove != MOVE_NULL
828 && (ss-1)->statScore < 23405
830 && eval >= ss->staticEval
831 && ss->staticEval >= beta - 32 * depth + 317 - improving * 30
833 && pos.non_pawn_material(us)
834 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
836 assert(eval - beta >= 0);
838 // Null move dynamic reduction based on depth and value
839 Depth R = (854 + 68 * depth) / 258 + std::min(int(eval - beta) / 192, 3);
841 ss->currentMove = MOVE_NULL;
842 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
844 pos.do_null_move(st);
846 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
848 pos.undo_null_move();
850 if (nullValue >= beta)
852 // Do not return unproven mate scores
853 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
856 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
859 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
861 // Do verification search at high depths, with null move pruning disabled
862 // for us, until ply exceeds nmpMinPly.
863 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
864 thisThread->nmpColor = us;
866 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
868 thisThread->nmpMinPly = 0;
875 // Step 10. ProbCut (~10 Elo)
876 // If we have a good enough capture and a reduced search returns a value
877 // much above beta, we can (almost) safely prune the previous move.
880 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
882 Value raisedBeta = std::min(beta + 189 - 45 * improving, VALUE_INFINITE);
883 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
884 int probCutCount = 0;
886 while ( (move = mp.next_move()) != MOVE_NONE
887 && probCutCount < 2 + 2 * cutNode)
888 if (move != excludedMove && pos.legal(move))
890 assert(pos.capture_or_promotion(move));
893 captureOrPromotion = true;
896 ss->currentMove = move;
897 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
899 [pos.moved_piece(move)]
902 pos.do_move(move, st);
904 // Perform a preliminary qsearch to verify that the move holds
905 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
907 // If the qsearch held, perform the regular search
908 if (value >= raisedBeta)
909 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
913 if (value >= raisedBeta)
918 // Step 11. Internal iterative deepening (~2 Elo)
919 if (depth >= 7 && !ttMove)
921 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
923 tte = TT.probe(posKey, ttHit);
924 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
925 ttMove = ttHit ? tte->move() : MOVE_NONE;
928 moves_loop: // When in check, search starts from here
930 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
931 nullptr , (ss-4)->continuationHistory,
932 nullptr , (ss-6)->continuationHistory };
934 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
936 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
937 &thisThread->captureHistory,
943 singularLMR = moveCountPruning = false;
944 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
946 // Mark this node as being searched
947 ThreadHolding th(thisThread, posKey, ss->ply);
949 // Step 12. Loop through all pseudo-legal moves until no moves remain
950 // or a beta cutoff occurs.
951 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
955 if (move == excludedMove)
958 // At root obey the "searchmoves" option and skip moves not listed in Root
959 // Move List. As a consequence any illegal move is also skipped. In MultiPV
960 // mode we also skip PV moves which have been already searched and those
961 // of lower "TB rank" if we are in a TB root position.
962 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
963 thisThread->rootMoves.begin() + thisThread->pvLast, move))
966 ss->moveCount = ++moveCount;
968 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
969 sync_cout << "info depth " << depth
970 << " currmove " << UCI::move(move, pos.is_chess960())
971 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
973 (ss+1)->pv = nullptr;
976 captureOrPromotion = pos.capture_or_promotion(move);
977 movedPiece = pos.moved_piece(move);
978 givesCheck = pos.gives_check(move);
980 // Calculate new depth for this move
981 newDepth = depth - 1;
983 // Step 13. Pruning at shallow depth (~170 Elo)
985 && pos.non_pawn_material(us)
986 && bestValue > VALUE_MATED_IN_MAX_PLY)
988 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
989 moveCountPruning = moveCount >= futility_move_count(improving, depth);
991 if ( !captureOrPromotion
994 // Reduced depth of the next LMR search
995 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
997 // Countermoves based pruning (~20 Elo)
998 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
999 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1000 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1003 // Futility pruning: parent node (~2 Elo)
1006 && ss->staticEval + 255 + 182 * lmrDepth <= alpha
1007 && thisThread->mainHistory[us][from_to(move)]
1008 + (*contHist[0])[movedPiece][to_sq(move)]
1009 + (*contHist[1])[movedPiece][to_sq(move)]
1010 + (*contHist[3])[movedPiece][to_sq(move)] < 30000)
1013 // Prune moves with negative SEE (~10 Elo)
1014 if (!pos.see_ge(move, Value(-(32 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1017 else if (!pos.see_ge(move, Value(-194) * depth)) // (~20 Elo)
1021 // Step 14. Extensions (~70 Elo)
1023 // Singular extension search (~60 Elo). If all moves but one fail low on a
1024 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1025 // then that move is singular and should be extended. To verify this we do
1026 // a reduced search on all the other moves but the ttMove and if the
1027 // result is lower than ttValue minus a margin then we will extend the ttMove.
