2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2022 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
22 #include <cstring> // For std::memset
36 #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, Root };
65 Value futility_margin(Depth d, bool improving) {
66 return Value(168 * (d - improving));
69 // Reductions lookup table, initialized at startup
70 int Reductions[MAX_MOVES]; // [depth or moveNumber]
72 Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {
73 int r = Reductions[d] * Reductions[mn];
74 return (r + 1463 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 1010);
77 constexpr int futility_move_count(bool improving, Depth depth) {
78 return improving ? (3 + depth * depth)
79 : (3 + depth * depth) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth d) {
84 return std::min((9 * d + 270) * d - 311 , 2145);
87 // Add a small random component to draw evaluations to avoid 3-fold blindness
88 Value value_draw(Thread* thisThread) {
89 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
92 // Skill structure is used to implement strength limit. If we have an uci_elo then
93 // we convert it to a suitable fractional skill level using anchoring to CCRL Elo
94 // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
95 // results spanning a wide range of k values.
97 Skill(int skill_level, int uci_elo) {
99 level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0);
101 level = double(skill_level);
103 bool enabled() const { return level < 20.0; }
104 bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
105 Move pick_best(size_t multiPV);
108 Move best = MOVE_NONE;
111 template <NodeType nodeType>
112 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
114 template <NodeType nodeType>
115 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
117 Value value_to_tt(Value v, int ply);
118 Value value_from_tt(Value v, int ply, int r50c);
119 void update_pv(Move* pv, Move move, Move* childPv);
120 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
121 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
122 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
123 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
125 // perft() is our utility to verify move generation. All the leaf nodes up
126 // to the given depth are generated and counted, and the sum is returned.
128 uint64_t perft(Position& pos, Depth depth) {
131 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
133 uint64_t cnt, nodes = 0;
134 const bool leaf = (depth == 2);
136 for (const auto& m : MoveList<LEGAL>(pos))
138 if (Root && depth <= 1)
143 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
148 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
156 /// Search::init() is called at startup to initialize various lookup tables
158 void Search::init() {
160 for (int i = 1; i < MAX_MOVES; ++i)
161 Reductions[i] = int((20.81 + std::log(Threads.size()) / 2) * std::log(i));
165 /// Search::clear() resets search state to its initial value
167 void Search::clear() {
169 Threads.main()->wait_for_search_finished();
171 Time.availableNodes = 0;
174 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
178 /// MainThread::search() is started when the program receives the UCI 'go'
179 /// command. It searches from the root position and outputs the "bestmove".
181 void MainThread::search() {
185 nodes = perft<true>(rootPos, Limits.perft);
186 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
190 Color us = rootPos.side_to_move();
191 Time.init(Limits, us, rootPos.game_ply());
194 Eval::NNUE::verify();
196 if (rootMoves.empty())
198 rootMoves.emplace_back(MOVE_NONE);
199 sync_cout << "info depth 0 score "
200 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
205 Threads.start_searching(); // start non-main threads
206 Thread::search(); // main thread start searching
209 // When we reach the maximum depth, we can arrive here without a raise of
210 // Threads.stop. However, if we are pondering or in an infinite search,
211 // the UCI protocol states that we shouldn't print the best move before the
212 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
213 // until the GUI sends one of those commands.
215 while (!Threads.stop && (ponder || Limits.infinite))
216 {} // Busy wait for a stop or a ponder reset
218 // Stop the threads if not already stopped (also raise the stop if
219 // "ponderhit" just reset Threads.ponder).
222 // Wait until all threads have finished
223 Threads.wait_for_search_finished();
225 // When playing in 'nodes as time' mode, subtract the searched nodes from
226 // the available ones before exiting.
228 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
230 Thread* bestThread = this;
231 Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
233 if ( int(Options["MultiPV"]) == 1
236 && rootMoves[0].pv[0] != MOVE_NONE)
237 bestThread = Threads.get_best_thread();
239 bestPreviousScore = bestThread->rootMoves[0].score;
240 bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
242 for (Thread* th : Threads)
243 th->previousDepth = bestThread->completedDepth;
245 // Send again PV info if we have a new best thread
246 if (bestThread != this)
247 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
249 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
251 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
252 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
254 std::cout << sync_endl;
258 /// Thread::search() is the main iterative deepening loop. It calls search()
259 /// repeatedly with increasing depth until the allocated thinking time has been
260 /// consumed, the user stops the search, or the maximum search depth is reached.
262 void Thread::search() {
264 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
265 // The former is needed to allow update_continuation_histories(ss-1, ...),
266 // which accesses its argument at ss-6, also near the root.
267 // The latter is needed for statScore and killer initialization.
268 Stack stack[MAX_PLY+10], *ss = stack+7;
270 Value alpha, beta, delta;
271 Move lastBestMove = MOVE_NONE;
272 Depth lastBestMoveDepth = 0;
273 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
274 double timeReduction = 1, totBestMoveChanges = 0;
275 Color us = rootPos.side_to_move();
278 std::memset(ss-7, 0, 10 * sizeof(Stack));
279 for (int i = 7; i > 0; i--)
280 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
282 for (int i = 0; i <= MAX_PLY + 2; ++i)
287 bestValue = delta = alpha = -VALUE_INFINITE;
288 beta = VALUE_INFINITE;
292 if (mainThread->bestPreviousScore == VALUE_INFINITE)
293 for (int i = 0; i < 4; ++i)
294 mainThread->iterValue[i] = VALUE_ZERO;
296 for (int i = 0; i < 4; ++i)
297 mainThread->iterValue[i] = mainThread->bestPreviousScore;
300 size_t multiPV = size_t(Options["MultiPV"]);
301 Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
303 // When playing with strength handicap enable MultiPV search that we will
304 // use behind the scenes to retrieve a set of possible moves.
