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) 2015-2018 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 {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razor and futility margins
70 constexpr int RazorMargin[] = {0, 590, 604};
71 Value futility_margin(Depth d, bool improving) {
72 return Value((175 - 50 * improving) * d / ONE_PLY);
75 // Futility and reductions lookup tables, initialized at startup
76 int FutilityMoveCounts[2][16]; // [improving][depth]
77 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
79 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
80 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
83 // History and stats update bonus, based on depth
84 int stat_bonus(Depth depth) {
85 int d = depth / ONE_PLY;
86 return d > 17 ? 0 : d * d + 2 * d - 2;
89 // Skill structure is used to implement strength limit
91 explicit Skill(int l) : level(l) {}
92 bool enabled() const { return level < 20; }
93 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
94 Move pick_best(size_t multiPV);
97 Move best = MOVE_NONE;
100 template <NodeType NT>
101 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
103 template <NodeType NT>
104 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
106 Value value_to_tt(Value v, int ply);
107 Value value_from_tt(Value v, int ply);
108 void update_pv(Move* pv, Move move, Move* childPv);
109 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
110 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
111 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
113 inline bool gives_check(const Position& pos, Move move) {
114 Color us = pos.side_to_move();
115 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
116 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
117 : pos.gives_check(move);
120 // perft() is our utility to verify move generation. All the leaf nodes up
121 // to the given depth are generated and counted, and the sum is returned.
123 uint64_t perft(Position& pos, Depth depth) {
126 uint64_t cnt, nodes = 0;
127 const bool leaf = (depth == 2 * ONE_PLY);
129 for (const auto& m : MoveList<LEGAL>(pos))
131 if (Root && depth <= ONE_PLY)
136 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
141 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
149 /// Search::init() is called at startup to initialize various lookup tables
151 void Search::init() {
153 for (int imp = 0; imp <= 1; ++imp)
154 for (int d = 1; d < 64; ++d)
155 for (int mc = 1; mc < 64; ++mc)
157 double r = log(d) * log(mc) / 1.95;
159 Reductions[NonPV][imp][d][mc] = int(std::round(r));
160 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
162 // Increase reduction for non-PV nodes when eval is not improving
164 Reductions[NonPV][imp][d][mc]++;
167 for (int d = 0; d < 16; ++d)
169 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
170 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
175 /// Search::clear() resets search state to its initial value
177 void Search::clear() {
179 Threads.main()->wait_for_search_finished();
181 Time.availableNodes = 0;
187 /// MainThread::search() is called by the main thread when the program receives
188 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
190 void MainThread::search() {
194 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
195 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
199 Color us = rootPos.side_to_move();
200 Time.init(Limits, us, rootPos.game_ply());
203 if (rootMoves.empty())
205 rootMoves.emplace_back(MOVE_NONE);
206 sync_cout << "info depth 0 score "
207 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
212 for (Thread* th : Threads)
214 th->start_searching();
216 Thread::search(); // Let's start searching!
219 // When we reach the maximum depth, we can arrive here without a raise of
220 // Threads.stop. However, if we are pondering or in an infinite search,
221 // the UCI protocol states that we shouldn't print the best move before the
222 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
223 // until the GUI sends one of those commands (which also raises Threads.stop).
224 Threads.stopOnPonderhit = true;
226 while (!Threads.stop && (Threads.ponder || Limits.infinite))
227 {} // Busy wait for a stop or a ponder reset
229 // Stop the threads if not already stopped (also raise the stop if
230 // "ponderhit" just reset Threads.ponder).
233 // Wait until all threads have finished
234 for (Thread* th : Threads)
236 th->wait_for_search_finished();
238 // When playing in 'nodes as time' mode, subtract the searched nodes from
239 // the available ones before exiting.
241 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
243 // Check if there are threads with a better score than main thread
244 Thread* bestThread = this;
245 if ( Options["MultiPV"] == 1
247 && !Skill(Options["Skill Level"]).enabled()
248 && rootMoves[0].pv[0] != MOVE_NONE)
250 for (Thread* th : Threads)
252 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
253 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
255 // Select the thread with the best score, always if it is a mate
257 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
262 previousScore = bestThread->rootMoves[0].score;
264 // Send again PV info if we have a new best thread
265 if (bestThread != this)
266 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
268 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
270 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
271 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
273 std::cout << sync_endl;
277 /// Thread::search() is the main iterative deepening loop. It calls search()
278 /// repeatedly with increasing depth until the allocated thinking time has been
279 /// consumed, the user stops the search, or the maximum search depth is reached.