1031 && !excludedMove // Avoid recursive singular search
1032 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1033 && abs(ttValue) < VALUE_KNOWN_WIN
1034 && (tte->bound() & BOUND_LOWER)
1035 && tte->depth() >= depth - 3
1038 Value singularBeta = ttValue - 2 * depth;
1039 Depth halfDepth = depth / 2;
1040 ss->excludedMove = move;
1041 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
1042 ss->excludedMove = MOVE_NONE;
1044 if (value < singularBeta)
1050 // Multi-cut pruning
1051 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1052 // search without the ttMove. So we assume this expected Cut-node is not singular,
1053 // that multiple moves fail high, and we can prune the whole subtree by returning
1055 else if (singularBeta >= beta)
1056 return singularBeta;
1059 // Check extension (~2 Elo)
1060 else if ( givesCheck
1061 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1064 // Passed pawn extension
1065 else if ( move == ss->killers[0]
1066 && pos.advanced_pawn_push(move)
1067 && pos.pawn_passed(us, to_sq(move)))
1070 // Last captures extension
1071 else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg
1072 && pos.non_pawn_material() <= 2 * RookValueMg)
1075 // Castling extension
1076 if (type_of(move) == CASTLING)
1079 // Add extension to new depth
1080 newDepth += extension;
1082 // Speculative prefetch as early as possible
1083 prefetch(TT.first_entry(pos.key_after(move)));
1085 // Check for legality just before making the move
1086 if (!rootNode && !pos.legal(move))
1088 ss->moveCount = --moveCount;
1092 // Update the current move (this must be done after singular extension search)
1093 ss->currentMove = move;
1094 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1095 [captureOrPromotion]
1099 // Step 15. Make the move
1100 pos.do_move(move, st, givesCheck);
1102 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1103 // re-searched at full depth.
1105 && moveCount > 1 + 2 * rootNode
1106 && (!rootNode || thisThread->best_move_count(move) == 0)
1107 && ( !captureOrPromotion
1109 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1111 || thisThread->ttHitAverage < 375 * ttHitAverageResolution * ttHitAverageWindow / 1024))
1113 Depth r = reduction(improving, depth, moveCount);
1115 // Decrease reduction if the ttHit running average is large
1116 if (thisThread->ttHitAverage > 500 * ttHitAverageResolution * ttHitAverageWindow / 1024)
1119 // Reduction if other threads are searching this position.
