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 = 600;
66 Value futility_margin(Depth d, bool improving) {
67 return Value((175 - 50 * improving) * d / ONE_PLY);
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn] / 1024;
75 return ((r + 512) / 1024 + (!i && r > 1024)) * ONE_PLY;
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 template <NodeType NT>
106 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
108 template <NodeType NT>
109 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
111 Value value_to_tt(Value v, int ply);
112 Value value_from_tt(Value v, int ply);
113 void update_pv(Move* pv, Move move, Move* childPv);
114 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
115 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
116 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
118 // perft() is our utility to verify move generation. All the leaf nodes up
119 // to the given depth are generated and counted, and the sum is returned.
121 uint64_t perft(Position& pos, Depth depth) {
124 uint64_t cnt, nodes = 0;
125 const bool leaf = (depth == 2 * ONE_PLY);
127 for (const auto& m : MoveList<LEGAL>(pos))
129 if (Root && depth <= ONE_PLY)
134 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
139 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
147 /// Search::init() is called at startup to initialize various lookup tables
149 void Search::init() {
151 for (int i = 1; i < MAX_MOVES; ++i)
152 Reductions[i] = int(1024 * std::log(i) / std::sqrt(1.95));
156 /// Search::clear() resets search state to its initial value
158 void Search::clear() {
160 Threads.main()->wait_for_search_finished();
162 Time.availableNodes = 0;
165 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
169 /// MainThread::search() is started when the program receives the UCI 'go'
170 /// command. It searches from the root position and outputs the "bestmove".
172 void MainThread::search() {
176 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
177 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
181 Color us = rootPos.side_to_move();
182 Time.init(Limits, us, rootPos.game_ply());
185 if (rootMoves.empty())
187 rootMoves.emplace_back(MOVE_NONE);
188 sync_cout << "info depth 0 score "
189 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
194 for (Thread* th : Threads)
196 th->bestMoveChanges = 0;
198 th->start_searching();
201 Thread::search(); // Let's start searching!
204 // When we reach the maximum depth, we can arrive here without a raise of
205 // Threads.stop. However, if we are pondering or in an infinite search,
206 // the UCI protocol states that we shouldn't print the best move before the
207 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
208 // until the GUI sends one of those commands.
210 while (!Threads.stop && (ponder || Limits.infinite))
211 {} // Busy wait for a stop or a ponder reset
213 // Stop the threads if not already stopped (also raise the stop if
214 // "ponderhit" just reset Threads.ponder).
217 // Wait until all threads have finished
218 for (Thread* th : Threads)
220 th->wait_for_search_finished();
222 // When playing in 'nodes as time' mode, subtract the searched nodes from
223 // the available ones before exiting.
225 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
227 Thread* bestThread = this;
229 // Check if there are threads with a better score than main thread
230 if ( Options["MultiPV"] == 1
232 && !Skill(Options["Skill Level"]).enabled()
233 && rootMoves[0].pv[0] != MOVE_NONE)
235 std::map<Move, int64_t> votes;
236 Value minScore = this->rootMoves[0].score;
238 // Find out minimum score and reset votes for moves which can be voted
239 for (Thread* th: Threads)
240 minScore = std::min(minScore, th->rootMoves[0].score);
242 // Vote according to score and depth
243 for (Thread* th : Threads)
244 votes[th->rootMoves[0].pv[0]] +=
245 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
247 // Select best thread
248 auto bestVote = votes[this->rootMoves[0].pv[0]];
249 for (Thread* th : Threads)
250 if (votes[th->rootMoves[0].pv[0]] > bestVote)
252 bestVote = votes[th->rootMoves[0].pv[0]];
257 previousScore = bestThread->rootMoves[0].score;
259 // Send again PV info if we have a new best thread
260 if (bestThread != this)
261 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
263 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
265 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
266 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
268 std::cout << sync_endl;
272 /// Thread::search() is the main iterative deepening loop. It calls search()
273 /// repeatedly with increasing depth until the allocated thinking time has been
274 /// consumed, the user stops the search, or the maximum search depth is reached.
276 void Thread::search() {
278 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
279 // The former is needed to allow update_continuation_histories(ss-1, ...),
280 // which accesses its argument at ss-6, also near the root.
281 // The latter is needed for statScores and killer initialization.
282 Stack stack[MAX_PLY+10], *ss = stack+7;
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, totBestMoveChanges = 0;
289 Color us = rootPos.side_to_move();
291 std::memset(ss-7, 0, 10 * sizeof(Stack));
292 for (int i = 7; i > 0; i--)
293 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
296 bestValue = delta = alpha = -VALUE_INFINITE;
297 beta = VALUE_INFINITE;
299 size_t multiPV = Options["MultiPV"];
300 Skill skill(Options["Skill Level"]);
302 // When playing with strength handicap enable MultiPV search that we will
303 // use behind the scenes to retrieve a set of possible moves.
