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 = 661;
69 Value futility_margin(Depth d, bool improving) {
70 return Value(198 * (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 + 520) / 1024 + (!i && r > 999);
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 > 17 ? -8 : 22 * d * d + 151 * d - 140;
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((23.4 + 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();
339 std::memset(ss-7, 0, 10 * sizeof(Stack));
340 for (int i = 7; i > 0; i--)
341 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
345 bestValue = delta = alpha = -VALUE_INFINITE;
346 beta = VALUE_INFINITE;
348 size_t multiPV = Options["MultiPV"];
350 // Pick integer skill levels, but non-deterministically round up or down
351 // such that the average integer skill corresponds to the input floating point one.
352 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
353 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
354 // for match (TC 60+0.6) results spanning a wide range of k values.
356 double floatLevel = Options["UCI_LimitStrength"] ?
357 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
358 double(Options["Skill Level"]);
359 int intLevel = int(floatLevel) +
360 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
361 Skill skill(intLevel);
363 // When playing with strength handicap enable MultiPV search that we will
364 // use behind the scenes to retrieve a set of possible moves.
366 multiPV = std::max(multiPV, (size_t)4);
368 multiPV = std::min(multiPV, rootMoves.size());
369 ttHitAverage = ttHitAverageWindow * ttHitAverageResolution / 2;
371 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
373 // In analysis mode, adjust contempt in accordance with user preference
374 if (Limits.infinite || Options["UCI_AnalyseMode"])
375 ct = Options["Analysis Contempt"] == "Off" ? 0
376 : Options["Analysis Contempt"] == "Both" ? ct
377 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
378 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
381 // Evaluation score is from the white point of view
382 contempt = (us == WHITE ? make_score(ct, ct / 2)
383 : -make_score(ct, ct / 2));
385 // Iterative deepening loop until requested to stop or the target depth is reached
386 while ( ++rootDepth < MAX_PLY
388 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
390 // Age out PV variability metric
392 totBestMoveChanges /= 2;
394 // Save the last iteration's scores before first PV line is searched and
395 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
396 for (RootMove& rm : rootMoves)
397 rm.previousScore = rm.score;
402 // MultiPV loop. We perform a full root search for each PV line
403 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
408 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
409 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
413 // Reset UCI info selDepth for each depth and each PV line
416 // Reset aspiration window starting size
419 Value previousScore = rootMoves[pvIdx].previousScore;
420 delta = Value(21 + abs(previousScore) / 128);
421 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
422 beta = std::min(previousScore + delta, VALUE_INFINITE);
424 // Adjust contempt based on root move's previousScore (dynamic contempt)
425 int dct = ct + (111 - ct / 2) * previousScore / (abs(previousScore) + 176);
427 contempt = (us == WHITE ? make_score(dct, dct / 2)
428 : -make_score(dct, dct / 2));
431 // Start with a small aspiration window and, in the case of a fail
432 // high/low, re-search with a bigger window until we don't fail
434 int failedHighCnt = 0;
437 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt);
438 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
440 // Bring the best move to the front. It is critical that sorting
441 // is done with a stable algorithm because all the values but the
442 // first and eventually the new best one are set to -VALUE_INFINITE
443 // and we want to keep the same order for all the moves except the
444 // new PV that goes to the front. Note that in case of MultiPV
445 // search the already searched PV lines are preserved.
446 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
448 // If search has been stopped, we break immediately. Sorting is
449 // safe because RootMoves is still valid, although it refers to
450 // the previous iteration.
454 // When failing high/low give some update (without cluttering
455 // the UI) before a re-search.
458 && (bestValue <= alpha || bestValue >= beta)
459 && Time.elapsed() > 3000)
460 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
462 // In case of failing low/high increase aspiration window and
463 // re-search, otherwise exit the loop.
464 if (bestValue <= alpha)
466 beta = (alpha + beta) / 2;
467 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
471 mainThread->stopOnPonderhit = false;
473 else if (bestValue >= beta)
475 beta = std::min(bestValue + delta, VALUE_INFINITE);
480 ++rootMoves[pvIdx].bestMoveCount;
484 delta += delta / 4 + 5;
486 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
489 // Sort the PV lines searched so far and update the GUI
490 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
493 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
494 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
498 completedDepth = rootDepth;
500 if (rootMoves[0].pv[0] != lastBestMove) {
501 lastBestMove = rootMoves[0].pv[0];
502 lastBestMoveDepth = rootDepth;
505 // Have we found a "mate in x"?
