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];
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(22.9 * std::log(i));
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, and select the best thread
243 int64_t bestVote = 0;
244 for (Thread* th : Threads)
246 votes[th->rootMoves[0].pv[0]] +=
247 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
249 if (votes[th->rootMoves[0].pv[0]] > bestVote)
251 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);
409 mainThread->stopOnPonderhit = false;
411 else if (bestValue >= beta)
413 beta = std::min(bestValue + delta, VALUE_INFINITE);
419 delta += delta / 4 + 5;
421 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
424 // Sort the PV lines searched so far and update the GUI
425 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
428 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
429 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
433 completedDepth = rootDepth;
435 if (rootMoves[0].pv[0] != lastBestMove) {
436 lastBestMove = rootMoves[0].pv[0];
437 lastBestMoveDepth = rootDepth;
440 // Have we found a "mate in x"?
442 && bestValue >= VALUE_MATE_IN_MAX_PLY
443 && VALUE_MATE - bestValue <= 2 * Limits.mate)
449 // If skill level is enabled and time is up, pick a sub-optimal best move
450 if (skill.enabled() && skill.time_to_pick(rootDepth))
451 skill.pick_best(multiPV);
453 // Do we have time for the next iteration? Can we stop searching now?
454 if ( Limits.use_time_management()
456 && !mainThread->stopOnPonderhit)
458 double fallingEval = (314 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
459 fallingEval = clamp(fallingEval, 0.5, 1.5);
461 // If the bestMove is stable over several iterations, reduce time accordingly
462 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
463 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
465 // Use part of the gained time from a previous stable move for the current move
466 for (Thread* th : Threads)
468 totBestMoveChanges += th->bestMoveChanges;
469 th->bestMoveChanges = 0;
471 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
473 // Stop the search if we have only one legal move, or if available time elapsed
474 if ( rootMoves.size() == 1
475 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
477 // If we are allowed to ponder do not stop the search now but
478 // keep pondering until the GUI sends "ponderhit" or "stop".
479 if (mainThread->ponder)
480 mainThread->stopOnPonderhit = true;
490 mainThread->previousTimeReduction = timeReduction;
492 // If skill level is enabled, swap best PV line with the sub-optimal one
494 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
495 skill.best ? skill.best : skill.pick_best(multiPV)));
501 // search<>() is the main search function for both PV and non-PV nodes
503 template <NodeType NT>
504 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
506 constexpr bool PvNode = NT == PV;
507 const bool rootNode = PvNode && ss->ply == 0;
509 // Check if we have an upcoming move which draws by repetition, or
510 // if the opponent had an alternative move earlier to this position.
511 if ( pos.rule50_count() >= 3
512 && alpha < VALUE_DRAW
514 && pos.has_game_cycle(ss->ply))
516 alpha = value_draw(depth, pos.this_thread());
521 // Dive into quiescence search when the depth reaches zero
523 return qsearch<NT>(pos, ss, alpha, beta);
525 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
526 assert(PvNode || (alpha == beta - 1));
527 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
528 assert(!(PvNode && cutNode));
529 assert(depth / ONE_PLY * ONE_PLY == depth);
531 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
535 Move ttMove, move, excludedMove, bestMove;
536 Depth extension, newDepth;
537 Value bestValue, value, ttValue, eval, maxValue;
538 bool ttHit, ttPv, inCheck, givesCheck, improving;
539 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
541 int moveCount, captureCount, quietCount;
543 // Step 1. Initialize node
544 Thread* thisThread = pos.this_thread();
545 inCheck = pos.checkers();
546 Color us = pos.side_to_move();
547 moveCount = captureCount = quietCount = ss->moveCount = 0;
548 bestValue = -VALUE_INFINITE;
549 maxValue = VALUE_INFINITE;
551 // Check for the available remaining time
552 if (thisThread == Threads.main())
553 static_cast<MainThread*>(thisThread)->check_time();
555 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
556 if (PvNode && thisThread->selDepth < ss->ply + 1)
557 thisThread->selDepth = ss->ply + 1;
561 // Step 2. Check for aborted search and immediate draw
562 if ( Threads.stop.load(std::memory_order_relaxed)
563 || pos.is_draw(ss->ply)
564 || ss->ply >= MAX_PLY)
565 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
566 : value_draw(depth, pos.this_thread());
568 // Step 3. Mate distance pruning. Even if we mate at the next move our score
569 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
570 // a shorter mate was found upward in the tree then there is no need to search
571 // because we will never beat the current alpha. Same logic but with reversed
572 // signs applies also in the opposite condition of being mated instead of giving
573 // mate. In this case return a fail-high score.