281 void Thread::search() {
283 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
284 Value bestValue, alpha, beta, delta;
285 Move lastBestMove = MOVE_NONE;
286 Depth lastBestMoveDepth = DEPTH_ZERO;
287 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
288 double timeReduction = 1.0;
289 Color us = rootPos.side_to_move();
291 std::memset(ss-4, 0, 7 * sizeof(Stack));
292 for (int i = 4; i > 0; i--)
293 (ss-i)->contHistory = this->contHistory[NO_PIECE][0].get(); // Use as sentinel
295 bestValue = delta = alpha = -VALUE_INFINITE;
296 beta = VALUE_INFINITE;
299 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
301 size_t multiPV = Options["MultiPV"];
302 Skill skill(Options["Skill Level"]);
304 // When playing with strength handicap enable MultiPV search that we will
305 // use behind the scenes to retrieve a set of possible moves.
307 multiPV = std::max(multiPV, (size_t)4);
309 multiPV = std::min(multiPV, rootMoves.size());
311 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
312 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
313 : -make_score(ct, ct / 2));
315 // Iterative deepening loop until requested to stop or the target depth is reached
316 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
318 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
320 // Distribute search depths across the helper threads
323 int i = (idx - 1) % 20;
324 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
325 continue; // Retry with an incremented rootDepth
328 // Age out PV variability metric
330 mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
332 // Save the last iteration's scores before first PV line is searched and
333 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
334 for (RootMove& rm : rootMoves)
335 rm.previousScore = rm.score;
337 // MultiPV loop. We perform a full root search for each PV line
338 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
340 // Reset UCI info selDepth for each depth and each PV line
343 // Reset aspiration window starting size
344 if (rootDepth >= 5 * ONE_PLY)
346 Value previousScore = rootMoves[PVIdx].previousScore;
348 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
349 beta = std::min(previousScore + delta, VALUE_INFINITE);
351 ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
353 // Adjust contempt based on root move's previousScore (dynamic contempt)
354 ct += int(std::round(48 * atan(float(previousScore) / 128)));
356 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
357 : -make_score(ct, ct / 2));
360 // Start with a small aspiration window and, in the case of a fail
361 // high/low, re-search with a bigger window until we don't fail
365 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
367 // Bring the best move to the front. It is critical that sorting
368 // is done with a stable algorithm because all the values but the
369 // first and eventually the new best one are set to -VALUE_INFINITE
370 // and we want to keep the same order for all the moves except the
371 // new PV that goes to the front. Note that in case of MultiPV
372 // search the already searched PV lines are preserved.
373 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
375 // If search has been stopped, we break immediately. Sorting is
376 // safe because RootMoves is still valid, although it refers to
377 // the previous iteration.
381 // When failing high/low give some update (without cluttering
382 // the UI) before a re-search.
385 && (bestValue <= alpha || bestValue >= beta)
386 && Time.elapsed() > 3000)
387 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
389 // In case of failing low/high increase aspiration window and
390 // re-search, otherwise exit the loop.
391 if (bestValue <= alpha)
393 beta = (alpha + beta) / 2;
394 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
398 mainThread->failedLow = true;
399 Threads.stopOnPonderhit = false;
402 else if (bestValue >= beta)
403 beta = std::min(bestValue + delta, VALUE_INFINITE);
407 delta += delta / 4 + 5;
409 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
412 // Sort the PV lines searched so far and update the GUI
413 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
416 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
417 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
421 completedDepth = rootDepth;
423 if (rootMoves[0].pv[0] != lastBestMove) {
424 lastBestMove = rootMoves[0].pv[0];
425 lastBestMoveDepth = rootDepth;
428 // Have we found a "mate in x"?
430 && bestValue >= VALUE_MATE_IN_MAX_PLY
431 && VALUE_MATE - bestValue <= 2 * Limits.mate)
437 // If skill level is enabled and time is up, pick a sub-optimal best move
438 if (skill.enabled() && skill.time_to_pick(rootDepth))
439 skill.pick_best(multiPV);
441 // Do we have time for the next iteration? Can we stop searching now?