305 multiPV = std::max(multiPV, (size_t)4);
307 multiPV = std::min(multiPV, rootMoves.size());
309 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
311 // In analysis mode, adjust contempt in accordance with user preference
312 if (Limits.infinite || Options["UCI_AnalyseMode"])
313 ct = Options["Analysis Contempt"] == "Off" ? 0
314 : Options["Analysis Contempt"] == "Both" ? ct
315 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
316 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
319 // Evaluation score is from the white point of view
320 contempt = (us == WHITE ? make_score(ct, ct / 2)
321 : -make_score(ct, ct / 2));
323 // Iterative deepening loop until requested to stop or the target depth is reached
324 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
326 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
328 // Age out PV variability metric
330 totBestMoveChanges /= 2;
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;
340 // MultiPV loop. We perform a full root search for each PV line
341 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
346 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
347 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
351 // Reset UCI info selDepth for each depth and each PV line
354 // Reset aspiration window starting size
355 if (rootDepth >= 5 * ONE_PLY)
357 Value previousScore = rootMoves[pvIdx].previousScore;
359 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
360 beta = std::min(previousScore + delta, VALUE_INFINITE);
362 // Adjust contempt based on root move's previousScore (dynamic contempt)
363 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
365 contempt = (us == WHITE ? make_score(dct, dct / 2)
366 : -make_score(dct, dct / 2));
369 // Start with a small aspiration window and, in the case of a fail
370 // high/low, re-search with a bigger window until we don't fail
372 int failedHighCnt = 0;
375 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
376 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
378 // Bring the best move to the front. It is critical that sorting
379 // is done with a stable algorithm because all the values but the
380 // first and eventually the new best one are set to -VALUE_INFINITE
381 // and we want to keep the same order for all the moves except the
382 // new PV that goes to the front. Note that in case of MultiPV
383 // search the already searched PV lines are preserved.
384 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
386 // If search has been stopped, we break immediately. Sorting is
387 // safe because RootMoves is still valid, although it refers to
388 // the previous iteration.
392 // When failing high/low give some update (without cluttering
393 // the UI) before a re-search.
396 && (bestValue <= alpha || bestValue >= beta)
397 && Time.elapsed() > 3000)
398 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
400 // In case of failing low/high increase aspiration window and
401 // re-search, otherwise exit the loop.
402 if (bestValue <= alpha)
404 beta = (alpha + beta) / 2;
405 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
410 mainThread->stopOnPonderhit = false;
413 else if (bestValue >= beta)
415 beta = std::min(bestValue + delta, VALUE_INFINITE);
422 delta += delta / 4 + 5;
424 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
427 // Sort the PV lines searched so far and update the GUI
428 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
431 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
432 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
436 completedDepth = rootDepth;
438 if (rootMoves[0].pv[0] != lastBestMove) {
439 lastBestMove = rootMoves[0].pv[0];
440 lastBestMoveDepth = rootDepth;
443 // Have we found a "mate in x"?
445 && bestValue >= VALUE_MATE_IN_MAX_PLY
446 && VALUE_MATE - bestValue <= 2 * Limits.mate)
452 // If skill level is enabled and time is up, pick a sub-optimal best move
453 if (skill.enabled() && skill.time_to_pick(rootDepth))
454 skill.pick_best(multiPV);
456 // Do we have time for the next iteration? Can we stop searching now?
457 if ( Limits.use_time_management()
459 && !mainThread->stopOnPonderhit)
461 double fallingEval = (314 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
462 fallingEval = clamp(fallingEval, 0.5, 1.5);
464 // If the bestMove is stable over several iterations, reduce time accordingly
465 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
466 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
468 // Use part of the gained time from a previous stable move for the current move
469 for (Thread* th : Threads)
471 totBestMoveChanges += th->bestMoveChanges;
472 th->bestMoveChanges = 0;
474 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
476 // Stop the search if we have only one legal move, or if available time elapsed
477 if ( rootMoves.size() == 1
478 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
480 // If we are allowed to ponder do not stop the search now but
481 // keep pondering until the GUI sends "ponderhit" or "stop".
482 if (mainThread->ponder)
483 mainThread->stopOnPonderhit = true;
493 mainThread->previousTimeReduction = timeReduction;
495 // If skill level is enabled, swap best PV line with the sub-optimal one
497 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
498 skill.best ? skill.best : skill.pick_best(multiPV)));
504 // search<>() is the main search function for both PV and non-PV nodes
506 template <NodeType NT>
507 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
509 constexpr bool PvNode = NT == PV;
510 const bool rootNode = PvNode && ss->ply == 0;
512 // Check if we have an upcoming move which draws by repetition, or
513 // if the opponent had an alternative move earlier to this position.