1123 // Decrease reduction if position is or has been on the PV
1127 // Decrease reduction if opponent's move count is high (~10 Elo)
1128 if ((ss-1)->moveCount > 14)
1131 // Decrease reduction if ttMove has been singularly extended
1135 if (!captureOrPromotion)
1137 // Increase reduction if ttMove is a capture (~0 Elo)
1141 // Increase reduction for cut nodes (~5 Elo)
1145 // Decrease reduction for moves that escape a capture. Filter out
1146 // castling moves, because they are coded as "king captures rook" and
1147 // hence break make_move(). (~5 Elo)
1148 else if ( type_of(move) == NORMAL
1149 && !pos.see_ge(reverse_move(move)))
1152 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1153 + (*contHist[0])[movedPiece][to_sq(move)]
1154 + (*contHist[1])[movedPiece][to_sq(move)]
1155 + (*contHist[3])[movedPiece][to_sq(move)]
1158 // Reset statScore to zero if negative and most stats shows >= 0
1159 if ( ss->statScore < 0
1160 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1161 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1162 && thisThread->mainHistory[us][from_to(move)] >= 0)
1165 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1166 if (ss->statScore >= -102 && (ss-1)->statScore < -114)
1169 else if ((ss-1)->statScore >= -116 && ss->statScore < -154)
1172 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1173 r -= ss->statScore / 16384;
1176 Depth d = clamp(newDepth - r, 1, newDepth);
1178 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1180 doFullDepthSearch = (value > alpha && d != newDepth), didLMR = true;
1183 doFullDepthSearch = !PvNode || moveCount > 1, didLMR = false;
1185 // Step 17. Full depth search when LMR is skipped or fails high
1186 if (doFullDepthSearch)
1188 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1190 if (didLMR && !captureOrPromotion)
1192 int bonus = value > alpha ? stat_bonus(newDepth)
1193 : -stat_bonus(newDepth);
1195 if (move == ss->killers[0])
1198 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1202 // For PV nodes only, do a full PV search on the first move or after a fail
1203 // high (in the latter case search only if value < beta), otherwise let the
1204 // parent node fail low with value <= alpha and try another move.
1205 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1208 (ss+1)->pv[0] = MOVE_NONE;
1210 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1213 // Step 18. Undo move
1214 pos.undo_move(move);
1216 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1218 // Step 19. Check for a new best move
1219 // Finished searching the move. If a stop occurred, the return value of
1220 // the search cannot be trusted, and we return immediately without
1221 // updating best move, PV and TT.
1222 if (Threads.stop.load(std::memory_order_relaxed))
1227 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1228 thisThread->rootMoves.end(), move);
1230 // PV move or new best move?
1231 if (moveCount == 1 || value > alpha)
1234 rm.selDepth = thisThread->selDepth;
1239 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1240 rm.pv.push_back(*m);
1242 // We record how often the best move has been changed in each
1243 // iteration. This information is used for time management: When
1244 // the best move changes frequently, we allocate some more time.
1246 ++thisThread->bestMoveChanges;
1249 // All other moves but the PV are set to the lowest value: this
1250 // is not a problem when sorting because the sort is stable and the
1251 // move position in the list is preserved - just the PV is pushed up.
1252 rm.score = -VALUE_INFINITE;
1255 if (value > bestValue)
1263 if (PvNode && !rootNode) // Update pv even in fail-high case
1264 update_pv(ss->pv, move, (ss+1)->pv);
1266 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1270 assert(value >= beta); // Fail high
1277 if (move != bestMove)
1279 if (captureOrPromotion && captureCount < 32)
1280 capturesSearched[captureCount++] = move;
1282 else if (!captureOrPromotion && quietCount < 64)
1283 quietsSearched[quietCount++] = move;
1287 // The following condition would detect a stop only after move loop has been
1288 // completed. But in this case bestValue is valid because we have fully
1289 // searched our subtree, and we can anyhow save the result in TT.
1295 // Step 20. Check for mate and stalemate
1296 // All legal moves have been searched and if there are no legal moves, it
1297 // must be a mate or a stalemate. If we are in a singular extension search then
1298 // return a fail low score.
1300 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1303 bestValue = excludedMove ? alpha
1304 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1307 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1308 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1310 // Bonus for prior countermove that caused the fail low
1311 else if ( (depth >= 3 || PvNode)
1313 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1316 bestValue = std::min(bestValue, maxValue);
1319 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1320 bestValue >= beta ? BOUND_LOWER :
1321 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1322 depth, bestMove, ss->staticEval);
1324 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1330 // qsearch() is the quiescence search function, which is called by the main search
1331 // function with zero depth, or recursively with further decreasing depth per call.