306 multiPV = std::max(multiPV, (size_t)4);
308 multiPV = std::min(multiPV, rootMoves.size());
310 complexityAverage.set(202, 1);
313 optimism[ us] = Value(39);
314 optimism[~us] = -optimism[us];
316 int searchAgainCounter = 0;
318 // Iterative deepening loop until requested to stop or the target depth is reached
319 while ( ++rootDepth < MAX_PLY
321 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
323 // Age out PV variability metric
325 totBestMoveChanges /= 2;
327 // Save the last iteration's scores before first PV line is searched and
328 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
329 for (RootMove& rm : rootMoves)
330 rm.previousScore = rm.score;
335 if (!Threads.increaseDepth)
336 searchAgainCounter++;
338 // MultiPV loop. We perform a full root search for each PV line
339 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
344 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
345 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
349 // Reset UCI info selDepth for each depth and each PV line
352 // Reset aspiration window starting size
355 Value prev = rootMoves[pvIdx].averageScore;
356 delta = Value(16) + int(prev) * prev / 19178;
357 alpha = std::max(prev - delta,-VALUE_INFINITE);
358 beta = std::min(prev + delta, VALUE_INFINITE);
360 // Adjust trend and optimism based on root move's previousScore
361 int tr = sigmoid(prev, 3, 8, 90, 125, 1);
362 trend = (us == WHITE ? make_score(tr, tr / 2)
363 : -make_score(tr, tr / 2));
365 int opt = sigmoid(prev, 8, 17, 144, 13966, 183);
366 optimism[ us] = Value(opt);
367 optimism[~us] = -optimism[us];
370 // Start with a small aspiration window and, in the case of a fail
371 // high/low, re-search with a bigger window until we don't fail
373 int failedHighCnt = 0;
376 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
377 bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
379 // Bring the best move to the front. It is critical that sorting
380 // is done with a stable algorithm because all the values but the
381 // first and eventually the new best one are set to -VALUE_INFINITE
382 // and we want to keep the same order for all the moves except the
383 // new PV that goes to the front. Note that in case of MultiPV
384 // search the already searched PV lines are preserved.
385 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
387 // If search has been stopped, we break immediately. Sorting is
388 // safe because RootMoves is still valid, although it refers to
389 // the previous iteration.
393 // When failing high/low give some update (without cluttering
394 // the UI) before a re-search.
397 && (bestValue <= alpha || bestValue >= beta)
398 && Time.elapsed() > 3000)
399 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
401 // In case of failing low/high increase aspiration window and
402 // re-search, otherwise exit the loop.
403 if (bestValue <= alpha)
405 beta = (alpha + beta) / 2;
406 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
410 mainThread->stopOnPonderhit = false;
412 else if (bestValue >= beta)
414 beta = std::min(bestValue + delta, VALUE_INFINITE);
420 delta += delta / 4 + 2;
422 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
425 // Sort the PV lines searched so far and update the GUI
426 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
429 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
430 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
434 completedDepth = rootDepth;
436 if (rootMoves[0].pv[0] != lastBestMove) {
437 lastBestMove = rootMoves[0].pv[0];
438 lastBestMoveDepth = rootDepth;
441 // Have we found a "mate in x"?
443 && bestValue >= VALUE_MATE_IN_MAX_PLY
444 && VALUE_MATE - bestValue <= 2 * Limits.mate)
450 // If skill level is enabled and time is up, pick a sub-optimal best move
451 if (skill.enabled() && skill.time_to_pick(rootDepth))
452 skill.pick_best(multiPV);
454 // Use part of the gained time from a previous stable move for the current move
455 for (Thread* th : Threads)
457 totBestMoveChanges += th->bestMoveChanges;
458 th->bestMoveChanges = 0;
461 // Do we have time for the next iteration? Can we stop searching now?
462 if ( Limits.use_time_management()
464 && !mainThread->stopOnPonderhit)
466 double fallingEval = (69 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
467 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 781.4;
468 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
470 // If the bestMove is stable over several iterations, reduce time accordingly
471 timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.63 : 0.73;
472 double reduction = (1.56 + mainThread->previousTimeReduction) / (2.20 * timeReduction);
473 double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();
474 int complexity = mainThread->complexityAverage.value();
475 double complexPosition = std::clamp(1.0 + (complexity - 326) / 1618.1, 0.5, 1.5);
477 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
479 // Cap used time in case of a single legal move for a better viewer experience in tournaments
480 // yielding correct scores and sufficiently fast moves.
481 if (rootMoves.size() == 1)
482 totalTime = std::min(500.0, totalTime);
484 // Stop the search if we have exceeded the totalTime
485 if (Time.elapsed() > totalTime)
487 // If we are allowed to ponder do not stop the search now but
488 // keep pondering until the GUI sends "ponderhit" or "stop".
489 if (mainThread->ponder)
490 mainThread->stopOnPonderhit = true;
494 else if ( Threads.increaseDepth
495 && !mainThread->ponder
496 && Time.elapsed() > totalTime * 0.43)
497 Threads.increaseDepth = false;
499 Threads.increaseDepth = true;
502 mainThread->iterValue[iterIdx] = bestValue;
503 iterIdx = (iterIdx + 1) & 3;
509 mainThread->previousTimeReduction = timeReduction;
511 // If skill level is enabled, swap best PV line with the sub-optimal one
513 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
514 skill.best ? skill.best : skill.pick_best(multiPV)));
520 // search<>() is the main search function for both PV and non-PV nodes
522 template <NodeType nodeType>
523 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
525 constexpr bool PvNode = nodeType != NonPV;
526 constexpr bool rootNode = nodeType == Root;
527 const Depth maxNextDepth = rootNode ? depth : depth + 1;
529 // Check if we have an upcoming move which draws by repetition, or
530 // if the opponent had an alternative move earlier to this position.
