2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Razor and futility margins
65 constexpr int RazorMargin = 661;
66 Value futility_margin(Depth d, bool improving) {
67 return Value(198 * (d - improving));
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d] * Reductions[mn];
75 return (r + 520) / 1024 + (!i && r > 999);
78 constexpr int futility_move_count(bool improving, Depth depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth d) {
84 return d > 17 ? -8 : 22 * d * d + 151 * d - 140;
87 // Add a small random component to draw evaluations to avoid 3fold-blindness
88 Value value_draw(Thread* thisThread) {
89 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
92 // Skill structure is used to implement strength limit
94 explicit Skill(int l) : level(l) {}
95 bool enabled() const { return level < 20; }
96 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
97 Move pick_best(size_t multiPV);
100 Move best = MOVE_NONE;
103 // Breadcrumbs are used to mark nodes as being searched by a given thread
105 std::atomic<Thread*> thread;
106 std::atomic<Key> key;
108 std::array<Breadcrumb, 1024> breadcrumbs;
110 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
111 // node for potential reductions. A free node will be marked upon entering the moves
112 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
113 struct ThreadHolding {
114 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
115 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
120 // See if another already marked this location, if not, mark it ourselves
121 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
124 (*location).thread.store(thisThread, std::memory_order_relaxed);
125 (*location).key.store(posKey, std::memory_order_relaxed);
128 else if ( tmp != thisThread
129 && (*location).key.load(std::memory_order_relaxed) == posKey)
135 if (owning) // Free the marked location
136 (*location).thread.store(nullptr, std::memory_order_relaxed);
139 bool marked() { return otherThread; }
142 Breadcrumb* location;
143 bool otherThread, owning;
146 template <NodeType NT>
147 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
149 template <NodeType NT>
150 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
152 Value value_to_tt(Value v, int ply);
153 Value value_from_tt(Value v, int ply);
154 void update_pv(Move* pv, Move move, Move* childPv);
155 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
156 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
157 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
158 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
160 // perft() is our utility to verify move generation. All the leaf nodes up
161 // to the given depth are generated and counted, and the sum is returned.
163 uint64_t perft(Position& pos, Depth depth) {
166 uint64_t cnt, nodes = 0;
167 const bool leaf = (depth == 2);
169 for (const auto& m : MoveList<LEGAL>(pos))
171 if (Root && depth <= 1)
176 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
181 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
189 /// Search::init() is called at startup to initialize various lookup tables
191 void Search::init() {
193 for (int i = 1; i < MAX_MOVES; ++i)
194 Reductions[i] = int((23.4 + std::log(Threads.size()) / 2) * std::log(i));
198 /// Search::clear() resets search state to its initial value
200 void Search::clear() {
202 Threads.main()->wait_for_search_finished();
204 Time.availableNodes = 0;
207 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
211 /// MainThread::search() is started when the program receives the UCI 'go'
212 /// command. It searches from the root position and outputs the "bestmove".
214 void MainThread::search() {
218 nodes = perft<true>(rootPos, Limits.perft);
219 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
223 Color us = rootPos.side_to_move();
224 Time.init(Limits, us, rootPos.game_ply());
227 if (rootMoves.empty())
229 rootMoves.emplace_back(MOVE_NONE);
230 sync_cout << "info depth 0 score "
231 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
236 for (Thread* th : Threads)
238 th->bestMoveChanges = 0;
240 th->start_searching();
243 Thread::search(); // Let's start searching!
246 // When we reach the maximum depth, we can arrive here without a raise of
247 // Threads.stop. However, if we are pondering or in an infinite search,
248 // the UCI protocol states that we shouldn't print the best move before the
249 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
250 // until the GUI sends one of those commands.
252 while (!Threads.stop && (ponder || Limits.infinite))
253 {} // Busy wait for a stop or a ponder reset
255 // Stop the threads if not already stopped (also raise the stop if
256 // "ponderhit" just reset Threads.ponder).
259 // Wait until all threads have finished
260 for (Thread* th : Threads)
262 th->wait_for_search_finished();
264 // When playing in 'nodes as time' mode, subtract the searched nodes from
265 // the available ones before exiting.
267 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
269 Thread* bestThread = this;
271 // Check if there are threads with a better score than main thread
272 if ( Options["MultiPV"] == 1
274 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
275 && rootMoves[0].pv[0] != MOVE_NONE)
277 std::map<Move, int64_t> votes;
278 Value minScore = this->rootMoves[0].score;
280 // Find out minimum score
281 for (Thread* th: Threads)
282 minScore = std::min(minScore, th->rootMoves[0].score);
284 // Vote according to score and depth, and select the best thread
285 for (Thread* th : Threads)
287 votes[th->rootMoves[0].pv[0]] +=
288 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
290 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
292 // Make sure we pick the shortest mate
293 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
296 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
297 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
302 previousScore = bestThread->rootMoves[0].score;
304 // Send again PV info if we have a new best thread
305 if (bestThread != this)
306 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
308 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
310 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
311 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
313 std::cout << sync_endl;
317 /// Thread::search() is the main iterative deepening loop. It calls search()
318 /// repeatedly with increasing depth until the allocated thinking time has been
319 /// consumed, the user stops the search, or the maximum search depth is reached.