1332 template <NodeType NT>
1333 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1335 constexpr bool PvNode = NT == PV;
1337 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1338 assert(PvNode || (alpha == beta - 1));
1345 Move ttMove, move, bestMove;
1347 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1348 bool ttHit, pvHit, inCheck, givesCheck, captureOrPromotion, evasionPrunable;
1353 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1355 ss->pv[0] = MOVE_NONE;
1358 Thread* thisThread = pos.this_thread();
1359 (ss+1)->ply = ss->ply + 1;
1360 bestMove = MOVE_NONE;
1361 inCheck = pos.checkers();
1364 // Check for an immediate draw or maximum ply reached
1365 if ( pos.is_draw(ss->ply)
1366 || ss->ply >= MAX_PLY)
1367 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1369 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1371 // Decide whether or not to include checks: this fixes also the type of
1372 // TT entry depth that we are going to use. Note that in qsearch we use
1373 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1374 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1375 : DEPTH_QS_NO_CHECKS;
1376 // Transposition table lookup
1378 tte = TT.probe(posKey, ttHit);
1379 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1380 ttMove = ttHit ? tte->move() : MOVE_NONE;
1381 pvHit = ttHit && tte->is_pv();
1385 && tte->depth() >= ttDepth
1386 && ttValue != VALUE_NONE // Only in case of TT access race
1387 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1388 : (tte->bound() & BOUND_UPPER)))
1391 // Evaluate the position statically
1394 ss->staticEval = VALUE_NONE;
1395 bestValue = futilityBase = -VALUE_INFINITE;
1401 // Never assume anything about values stored in TT
1402 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1403 ss->staticEval = bestValue = evaluate(pos);
1405 // Can ttValue be used as a better position evaluation?
1406 if ( ttValue != VALUE_NONE
1407 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1408 bestValue = ttValue;
1411 ss->staticEval = bestValue =
1412 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1413 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1415 // Stand pat. Return immediately if static value is at least beta
1416 if (bestValue >= beta)
1419 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1420 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1425 if (PvNode && bestValue > alpha)
1428 futilityBase = bestValue + 154;
1431 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1432 nullptr , (ss-4)->continuationHistory,
1433 nullptr , (ss-6)->continuationHistory };
1435 // Initialize a MovePicker object for the current position, and prepare
1436 // to search the moves. Because the depth is <= 0 here, only captures,
1437 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1439 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1440 &thisThread->captureHistory,
1442 to_sq((ss-1)->currentMove));
1444 // Loop through the moves until no moves remain or a beta cutoff occurs
1445 while ((move = mp.next_move()) != MOVE_NONE)
1447 assert(is_ok(move));
1449 givesCheck = pos.gives_check(move);
1450 captureOrPromotion = pos.capture_or_promotion(move);
1457 && futilityBase > -VALUE_KNOWN_WIN
1458 && !pos.advanced_pawn_push(move))
1460 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1462 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1464 if (futilityValue <= alpha)
1466 bestValue = std::max(bestValue, futilityValue);
1470 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1472 bestValue = std::max(bestValue, futilityBase);
1477 // Detect non-capture evasions that are candidates to be pruned
1478 evasionPrunable = inCheck
1479 && (depth != 0 || moveCount > 2)
1480 && bestValue > VALUE_MATED_IN_MAX_PLY
1481 && !pos.capture(move);
1483 // Don't search moves with negative SEE values
1484 if ( (!inCheck || evasionPrunable) && !pos.see_ge(move))
1487 // Speculative prefetch as early as possible
1488 prefetch(TT.first_entry(pos.key_after(move)));
1490 // Check for legality just before making the move
1491 if (!pos.legal(move))
1497 ss->currentMove = move;
1498 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1499 [captureOrPromotion]
1500 [pos.moved_piece(move)]
1503 // Make and search the move
1504 pos.do_move(move, st, givesCheck);
1505 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1506 pos.undo_move(move);
1508 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1510 // Check for a new best move
1511 if (value > bestValue)
1519 if (PvNode) // Update pv even in fail-high case
1520 update_pv(ss->pv, move, (ss+1)->pv);
1522 if (PvNode && value < beta) // Update alpha here!