514 if ( pos.rule50_count() >= 3
515 && alpha < VALUE_DRAW
517 && pos.has_game_cycle(ss->ply))
519 alpha = value_draw(depth, pos.this_thread());
524 // Dive into quiescence search when the depth reaches zero
526 return qsearch<NT>(pos, ss, alpha, beta);
528 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
529 assert(PvNode || (alpha == beta - 1));
530 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
531 assert(!(PvNode && cutNode));
532 assert(depth / ONE_PLY * ONE_PLY == depth);
534 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
538 Move ttMove, move, excludedMove, bestMove;
539 Depth extension, newDepth;
540 Value bestValue, value, ttValue, eval, maxValue;
541 bool ttHit, ttPv, inCheck, givesCheck, improving;
542 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
544 int moveCount, captureCount, quietCount;
546 // Step 1. Initialize node
547 Thread* thisThread = pos.this_thread();
548 inCheck = pos.checkers();
549 Color us = pos.side_to_move();
550 moveCount = captureCount = quietCount = ss->moveCount = 0;
551 bestValue = -VALUE_INFINITE;
552 maxValue = VALUE_INFINITE;
554 // Check for the available remaining time
555 if (thisThread == Threads.main())
556 static_cast<MainThread*>(thisThread)->check_time();
558 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
559 if (PvNode && thisThread->selDepth < ss->ply + 1)
560 thisThread->selDepth = ss->ply + 1;
564 // Step 2. Check for aborted search and immediate draw
565 if ( Threads.stop.load(std::memory_order_relaxed)
566 || pos.is_draw(ss->ply)
567 || ss->ply >= MAX_PLY)
568 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
569 : value_draw(depth, pos.this_thread());
571 // Step 3. Mate distance pruning. Even if we mate at the next move our score
572 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
573 // a shorter mate was found upward in the tree then there is no need to search
574 // because we will never beat the current alpha. Same logic but with reversed
575 // signs applies also in the opposite condition of being mated instead of giving
576 // mate. In this case return a fail-high score.
577 alpha = std::max(mated_in(ss->ply), alpha);
578 beta = std::min(mate_in(ss->ply+1), beta);
583 assert(0 <= ss->ply && ss->ply < MAX_PLY);
585 (ss+1)->ply = ss->ply + 1;
586 (ss+1)->excludedMove = bestMove = MOVE_NONE;
587 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
588 Square prevSq = to_sq((ss-1)->currentMove);
590 // Initialize statScore to zero for the grandchildren of the current position.
591 // So statScore is shared between all grandchildren and only the first grandchild
592 // starts with statScore = 0. Later grandchildren start with the last calculated
593 // statScore of the previous grandchild. This influences the reduction rules in
594 // LMR which are based on the statScore of parent position.
596 (ss + 4)->statScore = 0;
598 (ss + 2)->statScore = 0;
600 // Step 4. Transposition table lookup. We don't want the score of a partial
601 // search to overwrite a previous full search TT value, so we use a different
602 // position key in case of an excluded move.
603 excludedMove = ss->excludedMove;
604 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
605 tte = TT.probe(posKey, ttHit);
606 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
607 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
608 : ttHit ? tte->move() : MOVE_NONE;
609 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
611 // At non-PV nodes we check for an early TT cutoff
614 && tte->depth() >= depth
615 && ttValue != VALUE_NONE // Possible in case of TT access race
616 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
617 : (tte->bound() & BOUND_UPPER)))
619 // If ttMove is quiet, update move sorting heuristics on TT hit
624 if (!pos.capture_or_promotion(ttMove))
625 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
627 // Extra penalty for early quiet moves of the previous ply
628 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
629 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
631 // Penalty for a quiet ttMove that fails low
632 else if (!pos.capture_or_promotion(ttMove))
634 int penalty = -stat_bonus(depth);
635 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
636 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
642 // Step 5. Tablebases probe
643 if (!rootNode && TB::Cardinality)
645 int piecesCount = pos.count<ALL_PIECES>();
647 if ( piecesCount <= TB::Cardinality
648 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
649 && pos.rule50_count() == 0
650 && !pos.can_castle(ANY_CASTLING))
653 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
655 // Force check of time on the next occasion
656 if (thisThread == Threads.main())
657 static_cast<MainThread*>(thisThread)->callsCnt = 0;
659 if (err != TB::ProbeState::FAIL)
661 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
663 int drawScore = TB::UseRule50 ? 1 : 0;
665 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
666 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
667 : VALUE_DRAW + 2 * wdl * drawScore;
669 Bound b = wdl < -drawScore ? BOUND_UPPER
670 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
672 if ( b == BOUND_EXACT
673 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
675 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
676 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
677 MOVE_NONE, VALUE_NONE);
684 if (b == BOUND_LOWER)
685 bestValue = value, alpha = std::max(alpha, bestValue);
693 // Step 6. Static evaluation of the position
696 ss->staticEval = eval = VALUE_NONE;
698 goto moves_loop; // Skip early pruning when in check
702 // Never assume anything on values stored in TT
703 ss->staticEval = eval = tte->eval();
704 if (eval == VALUE_NONE)
705 ss->staticEval = eval = evaluate(pos);
707 // Can ttValue be used as a better position evaluation?