442 if ( Limits.use_time_management()
444 && !Threads.stopOnPonderhit)
446 const int F[] = { mainThread->failedLow,
447 bestValue - mainThread->previousScore };
449 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
451 // If the bestMove is stable over several iterations, reduce time accordingly
453 for (int i : {3, 4, 5})
454 if (lastBestMoveDepth * i < completedDepth)
455 timeReduction *= 1.25;
457 // Use part of the gained time from a previous stable move for the current move
458 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
459 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
461 // Stop the search if we have only one legal move, or if available time elapsed
462 if ( rootMoves.size() == 1
463 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
465 // If we are allowed to ponder do not stop the search now but
466 // keep pondering until the GUI sends "ponderhit" or "stop".
468 Threads.stopOnPonderhit = true;
478 mainThread->previousTimeReduction = timeReduction;
480 // If skill level is enabled, swap best PV line with the sub-optimal one
482 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
483 skill.best ? skill.best : skill.pick_best(multiPV)));
489 // search<>() is the main search function for both PV and non-PV nodes
491 template <NodeType NT>
492 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
494 // Use quiescence search when needed
496 return qsearch<NT>(pos, ss, alpha, beta);
498 constexpr bool PvNode = NT == PV;
499 const bool rootNode = PvNode && ss->ply == 0;
501 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
502 assert(PvNode || (alpha == beta - 1));
503 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
504 assert(!(PvNode && cutNode));
505 assert(depth / ONE_PLY * ONE_PLY == depth);
507 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
511 Move ttMove, move, excludedMove, bestMove;
512 Depth extension, newDepth;
513 Value bestValue, value, ttValue, eval, maxValue;
514 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
515 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
517 int moveCount, captureCount, quietCount;
519 // Step 1. Initialize node
520 Thread* thisThread = pos.this_thread();
521 inCheck = pos.checkers();
522 moveCount = captureCount = quietCount = ss->moveCount = 0;
523 bestValue = -VALUE_INFINITE;
524 maxValue = VALUE_INFINITE;
526 // Check for the available remaining time
527 if (thisThread == Threads.main())
528 static_cast<MainThread*>(thisThread)->check_time();
530 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
531 if (PvNode && thisThread->selDepth < ss->ply + 1)
532 thisThread->selDepth = ss->ply + 1;
536 // Step 2. Check for aborted search and immediate draw
537 if ( Threads.stop.load(std::memory_order_relaxed)
538 || pos.is_draw(ss->ply)
539 || ss->ply >= MAX_PLY)
540 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
542 // Step 3. Mate distance pruning. Even if we mate at the next move our score
543 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
544 // a shorter mate was found upward in the tree then there is no need to search
545 // because we will never beat the current alpha. Same logic but with reversed
546 // signs applies also in the opposite condition of being mated instead of giving
547 // mate. In this case return a fail-high score.
548 alpha = std::max(mated_in(ss->ply), alpha);
549 beta = std::min(mate_in(ss->ply+1), beta);
554 assert(0 <= ss->ply && ss->ply < MAX_PLY);
556 (ss+1)->ply = ss->ply + 1;
557 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
558 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
559 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
560 Square prevSq = to_sq((ss-1)->currentMove);
562 // Initialize statScore to zero for the grandchildren of the current position.
563 // So statScore is shared between all grandchildren and only the first grandchild
564 // starts with statScore = 0. Later grandchildren start with the last calculated
565 // statScore of the previous grandchild. This influences the reduction rules in
566 // LMR which are based on the statScore of parent position.
567 (ss+2)->statScore = 0;
569 // Step 4. Transposition table lookup. We don't want the score of a partial
570 // search to overwrite a previous full search TT value, so we use a different
571 // position key in case of an excluded move.