507 && bestValue >= VALUE_MATE_IN_MAX_PLY
508 && VALUE_MATE - bestValue <= 2 * Limits.mate)
514 // If skill level is enabled and time is up, pick a sub-optimal best move
515 if (skill.enabled() && skill.time_to_pick(rootDepth))
516 skill.pick_best(multiPV);
518 // Do we have time for the next iteration? Can we stop searching now?
519 if ( Limits.use_time_management()
521 && !mainThread->stopOnPonderhit)
523 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
524 fallingEval = clamp(fallingEval, 0.5, 1.5);
526 // If the bestMove is stable over several iterations, reduce time accordingly
527 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.97 : 0.98;
528 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
530 // Use part of the gained time from a previous stable move for the current move
531 for (Thread* th : Threads)
533 totBestMoveChanges += th->bestMoveChanges;
534 th->bestMoveChanges = 0;
536 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
538 // Stop the search if we have only one legal move, or if available time elapsed
539 if ( rootMoves.size() == 1
540 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
542 // If we are allowed to ponder do not stop the search now but
543 // keep pondering until the GUI sends "ponderhit" or "stop".
544 if (mainThread->ponder)
545 mainThread->stopOnPonderhit = true;
555 mainThread->previousTimeReduction = timeReduction;
557 // If skill level is enabled, swap best PV line with the sub-optimal one
559 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
560 skill.best ? skill.best : skill.pick_best(multiPV)));
566 // search<>() is the main search function for both PV and non-PV nodes
568 template <NodeType NT>
569 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
571 constexpr bool PvNode = NT == PV;
572 const bool rootNode = PvNode && ss->ply == 0;
574 // Check if we have an upcoming move which draws by repetition, or
575 // if the opponent had an alternative move earlier to this position.
576 if ( pos.rule50_count() >= 3
577 && alpha < VALUE_DRAW
579 && pos.has_game_cycle(ss->ply))
581 alpha = value_draw(pos.this_thread());
586 // Dive into quiescence search when the depth reaches zero
588 return qsearch<NT>(pos, ss, alpha, beta);
590 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
591 assert(PvNode || (alpha == beta - 1));
592 assert(0 < depth && depth < MAX_PLY);
593 assert(!(PvNode && cutNode));
595 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
599 Move ttMove, move, excludedMove, bestMove;
600 Depth extension, newDepth;
601 Value bestValue, value, ttValue, eval, maxValue;
602 bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
603 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
605 int moveCount, captureCount, quietCount;
607 // Step 1. Initialize node
608 Thread* thisThread = pos.this_thread();
609 inCheck = pos.checkers();
610 priorCapture = pos.captured_piece();
611 Color us = pos.side_to_move();
612 moveCount = captureCount = quietCount = ss->moveCount = 0;
613 bestValue = -VALUE_INFINITE;
614 maxValue = VALUE_INFINITE;
616 // Check for the available remaining time
617 if (thisThread == Threads.main())
618 static_cast<MainThread*>(thisThread)->check_time();
620 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
621 if (PvNode && thisThread->selDepth < ss->ply + 1)
622 thisThread->selDepth = ss->ply + 1;
626 // Step 2. Check for aborted search and immediate draw
627 if ( Threads.stop.load(std::memory_order_relaxed)
628 || pos.is_draw(ss->ply)
629 || ss->ply >= MAX_PLY)
630 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
631 : value_draw(pos.this_thread());
633 // Step 3. Mate distance pruning. Even if we mate at the next move our score
634 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
635 // a shorter mate was found upward in the tree then there is no need to search
636 // because we will never beat the current alpha. Same logic but with reversed
637 // signs applies also in the opposite condition of being mated instead of giving
638 // mate. In this case return a fail-high score.
639 alpha = std::max(mated_in(ss->ply), alpha);
640 beta = std::min(mate_in(ss->ply+1), beta);
645 assert(0 <= ss->ply && ss->ply < MAX_PLY);
647 (ss+1)->ply = ss->ply + 1;
648 (ss+1)->excludedMove = bestMove = MOVE_NONE;
649 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
650 Square prevSq = to_sq((ss-1)->currentMove);
652 // Initialize statScore to zero for the grandchildren of the current position.
653 // So statScore is shared between all grandchildren and only the first grandchild
654 // starts with statScore = 0. Later grandchildren start with the last calculated
655 // statScore of the previous grandchild. This influences the reduction rules in
656 // LMR which are based on the statScore of parent position.