574 alpha = std::max(mated_in(ss->ply), alpha);
575 beta = std::min(mate_in(ss->ply+1), beta);
580 assert(0 <= ss->ply && ss->ply < MAX_PLY);
582 (ss+1)->ply = ss->ply + 1;
583 (ss+1)->excludedMove = bestMove = MOVE_NONE;
584 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
585 Square prevSq = to_sq((ss-1)->currentMove);
587 // Initialize statScore to zero for the grandchildren of the current position.
588 // So statScore is shared between all grandchildren and only the first grandchild
589 // starts with statScore = 0. Later grandchildren start with the last calculated
590 // statScore of the previous grandchild. This influences the reduction rules in
591 // LMR which are based on the statScore of parent position.
593 (ss + 4)->statScore = 0;
595 (ss + 2)->statScore = 0;
597 // Step 4. Transposition table lookup. We don't want the score of a partial
598 // search to overwrite a previous full search TT value, so we use a different
599 // position key in case of an excluded move.
600 excludedMove = ss->excludedMove;
601 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
602 tte = TT.probe(posKey, ttHit);
603 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
604 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
605 : ttHit ? tte->move() : MOVE_NONE;
606 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
608 // At non-PV nodes we check for an early TT cutoff
611 && tte->depth() >= depth
612 && ttValue != VALUE_NONE // Possible in case of TT access race
613 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
614 : (tte->bound() & BOUND_UPPER)))
616 // If ttMove is quiet, update move sorting heuristics on TT hit
621 if (!pos.capture_or_promotion(ttMove))
622 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
624 // Extra penalty for early quiet moves of the previous ply
625 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
626 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
628 // Penalty for a quiet ttMove that fails low
629 else if (!pos.capture_or_promotion(ttMove))
631 int penalty = -stat_bonus(depth);
632 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
633 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
639 // Step 5. Tablebases probe
640 if (!rootNode && TB::Cardinality)
642 int piecesCount = pos.count<ALL_PIECES>();
644 if ( piecesCount <= TB::Cardinality
645 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
646 && pos.rule50_count() == 0
647 && !pos.can_castle(ANY_CASTLING))
650 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
652 // Force check of time on the next occasion
653 if (thisThread == Threads.main())
654 static_cast<MainThread*>(thisThread)->callsCnt = 0;
656 if (err != TB::ProbeState::FAIL)
658 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
660 int drawScore = TB::UseRule50 ? 1 : 0;
662 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
663 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
664 : VALUE_DRAW + 2 * wdl * drawScore;
666 Bound b = wdl < -drawScore ? BOUND_UPPER
667 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
669 if ( b == BOUND_EXACT
670 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
672 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
673 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
674 MOVE_NONE, VALUE_NONE);
681 if (b == BOUND_LOWER)
682 bestValue = value, alpha = std::max(alpha, bestValue);
690 // Step 6. Static evaluation of the position
693 ss->staticEval = eval = VALUE_NONE;
695 goto moves_loop; // Skip early pruning when in check
699 // Never assume anything on values stored in TT
700 ss->staticEval = eval = tte->eval();
701 if (eval == VALUE_NONE)
702 ss->staticEval = eval = evaluate(pos);
704 // Can ttValue be used as a better position evaluation?
705 if ( ttValue != VALUE_NONE
706 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
711 if ((ss-1)->currentMove != MOVE_NULL)
713 int bonus = -(ss-1)->statScore / 512;
715 ss->staticEval = eval = evaluate(pos) + bonus;
718 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
720 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
723 // Step 7. Razoring (~2 Elo)
724 if ( !rootNode // The required rootNode PV handling is not available in qsearch
725 && depth < 2 * ONE_PLY
726 && eval <= alpha - RazorMargin)
727 return qsearch<NT>(pos, ss, alpha, beta);
729 improving = ss->staticEval >= (ss-2)->staticEval
730 || (ss-2)->staticEval == VALUE_NONE;
732 // Step 8. Futility pruning: child node (~30 Elo)
734 && depth < 7 * ONE_PLY
735 && eval - futility_margin(depth, improving) >= beta
736 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
739 // Step 9. Null move search with verification search (~40 Elo)
741 && (ss-1)->currentMove != MOVE_NULL
742 && (ss-1)->statScore < 23200
744 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
746 && pos.non_pawn_material(us)
747 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
749 assert(eval - beta >= 0);
751 // Null move dynamic reduction based on depth and value
752 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
754 ss->currentMove = MOVE_NULL;
755 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
757 pos.do_null_move(st);
759 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
761 pos.undo_null_move();
763 if (nullValue >= beta)
765 // Do not return unproven mate scores
766 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
769 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
772 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
774 // Do verification search at high depths, with null move pruning disabled
775 // for us, until ply exceeds nmpMinPly.