708 if ( ttValue != VALUE_NONE
709 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
714 if ((ss-1)->currentMove != MOVE_NULL)
716 int bonus = -(ss-1)->statScore / 512;
718 ss->staticEval = eval = evaluate(pos) + bonus;
721 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
723 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
726 // Step 7. Razoring (~2 Elo)
727 if ( !rootNode // The required rootNode PV handling is not available in qsearch
728 && depth < 2 * ONE_PLY
729 && eval <= alpha - RazorMargin)
730 return qsearch<NT>(pos, ss, alpha, beta);
732 improving = ss->staticEval >= (ss-2)->staticEval
733 || (ss-2)->staticEval == VALUE_NONE;
735 // Step 8. Futility pruning: child node (~30 Elo)
737 && depth < 7 * ONE_PLY
738 && eval - futility_margin(depth, improving) >= beta
739 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
742 // Step 9. Null move search with verification search (~40 Elo)
744 && (ss-1)->currentMove != MOVE_NULL
745 && (ss-1)->statScore < 23200
747 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
749 && pos.non_pawn_material(us)
750 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
752 assert(eval - beta >= 0);
754 // Null move dynamic reduction based on depth and value
755 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
757 ss->currentMove = MOVE_NULL;
758 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
760 pos.do_null_move(st);
762 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
764 pos.undo_null_move();
766 if (nullValue >= beta)
768 // Do not return unproven mate scores
769 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
772 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
775 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
777 // Do verification search at high depths, with null move pruning disabled
778 // for us, until ply exceeds nmpMinPly.
779 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
780 thisThread->nmpColor = us;
782 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
784 thisThread->nmpMinPly = 0;
791 // Step 10. ProbCut (~10 Elo)
792 // If we have a good enough capture and a reduced search returns a value
793 // much above beta, we can (almost) safely prune the previous move.
795 && depth >= 5 * ONE_PLY
796 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
798 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
799 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
800 int probCutCount = 0;
802 while ( (move = mp.next_move()) != MOVE_NONE
803 && probCutCount < 2 + 2 * cutNode)
804 if (move != excludedMove && pos.legal(move))
808 ss->currentMove = move;
809 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
811 assert(depth >= 5 * ONE_PLY);
813 pos.do_move(move, st);
815 // Perform a preliminary qsearch to verify that the move holds
816 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
818 // If the qsearch held, perform the regular search
819 if (value >= raisedBeta)
820 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
824 if (value >= raisedBeta)
829 // Step 11. Internal iterative deepening (~2 Elo)
830 if (depth >= 8 * ONE_PLY && !ttMove)
832 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
834 tte = TT.probe(posKey, ttHit);
835 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
836 ttMove = ttHit ? tte->move() : MOVE_NONE;
839 moves_loop: // When in check, search starts from here
841 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
842 nullptr, (ss-4)->continuationHistory,
843 nullptr, (ss-6)->continuationHistory };
845 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
847 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
848 &thisThread->captureHistory,
853 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
854 moveCountPruning = false;
855 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
856 int singularExtensionLMRmultiplier = 0;
858 // Step 12. Loop through all pseudo-legal moves until no moves remain
859 // or a beta cutoff occurs.
860 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
864 if (move == excludedMove)
867 // At root obey the "searchmoves" option and skip moves not listed in Root
868 // Move List. As a consequence any illegal move is also skipped. In MultiPV
869 // mode we also skip PV moves which have been already searched and those
870 // of lower "TB rank" if we are in a TB root position.