321 void Thread::search() {
323 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
324 // The former is needed to allow update_continuation_histories(ss-1, ...),
325 // which accesses its argument at ss-6, also near the root.
326 // The latter is needed for statScores and killer initialization.
327 Stack stack[MAX_PLY+10], *ss = stack+7;
329 Value bestValue, alpha, beta, delta;
330 Move lastBestMove = MOVE_NONE;
331 Depth lastBestMoveDepth = 0;
332 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
333 double timeReduction = 1, totBestMoveChanges = 0;
334 Color us = rootPos.side_to_move();
336 std::memset(ss-7, 0, 10 * sizeof(Stack));
337 for (int i = 7; i > 0; i--)
338 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
342 bestValue = delta = alpha = -VALUE_INFINITE;
343 beta = VALUE_INFINITE;
345 size_t multiPV = Options["MultiPV"];
347 // Pick integer skill levels, but non-deterministically round up or down
348 // such that the average integer skill corresponds to the input floating point one.
349 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
350 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
351 // for match (TC 60+0.6) results spanning a wide range of k values.
353 double floatLevel = Options["UCI_LimitStrength"] ?
354 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
355 double(Options["Skill Level"]);
356 int intLevel = int(floatLevel) +
357 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
358 Skill skill(intLevel);
360 // When playing with strength handicap enable MultiPV search that we will
361 // use behind the scenes to retrieve a set of possible moves.
363 multiPV = std::max(multiPV, (size_t)4);
365 multiPV = std::min(multiPV, rootMoves.size());
367 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
369 // In analysis mode, adjust contempt in accordance with user preference
370 if (Limits.infinite || Options["UCI_AnalyseMode"])
371 ct = Options["Analysis Contempt"] == "Off" ? 0
372 : Options["Analysis Contempt"] == "Both" ? ct
373 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
374 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
377 // Evaluation score is from the white point of view
378 contempt = (us == WHITE ? make_score(ct, ct / 2)
379 : -make_score(ct, ct / 2));
381 // Iterative deepening loop until requested to stop or the target depth is reached
382 while ( ++rootDepth < MAX_PLY
384 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
386 // Age out PV variability metric
388 totBestMoveChanges /= 2;
390 // Save the last iteration's scores before first PV line is searched and
391 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
392 for (RootMove& rm : rootMoves)
393 rm.previousScore = rm.score;
398 // MultiPV loop. We perform a full root search for each PV line
399 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
404 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
405 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
409 // Reset UCI info selDepth for each depth and each PV line
412 // Reset aspiration window starting size
415 Value previousScore = rootMoves[pvIdx].previousScore;
416 delta = Value(21 + abs(previousScore) / 128);
417 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
418 beta = std::min(previousScore + delta, VALUE_INFINITE);
420 // Adjust contempt based on root move's previousScore (dynamic contempt)
421 int dct = ct + (111 - ct / 2) * previousScore / (abs(previousScore) + 176);
423 contempt = (us == WHITE ? make_score(dct, dct / 2)
424 : -make_score(dct, dct / 2));
427 // Start with a small aspiration window and, in the case of a fail
428 // high/low, re-search with a bigger window until we don't fail
430 int failedHighCnt = 0;
433 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt);
434 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
436 // Bring the best move to the front. It is critical that sorting
437 // is done with a stable algorithm because all the values but the
438 // first and eventually the new best one are set to -VALUE_INFINITE
439 // and we want to keep the same order for all the moves except the
440 // new PV that goes to the front. Note that in case of MultiPV
441 // search the already searched PV lines are preserved.
442 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
444 // If search has been stopped, we break immediately. Sorting is
445 // safe because RootMoves is still valid, although it refers to
446 // the previous iteration.
450 // When failing high/low give some update (without cluttering
451 // the UI) before a re-search.
454 && (bestValue <= alpha || bestValue >= beta)
455 && Time.elapsed() > 3000)
456 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
458 // In case of failing low/high increase aspiration window and
459 // re-search, otherwise exit the loop.
460 if (bestValue <= alpha)
462 beta = (alpha + beta) / 2;
463 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
467 mainThread->stopOnPonderhit = false;
469 else if (bestValue >= beta)
471 beta = std::min(bestValue + delta, VALUE_INFINITE);
476 ++rootMoves[pvIdx].bestMoveCount;
480 delta += delta / 4 + 5;
482 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
485 // Sort the PV lines searched so far and update the GUI
486 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
489 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
490 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
494 completedDepth = rootDepth;
496 if (rootMoves[0].pv[0] != lastBestMove) {
497 lastBestMove = rootMoves[0].pv[0];
498 lastBestMoveDepth = rootDepth;
501 // Have we found a "mate in x"?