776 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
777 thisThread->nmpColor = us;
779 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
781 thisThread->nmpMinPly = 0;
788 // Step 10. ProbCut (~10 Elo)
789 // If we have a good enough capture and a reduced search returns a value
790 // much above beta, we can (almost) safely prune the previous move.
792 && depth >= 5 * ONE_PLY
793 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
795 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
796 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
797 int probCutCount = 0;
799 while ( (move = mp.next_move()) != MOVE_NONE
800 && probCutCount < 2 + 2 * cutNode)
801 if (move != excludedMove && pos.legal(move))
805 ss->currentMove = move;
806 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
808 assert(depth >= 5 * ONE_PLY);
810 pos.do_move(move, st);
812 // Perform a preliminary qsearch to verify that the move holds
813 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
815 // If the qsearch held, perform the regular search
816 if (value >= raisedBeta)
817 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
821 if (value >= raisedBeta)
826 // Step 11. Internal iterative deepening (~2 Elo)
827 if (depth >= 8 * ONE_PLY && !ttMove)
829 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
831 tte = TT.probe(posKey, ttHit);
832 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
833 ttMove = ttHit ? tte->move() : MOVE_NONE;
836 moves_loop: // When in check, search starts from here
838 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
839 nullptr, (ss-4)->continuationHistory,
840 nullptr, (ss-6)->continuationHistory };
842 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
844 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
845 &thisThread->captureHistory,
850 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
851 moveCountPruning = false;
852 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
853 int singularExtensionLMRmultiplier = 0;
855 // Step 12. Loop through all pseudo-legal moves until no moves remain
856 // or a beta cutoff occurs.
857 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
861 if (move == excludedMove)
864 // At root obey the "searchmoves" option and skip moves not listed in Root
865 // Move List. As a consequence any illegal move is also skipped. In MultiPV
866 // mode we also skip PV moves which have been already searched and those
867 // of lower "TB rank" if we are in a TB root position.
868 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
869 thisThread->rootMoves.begin() + thisThread->pvLast, move))
872 ss->moveCount = ++moveCount;
874 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
875 sync_cout << "info depth " << depth / ONE_PLY
876 << " currmove " << UCI::move(move, pos.is_chess960())
877 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
879 (ss+1)->pv = nullptr;
881 extension = DEPTH_ZERO;
882 captureOrPromotion = pos.capture_or_promotion(move);
883 movedPiece = pos.moved_piece(move);
884 givesCheck = pos.gives_check(move);
886 // Step 13. Extensions (~70 Elo)
888 // Singular extension search (~60 Elo). If all moves but one fail low on a
889 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
890 // then that move is singular and should be extended. To verify this we do
891 // a reduced search on all the other moves but the ttMove and if the
892 // result is lower than ttValue minus a margin then we will extend the ttMove.
893 if ( depth >= 8 * ONE_PLY
896 && !excludedMove // Avoid recursive singular search
897 /* && ttValue != VALUE_NONE Already implicit in the next condition */
898 && abs(ttValue) < VALUE_KNOWN_WIN
899 && (tte->bound() & BOUND_LOWER)
900 && tte->depth() >= depth - 3 * ONE_PLY
903 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
904 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
905 ss->excludedMove = move;
906 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
907 ss->excludedMove = MOVE_NONE;
909 if (value < singularBeta)
912 singularExtensionLMRmultiplier++;
913 if (value < singularBeta - std::min(3 * depth / ONE_PLY, 39))
914 singularExtensionLMRmultiplier++;
918 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
919 // search without the ttMove. So we assume this expected Cut-node is not singular,
920 // that is multiple moves fail high, and we can prune the whole subtree by returning
921 // the hard beta bound.