658 (ss+4)->statScore = 0;
660 (ss+2)->statScore = 0;
662 // Step 4. Transposition table lookup. We don't want the score of a partial
663 // search to overwrite a previous full search TT value, so we use a different
664 // position key in case of an excluded move.
665 excludedMove = ss->excludedMove;
666 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
667 tte = TT.probe(posKey, ttHit);
668 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
669 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
670 : ttHit ? tte->move() : MOVE_NONE;
671 ttPv = PvNode || (ttHit && tte->is_pv());
672 // thisThread->ttHitAverage can be used to approximate the running average of ttHit
673 thisThread->ttHitAverage = (ttHitAverageWindow - 1) * thisThread->ttHitAverage / ttHitAverageWindow
674 + ttHitAverageResolution * ttHit;
676 // At non-PV nodes we check for an early TT cutoff
679 && tte->depth() >= depth
680 && ttValue != VALUE_NONE // Possible in case of TT access race
681 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
682 : (tte->bound() & BOUND_UPPER)))
684 // If ttMove is quiet, update move sorting heuristics on TT hit
689 if (!pos.capture_or_promotion(ttMove))
690 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
692 // Extra penalty for early quiet moves of the previous ply
693 if ((ss-1)->moveCount <= 2 && !priorCapture)
694 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
696 // Penalty for a quiet ttMove that fails low
697 else if (!pos.capture_or_promotion(ttMove))
699 int penalty = -stat_bonus(depth);
700 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
701 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
707 // Step 5. Tablebases probe
708 if (!rootNode && TB::Cardinality)
710 int piecesCount = pos.count<ALL_PIECES>();
712 if ( piecesCount <= TB::Cardinality
713 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
714 && pos.rule50_count() == 0
715 && !pos.can_castle(ANY_CASTLING))
718 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
720 // Force check of time on the next occasion
721 if (thisThread == Threads.main())
722 static_cast<MainThread*>(thisThread)->callsCnt = 0;
724 if (err != TB::ProbeState::FAIL)
726 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
728 int drawScore = TB::UseRule50 ? 1 : 0;
730 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
731 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
732 : VALUE_DRAW + 2 * wdl * drawScore;
734 Bound b = wdl < -drawScore ? BOUND_UPPER
735 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
737 if ( b == BOUND_EXACT
738 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
740 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
741 std::min(MAX_PLY - 1, depth + 6),
742 MOVE_NONE, VALUE_NONE);
749 if (b == BOUND_LOWER)
750 bestValue = value, alpha = std::max(alpha, bestValue);
758 // Step 6. Static evaluation of the position
761 ss->staticEval = eval = VALUE_NONE;
763 goto moves_loop; // Skip early pruning when in check
767 // Never assume anything about values stored in TT
768 ss->staticEval = eval = tte->eval();
769 if (eval == VALUE_NONE)
770 ss->staticEval = eval = evaluate(pos);
772 if (eval == VALUE_DRAW)
773 eval = value_draw(thisThread);
775 // Can ttValue be used as a better position evaluation?
776 if ( ttValue != VALUE_NONE
777 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
782 if ((ss-1)->currentMove != MOVE_NULL)
784 int bonus = -(ss-1)->statScore / 512;
786 ss->staticEval = eval = evaluate(pos) + bonus;
789 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
791 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
794 // Step 7. Razoring (~2 Elo)
795 if ( !rootNode // The required rootNode PV handling is not available in qsearch
797 && eval <= alpha - RazorMargin)
798 return qsearch<NT>(pos, ss, alpha, beta);
800 improving = ss->staticEval >= (ss-2)->staticEval
801 || (ss-2)->staticEval == VALUE_NONE;
803 // Step 8. Futility pruning: child node (~30 Elo)
806 && eval - futility_margin(depth, improving) >= beta
807 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
810 // Step 9. Null move search with verification search (~40 Elo)
812 && (ss-1)->currentMove != MOVE_NULL
813 && (ss-1)->statScore < 22661
815 && eval >= ss->staticEval
816 && ss->staticEval >= beta - 33 * depth + 299 - improving * 30
818 && pos.non_pawn_material(us)
819 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
821 assert(eval - beta >= 0);
823 // Null move dynamic reduction based on depth and value
824 Depth R = (835 + 70 * depth) / 256 + std::min(int(eval - beta) / 185, 3);
826 ss->currentMove = MOVE_NULL;
827 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
829 pos.do_null_move(st);
831 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
833 pos.undo_null_move();
835 if (nullValue >= beta)
837 // Do not return unproven mate scores
838 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
841 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
844 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
846 // Do verification search at high depths, with null move pruning disabled
847 // for us, until ply exceeds nmpMinPly.