503 && bestValue >= VALUE_MATE_IN_MAX_PLY
504 && VALUE_MATE - bestValue <= 2 * Limits.mate)
510 // If skill level is enabled and time is up, pick a sub-optimal best move
511 if (skill.enabled() && skill.time_to_pick(rootDepth))
512 skill.pick_best(multiPV);
514 // Do we have time for the next iteration? Can we stop searching now?
515 if ( Limits.use_time_management()
517 && !mainThread->stopOnPonderhit)
519 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
520 fallingEval = clamp(fallingEval, 0.5, 1.5);
522 // If the bestMove is stable over several iterations, reduce time accordingly
523 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.97 : 0.98;
524 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
526 // Use part of the gained time from a previous stable move for the current move
527 for (Thread* th : Threads)
529 totBestMoveChanges += th->bestMoveChanges;
530 th->bestMoveChanges = 0;
532 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
534 // Stop the search if we have only one legal move, or if available time elapsed
535 if ( rootMoves.size() == 1
536 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
538 // If we are allowed to ponder do not stop the search now but
539 // keep pondering until the GUI sends "ponderhit" or "stop".
540 if (mainThread->ponder)
541 mainThread->stopOnPonderhit = true;
551 mainThread->previousTimeReduction = timeReduction;
553 // If skill level is enabled, swap best PV line with the sub-optimal one
555 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
556 skill.best ? skill.best : skill.pick_best(multiPV)));
562 // search<>() is the main search function for both PV and non-PV nodes
564 template <NodeType NT>
565 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
567 constexpr bool PvNode = NT == PV;
568 const bool rootNode = PvNode && ss->ply == 0;
570 // Check if we have an upcoming move which draws by repetition, or
571 // if the opponent had an alternative move earlier to this position.
572 if ( pos.rule50_count() >= 3
573 && alpha < VALUE_DRAW
575 && pos.has_game_cycle(ss->ply))
577 alpha = value_draw(pos.this_thread());
582 // Dive into quiescence search when the depth reaches zero
584 return qsearch<NT>(pos, ss, alpha, beta);
586 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
587 assert(PvNode || (alpha == beta - 1));
588 assert(0 < depth && depth < MAX_PLY);
589 assert(!(PvNode && cutNode));
591 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
595 Move ttMove, move, excludedMove, bestMove;
596 Depth extension, newDepth;
597 Value bestValue, value, ttValue, eval, maxValue;
598 bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
599 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
601 int moveCount, captureCount, quietCount;
603 // Step 1. Initialize node
604 Thread* thisThread = pos.this_thread();
605 inCheck = pos.checkers();
606 priorCapture = pos.captured_piece();
607 Color us = pos.side_to_move();
608 moveCount = captureCount = quietCount = ss->moveCount = 0;
609 bestValue = -VALUE_INFINITE;
610 maxValue = VALUE_INFINITE;
612 // Check for the available remaining time
613 if (thisThread == Threads.main())
614 static_cast<MainThread*>(thisThread)->check_time();
616 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
617 if (PvNode && thisThread->selDepth < ss->ply + 1)
618 thisThread->selDepth = ss->ply + 1;
622 // Step 2. Check for aborted search and immediate draw
623 if ( Threads.stop.load(std::memory_order_relaxed)
624 || pos.is_draw(ss->ply)
625 || ss->ply >= MAX_PLY)
626 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
627 : value_draw(pos.this_thread());
629 // Step 3. Mate distance pruning. Even if we mate at the next move our score
630 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
631 // a shorter mate was found upward in the tree then there is no need to search
632 // because we will never beat the current alpha. Same logic but with reversed
633 // signs applies also in the opposite condition of being mated instead of giving
634 // mate. In this case return a fail-high score.
635 alpha = std::max(mated_in(ss->ply), alpha);
636 beta = std::min(mate_in(ss->ply+1), beta);
641 assert(0 <= ss->ply && ss->ply < MAX_PLY);
643 (ss+1)->ply = ss->ply + 1;
644 (ss+1)->excludedMove = bestMove = MOVE_NONE;
645 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
646 Square prevSq = to_sq((ss-1)->currentMove);
648 // Initialize statScore to zero for the grandchildren of the current position.
649 // So statScore is shared between all grandchildren and only the first grandchild
650 // starts with statScore = 0. Later grandchildren start with the last calculated
651 // statScore of the previous grandchild. This influences the reduction rules in
652 // LMR which are based on the statScore of parent position.
654 (ss+4)->statScore = 0;
656 (ss+2)->statScore = 0;
658 // Step 4. Transposition table lookup. We don't want the score of a partial
659 // search to overwrite a previous full search TT value, so we use a different
660 // position key in case of an excluded move.