1530 // All legal moves have been searched. A special case: If we're in check
1531 // and no legal moves were found, it is checkmate.
1532 if (inCheck && bestValue == -VALUE_INFINITE)
1533 return mated_in(ss->ply); // Plies to mate from the root
1535 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1536 bestValue >= beta ? BOUND_LOWER :
1537 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1538 ttDepth, bestMove, ss->staticEval);
1540 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1546 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1547 // "plies to mate from the current position". Non-mate scores are unchanged.
1548 // The function is called before storing a value in the transposition table.
1550 Value value_to_tt(Value v, int ply) {
1552 assert(v != VALUE_NONE);
1554 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1555 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1559 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1560 // from the transposition table (which refers to the plies to mate/be mated
1561 // from current position) to "plies to mate/be mated from the root".
1563 Value value_from_tt(Value v, int ply, int r50c) {
1565 return v == VALUE_NONE ? VALUE_NONE
1566 : v >= VALUE_MATE_IN_MAX_PLY ? VALUE_MATE - v > 99 - r50c ? VALUE_MATE_IN_MAX_PLY : v - ply
1567 : v <= VALUE_MATED_IN_MAX_PLY ? VALUE_MATE + v > 99 - r50c ? VALUE_MATED_IN_MAX_PLY : v + ply : v;
1571 // update_pv() adds current move and appends child pv[]
1573 void update_pv(Move* pv, Move move, Move* childPv) {
1575 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1581 // update_all_stats() updates stats at the end of search() when a bestMove is found
1583 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1584 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1587 Color us = pos.side_to_move();
1588 Thread* thisThread = pos.this_thread();
1589 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1590 Piece moved_piece = pos.moved_piece(bestMove);
1591 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1593 bonus1 = stat_bonus(depth + 1);
1594 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1595 : stat_bonus(depth); // smaller bonus
1597 if (!pos.capture_or_promotion(bestMove))
1599 update_quiet_stats(pos, ss, bestMove, bonus2);
1601 // Decrease all the non-best quiet moves
1602 for (int i = 0; i < quietCount; ++i)
1604 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1605 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1609 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1611 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1612 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1613 && !pos.captured_piece())
1614 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1616 // Decrease all the non-best capture moves
1617 for (int i = 0; i < captureCount; ++i)
1619 moved_piece = pos.moved_piece(capturesSearched[i]);
1620 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1621 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1626 // update_continuation_histories() updates histories of the move pairs formed
1627 // by moves at ply -1, -2, and -4 with current move.
1629 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1631 for (int i : {1, 2, 4, 6})
1632 if (is_ok((ss-i)->currentMove))
1633 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1637 // update_quiet_stats() updates move sorting heuristics
1639 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1641 if (ss->killers[0] != move)
1643 ss->killers[1] = ss->killers[0];
1644 ss->killers[0] = move;
1647 Color us = pos.side_to_move();
1648 Thread* thisThread = pos.this_thread();
1649 thisThread->mainHistory[us][from_to(move)] << bonus;
1650 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1652 if (type_of(pos.moved_piece(move)) != PAWN)
1653 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1655 if (is_ok((ss-1)->currentMove))
1657 Square prevSq = to_sq((ss-1)->currentMove);
1658 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1662 // When playing with strength handicap, choose best move among a set of RootMoves
1663 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1665 Move Skill::pick_best(size_t multiPV) {
1667 const RootMoves& rootMoves = Threads.main()->rootMoves;
1668 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1670 // RootMoves are already sorted by score in descending order
1671 Value topScore = rootMoves[0].score;
1672 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1673 int weakness = 120 - 2 * level;
1674 int maxScore = -VALUE_INFINITE;
1676 // Choose best move. For each move score we add two terms, both dependent on
1677 // weakness. One is deterministic and bigger for weaker levels, and one is
1678 // random. Then we choose the move with the resulting highest score.