572 excludedMove = ss->excludedMove;
573 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
574 tte = TT.probe(posKey, ttHit);
575 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
576 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
577 : ttHit ? tte->move() : MOVE_NONE;
579 // At non-PV nodes we check for an early TT cutoff
582 && tte->depth() >= depth
583 && ttValue != VALUE_NONE // Possible in case of TT access race
584 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
585 : (tte->bound() & BOUND_UPPER)))
587 // If ttMove is quiet, update move sorting heuristics on TT hit
592 if (!pos.capture_or_promotion(ttMove))
593 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
595 // Extra penalty for a quiet TT move in previous ply when it gets refuted
596 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
597 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
599 // Penalty for a quiet ttMove that fails low
600 else if (!pos.capture_or_promotion(ttMove))
602 int penalty = -stat_bonus(depth);
603 thisThread->mainHistory[pos.side_to_move()][from_to(ttMove)] << penalty;
604 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
610 // Step 5. Tablebases probe
611 if (!rootNode && TB::Cardinality)
613 int piecesCount = pos.count<ALL_PIECES>();
615 if ( piecesCount <= TB::Cardinality
616 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
617 && pos.rule50_count() == 0
618 && !pos.can_castle(ANY_CASTLING))
621 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
623 if (err != TB::ProbeState::FAIL)
625 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
627 int drawScore = TB::UseRule50 ? 1 : 0;
629 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
630 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
631 : VALUE_DRAW + 2 * wdl * drawScore;
633 Bound b = wdl < -drawScore ? BOUND_UPPER
634 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
636 if ( b == BOUND_EXACT
637 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
639 tte->save(posKey, value_to_tt(value, ss->ply), b,
640 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
641 MOVE_NONE, VALUE_NONE, TT.generation());
648 if (b == BOUND_LOWER)
649 bestValue = value, alpha = std::max(alpha, bestValue);
657 // Step 6. Evaluate the position statically
660 ss->staticEval = eval = VALUE_NONE;
666 // Never assume anything on values stored in TT
667 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
668 eval = ss->staticEval = evaluate(pos);
670 // Can ttValue be used as a better position evaluation?
671 if ( ttValue != VALUE_NONE
672 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
677 ss->staticEval = eval =
678 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
679 : -(ss-1)->staticEval + 2 * Eval::Tempo;
681 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
682 ss->staticEval, TT.generation());
685 improving = ss->staticEval >= (ss-2)->staticEval
686 ||(ss-2)->staticEval == VALUE_NONE;
688 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
691 // Step 7. Razoring (skipped when in check)
693 && depth < 3 * ONE_PLY
694 && eval <= alpha - RazorMargin[depth / ONE_PLY])
696 Value ralpha = alpha - (depth >= 2 * ONE_PLY) * RazorMargin[depth / ONE_PLY];
697 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
698 if (depth < 2 * ONE_PLY || v <= ralpha)
702 // Step 8. Futility pruning: child node (skipped when in check)
704 && depth < 7 * ONE_PLY
705 && eval - futility_margin(depth, improving) >= beta
706 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
709 // Step 9. Null move search with verification search
712 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
713 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
715 assert(eval - beta >= 0);
717 // Null move dynamic reduction based on depth and value
718 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
720 ss->currentMove = MOVE_NULL;
721 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
723 pos.do_null_move(st);
725 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
727 pos.undo_null_move();
729 if (nullValue >= beta)
731 // Do not return unproven mate scores
732 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
735 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
738 // Do verification search at high depths. Disable null move pruning
739 // for side to move for the first part of the remaining search tree.
740 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
741 thisThread->nmp_odd = ss->ply % 2;
743 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
745 thisThread->nmp_odd = thisThread->nmp_ply = 0;
752 // Step 10. ProbCut (skipped when in check)
753 // If we have a good enough capture and a reduced search returns a value
754 // much above beta, we can (almost) safely prune the previous move.
756 && depth >= 5 * ONE_PLY
757 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
759 assert(is_ok((ss-1)->currentMove));
761 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
762 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
763 int probCutCount = 0;
765 while ( (move = mp.next_move()) != MOVE_NONE
771 ss->currentMove = move;
772 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
774 assert(depth >= 5 * ONE_PLY);
776 pos.do_move(move, st);
778 // Perform a preliminary qsearch to verify that the move holds
779 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
781 // If the qsearch held perform the regular search
783 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
792 // Step 11. Internal iterative deepening (skipped when in check)
793 if ( depth >= 6 * ONE_PLY
795 && (PvNode || ss->staticEval + 128 >= beta))
797 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
798 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
800 tte = TT.probe(posKey, ttHit);
801 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
802 ttMove = ttHit ? tte->move() : MOVE_NONE;
805 moves_loop: // When in check, search starts from here
807 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
808 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
810 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
811 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
813 singularExtensionNode = !rootNode
814 && depth >= 8 * ONE_PLY
815 && ttMove != MOVE_NONE
816 && ttValue != VALUE_NONE
817 && !excludedMove // Recursive singular search is not allowed
818 && (tte->bound() & BOUND_LOWER)
819 && tte->depth() >= depth - 3 * ONE_PLY;
822 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
824 // Step 12. Loop through all pseudo-legal moves until no moves remain
825 // or a beta cutoff occurs.
826 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
830 if (move == excludedMove)
833 // At root obey the "searchmoves" option and skip moves not listed in Root
834 // Move List. As a consequence any illegal move is also skipped. In MultiPV
835 // mode we also skip PV moves which have been already searched.