922 else if (cutNode && singularBeta > beta)
926 // Check extension (~2 Elo)
928 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
931 // Castling extension
932 else if (type_of(move) == CASTLING)
937 && pos.rule50_count() > 18
938 && depth < 3 * ONE_PLY
939 && ss->ply < 3 * thisThread->rootDepth / ONE_PLY) // To avoid too deep searches
942 // Passed pawn extension
943 else if ( move == ss->killers[0]
944 && pos.advanced_pawn_push(move)
945 && pos.pawn_passed(us, to_sq(move)))
948 // Calculate new depth for this move
949 newDepth = depth - ONE_PLY + extension;
951 // Step 14. Pruning at shallow depth (~170 Elo)
953 && pos.non_pawn_material(us)
954 && bestValue > VALUE_MATED_IN_MAX_PLY)
956 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
957 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
959 if ( !captureOrPromotion
961 && !pos.advanced_pawn_push(move))
963 // Move count based pruning (~30 Elo)
964 if (moveCountPruning)
967 // Reduced depth of the next LMR search
968 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
971 // Countermoves based pruning (~20 Elo)
972 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
973 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
974 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
977 // Futility pruning: parent node (~2 Elo)
980 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
983 // Prune moves with negative SEE (~10 Elo)
984 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
987 else if (!pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
991 // Speculative prefetch as early as possible
992 prefetch(TT.first_entry(pos.key_after(move)));
994 // Check for legality just before making the move
995 if (!rootNode && !pos.legal(move))
997 ss->moveCount = --moveCount;
1001 // Update the current move (this must be done after singular extension search)
1002 ss->currentMove = move;
1003 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1005 // Step 15. Make the move
1006 pos.do_move(move, st, givesCheck);
1008 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1009 // re-searched at full depth.
1010 if ( depth >= 3 * ONE_PLY
1011 && moveCount > 1 + 3 * rootNode
1012 && ( !captureOrPromotion
1014 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1016 Depth r = reduction(improving, depth, moveCount);
1018 // Decrease reduction if position is or has been on the PV
1022 // Decrease reduction if opponent's move count is high (~10 Elo)
1023 if ((ss-1)->moveCount > 15)
1025 // Decrease reduction if move has been singularly extended
1026 r -= singularExtensionLMRmultiplier * ONE_PLY;
1028 if (!captureOrPromotion)
1030 // Increase reduction if ttMove is a capture (~0 Elo)
1034 // Increase reduction for cut nodes (~5 Elo)
1038 // Decrease reduction for moves that escape a capture. Filter out
1039 // castling moves, because they are coded as "king captures rook" and
1040 // hence break make_move(). (~5 Elo)
1041 else if ( type_of(move) == NORMAL
1042 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1045 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1046 + (*contHist[0])[movedPiece][to_sq(move)]
1047 + (*contHist[1])[movedPiece][to_sq(move)]
1048 + (*contHist[3])[movedPiece][to_sq(move)]
1051 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1052 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1055 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1058 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1059 r -= ss->statScore / 20000 * ONE_PLY;
1062 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1064 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1066 doFullDepthSearch = (value > alpha && d != newDepth);
1069 doFullDepthSearch = !PvNode || moveCount > 1;
1071 // Step 17. Full depth search when LMR is skipped or fails high
1072 if (doFullDepthSearch)
1073 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1075 // For PV nodes only, do a full PV search on the first move or after a fail
1076 // high (in the latter case search only if value < beta), otherwise let the
1077 // parent node fail low with value <= alpha and try another move.
1078 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1081 (ss+1)->pv[0] = MOVE_NONE;
1083 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1086 // Step 18. Undo move
1087 pos.undo_move(move);
1089 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1091 // Step 19. Check for a new best move
1092 // Finished searching the move. If a stop occurred, the return value of
1093 // the search cannot be trusted, and we return immediately without
1094 // updating best move, PV and TT.
1095 if (Threads.stop.load(std::memory_order_relaxed))
1100 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1101 thisThread->rootMoves.end(), move);
1103 // PV move or new best move?
1104 if (moveCount == 1 || value > alpha)
1107 rm.selDepth = thisThread->selDepth;
1112 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1113 rm.pv.push_back(*m);
1115 // We record how often the best move has been changed in each
1116 // iteration. This information is used for time management: When
1117 // the best move changes frequently, we allocate some more time.