848 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
849 thisThread->nmpColor = us;
851 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
853 thisThread->nmpMinPly = 0;
860 // Step 10. ProbCut (~10 Elo)
861 // If we have a good enough capture and a reduced search returns a value
862 // much above beta, we can (almost) safely prune the previous move.
865 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
867 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
868 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
869 int probCutCount = 0;
871 while ( (move = mp.next_move()) != MOVE_NONE
872 && probCutCount < 2 + 2 * cutNode)
873 if (move != excludedMove && pos.legal(move))
875 assert(pos.capture_or_promotion(move));
878 captureOrPromotion = true;
881 ss->currentMove = move;
882 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
884 [pos.moved_piece(move)]
887 pos.do_move(move, st);
889 // Perform a preliminary qsearch to verify that the move holds
890 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
892 // If the qsearch held, perform the regular search
893 if (value >= raisedBeta)
894 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
898 if (value >= raisedBeta)
903 // Step 11. Internal iterative deepening (~2 Elo)
904 if (depth >= 7 && !ttMove)
906 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
908 tte = TT.probe(posKey, ttHit);
909 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
910 ttMove = ttHit ? tte->move() : MOVE_NONE;
913 moves_loop: // When in check, search starts from here
915 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
916 nullptr , (ss-4)->continuationHistory,
917 nullptr , (ss-6)->continuationHistory };
919 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
921 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
922 &thisThread->captureHistory,
928 singularLMR = moveCountPruning = false;
929 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
931 // Mark this node as being searched
932 ThreadHolding th(thisThread, posKey, ss->ply);
934 // Step 12. Loop through all pseudo-legal moves until no moves remain
935 // or a beta cutoff occurs.
936 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
940 if (move == excludedMove)
943 // At root obey the "searchmoves" option and skip moves not listed in Root
944 // Move List. As a consequence any illegal move is also skipped. In MultiPV
945 // mode we also skip PV moves which have been already searched and those
946 // of lower "TB rank" if we are in a TB root position.
947 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
948 thisThread->rootMoves.begin() + thisThread->pvLast, move))
951 ss->moveCount = ++moveCount;
953 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
954 sync_cout << "info depth " << depth
955 << " currmove " << UCI::move(move, pos.is_chess960())
956 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
958 (ss+1)->pv = nullptr;
961 captureOrPromotion = pos.capture_or_promotion(move);
962 movedPiece = pos.moved_piece(move);
963 givesCheck = pos.gives_check(move);
965 // Calculate new depth for this move
966 newDepth = depth - 1;
968 // Step 13. Pruning at shallow depth (~170 Elo)
970 && pos.non_pawn_material(us)
971 && bestValue > VALUE_MATED_IN_MAX_PLY)
973 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
974 moveCountPruning = moveCount >= futility_move_count(improving, depth);
976 if ( !captureOrPromotion
978 && (!PvNode || !pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
980 // Reduced depth of the next LMR search
981 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
983 // Countermoves based pruning (~20 Elo)
984 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
985 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
986 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
989 // Futility pruning: parent node (~2 Elo)
992 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
995 // Prune moves with negative SEE (~10 Elo)
996 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
999 else if (!pos.see_ge(move, Value(-199) * depth)) // (~20 Elo)
1003 // Step 14. Extensions (~70 Elo)
1005 // Singular extension search (~60 Elo). If all moves but one fail low on a
1006 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1007 // then that move is singular and should be extended. To verify this we do
1008 // a reduced search on all the other moves but the ttMove and if the
1009 // result is lower than ttValue minus a margin then we will extend the ttMove.