661 excludedMove = ss->excludedMove;
662 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
663 tte = TT.probe(posKey, ttHit);
664 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
665 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
666 : ttHit ? tte->move() : MOVE_NONE;
667 ttPv = PvNode || (ttHit && tte->is_pv());
669 // At non-PV nodes we check for an early TT cutoff
672 && tte->depth() >= depth
673 && ttValue != VALUE_NONE // Possible in case of TT access race
674 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
675 : (tte->bound() & BOUND_UPPER)))
677 // If ttMove is quiet, update move sorting heuristics on TT hit
682 if (!pos.capture_or_promotion(ttMove))
683 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
685 // Extra penalty for early quiet moves of the previous ply
686 if ((ss-1)->moveCount <= 2 && !priorCapture)
687 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
689 // Penalty for a quiet ttMove that fails low
690 else if (!pos.capture_or_promotion(ttMove))
692 int penalty = -stat_bonus(depth);
693 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
694 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
700 // Step 5. Tablebases probe
701 if (!rootNode && TB::Cardinality)
703 int piecesCount = pos.count<ALL_PIECES>();
705 if ( piecesCount <= TB::Cardinality
706 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
707 && pos.rule50_count() == 0
708 && !pos.can_castle(ANY_CASTLING))
711 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
713 // Force check of time on the next occasion
714 if (thisThread == Threads.main())
715 static_cast<MainThread*>(thisThread)->callsCnt = 0;
717 if (err != TB::ProbeState::FAIL)
719 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
721 int drawScore = TB::UseRule50 ? 1 : 0;
723 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
724 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
725 : VALUE_DRAW + 2 * wdl * drawScore;
727 Bound b = wdl < -drawScore ? BOUND_UPPER
728 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
730 if ( b == BOUND_EXACT
731 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
733 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
734 std::min(MAX_PLY - 1, depth + 6),
735 MOVE_NONE, VALUE_NONE);
742 if (b == BOUND_LOWER)
743 bestValue = value, alpha = std::max(alpha, bestValue);
751 // Step 6. Static evaluation of the position
754 ss->staticEval = eval = VALUE_NONE;
756 goto moves_loop; // Skip early pruning when in check
760 // Never assume anything about values stored in TT
761 ss->staticEval = eval = tte->eval();
762 if (eval == VALUE_NONE)
763 ss->staticEval = eval = evaluate(pos);
765 if (eval == VALUE_DRAW)
766 eval = value_draw(thisThread);
768 // Can ttValue be used as a better position evaluation?
769 if ( ttValue != VALUE_NONE
770 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
775 if ((ss-1)->currentMove != MOVE_NULL)
777 int bonus = -(ss-1)->statScore / 512;
779 ss->staticEval = eval = evaluate(pos) + bonus;
782 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
784 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
787 // Step 7. Razoring (~2 Elo)
788 if ( !rootNode // The required rootNode PV handling is not available in qsearch
790 && eval <= alpha - RazorMargin)
791 return qsearch<NT>(pos, ss, alpha, beta);
793 improving = ss->staticEval >= (ss-2)->staticEval
794 || (ss-2)->staticEval == VALUE_NONE;
796 // Step 8. Futility pruning: child node (~30 Elo)
799 && eval - futility_margin(depth, improving) >= beta
800 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
803 // Step 9. Null move search with verification search (~40 Elo)
805 && (ss-1)->currentMove != MOVE_NULL
806 && (ss-1)->statScore < 22661
808 && eval >= ss->staticEval
809 && ss->staticEval >= beta - 33 * depth + 299 - improving * 30
811 && pos.non_pawn_material(us)
812 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
814 assert(eval - beta >= 0);
816 // Null move dynamic reduction based on depth and value
817 Depth R = (835 + 70 * depth) / 256 + std::min(int(eval - beta) / 185, 3);
819 ss->currentMove = MOVE_NULL;
820 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
822 pos.do_null_move(st);
824 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
826 pos.undo_null_move();
828 if (nullValue >= beta)
830 // Do not return unproven mate scores
831 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
834 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
837 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
839 // Do verification search at high depths, with null move pruning disabled
840 // for us, until ply exceeds nmpMinPly.
841 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
842 thisThread->nmpColor = us;
844 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
846 thisThread->nmpMinPly = 0;
853 // Step 10. ProbCut (~10 Elo)
854 // If we have a good enough capture and a reduced search returns a value
855 // much above beta, we can (almost) safely prune the previous move.
858 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
860 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
861 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
862 int probCutCount = 0;
864 while ( (move = mp.next_move()) != MOVE_NONE
865 && probCutCount < 2 + 2 * cutNode)
866 if (move != excludedMove && pos.legal(move))
868 assert(pos.capture_or_promotion(move));
871 captureOrPromotion = true;
874 ss->currentMove = move;
875 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
877 [pos.moved_piece(move)]
880 pos.do_move(move, st);
882 // Perform a preliminary qsearch to verify that the move holds
883 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
885 // If the qsearch held, perform the regular search
886 if (value >= raisedBeta)
887 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
891 if (value >= raisedBeta)
896 // Step 11. Internal iterative deepening (~2 Elo)
897 if (depth >= 7 && !ttMove)
899 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
901 tte = TT.probe(posKey, ttHit);
902 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
903 ttMove = ttHit ? tte->move() : MOVE_NONE;
906 moves_loop: // When in check, search starts from here
908 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
909 nullptr , (ss-4)->continuationHistory,
910 nullptr , (ss-6)->continuationHistory };
912 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
914 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
915 &thisThread->captureHistory,
921 singularLMR = moveCountPruning = false;
922 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
924 // Mark this node as being searched
925 ThreadHolding th(thisThread, posKey, ss->ply);
927 // Step 12. Loop through all pseudo-legal moves until no moves remain
928 // or a beta cutoff occurs.