532 && pos.rule50_count() >= 3
533 && alpha < VALUE_DRAW
534 && pos.has_game_cycle(ss->ply))
536 alpha = value_draw(pos.this_thread());
541 // Dive into quiescence search when the depth reaches zero
543 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
545 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
546 assert(PvNode || (alpha == beta - 1));
547 assert(0 < depth && depth < MAX_PLY);
548 assert(!(PvNode && cutNode));
550 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
552 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
556 Move ttMove, move, excludedMove, bestMove;
557 Depth extension, newDepth;
558 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
559 bool givesCheck, improving, didLMR, priorCapture;
560 bool capture, doFullDepthSearch, moveCountPruning, ttCapture;
562 int moveCount, captureCount, quietCount, improvement, complexity;
564 // Step 1. Initialize node
565 Thread* thisThread = pos.this_thread();
566 thisThread->depth = depth;
567 ss->inCheck = pos.checkers();
568 priorCapture = pos.captured_piece();
569 Color us = pos.side_to_move();
570 moveCount = captureCount = quietCount = ss->moveCount = 0;
571 bestValue = -VALUE_INFINITE;
572 maxValue = VALUE_INFINITE;
574 // Check for the available remaining time
575 if (thisThread == Threads.main())
576 static_cast<MainThread*>(thisThread)->check_time();
578 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
579 if (PvNode && thisThread->selDepth < ss->ply + 1)
580 thisThread->selDepth = ss->ply + 1;
584 // Step 2. Check for aborted search and immediate draw
585 if ( Threads.stop.load(std::memory_order_relaxed)
586 || pos.is_draw(ss->ply)
587 || ss->ply >= MAX_PLY)
588 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
589 : value_draw(pos.this_thread());
591 // Step 3. Mate distance pruning. Even if we mate at the next move our score
592 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
593 // a shorter mate was found upward in the tree then there is no need to search
594 // because we will never beat the current alpha. Same logic but with reversed
595 // signs applies also in the opposite condition of being mated instead of giving
596 // mate. In this case return a fail-high score.
597 alpha = std::max(mated_in(ss->ply), alpha);
598 beta = std::min(mate_in(ss->ply+1), beta);
603 thisThread->rootDelta = beta - alpha;
605 assert(0 <= ss->ply && ss->ply < MAX_PLY);
607 (ss+1)->ttPv = false;
608 (ss+1)->excludedMove = bestMove = MOVE_NONE;
609 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
610 (ss+2)->cutoffCnt = 0;
611 ss->doubleExtensions = (ss-1)->doubleExtensions;
612 Square prevSq = to_sq((ss-1)->currentMove);
614 // Initialize statScore to zero for the grandchildren of the current position.
615 // So statScore is shared between all grandchildren and only the first grandchild
616 // starts with statScore = 0. Later grandchildren start with the last calculated
617 // statScore of the previous grandchild. This influences the reduction rules in
618 // LMR which are based on the statScore of parent position.
620 (ss+2)->statScore = 0;
622 // Step 4. Transposition table lookup. We don't want the score of a partial
623 // search to overwrite a previous full search TT value, so we use a different
624 // position key in case of an excluded move.
625 excludedMove = ss->excludedMove;
626 posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
627 tte = TT.probe(posKey, ss->ttHit);
628 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
629 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
630 : ss->ttHit ? tte->move() : MOVE_NONE;
631 ttCapture = ttMove && pos.capture(ttMove);
633 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
635 // At non-PV nodes we check for an early TT cutoff
638 && tte->depth() > depth - (thisThread->id() % 2 == 1)
639 && ttValue != VALUE_NONE // Possible in case of TT access race
640 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
641 : (tte->bound() & BOUND_UPPER)))
643 // If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)
648 // Bonus for a quiet ttMove that fails high (~3 Elo)
650 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
652 // Extra penalty for early quiet moves of the previous ply (~0 Elo)
653 if ((ss-1)->moveCount <= 2 && !priorCapture)
654 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
656 // Penalty for a quiet ttMove that fails low (~1 Elo)
659 int penalty = -stat_bonus(depth);
660 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
661 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
665 // Partial workaround for the graph history interaction problem
666 // For high rule50 counts don't produce transposition table cutoffs.