1013 && !excludedMove // Avoid recursive singular search
1014 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1015 && abs(ttValue) < VALUE_KNOWN_WIN
1016 && (tte->bound() & BOUND_LOWER)
1017 && tte->depth() >= depth - 3
1020 Value singularBeta = ttValue - 2 * depth;
1021 Depth halfDepth = depth / 2;
1022 ss->excludedMove = move;
1023 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
1024 ss->excludedMove = MOVE_NONE;
1026 if (value < singularBeta)
1032 // Multi-cut pruning
1033 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1034 // search without the ttMove. So we assume this expected Cut-node is not singular,
1035 // that multiple moves fail high, and we can prune the whole subtree by returning
1037 else if ( eval >= beta
1038 && singularBeta >= beta)
1039 return singularBeta;
1042 // Check extension (~2 Elo)
1043 else if ( givesCheck
1044 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1047 // Passed pawn extension
1048 else if ( move == ss->killers[0]
1049 && pos.advanced_pawn_push(move)
1050 && pos.pawn_passed(us, to_sq(move)))
1053 // Castling extension
1054 if (type_of(move) == CASTLING)
1057 // Add extension to new depth
1058 newDepth += extension;
1060 // Speculative prefetch as early as possible
1061 prefetch(TT.first_entry(pos.key_after(move)));
1063 // Check for legality just before making the move
1064 if (!rootNode && !pos.legal(move))
1066 ss->moveCount = --moveCount;
1070 // Update the current move (this must be done after singular extension search)
1071 ss->currentMove = move;
1072 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1073 [captureOrPromotion]
1077 // Step 15. Make the move
1078 pos.do_move(move, st, givesCheck);
1080 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1081 // re-searched at full depth.
1083 && moveCount > 1 + 2 * rootNode
1084 && (!rootNode || thisThread->best_move_count(move) == 0)
1085 && ( !captureOrPromotion
1087 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1090 Depth r = reduction(improving, depth, moveCount);
1092 // Decrease reduction if the ttHit running average is large
1093 if (thisThread->ttHitAverage > 544 * ttHitAverageResolution * ttHitAverageWindow / 1024)
1096 // Reduction if other threads are searching this position.
1100 // Decrease reduction if position is or has been on the PV
1104 // Decrease reduction if opponent's move count is high (~10 Elo)
1105 if ((ss-1)->moveCount > 15)
1108 // Decrease reduction if ttMove has been singularly extended
1112 if (!captureOrPromotion)
1114 // Increase reduction if ttMove is a capture (~0 Elo)
1118 // Increase reduction for cut nodes (~5 Elo)
1122 // Decrease reduction for moves that escape a capture. Filter out
1123 // castling moves, because they are coded as "king captures rook" and
1124 // hence break make_move(). (~5 Elo)
1125 else if ( type_of(move) == NORMAL
1126 && !pos.see_ge(reverse_move(move)))
1129 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1130 + (*contHist[0])[movedPiece][to_sq(move)]
1131 + (*contHist[1])[movedPiece][to_sq(move)]
1132 + (*contHist[3])[movedPiece][to_sq(move)]
1135 // Reset statScore to zero if negative and most stats shows >= 0
1136 if ( ss->statScore < 0
1137 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1138 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1139 && thisThread->mainHistory[us][from_to(move)] >= 0)
1142 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1143 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1146 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1149 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1150 r -= ss->statScore / 16384;
1153 Depth d = clamp(newDepth - r, 1, newDepth);
1155 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1157 doFullDepthSearch = (value > alpha && d != newDepth), didLMR = true;
1160 doFullDepthSearch = !PvNode || moveCount > 1, didLMR = false;
1162 // Step 17. Full depth search when LMR is skipped or fails high
1163 if (doFullDepthSearch)
1165 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1167 if (didLMR && !captureOrPromotion)
1169 int bonus = value > alpha ? stat_bonus(newDepth)
1170 : -stat_bonus(newDepth);
1172 if (move == ss->killers[0])
1175 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1179 // For PV nodes only, do a full PV search on the first move or after a fail
1180 // high (in the latter case search only if value < beta), otherwise let the
1181 // parent node fail low with value <= alpha and try another move.
1182 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1185 (ss+1)->pv[0] = MOVE_NONE;
1187 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1190 // Step 18. Undo move
1191 pos.undo_move(move);
1193 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1195 // Step 19. Check for a new best move
1196 // Finished searching the move. If a stop occurred, the return value of
1197 // the search cannot be trusted, and we return immediately without
1198 // updating best move, PV and TT.
1199 if (Threads.stop.load(std::memory_order_relaxed))
1204 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1205 thisThread->rootMoves.end(), move);
1207 // PV move or new best move?
1208 if (moveCount == 1 || value > alpha)
1211 rm.selDepth = thisThread->selDepth;
1216 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1217 rm.pv.push_back(*m);
1219 // We record how often the best move has been changed in each
1220 // iteration. This information is used for time management: When
1221 // the best move changes frequently, we allocate some more time.
1223 ++thisThread->bestMoveChanges;
1226 // All other moves but the PV are set to the lowest value: this
1227 // is not a problem when sorting because the sort is stable and the
1228 // move position in the list is preserved - just the PV is pushed up.