929 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
933 if (move == excludedMove)
936 // At root obey the "searchmoves" option and skip moves not listed in Root
937 // Move List. As a consequence any illegal move is also skipped. In MultiPV
938 // mode we also skip PV moves which have been already searched and those
939 // of lower "TB rank" if we are in a TB root position.
940 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
941 thisThread->rootMoves.begin() + thisThread->pvLast, move))
944 ss->moveCount = ++moveCount;
946 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
947 sync_cout << "info depth " << depth
948 << " currmove " << UCI::move(move, pos.is_chess960())
949 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
951 (ss+1)->pv = nullptr;
954 captureOrPromotion = pos.capture_or_promotion(move);
955 movedPiece = pos.moved_piece(move);
956 givesCheck = pos.gives_check(move);
958 // Step 13. Extensions (~70 Elo)
960 // Singular extension search (~60 Elo). If all moves but one fail low on a
961 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
962 // then that move is singular and should be extended. To verify this we do
963 // a reduced search on all the other moves but the ttMove and if the
964 // result is lower than ttValue minus a margin then we will extend the ttMove.
968 && !excludedMove // Avoid recursive singular search
969 /* && ttValue != VALUE_NONE Already implicit in the next condition */
970 && abs(ttValue) < VALUE_KNOWN_WIN
971 && (tte->bound() & BOUND_LOWER)
972 && tte->depth() >= depth - 3
975 Value singularBeta = ttValue - 2 * depth;
976 Depth halfDepth = depth / 2;
977 ss->excludedMove = move;
978 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
979 ss->excludedMove = MOVE_NONE;
981 if (value < singularBeta)
988 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
989 // search without the ttMove. So we assume this expected Cut-node is not singular,
990 // that multiple moves fail high, and we can prune the whole subtree by returning
992 else if ( eval >= beta
993 && singularBeta >= beta)
997 // Check extension (~2 Elo)
999 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1002 // Passed pawn extension
1003 else if ( move == ss->killers[0]
1004 && pos.advanced_pawn_push(move)
1005 && pos.pawn_passed(us, to_sq(move)))
1008 // Castling extension
1009 if (type_of(move) == CASTLING)
1012 // Calculate new depth for this move
1013 newDepth = depth - 1 + extension;
1015 // Step 14. Pruning at shallow depth (~170 Elo)
1017 && pos.non_pawn_material(us)
1018 && bestValue > VALUE_MATED_IN_MAX_PLY)
1020 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1021 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1023 if ( !captureOrPromotion
1025 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1027 // Move count based pruning
1028 if (moveCountPruning)
1031 // Reduced depth of the next LMR search
1032 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1034 // Countermoves based pruning (~20 Elo)
1035 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1036 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1037 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1040 // Futility pruning: parent node (~2 Elo)
1043 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
1046 // Prune moves with negative SEE (~10 Elo)
1047 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1050 else if ( !(givesCheck && extension)
1051 && !pos.see_ge(move, Value(-199) * depth)) // (~20 Elo)
1055 // Speculative prefetch as early as possible
1056 prefetch(TT.first_entry(pos.key_after(move)));
1058 // Check for legality just before making the move
1059 if (!rootNode && !pos.legal(move))
1061 ss->moveCount = --moveCount;
1065 // Update the current move (this must be done after singular extension search)
1066 ss->currentMove = move;
1067 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1068 [captureOrPromotion]
1072 // Step 15. Make the move
1073 pos.do_move(move, st, givesCheck);
1075 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1076 // re-searched at full depth.
1078 && moveCount > 1 + 2 * rootNode
1079 && (!rootNode || thisThread->best_move_count(move) == 0)
1080 && ( !captureOrPromotion
1082 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1085 Depth r = reduction(improving, depth, moveCount);
1087 // Reduction if other threads are searching this position.