871 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
872 thisThread->rootMoves.begin() + thisThread->pvLast, move))
875 ss->moveCount = ++moveCount;
877 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
878 sync_cout << "info depth " << depth / ONE_PLY
879 << " currmove " << UCI::move(move, pos.is_chess960())
880 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
882 (ss+1)->pv = nullptr;
884 extension = DEPTH_ZERO;
885 captureOrPromotion = pos.capture_or_promotion(move);
886 movedPiece = pos.moved_piece(move);
887 givesCheck = pos.gives_check(move);
889 // Step 13. Extensions (~70 Elo)
891 // Singular extension search (~60 Elo). If all moves but one fail low on a
892 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
893 // then that move is singular and should be extended. To verify this we do
894 // a reduced search on all the other moves but the ttMove and if the
895 // result is lower than ttValue minus a margin then we will extend the ttMove.
896 if ( depth >= 8 * ONE_PLY
899 && !excludedMove // Avoid recursive singular search
900 /* && ttValue != VALUE_NONE Already implicit in the next condition */
901 && abs(ttValue) < VALUE_KNOWN_WIN
902 && (tte->bound() & BOUND_LOWER)
903 && tte->depth() >= depth - 3 * ONE_PLY
906 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
907 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
908 ss->excludedMove = move;
909 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
910 ss->excludedMove = MOVE_NONE;
912 if (value < singularBeta)
915 singularExtensionLMRmultiplier++;
916 if (value < singularBeta - std::min(3 * depth / ONE_PLY, 39))
917 singularExtensionLMRmultiplier++;
921 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
922 // search without the ttMove. So we assume this expected Cut-node is not singular,
923 // that is multiple moves fail high, and we can prune the whole subtree by returning
924 // the hard beta bound.
925 else if (cutNode && singularBeta > beta)
929 // Check extension (~2 Elo)
931 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
934 // Castling extension
935 else if (type_of(move) == CASTLING)
940 && pos.rule50_count() > 18
941 && depth < 3 * ONE_PLY
942 && ss->ply < 3 * thisThread->rootDepth / ONE_PLY) // To avoid too deep searches
945 // Passed pawn extension
946 else if ( move == ss->killers[0]
947 && pos.advanced_pawn_push(move)
948 && pos.pawn_passed(us, to_sq(move)))
951 // Calculate new depth for this move
952 newDepth = depth - ONE_PLY + extension;
954 // Step 14. Pruning at shallow depth (~170 Elo)
956 && pos.non_pawn_material(us)
957 && bestValue > VALUE_MATED_IN_MAX_PLY)
959 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
960 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
962 if ( !captureOrPromotion
964 && !pos.advanced_pawn_push(move))
966 // Move count based pruning (~30 Elo)
967 if (moveCountPruning)
970 // Reduced depth of the next LMR search
971 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
974 // Countermoves based pruning (~20 Elo)
975 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
976 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
977 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
980 // Futility pruning: parent node (~2 Elo)
983 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
986 // Prune moves with negative SEE (~10 Elo)
987 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
990 else if (!pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
994 // Speculative prefetch as early as possible
995 prefetch(TT.first_entry(pos.key_after(move)));
997 // Check for legality just before making the move
998 if (!rootNode && !pos.legal(move))
1000 ss->moveCount = --moveCount;
1004 // Update the current move (this must be done after singular extension search)
1005 ss->currentMove = move;
1006 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1008 // Step 15. Make the move
1009 pos.do_move(move, st, givesCheck);
1011 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1012 // re-searched at full depth.
1013 if ( depth >= 3 * ONE_PLY
1014 && moveCount > 1 + 3 * rootNode
1015 && ( !captureOrPromotion
1017 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1019 Depth r = reduction(improving, depth, moveCount);
1021 // Decrease reduction if position is or has been on the PV
1025 // Decrease reduction if opponent's move count is high (~10 Elo)
1026 if ((ss-1)->moveCount > 15)
1028 // Decrease reduction if move has been singularly extended
1029 r -= singularExtensionLMRmultiplier * ONE_PLY;
1031 if (!captureOrPromotion)
1033 // Increase reduction if ttMove is a capture (~0 Elo)
1037 // Increase reduction for cut nodes (~5 Elo)
1041 // Decrease reduction for moves that escape a capture. Filter out
1042 // castling moves, because they are coded as "king captures rook" and
1043 // hence break make_move(). (~5 Elo)
1044 else if ( type_of(move) == NORMAL
1045 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1048 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1049 + (*contHist[0])[movedPiece][to_sq(move)]
1050 + (*contHist[1])[movedPiece][to_sq(move)]
1051 + (*contHist[3])[movedPiece][to_sq(move)]
1054 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1055 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1058 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1061 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1062 r -= ss->statScore / 20000 * ONE_PLY;
1065 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1067 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1069 doFullDepthSearch = (value > alpha && d != newDepth);
1072 doFullDepthSearch = !PvNode || moveCount > 1;
1074 // Step 17. Full depth search when LMR is skipped or fails high
1075 if (doFullDepthSearch)
1076 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1078 // For PV nodes only, do a full PV search on the first move or after a fail
1079 // high (in the latter case search only if value < beta), otherwise let the
1080 // parent node fail low with value <= alpha and try another move.