1229 rm.score = -VALUE_INFINITE;
1232 if (value > bestValue)
1240 if (PvNode && !rootNode) // Update pv even in fail-high case
1241 update_pv(ss->pv, move, (ss+1)->pv);
1243 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1247 assert(value >= beta); // Fail high
1254 if (move != bestMove)
1256 if (captureOrPromotion && captureCount < 32)
1257 capturesSearched[captureCount++] = move;
1259 else if (!captureOrPromotion && quietCount < 64)
1260 quietsSearched[quietCount++] = move;
1264 // The following condition would detect a stop only after move loop has been
1265 // completed. But in this case bestValue is valid because we have fully
1266 // searched our subtree, and we can anyhow save the result in TT.
1272 // Step 20. Check for mate and stalemate
1273 // All legal moves have been searched and if there are no legal moves, it
1274 // must be a mate or a stalemate. If we are in a singular extension search then
1275 // return a fail low score.
1277 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1280 bestValue = excludedMove ? alpha
1281 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1284 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1285 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1287 // Bonus for prior countermove that caused the fail low
1288 else if ( (depth >= 3 || PvNode)
1290 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1293 bestValue = std::min(bestValue, maxValue);
1296 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1297 bestValue >= beta ? BOUND_LOWER :
1298 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1299 depth, bestMove, ss->staticEval);
1301 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1307 // qsearch() is the quiescence search function, which is called by the main search
1308 // function with zero depth, or recursively with further decreasing depth per call.
1309 template <NodeType NT>
1310 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1312 constexpr bool PvNode = NT == PV;
1314 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1315 assert(PvNode || (alpha == beta - 1));
1322 Move ttMove, move, bestMove;
1324 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1325 bool ttHit, pvHit, inCheck, givesCheck, captureOrPromotion, evasionPrunable;
1330 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1332 ss->pv[0] = MOVE_NONE;
1335 Thread* thisThread = pos.this_thread();
1336 (ss+1)->ply = ss->ply + 1;
1337 bestMove = MOVE_NONE;
1338 inCheck = pos.checkers();
1341 // Check for an immediate draw or maximum ply reached
1342 if ( pos.is_draw(ss->ply)
1343 || ss->ply >= MAX_PLY)
1344 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1346 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1348 // Decide whether or not to include checks: this fixes also the type of
1349 // TT entry depth that we are going to use. Note that in qsearch we use
1350 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1351 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1352 : DEPTH_QS_NO_CHECKS;
1353 // Transposition table lookup
1355 tte = TT.probe(posKey, ttHit);
1356 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1357 ttMove = ttHit ? tte->move() : MOVE_NONE;
1358 pvHit = ttHit && tte->is_pv();
1362 && tte->depth() >= ttDepth
1363 && ttValue != VALUE_NONE // Only in case of TT access race
1364 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1365 : (tte->bound() & BOUND_UPPER)))
1368 // Evaluate the position statically
1371 ss->staticEval = VALUE_NONE;
1372 bestValue = futilityBase = -VALUE_INFINITE;
1378 // Never assume anything about values stored in TT
1379 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1380 ss->staticEval = bestValue = evaluate(pos);
1382 // Can ttValue be used as a better position evaluation?