667 if (pos.rule50_count() < 90)
671 // Step 5. Tablebases probe
672 if (!rootNode && TB::Cardinality)
674 int piecesCount = pos.count<ALL_PIECES>();
676 if ( piecesCount <= TB::Cardinality
677 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
678 && pos.rule50_count() == 0
679 && !pos.can_castle(ANY_CASTLING))
682 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
684 // Force check of time on the next occasion
685 if (thisThread == Threads.main())
686 static_cast<MainThread*>(thisThread)->callsCnt = 0;
688 if (err != TB::ProbeState::FAIL)
690 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
692 int drawScore = TB::UseRule50 ? 1 : 0;
694 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
695 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
696 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
697 : VALUE_DRAW + 2 * wdl * drawScore;
699 Bound b = wdl < -drawScore ? BOUND_UPPER
700 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
702 if ( b == BOUND_EXACT
703 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
705 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
706 std::min(MAX_PLY - 1, depth + 6),
707 MOVE_NONE, VALUE_NONE);
714 if (b == BOUND_LOWER)
715 bestValue = value, alpha = std::max(alpha, bestValue);
723 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
725 // Step 6. Static evaluation of the position
728 // Skip early pruning when in check
729 ss->staticEval = eval = VALUE_NONE;
737 // Never assume anything about values stored in TT
738 ss->staticEval = eval = tte->eval();
739 if (eval == VALUE_NONE)
740 ss->staticEval = eval = evaluate(pos);
742 // Randomize draw evaluation
743 if (eval == VALUE_DRAW)
744 eval = value_draw(thisThread);
746 // ttValue can be used as a better position evaluation (~4 Elo)
747 if ( ttValue != VALUE_NONE
748 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
753 ss->staticEval = eval = evaluate(pos);
755 // Save static evaluation into transposition table
757 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
760 // Use static evaluation difference to improve quiet move ordering (~3 Elo)
761 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
763 int bonus = std::clamp(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000);
764 thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
767 // Set up the improvement variable, which is the difference between the current
768 // static evaluation and the previous static evaluation at our turn (if we were
769 // in check at our previous move we look at the move prior to it). The improvement
770 // margin and the improving flag are used in various pruning heuristics.
771 improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
772 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
775 improving = improvement > 0;
776 complexity = abs(ss->staticEval - (us == WHITE ? eg_value(pos.psq_score()) : -eg_value(pos.psq_score())));
778 thisThread->complexityAverage.update(complexity);
781 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
782 // return a fail low.
785 && eval < alpha - 348 - 258 * depth * depth)
787 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
792 // Step 8. Futility pruning: child node (~25 Elo).
793 // The depth condition is important for mate finding.
796 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta
798 && eval < 26305) // larger than VALUE_KNOWN_WIN, but smaller than TB wins.
801 // Step 9. Null move search with verification search (~22 Elo)
803 && (ss-1)->currentMove != MOVE_NULL
804 && (ss-1)->statScore < 14695
806 && eval >= ss->staticEval
807 && ss->staticEval >= beta - 15 * depth - improvement / 15 + 198 + complexity / 28
809 && pos.non_pawn_material(us)
810 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
812 assert(eval - beta >= 0);
814 // Null move dynamic reduction based on depth, eval and complexity of position
815 Depth R = std::min(int(eval - beta) / 147, 5) + depth / 3 + 4 - (complexity > 753);
817 ss->currentMove = MOVE_NULL;
818 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
820 pos.do_null_move(st);
822 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
824 pos.undo_null_move();
826 if (nullValue >= beta)
828 // Do not return unproven mate or TB scores
829 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
832 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
835 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
837 // Do verification search at high depths, with null move pruning disabled
838 // for us, until ply exceeds nmpMinPly.
839 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
840 thisThread->nmpColor = us;
842 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
844 thisThread->nmpMinPly = 0;
851 probCutBeta = beta + 179 - 46 * improving;
853 // Step 10. ProbCut (~4 Elo)
854 // If we have a good enough capture and a reduced search returns a value
855 // much above beta, we can (almost) safely prune the previous move.
858 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
859 // if value from transposition table is lower than probCutBeta, don't attempt probCut
860 // there and in further interactions with transposition table cutoff depth is set to depth - 3
861 // because probCut search has depth set to depth - 4 but we also do a move before it
862 // so effective depth is equal to depth - 3
864 && tte->depth() >= depth - 3
865 && ttValue != VALUE_NONE
866 && ttValue < probCutBeta))
868 assert(probCutBeta < VALUE_INFINITE);
870 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);
871 bool ttPv = ss->ttPv;
874 while ((move = mp.next_move()) != MOVE_NONE)
875 if (move != excludedMove && pos.legal(move))
877 assert(pos.capture(move) || promotion_type(move) == QUEEN);
879 ss->currentMove = move;
880 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
882 [pos.moved_piece(move)]
885 pos.do_move(move, st);
887 // Perform a preliminary qsearch to verify that the move holds
888 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
890 // If the qsearch held, perform the regular search
891 if (value >= probCutBeta)
892 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
896 if (value >= probCutBeta)
898 // if transposition table doesn't have equal or more deep info write probCut data into it
900 && tte->depth() >= depth - 3
901 && ttValue != VALUE_NONE))
902 tte->save(posKey, value_to_tt(value, ss->ply), ttPv,
904 depth - 3, move, ss->staticEval);
911 // Step 11. If the position is not in TT, decrease depth by 3.
912 // Use qsearch if depth is equal or below zero (~4 Elo)
918 return qsearch<PV>(pos, ss, alpha, beta);
925 moves_loop: // When in check, search starts here
927 // Step 12. A small Probcut idea, when we are in check (~0 Elo)
928 probCutBeta = beta + 481;
933 && (tte->bound() & BOUND_LOWER)
934 && tte->depth() >= depth - 3
935 && ttValue >= probCutBeta
936 && abs(ttValue) <= VALUE_KNOWN_WIN
937 && abs(beta) <= VALUE_KNOWN_WIN
942 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
943 nullptr , (ss-4)->continuationHistory,
944 nullptr , (ss-6)->continuationHistory };
946 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
948 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
955 moveCountPruning = false;
957 // Indicate PvNodes that will probably fail low if the node was searched
958 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
959 bool likelyFailLow = PvNode
961 && (tte->bound() & BOUND_UPPER)
962 && tte->depth() >= depth;
964 // Step 13. Loop through all pseudo-legal moves until no moves remain
965 // or a beta cutoff occurs.
966 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
970 if (move == excludedMove)
973 // At root obey the "searchmoves" option and skip moves not listed in Root
974 // Move List. As a consequence any illegal move is also skipped. In MultiPV
975 // mode we also skip PV moves which have been already searched and those
976 // of lower "TB rank" if we are in a TB root position.