1081 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1084 (ss+1)->pv[0] = MOVE_NONE;
1086 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1089 // Step 18. Undo move
1090 pos.undo_move(move);
1092 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1094 // Step 19. Check for a new best move
1095 // Finished searching the move. If a stop occurred, the return value of
1096 // the search cannot be trusted, and we return immediately without
1097 // updating best move, PV and TT.
1098 if (Threads.stop.load(std::memory_order_relaxed))
1103 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1104 thisThread->rootMoves.end(), move);
1106 // PV move or new best move?
1107 if (moveCount == 1 || value > alpha)
1110 rm.selDepth = thisThread->selDepth;
1115 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1116 rm.pv.push_back(*m);
1118 // We record how often the best move has been changed in each
1119 // iteration. This information is used for time management: When
1120 // the best move changes frequently, we allocate some more time.
1122 ++thisThread->bestMoveChanges;
1125 // All other moves but the PV are set to the lowest value: this
1126 // is not a problem when sorting because the sort is stable and the
1127 // move position in the list is preserved - just the PV is pushed up.
1128 rm.score = -VALUE_INFINITE;
1131 if (value > bestValue)
1139 if (PvNode && !rootNode) // Update pv even in fail-high case
1140 update_pv(ss->pv, move, (ss+1)->pv);
1142 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1146 assert(value >= beta); // Fail high
1153 if (move != bestMove)
1155 if (captureOrPromotion && captureCount < 32)
1156 capturesSearched[captureCount++] = move;
1158 else if (!captureOrPromotion && quietCount < 64)
1159 quietsSearched[quietCount++] = move;
1163 // The following condition would detect a stop only after move loop has been
1164 // completed. But in this case bestValue is valid because we have fully
1165 // searched our subtree, and we can anyhow save the result in TT.
1171 // Step 20. Check for mate and stalemate
1172 // All legal moves have been searched and if there are no legal moves, it
1173 // must be a mate or a stalemate. If we are in a singular extension search then
1174 // return a fail low score.
1176 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1179 bestValue = excludedMove ? alpha
1180 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1183 // Quiet best move: update move sorting heuristics
1184 if (!pos.capture_or_promotion(bestMove))
1185 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1186 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1188 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1190 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1191 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1192 && !pos.captured_piece())
1193 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1196 // Bonus for prior countermove that caused the fail low
1197 else if ( (depth >= 3 * ONE_PLY || PvNode)
1198 && !pos.captured_piece())
1199 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1202 bestValue = std::min(bestValue, maxValue);
1205 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1206 bestValue >= beta ? BOUND_LOWER :
1207 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1208 depth, bestMove, ss->staticEval);
1210 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1216 // qsearch() is the quiescence search function, which is called by the main search
1217 // function with zero depth, or recursively with further decreasing depth per call.
1218 template <NodeType NT>
1219 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1221 constexpr bool PvNode = NT == PV;
1223 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1224 assert(PvNode || (alpha == beta - 1));
1225 assert(depth <= DEPTH_ZERO);
1226 assert(depth / ONE_PLY * ONE_PLY == depth);
1232 Move ttMove, move, bestMove;
1234 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1235 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1240 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1242 ss->pv[0] = MOVE_NONE;
1245 Thread* thisThread = pos.this_thread();
1246 (ss+1)->ply = ss->ply + 1;
1247 bestMove = MOVE_NONE;
1248 inCheck = pos.checkers();
1251 // Check for an immediate draw or maximum ply reached
1252 if ( pos.is_draw(ss->ply)
1253 || ss->ply >= MAX_PLY)
1254 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1256 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1258 // Decide whether or not to include checks: this fixes also the type of
1259 // TT entry depth that we are going to use. Note that in qsearch we use
1260 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1261 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1262 : DEPTH_QS_NO_CHECKS;
1263 // Transposition table lookup
1265 tte = TT.probe(posKey, ttHit);
1266 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1267 ttMove = ttHit ? tte->move() : MOVE_NONE;
1268 pvHit = ttHit && tte->is_pv();
1272 && tte->depth() >= ttDepth
1273 && ttValue != VALUE_NONE // Only in case of TT access race
1274 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1275 : (tte->bound() & BOUND_UPPER)))
1278 // Evaluate the position statically
1281 ss->staticEval = VALUE_NONE;
1282 bestValue = futilityBase = -VALUE_INFINITE;
1288 // Never assume anything on values stored in TT
1289 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1290 ss->staticEval = bestValue = evaluate(pos);
1292 // Can ttValue be used as a better position evaluation?