1679 for (size_t i = 0; i < multiPV; ++i)
1681 // This is our magic formula
1682 int push = ( weakness * int(topScore - rootMoves[i].score)
1683 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1685 if (rootMoves[i].score + push >= maxScore)
1687 maxScore = rootMoves[i].score + push;
1688 best = rootMoves[i].pv[0];
1697 /// MainThread::check_time() is used to print debug info and, more importantly,
1698 /// to detect when we are out of available time and thus stop the search.
1700 void MainThread::check_time() {
1705 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1706 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1708 static TimePoint lastInfoTime = now();
1710 TimePoint elapsed = Time.elapsed();
1711 TimePoint tick = Limits.startTime + elapsed;
1713 if (tick - lastInfoTime >= 1000)
1715 lastInfoTime = tick;
1719 // We should not stop pondering until told so by the GUI
1723 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1724 || (Limits.movetime && elapsed >= Limits.movetime)
1725 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1726 Threads.stop = true;
1730 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1731 /// that all (if any) unsearched PV lines are sent using a previous search score.
1733 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1735 std::stringstream ss;
1736 TimePoint elapsed = Time.elapsed() + 1;
1737 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1738 size_t pvIdx = pos.this_thread()->pvIdx;
1739 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1740 uint64_t nodesSearched = Threads.nodes_searched();
1741 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1743 for (size_t i = 0; i < multiPV; ++i)
1745 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1747 if (depth == 1 && !updated)
1750 Depth d = updated ? depth : depth - 1;
1751 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1753 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1754 v = tb ? rootMoves[i].tbScore : v;
1756 if (ss.rdbuf()->in_avail()) // Not at first line
1761 << " seldepth " << rootMoves[i].selDepth
1762 << " multipv " << i + 1
1763 << " score " << UCI::value(v);
1765 if (!tb && i == pvIdx)
1766 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1768 ss << " nodes " << nodesSearched
1769 << " nps " << nodesSearched * 1000 / elapsed;
1771 if (elapsed > 1000) // Earlier makes little sense
1772 ss << " hashfull " << TT.hashfull();
1774 ss << " tbhits " << tbHits
1775 << " time " << elapsed
1778 for (Move m : rootMoves[i].pv)
1779 ss << " " << UCI::move(m, pos.is_chess960());
1786 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1787 /// before exiting the search, for instance, in case we stop the search during a
1788 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1789 /// otherwise in case of 'ponder on' we have nothing to think on.
1791 bool RootMove::extract_ponder_from_tt(Position& pos) {
1796 assert(pv.size() == 1);
1798 if (pv[0] == MOVE_NONE)
1801 pos.do_move(pv[0], st);
1802 TTEntry* tte = TT.probe(pos.key(), ttHit);
1806 Move m = tte->move(); // Local copy to be SMP safe
1807 if (MoveList<LEGAL>(pos).contains(m))
1811 pos.undo_move(pv[0]);
1812 return pv.size() > 1;
1815 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1818 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1819 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1820 Cardinality = int(Options["SyzygyProbeLimit"]);
1821 bool dtz_available = true;
1823 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1824 // ProbeDepth == DEPTH_ZERO
1825 if (Cardinality > MaxCardinality)
1827 Cardinality = MaxCardinality;
1831 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1833 // Rank moves using DTZ tables
1834 RootInTB = root_probe(pos, rootMoves);
1838 // DTZ tables are missing; try to rank moves using WDL tables
1839 dtz_available = false;
1840 RootInTB = root_probe_wdl(pos, rootMoves);
1846 // Sort moves according to TB rank
1847 std::sort(rootMoves.begin(), rootMoves.end(),
1848 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1850 // Probe during search only if DTZ is not available and we are winning
1851 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1856 // Clean up if root_probe() and root_probe_wdl() have failed
1857 for (auto& m : rootMoves)