1091 // Decrease reduction if position is or has been on the PV
1095 // Decrease reduction if opponent's move count is high (~10 Elo)
1096 if ((ss-1)->moveCount > 15)
1099 // Decrease reduction if ttMove has been singularly extended
1103 if (!captureOrPromotion)
1105 // Increase reduction if ttMove is a capture (~0 Elo)
1109 // Increase reduction for cut nodes (~5 Elo)
1113 // Decrease reduction for moves that escape a capture. Filter out
1114 // castling moves, because they are coded as "king captures rook" and
1115 // hence break make_move(). (~5 Elo)
1116 else if ( type_of(move) == NORMAL
1117 && !pos.see_ge(reverse_move(move)))
1120 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1121 + (*contHist[0])[movedPiece][to_sq(move)]
1122 + (*contHist[1])[movedPiece][to_sq(move)]
1123 + (*contHist[3])[movedPiece][to_sq(move)]
1126 // Reset statScore to zero if negative and most stats shows >= 0
1127 if ( ss->statScore < 0
1128 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1129 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1130 && thisThread->mainHistory[us][from_to(move)] >= 0)
1133 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1134 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1137 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1140 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1141 r -= ss->statScore / 16384;
1144 Depth d = clamp(newDepth - r, 1, newDepth);
1146 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1148 doFullDepthSearch = (value > alpha && d != newDepth), didLMR = true;
1151 doFullDepthSearch = !PvNode || moveCount > 1, didLMR = false;
1153 // Step 17. Full depth search when LMR is skipped or fails high
1154 if (doFullDepthSearch)
1156 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1158 if (didLMR && !captureOrPromotion)
1160 int bonus = value > alpha ? stat_bonus(newDepth)
1161 : -stat_bonus(newDepth);
1163 if (move == ss->killers[0])
1166 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1170 // For PV nodes only, do a full PV search on the first move or after a fail
1171 // high (in the latter case search only if value < beta), otherwise let the
1172 // parent node fail low with value <= alpha and try another move.
1173 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1176 (ss+1)->pv[0] = MOVE_NONE;
1178 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1181 // Step 18. Undo move
1182 pos.undo_move(move);
1184 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1186 // Step 19. Check for a new best move
1187 // Finished searching the move. If a stop occurred, the return value of
1188 // the search cannot be trusted, and we return immediately without
1189 // updating best move, PV and TT.
1190 if (Threads.stop.load(std::memory_order_relaxed))
1195 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1196 thisThread->rootMoves.end(), move);
1198 // PV move or new best move?
1199 if (moveCount == 1 || value > alpha)
1202 rm.selDepth = thisThread->selDepth;
1207 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1208 rm.pv.push_back(*m);
1210 // We record how often the best move has been changed in each
1211 // iteration. This information is used for time management: When
1212 // the best move changes frequently, we allocate some more time.
1214 ++thisThread->bestMoveChanges;
1217 // All other moves but the PV are set to the lowest value: this
1218 // is not a problem when sorting because the sort is stable and the
1219 // move position in the list is preserved - just the PV is pushed up.
1220 rm.score = -VALUE_INFINITE;
1223 if (value > bestValue)
1231 if (PvNode && !rootNode) // Update pv even in fail-high case
1232 update_pv(ss->pv, move, (ss+1)->pv);
1234 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1238 assert(value >= beta); // Fail high
1245 if (move != bestMove)
1247 if (captureOrPromotion && captureCount < 32)
1248 capturesSearched[captureCount++] = move;
1250 else if (!captureOrPromotion && quietCount < 64)
1251 quietsSearched[quietCount++] = move;
1255 // The following condition would detect a stop only after move loop has been
1256 // completed. But in this case bestValue is valid because we have fully
1257 // searched our subtree, and we can anyhow save the result in TT.
1263 // Step 20. Check for mate and stalemate
1264 // All legal moves have been searched and if there are no legal moves, it
1265 // must be a mate or a stalemate. If we are in a singular extension search then
1266 // return a fail low score.
1268 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1271 bestValue = excludedMove ? alpha
1272 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1275 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1276 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1278 // Bonus for prior countermove that caused the fail low
1279 else if ( (depth >= 3 || PvNode)
1281 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1284 bestValue = std::min(bestValue, maxValue);
1287 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1288 bestValue >= beta ? BOUND_LOWER :
1289 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1290 depth, bestMove, ss->staticEval);
1292 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1298 // qsearch() is the quiescence search function, which is called by the main search
1299 // function with zero depth, or recursively with further decreasing depth per call.
1300 template <NodeType NT>
1301 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1303 constexpr bool PvNode = NT == PV;
1305 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1306 assert(PvNode || (alpha == beta - 1));
1313 Move ttMove, move, bestMove;
1315 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1316 bool ttHit, pvHit, inCheck, givesCheck, captureOrPromotion, evasionPrunable;
1321 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1323 ss->pv[0] = MOVE_NONE;
1326 Thread* thisThread = pos.this_thread();
1327 (ss+1)->ply = ss->ply + 1;
1328 bestMove = MOVE_NONE;
1329 inCheck = pos.checkers();
1332 // Check for an immediate draw or maximum ply reached
1333 if ( pos.is_draw(ss->ply)
1334 || ss->ply >= MAX_PLY)
1335 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1337 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1339 // Decide whether or not to include checks: this fixes also the type of
1340 // TT entry depth that we are going to use. Note that in qsearch we use
1341 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1342 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1343 : DEPTH_QS_NO_CHECKS;
1344 // Transposition table lookup
1346 tte = TT.probe(posKey, ttHit);
1347 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1348 ttMove = ttHit ? tte->move() : MOVE_NONE;
1349 pvHit = ttHit && tte->is_pv();
1353 && tte->depth() >= ttDepth
1354 && ttValue != VALUE_NONE // Only in case of TT access race
1355 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1356 : (tte->bound() & BOUND_UPPER)))
1359 // Evaluate the position statically
1362 ss->staticEval = VALUE_NONE;
1363 bestValue = futilityBase = -VALUE_INFINITE;
1369 // Never assume anything about values stored in TT
1370 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1371 ss->staticEval = bestValue = evaluate(pos);
1373 // Can ttValue be used as a better position evaluation?