836 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
837 thisThread->rootMoves.end(), move))
840 ss->moveCount = ++moveCount;
842 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
843 sync_cout << "info depth " << depth / ONE_PLY
844 << " currmove " << UCI::move(move, pos.is_chess960())
845 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
847 (ss+1)->pv = nullptr;
849 extension = DEPTH_ZERO;
850 captureOrPromotion = pos.capture_or_promotion(move);
851 movedPiece = pos.moved_piece(move);
852 givesCheck = gives_check(pos, move);
854 moveCountPruning = depth < 16 * ONE_PLY
855 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
857 // Step 13. Extensions
859 // Singular extension search. If all moves but one fail low on a search
860 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
861 // that move is singular and should be extended. To verify this we do a
862 // reduced search on on all the other moves but the ttMove and if the
863 // result is lower than ttValue minus a margin then we will extend the ttMove.
864 if ( singularExtensionNode
868 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
869 ss->excludedMove = move;
870 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
871 ss->excludedMove = MOVE_NONE;
876 else if ( givesCheck // Check extension
881 // Calculate new depth for this move
882 newDepth = depth - ONE_PLY + extension;
884 // Step 14. Pruning at shallow depth
886 && pos.non_pawn_material(pos.side_to_move())
887 && bestValue > VALUE_MATED_IN_MAX_PLY)
889 if ( !captureOrPromotion
891 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
893 // Move count based pruning
894 if (moveCountPruning)
900 // Reduced depth of the next LMR search
901 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
903 // Countermoves based pruning
905 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
906 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
909 // Futility pruning: parent node
912 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
915 // Prune moves with negative SEE
917 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
920 else if ( depth < 7 * ONE_PLY
922 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
926 // Speculative prefetch as early as possible
927 prefetch(TT.first_entry(pos.key_after(move)));
929 // Check for legality just before making the move
930 if (!rootNode && !pos.legal(move))
932 ss->moveCount = --moveCount;
936 if (move == ttMove && captureOrPromotion)
939 // Update the current move (this must be done after singular extension search)
940 ss->currentMove = move;
941 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
943 // Step 15. Make the move
944 pos.do_move(move, st, givesCheck);
946 // Step 16. Reduced depth search (LMR). If the move fails high it will be
947 // re-searched at full depth.
948 if ( depth >= 3 * ONE_PLY
950 && (!captureOrPromotion || moveCountPruning))
952 Depth r = reduction<PvNode>(improving, depth, moveCount);
954 if (captureOrPromotion)
955 r -= r ? ONE_PLY : DEPTH_ZERO;
958 // Decrease reduction if opponent's move count is high
959 if ((ss-1)->moveCount > 15)
962 // Decrease reduction for exact PV nodes
966 // Increase reduction if ttMove is a capture
970 // Increase reduction for cut nodes
974 // Decrease reduction for moves that escape a capture. Filter out
975 // castling moves, because they are coded as "king captures rook" and
976 // hence break make_move().
977 else if ( type_of(move) == NORMAL
978 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
981 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
982 + (*contHist[0])[movedPiece][to_sq(move)]
983 + (*contHist[1])[movedPiece][to_sq(move)]
984 + (*contHist[3])[movedPiece][to_sq(move)]
987 // Decrease/increase reduction by comparing opponent's stat score
988 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
991 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
994 // Decrease/increase reduction for moves with a good/bad history
995 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
998 Depth d = std::max(newDepth - r, ONE_PLY);
1000 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1002 doFullDepthSearch = (value > alpha && d != newDepth);
1005 doFullDepthSearch = !PvNode || moveCount > 1;
1007 // Step 17. Full depth search when LMR is skipped or fails high
1008 if (doFullDepthSearch)
1009 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1011 // For PV nodes only, do a full PV search on the first move or after a fail
1012 // high (in the latter case search only if value < beta), otherwise let the
1013 // parent node fail low with value <= alpha and try another move.
1014 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1017 (ss+1)->pv[0] = MOVE_NONE;
1019 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1022 // Step 18. Undo move
1023 pos.undo_move(move);
1025 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1027 // Step 19. Check for a new best move
1028 // Finished searching the move. If a stop occurred, the return value of
1029 // the search cannot be trusted, and we return immediately without
1030 // updating best move, PV and TT.
1031 if (Threads.stop.load(std::memory_order_relaxed))
1036 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1037 thisThread->rootMoves.end(), move);
1039 // PV move or new best move?