977 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
978 thisThread->rootMoves.begin() + thisThread->pvLast, move))
981 // Check for legality
982 if (!rootNode && !pos.legal(move))
985 ss->moveCount = ++moveCount;
987 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
988 sync_cout << "info depth " << depth
989 << " currmove " << UCI::move(move, pos.is_chess960())
990 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
992 (ss+1)->pv = nullptr;
995 capture = pos.capture(move);
996 movedPiece = pos.moved_piece(move);
997 givesCheck = pos.gives_check(move);
999 // Calculate new depth for this move
1000 newDepth = depth - 1;
1002 Value delta = beta - alpha;
1004 // Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.
1006 && pos.non_pawn_material(us)
1007 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1009 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo)
1010 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1012 // Reduced depth of the next LMR search
1013 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0);
1018 // Futility pruning for captures (~0 Elo)
1019 if ( !pos.empty(to_sq(move))
1024 && ss->staticEval + 281 + 179 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1025 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1028 // SEE based pruning (~9 Elo)
1029 if (!pos.see_ge(move, Value(-203) * depth))
1034 int history = (*contHist[0])[movedPiece][to_sq(move)]
1035 + (*contHist[1])[movedPiece][to_sq(move)]
1036 + (*contHist[3])[movedPiece][to_sq(move)];
1038 // Continuation history based pruning (~2 Elo)
1040 && history < -3875 * (depth - 1))
1043 history += thisThread->mainHistory[us][from_to(move)];
1045 // Futility pruning: parent node (~9 Elo)
1048 && ss->staticEval + 122 + 138 * lmrDepth + history / 60 <= alpha)
1051 // Prune moves with negative SEE (~3 Elo)
1052 if (!pos.see_ge(move, Value(-25 * lmrDepth * lmrDepth - 20 * lmrDepth)))
1057 // Step 15. Extensions (~66 Elo)
1058 // We take care to not overdo to avoid search getting stuck.
1059 if (ss->ply < thisThread->rootDepth * 2)
1061 // Singular extension search (~58 Elo). If all moves but one fail low on a
1062 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1063 // then that move is singular and should be extended. To verify this we do
1064 // a reduced search on all the other moves but the ttMove and if the
1065 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1067 && depth >= 4 - (thisThread->previousDepth > 27) + 2 * (PvNode && tte->is_pv())
1069 && !excludedMove // Avoid recursive singular search
1070 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1071 && abs(ttValue) < VALUE_KNOWN_WIN
1072 && (tte->bound() & BOUND_LOWER)
1073 && tte->depth() >= depth - 3)
1075 Value singularBeta = ttValue - 3 * depth;
1076 Depth singularDepth = (depth - 1) / 2;
1078 ss->excludedMove = move;
1079 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1080 ss->excludedMove = MOVE_NONE;
1082 if (value < singularBeta)
1086 // Avoid search explosion by limiting the number of double extensions
1088 && value < singularBeta - 26
1089 && ss->doubleExtensions <= 8)
1093 // Multi-cut pruning
1094 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1095 // search without the ttMove. So we assume this expected Cut-node is not singular,
1096 // that multiple moves fail high, and we can prune the whole subtree by returning
1098 else if (singularBeta >= beta)
1099 return singularBeta;
1101 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1102 else if (ttValue >= beta)
1105 // If the eval of ttMove is less than alpha and value, we reduce it (negative extension)
1106 else if (ttValue <= alpha && ttValue <= value)
1110 // Check extensions (~1 Elo)
1111 else if ( givesCheck
1113 && abs(ss->staticEval) > 71)
1116 // Quiet ttMove extensions (~0 Elo)
1119 && move == ss->killers[0]
1120 && (*contHist[0])[movedPiece][to_sq(move)] >= 5491)
1124 // Add extension to new depth
1125 newDepth += extension;
1126 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1128 // Speculative prefetch as early as possible
1129 prefetch(TT.first_entry(pos.key_after(move)));
1131 // Update the current move (this must be done after singular extension search)
1132 ss->currentMove = move;
1133 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1138 // Step 16. Make the move
1139 pos.do_move(move, st, givesCheck);
1141 bool doDeeperSearch = false;
1143 // Step 17. Late moves reduction / extension (LMR, ~98 Elo)
1144 // We use various heuristics for the sons of a node after the first son has
1145 // been searched. In general we would like to reduce them, but there are many
1146 // cases where we extend a son if it has good chances to be "interesting".
1148 && moveCount > 1 + (PvNode && ss->ply <= 1)
1151 || (cutNode && (ss-1)->moveCount > 1)))
1153 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
1155 // Decrease reduction if position is or has been on the PV
1156 // and node is not likely to fail low. (~3 Elo)
1161 // Decrease reduction if opponent's move count is high (~1 Elo)
1162 if ((ss-1)->moveCount > 7)
1165 // Increase reduction for cut nodes (~3 Elo)
1166 if (cutNode && move != ss->killers[0])
1169 // Increase reduction if ttMove is a capture (~3 Elo)
1173 // Decrease reduction for PvNodes based on depth
1175 r -= 1 + 15 / ( 3 + depth );
1177 // Increase reduction if next ply has a lot of fail high else reset count to 0
1178 if ((ss+1)->cutoffCnt > 3 && !PvNode)
1181 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1182 + (*contHist[0])[movedPiece][to_sq(move)]
1183 + (*contHist[1])[movedPiece][to_sq(move)]
1184 + (*contHist[3])[movedPiece][to_sq(move)]
1187 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1188 r -= ss->statScore / 15914;
1190 // In general we want to cap the LMR depth search at newDepth. But if reductions
1191 // are really negative and movecount is low, we allow this move to be searched
1192 // deeper than the first move (this may lead to hidden double extensions).