1293 if ( ttValue != VALUE_NONE
1294 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1295 bestValue = ttValue;
1298 ss->staticEval = bestValue =
1299 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1300 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1302 // Stand pat. Return immediately if static value is at least beta
1303 if (bestValue >= beta)
1306 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1307 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1312 if (PvNode && bestValue > alpha)
1315 futilityBase = bestValue + 128;
1318 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1319 nullptr, (ss-4)->continuationHistory,
1320 nullptr, (ss-6)->continuationHistory };
1322 // Initialize a MovePicker object for the current position, and prepare
1323 // to search the moves. Because the depth is <= 0 here, only captures,
1324 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1326 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1327 &thisThread->captureHistory,
1329 to_sq((ss-1)->currentMove));
1331 // Loop through the moves until no moves remain or a beta cutoff occurs
1332 while ((move = mp.next_move()) != MOVE_NONE)
1334 assert(is_ok(move));
1336 givesCheck = pos.gives_check(move);
1343 && futilityBase > -VALUE_KNOWN_WIN
1344 && !pos.advanced_pawn_push(move))
1346 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1348 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1350 if (futilityValue <= alpha)
1352 bestValue = std::max(bestValue, futilityValue);
1356 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1358 bestValue = std::max(bestValue, futilityBase);
1363 // Detect non-capture evasions that are candidates to be pruned
1364 evasionPrunable = inCheck
1365 && (depth != DEPTH_ZERO || moveCount > 2)
1366 && bestValue > VALUE_MATED_IN_MAX_PLY
1367 && !pos.capture(move);
1369 // Don't search moves with negative SEE values
1370 if ( (!inCheck || evasionPrunable)
1371 && !pos.see_ge(move))
1374 // Speculative prefetch as early as possible
1375 prefetch(TT.first_entry(pos.key_after(move)));
1377 // Check for legality just before making the move
1378 if (!pos.legal(move))
1384 ss->currentMove = move;
1385 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1387 // Make and search the move
1388 pos.do_move(move, st, givesCheck);
1389 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1390 pos.undo_move(move);
1392 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1394 // Check for a new best move
1395 if (value > bestValue)
1403 if (PvNode) // Update pv even in fail-high case
1404 update_pv(ss->pv, move, (ss+1)->pv);
1406 if (PvNode && value < beta) // Update alpha here!
1414 // All legal moves have been searched. A special case: If we're in check
1415 // and no legal moves were found, it is checkmate.
1416 if (inCheck && bestValue == -VALUE_INFINITE)
1417 return mated_in(ss->ply); // Plies to mate from the root
1419 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1420 bestValue >= beta ? BOUND_LOWER :
1421 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1422 ttDepth, bestMove, ss->staticEval);
1424 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1430 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1431 // "plies to mate from the current position". Non-mate scores are unchanged.
1432 // The function is called before storing a value in the transposition table.
1434 Value value_to_tt(Value v, int ply) {
1436 assert(v != VALUE_NONE);
1438 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1439 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1443 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1444 // from the transposition table (which refers to the plies to mate/be mated
1445 // from current position) to "plies to mate/be mated from the root".
1447 Value value_from_tt(Value v, int ply) {
1449 return v == VALUE_NONE ? VALUE_NONE
1450 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1451 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1455 // update_pv() adds current move and appends child pv[]
1457 void update_pv(Move* pv, Move move, Move* childPv) {
1459 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1465 // update_continuation_histories() updates histories of the move pairs formed
1466 // by moves at ply -1, -2, and -4 with current move.
1468 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1470 for (int i : {1, 2, 4, 6})
1471 if (is_ok((ss-i)->currentMove))
1472 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1476 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1478 void update_capture_stats(const Position& pos, Move move,
1479 Move* captures, int captureCount, int bonus) {
1481 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1482 Piece moved_piece = pos.moved_piece(move);
1483 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1485 if (pos.capture_or_promotion(move))
1486 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1488 // Decrease all the other played capture moves
1489 for (int i = 0; i < captureCount; ++i)
1491 moved_piece = pos.moved_piece(captures[i]);
1492 captured = type_of(pos.piece_on(to_sq(captures[i])));
1493 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1498 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1500 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1501 Move* quiets, int quietCount, int bonus) {
1503 if (ss->killers[0] != move)
1505 ss->killers[1] = ss->killers[0];
1506 ss->killers[0] = move;
1509 Color us = pos.side_to_move();
1510 Thread* thisThread = pos.this_thread();
1511 thisThread->mainHistory[us][from_to(move)] << bonus;
1512 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1514 if (is_ok((ss-1)->currentMove))
1516 Square prevSq = to_sq((ss-1)->currentMove);
1517 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1520 // Decrease all the other played quiet moves
1521 for (int i = 0; i < quietCount; ++i)
1523 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1524 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1528 // When playing with strength handicap, choose best move among a set of RootMoves
1529 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1531 Move Skill::pick_best(size_t multiPV) {
1533 const RootMoves& rootMoves = Threads.main()->rootMoves;
1534 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1536 // RootMoves are already sorted by score in descending order
1537 Value topScore = rootMoves[0].score;
1538 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1539 int weakness = 120 - 2 * level;
1540 int maxScore = -VALUE_INFINITE;
1542 // Choose best move. For each move score we add two terms, both dependent on
1543 // weakness. One is deterministic and bigger for weaker levels, and one is
1544 // random. Then we choose the move with the resulting highest score.