1374 if ( ttValue != VALUE_NONE
1375 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1376 bestValue = ttValue;
1379 ss->staticEval = bestValue =
1380 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1381 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1383 // Stand pat. Return immediately if static value is at least beta
1384 if (bestValue >= beta)
1387 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1388 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1393 if (PvNode && bestValue > alpha)
1396 futilityBase = bestValue + 153;
1399 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1400 nullptr , (ss-4)->continuationHistory,
1401 nullptr , (ss-6)->continuationHistory };
1403 // Initialize a MovePicker object for the current position, and prepare
1404 // to search the moves. Because the depth is <= 0 here, only captures,
1405 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1407 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1408 &thisThread->captureHistory,
1410 to_sq((ss-1)->currentMove));
1412 // Loop through the moves until no moves remain or a beta cutoff occurs
1413 while ((move = mp.next_move()) != MOVE_NONE)
1415 assert(is_ok(move));
1417 givesCheck = pos.gives_check(move);
1418 captureOrPromotion = pos.capture_or_promotion(move);
1425 && futilityBase > -VALUE_KNOWN_WIN
1426 && !pos.advanced_pawn_push(move))
1428 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1430 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1432 if (futilityValue <= alpha)
1434 bestValue = std::max(bestValue, futilityValue);
1438 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1440 bestValue = std::max(bestValue, futilityBase);
1445 // Detect non-capture evasions that are candidates to be pruned
1446 evasionPrunable = inCheck
1447 && (depth != 0 || moveCount > 2)
1448 && bestValue > VALUE_MATED_IN_MAX_PLY
1449 && !pos.capture(move);
1451 // Don't search moves with negative SEE values
1452 if ( (!inCheck || evasionPrunable)
1453 && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
1454 && !pos.see_ge(move))
1457 // Speculative prefetch as early as possible
1458 prefetch(TT.first_entry(pos.key_after(move)));
1460 // Check for legality just before making the move
1461 if (!pos.legal(move))
1467 ss->currentMove = move;
1468 ss->continuationHistory = &thisThread->continuationHistory[inCheck]
1469 [captureOrPromotion]
1470 [pos.moved_piece(move)]
1473 // Make and search the move
1474 pos.do_move(move, st, givesCheck);
1475 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1476 pos.undo_move(move);
1478 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1480 // Check for a new best move
1481 if (value > bestValue)
1489 if (PvNode) // Update pv even in fail-high case
1490 update_pv(ss->pv, move, (ss+1)->pv);
1492 if (PvNode && value < beta) // Update alpha here!
1500 // All legal moves have been searched. A special case: If we're in check
1501 // and no legal moves were found, it is checkmate.
1502 if (inCheck && bestValue == -VALUE_INFINITE)
1503 return mated_in(ss->ply); // Plies to mate from the root
1505 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1506 bestValue >= beta ? BOUND_LOWER :
1507 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1508 ttDepth, bestMove, ss->staticEval);
1510 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1516 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1517 // "plies to mate from the current position". Non-mate scores are unchanged.
1518 // The function is called before storing a value in the transposition table.
1520 Value value_to_tt(Value v, int ply) {
1522 assert(v != VALUE_NONE);
1524 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1525 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1529 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1530 // from the transposition table (which refers to the plies to mate/be mated
1531 // from current position) to "plies to mate/be mated from the root".
1533 Value value_from_tt(Value v, int ply) {
1535 return v == VALUE_NONE ? VALUE_NONE
1536 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1537 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1541 // update_pv() adds current move and appends child pv[]
1543 void update_pv(Move* pv, Move move, Move* childPv) {
1545 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1551 // update_all_stats() updates stats at the end of search() when a bestMove is found
1553 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1554 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1557 Color us = pos.side_to_move();
1558 Thread* thisThread = pos.this_thread();
1559 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1560 Piece moved_piece = pos.moved_piece(bestMove);
1561 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1563 bonus1 = stat_bonus(depth + 1);
1564 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1565 : stat_bonus(depth); // smaller bonus
1567 if (!pos.capture_or_promotion(bestMove))
1569 update_quiet_stats(pos, ss, bestMove, bonus2);
1571 // Decrease all the non-best quiet moves
1572 for (int i = 0; i < quietCount; ++i)
1574 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1575 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1579 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1581 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1582 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1583 && !pos.captured_piece())
1584 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1586 // Decrease all the non-best capture moves
1587 for (int i = 0; i < captureCount; ++i)
1589 moved_piece = pos.moved_piece(capturesSearched[i]);
1590 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1591 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1596 // update_continuation_histories() updates histories of the move pairs formed
1597 // by moves at ply -1, -2, and -4 with current move.