1193 int deeper = r >= -1 ? 0
1194 : moveCount <= 4 ? 2
1195 : PvNode || cutNode ? 1
1198 Depth d = std::clamp(newDepth - r, 1, newDepth + deeper);
1200 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1202 // If the son is reduced and fails high it will be re-searched at full depth
1203 doFullDepthSearch = value > alpha && d < newDepth;
1204 doDeeperSearch = value > (alpha + 78 + 11 * (newDepth - d));
1209 doFullDepthSearch = !PvNode || moveCount > 1;
1213 // Step 18. Full depth search when LMR is skipped or fails high
1214 if (doFullDepthSearch)
1216 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode);
1218 // If the move passed LMR update its stats
1221 int bonus = value > alpha ? stat_bonus(newDepth)
1222 : -stat_bonus(newDepth);
1227 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1231 // For PV nodes only, do a full PV search on the first move or after a fail
1232 // high (in the latter case search only if value < beta), otherwise let the
1233 // parent node fail low with value <= alpha and try another move.
1234 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1237 (ss+1)->pv[0] = MOVE_NONE;
1239 value = -search<PV>(pos, ss+1, -beta, -alpha,
1240 std::min(maxNextDepth, newDepth), false);
1243 // Step 19. Undo move
1244 pos.undo_move(move);
1246 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1248 // Step 20. Check for a new best move
1249 // Finished searching the move. If a stop occurred, the return value of
1250 // the search cannot be trusted, and we return immediately without
1251 // updating best move, PV and TT.
1252 if (Threads.stop.load(std::memory_order_relaxed))
1257 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1258 thisThread->rootMoves.end(), move);
1260 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1262 // PV move or new best move?
1263 if (moveCount == 1 || value > alpha)
1266 rm.selDepth = thisThread->selDepth;
1271 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1272 rm.pv.push_back(*m);
1274 // We record how often the best move has been changed in each iteration.
1275 // This information is used for time management. In MultiPV mode,
1276 // we must take care to only do this for the first PV line.
1278 && !thisThread->pvIdx)
1279 ++thisThread->bestMoveChanges;
1282 // All other moves but the PV are set to the lowest value: this
1283 // is not a problem when sorting because the sort is stable and the
1284 // move position in the list is preserved - just the PV is pushed up.
1285 rm.score = -VALUE_INFINITE;
1288 if (value > bestValue)
1296 if (PvNode && !rootNode) // Update pv even in fail-high case
1297 update_pv(ss->pv, move, (ss+1)->pv);
1299 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1303 // Reduce other moves if we have found at least one score improvement
1306 && beta < VALUE_KNOWN_WIN
1307 && alpha > -VALUE_KNOWN_WIN)
1315 assert(value >= beta); // Fail high
1324 // If the move is worse than some previously searched move, remember it to update its stats later
1325 if (move != bestMove)
1327 if (capture && captureCount < 32)
1328 capturesSearched[captureCount++] = move;
1330 else if (!capture && quietCount < 64)
1331 quietsSearched[quietCount++] = move;
1335 // The following condition would detect a stop only after move loop has been
1336 // completed. But in this case bestValue is valid because we have fully
1337 // searched our subtree, and we can anyhow save the result in TT.
1343 // Step 21. Check for mate and stalemate
1344 // All legal moves have been searched and if there are no legal moves, it
1345 // must be a mate or a stalemate. If we are in a singular extension search then
1346 // return a fail low score.
1348 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1351 bestValue = excludedMove ? alpha :
1352 ss->inCheck ? mated_in(ss->ply)
1355 // If there is a move which produces search value greater than alpha we update stats of searched moves
1357 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1358 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1360 // Bonus for prior countermove that caused the fail low
1361 else if ( (depth >= 4 || PvNode)
1364 //Assign extra bonus if current node is PvNode or cutNode
1365 //or fail low was really bad
1366 bool extraBonus = PvNode
1368 || bestValue < alpha - 70 * depth;
1370 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + extraBonus));
1374 bestValue = std::min(bestValue, maxValue);
1376 // If no good move is found and the previous position was ttPv, then the previous
1377 // opponent move is probably good and the new position is added to the search tree.
1378 if (bestValue <= alpha)
1379 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1381 // Write gathered information in transposition table
1382 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1383 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1384 bestValue >= beta ? BOUND_LOWER :
1385 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1386 depth, bestMove, ss->staticEval);
1388 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1394 // qsearch() is the quiescence search function, which is called by the main search
1395 // function with zero depth, or recursively with further decreasing depth per call.