1040 if (moveCount == 1 || value > alpha)
1043 rm.selDepth = thisThread->selDepth;
1048 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1049 rm.pv.push_back(*m);
1051 // We record how often the best move has been changed in each
1052 // iteration. This information is used for time management: When
1053 // the best move changes frequently, we allocate some more time.
1054 if (moveCount > 1 && thisThread == Threads.main())
1055 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1058 // All other moves but the PV are set to the lowest value: this
1059 // is not a problem when sorting because the sort is stable and the
1060 // move position in the list is preserved - just the PV is pushed up.
1061 rm.score = -VALUE_INFINITE;
1064 if (value > bestValue)
1072 if (PvNode && !rootNode) // Update pv even in fail-high case
1073 update_pv(ss->pv, move, (ss+1)->pv);
1075 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1079 assert(value >= beta); // Fail high
1085 if (move != bestMove)
1087 if (captureOrPromotion && captureCount < 32)
1088 capturesSearched[captureCount++] = move;
1090 else if (!captureOrPromotion && quietCount < 64)
1091 quietsSearched[quietCount++] = move;
1095 // The following condition would detect a stop only after move loop has been
1096 // completed. But in this case bestValue is valid because we have fully
1097 // searched our subtree, and we can anyhow save the result in TT.
1103 // Step 20. Check for mate and stalemate
1104 // All legal moves have been searched and if there are no legal moves, it
1105 // must be a mate or a stalemate. If we are in a singular extension search then
1106 // return a fail low score.
1108 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1111 bestValue = excludedMove ? alpha
1112 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1115 // Quiet best move: update move sorting heuristics
1116 if (!pos.capture_or_promotion(bestMove))
1117 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1119 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1121 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1122 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1123 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1125 // Bonus for prior countermove that caused the fail low
1126 else if ( depth >= 3 * ONE_PLY
1127 && !pos.captured_piece()
1128 && is_ok((ss-1)->currentMove))
1129 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1132 bestValue = std::min(bestValue, maxValue);
1135 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1136 bestValue >= beta ? BOUND_LOWER :
1137 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1138 depth, bestMove, ss->staticEval, TT.generation());
1140 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1146 // qsearch() is the quiescence search function, which is called by the main
1147 // search function with depth zero, or recursively with depth less than ONE_PLY.
1148 template <NodeType NT>
1149 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1151 constexpr bool PvNode = NT == PV;
1153 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1154 assert(PvNode || (alpha == beta - 1));
1155 assert(depth <= DEPTH_ZERO);
1156 assert(depth / ONE_PLY * ONE_PLY == depth);
1162 Move ttMove, move, bestMove;
1164 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1165 bool ttHit, inCheck, givesCheck, evasionPrunable;
1170 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1172 ss->pv[0] = MOVE_NONE;
1175 (ss+1)->ply = ss->ply + 1;
1176 ss->currentMove = bestMove = MOVE_NONE;
1177 inCheck = pos.checkers();
1180 // Check for an immediate draw or maximum ply reached
1181 if ( pos.is_draw(ss->ply)
1182 || ss->ply >= MAX_PLY)
1183 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1185 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1187 // Decide whether or not to include checks: this fixes also the type of
1188 // TT entry depth that we are going to use. Note that in qsearch we use
1189 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1190 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1191 : DEPTH_QS_NO_CHECKS;
1192 // Transposition table lookup
1194 tte = TT.probe(posKey, ttHit);
1195 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1196 ttMove = ttHit ? tte->move() : MOVE_NONE;
1200 && tte->depth() >= ttDepth
1201 && ttValue != VALUE_NONE // Only in case of TT access race
1202 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1203 : (tte->bound() & BOUND_UPPER)))
1206 // Evaluate the position statically
1209 ss->staticEval = VALUE_NONE;
1210 bestValue = futilityBase = -VALUE_INFINITE;
1216 // Never assume anything on values stored in TT
1217 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1218 ss->staticEval = bestValue = evaluate(pos);
1220 // Can ttValue be used as a better position evaluation?