1383 if ( ttValue != VALUE_NONE
1384 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1385 bestValue = ttValue;
1388 ss->staticEval = bestValue =
1389 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1390 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1392 // Stand pat. Return immediately if static value is at least beta
1393 if (bestValue >= beta)
1396 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1397 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1402 if (PvNode && bestValue > alpha)
1405 futilityBase = bestValue + 153;
1408 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1409 nullptr , (ss-4)->continuationHistory,
1410 nullptr , (ss-6)->continuationHistory };
1412 // Initialize a MovePicker object for the current position, and prepare
1413 // to search the moves. Because the depth is <= 0 here, only captures,
1414 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1416 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1417 &thisThread->captureHistory,
1419 to_sq((ss-1)->currentMove));
1421 // Loop through the moves until no moves remain or a beta cutoff occurs
1422 while ((move = mp.next_move()) != MOVE_NONE)
1424 assert(is_ok(move));
1426 givesCheck = pos.gives_check(move);
1427 captureOrPromotion = pos.capture_or_promotion(move);
1434 && futilityBase > -VALUE_KNOWN_WIN
1435 && !pos.advanced_pawn_push(move))
1437 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1439 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1441 if (futilityValue <= alpha)
1443 bestValue = std::max(bestValue, futilityValue);
1447 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1449 bestValue = std::max(bestValue, futilityBase);
1454 // Detect non-capture evasions that are candidates to be pruned
1455 evasionPrunable = inCheck
1456 && (depth != 0 || moveCount > 2)
1457 && bestValue > VALUE_MATED_IN_MAX_PLY
1458 && !pos.capture(move);
1460 // Don't search moves with negative SEE values
1461 if ( (!inCheck || evasionPrunable)
1462 && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
1463 && !pos.see_ge(move))
1466 // Speculative prefetch as early as possible
1467 prefetch(TT.first_entry(pos.key_after(move)));
1469 // Check for legality just before making the move
1470 if (!pos.legal(move))
1476 ss->currentMove = move;
1477 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1478 [captureOrPromotion]
1479 [pos.moved_piece(move)]
1482 // Make and search the move
1483 pos.do_move(move, st, givesCheck);
1484 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1485 pos.undo_move(move);
1487 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1489 // Check for a new best move
1490 if (value > bestValue)
1498 if (PvNode) // Update pv even in fail-high case
1499 update_pv(ss->pv, move, (ss+1)->pv);
1501 if (PvNode && value < beta) // Update alpha here!
1509 // All legal moves have been searched. A special case: If we're in check
1510 // and no legal moves were found, it is checkmate.
1511 if (inCheck && bestValue == -VALUE_INFINITE)
1512 return mated_in(ss->ply); // Plies to mate from the root
1514 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1515 bestValue >= beta ? BOUND_LOWER :
1516 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1517 ttDepth, bestMove, ss->staticEval);
1519 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1525 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1526 // "plies to mate from the current position". Non-mate scores are unchanged.
1527 // The function is called before storing a value in the transposition table.
1529 Value value_to_tt(Value v, int ply) {
1531 assert(v != VALUE_NONE);
1533 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1534 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1538 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1539 // from the transposition table (which refers to the plies to mate/be mated
1540 // from current position) to "plies to mate/be mated from the root".
1542 Value value_from_tt(Value v, int ply, int r50c) {
1544 return v == VALUE_NONE ? VALUE_NONE
1545 : v >= VALUE_MATE_IN_MAX_PLY ? VALUE_MATE - v > 99 - r50c ? VALUE_MATE_IN_MAX_PLY : v - ply
1546 : v <= VALUE_MATED_IN_MAX_PLY ? VALUE_MATE + v > 99 - r50c ? VALUE_MATED_IN_MAX_PLY : v + ply : v;
1550 // update_pv() adds current move and appends child pv[]
1552 void update_pv(Move* pv, Move move, Move* childPv) {
1554 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1560 // update_all_stats() updates stats at the end of search() when a bestMove is found
1562 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1563 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1566 Color us = pos.side_to_move();
1567 Thread* thisThread = pos.this_thread();
1568 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1569 Piece moved_piece = pos.moved_piece(bestMove);
1570 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1572 bonus1 = stat_bonus(depth + 1);
1573 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1574 : stat_bonus(depth); // smaller bonus
1576 if (!pos.capture_or_promotion(bestMove))
1578 update_quiet_stats(pos, ss, bestMove, bonus2);
1580 // Decrease all the non-best quiet moves
1581 for (int i = 0; i < quietCount; ++i)
1583 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1584 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1588 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1590 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1591 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1592 && !pos.captured_piece())
1593 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1595 // Decrease all the non-best capture moves
1596 for (int i = 0; i < captureCount; ++i)
1598 moved_piece = pos.moved_piece(capturesSearched[i]);
1599 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1600 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1605 // update_continuation_histories() updates histories of the move pairs formed
1606 // by moves at ply -1, -2, and -4 with current move.
1608 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1610 for (int i : {1, 2, 4, 6})
1611 if (is_ok((ss-i)->currentMove))
1612 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1616 // update_quiet_stats() updates move sorting heuristics
1618 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1620 if (ss->killers[0] != move)
1622 ss->killers[1] = ss->killers[0];
1623 ss->killers[0] = move;
1626 Color us = pos.side_to_move();
1627 Thread* thisThread = pos.this_thread();
1628 thisThread->mainHistory[us][from_to(move)] << bonus;
1629 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1631 if (type_of(pos.moved_piece(move)) != PAWN)
1632 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1634 if (is_ok((ss-1)->currentMove))
1636 Square prevSq = to_sq((ss-1)->currentMove);
1637 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1641 // When playing with strength handicap, choose best move among a set of RootMoves
1642 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1644 Move Skill::pick_best(size_t multiPV) {
1646 const RootMoves& rootMoves = Threads.main()->rootMoves;
1647 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1649 // RootMoves are already sorted by score in descending order
1650 Value topScore = rootMoves[0].score;
1651 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1652 int weakness = 120 - 2 * level;
1653 int maxScore = -VALUE_INFINITE;
1655 // Choose best move. For each move score we add two terms, both dependent on
1656 // weakness. One is deterministic and bigger for weaker levels, and one is
1657 // random. Then we choose the move with the resulting highest score.