1119 ++thisThread->bestMoveChanges;
1122 // All other moves but the PV are set to the lowest value: this
1123 // is not a problem when sorting because the sort is stable and the
1124 // move position in the list is preserved - just the PV is pushed up.
1125 rm.score = -VALUE_INFINITE;
1128 if (value > bestValue)
1136 if (PvNode && !rootNode) // Update pv even in fail-high case
1137 update_pv(ss->pv, move, (ss+1)->pv);
1139 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1143 assert(value >= beta); // Fail high
1150 if (move != bestMove)
1152 if (captureOrPromotion && captureCount < 32)
1153 capturesSearched[captureCount++] = move;
1155 else if (!captureOrPromotion && quietCount < 64)
1156 quietsSearched[quietCount++] = move;
1160 // The following condition would detect a stop only after move loop has been
1161 // completed. But in this case bestValue is valid because we have fully
1162 // searched our subtree, and we can anyhow save the result in TT.
1168 // Step 20. Check for mate and stalemate
1169 // All legal moves have been searched and if there are no legal moves, it
1170 // must be a mate or a stalemate. If we are in a singular extension search then
1171 // return a fail low score.
1173 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1176 bestValue = excludedMove ? alpha
1177 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1180 // Quiet best move: update move sorting heuristics
1181 if (!pos.capture_or_promotion(bestMove))
1182 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1183 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1185 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1187 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1188 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1189 && !pos.captured_piece())
1190 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1193 // Bonus for prior countermove that caused the fail low
1194 else if ( (depth >= 3 * ONE_PLY || PvNode)
1195 && !pos.captured_piece())
1196 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1199 bestValue = std::min(bestValue, maxValue);
1202 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1203 bestValue >= beta ? BOUND_LOWER :
1204 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1205 depth, bestMove, ss->staticEval);
1207 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1213 // qsearch() is the quiescence search function, which is called by the main search
1214 // function with zero depth, or recursively with further decreasing depth per call.
1215 template <NodeType NT>
1216 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1218 constexpr bool PvNode = NT == PV;
1220 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1221 assert(PvNode || (alpha == beta - 1));
1222 assert(depth <= DEPTH_ZERO);
1223 assert(depth / ONE_PLY * ONE_PLY == depth);
1229 Move ttMove, move, bestMove;
1231 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1232 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1237 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1239 ss->pv[0] = MOVE_NONE;
1242 Thread* thisThread = pos.this_thread();
1243 (ss+1)->ply = ss->ply + 1;
1244 bestMove = MOVE_NONE;
1245 inCheck = pos.checkers();
1248 // Check for an immediate draw or maximum ply reached
1249 if ( pos.is_draw(ss->ply)
1250 || ss->ply >= MAX_PLY)
1251 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1253 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1255 // Decide whether or not to include checks: this fixes also the type of
1256 // TT entry depth that we are going to use. Note that in qsearch we use
1257 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1258 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1259 : DEPTH_QS_NO_CHECKS;
1260 // Transposition table lookup
1262 tte = TT.probe(posKey, ttHit);
1263 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1264 ttMove = ttHit ? tte->move() : MOVE_NONE;
1265 pvHit = ttHit && tte->is_pv();
1269 && tte->depth() >= ttDepth
1270 && ttValue != VALUE_NONE // Only in case of TT access race
1271 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1272 : (tte->bound() & BOUND_UPPER)))
1275 // Evaluate the position statically
1278 ss->staticEval = VALUE_NONE;
1279 bestValue = futilityBase = -VALUE_INFINITE;
1285 // Never assume anything on values stored in TT
1286 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1287 ss->staticEval = bestValue = evaluate(pos);
1289 // Can ttValue be used as a better position evaluation?