1397 template <NodeType nodeType>
1398 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1400 static_assert(nodeType != Root);
1401 constexpr bool PvNode = nodeType == PV;
1403 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1404 assert(PvNode || (alpha == beta - 1));
1409 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1413 Move ttMove, move, bestMove;
1415 Value bestValue, value, ttValue, futilityValue, futilityBase;
1416 bool pvHit, givesCheck, capture;
1422 ss->pv[0] = MOVE_NONE;
1425 Thread* thisThread = pos.this_thread();
1426 bestMove = MOVE_NONE;
1427 ss->inCheck = pos.checkers();
1430 // Check for an immediate draw or maximum ply reached
1431 if ( pos.is_draw(ss->ply)
1432 || ss->ply >= MAX_PLY)
1433 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1435 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1437 // Decide whether or not to include checks: this fixes also the type of
1438 // TT entry depth that we are going to use. Note that in qsearch we use
1439 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1440 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1441 : DEPTH_QS_NO_CHECKS;
1442 // Transposition table lookup
1444 tte = TT.probe(posKey, ss->ttHit);
1445 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1446 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1447 pvHit = ss->ttHit && tte->is_pv();
1451 && tte->depth() >= ttDepth
1452 && ttValue != VALUE_NONE // Only in case of TT access race
1453 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1454 : (tte->bound() & BOUND_UPPER)))
1457 // Evaluate the position statically
1460 ss->staticEval = VALUE_NONE;
1461 bestValue = futilityBase = -VALUE_INFINITE;
1467 // Never assume anything about values stored in TT
1468 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1469 ss->staticEval = bestValue = evaluate(pos);
1471 // ttValue can be used as a better position evaluation (~7 Elo)
1472 if ( ttValue != VALUE_NONE
1473 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1474 bestValue = ttValue;
1477 // In case of null move search use previous static eval with a different sign
1478 ss->staticEval = bestValue =
1479 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1480 : -(ss-1)->staticEval;
1482 // Stand pat. Return immediately if static value is at least beta
1483 if (bestValue >= beta)
1485 // Save gathered info in transposition table
1487 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1488 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1493 if (PvNode && bestValue > alpha)
1496 futilityBase = bestValue + 118;
1499 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1500 nullptr , (ss-4)->continuationHistory,
1501 nullptr , (ss-6)->continuationHistory };
1503 // Initialize a MovePicker object for the current position, and prepare
1504 // to search the moves. Because the depth is <= 0 here, only captures,
1505 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1506 // will be generated.
1507 Square prevSq = to_sq((ss-1)->currentMove);
1508 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1509 &thisThread->captureHistory,
1513 int quietCheckEvasions = 0;
1515 // Loop through the moves until no moves remain or a beta cutoff occurs
1516 while ((move = mp.next_move()) != MOVE_NONE)
1518 assert(is_ok(move));
1520 // Check for legality
1521 if (!pos.legal(move))
1524 givesCheck = pos.gives_check(move);
1525 capture = pos.capture(move);
1529 // Futility pruning and moveCount pruning (~5 Elo)
1530 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1532 && to_sq(move) != prevSq
1533 && futilityBase > -VALUE_KNOWN_WIN
1534 && type_of(move) != PROMOTION)
1540 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1542 if (futilityValue <= alpha)
1544 bestValue = std::max(bestValue, futilityValue);
1548 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1550 bestValue = std::max(bestValue, futilityBase);
1555 // Do not search moves with negative SEE values (~5 Elo)
1556 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1557 && !pos.see_ge(move))
1560 // Speculative prefetch as early as possible
1561 prefetch(TT.first_entry(pos.key_after(move)));
1563 ss->currentMove = move;
1564 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1566 [pos.moved_piece(move)]
1569 // Continuation history based pruning (~2 Elo)
1571 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1572 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold
1573 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
1576 // movecount pruning for quiet check evasions
1577 if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
1578 && quietCheckEvasions > 1
1583 quietCheckEvasions += !capture && ss->inCheck;
1585 // Make and search the move
1586 pos.do_move(move, st, givesCheck);
1587 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1588 pos.undo_move(move);
1590 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1592 // Check for a new best move
1593 if (value > bestValue)
1601 if (PvNode) // Update pv even in fail-high case
1602 update_pv(ss->pv, move, (ss+1)->pv);
1604 if (PvNode && value < beta) // Update alpha here!
1612 // All legal moves have been searched. A special case: if we're in check
1613 // and no legal moves were found, it is checkmate.
1614 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1616 assert(!MoveList<LEGAL>(pos).size());
1618 return mated_in(ss->ply); // Plies to mate from the root
1621 // Save gathered info in transposition table
1622 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1623 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1624 ttDepth, bestMove, ss->staticEval);
1626 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1632 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1633 // "plies to mate from the current position". Standard scores are unchanged.
1634 // The function is called before storing a value in the transposition table.
1636 Value value_to_tt(Value v, int ply) {
1638 assert(v != VALUE_NONE);
1640 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1641 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1645 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1646 // from the transposition table (which refers to the plies to mate/be mated from
1647 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1648 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1649 // and the graph history interaction, we return an optimal TB score instead.
1651 Value value_from_tt(Value v, int ply, int r50c) {
1653 if (v == VALUE_NONE)
1656 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1658 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1659 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1664 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1666 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1667 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1676 // update_pv() adds current move and appends child pv[]
1678 void update_pv(Move* pv, Move move, Move* childPv) {
1680 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1686 // update_all_stats() updates stats at the end of search() when a bestMove is found
1688 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1689 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1692 Color us = pos.side_to_move();
1693 Thread* thisThread = pos.this_thread();
1694 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1695 Piece moved_piece = pos.moved_piece(bestMove);
1696 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1698 bonus1 = stat_bonus(depth + 1);
1699 bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
1700 : stat_bonus(depth); // smaller bonus
1702 if (!pos.capture(bestMove))
1704 // Increase stats for the best move in case it was a quiet move
1705 update_quiet_stats(pos, ss, bestMove, bonus2);
1707 // Decrease stats for all non-best quiet moves
1708 for (int i = 0; i < quietCount; ++i)
1710 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1711 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1715 // Increase stats for the best move in case it was a capture move
1716 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1718 // Extra penalty for a quiet early move that was not a TT move or
1719 // main killer move in previous ply when it gets refuted.