1599 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1601 for (int i : {1, 2, 4, 6})
1602 if (is_ok((ss-i)->currentMove))
1603 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1607 // update_quiet_stats() updates move sorting heuristics
1609 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1611 if (ss->killers[0] != move)
1613 ss->killers[1] = ss->killers[0];
1614 ss->killers[0] = move;
1617 Color us = pos.side_to_move();
1618 Thread* thisThread = pos.this_thread();
1619 thisThread->mainHistory[us][from_to(move)] << bonus;
1620 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1622 if (type_of(pos.moved_piece(move)) != PAWN)
1623 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1625 if (is_ok((ss-1)->currentMove))
1627 Square prevSq = to_sq((ss-1)->currentMove);
1628 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1632 // When playing with strength handicap, choose best move among a set of RootMoves
1633 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1635 Move Skill::pick_best(size_t multiPV) {
1637 const RootMoves& rootMoves = Threads.main()->rootMoves;
1638 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1640 // RootMoves are already sorted by score in descending order
1641 Value topScore = rootMoves[0].score;
1642 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1643 int weakness = 120 - 2 * level;
1644 int maxScore = -VALUE_INFINITE;
1646 // Choose best move. For each move score we add two terms, both dependent on
1647 // weakness. One is deterministic and bigger for weaker levels, and one is
1648 // random. Then we choose the move with the resulting highest score.
1649 for (size_t i = 0; i < multiPV; ++i)
1651 // This is our magic formula
1652 int push = ( weakness * int(topScore - rootMoves[i].score)
1653 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1655 if (rootMoves[i].score + push >= maxScore)
1657 maxScore = rootMoves[i].score + push;
1658 best = rootMoves[i].pv[0];
1667 /// MainThread::check_time() is used to print debug info and, more importantly,
1668 /// to detect when we are out of available time and thus stop the search.
1670 void MainThread::check_time() {
1675 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1676 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1678 static TimePoint lastInfoTime = now();
1680 TimePoint elapsed = Time.elapsed();
1681 TimePoint tick = Limits.startTime + elapsed;
1683 if (tick - lastInfoTime >= 1000)
1685 lastInfoTime = tick;
1689 // We should not stop pondering until told so by the GUI
1693 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1694 || (Limits.movetime && elapsed >= Limits.movetime)
1695 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1696 Threads.stop = true;
1700 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1701 /// that all (if any) unsearched PV lines are sent using a previous search score.
1703 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1705 std::stringstream ss;
1706 TimePoint elapsed = Time.elapsed() + 1;
1707 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1708 size_t pvIdx = pos.this_thread()->pvIdx;
1709 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1710 uint64_t nodesSearched = Threads.nodes_searched();
1711 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1713 for (size_t i = 0; i < multiPV; ++i)
1715 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1717 if (depth == 1 && !updated)
1720 Depth d = updated ? depth : depth - 1;
1721 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1723 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1724 v = tb ? rootMoves[i].tbScore : v;
1726 if (ss.rdbuf()->in_avail()) // Not at first line
1731 << " seldepth " << rootMoves[i].selDepth
1732 << " multipv " << i + 1
1733 << " score " << UCI::value(v);
1735 if (!tb && i == pvIdx)
1736 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1738 ss << " nodes " << nodesSearched
1739 << " nps " << nodesSearched * 1000 / elapsed;
1741 if (elapsed > 1000) // Earlier makes little sense
1742 ss << " hashfull " << TT.hashfull();
1744 ss << " tbhits " << tbHits
1745 << " time " << elapsed
1748 for (Move m : rootMoves[i].pv)
1749 ss << " " << UCI::move(m, pos.is_chess960());
1756 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1757 /// before exiting the search, for instance, in case we stop the search during a
1758 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1759 /// otherwise in case of 'ponder on' we have nothing to think on.
1761 bool RootMove::extract_ponder_from_tt(Position& pos) {
1766 assert(pv.size() == 1);
1768 if (pv[0] == MOVE_NONE)
1771 pos.do_move(pv[0], st);
1772 TTEntry* tte = TT.probe(pos.key(), ttHit);
1776 Move m = tte->move(); // Local copy to be SMP safe
1777 if (MoveList<LEGAL>(pos).contains(m))
1781 pos.undo_move(pv[0]);
1782 return pv.size() > 1;
1785 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1788 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1789 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1790 Cardinality = int(Options["SyzygyProbeLimit"]);
1791 bool dtz_available = true;
1793 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1794 // ProbeDepth == DEPTH_ZERO
1795 if (Cardinality > MaxCardinality)
1797 Cardinality = MaxCardinality;
1801 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1803 // Rank moves using DTZ tables
1804 RootInTB = root_probe(pos, rootMoves);
1808 // DTZ tables are missing; try to rank moves using WDL tables
1809 dtz_available = false;
1810 RootInTB = root_probe_wdl(pos, rootMoves);
1816 // Sort moves according to TB rank
1817 std::sort(rootMoves.begin(), rootMoves.end(),
1818 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1820 // Probe during search only if DTZ is not available and we are winning
1821 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1826 // Clean up if root_probe() and root_probe_wdl() have failed
1827 for (auto& m : rootMoves)