1290 if ( ttValue != VALUE_NONE
1291 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1292 bestValue = ttValue;
1295 ss->staticEval = bestValue =
1296 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1297 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1299 // Stand pat. Return immediately if static value is at least beta
1300 if (bestValue >= beta)
1303 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1304 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1309 if (PvNode && bestValue > alpha)
1312 futilityBase = bestValue + 128;
1315 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1316 nullptr, (ss-4)->continuationHistory,
1317 nullptr, (ss-6)->continuationHistory };
1319 // Initialize a MovePicker object for the current position, and prepare
1320 // to search the moves. Because the depth is <= 0 here, only captures,
1321 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1323 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1324 &thisThread->captureHistory,
1326 to_sq((ss-1)->currentMove));
1328 // Loop through the moves until no moves remain or a beta cutoff occurs
1329 while ((move = mp.next_move()) != MOVE_NONE)
1331 assert(is_ok(move));
1333 givesCheck = pos.gives_check(move);
1340 && futilityBase > -VALUE_KNOWN_WIN
1341 && !pos.advanced_pawn_push(move))
1343 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1345 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1347 if (futilityValue <= alpha)
1349 bestValue = std::max(bestValue, futilityValue);
1353 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1355 bestValue = std::max(bestValue, futilityBase);
1360 // Detect non-capture evasions that are candidates to be pruned
1361 evasionPrunable = inCheck
1362 && (depth != DEPTH_ZERO || moveCount > 2)
1363 && bestValue > VALUE_MATED_IN_MAX_PLY
1364 && !pos.capture(move);
1366 // Don't search moves with negative SEE values
1367 if ( (!inCheck || evasionPrunable)
1368 && !pos.see_ge(move))
1371 // Speculative prefetch as early as possible
1372 prefetch(TT.first_entry(pos.key_after(move)));
1374 // Check for legality just before making the move
1375 if (!pos.legal(move))
1381 ss->currentMove = move;
1382 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1384 // Make and search the move
1385 pos.do_move(move, st, givesCheck);
1386 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1387 pos.undo_move(move);
1389 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1391 // Check for a new best move
1392 if (value > bestValue)
1400 if (PvNode) // Update pv even in fail-high case
1401 update_pv(ss->pv, move, (ss+1)->pv);
1403 if (PvNode && value < beta) // Update alpha here!
1411 // All legal moves have been searched. A special case: If we're in check
1412 // and no legal moves were found, it is checkmate.
1413 if (inCheck && bestValue == -VALUE_INFINITE)
1414 return mated_in(ss->ply); // Plies to mate from the root
1416 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1417 bestValue >= beta ? BOUND_LOWER :
1418 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1419 ttDepth, bestMove, ss->staticEval);
1421 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1427 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1428 // "plies to mate from the current position". Non-mate scores are unchanged.
1429 // The function is called before storing a value in the transposition table.
1431 Value value_to_tt(Value v, int ply) {
1433 assert(v != VALUE_NONE);
1435 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1436 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1440 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1441 // from the transposition table (which refers to the plies to mate/be mated
1442 // from current position) to "plies to mate/be mated from the root".
1444 Value value_from_tt(Value v, int ply) {
1446 return v == VALUE_NONE ? VALUE_NONE
1447 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1448 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1452 // update_pv() adds current move and appends child pv[]
1454 void update_pv(Move* pv, Move move, Move* childPv) {
1456 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1462 // update_continuation_histories() updates histories of the move pairs formed
1463 // by moves at ply -1, -2, and -4 with current move.
1465 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1467 for (int i : {1, 2, 4, 6})
1468 if (is_ok((ss-i)->currentMove))
1469 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1473 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1475 void update_capture_stats(const Position& pos, Move move,
1476 Move* captures, int captureCount, int bonus) {
1478 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1479 Piece moved_piece = pos.moved_piece(move);
1480 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1482 if (pos.capture_or_promotion(move))
1483 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1485 // Decrease all the other played capture moves
1486 for (int i = 0; i < captureCount; ++i)
1488 moved_piece = pos.moved_piece(captures[i]);
1489 captured = type_of(pos.piece_on(to_sq(captures[i])));
1490 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1495 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1497 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1498 Move* quiets, int quietCount, int bonus) {
1500 if (ss->killers[0] != move)
1502 ss->killers[1] = ss->killers[0];
1503 ss->killers[0] = move;
1506 Color us = pos.side_to_move();
1507 Thread* thisThread = pos.this_thread();
1508 thisThread->mainHistory[us][from_to(move)] << bonus;
1509 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1511 if (is_ok((ss-1)->currentMove))
1513 Square prevSq = to_sq((ss-1)->currentMove);
1514 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1517 // Decrease all the other played quiet moves
1518 for (int i = 0; i < quietCount; ++i)
1520 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1521 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1525 // When playing with strength handicap, choose best move among a set of RootMoves
1526 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1528 Move Skill::pick_best(size_t multiPV) {
1530 const RootMoves& rootMoves = Threads.main()->rootMoves;
1531 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1533 // RootMoves are already sorted by score in descending order
1534 Value topScore = rootMoves[0].score;
1535 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1536 int weakness = 120 - 2 * level;
1537 int maxScore = -VALUE_INFINITE;
1539 // Choose best move. For each move score we add two terms, both dependent on
1540 // weakness. One is deterministic and bigger for weaker levels, and one is
1541 // random. Then we choose the move with the resulting highest score.