1720 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1721 && !pos.captured_piece())
1722 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1724 // Decrease stats for all non-best capture moves
1725 for (int i = 0; i < captureCount; ++i)
1727 moved_piece = pos.moved_piece(capturesSearched[i]);
1728 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1729 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1734 // update_continuation_histories() updates histories of the move pairs formed
1735 // by moves at ply -1, -2, -4, and -6 with current move.
1737 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1739 for (int i : {1, 2, 4, 6})
1741 // Only update first 2 continuation histories if we are in check
1742 if (ss->inCheck && i > 2)
1744 if (is_ok((ss-i)->currentMove))
1745 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1750 // update_quiet_stats() updates move sorting heuristics
1752 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1755 if (ss->killers[0] != move)
1757 ss->killers[1] = ss->killers[0];
1758 ss->killers[0] = move;
1761 Color us = pos.side_to_move();
1762 Thread* thisThread = pos.this_thread();
1763 thisThread->mainHistory[us][from_to(move)] << bonus;
1764 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1766 // Update countermove history
1767 if (is_ok((ss-1)->currentMove))
1769 Square prevSq = to_sq((ss-1)->currentMove);
1770 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1774 // When playing with strength handicap, choose best move among a set of RootMoves
1775 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1777 Move Skill::pick_best(size_t multiPV) {
1779 const RootMoves& rootMoves = Threads.main()->rootMoves;
1780 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1782 // RootMoves are already sorted by score in descending order
1783 Value topScore = rootMoves[0].score;
1784 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1785 int maxScore = -VALUE_INFINITE;
1786 double weakness = 120 - 2 * level;
1788 // Choose best move. For each move score we add two terms, both dependent on
1789 // weakness. One is deterministic and bigger for weaker levels, and one is
1790 // random. Then we choose the move with the resulting highest score.
1791 for (size_t i = 0; i < multiPV; ++i)
1793 // This is our magic formula
1794 int push = int(( weakness * int(topScore - rootMoves[i].score)
1795 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1797 if (rootMoves[i].score + push >= maxScore)
1799 maxScore = rootMoves[i].score + push;
1800 best = rootMoves[i].pv[0];
1810 /// MainThread::check_time() is used to print debug info and, more importantly,
1811 /// to detect when we are out of available time and thus stop the search.
1813 void MainThread::check_time() {
1818 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1819 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1821 static TimePoint lastInfoTime = now();
1823 TimePoint elapsed = Time.elapsed();
1824 TimePoint tick = Limits.startTime + elapsed;
1826 if (tick - lastInfoTime >= 1000)
1828 lastInfoTime = tick;
1832 // We should not stop pondering until told so by the GUI
1836 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1837 || (Limits.movetime && elapsed >= Limits.movetime)
1838 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1839 Threads.stop = true;
1843 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1844 /// that all (if any) unsearched PV lines are sent using a previous search score.
1846 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1848 std::stringstream ss;
1849 TimePoint elapsed = Time.elapsed() + 1;
1850 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1851 size_t pvIdx = pos.this_thread()->pvIdx;
1852 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1853 uint64_t nodesSearched = Threads.nodes_searched();
1854 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1856 for (size_t i = 0; i < multiPV; ++i)
1858 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1860 if (depth == 1 && !updated && i > 0)
1863 Depth d = updated ? depth : std::max(1, depth - 1);
1864 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1866 if (v == -VALUE_INFINITE)
1869 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1870 v = tb ? rootMoves[i].tbScore : v;
1872 if (ss.rdbuf()->in_avail()) // Not at first line
1877 << " seldepth " << rootMoves[i].selDepth
1878 << " multipv " << i + 1
1879 << " score " << UCI::value(v);
1881 if (Options["UCI_ShowWDL"])
1882 ss << UCI::wdl(v, pos.game_ply());
1884 if (!tb && i == pvIdx)
1885 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1887 ss << " nodes " << nodesSearched
1888 << " nps " << nodesSearched * 1000 / elapsed;
1890 if (elapsed > 1000) // Earlier makes little sense
1891 ss << " hashfull " << TT.hashfull();
1893 ss << " tbhits " << tbHits
1894 << " time " << elapsed
1897 for (Move m : rootMoves[i].pv)
1898 ss << " " << UCI::move(m, pos.is_chess960());
1905 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1906 /// before exiting the search, for instance, in case we stop the search during a
1907 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1908 /// otherwise in case of 'ponder on' we have nothing to think on.
1910 bool RootMove::extract_ponder_from_tt(Position& pos) {
1913 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1917 assert(pv.size() == 1);
1919 if (pv[0] == MOVE_NONE)
1922 pos.do_move(pv[0], st);
1923 TTEntry* tte = TT.probe(pos.key(), ttHit);
1927 Move m = tte->move(); // Local copy to be SMP safe
1928 if (MoveList<LEGAL>(pos).contains(m))
1932 pos.undo_move(pv[0]);
1933 return pv.size() > 1;
1936 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1939 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1940 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1941 Cardinality = int(Options["SyzygyProbeLimit"]);
1942 bool dtz_available = true;
1944 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1945 // ProbeDepth == DEPTH_ZERO
1946 if (Cardinality > MaxCardinality)
1948 Cardinality = MaxCardinality;
1952 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1954 // Rank moves using DTZ tables
1955 RootInTB = root_probe(pos, rootMoves);
1959 // DTZ tables are missing; try to rank moves using WDL tables
1960 dtz_available = false;
1961 RootInTB = root_probe_wdl(pos, rootMoves);
1967 // Sort moves according to TB rank
1968 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1969 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1971 // Probe during search only if DTZ is not available and we are winning
1972 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1977 // Clean up if root_probe() and root_probe_wdl() have failed
1978 for (auto& m : rootMoves)
1983 } // namespace Stockfish