1658 for (size_t i = 0; i < multiPV; ++i)
1660 // This is our magic formula
1661 int push = ( weakness * int(topScore - rootMoves[i].score)
1662 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1664 if (rootMoves[i].score + push >= maxScore)
1666 maxScore = rootMoves[i].score + push;
1667 best = rootMoves[i].pv[0];
1676 /// MainThread::check_time() is used to print debug info and, more importantly,
1677 /// to detect when we are out of available time and thus stop the search.
1679 void MainThread::check_time() {
1684 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1685 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1687 static TimePoint lastInfoTime = now();
1689 TimePoint elapsed = Time.elapsed();
1690 TimePoint tick = Limits.startTime + elapsed;
1692 if (tick - lastInfoTime >= 1000)
1694 lastInfoTime = tick;
1698 // We should not stop pondering until told so by the GUI
1702 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1703 || (Limits.movetime && elapsed >= Limits.movetime)
1704 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1705 Threads.stop = true;
1709 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1710 /// that all (if any) unsearched PV lines are sent using a previous search score.
1712 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1714 std::stringstream ss;
1715 TimePoint elapsed = Time.elapsed() + 1;
1716 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1717 size_t pvIdx = pos.this_thread()->pvIdx;
1718 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1719 uint64_t nodesSearched = Threads.nodes_searched();
1720 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1722 for (size_t i = 0; i < multiPV; ++i)
1724 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1726 if (depth == 1 && !updated)
1729 Depth d = updated ? depth : depth - 1;
1730 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1732 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1733 v = tb ? rootMoves[i].tbScore : v;
1735 if (ss.rdbuf()->in_avail()) // Not at first line
1740 << " seldepth " << rootMoves[i].selDepth
1741 << " multipv " << i + 1
1742 << " score " << UCI::value(v);
1744 if (!tb && i == pvIdx)
1745 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1747 ss << " nodes " << nodesSearched
1748 << " nps " << nodesSearched * 1000 / elapsed;
1750 if (elapsed > 1000) // Earlier makes little sense
1751 ss << " hashfull " << TT.hashfull();
1753 ss << " tbhits " << tbHits
1754 << " time " << elapsed
1757 for (Move m : rootMoves[i].pv)
1758 ss << " " << UCI::move(m, pos.is_chess960());
1765 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1766 /// before exiting the search, for instance, in case we stop the search during a
1767 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1768 /// otherwise in case of 'ponder on' we have nothing to think on.
1770 bool RootMove::extract_ponder_from_tt(Position& pos) {
1775 assert(pv.size() == 1);
1777 if (pv[0] == MOVE_NONE)
1780 pos.do_move(pv[0], st);
1781 TTEntry* tte = TT.probe(pos.key(), ttHit);
1785 Move m = tte->move(); // Local copy to be SMP safe
1786 if (MoveList<LEGAL>(pos).contains(m))
1790 pos.undo_move(pv[0]);
1791 return pv.size() > 1;
1794 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1797 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1798 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1799 Cardinality = int(Options["SyzygyProbeLimit"]);
1800 bool dtz_available = true;
1802 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1803 // ProbeDepth == DEPTH_ZERO
1804 if (Cardinality > MaxCardinality)
1806 Cardinality = MaxCardinality;
1810 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1812 // Rank moves using DTZ tables
1813 RootInTB = root_probe(pos, rootMoves);
1817 // DTZ tables are missing; try to rank moves using WDL tables
1818 dtz_available = false;
1819 RootInTB = root_probe_wdl(pos, rootMoves);
1825 // Sort moves according to TB rank
1826 std::sort(rootMoves.begin(), rootMoves.end(),
1827 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1829 // Probe during search only if DTZ is not available and we are winning
1830 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1835 // Clean up if root_probe() and root_probe_wdl() have failed
1836 for (auto& m : rootMoves)