1545 for (size_t i = 0; i < multiPV; ++i)
1547 // This is our magic formula
1548 int push = ( weakness * int(topScore - rootMoves[i].score)
1549 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1551 if (rootMoves[i].score + push >= maxScore)
1553 maxScore = rootMoves[i].score + push;
1554 best = rootMoves[i].pv[0];
1563 /// MainThread::check_time() is used to print debug info and, more importantly,
1564 /// to detect when we are out of available time and thus stop the search.
1566 void MainThread::check_time() {
1571 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1572 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1574 static TimePoint lastInfoTime = now();
1576 TimePoint elapsed = Time.elapsed();
1577 TimePoint tick = Limits.startTime + elapsed;
1579 if (tick - lastInfoTime >= 1000)
1581 lastInfoTime = tick;
1585 // We should not stop pondering until told so by the GUI
1589 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1590 || (Limits.movetime && elapsed >= Limits.movetime)
1591 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1592 Threads.stop = true;
1596 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1597 /// that all (if any) unsearched PV lines are sent using a previous search score.
1599 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1601 std::stringstream ss;
1602 TimePoint elapsed = Time.elapsed() + 1;
1603 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1604 size_t pvIdx = pos.this_thread()->pvIdx;
1605 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1606 uint64_t nodesSearched = Threads.nodes_searched();
1607 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1609 for (size_t i = 0; i < multiPV; ++i)
1611 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1613 if (depth == ONE_PLY && !updated)
1616 Depth d = updated ? depth : depth - ONE_PLY;
1617 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1619 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1620 v = tb ? rootMoves[i].tbScore : v;
1622 if (ss.rdbuf()->in_avail()) // Not at first line
1626 << " depth " << d / ONE_PLY
1627 << " seldepth " << rootMoves[i].selDepth
1628 << " multipv " << i + 1
1629 << " score " << UCI::value(v);
1631 if (!tb && i == pvIdx)
1632 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1634 ss << " nodes " << nodesSearched
1635 << " nps " << nodesSearched * 1000 / elapsed;
1637 if (elapsed > 1000) // Earlier makes little sense
1638 ss << " hashfull " << TT.hashfull();
1640 ss << " tbhits " << tbHits
1641 << " time " << elapsed
1644 for (Move m : rootMoves[i].pv)
1645 ss << " " << UCI::move(m, pos.is_chess960());
1652 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1653 /// before exiting the search, for instance, in case we stop the search during a
1654 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1655 /// otherwise in case of 'ponder on' we have nothing to think on.
1657 bool RootMove::extract_ponder_from_tt(Position& pos) {
1662 assert(pv.size() == 1);
1664 if (pv[0] == MOVE_NONE)
1667 pos.do_move(pv[0], st);
1668 TTEntry* tte = TT.probe(pos.key(), ttHit);
1672 Move m = tte->move(); // Local copy to be SMP safe
1673 if (MoveList<LEGAL>(pos).contains(m))
1677 pos.undo_move(pv[0]);
1678 return pv.size() > 1;
1681 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1684 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1685 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1686 Cardinality = int(Options["SyzygyProbeLimit"]);
1687 bool dtz_available = true;
1689 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1690 // ProbeDepth == DEPTH_ZERO
1691 if (Cardinality > MaxCardinality)
1693 Cardinality = MaxCardinality;
1694 ProbeDepth = DEPTH_ZERO;
1697 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1699 // Rank moves using DTZ tables
1700 RootInTB = root_probe(pos, rootMoves);
1704 // DTZ tables are missing; try to rank moves using WDL tables
1705 dtz_available = false;
1706 RootInTB = root_probe_wdl(pos, rootMoves);
1712 // Sort moves according to TB rank
1713 std::sort(rootMoves.begin(), rootMoves.end(),
1714 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1716 // Probe during search only if DTZ is not available and we are winning
1717 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1722 // Assign the same rank to all moves
1723 for (auto& m : rootMoves)