1221 if ( ttValue != VALUE_NONE
1222 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1223 bestValue = ttValue;
1226 ss->staticEval = bestValue =
1227 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1228 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1230 // Stand pat. Return immediately if static value is at least beta
1231 if (bestValue >= beta)
1234 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1235 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1240 if (PvNode && bestValue > alpha)
1243 futilityBase = bestValue + 128;
1246 // Initialize a MovePicker object for the current position, and prepare
1247 // to search the moves. Because the depth is <= 0 here, only captures,
1248 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1250 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1252 // Loop through the moves until no moves remain or a beta cutoff occurs
1253 while ((move = mp.next_move()) != MOVE_NONE)
1255 assert(is_ok(move));
1257 givesCheck = gives_check(pos, move);
1264 && futilityBase > -VALUE_KNOWN_WIN
1265 && !pos.advanced_pawn_push(move))
1267 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1269 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1271 if (futilityValue <= alpha)
1273 bestValue = std::max(bestValue, futilityValue);
1277 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1279 bestValue = std::max(bestValue, futilityBase);
1284 // Detect non-capture evasions that are candidates to be pruned
1285 evasionPrunable = inCheck
1286 && (depth != DEPTH_ZERO || moveCount > 2)
1287 && bestValue > VALUE_MATED_IN_MAX_PLY
1288 && !pos.capture(move);
1290 // Don't search moves with negative SEE values
1291 if ( (!inCheck || evasionPrunable)
1292 && !pos.see_ge(move))
1295 // Speculative prefetch as early as possible
1296 prefetch(TT.first_entry(pos.key_after(move)));
1298 // Check for legality just before making the move
1299 if (!pos.legal(move))
1305 ss->currentMove = move;
1307 // Make and search the move
1308 pos.do_move(move, st, givesCheck);
1309 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1310 pos.undo_move(move);
1312 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1314 // Check for a new best move
1315 if (value > bestValue)
1321 if (PvNode) // Update pv even in fail-high case
1322 update_pv(ss->pv, move, (ss+1)->pv);
1324 if (PvNode && value < beta) // Update alpha here!
1331 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1332 ttDepth, move, ss->staticEval, TT.generation());
1340 // All legal moves have been searched. A special case: If we're in check
1341 // and no legal moves were found, it is checkmate.
1342 if (inCheck && bestValue == -VALUE_INFINITE)
1343 return mated_in(ss->ply); // Plies to mate from the root
1345 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1346 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1347 ttDepth, bestMove, ss->staticEval, TT.generation());
1349 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1355 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1356 // "plies to mate from the current position". Non-mate scores are unchanged.
1357 // The function is called before storing a value in the transposition table.
1359 Value value_to_tt(Value v, int ply) {
1361 assert(v != VALUE_NONE);
1363 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1364 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1368 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1369 // from the transposition table (which refers to the plies to mate/be mated
1370 // from current position) to "plies to mate/be mated from the root".
1372 Value value_from_tt(Value v, int ply) {
1374 return v == VALUE_NONE ? VALUE_NONE
1375 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1376 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1380 // update_pv() adds current move and appends child pv[]
1382 void update_pv(Move* pv, Move move, Move* childPv) {
1384 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1390 // update_continuation_histories() updates histories of the move pairs formed
1391 // by moves at ply -1, -2, and -4 with current move.
1393 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1395 for (int i : {1, 2, 4})
1396 if (is_ok((ss-i)->currentMove))
1397 (*(ss-i)->contHistory)[pc][to] << bonus;
1401 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1403 void update_capture_stats(const Position& pos, Move move,
1404 Move* captures, int captureCnt, int bonus) {
1406 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1407 Piece moved_piece = pos.moved_piece(move);
1408 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1409 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1411 // Decrease all the other played capture moves
1412 for (int i = 0; i < captureCnt; ++i)
1414 moved_piece = pos.moved_piece(captures[i]);
1415 captured = type_of(pos.piece_on(to_sq(captures[i])));
1416 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1421 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1423 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1424 Move* quiets, int quietsCnt, int bonus) {
1426 if (ss->killers[0] != move)
1428 ss->killers[1] = ss->killers[0];
1429 ss->killers[0] = move;
1432 Color us = pos.side_to_move();
1433 Thread* thisThread = pos.this_thread();
1434 thisThread->mainHistory[us][from_to(move)] << bonus;
1435 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1437 if (is_ok((ss-1)->currentMove))
1439 Square prevSq = to_sq((ss-1)->currentMove);
1440 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1443 // Decrease all the other played quiet moves
1444 for (int i = 0; i < quietsCnt; ++i)
1446 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1447 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1451 // When playing with strength handicap, choose best move among a set of RootMoves
1452 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1454 Move Skill::pick_best(size_t multiPV) {
1456 const RootMoves& rootMoves = Threads.main()->rootMoves;
1457 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1459 // RootMoves are already sorted by score in descending order
1460 Value topScore = rootMoves[0].score;
1461 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1462 int weakness = 120 - 2 * level;
1463 int maxScore = -VALUE_INFINITE;
1465 // Choose best move. For each move score we add two terms, both dependent on
1466 // weakness. One is deterministic and bigger for weaker levels, and one is
1467 // random. Then we choose the move with the resulting highest score.