1542 for (size_t i = 0; i < multiPV; ++i)
1544 // This is our magic formula
1545 int push = ( weakness * int(topScore - rootMoves[i].score)
1546 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1548 if (rootMoves[i].score + push >= maxScore)
1550 maxScore = rootMoves[i].score + push;
1551 best = rootMoves[i].pv[0];
1560 /// MainThread::check_time() is used to print debug info and, more importantly,
1561 /// to detect when we are out of available time and thus stop the search.
1563 void MainThread::check_time() {
1568 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1569 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1571 static TimePoint lastInfoTime = now();
1573 TimePoint elapsed = Time.elapsed();
1574 TimePoint tick = Limits.startTime + elapsed;
1576 if (tick - lastInfoTime >= 1000)
1578 lastInfoTime = tick;
1582 // We should not stop pondering until told so by the GUI
1586 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1587 || (Limits.movetime && elapsed >= Limits.movetime)
1588 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1589 Threads.stop = true;
1593 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1594 /// that all (if any) unsearched PV lines are sent using a previous search score.
1596 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1598 std::stringstream ss;
1599 TimePoint elapsed = Time.elapsed() + 1;
1600 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1601 size_t pvIdx = pos.this_thread()->pvIdx;
1602 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1603 uint64_t nodesSearched = Threads.nodes_searched();
1604 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1606 for (size_t i = 0; i < multiPV; ++i)
1608 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1610 if (depth == ONE_PLY && !updated)
1613 Depth d = updated ? depth : depth - ONE_PLY;
1614 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1616 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1617 v = tb ? rootMoves[i].tbScore : v;
1619 if (ss.rdbuf()->in_avail()) // Not at first line
1623 << " depth " << d / ONE_PLY
1624 << " seldepth " << rootMoves[i].selDepth
1625 << " multipv " << i + 1
1626 << " score " << UCI::value(v);
1628 if (!tb && i == pvIdx)
1629 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1631 ss << " nodes " << nodesSearched
1632 << " nps " << nodesSearched * 1000 / elapsed;
1634 if (elapsed > 1000) // Earlier makes little sense
1635 ss << " hashfull " << TT.hashfull();
1637 ss << " tbhits " << tbHits
1638 << " time " << elapsed
1641 for (Move m : rootMoves[i].pv)
1642 ss << " " << UCI::move(m, pos.is_chess960());
1649 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1650 /// before exiting the search, for instance, in case we stop the search during a
1651 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1652 /// otherwise in case of 'ponder on' we have nothing to think on.
1654 bool RootMove::extract_ponder_from_tt(Position& pos) {
1659 assert(pv.size() == 1);
1661 if (pv[0] == MOVE_NONE)
1664 pos.do_move(pv[0], st);
1665 TTEntry* tte = TT.probe(pos.key(), ttHit);
1669 Move m = tte->move(); // Local copy to be SMP safe
1670 if (MoveList<LEGAL>(pos).contains(m))
1674 pos.undo_move(pv[0]);
1675 return pv.size() > 1;
1678 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1681 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1682 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1683 Cardinality = int(Options["SyzygyProbeLimit"]);
1684 bool dtz_available = true;
1686 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1687 // ProbeDepth == DEPTH_ZERO
1688 if (Cardinality > MaxCardinality)
1690 Cardinality = MaxCardinality;
1691 ProbeDepth = DEPTH_ZERO;
1694 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1696 // Rank moves using DTZ tables
1697 RootInTB = root_probe(pos, rootMoves);
1701 // DTZ tables are missing; try to rank moves using WDL tables
1702 dtz_available = false;
1703 RootInTB = root_probe_wdl(pos, rootMoves);
1709 // Sort moves according to TB rank
1710 std::sort(rootMoves.begin(), rootMoves.end(),
1711 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1713 // Probe during search only if DTZ is not available and we are winning
1714 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1719 // Assign the same rank to all moves
1720 for (auto& m : rootMoves)