1468 for (size_t i = 0; i < multiPV; ++i)
1470 // This is our magic formula
1471 int push = ( weakness * int(topScore - rootMoves[i].score)
1472 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1474 if (rootMoves[i].score + push >= maxScore)
1476 maxScore = rootMoves[i].score + push;
1477 best = rootMoves[i].pv[0];
1486 /// MainThread::check_time() is used to print debug info and, more importantly,
1487 /// to detect when we are out of available time and thus stop the search.
1489 void MainThread::check_time() {
1494 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1495 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1497 static TimePoint lastInfoTime = now();
1499 TimePoint elapsed = Time.elapsed();
1500 TimePoint tick = Limits.startTime + elapsed;
1502 if (tick - lastInfoTime >= 1000)
1504 lastInfoTime = tick;
1508 // We should not stop pondering until told so by the GUI
1512 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1513 || (Limits.movetime && elapsed >= Limits.movetime)
1514 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1515 Threads.stop = true;
1519 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1520 /// that all (if any) unsearched PV lines are sent using a previous search score.
1522 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1524 std::stringstream ss;
1525 TimePoint elapsed = Time.elapsed() + 1;
1526 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1527 size_t PVIdx = pos.this_thread()->PVIdx;
1528 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1529 uint64_t nodesSearched = Threads.nodes_searched();
1530 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1532 for (size_t i = 0; i < multiPV; ++i)
1534 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1536 if (depth == ONE_PLY && !updated)
1539 Depth d = updated ? depth : depth - ONE_PLY;
1540 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1542 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1543 v = tb ? TB::Score : v;
1545 if (ss.rdbuf()->in_avail()) // Not at first line
1549 << " depth " << d / ONE_PLY
1550 << " seldepth " << rootMoves[i].selDepth
1551 << " multipv " << i + 1
1552 << " score " << UCI::value(v);
1554 if (!tb && i == PVIdx)
1555 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1557 ss << " nodes " << nodesSearched
1558 << " nps " << nodesSearched * 1000 / elapsed;
1560 if (elapsed > 1000) // Earlier makes little sense
1561 ss << " hashfull " << TT.hashfull();
1563 ss << " tbhits " << tbHits
1564 << " time " << elapsed
1567 for (Move m : rootMoves[i].pv)
1568 ss << " " << UCI::move(m, pos.is_chess960());
1575 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1576 /// before exiting the search, for instance, in case we stop the search during a
1577 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1578 /// otherwise in case of 'ponder on' we have nothing to think on.
1580 bool RootMove::extract_ponder_from_tt(Position& pos) {
1585 assert(pv.size() == 1);
1590 pos.do_move(pv[0], st);
1591 TTEntry* tte = TT.probe(pos.key(), ttHit);
1595 Move m = tte->move(); // Local copy to be SMP safe
1596 if (MoveList<LEGAL>(pos).contains(m))
1600 pos.undo_move(pv[0]);
1601 return pv.size() > 1;
1605 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1608 UseRule50 = Options["Syzygy50MoveRule"];
1609 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1610 Cardinality = Options["SyzygyProbeLimit"];
1612 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1613 if (Cardinality > MaxCardinality)
1615 Cardinality = MaxCardinality;
1616 ProbeDepth = DEPTH_ZERO;
1619 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1622 // Don't filter any moves if the user requested analysis on multiple
1623 if (Options["MultiPV"] != 1)
1626 // If the current root position is in the tablebases, then RootMoves
1627 // contains only moves that preserve the draw or the win.
1628 RootInTB = root_probe(pos, rootMoves, TB::Score);
1631 Cardinality = 0; // Do not probe tablebases during the search
1633 else // If DTZ tables are missing, use WDL tables as a fallback
1635 // Filter out moves that do not preserve the draw or the win.
1636 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1638 // Only probe during search if winning
1639 if (RootInTB && TB::Score <= VALUE_DRAW)
1643 if (RootInTB && !UseRule50)
1644 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1645 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1648 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1649 // we reset them to -VALUE_INFINITE
1650 for (RootMove& rm : rootMoves)
1651 rm.score = -VALUE_INFINITE;